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Unsaturated Polyester UP
INTRODUCTION
Unsaturated polyesters Polyester resins are thermoset materials that
first became available over 60 years ago. The principal advantage of
polyester resins is the ease in which they may be fabricated, a
consequence of polymerisation via a free radical reaction. Such
reactions are rapid and easily controllable, thus production cost is
relatively low.
Unsaturated polyester resins are very versatile materials. At room
temperature, the liquid resins are stable for months even years but can
be triggered to cure by a peroxide catalyst.
Styrene is by far the most commonly used diluent: it links the polyester
chains to form strong three-dimensional polymer network. The curing
reaction is exothermic, often providing adequate heat for a satisfactory
cure.
PROPERTIES
UP Resins are most commonly used as composite materials. These polymers
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.
UP Resins can be made very flame retardant and chemically resistant they
also give excellent weatherability. Dimensional stability and rigidity
is very good. Plus, of course, they can be easily reinforced with fibres
of various types as part of the moulding stage rather than having to use
pre-blended reinforcing agents.
These materials lend themselves to low volume applications with simple
tooling, by use of hand lay-up methods of fabrication, or for mass
production using injection moulding.
GRADES AVAILABLE
Typically UP Resins is processed by mixing a liquid UP resin with a
curing system. Reinforcement can be applied at this stage. FRP Resins
can be processed by a variety of processes as indicated below.
Processing Methods:
Contact Moulding
Compression Moulding
Injection Moulding
Continuous Lamination
Pultrusion
Transfer Moulding
Filament Winding
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
Tensile Strength n/a N/mm?
Notched Impact Strength n/a Kj/m?
Thermal Coefficient of expansion n/a x 10-6
Max Cont Use Temp n/a oC
Density n/a g/cm3
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.
RESISTANCE TO CHEMICALS
Dilute Acid ***
Dilute Alkalis ***
Oils and Greases **
Aliphatic Hydrocarbons *
Aromatic Hydrocarbons *
Halogenated Hydrocarbons *
Alcohols ***
KEY
* poor ** moderate *** good **** very good
APPLICATIONS
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).
A BRIEF HISTORY OF UNSATURATED POLYESTER
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.
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