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Youssef Hamidi, assistant professor of mechanical engineering at University of Houston-Clear Lake (UHCL, Houston, Texas, U.S.), is undergoing research into silk fiber for composites, in an effort to fill the growing demand for natural fiber-reinforced composites that combine strength, light weight and sustainability.

In his research, which can be found in  in the journal Materials, Hamidi claims that silk fibers are more ductile than traditional glass or carbon fibers, with high tensile strength, making them less brittle and better able to handle impact and stress. This, he says, raises the possibility of creating load-bearing, silk fiber-reinforced composites for automotive and other industries.

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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 research colleagues at the University of Oklahoma’s School of Aerospace and Mechanical Engineering (Norman, Okla., U.S.) started working with silk about a year ago.

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

Hamidi first worked with silk filaments straight from silkworm cocoons, but found it cumbersome. He soon discovered that silk fabric off the shelves worked best. However, he found that once the resin dried, it left tiny voids, or bubbles, within the resin and that the resin didn’t adhere fully to the fabric. Understanding how these voids form and how to remove them was the theme of Hamidi’s doctoral dissertation. Hamidi says that these are common problems in composites manufacturing that are often solved with the use of autoclaves to remove defects during the molding process. Since the idea behind Hamidi’s research is to create a low-cost alternative to high-cost manufacturing processes, however, he is committed to finding a solution that does not depend on a costly autoclave cure.

In his current research, Hamidi is tackling the resin/fiber adherence problem. For that, he has turned to sizing. Hamidi expects that applying the right sizing to the silk fabric will improve the resin adherence and reduce voids, resulting in silk fiber 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,” he says. “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.”