US1972055A - Impregnated sheet products - Google Patents

Description

Pa tented Au fzs, 193.4. 1,972,055

a UNITED STATES PATENT? OFFICE IMPBEGNATED snnn'r rnonucrs George A. RichterJBerlin, N. H., assignortoBrown Company, Berlin, N. H., a. corporation of No Drawing. Original application 1931, SerialNo. 580,496. Divided and'this 811- plication' July 28, 1932, Serial No. 625,531

1 Claim. (or. 91-70) The subject of this invention-is the manufacture of impregnated sheet products wherein novel types of prefabricated fibrous webs serve as foundations or carriers for the impregnant. More especially, the fibrous webs comprehended by the" present invention are those which are waterlaid,

i. e., deposited fromaqueous suspension, on ma- 'chinery of the papermaking class, such as a Fourdrinier or cylinder machine, and which, while capable of undergoing impregnation with aqueous dispersions of binders like rubber latex, are intended for impregnation with impregnants that can be introduced into the web in heat-liduefied condition and then caused toset or congeal as a continuous phase imparting theretosuch' desired properties as body, stiffness, strength, tear resistance, mouldability, water-repellency, and weathering capacity.

In making certamtypesor impregnated sheet products, it is customary practice to start with a' suitable prefabricated fibrous web and to immerse the web in a bath of heat-liquefied impregnant of sufficient fluidity to enter into and throughthe web and then to remove the web from the bath and cause the impregnant to set or harden to its normally solid state. Such, for example, is the practice in making waterproof sheet products for roofing, ring. and building PW, bath then allowed to stiffen in such shape.

In accordance with the present invention, I prepare a waterlaid fibrous web preeminently servlceable as-a carrier for an impregnant, and more especially one which is to be associated in a molten state with such web. There are factors whichI have found to be of great importance in preparing the web of the present invention, especially when comparatively short-fibered cellulose fibers, such as wood pulp, are employed as the raw material on account of their low cost and easy availability. Thus, I have found that one should not strive primarily for the securement of a strong web as womes off in dry condition from the paper machine. On the contrary, onemay advantageously deposit the web from the aqueous fiber suspension and then dry it with the application of little,

ifany, outside pressure either during form-' ing or drying. Other than the drawing together of the fibers on account of the action of surface tension during drying, the web may therefore, be

in practically the same condition of compactness as depositedikfrom aqueous suspension with its fibers only loosely interfelted. The web may hence be tender or lacking in much tensile strength, butlit may be fiuffy or bulky and so highly porous and absorbent that it can readily take up far more than its own weight in molten asphalt or: other liquid impregnant. In making the webs .of the present invention, I usesubstantially unbleached, preliberated cellulose pulps as raw material, preferably substantially unbleached wood pulps, such, as kraft or sulphite, or mixtures of such pulps, Preferably, also, these wood pulps, although substantially unbleached, have undergone refining to a higher alpha cellulose content than that associated with the raw pulp liberated from the wood. The refining treatment may be carried out in a suitable alkaline liquor under conditions to produce an unbleached product having an alpha cellulose content of, say, about 93% to 96%, as is now known to those skilled in the art.

bleached condition are preferably used in prac-. ti'cally unbeaten or unhydrated form in making the webs of the present invention, as beating not only shortens the fibers but generates cellulose hydrate and thereby detracts from the absorbency in the resulting web, even though it makes for greatest tensile strength in the web. As already indicated, however, I am not primarily interested in securing a web of initial high strength at the sacrifice of web absorbency, for reasons which will presently appear.

These raw or refined pulps in'substantially un- I have found that webs made as hereinbefore described are far superior to the kinds of webs heretofore used as the carriers for heat-liquefied impregnants. Heretofore, waterlaid webs wereprepared with a view toward securing considerable initial strength in the web at the expense of web absorbency. I have found, however, that the waterlaid webs of the present invention are so highly absorbent that, despite their low initial strength, they can be drawn continuously through one or more baths of heat-liquefied impregnants, e. g., molten asphalt, without much trouble on account of breakage. I attribute this success to the high rate at which the asphalt enters into and difiuses throughout the web, thereby serving to 2 I 1,972,055 bind together the fibers so that the web immediately upon entering the bath is so' greatly strengthened that it can endure the further stresses in its journey without rupturing. I attribute this success also to' the substantially unbleached state of thefibers and, if they have been refined, further to their high alpha cellulose content. In unbleached state, the fibers are notabLv freer from oxycelluloses. or other degraded celluloses which are unstable, especially under application of heat.

. This applies especially to raw pulps, but it holds true even in connection with refined pulps. Under the application of considerable 'heat, bleached pulpsmay lose much more of their strength than would be the case if the same pulps were'in substantially unbleached condition. Consequently when, as istrue of my web, it is made up of unbleached pulp fibers, it is not weakened appreciably in passing through a bath of hot, liquid impregnant. Moreoveniunbleached pulps invariably yield freeraqueous pulp suspensions than; do similar bleached pulps whose oxycellulose content apparently becomes more or less suspended as fine or colloidal particles in the water of suspension, especially under agitation. 'The unbleached pulp suspension which I useenhances the porosityof the resulting web, for freeness in a pulp sus-' pension is consonant with porosity in the webderived therefrom, whereas slowness in the pulp suspension makes for density in the web. when the web is made from unbleached, refined pulp, it is especially resistant to being weakened by heat on account" of the higher proportion of heat-resisting alpha cellulose in the refined pulp fibers. Moreover, the refined pulp fibers also conduce to a web of enhanced absorbency by virtue of the greater freeness possessed by refined pulps as compared with similar unrefined pulps.

Any one of the species of webs made in accordance with the present invention can imbibe more than 250% of its own weight in molten asphalt,

without having a superfluity of asphalt on its surfaces. This imbibition of asphaltmay take place even more rapidly than when webs of lower asphalt-holding capacity, such as heretofore made, are treated to the limit of their capacity. Ac-

cordingly, the webs of the present invention need be in,contact with the heat-liquefied asphalt for a shorter period of time, to attain a given asphalt content. There is thus less danger of scorching the fibers and causing their embrittlement.

Non-cellulosic fibers, especially those like wool and asbestos, which are more resistant to heat than cellulose fibers, may be blended with the aqueous suspension of cellulose pulp used in fabrieating my web. The cellulose pulp may be preliberated wood pulps or longer-fibered pulps, such as derived from manila, sisal, ramie, or the like, or mixtures of such pulps. Wood pulps offer the advantages of low cost and easy availability, but the blending with such pulps of the longer-fibered pulps derived from manila, sisal, ramie, or the like is desirable, inthat the web attainable from such a blend has higher initial strength. A portion of the pulp may be mercerized, as mercerized fibers appreciate the porosity and bulkiness of the web, although it is preferable to use a substantial portion, if not most, of the pulp in unmercerized condition, because of the difiiculty in realizing a as raw material a kraft pulp which has been refined to an alpha cellulose content of about 93% to about 96%, but which is in essentially unbleached condition. The pulp is suspended in practically unbeaten condition in water. to form a homogeneous dilute suspension, which is run of! on a'paper-making machine operated under conditions leading to a dry web having a compactness falling within the range of about 30 to 55. This means that, in some instances, there must be little, if any, pressure applied to the web during its traverse of both the wet and dry ends of the paper-making machine. Yet, in attaining felts having a compactness at the upper end of the range hereinbeforegiven, it may be necessary to compact or condense the web more or less during its fabrication on the paper-making machine. Webs or felts whose compactness lies at the upper end of the range are of great value in making bituminized sheets for fioorings, where high mouldability, stretchability, and yieldability under distorting or denting influences are undesirable qualities. Felts intended forbituminized roofings, on the other hand, may have a compactnesslying at the lower end of the range, since in such case it is desired that the felts imbibe during impregnation a maximum amount of bitumen or asphalt. The compactness values given are obtained by dividing the basis weight in pounds by the thickness in inches and multiplying by the factor 10 known in papermaking circles, represents the weight in pounds of 480 sheets whose dimensions are 24x36 inches, this being equivalent to 2880 square feet of sheet material. In other words, the compactness value really represents the weight of fiber per unit volume of sheet material. The dry starting web thus produced is tender and tears readily under much stress. Nevertheless, it can undergo bituminization satisfactorily as a continuous sheet by running it through one or more baths of molten asphalt, for instance, air-blown asphalt having a melting point of about 150 F. (ball and ring test) and heated to about 300 to 350 F. The stretch of web in contact with the fluid asphalt may be as short as 3 feet at a web speed of about 100 feet per minute to ensure a uniform and complete impregnation into a product containing about 300% or more by weight of asphalt, based on dry fiber, and yet possessing a dry surface, that is, one free from superfluous asphalt, after the continuous phase carried by the web has been allowed or caused to congeal or set. impregnation may be carried out under other conditions, but in each case thecontact of web with the fluid asphalt is coordinated with a web speed such that a-uniformly andcompletely impregnated product is secured.

The resulting product has high moisture andweather resistance, which properties are traceable to the high proportion of asphalt present therein as a continuous phase. The high asphaltto-fiber ratio reduces the cost of the finished product on a pound basis, because asphalt is considerably cheaper than fiber. The product has ioo The expression basis weight/f as a change in the material used as an impregnant, can serve in the manufacture of thermoplastic shoe stiffener material for cutting into shoe stiffener blanks, e. g., box'toe blanks. In place of the bath of asphalt, a suitable thermoplastic stiffening composition, like rosin, should be used at a temperature of, say, about 250 F. This temperature is suflicient to liquefy the rosin to a fluency ensuring quick and uniform impregnation of the web. If desired, the asphalt may be blended in the proportion of, say 50% with the rosin, in which case the temperature of the bath of mixed impregnant may be raised to, say, about 300 to 320 F. The impregnated web can be. cut up into shoe stiffener blanks, which are characterized by their excellent moulding properties under the application of heat moderate enough not to injure leather. Not only do such blanks lend themselves to moulding in heat-softened condition to the desired shape in the shoe, but they are superior to the article heretofore made. Their excellencefor this purpose is evidenced by the fact that they can be nicely moulded even about sharp edge comers while in heat-softened condition and then caused to set in such shape without subsequently tending to undergo deformation.

It is to be observed that the impregnated prod-" ucts for which the webs of the present invention serve as foundations are not in any way depreelated on account of the unbleached state of the fibers, as the fibers are masked by of the character of molten asphalt, rosin, or the like,- or, by aqueous'dispersions of binderasuchas rubber latex, asphalt, orcolored waxes, which are dried out in the'web to coalesce the dispersed particles as a continuous phase.

By the expression pro-liberated cellulosepulpi as used in the specification and in the appended 7 claim, I mean a pulp which, like wood pulp, has been liberated by a previous digestion of wood or other fiber-bearing raw material in any suitable liquor capable ofisolating the 'flbers as a pulp from the cementitious material binding them together in theraw material. It is thus seen that the expression is exclusive of cotton fibers or rag pulps which must undergo considerable beating to become converted into a suitable half-stock.

Besides, virgin cotton fibers carry waxes on their surfaces, which detract from their absorbency;

This application is a division of my application Serial No. 580,496, flied December 11, 1931, which in turn is a continuation-in-part of my application Serial No. 175,9}6, filed March 16, 1.927.

I claim:

1 A dried, porous, bulky, water-laid web made up preponderantly of preliber'ated, alkali-refined but substantially unbleached wood pulp having an alpha cellulose content of at least 93%, said web being impregnated with a bituminous imprcgnant initially distributed in heated condition in the web. I GEORGE A.