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Publication numberUS3141813 A
Publication typeGrant
Publication dateJul 21, 1964
Filing dateAug 23, 1961
Priority dateSep 1, 1960
Also published asUS3141812
Publication numberUS 3141813 A, US 3141813A, US-A-3141813, US3141813 A, US3141813A
InventorsJosef Gneisz, Vladimir Marek Bruno Stefan
Original AssigneeSchweizerische Viscose
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Process of impregnating paper containing synthetic fibers and an amineformaldehyde resin with copolyamide and polyacrylic acid ester
US 3141813 A
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Description  (OCR text may contain errors)

July 21, 964 B. s. v. MAREK ETAL 3,141,813

PROCESS OF IMPREGNATING PAPER CONTAINING SYNTHETIC FIBERS AND AN AMINEFORMALDEHYDE RESIN WITH COPOLYAMIDE AND POLYACRYLIC ACID ESTER Filed Aug. 23, 1961 AMlNE-FORMALDEHYDE P 0 AQUEOUS FIBRE SLURRY (WATER 5OLUBLE PQLY. a CONTAINING MAN MADE FIBRES DRYING DRIED WEB (CROSS LINKING AGENT) I AQUEOUS-ALCOHOLIC SOLUTION OF LINEAR COPOLYAMIDE |MPRE(3NF\TlNG-"- POLYACRYLIC ACID ESTER (HYnaoPHolslNc-a SUBSTANCE) IMPREGNATEDWEB DRYING DRIED PAPER (HOT CALENDERING) HNISHED PAPER y m A ttorneys United States Patent 3,141,313 PRQQESS 0F HMPREGNATHNG PAPER CQNTATN- ENG SYNTHETIC FIBERS AND AN AMINE- FQRMALDEHYDE RESIN WHTH (IQPGLYAMEBE AND PULYACRYLEC AtITD ESTER Bruno Stefan Vladimir Marek and Iiosef Gneisz, both of Emmenbruecke, near Lueerne, Switzerland, assignors to Societe de La Viscose Saisse, Emmenhrucire, Switzerland, a Swiss body corporate Filed Aug. 23, 1961, Ser. No. 133,305 Claims priority, application Great Britain Sept. 1, 1960 13 Claims. till. 162-135) This invention relates to the production of paper and more particularly to the production of resin-impregnated paper which includes synthetic fibers, is ink-fast, and has a high wet-strength and high tear-propagation resistance. For the purpose of the present process the term paper designates any paper or paper-like material prepared on a paper-making machine, such as paper, bonded fabric, non-Woven fabric and similar flat material.

Various methods of producing paper wherein synthetic resins are used have been described. Thus, it is known to coat paper with a polyacrylic ester of high molecular weight; such papers however do not have a very high wet-strength. It is also known to size paper with acrylamide polymers. Improved wet strength is achieved by incorporating melamine resins in the paper pulp. German Patent No. 902,967 describes the production of leather-like articles by binding a fiber mixture with polyacrylic acid derivatives. British patent specification No. 797,943 proposes the impregnation of paper with polyesters of dicarboxylic acids and N-aryl-dialkanolamines. In a known process for the manufacture of paper from synthetic fibers the fibers are coated with sparingly soluble cation-active or anion-active agents and a web is formed, the binder used being an N-alkoxypolyamide or a urea-formaldehyde condensate of high concentration. Further, it is known to produce a paper-like substance by coating a web of synthetic fibers with a polyacrylic ester and subjecting it to hot calendering; this process is somewhat expensive.

In our copending application Serial No. 133,304, filed August 23, 1961, a process for the production of paper is described in which the paper includes synthetic fibers, is ink-fast and has a high wet strength. The method employed is to add to an aqueous fiber suspension, which contains synthetic fibers, an amine-formaldehyde precondensate and optionally a water-soluble polyvalent metal salt, forming a web from such suspension, drying it, impregnating the web with an aqueous/alcoholic solution of a linear copolyamide ofhigh molecular weight, drying the web and, if desired, hot-calendering it.

In that application and in the present application the term synthetic fibers is used in the generally accepted sense of including both fibers made from natural polymers, as for example viscose fibers, and those made from synthetic polymers such as polyamides.

The synthetic fibers preferably have a titre of 1.5-3 denier and a staple length of 24 mm. in the case of fibers derived from natural polymers, and a titre of 1.5-3 denier and a staple length of 3-6 mm. in the case of fibers made from wholly synthetic polymers, and may be combined with the known substances used in making paper pulps, such as rags, bleached or unbleached wood pulp or the like. Preferred mixtures contain 20-80% by weight of synthetic fibers and 80-20% by weight of such other fibrous materials.

The copolyamides to be used in the aforesaid process should be readily soluble in aqueous alcohol, they should be colourless and, above all, their solutions should be stable and have little gelling tendency at room tempera- 3,l4l,8i3 Patented July 21, 1364 ice ture. Such copoyamides are good film-formers and melt at a relatively low temperature.

The reduced viscosity mentioned hereinafter is defined by the formula:

' ln 1; rel.

(where ln represents the natural or Napierian logarithm, 1 rel. the relative viscosity and c the concentration of the solution in grams per ccs. of solution), measured at 20 C. in a solution of the copolyamide of a concentration 0:0.2 gram copolyamide per 100 cc. of solution. The solvent is sulphuric acid of 93.7% strength.

Among the aforementioned copolyamides those are particularly suitable for use in the said process which can be prepared by polymerising a mixture of a salt of a diamine and a dicarboxylic acid with a lactam of an w-aminocarboxylic acid and/ or with an aminocarboxylic acid. The binary copolyamides of hexamethylene diammonium adipate and e-aminocaprolactam Within the range of 35-45% by weight of hexamethylene diammonium adipate and 65-55% by Weight of e-aminocaprolactam are particularly suitable. For the preparation of ternary copolyamides there may be used, for example, mixtures of 10-20% by weight of w-aminoundecyclic acid, 40-30% by weight of hexamethylene diammonium adipate and 50% by weight of e-aminocaprolactam. Such copolyamides should have, for example, a reduced viscosity of 1.10 to 1.30 and, according to the proportions used, a melting point of 165-185 C. After a preliminary partial swelling at room temperature they dissolve at 60 C. in 67% aqueous ethanol or methanol to give solutions of 8 to 12% strength. These solutions are clear and viscous and do not gel even after standing for several days at room temperature. On the other hand, a solution of 10% by Weight strength in 95% alcohol gels after a few hours. A film obtained from a 7.5% solution is clear, tough and stretchable; the films have a breaking length of 1.7 to 2.5 kilometers and an elongation at break of to 300%.

It has proved particularly advantageous to add to the paper pulp a reagent capable of cross-linking the copolyamide added to the paper base by the impregnation. For this purpose it is of advantage to use a cationic melamineformaldehyde precondensate having alfinity for the fiber, if desired in the presence of a small amount of aluminium sulphate. Such precondensates can be obtained by dissolving trirnethylolmelamine in water, adding formic or hydrochloric acid to this solution and letting it ripen for several hours until it displays a bluish opalescence. Such a solution is stable for 12 to 14 hours.

The cross-linking of the substratum with the copolyamide film and/or of the copolyamide alone may be achieved with the application of heat and under superatmospheric pressure. In this case a small amount of aluminium sulphate contained in the paper base acts as a mild catalyst.

For the purpose of further improving the wet strength or stability to Water respectively a small amount of a cross-linking substance, such as 1:6-hexamethylene-N:N- bis-(ethylene-urea) or formaldehyde donors and/or of a hydrophobising agent which likewise reacts chemically with the substratum and the copolyamide, such as octadecyl ethylene-urea, may be added to the impregnating liquor. Such additives must be compatible with the dissolved copolyamide, that is to say they must not precipitate from the copolyamide solution or be inactivated by it. When an imine is present no acidic catalyst should be present since otherwise the ethyleneimide derivatives would precipitate from the aqueous alcoholic solution or be inactivated by it.

It has now been discovered that the Elmendorf tear 3 of paper so manufactured is substantially improved by impregnating the web not only with the copolyamide of high molecular weight but also with a polyacrylic acid ester of high molecular weight.

According to the present invention, therefore, there is provided a process for the production of paper which comprises forming an aqueous suspension of fibers including synthetic fibers, adding thereto an amineforrnaldehyde precondensate, and optionally also a water-soluble polyvalent metal salt, forming a web from such suspension, drying it, impregnating the web with an aqueous alcoholic solution containing a linear copolyarnide of high molecular weight and a polyacrylic acid ester of high molecular weight, drying the web and, if desired, hotcalendering it. Both linear copolyamide and polyacrylic ester must be film-forming. (lo-pending application Serial No. 133,304 indicates that a useful range of reduced viscosities, as defined above, for the copolyamides is 1.10 to 1.30. The viscosity number of the polyacrylic esters may be between 1.5 and 3.5 as measured by forming a film from the commercial product, drying it in vacuo over phosphorus pentoxide, dissolving the film in a 1:9 mixture of ethanol and tetrachlorethane, determining the viscosities of a series of solutions of concentrations rang ing from 0.05 to 0.4 g. of polymer per 100 ml. of solution at a temperature of 20i0.05 C. using an Ostwald viscometer of 5 cc. capacity and a running time of 70-80 seconds for the solvent used and finally extrapolating the series to zero concentration; viscosity number lin1 (1 spec.)

where 1 spec. is the specific viscosity and c the concentration.

The invention further includes paper manufactured by the process just set forth.

It is to be understood that the materials used and the specific techniques described in our copending application Serial No. 133,304 filed August 23rd, 1961, and which have been referred to briefly above, may, taking into consideration the specific differences between the two processes, be employed in the process of the present invention.

To achieve a particularly high wet strength it has proved advantageous to add to the aqueous alcoholic solution a hydrophobising agent as well as a small amount of crosslinking agent, such as a formaldehyde donor (e.g. melamine resins or a diethylene urea derivative) and to press the impregnated sheet at an elevated temperature, for example on a hot calender. These measures have been described in our copending application Serial No. 133,304 filed August 23rd, 1961.

The process is illustrated in the accompanying drawing by a flow diagram in which the continuous lines indicate the main features of the invention and the discontinuous lines indicate subordinate features.

The following example illustrates the invention:

Example In a vat a suspension is prepared from 0.8 kg. of bleached and ground sulphate cellulose, 0.4 kg. of ground aspen wood pulp and 0.8 kg. of staple fiber of polyhexamethylene adipamide (staple length 4 mm., fiber titer 2 denier) and 150 liters of Water by stirring. This suspension is treated with a mixture of 40 grams of a melamine-formaldehyde precondensate (39% nitrogen) with 12 grams of formic acid, an aqueous solution of 62 grams of alum is run in, the whole is mixed and then made up with water to 200 liters. The pH value of this aqueous suspension in the stuff chest of the paper-making machine ranges from 5 to 6. This suspension is converted on a Fourdrinier paper-making machine into a paper web and dried. The moist paper web does not stick to the wet felt roller.

The weight in grams per square meter of the paper bases, prepared in the above-mentioned manner, was varied between 54 and 72. These paper bases are treated according to the following specifications to have in all cases a final weight of about 72 grams per square meter in the finished state:

(a) The first piece (1) is used only for measuring the properties of the paper base and is hot calendered for one second at 165 C.

(b) The second piece (2) is impregnated at room temperature while travelling at a rate of 3 meters per minute by passing it through a solution of 10% by weight of a copolyamide in 67% by weight aqueous ethanol. The copolyamide is prepared from 40% by weight of hexamethylene diarnmonium adipate and 60% of E-aminocaprolactam and has a melting point of 165 C. and a reduced viscosity (as hereinbefore defined) of 1.18. The paper is then dried on a drying cylinder under a pressure of 0.9 atmosphere gauge and hot calendered for one second at 165 C.

(c) The third piece (3) is impregnated according to the present process with a solution of 8.75% of the aforementioned copolyamide, 1.25% (calculated as solids) of a commercial non-ionic polyacrylic acid ester emulsion containing free carboxyl groups and having a solids content of 46.5% (pl-l value 6.0 to 6.5) in 67% by Weight aqueous ethanol, as described above under (12), and then further processed as described above. The product used was that sold by Rohm & Haas Company of Philadelphia under the name Rhoplex B15. It was the ethyl ester, had a saponification number of 530 mg. of KOl-I per gram of solids, 0.082 free carboxyl groups per structural (monomer) unit and a viscosity number of 2.55 measured as described above.

(d) The fourth piece (4) is further treated according to the process of our copending application Serial No. 133,304 filed August 23rd, 1961. For this purpose the paper is impregnated as described above under (15) with a solution of 10% by weight of the aforementioned copolyamide, 1.05% of an alkyl-modified hexamethylolmelamine, 0.5% of a hydrophobising agent based on octadecyl ethylene urea in 67% by weight aqueous ethanol, and then further treated as described above.

(2) The fifth piece (5) of the paper web is treated by the present process. The impregnation, drying and calendering are carried out as described under (b) above, except that the impregnating solution consists of 8.75% by weight of the aforementioned copolyamide, 1.25% of the polyacrylic acid ester emulsion used under (0) above, 1.05% of the hexamethylolmelamine used under (d), 0.5 of the hydrophobising agent used according to (a') in 67% aqueous ethanol.

Of all 5 papers, pieces having an identical weight of about 72 grams per square meter were measured. The results of the measurements are shown in the following table:

Break- Breaking Break- Wet Breaking Specimen ing elongalength, strength elonga- Elmen- N 0. length, tion dry, Wet, in in pen tion Wet, dorf dry, in in perkm. cent 1 in pertear 2 km. cent cent 0. 995 1. 66 0. 2 20 77. 5 4. 63 7. 34 1.72 37 8. l 244. 0 4. l1 8. 10 l. 67 40 8. 1 295. 0 5. 041 7. 2. 57 51 10. 2 192. 0 5. 26 8. 1O 2. 49 47 10. 3 275. 5

1 Wet strength= xioo.

breaking length, dry

2 Elmendorf tear, in the longitudinal machine direction, in gramscorreeted for a paper weight of grams per square meter.

As will be seen from the above table, the hot calendered paper base has only a restricted dry strength and a wet strength which is so low that it is diflicult to measure it with the available instruments. The Elmendorf tear is likewise relatively low. By a treatment of the paper with copolymer and polyacrylic compound as eifected for specimen (3), some increase in the wet strength and a considerable increase in the Elmendorf tear is achieved compared to specimen (2) not treated according to the present invention. The treatment applied to specimen (4), according to the process of our copending application Serial No. 133,304 filed August 23, 1961, aims primarily at improving the wet strength of the paper and this aim is actually achieved, but the Elmendorf tear is substantially reduced. The treatment of specimen (5) according to the present invention, with a combination of copolyamide, cross-linking agent and hydrophobising agent and copolyacrylic compound produces a paper of very high wet strength and Elmendorf tear and a relatively high stretchability.

It is remarkable that the treatments according to the present invention produce a chemical bond between the resins and the fibre material in spite of the extremely short condensation period of only one second, without adding any catalyst to the aqueous alcoholic binder solution. This fact is especially important since the polyacrylic acid ester emulsions and the ethylene imine derivatives in alcoholic solution are incompatible with acid catalysts and tend to precipitate when an acid catalyst is present in the impregnating solution.

The fact that the resins have actually entered into a chemical bond with the fibre material is shown by the following experiment:

A test sheet obtained by the treatment under (e), containing 40% by weight of polyamide fibers, was immersed for 5 hours in alcohol, then rinsed with fresh alcohol and dried at 60 C. The change in relative percent in the properties resulting from these treatments was as follows:

Breaking strength dry Increased by 2.7%. Breaking strength wet Increased by 1.0%. Elrnendorf tear Reduced by 8.0%.

Since the binders were applied in the form of an alcoholic solution, the breaking strength would be expected to show a decrease of at least 50% if the binder had not formed a chemical bond with the fibers.

Writing tests with ink, drawing ink, and coloured inks on papers prepared according to the present process as described under (c) and (8) revealed a very high ink fastness. In these tests specimens of extremely porous paper containing 80% of polyamide fibers were treated for 4 hours at room temperature with water, with hydrochloric acid of 4% strength and with sodium hydroxide solution of 4% strength, then washed and dried, after which it was possible to write on them without the ink running, and to print on them. Furthermore, the mechanical properties of the paper were not adversely aifected by such treatments.

It is also to be noted that the paper produced according to the present invention has excellent heat stability. Thus, the properties were found not to be adversely affected even after heating for several days at 105 C.

The papers produced according to the present invention may be die-stamped, creped and otherwise embossed.

We claim:

1. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amineformaldehyde precondensate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as filmforming constituent and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a polyacrylic ester, the total concentration of film-forming constituent in said solution being about 10% by weight and the concentration of copolyamide being about 8.75%.

2. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amine-formaldehyde precondensate and a water-soluble salt of a polyvalent metal is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as film-forming constituent and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a polyacrylic ester, the concentration of copolyamide being about 8.75% by weight and the concentration of polyacrylic ester being about 1.25% by weight.

3. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amineformaldehyde precondensate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as film-forming constituent and a cross-linking agent and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a polyacrylic ester, the concentration of said linear copolyamide in said solution being about 8.75% by weight and the concentration of said polyacrylic ester being about 1.25% by weight.

4. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amine-formaldehyde precondensate and aluminium sulphate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as film-forming constituent and a cross-linking agent and drying the impregnated sheet, the improvement which consists in including in the said aqueousalcoholic solution a further film-forming constituent consisting of a polyacrylic ester, the concentration of said copolyamide in said solution being about 8.75% by weight and the concentration of said polyacrylic ester being about 1.25% by weight.

5. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amineformaldehyde precondensate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as filmforming constituent and a hydrophobising substance and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a polyacrylic ester, said polyacrylic ester being about 1.25% by weight of said solution and said copolyamide being about 8.75% by weight of said solution.

6. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amine-formaldehyde precondensate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as film-forming constituent and a hydrophobising substance reactive chemically with both the material of the web and the copolyamide and drying the impregnated sheet, the improvement which consists in including in the said aqueousalcoholic solution a further film-forming constituent consisting of a polyacrylic ester, said polyacrylic ester being about 1.25 by weight of said solution and said copolyamide being about 8.75% by weight of said solution.

7. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amine-formaldehyde precondensate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as filmforming constituent and a octadecylethylene urea and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a poly- 7 acrylic ester, said polyacrylic ester being about 1.25% by Weight of said solution and said copolyamide being about 8.75% by weight of said solution.

8. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amine-formaldehyde precondensate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as film-forming constituent and a bifunctional ethylene imine derivative and drying the impregnated sheet, the improvement which consists in including in the said aqueous-ah coholic solution a further film-forming constituent consisting of a polyacrylic ester, the concentration of said copolyamide in said solution being about 8.75% by Weight and the concentration of said polyacrylic ester being about 1.25% by Weight.

9. In a process for the production of paper Which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amine-formaldehyde precondensate is formed into a Web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as filmiorming constituent and a formaldehyde donor and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a polyacrylic ester, the concentration of said copolyamide being about 8.75% and the concentration of said polyacrylic ester being about 1.25%, both by weight, of said solution.

10. Process according to claim 9, wherein the fibers used comprise 20-80% of synthetic fibers.

11. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing trimethylol melamine is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as film-forming constituent and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a polyacrylic ester, the concentration of said copolyamide being about 8.75% and the concentration of said polyacrylic ester being about 1.25%, both by weight of said solution.

12. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing an amine-formaldehyde precondensate is formed into a web and dried and the dried web is thereafter impregnated with an aqueous alcoholic solution of a linear copolyamide as film-forming constituent and drying and hot-calendering the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further fiim-forming constituent consisting of a polyacrylic ester, the concentration of said copolyamide being about 8.75% and the concentration of said polyacrylic ester being about 1.25 both by Weight, of said solution.

13. In a process for the production of paper which contains a proportion of synthetic fibers and in which an aqueous suspension of the fiber containing, based on the fiber, about 1% by weight of a polymethylol derivative of melamine is formed into a web and dried and the dried web is impregnated with an aqueous-alcoholic solution of a linear copolyamide as film-forming constituent and drying the impregnated sheet, the improvement which consists in including in the said aqueous-alcoholic solution a further film-forming constituent consisting of a polyacrylic ester containing free carboxyl groups, the total concentration of film-forming constituents in said solution being about 10% by Weight of which about seveneighths consists of the linear copolyamide and the remainder of the polyacrylic ester.

References Cited in the file of this patent UNITED STATES PATENTS 2,406,453 Charlton Aug. 27, 1946 2,540,352 Schenck Feb. 6, 1951 2,559,220 Maxwell July 3, 1951 2,983,627 Williams May 9, 1961 3,000,777 Williams Sept. 19, 1961 3,016,325 Pattilloch Jan. 9, 1962 FOREIGN PATENTS 873,700 Great Britain July 26, 1961 874,249 Great Britain Aug. 2, 1961

Patent Citations
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US2406453 *Mar 8, 1944Aug 27, 1946Ici LtdTreatment of paper or textile fabrics
US2540352 *Oct 27, 1945Feb 6, 1951Munising Paper CompanyMethod of making wet strength paper
US2559220 *Jul 31, 1942Jul 3, 1951American Cyanamid CoManufacture of cellulose products of improved wet strength
US2983627 *Aug 22, 1957May 9, 1961Portals LtdMethods of making webs of fibrous material
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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4392861 *Oct 14, 1980Jul 12, 1983Johnson & Johnson Baby Products CompanyTwo-ply fibrous facing material
US5512618 *Dec 27, 1994Apr 30, 1996Enviro-Chem, Inc.Includes an alcohol-ester coalescing agent
US6489040Feb 15, 2000Dec 3, 2002United States Gypsium CompanyWallboard with improved roll-up resistance
Classifications
U.S. Classification162/135, 162/166, 162/157.7, 162/157.3, 162/146
International ClassificationD21H17/55, D21H17/37, D21H23/00, D21H17/49
Cooperative ClassificationD21H17/49, D21H17/37, D21H23/00, D21H17/55
European ClassificationD21H17/49, D21H23/00, D21H17/37, D21H17/55