Vinyl esters are halfway between polyesters and epoxies as far as typical properties and toughness, and they are a step up on a cost basis. Their longer double bonded vinyl groups that link the ester groups together contribute a flexible nature to the resin. That flexible nature produces parts that withstand impacts and repeated flexing without developing cracks.

Vinyl esters provide excellent resistance to water, organic solvents and alkalis, but less resistance to acids than polyesters. Vinyl esters are stronger than polyesters and more resilient than epoxies. Molding conditions for vinyl esters are similar to those for polyesters.

Best Processes

Open Molding (Hand Lay-up/Spray-up), SMC/BMC, RTM. The best process for your product is a function of your tooling budget, part specification (such as dimensional tolerance and physical properties) and production volume.


Typically glass, but carbon is becoming a popular choice.

Special Design Considerations

MFG’s dedicated team of design engineers are available to help with your product design. MFG’s Technical Design Guide for FRP Composite Products and Parts provides in-depth information for product designers. You can download it on the bottom of this page.

Technical Details

Surface quality is not as good as with polyesters. The built-in toughness of vinyl esters makes smooth surfaces hard to accomplish. The double bonded nature creates shrinkage, which has a negative impact on surface quality.

The double bonded vinyl groups give the entire matrix a toughness that exceeds polyesters by about 2X depending on the test used. For example, typical tensile properties in SMC start at approximately 15,000psi and flexural properties exceed 30,000psi when using 28% glass reinforcement. As with polyesters, vinyl esters are not practical without additives, reinforcements and fillers. The next step up for toughness would be polyurethanes or epoxies.

As with polyesters, strength-weight ratio is very good and because of its low density (approximately 1.80 grams/cc) it often replaces metal parts that are close to 5X heavier.

Modulus (stiffness) is on a par – slightly less – than polyesters because of the toughness built in to the backbone of the ester linkage. Normal viscosity for vinyl esters is less than polyester, about 500 centipoise when cut in styrene monomer. Both polyester and vinyl ester resins will react and build in molecular weight to a solid waxy resin and be mostly unusable in that waxy state. Consequently, they are generally cut in styrene monomer for management as a liquid resin.

Cost Considerations

Next to polyesters, vinyl esters are the most economical for traditional fiberglass parts, and can be used with a variety of tooling and processes. If crack-resistance is important, this is your best value.