|University Federico II
of Naples, Italy
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These thermosetting resins are obtained by reacting phenol C6H5OH and phormaldehyde CH2O. Formaldehyde forms −CH2− bridges between two phenol molecules, producing chains. Linear chains are obtained when the reaction ratio is 1:1. Phenol, however, may also react with a third formaldehyde molecule. Whenever this happens, a branch is formed in the chain. One possible reaction sequence is shown in the top left scheme.
If there is a formaldehyde eccess, the branches grow in number and cross-react, until a single cross-linked network results.
The first step is preparation of a low molecular weight, generally linear pre-polymer. Adding more formaldehyde, the polymer cross-links and hardens, becoming a rigid, non deformable mass.
Phenol-formaldehyde resin was one of the earliest marketed synthetic polymers, with trade-name Bakelite. It has long been used as an insulator for switches and other electric devices. It has an unpleasant dark color, due to the presence of phenol groups. By secondary reactions, these happen sometimes to bind directly, without −CH2− bridges, forming condensed aromatic rings, that are black in color and are responsible for darkening of many products, such as rotting fruit or white cloth yellowed by time.
Phenol resin usage is declining nowadays, because of competition with modern plastics. The small production still present is mainly of cheap, low quality items, for instance pieces for games. For such applications one has to tolerate the dark, uneven color.
The most important application of phenol-formaldehyde resins is production of composite boards, such as plastic laminate. The boards are constituted of a relatively incoherent material, pasted by a phenol resin acting as a glue. Various materials can be used: wood chips, paper, cloth, sand, fiberglass. The phenol resin can make up just a small fraction of total, down to 10%. Composite boards are used mostly for workbench tops, but may also have higher prized usages: floors, doors, parts of machines, printed circuit boards.