Process for producing dry finished paperboard

#The present invention relates generally to the manufacture of coated, bleached paperboard, and more particularly, to the manufacture of such paperboard with a dry finishing technique.

#In the conventional manufacture of coated paperboard, the rawstock from the papermachine is surface sized and then machine calendered prior to coating. The surface sizing step is desired to enhance the strength characteristics of the paperboard, to apply a surface film which bonds surface fibers to the body of the paperboard itself and to achieve a holdout for the later applied coating. Meanwhile, the purpose of machine calendering is to achieve a surface smoothness before the coating operation.

#In a typical wet finishing process, the effectiveness of the calendering step is enhanced with the application of water from one or more water boxes or the like on the calender stack. The water application imparts good surface smoothness and printability to the paperboard, but not without some detrimental side effects. For instance, when the paperboard is wet finished on the calender, the caliper or bulk of the paperboard is reduced as the density is increased. This decrease in caliper is not ordinarily a problem, except that paperboard is sold according to area and not by weight. Thus, in order to achieve a designated final caliper to paperboard that is wet finished, the rawstock must be manufactured wtih a greater original caliper which increases raw material costs. In addition, wet finishing creates operating problems such as surface mottle of the paperboard due to the non-uniformity of water adsorption of the rawstock, web breaks and corrosion problems on the machine.

#As illustrated in the accompanying sheet of drawing, the paperboard rawstock W from the papermachine (not shown) is applied with a surface sizing at a size press 10. The size press illustrated is exemplary of a horizontal application but could comprise a vertical arrangement with any combination of rolls and blades or simply a blade application, if desired, as long as the size press formulation applied remains substantially on the surface of the paperboard. Both sides of the web W are applied with the size press formulation even though in general only one side is later coated. After sizing, the web is dried with driers shown schematically at 11,12 prior to going to the machine calender 13 for finishing. At the calender 13, elimination of the water box from one side of the paperboard, (the felt side) which is generally the uncoated side, is referred to as dry finishing one side (DFIS). Elimination of the water treatment from both sides of the calender is referred to as dry finishing two sides (DF2S). After the calenders, the web is coated, usually on one side, to produce a dry finished coated paperboard product.

#The following examples are illustrative of the present invention and are not to be construed as limiting the scope thereof.

#A series of trails were conducted to demonstrate the effectiveness of the use of a plasticizer in a size press formulation when manufacturing dry finished, coated paperboard. The effects of plasticizer concentration and the solids content of the size press formulation which controls the pick up were studied. The results are shown in Table I for the plasticizer propylene glycol.