Thermoplastic vs. Thermoset Resins

Differences in the two resins used in FRP composites

Colored polymer compounds.

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The use of thermoplastic polymer resins is extremely widespread and most of us come in contact with them in one form or another pretty much every day. Examples of common thermoplastic resins and products manufactured with them include:

  • PET (water and soda bottles)
  • Polypropylene (packaging containers)
  • Polycarbonate (safety glass lenses)
  • PBT (children's toys)
  • Vinyl (window frames)
  • Polyethylene (grocery bags)
  • PVC (plumbing pipe)
  • PEI (airplane armrests)
  • Nylon (footwear, clothing)

Thermoset vs. Thermoplastic Structure

Thermoplastics in the form of composites are most commonly not reinforced, meaning, the resin is formed into shapes that rely solely on the short, discontinuous fibers from which they are comprised to maintain their structure. On the other hand, many products formed with thermoset technology are enhanced with other structural elements—most commonly fiberglass and carbon fiber—for reinforcement.

Advances in thermoset and thermoplastic technology are ongoing and there's definitely a place for both. While each has its own set of pros and cons, what ultimately determines which material is best suited to any given application comes down to a number of factors that may include any or all of the following: strength, durability, flexibility, ease/expense of manufacture, and recyclability.

Advantages of Thermoplastic Composites

Thermoplastic composites offer two major advantages for some manufacturing applications: The first is that many thermoplastic composites have an increased impact resistance to comparable thermosets. (In some instances, the difference can be as much as 10 times the impact resistance.)

The other major advantage of thermoplastic composites is their ability to be rendered malleable. Raw thermoplastic resins are solid at room temperature, but when heat and pressure impregnate a reinforcing fiber, a physical change occurs (however, it isn't a chemical reaction that results in a permanent, nonreversible change). This is what allows thermoplastic composites to be re-formed and re-shaped.

For example, you could heat a pultruded thermoplastic composite rod and re-mold it to have a curvature. Once cooled, the curve would remain, which isn't possible with thermoset resins. This property shows tremendous promise for the future of recycling thermoplastic composite products when their original use ends.

Disadvantages of Thermoplastic Composites

While it can be made malleable through the application of heat, because the natural state of thermoplastic resin is solid, it's difficult to impregnate it with reinforcing fiber. The resin must be heated to the melting point and pressure must be applied to integrate fibers, and then, the composite has to be cooled, all while still under pressure.

Special tooling, technique, and equipment must be used, many of which are expensive. The process is much more complex and expensive than traditional thermoset composite manufacturing.

Properties and Common Uses of Thermoset Resins

In a thermoset resin, the raw uncured resin molecules are crossed linked through a catalytic chemical reaction. Through this chemical reaction, most often exothermic, the resin molecules create extremely strong bonds with one another, and the resin changes state from a liquid to a solid.

In general terms, fiber-reinforced polymer (FRP) refers to the use of reinforcing fibers with a length of 1/4-inch or greater. These components increase mechanical properties, however, although they're technically considered fiber-reinforced composites, their strength is not nearly comparable to that of continuous fiber-reinforced composites.

Traditional FRP composites use a thermosetting resin as the matrix that holds the structural fiber firmly in place. Common thermosetting resin includes:

  • Polyester Resin
  • Vinyl Ester Resin
  • Epoxy
  • Phenolic
  • Urethane
  • The most common thermosetting resin used today is a polyester resin, followed by vinyl ester, and epoxy. Thermosetting resins are popular because uncured and at room temperature, they're in a liquid state, which allows for convenient impregnation of reinforcing fibers such as fiberglass, carbon fiber, or Kevlar.

Benefits of Thermoset Resins

Room-temperature liquid resin is fairly straightforward to work with, although it requires adequate ventilation for open-air production applications. In lamination (closed molds manufacturing), the liquid resin can be shaped quickly using a vacuum or positive pressure pump, allowing for mass production. Beyond ease of manufacturing, thermosetting resins offer a lot of bang for the buck, often producing superior products at a low raw-material cost.

Beneficial qualities of thermoset resins include:

  • Excellent resistance to solvents and corrosives
  • Resistance to heat and high temperature
  • High fatigue strength
  • Tailored elasticity
  • Excellent adhesion
  • Excellent finishing qualities for polishing and painting

Disadvantages of Thermoset Resins

A thermosetting resin, once catalyzed, can not be reversed or re-shaped, meaning, once a thermoset composite is formed, its shape cannot be altered. Because of this, the recycling of thermoset composites is extremely difficult. Thermoset resin itself is not recyclable, however, a few newer companies have successfully removed resins from composites through an anaerobic process known as pyrolysis and are at least able to reclaim the reinforcing fiber.