What is a polymer?
A polymer is a substance whose molecules consist of long chains of the same repeated building block. That building block is called a monomer; joining them up is called polymerisation. Practically every plastic, adhesive, sealant, coating and membrane in construction is a polymer.
The name often gives the origin away: poly-ethylene is a chain of ethylene molecules, poly-styrene a chain of styrene, poly-vinyl chloride a chain of vinyl chloride.
Natural and synthetic
Polymers are not a chemical invention. Cellulose (wood), lignin, natural rubber, proteins and starch are natural polymers. Synthetic polymers have existed since the early 20th century.
Three families, three kinds of behaviour
| Type | Behaviour on heating | Examples in construction |
|---|---|---|
| Thermoplastic | Softens and hardens again; reshapeable and recyclable | PVC windows and pipes, polyethylene membrane, polystyrene insulation, polypropylene |
| Thermoset | Cures once, then stays hard until it decomposes | Polyester, epoxy, polyurethane foam, melamine |
| Elastomer | Stays rubbery and elastic | EPDM roofing, silicone sealant, rubber gaskets |
That distinction matters in practice. A thermoplastic distorts under heat load (a dark PVC section behind glass can bow this way), whereas a thermoset holds its shape but cannot be repaired or remelted.
Polymers inside building materials
They are present in far more places than the visible plastic items:
- Mortars and adhesives. Polymer-modified cement mortars (PCC and PC) owe their adhesion, flexibility and watertightness to added polymer dispersions.
- Paints and coatings. The binder in virtually every modern paint is a polymer — acrylic, alkyd, silicone resin.
- Concrete admixtures. Superplasticisers are polymers that push cement particles apart, so you get workable concrete with less water.
- Sealants and adhesives. Polyurethane, MS polymer, silicone.
- Insulation. EPS, XPS, PIR and PUR are all polymer foams.
What to watch
- UV resistance. Most polymers are broken down by ultraviolet light. External applications need UV-stabilised grades or covering.
- Thermal expansion. Plastics expand far more than stone, concrete and steel. Long plastic components need room to move.
- Fire behaviour. Many polymers are combustible and produce dense smoke. Look at the reaction-to-fire class, not just at the material.
- Service life and recovery. Thermoplastics are in principle readily recyclable; thermosets and composites much less so.
Related terms
- Plastic
- Elastomer
- Geopolymer
- PCC mortar
- Resin
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