Why cost should not be the consideration in the choice for industrial adhesives
If one were to consider the whole structure of a boat, the cost of the industrial adhesive is insignificant because it is more important to consider the value of high quality and the long term benefits of durability. Structural epoxy has many advantages over vinyl ester resins and polyester resins in terms of better adhesive properties. High adhesion is particularly important to ensure resistance to micro-cracking and when sandwich construction is used. The weakest link of a laminate is the bond between it and the core but the superior adhesion provided by structural epoxy greatly improves the strength between the core and the skins.
Advantages of structural epoxy in terms of mechanical properties
Based on tests handled on commercially available vinyl ester resins, polyester resins and epoxy resins, the following were the results:
- After a curing period of seven days, the tensile strength of epoxy resins was 20% to 30% higher than polyester resins and vinyl ester resins
- After the post cure, the difference in strength became greater
- A post-cured epoxy laminate exhibits tensile strength and stiffness that is almost double than a non-post cured polyester and vinyl ester laminates. It should be considered that boats wherein polyester resins were used are rarely post cured in a workshop unlike those that make use of epoxy resins
- Polyester and vinyl ester resins tend to shrink by 7% volumetrically but this will not be immediately obvious because the resins will continue to cure over a long period of time. In comparison, structural epoxy shrinks by less than 2% and is more stable with better cosmetics for a long period of time.
The strain the laminate can take before micro cracking takes place will depend on the toughness and adhesive properties of the resins. For more brittle resin systems like polyesters, micro cracking can occur long before laminate failure and it will severely limit the strains to which the laminates will be subjected to. In the marine environment, where there is water and moist air, the micro cracks in the laminate will considerably absorb water that can lead into an increase n weight, moisture attack on the resin, loss of stuff and eventually a drop in the ultimate properties. The main reason why epoxy resins are chosen over other industrial adhesives is its ability to withstand degradation from water penetration so that there will be no long term loss of mechanical properties.