Thermoplastic polymer resins are extremely common, and we come in contact with thermoplastic resins constantly. Thermoplastic resins are most commonly unreinforced, meaning, the resin is formed into shapes and have no reinforcement providing strength.
Examples of common thermoplastic resins used today, and products manufactured by 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 – Piping
- PEI – Airplane armrests
- Nylon – Footwear
Many thermoplastic products use short discontinuous fibers as a reinforcement. Most commonly fiberglass, but carbon fiber too. This increases the mechanical properties and is technically considered a fiber reinforced composite, however, the strength is not nearly as comparable to continuous fiber reinforced composites.
In general, FRP composites refers to the use of reinforcing fibers with a length of 1/4″ or greater. Recently, thermoplastic resins have been used with continuous fiber creating structural composite products. There are a few distinct advantages and disadvantages thermoplastic composites have against thermoset composites.
Advantages of Thermoplastic Composites
There are two major advantages of thermoplastic composites. The first is that many thermoplastic resins have an increased impact resistance of comparable thermoset composites. In some instances, the difference is as high as 10 times the impact resistance.
The other major advantage of thermoplastic composites is the ability reform. See, raw thermoplastic composites, at room temperature, are in a solid state. When heat and pressure impregnate a reinforcing fiber, a physical change occurs; not a chemical reaction as with a thermoset.
This allows thermoplastic composites to be reformed and reshaped. For example, a pultruded thermoplastic composite rod could be heated and remolded to have a curvature. This is not possible with thermosetting resins. This also allows for the recycling of the thermoplastic composite at end of life. (In theory, not yet commercial).
Properties and Benefits of Thermoset Resins
Traditional Fiber Reinforced Polymer Composites, or FRP Composites for short, use a thermosetting resin as the matrix, which holds the structural fiber firmly in place. Common thermosetting resin includes:
- Polyester Resin
- Vinyl Ester Resin
The most common thermosetting resin used today is a polyester resin, followed by vinyl ester and epoxy. Thermosetting resins are popular because of uncured, at room temperature, they are in a liquid state. This allows for convenient impregnation of reinforcing fibers such as fiberglass, carbon fiber, or Kevlar.
As mentioned, a room temperature liquid resin is easy to work with. Laminators can easily remove all air during manufacturing, and it also allows the ability to rapidly manufacture products using a vacuum or positive pressure pump. (Closed Molds Manufacturing) Beyond ease of manufacturing, thermosetting resins can exhibit excellent properties at a low raw material cost.
Properties of thermoset resins include:
- Excellent resistance to solvents and corrosives
- Resistance to heat and high temperature
- Fatigue strength
- Tailored elasticity
- Excellent adhesion
- Excellent finishing (polishing, painting, etc.)
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 creates extremely strong bonds with one another, and the resin changes state from a liquid to a solid.
A thermosetting resin, once catalyzed, it can not be reversed or reformed. Meaning, once a thermoset composite is formed, it cannot be remolded or reshaped. Because of this, the recycling of thermoset composites is extremely difficult. The thermoset resin itself is not recyclable, however, there are a few new companies who have successfully removed the resin through pyrolization and are able to reclaim the reinforcing fiber.
Disadvantages of Thermoplastics
Because thermoplastic resin is naturally in a solid state, it is much more difficult to impersonate reinforcing fiber. The resin must be heated to the melting point, and pressure is required to impregnate fibers, and the composite must then be cooled under this pressure. This is complex and far different from traditional thermoset composite manufacturing. Special tooling, technique, and equipment must be used, many of which are expensive. This is the major disadvantage of thermoplastic composites.
Advances in thermoset and thermoplastic technology are happening constantly. There is a place and a use for both, and the future of composites does not favor one over the other.