Typically, a waferboard panel comprises layers of wood flakes or wafers formed into a composite structure consolidated by the use of a resinous binder. The preparation of wafer board panels is complex, but broadly consists of two principal stages. The first stage comprises the preparation of the wafers and admixing thereof with the binder to form a loose layer or mat The second stage involves subsequent compression and curing of the resin, usually by the application of heat, to form the consolidated panel.
In the manufacture of wafer board it is a common practice to provide a three-layered structure, wherein the better quality flakes are placed on the outer layers of the mat with the poorer quality flakes forming the core of the mat. In practical terms, one would use a faster curing resin i.e. one having a higher temperature of polymerization in the core layer than in the outer layer. Once the core of the mat has been heated sufficiently so as to polymerize the resin, typically at a core temperature of approximately 140.degree. C., the press cycle is ended. A typical press time would be about five minutes.
In commercial operations, the platen temperature is generally set at between about 200.degree.-210.degree. C. to ensure that burning of the wood does not take place. It is known that the burning of wood, does not take place at a fixed temperature, but rather is dependent on the parametric properties of both temperature and time. So optimization of the process involves selection of the appropriate resin and its polymerization characteristics commensurate with the minimum press time and lowest platen temperature.
Particle board, strand board and wafer board (all termed composite board) all exhibit thickness swelling or dimensional instability upon exposure to moisture. Indeed, a measure of the dimensional stability of such composite boards is obtained by determination of such thickness swelling albeit under controlled conditions. Existing Canadian and U.S. standards for waferboard permit a thickness swelling in the range of between 20-25% as the acceptable norm (i.e. when the sample is soaked in cold water for 24 h.). However, one seeks to reduce the thickness swelling of these composite panels to at least that of solid wood which is of the order of 5 to 10%.
It is well known that the thickness swelling properties of waferboard panels may be reduced by increasing the resin content thereof. However this is a costly solution to the problem. It has become accepted in the industry to limit the quantity of resin used to that which is necessary to meet the requisite stability standards.
It is known that if wood is burned or charred, it will not exhibit the same degree of thickness swelling as does unheated raw wood. However, the burning of wood will deleteriously affect bending stiffness and strength.
Prior art processes which involve the injection of superheated steam into the mat are known and disclosed for example in U.S. Pat. No. 4,393,019. The objective of such processes is to polymerize the resin more rapidly thereby reducing the press time from five minutes to two minutes. It was found using these steam injection processes that the thickness swelling properties of the panel were somewhat improved.
Additionally, W. E. Hsu, in Canadian patent 1,213,707, taught that dimensional stability of composite board could be improved by subjecting the formed composite board to a secondary heat treatment. More specifically, the secondary heat treatment proposed heating the board from about 2 to 15 minutes at a temperature ranging from between 230.degree. C. to about 270.degree. C. These temperatures are well in excess of the normal press temperature of 205.degree. C. which is typically used in the manufacture of phenolformaldehyde bonded waferboard.