The papermaking bond between cellulose surfaces can be increased markedly by pretreatment of the surfaces in a corona plasma in oxygen or air. The effect is probably due to oxidative degradation of the molecules of cellulose near the surface. Similar increase in bonding can be achieved by treatment with ozone gas. This is the basis of the Paprizone process, in which paper made from mechanical pulp is brightened and strengthened by pretreatment with hydrogen peroxide and ozone.
When two surfaces of a thermoplastic such as polythene are pressed together at elevated temperature, autohesion can take place. Pretreatment of the surfaces in a corona plasma lowers the temperature at which autohesion occurs. This effect is probably not caused by surface oxidation, since it can be produced by both oxidising and non-oxidising plasmas. Its origin is uncertain, but a possible explanation is that the discharge causes electret formation in the polymer sheet, which promotes autohesion by facilitating interdiffusion of the polymer molecules on contiguous surfaces. Autohesion of thermoplastics will be most important in processes such as pressing and calendering of sheets containing the new synthetic fibres.
Preliminary results have indicated that effects similar to those described above may be obtained by treating cellulose and polythene surfaces in a microwave plasma. The advantage of using microwave energy is that the plasma can be made to fill the entire volume of the treatment chamber, rather than being restricted to a short spark as in the case of a corona discharge.
The thermally induced bond between cellulose and thermoplastic polymers is increased also by pretreatment of the surface in a corona plasma. The effect is usually most marked when both surfaces are treated, but treatment of the polymer alone will produce a good bond. For wood/polymer adhesion, it is possible to produce a bond equivalent to that obtained by use of a conventional plywood adhesive bd pressing a thin sheet of corona-treated polythene between two sheets of wooy veneer.
Hardboards can be made by hot pressing an air-dry mixture of wood fibre with a finely powdered polymer. Treatment of the polymer powder in a corona plasma,before mixing and pressing, produces a substantial improvement in both the strength and water resistance of the finished board. It is possible to make a board of pressure-refined aspen fibres and powdered polythene, which is as strong as conventional hardboards, but which shows about one third of their dimensional instability on soaking or boiling in water.