• Unsaturated Polyester UP

    Polyester resins are most commonly used as composite materials.  Innovation relating to these materials is driving the plastics industry into exciting new applications, most notably perhaps in the aerospace and construction markets.

    An Introduction

    UP Resins are made up of at least two separate components; reinforced fibre and embedding matrix.  Other additives may be used to improve properties or characteristics. Carbon, aramide or most commonly glass fibres may be used yielding FRP (Fibre Reinforced Plastic) or specifically GRP (Glass Reinforced Plastics) in the case of glass.  Materials basically bridge the gap between conventional, commodity plastics and specialist engineering plastics. Production takes place by introducing reinforcement while the resin is in an uncured, liquid state.  Such plastics are widely used in a host of applications where advantage may be taken of their good range of mechanical properties, corrosion resistance and low weight.

    The actual preparation of the first polyester resin is accredited to both Berzelius in 1847 and Gay-Lussac and Pelouze in 1883.  The unsaturated polyester resins used in today’s re-inforced plastics (RP) are combinations of reactive monomers. Carleton Ellis introduced the idea for the combination in the 1930’s. Ellis discovered that unsaturated polyester resins made by reacting glycols with maleric anhydride could be cured to insoluble solids simply by adding a peroxide catalyst. He applied for a patent on this idea in 1936.

    Ellis later discovered that a more useful product could be made by combining the unsaturated polyester alkyd with such reactive monomers as vinyl acetate or styrene, which makes it easier to add the catalyst and apply the resin.

    The first use of glass fibre reinforced polyester composites was in aircraft ducting, with the first non-military application in boat hulls.  The invention of the pultrusion line allowed development of unsaturated polyester based materials into new application areas such as oil wells, fishing rods and electrical insulation devices.  The filament winding process further expanded the potential market for unsaturated polyester resins.  This technique was initially developed for making military rocket cases and nozzle’s, however, potential for use in the making of pipes and storage vessels was soon recognised.

    A new polyester resin material called SMC (Sheet Moulding Compound) was formulated in the 1960s and following this BMC (Bulk Moulding Compounds) or DMC (Dough Moulding Compounds).  These products may be generically referred to as PMCs (polyester moulding compounds).  Compounds of this type contain chopped glass fibres and resin, they offer the advantage that, during formation, fibres and resins are able to flow under the action of heat and pressure in a hot press (or compression) moulding system.  Systems of this nature facilitate the moulding of complex details from simple material packs.  Cycle times tend to be between two and ten minutes.  High pressures and tooling costs associated with such processes mean those forming quantities of less than 10,000 tend not to be economic.

    Physical Properties

    The nature of the Unsaturated Polyester means that physical properties are dependant on the additional materials used, and the conditions of curing, for a specific application.   With such variety available it is best to refer to manufacturers for performance of individual grades.

    Ressistance to Chemicals
    Dilute Acid ***
    Dilute Alkalis ***
    Oils and Greases **
    Aliphatic Hydrocarbons *
    Aromatic Hydrocarbons *
    Halogenated Hydrocarbons *
    Alcohols ***


    KEY * poor ** moderate *** good **** very good


    Contact Moulding is the dominant UK processing technique for unsaturated polyesters. Contact moulding represents the more traditional (open mould systems) for processing polyester materials (i.e. hand and spray lay-up). The hand lay-up market represents approximately two thirds of all contact moulding. Compression moulding has grown in recent years, spurred by a move away from hand and spray lay up techniques.

    Highly automated processes, such as SMC/DMC Moulding and Resin Transfer Moulding and Pultrusion are also used to form items from UP Resins.



    UP Resins are widely used in a host of applications where advantage may be taken of their good range of mechanical properties, corrosion resistance and low weight.

    Un-reinforced versions are most commonly used for clear casting resins, coatings, buttons, body fillers, work-surfaces (such as polyester marble), polyester concrete (for applications such as road drainage) and in the manufacture of Gel Coats (applied to composite materials to improve the surface finish).

    The largest market for reinforced polyester resin (composite materials) is the building and sanitary ware market; here the material finds usage as structural parts (e.g. replacement of concrete clad steel), cladding panels, sheeting (e.g. for pre-fabricated buildings), roofing tiles, pipes and also for applications such as bathroom furniture (e.g. baths and shower trays).

    The Transport market makes significant usage of UP Resins.  Since the materials tend to lend themselves to lower volume applications it is most commonly used in aeroplanes, trucks, buses and coaches. Established markets include parts such as bumper beams, body panels, sunroof frames, catalytic converter heat shields, dashboard carriers, seat structures, battery supports and spring systems. In addition to weight reduction, polyester parts have the opportunity to reduce painting costs (should composite parts require painting, however, a conductive primer coat or in-mould coating, with lower paint transfer efficiency and lower rates of work means that part price overtakes costs associated with electro-galvanised steel). Polyester parts also provide good corrosion resistance properties.

    Rail cars, rolling stock and shipping containers are also manufactured using UP resin composites.  The marine market is also heavily reliant on UP Resins, principally for the manufacture of luxury boat hulls.

    Other markets for UP Resins include the caravan panel market and the material is widely used in storage vessels (where the chemical resistance of the material is highly valued).