Differences Between Polyester and Epoxy

WEST SYSTEM® Epoxy is—as the name states—an epoxy resin system. Epoxy is easy to use, has excellent properties, and is the go-to choice for repairs, high-performance composites, and wooden boat building. However, mass-production boats are primarily built with polyester. Let’s take a look at WEST SYSTEM Epoxy compared to polyester resin to understand why.

Production Process

Many production boat builders can produce a high-quality boat using polyester resins because polyester is well suited for use in a controlled manufacturing environment. These builders can control their shop temperature, have equipment to measure mix ratios with incredible accuracy, and follow a carefully engineered process the same way every time. Since mass-produced recreational boats require large volumes of resin, the slightly lower cost of polyester becomes an important factor.

The conditions for boat repair, wooden boat construction, and other DIY applications are often different. In these cases, controlling the shop environment is not always possible. Repairs and construction projects are often unique and more than just a matter of repeating the same process. For these types of projects, WEST SYSTEM Epoxy is an excellent choice. It offers accurate metering with pumps and is supported by a superior technical support team, providing unbeatable value.

Molecular Structure

The differences between WEST SYSTEM Epoxy and polyester resin begin with how the molecules react with each other during the curing process. The epoxy resin used in our products is made from a molecule that has a very strong backbone. The resin molecules then react with the hardener molecules in a process that creates a high degree of crosslinking between the resin and hardener molecules. This results in a strong 3-dimensional matrix.

Polyester resins consist of many long molecules with reactive sites at their ends. The catalyst, often MEKP, creates a chain reaction, causing these polyester molecules to begin linking to one another. This chain reaction enables polyester resins to cure quickly, but they will not have a molecular structure crosslinked in the same way as epoxy, and therefore, they are not as strong.

Curing Process

The different reaction mechanisms require different considerations when being used for laminating composites. The chain reaction in polyester requires a specific amount of catalyst, often measured in fractions of a percentage, and good temperature control. Many polyester boat manufacturers have dispensing equipment that precisely controls the amount of catalyst added to the resin to adjust cure speed. Like epoxy, temperature impacts the speed of the curing process. However, polyester has a much narrower temperature range. Builders have to control the shop temperature, particularly in the winter.

WEST SYSTEM Epoxy has a broader 5:1
or 3:1 mix ratio, allowing you to easily use the 300 Mini Pump set to dispense at the correct ratio. To adjust for temperature fluctuations, select the hardener speed most appropriate for the anticipated usage temperature.

Shelf Life

Polyester resins often require a promoter, which can be hazardous to use separately, so most manufacturers pre-blend it into the resin. Promoted polyester resin has a shelf life measured in months whereas WEST SYSTEM Epoxy Resin has a minimum 3-year shelf life. In addition, polyester resin often contains fillers to modify the viscosity whereas WEST SYSTEM Epoxy does not have any materials that will settle during storage, further improving shelf life.

Strength

The stronger backbone of epoxy resin provides high strength and fatigue resistance. This is why aerospace applications, high-performance sports equipment, and the highest levels of motorsports use epoxy resin. They can reduce weight by building thinner laminates with higher properties.

Epoxy resin is also a better adhesive. Our experience at Gougeon Brothers, Inc. is that adhesion is critical for a successful fiberglass repair. Using epoxy resin will provide a patch with a stronger bond than the original laminate, ensuring the patch will remain securely in place. The excellent adhesion of epoxy can also bond dissimilar materials, including wood, plastics, metals, and composites.

Cured Surface

Most polyester resins will not cure in the presence of air (air-inhibited). This characteristic results in a surface that often feels sticky because of the thin layer of unreacted polyester resin that remains on the surface. In a production setting, this is not a significant issue because when the next layer of fiberglass is applied, it inhibits the air, allowing the reaction to continue, forming a solid bond. This can become problematic when applying final finishes over a repair.

WEST SYSTEM Epoxy is not air inhibited and, provides a solid fully cured surface, even in thin films. This thorough cure also improves the adhesion to wood. Since there are small air pockets in wood, polyester resin does not completely cure at the wood interface, reducing the adhesion. WEST SYSTEM Epoxy’s legacy in wooden boat construction is partially due to its excellent adhesion to many different species of wood.

Moisture Resistance

Polyester resins can have an issue with osmotic blisters. Osmotic blisters can form on the bottom of boats built with polyester resin after they sit in the water for long periods, typically below the waterline. This issue results from water migrating through the gelcoat into the resin. The moisture will cause the polyester resin to break down in very small voids where the polyester did not completely react when catalyzed. This forms an acidic liquid that can continue to break down the polyester resin. This liquid contains compounds of the dissolved resin and is unable to pass through the semipermeable gelcoat. A blister is formed by pressure created during this osmotic process. WEST SYSTEM Epoxy will not break down from contact with water and has a very high resistance to absorbing water. These properties enable it to be used as a moisture barrier coat to prevent polyester hulls from developing osmotic blisters. See our Fiberglass Boat Repair & Maintenance manual.

V.O.C. and Shrink

Polyester resin often has a strong odor typically caused by the styrene included in the formulation. Polyester resin normally has up to 30% volatile organic compounds (V.O.C.). WEST SYSTEM Epoxy has extremely low V.O.C. content (less than 1%) and a very low order. This high amount of V.O.C. in polyester resin also results in significant shrinkage when it cures. It is common for polyester resin to shrink over 5% when it cures. This can create stress in bond lines and cause cosmetic issues in large laminations.

WEST SYSTEM Epoxy’s nearly 100% solids content results in extremely low shrink values during curing. Boat manufacturers make substantial efforts to manage the shrinkage of polyester resin when building boats, often by using additional layers of materials on the outside of the hull to mask the cosmetic issues resulting from resin shrinkage. When using WEST SYSTEM Epoxy, the resin will not change dimension during the curing process or after it has cured, providing a more stable surface and stronger bond lines.

When to use WEST SYSTEM

When repairing a mass-produced production boat, WEST SYSTEM Epoxy is a better choice than the polyester resin used when it was manufactured. The increased strength, adhesion, and moisture resistance will provide a better repair, and the simple mix ratio and low V.O.C. make it easy and more pleasant to use.