May 18th 2021
In the first two parts of this Back to Basic series we examined the three different types of common resins: Epoxy, Polyester & Vinyl Ester. In the last installment we take a look at which resin is your best choice depending on the part, process & other considerations.
Resin selection varies heavily upon what fabric is to be used and the final use capability needed out of the final composite part. Most common fabrics used are Carbon Fiber, Aramid (Kevlar), and Fiberglass (you can find out more about these fabrics here).
Questions during the design phase of any project when selecting a resin:
- How much time is required for layup and bagging processes?
- How large is the part?
- Is a repair being performed?
- Will the final part be painted or will be composite be exposed to water or sunlight?
- What temperatures will the part see during its life?
Layup & Bagging Process
Important factors of the layup process can may control what fabrics are used due to design constraints in the part. Any wet layup with a given resin is going to have a working time or “pot life” or gel-time. Though these times differ slightly, the “pot life” is a tested amount for a resin’s viscosity to double, this amount of time allows for layup to be complete and optimally for bagging processes to have neared completion (if using vacuum bagging). This time is extremely important to know for any project as layup & bagging processes should not be rushed through.
Size of the Part
For many large parts such as boat hull, kayak, large fairings etc; polyester or vinyl ester resins provide an advantage. While smaller can be achieved with Epoxy resin. Unless utilizing prepregs whose curing variation opens limits to a heat only cure process, providing layup times in the factor of days instead of hours.
Is This A Repair?
If selecting a resin to use for repair, epoxy will provide peace of mind when not knowing the molecular makeup of any component below. Epoxy resins can be used to provide a bond between properly prepped gelcoats and polyester / Vinyl Ester surfaces. Repairs with Poly/Vinyl Ester can be made to like surfaces but cannot be made on epoxy surfaces.
Paint & Water/Sunlight Exposure
Epoxy provides great protection from water however; additional measures need to be taken if the epoxy is bare (no paint) to a fabric on a part. Certain epoxies will “yellow” or degrade over time in sunlight due to UV damage. This can be mitigated or eliminated by using an epoxy resin with UV additives in it, or by applying a clear coat (automotive grade) over the finished parts which has UV protection in it
Service temperatures and glass transition points should be noted when in design of a part to be sure that a given resin will perform under needed conditions. Glass Transition (Tg) is one of the most important properties of any material. The Tg is the temperature in which the rigid cured polymer turns into a soft rubbery material. Yes, it can be exceeded in some senses however, as a thermoset, it will never return to the same material properties as before.
General Service temperatures of commonly used two component epoxies are around 250F. However, certain epoxies have been engineered to reach service temps up to 600F. Polyester and Vinyl Ester Resins are usually in the ballpark between 180-250F for a service life.