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Applications and Disposal of Polymers and Polymer Composites: A Review

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European Journal of Advances in Engineering and Technology, 2022, 9(3):65-89

Review Article ISSN: 2394 - 658X

Applications and Disposal of Polymers and Polymer Composites: A

Review
Isiaka Oluwole Oladele1, Samson Oluwagbenga Adelani1, Okikiola Ganiu Agbabiaka1, 2, and
Miracle Hope Adegun1,2
1
Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure PMB 704, Ondo
State, Nigeria
2
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology,
Hong Kong
iooladele@futa.edu.ng; adrenalsax@gmail.com
_____________________________________________________________________________________________

ABSTRACT
Polymers are ubiquitous materials that have found usage in almost all facets of modern life, primarily due to their low
cost, light weight, high specific strength, and modulus. Likewise, their flexibility and accurate control of structure and
properties have made them versatile materials for several applications in aerospace, marine, medical, automobile,
construction, packaging, and other industries. Current innovations in the polymer industries include the development
of fiber-reinforced polymers (FRPs), glass fiber-reinforced polymers (GFRP), carbon fiber-reinforced polymers
(CFRP), polymer-based nanocomposites, biomaterials, and other hybrid polymer-based composites with excellent
characteristics to meet high-tech application performances. GFRP and CFRP are identified as substitutes for metals in
transportation networks and other applications. With the growing population and industrialization, the global demand
for polymer composites will continue to increase and has projected to reach approximately 600 million tons in the next
two decades. Thus, this paper hereby reviews the various polymer fabrication techniques, advantages, and
disadvantages of polymer composites, applications, and various recycling techniques of polymers after usage. The
common challenges in the development of polymer-based composites are discussed alongside their future potentials.

Key words: polymer-based composites; applications; disposal; recycling

________________________________________________________________________________________

INTRODUCTION
The quest to develop durable materials with a combination of distinct properties such as high strength, lightweight,
good fracture toughness, and at a competitive cost, has led to optimum global interest in polymer-based composites [1–
3]. Polymer-based composites consist of a polymer matrix and reinforcement(s) and are easy to process at low
temperature due to their flexibility [4–6]. They also possess high specific strength and modulus, fatigue resistance, and
thermal stability [7–9]. However, the development of polymeric composites is based on the mixture of matrix and
reinforcement to enhance the polymer’s inherent properties such as strength and stiffness. By utilizing the traditional
processing route, it is possible to develop polymeric materials with desired properties to meet specific application
requirements by selecting a suitable matrix and reinforcement [10], where the matrix helps to firmly hold and protect
the reinforcement from environmental and/or mechanical damage. It also distributes evenly or transfer the externally
applied load through the fibers. Meanwhile, the reinforcements are the major load-bearing constituents because they
induce strength and stiffness, thereby making the developed composites exhibit enhanced mechanical properties, which
is usually higher than that of the polymer matrix [11]. Nowadays, polymer composites can be tailored towards any
application requirement at a low cost. The global consumption rate of polymers and polymer composites have been
increasing yearly with a prediction that the annual demand will reach an approximate value of 600 million tons in the
next two decades [4]. Based on this, there is a growing interest to meet this demand while minimizing cost.
Interestingly, the world has witnessed a lot of advancement in polymer composites, which include the use of glass fiber -
reinforced polymers (GFRP) and carbon fiber-reinforced polymers (CFRP) to replace metallic materials in
transportation networks and other applications [1,5]. Despite the numerous benefits that polymer-based composites are

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offering, their drawbacks such as low thermal resistance and high heat expansion coefficient have hampered their usage
in several potential applications [1]. Several attempts to resolve the aforementioned problems have led to intriguing
discoveries in terms of functionalities and performances. The recent advancement in nanotechnology has helped
formulate a design that has led to the right alignment of nanofillers or nanofibers of high modulus such as aramid,
silicon carbide (SiC), boron, and carbon to replace glass fibers and other relatively low-modulus fibers into a polymer
to develop very strong materials with enhanced properties and performances. These formulations have paved the way
for the fabrication of materials with alterable structural and load-bearing properties. Composites that are fabricated with
nanofibers offer good application performance due to their larger surface-to-mass ratio between the fiber and matrix
when compared to traditional fibers, and their use in aerospace, defense, and military industries cannot be
underestimated. Nevertheless, the polymers that are reinforced with nanofibers, nanoclays, carbon nanotubes (CNTs),
and graphene are used in making ballistics, gloves, boots, armor, smart textiles, and wastewater treatment applications
[12]. The alternative way of improving the mechanical properties of polymeric composites is to fabricate hybrid
polymer composites [13]. For instance, Bhajantri et al. [14] reported that a hybrid polymer-based composite of the right
composition, orientation, and distribution of fibers/fillers materials showed a significant improvement in mechanical
properties when compared to that of steel and can be used in specific tribological applications. Gururaja et al. [15]
tested and confirmed the application of hybrid polymer composites in various applications such as aircraft, defense,
marine, turbines, construction, and smart communications [10,11,16]. On the other hand, another major drawback of
polymer composites is the difficulty surrounding their disposal. The disposal of polymer composites after use is of great
concern today as their landfill and incineration pose serious environmental threat and pollution. Therefore, the recycling
of polymer composites is currently seen as the way forward to solve the disposal problem. Srebrenkoska et al. [16] and
El Abbassi et al. [17] have concluded that it is not all existing but a few polymeric composites that can be recycled and
be used for the production of a new class of composites with unprecedented properties. Notwithstanding, there are still
some challenges unresolved that are related to polymer recyclability. Matabola et al. [3] and Asmatulu et al. [18] have
reported that polymer composites recycling can be very challenging because they are made up of different materials
consisting of contaminants that can pose a huge setback towards the recycling process. Moreover, the presence of a
polymer crosslink is another major factor that hinders the recycling and processability of polymers. For instance,
thermoset resin, which is an example of a polymer, cannot be remolded because of its high cross-linkage. Given this,
our paper reviews some of the research conducted on polymer and polymer-based composites with a focus on their
fabrication techniques, applications, advantages and disadvantages, disposal, and their future trend.

POLYMERS AND POLYMER-BASED COMPOSITES

Polymers
Polymers consist of repetitive units of monomers that are combined to form giant molecular units. The units can be a
combination of carbon, hydrogen, oxygen, or nitrogen. Polymers possess distinct characteristics such as toughness,
viscoelasticity, and show great potential to transform into glassy and semi-crystalline structures rather than a crystalline
structure, which is attributed to macromolecules that have been found useful for mankind since time memorial.
Polymers can be processed by heat and pressure and shaped into different engineering components such as bearing,
bushes, gears, and many others. However, several studies have shown that the quantity of polymers used annually is
more than that of steel. Although, they are mostly used in diverse applications due to their lightweight, ductility, and
ease of processing. Unfortunately, polymers exhibit lower strength and modulus when compared to other material
counterparts such as metals and ceramics. In a bid to overcome the challenges, polymers are reinforced with appropriate
reinforcement to meet certain applications demand.

Classification of Polymers
Polymers can be classified into different groups based on the following: Firstly, by their origin such as natural, semi -
synthetic, synthetic; linear, branched, net-work/cross-linked; and addition and condensation polymers. Secondly, their
molecular force determines the functionality of the polymers. For example, elastomers, thermoplastic, and
thermosetting. Thirdly, by the number of their repetitive units such as pectin; homopolymers; and copolymers. Lastly,
by the type of polymerization growth such as chain growth and step-growth [19, 20]. The molecular structures of
polymers are shown in Fig. 1, while Table -1 compares selected polymers, their properties, and applications, of which
the majority of the polymers are used as matrixes to develop polymer-based composites.

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Fig. 1 Different structure of polymers molecules; (a) linear, (b) branched, (c) cross-linked, (d) interpenetrating, (e)
hyper-cross-linked structure [21,22]

Table – 1 The properties and applications of selected polymers [23]

Selected Polymers Tensile % Elastic Density Impact Applications
Strength Elongation Modulus [g/cm3] Strength
[MPa] [%] [MPa]
ThermoplasticsLow-density Polyethylene 21 800 276 0.92 4.9 Packing films, wire
High-density Polyethylene 38 130 1241 0.96 2.2 insulation, squeeze
Ultrahigh Polyethylene 48 350 690 0.934 16.2 bottles, tubing,
household items
Polyvinyl chloride (PVC) 62 100 4140 1.40 / Pipes, valves, fittings,
floor tile, wire,
insulation, vinyl
automobile roofs
Polypropylene (PP) 41 700 1517 0.90 0.50 Tanks, carpet fibers,
rope, packaging
Polystyrene (PS) 55 60 3103 1.06 0.2 Packaging, insulation
foams, lighting panels
appliance component,
egg cartons
Polyacrylonitrile (PAN) 62 4 4000 1.15 2.6 Textile fibers,
precursors for carbon
fibers, food container
Polymethyl methacrylate 83 5 3100 1.22 0.3 Windows,
(PMMA) windshields, coatings,
hard contact lenses,
lighted signs
Polychlorotrifluoroethylene 41 250 2070 2.15 1.4 Valve components,
gaskets, tubing,
electrical insulation
Polytetrafluoroethylene 48 400 550 2.17 1.6 Seals, valves, non-
(PTFE, Teflon) stick coating
Polyoxymethylene (POM) 83 75 3590 1.42 1.2 Plumbing fixtures,
(acetal) pens, bearings, gears,
fan blades
Polyester (PET) 72 300 4140 1.36 0.3 Fibers, photographic
films, recording tape,
boil-in-bag containers,
beverage container
Polycarbonate (PC) 76 130 2760 1.20 8.6 Electrical appliance,
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automobile
components, football
helmets, returnable
bottles
Polyamide (PA) (nylon) 83 300 3450 1.14 1.1 Bearings, gears,
fibers, rope
automotive and
electrical components
Polymide (PI) 117 10 2070 1.39 0.80 Adhesives, circuit
boards, fibers for
space shuttles
Polyetheretherketone 70 150 3790 1.31 0.90 High-temperature
(PEEK) electrical insulation
and coatings
Polyphenylene sulfide (PPS) 66 2 3310 1.30 0.3 Coatings, fluid-
handling components,
hair dryer components
Polyether sulfone (PES) 84 80 2410 1.37 0.9 Electrical components,
microwave oven
component
Polyamide-imide (PAI) 186 15 5030 1.39 2.2 Electronic
components,
aerospace and
automotive
applications
Thermosets Phenolics 62 2 9 1.27 / Adhesives, coatings,
Laminates
Amines 69 1 11 1.50 / Adhesives, cookware,
electrical moldings
Polyesters 90 3 5 1.28 / Electrical moldings,
decorative laminates,
polymer matrix in
fiberglass
Epoxies 103 6 4 1.25 / Adhesives, electrical
molding, matrix for
composites
Urethanes 69 6 / 1.30 / Fibers, coatings,
foams, insulations
Silicone 28 0 8 1.55 / Adhesives, gaskets,
sealants
Elastomers Polyisoprene 21 800 / 0.93 / Tires, golf balls, shoe
soles
Polybutadiene 24 / / 0.94 / Industrial tires,
toughening other
elastomers, tire’s inner
tubes, steam hoses,
weather stripping
Polyisobutylene 28 350 / 0.92 / Hoses
Polychloroprene (Neoprene) 24 800 / 1.24 / Hoses, cable sheathing
Butadiene-strene (BS or 21 2000 / 1.0 / Tires
SBR rubber)
Butadiene-acrylonitrile 5 400 / 1.0 / Fuels hoses and
(Buna-N) Gaskets
Silicones 7 700 / 1.5 / Gaskets, seals

Polymer-Based Composites
Polymer-based composites otherwise known as polymer matrix composites (PMCs) consist of a polymer as a matrix
together with fibers/fillers as reinforcement. PMCs has found a wide range of application in different sectors due to
their combination of unique physical, chemical, and mechanical properties such as good damping capacity, corrosion
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and fatigue resistance, low density, high strength, and stiffness. Their tendency to show a high level of resistance to
environmental degradation like water absorption, chemical attack, temperature, and impact forces, depends strongly on
the properties of the polymer matrix [1].

Classification of Polymer-Based Composites

Polymer-based composites can be classified based on the (i) polymer matrix and (ii) reinforcement source.
Classification Based on Polymer Matrix
The typical matrix used in the development of composite materials are polymers. About 75% of the total composites
produced today have polymers as their matrix, because they can be easily processed at a low temperature without
damaging the reinforcement [24]. The matrix can be classified into three, namely:
• Thermoset
• Thermoplastics
• Elastomeric/Rubber
Thermosetting is well known for its unique behavioral properties of making three-dimensional bonds after curing. For
instance, polyester, vinyl-esters, epoxies, bismaleimides, polyamides, phenol-formaldehyde, and isocyanates possess a
three-dimensional crosslinking structure that is caused by their curing process. Besides, thermosets are chemically and
dimensionally stable and possess good thermal stability and high resistance to cracking. They are commonly used as a
polymer matrix because of their lower cost and their ability to withstand elevated temperature. However, their major
drawback is that they are difficult to be remolded after being formed into shape. Thus, this poses a serious threat to their
recycling capability; however, they can be crushed and used as filler materials. Thermosets-based composites have a
wide range of commercial purposes in various sectors such as aerospace, sports, construction, automobiles, bio-medical,
and other real-life applications [11,25].
In contrast, thermoplastics can easily be softened and melted by heat and remolded into new shapes. Common examples
of thermoplastics are liquid crystals polymers (LCPs), polyamide-imide, polyetherimide (PEI), polyethylene sulphide
(PPS), polyesters, polypropylene (PP), and polyetheretherketone (PEEK). Nevertheless, in comparison to their
counterpart, thermoplastics are not chemically stable, the viscosity range of thermoplastic is 500 times higher than that
of uncured thermosets during melting and they soften quickly at high temperature, and they are a good matrix used in
the fabrication of composites for aerospace applications due to their dimensional and density stability. Moreover, they
show superior resistance to impact damage and cracking [1,19,25]. The merits and drawbacks of some selected
thermoplastic and thermosetting matrices are explained in Table -2.

Table – 2 Merits and demerits of some selected polymer matrices [25]

Polymer Matrix Merits Drawbacks
Polystyrene (PS) Resistance to moisture, chemical, and weather Brittle, flammable, and reduced impact
Good resistance to fatigue resistance
Polypropylene (PP) Good thermal resistance, Excellent resistance to Very difficult to compress,
chemical, lightweight and High cost and not readily available
Polyvinyl chloride Inexpensive, good tensile strength, optimum Poor resistance to ultraviolent rays and heat
(PVC) dimensional stability, fire retardant, and versatility
Natural rubber (NR) Increased resilience, low cost, resistance to water Dissolution and poor resistance to
hydrocarbon
Polyethylene (PE) Less cost, good ductility, high impact strength, and High thermal expansion, low resistance to
good resistance to fatigue weather, and flammable
Epoxy resin (EP) Good water resistance, good thermal and Hard to process, corrosive amine hardener
mechanical properties, less curing time, and
durability
Vinylester (VE) High resistance to chemical and good mechanical High curing shrinkage, requires post-curing,
properties expensive, high styrene content
Polyester (PES) Ease of use reduced cost Moderate mechanical properties, high
curing shrinkage
Phenolic (PHEN) fire retardant Hard to fabricate and high gas emission.
Polylactic acid (PLA) Good strength and modulus, low cost, and nontoxic Brittle, poor impact strength, low thermal
degradation
Starch (ST) Biodegradable, low cost Sensitive to water.
Cellulose (CL) Low cost, readily available, moderate impact and High moisture absorption and low
heat resistance, and eco-friendly decomposition

Elastomers are one of the early-discovered and largely used materials for different applications. Elastomers are a class
of polymers that exhibit extraordinary reversible ex-tension with low hysteresis and minimal permanent set. An
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elastomer is a material that can exhibit a rapid and large reversible strain in response to a stress. They are the type of
polymers that are relieved of molecular interactions, chain rigidity, and crystallinity constraints. Common
characteristics of elastomers are low modulus with poor abrasion and chemical resistance. Theoretical concepts have
been established for their thermodynamics and kinetics. This knowledge has been adopted to improve their properties
with the design of chemical and molecular structures, modification or control of crosslinking, blending, or additions of
fillers [26]. During the vulcanization process of elastomeric compounds and in other processing stages, different
chemicals called accelerators are used. These accelerators determine the type of network structure and, hence, the
material properties. The formulations of elastomers are built up for each constituent to meet some specific requireme nts
and contribute to the final properties [27]. However, despite the available choices of constituents, the desired properties
may still not be achieved, and often times, a combination of two or more rubbers has to be used in the preparation of
compounds for special properties. Thus, elastomeric materials are blended for properties improvement, better
processing, and lower cost.
For instance, natural rubber (NR) is the suitable choice when good tensile and tear strengths are desired since these
properties can be achieved due to the ability of NR to crystallize under stress [28]. On the other hand, polybutadiene
rubber (BR) is characterized by its superior abrasion resistance, so the blends of NR and BR will form a synergy of
excellent processing and physical properties of NR with the superior abrasion resistance of BR. This method is being
used in the industry for the production of tire treads and conveyor belts [29]. Since rubber mixtures are usually
multiphase systems, according to the adopted compounding mode, different distributions of the additives in each rubber
phase can be achieved, depending on the degree of affinity that each additive has towards each rubber. Usually,
competitive vulcanization occurs due to different rates of vulcanization and/or rates of diffusion of the additives in each
of the elastomeric phases. Hence, when two or more rubbers are blended, one of the expected challenges is the
difference in vulcanization rate for each rubber presents. NR is known to have a strong tendency to undergo reversion
as a result of thermo-oxidative degradation brought about by long heating times. Upon thermo-oxidative degradation,
which normally occurs via scission or depolymerization of the polymer molecules, NR gradually becomes softer and
sticky, and as a consequence, the maximum torque, which is the parameter related to the number of crosslinks,
decreases. One of the reasons for blending BR to other elastomers is the difficulty in processing while the lower price
of NR is one of the positive factors in favor of these blends.

Classification Based on Reinforcement Source

Polymer composites can be classified based on the reinforcement source into:
• Natural fiber
• Synthetic fiber
Fibers can either be natural or synthetic (advanced). Natural fibers are derived from renewable and carbon dioxide
neutral resources such as wood or plants, which are 100% biodegradable. Conventional natural fibers used in PMCs are
sisal, hemp, silk, wheat, jute, bamboo, kenaf, and coir. Whilst that of synthetic (advanced fibers) are Kevlar, nylon,
glass, latex, aramid, carbon, and alumina. The reinforcement in PMCs offers stiffness, toughness, hardness, and strength
as they form the major load-bearing constituent of the composites [5]. They exist as particles, fabrics, nanoparticles, or
fibers [1]. Fibers can either be natural or synthetic (advanced). Conventional natural fibers used in PMCs are sisal,
hemp, silk, wheat, jute, bamboo, kenaf, and coir. Whilst that of advanced fibers are Kevlar, nylon, glass, latex, aramid,
carbon, and alumina. Overall, natural fibers are advantageous over synthetic fiber. For instance, low cost, recyclability,
durability, formability, biodegradability, lightweight, readily available, and excellent mechanical properties, are what
identify natural fibers as eco-friendly replacement for synthetic fibers [10,13]. Generally, the fibers used to reinforce
polymers can be classified based on shape or orientation as follows: Continuous, discontinuous, unidirectional,
bidirectional, and random. Whichever shape/orientation is selected, the final mechanical properties of fiber-reinforced
polymer composites (FRPs) still depend on several other factors like modulus and strength of both fiber and matrix, and
the interfacial bonding between them. Likewise, the mechanical properties of FRPs can be tailored according to the
preferred orientation and direction of fibers. However, the ability of natural fibers to form aggregates due to
incompatibility issues with some hydrophilic polymers is another drawback to consider before using natural fibers. That
is why fiber surfaces are chemically treated to improve their surface characteristics such as wetting, adhesion, porosity,
and surface tension [25,30]. Kumar et al. [30] reported that surface modification treatment has positively influenced the
mechanical properties of natural fibers as it improves the surface roughness of the fibers to aid strong adhesion between
the matrix and the fiber phases. Notably, it is recommended that alkaline treatment is the most suitable treatment for
natural fibers. The versatility in the use of FRPs is not limited to construction and military, but also automobiles,
marine, biomedical, sport, and electronics applications [9,26,30]. Several automotive components are fabricated from
polymeric composites of polypropylene and polyesters matrixes, reinforced with fibers such as sisal, hemp, and flax
[2,31-32]. GFRPs are commonly used synthetic composites for engineering applica-tions due to their strength,
durability, and excellent resistance to chemicals, impact, and wear. However, its disposal and short lifetime are its
major challenges. On the other hand, CFRPs, boron fiber-reinforced polymers (BFRPs) and aramid fiber-reinforced
polymers (AFRPs) were later developed as a substitute for aluminum in aircraft and other applications requiring high

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technological performances [1,33]. Some selected fibers used in reinforcing polymer matrix, the fabrication techniques,
and their applications are shown in Table -3.

Table -3 Fibers used with selected polymer matrix, fabrication techniques, and their applications [34]
Fibers Used as
Polymer Matrix Fabrication Techniques Applications
Reinforcement
PP, epoxy resin,
Injection molding, filament winding, Fuel cells, armor, sports, lightweight
Polyether ether ketone Carbon
resin transfer (RTM), Pultrusion automotive product
(PEEK)
Polystyrene, (PS),
CVD, Pultrusion, hand/spray up Wind turbines, Gas tank,
epoxy, Polyaniline Graphene
method aircraft/automobile parts
(PANI)
PP, PS, epoxy resin Sisal Hand lay-up, compression molding Automobile body, roofing sheets
PE, PP, PU Hemp RTM, compression molding, Furniture, automotive
PLA, PP, Epoxy resin Kenaf Tooling, bearings, automotive parts Compression molding, Pultrusion
PP, Polyester, epoxy Flax Compressing molding Textile
Bullet proof vests, sockets
PP, Polyolefin, PLA Ramie Extrusion with injection molding
prosthesis, civil
Window/door frames, automotive
PU, PE Rice husk Compression/injection molding
structure
Hand lay-up, Compression/injection
Polyester, PP Jute Ropes, roofing, door panels
molding
Automobile structural components,
PP, epoxy, resin, PE Coir Extrusion, injection molding
roofing sheets, and insulation boards

Development of Bio-Composites
In recent times, more academic research and industrial development have begun to explore new ways of creating eco-
friendly materials for a variety of applications. Natural fibers offer the potential to deliver renewability, sustainability,
and better quality at a competitive cost in various industries. The ever-increasing volumes of scientific works in the
literature refer with interest to the potential of natural fibers in technological, economic, and ecological terms. This
enthusiasm tends to encourage more research on the use of natural fibers, and by extension, to the areas of human life
and socio-economic development for the fiber crop growers and their communities [35]. Thus, the use of agricultural
wastes has brought a paradigm shift in the way in which natural fibers are being addressed. What was usually referred to
as waste is now being called agricultural by-products due to the value that is been attached to these products. Though
there is little information on the growth of these natural fibers for commercial purposes, but it is envisaged that with the
trends of technological advancement, a new boom in the demand for natural fibers will translate to real improvement in
the quality of life of crop fiber growers and their communities in the future.
Bio-composites are made of biological materials as either polymer matrix and/or fibers/fillers as reinforcement. They
often mimic the structure of living materials and offer biocompatibility properties. Polyolefin thermoplastics such as
polypropylene and polyethylene are considered for fabricating bio-composites due to the difficulty in developing
biodegradable polymers. The biological reinforcements are fibers derived from plant sources such as hemp, sisal, kenaf,
jute, cotton, flax, or fibers that are processed as waste papers, recycled wood, farm by-products, and nanofibrils of
cellulose and chitin [10,11]. Bio-composites are biocompatible, sustainable, and environmentally friendly materials for
several applications ranging from biomedical, automobile, packaging, insulation, and construction industries. However,
their moisture sensitivity has limited their usage in high-humidity environments. Likewise, their mechanical properties
are still unsatisfactory and need to be improved for applications requiring high strength and toughness. Phuong et al.
[36] fabricated a sustainable, biodegradable, non-woven composite membrane from poly(lactic acid) PLA, bamboo fiber
dimethyl carbonate, and the developed membrane had porous structure (porosity of 0.719± 0.132) with tensile strengths
(32.7–73.3 MPa), which is comparable to traditional materials like polypropylene. Further analysis revealed that an
increase in the bamboo content causes improved mechanical stability, reduced swelling, and enhanced permeance up to
1068 ± 32 L/m2/h/bar in water. The result from the experiment affirmed that the bamboo/PLA membrane could be a
sustainable substitute to conventional membrane materials, thus reducing the demand for petroleum-based, non-
degradable polymers and toxic solvents being used in desalination, food processing, petrochemical, and pharmaceutical
industries.

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Development of Hybrid Nano-Composites
Hybrid composites are some of the recorded successes in the fabrication of polymer composites. The quest for a safer
environment and ecosystems and the industrial compliance with environmental policies have reduced the
overdependence on petroleum products, thereby encouraging emergent research towards the development of
biodegradable materials. Hybrid composites are produced by combining at least two different reinforcement phases
(fibers, fillers, and/or particulates) with a matrix. The reinforcements can both be natural or synthetic. Hybrid
composites are advantageous because of their flexibility in designs and can easily be tailored to desired mechanical
properties [13,34, 37]. Of recent, additive manufacturing industries have made use of polymeric composites to achieve
little or no waste production processes by making it possible to fabricate complicated parts that are in excessive demand
in high-tech applications like automotive, aerospace, biomedical, electronics, and robotics. Hence, FRPs are promising
materials to meet the future needs of next-generation materials and technologies [38,39]. Several studies and reviews
have been carried out on hybrid composites. Bhajantri et al. [14] studied the mechanical properties of polymer-based
hybrid composites based on glass fibers and other fillers as reinforcement in polyester resin. Their results showed that
the hybrid reinforcement significantly enhanced the tensile, impact, and flexural properties of the polyester resin. It was
therefore concluded that hybrid composites demonstrated better mechanical performance than conventional composites
at low cost. Nair et al. [39] studied the synthesis and characterization of hybrid polymer composites. From their study,
coir and human fibers were used to reinforce epoxy resin via hand layup technique. The mechanical characterization of
the developed composites showed that the tensile strength of hybrid composite increases from 16 MPa to 19 MPa while
the optimum flexural and impact strengths were 56 MPa and 6.8 MPa, respectively. From their analysis, synthetic fibers
can be used to replace eco-friendly fibers to develop hybrid composites of superior mechanical performance. Therefore,
it is an alternative way of improving the mechanical properties while minimizing the cost. In another work, Salih et al.
[40] investigated the influence of locally sourced rice husks and date palm fibers on the mechanical properties of hybrid
composites for construction purposes. Their results revealed that the fracture, compressive, tensile, and impact strengths
of the composites were improved as the fiber fractions of palm fibers increases at the expense of rice husks. A further
investigation showed that the reduction of palm fibers’ length to micrometer range increases the fracture toughness of
the polymer composite by 98%. The results from the analysis confirm that palm fiber better enhanced the mechanical
properties of polymer-based composites. Kaushic and Suresh [41] studied the mechanical behavior of hybrid composite
using E-glass and Kevlar as reinforcements. The sample formulation consists of 60%wt of epoxy and varying volume
fraction of E-glass and Kevlar fibers. Afterwards, the samples were fabricated by using a compression molding
technique. From their findings, the mechanical properties (tensile, impact, and flexural strengths) of the hybrid
composites were significantly improved by the addition of hybrid fibers. To conclude, the hybrid composite is suitable
for applications requiring high strength and toughness. Akash et al. [42] evaluated the mechanical properties of hybrid
composites reinforced with sisal and coir fibers, which were fabricated by the cold-pressing technique. By varying the
weight fraction of the individual fibers within an epoxy matrix, the hybrid composites were made to possess better
mechanical performance than pristine epoxy and other composite systems. However, the major issue related to hybrid
composites is their irregular water absorption behavior. For instance, the water absorption properties of hybrid
composites usually increase with an increase in the volume fraction of fibers. Recently, sisal and coir fibers were used
to fabricate a low-cost bio-composite, suitable for high-strength applications. The bio-composite consisting of a
polyethylene matrix was developed by Bazan and co-workers [43]. They combined varieties of natural fibers such as
coconut, basalt, and sawdust of varying proportions into a polyethylene matrix to fabricate polymer-based hybrid
composites. Upon further investigation, both the theoretical and experimental results showed that the mechanical
properties of polymer composites can be improved by combining different fibers together. Therefore, their work has
justified the effectiveness of using natural fibers as reinforcement towards the development of polymer-based
composites for intending applications. However, a study conducted to evaluate the superiority between basalt–wood and
basalt–coconut as reinforcing fibers/fillers showed that basalt–wood-reinforced composites possess higher strength
(50%) and stiffness (65%) over basalt–coconut-reinforced composites. From the SEM images in Fig. 2a,b, there is a
clear difference in the interfacial bonding between the fibers and matrixes for each composite. In Fig. 2a, the wood flour
consists of well-embedded hollow cylindrical cells and strung that are parallel to one another within a polyethylene
matrix. From the image, it can be seen that a good surface bonding existed between the fibers and matrix. Whilst in Fig.
2b, it was observed that the surface adhesion or roughness between the coconut–basalt fibers and matrix is weak. Thus,
it justifies the reason that the hybridization of wood flour and basalt fibers within a polymer matrix possesses better
mechanical properties than that of coconut- and basalt-based composites. To conclude, the strength and stiffness of
hybrid composites made from wood flour and basalt fibers increases by 50% and 65%, respectively, compared to
composites reinforced with basalt and coconut hybrid fibers. Jose et al. [44] investigated the mechanical properties of a
hybrid composite reinforced with coir and wood dust particulates. Scanning electron microscopy as shown in Fig. 3a
and b show an even distribution and good interfacial bonding of the coir and wood in the particulates to the polymer
matrix, and this causes increases in tensile strength and flexural strength with an increase in concentration of the fibers
and coir particulates

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Fig. 2 SEM images of polyethylene-based hybrid composites consisting of (a) wood flour and basalt fibers and (b)
coconut and basalt fibers [43]

Fig. 3 SEM images of (a) coir and (b) wood dust particulate reinforced hybrid polymer composites [44]

According to the review paper published by EL-Wazery [13], it was concluded that polymer-based hybrid composites
can be used for diverse applications requiring low density, excellent rigidity, and strength in relation to nanotechnology,
orthopedics, wind power generation, transportation, construction, chemistry industries, and smart technology. Most
especially, the application of hybrid composites in nanotechnology has paved the way for the use of particulates in their
nano-scale range as reinforcement to develop these composites of targeted mechanical properties. As a result, it has
given them room to exhibit a good surface–volume ratio to afford better interfacial bonding between the reinforcement
and polymer matrix [1,4]. For instance, if a polymer composite consists of well-dispersed nanoparticles, it will produce
a polymer nanocomposite of excellent compressive strength, fatigue, and fracture resistance. The reason is that an
improved infiltration and impregnation of resin will be made possible when the reinforcing particles do not agglomerate
as a result of the sizing effect, and thus justify the importance of polymer nanocomposites in high-tech applications [8].
Nevertheless, the processing techniques for developing nanocomposites are in-situ and ex-situ polymerization, melt
intercalation, template synthesis, and sol-gel processing [35]. Several research works have reported the use of certain
polymers such as polyamides, polypropylene, polyethylene, epoxy, acrylics, and polyurethanes, as a matrix towards the
development of nanocomposites for different engineering purposes. Recently, the research focus has been on the
development of biodegradable polymeric materials. Some polymers are being reinforced with suitable reinforcing
fillers/fibers to improve their biodegradability to ensure a sustainable environment [44–47]. Rajak et al. [35] showed in
their studies that carbon nanotubes (CNTs) polymer composites are of good mechanical, thermal, electrical, and
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magnetic properties. Satis and co-workers [45] investigated the morphology and thermal stability of epoxy reinforced
multi-walled CNTs. From their findings, it was observed that the thermal stability of the epoxy was improved due to
CNTs addition. The SEM morphology of the epoxy composite as shown in Fig. 4 has revealed that good interfacial
bonding existed between the epoxy matrix and CNTs, as well as structural homogeneity. Thus, it was the reason that the
strength and toughness of the epoxy composites were improved. In summary, multi-walled CNTs are a suitable
reinforcement for enhancing the mechanical properties and thermal stability of epoxy-based composites. Furthermore, it
is noteworthy to mention that polymer-based nanocomposites possess better mechanical, thermal, and flame-retardation
properties than conventional composites due to the higher surface area that exists between the nanofiller and matrix
[46].

Fig. 4 SEM morphology of the fractured surface of Epoxy-reinforced multi-walled carbon nanotubes (MWCNTs) [45]

Advantages and Disadvantages of Polymer-Based Composites

Polymer-based composites offer numerous advantages ranging from a reduction in fuel consumption as a result of less
weight associated with excellent specific strength and stiffness, as well as being less energy-intensive for their
production. Other advantages include good fracture toughness, top-notch resistance to corrosion and fatigue, good
damping capacity and high tolerance to damage, high resistance to impact and abrasion, flexibility in design, and low
processing and maintenance cost. All these and many others are the important advantages of polymer composites.
However, the challenges limiting the use of polymeric composites are due to their low strength, low thermal resistance,
low glass transition temperature (T g), and a high coefficient of thermal expansion between the polymer matrix and
reinforcement.

FABRICATION TECHNIQUES OF POLYMER-BASED COMPOSITES

The underlying principle behind the development of composites is based on the application of heat and pressure to cure a
mixture of polymer and reinforcement into the desired shape and geometry. The process involves the following: Material
sourcing, sorting and pre-treatment, fiber impregnation, forming into shapes, curing, and finishing [48]. Nevertheless, the
commonest fabrication techniques of polymer-based composites are discussed below.

Wet/Hand Lay-Up
This is the most commonly used technique in the development of small and large components. The process is usually
carried out in an open mold by placing preform fibers either in woven, knitted, or stitched fabrics, followed by resin
impregnation, which is carried out by using brush, nip, roller, or impregnator. The process of resin impregnation is to
force a resin into fabrics to form laminates followed by curing under atmospheric conditions. However, some of the
advantages of the process are low tooling cost, ease of processing, and flexibility. In contrast, the major challenges to be
faced during the process are the quantity of viscous resin used and the non-uniformity in its distribution [35,49].

Spray up
The similarity between hand lay-up and spray-up technique is that they make use of the open mold and roller for exerting
pressure when needed. In the case of the spray-up processing technique, chopped fibers are held in a gun and fed into a
spray of resin. Afterwards, it is deposited inside the mold and allowed to cure. Although the spray-up process is faster
than the hand lay-up process, it is not suitable for manufacturing parts requiring high structural and dimensional tolerance
[49].

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Vacuum Bagging
Vacuum bagging involves the use of flexible diaphragm like polyvinyl alcohol or nylon polythene that requires the use of
a hand lay-up process to first make the laminates before spreading a plastic film over them. Thereafter, pressure is
applied to improve its consolidation while the trapped air is extracted using a vacuum pump. To compare and contrast
vacuum bagging and hand lay-up, the former can be used to develop composites consisting of high fiber loading with
lesser void and volatile constituents. However, vacuum bagging is a capital-intensive technique that requires highly
skilled operators. Hence, it is the reason that the cost of production under vacuum bagging is much higher than that of the
hand lay-up process [35].

Pultrusion
Pultrusion is an automated technique used for processing composite materials into continuous and constant cross-sections
in the form of rods, tubes, and dies. During the process, strands of continuous fibers are pulled through a resin bath,
which is later allowed to pass through a heated die. Hence, the final product has the same shape as the die, which helps to
complete the impregnation process. The pultrusion technique is a rapid process that gives room for mass production of
desired products with good fiber control. However, the process is limited only to near-shaped or net cross-section
components [35].

Filament Winding
Filament winding is a technique used for fabricating hollow, circular, and oval shape components such as pipes, tubes,
cylinders, large tanks, and many others. During the process, flexible fibers are made to pass through a resin bath and
wounded into different orientations through a feeding mechanism via rotating mandrel to form a desired component,
which is then allowed to cure in an oven or autoclave. The major advantage of the filament winding process is that it is
very fast and economically viable. Additionally, it can be used to create complex patterns with fibers for load-bearing
applications. However, its processing requires resin with low viscosity, and it is limited to convex-shaped components
only [35].

Resin Transfer Molding

Resin transfer molding (RTM) is a processing technique that involves the arrangement of fibers into desired preform or
orientation. During the process, the fibers were first held together by using a binder and pre-pressed into a mold shape.
Afterwards, another similar mold was clamped over the first mold while a resin is forced into the cavity by pressure. The
variance of this process is called vacuum-assisted resin transfer molding (VARTM), whereby a vacuum is applied to the
cavity to aid the impregnation of resin into the fibers. The process can also be carried out at atmospheric or elevated
temperatures under closed conditions to ensure good environmental control [35]. Finally, the process is cost-effective and
suitable for the development of complex shapes and geometry.

Extrusion
This technique is a continuous process employed by industries to incorporate fibers/particles into a polymer matrix to
produce polymer composites. In extrusion, a mixture of molten polymer and fiber/particles are continuously passed
through a die with a predetermined shape in a flow state. It is one of the fastest developing techniques for the fabrication
of polymer composites due to the wide range of advantages it offers such as continuity, short time of processing, good
mixing efficiency, high capacity, good quality of products, diversity, and versatility. Extruders are classified into single
screw and twin-screw extruders, single screw extruders are being used when a low mixing effect is required, but the
configuration of single-screw extruders offers better alignment of fibers in the polymer matrix at higher temperature.
However, an excellent mixing effect and homogenous distribution of fibers or nanoparticles in polymer melt can be
achieved by using twin-screw extruders. The process parameters for the extrusion process are screw profile, speed,
residence time, and temperature, and optimization of these process parameters plays a vital role in the final properties of
the composites produced. Therefore, the thermal stability of the fiber should be considered while adjusting the process
temperature, and process parameters should be correctly defined to enhance the aspect ratio of the fibers as well as
reducing possible damages to the fibers [50].

Melt Blending
This is the most preferred technique of preparing polymer nanocomposites of either thermoplastics or elastomeric
polymer matrix and it is becoming more popular owing to its significance in industrial applications. In melt blending, the
polymer is melted at a temperature above its softening point and mixed with the desired quantity of intercalated
nanoparticles using an extruder. This process is carried out in the presence of an inert gas such as neon, argon, or
nitrogen. Moreover, the polymer may be dried and mixed with intercalant before heating in a mixer and subjected to
shear force sufficient to form the desired polymer nanocomposites. The merits of melt blending over in situ intercalative
polymerization of polymer solution intercalation are that the process is environmentally friendly due to absence of

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organic solvents, it provides better mixing of polymers and clay-nanofillers compared to solution-blending techniques,
and its compatibility with current industrial processes such as extrusion and injection molding. Important operation
parameters for this process are melting conditions such as feed rate, temperature, scree speed, mixing time, oxidative
environment, die pressure, materials grades, and the chemical nature of the nanoclays [51,52].

Solution-Blending
This is a solvent-based method in which solubilized polymers or pre-polymers are mixed with fillers under continuous
agitation by mechanical stirring, and the polymers are firstly dissolved using appropriate solvent such as water,
chloroform, alcohol, or toluene. The polymer chains are intercalated, and the solvent is displaced within the interlayer
when the polymer and fillers are mixed in the solution. The solvents are removed by either vaporization or precipitation,
and the intercalated sheets are reassembled yielding polymer/nanocomposites. Entropy gained from desorption of the
solvent molecules acts as the driving force for the intercalation in the process. The whole process involves three stages,
namely the dispersion of nanoparticles in a polymer solution, controlled removal of solvent, and casting of the composite
film. Most times, the polymers are heated, or the pH is adjusted to enhance the formation of film and improve film
properties. Lately, solution blending is often used in preparing thermoplastic and epoxy composites at the laboratory
scale due to its ease of operation, optimum particle dispersion, coupling chemical reactions, and no special equipment is
required. A composite with excellent properties can be obtained with careful selection of appropriate solvent and
controlled fabrication techniques. The drawbacks of solution bending include the fact that intercalation can only take
place for certain combinations of polymer, clay, and solvents, a large quantity of organic solvents is required, and this
may not be environmentally friendly and economically viable [52,53].

In Situ Polymerization
This was the first method used to synthesis polymer clay nanocomposites based on the use of nylon 6. The layered
silicate is swollen within a solution of liquid monomer so that the formation of polymer can take place between the
intercalated sheets. The polymerization can be initiated either by heat or radiation by diffusing a suitable initiator or used
of organic initiator or catalyst that has been fixed inside the interlayer before the swelling stage. This method gives room
for versatility in molecular designs of polymer matrix and provides an efficient approach to the synthesis of varieties of
polymer/nanocomposites with a range of properties, and flexible tuning of matrix composition and structure can be
achieved through this method [52].

Sol-Gel
In the sol-gel method, organic molecules and monomers are embedded on the sol-gel matrix and organic groups are
introduced to form chemical bonds. This leads to in situ formation of a sol-gel matrix within the polymer and the
generation of organic or inorganic networks. The nanoparticles are synthesized within the polymer matrix by using an
aqueous solution containing polymer and silicate building blocks. Nucleation and growth of the inorganic host crystal are
aided by the polymers, and the polymers get trapped within layers as they continue to grow [51]. The process is limited
to polymers with hydrogen bond acceptor groups with the ability to form hydrogen bonds with hydroxyl groups on the
inorganic filler surface [54]. Table- 4 present the advantages and the disadvantages of each polymer fabrication method.
More research still needs to be carried out to investigate comparatively the influence of these production methods on the
properties of the developed products. This is necessary since each of these techniques will bring about different structures
and, thus, varying properties. Moreover, the influence of these manufacturing processes on new advanced materials
needs to be ascertained.

Table -4 Merits and demerits of selected polymer-based composites fabrication techniques

Fabrication
Merits Demerits
Technique
Low tooling cost, ease of processing
Wet/Hand
and flexibility, any combination of Non-uniformity in resin distribution
layup
fiber and polymer matrix can be used
Difficult to control fiber volume, styrene emission, not suitable for
Faster than hand lay-up process,
manufacturing parts requiring high structural and dimensional
Spray up economical for producing small to
tolerance, does not provide good surface finishing on both sides of
large parts
the product, can only use short fibers
Composite of high fiber loading with
Vacuum
lesser void and volatile constituents Capital intensive, highly skilled operators required,
bagging
can be produced
Fast and give room for mass
Pultrusion Limited to only near shaped or net-cross-section components
production
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Requires resin with low viscosity, limited to convex shaped
Filament Fast and economically viable, resin
components, rough external surface of component, cost of mandrel
winding content can be controlled.
for large components can be high
Cost-effective and suitable for
Resin Transfer fabrication of complex shapes and Sizes of part are limited to the type of mold, tool offset is required
Molding geometry, lower emission, high- for high production rates
quality finishing,
Extrusion Low cost, flexibility Size variances, product
Fast, simple, and environmentally
Melt Blending Poor dispersion of particles in matrix
benign
Solution
High particle dispersion, High cost of solvent, not environmentally friendly
Blending
Insitu Faster polymerization, easy to
Uses expensive equipment,
Polymerization AUTOMATE
Low equipment cost, reduce fiber
Sol-gel damages due to low processing Limited to polymers having hydrogen-bond acceptors,
temperature

APPLICATIONS OF POLYMER-BASED COMPOSITES

The applications of polymer-based composites are prominent in areas like aerospace, automobiles, construction, marine,
and biomedical, due to the promising advantages of being lightweight in combination with their satisfactory mechanical
properties such as specific stiffness and strength, good corrosion, and fatigue resistance.

Marine
One of the basic objectives of manufacturing marine structures is to produce components of low density and high specific
strength. Properties like lightweight and good corrosion resistance have made it possible for polymer composites to be
useful for boat making, marine construction, and the fabrication of components such as the bulkhead, deck, mast, and
craft [9]. Other important engineering properties, for example, fatigue resistance and temperature stability together with
low maintenance cost, have made CFRP (an example of a polymer-based composite) a suitable material in various
marine sectors, and they are used for fittings and internal equipment such as valves, ducts, pumps, heat exchangers,
pipes, naval vessels, small ships, superstructures, heat exchangers, bulkheads, machinery, propellers, propulsion shafts
for warship equipment, etc. [49].

Automobiles
Automobile industries use a lightweight material to maximize fuel efficiency and reduce environmental pollutions
generated through emissions. These have made polymer composites an important material in the automobile industry.
Examples of automobile parts produced using polymer-based composites are engine frames, dashboards, door panels,
interior structures, and storage tanks. They are made possible by reinforcing polymers with natural or synthetic fibers.
Examples of commonly used fibers are sisal, jute, bamboo, cotton, glass, flax, carbon, and hemp [34–35,46]. Rajak et al.
[35] reported the use of glass/carbon matrix thermoplastic to manufacture an automobile bumper beam due to its
exceptional impact strength and lightweight compared to conventional glass thermoplastic bumper beam. Kim and co-
workers investigated the application of polymer composites made of polyamide matrix and alumina fibers (60 wt.%) for
automotive lamp reflector purposes and reported that the composite exhibited a remarkable thermal conductivity when
compared with fog lamp reflectors that are made from conventional polybutylene terephthalate [54].

Aerospace
The aerospace industry is the largest consumer of polymer composites, as weight reduction and safety are the major
considerations in designing aircraft structures. Several studies have shown that there is about 50% and 20% weight
reduction and cost cut, respectively, when polymer composites are used for fabricating aircraft structural parts compared
with metals. Examples of such parts are wings, stabilizers, fuselages, floor beams, rotor blades, and rudders, among
others. Since the 1950s, the aerospace industry.has been adopting glass-fiber-reinforced polymers (GFRPs). Their
relatively low cost and high specific strength make them attractive in the aerospace industry to date. Another polymer-
based composite thatis commonly used for fabricating aircraft components is carbon-fiber reinforcement polymers
(CFRPs). Other potential replacements for aluminum (Al) in aircraft structures are BFRPs and AFRPs [1,6,55]. Research
has shown that several natural fibers-reinforced thermosets and thermoplastics meet performance requirements of aircraft
interior panels such as specific weight, good resistance to heat and flame, ease of maintenance, and recoverability. The
biodegradability and cost-effectiveness of the composites are added advantages for the industry [34]. Kesarwani [33]

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reported the use of CFRPs in the wing box, upper deck floor, and rear pressure bulkhead of A380 aircraft as shown in
Fig. 5, causing a weight reduction compared to when an aluminum alloy was used. The work further emphasizes the
importance of polymer composites to solve problems related to an increase in fuel cost, environmental instability, and
maintenance in the aviation industries.

Fig. 5 Schematic showing the composite structure of A380 aircraft [56]

Medical
Strength and biocompatibility are important properties requirements for polymer-based composite materials in the
dentistry and orthopedics field. This will enable them to act as a good replacement for metals in the fabrication of
medical implants and devices. Problems associated with the use of metallic implants are metal allergy, which is caused
by ionic reactions, and the inability of the metallic implant to match the elastic stiffness of the body part being attached to
it. Fatigue is also a big issue while using metal as an implant. However, the aforementioned problems can be subdued if
the polymer composite is used because its stiffness can be adjusted to the intending body part, thereby annulling any
negative reaction caused by stiffness imbalance [49, 56]. The wide range of fiber-reinforced polymer composites used for
prostheses, correction of bone defects, and producing dentures and sutures. Gunatillake and Adhikari [57] reviewed the
application of synthetic biodegradable polymers in tissue engineering. They reported a vast number of biodegradable
polymers with structures tailorable to desired mechanical properties for medical applications. Finally, they discussed the
use of polymer composites as precursors like polyols, injectable polymer systems, and scaffolds, for tissue engineering
applications. Polymers reinforced with clay-nanofillers have successfully been used to fabricate scaffolds in cell-
transplantation and neural tissue engineering. The scaffolds are made to exhibit a high degree of porosity,
biodegradability, and biocompatibility. The quantity of nanoclays used in making the scaffolds hasa significant effect on
their elastic modulus and storage capacity. However, studies have shown that scaffolds made from polymer-nanoclays
exhibit high antibacterial properties, which in turn contributes to their rapid degradation rate [52].

Construction
Polymer composites have found application in the construction of bridges, fire-resistance concrete, concrete beams, deck
panels, pile materials, earthquake-resistance columns, and sensors used for structural health monitoring technology.
Benefitting from the lightweight, corrosion resistance, durability, and earthquake failure resistance, polymer composites
are now being harnessed in construction sectors. In contrast to most common construction materials, fiber-reinforced
polymer composites offer increased strength-weight-ration and increased stiffness-to-weight ratio [49].

Military
Over the decades, there has been an increase in the use of nanomaterials in the defense and military sectors with efforts to
improve the performance of military devices and ensuring the comfort and safety of military personnel. Polymer
nanocomposites are used in various defense and military sectors, and they are being used in the production of military
devices, materials, and structures that are lighter, smaller, and of good strength. They are suitable materials used in the
production of body armor, smart textiles, gloves, and boots [49].

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Food Packaging
The unique combination of properties of polymer nanocomposites such as being a good barrier to many gases and good
mechanical, optical, and thermal properties have made these composites good materials for food packaging. Over the
decades, many polymer nanoclay composites have been developed with less permeability to gases and enhanced thermal
and mechanical properties. Okpala [46] has reported the use of nanofiller-reinforced polymer composites for strong
containers and packaging of both consumable and non-consumable substances owing to their lightweight and good
abrasion resistance.

Wastewater Treatment
The adsorption technique is used for the removal of pollutants from water.With water pollution and treatment of
wastewater from various industry becoming a crucial problem due to the presence of toxic substances and the complex
mixture of water and organic substance that can be difficult to treat, quite a number of adsorbents have been produced
using advanced polymer/nanocomposites in recent years in treating these wastewaters. Polymer nanoclay composites
show high adsorption capacity and a good life cycle for water treatment due to their ease of processing, effective cation
exchange, low cost, and toxicity. They are capable of removing various pollutants from aqueous solutions and effective
in the treatment of water [52]. Alamaar et al. [58] developed polybenzimidazole (PBI), graphene oxide (GO), and
reduced graphene oxide (rGO) nanocomposite membranes through blade coating and phase inversion techniques, for the
treatment of wastewater produced from the oil and gas industry. The experiment shows that the addition of a small
weight percent of GO into the PBI matrix gives effective oil-removal efficiency up to 99.9%; moreover, the presence of
GO also improved the mechanical stability of the membrane. It was concluded from the work that the nanocomposites’
membrane shows promising performance for wastewater treatment under harsh industrial conditions, and the membranes
can de-oil high-salinity emulsions for reuse.

Miscellaneous
Polymer composites have found several applications in many sectors ranging from the household as shown in Fig. 6 to
advanced materials such as panels, insulators, hydrogels, bio-boards, connectors in electrical industries, fabrication of
golf clubs, fishing roads, tennis rackets in sports, windmills, etc. Phuong et al. [36] reported the use of bio-polymer
composites in the developments of biodegradable membranes used in different industrial sectors with a wide range of
applications in food processing, petrochemical, desalination, and pharmaceutical industries. These composites are
promising substitutes for non-renewable, petroleum-based polymer materials and toxic solvents currently used.

Fig. 6 Selected polymer-based household products in Akure, Nigeria

DISPOSAL OF POLYMER AND POLYMER-BASED COMPOSITES

Due to the versatility of the use of polymeric composites, they are readily available in large amounts globally. In
developing countries, where there is no stringent measure to curtail the indiscriminate disposal of wastes from initial
products, they are usually a menace to the global community as shown in Fig. 7. Most times, they are burnt, and after
burning, the scar left behind will still not be conducive to an ideal environment since the entire polymeric product will
not be burnt, as shown in Fig. 8. Activities from these parts of the world contribute to the global challenges in the oceans
and atmosphere as these pollutants will end up in such places. Hence, there is a need to proffer global solutions that will
discourage the illicit act.

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Fig. 7 Refuse dumping site in Akure, Nigeria, with a lot of polymer and polymer-based composite products

Fig. 8 Refuse dumping site in Akure, Nigeria, after burning the polymeric waste products

Over the years, a linear increment in the production and consumption of polymers and polymeric composites has been
recorded due to their numerous applications in different fields. However, our manufacturing industries need to consider
the end-of-life disposal of their products following the existing environmental policies and waste management
regulations, to achieve a sustainable and eco-friendly environment. It is tenable because an increase in the production and
usage of these materials will create wastes that must be handled in the future [3]. Landfill, incineration, and recycling are
the common methods of disposing of polymers and polymer-based composite wastes [16,17]. Incineration is associated
with global warming problems, while landfills contributed to waste in every environment as there are land shortages.
Some of the polymer wastes are also transported to oceans, which adversely affect the marine life and humans who later
consume them via the food chain. The prohibition of waste storage by Germany and the introduction of additional taxes
by Sweden, which is projected to extend to other countries soon, and the existence of other regulations that are aimed at
forming effective material recycling and energy recovery processes in line with environmental and economic
consideration are against the use of landfill and incineration as disposal methods. The proposition of the European
Commission (EU) tagged ‘Plastic Strategy’ projected that, by 2030, recyclable and reusable plastic will be used for all
forms of packaging, to ensure the recovery of waste polymers and their reclaim towards the development of new
materials [4]. Researchers have reported that recycling is the best material disposal method by putting environmental
effects into consideration. The underlying principle of recycling is carried out at different stages such as separation,
shredding, and chemical or mechanical treatment, followed by drying, reforming, and development into a final product.
Compared to other materials like steel and aluminum that have effective recycling processability, this is not so for
polymer composites whose recycling processing is more complex because they are made from heterogeneous materials
like fibers, organic matrixes, and additives [59].
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Asmatulu and co-workers [18] reported that the presence of heterogeneity in the materials used for polymer composites
results in contamination, which is one of the major challenges encountered during the recycling process. In addition, the
cost of collecting, classifying, and separating the material scraps needs to be considered due to economic reasons. The
encouragement of developing eco-friendly materials like polymer composites has helped to improve the efficiency of the
existing recycling process and maximizing their energy requirements [3]. Several recycling technologies have been
developed to handle the several tons of composite wastes produced annually. The technologies can be grouped into
mechanical, thermal, and chemical methods, of which nearly all of these methods involve the reduction of the waste
materials into smaller sizes [35].
The mechanical recycling method, which is among the most investigated technologies, is known as the economically
viable method for recycling polymers and polymer-based composites. The method involves stepwise material reduction
via mechanical processes such as cutting, crushing, shredding, grinding, and milling. All the unwanted components such
as metals and impurities are separated before grinding and milling. After which, the material is classified into different
sizes, ranging from powder to various fiber lengths, which are subsequently used as reinforcement, which is then mixed
with virgin materials to develop new composite components for other applications. The mechanical method of recycling
has been used for CFRPs, GFRPs, and many other reinforced composites [18,59]. Srebrenkoska et al. [16] reported the
use of recycled rice-hull-filled poly(lactic acid) (PLA) bio-composites for the development of new environmentally
friendly materials with suitable thermal and mechanical properties. The flexural strength and modulus of the recycled
bio-composites can be compared to that of traditional formaldehyde wood fiberboards used for indoor constructions. As
shown in Fig. 9, the scanning electron microscope (SEM) micrographs of cryogenically fractured samples of recycled
biodegradable PLA-based composites revealed the surface bonding between the reinforcing fillers and matrix before and
after it was subjected to external loading. The SEM images show the similarity between the surface morphology of the
virgin and that of the recycled composites. From the analysis, it indicates that the recycling process does not affect the
surface adhesion between the filler and matrix, and that explains why the recycled composites showed a comparable
mechanical and thermal property with the virgin composites.

Fig. 9 SEM images of cryogenic-fractured surfaces of: (a) Virgin poly(lactic acid) (PLA)-based composite (b) x1
recycled PLA-based composite, and (c) x2 recycled PLA-based composite [16]

Das et al. [60] reinforced mechanically recycled polypropylene (RPP) derived from post-consumer plastic products with
jute caddies. The composite was developed using the solution impregnation method, in which silane coupling agents of
two different respective weights of 2 wt.% and 6 wt.% were used and treated with 6 vol.% Vinyltrimethoxysilane
(VTMO) to improve the interfacial interaction between the RPP matrix and jute caddies. It was reported that the flexural
strength and modulus were improved by 7.4% and 22.7%, respectively. The surface treatment method reduced the water
absorption behavior of the composites while the thermal stability of the composites was enhanced. Adhikary [61]
investigated the mechanical properties, dimensional, and thermal stability of wood flour recycled polymer composites
(WPCs) panels as building materials. The WPCs were made from recycled high-density polyethylene (rHDPE) and
polypropylene (rPP), while sawdust (Pinusradiata) was used as filler. From the result, both matrices showed superior
dimensional stability and mechanical performance when compared with the virgin materials. Meanwhile, the addition of
3wt.%maleated polypropylene (MAPP) as a coupling agent and 50% wood flour as a filler increased the tensile strength
of WPCs made from rHDPE and rPP by 60% and 35%, respectively. Hugo et al. [62] studied the mechanical and thermal
properties of recycled polymer composites for structural application. The blend of pulverized recycled amorphous and
semi-crystalline polymers of 10 mm size was reinforced with different fillers. In their work, the developed hybrid
composites possess excellent strength and modulus after incorporating a small quantity of mica blended with polymer
reinforced with glass fiber. Their studies have shown that the hybrid composites exhibited appreciable properties at a
reduced cost. In another work, Keskisaari et al. [63] investigated the mechanical properties of thermoplastic composites
developed from recycled high-density polyethylene (HDPE) and varieties of fillers. Composites developed with primary
sludge as filler showed excellent mechanical properties. Benoit [64] in his investigation mechanically reinforced rHDPE

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with 15wt.% flax fibers. The mechanical analysis shows that flax fiber is a good reinforcement alongside recycled matrix
and concluded that a higher percentage of the mechanical properties was retained in the composite after recycling when
compared with the virgin matrix used in the fabrication of the composites. Martikka and Kärki [65] reported that the
addition of suitable compatibilizers to mechanically recycled plastic provides an economically viable means of recycling
mixed waste plastics to be used for manufacturing wood mixed polymer composites. About 50% improvement in the
mechanical properties and moisture resistance of the composite was reported, which was attributed to the presence of
compatibilizers in the wood-mixed waste plastic composites. Hirayama et al. [66] uses chopped carbon fibers of 4.5 mm
and 3.0 mm length recovered from aerospace waste materials to fabricate polypropylene-based composites. The recycled
carbon fibers can reinforce the composite despite the complex phase structure and low reduced adhesion relationship that
exist between the carbon fibers and polypropylene matrix. The European Research Centers on Mathematics (ERCOM)
located in Germany and Phoenix Fiberglass in Canada are examples of industrial-scale development of mechanical
recycling. Their technology helps to recycle automobile waste by shredding, milling, and classification, even down to
powdery products used in the production of new sheet materials. However, the two companies have stopped operating
due to economic reasons. Alternatively, the European Composites Recycling Services (ECRC) was founded in 2003 to
follow the European Union policies on end-of-life vehicles. The ECRC adopts a cost-effective closed-circuit technology
to achieve shredding and grinding [67]. Tuner et al. [68] use a novel recycling method developed by the University of
Sheffield and Qatar University called polymer reformative engineering in recycling plastic waste. The process can handle
a large quantity of industrial and domestic waste, and reform them into structural components for various engineering
applications such as railway sleepers, noise barriers, flood defenze, communication, and utility poles. The recycled
composite is passed through a novel high-speed filtering operation, which makes it a faster and more efficient process
than conventional methods. Furthermore, the composites developed via this route are reported to have good mechanical
and thermal properties. The composites can be potentially used as a replacement for concrete, tropical hardwood, or
treated softwood in some applications. However, a very promising recycling process for thermoset composites was
designed by Pickering in his studies in which a combination of mechanical and thermal techniques were used, whereby
the reduction of scrap size used to produce recyclate was achieved by mechanical means. Meanwhile, the thermal
processing route was used to achieved disintegration of scraps into fibers and recovers the energy. The thermal recycling
technique involves the application of heat to decompose composite wastes into different products such as solid, liquid,
and gas. The various techniques of thermal recycling are combustion, fluidized bed combustion, and pyrolysis. These
methods recover fibers, fillers, and inserts but no recovery of monomers that can be re-used as a matrix because thermal
recycling is carried out at a temperature around 450 °C–750 °C depending on the types of material and resin at these
temperatures, which volatilize into lower molecules of gases such as carbon dioxide (CO 2), hydrogen (H2), and methane
(CH4) [63,67]. A good merit of the thermal recycling technique is the ability to generate energy from the liquid and gas
product during the process [69]. Combustion and incineration do not bring about recovery of materials. That is why it is
not referred to as a recycling technique. It involves the burning of scraps to produce an energy source that can be
converted into other forms of energy such as mechanical and electrical energy. Polymer composites reinforced with glass
fibers can be used as recovery materials and be converted into energy in cement kilns, especially those inorganic short
fibers whose recovery from the resin composites seems difficult. About 10% of the fuel used in a cement kiln can be
replaced with GCFR, which has immensely reduced the production cost of cement. However, the quality of the cement
produced is affected by the presence of boron E-glass fibers. Notwithstanding, based on reports from the European
Recycling Service Company (ERSC) and the European Composites Industry Association (ECIA), the composites
industries in Europe still recommend the cement kiln route to be the eco-friendliest means of managing glass fibers
reinforced thermoset wastes even though the process is less economically viable in comparison to landfill processes
[18,67]. Pyrolysis is carried out either in the presence or absence of air, and the latest trend involves the presence of
steam. The products of the decomposition of the matrix are oil, gases, and solids, which are used as filler, fibers, and
char. It entails degradation and conversion into oils, gases, and a solid portion, which can be used as reinforcement to
fabricate composites. Post-treatment is required to purify the fibers contaminated with char. For glass fiber-reinforced
polymers, special attention must be given to control temperature and residence duration in pyrolysis to ensure full
decomposition and pollutant-free recovered fibers. The technique can be used for the recycling of both GFRPs and
CFRPs because it is used for the development of recycled GFRPs on an industrial scale. However, studies have
confirmed that deterioration up to about 50% of the mechanical properties of glass fibers happen while recycling at
higher temperatures. Thus, it is suggested that the minimum temperature for processing GFRPs during recycling should
be set at 450 °C. Despite carbon fibers’ minimal sensitivity to temperature, they are contaminated with the remains of the
decomposed resin, which looks like char and inhibits the required interfacial bonding between the recycled fibers and
resins. To enhance the properties, there is a need for further treatment [67,69–70]. The pyrolysis carried out between the
temperatures 450 °C and 550 °C shows reasonable retained strength for carbon fibers while the glass retains no more
than 50% of its mechanical properties at a reduced temperature of 400 °C. The blend and mixture of different recycled
fibers from different stocks of varying properties can help to achieve good mechanical properties comparable to that of
virgin fibers. This strategy is usually carried out in the industries to reduce the rate at which the properties of the recycled
fiber are is varied, suggesting that the properties of the pyrolysis recycled fiber is up to 90% of the virgin fibers [69].
Szpieg [71] used fluffy, short carbon fibers recycled through a pyrolysis technique from aircraft parts, and transformed
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them into preforms, which are used to reinforce the recycled polypropylene matrix to produce recycled CF/PP
composites. The interfacial bonding, stiffness, and strength of the composites were enhanced due to the addition of
maleic anhydride polypropylene grafted to the recycled polypropylene. Shuaib et al. [72] reported that the microwave-
assisted pyrolysis (MAP) technique offers better retention strength for carbon fibers than the fluidized bed technique.
Moraes et al. [73] uses the MAP technique to recover laminated glass fibers and their results showed about 3 wt.%
humidity, 68 wt.% weight loss, and about 76% reduction in the tensile strength of the composites developed by recycling
fiberglass compared with virgin fiberglass. Witik and co-workers [74] evaluated the life cycle assessment (LCA) of both
pyrolysis and incineration for end-of-life treatment of CFRPs and their results showed that pyrolysis is a very efficient
recycling method for carbon fibers when environmental impact is more important than energy recovery. Abdou et al. [75]
recycled polymer composites obtained from industrial waste for carbon fiber recovery by pyrolysis, and a thorough
evaluation of the effect of temperature, time, and atmosphere condition on the recovered carbon fibers was carried out.
From the result, they inferred that at a temperature of 550 °C for a duration of 1 h, it was possible to obtain free carbon
fibers from the polymer matrix, and the fibers recovered are free of pores, material fracture, and carbonization. The
results showed that pyrolysis with sample communication can be applied in the disposal of polymer composite.
Nevertheless, pyrolysis is an established process for the recovery of useful products from GFRPs scraps. It is also known
as the basic optimized process that uses a hydraulic guard to ensure safety processes and energy-saving via the recovery
of useful products like clean and reusable fibers, oil fractions with minimal sulphur, halogen, and heavy metals. The oil
fraction can be directly used as fuel without further purification or inclusion of other raw materials [76]. Wu et al. [70]
demonstrated catalytic pyrolysis in molten Zinc chloride (ZnCl 2) via the recovery of carbon fibers from epoxy composite
wastes. The enhanced recovery process with full de-polymerization of the epoxy matrix at a temperature of 360°C in 1 h
20 min under standard temperature was reported due to the complete solubility effect of the molten ZnCl 2 on the epoxy
matrix. The catalytic fracture of the C-N bonds is by the Zn2+ion. Reclaimed carbon fibers from the catalytic process with
destroyable graphitization structure have surfaces almost the same as virgin fibers with a tensile strength greater than that
of the processed fiber in atmospheric conditions. Property retention of about 95% in comparison with new fibersoccurs,
as well as a reasonable reinforcing effect on the flexural property and inter-laminar strength of epoxy composites
reinforced with carbon fibers, whose performance can be likened to that of the composites reinforced with T700 carbon
fibers. Studies have shown that microwave thermolysis is a fast and efficient process for the recovery of clean carbon
fibers and also saves energy better than conventional thermal recycling processes [77]. Adherent Technologies, Inc.’s
(USA) optimization of the pyrolysis process has led to the development of a catalytic pyrolysis process operated at a
reduced temperature of 200 °C to recover clean fibers with tensile strength ranging from 83% to 99% of that of virgin
fibers [78]. The fluidized bed recycling (FBR) process has been studied and used for recovering high-grade glass and
carbon fibers from wastes glass and carbon-reinforced composites. Compared to other common techniques, the FBR
process shows a moderately high rate of recovery of filler and resin materials. It involves the use of a bed made of silica
sand, operated at a temperature of 450°C–500°C. The bed is fluidized with oxidants made of hot air to ensure fast heating
of the materials and the recovery of the fibers from the resin. Due to the volatility of the organic part of the scrap
composite by the action of the hot air, they are transferred with silica particles in the air streams, followed by separation
using appropriate classifying and recovery media. However, the recovery of energy from the organic portion of the resin
can be achieved by further combustion at a temperature of 1000 °C to produce a pure flue gas [18,59]. Several methods
of the fluidized bed combustion process have been developed. The University of Nottingham developed the fluidized bed
combustion technique to remove resin from the waste composite to be used as a source of energy to recover glass and
carbon fibers to be used as reinforcement in the fabrication of new composites. Meanwhile, the University of Hamburg
has utilized the process to recycle fibers to be used as reinforcement for the manufacture of composite and recovery of
secondary fuel [58]. The FBR process is of great interest today because of its capability to process and recycle mixture
and contaminants. It is also recommended as the best processing method for end-of-life waste polymer composites.
Nevertheless, some of the shortcomings of this method are that the process causes more degradation to carbon fibers and
has gases as the only by-product from the resin part, whereas pyrolysis offers less degradation, and still gives room for
the recovery of other valuable products containing oil [67,69]. Pender and Yang [79] experimented on the influence of
metal catalysts (CuO) nano-powders on fluidized bed recycling of epoxy composite reinforced with glass fiber. The
integration of CuO nano-powders as a catalyst for the process helps to reduce the recycling temperature of the
composites and increases the rate of de-polymerization of the epoxy. About 59% of glass fibers were recovered at a
reduced temperature of 400 °C. The mechanical analysis showed no adverse effect on the CuO nano-powders. The
chemical recycling technique involves selective chemical dissolution of composite waste in chemical solutions of
solvents such as acid, bases, alcohol, and washing liquids. The chemical solvents depolymerize the matrix to recover
clean fibers and fillers, which are further used as material for various purposes. Mechanical treatment such as
grinding/disintegration of composite waste is first carried out to increase the surface area between the solid waste and
chemical solution, as this helps to increase the rate of diffusion and dissolution. The dissolution process is classified as
solvolysis and hydrolysis based on the solvent used. For instance, water is used in hydrolysis while the organic solvent is
used in solvolysis [18,67]. On the other hand, hydrolysis is a chemical recycling process in which chemical de-
polymerization of the composite is achieved by dissolving the solid composite in water. Whereas solvolysis involves the
degradation of organic components of the composite via a chemical reaction between the ester linkages and solvent at the
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prevailing concentration and time, taking into consideration the process parameters that influence the rate of the process
such as temperature, rate of agitation, pressure, and catalyst [18]. Chemical recycling techniques produce uniform, long
fibers, with surfaces free of resin and good mechanical proportion retention compared to other techniques because the
process is not rapid and has been used in the recycling of carbon fibers. However, recovery of fibers with high
mechanical properties and low environmental impact can be achieved by optimizing the use of supercritical fluids
[18,61]. For instance, solvolysis and glycolysis can depolymerize epoxy resins into their monomers for use as chemical
feedstock [67]. Water seems to be the most commonly used solvent because it is neat and environmentally friendly, and
can be used with other solvents like alcoholic, amine, phenolic, and less-acidic catalysts [68,80]. Knight [80] uses sub-
critical and supercritical water to recycle woven CFRP on a scale that is 30% higher than the already reported scale. The
experiment shows that up to 99% of highly cross-linked resins are separated from an aerospace-grade composite system
with the recovery of single-filament woven fibers with 100% retention of tensile strength and modulus, which are later
used in the manufacture of reclaimed fiber composites with up to 95% and 98% retention of flexural strength and flexural
modulus, respectively. Shibata and Nakagawa [81] use a benzyl alcohol solvent with tri-potassium phosphate as a
catalyst to dissolve epoxy resin from a used tennis racket in the recovery of carbon fiber. The composite waste was held
in the solution for 10 min at a temperature of 200 °C. The recycled carbon fiber-reinforced polymer developed from
recovered carbon fiber has mechanical attributes thatrelate to that made with virgin carbon-fiber non-woven fabric. In
addition, the developed composite shows that the degraded epoxy, when solidified, can be used as recycled epoxy. Jiang
et al. [82] studied the use of supercritical propanol in the recycling of a carbon fiber/epoxy resin composite. The process
was carried out in a reactor for 10 min, at 300° and 50 bars, and the recycled carbon fiber shows comparable mechanical
properties with fresh carbon fibers, despite a noticeable reduction in the interfacial bonding with epoxy resin due to the
reduction in surface oxygen. Liu et al. [83] developed a mild chemical process of recycling aerospace fiber/epoxy
composite waste. Despite the difficulty in the use of a chemical method to recycle the CFRP with a high glass transition
temperature of above 200°C, the research group use ZnCl2/ethanol catalyst in the process. The efficient degradation of
the chemical bonds of the polymer was reported to be the result of the good coordinating effect of ZnCl 2, which has C-N
bonds and the swelling capability of ethanol to yield clean fibers and depolymerized polymers.
Table – 5 Control measure for polymer-based materials disposal
(i) To carry out recycling of waste
products
Expectation from RESEARCHERS: (ii) To develop alternative sources of
plastic materials
(i) Stop indiscriminate dumping of
polymer waste products
Expectation from CONSUMERS (ii) Do not leave everything for the
Government or others.
Government should be committed to
good environment and take action on
Expectation from the GOVERNMENT:
policies that can ensure clean
environments.
We should show commitment to
Expectation from INDIVIDUAL: whichever of the abovementioned
classes/categories as it relate to us.

Future Benefits of controlled Disposal Systems

 The waste serves as raw materials for new ones.
 Job and wealth creation opportunity.
 Socio-economic activities will increase, and many people will want to stay in a well-organized
environment/place.

PAST, CURRENT, AND FUTURE POTENTIALS OF POLYMER AND POLYMER-BASED

COMPOSITES
Modernization has led to the discovery of new materials that are useful for mankind. History has told us about the
different ages (stone, bronze, iron, etc.), with each of these ages depicting the kind of materials use by human at that
time. Although, biological polymers such as Deoxyribonucleic Acid (DNA) and proteins have been in existence from
inception. The absence of detailed knowledge about how synthetic polymers are developed and the existence of long-
chain molecules limit their adoption. The birth of of the polymer occurred over 100 years ago. The generic idea about
polymers or macromolecules started replacing pre-existing theories that infer that macromolecules were formed from
aggregates of small molecules. Along this line, it recorded an increase in the development of numerous synthetic fibers,
plastics, and elastomers,as did, the formulation of techniques for manufacturing high molecular weight polymers from
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monomers [84]. Ever since then, polymers have remained the material of much interest to date. Their unique properties
contributed to this as they have found numerous applications ranging from domestic use such as piping, food packaging,
and waste bags, to high-tech application like aerospace parts, military, computers, medicals, and information technology
[3]. The USA’s exordium of sweeping reforms of composite development technologies in the 1950s formally marks the
start of the development of polymer-based composite, of which 1955 – 1963 and 1964 – 1973 were tagged as the
foundation and sustaining periods, respectively. During this time, there was an increase in the cost of oil from 1974 to
1983. This transition period was a decade in which a reduction in the growth of PMCs was experienced due to expensive
raw material, reduction in the cost of fuel, and competitions from other materials companies. These are the reasons that
the PMCs industries delved into new application of products and versatility of the applications of PMCs to enhance the
industrial economic viability. Between 1984 and 1993, various studies ascertained the usability of PMCs in global
industries on the basis of safety, durability, and service integrity in combination with improvement in the manufacturing
technologies [85]. In present day, the world has witnessed a lot of research on polymers and polymer-based composites.
Some of the recent advancements in the PMCs field include but are not limited to the design of cost-efficient fabrication
techniques, development of ecological and bio-composites from a blend of natural and synthetic polymers, and
hybridization of two or more reinforcements in one matrix to manufacture hybrid composites with improved functional
and structural performances in various high tech applications [86]. Advancement in nanocomposites to improve
mechanical, optical, nonlinear optical, photochemical, and electrical properties of polymer-based composites,
development of multifunctional, smart, and intelligent materials, use of polymer-based composites in powder-based
additive manufacturing processes, and products of all these advances have found application in different fields such as
medical, petroleum, automotive, aerospace, military, sports, energy generation and storage, construction, etc. [6,7,13,87–
92]. With the emergence of various developments in diverse fields, the future trends of polymers and polymer composite
industries revolve around the development of multiphase polymer blends, organic and inorganic hybrid composites, and
the design of more novel production techniques and models to study the effect of residual stresses on resins. Advanced
research will continue on the development of intelligent materials, how the present challenges of polymeric materials
overcome the fabrication of polymers of high molecular weight without chain entanglement, excellent ductility,
appreciable glass transition temperature, good impact strength, reasonable modulus, excellent optical clarity, and
electrical conductivity. It is projected that an upgrade in the existing polymeric composite will lead to the fabrication of
composites with effective damping capacities and lightweight properties [93]. With the increasing interest in the
development of secondary materials rather than creating new ones, recycling of polymer/polymer-based composites has
become pertinent. Availability of these wastes as raw materials globally will aid the widespread development of its
secondary materials [94]. Hence, it is expected that from macro- to nano-levels, more research efforts should be tailored
towards improving the properties of both virgin and recycled polymers and polymer-based composites.

CONCLUSIONS
This review paper discussed the properties, fabrication methods, modernized applications, and disposal of polymers and
polymer composites. Polymers have a combination of unique properties such as lightweight, low cost, recyclability, and
ease of fabrication that have boosted their relevance in all aspects of modern life. These inherent properties have made
polymers the most preferred in all areas of applications. Hence, there is a global rise in demand for polymer products
because they can act as substitutes for metallic materials in numerous applications. Interestingly, the global demand for
polymers and their composites is expected to reach approximately 600 million tons in the next two decades. Further
advancement in polymeric materials such as fiber-reinforced composites, nanocomposites, and bio-composites is
inevitable. Polymer-based composites are promising to meet the current and future demands of the industries where
performance, sustainability, and eco-friendliness are key requirements. However, the challenges faced by the industries
are the difficulties associated with their disposal. Most especially, the serious threat they pose to lives and the
environment. Studies have shown that polymer recycling is an effective means of solving any environmental problem
associated with polymer disposals after use. In essence, polymer and polymer-based composites will continue to be
relevant, considering the continuous worldwide thirst for economically viable, sustainable, and eco-friendly materials
with unprecedented properties for high-tech applications. Given the foregoing, the current developments in the areas
where polymers are being used, the future trend of polymeric composites will involve the fabrication of multiphase
polymer blends, organic and inorganic hybrid composites, the design of novel and rational production techniques like
additive manufacturing, and the development of models to evaluate the influence of residual stresses on resins, and much
more research is expected on their usage as intelligent materials as well.

Conflicts of Interest: The authors declare no conflict of interest.

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