Crossed-linked pulp and method of making same

Details Crossed-linked pulp and method of making same Crossed-linked pulp and method of making same

2001-04-11

2003-09-16

Griffin, Steven P. (Department: 1731)

Paper making and fiber liberation

Processes and products

Synthetic fiber

C162S168100, C008S120000, C008S493000, C604S376000

Reexamination Certificate

active

06620293

ABSTRACT:

This invention relates to cross-linked cellulose pulp sheets having low knot and nit levels and excellent absorbency and wet resiliency properties. More particularly, this invention relates to the cross-linking of cellulosic pulp fibers in sheet form and a method making cross-linked cellulose pulp sheets having performance properties which are equivalent or superior to those comprised of fibers which are cross-linked in fluff or individualized fiber form.
BACKGROUND OF THE INVENTION
Within the specialty paper market there is a growing need for high porosity, high bulk, high absorbency pulps with superior wet resiliency. The filter, towel, and wipe industries particularly require a sheet or roll product having good porosity, absorbency and bulk, which is able to retain those properties even when wet pressed. A desirable sheet product should also have a permeability and/or absorbency which enables gas or liquid to readily pass through.
Pulps are cellulose products composed of cellulose fibers which, in turn, are composed of individual cellulose chains. Commonly, cellulose fibers are cross-linked in individualized form to impart advantageous properties such as increased absorbent capacity, bulk, and resilience to structures containing the cross-linked cellulose fibers.
I. CHEMICALS AS CROSS-LINKING AGENTS
Cross-linked cellulose fibers and methods for their preparation are widely known. Common cellulose cross-linking agents include aldehyde and urea-based formaldehyde addition products. See, for example, U.S. Pat. Nos. 3,224,926; 3,241,533; 3,932,209; 4,035,147; and 3,756,913. Because these commonly used cross-linkers, such as DMDHEU (dimethyloldihydroxy ethylene urea) or NMA (N-methylol acrylamide), can give rise to formaldehyde release, their applicability to absorbent products that contact human skin (e.g., diapers) has been limited by safety concerns. These cross-linkers are known to cause irritation to human skin. Moreover, formaldehyde, which persists in formaldehyde-cross-linked products, is a known health hazard and has been listed as a carcinogen by the EPA. To avoid formaldehyde release, carboxylic acids have been used for cross-linking. For example, European Patent Application EP 440,472 discloses utilizing carboxylic acids such as citric acid as wood pulp fiber cross-linkers.
For cross-linking cellulose pulp fibers, other polycarboxylic acids, i.e., C
2
-C
9
polycarboxylic acids, specifically 1,2,3,4-butanetetracarboxylic (BCTA) or a 1,2,3-propane tricarboxylic acid, preferably citric acid, are described in EP 427, 317 and U.S. Pat. Nos. 5,183,707 and 5,190,563. U.S. Pat. No. 5,225,047 describes applying a debonding agent and a cross-linking agent of polycarboxylic acid, particularly BCTA, to slurried or sheeted cellulose fibers. Unlike citric acid, 1,2,3,4-butane tetracarboxylic acid is considered too expensive for use on a commercial scale.
Cross-linking with polyacrylic acids, is disclosed in U.S. Pat. No. 5,549,791 and WO 95/34710. Described therein is the use of a copolymer of acrylic acid and maleic acid with the acrylic acid monomeric unit predominating.
Generally, “curing” refers to covalent bond formation (i.e., cross-link formation) between the cross-linking agent and the fiber. U.S. Pat. No. 5,755, 828 discloses using both a cross-linking agent and a polycarboxylic acid under partial curing conditions to provide cross-linked cellulose fibers having free pendent carboxylic acid groups. The free carboxylic acid groups improve the tensile properties of the resulting fibrous structures. The cross-linking agents include urea derivatives and maleic anhydride. The polycarboxylic acids include, e.g., acrylic acid polymers and polymaleic acid. Importantly, the cross-linking agent in U.S. Pat. No. 5,755,828 has a cure temperature, e.g., of about 165° C. The cure temperature must be below the cure temperature of the polycarboxylic acids so that, through only partial curing, uncross-linked pendent carboxylic acid groups are provided. The treated pulp is defiberized and flash dried at the appropriate time and temperature for curing.
Intrafiber cross-linking and interfiber cross-linking have different applications. WO 98/30387 describes esterification and cross-linking of cellulosic cotton fibers or paper with maleic acid polymers for wrinkle resistance and wet strength. These properties are imparted by interfiber cross-linking. Interfiber cross-linking of cellulose fibers using homopolymers of maleic acid and terpolymers of maleic acid, acrylic acid and vinyl alcohol is described by Y. Xu, et al., in the Journal of the Technical Association of the Pulp and Paper Industry, TAPPI JOURNAL 81(11): 159-164 (1998). However, citric acid proved to be unsatisfactory for interfiber cross-linking. The failure of citric acid and the success of polymaleic acid in interfiber cross-linking shows that each class of polymeric carboxylic acids is unique and the potential of a compound or polymer to yield valuable attributes of commercial utility cannot be predicted. In U.S. Pat. No. 5,427,587, maleic acid containing polymers are similarly used to strengthen cellulose substrates. Rather than intrafiber cross-linking, this method involves interfiber ester cross-linking between cellulose molecules. Although polymers have been used to strengthen cellulosic material by interfiber cross-linking, interfiber cross-linking generally reduces absorbency.
Another material that acts as an interfiber cross-linker for wet strength applications, but performs poorly as a material for improving absorbency via intrafiber cross-linking is an aromatic polycarboxylic acid such as ethylene glycol bis(anhydrotrimellitate) resin described in WO 98/13545.
One material known to function in both applications (i.e., both interfiber cross-linking for improving wet-strength, and intrafiber cross-linking for improved absorbent and high bulk structures) is 1,2,3,4-butane tetracarboxylic acid. However, as mentioned above, it is presently too expensive to be utilized commercially.
Other pulps used for absorbent products included flash dried products such as those described in U.S. Pat. No. 5,695,486. This patent discloses a fibrous web of cellulose and cellulose acetate fibers treated with a chemical solvent and heat cured to bond the fibers. Pulp treated in this manner has high knot content and lacks the solvent resiliency and absorbent capacity of a cross-linked pulp.
Flash drying is unconstrained drying of pulps in a hot air stream. Flash drying and other mechanical treatments associated with flash drying can lead to the production of fines. Fines are shortened fibers, e.g., shorter than 0.2 mm, that will frequently cause dusting when the cross-linked product is used.
II. PROCESSES IN CROSS-LINKING CELLULOSE FIBERS
There are generally two different types of processes involved in treating and cross-linking pulps for various applications. In one approach, fibers are cross-linked with a cross-linking agent in individualized fiber form to promote intrafiber crosslinking. Another approach involves interfiber linking in sheet, board or pad form.
U.S. Pat. No. 5,998,511 discloses processes (and products derived therefrom) in which the fibers are cross-linked with polycarboxylic acids in individualized fiber form. After application of the crosslinking chemical, the cellulosic material is defiberized using various attrition devices so that it is in substantially individualized fibrous form prior to curing at elevated temperature (160-200° C. for varying time periods) to promote cross-linking of the chemical & the cellulose fibers via intrafiber bonds rather then interfiber bonds.
This mechanical action has its advantages. In specialty paper applications, “nits” are hard fiber bundles that do not come apart easily even when slurried in wet-laid operations. This process, in addition to promoting individualized fibers which minimize interfiber bonding during the subsequent curing step (which leads to undesirable “nits” from the conventional paper pulps used in this technology), also promotes curling and twisting of the fibers whi

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