Reinforcements can be oriented to provide tailored properties in the direction of the loads imparted on the end product.
Many materials are capable of reinforcing polymers. Some materials, such as the cellulose in wood, are naturally occurring products. Most commercial reinforcements, however, are man-made. There are many commercially available reinforcement forms to meet the design requirements of the user. The ability to tailor the fiber architecture allows for optimized performance of a product that translates to weight and cost savings.
Although many forms of fiber are used as reinforcement in composite laminates, glass fibers account for more than 90 percent of the fibers used in reinforced plastics because they are inexpensive to produce and have relatively good strength-to weight characteristics.
- Glass Fibers: Based on an alumina-lime-borosilicate composition, “E” or “E-CR” glass produced fibers are considered the predominant reinforcements for polymer matrix composites due to their high electrical insulating properties, low susceptibility to moisture and high mechanical properties. E-CR glass is further distinguished from E-glass by having superior corrosion resistance properties. Other commercial compositions include “S” glass, with higher strength, heat resistance and modulus, H-glass with higher modulus, and AR glass (alkali resistant) with excellent corrosion resistance. Glass is generally a good impact resistant fiber but weighs more than carbon or aramid. Glass fibers have excellent mechanical characteristics, stronger than steel in certain forms. The lower modulus requires special design treatment where stiffness is critical. Glass fibers are transparent to radio frequency radiation and are used in radar antenna applications.
- Carbon Fibers: Carbon fibers are made from organic precursors, including PAN (polyacrylonitrile), rayon, and pitches, with the latter two generally used for low modulus fibers. The terms “carbon” and “graphite” fibers are typically used interchangeably, although graphite technically refers to fibers that are greater than 99 percent carbon composition, versus 93-95 percent for PAN-based carbon fibers. Carbon fiber offers the highest strength and stiffness of all the reinforcement fibers. High temperature performance is particularly outstanding for carbon fibers. The major drawback to PAN-based fibers is their high relative cost, which is a result of the cost of the base material and an energy-intensive manufacturing process. Carbon fiber composites are more brittle than glass or aramid. Carbon fibers can cause galvanic corrosion when used next to metals. A barrier material such as glass and resin is used to prevent this occurrence.
- Aramid Fibers (Polyaramids): The most common synthetic fiber is aramid. Aramid fiber is an aromatic polyimid that is a man-made organic fiber for composite reinforcement. Aramid fibers offer good mechanical properties at a low density with the added advantage of toughness or damage/impact resistance. They are characterized as having reasonably high tensile strength, a medium modulus, and a very low density as compared to glass and carbon. Aramid fibers are insulators of both electricity and heat and increase the impact resistance of composites. They are resistant to organic solvents, fuels and lubricants. Aramid composites are not as good in compressive strength as glass or carbon composites. Dry aramid fibers are tough and have been used as cables or ropes, and frequently used in ballistic applications. Kevlar® is perhaps the best known example of aramid fiber. Aramid is the predominant organic reinforcing fiber replacement for steel belting in tires.
- New Fibers: Polyester and nylon thermoplastic fibers have recently been introduced both as primary reinforcements and in a hybrid arrangement with fiberglass. Attractive features include low density, reasonable cost, and good impact and fatigue resistance. Although polyester fibers have fairly high strengths, their stiffness is considerably below that of glass. More specialized reinforcements for high strength and high temperature use include metals and metal oxides such as those used in aircraft or aerospace applications.
Regardless of the material, reinforcements are available in forms to serve a wide range of processes and end-product requirements. Materials supplied as reinforcement include roving, milled fiber, chopped strands, continuous, chopped or thermoformable mat. Reinforcement materials can be designed with unique fiber architectures and be preformed (shaped) depending on the product requirements and manufacturing process.
- Multi-End and Single-End Rovings: Rovings are utilized primarily in thermoset compounds, but can be utilized in thermoplastics. Multi-end rovings consist of many individual strands or bundles of filaments, which are then chopped and randomly deposited into the resin matrix. Processes such as sheet molding compound (SMC), preform and spray-up use the multi-end roving. Multi-end rovings can also be used in some filament winding and pultrusion applications. The single-end roving consists of many individual filaments wound into a single strand. The product is generally used in processes that utilize a unidirectional reinforcement such as filament winding or pultrusion.
- Mats & Veils: Reinforcing mats and non-woven veils are usually described by weight-per-unit-of-area. For instance, a 2 ounce chopped strand mat will weigh 2 ounces per square yard. The reinforcement type, the fiber dispersion, and amount of binder that is used to hold the mat or veil together dictate differences between mat products. In some processes such as hand lay-up, it is necessary for the binder to dissolve. In other processes, particularly in compression molding, and pultrusion the binder must withstand the hydraulic forces and the dissolving action of the matrix resin during molding. Therefore, from a binder point of view, two general categories of mats or veils are produced and are known as soluble and insoluble binders.
- Woven, Stitched, Braided & 3-D Fabrics: There are many types of fabrics that can be used to reinforce resins in a composite. Multidirectional reinforcements are produced by weaving, knitting, stitching or braiding continuous fibers into a fabric from twisted and plied yarn. Fabrics can be manufactured utilizing almost any reinforcing fiber. The most common fabrics are constructed with fiberglass, carbon or aramid. Fabrics offer oriented strengths and high reinforcement loadings often found in high performance applications. Fabrics allow for the precise placement of the reinforcement. This cannot be done with milled fibers or chopped strands and is only possible with continuous strands using relatively expensive fiber placement equipment. Due to the continuous nature of the fibers in most fabrics, the strength to weight ratio is much higher than that for the cut or chopped fiber versions. Stitched fabrics allow for customized fiber orientations within the fabric structure. This can be of great advantage when designing for shear or torsional stability.
- Unidirectional: Unidirectional reinforcements include tapes, tows, unidirectional tow sheet and roving (which are collections of fibers or strands). Fibers in this form are all aligned parallel in one direction and uncrimped, providing the highest mechanical properties. Composites using unidirectional tapes or sheets have high strength in the direction of the fiber. Unidirectional sheets are thin and multiple layers are required for most structural applications. Typical applications for unidirectional reinforcements include highly loaded designed composites, such as aircraft components or race boats.
- Prepreg: Prepregs are a ready-made material made of a reinforcement form and polymer matrix. Passing reinforcing fibers or forms such as fabrics through a resin bath is used to make a prepreg. The resin is saturated (impregnated) into the fiber and then heated to advance the curing reaction to different curing stages. Thermoset or thermoplastic prepregs are available and can be either stored in a refrigerator or at room temperature depending on the constituent materials. Prepregs can be manually or mechanically applied at various directions based on the design requirements.
- Milled: Milled fibers are chopped fibers having very short fiber lengths (usually less than 1/8”). These products are often used in thermoset putties, castings, or syntactic foams to prevent cracking of the cured composition due to resin shrinkage.