JEC COMPOSITES MAGAZINE - Issue #49 - June 2009 - 64

KNOW-HOW Release agents …/… Mould release system for the marine industry final mechanical properties of the part. The reactivity of the resin is directly proportional to the number of double bonds predisposed to react with the free radicals. The number of double bonds in the resin molecular chain dictates the degree of cross-linking of the cured polymer. The final physical properties of the cured polymer (heat distortion temperature, hardness, elongation at break, etc.) are affected by the achieved degree of cross-linking. As the curing stage begins, the molecular chains start to bundle together and the whole matrix gradually shrinks. The shrinkage of unsaturated polyester resins is volumetric and can be very significant (up to 10%). This effect is a major problem because it may cause unexpected defects in the moulded composite parts such as warpage, distortion, rippled surface, etc. The more abrupt the shrinkage is, the greater the chance of severe warpage of the part. The effect of resin shrinkage on composite deformation is very complex because of the anisotropic properties induced by the fibres. The effects of resin shrinkage on gelcoats are also very important and can lead to a severe pre-release of the coating, adversely affecting the aesthetics of the part. The main chemical factors involved are: - High-reactivity resin: a high-reactivity resin will produce a very fast reaction that may lead to severe pre-release. - High-reactivity peroxide: a high-reactivity peroxide may also lead to increased shrinkage resulting in pre-release. - Abrupt temperature changes: the surrounding temperature affects resin shrinkage. Pre-release is normally more common in summer than in winter. - Pigments: inorganic pigments such as titanium dioxide tend to offset resin shrinkage due to a high filler content; however, organic pigments do not contain fillers and cannot counteract matrix shrinkage. - Excessive quantity of promoter (cobalt octoate): excess promoter also leads to premature part release. Physical factors include: - Excessive resin thickness: this effect is more acute with gelcoats and produces a differential shrinkage that may result in severe pre-release. Thick sections always cure faster than thin ones. It is likely to take place in corners where a higher quantity of gelcoat is deposited. - Mould geometry: concave moulds (female moulds) are more prone to pre-release than convex moulds (male moulds). Mould size also matters as shrinkage is more noticeable when working with mould structures of over 8 m long. Fig. 3: Pre-release effect in a concave surface. Analysis of pre-release phenomena The pre-release of unsaturated polyester resins is a complex phenomenon that is not yet well understood. There are many chemical and physical factors that may trigger this process. To simplify the description of this phenomenon, we can say that “pre-release is the result of a higher than usual chemical reaction speed”. In other words, the chemical reaction between the resin and the monomer progresses at a much faster pace than it should. The consequences of such a fast reaction are serious as the polymer shrinks abruptly and the part self-releases without replicating the mould surface. In most cases, the part is rejected without any possibility for repair. This excessive reaction speed can be produced by many factors, some being chemical and others being physical. Fig. 4: Shrinking forces on mould geometry. Ensuring good release When attempting to achieve successful moulding results, two aspects need to be considered: - The material where the mould release system is to be applied, the tool. - The material to be applied over the mould release system, the resin. Three basic steps must be followed to ensure successful release: cleaning, sealing, and proper application of the release agent. Cleaning the mould A clean tool is important to ensure a good-quality moulded part. 64 jec composites magazine / No49 June 2009

JEC COMPOSITES MAGAZINE - Issue #49 - June 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #49 - June 2009