Overview of Composite Materials
Resins for Composites
In composites, the resin (or plastic) is known as the matrix and serves two critical functions: 1) transfers the load to the reinforcement fiber and 2) protects the fiber from environmental effects. There are two family groups of resins that comprise what we call plastic materials - thermosets and thermoplastics.
These “plastic” resins are made of polymers consisting of long chain-like molecules. The term mer is a suffix, which means segment, and the word polymer literally means many segments or a repeating chain of molecular units. When these polymers are joined, the process is called crosslinking. Thermoset resins and thermoplastic resins differ in molecular structure – thermosets are crosslinked and thermoplastics are not crosslinked.
Thermoset resins are converted from a liquid to a solid using an initiator or heat – the process is irreversible. Thermoplastic resins are melted and formed and can be re-melted and re-formed – the process is reversible. Think of it this way, a thermoset resin is cake and a thermoplastic resin is fudge. The ingredients are similar and both start out as a liquid. However, once the cake is baked, it rises and solidifies and cannot be turned back into a liquid. On the other hand, fudge solidifies when cooled, and can be re-melted by heating. This illustrates the difference between thermosets and thermoplastics.
Typical household plastics are thermoplastics and consist of nylon, polystyrene, polyethylene, acrylic, and many other plastic compounds. Familiar thermosets are polyester, vinyl ester and epoxy resins.
Resins are selected on the basis of performance, adaptability to the molding process, and cost. There are a number of types of thermoset resins with varied characteristics and performance abilities. Following are resins used in composites molding:
Most Common Resins:
- Unsaturated polyester
- Vinyl ester
Less Common Resins:
Polyester resins are the most commonly used resin systems in FRP fabrication because they are low cost and the cured physical properties meet many of the needs in the commercial composite industry.
Vinyl ester resins are used where either superior corrosion resistance or toughness are required properties. Vinyl esters are formulated by reacting epoxy resin with methacrylic acid, forming a polymer that has characteristics like both polyester and epoxy. Vinyl ester resins are cured and handle very similarly to polyester resins but have a higher cost.
Monomer - The monomer serves several purposes in the resin system. First, it co-reacts with the backbone polymer in a resin system when polymerization (crosslinking) takes place. Second, the monomer also reduces the viscosity of the polymer to provide a workable liquid product, acting as a diluent. Styrene is termed a reactive diluent because it takes part in the curing process. Styrene and methyl methacrylate (MMA) are the most common monomers used with polyester and vinyl ester resin systems.
Most of the styrene monomer in a resin system is captured in the crosslinking reaction. However, a small portion of the monomer may evaporate before curing takes place. This is the characteristic smell of polyester and vinyl ester resins. Lowering styrene emissions is an objective of the composites industry to reduce environmental impact and worker exposure to styrene.
Curing Resins – Resin must cure (harden) in a way that is compatible with the fabrication process. Some parts are small and can be laid-up quickly. The faster a resin cures, the quicker the turnaround is on the molds and the greater the production rates. Other parts may involve large lay-ups where more time is required for the lamination process. In compression molding, pultrusion and sometimes RTM, heated molds provide rapid curing.
Another aspect of curing resin is the physical properties of the cured laminate are determined by the efficiency of the cure. The hardness of the laminate is affected by the curing process as well as the chemical resistance of the laminate surface. Thick laminates also require special attention. Resin exotherm must be controlled in order to prevent excessive shrinkage and laminate warping.
Initiators/Promoters/Inhibitors Initiator is the correct technical term for the product commonly called the catalyst in the composites industry. In technical terms, a catalyst causes a chemical reaction but does become part of the reaction. An initiator initiates or speeds up a reaction but becomes consumed in the process. In the case of polymerizing polyester resins, the initiator becomes part of the crosslinked polymer. Increasing the amount of initiator added to the resin will increase the rate of cure.
An essential factor in maintaining control over the curing process revolves around selecting the correct initiator. There are several types of initiators used to cure polyester and vinyl ester resins: ketone peroxides, acetylacetone peroxides, benzoyl peroxides, cumine hydroperoxides.
There are a number of additives that are used to modify and enhance resin properties. These additives include:
- Fire Retardants
- UV Inhibitors
- Conductive Additives
Fillers - Adding inert fillers to resin will modify the properties and can reduce cost. Types of fillers include:
- Mineral fillers
- Calcium carbonate
- Calcium sulfate
- Organic fillers
- Wood flour
- Walnut shells
- Corn cobs
- Solid glass spheres
- Hollow glass spheres
- Ceramic spheres
- Thermoplastic spheres
- Phenolic spheres
Pigments and Colorants - Pigment dispersions and color pastes can be added to resin or gel coat for cosmetic purposes or to enhance weatherability.
Fire Retardants - Most thermoset resins are combustible and create toxic smoke when burned. In critical applications such as aircraft, train interiors or mine equipment, reducing fire hazards is important. Fire retardant additives such as alumina trihydrate and antimony trioxide reduce flame spread and smoke generation of burning composites.
Suppressants/Film Formers - In order to reduce styrene emissions, suppressant additives can be used to block evaporation. These wax-based materials form a film on the resin surface and reduce the loss of styrene. Additionally, many polyester resins remain tacky on the surface after curing. This is due to air inhibition, which prevents a very thin surface layer from properly curing. The addition of a film former, such as paraffin wax, excludes the air from the surface and allows a non-tacky sandable surface.
UV Inhibitors - In the event that a non-gel coated resin will be exposed to sunlight, the addition of a UV inhibitor will slow the surface degradation.
Conductive Additives - Composite laminates (except carbon fiber) are inherently non-conductive. In some cases it is necessary to make a laminate conductive to reduce static charge or to enable electrostatic painting. Carbon black, carbon fibers, metallic fibers, or metallized glass can be used to create an electrically conductive laminate.
Gel coat is a specialized polyester resin that is formulated to provide a cosmetic outer surface on a composite product and to provide weatherability for outdoor products. Gel coat is not paint. Paint contains solvents that must evaporate for the paint to dry. The “solvent” in gel coat is styrene monomer and/or methylmethacrylate (acrylic) which crosslinks during curing.
The primary function of fiber reinforcements is to carry load along the length of the fiber to provide strength and stiffness in one direction. Reinforcements can be oriented to provide tailored properties in the direction of the loads imparted on the end product.
Reinforcements can be both natural and man-made. 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. The most common fiber reinforcement is glass fiber. Other fiber reinforcements are carbon and aramid.
Glass fiber is the least expensive of all reinforcements. Glass fiber (also called fiberglass) is used in more than 90 percent of manufactured composites. Composites made of polyester resins and glass fibers are so common, in fact, the term “fiberglass” is often used for the composite material itself such as “fiberglass boat”. Glass fibers, however, are only one part of a composite—they do the reinforcing.
Glass fibers come in several varieties, designated S-, A-, C-, or E-glass. Each variety has special characteristics. S-glass is exceptionally strong. C-glass is extremely resistant to corrosion and chemical attack. A-glass has good resistance to chemicals. E-glass does not conduct electricity. Though economical, glass fiber is relatively heavy. Of the common synthetic reinforcements, it has the least efficient strength-to-weight ratio.
Carbon Fiber is a very strong fiber and extremely stiff. It is lighter in weight than glass fiber. Carbon fibers come in several varieties and strengths and are the most expensive kind of fiber reinforcements. They are typically used in airplanes and spacecraft. Carbon fiber reinforced composites are also used in products such as bicycle frames, tennis rackets, skis, and golf club shafts.
Aramid Fiber resists impact. It is used extensively in bulletproof vests and body armor. Racing drivers wear aramid suits that help protect them from burns in fiery, high-speed crashes. Aramid is commonly known as KevlarTM, produced by DuPont. Aramid fibers cost is between glass and carbon. Aramid is more difficult to work with than glass and has a tendency to absorb moisture.
Other fibers - Different fibers can be combined to make a composite cost less or perform better. Composites that are made of more than one fiber are called hybrid composites. Fibers with special characteristics are used when a composite must be exceptionally strong or heat-resistant—for high-performance military aircraft, for instance, or aerospace applications. These materials are quite expensive. Examples include boron (an extremely hard natural element) and ceramics (hard, manufactured materials that can withstand high heat and harsh chemicals).
In most composite products, the fiber reinforcements are bundled together for strength. Fibers are assembled in various patterns called fabrics. Typical forms include:
Because each pattern carries loads differently, how the fibers are placed or assembled is important to engineers and designers. The cost of each of these forms also varies, depending on the amount and the quality of the fiber used.
Core materials are widely used in composites to make stiff, lightweight products. Typical core materials include balsa (wood from the balsa tree), polyurethane foam and PVC (polyvinyl chloride) foam (both manufactured in chemical processes), and honeycomb. These materials are light and strong.
Core materials are used to make sandwich construction. Using sandwich construction, a core material is placed between two outside surfaces (called “face skins”) of fiber reinforcements. This sandwich is bonded together with an adhesive or glue. As the core is made thicker, the stiffer the sandwich panel. Sandwich construction is used in commercial aircraft flooring because it is lightweight, strong, stiff, and economical.