Resin choice
The use of resin in a composite profile plays two main roles — to bind the reinforcement together and to keep it in the correct position to deliver the composite’s mechanical properties. Depending on the type of resin that is used, it also helps to deliver a variety of benefits including corrosion or temperature resistance and electrical insulation. Some of the most typical resins used alongside carbon fiber and fiberglass reinforcements are polyester, vinyl ester, polyurethane and epoxy.
Polyester (UPE)
Polyester resins are one of the most widely used resin types, particularly in the marine industry. They often come in unsaturated form, which makes the resin a thermoset that is cured from a liquid to solid state when it is under the right conditions. Most polyester resins consist of a polyester solution and a monomer, which is typically styrene. The styrene helps reduce the resin’s viscosity and make it easier to handle, while enabling it to cure from a liquid to a solid by cross-linking the molecular chains of the polyester — a process known as polymerization.
As a result, polyester resins can be molded without the need to apply pressure and are often referred to as ‘low pressure’ resins. Polyester resins have good chemical resistance properties, performing well in weak alkalis and even better in weak acidic conditions.
Typical mechanical properties of the polyester/fiberglass profiles are described in the following table.
| Unit | Structure = UCU | Structure = U | |
| Typical glass content | [%-vol] | 55-58 % | 58-65 % |
| Tensile strength | [Mpa] | 600-900 | 1000-1200 |
| Tensile modulus | [Gpa] | 38-42 | 42-45 |
| Flexural strength | [Mpa] | 700-1000 | 1000-1300 |
| Flexural modulus | [Gpa] | 35-40 | 40-43 |
| Coefficient of thermal expansion | [10-6/K] | 9-11 | 6-8 |
| Density | [kg/dm3] | 1.9-2.0 | 1.9-2.0 |
Structure: U = unidirectional, C = cross-wound
Polyester resins can be modified so that they are flame-retardant or self-extinguishing.
Vinyl esters (VE)
Vinyl ester resins combine the best features of polyester and epoxy resins into one solution. They are typically stronger than polyesters and more resistant than epoxies, as they are a hybrid form of polyester resin that has been strengthened by epoxy.
Vinyl esters are more tolerant of stretching than polyesters, making them more able to absorb impact without damage and less likely to show stress cracking. The resin also has fewer open sites in its molecular chain, which makes it more resistant to water penetration.
Epoxies (EP)
Epoxy resins are another variety of thermoset resin, which are commonly used with high performance reinforcements such as carbon or a high volume of glass fibers. However, unlike polyester resins, epoxies are cured using a hardener rather than a catalyst.
The hardener, often an amine, cures the epoxy during an additional reaction that involves both materials. The chemistry of the reaction creates two epoxy sites that bind to each amine site, forming a complex three-dimensional structure. For this reaction to take place accurately, it is essential that the correct mix ratio of resin and hardener is obtained. If the amine and epoxy molecules are not well mixed, unreacted resin or hardener could remain in the matrix and affect the properties of the composite.
Epoxy resins offer a number of beneficial properties to composites. For example, their low shrinkage during the curing process minimizes the risk of internal stresses, and epoxy resins also enable a high electrical insulation and good chemical resistance.
Polyurethanes (PU)
During the pultrusion process, polyurethane can be an excellent alternative to epoxy for mechanical high-performance applications such as springs, thanks to the possibility to use a high fiber volume content. This high fiber volume content also delivers high stiffness, allowing the use of thinner wall sections to help save weight. Polyurethane resins are often noted for their chemical and oil resistance, and they are typically very good at resisting abrasion and are not easily torn.
The toughness of the resin is superior compared to polyester and vinyl esters, and screw retention is outstanding even without the use of crosswise reinforcement. In addition, the resin is compatible with both standard fiberglass and high-performance carbon fiber grades.
Resin additives
Resins don’t always work alone, and additives can often be added into the matrix to offer even more benefits. Generally, additives are incorporated into the mix for three reasons: to reduce costs, provide extra properties or to benefit the overall manufacturing process.
Price-reducing additives, or fillers, help manufacturers reduce the amount of expensive product they need to use and decrease the price of the finished product as a result. Additives related to a composite’s function can include the addition of pigments, which alter the color of the finished product. This is particularly useful if the end user is receiving a number of composite solutions that will fit together to make one final product, as the user can easily identify where each part belongs based on its color.
Process-related additives can directly benefit the pultrusion process as well as the cured profile. For example, additives can be used to avoid shrinkage during the curing process, which prevents the chance that cracks will appear in the composite while also helping to improve its internal stress.