Coating processes – With post-treatment of coating or coating material – Solid treating member or material contacts coating
Reexamination Certificate
2000-09-06
2003-03-11
Bareford, Katherine A. (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Solid treating member or material contacts coating
C427S365000, C427S366000, C162S136000, C162S184000, C162S206000
Reexamination Certificate
active
06531183
ABSTRACT:
TECHNICAL APPLICABILITY AND FIELD OF INVENTION
The present invention relates to a method of manufacturing a high-gloss, high-bulk paper product using a coating material comprising a particulate plastic pigment. The method of the invention permits calendering a base stock coated with such a coating material at lower roll temperatures than used in conventional gloss or soft calenders, and at lower nip loads than used in conventional supercalenders. As a result, densification of the resulting coated paper product is reduced, and a product having high gloss is obtained.
BACKGROUND OF THE INVENTION
In papermaking, the finishing operation may be a calendering process, in which a paper web is passed between the nips formed between one or more pairs of rolls and the surface of the web is thereby flattened to form a smooth surface. Simultaneously, the thickness, or caliper, of the paper web is reduced and the web is densified. The density of the resulting product is usually calculated as:
Density=Basis Weight/Caliper
where the basis weight is the weight of a ream, in pounds, and the caliper is the thickness of the web, measured in thousandths of an inch, or points. Calendering generally reduces caliper, and, as a result, a higher density is obtained in the finished paper product. Bulk is inversely related to density, therefore when the density is increased, the bulk of the finished paper product will be reduced.
Calendering may generally be accomplished using a gloss calender, soft calender or supercalender. The gloss calender is typically comprised of a hard, non-resilient, heated roll made, for example, of steel, positioned proximally to a soft roll so as to form a narrow gap or nip. As the web passes through the nip it is exposed to a nip load in the range of from about 100 to about 900 pounds per lineal inch (pli). Nip pressures in this type of device are usually in the range of less than about 2000 pounds per square inch (psi). A wide range of processing temperatures can be used in a gloss calender, with the typical maximum temperature being in the range of about 450° F. U.S. Pat. No. 5,118,533, for example, discloses a gloss calender in which the metal roll is held at a temperature of about 100-500° C. (212-932° F.). This temperature produces a high gloss finish on the surface of the web as it is passed through the nip, while the lower pressure used in a gloss calender causes less densification of the web, in comparison to a conventional supercalender.
The finishing effect achieved using the gloss calender, however, is not as smooth or as flat, and therefore not as glossy, as the surface produced using an apparatus capable of applying higher pressure. It is therefore often useful to increase the nip load or the roll temperature, or both, to plasticize and smooth the surface layers of the paper. Such modifications are incorporated, for example, in the design and operation of the conventional soft calender. The soft calender is usually constructed as having one to two nips per coated side, or as a two- or four-nip device, with each nip being formed between a heated hard roll and an unheated soft roll.
Alternatively, supercalendering may be used as the finishing operation. In such a process, the web is sequentially passed between a series of nips formed between the vertically stacked rolls of a supercalender. The supercalender typically comprises a frame having an upper roll and a lower roll between which are positioned intermediate rolls. The rolls of the supercalender may be heated hard rolls or unheated soft rolls, in serial or alternating arrangement. The nips formed between the rolls are typically shorter than those of a soft calender or gloss calender. The maximum temperature of the heated rolls in the supercalender is usually up to about 250° F. As the web is passed through each nip, the web is compacted to form paper of substantially uniform density and high gloss by virtue of the repeated pressurization and heat exposure. The high pressure however also causes a reduction in bulk. In a supercalender, the nips are loaded initially by gravity, i.e., gravitational forces acting on the weight of the rolls themselves produce a distribution of the weight from the upper nip to the bottom nip that is substantially linear and increasing. This has the consequence that the load present in the bottom nip actually determines the minimum loading capacity of the calender.
Some paper and paperboard grades are sold by area, and, accordingly, a lower density sheet will give more surface area per ton of paper, which is advantageous for both the manufacturer and the end user. Thus it will be appreciated that a manufacturing method that will provide the desired surface finish on the base stock without substantially affecting its bulk is desirable. Where it is desirable to maintain more bulk in the finished product, using a conventional supercalender has typically been a disadvantage because such a process requires relatively high initial nip loads and corresponding nip pressures, which are at least maintained and, more often, increased as the web moves through the series of rolls. In this regard, a typical 10-12 roll supercalender device will produce a minimum load on the bottom nip in excess of about 1000 pli which could translate to a nip pressure greater than about 2500 psi depending upon the nip width. Moreover, in order to achieve some calendering potential from the upper nips, additional external load must be applied. For example, where the initial nip load may be about 1000 pounds per linear inch (pli) as the web enters the first nip, it is then exposed to subsequent nip loads at each of the successive intervening nips before passing through a final nip at a cumulative nip load of about 2000-3000 pli, which reflects the mass of each of the preceding rolls. As a result of this amount of pressurization in combination with heat, the web is highly densified to form a paper product having a high gloss surface. Since the pressure created by the extra loading at the nip is an important factor in achieving high gloss and smoothness, the result is a good finish for the web, but at the expense of an increase in density and loss of bulk.
In high throughput finishing operations, it is generally more efficient to use a supercalender to achieve the desired high gloss effect. However, as mentioned above, using the conventional supercalender exposes the base stock to linearly rising nip loads that may result in a glossy but highly densified product of reduced bulk. For example, U.S. Pat. No. 4,624,744 discloses a process that involves finishing a paper web at a nip pressure of at least about 2000 psi using a smooth metal finishing roll and a resilient backing roll, wherein the metal finishing roll is heated to a temperature sufficient to mold the web beneath its surface, generally referred to as substrata thermal molding. A comparison between supercalendering and gloss calendering is reported in the article entitled “Supercalendering and Soft Nip Calendering Compared”, by John D. Peel, TAPPI Journal, October 1991, pp. 179-186.
A recent development in the calendering art addresses the problem of increasing linear loads at the successive nips in a supercalender. U.S. Pat. No. 5,438,920 describes a modified calender which is comprised of a series of rolls similar to a conventional supercalender, but in which the loading at each nip can be controlled by way of relief means that partially or completely relieve the nip loads produced by the masses of the intermediate rolls. In this regard, as the web passes through this calender, there is less variation in the nip load and nip pressure that is applied at each nip. As a result, there is less reduction in the bulk of the finished paper. This patent does not, however, teach or suggest making a high gloss paper of reduced bulk. Laid-open Canadian Patent Application 2238466AA, filed Dec. 20, 1998, teaches using another type of modified calender with reduced nip loads at each nip to make an ultra-light weight coated (ULWC) paper, which is a high-bulk glossed
Cason David B.
Petro S. Craig
Renvall Stig V.
Sastri Bhima S.
Bareford Katherine A.
Brown D. L.
MeadWestvaco Corporation
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