Fabrics as Reinforcement Material in Composites

Introduction

As the name suggests, a composite material consists of a combination of various—at least two—base materials. Besides the resin matrix, the reinforcement is an essential component of composite materials, also known as composites. The resin matrix serves to embed and support the fabric, primarily fulfilling two tasks: transferring mechanical loads to the fibers and protecting the reinforcement from external environmental influences, such as chemicals. Mainly, epoxy resins, phenolic resins, silicone resins, and melamine resins are used for composites. In addition to the resin systems used, the fibers employed also play a crucial role in determining the properties of composite materials. Fibers are twisted, spun, or laid into yarns, and these yarns are then woven into fabrics. These fabrics, in turn, reinforce the composites. This brief introduction to the topic of fabrics as reinforcement material in composites aims to build up from the small fiber to the larger yarn and finally to the fabric.

Fibers as Building Blocks of Yarns

Fibers can be made from various materials, each with different properties and uses. The most common fiber materials for composites are glass, aramid, carbon, synthetic fibers like polyethylene, and natural fibers such as cellulose and cotton.

Glass Fibers

Glass fibers are inexpensive to produce and mechanically very stable. This combination makes glass fibers the most commonly used fibers for composites. Only in applications where extreme lightweight construction requirements exist, such as in aviation, are lighter fibers like carbon fibers preferred. Glass fibers are divided into E-glass and S-glass. The “E” in E-glass stands for “electric.” These fibers are mainly used for composites in the field of electrical insulation and account for 90% of the various types of glass fibers. The “S” in S-glass stands for “strength.” These fibers have increased strength due to a different composition. There are also other types of glass fibers like acid-resistant ECR, temperature and moisture-resistant R-glass, or Q-glass for very high temperatures. However, these types of glass fibers are more niche products.

Carbon Fibers

Carbon fibers have excellent mechanical properties at very low weight or density. However, these fibers are very expensive compared to glass fibers. Therefore, carbon fibers are only used where extreme requirements for the material’s density exist. These markets are primarily aviation but increasingly also ground mobility applications, motorsports, and more commonly, consumer sports equipment such as racing bicycles and mountain bikes. Carbon fibers are mainly categorized by properties such as strength, stiffness, or modulus of elasticity.

Aramid Fibers

Aramid is a portmanteau composed of “aromatic polyamide.” Therefore, it is a polymer and technically also a synthetic fiber. However, aramid fibers are particularly significant for their extensive use in ballistic composite materials. Aramid fibers exhibit an extremely high tensile strength, allowing them to absorb a large amount of energy. This enhances their ballistic capabilities, as aramid-based composite materials can absorb a significant amount of kinetic energy from projectiles, thereby stopping them effectively.

Synthetic Fibers

Other synthetic fibers used as reinforcement in composite materials include polyester fibers, polyamide fibers, and polyethylene. These fibers have different properties, resulting in a variety of composite materials suitable for numerous applications.

Natural Fibers

The primary natural fibers used as reinforcement in composite materials are cellulose fibers and cotton fibers. Cellulose, derived from trees as a natural product, cannot be spun into yarn but is used in paper form with an unoriented fiber alignment. Cotton fibers, on the other hand, are spun into yarns and further processed into fabrics. Natural fibers also have low density but do not possess the mechanical strength of glass fibers and carbon fibers. Cotton fibers, however, have a low friction coefficient, making composites based on cotton fabrics suitable for applications such as slide bearings, slide rails, and ball bearing cages.

Yarns

Yarns are defined according to DIN 60900 as “a collective term for linear textile structures” — in other words, long and thin composites made from multiple fibers. These intermediate products are produced from the fibers described above and form the basis of every fabric. Yarns are made from multiple fibers, which are then woven into fabrics. Two different types of yarns are distinguished. Staple fiber yarns consist of fibers of finite length, which are twisted together and primarily held together by friction. Continuous fibers, theoretically infinitely long, can also be used to create filament yarns. These fibers do not need to be twisted together; they can simply lie next to each other, forming a loose composite. Filament yarns cannot be produced from natural fibers, as they always have a limited length.

Sizing

For further processing, a sizing agent is applied to the surface of the yarn. This agent smooths the surface of the yarn and increases its resistance. This process step is necessary so that the yarn can withstand mechanical forces such as tension, bending, and especially frictional forces during weaving without failing. Typically, after weaving, the sizing agent is removed through washing with acids, sulfates, thermally, or enzymatically. This process is called desizing. However, in some cases, the sizing agent is left on the fabric as an adhesion promoter between the resin matrix and the yarn. In this case, the sizing agent must be chosen to be compatible with the resin. For epoxy resins, for example, silane sizing agents are used.

Fabrics

A textile fabric is formed when threads from two different directions are interlaced to create a largely solid composite. Typically, the threads intersect at an approximately right angle (under 90°). There are warp threads in one direction and weft threads in the other. Warp threads run longitudinally, parallel to the edge of the fabric, and weft threads run transversely, parallel to the fabric edge. The connection of warp and weft threads is primarily achieved through friction. Therefore, fabrics must be tightly woven to achieve sufficient shear strength. Nowadays, fabrics are industrially produced on mechanical looms. Fabrics can be categorized in various ways, but for technical fabrics used in the production of composite materials, categorization by weave pattern is most meaningful.

Roving

Roving is a multifilament yarn. “Multi” stands for many, and “filament” refers to continuous fibers. Therefore, Roving is a yarn consisting of many theoretically infinitely long fibers that lie more or less loosely next to each other. Roving is not twisted or spun. However, there are Rovings that have a slight twist for stabilization, typically less than 10 twists per meter. Rovings, like all yarns based on continuous fibers, are only made from synthetic fibers. The most common fibers used are glass fibers, carbon fibers, and aramid fibers. Rovings are characterized by the number and fineness of individual filaments. For example, “3K” corresponds to 3000 filaments per yarn, and “800 tex” indicates a fineness of 800 grams per 1000 meters of filament.

Plain Weave

The simplest type of weave is plain weave. Here, warp and weft threads alternate regularly, so that each warp thread passes alternately over and under each weft thread. Plain weave is suitable for flat laminates but is not very drapable. Drapability refers to the ability to form curved contours with the fabric without causing folds, which is essential for three-dimensionally shaped composite components, such as those manufactured in Resin Transfer Molding (RTM) processes.

Twill Weave

Twill weave is also widely used. In twill weave, the weft thread passes over one warp thread and then under two or more warp threads in succession. In the next row, this pattern shifts by one warp thread. This creates a surface texture with a visible diagonal pattern known as a twill line. Twill weave is much more drapable but is also suitable for stiff flat laminates.

Atlas Weave

Atlas weave creates an uneven front and back side of the fabric. In this weave, the weft thread passes over one warp thread and then under more than two warp threads. This weave is also highly drapable and forms stiff laminates.

Unidirectional Fabric

Unidirectional fabric, strictly speaking, is not a weave but rather a fabric where all yarns are oriented in one direction. This maximizes fiber volume in that direction, resulting in excellent mechanical properties along that axis. The material behavior is highly anisotropic, meaning it varies significantly with direction. The laid yarns are often minimally fixed in the transverse direction with fewer, smaller yarns.

Satin Weave

In satin weave, threads in the warp direction, and sometimes also in the weft direction, pass over and under each other in a manner that creates a twisted effect. These weaves are highly drapable and suitable for creating very voluminous laminates.

Roving Fabric

Any type of fabric based on roving yarn is referred to as roving fabric. The most common weaves are plain weave and twill weave. Satin weave is also possible. Unidirectional roving fabric is also widely used to maximize reinforcement in one direction. Fabrics with many twists in the yarn, such as satin weaves, are more challenging to manufacture and are therefore not widely used industrially.

Conclusion

Fibers are fundamental components of many fiber composite materials. They are utilized in various forms—as fibers, yarns, or fabrics—playing a crucial role in enhancing mechanical properties. The excellent characteristics of different natural and synthetic fibers are indispensable for the development of modern fiber composite materials, also known as composites, as we know them today.