US 3157566 A
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Nov. 17, 1964 D. A. BRAFFORD METHOD OF INCORPORATING RESIN IN MOLDED PULP PRODUCTS Filed July 17, 1962 oncza A. lfafof'd B INVENTOR.
Y M da cy laoRNEi/s United States Patent O 3,157,566 lviE'HD 9F EQCRPRATNG RESBN iN MGLDED PULP PRDUCT S Dond A. Brantford, Beloit, Wis., assigner to leloitCorporation, Beloit, Wis., a corporation of Wisconsin Filed luiy 17, 1962, Ser. No. 210,547 7 Claims. (Ci. 162-146) This invention relates to an improved method of forming molded pulp articles, and more particularly, to an improved method of eecting vresin impregnation 'of articles formed by the deposition of brous pulp onto a porous forming surface.
Although the instant invention may have a number of uses, it is particularly adapted to uses which relate generally to the paper treating art, and the instant invention will be described primarily in connection therewith. In general, in the paper making or forming art, the fibrous pulp particles in a dilute aqueous slurry (commonly referred to as stoc are formed into the desired shape initially by forcing the slurry through a porous forming surface, which might be a forming wire or a porous mold form of one shape or another, so as to deposit a layer of such iibers on the forming surface. The instant invention may be used in the formation of a sheet on a forming Wire or a molded pulp article on a porous mold surface; and the instant invention is particularly adapted for use in the formation of relatively thick Walled pulp articles of generally cylindrical or shell-like shape having a relatively large cylinderV length-to-diameter ratio.
Heretofore, it was known that the wet strength and other properties of paper might be increased by the incorporation in the dilute aqueous stock of certain small amounts of water-soluble resins, but this procedure involved the possibility of loss of the usefulness of some of the dissolved resin, which would be carried away by the aqueous component or white water which actually liowed through the pulp layer and the porous forming surface. It was also known heretofore that a formed paper sheet or other article might be impregnated with a suitable resin by dipping the paper sheet or other article in a suitable resin solution, but particularly in the case of relatively thick paper sheets, this often resulted in a failure to impregnate the inner portion of the paper sheet or other article. This procedure also tended in many cases to impart a glue-like surface on the article, which in many cases resulted in a sticky surface. The resin deposited on the surface of the article in this procedure would often tend to block further penetration of the inner portion of the article. These various diliiculties in the prior art were greatly magniiied, when it was desired to introduce a relatively high concentration of resin in the paper article. This was particularly so in the case of attempting to impregnate the paper article by dipping the same in an organic solvent of the resin, because such solutions having relatively high concentrations of resins often became too viscous to penetrate the article satisfactorily.
The instant invention relates to an improved method of incorporating resins in molded pulp products. The
instant invention also provides an improved method of encapsulating various types of all of the fibers in a molded pulp product with a suitable synthetic resin to provide protection for the individual fibers while at the sameV time effecting a measure of improvement in the strength properties of the molded pulp product.
it is, therefore, an important object of the instant in- Other and further aspects, objects and advantages of the present invention will become apparent from the detailed discussion thereof as set forth hereinafter, and the drawing attached hereto which shows a self-explanatory flow sheet for the instant method.
In general, the instant invention consists in a method of producing molded pulp articles, which comprises dispersing finely divided particles or fibers, i.e. resin linters of a water insoluble synthetic resin in dilute aqueous stock containing cellulosic iiber, forcing such stock through a porous forming surface to deposit a layer of mixed particles and ibers on the forming surface, etecting removal of Water from said layer to substantially dry the same, next contacting said layer with a solvent for said particles to effect in situ solubilization of the resin particles and deposition of the resin in film form onto said fibers, and then removing substantially all of the solvent from said layer.
An initial step in the practice of the instant invention involves the provision of or the preparation of dilute aqueous stock containing cellulosic fibers, with finely divided particles of a water insoluble synthetic resin dispersed therein. The fluid stock employed is preferably composed of water with bers in suspension therein in a relatively dilute consistency of 0.01% to 2%, and preferably a consistency of 0.05% on a dry liber basis. (As used herein, the term percent means percent by Weight unless otherwise specified.) The fibers in the stock are essentially cellulosic fibers, but they may contain certain conventional additives such as clay, pigments, proteins, etc.
The cellulosic bers may consist entirely of the conventional wood pulp fibers as in the case, for example, of kraft pulp, or the dilute aqueous stock may contain a mixture of conventional wood fibers and other cellulosic" iibers such as cotton linters. The cotton linters used in the practice of the instant invention may be in their natural cellulose form, or they may be modified by partial or complete etheriiication or esteriiication, in accordance with conventional procedures. Such cotton linters may thus comprise in whole or in part cellulose ethers such as methyl cellulose, ethyl cellulose, etc. or cellulose esters such as cellulose acetate, cellulose nitrate, etc. In
a preferred process of the instant invention the cellulosic` fibers may comprise 25% to 75% wood iibers and 25% to cotton linters (particularly good results being obtained using approximately 40% kraft fibers and approximately 60% cotton linters). Such stock compositions have been found particularly useful in the formation of cylindrical molded pulp products having a relatively high cylindrical length-to-diameter ratio.
The synthetic resin is dispersed inthe dilute aqueous stock in the form of finely divided particles by any of a number of conventional methods, which include merely dispersing the resin in powdered form in the stock or initially forming a water base paste, which may be ball milled to effect a desired initial suspension in the paste which, in turn, is dispersed in the dilute aqueous stock.
An important feature of the instant invention resides in the concept of using a water insoluble (or substantially Water insoluble) synthetic resin for this purpose. The resin is reduced to a line particle Sile in powder or-dispersion paste form, and it is dispersed throughout the dilute aqueous stock in concentrations which may range from as little as about 1% to as'much as about 100% o'n a dry fiber basis. In the practice ofthe instant invention it is possible to incorporate relatively high proportions of the synthetic resin particles in the molded pulp articles and preferably the resin concentration is equivalentrto from about 5 to about 25% on a dry iiber basis. Alternatively, the resin in the form of fine fibers, as from spinnerettes, may be blended with the aqueous cellulosic liber stock.
Itis generally understood that the cellulosic fibers in paper stock are negatively charged, and in the practice of the instantinvention it has been found advantageous to use resins which also carry a negative charge in the dilute basis) are used. In the practice of the instant invention,
however, the incorporation ofthe finely divided particles of the water insolublesynthetic resin, even at such higher consistencies, tends to prevent liocculation of the fibers. This is particularly important for the reason that the flocculatiou of the `fibers at such higher consistencies characteristically results in poor formation of heavier thick sheets or moldedrpulp products.
YNext, the stock (With the resin dispersed therein) is conveyed Ato and forced through a porous forming surface to deposit a layer of Vmixed resin particles and cellulosic fibers onrthe forming surface. This step involves generallyfconventional web or paper formation Aon a wire' or on a suitable porous mold Wall or surface.V As previously indicated, the molding surfacemay be a forming wire for sheet formation, but preferably it is a mold for the formation of generally cylindrical articles having relatively thick Walls lin the ultimately formed molded pulp article. 4The mold Wall may be formed of a material such as individual glass beads coated with anV epoxy resin to bind the .beads together into the mold shape'.v A description of a preferred form of mold material and the method ofmaking `such material is provided in copend-Y ing patent application `of Modersohn and Hornbostel, entitled Moldf U.S. Serial No. 89,451, led February 15, The individual resin particles, being substantially insoluble in water, Vare not carried through the porous forming surface and, instead,.the'iibers forming on the mold or Wire surface tend to trap even the finest particle Y sizes of-resin so that the resulting molded pulpproduct consistsessentiallyof a `layerof mixed particles and fibersV on-the forming surface, WithV the particles thoroughly 4 involves Washing the pulp layer Yon the forming surface with a substantially anhydrous Water-miscible solvent Yin which the resin is substantially insoluble. In the case of the preferred use of polyvinyl-aldol resins, a substantially anhydrous alcohol such as methanol, ethanol, ethylene glycol, etc., is preferably used. Such alcohois have a comparatively high attraction for water and cannot ordinarily be obtained by economically practical processes in a total'- ly anhydrous state. Substantially vanhydrous methanol orY ethanol, however, which may contain onlyS to water, is still capable of taking on substantial quantitiesk of water and such an alcoholic system acts as an anhymixed or incorporatedvthroughout the entire body of this pulp layer. v Y
Y After the Vpulp layer has been formed in the desired thickness on the porous forming surface,V it is removed from Contact With the dilute aqueous stock, but the resulting pulp product at this stage still contains a substantial Vamount of Water. It will be appreciated thatthe preferred liuid stock'for usefin the practice of the instant invention is; an laqueous suspension of thercellulosic bers on the resin particles, but the use ofjother than an aqueous vehicle is withinA the broad concept ofthe invention;V The aqueous stock system is preferred for use in the practice Vof the instant invention for obvious economic reasons;
butin certain special cases an alcoholic or'aqu'eousgalcoe holic vehicle, forfexample, might beused'in the practice 'of i the instant invention, so long as the cellulosic fibers and 1 the resin particles are both substantially insoluble in such vehicle.Y YIn any event, once the molded pulp article has been lformed with the desired VVweb or paper thickness, and
Ythe molded pulp article has been separated from VYthe stock, Y
thestock vehicle retained `in the molded pulp article is preferablyremovedto a substantial extent. In the case of aqueousfvehicle, it is desirable vto eliect removal of vi/'afterV from the pulp layer VVon the forming surface to Y. substantially-'Vdryrthe layer; e Such' drying or vehicle removaljmay be accomplishedY by`any of a numberfofconventionalmethods, Ybut particularly `in'thecase ofthe formation'of'generallycylindrical molded pulprarticles in a device having the ,desiredmoldshapea it is'desirable to effect the removal of Water V(or other stock vehicle) while the molded pulp'article is retained'on the porous forming f In such situations, conventional drying means surface.
Y' ,Y arefnot ordinarily preferred; andthe preferred process the Wet sheet,.and 91% fdenatured 'ethyl alcohol Wasj 'i drous or substantially anhydrous system, in that it maybe used to Wash the pulp layer (ordinarily using the alcohol in a total amount ranging from about 5 to 2,0 times the weight of the layer itself) to effect substantial removal of the Water from the layer. AV nominal amount of Water is retained in the layer, and this has been found to be useful in the ultimate bonding of the resin to the bers.
After substantially drying the pulp layerY (or removing substantially all of the vehicle), the resulting layer is still in the formof mixed resin particles and cellulosic fibers. This layer is then contacted with a solvent for the resin particles to effect in situ solubilization of the resin particles and deposition* of the resin in iilrn form onto the fibers.V Preferably, this'is done by washing the pulp layer or molded pulp Varticle while Vit is still retained on the porous forming surface with a suitable solvent. The 'cellulosic fibers are, of course, substantially insoluble in this solvent, butthe resin particles are soluble therein.: In the case of the preferred polyvinyl-aldol resins, the. solvent used may be ajmixture.y of toluene and ethanol (c g. a blend'of 30-60% toluene and 40-70% ethanol).
lt'will be appreciated thatV excessive Vwashing of the pulp layerl at this time with such aV solvent might tend to leach a out an appreciable quantity of the resin, but washing ofV the pulp layer with a nominal amount of such solvent to carry out an initial air drying procedure in order to Y elect suliicient integration of the molded pulp sheet or article to permit its removal from Vthe porous forming i surface, followedV by a more extensive, air drying procedu're to remove substantially all of the solvent.Y Heat mayY also be employed, particularly oncerthe pulp layer has been removed from the porous forming surface, in
order to complete the'rernoval of the solventrand to as?V sist in the formation of a bond between the'bers via the Y resin films. y I Y y.
The invention mayY be demonstrated as follows:
Y Demonstration A Y VTAPlPl hand sheets are formed according tofspecilica- Y tion vT205M-58. The pulp usedis bleached kraft Vmade upto 1% consistency in a beater'without refining.; An f .amount of polyvinyl-formal resin in the form of a free owing Vpovvder (Formvar 7/95S)f 'equivalent to 25% of the fiber contentof'the Astock (on va dry. liber basis) was dispersed in the stock in Athebeater. After theneyvly formed WietvsheetV Was made accordingto the''specifica-V tion, va piece of Fourdrinier'Wir-e was placed on topfof flooded over the wire in a quantity equal to about five times theweight of the wet sheet., Vacuum was applied i Y Yto thehand sheet to'draw the'sol'vent through jthe sheet. 1
Next, the vvire was flooded with about tive times. Y the Weight of the resulting sheet Vof 'anorganic solvent @for the resin'consistingV of 30% toluene .and '770% ethanol;
' and after lthis solvent had Acompletely impregnated'thefa-t, 'j'
sheet, it was removedin part/bythe application of vac? A number of variations are possible in the procedure of Demonstration A. For example, if the procedure of Demonstration A is repeated except that the ber consistency employed in the initial stock is 2%, there would ordinarily be a tendency to obtain fiocculation in the stock, but it will be noted that the incorporation of the resin particles in the beater tends to minimize occulation.
Also, the amount of resin employed may be reduced to 10% and 5%, respectively, of the ber on a dry ber basis, with successive decreases in the ultimate strength (c g. the tensile strength factor) in the iinal sheet, but even using 5% resin it will be noted that a substantial increase in strength is obtained in the practice of the invention.
It will also be noted that comparable results are obtained using an organic solvent in the last step which consists essentially of 40% ethanol and 60% toluene.
Demonstration C A procedure is carried out that is the same as that of Demonstration A, except that 75% of the kraft bers are replaced with cotton linters, and it is found that substant'ally the same results are obtained. In this procedure the cotton linters used are natural cotton bers (Le. cellulose), but comparable results are obtained in this procedure using in place of the natural cotton bers ethered cellulose bers (ie. composed of methyl cellulose or ethyl cellulose) and/or esteried cellulose (i.e. cellulose acetate and/or nitrated cellulose bers).
Demonstration D Stock is processed in a beater to obtain 0.05% ber consistency kraft bers and 75% cotton linters). An amount of the aforementioned polyvinyl-formal resin equal to 20% of the ber content of the stock (on a dry ber basis) was formulated into an aqueous paste in a ball mill and then added to the stock in the beater to disperse the resin particles throughout the stock uniformly. The stock is then forced through a generally cylindrical porous female mold formed of glass fiber beads bound together by an epoxy resin (in the manner hereinbefore described) in order to form on the cylindrical forming surface of the mold a generally cylindrical Wet molded pulp article. A porous male mold is -then pressed down on the pulp article on the female mold, and 91% ethanol in an amount equal to iive to ten times the Weight of the molded pulp article is passed through the molded pulp article md both the male and female molds to substantially dry the molded pulp article. Next, an organic solvent consisting essentially of 40% ethanol and 60% toluene, in an amount equal to about 5 to 10% of the Weight of the dry molded pulp article is ushed through the article (and through the male and female molds), to effect in situ solubilization of the resin particles and deposition of the resin in film form onto the bers. Warm dry air is then forced through the molded pulp article (and the male and female molds) to effect at least partial removal of the solvent, and the mold members are then separated and the molded pulp article removed therefrom and placed in an oven for further heating and drying to complete the removal of the solvent. The resulting product is found to have a resin lilm substantially encapsulat- 6 ing the bers therein and elfecting increased strength in the molded article.
Again, the cotton linters employed in the demonstration of the previous paragraph are natural cotton bers, but a comparable improvement is obtained if these cotton linters are esteried (eg. to form cellulose acetate or cellulose nitrate), and it is found that the esteried cellulose bers are protected and encapsulated by the resin lm so as to obtain a particularly desirable result.
Demonstration E A procedure is carried out that is the same as that described in Demonstration D except that the resin particles used are formed of polyvinyl acetate resin, and the results obtained are comparable to those described in Demonstration D.
Demonstration F A procedure is carried out that is the same as that of Demonstration D, except that the resin particles used are in the form of a finely ground co-condensate of toluenesulfonamide, melamine and formaldehyde (eg. as described in Example 6 of U.S. Patent No. 2,809,954), and the organic solvent used for the particles to elfect in situ solubilization is acetone instead of the alcoholictoluene solvent, and it is found that comparable results are obtained.
In the pratcice of the instant invention, the resin particles used are preferably thermoplastic resins which are at least reasonably readily soluble in an organic solvent (eg. alcoholic-toluene, acetone, etc.) for use in elfecting in situ solubilization in accordance with the end step of the process. Also, the resins used in the practice of the instant invention should be substantially insoluble in any solvent that is used in the removal of the stock vehicle, i.e., the substantially anhydrous alcohol that is preferably used in the removal of water. In addition, the resins used in the practice of the instant invention are substantially insoluble in the stock vehicle (Le. Water). The resins which may be used in the practice of the instant invention include the Water insoluble acrylic resins (such as the polyacrylamides and the polyacrylic esters such as polymethyl methacrylate, polyethyl acrylate, etc.), the various Water insoluble polyvinyl resins, and other Water insoluble thermoplastic resins such as the toluenesulfonamide-formaldehyde resins, with or without modication by cocondensation with comparatively small amounts of polyamides such as urea, melamine, etc. Preferably, however, the resins used in the practice of the instant invention are of the polyvinyl type and the best results are obtained using a polyvinyl-aldol resin. The most common of these resins are polyvinyl formal, polyvinyl acetal and polyvinyl butyral; and the best results are obtained using polyvinyl-formal. Other polyvinyl resins which may be used in the practice of the invention include polyvinyl acetate, polyvinyl chloride-acetate, etc. since polyvinyl alcohol is Water-soluble, it is not used as such, but when modied by reaction with an aldehyde to form a polyvinyl-aldol such as polyvinyl-formal, the resin may be used.
Demonstration G Water insoluble powdered phenol formaldehyde resins i are also available for use in the practice of the instant invention (ie. advanced B-stage phenolic resins). The procedure of Demonstration A is repeated, except that the synthetic resin used is powdered Water insoluble phenol formaldehyde resin, and the iinal rinsing or Washing step With a solvent for the phenolic resin is carried out using an amount of dimethyl formamide that is equal to about three times the Weight of the dried paper sheet, and it is found after removal of the dimethyl formamide that a signicant improvement in strength is obtained in the resulting paper sheet.
It will be understood that modifications and variations may be elfected without departing fromV the spirit and scope of the novel concepts of the present invention.
`I claim as my invention: Y
l. A method of producing molded pulp articles, which comprises dispel-sing finely divided particles of a water insolublersynthetic resin in dilute aqueous stock containing cellulosic ibers, forcing such stock through a porous forming surface to deposit a layer of mixed particles and fibers on the forming surface,'effecting removal of Water from said layer to substantially dry the same by Washing said layer'with a substantially anhydrous Water-miscible solvent in which the VVresin is substantially'insoluble, nextV contacting said layer with a solvent for said particles to eect in situ solubilization of the resin particles and deposition of the resinin film form onto said fibers, and then removing substantially allV of the solvent from said layer,y the amount of ,saidssubstantially anhydrous solvent used in the aforesaidwashing step being substantially Vto times the Weight of the layer.
n 2. VA method of producing molded pulp articles, Vwhich comprises dispersing finely divided particles of a water insoluble synthetic resin in ,dilute aqueous stock containing cellulosic fibers having a ber consistency of 0.0l% to V2% and using resin in a vconcentration equivalent to from 1% to 100% on dry fiber basis, forcing'such stock through a porous forming surface to deposit a layer of mixed particles and fibers on the forming surface, effecting removal of water from said layer to substantially dry the same by Washingrsaid layer with a substantially anhydrous Y water-miscible solvent in which theresin is substantiallyV insoluble, nextcontacting said layer With a solvent for said particles to effect in situ solubilization of the resin particles and deposition ofthe resinV in iilm form onto said Viibers, and thenremoving substantially all of the solvent from saidlayer, the amount of said substantially anhydrous solvent used in the aforesaid washingY step being substantially 5V to 20 times the weight of the layer.
3. A method of producing moldedv pulp articles, which comprises dispersing finely divided Vparticles ofV a water insoluble polyvinyl resin in dilute aqueous stock y.containing cellulosic fibers, forcing such stock through a porous forming surface to deposit a layer of mixed Vparticles and fibers onthe forming surface, effecting removal of'water lfrom said layerto substantially dry the same by Washing said Vlayer with 5 vto A20 times its Weight of a substantially anhydrous Water-'miscible solvent in which the resin is substantially insolube, next contacting said layer with a solvent for saidparticles to effect in situ solubilization of Vthe,resirnrparticles and deposition of the resin in film tacting said layer with a solvent for said particles to effectr` in situ solubilizaton of the resin particles and deposition rof theresin in iilm forrnV onto said fibers, and then reof mixed particles and fibers on the forming surface,
effecting removal of Water from said layer to substantially dry the saine by passing substantially anhydrous ethanolV through said layer in amounts Vof substantially 5v to V2,() times the Weight of the layer, next contacting said layer with a solvent for said particles to effect in situ solubilization of the resin particles ,and deposition of the resin inV film form onto said fibers, and thenremoving substantially all of the solvent from said layer. Y
6. A method of producing molded pulp articles, which comprises dispersing finely divided particles of a Water insoluble synthetic resin inV diluteaqueous stock containing cellulosic fibers comprising 25% to 75% Wood fibers and 25% to 75 cotton linters, forcing suchstock through a porous forming surface to deposit a layer of mixedV particles and fibers on the forming surface, effecting rernoval of water from said layer to substantially dry the same by Washing said layer with 5 to 20 times its Weight 'of a substantially anhydrous water-miscibley solvent'in which the resin is substantially insoluble, next contacting said layer with a solvent for said particles to effect in situ ysolubilization of the resin particles and deposition of the i resin in iilm form onto Vsaid fibers, and then removing form Vonto said fibers, and then removing'substantially all of the solvent from said layer. i, Y
f4.' A method of producing molded pulp articles, which comprises dispersing iinely'divided particles of a water insolublepolyvinyl-aldol resinV in dilute aqueous stock ,containing cellulosic fibers havingna fiber consistency of VY0,01% to 2% and using resin `in a concentration equiva-k lent to from 1% to y100% .on dry fiber basis, forcing'such ,Y stock' through a porous forming surface to deposit a Y layer of mixed particles and fibers on the formingsurface,
effecting removal of water from said layer to substantially K dry the same byV washing said layer with 5 to 20times its Y, V Weight of a substantiallyranhydrous ivater-miscible solvent inY ivhichthe'resin isr substantially insoluble,V nextrconsubstantially all of the Vsolvent from said'layer.
7. A method of producing molded pulp articles, which comprises providing a dilute fluid stock containing' cellulosic fibers and finely divided synthetic resin particles in a vehicle in which both'the particles and the fibers are s insoluble, forcing such stock through a porous forming Y surface to deposit a layer of mixed particles and fibers`v on the forming surface, Veecting removal ofthe vehicleYV from said layer by washing such layer with substantially 5 to 20 times the layer weight of a solvent that is initially substantially free of the vehicle but'ismiscible with theY vehicle and in which theparticles and'iibers of such layer are Vsubstantially insoluble, next contacting said layer `with aV solvent for said particles to effect in situ solubilization f of the resin particles and depositionof the resin in film form onto said fibers, and then removing substantially al of the solvent from said layer.
References Citedin the le of'this patent UNITED STATES PATENTS 1,829,585 Y Dreyfus et a1. oct. 27, i931 1,848,732 *Y Lionne Marl 8,1932 Y Y Y FOREIGN PATENTS i Y 1 565,980 Great Britain nec. 7, 1944 1,083,697 France' v .r. r lune 30,'1954