US3492081A - Method of treating paper with isocyanates blocked with cyclohexanol - Google Patents

Description

United States Patent 3,492,081 METHUD 0F TREATING PAPER WITH ISOCYA- NATES BLUCKED WlTH CYtILOHEXANUL Alfred .l. Morak, Appleton, Wis., assignor, by mesne assignments, to Container Corporation of America, Chicago, 111., a corporation of Delaware No Drawing. Filed June 8, 1965, Ser. No. 462,437 Int. Cl. D06m 13/54, 13/34 US. Cl. 8-1161 Claims ABSTRACT OF THE DISCLOSURE Treating cellulosic paper with blocked isocyanates and thereafter heating to produce the isocyanate in situ and effect cellulose cross-linking.

The present invention relates generally to paper and paper products, and more particularly it relates to a method of treating paper and paper products to improve the properties of the paper, and to paper treated in accordance with such a method.

Paper and paper products, referred to generally herein as paper, may be made from pulp manufactured by several known pulping processes, the best known of which are the kraft or sulfate process, the sulfite process, the soda or caustic process, the semi-chemical processes, and the ground Wood process. The pulp obtained from the pulping process is formed into sheets or other shapes suitable for the desired end use in accordance with known procedures.

The paper obtained from the various processes may have diiferent characteristics and properties depending on the process employed in pulp preparation and any treatments to which the pulp or paper is subjected to improve the properties thereof.

Processes are known for treating pulp and paper and/ or modifying their characteristics. These processes are generally intended to improve one or more of the physical or mechanical properties of paper, such as wet strength, bursting strength, tensile strength, ring compression, stifiness and folding endurance. The properties of paper may be improved by coating or impregnating the paper with a suitable impregnant, or by chemically reacting the cellulose constituent of the paper with a suitable chemical. Generally, the various processes that are known for improving the properties of paper improve only one of the properties of the paper, and may adversely affect other properties of the paper. In this connection, many of the processes for treating paper, particularly those Where the cellulose is reacted with a chemical compound, produce undesirable side reactions which may destroy the fiber characteristics of the paper or which may adversely affect other properties of the paper. Further, many such chemical processes require the use of a catalyst which remains in the treated paper and which, in some instances, must be removed from the paper in order to prevent subsequent undesired chemical reaction between either the paper or the catalyst.

It is generally known that cellulose may be cross-linked with a diisocyanate, the diisocyanate reacting with the free hydroxyl groups of the cellulose. Cloth and yarn fibers have been cross-linked with diisocyanates to improve water repellency by immersion in a solution of a diisocyanate dissolved in a suitable solvent. It is also known to treat paper by immersion in a solution of a diisocyanate to improve the wet strength of the paper. However, liquid phase cross-linking of paper with a diisocyanate generally requires the use of a suitable catalyst, such as pyridine in order to obtain desired results. Also,

the liquid phase cross-linking of paper with a solution of diisocyanates may produce undesired side reactions between the diisocyanate and impurities present in the paper, and it is diflicult to obtain even distribution of the diisocyanate cross-linking agent on the surface of the paper since the diisocyanate is highly reactive and reacts generally with the first hydroxyl radical with which it comes into contact.

It is a principal object of the present invention to provide paper having improved properties. A further object is to provide a method of treating paper to improve substantially all of the mechanical properties of the paper. An additional object is to provide a method of treating paper which does not require the use of a catalyst. Yet another object is to provide a method of treating paper which is convenient and economical and which may be employed to improve the mechanical properties of substantially all types of paper and paper products.

These and other objects of the invention will become apparent from a study of the following detailed description.

Generally, the present invention is directed to a method of treating paper to improve the mechanical properties thereof comprising, impregnating the paper with a nonreactive adduct of an organic diisocyanate and a blocking agent, and heating the paper to decompose the adduct to the corresponding diisocyanate, which diisocyanate cross-links the cellulose constituents of the paper; and to a product obtained by such a method. The adducts of the organic diisocyanate and the blocking agent have decomposition temperatures below about C., at which temperatures the adducts decompose to the corresponding diisocyanates. The impregnated paper is heated to a suitable temperature above the decomposition temperature of the adduct, whereupon the diisocyanate causes cross-linking of the cellulose constituents of the paper.

The method of the present invention may be carried out to effect cross-linking of paper and to provide a paper product having substantially improved bursting strength, tensile strength, ring compression and stiffness, a measured in accordance with conventional testing procedures.

The paper may be treated before or after being converted, and in some instances it may be desirable to treat the paper while in pulp form. The method of the present invention allows the diisocyanate to be evenly distributed throughout the paper while in the form of an unreactive adduct which insures even distrubution of the crosslinking agent throughout the paper and uniform crosslinking When the adduct is decomposed. The adducts are easier to handle than are diisocyanates since they are not reactive with water and other compounds containing active hydrogens, and are usually less toxic than the diisocyanates. The method may also be conveniently carried out in the absence of a catalyst which is generally considered to be necessary for conventional liquid phase crosslinking of cellulose.

The nonreactive adducts may be prepared by reacting an organic diisocyanate with a blocking agent. For purposes of the present invention, the term nonreactive adduct is defined as a reaction product, which is not reactive with an active hydrogen, of an organic diisocyanate and a blocking agent selected from the group consisting of ROH, RCOOH, R R NH and MSO H, where R, R and R may be hydrogen, an alkyl radical of from one to 18 carbon atoms, or an aryl radical or a hydrogenated aryl radical having from 6 to 12 carbon atoms, and M is a cation. The alkyl and aryl radicals may be unsubstituted or may be substituted, for example with methyl, methoxy, nitro or amino radicals. Specific examples of blocking agents that may be employed within the scope of the present invention include, but are not limited to ethanol, cyclohexanol, p-methoxyphenol, p-

cresol, guaiacol, p-nitrophenol, acetic acid, sodium bisulfite, and ammonium bisulfite.

In order that the nonreactive adduct will not react with the free hydroxyl groups of the cellulose, the diisocyanate is preferably reacted with at least a stoichiometric amount of the blocking agent to insure that both isocyanate groups of the diisocyanate will be blocked.

Any suitable organic diisocyanate may be employed, and the diisocyanate may be an aliphatic diisocyanate or an aromatic diisocyanate as desired. Suitable examples of aliphatic and aromatic diisocyanates which may be employed within the scope of the present invention include: ethylene diisocyanate, propylene diisocyanate, butylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, decamethylene diisocyanate, trans-vinylene diisocyanate, cyclohexylene diisocyanate, o-phenylene diisocyanate, tolylene 2,4 diisocyanate, tolylene 2,6- diisocyanate, diphenylmethane 4, 4 diisocyanate, naphthalene diisocyanates. Tolylene diisocyanates and hexamethylene diisocyanates have been found to be particularly desirable. It is apparent that other diisocyanates are also contemplated.

The nonreactive adduct may be prepared by dissolving the blocking agent in a suitable solvent, such as reagent grade chloroform, in the presence of a catalyst such as 0.5 percent by Weight of the diisocyanate of triethylamine. For some adducts neither solvent nor catalyst are necessary. At least a stoichiometric amount of diisocyanate diluted with chloroform is then added dropwise to the solution of the blocking agent.

The reaction is usually slightly exothermic. After the reaction has been completed, the solution of the adduct is refluxed for two or three hours and cooled. Petroleum ether is added to the solution to cause precipitation of the adduct which is purified by recrystallization. It is apparent that other methods of preparing the adduct may be employed, and the particular method of manufacturing the adduct is not considered to be an essential part of the invention.

The adduct is dissolved in a suitable solvent such as pyridine, chloroform, acetone, alcohols, etc., and the paper to be treated is immersed in the solution. Alternately, the solution may be sprayed onto the paper, or any other convenient method may be employed to insure that the paper is impregnated with the adduct. It is generally desired to saturate the paper. No particular length of immersion of the paper in the solution of the adduct is necessary, so long as the paper is evenly impregnated.

The adduct solution preferably has a concentration such that the impregnated paper contains at least about 5 percent by weight, dry basis, of the diisocyanate preferably at least about percent. At levels much greater than about 12 percent, the improvement obtained does not warrant the use of additional amounts of the adduct.

The impregnated paper is then heated to a temperature that is above the decomposition temperature of the adduct to cause decomposition of the adduct to the corresponding diisocyanate which reacts with the free hydroxyl radicals of the cellulose to cause cross-linking of the paper.

The paper is not heated to a temperature above about 200 C., since at temperatures above 200 C. the paper may be adversely affected by the heat treatment. It has been found that most adducts will decompose at a temperature between about 100 C. and about 200 C., and accordingly, it is usually not necessary to heat the impregnated paper to a temperature outside this range. However, it is apparent that higher or lower tempertaures may be employed to cause decomposition of particular adducts.

It can be seen from the foregoing that the particular temperature to which the impregnated paper is heated is not critical, so long as the paper is not adversely affected by heating at the selected temperature.

Also, it has been found that in some cases it is preferable to retain the blocking agent within the paper since the mechanical properties of the paper are improved to some extent by the presence of the blocking agent in the paper as an impregnant, although the improvement obtained by the presence of the blocking agent is less per unit than the improvement obtained by cross-linking of the paper with the diisocyanate. Accordingly, in a preerred form of the method, the impregnated paper is heated to a temperature which causes decomposition of the adduct, but which does not cause substantial vaporization of the blocking agent. In order to retain the greatest amount of the blocking agent within the paper, it has been found to be desirable, in some instances, to carry out the heating of the paper in an autoclave at higher pressures in order to retain volatile materials within the paper.

The exact mechanism by which the diisocyanates liberated upon decomposition of the adduct are reacted with the cellulose constituents of the paper is not altogether understood. It may be that the reaction involves a two step process with an initial unimolecular splitting, or the reaction may involve a direct bimolecular displacement. The precise mechanism by which the reaction proceeds is not believed to be important in understanding the present invention. It is apparent, however, that a reaction does occur and that the diisocyanate does cross-link the cellulose constituents of the paper as is borne out by the experiments set forth below.

The impregnated paper is heated for a period of time sufficient to effect substantially complete decomposition of the nonreactive adduct. It is not believed that the length of the heating of the impregnated paper is particularly critical. However, for best results, the heating should be continued for at least one-half hour, preferably between about one-half hour and about 2 hours. Additional heating beyond about 2 hours does not appear to provide any particular additional improvement in the mechanical properties of the paper.

In carrying out various experiments, the results of which are set forth below to determine the improvement of the mechanical properties of the paper by the method of the present invention, the bursting strength was measured in accordance with TAPPI Method No. T403; the tensile strength was measured with an Instron Tensile Tester utilizing a one inch wide specimen of five inches in length as a test span, excepting the samples of Table III where a three inch test span was employed, and a cross-head speed of one inch per minute was employed; the ring compression was determined in accordance with ASTM Method No. D1164; and the Taber Initial Stiltness was determined in accordance with TAPPI Method No. T-489 at 73 F.

The paper samples employed in the experiments were taken from an 8 /2 inch wide roll obtained from a production roll of linerboard. Several samples of untreated hnerboard were tested and the properties thereof were averaged in order to obtain a suitable control.

The adducts of the diisocyanate and the blocking agent were prepared in accordance with the previously described method. In some cases, the blocking agent provides a solvent function as well as a reactant in the formation of the adduct, as for example in the use of ethanol as the blocking agent. For cyclohexanol adducts chloroform was used as a solvent. For p-nitrophenol adducts a 1:1 mixture of acetone and ethanol was employed as a solvent; the acetic acid adducts were prepared employing a petroleum ether solvent, the sodium bisulfite and ammonium bisulfite adducts were utilized with an aqueous solvent, and the remaining adducts were prepared utilizmg acetone as the solvent.

The paper samples were immersed in the adduct solution in a flat stainless steel tray. About 22 milliliters of solvent containing the calculated amount of solute was placed in the trap and the paper was allowed to absorb the entire amount of the solution. The impregnated paper was then air dried under a hood for 30 minutes. Nitrogen determinations, made in accordance with the A.O.A.C. method showed that the paper contained substantially all of the theoretical amount of adduct and random sampling indicated that the paper was uniformly impregnated With the adduct. The impregnated paper samples were then cured in a forced draft oven at a temperature of 147 C. Five minutes was allowed for the samples to reach an equilibrium in the oven at 147 C., which point was considered zero time for the heat treatmerit.

There is set forth in Table I the results of experiments carried out to determine the improvement of the mechanical properties of paper impregnated with a nonreactive adduct of a diisocyanate followed by heat treatment of the impregnated paper to effect decomposition of the adduct to the corresponding diisocyanate which causes provement that is effected by the heat treatment alone (sample 2) and over the improvement that is provided by treatment of the paper with a diisocyanate alone (samples 3, 4, 5, and 6) and the blocking agent alone (samples 8, 10, 12, 14 and '16). All of the treatments except samples 18 and 19 were carried out by heat treating the impregnated samples at 147 C. for threequarters of an hour. All of the samples which included treatment of the paper with an adduct of a diisocyanate substantially improved the stiffness and ring compression of the linerboard. The cyclohexanol adduct appears to provide the best ring compression and stiffness whereas the p-nitrophenol produces a somewhat lower ring compression than the other phenol adducts.

Table II illustrates the improvement obtained when varying concentrations of the adduct are present in the impregnated paper.

TABLE II Instron tensile strength (lb./in). Ring Compression, lbs. Taber Initial Stiffness gtmstiphg 50% Rel. Hum. 50% Rel. Hum. 80% Rel. Hum. 50% Rel. Hum. 80% Rel. Hum.

reng

Sample Treatment p.s.i.g. M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D

25.. Control, untreated 94. 5 80. 6 37. 5 107. 0 74. 2 72. 8 46.4 80. 0 24. 0 76. 0 22. 0 6 Control, hr., 85. 0 36. 6 110. 9 78. 6 84. 1 54. 5 92. 0 27. 0 83. 0 19. 6 103. 4 40. 6 134. 0 87. 6 97. 4 64. 8 105. 6 34. 2 89. 0 25. 6

28 IOfiZ p-fnethoxyphenol-TDI, hi1, 95.7 102. 6 42.9 141. 8 101. 6 106.8 76.0 114. 8 40.4 91.0 26. 7 29 15.14;? g-tnethoxyphenol-TDI, hr., 92.0 106.4 44.8 144. 4 109. 5 107. 8 78. 6 127. 6 41.9 82.1 27. 8 19163, Imethoxyphenol-TDI, hr., 95.9 116. 2 42. 7 155. 4 119.6 122.9 94. 2 111.3 46. 4 102.9 30. 5 31 5.3% p-eresol-TDI, hr., 147 Cg.. 88.0 101.5 40. 3 126.7 88.4 96. 6 67.0 104. 5 32.9 80.0 23.1 32. 10.1% p-cresol-TDI, A l11'., 147 0.- 92. 9 104. 8 41.3 128. 3 91.5 98. 5 73. 7 101.7 36.0 81. 6 25. 4 33. 14.4% p-cresol-TDI, hr., 147 0.... 92. 2 106.2 41. 5 145. 9 103. 7 119. 7 77. 8 104. 5 37. 1 89. 2 27. 2 34. 18.3% p-cresol-TDI, hr., 147 (3-- 91. 6 106.1 43. 1 158. 9 104.0 117. 6 81. 0 112. 6 39. 0 89.7 27.6 35. 5.7% guaiacol-TDI, hr., 147 C 89. 5 102. 9 42. 0 115. 4 87. 3 91. 9 61. 4 91.2 34. 0 80. 6 22. 7 36. 10. 8% gualacol-TDI, hr., 147 C 87. 0 104. 2 40. 2 133. 2 91. 4 101. 2 70. 3 104. 5 33. 6 77. 1 22. 8 37. 14.5% gua 1acol-TDI hr., 147 C... 97. 7 102.2 43. 4 144.2 83.2 104. 9 77. 2 103. 4 34.2 79. 7 24.0 38 19.6% gualacol-TDI, hr., 147 C 89.4 110. 1 44.5 164. 9 110.6 120. 6 85. 5 114. 1 39. 3 90.6 25. 9

cross-linking of the cellulose constituents of the 'paper. The various amounts of the adducts employed in the experiments set forth in the tables were calculated as percent by weight, dry basis, of the paper sample.

It can be seen that, in general, the mechanical prop erties of the paper are improved to a greater extent as the amount of adduct in the paper increases. However,

TABLE I Instron tensile strength (lb./in). Ring Compression, lbs. Taber Initial Stiffness slgurstfig Rel. Hum. 50% Rel. Hum. 80% Rel. Hum. 50% Rel. Hum. 80% Rel. Hum.

reng Sample Treatment p.s.i.g. M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D

1 Control, untreated 94. 5 80. 6 37. 5 107. 0 74. 2 72. 8 64. 4 80. 0 24. 0 76. 0 22.0 2 Control, hr. 147 C... 85. 0 36. 6 119. 9 78. 6 84. 1 54. 5 92. 0 27. 0 83. 0 19. 6 3 9.58% IDI hr., 147 C. 107. 4 116.0 47. 8 121.4 84.1 97.4 64. 8 4 10.32% TDI, 4 hr., 147 C. 91. 9 107. 9 44. 2 127. 8 86. 2 90. 6 58. 9 5 9.6% HMDI hr., 147 0.. 106. 2 117. 8 46. 8 136. 0 90. 0 94.8 66. 1 86. 6 32. 1 79. 3 22. 4 6 10.3% HMDI hr., 147 C 122. 6 139. 5 58. 2 161. 1 115. 6 116. 8 79.8 93. 7 40. 2 84.0 31. 0 7 19. 5? g-methoxyphenol-TDI, hr., 100.0 99. 4 44.0 165.4 131. 7 121. 8 95. 9 143.0 45.0 112.6 35. 2

14 12.5% p-methoxyphenol, hr., 147 C 119. 2 79. 0 80. 6 52. 7 89. 0 25. 0 81. 5 20. 4 17. 7% cyclohexanol-TDI, 4111:, 147 C. 101. 0 107. 0 44. 0 180. 1 137. 1 139. 4 107. 2 132. 0 48. 0 119. 6 37. 2 10.3% oyclohexanol, h1'., 147 C 118. 6 77. 1 83. 8 53. 5 88. 0 25. 0 80. 4 20. 6 18.3% p-cres0l-TDI, hr., 147 C. 155. 5 112. 1 118. 6 93. 8 135. 0 42. 0 91.3 27. 1 11 0 p-cresol, hr., 147 C 115. 1 75.6 76.3 49. 6 95. 0 25.0 75. 2 19. 3 19.5 a guaiacol-TDI, hr., 147 0.. 96.0 95. 0 41. 9 163. 5 126. 5 122.2 95.0 115.0 44. 0 106. 2 32. 2 13.4% guaiacol, hr., 147 C 117. 4 79. 6 79. 9 52. 6 80. 0 24. 0 79. 0 20. 3 18% p-nitrophenol-TDI, hr., 147 C 87.3 86. 5 37. 8 131.8 85. 8 78. 1 50. 6 106. 4 33. 1 82. 4 21. 8 13.5% p-nitrophenol, hr., 147 C 113. 0 91.0 42. 7 132. 2 80.7 105.0 30.0 14.6% acetic acid-HMDI, hr., 147 C- 86. 1 105. 8 45. 3 136. 4 89. 6 84. 6 50. 2 94. 3 33. 4 77. 8 20. 7 8.6% NaHSO -HNDI, 1% hr., 147 C- 92. 7 95. 3 41. 1 130. 0 86. 0 75.4 51. 6 95.2 35. 6 73. 1 22.1 75. 1 95. 0 38. 8 134. 6 94. 6 86. 0 64. 2 95. 7 33. 7 76. 8 22. 2

1 tolylene diisocyanate. 2 hexamethylene diisocyanate.

Table I illustrates the substantial improvement in the the improvement per unit of nitrogen addedto the paper properties of the paper obtained by the method of the decreases as the amount of adduct present in the paper present invention. Substantially all of the mechanical properties of the paper are improved by impregnating the paper with a nonreactive adduct of a diisocyantte and a blocking agent followed by decomposition of the adduct.

increases even though the improvement of the mechanical properties of the paper increases. The improvement in the mechanical properties of the paper increases as the amount of adduct is increased from about 5 percent to about 20 The properties of the paper are improved over that impercent in substantially all cases.

7 8 Table III illustrates the effect of the length of heat Various of the features of the invention are set forth treatment of the impregnated paper. in the following claims:

TABLE III Instron Tensile Ring Compression, lbs. Taber Initial Stiffness Strength (lb./in.)

50% Rel. Hum. 50% Rel. Hum. 80% Rel. Hum. 50% Rel. Hum. 80% Rel. Hum.

Sample Treatment M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D.

44 Control, hr., 147 97. 6 43. 8 110. 4 73.0 68. 6 44. 4 99. 0 27. 0 79. 0 20. 0 45 Control, 4 hr., 147 0 98.0 44. 0 107.1 72.4 81. 6 46.2 102.0 27.0 82.0 22.0 46 Control, 12 hr., 147 C 99. 9 44. 6 113.4 76.1 81. 3 52.2 103. 0 26. 0 82. 0 21.0 47 7.9 7 g-methox henol-TDI, hr., 104.7 45.9 126.0 78.4 99.3 68.1 109.0 37.0 97.0 28.0 48 7.9% gmethoxyphenol-TDI, 4 hr., 115.1 46.3 127.2 90.0 94.4 66.8 115.0 32.0 89.0 26.0

147 49 7.9:; (pj-methoxyphenol-TDI, 12 hr., 117.3 47.4 131.6 94.2 97.8 67.0 110.0 32.0 88.0 24.0

1 1 50 145 75 (p-methoxyphenol-TDI, hr., 118.1 49.6 151.5 107.4 112.2 78.2 140.0 48.0 105.0 36.0 51 145g Op-methoxyphenol-TDI, 4 hr., 109.7 47.1 137.6 96.8 98.1 67.4 124.0 33.0 99.0 28.0 52 14.7% 3-methoxypl1enol-TDI, 12 hr., 116.0 49.5 140.8 93.3 99.3 66.2 110.0 33.0 92.0 26.0

It can be seen that increasing the length of the heat I claim: treatment of the impregnated paper does not produce a 1. A method of treating cellulosic paper to improve its corresponding increase in the improvement of the memechanical properties comprising the steps of contacting chanical properties of the paper. It was also determined the paper with a treating agent consisting of an adduct that the amount of nitrogen present in the samples deof an organic diisocyanate which cross-links cellulose and creased upon extended periods of heat treatment. This a blocking agent comprising cyclohexanol, substantially is believed to be due to the volatility of the blocking all of the isocyanate groups of the adduct being reacted agents, and at least a partial vaporization of the diwith the blocking agent so that the adduct is not reactive isocyanate during longer heat treatments. with the free hydroxyl groups of the paper, and thereafter TABLE Iv Instron Tensile Strength (lb./in.) Ring Compression, lbs. Taber Initial Stiffness SBursting 50% Rel. Hum. 50% Rel. Hum. 80% Rel. Hum. 50% Rel. Hum. 80% Rel. Hum.

trength, Sample Treatment p.s.i.g. M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D. M.D. C.D.

58 Control, hr., 147 (1 94.3 97.2 38.5 108.6 78.9 76.3 53.9 67.0 26.0 67.0 18.0 59 13.0%guaiacol-TDI,%hr.,147 95.6 101.0 43.5 141.8 95.4 113.8 81.5 115.0 49.0 103.0 35.0 60. 14.8% p-cresol-TDI, %hr., 147 107.9 102.6 41.2 133.9 100.0 108.8 86.6 116.0 37.0 98.0 42.0 61 18.8% NaHSO -HMDI, 1% hr., 147 0. 76.2 89.2 37.1 134.4 93.7 92.5 63.9 114.0 50.0 84.0 24.0

Table IV ilustrates the results attained when the samheating the paper to a temperature above the decomposiples were cured under pressure in an autoclave as Option temperature of the adduct, to effect cross-linking of posed to a forced draft oven. The pressure within the the cellulose through the liberated organi diisocyanate. autoclave was maintained at about p.s.i.g. It will be 2. The method of claim 1 in which the organic diseen that the curing of the samples in an autoclave reisocyanate comprises tolylene diisocyanate. tains a greater percentage of volatile matter within the 3. The method of claim 1 in Which the organic diisopaper as an impregnate, and accordingly, the total im- 50 cyanate comprises hexamethylene diisocyanate. provement of the mechanical properties of the paper is 4. The method of claim 1 in which the paper is heated in rea d, to a temperature of between about 100 C. and 200 C.

It can be sen that a method has been provided which for a period of at least about /2 hour. improves substantially all of the mechanical properties 5. The method of claim 4 in which the paper is heated of paper. The method is particularly convenient in that at a pressure of at least about 50 p.s.i.g. the nonreactive adduct of diisocyanate and a blocking agent are easy to handle and allow even distribution of References it d :fie adduct throughbout the plapertbeinlgotrfated. 1Further, UNITED STATES PATENTS e process may e came on wr u emp oying a catalyst for the cross-linking reaction and the actual Schlrm "gigg gcross-linking reaction takes place in the absence of a sol- 3040002 6/1962 i i e a vent, the solvent having been vaporized from the paper 3233962 2/1966 8 'lfz 111 2:331 f fggz below the dewmpwtlon 3,238,010 3/1966 Habib et a1. 8-116.2 XR

Various features of the invention have been set forth GN ATENTS with particularity in order to describe the invention. Al- 716,233 9/ 1954 Great Britain ternative embodiments within the skill of the art are contemplated. For example, adducts of diisocyanate with GEORGE LESMES Pnmary Exammer other blocking agents are contemplated and heating the I. CANNON, Assistant Examiner the impregnated paper at temperatures other than those set forth in the tables in contemplated as within the scope of the invention. 117l 54; 162-157 Patent No.

Inventor(s) It is 3 ,492 ,081 Dated January 27 1 Q70 Alfred J. Morak certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2,

Table 1,

Table I,

Table I,

Table I,

Table I,

Table I,

Column 5,

Table II,

Table II Table II,

Table II Column 7,

line 40 "a" should be --as--. line 47, "distribution" is misspelled.

Sample 1, Ring Compression lbs. 80% Rel. Hum. C.D.

"64.4" should be --46.4--.

Sample 3, Treatment, "IDI" should be "TDI" Sample 11, Bursting Strength psig, "80. 7"

should be -87.0.

Sample 18, Treatment, "HNDI" should be -HMDI-.

Sample 19 Instron-Tensile Strength lb/in, 50%

Rel. Hum. M.D. "95.0" should be --94.0-.

Sample 19 Ring Compression 50% Rel. Hum. C.D.

"94.6" should be 94. 3--.

line 73, "diisocyanate" is misspelled.

Sample 31, Treatment, "147Cg. should be l47C.--

Sample 32, Taber-Initial Stiffness 80% Rel. Hum.

M.D. "81.6" should be -86.l-.

Sample 37, Treatment, "14.5%" should be --l5. 4%--.

Sample 37, Taber-Initial Stiffness 50% Rel. Hum.

M.D. "103.4" should be -l03.5-.

"seen" line 53 is misspelled.

Inventor(s) Alfred J. Morak Column 7, line 70 the word "the" is repeated.

Column 7, line 71, "in" (second occurrence) should be SIGNED AND SEALED JULl4|97D SEAL Edmra It Gounissiom of Paton"