US3238010A - Method of reacting cellulose paper and specific non-ionic latices containing hydrogen and hydroxy groups in the polymer chain with polyisocyanate adducts - Google Patents

Description

United States Patent M METHQD 0F REACTING CELLULUSE PAPER AND SPECIFIC NON-HUNMI LATICES CQNTAINING HYDROGEN AND HYDRGXY GROUPS IN THE PGLYMER CHAHN WITH PULYISOCYANATE ADDIUCTS Emile E. Hahih, Boston, and Melvin Nimoy, Hyde Park, Mass, assiguors to W. R. Grace dz (10., Cambridge, Mass, a corporation of Connecticut No Drawing. Uriginal application .lune 2, 1959, Ser. No. 317,488. Divided and this application Mar. 31, 196i, Ser. No. 102,085

9 Claims. (Cl. 8-4156) This application is a divisional application of copending application Serial No. 817,488, filed on June 2, 1959, and now abandoned.

This invention relates to a method of treating paper to improve the physical properties of the same.

It is common knowledge that the physical properties of an ordinary sheet of paper are drastically impaired when the sheet is wet. It is also known that the physical properties of the same sheet would be phenomenally increased even when the sheet is wet, if the paper is impregnated with a polymeric latex, dried, and then immersed into an organic solution containing an isocyanate. In theory, it is thought that the isocyanate, which is used, functions to cross-link the elastomeric polymer to the paper by means of urethane bridges thereby mechanically bonding the fibers of the paper to the polymer. It is also thought that this cross-linking decreases the hydrophilic nature of the fibers of the paper so that they absorb less water. In any case, it has been found that a sheet of paper, which has been treated in this manner, exhibits an enhanced degree of tensile strength and abrasion resistance even though the sheet may be wet.

The basic difliculty with this process is that it was found necessary, in each and every case, to dissolve the isocyanate in a non-reactive solvent in order to use the isocyanate in the treatment of paper. If this is not done, the isocyanate, when used for treatment, reacts predominantly at a fairly rapid rate, with the active hydrogens present in the latex system, forming insoluble di-urea compounds. When this happens, a substantial quantity of the isocyanate is consumed and thereby becomes unavailable to cross-link the elastomeric polymer and the paper.

The use of solvents has been found to be a serious hindrance to the commercialization of this process for reasons of economics and safety. This treatment is expensive because the solvent absorptivity of impregnated type paper may be as high as 100% based on the weight of the paper even after squeezing out the excess with pressured rolls. Needless to say, this treatment could be quite expensive in view of the amount of solvent that is absorbed unless care is taken to recover the absorbed solvent. A suitable solvent recovery system would be expensive to construct and to operate especially in view of the large quantities of paper that wolud be involved if this method of treatment was used with a high speed paper machine. A further deterrent to the use of this treatment is the fire hazard associated with the use of large amounts of solvent. Another deterrent is that this treatment is essentially a two-stage process in which the latex treated paper must be removed to another machine and treated with the isocyanate solution in a separate 3,238,0W Patented Mar. ll, 1966 operation. It is evident that the use of such a treatment be deterred except where absolutely necessary.

We have developed a method whereby paper may be impregnated with latex and treated with isocyanate without the use of solvent and without the necessity of a twostep process. In this method the isocyanate is stabilized to the presence of active hydrogens by reacting the isocyanate with an agent which renders the isocyanate group inactive at ordinary temperatures but which permits the isocyanate to be regenerated and thereby become reactive when heated to a temperature above about C. Since we are able to add the isocyanate, in unreactive form, to the latex, we have been able to impregnate the paper with the polymeric latex, as well as the isocyanate, in a single operation. This treatment imparts the same properties to paper as does a treatment with latex, followed by a subsequent treatment with an isocyanate dissolved in an organic solvent. The difliculties which industry has heretofore encountered with the utilization of a solvent solution are eliminated.

In general, our process comprises treating a sheet of paper with a latex containing a polyfunctional isocyanate in unreactive form, and then regenerating the isocyanate in active form, while in contact with the paper. For claritys sake, all polyfunctional isocyanates are herein referred to as isocyanate.

More particularly, our process comprises impregnating a sheet of paper with a latex containing an isocyanate in unreactive form, drying the paper at a temperature below 140 C., and heating the impregnated paper to a temperature above 140 C. to regenerate the isocyanate in a form which is reactive to both the cellulosic fibers in the paper and the elastomeric chain in the latex.

In the practice of the invention, an isocyanate in unreactive form is incorporated into the aqueous latex most generally by means of an emulsion. The isocyanate is one in which the reactive groups of the isocyanate molecule have been rendered inactive at ordinary temperatures by the formation of a chemical complex, which complex when heated, decomposes to regenerate the isocyanate molecule in the freely reactive form. Paper is thus impregnated with latex containing this isocyanate in unreactive form. Subsequently, this treated sheet is dried and then is heated and the isocyanate groups contained therein are thereby regenerated. It is known that cmpounds which contain free isocyanate groups are, most generally, very reactive compounds and that the free isocyanate groups of these types of compounds will react with practically any active hydrogen compound, i.e. a compound containing a hydrogen which may normally be replaced with sodium. The free isocyanate groups even react with the hydrogen and hydroxyl groups present in the cellulosic chain of the paper and in the elastomeric chain of the polymer. As a result of such a reaction the cellulosic fibers appear to be interconnected to each other by means of elastorneric chains which tie the fibers into a network wherein the elastomeric chains are bonded to the cellulosic fibers by means of urethane bonds.

ISOCYANATES There are many organic molecules containing isocyanate groups which will react with the polymer in the latex and with the cellulose fibers in the paper. In general, any of the polyisocyanates in the following classes, due to their chemical nature are reactive With groups containing e: an active hydrogen. Suitable isocyanates include the aliphatic diisocyanates such as ethylene, trimethylene, tetramethylene, pentamethylene, hexamethylene, propylene- 1,2 butylene-1,2, butylene-2,3, butylene-1,3, ethylidene, and butylidene diisocyanate; the aromatic diisocyanates such as m-phenylene, p-phenylene, 4,4-diphenyl, 1,5- naphthalene, and 1,4-naphthalene diisocyanate; the cycloalkylene diisocyanates such as cyclopentylene-l,3, cyclohexylene-l,4, and cyclohexylene-1,2; the aliphatic aromatic diisocyanates such as 4,4-diphenylene methane, 2,4- tolylene, 4,4-tolidene, and 1,4-xylylene diisocyanate; the nuclear substituted aromatic isocyanates such as dianisidene, 4,4'-diphenyl ether and chloro-diphenylene diisocyanate; the triisocyanates such as 4,4,4"-triisocyanato triphenyl methane, 1,3,5-triisocyanato benzene, and 2,4,6- triisocyanato toluene; and tetraisocyanates such as 4,4- dimethyl-diphenyl methane and 2,2,5,5-tetraisocyanate.

INHIBITING AGENTS As pointed out above in the practice of our invention the active isocyanate group is reacted with a chemical agent which renders the isocyanate group inactive at ordinary temperatures, but which degenerates thereby regenerating the isocyanate, in active form, upon heating to temperatures above about 140 C. There are many chemical reagents which will accomplish this result. These include aceto-acetic ester, diethylmalonate; mercaptans such as 2-mercapto-benzothiazole, lactams such as epsilon-caprolactams, delta-valerolactam, gamma-butyrolactam, and beta-propiolactam; imides such as succinimide and phthalimide; tertiary alcohols such as ter-amyl, tertiary butyl, dimethyl ethenyl carbinol, dimethyl phenyl carbinol, methyl diphenyl carbinol, triphenyl carbinol, 1- nitro tertiary butyl carbinol, l-chlorotertiary butyl carbinol, and triphenyl silinol; secondary aromatic amines such as diphenyl-amine; diaryl compounds such as diphenylamine, o-ditolyl amine, m-ditolyl amine, p-ditolyl amine, N-phenyl toluidine, N-phenyl xylidene, xylidene, carbazole, phenyl alpha naphthylamine, and phenyl beta naphthylamine; dimers of aromatic mono-isocyanates; bisulfite addition products, phenols and cresols.

POLYMERIC LATEX The polymeric latices used in this treatment must be non-ionic in nature. It has been found that the stability of the blocked isocyanate to premature cleavage is excellent in non-ionic latices and poor in either anionic or cationic latices. The polymeric systems, which are used, however, must contain labile hydrogen or hydroxy groups with which the isocyanate, when regenerated in active form, may react. The polymeric latices which are preferably used are polymers which can be made in non-ionic systems. Examples of non-ionic latices are those containing polyvinyl acetate, copolymers of vinyl acetate, and copolymers of butadiene-acrylonitrile such as that commercially available under the trade name of Hycar 1872.

ACTIVATOR The reactivity of the polymeric system may be further enhanced by increasing the number of labile hydrogen or hydroxy groups present in the system. This may be accomplished, in the case of polyvinyl acetate, by a partial hydrolysis of the system. It may also be accomplished, in the case of both polymers and copolymers of vinyl acetate, by introducing reactive groups into the molecule during the polymerization of the monomer, such as by incorporating hydroxy bearing emulsifying agents into the reactive charge during the emulsion polymerization of the monomer. It has been noted that only about 50% of such hydroxy bearing material has been recovered after the polymerization reaction has been completed indicating that the hydroxy bearing material, which has not been recovered, had been incorporated into the polymer. There are a number of hydroxy bearing materials which may be used for this grafting technique, such as polyvinyl alcohol, hydroxy-ethyl cellulose and carboxymethyl cellulose.

EXTENDERS It has also been found that these polymeric latices may be economically extended with emulsions of polymerized petroleum resins, low molecular weight oils, or other extenders which will not filter out during impregnation of paper. We have preferentially used emulsions of the polymerization types of petroleum resins such as those commercially available under the trade name of Piccopalc N-2.

PAPER The paper which is to be treated by this method, should be sufficiently porous that the ingredients of the impregnating composition will not be filtered out by the paper during treatment. The minimum degree of necessary porosity is controlled by the particle size of the particular polymer present in the latex. Particle size, of course, varies depending not only on the method of producing the polymer but also on the specific monomers used in the process. Since the method of producing suitable porous papers is an art itself which is well known to the paper industry it is not considered necessary to consider this subject in detail.

DISPERSION OF THE ISOCYANATE IN THE LATEX The isocyanate material in the unreactive or blocked state is introduced into the latex in the form of a dispersion or of an emulsion in water. A dispersion in water may be formed by ball milling the blocked isocyanates in the presence of water and a dispersion agent. We prefer, however, to form an emulsion of the blocked isocyanate since we have found that when added as an emulsion the isocyanate is more intimately mixed with the latex. A rather satisfactory emulsion may be formed by dissolving the blocked isocyanate in ethyl acetate, and then adding the solution to a warm aqueous solution containing sodium alkyl naphthalene sulfate, alkyl aryl sodium sulfonate, and ammonium caseinate. In the emulsion the ammonium caseinate functions as a stabilizer for the emulsion, the sodium naphthalene sulfate functions as a wetting agent for the system, and the alkyl aryl sodium sulfonate functions as a dispersing agent for the organic phase.

TREATMENT OF PAPER The paper may be impregnated with the impregnation compound in a number of ways. These include beater impregnation, wet web impregnation and dry web impregnation. For convenience the examples that follow were made by dry web impregnation by passing paper through a bath containing the latex solution in which the blocked isocyanate was dispersed. After impregnation, the paper was further processed by passing the same through a conventional press or calender in order to squeeze out the excess impregnant. Subsequently, this paper was heated at a low temperature to substantially dry it and then heated to a temperature between about 100 C. and 180 C.

p for a period up to 10 minutes to regenerate the isocyanate and cross-link the polymer and paper.

The following examples show how various blocked isocyanates are dispersed in a vinyl acetate copolymer latex and how these dispersions are used to treat paper. All formulations given herein are on the dry base for all ingredients. The treated sheets were then conditioned by maintaining them at a temperature of about F. and a relative humidity of about 50% for 14 to 16 hours. After conditioning, the treated sheets were tested to determine their tensile strength.

Tensile strength-The treated sheets were tested on a Scott tensile tester, Model X-3. These measurements were made on /2" x 4" strips of treated paper with a one inch gap between the jaws. Elongation was made at the rate of 12" per minute and the tensile was reported as pounds (corrected to an equivalent 1" strip). Prior to the wet tensile test, the paper was soaked in water at 70 C. for 24 hours.

In Example I, a blocked isocyanate was emulsified and then dispersed in a vinyl acetate copolymer latex which was later used to treat paper.

Example I 50 parts by weight of the reaction product of toluene, 2,4-diisocyanate, trimethylol propane and phenol were ground into a coarse powder and disolved in ethyl acetate. This solution was then emulsified in another solution containing 3 parts by weight of ammonium caseinate, 5 parts by weight of sodium alkyl naphthalene sulfate, and 0.5 part by weight of alkyl aryl sodium sulfonate dissolved in 23 parts by weight of warm water. The emulsion was then homogenized and stirred until the temperature of the emulsion was below about 110 F.

Various quantities of the emulsion were then added to an aqueous latex of non-ionic vinyl acetate copolymer which was the reaction product of the emulsion polymerizaiton of 77 parts by weight of vinyl acetate and 23 parts by weight of dibutyl maleate. The resulting mixture was then stirred until it was uniform.

8" x 10 sheets of impregnated base paper were then totally immersed into this liquid mixture, drained, placed between sheets of blotting paper and then passed through a roller to simulate the squeeze rolls used in commercial practice. The sheets were dried, and then heated to a temperature of about 170 C. for 2 minutes to insure complete reaction between the paper and the isocyanate.

Table I sets forth the results obtained when the amount of blocked isocyanate in the system is varied. The papers were impregnated at a level of about 20 parts by weight of polymer per 100 parts by weight of paper. It is clear that the wet tensile strength and wet elongation of the impregnated paper increase as the amount of blocked isocyanate in the impregnant is increased. The increase in wet tensile is quite rapid as the amount of isocyanate is increased to 20 parts by weight. The optimum amount of isocyanate in the impre'gnant is between and parts by weight. The wet tensile strength of a sheet treated with such an impregnant would be 8 to 9 times that of a control sheet which was not treated with isocyanate. Also, the effect on elongation closely follows that of the tensile.

1 Expressed in parts by weight per 100 parts by weight of polymer. 2 Expressed as pounds per inch. 3 Expressed as percent.

Example 11 10 parts by weight of the reaction product of toluene 2,4-diisocyanate, trimethylol propane and phenol were emulsified as set forth in Example I. This emulsion was then added to 100 parts by weight of a mixture which contained varying ratios of a vinyl acetate copolymer latex (same as that used in Example I) and a resinous material commercially available under the tradename of Piccopale N-2. Sheets of impregnated base paper were then treated, conditioned and tested as heretofore set forth.

The effects on the physical characteristics of a treated sheet were observed and listed in Table II. The level of total impregnant absorbed by the paper was 20 parts by 6 weight per 100 parts by weight of paper. It appears from this data that 60% to 70% extenders may be added to the impregnant without severally affecting the tensile strength or elongation of the sheet.

TABLE II Tensile Strength 2 Latex 1 Extender 1 Elongation, 3

Dry Dry Wet 1 Expressed as parts by weight of impregnant. 2 Expressed as pounds per inch. 3 Expressed as percent.

Example 111 5 parts by weight of the reaction product of toluene 2,4-diisocyanate, trimethylol propane, and phenol was emulsified as set forth in Example I. This emulsion was then added to 100 parts by weight of a polyvinyl acetate latex (same as that used in Example I). Sheets of impregnated paper were then treated at various levels of impregnation, conditioned and tested as heretofore set forth.

The results were then set forth in Table III. It appears that the wet and dry physical properties of a treated sheet are increased as the level of impregnation is raised.

TABLE III Tensile Strength 3 Elongation 4 Impregnation Amount Level 1 Blocked Isocyanatc 2 Dry Wet Dry Wet v hundred parts by weight of paper.

1 Expressed as parts by weight of impregnant per parts by weight of paper.

Example IV Varying amounts of the reaction product of toluene 2,4-diisocyanate, trimethylol propane, and phenol were dispersed in 100 parts by weight of a vinyl acetate copolymer latex by means of a ball mill. The latex used was the same as that described in Example I. Sheets of impregnation base paper were then treated with this dispersion, conditioned and tested as heretofore described.

In Table IV, the wet tensile strength of sheets treated by the emulsion technique (set forth in Example I) were compared with sheets treated by the technique used in Example IV. The significant difference between these two methods is the manner in which the blocked isocyanate is dispersed in the latex. These papers were impregnated at a level of about 22 parts by weight per It appears that no matter which technique is used the wet tensile strength of 21 treated sheet is greater than that of a control sheet which did not contain isocyanate. Also, the method of dispersing the isocyanate in the latex appears to have a significant effect on the Wet tensile strength of the sheet. The wet tensile strength of a sheet treated according to procedure set forth in Example I is greater than a sheet treated by this procedure because the blocked isocyanate is more finely distributed in the latex when it is in emulsified form.

1 Expressed as parts by weight per 100 parts by weight of polymer. 1 Expressed as pounds per inch width.

In Example V, the diphenylamine adduct of toluene 2,4-diisocyanate was prepared, emulsified and then dispersed in a vinyl acetate copolymer latex which was later used to treat paper.

Example V 17.4 grams toluene 2,4-diisocyanate were dissolved in 1000 ml. of toluene, 34 grams of diphenylamine were then added to this solution and it was stirred for 2 hours and then allowed to stand for 12 hours. The toluene was then distilled from the liquid at which time the liquid was cooled to 20 C. and filtered to separate the precipitate.

4 parts by weight of the precipitate (which was the diphenyl amine adduct of toluene 2,4-diisocyanate) was ground into a coarse powder and dissolved in 6 parts by weight of ethyl acetate. This solution was then dispersed in another solution containing 2 parts by weight of ammonium caseinate, 0.5 part by weight of sodium alkyl naphthalene sulfate, and 0.05 part by weight of alkyl aryl sodium sulfonate dissolved in 10 parts by weight of warm water. The dispersion was then homogenized and stirred until the temperature of the emulsion was below about 110 F.

4 parts by weight of this isocyanate emulsion (parts in this case meaning emulsion solids) was then added to a non-ionic vinyl acetate copolymer latex containing 100 parts by weight of polymer. This latex was the same as that utilized in Example I. The sheets were tested for wet strength (as in Example I), conditioned and tested as heretofore described and the results appear in Table V. It clearly appears that the wet tensile of an impregnated sheet increases as the level of impregnation is raised.

TABLE V Sample Blocked Impregna- Wet Tensile 3 Isoeyanate 1 tion Level 2 1 Expressed as parts by weight per 100 parts by weight of polymer. 2 Expressed as parts by weight per 100 parts by weight of paper. 3 Expressed as pounds per inch.

sulfate and 0.2 part by weight of alkyl aryl sodium sulfonate dissolved in 40 parts by weight of warm water. The dispersion was homogenized and stirred until the temperature of the emulsion was below about 110 F.

6.5 parts by weight of this isocyanate emulsion and 0.5 part by weight of a modified sodium polyacrylate were then added to an aqueous vinyl acetate copolymer latex containing 100 parts by weight of polymer and the resulting mixture was stirred until uniform. The latex is the same as that used in Example I. The sheets were treated (as set forth in Example I), conditioned, and tested as heretofore described and the results appear in Table VI. These results show the same general trend as that set forth in Table V.

TABLE VI Sample Blocked Impregna- Wet Tensile 3 Isocyanate 1 tion Level 2 1 Expressed as parts by weight per 100 parts by weight of polymer. 2 Expressed as parts by weight per 100 parts by weight of paper. 3 Expressed as pounds per inch.

In Example VI, the morpholine adduct of 4,4-methylene di-o-tolylisocyanate was prepared, emulsified, and then dispersed in a vinyl acetate copolymer latex which was later used to treat paper.

Example VII 28 grams of 4,4-methylene di-o-tolylisocyanate and grams of morpholine were mixed in 200 m1. of methyl ethyl ketone. The exothermic reaction which resulted was maintained at 40 C. after which a yellowish white precipitate was filtered off.

2 /2 parts by weight of this precipitate (which was the morpholine adduct of 4,4-methylene di-o-tolylisocyanate) was dissolved in 47 /2 parts by weight of methyl Cellosolve. This solution was then added to 50 parts by weight of water and the isocyanate was precipitated in the form of a fine dispersion.

7 parts by weight of this dispersion and 0.5 part by weight of a modified sodium polyacrylate (both on the dry basis) were added to a vinyl acetate copolymer latex containing parts by weight polymer and the resulting mixture was stirred until uniform. The latex used was the same as that used in Example I. The sheets were treated (as in Example I), conditioned, and tested as heretofore described and the results appear in Table VII. The same trend is shown that appears in Tables V and VI although a different isocyanate has been used.

TABLE VII Sample Blocked Imprcgna- Wet Tensile 3 Isoeyanate 1 tion Level 2 1 Expressed as parts by weight per 100 parts by weight of polymer. 2 Expressed as parts by weight per 100 parts by weight of paper. 3 Expressed as pounds per inch.

The following is a list of other isocyanates which have been found suited for use in this procedure. The wet tensile strengths obtained on paper which had been treated with a vinyl acetate copolymer latex containing one of these individual isocyanates have been found to be very similar to the results heretofore set forth in the various tables.

Recom- Blocked Isocyanate mended Proportion 1. Bis phenyl adduct of methylene bis(4-phenyllsocyanate) 8. 3 2. Reaction product of phenol and triphenyl methane triisocyanate 6. 7 3. Reaction product of morpholine and triphenyl methane triisocyanate 6. 5 4. Reaction product of diphenylamine and triphenyl methane triisocyanate 9.0 5. Reaction product of aceto-acetic ester and triphenyl methane triisocyanare 7. 9 6. Morphollne adduct of methylene bis(4-phenylisocyanate). 6. 4 7. Aceto-acetic ester adduct of methylene bis(4-phenylisocyanate). 7. 7 8. Diphenylamine adduct of methylene bis(4-phenylisocyanate 9.1 9. Phenol adduct of 4,4-methy1ene di-o-tolylisocyanate 7. l0. Aceto-acetic ester adduct of 4,4-methylene di-o-tolyliso cyanate. 8.4 11. Diphenylamine adduct of 4,4-methylene di-o-tolylisocy'mate 9.

1 Parts by weight per 100 parts by weight of polymer.

In many instances it is highly desirable to be able to modify the properties of a treated sheet to meet a particular requirement. For example, for dynamic uses, the tensile-product of the paper is of prime importance rather than the tensile alone. The tensile-product is the product of the tensile per inch width of the paper multiplied by the percent elongation. High tensile-products may be accomplished by varying the ingredients in the formulation which is used to treat the paper. To be more specific, it has been found that the elongation and tensile strength of a treated sheet may be varied when synthetic polymeric systems, such as butadiene-styrene and butadiene acrylonitrile are incorporated into the compound. In such instances, however, the addition polymer must be non-ionic in nature. If, however, it is not possible to use a non-ionic system for the polymer, a system should be used wherein each of the components, in and of itself, is stable. The component which initially functions as a carrier for the isocyanate must be non-ionic in nature, while the other component may contain a material which is ionic in nature. When mixed, these two components will form an unstable system due to the effect of the ionic component upon the blocked isocyanate. Before destabilization has occurred to any great extent, this fluid system is used to treat paper in the manner heretofore described. Normal take up of the compound by the paper during treatment will consume the compound before substantial destabilization takes place and further destabilization will then proceed after the polymer is in place in the paper.

A typical two component system is set up in Example VIII.

Example VIII Component A.4 parts by weight of the reaction product of toluene 2,4diisocyanate, trimethylol propane and phenol were emulsified in a manner similar to that in Example I. This emulsion and 0.2 part by weight of a modified sodium polyacrylate were then dispersed in 30 parts by weight of a vinyl acetate copolymer latex. This latex was the same as that in Example I.

Component B.0.5 part by weight of ammonium caseinate was added, with stirring, to 70 parts by weight of a 50:50 butadiene styrene synthetic rubbery polymeric latex. Subsequently 2 parts by weight of an antioxidant and 0.3 part by weight of the sodium salt of tetraacetic acid ethylenediamine was added to this liquid. The systern was mixed and 1% of ammonia (based on the wet wet of the system) was added thereto with stirring.

Component A was uniformly dispersed in Component B and the resulting fluid mixture was used to treat paper in the manner heretofore described.

The paper was then tested and it was found that the tensile product was even greater than that obtained when a similar paper was treated with the basic system heretofore described (which contained only polymers or copolymers of vinyl acetate). To verify this increase, a series of fluid compounds prepared according to the procedure set forth in Example VIII were used to treat paper. In each case, including the controls, the amount of polymeric material present in the individual compound was varied. Also, in the case of the control strips, the use of isocyanate was omitted from the treatment. The amount of isocyanate used is based upon parts by weight in a latex having 100 parts by weight of polymer. Sheets treated with these compounds were tested for tensile strength and a summary of the results is set forth in Table VIII. It appears that an increase in wet tensile strength is obtained when a 50:50 copolymer of buta diene-styrene synthetic polymer is substituted for the vinyl copolymer acetate latex in the basic formulations. As is also apparent, a higher level of wet tensile may be obtained when mixtures of the vinyl acetate copolymer with 50:50 butadiene styrene copolymer are used.

One of the basic advantages of the two component system is that destabilization of the blocked isocyanate is initiated at relatively low temperatures. The mixed system, in any case, however, is stable for at least 24 hours after mixing. Therefore, the paper may be impregnated with the polymeric system (including the isocyanate) and destabilization will take place while the polymer is in place in the paper. It has been found that when a twocomponent system is used that a satisfactory cure may be obtained at temperatures as low as about 120 F. in as little as 3 days. This is especially adaptable to paper making practices because the paper may be treated, rolled warm, and held for several days prior to shipment.

In the following Table IX a comparison of the wet tensile is made on identically treated samples except that one was cured at 350 F. for 2 minutes, while the other was cured at 120 F. for 3 days. In any case the paper was treated with material prepared according to Example VIII with the exception that the amount of isocyanate used and the impregnation level were varied as indicated. It appears that between to of the wet tensile obtained by a high temperature cure is achieved even when a low temperature cure is used.

TABLE IX Parts by Impregnation Wet Tensile, weight Isocy- Level, parts pounds per Elongatlons anate Used by weight of square inch per 100 parts Impregnant by weight per 100 parts of Polymer by weight 350 F. F. 350 F. 120 F.

of Paper As is apparent from the aforesaid data, the physical properties of this treated paper are improved for both dry and wet applications. The great increase in dry elongation results in paper having a very high tensile-product, a property highly desirable for dynamic uses such as in multiwall bags. The phenomenal increase in wet strength makes the paper useful for packaging wet products which can not otherwise be packed successfully in paper bags. These improvements in the physical properties of paper are of extreme importance to industry. It is known that today over one and one-half billion pounds of paper are required for the normal yearly production of multiwalltype bags. This is primarily due to the fact that it has been found necessary to utilize as many as 6 plies of paper in the production of these type bags. As many as 6 plies are required in order to insure that the sidewall has the desired degree of strength. However, by utilizing paper treated in the aforesaid manner, this poundage requirement could be substantially reduced. This could be done by substantially reducing the number of plies and consequently the weight required in this type paper bag. By utilizing paper treated in the aforesaid manner, these bags may be produced with as little as 4 plies and, in many cases, with as little as 2 plies. In many cases, even though the number of plies of such bags are reduced, the wet strength of these bags could be increased anywhere from four to five hundred percent. This is only one particular instance in which the present treatment is useful, but this is only by way of illustration and this invention should not be limited thereto.

We claim:

1. A method of improving the physical properties of cellulose paper which comprises: impregnating said paper with a composition consisting essentially of a substantially non-ionic polymeric vinyl acetate latex wherein the chain portion of the vinyl acetate polymer contains hydrogen and hydroxy groups as substituents on said chain, said groups being capable of reacting with isocyanate groups, and an aqueous emulsion of a polyisocyanate adduct which is stable in the presence of the non-ionic latex and non-reactive until heated to a temperature which regenerates free isocyanate groups in reactive form, substantially drying the treated paper at a temperature below the temperature at which free isocyanate groups are regenerated, and heating the dried paper to a temperature above about 140 C. for a period of time sufficient to provide free isocyanate groups which react with the hydrogen and hydroxy groups in the paper and the polymer.

2. A method of improving the physical properties of cellulose paper which comprises: impregnating said paper with a composition consisting essentially of a substantially non-ionic polymeric vinyl acetate latex wherein the chain portion of the vinyl acetate polymer contains hydrogen and hydroxy groups as substituents on said chain, said groups being capable of reacting with isocyanate groups, and an aqueous emulsion of between 2.5 and 123 parts by weight based on 100 parts by weight of said polymeric latex solids, of a polyisocyanate adduct selected from the groups consisting of the phenol adducts of triphenyl methane triisocyanate, toluene-2, 4-diisocyanate, and 4,4-methylene di-o-tolylisocyanate; the aceto-acetic ester adducts of methylene bis(4-phenylisocyanate), triphenyl methane triisocyanate, toluene-2,4-diisocyanate, and 4,4-methylene di-o-tolylisocyanate; the morpholine adducts of triphenyl methane triisocyanate, 4,4-methylene di-o-tolylisocyanate, and methylene bis(4-phenylisocyanate); and the adduct of toluene-2,4-diisocyanate, trimethylol propane and phenol; said adduct being stable in the presence of the non-ionic latex and non-reactive until heated to a temperature which regenerates free isocyanate groups in reactive form, substantially drying the treated paper at a temperature below the temperature at which free isocyanate groups are regenerated, and heating the dried paper to a temperature above about 140 C. for a period of time sufiicient to provide free isocyanate groups which react with the hydrogen and hydroxy groups in the paper and the polymer.

3. The method according to claim 2 wherein the polymeric component of the latex is comprised of a vinyl acetate-dibutyl maleate copolymer.

4. The method according to claim 3 wherein between 2.5 and 50 parts by weight per 100 parts by weight of said copolymer latex solids of a polyisocyanate adduct are used.

5. The method according to claim 4 wherein the adduct is the diphenylamine adduct of toluene-2,4-diisocyanate.

6. The method according to claim 4 wherein the adduct is the aceto-acetic ester adduct of toluene-2,4-diisocyanate.

7. The method according to claim 4 wherein the adduct is the morpholine adduct of 4,4'-methylene di-o-tolylisocyanate.

8. The method according to claim 4 wherein the adduct is the adduct of toluene-2,4-diisocyanate, trimethylol propane and phenol.

9. A method of improving the physical properties of cellulose paper which comprises: impregnating said paper with a composition comprised of between 50 to 100 parts by weight of substantially non-ionic vinyl acetate copolymer latex solids wherein the vinyl acetate chain portion of said copolymer contains hydrogen and hydroxy groups as substituents on said chain capable of reacting with isocyanate groups, an aqueous emulsion of about 4 parts by weight of the adduct of toluene-2,4-diisocyanate, trimethylol propane and phenol, said adduct being stable in the presence of the non-ionic latex and non-reactive until heated to a temperature which regenerates free isocyanate groups in reactive form, and up to 50 parts by weight of a substantially non-ionic butadiene-styrene copolymer latex; substantially drying the treated paper at a temperature below the temperature at which free isocyanate groups are regenerated; and heating the dried paper to a temperature in the range of about 120 F. to 350 F. for a period of time ranging from about 2 minutes to 3 days to provide free isocyanate groups which react with the hydrogen and hydroxy groups in the paper and the vinyl acetate copolymer.

References Cited by the Examiner UNITED STATES PATENTS 2,570,253 10/1951 Lindquist 260-7 8.5 2,897,094 7/1961 Hayes et a1 117l55 2,994,671 8/1961 Thompson 26029.7 2,994,672 8/1961 Geerdes 117155 3,001,957 9/1961 Kray et al l17155 3,005,728 10/1961 Bridgeford 8116.2 X 3,092,601 6/1963 Sullivan et a1 117-155 NORMAN G. TORCHIN, Primary Examiner.

MORRIS O. WOLK, Examiner.

Claims (2)

1. A METHOD OF IMPROVING THE PHYSICAL PROPERTIES OF CELLULOSE PAPER WHICH COMPRISES: IMPREGNATING SAID PAPER WITH A COMPOSITION CONSISTING ESSENTIALLY OF A SUBSTANTIALLY NON-IONIC POLYMERIC VINYL ACETATE LATEX WHEREIN THE CHAIN PORTION OF THE VINYL ACETATE POLYMER CONTAINS HYDROGEN AND HYDROXY GROUPS AS SUBSTITUENTS ON SAID CHAIN, SAID GROUPS BEING CAPABLE OF REACTING WITH ISOCYANATE GROUPS, AND AN AQUEOUS EMULSION OF A POLYISOCYANATE ADDUCT WHICH IS STABLE IN THE PRESENCE OF THE NON-IONIC LATEX AND NON-REACTIVE UNTIL HEATED TOA TEMPERATURE WHICH REGENERATES FREE ISOCYANATE GROUPS IN REACTIVE FORM, SUBSTANTIALLY DRYING THE TREATED PAPER AT A TEMPERATURE BELOW THE TEMPERATURE AT WHICH FREE ISOCYANATA GROUPS ARE REGENERATED, AND HEATING THE DRIED PAPER TO A TEMPERATURE ABOVE ABOUT 140*C. FOR A PERIOD OF TIME SUFFICIENT TO PROVIDE FREE ISOCYANATE GROUPS WHICH REACT WITH THE HYDROGEN AND HYDROXY GROUPS IN THE PAPER AND THE POLYMER.

9. A METHOD OF IMPROVING THE PHYSICAL PROPERTIES OF CELLULOSE PAPER WHICH COMPRISES: IMPREGNATING SAID PAPER WITH A COMPOSITION COMPRISED OF BETWEEN 50 TO 100 PARTS BY WEIGHT OF SUBSTANTIALLY NON-IONIC VINYL ACETATE COPOLYMER LATEX SOLIDS WHEREIN THE VINYL ACETATE CHAIN PORTION OF SAID COPOLYMER CONTAINS HYDROGEN AND HYDROXY GROUPS AS SUBSTITUENTS ON SAID CHAIN CAPABLE OF REACTING WITH ISOCYANATE GROUPS, AN AQUEOUS EMULSION OF ABOUT 4 PARTS BY WEIGHT OF THE ADDUCT OF TOLUENE-2,4-DIISOCYANATE, TRIMETHYLOL PROPANE AND PHENOL, SAID ADDUCT BEING STABLE IN THE PRESENCE OF THE NON-IONIC LATEX AND NON-REACTIVE UNTIL HEATED TO A TEMPERATURE WHICH REGENERATES FREE ISOCYANATE GROUPS IN REACTIVE FORM, AND UP TO 50 PARTS BY WEIGHT OF A SUBSTANTIALLY NON-IONIC BUTADIENE-STYRENE COPOLYMER LATEX; SUBSTANTIALLY DRYING THE TREATED PAPER AT A TEMPERATURE BELOW THE TEMPERATURE AT WHICH FREE ISOCYANATE GROUPS ARE REGENERATED; AND HEATING THE DRIED PAPER TO A TEMPERATURE IN THE RANGE OF ABOUT 120*F. TO 350*F. FOR A PERIOD OF TIME RANGING FROM ABOUT 2 MINUTES TO 3 DAYS TO PROVIDE FREE ISOCYANATE GROUPS WHICH REACT WITH THE HYDROGEN AND HYDROXY GROUPS IN THE PAPER AND THE VINYL ACETATE COPOLYMER.