1. Introduction
During the last several decades, fiber-reinforced polymer composites have experienced remarkable growth in various sectors, ranging from packaging and sporting goods to automotive and aerospace industries. This increased usage is essentially due to their lightweight, higher mechanical properties, and superior corrosion resistance compared to conventional materials [
1,
2].
Recently, growing environmental awareness has led to stricter policies regarding sustainability and encouraged industry to pursue ecofriendly products [
3,
4,
5]. In this context, natural fibers have attracted increased attention over the past several years as alternatives to traditional reinforcements, namely glass, carbon, and aramid fibers. Currently, glass fibers are the most commonly used reinforcement in composites [
3], since they offer a stable supply chain and relatively low-cost products with high mechanical performance. However, these inorganic fibers introduce several drawbacks, including non-biodegradability, high abrasion of processing equipment, and potential dermal and respiratory irritations [
6]. In contrast, natural fibers offer a lower density, less abrasiveness, as well as promising biodegradability and sustainability [
4,
5,
7]. For instance, plant-based natural fibers such as sisal, flax, jute, and hemp have been widely investigated in the literature as potential low-cost, ecofriendly alternatives to synthetic fibers [
5,
6,
7]. Nonetheless, composites reinforced with plant fibers exhibit lower mechanical performance compared to those reinforced with glass fibers, which limits their use in structural applications [
4,
8,
9]. Furthermore, plant fibers tend to exhibit thermal instability at elevated temperatures, lower impact strength, and mechanical degradation during processing. Despite these drawbacks, the commercial use of plant fiber composites in non-load-bearing applications has significantly increased, predominantly in the automotive industry [
3,
10].
In contrast to fibers extracted from plants, silk is an animal-based fiber that offers attractive features such as low density, flame resistance, and high elongation even at low temperatures [
4,
8,
11]. More importantly, silk exhibits higher mechanical performance than plant fibers, and, in some cases, comparable specific mechanical properties to glass fibers [
4,
8,
9]. Silk denotes a group of protein-based fibers, called fibroin, produced by several arthropods like silkworms, spiders, and scorpions [
3,
12]. Fibroin generally has an irregular, almost-triangular cross-section, with a width in the range of 8 to 13 µm [
13]. Owing to its biocompatibility and bioresorbable properties as well as high strength and toughness, silk fibers are used in a variety of clinical applications, such as braided suture threads for surgical procedures and scaffolds for cartilage and bone repair [
13,
14]. Aside from biomedical applications, silk from the cocoons of the domesticated mulberry silkworm,
Bombyx mori, is of particular interest in textile industry due to its availability [
9,
14,
15]. Silk cocoons are generally degummed, spun into rovings and yarns, then woven into textile fabrics [
14,
15]. These woven silk fabrics may be used as a woven reinforcement in composites for structural applications [
14,
15,
16]. In addition, some researchers have explored using abundant silk waste from textile industry to reinforce polymer composites [
17,
18,
19].
Despite these promising features, silk fibers have received only limited interest as a reinforcement for polymer composite products, and practically no commercial use exists beyond biomedical applications [
4,
9,
11,
13,
15]. One plausible explanation for this limited use is the higher cost of silk compared to plant fibers in a very cost-competitive environment, especially for nonstructural composite parts. While silk might be more expensive than conventional reinforcements, waste silk fabric can be processed and utilized in composite laminates in a cost-effective manner [
17,
18,
19]. Another possible limiting factor of silk fibers seems to be the incompatibility between the hydrophilic natural fibers and the hydrophobic polymer matrix that requires some form of surface treatment to improve the interfacial bonding [
3,
6]. In addition, silk is known to be prone to environmental factors, such as moisture and UV radiation, that significantly alter the mechanical performance of the fibers [
20,
21]. Therefore, silk fibers and fabrics might require special storage and transportation conditions for best performance. Nevertheless, the high specific properties of silk fibers make it a suitable replacement for glass fibers in composite applications where lightweight and energy-absorbance are important, such as automotive, aerospace, and wind turbine structures [
15,
16].
The limited available literature on silk-reinforced composites mainly investigated either discontinuous silk fiber reinforced thermoplastics, or continuous silk fiber reinforced thermosetting composites. Due to their recyclability, discontinuous natural fibers, also referred to as short fibers, were traditionally used to reinforce injection-molded thermoplastics, particularly polypropylene [
22]. More recently, short fiber silk composites were used to reinforce biodegradable thermoplastics, such as polylactic acid (PLA) [
22], poly vinyl alcohol (PVA) [
23], and polybutylene succinate [
24]. For instance, Ho et al. [
22] manufactured PLA composite reinforced with 5 wt.% short silk fibers by injection molding, and reported a 27% and 2% improvement over PLA in tensile and flexural moduli, respectively. Considerably higher improvements were reported in several mechanical properties for silk reinforced gelatin composites over neat gelatin [
25], including a 260% increase in tensile strength, a 4-fold rise in tensile modulus, a 320% improvement in bending strength, a 450% increase in bending modulus, and a 260% improvement in impact strength. Although these improvements achieved using silk fibers are significant for certain ecofriendly applications, the obtained mechanical performance remains inferior to glass reinforced composites.
On the other hand, structural composite laminates, intended for energy-absorbing structures, are often fabricated using woven textile fabrics and stiffer thermosetting polymers [
3,
8,
9,
15,
26,
27]. Epoxy resins are frequently used owing to their lower cost, higher processability, higher mechanical properties, good adhesive performance, and chemical resistance [
11,
15]. For instance, Oshkovr and coworkers attempted to use woven silk/epoxy composite square tubes as energy-absorbers and evaluated their crashworthiness [
8,
26]. However, catastrophic failures were reported under compression tests in both studies. Although impressive single fiber properties might be reported in the literature [
3,
8,
21], the actual improvement over unreinforced epoxy may be limited by defects in the silk fabric, such as fiber misalignment and waviness inadvertently introduced during weaving. Furthermore, Yang et al. [
15] investigated the tensile, flexural, interlaminar shear, impact, dynamic, and thermal properties of the silk/epoxy composites at 30%, 40%, 50%, 60%, and 70% fiber contents. A linear increase of most properties was observed with increasing fiber content between 30% and 70%. Optimal tensile properties were observed at 70% fiber content, with 145%, 130%, and 70% improvement over neat epoxy in tensile stiffness, ultimate stress, and ultimate strain, respectively. In addition, impact strength was observed to increase significantly only for fiber contents above 60%. The same research group studied silk/epoxy laminates for two natural silk varieties:
Bombyx mori and
Antheraea pernyi silk [
27]. The authors reported that at 60% fiber content, both silk types showed a 2-fold increase in both specific tensile modulus and strength compared to the unreinforced epoxy resin. For the 60%
Antheraea pernyi silk/epoxy laminates, the breaking energy was found to be 11.7 MJ/m
3, an order of magnitude higher than the 1.1 MJ/m
3 measured for neat epoxy. Moreover, a 3-fold increase in specific flexural strength was also reported, reaching 316 MPa/g·cm
−3.
More recently, Shah et al. [
3,
7,
10] attempted to make a case for silk as a reinforcing agent in composite laminates by comparing their mechanical performance to flax- and glass-reinforced composites. The authors fabricated silk/epoxy laminates with nonwoven silk preform at 36% fiber content and woven silk fabric at 45% fiber content. The authors reported tensile and flexural specific strengths of ~90 MPa/g·cm
−3 and ~170 MPa/g·cm
−3, respectively [
3]. These values were comparable, although not necessarily superior, to those of glass/epoxy laminates. Other researchers incorporated silk into glass reinforced composites in the pursuit of hybrid composites with improved impact properties [
16,
17,
28,
29,
30]. For example, Zhao et al. [
16] investigated silk fabric/glass mat/polyester hybrid laminates at 14.5% and 2.4% fiber content of glass and silk, respectively. However, the authors reported practically no effect of the limited silk fabric presence on impact and flexural properties.
Surprisingly, two important aspects were not addressed in the available literature on silk reinforced composites. First, silk fabric is mostly used as received and surface treatment is seldom attempted to improve the silk/epoxy interface [
31,
32,
33]. Generally, fiber sizing can be used to tune the bulk properties of composite laminates [
34,
35]. Surface treatment of natural fibers has been shown to significantly improve the properties of composite laminates [
36,
37,
38,
39]. 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]. Presence of these defects, in turn, is known to significantly degrade the mechanical performance of composites [
41]. A few other studies [
9,
15,
27] used hand lay-up followed by hot pressing in order to increase the fiber content of the laminates, and thus improve the composite performance.
As described in the previous paragraph, remarkable improvements over neat epoxy were only achieved for silk/epoxy composites with fiber contents of 60% and higher [
15,
27]. Studies conducted at low or moderate fiber contents did not yield considerable improvement in mechanical performances. 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.
In order to investigate the suitability of silk as an alternative reinforcement to glass fibers in polymer composites, the processability of silk reinforced composites was verified by fabricating silk/epoxy laminates using VARTM. In addition, the effects of manufacturing process and microstructural parameters such as post-fill external pressure and silk fabric anisotropy on the process-induced microstructure of silk epoxy laminates were studied. The mechanical performance of the fabricated laminates was also compared with those of neat epoxy and glass/epoxy laminates. Finally, the effect of storage conditions of silk fibers and moisture absorbed by silk on the manufacturing and performance of silk/epoxy composites was investigated.
2. Materials and Methods
2.1. Materials
INF 114 epoxy (PRO-SET) was used as the resin and polyamine INF 211 (PRO-SET) was chosen as the hardener. Before laminate fabrication, the resin and curing agent were mixed for 5 min at 350 rpm at a ratio of 100:27.4 by weight and degassed for 10 min under vacuum.
A woven silk fabric (Satin Ahimsa, Aurora Silk, Inc., Portland, OR, USA) was used in this study. The silk was produced from degummed cultivated Bombyx mori mulberry silk and had an areal density of 81.4 g/m2. For each laminate, twelve layers of 152 mm-wide × 203 mm-long (6″ × 8″) silk fabric were prepared, stacked, and placed on the mold before infusion. Preparation of silk layers involved cutting the fabric to the desired size, ironing to remove wrinkles, and then drying in a vacuum oven at 50 °C. Since silk fabric exhibited an unbalanced weave pattern, laminates were fabricated with layers cut along both planar directions to investigate the effect of fabric anisotropy on the fill time and mechanical performance. Hence, two separate sets of laminates were investigated for each case: one set with fabric layers cut such that its length is parallel to the roll direction (longitudinal), and another set cut such that its length is perpendicular to the roll direction (transverse).
2.2. Manufacturing Procedure
An improved variant of vacuum-assisted resin transfer molding (VARTM) was used to fabricate silk/epoxy laminates.
Figure 1 illustrates the experimental molding setup which can facilitate external air pressure on a typical VARTM mold to increase the limited compaction pressure in conventional VARTM (i.e., higher than 1 atm) [
42]. Depending on the applied pressure, the fiber volume fraction in fabricated laminates can be significantly increased, which yields high-quality laminates with improved mechanical properties.
As seen in
Figure 1, 12 layers of woven silk fabric (i.e., preform) were sealed with a vacuum bag and the epoxy/hardener mixture was infused into the mold from the inlet resin reservoir towards the vacuum source (i.e., exit). In order to reduce the effect of ambient temperature fluctuations, the mold temperature was kept constant at 30 °C. At this temperature, the viscosity of the resin was in the range of 180 to 200 mPa s. After the preform was completely wetted, infusion was continued for an additional 5 min (i.e., resin flushing) to mobilize and remove the process-induced voids with resin outflow from the exit gate [
42]. Once the resin flushing was completed, the inlet port was closed to remove the excess resin from the exit of the mold (i.e., resin bleeding). In certain fabrication scenarios an external chamber pressure of 300 kPa (~44 psi) was applied during the post-filling stage to further compact the preform and improve the mechanical properties. All fabricated laminates were cured at 60°C for 8 hours after resin gelation occurred in 5 h at 30 °C.
Table 1 presents the various fabrication scenarios used in this study and lists the laminate designations, reinforcement types, and fabric/lay-up details as well as the thicknesses of the fabricated laminates. For comparison purposes, neat epoxy samples (E) were manufactured by gravity casting and cured following the same cure schedule. In addition, glass/epoxy laminates (G), fabricated using the same resin in a recent study by our research group [
43], were considered. Five different fabrication scenarios for silk composites (S) were performed. These scenarios were designed to assess the effects of fabric anisotropy, external pressure, and fabric moisture on the properties of the fabricated laminates. Furthermore, fabric anisotropy effects on wetting characteristics such as fill time, critical in defining the manufacturing cycle, were also investigated.
Figure 2a shows a representative image of the fabric and the disposition of longitudinal (
y) and transverse (
x) directions with respect to the fabric roll. Additionally,
Figure 2b shows that the silk fabric was tightly woven with low porosity between the fibers with several silk threads in orthogonal directions. Abbreviations “L” and “T” were used to indicate whether the infusion was performed along the longitudinal or transverse directions of the fabric roll, respectively. In contrast, symbols
x and
y, depicted in
Figure 2, were exclusively used in the designation to indicate testing directions of the composite samples. In addition, “P” indicates that external pressure was applied during the post-filling stage. “M” designates that the silk fabric was exposed to moisture prior to molding. For each fabrication scenario, two identical, 152 mm × 203 mm laminates were fabricated to ensure the repeatability of fabrication procedure.
2.3. Sample Preparation
Each molded laminate was sectioned using a diamond saw into several rectangular specimens. According to ASTM standards (D792, E1131, D790, D7028, and D2344), five samples for density measurement, five for thermogravimetric analysis, twelve for flexural testing (six samples along the flow direction (y), and six along (x) perpendicular to the flow), ten for SEM imaging (five along and five perpendicular to the flow direction) were cut in particular dimensions. When the sample thickness allowed, ten rectangular samples were cut for short beam tests (five along and five perpendicular to the flow direction).
2.4. Density Measurement and Volume Fraction Determinations
The density of five samples from each laminate was measured according to ASTM D792. Similarly, the density of the neat epoxy (i.e., the matrix) was measured as
ρEpoxy = 1.140 ± 0.001 g/cm
3. The density of silk fibers, on the other hand, was measured using a gas pycnometer (AccuPyc II 1340) as
ρSilk = 1.361 ± 0.002 g/cm
3. Using both densities of silk and epoxy, as well as the measured density of each fabricated composite laminate,
ρLaminate, both fiber volume fraction,
Vf, and resin volume fraction,
Vr can be calculated as
where
MLaminate is the measured mass of the laminate and
MSilk is the measured mass of the twelve silk fabric layers. Using both fiber and resin volume fractions, void volume fraction,
Vν, can be calculated for each laminate as
2.5. Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was used to verify the thermal stability and identify the maximum temperature at which the silk/epoxy composite laminates can be used. TGA thermograms were obtained by using a Thermogravimetry-Differential Scanning Calorimetry (TG-DSC) instrument (TA Instruments Q50, New Castle, DE, USA) at 10 °C/min heating rate in a nitrogen atmosphere.
2.6. Mechanical Testing
Flexural tests were performed according to the ASTM D790 standard to measure the flexural properties of each laminate. As mentioned earlier, specimens from each laminate were used to characterize the flexural properties along the longitudinal (y) and transverse (x) directions. Short beam tests were also executed on rectangular samples along the y- and x-directions cut from each silk/epoxy laminates. Interlaminar shear strength (ILSS) values were obtained in accordance with ASTM D2344. The mechanical testing was carried out at a rate of 2 mm/min.
2.7. Dynamic Mechanical Analysis
A dynamic mechanical analyzer (DMA) (TA Instruments DMA-Q800, New Castle, DE, USA) was used to measure the glass transition temperature, Tg, of selected configurations of the composites, namely neat epoxy (E), glass/epoxy (G), silk/epoxy impregnated along the transverse direction (S/T), and moist silk/epoxy impregnated along the transverse direction (S/T/M). For these particular measurements, 51 mm × 12.7 mm (2″ × 0.5″) specimens were prepared. The storage modulus, loss modulus, tanδ, and glass transition temperatures were determined for each sample.
2.8. Scanning Electron Microscopy (SEM) Imaging
SEM imaging was performed for the analysis of the microstructure of composite laminates. Specimen cut from the laminates were mounted in an acrylic resin to expose the through-the-thickness cross-section. Once polished, the specimens were sputter coated with 5 nm of gold/palladium to avoid charge build-up during SEM imaging. SEM images at different magnifications were obtained using a Zeiss Neon 40 EsB microscope (Carl Zeiss AG, Oberkochen, Germany). Additionally, fracture surfaces of the flexural test samples, as well as the silk preforms, were examined by SEM.