When stretched, auxetic materials tend to expand rather than contract, which gives them a multitude of functional uses. Auxetic materials possess a negative Poisson’s ratio, and their auxetic properties are a result of their unique structure, which when stretched resembles a honeycomb shape. A research focus in the textile industry is the creation of materials with auxetic properties at the fiber, yarn, and fabric levels.
At the yarn and fabric levels, auxetic materials can be produced in two ways. Either the yarn/fabric can be produced with induced auxetic geometry using standard fibers and yarns during the production process (for example, spinning, weaving, or knitting) or by using fibers that themselves possess auxetic properties.
The first laboratory attempt to create materials with auxetic properties was conducted in the early 1990s by Alderson & Evans. In this attempt, ultra-high molecular weight polyethylene was produced, which had a Poisson’s ratio of -1.2. This auxetic polymer was composed of a network of rectangular nodes connected by freely hinged, inextensible fibrils/rods in a microporous node-fibril structure. The material was produced using a three-stage thermal process.
Since the initial experiments, there has been a growing body of studies on manufacturing varied materials with auxetic properties. Anderson et al. used a modified industrial scale conventional melt spinning technique to produce auxetic polypropylene and polyethylene. In 2002, Ravirala et al. produced auxetic polyamide and polyester fibers by modifying an earlier auxetic polypropylene fiber fabrication method. He et al. in 2005 produced auxetic liquid crystalline polymers.
Work in the last decade by Hook, Sloan, Ge, and Ng, amongst others, has developed and improved helical auxetic yarns (HAY.) These auxetic materials consist of a thick yarn core and finer, stiffer helically wound wrap yarn which under tension starts to uncurl, with the core yarn moving laterally. This increases the width of the yarn and achieves the negative Poisson’s ratio characteristic of auxetic materials. The auxetic yarn undergoes a resulting lateral extension.
Producing Low-Cost Auxetic Fibers Using Conventional Fibers and Yarn
Another way to produce low-cost auxetic fibers is to manufacture auxetic fabrics using conventional fibers and yarn. By inducing a unique fabric structure, a multi-functional auxetic material can be manufactured. Numerous studies have been published which have developed these low-cost auxetic fabrics.