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Unsaturated polyester resin (UPR)
0.8-3 usd/kg
Unsaturated polyester resin (UPR) is a relatively low cost and durable thermosetting plastic made up of two parts: liquid resin and catalyst (hardener). The principal difference between these systems and thermoplastic polyesters, such as polyethylene terephthalate (PET), is the inclusion of double bonds in the polymer chain, which allows for permanent cross-linking and the formation of a rigid thermoset structure. This has many advantages: the resin starts off in liquid state and so suitable for casting, pouring, coating, composite laminating and so on; very good resistance to chemicals and weathering; and high adhesive strength with excellent wetting (ability to coat a solid surface, like glass fibre). However, on is own it is quite brittle and so tends to be used in combination with fibre or mineral reinforcement. Also, as a thermoset, it is not practical to recycle. This problem is especially acute in the production of wind turbine blades, where thousands of tons of this material are being consumed each year and will inevitably end up in landfill.
Unsaturated polyester resin contains some harmful chemistry, such as individual ingredients like styrene, a known carcinogen, as well as catalysts like methyl ethyl ketone peroxide (MEKP) that can cause skin damage and blindness.
There are two main families: ortho (based on phthalic anhydride) and iso (based on isophthalic acid). Ortho types are the most common and used in everything from boat hulls to bathtubs and countertops. Iso types are higher priced and higher performance. They are used in applications that require higher levels of scratch resistance, clarity and weather resistance. For example, iso resins are used in gel coats, which provide the outer decorative and protective layer on FRP structures. It is painted onto the mould before the fibreglass is applied behind. So when demoulded, it becomes the visible layer.
Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Low circularity potential
Potentially toxic in use
Hazardous end of life
Microplastics
Unsaturated polyester resin (UPR) is predominantly used as the matrix in fibre and mineral reinforced composites. Its is the most common thermoset composite material as a result of its relatively low cost and balance of mechanical and physical properties – so much so, that composites made with polyester are often simply referred to as fibreglass, fibreglass reinforced plastic (FRP) or glass fibre reinforced plastic (GRRP). It is not as strong as epoxy resin, or as versatile as polyurethane or acrylic, but offers a good balance of resistance to wear, weathering, water and chemicals.
Applications span marine, industrial, construction, furniture and transport. It is used to make things like boat hulls (yachts, dinghies), transport interiors (paneling, profiles and furniture), structural parts in transport and aircraft, countertops for kitchens and bathrooms, and building cladding and siding.
As the matrix it makes up a large proportion of the finished material, binding the reinforcement together and maintaining the shape of the part by transferring applied loads to the reinforcement. It also plays an important role in protecting the reinforcement from the elements, as well as abrasion, water and chemicals.
It is compatible with all types of composite forming, including casting, hand lay-up, compression moulding, dough moulding compound (DMC), sheet moulding compound (SMC), spray moulding and resin infusion (vacuum infusion).
Polyester resin cannot be recycled like thermoplastics. It cures in a one-way reaction and so is not suitable for mechanical recycling and melt-processing. Durat composite material is a solid surface material used in sinks, bathtubs, signage and countertops, for example. It is made with 30% post industrial recycled (PIR) plastic material. According to the company’s environmental product declaration (EPD), this material has a carbon footprint of 3.35 kgCO2/kg.
