The term 'fabric' describes a structure in which overlapping or connected pieces of material work together to reinforce one another. The traditional material that comes most quickly to mind when we think of fabric is cloth, which is woven from strands of textile fibers for garments and clothing. The overlapping structure of flexible threads makes makes clothing durable, while at the same time allowing it to be flexibile and comfortable. However, this is not the only way that fabric structures can be used; fabric woven out of appropriate materials is also used to create structural machine components. In this case, the inherent flexibility of the fabric structure is used during the molding process to fit into complex shapes, and the durability of the material (along with epoxy and other added filler medium) helps maintain that shape after forming.
Fabrics for both clothing and composites (that is, fabric plus adhesive filler medium) are woven from strands of fibers and films precisely placed within the weave to give the fabric a particular structure. Different weave styles exist to give each unique fabric its basic characteristics and define its purpose. The strands incorporated into the weave are grouped into two basic categories: warp strands and weft strands. The warp strands run continuously along the full predetermined length of the fabric; they are held together by weft strands which cross the width of the fabric, passing over or under the warp strands in a particular pattern that establishes the weave style.
There are several weave styles and variations of each style used to construct fabrics. This article references weave styles that were originally used for cloth construction, but the variations within are what allow the weaves to offer much more than just garment and cloth construction. Composite fabrics used for structural applications have many weave styles of their own but most are variations of the same weaves used in clothing and textile industries. At DexMat we are experimenting with the patterns discussed below to better develop fabrics from our Galvorn yarns and films for the wearables, composites, and other fabric applications; we hope to find the optimal configuration and performance for each application considered.
The plain weave is the most basic weave, consisting of a 1 x 1 pattern produced by a weft strand going over and then under each alternating warp strand. This weave has the highest frequency of over and under passes and is considered to be strong from a cloth standpoint but not the best option for all composite use. This weave is very stable when being handled because of the frequent intersections of weft and warp making it dense and less likely to fray at the ends. The overall structure will be less pliable and harder to use as a composite that has complex shapes and 3D features, but works very well for flat surfaces. When considering use as a garment or wearable, the plain weave is commonly used for linens and other simple cloth formats.
The twill weave is a multi-functional weave established by two or more weft strands alternating over and under one or more warp strands. In the base version with two weft strands alternating over one warp strand we would call it a 2 x 2 twill weave. This same process can be expanded into a 4 x 4 twill weave in which four weft strands alternate over and under one or more warp strands and so on. The twill weave forms a very distinct design with a clear diagonal line across the fabric, but will have a smooth face with opposite design on the backside. The structure offers a more durable and heavy fabric with less tendency to wrinkle compared to a plain weave. This is a very pliable fabric, and drapes well into complex molds and comfortable clothing. A twill weave is more susceptible to fraying at the edges and needs to be handled with care. The twill weave is most commonly used as denim, flannel, and jersey; for the automotive industry it is a viable option for complex 3D molded composite parts.
The Dual Twill weave is similar to the twill weave but made with multiple materials for producing hybrid fabrics. It is established by two weft strands alternating over and under the same two warp strands like a 2 x 2 twill, but both the weft and the warp strands include two different fabric types (for example, in the image below every other warp strand and every other weft strand is composed of an alternate material). This weave could provide the ability to combine carbon nanotube (CNT) fibers with either a standard textile strand or a carbon fiber strand in order to achieve a conductive pathway in a wearable garment or an electrically enhanced composite fabric.
The basic structure of the satin weave is established with each weft strand wrapped over 3 warp strands and then under one warp strand. Another notable feature of the satin weave is the continuous warp strands along the fabric similar to the way the weft strands are used back and forth across the fabric. The weave is highly adaptable, and flexibility can be increased with a common variation of crossing over four or even six warp strands at once. This type of weave has a low frequency of interruptions of the weft and warp strands resulting in a smooth and flexible structure. Because of the relatively long runs of fibers without a transition (i.e., a cross between the warp and weft), this type of fabric is known to snag easily when handling and requires special attention and care. Silk and nylon fibers are commonly used with this construction to create an elegant feel in high end finished garments or cloth. Carbon fiber composite manufacturers are very fond of this weave as well because it is even more pliable than twill, allowing it to be molded into very complex curved forms and 3D shapes.
The jacquard weave is, at its most simple, an alternate version of the 1 x 1 plain weave but made on a special loom to create very intricate patterns. The jacquard looms used to make this weave operate with either electronic controllers or mechanical punch cards to lift individual warp strands at precise times to create images or intricate patterns in the fabric. Flexibility and durability are variable with this weaving process depending on the design intended and how many intersections the weft strands have to make to achieve the desired pattern. The jacquard weave is popular for drapery and upholstery. This weave is a less common in the composites space, but is very well suited for finished products that require intricate designs on the face of the composite made from carbon fiber or similar materials. Another aspect to this weave is the ability to incorporate CNT strands to establish a woven circuit or directed section to yield high wear resistance or electronic interfacing components.
Fabrics with Galvorn CNT Fibers and Films
Given the collection of weaves listed above, it's evident that there is not a 'one weave fits all' solution. Most weaves suitable for clothing are going to have different variations of construction that are designed to help with the flexibility of the cloth and the cosmetic appearance of the finished product. When considering a weave for composites it's more important that the weave has a higher degree of basic mechanical strength but is also able to maintain flexibility for forming into various machine parts.
CNT weaves for cloth fabrics
Because CNT yarn can be handled like a textile thread, it can be woven together with conventional yarns to make clothing that has complex electronic functionality without sacrificing comfort or weight. CNT yarns and films might be used to create wearable devices that range from heating elements in clothing, to power generation and feedback for our mobile devices, to wearable skin contact electrodes that monitor our health and wellness. Material of this kind can be woven using a jacquard loom to add capacitive touch sensing pads, like the one demonstrated here, directly into the fabric; any parts of the design of a piece of clothing might then double as built-in touch-sensitive buttons. Other applications might exploit the purely mechanical properties of aligned CNT woven fabric, such as use of the dual twill weave to make cut-resistant gloves and lightweight, flexible bulletproof clothing from CNT fibers or other high strength polymers such as Kevlar. Each application will demand its own variation of flexibility and overall strength, requirements that will rely on the chosen weave style or combination of patterns in layers for an optimal performance.
CNT weaves for Composite fabrics
CNT yarns and films can also be woven into a stand-alone fabric to use for composite construction, such as the prototypes shown here. Below is a woven fabric sample that is a plain weave made from 1 cm wide Galvorn CNT film. Fabrics suitable for composite materials can also be made out of CNT fiber (see the image above in the previous section). This 100% CNT fabric could potentially take the place of traditional carbon fiber composites, with the added benefit of increased electronic conductivity. Applications that need high strength and lightweight composites could also turn to composites made from CNT yarns and films to add in EMI shielding, utilizing the conductivity of the CNTs to block and limit electromagnetic signal interference.
All of the fabric weaves listed here and the applications mentioned serve only as a basic introduction to the possibilities of woven materials made from CNT fibers and films. Galvorn fibers and films are conductive, durable, flexible, and lightweight, which makes them a perfect fit for the emerging field of wearable electronics and makes them highly adaptable for continuing the evolution of electronic interfacing for composite materials.