Unsaturated polyester resins (UPRs) are thermosetting polymers that are manufactured through the polycondensation reaction of maleic anhydride or fumaric acid with glycols. UPRs contain unsaturated carbon-carbon double bonds in their backbone that allow them to undergo addition polymerization and crosslinking when cured with styrene monomer and peroxide initiators. This curing process results in the formation of a highly crosslinked network thermosetting plastic with useful mechanical, thermal, and chemical resistance properties.
Typical Resin Components and Production
The main reactants used in producing UPRs include phthalic anhydride, maleic anhydride, propylene glycol, and ethylene glycol. Other ingredients like isophthalic acid and diethylene glycol may also be used. In the production process, these reactants are mixed and heated to promote esterification reactions and create low molecular weight oligomers. Styrene monomer is then added which acts as a reactive diluent during curing. Organic peroxides like methyl ethyl ketone peroxide (MEKP) or benzoyl peroxide are also added as curing agents.
Curing and Properties of Cured Resins
When Unsaturated Polyester Resins mixtures are cured, the initiator compounds start generating free radicals that trigger polymerization of the styrene monomer first. This leads to gelation where individual molecules join to form a continuous rigid structure. Further curing reactions occur between the unsaturated groups in polyester chains, crosslinking them tightly and forming the final thermoset network plastic. Cured UPRs exhibit properties like good mechanical strength, chemical resistance, dimensional stability and relatively low cost which make them suitable for various applications.
Major Applications of Unsaturated Polyester Resins
Some of the most common applications of UPRs include:
- Construction of Fiberglass Products: UPRs are predominantly used to produce fiber reinforced plastic (FRP) items like boats, water tanks, swim pools, baths etc. through open molding techniques. The glass fibers impart strength and rigidity.
- Production of Sanitaryware: UPR sheets are used to manufacture bath items, toilet bowls, sinks etc. through casting and centrifugal molding. Their non-porous surface makes them suitable for sanitary applications.
- Electrical Applications: Phenolic-blended UPR cast resins are employed to manufacture switchboards, distribution boxes and other industrial electrical components due to their good dielectric and arc resistance.
- Tooling Applications: Unsaturated polyester resins casting resins find application in tooling where master patterns or molds are produced for injection molding of plastic items like containers, lids etc.
- Decorative Laminates: Decorative paper or foil overlays impregnated with UPR are hot pressed to create high-pressure laminates used for kitchen countertops, table/furniture tops etc.
- Transportation Industry: Pultruded FRP structural profiles made from UPR composites are used in rail cars, trucks and shipbuilding for their light weight and corrosion resistance properties.
Continuous Innovation and New Applications
Research into developing bio-based polyols from plant oils is allowing production of more sustainable UPR formulations. Additives are enabling properties like fire retardancy and UV resistance in UPR composites for solar panel frames, building panels etc. Nanoclay and nanofiller reinforced UPR nanocomposites with enhanced mechanical/barrier properties find expanding uses. 3D
rinting of UPR resins is an emerging application as well. With continuous innovation, the applications of UPRs in various industries will further grow.
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