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frequently found in many high-strength adhesives. This is due to their chemical com- position and the presence of polar hydroxyl and ether groups. As epoxies cure with low shrinkage the various surface contacts set up between the liquid resin and the adherends are not disturbed during the cure. The adhesive properties of epoxy are especially useful in the construction of honeycomb-cored laminates where the small bonding surface area means that maximum adhesion is required. The strength of the bond between resin and fibre is not solely dependent on the adhe- sive properties of the resin system but is also affected by the surface coating on the reinforcement fibres. This ‘sizing’ is discussed later in section 5.2.2.5. 5.1.4.2 Mechanical Properties Two important mechanical properties of any resin system are its tensile strength and stiffness. Figure 22 shows results for tests carried out on commercially available poly- ester, vinylester and epoxy resin systems cured at 20°C and 80°C. Figure 22 – Comparison of resin tensile strength & modulus 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 bi-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. GTC-6-0417 - 21PDF Image | GUIDE TO COMPOSITES
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