US3179342A

US3179342A - Method for producing leather fiber slurry

Info

Publication number: US3179342A
Application number: US217775A
Authority: US
Inventors: Harland H Young; Edward J Majka; Richard H Eshbaugh
Original assignee: Swift and Co Inc
Current assignee: JBS USA LLC
Priority date: 1957-01-08
Filing date: 1962-07-23
Publication date: 1965-04-20
Anticipated expiration: 1982-04-20

Description

United States Patent Ofifice 3,179,342 Patented Apr. 20, 1965 3,179,342 METHOD FGR PRQDUCING LEATHER FTBER SLURRY Harland H. Young, Western Springs, Edward J. Mailra,

Chicago, and Richard H. Eshbaugh, Hinsdale, 111., assignors to Swift & Company, Chicago, 111., a corpora tion of Illinois No Drawing. Original application Jan. 8, 1957, Ser. No, 632,984, now Patent No. 3,116,269, dated Dec. 31, 1963. Divided and this application July 23, 1962, Ser.

No. 217,775 r 3 Claims. (Cl. 241-4) This application is a division of application Serial No. 632,984 filed January 8, 1957, now Patent No. 3,116,200 granted December 31, 1963.

This invention relates to a reconstituted leather product and its method of manufacture. The product in its preferred form is a sheet of reconstituted leather fibers which has characteristics closely approximating the leather properties of color, odor, flexibility, surface appearance, and warmth of feel or hand.

The idea of reconstituting leather and particularly leather scraps into a usable sheet form is old and attempts have been made over the years to reduce animal leather to a pulp form and then to reconstitute the fibers into a sheet resembling a hide. Various approaches have been employed, including processes which employ some of the techniques of the paper and hardboard industries. The procedures utilized in the past fall generally into five distinct classes:

Class I.-Leather, with or without other fibrous materials, is finely comminuted and handled through conventional paper or board making machinery without any added binders.

Class II.-Leather processed as in Class I has added to it such conventional proteinaceous binding materials as glue, casein, blood or hydrolyzed leather, and then is formed under heat and pressure into sheet or other molded form.

Class III.--Leather fibers, alone or admixed with other fibers, are worked into a dough form with various mastics, such as rubber, resin, rosins, oil, grease bitumin, asphalt and the like, followed by molding into the desired shape. This technique employs organic solvents or even water to make a semi-fluid workable mass.

Class IV.-Leather fibers with or without other fibers, are formed into a porous felt, sometimes with the aid of paper making techniques, which felt is then dried prior to saturation with solutions or emulsions of rubber, resin, waxes, asphalt, and other materials of like nature.

Class V.--Leather fibers, with or without other fibrous extenders, are used as fibrous re-enforcement for plastic sheeting made of any of many plastic materials, e.g. polyvinyl chloride, polyvinylidene chloride, rubber, polyisobutylene, polyethylene, and other thermoplastic materials.

Defects have been present in all of the foregoing leather-containing products, to the extent that only those products falling in Class V have become commercial realities. The unsized, or unbonded, fabrics or felts of Class I have a very low resistance to a tearing or rupturing force because of the short fiber lengths and the absence of the fiber-interlocking characteristic of natural leather. The addition of a size or binder as in Class II does not correct these defects and, in addition, produces a stiff sheet incapable of sharp angle bends. The product of Class II also has poor resistance to tearing and resembles dried, untanned, and unplasticized hide.

The leather-containing products of Class III above, contain more mastic or plasticized binders than they do fibers and since such mastics, except for bitumin and asphalt, are expensive, they have not been widely used.

The more promising mastic sheets of this type have not used leather fibers at all but have made use of rags, cotton, wood, or paper pulp. High-grade, heavy-duty, asphalt roofing paper belongs in this group.

The products of Classes 1V and V have been made commercially and have considerable flexibility, high strength and excellent resistance to moisture and solvents generally. They are little more than fiber reinforced plastic sheetings, however, and in addition to being expensive they possess the cold feel and the texture of plastic sheeting. Products of this type have a continuous resin film and hence are impervious and do not breathe as does leather. In the Class V product the leather fibers are used solely for the reinforcement of the plastic material and is used for flooring tiles, luggage and upholstery. However, it does not resemble leather and is not used as a substitute for leather in luxury items.

We have discovered that with suitable modifications and some additional techniques, the practices of the hardboard and paper industries, together with the use of a particular bonding resin, namely, polyvinyl acetate, and certain plasticizers, it is possible to manufacture a reconstituted leather product with properties and characteristics unattainable in the many products of the past. The raw material used in the manufacture of our improved reconstituted leather product is mineral tanned leather, the scraps of which at the present time have little value. These leather scraps or hide pieces, if desired, properly processed may be reconstituted into sheet form of predetermined dimensions and desirable properties. The re constituted leather sheet has many characteristic leather properties relating to color, odor, flexibility, surface appearance, and warmth of feel. The sheet, unlike most of the products of the prior art, is composed of a maximum amount of leather fibers, with a minimum of binder or resinous adhesive. The sheet is free from the characteristic resinous varnish-like, tacky, or rubbery surface commonly found in reconstituted leather products using a thermoplastic substance as the binding material. The reconstituted leather features a porosity, or moisture vapor permeability approaching that of leather, which is unattainable in products where the resin is the continuous phase of the sheet.

Attempts have been made before to convert tanned leather scrap into an aqueous fibrous pulp or stock and mat that aqueous pulp to form a sheet. The leather fibers have been invariably excessively hydrated, with the result that the ultimate sheets have proven to be objectionably brittle. Leather placed in water will take up an amount of moisture which may be called free water. A highly hydrated leather contains not only free water, but in addition it will carry a relatively large amount of combined or bound water. Hydrated leather comprises a hydrophilic, though insoluble, protein, which tends to gelatinize when swollen by the bound water. With the practice of the method of our invention for producing a fibrous slurry, the fibers in the resulting pulp slurry are not excessively hydrated even though suspended in an aqueous medium. As pointed out above, this is highly advantageous since a reconstituted leather sheet made from excessively hydrated hide fibers is brittle. In addition, a pulp made up of highly hydrated fibers has a slow felting rate, i.e., a longer time is required to drain the water from the fibrous slurry in forming the mat, and in addition it is dimcult, following felting, to express water from a Wet mat made up of hydrated pulp. This is reflected in the tendency of a highly hydrated pulp to flow or spread under pressure rather than to lose free water from the mat. In our preferred method for producing a fibrous slurry or stock from mineral tanned leather pieces, the leather pieces suspended in water are passed through a milling zone and within that zone they are subjected to a 3 brief but intense rubbing or shearing action by'closely spaced and serrated surfaces. The serrated surfaces effeet a shearing force upon the leather pieces therebetween, milling the water-carried leather pieces to a fibrous slurry or pulp. The leather is placed in a slurry or fibrous stock form in a short interval of time, requiring only a few seconds, generally under ten seconds of the intense milling is adequate. Prolonged milling of the leather is principally. responsible for the hydration of the fibers and should be avoided for the best results.

We have found that only the mineral tanned leathers, e.g. chrome, alum, zirconium and iron tanned, are suitable for use in the manufacture of our improved composition. Chrome tanned scrap will normally be used because of its large volume and availability. Iron, alum and zirconium tanned leathers are not available in amounts sufficient to produce commercial quantities of scrap. There is a profound difference between mineral and bark tanned leathers. The former mills readily into well-defined fibers or fiber bundles, whereas the latter on milling yields an excessively hydrated pulp that conglomerates into slimy, stringy masses that felt very slowly. Reasonable felting time is dependent upon the freeness of the stock and is a commercial necessity. We are of the opinion that a bond is formed between the chromium or other metal ion fixed on the protein fiber and the acetate radical bonded to the polyvinyl nucleus. This bond partially explains the superiority of polyvinyl acetate as a binder for mineral tanned leather.

The lowest moisture content to which the leather has been dried in its previous handling is of significance in the preparation of the aqueous fibrous stock. The leather which has an initial or equilibrium moisture content within the range of l-25% is most suitable for the preparation of stock having discrete individual fibers or fiber bundles. It has been our experience that a leather which has say a moisture content at time of milling of 50% may not be readily disintegrated to provide a stock of suitable fineness without the danger of hydrating that leather to the point where it will take up an objectionable amount of bound or combined water. Actually this critical moisture content is the lowest moisture content to which the chrome or other mineral tanned leather has been subjected, rather than its actual moisture content at the time of milling. That is to say, a leather which has been pre-dried to a moisture level within the recommended range of -25% and held there to reach an equilibrium may be placed in water prior to milling. This leather will then of course have a moisture content outside of the range, but this is not objectionable providing the leather has not been held for a period of time long enough to permit a large amount of the water to become bound, thus excessively hydrating the leather. Hence it is our recommendation that the leather used in our process have an initial moisture content within the range of 1025% as such leather may be readily milled to provide the necessary fine and less hydrated fibers needed for reconstituted leather sheets having greater strength. If dried to a moisture content below 10% there is danger of embrittlement and ultimate production of shorter fibers leading to poor strength in the final sheet. If not dried to the preferred range, then coarser mill settings are required. Coarser mill settings give coarser fibers and fiber bundles which make weaker leather sheets.

At the outset of our process we prepare a pulp or Water slurry of the fibers described above. The resin, preferably with a plasticizer, is introduced into the slurry and time allowed to permit precipitation of the resin and plasticizer onto the individual fibers and fiber bundles. The slurry is then felted, draining off most of the water, to form a fibrous mat. The mat is generally cold pressed to remove more of the water and the resulting cold pressed felt is then dried to further adjust the moisture content. Following drying the felt is hot pressed to activate the thermol plastic resin. The sheet at this point may, if desired, be subjected to further finishing.

It is essentially for the production of a good reconstituted leather sheet that the plasticizer or plasticizers used be thoroughly mixed with the resin before addition of the resin-plasticizer mixture to the slurry. If the materials arenot thoroughly mixed, the finished product will exhibit an unsatisfactory finish in having stained or spotty surfaces, and in addition the strength of the product will be impaired.

The aqueous fibrous stock must have a reasonable felting time in order to provide a commercially acceptable process. This may be had to some extent by raising the temperature of the fibrous stock before felting. We have discovered that the freeness of the stock may be appreciably increased through the use ofcertain anionic surfactants added to the stock prior to felting. The preferred class of anionic surfactant comprises sulfated fatty materials whether derived from vegetable, animal or marine sources.

It is critical that the moisture content of the mat from the felting operation be adjusted to a moisture conent within the range of 10-30% before hot pressing to activate the polyvinyl acetate resin. Moisture contents in excess of 30% give an unsatisfactory bond, and moisture contents of less than 10% at the time of hot pressing result in a relatively dry board exhibiting an objectionably chalky exterior. In our preferred method the wet felt is cold pressed at any suitable pressure remembering that excessively high pressures, rapidly applied, may cause flowing of the felt with resultant fissures or spreading. High pressures are satisfactory when gradually increased as the water is expresed. Following cold pressing the leather sheet is air dried to complete adjustment of its moisture content to within the recommended range.

It is recommended that the mat, after cold pressing and drying, be hot pressed at pressure within the range of 3001000 psi. at a temperature of 300 F. Usually a pressing time of 5 minutes is sufiicient for a moderate pressure of 600 psi. and a temperature of F. With the higher pressures and temperatures the time of pressing will drop accordingly and sometimes a pressing interval of as little as /2 minute will sufiice. If pressures and temperatures at the lower end of the range are employed, the time should be proportionately increased. A reconstituted leather sheet may be manufactured with pressures somewhat in excess of 1000 psi. but very short times of pressing would necessarily be resorted to. Pressures less than 300 p.s.i. could be employed if excessively long periods of pressure are not objectionable and gen erally, if such low pressures are used, it is best to increase the amounts of resin and plasticizer incorporated in the sheet.

The reconstituted leather sheet of our invention in its preferred form comprises chrome leather fibers bonded by plasticized polyvinyl acetate. The total amount of resin and plasticizer is less than the amount of leather fibers, and preferably the plasticizing composition should contain an aromatic hydrocarbon sulfonamide such as an N-substitutedtoluene sulfonamide. It is characteristic of our composition that the plasticized resin does not constitute the continuous phase. An aromatic hydrocarbon sulfonamide, when used in conjunction with a plasticizer incorporated for low temperature flexibility, will impart a feel very similar to that of natural leather and will in addition appreciably enhance the bursting and tearing strength of the sheet. In our preferred reconstituted leather sheet the mineral tanned leather fibers make up 60 to 85% of the composition, with 10 to 25% of the polyvinyl acetate resin, and 5 to 15% of the of one table should not, without caution, be compared critically with that of another because of variations in testing. The Mullen test has been used as a general measure of strength and it should be remembered that the results of the Mullen test vary with temperature, humidity and thickness of the sheet. Most of the Mullen test data has been run on samples conditioned at 72 F. and 50% relative humidity. However, several of the tests have been run under other conditions which were constant for the experiments reported but not necessarily the same conditions that existed during other tests. Some differences arise because of the variation in sheet thickness from test to test. The extent of fiber reduction, the nature of the raw materials, and other variables make it necessary to generally limit comparison of results within the same experimental run. It is no coincidence that the known art and present commercial reconstituted leather products feature vegetable tanned leather scrap as the raw material in spite of the fact that chrome tanned scrap is much more abundant and available. It is our opinion that this is due to the fact that a satisfactory sheet of reconstituted mineral tanned leather has not been previously prepared.

The properties of the leather itself are responsible for the various techniques which we use. The ease with which the leather is reduced to fiber and the type of fiber produced in pulp form is vastly different depending upon the tanning method used. Vegetable tanned leather, if dry milled, does not act too differently from chrome tanned stock. In wet milling, however, the difference is great; vegetable tanned stock produces a ropy fiber which tends to congeal into long ropy masses which are highly hydrated, whereas chrome stock produces individual fibers that tend to remain in free suspension over a wider range of conditions. Vegetable tanned stock will wet more rapidly than does chrome stock and this is probably due to its greater content of fat liquoring components. Whereas one would expect fats, resins, waxes, and other water repellants of this type to increase the moisture resistance of leather fiber the reverse is true. Therefore chrome tanned stock characterized usually by an absence of fat liquor or other lubricant does not wet readily and when reconstituted into sheet form after pulping will not rehydrate unless soaked in water for a prolonged period of time. When we use a thermoplastic resin as a binder for chrome leather fiber we then find a greater ease of hydration which is a function of the amount of resin and the amount of plasticizer. In other words the combination of resin and plasticizer which we incorporate into our reconstituted sheet functions not only as a binder but a fat liquoring agent and lubricant as well. The proper choice of resin and plasticizer is therefore critical in the control of the surface feel and other physical properties of our reconstituted sheet.

There .are a number of pulp forming machines in addition to the conventional hammer or stone mills which have been used in the past in the manufacture of aqueous stock in the paper making industry. We have found that one type of machine is much more suitable than the others in the preparation of a leather fiber pulp. The type of machine which is particularly adapted to our needs is a machine which features serrated shearing surfaces with provisions for adjusting the clearance between the two surfaces. In general, then, we require a mill which has serrated plates, one shearing against the other at a predetermined clearance. We prefer to use the Bauer type mill alone or, in the case where a very fine pulp is desired, in conjunction with a Jordan mill particularly when very thin drapery stock is desired. The larger pieces of the chrome or other mineraltreated leather scrap should be first processed by a hammer mill or shredder or some other rough cutter to reduce the scrap in size so that it can be fed to the Bauer mill with some uniformity of feed. A suitable size of leather piece is that characteristic of the industrial leather waste known as chrome shavings. There are several types of Bauer mills available and in each type of mill there is a rotation of one disc with respect to another. One type of Bauer mill has a fixed disc against which a like disc rotates. Still another type has both discs rotating in opposite directions. The leather pieces suspended in water or with a stream of water are introtroduced into the milling zone defined by the two closely spaced discs at their axis of rotation and are moved outwardly with the assistance of centrifugal force to the periphery of the milling zone. During its brief passage through the mill the materal is subjected to an intense shearing or rubbing force supplied by the closely spaced revolving serrated discs and by the leather rubbing against itself. This action mills the water carried leather pieces to an aqueous stock of leather fibers. The dwell time of the leather pieces in the milling zone is very brief, an average of two to ten seconds depending upon the diameter of the discs.

In the milling or grinding step water is used in order to float the stock out of the grinding zone after it has reached the desired fineness. While the amount of water used can be varied, it must be sufiicient at all times so as to force the fibers out of the mill before severe heating can take place. It has been our experience that satisfactory fiber characteristics are obtained only when the weight of the water through the mill with the fiber is at least 4 times and preferably 10 to 20 times the dry weight of the scrap leather fed. Amounts of water less than 4 times the amount of leather results in an "objectionable heating of the fibers during milling, A single passage of the material through the mill will place the leather fibers in a suitable form for subsequent processing. Multiple passages through a mill or milling through subsequent or consecutive zones may be resorted to but is not normally needed, and generally extensive milling should be avoided as it will cause an objectionable hydration and/ or breaking of the fibers.

Chrome leather scraps vary greatly in their moisture content, from 60% moisture down to as low as 5%, depending upon whether the leather scrap is fresh from the tannery or whether it has been stored a sufficiently long time so as to dry out. The lowest moisture content to which the leather has been dried in its previous handling is a critical factor in determining the proper clearance between the shearing surfaces of the mill. We know from experience that chrome scrap leather which has been thoroughly dried, i.e. to less than 15% moisture content, will provide a satisfactory fibrous pulp upon milling in an 8 diameter disc Bauer mill where the discs are set 0.003"-0.005 apart. Bauer mills are classified by the size of their shearing disc and are available with 8", 24", 30 and 36" discs. Obviously, the dwell time of the leather in a larger mill is somewhat greater than in a smaller mill and accordingly the clearance between the discs should be increased for the larger miils. A satisfactory leather slurry is an aqueous fibrous stock which exhibits suitable freeness and one which may be felted in reasonable time with commercial equipment. A leather scrap fresh from the tannery will generally contain from 60% moisture and if milled with a setting of 0.003"- 0.00'5 in an 8" disc Bauer mill (as the dried leather scrap above) will produce a pulp that will felt so slowly as to be impractical. Therefore, stock having such high moisture content must be milled at a coarser setting, for example, 0.025"0.030". If, however, the high moisture stock is predried to a moisture level of 15 then resoaked prior to milling, it behaves as dry stock and may be milled with a setting of 0.003"0.005 in an 8" disc Bauer mill. The disc clearance in either a Bauer or Jordan mill is a function of the lowest moisture con tent to which the chrome leather scraps have been subjected rather than to its moisture content at the time of milling. A reconstituted leather sheet composed of finely milled fibers has a greater strength than a sheet TABLE I Minimum moisture content to which leather scrap has been reduced, per- Optirnum clearances between discs for Bauer Mills of various sizes. Disc diameters and clearances given in niches.

cent

The freeness of a felt increases while strength decreases when the mill clearances are increased. The time required to drain the water from the fibrous slurry in the formation of'the leather sheet is known as the felting time. Time for felting is a matter of choice, but obviously in a commercial operation it may not be unreasonably long and for this reason some strength will necessarily be sacrificed to obtain an acceptable felting time. In some instances slower felts which produce much stronger sheets may be desirable for certain uses and if so, felting capacity must be increased in order to maintain production. The following example illustrates the relationship between mill clearances, felting time, and product strength.

Example I I Chrome leather shavings, initially having a 50% moisture content, were dried in air until the moisture level reached 25%. This material suspended in water was run through an 8 Bauer mill at the several disc settings shown in Table II below and for each mill clearance one portion of the fibrous slurry was felted into a control sheet and the other portion, prior to felting, was treated with a polyvinyl acetate resin-plasticizer mix. Both the control and the resin bonded sheet were cold pressed at 300 p.s.i. to remove free water and dried to a moisture content of 20%. The sheets were hot pressed at 600 p.s.i. for two minutes at a temperature of 140 F. The sheets were held after hot pressing for a period of time at room temperature to condition. Table II shows the time required for felting the control and the the resin bonded sheet for each Bauer mill setting and the Mullen test data on the two completed boards. The time of felting is in seconds and is described as freeness in the table. per square inch. Freeness is the time in seconds required to pull the free water from a standard volume of the slurry at a vacuum of Hg.

The Mullen test data is in pounds significant.

. s! increased by a rise in temperature of the slurry. The following example illustrates this.

Example II Chrome tanned shavings which had an initial moisture content of 12% were processed as in Example I using the various clearances indicated in Table III below between the shearing discs of an 8 Bauer mill. Samples of fibrous stocks were heated to temperatures of either F. or 140 F.

Here again it is seen that freeness is poorest and strength is the greatest at close clearances of the mill discs. As these clearances increase, freeness increases and strength decreases. The data of Table III indicates that heating the fibrous slurry prior to felting increases freeness to the point where advantage can be taken of the greater strength resulting from the closer mill settings. In the milling of chrome leather scrap containing 12% moisture, the mill settings for an 8" Bauer mill may be in a range of 0.004" to 0.008 if the pulp slurry is heated to 140 F. before felting. If the slurry is not heated then mill settings of 0.010" or greater should be used to obtain a practical felting time which will result in an accompanying decrease in strength.

Example III The work of this example illustrated the effect of the moisture content of the mineral'tanned scrap on milling technique. Chrome shavings direct from the tannery and containing 51.1% moisture were air dried in a tumbling barrel. Samples were withdrawn after 0, 45, 90, 120, 150, 160, 180, and 200 minutes of drying time and were found to have moisture contents respectively of 55.1%, 48.5%, 44.4%, 37.2%, 30.6%, 24.8%, 14.6%, and 8% after reaching equilibrium. Each sample was passed through an 8" Bauer mill at the three disc clearances of 0.005", 0.015", and 0.025". The ratio of water to fiber through the mill in each instance was approximately 10:1. There was some little difference in dwell time among the several samples because the higher moisture stock runs through more rapidly than does lower moisture stock; however, this was not considered to be Freeness (time required to form the felt from the fibrous stock)- was measured as seconds re- .Resin bonded, Mullen -11-- 424 psi...

From our experience, six minutes felting time is com- .mercially unacceptable except where an exceptionally high strength felt is desired. We consider a 2-3 minute. felt to be more practical. As will be seen from an examination of Table II, a -second felt may be had .by milling 25% moisture level chrome shavings through an 8" Bauer mill with a disc clearance of 0.010".

'for two days.

permitted to dry to equilibrium at room temperatures 7 They were then hot pressed for 3 minutes at F. and600 p.s.i. The finished sheets were conditioned at 50% relative humidity and 72 F., and

The freeness of the fibrous slurry in felting may be 7 5 on weighing were found to contain 16.5% moisture.

The times of felting and results of the Mullen strength not uniformly dispersed throughout the fiber, which will tests are in Table IV. result 1n an mferior reconstituted leather sheet. For

TABLE IV Moisture Weight of Freeness content of Clearance Freeness conditioned Mullen inseconds Avg. leather scrap ofdises in seconds felts at 16.5% test Mullen, p.s.l.

moisture, gm.

0. 005 540 85. 7 144 3. 7 0. 015 390 .86. 79 4a 9 4. 3 0.025 450 90. 8 107 4. 2 0. 005 300 78. 128 2. 3 0. 015 210 80.0 81 2. 6 2. 5 0.025 180 82.2 70 2.6 0. 005 240 85. 9 101 2. 4 0. 015 180 80. 5 70 2. 6 2. 4 0. 025' 105 87. 3 58 2. 1 0. 005 300 80. 4 140 2.1 0.015 150 83.8 85 1. 8 1. 8 0. 025 90 83. 5 66 1. 4 0. 005 240 86. 2 118 2. 0 0. 015 80 87. 8 07 1. 2 1. 5 0.025 t 60 88. 1 53 1.1 0. 005 300 72. 4 190 1. 0 0. 015 135 83. 5 97 1. 4 1. 4 0.025 70 83.4 75 .9 0. 005 300 83. 1 183 1. 7 0. 015 150 84. 7 112 1. 3 1. 4 0. 025 120 83. 6 05 1. 3 0. 005 510 82. 4 180 2. 8 0. 015 210 83. 0 114 1. 8 1. 9 0. 025 90 82. 2 s3 1. 1

Variations in the pH of the fibrous slurry at the time of felting has no significant effect on the strength of the board as evidenced by the Mullen test, providing excessive acidity or alkalinity is avoided. The normal pH of the slurry out of the mill is in the range of 3.5 to 5.0.

It may sometimes be desirableto mix other fibers with the leather pulp and where this is done such other fibers should be first milled according to that method which is most applicable. Other fibrous pulps can be prepared from wood, rags, linters, straw, etc., in the ways commonly used in the paper pulp industry for these materials. When incorporating non-leather fibers as extenders, simple mixing of the fiber slurries should not be resorted to. Mixing is best effected by running the combined pulps through a subsequent milling during which the discs are set relatively wide apart in order to avoid excessive fiber reduction. The addition of nonleather fibers as extenders reduces the true leather feel, warmth, flexibility, and appearance to the extent that such fibers are incorporated in the sheet.

After milling and in the instance where extenders are used, after thorough mixing of the combined fibers the fibrous stock may be used immediately or stored for future use for a reasonable length of time (few. days). If storage space is limited, the milled slurry effluent may be screened and pressed to remove excess water and stored at a consistency up to fiber solids. More concentrated slurries do not stir out well and may require remilling when diluted. Excessive milling should be avoided since it may shorten fiber lengths with corresponding reduction in strength of the final product and increase hydration of the fibers thereby making the end product more brittle.

The fibrous content of the aqueous leather, stock to which the resin-plasticizer mix is added may be varied over a wide range, generally from about 1% to 10%, being limited by the time required for complete precipitation of the resin and plasticizer and the difliculty of mixing heavy pulp slurries uniformly. It has been our experience that a slurry containing, say, only 0.5% leather fibers requires an excessively long time for complete precipitation and adsorption of the resin-plasticizer emulsion. If there is a milky efiiuent from the felting operation that is evidence that the expensive resin-plasticizer mix has not been Wholly precipitated on the fibers. Pulp slurries of 10% fiber content are difiicult to agitate, and even though the resin will precipitate rapidly it is best results we recommend slurries containing from 1% to 6% dry fiber. Resin precipitation is normally completed in a few minutes but in case of rather dilute slurries continued agitation for perhaps 15-30 minutes may be required. Normally the leather fibers Will have sufiicient tanning agents, chrome or other mineral tanning material, to effect precipitation of the resin from its emulsion, but in the event the fibers are insufiicient in tanning agents additional precipitants may be added, particularly when working with very dilute fiber slurries. Satisfactory precipitants include the syntans, bark tan nins, and the soluble salts of aluminum, iron and chromium. The normal pH of the fibers of pulp, which will be in the range of 3.5 to 5.0, is suitable for these precipi tants. The pH range may be varied without impairment of the strength of the final sheet within the range of about 2.0 to 9.0. However, beyond these extremes the tanning agents have a tendency to leave the leather with resulting decrease in the freeness of the felt stock due to gelatinization.

The free water of the slurry may be separated from the leather fibers through the use of any of various available felting machines. The equipment used in the manufacture of felts for the subsequent processing into paper, hardboard, pasteboard, fibreboard, insulation board, and the like, is suitable for our purpose. Batch filters of the Chapman Box type which involve flotation of the slurry over a flooded screen bottom, followed by evacuation under vacuum of the water through the screen, leaving a felt thereon, may be used. Continuousfelting machinery such as the Oliver continuous felter is also adaptable to our processing. The Oliver felter features a rotary drum provided with an outer screen surface and an inner mechanism for evacuation. This drum rotates partially submerged in the fibrous stock and when the felt reaches the desired thickness it is removed from the screen as a continuous felted sheet. The speed of the felting drum may be varied with the felt thickness desired.

The felt is freed of excess water by cold pressing on a screen. Cold pressing may be accomplished by hydraulic pressing with large fiat surfaces or byrunning the felt through squeeze rolls. The pressure applied and the rate at which the pressure is applied must be correlated with felt freeness to extract water without damage to the felt. Generally speaking, when using large flat surfaces to press the board, pressures in the range of to 500 p.s.i. are satisfactory. Pressures applied too quickly may cause fissures or undue spreading or flowing of the felt.

.short interval.

of the leather pieces to form a fibrous slurry, greater strength may be imparted to the finished reconstituted leather sheet by more finely milling the fibers. However, pulp slurries containing fine fibers in contrast to more coarse fibers require longer periods of time in the felting operation to separate the free water. As mentioned earlier, heating of the slurry prior to felting is known to increase the freeness to some extent. We have found that the freeness of the slurry can be increased appreciably through use of certain anionic surfactants to the point where advantage can be taken of the greater strength resulting from closer mill settings of the Bauer mill. The anionic surfactant may be added to the fibrous slurry before or after the incorporation of the resin-plasticizer mix. The one particular type of anionic surfactant which we have found to be satisfactory is sulfated fats, Whether derived from vegetable, animal or marine sources. This group of anionic surfactants has a marked accelerating effect affording a reduction of from 50 to 80% in felting time for leather fiber slurries. Cationic surfactants are unsuitable and will generally decrease freeness without appreciably increasing the strength of the finished sheets and can be generally expected to increase felting times from 30 to 60%. For the most part non-ionic surfactants behave like the cationic surfactants in that theyincrease. felting time though to a lesser degree.

We prefer to use a continuous drier featuring a flow of Warm air countercurrent to the movement of the felt.

A drier of this type minimizes case hardening and produces a felt with uniform moisture content at a level suitable for hot pressing. Case hardening of the fibers or felt may result if an excessive temperature difference and drying rate should be used. The felt should not be heated above 260 F. It has been our experience that a stream of warmair at a temperature within the range of 120 F.

and 250 F. applied to the felt for /2 to 5 hours in a tunnel drier will produce a felt ready for hot pressing having a moisture content within the range of 12 to 25 The felt from the cold pressing operation, if held at room temperature and at a relative humidity within the range of 20 to 90% will in time, usually in excess of 16 hours, reach a moisture content within the recommended range of 30%. However, such long periods of time are not commercially feasible.

Conventional systems of applying heat and pressure are satisfactory provided allowances are made for suitable or it may be a series of paired rollers arranged as in the paper-making industry. In any event, the press mechanism should be capable of developing a pressure of 300 to 1000 psi. for a period of time from /2 to 10 minutes (usually 5 minutes is adequate) with provisions for maintaining the temperature of the pressing surfaces from 120 to 300 F. Temperatures in excess of 300 F. are avoided because spotting or stains due to migration of the polyvinyl acetate binder and plasticizer will occur at such I elevated temperatures unless the pressure is applied for a If a formula of resin and plasticizer is used which is designed to produce a softer or more flexible finished felt, it becomes necessary to decrease the temperature, pressure, and the time under pressure. The variables of time, temperature, and pressure should be integrated so as to provide a pressing cycle which will give maximum strength and most leatherlike properties at a minimum cost of equipment, labor, power and other expenses.

Obviously, if one Were to use hot rolls or calender rolls typical of papermaking machinery, where exposure to temperature is very short or instantaneous, then temperatures up to 350 F. may be used. Conversely, one could press at F. for hours using very high pres sures, e.g. 3000 p.s.i., and get a satisfactory sheet. However, modern production methods would generally demand the following ranges in order to minimize production costs.

Pressures 300 to 1000 lbs. per square inch.

Temperatures F. to 300 F. Times /2 min. to 10 min. Moisture content 10% to 30%.

The following Example V demonstrates the wide variations in pressure, temperature, and time that may be selected.

Example V Chrome leather shavings with a moisture content of 22% were milled in an 8" Bauer mill set to a disc clearance of 0.015". The leather scrap was fed to the mill suspended in 20 lbs. of Water for each lb. of leather. The pulp was gathered in a drum and found to contain 11.5 lbs. of dry fiber. Additional water was added to adjust the fiber solids content to 3%. 3.5 lbs. of polyvinyl acetate emulsion (50% solids) was agitated for 15 minutes while adding 1 lb. of water and a clear solution containing 0.5 lb. of dibutyl phthalate, 0.5 lb. of N-ethyl toluene sulfonamide, and 0.5 lb. chlorinated biphenyl (60% chlorinated). When the plasticizer and the added water had 'been incorporated in the polyvinyl acetate emulsion, the

mixture was stirred for an additional 15 minutes and the completed resin-plasticizer mixture was held for 24 hours. The following day the aged resin-plasticizer binder was added slowly to the pulp slurry at 65 F. while the slurry was vigorously agitated. Agitation was continued for 30 minutes at which time the supernatant liquor cleared, indicating complete adsorption or precipitation of the binder on the leather fiber. Solids content of the final slurry was checked and found to be 3.8%. Separate batches were weighed out and drained to form wet felts. The felts were held in a drier supplied with a stream of air at 120-440 F. for 5 hours. The dried felts were held for another 24 hours at room temperature and at the end of that period were found to have a moisture content of 18.8%. The several felts were then pressed under the conditions of time, temperature and pressure as shown in Table VI following. The Mullen tests were run after the hot pressed felts had been conditioned 16 hours at room conditions of 72 F. and 50% relative humidity.

' TABLE VI Pressure Temp, Time in Mullen Appearance of felt in lbs/in. F. minutes test 3. 0 340 Satisfactory. 2. 0 355 Do. 1.0 325 Do. 0. 5 300 Slightly stained. 220 0.25 310 potted. 240 0.1 270 Do. 140 4. 0 300 Satisfactory. 160 3.0 290 Do. 180 2. 0 310 D0. 140 5.0 280 Do. 160 3. 5 260 Do. 180 2.0 265 Do.

Spotting or stains due to migration of the binder prevail when temperatures near 200 F. are used. This is accompanied by some decrease in strength due probably 'to migration of the binder to the surface. This migration is also greatest at higher pressures and longer times. It can be seen also that strength increases with pressure and that the pressure and temperature are more critical than time.

The finishing of the grained surface may be achieved by any of the various techniques now used in the leather 13 industry. Embossing by heat and pressure may be done before or after applicationof coloring or surface finishing. The embossing plates may be used in a flat press or as part of a pressure roller system. The moisture content of the reconstituted leather sheet should be in the same optimum range of -30% as recommended for hot pressing. Hence, if the moisture content of the reconstituted leather sheet should be below 10%, it is recommended that it be rehumidified before imprinting the desired grain.

If a smooth, satin surface instead of an embossed grain is desired, the sheet may be calendered with hot rolls or smooth plate in a hot press in the fashion used for conventional leather. The preferred temperature range for finishing is in the range of ISO-200 F. and the recommended pressure range is 300-600 p.s.i. The time required for this plating operation may vary from substantially instantaneous at l80200 F. to 30 seconds at 150- 180 F. The moisture content, like that recommended for embossing, is in the range of 10-30% for best results. If excessive temperatures, pressures or times are employed, the surface of the leather sheet tends to become glazed or resinous due to migration of the plasticized polyvinyl acetate resin to the surface. Migration of the resin to the surface mars the soft, leather-like finish that can be had with the use of proper temperature, pressure and time. Typical leather finishing steps such as dyeing, pigmentation, lacquering, enamelling, waxing, or polishing can be effected by the methods now in use by leather finishers. However, an advantage afiorded by our method of manufacture from pulp slurries is the opportunity to produce intense colors by dyeing of the fibers in slurry form or dispersion of several pigments together in the slurry form prior to felting. Interesting mottled or marble effects can be had by mixing different colored slurries together prior to felting. The several following examples illustrate some of the various conditions and materials which may be utilized in producing a reconstituted leather sheet in accordance with the teachings of our process.

Example VI heavy slurry was diluted with water to a consistency of 24 gms. of a (50% solid) polyvinyl acetate emulsion were diluted with 4 gms. of water. The diluted emulsion was heated to 90 F. and to 'this was added a clear solution containing 4 gms. of dibutyl phthalate, 4 gms. of N-ethyl sulfonamide of mixed orthoand paratoluene, and 4 gms. of a 60% chlorinated biphenyl. The polyvinyl acetate emulsion was continuously agitated while the plasticizer solution was poured slowly into it. Mixing was continued for 30 minutes and the resin-plasticizer mix was then allowed to stand for 24 hours.

The aged resin-plasticizer mix was added to 8.8 lbs. of the pulp slurry (65? F.). The slurry with the added plasticizer-resin mix was agitated for minutes at which time precipitation was completed as evidenced by a clear, non-milky, supernatant liquor.

The slurry was then poured over a screen of a felting box and after being dispersed uniformly was evacuated to form a felt. The Water was drained from the mat in approximately 3 minutes. Free water was expressed by cold pressing at 300 p.s.i. and the felt dried over night in air at 80 F. The dry felt, which had a moisture content of 16%, was hot pressed for 3 minutes at 140 F. and 600 p.s.i. The following day the finished sheet was plated by pressing instantaneously at 180 F. and 600 'p.s.i. The final reconstituted leather sheet showed an average Mullen test of 470 lbs. per square inch.

14 Example VII This example was handled as in the previous example with the following variations:

Moisture content of the scrap "percent 15 Disc clearance 0.005" Ratio of water to leather 15:1 Consistency of slurry after adjustment "percent 3 Weight of slurry used lbs 5.9

Resin emulsion used: 26 gms. polyvinyl acetate (50% solids) Plasticizers used:

1 gm. butyl benzyl phthalate;

7 gms. N-ethyl toluene sulfonamide; and

4 gms. 60% chlorinated biphenyl Resin-plasticizer mixture aging period days 2 Time for complete resin precipitation a rnins 30 Temperature of felting slurry F 70 Felting time (freeness) mins. 4 Moisture in dried felt percent 17 Mullen test p.s.i 520 Example VIII This example was handled as in Example VI With the following variations:

Moisture content of the scrap percent 55 Clearance between discs 0.020" Ratio of water to leather 25:1 Consistency of slurry after adjustment percent 1% Weight of slurry used lbs 11.7

Resin emulsions used:

22 gms. polyvinyl acetate (50% solids) and 2 gms. polyvinyl chloride Plasticizers 4 gms. dipropylene glycol dibenzoate;

4 gms. N-ethyl toluene sulfonamide; and

4 gms. O-nitro-biphenyl Resin plasticizer mix was aged for 2 days Fiber slurry and binder stood overnight prior to felting.

Temperature of slurry at time of felting F-.. 150 Felting time (freeness) min 1% Temperature of drying air F-.. Drying time hrs 4 Moisture content at time of hot pressing percent 24 Mullen test p.s.i 330 Example IX This example was run as Example V1 with the following variations;

Alum leather shavings (55%) moisture) were dried, to percent 32 Clearance of discs 0.010 Ratio water to leather 20:1 Consistency of slurry after'adjustment percent 3 /2 Weight of slurry for felting s lbs-.. 5.0

Resin emulsion used:

23 gms. polyvinyl acetate (50 solids);

1 gm. polyamide resin Plasticizers:

2 gms. butylbenzyl phthalate;

2 gms. dibutyl phthalate 8 gms. N-ethyl toluene sulfonamide; and

2 gms. 60% chlorinated biphenyl Aging period of resin plasticizer mix days 2 Precipitation time hr /2 Temperature of slurry at time of felting F 140 Time of felting (freness) min 1% Temperature of drying air F Time of drying u hrs.-- 3 /2 Moisture content of felt at time of hot pressing percent 20 Plating time seconds 5 Mullen test p.s.i 340 Example X This example was run the same as Example VI except for the following variations:

Chrome leather shavings (55% moisture) were dried to percent 24 Clearance between discs 0.015" Ratio of water to leather 18:1 Consistency of pulp slurry after adjustment percent 5 Weight of slurry used lbs 3.5

Resin emulsion used:

22- gms. polyvinyl acetate emulsion (50% solids); 24 gms. polyacrylamide solution) Plasticizers:

1 gm. diotcyl phthalate; 2 gms. dibutyl phthalate; 5 gms. N-ethyl toluene sulfonamide; and 4 gms. ortho nitro diphenyl Slurry plus binder stood for 1 hour before felting.

Temperature of slurry at time of felting F 100 Time of felting (freeness) rnin 2 /2 Temperature of drying air F 140 Time in drier hrs 3 /2 Moisture content of felt at time of hot pressing percent 19 Temperature of press F 160 Time in press mins 2 Plating temperature a" F 170 Plating time sec Mullen test p.s.i 375 Example XI for the following variations:

Chrome leather shavings (55% moisture) were dried to percent 16 Ratio of water to leather 16:1 Consistency of slurry after adjustment percent 4 .Weight of slurry for felting lbs 4.4

Resin used: gms. polyvinyl acetate emulsion solids) Plasticizers used:

1 gm. diethyl phthalate; 2 gms. dibutyl phthalate; 6 gms. N-ethyl toluene sulfonamide; and V 3 gms. 50% chlorinated biphenyl Precipitation time min 90 Temperature of slurry at time of felting F 58 Time of felting (freeness) rnin 3 Temperature of drying air F 150 Drying time hours 2 Moisture content of felt at time of hot pressing percent 22 Temperature of press F 165 Pressure in press p.s;i 300 Time in press min 2 Plating temperature F 175 Mullen test p.s.i 440 Example XII This example was run the same as Example VI except for the following variations:

Chrome leather shavings moisture) were dried to percent 28 Clearance between discs inch 0.010

Ratio of Water to leather 19:1 Consistency of slurry after adjustment percent 4 Weight ofslurry used lbs. 4.4

Plasticizers used:

. 7 /2 gms. ortho nitro biphenyl; and

7 /2 gms. N-ethyl toluene sulfonamide Aging time of resin-plasticizer mix days 2 Temperature of slurry at time of felting F Felting time (freeness) min 1% Temperature of drier air F Time in drier hours 3 Moisture content of felt at time of hot pressing percent 25 Pressure in press p.s.i 500 Time in press min 3 /2 Plating temperature F Plating time -sec 30 Mullen test p.s.i 420 Example XIII This example was run the same as Example VI except for the following variations:

- Iron leather shavings (55% moisture) were dried to percent 18 Clearance between discs inch 0.008 Ratio of Water to leather 20:1 Consistency of slurry after adjustment percent 3 Weight of slurry used lbs 5.9

Plasticizers used:

4 gms. ortho nitro biphenyl;

, 4 gms. N-ethyl toluene sulfonamide; and

4 gms. 60% chlorinated biphenyl Precipitation time hours 1 Temperature of slurry at time of felting F 60 Felting time (freeness) min 2 Temperature of drying air F 140 Drying time hrs 2 /2 Moisture content of felt at time of hot pressing "percent" 21 Temperature of press F 5 Pressure in press p.s.i 300 Time in press min 2 Plating temperature F Mullen test p.s.i 405 The reconstituted sheet leather of our invention has many uses without surface finishing. gasket material or as the inner crown material for metal bottle caps. These uses capitalize upon the resiliency of the material. When finished with suitable coloring or embossing or enamelling or lacquering, all finishes used conventionally in leather working, various degrees of lustre, water-proofing, scuff resistance, and other properties can be had. The finished product has the general appearance of natural leather and is suitable for most any use Where resistance to repeated sharp flexing is not required. We do not recommend it for use as shoe uppers or soles as leather used for these purposes should have high flexibility. .It is suitable, however, for inner soles and sock linings. The material is especially desirable as decorative wall tile or even flooring. It is excellent as a resilient underlayment for composition flooring material or hardwood floorings. Other uses are for low cost clothing accessories, for example, handbags and belts, where color variety is desired and extreme wearability is not required. In the automotive industry the reconstituted leather sheet may be incorporated in door and crash panels as well as upholstery for the inside car roof and rear window shelves. This material may be laminated to a rigid structure of Wood, fiber board or metal in the manufacture of luggage, providing a leather appearance at low cost. In the furniture field it is useful as leather inlay for coffee, cocktail and card tables,

It can be used as 17 breakfront panels, and backs of television cabinets after perforation.

In addition to wall tile our product can be made in various thicknesses. Sheets of A thickness are suitable as drapery material or wallpaper, laminates for wallboard or acoustical tile, and inexpensive finished laminates for flush doors in harmony with various decorative schemes.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. An improved method of producing a fibrous slurry characterized by fibers having a relatively low degree of hydration from mineral tanned leather which comprises: passing leather having an initial moisture content of 1025% in piece form through a milling zone suspended in water in a ratio of at least four parts Water to one part of the leather and within that zone subjecting the leather pieces to an intense rubbing action provided by closely spaced and serrated surfaces, said surfaces having a rotative motion relative to each other so as to efiect a rubbing action upon the leather pieces therebetween, thereby milling said water carried leather pieces to an aqueous fibrous slurry, with the leather pieces being placed in a fibrous slurry form within ten seconds of millmg.

2. An improved method of producing a fibrous slurry characterized by containing fibers having a relatively low degree of hydration from mineral tanned leather having an initial moisture content in the range of 10-25% which comprises: introducing the leather in piece form suspended in at least six parts of water to one part of leather to the center of a milling zone defined by two closely spaced, serrated surfaces, which surfaces have relatively rotative motion about their common axes, moving the leather pieces outwardly of the common axis through the milling zone with the assistance of centrifugal force and simultaneously subjecting said leather pieces to an intense shearing force provided by the closely spaced surfaces, thereby milling said aqueous carried leather pieces to a fibrous slurry form by the time the leather reaches the outer periphery of the milling zone.

3. An improved method of producing a fibrous slurry characterized by fibers having a relatively low degree of hydration from mineral tanned leather which comprises: drying mineral tanned leather to less than 15% moisture content; passing said leather in piece form through a milling zone suspended in water in a ratio of at least four parts water to' one part of the leather and within that zone subjecting the leather pieces to an intense rubbing action provided by serrated surfaces spaced about 0.003- 0.005 inch apart, said surfaces having a rotative motion relative to each other so as to effect a rubbing action upon the leather pieces therebetween, thereby milling said water-carried leather pieces to an aqueous fibrous slurry, with the leather pieces being placed in a fiber slurry form within 10 seconds of milling.

References Cited by the Examiner UNITED STATES PATENTS 1,984,869 12/34 Farley et al 241-21 2,03 5,994 3/ 36 Sutherland 24l--2l 2,15 6,320 5/39 Sutherland 241-28 1. SPENCER OVERHOLSER, Primary Examiner.

Claims

1. AN IMPROVED METHOD OF PRODUCING A FIBROUS SLURRY CHARACTERIZED BY FIBERS HAVING A RELATIVELY LOW DEGREE OF HYDRATION FROM MINERAL TANNED LEATHER WHICH COMPRISES: PASSING LEATHER HAVING AN INITIAL MOISTURE CONTENT OF 10-25% IN PIECE FORM THROUGH A MILLING ZONES SUSPENDED IN WATER IN A RATIO OF AT LEAST FOUR PARTS WATER TO ONE PART OF THE LEATHER AND WITHIN THAT ZONE SUBJECTING THE LEATHER PIECES TO AN INTENSE RUBBING ACTION PROVIDED BY CLOSELY SPACED AND SERRATED SURFACES, SAID SURFACES HAVING A ROTATIVE MOTION RELATIVE TO EACH OTHER SO TO EFFECT A RUBBING ACTION UPON THE LEATHER PIECES THEREBETWEEN, THEREBY MILLING SAID WATER CARRIED LEATHER PIECES TO AN AQUEOUS FIBROUS SLURRY, WITH THE LEATHER PIECES BEING PLACED IN A FIBROUS SLURRY FORM WITHIN TEN SECONDS OF MILLING.