Silk fibres can make composites more resilient to impact and stress, according to a University of Houston-Clear Lake assistant professor. This implies creating load-bearing composites that could replace much of the steel used in cars and other manufactured goods is possible. Traditionally, composites consist of resins reinforced with carbon or glass fibres.

The demand for natural fibre-reinforced composite materials is expected to surge through 2023, market research firm Modor Intelligence reported, a demand driven by the automotive industry as it races to meet US government standards for more fuel-efficient vehicles by 2025. It will require composites that are more lightweight, recyclable, tougher and cost-effective to produce. University of Houston-Clear Lake Assistant Professor of Mechanical Engineering Youssef Hamidi’s ongoing research into this field shows promising results. Hamidi is seeking funding to continue his research.

Traditionally, composites consist of resins reinforced with carbon or glass fibres. In recent years, manufacturers have worked with plant fibres — cotton, jute, hemp and others — in attempts to create composites from sustainable and biodegradable materials. Those attempts have come with trade-offs in either manufacturing costs or in the materials’ “mechanical properties” — hardness, impact resistance, the ability to withstand stress, strain, deformation and other factors.

Hamidi, who is experimenting with using silk to reinforce composites, explained the process and his progress in an article published recently in Materials, a renowned scientific journal on materials science.

Carbon or glass fibres in traditional advanced composites give the material strength and are sometimes costly to produce, depending on usage, Hamidi explained. But the fibres are often shatterable, which contribute to the composites’ brittleness. In contrast, he said, silk fibres are ductile, which by their nature would make composites more resilient to impact and stress. It raises the possibility of creating load-bearing, silk-fibre composites that could replace much of the steel used not only in cars, but in other manufactured goods.

Hamidi, who joined UHCL’s mechanical engineering faculty in 2018, has been researching composite materials since 2000, mainly working out ways to reduce process-induced defects. He and his colleagues at the University of Oklahoma’s School of Aerospace and Mechanical Engineering in Norman, Okla, started working with silk about a year ago.

“I was thinking about what would be a good fit,” he said. “In most (bio-based) applications, people are using short, plant-based fibres. But silk has higher properties. It’s readily available. There’s no shortage of it.”

Among silk’s notable properties, Hamidi says, are its tensile strength and the amount of elongation it allows before it breaks, which would make silk-fibre composites far less brittle than those made with glass, carbon or other natural fibres. “That’s a nice feature if you are using the composite in some application where impact is expected,” he said, using car bumpers or fenders as examples.

In composite materials, before resin is applied, untwisted bundles of continuous filaments, called “tows,” are woven or knit into “preforms” — three-dimensional fabric forms designed to conform to a specific shape. Hamidi first worked with silk filaments, straight from silkworm cocoons, but found it cumbersome. He soon discovered that silk fabric — straight off the shelves — worked best.

However, he found that once the resin dried, it left tiny voids, or bubbles within the resin. Furthermore, the resin didn’t adhere fully to the fabric. He added that it’s a common problem in composites manufacturing, one that diminishes the finished molding’s integrity. Manufacturers solve the problem by using large, expensive autoclaves to apply intense compression to the composite during the molding process to remove the defects.

“Autoclaves cost an arm and a leg,” Hamidi said. Since the idea is to create a low-cost alternative to high-cost manufacturing processes, Hamidi wants to find a solution that would deliver “a decent composite at a fraction of the price.”

Understanding how these voids form and how to remove them was the theme of Hamidi’s doctoral dissertation. He is currently tackling the resin/fibre adherence problem. For that, he turned to a solution known to tailors, seamstresses and others who work with textiles: sizing.

In textiles, sizing is a plastic-based mixture that adds stiffness and finish to synthetic fibres, like starch does to natural fibres. The principle is similar for composite materials, although the chemistry is different. Hamidi expects that applying the right sizing first to the silk fabric will improve the resin adherence and reduce voids, which would result in silk composites with significantly improved performance.

He’s currently working with UHCL chemistry professors on coming up with a better sizing compound. “We need to find the right sizing, one that improves the bonding. Normally, the mechanical properties of the composite are defined by that bonding. We can change the bulk of mechanical properties of the composite by changing the chemical composition of the sizing and how it is applied.” (SV)