1. Field of the Invention
This invention relates to a process for repulping wet strength paper and more particularly it relates to such a process incorporating in the paper a cationic thermosetting resin.
2. Description of the Prior Art
Wet strength resins are often added to paper including paperboard at the time of manufacture. In the absence of wet strength resins, paper normally retains only 3% to 5% of its strength after being wetted with water. However, paper made with wet strength resin generally retains at least 10%-50% of its strength when wet. Wet strength is useful in a wide variety of paper applications, some examples of which are toweling, milk and juice cartons, paper bags, and liner board for corrugated containers.
As stated in Handbook for Pulp and Paper Technologists, Gary A. Smook, Angus Wilde Publications, 1992: "Paper has traditionally been defined as a felted sheet formed on a fine screen from a water suspension of fibers. Current paper products generally conform to this definition except that most products also contain non-fibrous additives. Dry forming methods are now utilized for the manufacture of a few specialty paper products. Pulp is the fibrous raw material for papermaking. Pulp fibers are usually of vegetable origin, but animal, mineral, or synthetic fibers may be used for special applications. The distinction between paper and paperboard is based on product thickness. Nominally, all sheets above 0.3 mm thickness are classed as paperboard; but enough exceptions are applied to make the distinction somewhat hazy."
Because of increased commercial emphasis on developing paper products based on recovered cellulose, there is growing interest in developing paper which is readily repulpable. Paper and paperboard waste materials are difficult to repulp in aqueous systems without special chemical treatment when they contain wet strength resins.
Improving the repulpability of paper containing wet strength resins has generally been achieved by modifying the repulping conditions.
In U.S. Pat. No. 2,872,313, House et al. teach the use of hypochlorite salts to repulp paper containing polyaminoamide/epichlorohydrin wet strength resins. Miller, in U.S. Pat. No. 3,427,217, teaches the use of oxidizing salts such as sodium hypochlorite, ammonium persulfate and others, to repulp wet strength paper containing resins such as polyaminoamide/epichlohydrin resins, urea-formaldehyde and melamine-formaldehyde resins, etc. Although the hypochlorite salts effectively repulp wet strength paper, they are also chlorinating agents under some conditions and can form environmentally undesirable chlorine-containing degradation products in the process effluents.
Schmalz, in TAPPI, 44, no. 4, pp 275-280, April 1961, teaches the repulping of polyaminoamide/epichlorohydrin resin-containing paper with hypochlorite, or with strong alkali. Although the alkali method will not chlorinate organic byproducts, it is a relatively slow process.
Espy, in European Patent Application Publication No. 585,955-A, discloses a composition for repulping paper containing polyaminoamide/epichlorohydrin wet strength resins in an aqueous slurry, which comprises treatment with a non-chlorinated oxidizing agent and an alkali that is a water-soluble buffering salt capable of maintaining a pH of 7-12 in the aqueous reaction mixture.
Caropreso et al., in PCT International Publication No. WO 94/20682 disclose a composition containing a persulfate and a carbonate, bicarbonate or sesquicarbonate, which composition is suitable for oxidizing wet strength resin used in wet strength paper. The combination is stated to decrease the time required to repulp wet strength paper.
In U.S. Pat. No. 5,330,619, Henry et al. teach a method for treating fibrous sheet materials, such as paper or paperboard containing wet strength resin, which comprises reacting the fibrous materials with an enzyme to hydrolyze the resin and thereby improve repulping of the fibrous materials.
Copending application filed Jun. 3, 1996 (Hercules' Docket No. Dasgupta Case 12 "Process of Repulping Wet Strength Paper" Sunil P. Dasgupta) discloses a process for repulping paper by providing paper comprising at least one coacervate complex made from at least one cationic polymer and at least one anionic polymer and repulping the paper to obtain recycled pulp fibers.
Since many of the repulping processes used for wet strength paper result in formation of environmentally undesirable chlorine-containing degradation products, involve strong oxidizing agents, or proceed slowly, there is a need for improved methods for making wet strength paper that will be readily repulpable without significantly lowering the wet and dry strength properties of the paper.