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Resin Systems

Any resin system for use in a composite material will require the following properties:

1. Good mechanical properties 
2. Good adhesive properties 
3. Good toughness properties 
4. Good resistance to environmental degradation

Mechanical Properties of the Resin System 

The figure below shows the stress / strain curve for an 'ideal' resin system. The curve for this resin shows high ultimate strength, high stiffness (indicated by the initial gradient) and a high strain to failure. This means that the resin is initially stiff but at the same time will not suffer from brittle failure.



It should also be noted that when a composite is loaded in tension, for the full mechanical properties of the fibre component to be achieved, the resin must be able to deform to at least the same extent as the fibre. The figure below gives the strain to failure for E-glass, S-glass, aramid and high-strength grade carbon fibres on their own (i.e. not in a composite form). Here it can be seen that, for example, the S-glass fibre, with an elongation to break of 5.3%, will require a resin with an elongation to break of at least this value to achieve maximum tensile properties.



Adhesive Properties of the Resin System 

High adhesion between resin and reinforcement fibres is necessary for any resin system. This will ensure that the loads are transferred efficiently and will prevent cracking or fibre / resin debonding when stressed.

Toughness Properties of the Resin System 

Toughness is a measure of a material's resistance to crack propagation, but in a composite this can be hard to measure accurately. However, the stress / strain curve of the resin system on its own provides some indication of the material's toughness. Generally the more deformation the resin will accept before failure the tougher and more crack-resistant the material will be. Conversely, a resin system with a low strain to failure will tend to create a brittle composite, which cracks easily. It is important to match this property to the elongation of the fibre reinforcement.

Environmental Properties of the Resin System 

Good resistance to the environment, water and other aggressive substances, together with an ability to withstand constant stress cycling, are properties essential to any resin system. These properties are particularly important for use in a marine environment.

Published courtesy of David Cripps, Gurit


Resin Systems

2.1. Resin Types

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.
2.2. Polyester Resins

Polyester resins are the most widely used resin systems, particularly in the marine industry.
2.3. Vinylester Resins

Vinylester resins, although similar to polyesters, exhibit better resistance to water and many other chemicals.
2.4. Epoxy Resins

Epoxy resins represent some of the highest performance resins and generally out-perform most other resin types in terms of mechanical properties and resistance to environmental degradation.
2.5. Gelation and Curing

Introduces catalyst or hardeners to resins and speed of cure.
2.6. Adhesive Properties

Discusses the adhesion properties of different resin systems.
2.7. Strength & Stiffness

The important mechanical properties of any resin system are its tensile strength and stiffness.
2.8. Micro-Cracking

Before the ultimate strength is achieved, a laminate will reach a stress level where the resin will begin to crack away from those fibre reinforcements not aligned with the applied load, and these cracks will spread through the resin matrix.
2.9. Fatigue Resistance

Composites show excellent fatigues resistance in general, when compared with most metals.
2.10. Water Ingress

All resins absorb some moisture but most importantly, it is how the absorbed water affects the resin and the loss in mechanical properties.
2.11. Osmosis

The use of resin rich layers next to the gel coat are essential with polyester resins to minimise degradation through osmosis.
2.12. Resin Comparison

The main advantages and disadvantages of resin types.
2.13. Other Resins

Explores alternative resin systems and their properties.
2.14. Release Agents

Optimisation of cycle time and consistency can be determined by the use of release agents.