AUTEX Research Journal, Vol. 17, No 3, September 2017, DOI: 10.
1515/aut-2016-0010 © AUTEX
AN EXPERIMENTAL STUDY OF THE COMPRESSION PROPERTIES
OF POLYURETHANE-BASED WARP-KNITTED SPACER FABRIC COMPOSITES
Si Chen, Xue-pei Zhang, Hong-xia Chen, Xiao-ping Gao
College of Light Industry and Textile, Inner Mongolia University of Technology, Hohhot, China
Correspondence to: Si Chen email: ansn9119@126.com
Abstract:
The present work has reported the compression properties of polyurethane-based warp-knitted spacer fabric
composites (PWSF). In order to investigate the effect of structural parameters of fabric on the compression
performance of composites, a series of warp-knitted spacer fabrics (WSF) with different structural parameters
including spacer yarn inclination angle, thickness, fineness of spacer yarns, and outer layer structure have been
involved. The produced composites have been characterized for compression properties. The energy-absorption
performance during the compression process has been determined as a function of the efficiency and the
compression stress obtained from compression tests. The results show that the composites based on spacer
fabrics having smaller spacer yarns inclination angle, higher fabric thickness, finer spacer yarn, and larger mesh
in outer layers perform better with respect to energy-absorption properties at lower stress level, whereas at higher
stress level, the best energy-absorption abilities are obtained in case of spacer fabrics constructed of larger spacer
yarn inclination angle, lower fabric thickness, coarser spacer yarn, and smaller mesh in surface layers.
Keywords:
Compression properties, Energy-absorption capacities, Structural parameters, Warp-knitted spacer fabrics
1. Introduction Vuure [9] has developed a unit-cell model of core properties
of composite panels based on spacer fabrics, and the finite
Recently, textile fabrics are commonly used as alternative element method calculations of the compression responses of
low-cost reinforcement for structural applications, such as these novel composites have been performed.
automotive, packaging, building products, furniture, and
consumer goods [1]. The various applications are demanding The present work reports the compression properties
complex-shaped fabrics to meet the requirements of such of polyurethane-based WSF composites for cushioning
domains [2]. Warp-knitted spacer fabrics (WSF) are one of the applications. With an attempt to discuss the effect of fabric
complex-shaped 3D constructions composed of two separate structural parameters on compression properties of the novel
fabric layers connected by spacer yarns. The spacer yarns have composites, a series of WSF with different parameters (such
different inclination angles depending on the different lapping as spacer yarn inclination angle, thickness, diameter of spacer
movements of guide bars. And the thickness of spacer fabrics yarn, and outer layer structure) were fabricated on a double-
can also be designed. Moreover, WSF composites exhibit needle-bar warp knitting machine of E18. It is expected that
greater drapability and compression resistance capacities as a regular pattern for tailoring WSF composites with favorable
compared to other textile-reinforced composites. All of these compression responses could be found from this study.
advantages make WSF to obtain great potential to be used for
the reinforcement of composites.
2. Experiment part
In some of the published studies, the compression behaviors
of composites reinforced by knitted spacer fabrics have been 2.1. Samples
examined [3-7]. These composites were produced based on
an unsaturated resin. It can be found in these studies that the Warp-knitted spacer fabrics
structural parameters of spacer fabrics have obvious influences
on the compression properties of these composites. Also, a Eight WSF with different structural parameters were involved
type of sandwich composite panels based on WSF, which have in this study. They were produced on a double-needle-bar
a considerably lightweight core, has been fabricated by using Raschel warp knitting machine of E18 (Wuyang CO. LTD,
modified vacuum assisted resin transfer molding (VARTM) Jiangsu, China). The PET monofilament of 0.2 mm (0.16 mm)
technology [8]. This study shows that the panels with higher in diameter was used as spacer yarns, and 300D/96F PET
cross-thread density and finer yarns exhibit high facing bending multifilament was used to knit the surface layers of fabrics.
stress and core shear stiffness. And coarser cross-thread used Three different structures, that is, Chain+Inlay, Rhombic Mesh,
in the core results in better compression responses. Recently, and Hexagonal Mesh, were involved for knitting surface layers.
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The chain notations for each outer layer structure are listed in Preparation of the composites
Table 1. Three types of spacer yarns were used to connect the
two outer layers with different inclination angles. The details The warp-knitted spacer fabric composites (hereinafter referred
of spacer yarns are presented in Table 2. By considering to briefly as PWSF, Figure 2) were produced by impregnating
the different types of spacer yarns, different thickness, and these warp-knitted spacer fabrics with a semi-rigid polyurethane
different outer layer structures, eight WSF were produced. The foams, consisting of a mix of 1.21 g/cm3 isocyanate and 0.78
structural parameters of these WSF are shown in Table 3. The g/cm3 polyol, in a 43.7/100 isocyanate–polyol mixing ratio (by
front elevation and right view of WSF1, which is chosen as the weight). Production of the composite samples was carried out
representative of eight samples, are provided in Figure 1. in a mold whose and bottom surfaces can be heated by a water
Table 1. Chain notations for outer layer structures
Structure Chain notation
GB1:0-0 0-0/5-5 5-5// fully threaded GB2:1-0 0-0/1-0 0-0// fully threaded GB5:0-0 1-0/0-0 1-0// fully
Chain+Inlay
threaded GB6:0-0 5-5/5-5 0-0// fully threaded
GB1:1-0 0-0/1-2 2-2/2-3 3-3/2-1 1-1// 1 fully 1 empty threaded GB2:2-3 3-3/2-1 1-1/1-0 0-0/1-2 2-2// 1
fully 1 empty threaded
Rhombic Mesh
GB5:1-0 1-0/0-0 1-2/2-2 2-3/3-3 2-1// 1 fully 1 empty threaded
GB6:2-3 2-3/3-3 2-1/1-1 1-0/0-0 1-2// 1 fully 1 empty threaded
GB1: (1-0 3-3/3-2 1-1)×2/(1-0 3-3/3-2 4-4)/(5-4 3-3/3-2 4-4)×2/5-4 3-3/3-2 2-2//
2 empty 2 thread
GB2: (5-4 3-3/3-2 4-4)×2/(5-4 3-3/3-2 1-1)/(1-0 3-3/3-2 1-1)×2/1-0 3-3/3-2 4-4//
Hexagonal Mesh
2 empty 2 thread
GB5: (1-1 1-0/3-3 3-2)×3/(4-4 5-4/3-3 3-2)×3// 2 empty 2 thread
GB6: (4-4 5-4/3-3 3-2)×3/(1-1 1-0/3-3 3-2)×3// 2 empty 2 thread
Table 2. Details of spacer yarns
Symbol Diameter(mm) Lapping movement
GB3:1-0 3-2/3-2 1-0// 1 full 1 empty
I 0.2
GB4:3-2 1-0/1-0 3-2// 1 empty 1 full
GB3:1-0 4-3/4-3 1-0// 1 full 1 empty
II 0.2
GB4:4-3 1-0/1-0 4-3// 1 empty 1 full
GB3:1-0 4-3/4-3 1-0// 1 full 1 empty
III 0.16
GB4:4-3 1-0/1-0 4-3// 1 empty 1 full
Table 3. Structural parameters of warp-knitted spacer fabrics
Course-wise Wale-wise
Thickness Area density Bottom
Specimen density density Top layer Spacer yarn
(mm) (g/m2) layer
(w/5cm) (c/5cm)
WSF1 7.68 34.95 28.25 911.8 C I C
WSF2 7.72 35.35 27.86 881.7 C II C
WSF3 7.69 34.17 28.2 760.3 C III C
WSF4 6.12 33.42 27.05 820.3 C I C
WSF5 10.62 35.12 28.05 1027.1 C I C
C: 35.75 C: 28.50
WSF6 7.58 829.1 C I H
H: 36.20 H: 28.57
WSF7 7.61 34.65 27.56 756 R I H
WSF8 7.66 35.32 29.18 778.2 H I R
“C,” “H,” and “R” represent Chain+Inlay, Hexagonal Mesh, and Rhombic Mesh, respectively.
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bath heating system. Furthermore, the height of the mold can diameter. The specimens were pressed to a deformation with
be easily adjusted according to the thickness of corresponding 60% of the initial thickness at a load speed of 1 mm/min in an
fabrics. The isocyanate–polyol mixing liquid was infused environment of 23°C and 65% relative humidity. Five repeats
through the core of spacer fabrics in the warp direction. At the were carried out for each specimen. Each compression stress–
same time, the temperature of the water was maintained at strain curve provided is an average of the five experimental
40°Cduring the infusion process, which was carried out for 8 results.
min, in order to maintain the cured polyurethane foam. After
foaming, all the specimens were placed for 24 hours at room
temperature, until the polyurethane foam was stable. The eight 3. Result and discussion
types of composites produced have been listed in Table 4.
3.1. The stress–strain curves and energy-absorption
2.2. Compression experiments efficiency diagrams
The composites were characterized for compression properties A typical compression stress–strain curve of composite
according to the Chinese standard GB/T8168-2008 using (PWSF1) is shown in Figure 3. According to the published
HuaLong Compression Instrument (Shanghai, China). The results [4,10-12], the compression stress–strain curve can be
diameter of the two compression circular platens is 100 mm. split into three stages: linear stage (stage I), plastic plateau
The shape of all the specimens was circle with 60 mm in stage (stage II), and densification stage (stage III). At the
(a) (b)
Figure 1. The front (a) and right (b) view of WSF1
Figure 2. The real appearance of PWSF
Table 4. Details of composites
Finished properties of polyurethane foam Fiber volume
Thickness Weight ratio of
Sample Elastic modulus fraction
(mm) Density (g/cm3) fabric/foam (%)
(MPa) (%)
PWSF1 7.68 0.795 12.09 7.33 18.2/81.8
PWSF2 7.72 0.795 12.09 6.57 17.3/82.7
PWSF3 7.59 0.795 12.09 6.32 17.8/82.2
PWSF4 6.12 0.795 12.09 8.91 16.8/83.2
PWSF5 10.62 0.795 12.09 7.26 19.2/80.8
PWSF6 7.58 0.795 12.09 6.92 17.9/82.1
PWSF7 7.61 0.795 12.09 2.25 18.3/81.7
PWSF8 7.66 0.795 12.09 6.64 17.6/82.4
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initial stage (stage I), the compression curve exhibits a linear point, the efficiency begins to decrease because of the swift
behavior because of the compression of loose foams and their densification of composites. According to the above analysis,
poor constraint capacities for fabrics. When the composite it is demonstrated that the efficiency diagrams are useful
is further compressed, the curve goes to a plateau region in tools to determinate the preferable energy-absorption region
which a relatively constant stress is exhibited. Then, there is of composites. Therefore, the energy-absorption efficiency
a rapid increase in the stress values at stage III, because the diagrams will be used throughout this study to discuss the effect
entire composite becomes densified at this stage. of fabric structural parameters on the compression behaviors
of composites.
Figure 3. Typical stress–strain curve of composite
Figure 4. The compression stress–strain curves of composites
Referring to Figure 3, it can be observed that a nearly constant
stress is obtained with a large deformation at stage II. This
specific behavior is just the requirement of cushioning materials.
From the beginning of stage I to the end of stage II, the area
under stress–stain curve represents the ideal energy absorbed
region where the stress values are low and nearly constant. It
is important to point out that at stage III, the energy absorbed
by composite is low, but the stress values are high. In these
circumstances, cushioning materials should be used to absorb
energy before reaching their densification stages in order to
avoid high compression stress. The compression stress–strain
curves for all the composites are listed in Figure 4.
Although the energy-absorption behaviors can be shown
in stress–strain curves, in order to have better view of the
energy-absorption process, the energy-absorption efficiency E
is involved. The energy-absorption efficiency E is defined as
the ratio of the energy absorbed by a real material at a given Figure 5. The energy-absorption efficiency diagrams of composites
strain and energy absorbed by an ideal one that transmits the
same but constant force at the same strain [13]. The absorption 3.2. The influence of fabric structural parameters on
efficiency E can be expressed by Equation (1): compression behaviors
å
=
E
W
=
∫0 σ (ε )d ε (1) Inclination angle of spacer yarn
σ σ
where W is the absorbed energy per unit volume and σ is the Spacer yarns are used to connect two fabric outer layers.
stress at the strain ε . The inclination angle (ɑ, Figure 6) is defined as the angle
between spacer yarns and outer layers. In order to investigate
The energy-absorption efficiency diagrams are shown in Figure the effect of inclination angle on the compression behaviors
5. From energy-absorption efficiency diagrams, it is clearly of composites, two specimens (PWSF1 and PWSF2) with
observed that a similar variation trend as that of stress–strain different inclination angles were used for comparison study.
curves is exhibited. Furthermore, the maximum efficiency
points that can be regarded as the critical points between Figure 6 shows the global arrangements of spacer yarns
stage II and stage III are obtained at the end stage II. After this of type I and II. In Figure 6, the X-, Y-, and Z-axes indicate
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the direction of weft, wale, and thickness of spacer fabric, Thickness
respectively. All the dots in the figure represent the junctions
between spacer yarns and fabric outer layers. The red lines Three specimens (PWSF1, PWSF4, and PWSF5) with the
and green lines represent the spacer yarns carried by GB3 same type of spacer yarn and surface layer density were
and GB4, respectively. In order to have a better view of spacer involved in this section. However, the thickness of these
yarns’ arrangements, three adjacent courses are put in three specimens is different.
planes: the white dots, representing the first course, are placed
in the first plane, while the black and blue dots, representing It is interesting to note that PWSF4 has the highest cushioning
the second and third courses, respectively, are placed in the performance (Figure 4). For PWSF1 and PWSF5, at linear and
subsequent two planes. However, it is necessary to point plateau stages, the compression resistance decreases as the
out that the blue dots placed in the third plane represent the thickness increases; after the strain reaches about 50%, the
beginning of a new cycle process. situation is inversed. At the end of plateau stage, the stress
values of PWSF1 drops obviously, while the stress value of
Obviously, it can be found from Figure 6 that the inclination PWSF5 has a slight increase. The efficiency–stress diagrams
angle decreases as the lapping movement of guide bars (GB3 are shown in Figure 5. It can be found that PWSF1 exhibits the
and GB4) increases. The sequence of inclination angle for highest efficiency, while the efficiency of PWSF4 is the lowest.
these two specimens is PWSF1 > PWSF2. However, PWSF4’s stress values at maximum efficiency are
the highest, whereas PWSF5 achieves its maximum efficiency
If the performances of PWSF1 and PWSF2 under compressive at the lowest stress values. PWSF1 has the middle values of
loading are compared, it is clear from Figure 4 that the stress at its maximum efficiency. In these circumstances, the
compression resistance increases as the inclination angle specimens made by different thickness have their own ranges
increases. The specimen made from larger inclination angle of applications. It is inappropriate to simply compare the
has higher plateau values than the specimen with smaller energy-absorption capacities of specimens made with different
inclination angle. It is indicated that for a given amount of energy thickness. It is necessary to take into consideration the amount
to be absorbed, the specimen with larger inclination angle will of energy to be absorbed and the stress level to be allowed
have higher stress values because of its higher plateau values. when selecting the thickness of specimens. Furthermore,
From Figure 5, it can be found that the maximum efficiency the efficiency diagrams can be a good tool for optimizing the
decreases as the inclination angle decreases. At lower stress thickness of composites.
level, less than about 0.3 MPa, it can be clearly observed that
PWSF2 has higher efficiency. However, after this stress level, Fineness of spacer yarn
its efficiency reverses. The behavior of PWSF1 is the inverse
of PWSF2. These results show that at lower stress level, the Two specimens (PWSF2 and PWSF3) with the same lapping
specimen made from smaller inclination angle has better movement of spacer yarn, but with different spacer yarns’
performance on the energy-absorption capacities. In contrast, diameters (0.2 mm and 0.16 mm), were involved in this regard.
the specimen constructed with larger inclination angle is more These two specimens also have the similar thickness and outer
appropriate for absorbing energy at higher stress level. layer density.
(a)
(b)
Figure 6. The global arrangements of spacer yarns: (a) type I and (b) type II
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From Figure 4, it can be seen that the specimen made from at lower stress level, less than about 1.3 MPa, PWSF8 has
coarser spacer yarn has better cushioning performances the highest energy-absorption efficiency, whereas PWSF7’s
and higher plateau values. The maximum efficiency values efficiency is the lowest. However, when the stress is higher
increase with the increasing spacer yarns’ diameter, as shown than 1.3 MPa, the situation is inversed. The earlier results
in Figure 5. The stress values at maximum efficiency point are demonstrate that the specimen made from hexagonal mesh
slightly higher for specimen made from coarser spacer yarn. It in both surface layers is more suitable for absorbing energy at
can also be observed that the specimen constructed of finer low stress level, while the specimen constructed of Rhombic
spacer yarn has higher efficiency when the stress is less than Mesh in both outer layers exhibits preferable cushioning
0.5 MPa, indicating that the specimen made from finer space behaviors at lower stress values. Thus, varying the structures
yarn is suitable for absorbing energy at lower stress level. of outer layers can be another method to adjust the energy-
However, the specimen made from coarser spacer yarn can absorption properties of composites.
absorb more energy in a range of stress level higher than 0.5
MPa. Thus, it can be concluded that the specimen made from
finer spacer yarn is suitable for lower-stress energy-absorption 4. Conclusions
use, whereas the specimen constructed of coarser spacer yarn
can be used for absorbing energy at higher stress level. In this study, the compression behaviors of WSF composites
developed for cushioning applications have been thoroughly
Outer layer structure investigated. The following conclusions were established
through this work:
In this section, four types of outer layer structures (PWSF1,
PWSF6, PWSF7, and PWSF8) were involved for comparison As a new type of cushioning materials, WSF composites not
study. These specimens have the same type of spacer yarn and only have excellent strength but also have outstanding energy-
nearly the same thickness. The outer layer structures can be absorption capacities. Furthermore, their energy-absorption
divided into three categories: two-side outer layers with close capacities can be easily tailored depending on specific end-
structure (PWSF1), one-side outer layer with open structure/ use requirements by altering the structural parameters of WSF.
one-side surface layer with close structure (PWSF6), and both
outer layers with open structures (PWSF7 and PWSF8). Figure The structural parameters of the fabric have obvious influences
7 shows each of these outer layer structures. It is known that the on the energy-absorption capacities of composites. The
surface layer structures could slightly influence the outer layer specimens with smaller spacer yarn inclination angle, higher
density and spacer yarn’s inclination angle, although these fabric thickness, finer spacer yarn, and larger mesh in outer
parameters were kept constant during the knitting process. layers are suitable for absorbing energy at lower stress level
with higher efficiency. In contrast, the specimens constructed
Referring to Figure 4, it is clearly observed that PWSF7 of larger spacer yarn inclination angle, lower fabric thickness,
and PWSF8 exhibit the highest and lowest compression coarser spacer yarn, and smaller mesh in surface layers can
resistance, respectively. PWSF1 and PWSF6 have middle be used to absorb energy at higher stress level.
compression resistance abilities. The plateau values of
PWSF8 are lower than that of the other specimens, which
indicates that the maximum efficiency of PWSF8 can be Acknowledgement
obtained at a lower stress values. On the contrary, PWSF7
will achieve its maximum efficiency at a higher stress level This work was supported by The Foundation of Inner Mongolia
because of its higher plateau values. As shown in Figure 5, University of Technology (ZD201620) and The National Natural
it can be revealed that the specimens (PWSF7 and PWSF8) Science Foundation of China (11462016).
with open structure in two-side outer layers also perform
the highest and lowest stress values, respectively, at the References
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