Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
1999-12-14
2002-05-07
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S109000, C162S111000, C162S112000, C162S179000, C162S181100, C162S148000
Reexamination Certificate
active
06383336
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to particulate filler-containing tissue products such as bath tissue, facial tissue and towels, and methods of making the same. In addition to the particulate filters which are generally employed to improve the opacity of the paper-containing products, the present invention employs certain additives and, more specifically, incorporates alkyl amides into such tissue products. The additives reduce the negative impact on the tissue's softness and strength often caused by the presence of the particulate fillers.
BACKGROUND OF THE INVENTION
The most common approach for improving the opacity of paper products involves the incorporation of various particulate fillers, such as pigments, into the products. Pigments generally include inorganic particulate fillers such as kaolin clay, calcium carbonate or titanium dioxide. Other fillers known in the art include talc, zirconium dioxide, zinc oxide, calcium silicate, aluminum silicate, calcium sulfate, alumina trihydrate, and mixtures of such materials. They can be applied either in the wet end of a tissue-making process or as a coating, or other dry-end additive, to an already-formed tissue web. Much research has been devoted to the use of such particulate fillers for the purpose of increasing the opacity and brightness primarily in newsprint, bible and directory papers and fine papers.
Usually, these particulate fillers are applied in the wet end of the papermaking process by flocculating the filler with a cationic starch and using a cationic retention aid at the outlet of the fan pump. Flocculant size is often an important aspect of maintaining desirable opacity levels and strength in tissue products. If the flocculent particles are too large, good retention is achieved but with a significant loss of strength and poor opacity due to the reduction of air-filler and fiber-filler interfaces. On the other hand, if the flocculent particles are too small, retention is poor even though less strength is lost and greater opacifying efficiency is obtained. Complete books such as R. W. Hagemeyer, ed.,
Pigments for Paper
, TAPPI Press, 1997 have been written regarding use of pigments in the paper industry where their primary use is to increase opacity and brightness in products of reduced basis weight. These references describe both the products and processes by which the fillers are incorporated. Kaolin clay is one of the most widely used particulate fillers used to improve the opacity of various paper products, including tissues. Kaolin is particularly attractive due to its very low cost, which is currently about $0.07/lb. The opacifying power of kaolin clay is considerably poorer than that of titanium dioxide, but the much higher cost of titanium dioxide (currently about 10-20 times the cost of Kaolin) often offsets any drawback in opacity efficiency.
Kaolin is comprised of aluminum silicate and is commercially available in two primary forms called hydrous and calcined. Natural kaolin, referred to as “hydrous” kaolin, has the chemical structure Al
2
(OH)
4
Si
2
O
5
. Subjecting natural kaolin to temperatures in excess of 450 C. results in a loss of water and the rearrangement of its basic crystalline structure. Such kaolin is referred to as “calcined” kaolin and has the chemical structure Al
2
O
3
SiO
2
. Calcined kaolin is advantageous over hydrous kaolin in that it results in higher brightness. However, a disadvantage of calcined kaolin is that it is more abrasive than hydrous kaolin.
Kaolin has a structure which allows the crystal lattice to form thin platelets that adhere together to form “stacks” or “books”. During processing, some separation into individual platelets does occur. Each clay platelet is a multilayer structure of aluminum polysilicate. Each basic layer contains a face consisting of a continuous array of oxygen atoms uniting the edges of the polysilicate sheet structure. The other face consists of octahedral alumina structures joined by hydroxyl groups, which, in essence, forms a two-dimensional polyaluminum oxide structure. The aluminum and silicon atoms are bound by oxygen atoms sharing the tetrahedral and octahedral structures. Imperfections in the assembly are primarily responsible for the anionic charge that the natural clay particles possess while in suspension. Other divalent, trivalent, and tetravalent cations substitute for aluminum with the consequence that some of the oxygen atoms on the surface become anionic and form weakly dissociated hydroxyl groups.
Kaolins also possess a cationic character. If this cationic character is not satisfied with solution anions, the clay will satisfy its own charge balance in that the crystal structure orients itself edge-to-face and forms thick dispersions. To remedy this, polyacrylate dispersants capable of ion exchange with the cationic sites are often added to the kaolin. Kaolin clay is usually purchased as a solid powder incorporated with a polyacrylate dispersant.
Titanium dioxide, although it is more expensive than kaolin, exhibits a greater opacifying power than kaolin. The greater opacifying power of Titanium oxide relative to Kaolin means that lower levels of filler are required to produce a given opacity. This, in turn, may provide a greater capability of making a filled product at a given opacity with a higher degree of softness because less filler is used.
Among the types of titanium dioxide available are Anatase and Rutile. Anatase titanium dioxide has more opacifying power than Rutile, but it is also more abrasive and expensive.
As mentioned above, cationic starches are commonly used to agglomerate the kaolin clay or other filler particles. It is believed that the cationic starch becomes insoluble after binding to the anionically-charged filler particles. The goal of agglomeration is having the filler covered with the bushy starch molecules. The starch molecules provide a cationic surface for the attachment of more filler particles, causing an increase in agglomerate size.
The size of the starch filler agglomerates is an important factor in obtaining the optimal balance of strength and optical properties. Agglomerate size is controlled by the rate of shear supplied during the mixing of the starch with the filler. The agglomerates, once formed, are not overly shear sensitive, but they can be broken down over an extended period of time or in presence of very high shear forces.
The charge characteristic of the starch is significant as well. Since starch is usually employed at an amount of less than 5% by weight of filler, the filler-starch agglomerates possess a negative charge. In this case, a cationic retention aid is utilized.
Higher levels of starch are sometimes employed. In these instances, the filler-starch agglomerates may actually possess a net positive charge and would, thus, require the use of an anionic retention aid.
Various anionic and cationic retention aids are known in the art. Generally, the most common anionic retention aids are charged polyacrylates, whereas the most common cationic retention aids are charged polyacrylamides. These retention aids agglomerate the suspended particles through the use of a bridging mechanism. A wide range of molecular weights and charge densities are available. In general, high molecular weight materials with a medium charge density are preferred for flocculating particulate fillers. The filler retention aid flocs are easily broken down by shear forces and are usually added after the fan pump.
Nonparticulate fillers may also be employed. One such class of nonparticulate fillers includes thermoplastic microspheres. Such non-particulate fillers are generally applied as a coating in a post-treatment operation; however, they may be applied in the wet end. When applied in the wet end, these non-particulate fillers may have the same deleterious impact on strength and softness as do particulate fillers.
While particulate, as well as non-particulate fillers, may be incorporated into tissue products in order to render the products more opaque, several drawbacks ex
Chin Peter
Kimberly--Clark Worldwide, Inc.
Nelson Mullins Riley & Scarborough
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