US3135590A - Dimensionally stabilized paper and coated abrasives made therefrom - Google Patents

Description

United States Patent 3,135,590 DHMENMONALLY STABHJIZED PAPER AND COATED ABRASIVES MADE Tl-EREFROM John E. Campbell, Needham, Mass, Lester A. Hansen,

Averill Park, and Charles E. Lanyon, Troy, N.Y., and

Nicholas E. Oglesby, deceased, late of Troy, N.Y., by

Louise S. Oglesby, executrix, Troy, N.Y., assignors to Norton tlompany, Troy, N.Y., a corporation of Massachusetts No Drawing. Filed July 12, 1957, Ser. No. 671,669

8 Claims. (Cl. 51-298) This invention relates to the manufacture of paper with controlled expansions and contractions or controlled changes in dimensions with gain or loss of moisture as occurs, for instance, in exposure to the variable humidities encountered in storage, handling, and use, and to the use of such paper as a base for various coatings.

It is an object of the invention to prepare papers which may be coated with various materials, such as resins, or varnishes, or other coatings and adhesives to form substantially non-curling products useful for wall coverings, artificial leather and the like.

More specific objects of the invention are to produce backings for use in the manufacture of coated abrasives which are non-curling, or flat, or which have controlled degree of curl and the manufacture of coated abrasives with such backings.

In the patent to Nicholas E. Oglesby, No. 2,136,150, issued November 8, 1938, a general theory and explanation of the curling of coated abrasives has been given. As applied to both coated abrasives made with a glutinous binder and those made with synthetic resin binder, the

phenomenon of curling can be attributed to unequal ex-' pansions and/or contractions of the adhesive-abrasive coating and the paper or other supporting backing with changes in atmospheric moisture, which in turn result in a change in the quantity of moisture held within the cellulosic backing and the glutinous or other hygroscopic adhesive if used. In the case of a glutinous or other hygroscopic binder, the dimensions of both the backing and the abrasive-adhesive coating change appreciably with changes in the atmospheric moisture because of the resulting changes in the moisture content of the backing and of the glutinous or other hygroscopic adhesive.

In the case of coated abrasives made with a synthetic resinous binder such as the phenolaldehyde resins, the alkyd resins and the urea resins, there is little, if any, change in the dimensions of the abrasive-binder coating with changes in atmospheric moisture, once the resinous adhesive has been thoroughly cured and rendered immiscible with water and resistant to its vapor. The paper backing or other cellulosic backing on the other hand swells or attempts to swell as usual with gain or loss of moisture.

In the patent to Nicholas E. Oglesby, No. 2,136,150, issued November 8, 1938, a method of decreasing and controlling the curl of synthetic resin bonded coated abrasives has been disclosed and consists of using paper with a high cross-to-length strength ratio which in turn has a low differential of expansion and contraction in the cross and machine directions as compared with prior art paper, which has ordinarily had a high differential of expansion and contraction between the cross, and the length or machine direction of the Web.

3,135,590 Patented June 2, 1964 The use of the papers and procedures described in U.S. Patent 2,136,150 have been a distinct advance over the prior art and highly satisfactory for use in the manufacture of coated abrasives having a binder consisting of hide glue. a

In recent years a large part of so-called glue bond sandpaper has been made under U.S. Patent 2,322,156, issued to Nicholas E. Oglesby on June 15, 1943, and has a binder of hide glue modified by a large percent by volume of inert filler. The expansion and contraction of such modified binders when exposed to variable humidities is much less than in the case of unmodified hide glues and while the papers and procedures of U.S. Patent 2,136,150 are satisfactory for backings of coated abrasives made under U.S. Patent 2,322,156 for most purposes, backings having smaller changes in the cross direction of the paper with varying humidities are desirable for some purposes and such backings are provided by the present invention.

The procedures of U.S. Patent 2,302,711, issued to Nicholas E. Oglesby and Franklin Strain on November 24, 1942, have been successful in eliminating the objectionable sand concave curl of coated abrasives made with nonhygroscopic binders, such as the binders of varnishes and synthetic resins, including phenol-formaldehyde resins, urea-formaldehyde resins and alkyd resins. However, the procedures of this patent leave room for further improvement in that a coated abrasive having its cellulosic backing shrunk or adjusted as taught in the patent to be flat or have a desired degree of convex curl at about relative humidity may curl more than is desired when at a suficiently low humidity, such as a humidity of 20% or less.

The preparation and use of backings in the manufacture of coated abrasives according to the present invention, enables one to control the swing or change in curling as desired or required for various operations upon which coated abrasives are used.

One of the most successful of the procedures of U.S. Patent 2,302,711 is that covered by claim 4 of the patent and this procedure is very useful in combination with the present inventions.

In the prior art many attempts have been made to solve the problems of curling of coated abrasives by treating the paper backings, both of the Fourdrinier and cylinder types, with various oils, resins, varnishes, and rubber. The theory back of many of these attempts was that such treatments waterproof and moistureproof the paper and fibres and prevent them from swelling or shrinking due to changes in moisture content of the atmospheric. It has not been found possible to prevent the swelling and contracting of the fibres and paper by such means without individual fibre penetration and serious embrittlement and weakening of the paper in the cases of the harder resins; or without the use of excessive treating material in the cases of rubber and the like.

In U.S. Patent 2,226,553, issued December 31, 1940, to Bert S. Cross, treatments consisting of incorporating in the backings rubber and resin are disclosed for the purposes of increasing fibre adhesion and ply adhesion of the backings and of decreasing peeling of the coatings from the backings. Such treatments are also disclosed to increase flexibility and strength and to decrease brittleness.

While incorporation of rubber or resin or combinations of the two in the backings as in the prior art were successful in changing or controlling the hardness or softness, ply and fibre adhesions, flexibility and strength of the coated abrasive backings, only slight improvement in curling of coated abrasives made therewith was attained without the use of excessive amounts of rubber and/or resin, since the improvement in curl by such materials is roughly dependent upon and proportional to the amount of paper fibres replaced by such materials. Thus, the desired improvement in curl of the usual coated abrasive papers by such backing treatments was not accomplished Without loss of some of the desirable properties of paper backings due to the necessity of using large amounts of such modifying agents for obtaining the desired improvement in curl, with the result that the major fraction of the backing composition by volume did not consist of the usual cellulosic papermaking fibres which are responsible for the characteristics of paper.

We have discovered that small amounts of certain nonhydroscopic fibers which do not swell or contract substantially when exposed respectively to high or low humidities, as for instance glass fibres of the correct type or equivalent fibres, may be added to the usual cellulosic papermaking fibres to produce paper, as for example coated abrasive paper backings, having surprisingly reduced cross dimensional changes with variations in humidity to which they are exposed after coating and therefore greatly proved curling characteristics.

The unexpectedly large reductions in cross expansions and contractions obtained by replacing small fractions by volume of the usual cellulosic papermaking fibres in a furnish with such non-hygroscopic fibres is obtained not only with furnishes otherwise consisting essentially of cellulosic papermaking fibres, but also with furnishes containing in addition thereto such modifying agents as one or more of the rubbers or rubber and resins added according to the known art.

In cases where only small amounts of resin are added to the rubber containing papermaking stock, or where rubber without resin is used we may, if required, add papermakers alum to the stock before formation of the sheet in order to aid in the retention of rubber binder in the sheet.

In cases where very small fractions of glass fibre or equivalents are adequate to produce the modifications required, the ply adhesion, fibre adhesion, tear strength, folding endurance and other characteristics of the paper are not substantially impaired as compared with paper in which such fibres are not used.

in the cases of coated abrasives having water insensitive i synthetic resin grit holding binders, greatly reduced cross dimensional changes with variation in humidity are desirable, and We may use up to 2 to about 50% by weight of glass or equivalent fibres in the furnish to bring about the desired dimensional stabilization in cross dimenisonal changes of the paper with vaniations in moisture content.

In general, according to the reduction in variations of dimensional changes of the paper with variations in humidity desired, we replace from about 2% to about 40% by volume of the usual cellulosic papermaking fibres in a furnish with suitable glass and/ or equivalent non-hygroscopic fibres.

In cases where about 2% or more by volume of the cellulosic papermaking fibres are replaced by such nonhygroscopic fibres, desirable properties of the paper such as ply adhesion, fibre adhesion, tearing strength and folding endurance are impaired according to the amount of cellulosic fibres replaced with the non-hygroscopic fibres, and we add binders of resin and/ or rubber to the furnish to restore or improve suchcharacteristics. This is especially true of the paper or the ply of a multi-ply paper, to which the abrasive grits are attached in making coated abrasives.

We find that binders, such as the phenol-formaldehyde, melamine-formaldehyde and urea-formaldehyde resins customarily added to paper fibres in a beater as known in the art, may be used to improve the adhesion and the properties of the paper in general if not used in large enough quantities to embrittle the paper too much. These resins also permit a larger retention of elastomer in the resulting product.

We prefer to use, especially where a high degree of cross dimensional stability to moisture changes is required, a combination binder comprising a suitable resin and a suitable rubber of the types usually added in a beater. Suitable rubbers include the latices ordinarily added to paper by the beater process, for example natural rubber, neoprene, rubber formed of a copolymer of butadiene and an unsaturated compound, the preferred unsaturated compounds being aromatic, such as styrene, or aliphatic, such as the unsaturated nitriles (e.g. acrylic nitrile, alpha-mcthacrylic nitrile, alpha-ethacrylic nitrile, alphaisopropyl-acrylic nitrile), isobutylene, and methacrylates such as methyl methacrylates, their equivalents and homelogs.

A preferred latex is one formed of the copolymer of butadiene and acrylonitrile in which the acrylonitrile content is 35% to 70% of the weight of the copolymer. A copolymer in which the acrylonitrile content is 35 to 40% is highly satisfactory.

Such a combination binder, when incorporated in paper or a paper furnish containing glass fibres in the beater as taught herein, may be used to obtain paper suitable for coated abrasives backings and other coating purposes and having improved curling characteristics, ply adhesion, fibre adhesion, tensile strength, tear strength, bursting strength, folding endurance and wet strength but which is essentially a paper product with the desirable characteristics of paper.

While we may first form the paper and then saturate it with the resin, rubber or resin-rubber combination in solution or emulsion form to produce the improved paper products hereof, we generally prefer to add the rubber or rubber and resin latex (or emulsion) to the fibres in the beater during the manufacture of the paper by the usual methods known to those skilled in the art. Beater methods are especially indicated where small amounts of rubber and resin are to be added.

While glass fibres of the proper types as hereinafter described, are highly satisfactory for our use, other kinds of fibres are also used by us.

For example fibres of proper lengths and diameters of Dacron consisting of highly polymeric linear terephthalic esters disclosed in U.S. Patent 2,465,319, issued to John Rex Whinfield on March 22, 1949, are equivalent to glass fibre and highly satisfactory for use in carrying out our inventions as disclosed and claimed herein.

While less effective when used in equal quantities, certain asbestos fibres are useful in carrying out our invention. Asbestos fibres equal to or better than other asbestos fibres tried by us are of the crysotile type.

Synthetic fibres sold under the trade names of Dynel and Vinyon are less effective pound for pound than glass or Dacron but about as effective as the better asbestos fibres for carrying out our inventions.

The non-hydratable fibres useful in carrying out our inventions at equilibrium with 65% relative humidity and 70 F. contain no more than about one percent in excess of the moisture content of the same fibres at the same temperature in equilibrium with 20% relative humidity, and the individual non-hydratable fibres as compared with cellulosic papermaking fibres are substantially free from volume changes with humidity changes throughout the range 20% to 65 relative humidity at 70 F.

The moisture regain test is a valuable guide in selecting our non-hydratable fibres. As measured by this test, our fibres have a moisture regain of not to exceed 1%.

Our non-hydratable fibres have average diameters of 0.0001 to 25 microns and average lengths at least fifteen times as great as their average diameters.

A-mdl. I

Suitable fibres for our use are conveniently prepared by cutting and adding to the beater with the cellulosic fibres either suitable non-hydratable textile grade fibres, or non-hydratable fibres made or prepared for papermaking.

Our methods are successful in making paper backings of the Fourdrinier and cylinder types both of which types are used extensively in such products as coated abrasives.

Since Fourdrinier papers are made in a single ply on the papermaking machine, the non-hydratable fibres and other modifying agents, such as resin and/or rubber, if used, are usually added in the beater and extended throughout the thickness of the formed Fourdrinier sheet. However, this is not always the case as resin and/or rubber, in a form capable of some penetration of the paper, may be added as sizes to either or both sides of the formed paper.

Our procedures are particularly adaptable to the manufacture of cylinder paper backings having two or more plies. Presently the most important paper backings for coated abrasives are four or five ply cylinder papers.

In using our methods to make cylinder paper backings, the non-hydratable fibres with or without rubber or rubber and resin may be added to all of the plies in the paper or to any one or more plies.

For example, in making cylinder paper backings with three, four or five plies, it is sometimes advantageous to omit the non-hydratable fibres from the ply to which the abrasive grains are applied, thereby producing a ply having greater ply and fibre adhesion than would otherwise be the case, and making the abrasive coating harder to shed or peel from the backing. Where the coated abrasives are to be used in the form of belts, especially unless substantial amounts of resin and/or rubber are included in the plies, we obtain better belt joints by omitting the non-hydratable fibres from both the front (or coat) and back (or print) plies.

In both these Fourdrinier and cylinder types of backing we find it advisable to replace up to 40% by volume of the cellulosic papermaking fibres with glass or equivalent fibres, the extent of the replacement being made according to the reduction in dimensional changes of the backing required which in turn depends upon the dimensional changes in the abrasive-binder coating.

PAPERMAKING Example 1 This example will describe the formation of a 130 pound four ply cylinder paper that is typical of our I invention.

fibre (composed of a mixture of Wood and rope fibres) are thoroughly mixed in a vat in an aqueous media. To this cellulose fibre dispersion 110 pounds of glass fibres A inch long, 8-10 microns in diameter, which can be purchased from Owens-Corning Fiberglas Corporation are then added to the cellulosic fibre dispersion and the whole thoroughly mixed. To this cellulose fibre-glass fibre mixture 44 pounds of melamine formaldehyde resin (solids) such as (Parez 607) sold by American Cyanamid & Chemical Co. are added as an acid colloid in a hydrochloric acid media made up of 17 pounds of 20 B. HCl and 29.6 pounds of water. Thence 340 pounds of rubber (850 pounds of Hycar latex 1562, a butadieneacrylonitrile copolymer, manufactured by B. F. Goodrich Chemical Company) are added to the cellulose fibrefibreglas-resin mixture. All of these components are thoroughly mixed throughout to form a homogeneous mass. To this mass is added a 10% solution of papermakers alum to precipitate the rubber at a pH of 69 approximately 4.5. After the whole is thoroughly mixed it is processed on the cylinder papermaking machine and dried on heated cans to form the novel dimensionally stable paper of this invention.

Examples 2 and 3 Two ply cylinder papers having a weight of about 70 pounds per ream were made in the manner specified in Example 1, but, with an increased percentage of nonhydratable fibre added. The components are added in the manner specified in the preceding example, but in the following proportions by weight (dry basis) Cellulose Parez Glass Wt. (2 Example Fibre Hycar 607 Fibre ply),

#lream Since, in the papermaking process, all the materials added before the sheet is formed are not completely retained in the sheet, the composition of the formed sheet will not correspond exactly to the composition of the fibre, rubber resin suspension before formation of The papers made according to the above examples were of suitable physical characteristics for the formation of coated abrasives. In particular, the dimensional stability was greater in each case than the dimensional stability of a control which was a prior art paper, containing cellulosic fibres only, of good dimensional stability.

For purposes of quantitative comparison we define dimensional stability as the percent cross axis (cross to the machine direction of the paper) dimensional change of the control paper (Pc) minus percent cross axis dimensional change of the experimental paper (Pe) divided by the percent cross axis dimensional change of the control paper (Pc) with the resultant quotient multiplied by 100, i.e.

with measurements made at 65% RH. (70 F.) and 20% RH. F.).

The papers of Examples 1, 2, and 3 had dimensional stabilities respectively of 46%, 59% and 39%.

EXAMPLE OF THE MANUFACTURE OF A COATED ABRASIVE In the manufacture of non-waterproof sandpaper according to this example of our inventions we employ as a backing the dimensionally stable sheet described in Example No. 1. This paper backing is coated with a 34% solution of 114 millipoise glue. This adhesive is placed in adhesive heating apparatus of a sandpaper machine and brought to a temperature of F. The adhesive was applied to the backing by the brush coat method to give a maker weight of about 2.84 lbs. per sandpaper ream to the prepared backing previously described, after which 10.36 lbs. per sandpaper ream of grit aluminum oxide abrasive grain is applied to the maker coat of the adhmive on the backing by conventional means. We have found the electrostatic methods disclosed and claimed in US. Patent 2,318,570 highly efficient in the application of the grain.

The web coated with the making coat of adhesive and the abrasive grain is then moved into a conventional 7 sandpaper drying room and dried at any convenient temperature preferably about 100 F.

After the binder has been so dried, the abrasively coated web is moved through a conventional sandpaper sizing machine where the sizing coat of adhesive is added. In this example the sizing coat of adhesive comprises 60% of suitable film forming phenolic resin such as an alkali catalyzed condensation product of phenol and formaldehyde, and 40% water. Suitable resins are available from Bakelite Corp, Varcum Chemical Co., Monsanto and other suppliers. The sizing adhesive was applied at a temperature of 80 F, more or less, these temperatures not being critical and subject to variation according to other operating conditions which will be appreciated by those skilled in the art.

The amount of size added is controlled partly by weight but, to a greater extent, by the ability of the skilled operator to judge the size. Curable contents of a sizing solution of about 2.86 lbs. per sandpaper ream are usually reasonably satisfactory. After this sizing operation the web is passed into a conventional sandpaper drying room and heated to cure the resin. A satisfactory cure of the binder after the sizing operation may be obtained by heating for 100 minutes at 140 F. The material is then given a final cure which consists of 110 minutes at 200 F. Those skilled in the art will appreciate that a number of variations may be made in the time and temperature of the curing cycle.

The sandpaper produced by this example was subiected to a curl correction such as described in US. Patent 2,302,711, and mechanical flexing in the conventional manner known in the coated abrasive art.

The finished product was then evaluated and compared with a similar product made with the same binder and grit, and treated in the same manner, but made with a conventional non-cupping paper of the prior art. In terms of general properties such as cutting life, rate of out, etc. the product of our invention was at least equal to or better than the standard product.

However, in terms of product curl the sandpaper of coated abrasive, Example 1, was much superior to the control. In testing the curl of the product rectangular samples of the control and of my improved product, strips 6" wide, were first exposed and allowed to come to equilibrium in an atmosphere having a humidity of 65% and temperature of 70. The curl of each of the two products was measured by measuring the deviation from planarity of each material. The samples were then aliowed to come to equilibrium in an atmosphere at 20% RH. and 90 F., and again curl measurements were taken. The following table shows the comparison of the two products, the curl being an average value of the deviation from planarity measured in sixteenths of an inch.

In measuring the curl of the paper, a measurement is taken at each corner of the sample and a measurement is taken at the center of each long side of the sample. The plus sign refers to a sand convex curl, all the curl in the paper being about the long axis (machine direc tion) of the sheet.

Although we have used glass as the non-hydratable fibres in the preceding examples, other non-hydratable fibres of length and diameter suitable for paper making have been found to be satisfactory and may be substituted therefor, as previously mentioned in this specification. The following table compares results obtained in 8 the use of varying proportions of glass and Dacron fibres:

Percent Percent Percent Percent Percent Glass by Glass by Dacron Dacron" Stabilivol. wt. by vol. by Wt. zation Although we have used a butadiene-acrylonitrile latex as the rubber ingredient in the above examples, we have found that other synthetic or natural rubber latices such as referred to above in this specification used in amounts from 5 to 25% by weight are satisfactory.

Similarly, we have found that as the resin ingredient of our paper instead of the melamine formaldehyde resin of the examples, we may use phenol-formaldehyde and urea formaldehyde resins customarily used in the paper making art. We find that satisfactory results may be obtained upon the use of 0.5 to 5% by weight of the melamine-formaldehyde or equivalent resin.

Although we have described our paper as being of particular value in the manufacture of coated abrasives, obviously such paper has many other useful applications where a combination of a controlled amount of dimensional stability in combination with high resistance to splitting, resistance to peeling, tear resistance and folding endurance.

In general, we have found that satisfactory papers from the point of view of dimensional stability have a variation in their width dimension under conditions of humidity and temperature between 65% RH. at 70 and 20% RH. at 90 of not more than 0.8%. Where the ratio exceeds this value we find that the papers are no more satisfactory from the point of view of dimensional stability and anti-curl properties when used in coated abrasives than the best available prior art papers suitable in other properties for the making of coated abrasives.

The percent dimensional change in the width direction of various machine made papers made according to our invention is as follows:

Paper of Example 1 0.39% (10% glass fibre) Paper of Example 2 0.25% (16% glass fibre) Paper of Example 3 0.37% 8% glass fibre) We claim:

1. A coated abrasive comprising a flexible paper backing with abrasive grains adhesively anchored to one side of said backing, the fibre content of said paper consisting a substantially homogeneous mixture of at least by volume, of the total fibre volume, of hydratable ccllulosic paper making fibres and not more than 40% by volume of said total fibre volume being non-hydratablc fibres having a moisture regain value up to about 1% selected from the group consisting of papermaking and textile fibres, said non-hydratable fibres having substantially the same dimensions at a temperature of 70 F. throughout the relative humidity range of 20% to at F., and in which the percent by volume of the non-hydratable fibres is not less than about 2% by volume based on the total volume of the paper sheet.

2. A coated abrasive comprising a flexible paper backing containing paper with abrasive grains adhesively anchored to one side of said backing, the fibre content of said paper consisting of a substantially homogeneous mixture of at least 60% by volume, of the total fibre volume, of hydratable cellulosic papermaking fibres and not more than 40% by volume of said total fibre volume being non-hydratable fibres having a moisture regain value up to about 1% selected from the group consisting of papermaking and textile fibers, said non-hydratable fibres having substantially the same dimensions at a temperature of 70 F. throughout the relative humidity range of to 65%, the average diameters of said non-hydratable fibres ranging from 0.1 miliimicron to microns and the average lengths thereof being at least 15 times as great as the average diameters, said paper in its width direction varying by not more than 0.8% throughout the relative humidity range of 20% at 90 F. to 65% at 70 F.

3. The article of claim 2 containing paper fibre binder in an amount of 2 to of the total of the volumes of all of the ingredients of the said paper, and in an amount suificient to overcome at least such losses in elasticity, resistance to splitting, resistance to peeling, tear resistance and folding endurance, that in the absence of said fibre binder would be imparted to the paper by the nonhydratable fibres present.

4. The article of claim 3 in which the fibre binder component comprises a rubbery constituent chosen from the group consisting of natural and synthetic elastomers.

5. The article of claim 3 in which the fibre binder components comprises a resinous binder selected from the group consisting of melamine formaldehyde resins, urea formaldehyde resins, and phenol formaldehyde resins, and a rubbery constituent chosen from the group consisting of natural and synthetic elastomers.

6. The article of claim 2 in Which said non-hydratable fibres are glass fibres.

7. The article of claim 2 in which said non-hydratable fibres are formed of a polymeric linear terephthalic ester.

8. A paper sheet having a fibre content comprising a substantially homogeneous mixture of at least by volume, of the total fibre volume, of hydratable cellulosic papermaking fibres and up to 40% by volume, of the total fibre volume, of non-hydratable fibres having a moisture regain value up to about 1% selected from the group consisting of papermaking and textile fibres, said nonhydratable fibres having substantially the same dimensions at a temperature of 70 F. throughout the relative humidity range of 20% to the average diameters of said non-hydratable fibres ranging from 0.1 millimicron to 25 microns and the average lengths thereof being at least 15 times the average diameters, said paper sheet in its Width direction varying by not more than 0.8% throughout the relative humidity range of 20% at 90 F. to 65% at F.

References Cited in the file of this patent UNITED STATES PATENTS 1,829,585 Dreyfus et al. Oct. 27, 1931 2,232,389 Iurlrat Feb. 18, 1941 2,322,156 Oglesby June 15, 1943 2,526,125 Francis Oct. 17, 1950 2,582,840 Maxwell Jan. 15, 1952 2,653,870 Kast Sept. 29, 1953 2,673,146 Knzrnick Mar. 23, 1954 2,694,630 Landes et al. Nov. 16, 1954 2,702,241 Hawley et al. Feb. 15, 1955 2,711,365 Price et al. June 21, 1955 2,740,725 Ball Apr. 3, 1956 2,745,224 Erickson May 15, 1956 2,880,080 Rankin et al. Mar. 31, 1959

Claims (1)

1. A COATED ABRASIVE COMPRISING A FLEXIBLE PAPER BACKING WITH ABRASIVE GRAINS ADHESIVELY ANCHORED TO ONE SIDE OF SAID BACKING, THE FIBRE CONTENT OF SAID PAPER CONSISTING A SUBSTANTIALLY HOMOGENEOUS MIXTURE OF AT LEAST 60% BY VOLUME, OF THE TOTAL FIBRE VOLUME, OF HYDRATABLE CELLULOSIC PAPER MAKING FIBRES AND NOT MORE THAN 40% BY VOLUME OF SAID TOTAL FIBRE VOLUME BEING NON-HYDRATABLE FIBRES HAVING A MOISTURE REGAIN VALUE UP TO ABOUT 1% SELECTED FROM THE GROUP CONSISTING OF PAPERMAKING AND TEXTILE FIBRES, SAID NON-HYDRATABLE FIBRES HAVING SUBSTANTIALLY THE SAME DIMENSIONS AT A TEMPERATURE OF 70%F. THROUGHOUT THE RELATIVE HUMIDITY RANGE OF 20% TO 65% AT 70*F., AND IN WHICH THE PERCENT BY VOLUME OF THE NON-HYDRATABLE FIBRES IS NOT LESS THAN ABOUT 2% BY VOLUME BASED ON THE TOTAL VOLUME OF THE PAPER SHEET.