Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-02-29
2001-11-13
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168300, C162S158000
Reexamination Certificate
active
06315866
ABSTRACT:
TECHNICAL FIELD
This invention concerns a method of improving the dry strength of paper products using a single cationic dispersion polymer strength additive.
BACKGROUND OF THE INVENTION
Various properties of paper products, including strength, opacity, smoothness, porosity, dimensional stability, pore size distribution, linting propensity, density, stiffness, formation and compressibility are primarily are to a large extent dependent on the bonds which exist between the cellulosic fibres in the paper. The bonding capability of these fibers is enhanced by the mechanical beating or refining step(s) of the papermaking process, during which the fibers are made more flexible and the available surface area is increased.
The strength of paper products is a property having three categories, referred to as dry strength, wet strength or rewetted strength, and wet web strength. Dry strength is the tensile strength exhibited by the dry paper sheet, typically conditioned under uniform humidity and room temperature conditions prior to testing. Wet strength, or rewetted strength, is the tensile strength exhibited by a paper sheet that has been fully dried and then rewetted with water prior to testing. Wet web strength is strength of a cellulosic fiber mat prior to drying to a paper product.
Strength resins are polymers generally added at the wet end of the papermaking process to the cellulosic slurry, prior to the formation of the paper mat or sheet, to improve the strength characteristics of the paper product. Strength resins are generally believed to work by supplementing the number of interfiber bonds.
Dry strength additives are used to increase the dry strength of various paper products including paper, paperboard, tissues and others. Dry strength additives are particularly useful in the manufacture of paper products from recycled fibers, as recycling is known to have a weakening effect on the resulting paper. In addition, dry strength additives should reduce the amount of refining required to achieve a given dry strength for a given pulp, and the corresponding energy consumption required for refining and should not adversely affect the drainage rate of the cellulose web on the papermaking machine.
Various polyacrylamides have been used as wet-end additives to increase dry strength of paper products. The polyacrylamides are particularly useful as dry strength additives as they contain primarily amide groups which can form hydrogen bonds so that the interfiber bonds in the paper sheet increase without the increase in wet strength caused by covalent crosslinks. The polyacrylamides are generally also water-soluble which leads to a uniform distribution in the stock and a uniform adsorption on the surfaces of the fibers. See Paper Chemistry, An Introduction, D. Eklund and T. Lindstrom, DT Paper Science Publications, Grankulla, Finland (1991).
The use of certain cationic copolymers of acrylamide and &agr;,&bgr;-unsaturated quarternary ammonium compounds as dry strength additives is disclosed in U.S. Pat. No. 2,884,057.
A polyelectrolyte complex comprising at least one high molecular weight, low charge, water soluble cationic polymer and at least one anionic polymer for increasing the dry strength of paper is disclosed in U.S. Pat. No. 5,338,406. Copolymers of acrylamide and diallyldimethylammonium chloride or methacryloyloxyethyltrimethylammonium chloride are recited as preferred cationic polymers.
A papermaking process that utilizes mixtures of wet strength agents and dry strength agents, including cationic acrylamide-diallyldimethylammonium halide copolymers, to produce paper having increased wet strength without compromising dry strength is disclosed in PCT/US98/01980.
However, the need still exists for a sole treatment agent for increasing the dry strength of paper products.
SUMMARY OF THE INVENTION
I have discovered that the dry strength of paper products may be improved by using certain cationic dispersion polymers as the sole strength additive.
Accordingly, in its principal aspect, this invention is directed to a method for improving the dry strength of a paper product comprising
a) adding to an aqueous cellulosic papermaking slurry an effective amount of a single cationic dispersion polymer strength additive, wherein the cationic dispersion polymer is prepared by polymerizing in an aqueous solution of a polyvalent anionic salt in the presence of a dispersant:
i. a cationic diallyl-N,N-disubstituted ammonium halide monomer of formula
(H
2
C═CHCH
2
)
2
N
+
R
1
R
2
X
−
wherein R
1
and R
2
are independently C
1
-C
20
alkyl, aryl or arylalkyl and X is an anionic counterion and
ii. an acrylamide monomer of formula
wherein
R
3
and R
4
are independently hydrogen, C
1
-C
10
alkyl, aryl or arylalkyl;
R
5
is hydrogen or methyl and
R
6
and R
7
are independently hydrogen or C
1
-C
10
alkyl;
b) draining the slurry to form a sheet; and
c) drying the sheet.
The cationic dispersion polymer is water-soluble and dissolves rapidly resulting in a homogeneous distribution of the polymer over the cellulose fiber web. After drying, the cellulose fiber web possesses materially greater dry strength than that possessed by untreated cellulose fiber webs. This polymer deposition is irreversible and the polymer will not be removed by any subsequent step to which the fibers are normally subjected in the manufacture of paper.
The cationic dispersion polymer of this invention is useful for increasing the dry strength of a number of paper products including Kraft, tissue, testliner, duplex topside white paper, cardboard and shaped or molded paperboard. Use of the cationic polymers of this invention as dry strength additives is particularly advantageous as the use of additional strength agents, such as wet strength additives is not required.
In addition to increasing dry strength, the cationic dispersion polymers described herein confer additional advantages to the papermaking process including improved retention and drainage, reduction of refining time resulting in lower production costs, improved sheet formation and increased paper sheet brightness. The cationic dispersion polymers also permit the use of less costly, lower quality grade of recycle furnish in the furnish mixture while still achieving the desired paper characteristics.
DETAILED DESCRIPTION OF THE INVENTION
“Dry strength additive” means an additive that, when added to the papermaking process, increases the dry strength of the paper by about 10 percent or more.
“Monomer” means a polymerizable allylic, vinylic or acrylic compound.
“Cationic diallyl-N,N-disubstituted ammonium monomer” means a compound of formula (H
2
C═CHCH
2
)
2
N
+
R
1
R
2
X
−
wherein R
1
and R
2
are independently C
1
-C
20
alkyl, aryl or arylalkyl and X is an anionic counterion. Diallyl-N,N-disubstituted ammonium monomers are well-known and commercially available from a variety of sources. Representative cationic diallyl-N,N-disubstituted ammonium halide monomers include N-methyl-N-ethyl-N,N-diallyl ammonium chloride and diallyldimethyl ammonium chloride (DADMAC). A preferred cationic diallyl-N,N-disubstituted ammonium monomer is DADMAC.
“Anionic counterion” means any organic or inorganic anion which neutralizes the positive charge on the quaternary nitrogen atom of the cationic diallyl-N,N-disubstituted ammonium monomer. Representative anionic counterions include halogen, sulfate, phosphate, monohydrogen phosphate, nitrate, and the like. A preferred anionic counterion is halogen.
“Acrylamide monomer” means a monomer of formula
wherein R
3
and R
4
are independently hydrogen, C
1
-C
10
alkyl, aryl or alkylaryl; R
5
is hydrogen or methyl and R
6
and R
7
are independently hydrogen or C
1
-C
10
alkyl. Representative acrylamide monomers include acrylamide, (meth)acrylamide, ethyl hexyl (meth)acrylamide, diethylaminopropyl (meth)acrylamide, dimethylaminohydroxypropyl (meth)acrylamide, N-isopropyl (meth)acrylamide, N-tert-butyl (meth)acrylamide, C
1
-C
10
N-alkyl acrylamide, C
1
-C
10
N-alkyl methacrylamide, N-aryl acrylamid
Breininger Thomas M.
Fortuna Jose
Martin Michael B.
Nalco Chemical Company
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