Polyurethanes and Bakelite

Polyurethanes

Polyurethane is formed by step-growth addition polymerisation of a diisocyanate with a diol (polyol). For example, toluene-2,4-diisocyanate (TDI) or methylene diphenyl diisocyanate (MDI) reacts with ethylene glycol or a polyether polyol:

n OCN–R–NCO + n HO–R'–OH → [–CO–NH–R–NH–CO–O–R'–O–]ₙ
 (diisocyanate)   (diol)         polyurethane

The urethane link (–NH–CO–O–) joins the units. No small molecule is lost. Water added in the mixture reacts with –NCO to give CO₂, which blows the polymer into a foam. With a higher functionality polyol a cross-linked polymer is obtained.

Uses:

• Flexible foams for mattresses, cushions and car seats.
• Rigid foams for thermal insulation of refrigerators and buildings.
• Paints, varnishes and coatings, adhesives and sealants.
• Elastic fibres (spandex/Lycra), shoe soles, rollers, wheels and synthetic leather.
• Medical uses such as catheters and artificial heart parts.

Bakelite

Bakelite is a phenol–formaldehyde resin, the first fully synthetic thermosetting plastic. It is made by condensation polymerisation of phenol and formaldehyde (methanal).

1. Phenol reacts with formaldehyde (acid or base catalyst) to give o- and p-hydroxymethyl phenols: C₆H₅OH + HCHO → HOCH₂–C₆H₄–OH
2. These condense with more phenol with loss of water, forming a linear, low molecular mass prepolymer (novolac with an acid catalyst and excess phenol; resole with a base catalyst and excess formaldehyde).
3. On heating with a hardener (hexamethylenetetramine for novolac) or just heat for resole, –CH₂– bridges form between the rings at the o- and p-positions, giving a hard, cross-linked three-dimensional network.

   OH         OH         OH
   |          |          |
  (C₆H₃)–CH₂–(C₆H₃)–CH₂–(C₆H₃)–   (rings joined by –CH₂–;
         |            |            more CH₂ bridges cross-link
        CH₂          CH₂           the chains in 3-D)

n C₆H₅OH + n HCHO → [C₆H₃(OH)–CH₂]ₙ + n H₂O (simplified)

Properties and uses: Bakelite is hard, rigid, heat resistant, a good electrical insulator, and does not soften on reheating. It is used for:

• Electrical switches, plugs, sockets, switch-boards and insulating parts of computers and appliances.
• Handles of cookers, pans and irons.
• Laminates (e.g. phenolic printed circuit boards), brake linings, and adhesive for plywood.
• Telephone cases, radio cabinets, billiard balls, gears and bearings.