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.
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.
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
Fatigue Resistance and Micro-Cracking:
strain that a laminate can take before micro cracking depends strongly on the toughness and adhesive properties of the resin system.
For relatively more brittle resin systems, such as many polyesters, this point occurs a long way before laminate failure, and so severely limits the strains to which such laminates can be subjected.
In an environment such as water or moist air, the microcracked laminate will absorb considerably more water than an uncracked laminate.
This will then lead to an increase in weight,moisture attack on the resin and fiber sizing agents, loss of stiffness and with time, an eventual drop in ultimate properties.
The superior ability to withstand cyclic loading is an essential advantage of epoxies vs. polyester resins. This is one of the main reason epoxies are chosen almost exclusively for aircraft structures.
Degradation from Water Penetration:
An important property of any resin, particularly in water conatct, is its ability to withstand degradation from water penetration.
All resins will absorb some moisture, adding to a laminate’s weight,but what is more significant is how the absorbed water affects the resin and resin/fiber bond in a laminate, leading to a gradual and long-term loss in mechanical properties.
Both polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures.
As a result, a thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength after immersion over period of one year, whereas an epoxy laminate immersed for the same period will retain around 90%
water is then drawn through the semi-permeable membrane provided by the gelcoat in an attempt to dilute this solution.
This water increases the fluid pressure in the cell.
Eventually the pressure will distort or burst the gel coat, leading to a characteristic “chickenpox” surface.
To delay the onset of osmosis, it is necessary to use a resin that has both a low water transmission rate and a high resistance to attack by water.
A polymer chain having epoxy linkages in its backbone is substantially better than polyester or vinyl ester systems at resisting the effects of water.