192 Chapter 8
To produce polyurethane foam, water is added to the polyol and isocyanate
starting materials. The water molecules react with the cyanate groups to form
amine groups (NH 2 ) and carbon dioxide gas. The amines continue to react
with isocyanate groups to form urea linkages (HNCONH) between the
chains, instead of the urethane groups created in the absence of water. As
polymerisation progresses and molecular weight increases, carbon dioxide
gas becomes trapped in the increasingly viscous liquid polymer. The trapped
bubbles comprise cells in the polyurethane foam.
The physical properties of the final polyurethane are determined by those
of the raw materials. The physical form in which polyurethanes are produced,
whether as fibres or expanded foams, depends on the chemical formulae of
both the isocyanate and the alcohol or components; these control the molec-
ular weights and extent of cross-linking in the resultant polymer. Polyglycols,
such as a polyethylene glycol with a molecular weight of around 2000,
result in a higher molecular weight polyurethane than if a diol was used.
Polyurethanes are noted for their resistance to most organic solvents and high
tensile strength. Due to the toxicity of isocyanate vapour, one of the raw mate-
rials, good ventilation and safety equipment are necessary when handling these
materials.
Most polymerisation reactions do not progress to completion, resulting in the
residues of starting materials in the plastic. Approximately 1–3% of residual
monomer is found in acrylics, PVC, polystyrene, polycarbonates, polyesters,
polyurethanes and formaldehyde polymers immediately after production.
Monomers with boiling points lower than ambient are likely to have evapo-
rated before the final product is used, while those with higher boiling points,
including styrene monomer and terephthalates used in saturated polyesters,
off-gas slowly from the plastic formulation and can often be detected by
odour.
2.2 Additives
The chemical and physical properties of polymers can be changed considerably
by incorporating additives soon after manufacture. The type and quantity of
additives allow many different products to be produced from the same polymer.
HOROH + OCNR 1 NCO + HOROH + OCNR 1 NCO
polyol polyisocyanate-OROOCNHR 1 NHCOOROOCNHR 1 NCHO-
polyurethaneReaction Scheme 3Preparation of polyurethane from polyol and polyisocyanate