Epoxy vs Polyester - Tartan Yachts - #1

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The Advantages of Epoxy Resin versus Polyester in Marine Composite Structures Introduction In any high-tech structural application, where strength, stiffness, durability and light weight are required, epoxy resins are seen as the minimum standard of performance for the matrix of the composite. This is why in aircraft and aerospace applications, as well as offshore racing boats, epoxies have been the norm for years. However 95% of pleasure boats under 60 feet today are still made with polyester resin. The main consideration for materials selection for most composite builders is cost, with performance and more importantly value for money often being a secondary consideration. As a general rule epoxy resins are twice as expensive as vinyl ester resins and vinyl ester resins are twice as expensive as polyesters. Since the resin can constitute 40 to 50% of the weight of a composite component, this price difference is seen as having a significant impact on the cost of the laminate. However, when considered against the cost of the whole structure (the boat) the cost is relatively insignificant, and the value of higher quality and long term gain of better durability (therefore better resale value) can be tremendous. What contributes to this better value…..? Epoxy resins have performance advantages over polyester and vinyl esters in five major areas: s Better adhesive properties (the ability to bond to the reinforcement or core) s Superior mechanical properties (particularly strength and stiffness) High adhesion is especially important in resistance to micro-cracking (see later) and when using sandwich construction. The bond between the core and the laminate is usually the weakest link of the laminate, and the superior adhesive properties of the epoxy resin greatly increase the strength of the interface between skins and core. Mechanical Properties Two important mechanical properties of any resin systems are its tensile strength and stiffness. The figure below shows results of tests carried out on commercially available polyester, vinyl ester and epoxy resin systems, either cured at room temperature or post cured at 175°F. After a cure period of seven days it can be seen that the tensile strength of the epoxy resin is 20 to 30% higher than those of polyester and vinyl ester. More importantly, after post cure the difference becomes ever greater. It is to be noted that boats built with polyester resins are rarely post cured in the workshop while boats built with epoxy quite often are. However, in practice all boats can quite often see “natural” post cures – particularly dark coloured surfaces under a Caribbean sun! The consequences are two fold: Structurally A post-cured epoxy laminate will exhibit tensile strength and modulus (stiffness) close to double that of a non-post cured polyester or vinyl ester laminate. Adhesive Properties Cosmetically Polyester and vinyl ester resins shrink up to 7% volumetrically and because the resin continues to cure over long periods of time this effect may not be immediately obvious. This cure accounts for the print through effect observed on a lot of older polyester boats. In comparison, epoxies shrink less than 2% and an epoxy laminate will be a lot more stable and have better cosmetics over a long period of time than a polyester one. Epoxy resins have far better adhesive properties than polyester and vinyl ester resins. However many times have you known a polyester car body filler fall off a ding repair? The superior adhesion of epoxy is due to two main reasons. The first is at the molecular level, where the presence of polar hydroxyl and ether groups improves adhesion. The second is at the physical level - as epoxies cure with low shrinkage, the various surface contacts set up between the liquid resin and the reinforcement are not disturbed during cure. The result is a more homogenous bond between fibers and resin and a better transfer of load between the different components of the matrix. Comparative Tensile Strength of Resins s Improved resistance to fatigue and micro cracking s Reduced degradation from water ingress (diminution of properties due to water penetration) s Increased resistance to osmosis (surface degradation due to water permeability) Comparative Stiffness of Resins