Can thermoplastic fiberglass also be made into running shoe insoles?

  Everyone knows that the soles of running shoes usually have three layers: the durable outer outsole, the midsole that absorbs most of the impact, and the inner sole that is in the most direct contact with the foot.

  In order to create the lightest and most durable sports shoes, many carbon fiber composite midsoles and other shoe components have emerged and these products have been launched. These carbon fiber composite reinforced shoes are usually targeted at elite athletes, providing runners with top stride and speed improvements.

  But carbon fiber is also very stiff, which can negatively impact a runner's feet, legs, and muscles, explains Gordon Brown, president of sporting goods developer Flexi-StiX LLC (Anderson, SC, USA). He believes that fiberglass is a better, more flexible, and more forgiving material choice for the average runner, improving running shoe performance by providing a smoother, more consistent return of energy to the runner's foot.

Brown and Milliman recently developed and commercialized their latest innovation, a patent-pending running shoe insole made of unidirectional (UD) continuous fiberglass reinforced thermoplastic prepreg sandwiched between two layers of neoprene foam. Brown and Milliman began making prototype continuous fiberglass/epoxy midsoles in the 1990s. For running shoes, the goal was maximum cushioning and a spring-like effect that returned energy to the runner. These midsoles are shaped roughly like two sine waves, one on top of the other, wet-laid in an aluminum mold, pressed, and baked to cure. "These midsoles worked, they were flexible, they lengthened the stride, they created a spring effect, but they were labor intensive and ultimately not commercially viable," Brown said. The team decided to switch to a fiberglass composite insole to offer cost benefits to consumers without having to redesign the entire shoe. The first insole design involved laying down tiny pultruded fiberglass/epoxy strips along the width of the insole. There was a placement that allowed them to flex in the middle, allowing for a smooth and dynamic motion that lengthened the stride with a more gentle spring-like effect. "That design was also successful in practice, but it was also labor intensive to manufacture," Brown said. He noted that the development and increasing availability of UD thermoplastic prepregs over the past few years "has been a game changer." He explained that the game-changing nature of these materials is twofold. First, the UD and continuous orientation of the fibers, if aligned across the width of the shoe as in the original pultruded fiberglass rod design, can lengthen a runner's stride in this way. "For running shoes, it's all about increasing forward motion," Milliman said. When a runner presses down on the insole with their foot, a regular neoprene insole absorbs the energy applied, but the fiberglass composite insole reflects the energy back to the runner, providing a more controlled spring effect and lengthening the stride. And using a thermoplastic instead of a thermoset resin allows for a lower modulus, which makes the insole more comfortable for the wearer and also helps the insole conform to the foot. "The heat from the foot warms the thermoplastic slightly, enough for it to form to the shape of the foot," Brown says. After hand-building several prototypes, Brown and Milliman worked with materials partner Avient Inc. (Avon Lake, OH, U.S.) to produce sheets of the sandwich material consisting of three layers-a layer of fiberglass/thermoplastic prepreg between two layers of neoprene foam. The sheets are then die-cut into insoles that match a variety of shoe sizes and can replace or be placed underneath existing insoles. Available in two different thicknesses (5 or 8 mm), depending on the cushioning the customer requires