In the last few decades, fiber reinforced composites have been established as the materials of choice for lightweight applications in a large spectrum of applications ranging from aerospace, defense, and marine industries to automotive products and consumer goods. With the growing shift to sustainable resources, natural fibers, especially plant fibers, received increased interest throughout the years. Among these natural fibers, silks stand out with low stiffness and a high failure strain, unlike conventional fibers such as carbon or glass. Although gaining traction as a natural alternative reinforcement, silk still has little to no commercial uses despite its higher performance. Besides its higher mechanical properties and lightweight, silk exhibits other attractive properties such as improved flame retardancy and biodegradability.
To take advantage of these features, proper fiber/matrix adhesion must be achieved. Such silk/matrix bonding can be inferred from the silk/resin affinity during composite manufacturing. In this study, the affinity/wettability of several silk/resin systems were analyzed via static contact angles using imageJ software to determine candidates for silk reinforced composite laminates with better adhesion. To this end, a combination of four silk fibers and three resin systems were investigated. The investigated silk fibers were Ahimsa, Charmeuse, Habotai, and Tussah; and the resins included a vinyl ester (Hydrex) and two epoxies (INF114 and INR). For Tussah fibers, initial contact angles were consistently one of the lowest. However, these fibers exhibited a higher contact angle over time compared to the other silk fibers studied. Conversely, Ahimsa silk fibers showed the highest initial contact angle, then quickly dropped to com-plete wetting. Habotai fibers dropped towards complete wetting quickly, however, consistently slowed considerably shortly after. Charmeuse fibers performed similarly to Ahimsa fibers with Hydrex, however was considerably slower to wetting with the other resins. Among the investigated resins, Hydrex showed the best affinity to silk fibers with the majority of the lowest initial contact angles and the fastest to complete wetting. INF114 consistently receded at a slower, albeit steady, rate until reaching complete wetting apart from Tussah. INR showed the highest initial contact angles and never reached complete wetting after an hour for two of the four silks investigated. Therefore, the best silk/resin affinity was observed for the Ahimsa and Charmeuse silk fibers and the Hydrex vinyl ester resin. In future work, silk composites with these constituents would be investigated.
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Effects of Silk Humidity Exposure on Silk/Resin Affinity for Silk Reinforced Composites
Thanks to its comparable specific mechanical properties to glass fibers, silk is a natural fiber that can be used as an eco-friendly alternative to synthetic reinforcing fibers in composite materials. Compared to natural fibers, especially plant fibers, silk enjoys higher mechanical performances, lower density, and higher elongation even at low temperatures, silk also exhibits other attractive qualities like flame resistance and being naturally continuous. However, silk is known to be prone to moisture absorption from surrounding humid environments. Moisture absorption may alter the silk/resin dynamics during composite manufacturing, and later lead to prem-ature degradation in the composite thermomechanical properties. This study investigates the effect of humidity on silk/resin wettability using two different resins (one epoxy and one vinyl ester) and three different silk architectures. Silk fibers are first exposed to different relative humidity environments. Subsequently, the affinity of the conditioned silk to a set of resins is assessed through measurements of silk/resin contact angle over time. Different silk/resin systems were observed to have contrasting responses to humidity exposure. While some silk/resin systems, such as Ahimsa/epoxy, did not show any change after humidity exposure. Other combinations showed tremendous susceptibility of silk/resin affinity to prior exposure of silk to humidity. For instance, although starting at virtually the same initial hydrophobic contact angle of ~123 degrees, Habotai silk/epoxy samples had contrasting wetting times. While the dried Habotai silk reached full wetting after around 5 minutes, the silk samples exposed to humidity took around 1 hour to reach full im-pregnation. These findings demonstrate the importance of humidity exposure control in silk reinforced composites.
Keywords: Natural-Fiber Composites, Contact Angle, Silk, Wettability, Humidity.
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- Award ID(s):
- 1928622
- NSF-PAR ID:
- 10341620
- Date Published:
- Journal Name:
- American journal of advanced research
- Volume:
- 5
- Issue:
- December
- ISSN:
- 2572-8830
- Page Range / eLocation ID:
- 7-12
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5281/zenodo.580785
