Fiber-reinforced polymer (FRP) profiles are diverse composite materials used across a range of industries worldwide. From aerospace to telecommunications to architecture and construction and everything in between, FRPs are fast outperforming traditional materials.
Let’s take a look at the differences between thermoset and thermoplastic resins and the impact your choice of resin has on end products.
The Pultrusion Process
Pultrusion is a method of FRP manufacturing that pulls continuous fiber materials through a resin bath for reinforcement. The liquid resin permeates the material and impregnates it with a range of technical and performance properties dependent on the resin used. Once saturated, the material is cured (polymerized), and your FRP is ready for deployment.
For more information on the pultrusion process used at Tencom, read our guide.
Thermoplastic vs. Thermoset Resins
Thermoplastic polymer resins are commonly used in the manufacture of profiles. We come in contact with thermoplastic resins constantly.
Between 2019 and 2024, the thermoplastic composites market is projected to grow at a compound annual growth rate (CAGR) of 5.2% and reach US$ 36 billion by 2024.
Thermoplastic resins are most commonly unreinforced, meaning the resin is formed into shapes and does not have strength reinforcements. Such examples of common thermoplastic resin products include:
- Polyphenylene Sulfide (PPS)
- Polyvinylidene Fluoride (PVDF)
- Polyether ether ketone (PEEK)
- Polypropylene (PP)
- Polystyrene (PS)
- Polyamide (PA)
In general, FRP composites refer to the use of reinforcing fibers with a length of 1/4″ or greater. There are a few distinct advantages and disadvantages of thermoplastic composites.
Advantages
The major advantage of thermoplastic composites is that many thermoplastic resins exhibit greater impact resistance than thermoset resins.
In some instances, the difference can be as high as 10 times the impact resistance.
Disadvantages
Because thermoplastic resins are naturally in a solid state, it is much more difficult to impregnate the 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. Another disadvantage of thermoplastic composites is that expensive tooling, techniques, and equipment are required.
Thermoset Resins
Traditional FRP composites use a thermosetting resin as the matrix, which holds the structural fiber firmly in place. Between 2016 and 2021, the thermoset composites market is expected to reach US$ 57.98 billion, at a CAGR of 6.67%. This is the larger market of the two types discussed here. Common applications are in civil engineering, appliances and electrical, and commercial and residential construction.
Within the class of thermoset resins, there are numerous resins available. These include (but are not limited to):
- Saturated and Unsaturated (UP) Polyester
- Vinyl Ester
- Epoxy
- Phenol-Formaldehyde (PF, Phenolic)
- Polybenzimidazole (PBI)
- Urea Formaldehyde (UF)
The most common thermosetting resin used today is polyester resin, followed by vinyl ester, epoxy, and polyurethane (PU). Thermosetting resins are popular because, when uncured and at room temperature, they are in a liquid state. This allows for convenient impregnation of reinforcing fibers such as fiberglass, carbon fiber, or Kevlar.
Applications
Common examples of thermoset plastics and polymers include epoxy, silicone, polyurethane, and phenolic. In addition, some materials, such as polyester, can be available in both thermoplastic and thermoset forms.
Compared to epoxy, unsaturated polyester, or vinyl ester resins, polyurethane (PU) resins are relatively new materials for pultrusion. They are a combination of polymers consisting of polyol and isocyanate components. They have excellent adhesion to various substrates and high elasticity.
Polyurethane fiberglass pultrusions have been extensively tested against traditional resins for tensile, compression, and flexural strength. In terms of compression strength, PU performed on par with other resin types.
However, PU showed marked advantages in tensile and flexural strength, especially when measured under stresses applied in the important 90-degree direction.
For instance, polyurethane fiberglass pultrusions withstood 48.7 MPa of 90-degree tensile stress. Vinyl ester pultrusions, by contrast, only withstood 40.5 MPa of stress.
Pultrusion is a manufacturing technique that provides for the continuous production of fiber-reinforced profiles. It uses continuous rovings, usually in glass, carbon, basalt, or aramid fibers, which are impregnated with a liquid matrix resin.
The resulting composite material then passes through a heated die, where it crosslinks into the desired profile. The pultrusion machine incorporates a pull mechanism ─ such as grippers ─ located downstream of the die that exerts a continuous pull force on the hardened profile. After cooling, the profiles are cut into customized cross-sections.
Thermoset resins have attributes which include:
- Resistance to solvents and corrosives
- Resistance to heat and high temperature
- Fatigue strength
- Customized elasticity
In a thermoset resin, the uncured resin molecules are crosslinked via a catalyzed chemical reaction. This reaction, which is often exothermic, allows the resin to form very strong bonds with itself and to change state from liquid to solid.
A thermosetting resin, once catalyzed, cannot be reversed or reformed. This means that once a thermoset composite is formed, it cannot be remolded or reshaped. This makes the recycling of thermoset composites extremely difficult. The thermoset resin itself is not recyclable; however, a few new companies have successfully removed the resin through pyrolysis and reclaimed the reinforcing fiber.
The Takeaway
Tencom has produced pultruded thermoset products for ultra-high temperatures, achieving a Glass Transition Temperature (Tg) of 575°F (302 °C).
This makes it the highest Tg pultruded material ever tested and further confirms the tolerance of thermoset composites.
Other examples of thermoset end products include fiberglass poles for tree and vine support, telescoping poles, and crossbars; electrical components such as terminal boards, arc shields, and standoff insulators; and common-use items like fans, ducts, and pumps.
In general, advances in thermoset and thermoplastic technology are constantly evolving. There is a place and a use for each. The diversity in resin application and its impact on end products is very evident when one analyzes pultrusion manufacturing and the array of resins available.
Regardless of your project, by tailoring the resins used, you can find an FRP profile that meets your design and performance specifications.
If you’re interested in learning what resins are available for your next project, contact our team of experts.
