Vacuum pressure process
A process for the impregnation of permeable wood – the wood must have a moisture content of less than 30% (fibre saturation point) to be ready for treatment.
The best protection is achieved using the so-called full-cell method, in which the pressure phase is maintained until the wood cannot absorb any more treatment solution. The most important staged for successful treatment results are the initial vacuum, the flooding of the vessel under vacuum and the pressure phase.
Powerful vacuum pumps evacuate the air out of the treatment vessel and the wood structure. The stronger the vacuum, the more preservative solution can be absorbed. While the vessel is being flooded with preservative solution, the vacuum must be maintained. Then, pressure pumps force the solution deep into the wood under extremely high pressure. The final vacuum ensures that the wood can be removed from the treatment plant without dripping.
Wood species are very different in their permeability levels. This means that much longer vacuums and pressure phases must be selected for spruce, for instance, than for pine.