1. Field of the Invention
This invention relates to sheet structures having improved localized ply-delamination performance and compression performance and processes for making same. These sheet structures include papers and pressboards that can be suitable for electrical insulation, composite structures, and other applications.
2. Description of Related Art
Thick sheet structures containing aramid fibrids are usually formed by making a ply of material using wet-lay technology followed by hot compression or densification of multiple plies of the material. Such multiple-ply sheet structures, having a final average or nominal thickness of up to about 0.9-1.0 mm, are typically referred to as paper; if the final average or nominal thickness is 0.9-1.0 mm or greater, the sheet structure is typically called pressboard.
If the adhesion between the ply layers (i.e. the ply adhesion) is not both adequately high and uniform in the final sheet structure, additional processing steps conducted on the sheet structure such as slitting into narrow strips and/or punching of small parts can cause delamination of the sheet structure and the loss of the part. In particular, the use of high-speed punching operations combined with the desire for smaller punched parts requires improved localized ply adhesion. These punching operations also require a sheet structure that is flat and not warped; otherwise it is impossible to make final parts of the precise size and necessary shape.
Exemplary processes for thermally laminating ply layers to make aramid pressboard are disclosed in U.S. Pat. No. 4,752,355 to Provost, and U.S. Pat. Nos. 5,076,887 and 5,089,088 to Hendren. All of these processes require the removal of moisture from the plies prior to the thermal lamination at a temperature of from 270 to 320 degrees C. U.S. Pat. No. 4,481,060 to Hayes for the making of thick papers is illustrative of the need to fully dry the plies prior to lamination, the conventional thinking being that any excessive moisture contained in the plies would flash once the heated sheet structure exited the high compression zone, causing areas in the sheet to delaminate, and creating what are known in the art as “blisters”, making the paper or pressboard unusable. This effect is illustrated by U.S. Pat. No. 4,515,656 to Memeger wherein a coherent, expanded, highly-voided sheet, normally unacceptable for pressboard, is made by increasing the water content in the sheet to at least 60% by weight, heating the wet sheet under pressure and temperature to vaporize the water rapidly and simultaneously expand the sheet. This process forms random expanded macroscopic cells within and between the plies.
However, Unexamined Japanese Patent Publication Showa 54-50613 discloses a method for producing an aromatic polyamide paper laminated material wherein water, or a mixture of water and an organic solvent soluble to water, is added to aromatic polyamide paper to increase the moisture content of the paper up to 6 to 30% by the weight. Several of these wetted papers are then layered together and the assembled layers are then first compressed at normal (room) temperature, follow by heating of the layers to a low temperature while the compression of the assembled layers is maintained. The low temperature is below what is called in the publication the “melting temperature of the aromatic polyamide” and is in the range of 150 to 230 C, preferably the range of 170 to 190 C. This low temperature thermal pressing is followed by cooling to 100 C or lower while maintaining the pressure on the laminated material. The first two steps, involving a first compression step in an unheated press at room temperature, followed by the second step that adds subsequent gentle heating at low temperature, is said to produce a laminated material at a low temperature compared to other processes.
Unfortunately, this process creates a laminated material having high compressibility, as the examples in the publication reveal; the laminated material has a compressibility in the range of 15 to 23 percent. This material is too highly compressible and not rigid enough to be suitable as electrical insulation such as spacers and/or sticks, or other structural components that require minimum compressibility and compression set.
So, what is needed is an improved method of making a dense sheet structure, such as thick papers and pressboard, having improved ply adhesion and adequate compressive properties.