Review

Natural and Synthetic Polymer Fillers for Applications in 3D

Printing—FDM Technology Area
Bogna Sztorch 1 , Dariusz Brzakalski
˛ 2 , Daria Pakuła 2 , Miłosz Frydrych 2 , Zdeno Špitalský 3 and
Robert E. Przekop *1,

1 Centre for Advanced Technologies, Adam Mickiewicz University in Poznań, 10 Uniwersytetu Poznańskiego,
61-614 Poznań, Poland
2 Faculty of Chemistry, Adam Mickiewicz University in Poznań, 8 Uniwersytetu Poznańskiego,
61-614 Poznań, Poland
3 Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 845 41 Bratislava, Slovakia
* Correspondence: rprzekop@amu.edu.pl or r.przekop@gmail.com

Abstract: This publication summarises the current state of knowledge and technology on the possibil-
ities and limitations of using mineral and synthetic fillers in the field of 3D printing of thermoplastics.
FDM technology can be perceived as a miniaturised variation of conventional extrusion processing
(a microextrusion process). However, scaling the process down has an undoubtful drawback of
significantly reducing the extrudate diameter (often by a factor of ≈20–30). Therefore, the results
produced under conventional extrusion processing cannot be simply translated to processes run with
the application of FDM technology. With that in mind, discussing the latest findings in composite
materials preparation and application in FDM 3D printing was necessary.

Keywords: FDM; filament; thermoplastic; composite; additive manufacturing; particles; nanofiller;

fibers; fibres; micro extrusion
Citation: Sztorch, B.; Brzakalski,
˛ D.;
Pakuła, D.; Frydrych, M.; Špitalský,
Z.; Przekop, R.E. Natural and
Synthetic Polymer Fillers for 1. Short Introduction about 3D Printing by FDM/FFF Technique and Role of Fillers
Applications in 3D Printing—FDM
The principle of additive technologies involves applying the material (polymers:
Technology Area. Solids 2022, 3,
resin or thermoplastic polymers; ceramic paste, metal) one layer at a time until a three-
508–548. https://doi.org/10.3390/
dimensional object, according to the computer-designed model, is obtained. The beginning
solids3030034
of the additive prototyping technology dates back to the second half of the 1980s, which
Academic Editor: Dino Leporini resulted in the world’s first patent on stereolithography (SLA) [1]. The fused deposition
modeling (FDM) method was developed in 1989 by the Israeli–American company Strata-
Received: 5 August 2022
sys, which is among the largest players in the 3D printing market. The FDM technology
Accepted: 26 August 2022
Published: 16 September 2022
itself accounts for about 40% of the world market. The technology is dedicated to low-
volume production; its advantage over traditional techniques such as injection molding or
Publisher’s Note: MDPI stays neutral extrusion is the reduced waste index, lower energy consumption [2,3], and the possibility
with regard to jurisdictional claims in of obtaining complicated structures without the need to use additional, expensive tools [4].
published maps and institutional affil-
In 2021, the global market for additive technologies was estimated at USD 13.84 billion.
iations.
The overall growth of the 3DP market was influenced by the global COVID-19 pandemic,
logistics problems and supply chain suspension. This resulted in delays or complete discon-
tinuation of production in many companies in Europe and the world. Current projections
Copyright: © 2022 by the authors.
are a cumulative annual growth rate (CAGR) of 21% between 2022 and 2030 [5]. The largest
Licensee MDPI, Basel, Switzerland.
global producers and distributors in the additive techniques industry are North America
This article is an open access article
(USA, Canada, Mexico), Europe (Great Britain, Germany, France, Italy, Spain), Asia (China,
distributed under the terms and Japan, India), and South Africa. Additive manufacturing methods using the FDM technique
conditions of the Creative Commons bring significant benefits in rapid prototyping, structural design and object modeling. The
Attribution (CC BY) license (https:// interest in the development of additive technologies is related to the growing demand for
creativecommons.org/licenses/by/ new prototype applications in various industries such as medicine [6] and biomedicine [7],
4.0/). aerospace [8], automotive [9], education [10], and rapid prototyping [11].

Solids 2022, 3, 508–548. https://doi.org/10.3390/solids3030034 https://www.mdpi.com/journal/solids

Solids 2022, 3 509

FDM is one of the simplest and most widely used 3D printing technologies. The
method uses thermoplastic polymers, heated in the printhead above their melting point
and then extruded from a nozzle on a movable printer table layer by the pressure of
the filament fed into the printhead by layer [12]. FDM (fused filament fabrication—FFF)
technology is characterised by low complexity and the easy availability of raw materials
(filaments). The most commonly used polymers in the FDM technique include: polylactide
(PLA) [13,14], glycol-modified poly (ethylene terephthalate) (PET-G) [15], amorphous
acrylonitrile-butadiene-styrene polymer (ABS) [14,16,17], polyamide 12 (PA12) [18] and
others such as polystyrene (PS), polyethylene (PE), poly(ethylene terephthalate) (PET),
polycarbonate (PC), polycaprolactone (PCL), polyetheretherketone (PEEK), nylon and
thermoplastic urethane (TPU) [19].
Various auxiliary agents can be used in FDM technology to improve materials’ pro-
cessing and functional properties. The most frequently used functional additives include:
fillers, plasticisers, pigments, lubricants, flame retardants, stabilisers and chemical mod-
ifiers of material properties. Introducing these additives into the polymer matrix pos-
itively affects their physicochemical (thermal, electrical, rheological, hydrophobic) and
mechanical properties, as well as improves the aesthetic values. Using natural, mineral, or
synthetic filler often reduces the price of end products, depending on the material used (see
Section 3). It often positively affects mechanical and thermal properties, as well as thermal
or electrical conductivity/resistivity, depending on the filler choice and target function-
ality [20]. The fillers used in the FDM technology can be divided into: carbon materials
(carbon black, graphene, nanotubes, carbon fibres), ceramic and metal powders, glassy and
fibrous fillers (renewable raw materials such as hemp, kenaf, flax, jute [21], cellulose [22],
bamboo, coconut [23], and others) mainly used to reinforce the structure and improve
mechanical properties; mineral ones (titanium white, mica, metal powders, graphite, talc,
chalk, diatomaceous earth), characterised by thermal, chemical and UV resistance; and
biofillers (coffee grounds [24], wood flour [25]).

2. Classification of the Fillers

Fillers may be divided into classes based on several criteria, including:
- Origin;
- Chemical composition;
- Shape;
- Size and aspect ratio;
- The effects they exert on the matrix material (intended function) or their price relative
to the matrix polymer.
Each of these classification methods will be briefly discussed hereinafter.
Based on the origin, we can divide fillers into natural and synthetic ones (Figure 1) [26].
Natural fillers are obtained from environmental resources (raw materials) and suitable for
use after the necessary steps of preparation, such as washing, milling, sieving, drying, etc.
Examples are bentonite, montmorillonite, and other types of clay. Among natural fillers,
biofillers take a special place, as these are the products of (micro)organisms’ bioprocesses
and are considered a more renewable resource [27]. Diatomite earth may be an example
of, from a chemical point of view, an inorganic (mineral) biofiller (biosilica doped with
compounds of several other elements). Meanwhile, plant- (kenaf, ramie) or animal-derived
fibres (cotton, silk) are organic biofiller. Synthetic fillers are products of either material
synthesis or heavy chemical processing of raw materials, such as metal ores or biomaterials.
The first group is represented by precipitated calcium carbonate or precipitated silica,
while the latter is by TiO2 produced via the sulfate process or synthetic silk, e.g., from the
viscose method.
Solids 2022,33, FOR PEER REVIEW
Solids2022, 5103

Figure1.1.The
Figure Thedivision
division of
offillers
fillersaccording
accordingto
tothe
theorigin
originof
ofthe
thematerial
material(thick
(thickframe)
frame)with
withexamples
examples
(thin
(thinframe)
frame)and
andchemical
chemicalformulas
formulas(dotted
(dottedline).
line).

Based
Basedon onthethecomposition,
composition,fillers
fillerscancan
be be
divided
dividedintointo
organic and and
organic inorganic typestypes
inorganic [28].
As it may
[28]. As itbemay
perceived as intuitive,
be perceived inorganic
as intuitive, fillers arefillers
inorganic composed of the subgroups
are composed of oxides
of the subgroups
(TiO 2 , Fe 3 O4 ), hydroxides (Al(OH) 3 , Mg(OH) 2 ), salts (CaCO
of oxides (TiO2, Fe3O4), hydroxides (Al(OH)3, Mg(OH)2), salts (CaCO3, BaSO4, BaTiO3 , BaSO 4 , BaTiO 3 ), metals
3),
(boron, steel), and silicates (talc, mica). On the other hand, organic ones
metals (boron, steel), and silicates (talc, mica). On the other hand, organic ones are rep- are represented
by naturalby(cellulose,
resented kenaf, ramie,
natural (cellulose, kenaf,silk) polymers,
ramie, of which of
silk) polymers, many are many
which characterised by
are charac-
intrinsic fibrillar structure, and synthetic ones, such as polyamide or
terised by intrinsic fibrillar structure, and synthetic ones, such as polyamide or polyester. polyester. They
are used to produce fibres and sheets for fabricating fibre-reinforced thermoplastics or
They are used to produce fibres and sheets for fabricating fibre-reinforced thermoplastics
laminated objects. Silicone resins also fall into this group, allowing for the control of
or laminated objects. Silicone resins also fall into this group, allowing for the control of
the dynamic mechanical properties of composites, such as loss modulus or loss factor.
the dynamic mechanical properties of composites, such as loss modulus or loss factor.
Interestingly enough, carbon fillers (carbon fibres, graphite fibres and flakes, CNTs) are also
Interestingly enough, carbon fillers (carbon fibres, graphite fibres and flakes, CNTs) are
considered organic fillers in composites engineering, despite being considered inorganic
also considered organic fillers in composites engineering, despite being considered in-
carbon materials in some other fields [29].
organic carbon materials in some other fields [29].
Depending on the size of the filler, fillers may be divided as presented in Figure 2,
Depending on the size of the filler, fillers may be divided as presented in Figure 2,
which comes from the classification by Bayne and Heymann for dental composite sys-
which comes from the classification by Bayne and Heymann for dental composite sys-
tems [30].
tems [30].
Wypych provided a detailed breakdown of the number of fillers falling into each size
category [31]. However, it should be noted that virtually no filler of practical application
is of one specific size; therefore, in the plastics industry, a common characteristic of a
given filler is its size distribution, which quantitatively represents the share of particles
of each size in the material’s sample [32]. Another common practice is to provide the
critical particle size diameters, such as d50 , where particle size is sorted in ascending order
and the value is given when a cumulative percentage of the sample reaches 50%; in other
words, it is a median of the particle size. In the same manner, d10 or d90 parameters are
usually provided. However, particle size may be represented by a single (median) or a set
of individual values, as mentioned above, only in a case where the filler particles have even
 Solids 2022, 3 511

Solids 2022, 3, FOR PEER REVIEW 4

shapes, such as spherical or cubical. In other cases, more parameters are required. This
leads to another criterion of classification, which is the filler shape.

Figure 2. Filler size categories by Bayne and Heymann.

Figure 2. Filler size categories by Bayne and Heymann.
Considering that the filler is not completely random in shape (which is one of the
Wypych
possible shapesprovided a detailed
per se), fillers may bebreakdown of the
classified into numberthat
subgroups of fillers
define falling into
different each
shapes,
sizeshare
yet category [31]. However,
a similar aspect ratio it (AR),
shouldwhich
be noted that virtually
is critical from theno fillerofofview
point practical
of the appli-
me-
cation is of one specific size; therefore, in the plastics industry, a common
chanical reinforcing action, light reflectance/scattering properties, anisotropy effects, or characteristic of
a given filler is its size distribution, which quantitatively represents
melt rheology they exert on the composites they will produce among introduction into the the share of particles
of eachpolymer.
matrix size in On the this
material’s
basis, we sample [32]. Another
can distinguish common filler
the following practice
shapesis to provide
(Figure the
3) [28]:
-critical particle size diameters, such as d 50, where particle size is sorted in ascending order
Particulate: spherical, cubical, and others of AR ≈1 (calcite, spherical silica);
-and the value
Blocks is given
of AR up towhen a cumulative percentage of the sample reaches 50%; in other
≈4 (barite);
-words,
Plates of AR 4–30 (kaolin,particle
it is a median of the talc); size. In the same manner, d10 or d90 parameters are
-usually
Flakes of AR 50–200 (aluminium,size
provided. However, particle may
mica, be represented
graphite, by a single (median) or a set
montmorillonite);
-of individual
Nanosheets values,
of ARas> mentioned
1000 (boronabove,
nitride,only in a case
graphene) where the filler particles have
[33,34];
-evenFibres,
shapes, such as spherical
nanotubes, nanowires, or cubical.
nanorods, In nanowhiskers;
other cases, more AR parameters
of 20–1000+ are required.
(glass fibres,
This basalt
leads tofibres,
anothercarbon
criterionnanotubes
of classification, which is the filler shape.
Ag nanowires, TiO2 nanorods, cellulose
Considering that
nanowhiskers) the filler is not completely random in shape (which is one of the
[35];
Solids 2022, 3, FOR PEER REVIEW -possible
Other, shapes
complex pershapes,
se), fillers may be
including classified
porous into subgroups
and mesoporous that define
powders, different
urchin-like 5hy-
shapes, yet share apowder,
droxyapatite similarnanocluster
aspect ratio powders,
(AR), which is critical from
tetrapod-like the point
whiskers, of viewfibres,
core-sheath of the
mechanical reinforcing
or microcapsules action, light reflectance/scattering properties, anisotropy effects,
[36].
or melt rheology they exert on the composites they will produce among introduction into
the matrix polymer. On this basis, we can distinguish the following filler shapes (Figure
3) [28]:
- Particulate: spherical, cubical, and others of AR ≈1 (calcite, spherical silica);
- Blocks of AR up to ≈4 (barite);
- Plates of AR 4–30 (kaolin, talc);
- Flakes of AR 50–200 (aluminium, mica, graphite, montmorillonite);
- Nanosheets of AR > 1000 (boron nitride, graphene) [33,34];
- Fibres, nanotubes, nanowires, nanorods, nanowhiskers; AR of 20–1000+ (glass fibres,
basalt fibres, carbon nanotubes Ag nanowires, TiO2 nanorods, cellulose
nanowhiskers) [35];
- Other, complex shapes, including porous and mesoporous powders, urchin-like
hydroxyapatite powder, nanocluster powders, tetrapod-like whiskers, core-sheath
fibres, or microcapsules [36].

Figure 3. Different shapes of the filler applied in polymer composite design and fabrication.
Figure 3. Different shapes of the filler applied in polymer composite design and fabrication.

The filler shape may be due to its intrinsic characteristics, where filler processing
and pretreatment covers only its purification and milling/deagglomeration (natural fill-
ers, such as talc, plant fibres), a result of the heavy processing and chemical treatment of
natural material to structurise it in order to obtain the desired properties, such as particle
Solids 2022, 3 512

The filler shape may be due to its intrinsic characteristics, where filler processing and
pretreatment covers only its purification and milling/deagglomeration (natural fillers, such
as talc, plant fibres), a result of the heavy processing and chemical treatment of natural
material to structurise it in order to obtain the desired properties, such as particle size,
aspect ratio, or surface physicochemistry (nanoclays, cellulose (nano)fibres). This is a
somewhat controllable outcome, of processing conditions when preparing synthetic fillers
(TiO2 nanoparticles via sulphate or chloride process, fumed silica), or an effect of thorough
nanoengineering design (nanowhiskers, carbon nanotubes).
Fillers may serve one of several roles in the composite they are applied in, and always
change several characteristics of the polymer matrix they have been introduced into. The
common functions of the fillers are (Figure 4) [31,33,37]:
- Mechanical reinforcement of the polymer matrix (increasing Young’s modulus, tensile
strength, toughness, impact resistance, abrasion resistance, hardness);
- Improving thermal and thermomechanical behaviour (increasing glass transition
temperature and heat deflection/softening temperature, storage modulus or damping
factor, reducing thermal conductivity);
- Reducing heat expansion coefficient or warping effect;
- Improving insulating properties (increasing breakdown strength or modifying dielec-
tric constant), or conductive properties (conductive fillers);
- Modifying gas permeability/barrier properties;
- Inducing polymer crystallisation;
- Reducing polymer flammability;
- Modifying melt rheology;
- Modifying surface properties (roughness, hydrophobicity);
Solids 2022, 3, FOR PEER REVIEW - Improving UV and weathering resistance; 6
- Adding colour or opacity.

Figure 4. Different functions of fillers in polymer composites.

Figure 4. Different functions of fillers in polymer composites.
Fillers designed to fulfil the above-mentioned roles are often called functional fillers.
Finally, a critical aspect of the filler application is its price in relation to the matrix poly-
3. Statistical Presentation of the Application of Fillers in Materials for 3D Printing
mer and the overall process economy when producing a chosen composite. If the main
Based on the SCOPUS Database
application thereof is to reduce the fabrication price of the resulting product or reduce the
The number
polymer of publications
consumption on fillers
and all the other in are
effects plastics for 3Dsecondary,
considered printing has
suchgrown inten-
material will
sively in recent years. Based on
be referred to as an extender [31]. Scopus database analysis, one can see a rapid growth in
2016 (Figure 5). In the years 2004–2015, the number of publications found with the key-
words ‘filler’, ‘fillers’, and ‘3D printing’ is 20, while in 2016 alone this number is 23, which
shows a breakthrough moment for research related to fillers in 3D printing materials.
This growing trend is related to the development of additive technologies, and thus the
search for new solutions that will guarantee the improvement of the physicochemical
properties of materials while reducing production costs. The total number of scientific
articles in the period 2004–2022 with the keywords ‘filler’, ‘3D printing’ was 865 (in-
Solids 2022, 3 513
Figure 4. Different functions of fillers in polymer composites.

3. Statistical Presentation of the Application of Fillers in Materials for 3D Printing

3. Statistical
Based on the Presentation of the Application of Fillers in Materials for 3D Printing
SCOPUS Database
Based on the SCOPUS Database
The number of publications on fillers in plastics for 3D printing has grown inten-
sivelyThe numberyears.
in recent of publications on fillers
Based on Scopus in plastics
database for 3D one
analysis, printing has agrown
can see rapid intensively
growth in
2016 (Figure 5). In the years 2004–2015, the number of publications foundgrowth
in recent years. Based on Scopus database analysis, one can see a rapid with thein key-
2016
(Figure 5). In the years 2004–2015, the number of publications found with the keywords
words ‘filler’, ‘fillers’, and ‘3D printing’ is 20, while in 2016 alone this number is 23, which
‘filler’, ‘fillers’, and ‘3D printing’ is 20, while in 2016 alone this number is 23, which shows a
shows a breakthrough moment for research related to fillers in 3D printing materials.
breakthrough moment for research related to fillers in 3D printing materials. This growing
This growing trend is related to the development of additive technologies, and thus the
trend is related to the development of additive technologies, and thus the search for
search for new solutions that will guarantee the improvement of the physicochemical
new solutions that will guarantee the improvement of the physicochemical properties of
properties of materials while reducing production costs. The total number of scientific
materials while reducing production costs. The total number of scientific articles in the
articles in the period 2004–2022 with the keywords ‘filler’, ‘3D printing’ was 865 (in-
period 2004–2022 with the keywords ‘filler’, ‘3D printing’ was 865 (including ‘filler’ and
cluding ‘filler’ and ‘FDM’-214 articles; ‘filler’ and ‘FFF’-80 articles).
‘FDM’-214 articles; ‘filler’ and ‘FFF’-80 articles).

Figure 5.
Figure Scopus key
5. Scopus key words:
words: filler,
filler, fillers,
fillers, 3D
3D printing;
printing; natural
natural filler,
filler, 3D
3D printing.
printing.

For both
For both synthetic
synthetic and
and natural
natural fillers,
fillers, the
the number
number of
of scientific
scientific articles
articles increases
increases after
after
2016. In
2016. In 2020
2020 and
and 2021,
2021, the
the sum
sum of
of publications
publicationsaccording
accordingto
to the
the keywords
keywords‘natural
‘naturalfiller’
filler’
with ‘3D printing’, and ‘mineral filler’ with ‘3D printing’ is 54, while for the keywords
with ‘3D printing’, and ‘mineral filler’ with ‘3D printing’ is 54, while for the keywords
‘synthetic filler’ and ‘3D printing’ the total is 14. From the obtained data, it can be seen
‘synthetic filler’ and ‘3D printing’ the total is 14. From the obtained data, it can be seen
that the number of articles describing natural fillers is 285% higher than for synthetic fillers.
that the number of articles describing natural fillers is 285% higher than for synthetic
This shows the current trends in both science and economy, which focus on sustainable
development, green economy, and green chemistry. As a result, looking for solutions based,
among others, on waste- or by-products that can be used as natural fillers for plastics is a
continuous trend.

4. Mineral and Natural Fillers in 3D Printing Thermoplastics

4.1. Diatoms
One group of natural fillers that are still gaining in popularity are diatoms. They are
single-celled eukaryotes belonging to one of the most numerous groups of algae. Their
silica shells make them peculiar, having unique ordered shapes impossible to recreate
with the help of structures produced by engineering methods [38]. In addition, they are
characterised by high porosity, non-flammability, low thermal conductivity, odorlessness,
low moisture level, and resistance to acids [39].
The first literature reports on the use of diatoms in the FDM printing technique
are attributed to Aggarwal et al. [40], who used them as a filler for PLA matrix. The
obtained results showed a slight deterioration in the mechanical properties; however, the
consumption of PLA was reduced by approx. 10%, while using a fraction of the production
costs. The phenomenon of matrix nucleation in the presence of diatomaceous earth was
also characterised. Moreover, by designing and obtaining spatial structures containing
diatomaceous earth protruding from the printed object, it allowed the immobilisation of
chemicals and imparting of surface properties (e.g., antibacterial, antiviral).
Solids 2022, 3 514

Examples of research using diatoms as a filler in 3D printing are the works of R.

Han et al. [41,42]. These works are focused on finding applications of composites consist-
ing of poly(DL-lactide-co-glycolide), PDLGA, with diatoms in tissue engineering. The
tests showed an increase in the compressive strength of the obtained material. Addition-
ally, the degradation of the material was compared, which showed that its modification
with diatoms improves the durability of the composite. Thermal degradation also indi-
cated a similar relationship-diatoms inhibited the thermal degradation of PDLGA. These
studies confirmed the potential application of the composite as a bone-filling additive in
3D-printed scaffolds.
Dobrosielska et al., in her work, focused on the influence of diatom fraction size on
the mechanical and functional properties of PLA composites intended for and tested under
3D printing conditions [43]. The obtained composites, as in the previous works, were
characterised by improved thermal stability and increased mechanical strength. It was also
found that the addition of a filler to the material causes the material to have higher melt
flow. Due to the fraction used, different properties of the final composite were obtained.
The diatom fraction <40 µm had higher hydrophobicity, and the conditioning of samples
in water resulted in an improvement in mechanical and rheological parameters. Higher
mesh fraction showed greater thermal stability and tensile strength. Such composites are
an emerging subject in the area of biocomposites material research and, therefore, there are
not many reports in the literature at the moment.

4.2. Calcite
Calcite is the most thermodynamically stable anhydrous crystalline polymorph of
calcium carbonate [44]. Due to its properties such as biological passivity, low toxicity, and
good dispersion in the polymer matrix, CaCO3 has been used in composites [45,46].
The use of calcite as a polymer filler for 3D printing is known in the literature. An
example of such work is [47], who used CaCO3 and Ca3 (PO4 )2 as an addition to the com-
posite scaffold structures produced by the FDM technique, in the field of tissue engineering.
The obtained results show that with proper loading, the rate of polymer degradation can
be controlled, which is important when using it during bone tissue regeneration.
CaCO3 in a polypropylene matrix has been studied exhaustively. The filler imparts
a significant reduction in the shrinkage of the printed material [48]. Other studies on the
same matrix focused on the use of different anhydrous crystalline CaCO3 polymorphs and
on determining their properties [49]. It was shown that in the polypropylene matrix, only
the addition of aragonite showed an improvement in mechanical properties, that is, tensile
stress, by 12%.
This is in line with other data from the literature, which suggest that aragonite has a
better effect on mechanical properties as a filler in polyvinyl chloride or polypropylene than
calcite [50]. The remaining types of calcium carbonate either did not cause major changes in
properties or worsened them. The research [51,52] showed an increase in Young’s modulus,
tensile strength, and tensile yield strength when using calcite nanoparticles in a PP matrix.
An example of research examining the influence of calcite on the tribological properties
of polymers is the work of Sudeepan, J. et.al. [53], where ABS was used as the matrix. It
has been shown that the addition of 5% CaCO3 while adjusting the appropriate process
parameters (35N load, 120 rpm speed) improves the tribological properties by more than
70% compared to the reference sample.
The work of Kotlarz et al. [54] focuses on the use of three-component composites
consisting of a PLGA matrix, poly(2-oxazoline) amphiphilic polymer, and CaCO3 filler as
scaffolds. Homogeneous printable composites were obtained, which contained as much
as 30% of the filler in the composition. CaCO3 and the amphiphilic polymer showed
a synergistic effect on the wettability of the produced scaffolds. Moreover, the mineral
addition in the degradation process caused the formation of micropores, which accelerated
the degradation of the material. The novelty is that the obtained scaffolds with potential
application in tissue engineering are manufactured in a one-step process.
Solids 2022, 3 515

Composites used in additive techniques consisting of PLA and CaCO3 matrix were
the subject of several scientific papers [55–58]. Mostly, the applications were limited as
a substitution of complex bone defects or scaffolds in tissue engineering [59–61]. The
obtained composites were characterised by the easier thermoforming of the material, a
change in surface properties and in mechanical parameters compared to neat PLA-they
were more brittle and stiff.

4.3. Natural Fibres

4.3.1. General Considerations
Natural fibres’ use in additive manufacturing technologies for green composites has
been discussed previously in several other reviews, including recent ones, e.g., of Wang [62],
Shanmugam [63], Subramaniyan [64], Muthe [65], Khalifa [66], Suárez [67], Mazzanti [4],
Rajak [68], Mohan [69], Ilyas [70], Badgar [71], Wasti [72], or chapters by Velu et al. [73],
Murawski et al. [74], and Gharde et al. [75]. Some reviews are specifically focused on
theb3D printing of natural fibre-reinforced composites, that is, by Le Duigou [76], Zhao [77],
Balla [78], Li [79], Tonk [80], Deb [81], Ganguly [82], Kopparthy [83], Mangat [84], Sekar [85],
Mazzanti [86], Suriani [87], Shahinur [88], Rajendran Royan [89], Ahmed [90], Aida [91],
Devarajan [92], Lee [21], and Rajeshkumar [93]. Therefore, due to the detailed exploration
of this subject, only the most recent works will be discussed to provide an up-to-date review
on natural fibres in FDM.
Natural fibres are commonly derived from plant matter, including wood, straws,
husks, kernels, and various weeds, and falling into a group of lignocellulosic biomass
exhibiting a significant variability in terms of structure and quantitative composition of
cellulose, hemicellulose and lignin compounds comprising this group [94]. Popular natural
fibres commonly used or tested as fillers are, among others, flax, jute, kenaf, bamboo, coir,
cotton, harakeke or hemp, in addition to various wood-derived fibres. The present results
show that the application of natural fibres as fillers for FDM printing technology provides
satisfactory, reproducible results, although long-term part performance is still a challenge.
Mastura et al. have performed computational analysis by integrating the Analytic
Hierarchy Process and Analytic Network Process, comparing several natural fibres and
assessing them on the basis of criteria such as production rate, raw cost, density, mechanical
parameters, or cellulose/hemicellulose/Lignin content [95]. In conclusion, by the model
applied for the study, flax was found to be the most promising material, followed by kenaf
and sisal.

4.3.2. PLA-Based Composites

Duigou, Fruleux et al. investigated the water- and moisture-induced shape-change
properties of PLA-, PBS-, PBSA-, and PBAT-based composites reinforced with continuous
flax fibre, showing an example of novel Hygromorth biocomposites (HBC) [96,97]. The
authors reported a relationship between matrix stiffness and composite hygroscopic strain
to be a linear decreasing trend. The shape change was an effect of the filler fibre expansion
caused by water uptake. Duigui et al. also provided optimisation study on 3D printing
parameters for continuous flax fibre-reinforced PLA composites, including layer height,
interfilament distance, number of trips, number of layers, and their relationship with the
final printing accuracy, mechanical parameters, and microstructure [98].
Stoof et al. tested hemp and harakeke fibres as reinforcing fillers for PLA composites
at 0–30% loading [99]. Both types of fibres were alkali-treated at elevated temperatures
of 160 ◦ C and 170 ◦ C, respectively (for harakeke, with an addition of sodium sulphite).
Both fillers’ addition resulted with decreasing tensile strength at higher loadings, especially
hemp, and increasing Young’s modulus. Harakeke was found to be a more effective
reinforcing material; however, for both types of fibres, high filler loading resulted in
extrusion and 3D printing issues, such as die blockage, printed object defects, and poor
layer-to-layer adhesion.
Solids 2022, 3 516

Yaguchi et al. developed a machine setup for the direct application of continuous
natural fibres under conditions of FDM printing, allowing for the use of standard printing
filament, while producing composite extrudate in situ [100]. They presented the technology
using PLA as the matrix and hemp fibres as the reinforcing phase. Additionally, ageing
studies under UV irradiation, moisture, and simulated soil composting conditions were
performed. Interestingly, the PLA used for the study, as well as the composites made
thereof, showed little-to-no degradation under the tested conditions after an initial stage.
Cai et al. also used a setup for introducing continuous fibres and used ramie fibres (called,
in their work, in-situ impregnation 3D printing technique), also utilising machine learning
methodology to refine the printing conditions towards obtaining satisfactory dynamic
strength of the objects and link the printing parameters, especially layer thickness and
hatch spacing, with the objects’ strength characteristics [101]. An increase in layer thickness
caused an increase in void content and reduced the sample strength, while reducing layer
thickness and hatch spacing resulted in increased fibre content in the composite and its
higher dynamic strength.
Regalla et al. reported the analysis of application of PLA reinforced with carbon fibres,
jute fibres, and coir (coconut) fibres for additive manufacturing of sockets for below-knee
amputation prosthetics [102]. Natural fibres provided a small increase in the composites’
tensile properties, significantly smaller, however, than carbon fibres. Finite element static
loading simulation allowed for correct prediction of the stress fracture site in the element.
Jamadi et al. produced PLA composites with untreated and chemically treated kenaf
fibres [103]. The treatment involved an alkali wash (NaOH in water solution) followed by
the application of various amounts of APTES in a water-methanol solution. The results
showed significant improvement of matrix-filler adhesion at up to 1% silane loading, as
visible from tensile and flexural tests and SEM imaging (reduced fibre pull-out action),
resulting in actual composite reinforcement.
Taborda-Rios et al. presented an optimisation process based on Tamaguchi’s Design of
experiments (DOEs) methodology for 20% bamboo fibre-containing PLA composites [104].
By this experimental design, deposition geometry, layer thickness, and fill density were
selected as individual factors, the impact of which was assessed individually. The thorough
statistical analysis allowed for the quantitative assessment of each factor; however, the
most important conclusion of the study was the ineffective composite reinforcement due to
lack of matrix-filler adhesion, with complete adhesive failure between the two.
Rafiee et al. described the fabrication and 3D printing of PLA composites reinforced
with 0–10% of birch fibres and plasticised with 0–10% of PEG 2000 [105]. The typical
problems of composite preparation were reported, especially the screw extrusion com-
pounding, which in this case was conducted simultaneously with filament fabrication on
a single-screw filament fabricating extrusion machine, which emphasises the importance
of proper material compounding prior to filament extrusion, preferably on a twin-screw
machine. Additionally, the authors encountered issues with the FDM process itself, includ-
ing filament brittleness and nozzle clogging, as well as the process optimisation, which are
the fundamental factors challenging the research involving FDM printing and are rarely
expressed enough. The conclusion of the study was the filler not contributing positively to
the mechanical properties of the composite, while small addition of PEG plasticised the
filament, as visible from the strain-stress curves.
Dogru et al. studied the ageing effects of untreated hemp fibre-reinforced PLA 3D-
printed under different infill patterns [106]. It was observed that delamination of print
layers in Z axis was visible as short as one week into the ageing experiment and that, due
to the lack of treatment of the fibres, inhomogeneity and matrix–filler fusion failure effects
were observed in the material despite the use of a twin-screw extrusion machine. On top
of that, a significant loss of tensile strength occurred over the three-week ageing study.
Similarly, Kesentini et al. investigated the effects of water uptake by flax fibre-reinforced
PLA composites over a 45-day period on their mechanical properties [107]. The Young’s
modulus and ultimate tensile strength decline was observed and visible as quickly as after
Solids 2022, 3 517

three days, while strain at rupture and loss factor were increasing, showing that the PLA
degradation resulted in its plasticisation. The absorption of water by composite was stated
to fit the Fick diffusion model.
Mazur et al. produced PLA composites reinforced with wood, bamboo and cork
fibres under different infill ratios and studied the impact of post-process heat-induced
crystallisation and saline water degradation on their characteristics [108]. It was proven
that heat treatment at 85 ◦ C induced as much as 27% crystallinity level, while as-received
3D samples showed no more than ≈5% crystallinity, as PLA is well-known for its poor
crystallisation behaviour; the treatment positively affected the tensile, flexural, and impact
strength of the composites, while reducing elongation at rupture. Additionally, mechanical
properties of PLA and composites thereof were provided at 80 ◦ C to visualise the loss of
structural characteristics above the Tg point of the matrix polymer.
Mangat et al. studied the effect of PLA reinforcement using silk and sheep wool [109].
The printing parameters, including the number of laminates, infill density, and raster
angle, were studied in detail. On top of that, the printed structures were assessed towards
biocompatibility with Madin-Darby bovine kidney epithelial fibroblast cells. The authors
stated that the addition of fibres positively stimulated cell growth. The addition of reinforc-
ing phase, however, was performed manually and done between the 3D-printed layers,
resulting in a laminate structure.
Sekar et al. produced micro-perforated panels of controlled porosity from PLA/wood
fibre composite for sound absorption application [110,111]. Changing the perforation
volume and pore size allowed for the limited tuning of the sound absorption coefficient
over the frequency range.

4.3.3. Other Polymer Matrices Reinforced with Natural Fibres

Balla et al. described composites based on Hytrel 4056 thermoplastic polyester elas-
tomer reinforced with soybean hulls after different pretreatment procedures, including dry
blending, wet shear mixing, single- and two-stage hydrolysis [112]. The acid hydrolysis pro-
cedure resulted in significant reduction of samples’ porosity in terms of pore fraction and
size, as well as improved the composites’ toughness and increased their Young’s moduli.
Ariel Leong et al. reported trials on printability of 0–10% corn husk fibre-reinforced
composites utilising recycled polystyrene foam (rPS) as a feedstock matrix [113]. H2 O2
treatment was applied to the fibres, which was proven to successfully remove lignin content.
The fibre content of 2.5% caused a 38% reduction in MFI when compared to the neat rPS.
Printing and layer adhesion issues both for neat rPS and the composites were observed,
with 10% composition being found to be unprintable and the composite filament being
brittle. The addition of the filler also negatively affected the thermal stability and tensile
properties of the composites, besides elongation at rupture for 2.5% composition. Costa
et al. utilised alkali treatment followed by H2 O2 oxidation method when preparing ABS
composites reinforced with pine cone fibres [114]. The XRD and FT-IR measurements
confirmed removal of lignin and hemicellulose from cellulose when comparing H2 O2 -
treated samples with the raw material or alkali-treated-only samples. This procedure has
resulted in the improved thermal stability of the fibres, as well as matrix-filler compatibility,
as suggested by SEM imaging and material density measurements. The mechanical testing
data were not provided.
Ahmad et al. prepared ABS-based composite with 5% of oil palm fibres and provided
tensile and flexural characteristics of the obtained 3D-printed specimens; however, only
two specimens were used per experiment [115]. The filler did not provide reinforcing action
and microscopic analysis showed layer-to-layer adhesive failures on top of stress cracking
of the material itself. Han et al. studied ABS/kenaf composites at 0–10% loading [116]. The
materials suffered from poor layer-to-layer adhesion, which resulted in parts delamination
during flexural tests, and all the systems obtained were characterised by toughness lower
than that of the matrix ABS.
Solids 2022, 3 518

Balla et al. investigated soybean hull fibres after various pretreatment procedures
as fillers for thermoplastic copolyester (TPC) [117]. The fibres were ground under dry
and wet conditions, and either used as such or further treated by a single- or two-step
sulfuric acid solution hydrolysis. The study concluded that acid hydrolysis is an effective
means to promote matrix-biofiller adhesion, which in turn reduced sample porosity and
increased Young’s modulus; however, no significant improvement of tensile strength
was observed, which, in combination with reduced elongation at rupture, translated into
reduced sample toughness.
Carrete et al. proposed a methodology for recycling waste textile materials as a source
of cellulose fibres and PET from drinking water bottles (unused ones) as a feedstock material
for fabrication of composite filaments [118]. For the adhesion-promoting step, cellulose
was first hydrolyzed in a H2 SO4 -HCl mixture and, after neutralisation, further treated with
APTES. A significant increase of impact resistance was observed, and the SEM imaging
allowed for confirmation of the transition from brittle to ductile mode when comparing
neat PET and modified cellulose-reinforced PET. Additionally, a two-fold increase of melt
flow index was registered.
Morales et al. studied recycled polypropylene (rPP) reinforced with untreated rice husk
fibres (0–10%) [119]. The material containing 10% of filler was characterised by water uptake
about a magnitude of power higher than neat rPP, which is typical for NFRPs. Additionally,
the water uptake caused significant swelling, which is in line with Le Duigou’s findings [96].
The filler addition caused the reduction of samples’ crystallinity when compared to neat
rPP, especially at 5%, and, depending on printing orientation, caused significant changes to
the composite mechanical properties. At 0◦ , all the parameters (tensile strength, elongation
at rupture, Young’s modulus) were decreased, while at 90◦ , a reinforcing action of the
filler was reported, with both 5% and 10% composite providing improved toughness. The
authors observed a reduced warping effect as well. They also tested cocoa bean shell
fibres at 5% w/w as a filler for rPP [120]. The fibres caused a drop in material crystallinity
due to amorphous lignin and hemicellulose content, not providing any nucleation to rPP.
SEM imaging allowed the identification of particles fracture, filler-matrix debonding and
matrix fractures as different failure mechanisms. The filler, however, reduced warping
effect by 67%, while causing only minor swelling when the material was exposed to water.
A significant fall of tensile properties was observed in 0◦ raster orientation, due to poor
reinforcing action, while in 90◦ orientation, a small improvement was observed, showing
increased layer-to-layer adhesion.

4.4. Other Natural Fillers

Other examples of natural fillers, i.e., wood flour, rice husk, cocoa shell waste, egg
shell cork powder, and rachis from chicken feathers used in the processing of plastics for
additive technologies are also known in the literature. They constitute, i.a., by-products or
waste from various sectors of the economy. Their use in polymer composites allows the
reduction of the production costs of new polymer-based materials without deteriorating
the physicochemical properties, while being in line with the trend of sustainable economic
development due to the reuse of waste. There are reports on using wood in various forms
as a filler for plastics used in additive technologies. Examples are given on the use of wood
flour in materials for 3D printing [121–123].
The first of these works describes polylactic acid (PLA) composites with 5 wt.% wood
flour content. The produced material exhibited increased initial deformation resistance
in comparison to neat PLA. The second work concerns the use of wood flour in another
material, thermoplastic polyurethane. The conducted mechanical tests showed a decreased
tensile strength with an increase in the filler content. The tensile strength showed lower
values for the filler content of 20 wt.%, while they increased with filler loading above
20 wt.%. SEM and FT-IR analysis confirmed good compatibility between the filler and
the matrix after modification with the EPDM-g-MAH compatibiliser, as well as higher
elongation at break. Other papers also describe the effect of plasticiser additives on changes
Solids 2022, 3 519

in the properties of composites with wood-based fillers. Xie et al. studied the use of a
combination of plasticisers (glycerol and tributyl citrate in various ratios). Kariz et al.
investigated the influence of the wood content in the filament on the properties of 3D-
printed materials. The authors obtained six filaments with a wood content from 10 wt.% to
50 wt.% in polylactide. Filaments with 10 wt.% content show an increase in tensile strength
from 55 MPa to 57 MPa, but higher wood contents decreased this parameter. Material with
50 wt.% wood has achieved a tensile strength of 30 MPa. Higher wood loadings changed
the surface morphology of composites. The surfaces were rougher and also had more
defects and voids. The studies describe the possible use of wood in PLA, but in order to
obtain better properties, the process should be optimised [124].
Le Guen et al. presented the preparation of PLA filaments with the addition of rice
husk (10 wt.%) and wood powders (10 wt.%) by the twin-screw extrusion method. The
biofillers have been characterised in terms of their physicochemical properties (including
particle size, density, solid-state NMR, or ash content). When comparing the complex
viscosity of neat PLA to the biofiller-containing compositions, the introduction of wood
powder increased the complex viscosity of the compound, whereas rice husk powder
decreased it. The obtained results were explained on the basis of the higher specific surface
area compared to wood flour and the presence of silica in rice husks. The authors of
the work noted that the mechanical properties of the materials depended on the printing
direction, while the type of filler used did not significantly change the strength parameters.
For example, storage modulus values at 30 ◦ C for neat PLA printed at 0◦ (the longitudinal
direction) was 2.66 (0.22) GPa, for PLA-wood 2.31 (0.07) GPa, and for dla PLA–rice husk
2.50 (0.15) GPa [125].
Tran et al. described the process of biofilament production based on cocoa shell waste,
which is a by-product in the chocolate production industry, in a poly(ε-caprolactone)-
PCL matrix. The authors of the study obtained composites with a waste content of up
to 50%. The conducted research showed that the addition of micronised cocoa shells did
not significantly change the crystal structure of the polymer and the Young’s modulus.
The scanning electron microscopy confirmed the very good dispersion of cocoa shells in
the polymer matrix without the need to use compatibilisers or other chemical modifiers.
The obtained material can potentially be used in 3D printing in home and biomedical
applications [126]. Another example of the use of shells as fillers is described in the work
by Girdis et al. [127]. The paper examines the use of Macadamia nut shells, a by-product of
forestry and agriculture, as a potential alternative to wood polymer composite filaments.
The composites consisted of 19–29 wt.% of biomass in the acrylonitrile–butadiene–styrene
(ABS) matrix. Samples with 19 wt.% of macadamia shells had a higher tensile stress than
commercially available wood-filled material, and were also lighter by more than 30%
compared to commercial wood-filled material. However, along with the increasing amount
of shells in the ABS matrix, a decrease in strength for all samples was noticeable.
Brites et al. presented the use of cork powder, which is the largest by-product of
cork processing, as a natural filler for applications in 3D printing. The work compares the
physicochemical properties of high-density polyethylene (HDPE) composites obtained with
natural fillers, i.e., neat cork powder, as well as cork powder being a waste material obtained
from the production of floor coverings, containing impurities, e.g., varnish, wood fibre,
polyvinyl chloride and polyurethane. HDPE-g-MA was additionally used as a coupling
agent. The research of the density shows that a larger weight amount of waste cork powder
can be added to the polymer than the neat cork powder [128]. In terms of mechanical
properties, all cork polymer composites (CPC) had a lower tensile strength than neat HDPE
and a decrease of strain at rupture.
Sankaravel et al. investigated the properties of a PLA/egg shell biocomposite for use
in the FDM technology, which could potentially be applied in tissue engineering. The filler
content in PLA and the porosity of the material was up to 12 wt.% and 60%, respectively.
Loading of up to 10 wt.% of the eggshell allowed for accurate printing, while the weight
percent of 12% of the filler increased the brittleness of the filament. The composite materials
Solids 2022, 3 520

showed a better osteoregenerative performance in vitro with increasing egg shell content.
The composite materials also had a higher compressive strength parameter (the highest
value obtained for 10 wt.% of the filler), while 12 wt.% caused printing issues (nozzle
clogging, poor flow). The increase in the content of eggshells allowed the reduction of the
acidic reaction of PLA by changing the degradation rate and bioactivity [129].
Flores-Hernandez et al. developed a PLA-based composite reinforced with rachis from
chicken feathers. The materials were characterised by the content of rachis in the range
of 0.5–1 wt.% and 5–10 wt.%. Rachis was modified with NaOH and ground before being
incorporated into the plastic. The thermomechanical tests carried out showed a significant
improvement of the storage modulus (195%) at 1 wt.% rachis as compared to neat PLA.
The analyses showed that the addition of small amounts of keratin materials improve the
thermomechanical and thermal conductivity properties of the polymer [130]. Due to the
increasing amount of waste in the economy, new solutions are sought that will allow for
their reuse.
In the work [131] two lignocellulosic fillers were obtained from Opuntia ficus indica
(cladodes from Opuntia ficus indica (OFI)) and Posidonia oceanica (scraps were collected
on the Palermo coast), which were used to prepare PLA-based composites. The filler
materials were pre-washed and ground before further processing. The research shows that
it is possible to replace the polymer with 20% filler, thus reducing production costs without
significantly changing processability and mechanical properties. A commonly used filler in
PLA matrix is biodegradable starch, which consists of amylose and amylopectin. Due to
the poor adhesion between PLA and starch, appropriate plasticisers [132,133] and chemical
modifications are often explored [134]. Increasing the interactions between starch and the
matrix allows the improvement of the properties of, among others, impact strength and
other mechanical properties.

5. Synthetic Fillers in 3D Printing of Thermoplastics

5.1. TiO2
Hossain et al. assessed the printability of binary blends of PLA with HDPE and
multicomponent blends consisting of PLA and recycled plastics, and 1% of TiO2 as an
additive [135]. The recycled plastic comprised a mixture of several polymers, including PET,
ABS, PLA, PP, HDPE, and PS. The FDM process parameters were optimised via machine
learning protocol. Both the addition of TiO2 and recycled plastics resulted in a requirement
for running the process at higher temperatures (210 ◦ C and 223 ◦ C, respectively) when
compared to neat PLA. The addition of TiO2 caused a significant rise in the composite
water uptake, from ≈1% for the blends to 6.35% and 7.39% for TiO2 -containing binary and
multicomponent systems, respectively. In addition, the pigment presence resulted in surface
imperfections of the printed samples. The non-pigmented blends showed elongation at
rupture of up to 22%, superior to that of usual neat PLA; however, the addition of TiO2
caused a large drop in this feature.
Olam et al. prepared PLA-based composites with varying amounts (0–4%) of TiO2
and either natural or synthetic hydroxyapatite [136]. By means of SEM imaging, they
observed that the addition of TiO2 caused raster gap reduction in printed samples, and EDX
analysis allowed the confirmation of the high homogeneity of the samples. Furthermore,
a significant rise of glass transition temperature was observed, from 52 ◦ C for the neat
PLA up to 71 ◦ C for the material containing 4% of TiO2 and 1% of synthetic HA. The same
material showed improved tensile properties of both the tensile strength (by ≈20%) and
strain at rupture, as well.
Singh et al. developed PA6 composite filament loaded with 30% of TiO2 [137]. The
authors did not perform 3D printing experiments, but they studied in detail the twin-screw
extrusion process and optimised it towards the minimal porosity of the filament surface
and mechanical parameters. Furthermore, an increase of MFI from 24.6 (neat PA6) to
39 g/10 min was observed. Soundararajan et al. studied mechanical properties of PA6
composites containing 0–30% of TiO2 [138]. Progressive increase of tensile strength and
Solids 2022, 3 521

Young’s modulus were observed, as well as a decrease of elongation at rupture, when

using standardised tensile dumbbells prepared by 3D printing. The composites were more
resistant to tribological wear measured by pin on disc method, independently on the load
applied, the wear rate being reduced by as much as 66% for 30% TiO2 loading.
Tiwary et al. studied ABS/TiO2 composites of 5–15 wt.% loading. Satisfactory level
of tensile strength at 20.5 MPa was reported at 10% TiO2 loading [139]. Additionally, the
authors explored the photodegradability of the materials, correlating the sample mass loss
with the UV source power and titanium white mass fraction. The mass loss, however,
was minute, as the experiment was performed for 4 h only. Vidakis prepared ABS/TiO2
composites (0–10% loading) and observed, under optimal conditions, an increase of tensile
strength by 7% and flexural strength by 12% (at 2.5% loading), and Vickers microhardness
by 6% (at 10% loading) [140]. The filler addition did not affect the thermal degradation
profile of the composites at any loading. In another work by the abovementioned group,
polypropylene (PP)/TiO2 composites were studied at 0–4% filler loading [141]. Small
reinforcing effects were observed under tensile and flexural load, revealing that for flexural
properties, the highest (4%) loading was the most beneficial, while for tensile properties,
impact resistance, and microhardness this was 2% loading. Besides 4% loading, all the
samples exhibited increased damping factor (tanδ) values from the DMTA study.
Car et al. explored the applicability of various thermoplastic materials as matrices
for the 3D printing of monoliths for the catalytic oxidation of volatile organic compounds
(VOCs) [142]. The catalytic activity experiments were not included in the work, the investi-
gation being focused on the composite preparation and additive manufacturing, together
with basic (TGA, tensile properties) material testing. LDPE, HDPE, PETG, HIPS, ABS,
PC/ABS blend Z-PCABS, ABS-based Z-ULTRAT, and thermoplastic polyester-based Z-
GLASS (the last three materials being branded by Zortrax) were tested. On the basis
of TGA, Z-PCABS, Z-PETG, and Z-GLASS-based systems were selected for 3D printing
experiments. It was observed that, due to thermoplastic properties of the matrices used,
the monoliths are suitable for photocatalysis, and not thermal oxidative catalysis, as the
monoliths underwent heat deformation above 170 ◦ C. Finally, Z-GLASS was chosen as the
matrix for this purpose on the basis of its transparency, which was seen as beneficial from
the point of view of photocatalysis. It was found that TiO2 addition (in 1–10% ratio loading)
does not affect the thermal stability of the composites significantly, although at the highest
loadings, a decline of mechanical properties occurred.
Sevastaki et al. developed PS/TiO2 composite photocatalysts with 20% and 40%
nanoparticle loading, using recycled polystyrene [143]. The photocatalysts were then tested
in a drug UV-A photodegradation study of acetaminophen (paracetamol), where three
repeated runs were performed for each catalyst, allowing for the decomposition of ≈60%
of the drug in each run. Similarly, Viskadourakis et al. prepared PS/TiO2 photocatalytic
scaffolds that allowed for the full degradation of methylene blue within 60 min of an
experiment and showed activity under conditions of repeated reaction cycles [144]. Using
a similar strategy, Sangiorgi et al. prepared photocatalytic, filter-like scaffolds containing
15 and 30% of PEI-stabilised TiO2 nanoparticles embedded in the PLA matrix [145]. The
scaffolds effectively catalysed the photodegradation of methyl orange. The impact of
scaffold geometry and printing infill was also discussed. Zhou et al. developed a 3D-
printed compact flow reactors from PLA containing 5% of TiO2 as a photocatalytic phase,
for water purification purposes [146]. Methylene blue and phenol were used as model
molecules, and the reactors were proven to be effective under single pass and circulating
flow conditions, with no apparent loss of photoactivity. The critical step in obtaining
the high activity of the catalytic phase was UV-A pretreatment of the composite. Li et al.
utilised aqueous-phase plasma-aided grafting method, based on cold plasma discharge,
to graft TiO2 and ZnO particles on the surface of 3D-printed PLA, following a number of
different strategies: direct grafting of nanoparticles onto an activated surface (proven only
feasible for ZnO); grafting of core-shell particles coated with either poly(acrylic acid) or
methacrylic acid (for ZnO), or a variation of the same using vinylphosphonic acid (for TiO2 );
Solids 2022, 3 522

and grafting the unmodified particles to plasma-activated PLA surface via an intermediate
phase of poly(vinylphosphonic acid) (PVPA) coating [147]. The obtained photocatalysts
were then assessed towards the degradation of Rhodamine B dye as a model molecule of
organic pollutants. The PVPA coating method was proven to be effective for both types of
NPs, and discussed as an efficient means for grafting a high variety of inorganic particles.
All the photocatalysts were proven effective, and the ZnO-based system was additionally
tested in a three-run experiment with only slight reduction of the photoactivity. McQueen
et al. studied the photocatalytic activity of PLA/TiO2 composites for decomposition of
4- and 5-member polycyclic aromatic hydrocarbons (PAHs) [148]. The elimination of
PAHs was quantified over the time of 6 to 24 h, and proven to undergo this via both
photodegradative and photocatalytic pathways, with the latter (PLA/TiO2 composite-
assisted) being significantly faster and providing a reduction in PAHs concentration to
non-detectable levels within 24 h. The effectiveness of PAHs removal was additionally
demonstrated, as the photo-treated water became non-toxic to Ceriodaphnia dubia, a species
of water fleas.
Kumar et al. proposed a methodology of multimaterial printing utilising PLA and
PA6/TiO2 materials, preparing layered composites [149]. A PA6/TiO2 ‘shell’ provided
improved wear resistance under pin on disc tests and increased elongation at rupture, but
did not protect the PLA phase from cracking. Furthermore, void formation and debonding
between the polymer phases were observed by SEM.
Nájera et al. assessed the applicability of PLA/PCL blend doped with TiO2 of various
medium particle sizes as composites for bone tissue grafts [150]. The composites did not
show complete polymer melt blending, as stated from DSC analysis; however, the addition
of small- and medium-sized TiO2 particles resulted in improved mechanical properties of
the composites when compared to the polymer blend, both in terms of tensile strength and
elongation at rupture. A comparison with neat polymer components was not provided.
The composite was characterised by biocompatibility with mouse calvarial preosteoblast
cells MC3T3-E1.
Braunstaien et al. investigated the antimicrobial properties of ZnO- and TiO2 -filled
PLA-based composites plasticised with poly(ethylene glycol)s (PEGs) of different molec-
ular weights (1 kDa, 2 kDa and 10 kDa) [151]. The incorporation of TiO2 did not affect
composite thermal stability, while ZnO caused a significant drop of onset temperature
(>50 ◦ C considering T5% parameter), and PEG additives caused a drop of Tg (below the
measurement temperature threshold of 25 ◦ C). PEGs addition also significantly increased
strain at rupture (from ≈6% up to ≈105%), drastically reducing tensile strain and Young’s
modulus, however. Additionally, ZnO and TiO2 were proven to exhibit antimicrobial
properties, while PEGs did not affect the PLA biodegradability. Horst et al. verified the
printability of natural oleo-gum resins as from styrax benzoin, myrrha, and olibanum as
substrates for FDM-based 3D printing [152]. In addition, the materials were loaded with
10% of either TiO2 , Cu2 O or MoO3 nanoparticles. The resins showed some antibacterial
properties against S. aureus, E. coli, P. aeruginosa, and C. albicans, while the addition of
any type of chosen NPs resulted in the improvement of this property.
Ding et al. assessed the applicability of PEEK-based composites with TiO2 and Fe2 O3
as pigmenting fillers [153]. Most of the systems produced exhibited tensile strength and
modulus comparable with or higher than that of PMMA resin used as a standard for
comparison, while having greatly improved flexural strength (even by three-fold) and
significantly higher flexural modulus. This finding showed a potential for the practical
application of such materials as dental composites. Dong et al. developed methodology for
producing tissue-simulating phantoms with FDM technology, using low-melting gel wax
loaded with TiO2 and graphite as a scattering and absorbing agents, respectively [154]. The
FDM system was equipped with a mixing system that allowed application of the scatter-
ing and absorbing components in a required ratio, thus providing the optical properties
required to mimic the simulated tissue in a satisfactory manner. The method allowed for
the 3D printing of a brain phantom on the basis of simplified MRI slice images.
Solids 2022, 3 523

Castro et al. developed high-permittivity, cyclo-olefin polymer-based (COP) com-

posites filled with either TiO2 , MgCaTiO3 , or Ba0.55 Sr0.45 TiO3 fillers, for application in
RF/microwave devices [155,156]. The obtained composites were characterised by high
(>4.5) εr parameters, and besides the case of Ba0.55 Sr0.45 TiO3 , very low-loss tangent tanδd .
ABS as a matrix, on the other hand, provided less satisfactory electromagnetic characteristics.

5.2. Glass Fibres

The use of glass fibres (GF) in the composite materials industry dates back to the late
1930s. This was related to the optimisation of the commercial fibre production process for
use in composite materials by Owens-Illinois Glass Company. The second event influencing
the progress of glass fibre-reinforced polymers was the development by DuPont of resins
that could be modified with glass fibres [157]. Currently, the most important branch of
industry, using the largest amounts of glass fibres produced, is the thermal insulation
market. The second most important and biggest market in terms of GF usage is the
composite materials sector, in which GF is the reinforcement of polymers [158]. The
development of additive techniques and the need for composite materials with better
mechanical performance have made glass fibres also used in 3D printing [159].
Glass fibres are used as reinforcements in, among others, PP, ABS, PEEK, PLA, and
polyamides. Shulga et al. presented the results of micro-CT analysis and mechanical tests
of materials based on polypropylene (PP) reinforced with short glass fibres. The samples
were printed with the FFF method in different raster orientations (90, 90 and 45, 135), with
different thicknesses of the layers. The presented results show the significant influence
of the orientation of the short glass fibre on the tensile strength, modulus, and stress at
rupture [160].
Sodeifian et al. analysed the mechanical, rheological, and crystallinity properties of
PP/GF and PP/GF composites with maleic anhydride-grafted polyolefin (POE-g-MA) as a
coupling agent. PP/GF materials were characterised by high brittleness; the addition of
POE-g-MA increased the flexibility of the composites. Tensile strength analysis showed
that the addition of GF increases strength and modulus, while reducing flexibility. The
addition of POE-g-MA increases flexibility, but lowers the ultimate tensile strength [161].
Carneiro et al. compared the properties of neat polypropylene and commercially available
polypropylene reinforced with 30 wt.% of glass fibres. The composite was characterised by
higher modulus and tensile strength parameters in relation to neat PP by approximately
30% and 40%, respectively [162].
Mohan Kumar et al. investigated the effect of the addition of short glass fibre on
the mechanical properties of composites in the ABS matrix. The authors suggest strong
interactions between the matrix and the fibres, which resulted in an increase in the Young’s
modulus and strength of the samples, with a simultaneous decrease in the value of elonga-
tion at break [163]. Similar results were obtained by Zhong et al. [164], who conducted a
series of experiments of glass fibre-reinforced materials on an ABS matrix. The analysis
showed that glass fibres (30% by weight) significantly improve the strength of the ma-
terial, but significantly increase brittleness, which results in difficulties in its processing.
The obtained materials could not be used for the FDM method. Only the addition of a
plasticiser and compatibiliser (LLDPE) allowed for the effective use of the material in the
FDM technique.
Ranganathan et al. [165] described the influence of glass fibres and glass beads on
the thermal properties of composites made of polyamide 6 (PA6). Composite samples
were obtained by the FDM method. The analyses show that the introduction of glass fibre
into the polymer increases thermal conductivity and heat distortion temperature. For 30
wt.% of glass fibre, an increase in values of 140% and 130% was obtained, respectively,
for heat distribution temperature and thermal conductivity in relation to neat polymer.
Luke et al. investigated the effect of continuous glass fibre content and orientation on the
mechanical properties of reinforced nylon. The materials tested in terms of mechanical
properties contained 9.5%, 18.9%, 28.4% by volume of fibres with an orientation between
Solids 2022, 3 524

0◦ and 90◦ (with an increment of 15◦ ). The tensile strength parameter increased with the
fibre content in the polymer matrix. For the lowest fibre content (9.5% by volume), 122 MPa
was obtained, while for the highest, i.e., 28% by volume, this parameter was 291 MPa.
The change of the fibre angle from 0◦ (parallel to the tensile stress) to 15◦ lowered the
tensile strength value for the 28% vol. fibres by 78% (from 291 MPa to 64 MPa), which
indicates the significant influence of the fibre orientation in the polymer [166]. Dickson
et al. also investigated the effect of GF on nylon properties (proprietary grade provided
by the manufacturer). Mechanical strength tests show that the GF additive increased the
tensile strength parameter by 91 MPA for GF-concentric and by 151 MPa for GF isotropic
compared to neat Nylon, with a simultaneous decrease in elongation at break. The flexural
strength values increased significantly by 123.81 and 154.77 MPa for GF-concentric and
GF isotropic, respectively, compared to the reference. The authors of the study note
that with the increase in the volume fraction of fibres, larger amounts of air are also
introduced into the matrix, which affects the mechanical properties. The best results were
obtained for materials with a filler content of up to 22.5%. Samples with fibre content up
to 33 vol.% showed only a slight improvement in mechanical properties [167]. Chabaud
et al. described the hydromechanical properties of GF/PA composites, which are to be a
potential structural application. The authors of the work characterised the materials by
sorption, hygro-expansion, and mechanical properties in a wide range of relative humidity
(10–98%) [168].
There are also examples in the literature of reinforcing polylactide (PLA) with glass
fibres. A recovery of glass fibres from wind turbines has been described, together with
production of PLA/GF granulates using a twin-screw extruder, and then obtaining a
composite filament on a single-screw extruder. The samples for analysis were prepared
by the FFF method. The performed mechanical tests showed an 8% increase in Young’s
modulus compared to the reference sample of unmodified PLA; however, the authors did
not report a significant improvement in tensile strength. The SEM images of the samples
showed broken fibres, which is the result of the load transfer between the PLA matrix and
GF [169]. Li et al. also described the effect of GF additive on PLA. It can be seen from the
conducted analyses that the addition of 4 wt.% glass fibres improve the hardness, but on
the other hand lowers the toughness. The addition of 8 wt.% of glass fibre significantly
increased the brittleness of the material and its hardness [170]. The next work [171]
describes not only the properties of the obtained PLA/continuous glass fibre composites,
but also the innovative method of 3D printing. The first step involved preparing the
filament with melt impregnation, and then printing the materials on a modified 3D printer.
The process was optimised in terms of the amount of fibres in the matrix, the amount of
compatibiliser (PLA grafted with maleic acid glycoside), and the printing parameters, i.e.,
nozzle diameter, edge width, layer thickness, printing speed, and temperature. Optimised
parameters allowed composites with 45% GF content to be obtained. The addition of a
compatibiliser increased the forces of interfacial interactions between the polymer matrix
and glass fibres. The obtained materials (45 wt.% of GF) were characterised by high values
of flexural strength and flexural modulus (respectively 313 MPa; 21.5 GPa). Another work
shows that the addition of 4 vol.% of glass fibres to the PLA matrix increases the tensile
strength parameter by 57% [172].
Another polymer known in the literature that has been reinforced with glass fibres
is PEEK (heat-resistant polyetheretherketone). Reinforced composites with 5 wt.% of GF
were obtained. The authors of the study also investigated the influence of the nozzle
temperature, table temperature, printing speed, and layer thickness on the mechanical
properties (tensile strength, flexural strength, and impact strength) of materials. The
conducted tests showed an increase in the tensile and flexural strength parameters of glass
fibre-reinforced composites due to the “pinning” effect of the fibres compared to neat
PEEK. However, the impact strength of materials with the addition of GF is lower, which
was explained by the authors on the basis of morphological changes (creating a porous
structure) and degradation of the molecular chain [173].
Solids 2022, 3 525

5.3. Carbon Fibres

5.3.1. General Considerations
Carbon fibres (CFs) are fibres which contain 90–100 wt.% carbon atoms arranged
in planar hexagonal networks [174]. They can be produced from different polymeric
precursor materials, such as polyacrylonitrile (PAN), cellulose, pitch, and poly(vinyl chlo-
ride) [175]. PAN precursors account for 90% of the carbon fibre market [176]. Contemporary
carbon fibre production from PAN requires solution spinning PAN into fibres, thermo-
oxidatively stabilising the fibres in the air in the temperature range of 200–400 ◦ C, and
finally carbonising the spun fibres at high temperature in the range of 1000–1700 ◦ C. Ultra-
high-performance carbon fibres can be created by graphitising the fibres at temperatures
ranging from 2500 ◦ C to 3000 ◦ C after carbonisation [174].
From a macro perspective, carbon fibres are very thin filaments (about 5–10 µm
in diameter), which are just barely visible to the human eye. Due to their high elastic
modulus (up to 940 GPa) and tensile strength (up to 5.7 GPa), as well as low density,
these fibres, along with different polymer matrices, form composite materials called carbon
fibre-reinforced polymer (CFRP) composites, that can be formed into rigid components
with tailored directional properties for great weight savings compared to conventional
engineering metals [174]. CFRP found applications in the automotive and aerospace
industries, as well as wind turbines, etc. [177]. Therefore, filaments for 3D printing with
CFs have become a subject of interest.

5.3.2. PLA-Based Composites

As mentioned above, CFs are used mainly for reinforcing and reducing the weight
of the final product. For the same purpose, they are used also in 3D printing. One
needs to keep in mind that there are a lot of factors affecting the final properties of 3D-
printed models. El Magri et al. [178] observed the effect of printing properties on the
mechanical properties of CF-reinforced PLA parts. Mechanical properties showed an
improvement in Young’s modulus and tensile strength as a function of nozzle temperature
and reached their maximum at 230 ◦ C. Maximum values of 2622 MPa and 3553 MPa for
the Young’s modulus were thus obtained, respectively, for PLA and PLA-CF. The addition
of CFs increased crystallinity slightly, which was explained by the alignment of CFs in
the direction of 3D printing as well as the orientation of polymeric chains toward CFs
during the cooling process. However, increasing nozzle temperature led to a progressive
decrease in the crystallinity level for PLA. For PLA-CF, the same tendency is observed in
the 180–220 ◦ C range. The optimisation of printing parameters on mechanical properties of
CF-reinforced composites was done also for ABS [179] or other polymers and the effects
of the fibre orientation, stacking sequence, matrix infill pattern, infill density, raster angle,
layer thickness, build orientation, platform/printing temperature, deposition speed, or
annealing temperature is summarised in [180].
Ansari and Kamil [181] used CFs for increasing impact strength and hardness of
PLA, with the maximum hardness increasing up to 79.6 (Shore D), while the maximum
impact strength was 114 J/m, which was 38% and 2.85× higher than the neat polymer,
respectively. The addition of milled carbon fibres increased the thermal conductivity of
PLA [182]. The enhanced thermal performance was useful in determining whether the
3D-printed parts had any porosity. Bakis et al. [183] described alignment in filament
in the measured tensile moduli and strength of feedstock and unidirectionally printed
material. From the results presented for stiffnesses, it can be concluded that the short CFs
increased the tensile modulus E1 (respective to the printing direction) of the reinforced
PLA/CF by about 2.2 times in comparison to the same property of the neat PLA. The tensile
modulus E2 (transverse to the printing direction) and the shear modulus G12 (respective
to the plane of printing) were also increased by the addition of the short fibres, 1.25 and
1.16 times, respectively, therefore not as much as E1 was. Moreover, the Poisson coefficients
were provided to show the mechanical anisotropy of the prepared materials [184]. The
mechanical properties can be improved by the annealing of the 3D-printed samples [185]
Solids 2022, 3 526

or the decreasing of layer thickness [186]. Al Zahmi et al. [187] used recycled PLA for
low-cost applications. They found the recycled PLA showed a higher level of ductility
by 15% when compared to the commercial PLA filament. For the CF/PLA, the CF has an
opposite impact on the ductility: it decreased the strain at the failure by 43% and 30% for
10% and 20% CF, respectively.

5.3.3. ABS-Based Composites

ABS with 5% of CF exhibits a relatively high porosity (up to 19%) and prevents it
from being mechanically and structurally stable comparable to neat ABS or ABS reinforced
with CNTs [188]. Tekinalp et al. [189] compared 3D-printed and compression-moulded
samples of ABS with chopped CF (up to 40%). The tensile strength and modulus of the
3D-printed samples increased by 115% and 700%, respectively. Indeed, 3D printing yielded
samples with very high fibre orientation in the printing direction (up to 91.5%), whereas the
compression moulding process yielded samples with significantly lower fibre orientation.
Microstructure–mechanical property relationships revealed that although a relatively high
porosity is observed in 3D-printed composites as compared to those produced by the
conventional compression moulding technique, they both exhibited comparable tensile
strength and modulus. This phenomenon is explained based on the changes in fibre
orientation, dispersion and void formation. Ning et al. [190] compared different content
and length of CFs in ABS plastic; adding CFs could increase tensile strength and Young’s
modulus, but may decrease toughness, yield strength, and ductility. The specimens with
150 µm carbon fibre had larger tensile strength and Young’s modulus than those of 100 µm.
Compared with the CFRP composite specimen with 100 µm carbon fibre, the 150 µm ones
had smaller toughness and ductility. There was no significant difference in yield strength
value between these two kinds of specimens. The addition of CFs to ABS increased the
thermal conductivity, decreased the coefficient of thermal expansion (CTE), and greatly
reduced the distortion of the parts [191,192].

5.3.4. Polyamide-Based Composites

de Toro et al. [193] compared PA6 composites filled with 20 wt.% of short CFs prepared
by 3D printing and injection moulding (IM). Both the 3D printing and IM processes led
to the breakage of fibres, making them shorter by up to 24%. Tensile tests of printed parts
showed worse results than for the IM parts, but differences did not exceed 21% for either
yield strength, tensile strength or Young’s modulus. The compression tests revealed a
more similar behaviour of 3D-printed parts and IM parts (only 4% improvement). Printed
samples had higher stiffness values than the IM parts. The reinforcing effect was also
confirmed for PA when 15% of short CFs had the highest values of both tensile, 90.8 MPa,
and flexural strength, 114 MPa [194]. Furthermore, it was observed that the increase
in the infill density determines the decrease in the glass transition temperature. The
thermal conductivity of PA12 filled with CNTs grafted on chopped CFs was improved
only slightly [195]. Comparing the fibre reinforcing, it was found that the nylon composite
(based on the proprietary nylon grade provided by the manufacturer) strength was in the
following order: carbon fibre > glass fibre > Kevlar fibre [167]. These results were also
confirmed by the other authors [159,196,197].

5.3.5. PETG-Based Composites

Another studied polymer with CF used for 3D printing is PETG. Špitalský’s group
studied native polymer matrix and cheaper recycled matrix filled with CF up to 20 wt.%
when they observed that replacing a virgin matrix with a recycled matrix does not signif-
icantly change the properties of the filament, and just causes a price reduction. The best
sample had a 124% higher E than the neat polymer matrix. As the increase in CF content
also increased the tensile stress at yield and rupture, the material became more resistant to
permanent (plastic) deformation. A decrease in the εB of the prepared CF composites was
observed. The same effect was observed for nanoindentation measurements. The Er was in-
Solids 2022, 3 527

creased with the increasing loading of CF up to 150% compared to pure polymer matrix, but
H remained constant at the same value of about 140 MPa [198]. With similar reinforcement
of PETG, microscopic analysis revealed a 12% of void spots and fibre alignment according
to the deposition path [199]. In another work [200], the experimental results revealed
flexural stress of 66.9 MPa for neat PETG, and 79.2 MPa for the PETG/CF composite. These
results were better for CF than for Kevlar fibres. Furthermore, it was shown that, compared
to neat PETG, when carbon fibres were added, the thermal conductivity of the composite
increased by around 5%. The opposite results were observed by Valdez where the decrease
of compressive properties was measured [201]. This effect was also confirmed by cyclic
compression [202].

5.3.6. PEEK-Based Composites

There are also other polymer matrices studied for 3D printing by the FFF/FDM method
that use CFs. In the case of PEEK, the addition of short fibre led to the porosity increase
of composites, and the ductility of the material is also reduced; the maximum tensile
strength of 5 wt.% CF/PEEK in this experiment is 94 MPa, which is about 19% higher than
that of 3D printing neat PEEK. Both tensile and flexural strength of CF/PEEK decreased
monotonically with the increase of fibre weight percentage in the range of 5 to 15 wt.%.
The thermal stability of composites was also improved compared with that without the
addition of fibre [203]. Based on the impact properties of 3D-printed test samples from CF
composites, HIPS material is considered semi-plastic and PC as plastic [204]. PC filament
with short CFs has a decreasing effect on the ductility and toughness with an increasing
concentration of CFs. Opposite to this, a significant increase in the strength and modulus is
observed due to the strong adhesion between the fibre and matrix [205].

5.3.7. Continuous CF-Reinforced Systems

The independent group of CF-reinforced polymer composites for 3D printing is an
area of continuous CF composites which has been developed during the last 10 years [206].
Because there are several differences in the feeding of filaments (towpreg extrusion, in situ
impregnation and co-extrusion with towpreg) compared to “common“ feeding in FFF 3D
printing, we will not focus on this area. It is available for PLA [207], epoxy [208], PEEK [209],
or PA [210]. More details can be found in several reviews dedicated to continuous CF-
reinforced polymers manufactured by fused filament fabrication, e.g., [211]. One example
is the comparison of CFs in nylon. The tensile strength and stiffness of the continuous
fibre printed parts were 986 MPa and 64 GPa, respectively, which is more than an order
of magnitude higher than the short fibre-reinforced Nylon printed parts (33 MPa and
1.9 Gpa) [212].

5.4. Basalt Fibres

Sang et al. tested PLA-based systems with varying loading of APTES-modified basalt
fibres (BFs) (5–20%) of different fibre lengths, and additional systems with carbon fibres
(CFs) as the reinforcing phase. The FDM-fabricated samples were then compared to the
ones prepared by compression moulding [213]. BF-reinforced composites showed much
less severe rise in the melt viscosity than CF-reinforced ones, which translated to better
printing feasibility and less microstructural defects in the produced specimens. In other
work, they investigated PLA-based composites and composite blends with various amounts
(17–34%) of polycaprolactone (PCL) and constant amount of 15% w/w APTES-modified BFs
for 3D printing of honeycomb structures of different unit cell shapes for energy-absorbing
applications [214]. The impact of polymer blending on the mechanical, thermal and
thermomechanical properties of the composites and the effect of honeycomb unit cell on
the mechanical performance of the systems were studied. The addition of PCL reduced the
brittleness of PLA, improved the ductility and elasticity of the composite, and facilitated
matrix–filler adhesion; the PCL-PLA/BF composite honeycombs showed elastic response
to compression and superior energy absorption when compared to PLA/BF systems.
Solids 2022, 3 528

Yu et al. studied PLA-based composite containing 15 wt.% of BFs [215]. By means

of 3D X-ray microscopy combined with computer tomography (CT) simulation analysis,
they investigated the microstructure anisotropy of the 3D-printed composite. The work
proved the orientation of the fibres are in accordance with the raster orientation during
the printing process and that the shear stress generated in the nozzle enforces the uniform
distribution of the fibres within the printed composite piece. The important conclusion
was that changing the printing direction allows the tuning of the mechanical properties
of the sample. In another work, they analysed the matrix–filler–void anisotropy effects
of composites containing 5–20% of the filler [216]. The works discussed the inner- and
inter-filament void formation mechanisms and modes, as well as their impact on the
material rigidity, and emphasised the positive impact of the BF filler on the reduction of
the inter-voids formation, while the inner-voids formation may be reduced by improving
the matrix-filler interface.
Kurniawan et al. tested several methods for improving BF-PLA interaction [217].
As a control sample, commercially available, silane-treated basalt fibres were used. For
BF modification, pyrolytic desizing at 450 ◦ C, and subsequent treatment with either 3-
glycidoxypropyltrimethoxysilane or plasma-polymerised acrylic acid vapours was per-
formed. For another approach, a compatibiliser was prepared in a form of maleic anhydride-
grafted PLA (MAPLA), by reactive extrusion with utilisation of dicumyl peroxide as a
free radical initiator. It was noted that desizing reduced the PLA-BF compatibility, while
silanisation and plasma-assisted acrylic acid grafting significantly increased tensile strength
of the composites. On the other hand, MAPLA addition resulted in decrease of composite
storage modulus; however, due to the preliminary character of the presented study, the
optimisation of MAPLA loading or maleic acid to PLA ratio during the grafting process
was not explored.
Balaji et al. discussed the applications of basalt fibres in composites for the automotive
industry, as well as examples of approaches towards their use in AM technologies, together
with the strategies of surface modification of the filler towards the improved mechanical
properties of the 3D-printed composites [218].
Zotti et al. tested PLA/BF composites containing 0–30% basalt fibres, KV-12 type, sized
by the manufacturer with a proprietary silane agent. The fibres were used independently
or together with 5–15% of either sepiolite or talc as a secondary filler [219].
Arslan et al. prepared ABS composites filled with 0–30 wt.% of APTES-treated
BFs [220]. While BFs addition resulted in increasing the material’s tensile strength by
>20% and flexural strength by >60% for the highest filler loading, the composite suffered
from reduced strain at rupture, as is common for filled plastics. BFs also slightly improved
thermal stability measured by thermogravimetry, while decomposition Tmax was virtually
unaffected, as well as Tg from DMTA.
Coughlin et al. prepared ABS composites containing 0–60 wt.% of BF [221]. Due to
high filler loading, 60%-filled composite was found not suitable for FDM application, both
the filament being found to be brittle to spool and feed, and the FDM process failing due to
nozzle clogging, feeding issues, and printed part warping.
Dowling et al. investigated the effect of air atmospheric plasma treatment of continu-
ous basalt fibre (CBF) surface for the 3D printing of reinforced polypropylene (PP) [222].
The filler was sized with a proprietary silane coupling agent by the manufacturer, the sizing
agent claimed to be chosen for improved compatibility with PP. The plasma treatment
provided increased modulus and strength (by means of flexural and short beam shear
testing) of the composite. The effect was explained on the basis of enhanced interfacial bond
strength, supported by computer tomography and optical microscopy. As the plasma treat-
ment was performed at high feeding speed of CBF and in-line during filament extrusion, it
is possible that PP grafting was occurring at the plasma-activated surface.
Solids 2022, 3 529

5.5. Nanosilica (Aerosil, etc.)

Nanometric silica is used as a filler for composite materials, mainly due to the im-
provement of their mechanical properties and very high thermal resistance. Its biggest
disadvantages include the possibility of the formation of agglomerates caused by the in-
teraction of the surface hydroxyl groups and the weak interaction of the polar surface of
the filler itself with the matrix. The simplest method of preventing this phenomenon is the
modification of silica with adhesion promoters, which results in better dispersion of the
filler and influences its higher chemical affinity to the polymer matrix [223,224].
In the work of Wu et al. [225], nanosilica was modified with APTES and then incor-
porated into the PLA matrix; it was shown that the best results of filler dispersion are
obtained for its low content of up to 1%. The influence of the chemical treatment on the
improvement of the mechanical properties of the composites was found, in this case also
the best results were observed at low loading of nanosilica; additionally, modified SiO2
improved the thermal stability of the systems.
Gong et al. modified nanosilica using bis-(3-triethoxysilylpropyl)tetrasulfide (TESPT)
as a silanising agent in order to improve the dispersion of nanoparticles in the matrix, and
then such modified filler was added to PLA/natural rubber thermoplastic vulcanizate
(TPV). In addition to improving dispersion, the reduction in surface tension also made it
possible to improve the compatibility between the two phases. The modification improved
the impact strength and mechanical properties [226]. The same authors investigated the
effect of 3-methacryloxypropyltrimethoxysilane (MATMOS)-modified nanosilica in the
same type of polymer matrix, and once again the improvement of modifier compatibility in
the matrix and over 38-fold improvement in impact strength compared to neat PLA were
observed [227].
Kodali et al. [228] tested polycarbonate aerosil composites with 0.5, 1 and 3 wt.% filler
loading. The research showed an increase in thermal stability and tensile strength-the
best effects were observed for the lower loadings. This effect can be explained by the fact
that nanosilica improves the mechanical properties only in a certain favourable content,
and exceeding it causes an increase in the brittleness of the material due to discontinuities
within polymer volume and, as a result, the observed decrease in the tested parameters.
In subsequent published studies [229], PLA/nanosilica filament with a grain size of
20 nm was produced. Systems with a 2, 4, 6 and 8 wt.% filler loading were prepared. In this
work, the highest increase in tensile and flexural strength, compression, and hardness at 8%
filling was observed, additionally, a reduction in the coefficient of friction was observed.
Sharma and Singholi investigated the mechanical properties of MATMOS-modified
nanosilica (50 nm average grain size) in 0–6 wt.% loading for PLA/wood composite. Im-
provements in mechanical properties, such as an increase in flexural strength, compression
strength, as well as increase in hardness, were observed. The most favourable results were
achieved with the addition of 2% silica and were, respectively, 16.6%, 60% and 60%. An
increase in thermal stability was also observed for all tested samples [230]. In other work,
the same authors investigated the effect of MATMOS-modified nanosilica and nanoalumina
on the properties of PLA/wood composite. Filaments containing 2% of each filler and a
mixture of 2% nanosilica + 2% nanoalumina were produced. Addition of fillers improved
the hardness, compressive strength, and flexural strength, as well as thermal stability [231].
One of the biggest disadvantages of PLA is its high hygroscopicity. The research
carried out in the paper [232] involved the use of proprietary silanol-treated nanosilica at
1 and 3 wt.%, unmodified silica was used as a reference system. The modifications carried
out showed a reduced moisture absorption by the composite. Better results were obtained
for 1% loading. Higher tensile strength and Young’s modulus was also obtained for each
modification.
Vidakis et al. investigated the influence of nanosilica on the properties of the isotactic
PP homopolymer. The addition of the filler improved the mechanical properties for both
1, 2 and 4% loading, and rheological tests showed that the addition of silica did not
deteriorate the processability of the composite [233]. The authors also conducted research
Solids 2022, 3 530

on the preparation of SiO2 /PLA composites and observed the influence of the filler on the
mechanical properties, the highest increase was observed for a composite containing 1% of
filler, and a decrease in mechanical strength was observed with the increase in its content.
In the study, antibacterial measurements were carried out, and mild antibacterial properties
against S. aureus were observed for composites with 4% loading [234].

5.6. Carbon Materials-Graphene, Graphite, Soot

Carbon forms many allotropes. Its structure determines its properties. A three-
dimensional allotrope of carbon-graphite, is known from ancient times and consists of
individual layers of another one-dimensional carbon allotrope–graphene [235], while
zero-dimensional fullerenes (carbon molecules that are characterised by a symmetrical
closed-cage structure) and one-dimensional nanotubes (CNTs) were discovered in the last
century. Another known carbon structure for polymer composites is known as carbon
black (CB)-grape-like aggregates of highly fused spherical particles [236]. Members of
the carbon particle family have shown unique features and have been widely exploited
in biomedicine, agriculture, food industry, electronics, plastic industry, energy industry,
etc. [237]. The carbon-filled composites with high electronic conductivity are attractive
in many potential applications, especially in the field of electronics, including energy
storage devices, sensors, and electrically conductive structures. Compared to electronics
and electrochemical devices manufactured by conventional manufacturing techniques,
such as slurry casting and coating nanomaterials on the substrate, 3D printing provides
a simplistic, rapid, and low-cost approach to producing these electronics with complex
structures. In addition, 3D printing allows manipulating the structure of the components
by overcoming the limitations of electronics and electrochemical devices (i.e., the trade-off
between the power and energy of conventional electrodes, a lack of scalability and the
high cost of producing high-resolution sensors). As 3D printing for these products has
been actively studied, they exhibit better performance in terms of high areal energy and
power density, and high sensitivity, compared to the conventional electrodes and sensors,
respectively [238].
Biodegradable PLA may be used in combination with conductive carbon filler for
different 3D applications. In the work of Foster [239], PLA filament loaded with ≈8 wt.%
graphene was prepared for the 3D printing of disc electrodes for energy storage devices.
The addition of graphene enhanced the thermal properties of the filament. It was applied
as freestanding lithium-ion anodes and solid-state graphene supercapacitors. Additionally,
this 3D electrode (3DE) platform has been analysed for its ability to create hydrogen via
the hydrogen evolution reaction, in which these 3DEs exceed expectations and exhibit
an extremely competitive onset potential compared to that of a platinum electrode. The
addition of graphene also improved nanohardness by 18%, increased the elastic modulus of
PLA/graphene by 11% and resistance to displacement (25%), suggesting there is a higher
resistance to plastic deformation in the reinforced samples. This behaviour can be attributed
to the effective transfer of stress to graphene reinforcement. During the unloading period,
PLA/graphene can recover 43% of the elastic deformation experienced, while PLA was
able to recover 25%, indicating that the presence of graphene in the filament decreases
the permanent deformation. Wear resistance was increased by 14% in PLA/graphene as
compared to the neat PLA [240]. In the other work, it was observed that PLA/graphene
nanoplatelets (GNP) composite samples showed the best performance in terms of tensile
and flexural stress (about 1.7 times higher than PLA). In addition, PLA/graphene composite
samples showed the highest interlaminar shear strength (about 1.2 times higher than neat
PLA samples). However, the addition of GNPs tended to reduce the impact strength
of the PLA/graphene composite samples (PLA samples exhibited an impact strength
about 1.3 times higher than PLA/graphene composites). Furthermore, the addition of
graphene nanoplatelets did not affect, in general terms, the dimensional accuracy of the
PLA/graphene composite specimens. In addition, PLA/graphene composite samples
showed the best performance in terms of surface texture, particularly when parts were
Solids 2022, 3 531

printed in flat and on-edge orientations [241]. The addition of carbon materials enhances
the thermal conductivity of the material, and this fact is used also for welding of 3D-printed
samples. The addition of CNTs and microwave irradiation was shown to improve the weld
fracture strength by 275% [242]. The Ivanov group observed a small synergistic effect in
the GNP/MWCNT/PLA bi-filler hybrid composites when combining GNPs and CNTs at a
ratio of 3% GNP: 3% CNT and 1.5% GNP: 4.5% CNT, showing higher electrical conductivity
in comparison to the systems incorporating individual CNTs and GNPs at the same overall
filler concentration. This improvement was attributed to the interaction between CNTs and
GNPs, limiting GNP aggregation and bridging adjacent graphene platelets, thus forming
a more efficient network (percolation effect). The thermal conductivity increases with
higher filler content; this effect was more pronounced for the mono-filler composites based
on PLA and GNP due to the ability of graphene to better transfer the heat [243]. In the
work of Foo et al. [244], another 3D-printed electrode from PLA/graphene composite was
prepared using a commercial 3D printer. The as-fabricated supercapacitor provided a
good capacitance performance, with a specific capacitance of 98.37 F·g−1 . They had a
photocurrent response that exceeded expectations (≈724.1 µA) and a lower detection limit
(0.05 µM) than an ITO/FTO glass electrode. In another work, a graphene/PLA filament
was used to print a disc electrode, which demonstrated an unexpectedly high catalytic
activity towards the hydrogen evolution reaction (−0.46 V vs. SCE) upon the 1000th cycle,
such potential being the closest observed to the desired value of platinum at (−0.25 V vs.
SCE) [239]. Chen et al. [245] prepared filaments blending TPU with PLA (with TPU to PLA
ratio fixed at 7:3.) with the addition of graphene oxide (GO). They observed compression
modulus constantly increasing with the loading of GO up to 167% for 5 wt.% GO. Tensile
modulus and yield point have increased by 75.50% and 69.17%, respectively, at 0.5 wt.%
loading of GO, which indicates that the addition of GO has significantly improved the
tensile strength of the materials. However, when GO loading was further increased, both
the tensile modulus and yield point decreased. The TGA and DSC data both demonstrated
that the thermal stability of the TPU/PLA blend has been significantly improved by the
addition of GO, with a 90 ◦ C increase in degradation temperature as well as the formation
of better crystalline structures. Cell culturing results revealed the excellent cell viability of
3D-printed scaffolds, indicating that addition of limited amounts of GO has no obvious
toxicity to cell growth, and a small amount of GO is beneficial for cell proliferation.
TPU filaments can be used for 3D printing of the multiaxial force sensors. The sensor
had two components-a structural and a sensing part-both of which were fabricated by
3D printing with different functional material filaments. The structural part was printed
with the commercial TPU filament and the sensing part was printed with CNT/TPU
nanocomposite filament with a piezoresistivity on the surface of the structural part and
an average resistivity of 0.143 Ω·m [246]. With the increased addition of MWCNTs, the
material strength, initial elastic modulus, and electrical conductivity all increased. The
increased modulus in the presence of MWNCTs greatly enhanced the printing capability of
soft TPU by increasing the resistance to buckling. Very modest decreases were observed
in the elasticity modulus, only ≈14.4%. This indicated an excellent adhesion between the
TPU layers during the fusion process. As a result of such adhesion, no degradation was
observed in the electrical conductivity, in both through-layer and cross-layer directions,
of the nanocomposites after printing. The piezoresistivity gauge factors for the FDM 3D-
printed and bulk samples were found to be similar, indicating that the printed samples
performed with no decay under applied strains as large as 100%. It was shown that different
MWCNT contents can provide different ranges of flexibility and sensitivity, tunable for
particular applications [247]. Xiang et al. 3D-printed 1.5 wt.% TPU/CNT nanocomposites
exhibiting higher tensile modulus (20.3 MPa) and strength (16.5 MPa) and comparative
elongation (710%) relative to the printed neat TPU sample. Compared with the printed
3 wt.% TPU/CNT nanocomposite, the resistivity of TPU/CNT composite containing
CNTs surface-modified with 1-pyrenecarboxylic acid (PCA) decreased by 37 times in the
cross-layer direction at the same CNT loading. Highly flexible strain sensors exhibited
Solids 2022, 3 532

high sensitivity (GF = 117 and 213 at a strain of 250% for the printed sensor of 1.5 wt.%
CNT/TPU) and a large detectable strain. Furthermore, the sensor performed well in
the frequency range of 0.01–1 Hz and showed the capability to monitor the strains with
different frequencies. In addition, this sensor showed excellent stability during cyclic strain
testing up to 1000 cycles [248].
In the case of ABS, it was demonstrated that CNT content has a significant influence
on the mechanical properties and electrical conductivity of 3D-printed samples. Printed
samples obtained from high CNT content composites exhibited an improvement in the
tensile strength of 12.6% and resistivity of 0.15 Ω·m for 9.09 wt.% loading of CNTs [249].
In the work of Sezer [250], the tensile strength increased by 206% for 10% MWCNTs load-
ing as compared to the neat polymer. There was almost a linear increase for 3, 5, 7 and
10% MWCNTs loading with a maximum achieved at 10% (668% increase over neat ABS).
The highest electrical conductivity value achieved for the 10% MWCNTs loading was
232 × 10−2 S/cm. MFI values dramatically decreased with MWCNT loading, reaching
0.03 g/10 mm value for 10% loading, explaining the nozzle clogging problem during the
3D printing process using filaments with higher MWCNT filler rates. Thermal properties
of neat ABS were significantly influenced by the addition of CNTs. While a decrease of
specific heat cp (−11%) was observed, thermal diffusivity α (+55%) and conductivity λ
(+30%) increased with a CNTs amount of 6 wt.% for bulk materials at 25 ◦ C [251]. The
SWCNT-filled (5 wt.%) composite demonstrated improvements of 31 and 93% in tensile
strength and modulus, respectively. The SWCNTs provided more reinforcement than the
CFs, and alignment was beneficial to the strength and modulus [252]. Another filament
with graphene prepared by in-situ reduction of GO in ABS by hydrazine reached the value
of conductivity 6.4 × 10−5 S·m−1 (at 3.8 wt.% loading of graphene). After it was 3D-printed
into a 10 mm × 10 mm × 1 mm rectangular model, its measured conductivity decreased
to 2.5 × 10−7 S·m−1 . In FDM, the internal voids among adjacent stacked filaments may
account for the falloff in conductivity. The highest graphene-loaded printable composite
(5.6 wt.%) bears a conductivity of 1.05 × 10−3 S·m−1 . The storage modulus results showed
that G/ABS composites are higher than neat ABS polymer in the glass transition regime
between 102 ◦ C and 113 ◦ C. The DMA results of G/ABS composites demonstrated a more
elastic behaviour as compared with neat ABS, suggesting enhanced stiffness of the material.
All samples including ABS and its graphene composites exhibited less than 1% expansion
under heating from RT to Tg [253]. Upon addition of 4 wt.% of GNP, the elastic modulus
increased by about 30% compared to unfilled ABS. Due to the orientation of polymer
during extrusion, in the entire temperature range, the storage modulus of extrudate is
higher than that of compression-moulded samples and FDM-printed parts. Due to the
GNP addition in the ABS matrix, the storage modulus of CM, extrudate and FDM parts
increases by about 30–50% with respect to the neat ABS below the Tg . The effect of GNP
nanofiller is manifestly more evident above Tg . In fact, the storage modulus of composite
materials at 125 ◦ C is more than twice that of neat ABS for all the investigated samples. The
coefficient of linear expansion (CLTE) values of neat ABS up to 50 ◦ C are in the range of
60–75 × 10−6 /K. After the addition of graphene nanoplatelets, CLTE was remarkably
reduced with values in the range 44–66 × 10−6 /K, which means better dimensional ther-
mal stability of the material in all the temperature intervals [254]. In the case of graphite
application in ABS composites production, due to the layered structure of the filler, the
reinforcement of the mechanical properties for recycled ABS with graphite is not so signifi-
cant [255]. In the work of Aumnate et al., ABS/GO (graphene oxide) composite filaments
were produced using dry mixing and solvent mixing methods before further melt being
extruded, to investigate the proper way of dispersing GO within the ABS matrix. From the
typical stress-strain tensile curves and corresponding statistical data, the elongation at break
value decreased with the GO loading. The elongation at break value of neat ABS was de-
termined as 5.8%, while the value for ABS/GO composite was 2.9%. However, by adding
2 wt.% of GO, the tensile strength and Young’s modulus of ABS were enhanced [256].
For ABS with 15 wt.% of carbon black (CB), it can be concluded that the resistivity of
Solids 2022, 3 533

the printed samples increased remarkably. Specifically, the resistivity increased from
29.06 Ω·m ± 0.77 Ω·m for the filaments to 42.59 ± 1.94 Ω·m and 101.60 ± 6.85 Ω·m for the
cubic samples in the horizontal direction and the vertical direction, respectively [257].
The addition of CNTs showed a shift of the melting and crystallisation peaks towards
higher temperatures compared to plain PEEK but had almost no effect on tensile prop-
erties [258]. PEEK/MWCNT (multi-walled carbon nanotubes) nanocomposites revealed
an electrical percolation threshold taking place between 2 and 3 wt.% loading of CNTs.
The incorporation of GNPs induced a further increase in the electrical conductivity levels
attained, albeit moderate. Combinations of 3 wt.% CNT with higher loadings of GNPs, or
of 4 wt.% CNTs with lower loadings of GNPs, showed consistent electrical conductivities
of approximately 10 S/m. Interestingly, the incorporation of GNPs into the matrix had
a less adverse effect on the processability than that of MWCNTs. The addition of MWC-
NTs/GNPs to PEEK reasonably improved Young’s modulus and the yield strength, while
reducing the ductility of PEEK filaments. The thermal conductivity was also enhanced.
The 3D-printed tensile bars showed an improved modulus and a higher ultimate tensile
strength, but a much smaller elongation at break as compared to the extruded filaments. A
loss of electrical conductivity of two orders of magnitude was observed from filament to
raft and to 3D-printed part [259].
Zhu et al. [260] compared samples prepared by 3D printing and compression mould-
ing of PA12 with graphene nanoplatelets. The GNPs were overall uniformly dispersed
in the PA12 matrix at lower contents (2, 4 and 6 wt.%), while GNPs agglomerates could
not be avoided even at the lowest level of content due to the large size of utilised GNPs
which were difficult to disperse. The optimal GNPs content of 6 wt.% was determined by
the slightly decreased crystallinity which implied higher printing accuracy, but also higher
thermal stability, improved thermal conductivity (+51.4%), and improved elastic modulus
(+7%), along with an appropriate MFI value, than that of compression-moulded parts. In
another work, multiscale thermoplastic composites consisting of proprietary polyamide
copolymer (Pac), copoly(ester amide) (Vectra B950) copolymer reinforced with thermotropic
liquid crystalline polymer (TLCP), and carbon nanotubes (CNTs) were generated. The
copolymer was reported to be a random copolymer resulting from a reaction of 60 mol.%
2-hydroxy-6-naphthoic acid, 20 mol.% terephthalic acid, and 20 mol.% aminophenol. The
most improvement in tensile properties due to the 1 wt.% addition of CNTs was observed
to be for the 20 wt.% TLCP/1 wt.% CNTs/PAc samples. The resulting composite filaments
exhibited 225% and 80% improvement in the tensile modulus and strength, respectively,
compared to the composites without CNTs. In addition, 40 wt.% TLCP/1 wt.% CNT/PAc
3D-printed specimens with filaments laid parallel to the printing direction exhibited excel-
lent tensile modulus and strength of 38.92 GPa and 127.16 MPa, respectively. The measured
tensile modulus of 40 wt.% TLCP-reinforced composite is even higher than the reported
69 wt.% long glass fibre or 26 wt.% long carbon fibre-reinforced nylon in fused filament
fabrication with the same printing pattern [261]. In the work of Zhang et al. [195] CNTs
and GNPs/CNTs were used to modify PA12 to improve its thermal conductivity.Moreover,
7 wt.% CNTs improved the thermal conductivity (0.44 W/mK v.s. 0.28 W/mK for neat
PA12) and electrical conductivity of the PA12, but the filler agglomeration was observed.
PA12 with 15 wt.% of GNPs and 1 wt.% CNTs demonstrated the most improved thermal
conductivity (0.73 W/mK), and a processability assessment for FFF 3D printing technology
was conducted to determine the processing window for the selected composition.
Gnanasekaran et al. suggested that at least ≈0.49 wt.% of CNTs (≈0.31 vol.%) and ≈5.2
wt.% of graphene (ϕc ≈ 3.3 vol.%) are required for the fabrication of PBT-based conductive
filaments for the FDM method [262]. The addition of conductive fillers improved the
thermal stability, as both the onset and the maximum degradation temperature shifted
to higher values. The presence of the conductive fillers facilitated heat conduction and
acted as a barrier to inhibit the emission of decomposition products during combustion.
The PBT/CNT composite exhibited higher thermo-oxidative stability than the PBT/G
composite. The stiffness of the PBT/CNT 3D-printed monolayer was significantly higher
Solids 2022, 3 534

than that of the PBT/G composite; more precisely, the storage modulus was 28% higher
for the PBT/CNT at 25 ◦ C. In the work of Spitalsky [198] PETG filaments were filled with
expanded graphite (EG) up to 10 wt.% and compared native and recycled polymer matrices.
With increasing content of EG, the E increased, so that the material became stiffer. Tensile
stress at yield (σY ) and tensile stress at break (σB ) did not change significantly with the
addition of EG, but for the highest concentration of filler (10 wt.%) they dropped sharply.
The same effect was confirmed from nanoindentation measurements. EG had a synergic
effect on mechanical properties with CF, however. The value for the thermal expansion
coefficient slightly decreased from the value for virgin PETG, ≈60 × 10−6 K−1 up to
≈51 × 10−6 K−1 . The wettability analysis showed that the addition of EG to PETG resulted
in reduced contact angle values.
Kwok et al. prepared conductive filaments dedicated for 3D printing of electrical
circuits and sensors, that were made from PP and CB [263]. The results suggested a
percolation threshold below 11.3 wt.% (5.2% by volume). The effect of UV irradiation
on the electrical resistance of the printed plastic wires was observed with no statistically
significant difference before and after the UV stress test. The conductive thermoplastic
composites remained stable for use at 12 V AC for at least one week without a drift in
their electrical resistance. Between 60 V and 120 V, resistive heating melted and distorted
the shape of the tested filaments and greatly increased the resistance of the printed arrays
after stress. Thus, such prepared conductive filaments are unsuitable for applications
that require a relatively high voltage (>60 V, AC) supply, and significant current. All
the printed conductive filaments tested here displayed Positive Temperature Coefficient
(PTC) effect, in which an increase in temperature led to an increase in electrical resistance.
Han et al. prepared the filament of PP with an addition of maleic anhydride-grafted
polypropylene (MAPP, 16 wt.%), thermotropic liquid crystalline polymer (TLCP) and
CNTs [264]. That PP material exhibited a 265% improvement in the tensile modulus of the
filaments. The tensile modulus of the filament with 1 wt.% CNT-reinforced composite was
competitive with 9 wt.% continuous carbon fibre-reinforced nylon composite materials
in FFF. Barrett’s group prepared 3D-printed sensing strips, with MWCNTs loadings up
to 15% mass, whose function was reversible vapour sensors with the strongest responses
arising with organic compounds capable of readily intercalating, and subsequently swelling
the PVDF matrix (acetone and ethyl acetate). A direct correlation between MWCNTs
concentration and resistance change was also observed, with larger responses (up to
161% after 3 min) generated with decreased MWCNT loadings. 3D-printed material had
excellent conductivity of ≈3 × 10−2 S/cm. Samples composed of three layers were also
moderately more conductive (159 ± 4.8 Ω) than their single-layer counterparts (95 Ω) due
to an increase in cross-sectional area [265]. PVA with graphene was used for printing
parts with enhanced electromagnetic interference (EMI) shielding capability via FDM. The
EMI shielding efficiency (SE) of the printed PVA/GNP parts with a thickness of 2.43 mm
reached 26−32 dB in the frequency range of 8−12.4 GHz, which can completely satisfy the
requirements for practical applications. The addition of 8 wt.% graphene to PVA increased
the water contact angle from 31.4◦ to 50.9◦ and the mechanical properties, yielding a
Young’s modulus of 49.1 MPa, the ultimate tensile stress of 10.6 MPa, and elongation
at break of 128.4% [266]. EVA tensile sensor from filament filled with graphite up to
50 wt.% had maximum electrical conductivity 2.3 × 10−4 S cm−1 , which was 539 times
higher compared to EVA, but resulted in the decrease in tensile properties and increasing
compression properties of 172% [267].

5.7. Other Fillers

Castles et al. presented the production of a series of composites containing different
contents of BaTiO3 microparticles in an ABS matrix, which can be used for 3D printing.
Microwave dielectric properties of printed materials with a content of 70 wt.% BaTiO3 has
been characterised using a 15 GHz split post dielectric resonator (SPDR). The materials
exhibited relative permittivities in the range of 2.6–8.7 and Loss tangents in the range
Solids 2022, 3 535

of 0.005–0.027 [268]. Another work on BaTiO3 in the ABS matrix presents the process
of producing composites, as well as their mechanical and dielectric characteristics [269].
Nanocomposites were produced with a filler content of up to 35 vol.% BaTiO3 (74.2 wt.%).
Due to the brittleness of the material, composites with a 40 vol.% were not printable. With
a content of 45 and 50 vol.% the composites showed stick-slip behaviour and could not
be extruded in a reproducible manner. The mechanical characteristics showed that the
brittleness of the material increased with the increase of the filler content. Composites
with 35% by volume of BaTiO3 were characterised by the tensile strength parameter of
13.7 MPa, while the value for neat ABS was 25.5 MPa. Similar results were obtained for the
flexural strength tests. For samples of ABS containing 35 vol.% of the filler an increase in
relative electric permeability to 11.5 at 200 kHz was observed, which was an increase of
8.42 compared to pure polymer.
Another method is to use BaTiO3 nanowires in the PLA matrix, which are piezoelec-
tric nanocomposites [270]. The conducted electromechanical studies show that proper
alignment of nanowires significantly affects the efficiency of energy collection. Optimum
power output can be increased up to 8 times for composites. Kim et al. prepared and
characterised the BaTiO3 /PVDF nanocomposite produced by the FDM method. BaTiO3
agglomeration effect in the polyvinylidene fluoride matrix was explained on the basis of the
higher density of the filler compared to the matrix. The use of the FDM process allows for
better dispersion and homogeneity. The BaTiO3 /PVDF composite 3D-printed with FDM
exhibited a piezoelectric response three times higher than the solvent-casted nanocom-
posites [271]. Another work by the same team also presents 3D printing of BaTiO3 in a
poly(vinylidene) fluoride matrix as piezoelectric sensors [272]. The work by Paspali et al.
presents the influence of the nanostructure on the mechanical properties of the printed
polylactide/nanoclay composites by the FFF method. Commercially available sodium
bentonite clays are modified with bis(hydrogenated tallow alkyl) dimethyl ammonium
salt. The conducted mechanical tests show that the addition of 5 wt.% modified clay to
PLA increased the tensile modulus by 10%, while reducing the tensile strength value by
15% [273]. C.H. Lee et al. investigated the effect of the powder dosing rate on the mechani-
cal properties of the ABS-CuFe2 O4 composite. The amount of filler was controlled by the
speed of the powder dispenser. The conducted mechanical tests show that the addition of
the filler at high speed to the ABS matrix reduces the strength from 17.1 MPa to 14.6 MPa,
which is related to the agglomeration of the filler, as well as the creation of voids in the
tested materials [274].

6. Conclusions
Within the last couple of years, a significant growth of interest in FDM printing com-
posites with fillers of both natural and synthetic origin has been observed. Based on the
characteristics of the fillers selected, printable systems of a number of new features may
be obtained, from increased mechanical and thermal properties, to photocatalytic reactors,
to biomedical or microelectronic and electromagnetic devices. Although the development
of these novel materials is the testament to the Additive Technologies being one of the
pillars of the Fourth Industrial Revolution, it is still a matter of heavy experimentation
and optimisation until a large fraction of the assumed applications will be refined to the
form of sophisticated solutions ready for the world market. Nonetheless, since the first
development of the FDM technology in 1989, it has become a platform for fabricating
special purpose-oriented materials, the number of applications of which being seemingly
unlimited. The discussed review presented that a direct transfer from conventional pro-
cessing technologies (extrusion, injection moulding) to FDM (which can be perceived as a
microextrusion technology) does not always provide similar or expected results.. Therefore,
it is fully justified to perform independent material studies solely dedicated to the FDM
composite area.

Author Contributions: Conceptualisation, R.E.P. and B.S.; methodology, R.E.P.; validation, B.S. and
R.E.P.; formal analysis, B.S., R.E.P. and D.B.; resources, B.S.; data curation, M.F. and D.P.; writing—
Solids 2022, 3 536

original draft preparation, R.E.P., B.S., D.B., M.F., D.P. and Z.Š.; writing—review and editing, R.E.P.;
visualisation, D.B., M.F. and D.P.; supervision, R.E.P. and B.S.; project administration, B.S.; funding
acquisition, B.S. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by National Centre for Research and Development, Poland,
under the LIDER X project (LIDER/01/0001/L-10/18/NCBR/2019).
Data Availability Statement: The data used for the study was provided from Scopus database analysis.
Acknowledgments: Scientific research in the field of designing organosilicon modifiers of thermo-
plastic properties for the incremental FDM technique financed from the sources of the National Centre
for Research and Development under the LIDER X project (LIDER/01/0001/L-10/18/NCBR/2019).
Z.Š. would like to thank the Slovak Grant Agency for financial assistance, project VEGA 2/0051/20.
Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

ABS Acrylonitrile–butadiene–styrene terpolymer

AM Additive Manufacturing
APTES 3-aminopropyltriethoxysilane
AR Aspect ratio
BF Basalt fibres
CAGR Cumulative annual growth rate
CB Carbon black
cFF Continuous flax fibres
CLTE Coefficient of thermal expansion
CNFRB Continuous natural fibre-reinforced biocomposites
CNT Carbon nanotubes
COP Cyclo-olefin polymer
CPC Cork polymer composites
DMA Dynamic Mechanical Analysis
DMTA Dynamic Mechanical Thermal Analysis
DSC Differential Scanning Calorimetry
EDX Energy-dispersive X-ray spectroscopy
EMI Electromagnetic interference
EPDM-g-MAH Ethylene-propylene-diene monomers polymer grafted with maleic anhydride
EVA Ethylene-vinyl acetate copolymer
FDM Fused Deposition Modelling
FF Flax fibres
FFF Fused Filament Fabrication
FRP Fibre-reinforced plastic
FT-IR Fourier-Transform Infrared
FTO Fluorine-doped tin oxide
GF Glass fibres
GNP Graphene nanoplatelet
GO Graphene oxide
HA Hydroxyapatite
HBC Hygromorph biocomposite
HDPE High-density polyethylene
HDPE-g-MA High-density polyethylene grafted with maleic anhydride
ITO Indium-doped tin oxide
LDPE Low-density polyethylene
LLDPE Linear low density polyethylene
MAPLA Maleic anhydride-grafted PLA
MAPP Maleic acid-grafted polypropylene
MATMOS 3-methacryloxypropyltrimethoxysilane
MFI Melt-flow index
Solids 2022, 3 537

MWCNT Multi-wall carbon nanotubes

NFRP Natural fibre-reinforced plastics
PA12 Polyamide 12
PA6 Polyamide 6
Pac Polyamide copolymer
PAH Polycyclic aromatic hydrocarbon
PAN Polyacrylonitrile
PBAT Poly(butylene adipate terephthalate)
PBS Poly(butylene succinate)
PBSA Poly(butylene succinate adipate)
PBT Poly(butylene terephthalate)
PC Polycarbonate
PCL Polycaprolactone
PEEK Polyetheretherketone
PEG Poly(ethylene glycol)
PEI Polyethyleneimine
PET Poly(ethylene terephthalate)
PETG Poly(ethylene terephthalate) modified with cyclohexyl-1,4-dimethanol
PLA Poly(lactic acid)
PLGA Poly(lactic-co-glycolic acid)
PMMA Poly(methyl methacrylate)
POE-g-MA Polyolefin grafted with maleic anhydride
PP Polypropylene
PDLGA poly(D,L-lactic-co-glycolic acid)
PTC Positive temperature coefficient
PVA Poly(vinyl alcohol)
PVDF Poly(vinylidene difluoride)
PVPA Poly(vinylphosphonic acid)
rABS Recycled ABS
rPP Recycled polypropylene
rPS Recycled polystyrene
RT Room temperature
SCP Saturated calomel electrode
SEM Scanning Electon Microscopy
SPDR Split post dielectric resonator
SWCNT Single-wall carbon nanotubes
Tg Glass transition temperature
TG Thermogravimetry
TLCP Thermoplastic liquid crystalline polymer
Tmax Temperature of maximal rate of mass loss from TG
TPU Thermoplastic polyurethane
TPU Thermoplastic polyurethane
XRD X-ray Diffractometry

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