Epoxies are two component systems, wherein the resin and the hardener ratio must be accurate. The reaction is exothermic, i.e. heat is released. When using epoxies the surrounding temperature should be more than 15°C and the relative humidity 40-80%. Generally, the final hardness is achieved through post-curing.
The advantages of epoxy include a low shrinkage rate, good bonding strength, high elongation at break and Young’s modulus, low water absorption, and good adaptability. Epoxies are versatile and are available with different solvent grades. In turn, the disadvantage of epoxies is their high cost. Epoxy might cause allergic reactions, so it’s important to cover your skin while working.
Thanks to its good adaptability and low water absorption, epoxy resin is the primary choice for repairing reinforced plastics, e.g. in boat reparation.
Polyester resins are widely used in reinforced plastics industries due to its beneficial price. The resins are unsaturated, comprised of polyester and styrene, and are cured at room temperature with a peroxide hardener. The curing reaction is exothermic, i.e. heat is released. The curing takes place in three stages: coagulation (gelation), curing, and post curing. The gelation time is dependent on the resin´s working time. Post-curing should be performed at 60-120 °C to achieve optimal resin properties.
There are different kind of polyester resins for different applications – for example, vinyl ester resin is applied in chemical containers and pipe manufacturing. Vinyl ester has better thermal and chemical resistance than the basic polyester resin, but they are usually only available in large amounts. Gelcoat and topcoat are also polyester based resins.
Gelcoats protect the laminate from weather, moisture, stresses caused by chemical exposure, as well as they give the laminate a finished surface. Gelcoats are painted or injected into the mould before the lamination process begins. The recommended application temperature is 18-22°C. Normally, the gelcoat layer weighs 550-850 g/m2, wherein the hardened surface has a thickness of 0,4-0,7 mm. It may not be applied onto an under-cured, cold, or damaged (scratches, holes, cracks, etc.) mould surface, because there can be a possibility of chemical or mechanical bondage (despite the usage of release agent).
Lamination of polyester resin should be applied on the gelcoat surface when the gelcoat is still soft. It can’t be spread directly on damaged surfaces because it doesn’t cure fully under the influence of air. A convenient coating for reparation purposes is topcoat, which is practically gelcoat, but it contains wax solution.
Topcoat is a paraffin-based surface paint that cures under the influence of air. Topcoat should by spread as soon as possible after the laminate is completed. If it is done later, the laminate surface must be thoroughly sanded and cleaned before it is applied so as to ensure adhesion. The consumption of topcoat is approximately 600 g/m2
Topcoat may not be applied on epoxy resin due to poor adhesion.