The production of substrates comprising cellulosic fibers and a thermosetting resin composition useful in supporting decorative layers in the formation of high pressure decorative thermoset plastics laminates is well known. Conventionally, said substrates comprise a plurality, i.e. about 2-10, of paper core sheets impregnated with a liquid thermo-setting resin composition, said core sheets being prepared by treating a web of paper, prepared by a wet-laying process, with a solution or dispersion of a thermosetting resin composition in a volatile solvent, drying said treated web to reduce the volatile matter content to a desired level and cutting said treated, dried paper web into sheets of the required dimensions.
In order to provide satisfactory handling and usage properties in said laminates, they are conventionally produced in thicknesses of from about 0.5 mm to about 2.0 mm, this thickness range being achieved primarily by superimposing a plurality of said paper core sheets. Whilst it would clearly be advantageous to use a single core sheet to provide the substrate for the laminate, there are problems of manufacture and processing associated with the production and resin-impregnation of wet laid paper sheets having a basis weight significantly greater than about 256 gsm (grammes per square meter) and a thickness of about 0.27 mm.
Further, it is desirable for environmental and energy conservation reasons, to obviate the drying stage necessary with conventionally produced resin composition treated paper substrates. Attempts have been made in the past to avoid this drying step by providing a wet-laid paper containing a thermo-setting resin composition in solid particulate form as a sheet of the laminate substrate and formed during the paper making process from an aqueous slurry comprising the paper fibers and the particulate resin.
However, this process has not found wide commercial acceptance because of problems arising from the propensity of the liquid phase to convey the resin particles through the forming wire.
Wet-laid papers, while generally producing high pressure decorative laminates of excellent properties, are known to have a propensity to stress-crack under conditions of low relative humidity. Therefore, conventional high pressure decorative laminates, after a period of time well within their expected life-times, may undergo a marked deterioration in their aesthetic appearance and utility. For this reason, conventional high pressure decorative laminates have not found successful commercial utility in many areas where low relative humidity is a prevalent condition especially where the laminates are first subjected to modification such as by notching, cutting or other treatment whereby sharp corners are cut into their cross section.
Wet-laid papers also exhibit a variation in at least some of their physical properties depending upon whether the properties are measured in the direction of travel of the machine wire upon which the paper was formed, or transversely of it. This variation in properties is due to the non-random orientation of the fibers in the paper due to the alignment of fiber caused by the flow of the liquid phase onto and through the wire and by surface tension effects. Laminates produced from substrates comprising said wet-laid papers also exhibit this direction dependent variation in at least some of their physical properties and although this is not generally disadvantageous, there are some applications where a laminate exhibiting less variation in physical properties according to the direction of measurement is preferred.