US3255072A - Method of preparing cellulosic paper containing acrylamide copolymer and product thereof - Google Patents

Description

United States Patent 3,255,072 METHOD OF PREPARING CELLULOSIC PAPER CONTAINING ACRYLAMIDE COPOLYMER AND I PRODUCT THEREOF David P. Sheetz and Charles G. Humiston, Midland, Mich, assignors to The Dow Chemical Company, Midland, Mich. a corporation of Delaware No Drawing. Filed July 29, 1963, Ser. No. 298,435 6 Claims. (Cl. 162168) This invention is concerned with cellulosic webs having improved wet and dry strengths and is particularly directed to paper having incorporated therein a small but effective amount of a polymer containing pluralities of primary amino and amide groups and to a method for the production of paper so treated.

In the past, various materials have been applied to fibrous products, especially to aqueous dispersions of cellulosic fibers prior to the formation of felted webs therefrom, in order to improve the dry strength thereof. For example, Canadian Patent 477,265 describes the use of hydrolyzed polyacrylamide to improve the dry strength of paper. Optimization in the use of this material is highly dependent upon the presence of controlled amounts of alum. Similarly, naturally occurring materials such as starch and locust beam gum produce moderate dry strength improvements when employed conjunctively with'alum. It would be desirable to have available a dry strength additive which could be employed independently of alum if such were desired. This might be the case as, for example, when preparing paper from a neutral or alkaline stock.

Other dry strength additives of the prior art include those of United States Patent 2,884,057 wherein the use of poly-quaternary ammonium compounds is suggested for producing paper of improved dry strength. It is characeteristic of the latter additives that they have little, if any, effect on the wet strength of the paper.

Another patent of interest in connection herewith is United States Patent 2,838,397. This patent describes the use of certain nitrogenous polymers as :filler rentention aids in the manufacture of paper. As will be elaborated more fully hereinafter the practice of these teachings interferes with and substantially diminishes the results obtained with the unique paper additives of the present invention.

It would be most desirable and it is an object of the present invention to provide a water-soluble, and thus easily applied, agent to enhance both the wet and dry strengths of paper. Particularly, it is an object to provide a dry strength additive of which small amounts impart to wet felted cellulosic products superior improvements in dry strength and moderate wet strength properties, i.e., the latter being such that repulping or broke recovery problems are avoided. A further object is to provide a polymeric strength additive for paper which does not require for its effective use the presence of alum in the furnish. Other objects and benefits will become apparent from the following specification and claims.

In accordance with the present invention, it has been discovered that enhanced dry and wet strengths are produced in paper by incorporating in cellulosic slurries employed to prepare the paper a small proportion of a watersoluble polymer containing intralinear units of the general formulae:

a i Tit TY In the above formulae, R and R are independently selected from the group consisting of methyl and hydrogen, n is an integer from 2 to 3 inclusive and x and y are the number of monomer units in the polymer. The x/y ratio, or mole ratio of Formula A to Formula B, in the finished polymer can vary from about 50 to about 1.5. In a preferred copolymer of acrylamide and 2- aminoethyl methacrylate, this range of mole ratios corresponds to interpolymers of from about 97 to about 45 Weight percent of acrylamide and correspondingly from about 3 to about 55 weight percent of Z-aminoethyl methacrylate. While the described polymers can be employed as the free base, the mineral acid salts of such polymers are preferred. The amount of the polymeric additive used is that amount suificient to produce a significant increase in the dry strength properties of the paper.

A'unique feature of the above polymers is their performance without the presence of alum. Although the presence of alum does not detract from the effectiveness of the above polymers, its presence is definitely not required for superior results. An additional unique feature of the above polymers resides in the fact they perform as intended in a substantially improved fashion in the absence of inorganic paper fillers. Thus, for maximum dry strengths in accordance with the invention the polymers should be utilized with cellulosic pulps substantially free of filter materials, e.'g., calcium carbonate, clay and pigments.

Fundamentally, the polymers eifective in the invention are water-soluble copolymers derived by polymerization through the ethylenic double bond of at least one monomer selected from primary aminoalkyl esters of acrylic and methacrylic acids wherein the alkyl group contains 2 or 3 carbon atoms, and mineral acid salts of such esters with at least one monomer of the group of acrylamide and methacrylamide.

The effective polymers may also contain small amounts, e.g., up to about 25 percent by weight of the total polymer, of other suitable monoethylenically unsaturated acrylic monomers having up to 7 carbons. Examples of such optional monomers are N-methylolacrylamide, acrylonitrile, methacrylonitrile, ethyl m-hydroxymethylolacrylate and alkyl and hydroxyalkyl esters of acrylic and'methacrylic acids wherein the alkyl group contains from 1 to 3 carbon atoms. While they are not necessary for effective results, the presence of the optional acrylic monomers in the interpolymer does not detract from its general performance capabilities for the purposes of the inven-tion.

For use in accordance with the invention, the aminoalkyl and amide acrylic copolymer should be of moderately high molecular weight, that is, such copolymer should have a Staudinger molecular weight of at least about 5,000. Above this limit, increasing molecular weights, while giving generally better results, have a comparatively small effect on the strength properties of the finished paper as compared to the magnitude of the overall strength improvement. In practice, the relative viscosity of a solution of such polymers dissolved in 2 percent aqueous sodium chloride is a convenient criterion of the polymer molecular weight. Good results have been obtained when employing polymer agents characterized by relative viscosities from as little as 1.2 up to as much as 9 or more, the latter limit corresponding to an approximate Staudinger molecular weight of about 1,000,- 000. The relative viscosity is the viscosity of an aqueous solution containing 0.5 percent by weight of the polymer and 2 percent by weight sodium chloride divided by the viscosity of the 2 percent sodium chloride solution, the viscosity measurements being made at 25 C. with; a Ubbelohde viscosimeter.

The aminoalkyl ester monomers described herein are unstable in alkaline solutions and are, therefore, usually polymerized in the form of their mineral acid salts. Suitable mineral acids include hydrochloric, hydrobromic, sulfuric, nitric, phosphoric and the like acids. Generally, any mineral acid can be used that has a pK below about 4. The copolymers are readily prepared by carrying out the polymerization of monomers of each Formula A and Formula B in an aqueous solution of the monomers, employing a water-dispersible catalyst, e.g., 'an organic or inorganic peroxide, capable of yielding free radicals. When it is desired to introduce a minor proportion of an optional water-insoluble comonomer, such as an alkyl methacrylate, polymerization may be effected in an organic solvent system such as a lower alkanol. Polymerization modifiers such'as thiourea, Z-mercaptoethanol or the like, can be incorporated in the polymerization recipe if desired. Also, the concentration of catalytic metal cations, if any, in the reaction mixture can be controlled by suitable additions of a chelating agent such as a salt-of ethylenediaminetetracetic acid or of diethylenetriarnjinepentacetic acid. A preferred catalyst system consists of a small percentage of hydrogen peroxide and thiourea together with a few parts per million of copper ion.

A recipe for preparation of a preferred copolymer of the invention is as follows:

Ingredient: Percent by weight 2-aminoethy1 methacrylate hydrochloride 5 Acrylamide 20 Thiourea 0.2 Cupric ion p.p.m. 5 Hydrogen peroxide 0.4 Water, to make 100 P.p.m. indicates parts by weight per million parts of total charge.

tain from as little as 0.01 percent up to much as percent or more by weight polymer solids. Generally, the dilute solution of the copolymer is added with suitable mixing to paper pulp at any point'prior to formation of the paper web. While it may be added prior to com- I pletion of the beating and/ or refining steps, it is preferred polymer in the form of a viscous aqueous solution there- In large scale reactions, cooling of the reaction mixture may be required to avoid runaway reactions. In general, it is desirable to regulate the rate of reaction so that a peak temperature in the range of from 80 to 100 C is reached. Alternatively, the reaction rate may be controlled by introducing an aqueous solution of the monomers gradually or portionwise into a reaction vessel containing a solution of the polymerization catalyst. Oxygen appears to have an inhibitory effect on the polymerization reaction so that for rapid initiation of polymerization, it is desirable to purge the reaction mixture with an inert gas such as nitrogen prior to addition of the hydrogen peroxide. Although a dry product can be recovered by conventional separatory processes, e.g., drying on rolls or precipitation in solution with a water-miscible non-solvent for the polymer such as methanol, the copolymers prepared in the above manner are preferably maintained in aqueous solution form until used.

In carrying out .the invention, an aqueous stock solution of the copolymer is mixed with sufficient water to prepare a dilute solution, e.g., such solutions usually conto incorporate the copolymer into a turbulent stream of the pulp after the latter two operations. For best results the pH of the treated fiber suspension is maintained within the range from about 4 to about 10. In addition to the foregoing modes of operation, it is also possible to incorporate the polymeric additament into a preformed paper web as by spraying it with or dipping it in a solution of the copolymer and thereafter drying the coated a er.

p Vhen the polymeric addit-ament of the invention is added to the paper pulp prior to formation of the paper web, it is desirable, but not essential for good results, that it be added before completion of the addition of other paper-making ingredients. For optimum results, however, it is essential to avoid the use of fillers which have a deleterious effect on the dry strength of the finished paper product. Further, while alum can be used with the copolymers of the invention, it is not required for good performance. If alum is employed, it may be added either before or after the addition of the copolymer.

In one conventional mode of operation, the copolymer solution is introduced at the inlet side of the fan pump on a conventional papersheeting machine, such as a Fourdrinier. Alternatively, the copolymer solution may be mixed with the pulp suspension as it enters the headbox, provided there is sufficient agitation in the headbox assembly to provide lfOI thorough mixing of the polymer with the pulp prior to formation of the paper web. A sufiicient amount of the copolymer solution is employed to provide from about 0.01 to about 3 percent, preferably from about 0.2-5 to about 1.5 percent, by weight of the polymer, based on the weight of cellulosic fiber in the pulp suspension.

Fol-lowing the addition of the copolymer, the treated pulp suspension may be employed for the preparation of paper in conventional fashion, as, for example, by the use of handscreens or by modern, high-speed paper-making machinery. The paper web so formed may be pressed and dried in conventional fashion.

The following examples illustrate the invention but arenot to be construed as limiting the same.

Example I 360 grams of unbleached, air-dried soda pulp from hardwoods was distributed in 23 liters of water in a Valley laboratory beater and circulated until thoroughly dispersed. Thereupon a 5,500 gram load was placed on the beater bedplate arm and the pulp was beaten to a freeness (Canadian Standard) of about 400. 325 milliliter portions of the beaten pulp suspension were diluted with 2 liters of water to produce a furnish for test handsheets. In each determination, at least one such portion of the suspension was employed without further treatment to prepare a h andsheet as an untreated check. When treated paper was prepared, a suflicient amount of one of the aminoethyl acrylate and acry-lamide copolymers (hereinafter called lgenerically amino-amido acrylic copolymers) was added as an aqueous solution to the beaten and diluted fiber suspension to provide the desired proportion of copolymer to cellulosic fiber on a dry basis. Handsheets were made from the treated and untreated furnishes essentially in accordance with TAPPI Standard Method T205 m-53 using a Williams 10 inch by 12 inch mold modified to employ a sample size as specified above. After the [furnish had drained through the wire of the h-andsheet mold to form a wet paper sheet, the wet sheet on the screen was placed between blottcrs and thereafter the combination was placed in a 12 inch by 12 inch hydraulic press .and pressed lightly until the wet sheet adhered to the adjacent blotter. The screen was then removed and replaced with a polished metal plate and the latter was backed with additional blotters. The resultant stack was then placed in the hydraulic press and compressed for 3 minutes at a pressure of 60 pounds per square inch. This procedure caused the moist sheet to adhere to the metal plate. The blotters were then separated from the plate with adherent sheet and the latter was clamped in a frame and dried in a forced air oven at 110 C. for 15 to 30 minutes. Thereafter, the dried handsheet was separated from the plate and conditioned in a chamber maintained at 70 CF. and 62 percent relative humidity for at least 48 hours before testing for strength properties.

Following the foregoing procedure, a series of handsheets was prepared without additaments and dried for 30 minutes at 110 C. to serve as untreated checks. In similar manner, another series of handsheets was prepared, each sheet containing one percent, by weight of the dry cellulosic fiber, of one polymer of a series consisting of copolyrners of Zaminoethyl methacrylate hydrochloride with varying proportions of acrylamide. A homopolymer of 2-aminoethyl methacrylate was also run for purposes of comparison. All the resulting handsheets were conditioned as described above and tested for tensile strength, dry bursting strength and basis weight according to TAPPI Standard Method T220 m-5 3. Wet burst strengths were determined by TAPPI Standard Method after soaking test portions of handsheets in deionized water for 24 hours. All test results were corrected to a constant basis weight of 50 pounds. The results are summar'ued in Table 1 wherein the [figures in the Treatment column From the foregoing it will be observed that the copolymer produces superior dry strength coupled with a moderate increase in wet strength. Only in this latter regard do any of the amino-amide acrylic copolymers fall below the performance level of the homopolymer of AEM. The wet strength improvement of the copolymer, particularly in comparison to the wet strength performance levels achieved with the homopolymer, when up to nearly 95 weight percent acrylamide is incorporated into the copolymer is surprising in view of the fact that polyacrylamide itself gives essentially no wet. or dry strength improvement under these test conditions, i.e., in the absence of alum.

Example 2 In a manner similar to the procedure of Example 1, several test handsheets were prepared in which one percent by weight of a polymeric additive was incorporated into the finished paper. The polymers added included a co polymer of the invention containing 80 weight percent acrylamide and 20 weight percent Z-aminoethyl methacrylate hydrochloride and for the purposes of comparison poly N,N-dimethyl2-aminoethyl methacrylate hydrochloride, poly-N-methyl-Z-aminoethyl methacrylate hydrochloride, poIy-Z-aminoethyl methacrylate hydrochloride and polyacrylamide.

After drying at 110 C., the handsheets were placed in a constant temperature and humidity chamber at F. and 62% relative humidity. The sheets were thus conditioned for at least 48 hours before being subjected to tests to determine their strength properties according to TAPPI Standard T220 m-53. Results of these tests are tabulated below in Table 2 according to the prior polymer treatment of the handsheet.

refer to the proportions by weight of the indicated monomers combined in the copolymer employed and the values indicated for strength property measurements are averages of several determinations. [For example, 95/5 AA/AEM represents a copolymer of 95 parts by weight of acrylamide with 5 parts by weight of 2-aminoethyl methacrylate hydrochloride and 0/100 AA/AEM represents the homopolymer of Z-aminoethyl methacrylate hydrochloride.

From the foregoing it will be observed that the copolymer of the invention provided excellent improvement in the dry and tensile strengths of the finished paper as compared to the homopolymers with simultaneous substantial improvement in wet strengths.

Example 3 Similar determinations to those in Example 1 were carried out with c'opolymers of percent by weight of TABLE 1.EFFECT OF COPOLYMER COMPOSITION Dry Burst Increase Wet Burst Wet/Dry Breaking Increase Treatment Factor Over Control Factor Control Length Over Control (Percent) (Percent) (Meters) (Percent) Untreated checks"... 22. 9 1 3, 758

acrylamide with 20 percent of Z-amihoethyl methacrylate hydrochloride. The copolymers were added to the indicated cellulosic pulp in the amount of 1 percent by weight of the dry fiber content. The copolymers employed varied in molecular weights as evidenced by a range of viscosities of from 5 27 to 23,450 centipoises for aqueous 25 percent by Weight solutions thereof at a temperature of 25 C. This latter property is indicated below as the Viscosity Characteristic. The dry strength measurements are summarized in the following table wherein the pulp type is indicated as follows: H=unbleached hardwood soda pulp beaten to a Canadian Standard freeness of 390, and P=bleached chemical pulp having ahigh content of alpha-cellulose.

Viscosity Pulp Dry Burst Characteristic Type Factor (c'entipoises) 23, 450 H 37. 5 15, 450 H .36. 4 4, 050 H 38. 4 23, 450 P 44. 4 15, 450 P 43. 7 8, 620 P 41. 7

Example 4 To illustrate the advantage of employing the copolymer of the invention in the substantial absence of filler and its tolerance of the presence of alum, a copolymer of the invention was employed in the preparation of a series of handsheets containing in addition to the polymer portions of (1) alum and (2) alum and clay filler. The test sheets were prepared and tested in a manner similar to that of Example 1 except that they were cured for 20' minutes at 100 C. Also, the pulp employed was a bleached softwood kraft beaten to 430 milliliters Canadian Standard freeness. The percent loading of the various paper-making ingredients in the test handsheet based on the dry fiber con-tent of the sheet and the results of the test operations including the basis weight, dry burst and tensile factors corrected to a constant basis weight of 50 pounds are reported for each run in the following 1 A copolymer containing 80% by weight acrylamide and 20% by weight 2-an1inoethyl methacrylate hydrochloride.

2 Lbs. per 500 sheets having dimensions of 25 x 40".

Example 5 In further determinations similar to Example 2 above, it was shown that the cop-olymer of 80 percent by weight of acrylamide with 20 percent by weight of Z-aminoethyl acrylate hydrochloride, when employed for treating unbleached hardwood soda pulp in amounts of from 0.5 to 1.5 percent by weight of the cellulose, resulted in increased strength of the paper produced therefromof from about 32 to 45 percent when measured by the dry burst test.

Paper prepared as in the above examples was analyzed for nitrogen content to determine the retention of the active polymer in the finished paper. It was found that the percent of the added polymer retained in the finished paper varied from about 10 to 80 percent of the polymeric agent added, depending upon such factors as the molecular weight of the polymer, the proportions of aminoalkyl ester moieties in the particular polymeric agent employed and the type and prior treatment of the cellulosic pulp. Particularly favorable retention was obtained with the copolymers prepared from to parts by weight of acrylamide and correspondingly from 20 to .10 parts by weight of Z-aminoethyl methacrylate hydrochloride.

In a manner similar to that of the foregoing, comparable improvements in the dry and wet strengths of paper products are accomplished by substituting for the polymeric additives in Example 2, interpolymers under the invention of 2-aminoethyl acrylate hydrochloride and methacrylamide, which interpolymers also contain small amounts of other acrylic monomers such as N-methylolacrylamide, acrylonitrile, methacrylonitrile, ethyl ochydroxymethylolacrylate, methyl methacrylate, hydroxyethyl methacrylate,'hydroxypropyl methacrylate and hydroxyethyl acrylate, said interpolymers containing no more than about 25 percent by weight of the latter optional acrylic moieties.

What is claimed is:

1. A method for preparing paper having improved strength properties which consists essentiallyof the steps of forming an aqueous suspension of cellulosic fibers, incorporating in such suspension an amount within the range from about 0.01 to about 3 percent by weight of the dry cellulose fibers of a water-soluble polymeric agent containing intralinear units of the general formulae:

R lay l )CnH2u-NH2 wherein R and R are independently selected from the group consisting of methyl and hydrogen, n is an integer from 2 to 3 inclusive and x and y are the numbers of the monomer units in the polymeric agent, the ratio of x/y being within the range from about 50 to about 1.5 and forming a web by wet laying said fibers.

2. A method according to claim 1 wherein the polymeric agent is selected from the group consisting of copolymers containing in polymerized form from about 97 to about 45 percent by weight of acrylamide and correspondingly from about 3 to about 55 percent by weight of Z-aminoethyl methacrylate and mineral acid salts of the foregoing copolymers.

3. A method for preparing paper having improved strength properties which comprises the steps of forming an aqueous suspension of cellulosic fibers substantially free of an inorganic filler, incorporating in such suspension an amount from about 0.01 to about 3 percent by weight of the dry cellulose fibers of a water-soluble polymeric agent containing intralinear units of the general formulae:

H I l Tiff J.

and

and

wherein R and R are independently selected from the group consisting of methyl and hydrogen, n is an integer from 2 to 3 inclusive and x and y are the numbers of the monomer units in the polymeric agent, the ratio of x/ y being within the range from about 50 to about 1.5 and forming a web by wet laying said fibers.

4. A method according to claim 3 wherein the polymeric agent is selected from the group consisting of copolymers containing in polymerized form from about 97 to about.

and

10 wherein R and R are independently selected from the group consisting of methyl and hydrogen, n is an integer from 2 to 3 inclusive and x and y are the numbers of the monomer units in the polymeric agent, the ratio of x/y being within the range from about to about 1.5 and forming a web by wet laying said fibers.

6. A paper product as in claim 5 wherein the polymeric agent is selected from the group consisting of copolymers containing in polymerized form from about 97 to about 45 percent by weight of acrylamide and correspondingly from about 3 to about percent by weight of Z-aminoethyl methacrylate and mineral acid salts of the foregoing copolymers.

References Cited by the Examiner UNITED STATES PATENTS 2,838,397 6/1958 Gruntfest et a1 162168 2,972,560 2/1961 Stilbert et al 162168 3,007,887 11/1961 Essig 162168 3,084,093 4/1963 Humiston 162168 FOREIGN PATENTS 477,265 9/1951 Canada.

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. WOLK, Examiner.

S. L. BASHORE, Assistant Examiner.

Claims (1)

1. 3. A METHOD FOR PREPARING PAPER HAVING IMPROVED STENGTH PROPERTIES WHICH COMPRISES THE STEPS OF FORMING AN AQUEOUS SUSPENSSION OF CELLULOSIC FIBERS SUBSTANTIALLY FREE OF AN INORGANIC FILLER, INCORPORATING IN SUCH SUSPENSION AN AMOUNT FROM ABOUT 0.01 TO ABOUT 3 PERCENT BY WEIGHT OF THE DRY CELLULOSE FIBERS OF A WATER-SOLUBLE POLYMERIC AGENT CONTAINING INTRALINEAR UNITS OF THE GENERAL FORMULA: