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Strength & Stiffness

  • Thursday, 24th January 2019
  • Reading time: about 2 minutes

Two important mechanical properties of any resin system are its tensile strength and stiffness. The two figures below show results for tests carried out on commercially available polyester, vinylester and epoxy resin systems cured at 20°C and 80°C.

After a cure period of seven days at room temperature it can be seen that a typical epoxy will have higher properties than a typical polyester and vinylester for both strength and stiffness. The beneficial effect of a post cure at 80°C for five hours can also be seen.

Also of importance to the composite designer and builder is the amount of shrinkage that occurs in a resin during and following its cure period. Shrinkage is due to the resin molecules rearranging and re-orientating themselves in the liquid and semi-gelled phase. Polyester and vinylesters require considerable molecular rearrangement to reach their cured state and can show shrinkage of up to 8%. The different nature of the epoxy reaction, however, leads to very little rearrangement and with no volatile by-products being evolved, typical shrinkage of an epoxy is reduced to around 2%. The absence of shrinkage is, in part, responsible for the improved mechanical properties of epoxies over polyester, as shrinkage is associated with built-in stresses that can weaken the material. Furthermore, shrinkage through the thickness of a laminate leads to ‘print-through’ of the pattern of the reinforcing fibres, a cosmetic defect that is difficult and expensive to eliminate.

Published courtesy of David Cripps, Gurit

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