Composites Overview - Resin

Resins are the matrix material for composites that hold the reinforcing fibers together and protect the fibers from impact and environmental conditions. While the fibers provide the primary strength and stiffness in continuous fiber composites, the resin choice determines the operating capabilities (temperature, fluid exposure, handling) of the composite. In discontinuous fiber composites, the resin becomes a parameter in the strength capability.

Polymer matrix resins fall into two categories; thermoset and thermoplastic. While both resin types are chemically comprised of molecular chains, thermoset chains crosslink during the cure reaction (set off by heat, catalyst, or both) and set into a final rigid form. (An analogous everyday example is a cooked egg.) Thermoplastic resins are long, linear molecular chains processed at higher temperatures and are capable of being reheated and reshaped more than once. (An analogous example is chocolate.)

The choice of thermosets and thermoplastics depends on operating environment, cost, cure time, processing temperature, handleability, cycle time and physical properties.

Thermosets
The list of thermoset resins primarily consists of:

• Polyester - basic structural properties, low cost, good processability
• Vinyl Ester - polyester family, higher mechanical and chemical properties, higher cost than polyester
• Phenolic - excellent flame resistance, low smoke generation, low cost, good processability, secondary structure
• Epoxy - primary thermoset resin for advanced composite applications, high strength, good chemical resistance, good hot/wet performance, primary and secondary aircraft structure
• Cyanate Ester - good strength, toughness, and processability, higher temperature service than epoxies
• Bismaleimide - polyimide family, good mechanical properties, excellent hot/wet properties, harder to process than epoxies
• Polyimide - good mechanical strength, excellent thermal stability, harder to process than BMI, relatively brittle

Thermoplastics
Thermoplastic resins are grouped in two categories.

• Amorphous Thermoplastics - tougher, more flexible
  • Acrylonitrile-Butadiene-Styrene (ABS)
  • Polystyrene (PS)
  • Polycarbonate (PC)
  • Polyetherimide (PEI)

 

• Crystalline Thermoplastics - stronger, stiffer, sharper melting point
  • Polypropylene (PP)
  • Polyethylene
  • Nylon/Polyamide (PA)
  • Polyetheretherketone (PEEK)
  • Polyethersulfone (PES)

This qualitative chart shows the relative differences in operating temperature and toughness for the various types of resins.