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Manufacturing Silk/Epoxy Composite Laminates: Challenges and Opportunities

Authors:
Youssef K. Hamidi at University of Houston - Clear Lake
Youssef K. Hamidi
M. Akif Yalcinkaya at University of Oklahoma
M. Akif Yalcinkaya
E Guloglu
E Guloglu
E Guloglu
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Maya Pishvar at California State University, Northridge
Maya Pishvar
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Abstract

Application of natural fibers in polymer composites has been gaining popularity in several industries pursuing environmentally friendly products. Among the natural fibers with proven potential applications, silk fibers have recently received considerable attention from researchers. Silk fibers provide higher mechanical properties compared to other commonly used natural fibers such as sisal, jute, and hemp. Silk may also exhibit comparable specific mechanical properties to glass fibers. However, silk composite laminates are rarely used in commercial products due to a number of fabrication challenges. This paper investigates such challenges for silk/epoxy laminates, especially issues related to manufacturing and preform architecture. First, challenges arising from preform architecture (i.e., random and woven preforms) are presented. Unlike glass fibers for which random mats are easier to manipulate, handling random silk preform proves to be more challenging, particularly compared to woven silk fabrics. The random silk/epoxy laminates show higher thickness variation and lower compaction, yielding lower fiber content. Second, fabrication of laminates by vacuum bag/wet lay-up and vacuum assisted resin transfer molding (VARTM) processes are presented. VARTM is found to be more appropriate for silk/epoxy laminate fabrication, as it allows a uniform impregnation of the silk preform, yielding higher part quality and limited void formation. Moreover, applying 0.21 MPa (30 psi) external pressure to the VARTM laminates allows to increase the fiber content of both random and woven silk/epoxy laminates from ~17 and ~30% to ~21 and ~33%, respectively. In contrast, wetting of silk preform during wet lay-up process, which is operator dependent, is difficult to achieve; and the produced laminates have high void content. Furthermore, SEM images show a weak silk/epoxy adhesion in laminates fabricated without external pressure. Finally, the mechanical performance of these laminates is assessed. The woven silk/epoxy laminates fabricated by pressurized VARTM exhibits the highest improvement in the specific flexural strength and modulus over pristine epoxy with 30 and 65% increase, respectively.
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Paper G06-229 1
34th International Conference of the Polymer Processing Society, Taipei, Taiwan, May 21st-25th 2018
Manufacturing Silk/Epoxy Composite Laminates:
Challenges and Opportunities
Youssef K. Hamidi a,b,*, M. Akif Yalcinkaya a, Gorkem E. Guloglu a,
Maya Pishvar a, Mehrad Amirkhosravi a, M. Cengiz Altan a
a School of Aerospace and Mechanical Engineering, University of Oklahoma, USA
b Ecole Nationale Supérieure des Mines de Rabat, Morocco
Email: hamidi@ou.edu
Abstract: Application of natural fibers in polymer composites has been gaining popularity in several industries pursuing
environmentally friendly products. Among the natural fibers with proven potential applications, silk fibers have recently
received considerable attention from researchers. Silk fibers provide higher mechanical properties compared to other
commonly used natural fibers such as sisal, jute, and hemp. Silk may also exhibit comparable specific mechanical
properties to glass fibers. However, silk composite laminates are rarely used in commercial products due to a number of
fabrication challenges. This paper investigates such challenges for silk/epoxy laminates, especially issues related to
manufacturing and preform architecture. First, challenges arising from preform architecture (i.e., random and woven
preforms) are presented. Unlike glass fibers for which random mats are easier to manipulate, handling random silk
preform proves to be more challenging, particularly compared to woven silk fabrics. The random silk/epoxy laminates
show higher thickness variation and lower compaction, yielding lower fiber content. Second, fabrication of laminates by
vacuum bag/wet lay-up and vacuum assisted resin transfer molding (VARTM) processes are presented. VARTM is found
to be more appropriate for silk/epoxy laminate fabrication, as it allows a uniform impregnation of the silk preform,
yielding higher part quality and limited void formation. Moreover, applying 0.21 MPa (30 psi) external pressure to the
VARTM laminates allows to increase the fiber content of both random and woven silk/epoxy laminates from ~17 and
~30% to ~21 and ~33%, respectively. In contrast, wetting of silk preform during wet lay-up process, which is operator
dependent, is difficult to achieve; and the produced laminates have high void content. Furthermore, SEM images show a
weak silk/epoxy adhesion in laminates fabricated without external pressure. Finally, the mechanical performance of these
laminates is assessed. The woven silk/epoxy laminates fabricated by pressurized VARTM exhibits the highest
improvement in the specific flexural strength and modulus over pristine epoxy with 30 and 65% increase, respectively.
Keywords: Silk fibers, natural fiber composites, fabrication, mechanical properties.
PACS: 81.05.Qk, 81.20.Hy, 81.70.Bt
INTRODUCTION
Over the past few years, environmental awareness and stricter environmental policies have forced industries,
including automotive and construction, to pursue alternative eco-friendly materials, thus leading to increased interest
in natural materials [1-3]. Driven by their promising biodegradability, natural fiber reinforced composites have
attracted wide attention. For instance, the low cost, low density, and sustainable nature of plant fibers, such as sisal,
jute, and hemp, made them attractive in comparison to the commonly used reinforcing fiber, E-glass, despite their
lower properties. Therefore, a significant growth in the commercial use of plant fiber reinforced composites has
been recently observed, mainly in the automotive industry [4].
In contrast, silk fibers have received limited scientific interest as a reinforcement for composites. Yet, silk
filaments exhibit higher mechanical performance than plant fibers and offer naturally continuous length. In addition
to a considerably higher toughness, its lower density makes silk a potential reinforcement to produce composites
with specific mechanical properties comparable to those reinforced with glass fibers, as suggested in recent studies
[4-5]. For example, Yang et al. [5] fabricated woven silk/epoxy composites using a wet lay-up process followed by
vacuum bagging and hot pressing at high pressure levels, yielding laminates with fiber volume fractions as high as
70%. The authors investigated the tensile, flexural, interlaminar shear, impact, dynamic and thermal properties of
the silk/epoxy composites and reported a linear increase of almost all properties with increasing fiber content
between 30 and 60%. The reported specific mechanical properties compared well with those of glass/epoxy
composites [5]. Similarly, Shah et al. [4] prepared nonwoven and woven silk/epoxy composites using vacuum
assisted resin transfer molding (VARTM), with fiber volume fractions of 36 and 45%, respectively. The authors
reported the tensile and flexural specific strengths (~90 MPa/g cm-3 and ~170 MPa/g cm-3) to be comparable,
although not necessarily superior, to those of glass/epoxy laminates. These findings propose silk fibers as potential
sustainable alternative reinforcement materials to glass fibers in structural applications.
Paper G06-229 5
34th International Conference of the Polymer Processing Society, Taipei, Taiwan, May 21st-25th 2018
Random silk/epoxy laminates showed no improvement in flexural strength over the neat epoxy. While practically
no change is observed for in absolute numbers for both laminates fabricated with wet lay-up and VARTM without
external pressure, a slight decrease is registered for the specific flexural strength as the density of the laminates
increases. A net increase of 26-48% in flexural modulus is observed over neat epoxy, with a concurrent decrease
(~50%) in strain to failure. These results are attributed to the observed weak silk/epoxy interface (Figure 4), as well
as excessive voidage [9]. Applying a 0.21 MPa external pressure, however, yields a significant increase in
mechanical performance. For instance, flexural strength increases by 26% to reach 150.7 MPa. Woven silk
fabric/epoxy laminates, on the other hand, showed improved mechanical performance, which is expected given their
higher fiber content. At 30% fiber content, the woven silk/epoxy laminate fabricated using VARTM without
external pressure displayed a 23% increase over neat epoxy, reaching a comparable performance to the pressurized
random silk/epoxy laminate. The woven silk/epoxy laminates fabricated by pressurized VARTM exhibited the
highest improvement in the specific flexural strength and modulus over pristine epoxy with 30 and 65% increase,
respectively. Comparing the obtained properties of both silk preforms further confirms the superiority of woven silk
as a reinforcement for epoxy composites. Furthermore, the fractured surfaces of these laminates, depicted in Figure
5, show mostly pulled silk fibers, which implies that an appropriate surface of silk fibers may significantly improve
silk/epoxy adhesion, yielding further improvement of the composite performance.
FIGURE 5. Representative SEM images obtained at 500X magnification from fractured surfaces of silk/epoxy laminates
manufactured with: (a) random silk mats without external pressure and (b) woven silk fabric pressurized at 30 psi.
CONCLUSION
This paper investigated fabrication challenges for silk/epoxy laminates, especially issues related to
manufacturing and preform architecture. Woven preform architecture was observed to be more convenient for
structural silk composites, as handling random silk preform proved to be more challenging. The random silk/epoxy
laminates showed higher thickness variation and lower compaction, yielding lower fiber content. Furthermore,
vacuum assisted resin transfer molding (VARTM) is found to be more appropriate for silk/epoxy laminate
fabrication, as it allows a uniform impregnation of the silk preform, yielding higher part quality and limited void
formation. Silk preform wetting in wet lay-up, on the other hand, is difficult to achieve; and the produced laminates
contain excessive voids. In addition, applying an external pressure as low as 0.21 MPa (30 psi) to the VARTM
laminates yielded an increase in the fiber content of both random and woven silk/epoxy laminates from ~17 and
~30% to ~21 and ~33%, respectively. Furthermore, SEM images showed weak silk/epoxy adhesion in laminates
fabricated without external pressure. Regarding the mechanical performance, the woven silk/epoxy laminates
fabricated by pressurized VARTM exhibited the highest improvement in the specific flexural strength and modulus
over pristine epoxy with 30 and 65% increase, respectively. These performances can be further enhanced using a
suitable surface treatment or a sizing applied to silk fibers at higher fiber contents.
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(a)
(b)
20 μm
20 μm
... Second, no attempt was made to investigate the effect of manufacturing processes and relevant process parameters on the produced silk composites. In fact, most of the reported investigations employed a rather simple, hand lay-up method to manufacture silk/epoxy laminates [16,17,30,40]. While simple fabrication methods such as hand lay-up can be attractive for their relative ease and low cost, they are operator-dependent, prone to process-induced defects, and often result in low-quality composite parts with higher void occurrence [40]. ...
... In fact, most of the reported investigations employed a rather simple, hand lay-up method to manufacture silk/epoxy laminates [16,17,30,40]. While simple fabrication methods such as hand lay-up can be attractive for their relative ease and low cost, they are operator-dependent, prone to process-induced defects, and often result in low-quality composite parts with higher void occurrence [40]. Presence of these defects, in turn, is known to significantly degrade the mechanical performance of composites [41]. ...
... Consequently, investigating more appropriate manufacturing processes for silk/epoxy composite applications, such as variants of liquid composite molding (LCM), might be of interest. Only a couple of articles used vacuum-assisted resin transfer molding (VARTM) to manufacture silk/epoxy laminates [3,40]. In fact, Shah et al. [3] were able to achieve comparable mechanical performance ...
Silk as a Natural Reinforcement: Processing and Properties of Silk/Epoxy Composite Laminates
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With growing environmental awareness, natural fibers have recently received significant interest as reinforcement in polymer composites. Among natural fibers, silk can potentially be a natural alternative to glass fibers, as it possesses comparable specific mechanical properties. In order to investigate the processability and properties of silk reinforced composites, vacuum assisted resin transfer molding (VARTM) was used to manufacture composite laminates reinforced with woven silk preforms. Specific mechanical properties of silk/epoxy laminates were found to be anisotropic and comparable to those of glass/epoxy. Silk composites even exhibited a 23% improvement of specific flexural strength along the principal weave direction over the glass/epoxy laminate. Applying 300 kPa external pressure after resin infusion was found to improve the silk/epoxy interface, leading to a discernible increase in breaking energy and interlaminar shear strength. Moreover, the effect of fabric moisture on the laminate properties was investigated. Unlike glass mats, silk fabric was found to be prone to moisture absorption from the environment. Moisture presence in silk fabric prior to laminate fabrication yielded slower fill times and reduced mechanical properties. On average, 10% fabric moisture induced a 25% and 20% reduction in specific flexural strength and modulus, respectively.
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... The fracturing of a brittle thermoset polymer matrix, with a far lower strain to failure compared to the silk fibres, will lead to complete composite failure, thus not allowing the silk fibres to reach their full potential. This is further supported in the bibliographic data presented in Table 2, where significantly lower strain at failure for thermoset matrix composites is often observed compared to their thermoplastic matrix counterparts [27][28][29][30][31]. On the other hand, although the resulting ductility of the composite is remarkably lower, the flexural strength of epoxy matrix composites is superior to this for thermoplastic composites, hypothesized due to the absence of damage development in the compressive region of the composites since thermoset matrices are often characterized by high stiffness, providing more support to the fibre against compressive loads and preventing fibre kinking [32]. ...
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In this study, the tensile and bending properties of silk fibre composites using three (0, 90) woven fabrics with different architectures are investigated. The tensile results show that the silk composites can achieve high strain to failure (more than 20%) and toughness (up to 13 MJ/m3), which can be further manipulated based on the architecture of the fabrics, thus providing more tailored properties and design freedom in applications. XCT and SEM characterization are used to investigate and explain the outstanding toughness of these composites. In tension, a high density of microcracking was observed away from the failed location, which could explain the intrinsic high ductility and energy absorption of silk fibre composites by means of damage spreading throughout its volume in contrast to inherently brittle materials. In bending, significantly lower properties were observed with the more striking being the strain at failure reaching only 30% of the tensile value, thus limiting the potential of silk fibres in bending-dominated loading configurations. XCT revealed that the lower performance is due to failure on the compressive side of the composites, where a clear characteristic kink-band was observed in all composites subjected to bending, while there was no visible damage on the side under tension. This behaviour is also linked to the soft HDPE polymer matrix used, which provides little resistance to fibre buckling.
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... Given the particular stress-strain behaviour of silk fibre, with a modest E-modulus, but a very high strain to failure, it is perhaps surprising that a remarkable number of researchers have focused on thermoset matrices, particularly epoxy [1,2,4,5,15,16,17], with a strain to failure below the strain to failure of the silk fibre. This would normally mean that in these composites the matrix will fail first and the question is if the silk fibre can then keep on carrying the load to reach its full potential. ...
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  • Jan 2016
  • 447-470
  • N Saba
  • M Jawaid
  • O Y Alothman
  • M T Paridah
  • A Hassan
N. Saba, M. Jawaid, O. Y. Alothman, M. T. Paridah and A. Hassan, J. Reinf. Plastics & Compos. 35, 447-470 (2016)