Epoxy & Polyester Systems

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Although there are many different types of resin in use in the composite industry, the majority of structural parts are made with three main types, namely polyester, vinylester and epoxy. These are termed thermosetting resins and once cured, thermosets will not become liquid again if heated, although above a certain temperature their mechanical properties will change significantly. This temperature is known as the Glass Transition Temperature (Tg), and varies widely according to the particular resin system used, its degree of cure and whether it was mixed correctly. Above the Tg properties such as resin stiffness drop sharply, and as a result the compressive and shear strength of the composite does too. Epoxy resins are more expensive than vinyl ester or polyester resins, but are the most widely used for high performance structures as they have performance advantages over polyester and vinyl esters in five major areas:

  • Better adhesive properties (the ability to bond to the reinforcement or core)
  • Superior mechanical properties (particularly strength and stiffness)
  • Improved resistance to fatigue and micro cracking
  • Reduced degradation from water ingress (diminution of properties due to water penetration)
  • Increased resistance to osmosis (surface degradation due to water permeability)