In the manufacture of paper products, particularly tissue sheets, it is generally desirable to provide a final product with as much bulk as possible without compromising other product attributes. Many papermaking machines utilize a process known as “wet-pressing.” Fundamentally, in “wet-pressing,” a large amount of water is removed from the newly-formed web of paper by mechanically pressing water out of the web, while the web is supported on a papermaking felt, in a pressure nip. The pressure nip is formed between the pressure roll and a Yankee dryer surface as the web is transferred from a papermaking felt to the Yankee dryer. The web may then be creped to soften it and provide stretch to the resulting tissue sheet. A disadvantage of the pressing step is that the pressing step may densify the web, thereby decreasing the bulk and absorbency of the tissue sheet. The subsequent creping step may only partially restore the desired sheet properties. This wet-pressing step, while an effective dewatering means, may compress the web and causes a marked reduction in web thickness and hence bulk.
Papermaking felts may be used to remove the water expelled from the web during the wet-pressing operation. One improvement to conventional felts is the application of a pattern to the felt. The pattern is imprinted into the tissue sheet, thereby producing a corresponding high density pattern in the paper. Generally, in the past, the corresponding high density pattern occurs in the X-Y direction, i.e., within the plane of the paper, in almost all cases, the tensile strength of the paper increases with its density.
One manner in which to apply a pattern layer to a papermaking felt is described in U.S. Pat. No. 5,693,187 issued to Ampulski et al. The pattern layer is created by applying a liquid precursor, typically a curable resin, to the felt. Prior to curing, this liquid precursor permeates the felt. The desired portion of the resin is cured, typically through a patterned mask, to form a solid pattern layer. Any excess liquid resin is removed. Such permeation of the liquid precursor into the felt joins the patterned layer to the felt upon curing.
However, this approach, without more, does not control where the liquid precursor, and hence ultimately after curing, the patterned layer permeates the felt. If too much of the liquid which forms the patterned layer permeates the felt and later cures, the felt becomes impermeable. An impermeable felt is undesirable because it does not allow for water removal from the wet web which is in contact with the felt.
Other patterned papermaking felts are generally made with various hardnesses of yarns woven into the felt material as generally disclosed in U.S. Pat. No. 4,533,437 to Curran et. al. The hard yarns in the felt, when pressed against the web during drying, provide for varying densities in the resulting paper produced. The approach disclosed in Curran et. al. is limited by the indirect contact of the yarns with the sheet and the patterns that can be woven using the yarns. Hence, the felts generally disclosed in Curran et al. have only limited ability to influence sheet bulk and are unable to impart aesthetically pleasing patterns to the sheet.