Composite Materials FAQ: Resins


Polyester resin is a single part thermoset resin, the least expensive type of resin on the market and is also the weakest of the three main resin types. A thermoset is a type of plastic that once cured, is not able to be remolded through the use of heat. I say it is a single part resin because it will begin to cure at an extreemly low speed from the moment it is produced. It has a shelf life of approximately 9 months depending on the temperature of the area in which it is being kept. When used, a catalyst, generally methyl ethyl keytone peroxide (MEKP), is mixed with the resin to speed the curing. The more MEKP that is added to the resin, the faster it will cure. Most manufacturers recommend adding 1% MEKP per volume of resin. The curing time relates directly to the surrounding temperature. Attempting to cure resin at a temperature below 70 F may require an increased amount of MEK. Polyester resin gives off some hefty fumes so be sure to use this only in a well ventilated area and preferably with a ventilator.

Polyester resin is recommended for use with fiberglass fabric and mat. Both E-glass and S-glass are compatable. It can be used with both fabric and chopped strand fiberglass mat because the styrene binder that holds the fiberglass strands in the mat together dissolves in the polyester resin. This allows the fiberglass mat to conform to complex part shapes. Polyester is NOT recommeded for use with carbon fiber. While it can be used in a pinch, if you are spending the money on carbon fiber, stick with a higher grade resin such as epoxy. If you have purchased a carbon fiber part and you notice spider cracking in the part, it is most likely due to the use of a polyester resin.


Vinylester resin is a single part thermoset resin that is stronger than polyester but weaker than epoxy resin. It also has a price that falls between the two. Vinylester resin has properties similar to that of polyester in that it is compatible with chopped strand mat but not with carbon fiber. It can also be used with E-glass and S-glass cloth.


Epoxy resin is the strongest and most expensive of the three resins. Its strength far exceed that of polyester and vinylester resins. Whenever you purchase a carbon fiber part, you should always inquire to the type of resin that was used to make it. If they do not use epoxy resin then do not buy it. The part will be much weaker than an epoxy part. Epoxy is not compatible with chopped strand fiberglass mat but it can be used with any type of reinforcement cloth including fiberglass cloth, carbon fiber, and more exotic reinforcements such as Kevlar.


Epoxy is a two part curing system. One part is the resin while the second portion is the hardener. The two must be mixed according to the manufacturer's recommended ratio to ensure that a full chemical reaction will take place. Fast, medium, and slow curing mixes resin systems are available. Formulations that cure at different temperatures are also available. Other properties of the resin can be added/modified such as flexibility, heat resistance, and UV resistance, though special additives. The speed epoxy resin curing can be increased through the use of heat, UV light, and even microwaves.


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