What is polyester?
Polyester is a synthetic resin that cures chemically into a hard, unmeltable solid once a hardener is added. In construction it is almost always used in reinforced form: unsaturated polyester with glass fibre, known as GRP (glass reinforced plastic).
It is a thermoset: once cured it cannot be remelted or reshaped, unlike thermoplastics such as PVC and polyethylene.
Where it is used in building
- Gutters and gutter linings. Applied seamlessly into an existing gutter, a widely used repair method for lead, zinc or timber gutters.
- Rooflights, domes and profiled sheeting. Translucent polyester sheet on verandas, farm buildings and stores.
- Watertight tanks and trays. Planters, roof terrace trays, fountains, bunding.
- Prefabricated facade elements and ornament replicas. Light, dimensionally stable copies of stone or plaster ornament for restoration work.
- Chemical anchoring and injection. Polyester resin anchors and capsule anchors in masonry and concrete.
- Drain relining. A resin-impregnated liner is drawn into an existing pipe and cured to form a new inner pipe.
Properties
Advantages: light, strong for its weight, completely watertight, readily formed to shape, immune to rot and corrosion, available in practically any colour and in translucent grades.
Disadvantages:
– UV sensitive without protection. Unprotected polyester discolours, weathers and eventually exposes the glass fibres (“fibre blooming”). A topcoat or gelcoat with UV filter is not optional outdoors.
– Strong odour during application; the styrene vapour demands good ventilation and personal protection.
– Brittle at low temperature and sensitive to impact.
– Thermal expansion considerably higher than that of concrete and steel; long elements need movement joints.
– Fire behaviour. Standard polyester is combustible; applications with fire requirements need specially modified grades.
Application tips
- Temperature. Polyester cures poorly or not at all below about 10 °C. Do not work in the cold, or use a hardener suited to it.
- Dry substrate. Moisture in the timber or concrete beneath causes blistering and loss of adhesion.
- Hardener dosage. Too much hardener means too fast a cure, internal stresses and cracking; too little leaves a tacky layer that never hardens. Weigh it, do not guess.
- Layers within the overlaminating window. While the previous layer has not fully cured, the next bonds chemically. After that, abrading is necessary.
Related terms
- Plastic
- Resin
- Composite
- Profiled sheeting
- Rooflight
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.