The Benefits of Epoxy vs Polyester Resins in Composite Structures

Composites are essentially two parts; the fibre and resin which bonds together creating the ‘matrix’.

Carbon fibre and fibreglass are talked about a lot, but consideration of the correct resin system is as important as fibre selection. Below we will cover some of the key resin varieties and why we choose to use epoxy resin (in a pre-impregnated format) for our products.

Epoxy Resin

Epoxy resins are the superior resin for high-tech structures that require strength, stiffness, durability, and lightweight. This is why aircraft, aerospace applications, and offshore racing boats have been using epoxies for years over other resins. However, most pleasure boats under 60 feet are still made with cheaper polyester resin due to the cost being the main consideration for composite boat builders.

Epoxy resins are twice as expensive as vinyl ester resins and vinyl ester resins are twice as expensive as polyesters. Since the resin can make up 40 to 50% of the weight of a composite component, the cost has a significant impact on the cost of the final component. However, when considering the cost of the whole structure, the resin cost is relatively insignificant compared to the long-term gains in better durability and higher quality. Epoxy resins are not only superior but are essential to meet the performance demand of aerospace, mining, or other components that are exposed to the most challenging structural environments.

The reason why epoxy resins are superior to polyester and vinyl ester resins is because of the following qualities:

- superior adhesive properties meaning it has a much stronger bond to the reinforcement material and core materials,

-better mechanical properties, particularly in strength and stiffness,

- improved resistance to fatigue and micro-cracking from impact,

-reduced degradation from water ingress, and

- increased resistance to osmosis, meaning the surface is less likely to be degraded compared to polyester and vinyl ester resins.

Now let’s discuss each point in more detail:

Superior Adhesive Qualities

The superior adhesive quality of epoxy resins comes from the molecular and physical levels, as epoxies cure with low shrinkage, leading to better homogenous bonding between fibers and resin and better transfer of load between the different components of the matrix. At Composite Components we are often using a sandwich construction for weight and structural benefits. This is why Epoxy resins are key to ensuring that the potentially weakest point (the bond between the laminate and core) has the strongest possible bond.

Better Mechanical Properties

Tests show that epoxy resins have 20 to 30% higher tensile strength than polyester and vinyl ester resins, especially when post-cured in an oven at 79°C. Seven days after a cure the tensile strength of the epoxy is 20-30% higher than those of polyester and vinyl ester. More often than not, Composite Components also ‘post-cure’ their parts in an oven to create an even stronger part. The benefits of a post-cure are both structural and cosmetic. Structurally, a post-cured epoxy laminate component has close to double tensile strength and stiffness to that of a polyester or vinyl ester laminate that has not been post-cured.

Additionally, polyester and vinyl ester resins shrink up to 7% volumetrically, leading to print-through effects, while epoxies shrink less than 2% and have better cosmetics over a long period of time.

 Improved Fatigue resistance

When designing a composite laminate, it is important to consider not only the ultimate strength of the resin matrix but also its ability to resist fatigue and micro-cracking. Micro-cracking occurs when the stress level in the laminate causes the resin to crack away from the fiber reinforcements. This can lead to moisture absorption and a decrease in stiffness and overall properties over time. Brittle resin systems, like many polyesters, are particularly susceptible to micro-cracking and are limited in the strains they can withstand. Epoxy resins have superior adhesive and toughness properties that allow them to withstand cyclic loading and resist micro-cracking better than polyester resins. This is why epoxy resins are often chosen for high-performance applications like aircraft structures.

Reduced degradation from water ingress

The ability of a resin to withstand water penetration and degradation is an important property, especially in marine environments. All resins absorb some moisture, 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. Polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures. A thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength after immersion for one year, whereas an epoxy laminate immersed for the same period will retain around 90%.

Increased resistance to osmosis

Osmosis occurs in all laminates in a marine environment due to the low quantities of water that pass through them in vapor form. As this water passes through, it reacts with any hydrolysable components inside the laminate to form tiny cells of concentrated solution. The osmotic pressure generated draws more water through the semi-permeable membrane provided by the gel coat in an attempt to dilute this solution, increasing 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.