Resins used in the production of composite materials are generally classified into two categories: thermoplastic and thermoset resins.
Resin
Thermoset Resins
There are various manufacturing methods available for thermoset composites, such as hand lay-up, resin transfer molding, RTM, and autoclaving. Thermoset resins harden when heat is applied over a specific period of time. However, once a thermoset resin is heated or cured, the process is irreversible, meaning the resin cannot be reshaped after the heating or curing process. When a thermoset resin is heated, chemical bonds (three-dimensional (3-D) bonds, also referred to as crosslinking or polymerization reactions) are formed between resin molecules. Therefore, after curing, thermoset resins retain their strength and shape at high temperatures and do not return to a liquid state when reheated. However, above a certain temperature, a dramatic reduction in the mechanical properties of thermoset resins is observed. This temperature is called the glass transition temperature (Tg). Tg varies for each thermoset resin depending on specific curing cycle parameters such as time, temperature, and pressure. Since thermoset resins remain in a liquid state at low temperatures before curing, continuous fiber reinforcements can be embedded into the resin matrix. For this reason, thermoset composites are versatile and can be diversified to use different fiber reinforcements. Examples of thermoset resins used in fiber-reinforced composites include epoxy, vinyl ester, furan, cyanate ester, bismaleimide, phenolic resin, and unsaturated polyester.
Epoxy resins are the most commonly used resins. Although epoxy is more expensive than other polymer matrices, it is the most popular PMC matrix. More than two-thirds of polymer matrices used in space applications are epoxy-based. The main reasons epoxy is the most used polymer matrix material include:
- High strength
- Low viscosity and low flow rates, which ensure good wetting of fibers and prevent misalignment of fibers during processing
- Low volatility during curing
- Low shrinkage rates, reducing the tendency of the bond between epoxy and reinforcement to shear
- Configurable in more than twenty classes to meet specific property and processing requirements.
Thermoplastic Resins
Thermoplastic composites can be processed using different manufacturing methods. For example, autoclaving, diaphragm molding, and compression molding are among the most commonly used techniques. Thermoplastic resins soften when heated and harden and solidify when cooled. A thermoplastic resin system can be heated and cooled as needed without causing a chemical change in the resin's molecular structure. This means the manufacturing process of thermoplastic resin composites is reversible. Examples of commonly used thermoplastic resins include PET, polypropylene, polycarbonate, PBT, vinyl, polyethylene, PVC, PEI, and nylon 11. Thermoplastic composites, particularly those with short fiber reinforcement, are widely used in the composite industry, and these thermoplastic composites typically contain glass or carbon fiber as reinforcement. However, in the past decade, it has been possible to observe long-fiber thermoplastic composite applications, particularly in the automotive industry. These materials offer advantages in terms of material structure and manufacturing processes, such as adjustable and reproducible fiber length distribution, high efficiency, and short processing times.
