Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-05-16
2002-08-06
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S500000, C524S519000, C524S538000, C524S551000, C524S555000, C162S164100, C162S164300, C162S164600, C162S168200
Reexamination Certificate
active
06429253
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to mixtures of polymers with improved stability which may be used in a papermaking process to provide paper which is readily repulpable, yet still exhibits adequate wet and dry strength.
Paper is typically manufactured with chemical additives which tend to improve various paper properties e.g. sizing, wet strength, dry strength, etc. Additives which provide wet strength (wet strength agents) may be classified as being either “permanent” or “temporary,” based on the permanence of the wet strength they provide. Temporary wet strength agents are generally distinguished from permanent wet strength agents in that they provide a certain degree of wet strength immediately e.g. 5-40 seconds after the paper is wetted, but a good portion e.g. 30-75% of this immediate wet strength is lost after 30 minutes soaking in water, depending on the soaking conditions. In contrast, the immediate wet strength of a paper treated with a permanent wet strength agent tends to decay much more slowly and may often be considered permanent for many practical purposes.
A number of chemical treatments have been used to impart wet strength to paper, including polymers based on melamine-formaldehyde (MF) e.g. those disclosed in U.S. Patent No. 4,461,858, as well as synthetic cationic polymers based on polyamide epichlorohydrin, polyamine epichlorohydrin, and polyamide-amine epichlorohydrin (collectively PAE). For instance, wet strength agents based on PAE are disclosed in U.S. Pat. Nos. 2,926,116; 2,926,154; 3,733,290; 4,566,943; and 4,722,964. Specific temporary wet strength agents are disclosed in U.S. Pat. Nos. 3,556,932 and 4,605,702. All of the foregoing patents are hereby incorporated herein by reference.
As a result of the heightened awareness and increased demand for paper products containing recovered cellulose fiber, efforts have been undertaken to develop paper products which are more readily recyclable. Commercially available wet strength paper products are often difficult to repulp because they utilize relatively high levels of permanent wet strength agents. Although the formulation of paper with temporary wet strength agents would appear to be a solution to this problem, in actual practice difficulties may be encountered because the immediate wet strength obtained with temporary wet strength agents is often disadvantageously less than that obtained with permanent wet strength agents. Also, delays in repulping may be encountered because of the time necessary for the wet strength to decay. In addition, since temporary wet strength agents typically contain reactive functional groups, they may have poor stability as evidenced by a tendency to gel or become water-insoluble on storage. Polymers solutions which have gelled, or a in the late stages of the gelling process, are no longer pourable and thus may present handling difficulties. While gelling may be partially mitigated by reducing the polymer solids of the polymer solution, this presents commercial disadvantages such as increased shipping and storage costs.
In the past, paper having greater repulpability has been produced using commercially available blends of permanent and temporary wet strength agents, and U.S. Pat. Nos. 5,427,652 and 5,466,337 disclose blends of permanent and temporary wet strength agents. However, in some cases the stability of such a blend may be disadvantaged by the inclusion of a reactive temporary wet strength agent. Moreover, there is in most cases an expectation that blending will compromise some other desirable property that is provided by one or the other component. In general, the expectation is based on the well-known “rule of mixtures,” which states that any particular property of a mixture is a weighted average of the properties of the individual components making up the mixture, see e.g. “Predicting the Properties of Mixtures: Mixture Rules in Science and Engineering,” Lawrence E. Nielson, Marcel Dekker, Inc. 1978, pp. 5-9, as well as U.S. Pat. Nos. 5,496,295; 5,476,531; 5,277,245 and 4,926,458.
It is therefore an object of the instant invention to provide novel wet strength compositions that remain pourable for extended periods of time, methods for utilizing said compositions in papermaking, as well as paper having reduced wet strength (and therefore increased repulpability) without having unduly compromised dry strength, or, in the case of multi-ply paperboard, without having unduly compromised dry ply bonding strength
SUMMARY OF THE INVENTION
It has now been found that compositions comprised of wet strength agents and dry strength agents may, when prepared according to the teachings herein, remain pourable for extended periods of time. It has also been found that effective proportions of wet strength agents and dry strength agents may, when used in papermaking according to the teachings herein, provide paper having lower wet strength without unduly compromised dry strength. Therefore, according to the instant invention, there is provided paper comprised of (a) cellulosic fibers, (b) a polymeric cationic wet strength agent, and (c) a synthetic polymeric cationic dry strength agent different from said wet strength agent, having from about 1 to about 15% of cationic recurring units, by mole based on total moles of recurring units; wherein the amounts of said (a), (b) and (c) are effective to provide said paper with an immediate wet strength that is less than the immediate wet strength of a comparable paper in which only (b) is used in place of (b) and (c); and wherein the amounts of said (a), (b) and (c) are effective to provide said paper with a dry strength that is greater than the expected dry strength based on the rule of mixtures.
In another embodiment of the instant invention, there are provided compositions comprised of (a) a polymeric cationic wet strength agent, (b) a synthetic polymeric cationic dry strength agent different from said wet strength agent, having from about 1 to about 15% of cationic recurring units, by mole based on total moles of recurring units, and (c) water, wherein the weight ratio of said (a) to said (b) is in the range of about 1:4 to 4:1, and wherein a sample of said composition, prepared by mixing (a) and (b) in water to provide a 15% solids composition, by weight based on total weight, remains pourable for at least about 25 days after preparation when stored at about 35° C.
In another embodiment of the instant invention, there are provided methods comprising (a) providing a paper stock, (b) mixing (i) a polymeric cationic wet strength agent, and (ii) a synthetic polymeric cationic dry strength agent different from said wet strength agent, with said paper stock to form an admixture, (c) forming a web from said admixture, and (d) forming a paper from said web; wherein said (ii) has from about 1 to about 15% of cationic recurring units, by mole based on total moles of recurring units; wherein the amounts of said (i), (ii) and paper stock are effective to provide said paper with an immediate wet strength that is less than the immediate wet strength of a comparable paper in which only (i) is used in place of (i) and (ii), and wherein the amounts of said (i), (ii) and paper stock are effective to provide said paper with a dry strength that is greater than the expected dry strength based on the rule of mixtures.
DETAILED DESCRIPTION OF THE INVENTION
As used herein, “paper” is a general term that includes sheet-like masses and molded products made from fibrous cellulosic materials which may be derived from both natural and/or synthetic sources. Paper may be prepared from any aqueous suspension of cellulose fiber and may contain other fibrous matter such as organic, inorganic, or synthetic fibers. Specific examples of paper include printing and writing papers, absorbent papers, tissue, towel, paperboard, linerboard medium, container board, or boxboard, any of which may be coated or uncoated. Paper may be formed from cellulosic fibers derived from any fiber source including, but not limited to, any bleached or unbleached hardwood
Guerro Gerald J.
Lawrence Leigh Ann
Bayer Corporation
Gil Joseph C.
Henderson Richard E. L.
Zitomer Fred
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