1. Field of the Invention
The present invention relates to the field of treating sheet materials pressed against rotary drums after being treated with heat and/or moisture and, more particularly, to an apparatus for controlling the surface gloss of a calendered paper sheet.
2. Background of the Related Art
One of the aspects by which sheet materials are graded is the gloss of the sheet surface. For example, paper may be categorized into various grades of paper having different degrees of surface gloss for various applications. Bulk paper is typically produced in a continuous sheet that is wound in a roll. The paper roll may have a dimension in the cross-direction, that is, across the width of the sheet, of 25 feet or more. The continuous sheet may then be unrolled and cut into individual sheets of the desired size. The consistency of the surface gloss of the individual sheets depends upon the uniformity of the surface gloss of the original bulk paper roll. Thus, it is very important to have a uniform surface gloss across the continuous bulk paper roll.
Paper production of a bulk paper roll typically involves a calendering process that includes pressing the paper sheet material between calender rolls to obtain the desired characteristics. For example, subjecting a paper sheet to the calendering process can change its density, thickness, smoothness, and gloss. Gloss is conventionally controlled by applying steam to the surface of the paper, followed by pressing the paper sheet between a series of calender rolls. Typically, the series of calender rolls are arranged in a stack, alternating between hard, polished steel rolls and soft, resilient rolls made of cotton. The paper absorbs the heat and moisture of the steam, and paper fibers at the sheet surface are softened. As the polished steel roll comes into contact with the paper surface that has been treated with steam, the paper surface is smoothed and pressed flat by the pressing and rubbing action of the hard steel roll against an adjacent soft roll, producing a glossy finish on the surface of the paper facing the steel roll. The process is not unlike treating a laundered shirt with a steam iron and ironing board to produce a smoother, brighter, and more glossy shirt surface. The degree of gloss is dependent on the amount of heat, moisture and pressure applied. Coatings may also be applied to the sheet material to further control the degree of gloss.
A common problem encountered in making a glossy finish on a paper surface using a steam treatment is the non-uniformity of the glossy finish. As noted, it is important to have a uniform surface gloss across the entire bulk paper roll. Variations in the glossy finish may be a result of localized variations in the amount of steam applied to the paper and other variables in the calendering process, such as localized temperature variations and changes in the calender roll pressure. It is known to independently vary the amount of steam directed at the sheet material in different sections of the sheet in the cross-direction of the paper surface, thereby obtaining a more uniform gloss finish. The independent variation may be achieved by a sectionalized steam shower comprising a plurality of steam nozzles distributed across the paper sheet in the cross direction. This is discussed, for example, in commonly assigned U.S. Pat. No. 4,786,529 to Boissevain.
A potential problem with many systems that regulate the amount of steam applied to the surface of paper lies with the fact that steam is used. Saturated steam is normally employed in such systems so that, upon contact with the sheet, the condensing steam will liberate a substantial amount of heat energy to the sheet. For saturated steam at a given pressure, the relationship between moisture and heat is fixed. Thus, for a given volume and flow of steam applied to a paper surface, there will be a given amount of heat and water in the steam. For example, it has previously been discussed that a coating may be applied to the paper in order to control the gloss of the surface finish. With some coatings, applying steam to the paper beyond a given amount causes the coating to separate from the sheet surface. In this situation, the amount of steam applied must then be decreased. Depending on the coating and paper involved, the coating may become separated from the sheet because of either too much moisture or too much heat. However, decreasing the amount of steam applied to the paper also decreases the amount of both heat and moisture applied to the paper. Decreasing both heat and moisture may adversely affect other aspects of the paper quality, such as smoothness. Accordingly, the present inventors have realized that it would be desirable to control the separation of the coating from the paper while maintaining other aspects of paper quality, such as the smoothness or gloss of the paper surface, by separately and independently controlling the amount of heat and moisture applied to the paper.
Another problem associated with applying steam to paper in order to control surface gloss is condensate dripping. When steam is directed at a paper surface, not all of the steam is deposited on the paper. Due to the high temperature of the steam and the speed of the steam flow, the steam has a great deal of kinetic energy. Some of the steam bounces off of the surface of the paper and condenses on the machinery associated with the calendering process. When enough of the steam has condensed, water droplets may form and drip onto the continuous paper sheet making its way through the calender stacks. This produces a localized variation in the amount of moisture applied to the paper sheet. This again results in a localized variation in the gloss finish of the sheet material. In addition, steam may condense on a cooler portion of paper surface or on the steel roll itself, again affecting amount of moisture applied to the paper surface and the surface gloss of the paper.
In addition to condensate dripping, another problem associated with applying steam to the paper surface is that the steam, intended to effect gloss, also tends to heat the paper which, in turn, heats the steel calender roll When the steel calender roll is heated, it expands slightly. This changes the pressing action at the location where the steel roll meets the soft roll, also referred to as the nip. As noted previously, the calendering process changes the density, thickness, smoothness, and gloss of the paper sheet. Thus, unintentional changes the pressing action of the steel roll against the soft roll results in changes to the calendering process that cannot be accurately predicted and that may adversely affect paper quality.