There are many characteristics of tissue products such as bath and facial tissue that must be considered in producing a final product having desirable attributes for the product's intended purpose. Improved softness of the product has long been one major objective, particularly for the success of premium products. In general, the components of “softness” include stiffness and bulk (density), with lower stiffness and higher bulk (lower density) generally improving perceived softness. While enhanced softness is generally desirable for all types of tissue products, it has been especially challenging to achieve.
The throughdrying process is well known in the art and is used extensively in the manufacture of all types of tissue products having desirable softness characteristics. Throughdrying provides a relatively noncompressive method of removing water from a web by passing hot air through the web until it is dry. More specifically, a wet-laid web is transferred from the forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until dry. The web is typically conveyed on the fabric through passage of one or more throughair dryers (“TAD”) where the web is dried to its final dryness state. The resulting dried web is typically softer and bulkier than a conventionally dried uncreped sheet because fewer bonds are formed in the web and the web is less compressed. Such throughdrying processes are described in detail, for example, in U.S. Pat. Nos. 6,149,767 and 6,331,230 B1.
However, from a manufacturing perspective, the throughdrying process is relatively energy intensive and therefore expensive compared to conventional wet pressing processes. The energy consumption issue is compounded by the fact that a significant amount of heat is vented to atmosphere during the process and, thus, wasted.
The present invention relates to an improvement in throughdrying processes that will significantly enhance the energy efficiency of such processes.