Manufacturing silk/epoxy composite laminates: Challenges and opportunities

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.

Silk as a Natural Reinforcement: Processing and Properties of Silk/Epoxy Composite Laminates

“…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 to glass/epoxy laminates at a fiber content of only 45%.…”

“…A lower occurrence of process-induced defects within the silk composites is believed to play a significant role in reaching this performance. More recently, our research group investigated fabrication challenges for silk/epoxy laminates [40], which showed that compared to hand lay-up, VARTM was more appropriate for silk/epoxy laminate fabrication as it allows uniform impregnation of the silk preform by the liquid resin, yielding higher part quality and reduced void formation.…”