CA1101590A - Adhesive compositions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Adhesives consisting essentially of polyvinyl alcohol or ethylene/vinyl alcohol copolymer, a crystalline solvent for the polymer component (such as urea, thiourea, acetamide, ammonium formate, etc.) and a viscosity reduc-ing diluent (such as water, glycols, etc.) are disclosed.
The adhesives are particularly suitable for cellulosic substrates and provide excellent ambient temperature water resistance while being fully repulpable in hot water.

Description

BACKGROUND OF THE INVENTION

(1) Field of the Invention .
This invention relates to polyvinyl alcohol-based compositions and more particularly to adhesives containing polyvinyl alcohol or ethylene/vinyl alcohol copolymer, a crystalline solvent for the polymer and a viscosity reducing diluent.

(2) Description of the Prior Art _ _ . _ _ Japanese Patent Publication No. 12851/72 describes the preparation of shaped articles of polyvinyl alcohol. The process involves -the preparation of an up to 20 percent by weight aqueous polyvinyl alcohol solution and the addition of 5-55 percent by weight, based on the weight of the aqueous solution, or urea thereto. Thereafter, uniform stress is applied to the aqueous solution by mec~an-ical stirring. The polyvinyl alcohol/water/urea blend employed in this process contains 0.1 to 19 percent poly-vinyl alcohol, 55 to 95 percent water and 5 to 36 percent ; i~;
urea, all based on the total weight of the solution. The shaped articles prepared by this process are disclosed tobe useful as highly tenacious nonwoven fabrics, as fluff for mattress wadding or as filaments obtained by splitting ~;
the shaped article. There is no disclosure of the suit-ability of either the aqueous solutions or the shaped articles as adhesives.
U.S. 3,313,637 discloses models which may be separated readily from their molds. The models consist of a major portion by weight of a finèly divided crystalline material (such as urea), a minor portion by weight of a finely divided organic material (such as polyvinyl alcohol) r~ ~ :

5~

and a liquld solvent for the crystalline material (such as water). Example 1 disclos~s the use of a composition com-prising 85 percent urea, 10 percent polyvinyl alcohol and 5 percent water. There is no disclosure whatsoever of adhesive use generally or as a hot melt adhesive specifi-cally; on the contrary, it is emphasized that the model after pressing can be readily extracted from the matri~
in which it was formed.
SUMMARY OF THE INVENTION
According to the present invention there is provided an adhesive composition consisting essentially of about 2 to 35 percent by weight of a polymer selected from the group consisting of polyvinyl alcohol and ethylene/
vinyl alcohol copolymers containing at least 50 mol percent vinyl alcohol, about 10 to 80 percent by weight of at least one solvent for said polymer, which solvent is crystalline at 40C. and is selectecL from the group con-sisting of urea, solid alkyl substituted ureas containing a total of up to 9 carbon atoms, thiourea, biuret, ~~caprolactam1 solid aliphatic amides containing up to 6 carbon atoms, solid polyhydric compounds and ammonium carboxylate salts, about 5 to 80 percent by weight of at least one viscosity reducing diluent selected from the group consisting of water, li~uid polyhydric compounds r liquid alkyl substituted ureas having up to 9 carbon atoms in the alkyl group, liquid aliphatic amides containing up to 6 carbon atoms, and dimethyl sulfoxide, and 0 to about 60 percent by weight extender selected from the group con-sisting of starchr dextrin, clay, silica, carbon black, talc, calcium carbonate, barium sulfate and vinyl polymer ,~r~

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latices, provided that when the viscosity reducing diluen-t is present in an amount of above 45 percent by wei~ht the adhesive composition is a suspension and the polymer is polyvinyl alcohol which has been subjected to a heat treat-ment at a temperature of 70~C.-190C. disperse~ in a liquid solvent comprising 42-100% by weight of methanol, 0-13~ by weight of water, and 0~45% by weight of a solvent from the gxoup consisting of 2 to 5 carbon monohydric alcohols, esters, ketones, ethers, hydrocarbons and chlorohydrocarbons in which said polyvinyl alcohol is insoluble, to reduce the cold water solubles content of said pol vlnyl alcohol while maintaining its solubility in 80C.-100C. water.
According to the present invention there are ~;
further provided structures comprisin~ cellulosic substrates bonded together by means of the adhesive composition of this invention.
In the context of the present invention the phrase I'consisting essentially of" means that only unspecified ingredients which do not materially affect the basic and novel characteristics of the present invention canbe included in the ~dhesive composition. Such ingredients, e.g., can be ~etting a~ents, curing agents, etc.
- DETAILED DESC~IPTION OF THE INVENTION
~dhesives based upon polyvinyl alcohol (PVA) and ~;
eth~lene/vinyl alcohol (E/VOH) c~polymerscontaining atleast 50 mole percent VOH,which adhesivescan beapplied byconven-tionalcoating,sprayingor hotmelttechniques havebeen developed by thepresentinvention. Thesecompositions,whichmayor may not be homogeneous,constitute several new types of adhesi~e systems for bondingcellulosic materials. Allof the adhesive L5~

compositions contain at least the pol~mer, a crystalline solvent for the polymer and a viscosity reducing diluent.
A variety of auxiliary extenders, such as starch, dextrin, clay, silica, carbon black, talc, calcium carbon~
ate, barium sulfate, as well as vinyl polymer latices (such as polyvinyl acetate emulsion and vinyl acetate/
ethylene copolymer emulsion) and also mixtures of these materials can be incorporated into the adhesive blends to achieve a broad range of application and adhesive proper-ties.
The pol~er component func~ions as the binde~,contributing to adhesive bond strength and water resis-tance. The crystalline solvent for the polyvinyl alcohol or ethylene/vinyl alcohol functions as a solvent for the polymer when compositions of the present invention are prepared as hot melt adhesives. It also acts as a viscos-ity depressan~, and impar-ts "set" or "quick tack" to some of the compositions described later. When compositions are applied as hot melts the viscosity reducing diluent's role is as a viscosity and melt point depressant and as such it enhances the molten adhesive's handleability and melt stability.
The adhesive compositions described herein may be prepared in a variety of ways so that they can be applied to ce~lulosic materials over a broad temperature range depending on the physical form o~ the adhesive.
Specifically, adhesive compositions can be applied from room temperature (about 20C.) to as high as 135C~, or any temperature in between~
When used as a molten or hot melt adhesive the 5~

composition of the present invention is homogeneous and undergoes rapid "set" upon cooling. This "set" character-istic results in "quick tack" performance, which is a unique feature of the molten, homogeneous adhesive system of tlle present invention, contrasting sharply with con-ventional polyvinyl alcohol adhesives which, upon cooling, give weak, non-tacky bonds. "Tack" is defined by Skeist (Ref. Handboo'~ of Adhesives, p. 63, Reinhold, 1962) as the "stickiness" an adhesive exhibits giving it the ability to adhere upon contact. When the adhesive has this character-istic to a high degree it is said to have "guic~ tack".
For the purpose of this discussion "quick tack" is defined as the time in seconds required for the adhesive to develop a ~iber-tearing bond to a cellulosic substrate in a stan~
dard T-peel test. ~or a corrugating or case sealing adhesive a "quick tack" of 1-3 seconds is the preferred range with a 0.5-10 second range being acc~ptable. The molten homogeneous adhesives systems of tnis invention exhibit quick tack falling within the acceptab]e range.
Adhesives containing urea or thiourea as the principal crystalline solvent for the polymer exhi~it "quick tack"
falling within the preferred range.
The molten homogeneous adhesives of the present invention can be distinguished readily from blends contain-ing only one of the two essential solvent and diluent com-ponents (i.e., ones that contain only the crystalline sol-~ent or the viscosity xeducing dilùent). Such two compo-nent systems are characterized by very poor melt stability (for example, PVA/urea blends), excessive and unstable application viscosities (e.g., ~VA/urea or PVA/thiourea IL5~

blends~, and failure to "set" by crystallization upon cool-ing (e.g., PVA/water or PVA/N-methyl pyrrolidone blends).
Similarly, solutions containing 20 percent or more by weight of polyvinyl alcohol in conventional solvents (e.g., water, dimethyl sulfoxide, N-methyl pyrrolidone) exhibit excessively high viscosities and are highly prone to undergo gelation upon cooling.
When the adhesive blends described above are kept in the molten state, usually at 100C.-135C., for extended periods of time storage in tightly covered con-tainers is recommended to prevent 105s of viscosity reduc-ing diluent. If excessive loss of diluent occurs, the melt can become progressively more aerated due to the evolution of ammoniaresulting from the reaction o~ poly--vinyl alcohol and urea as the temperature increases.
-Adhesive blends can be prepared which are not homogeneous melts and which can be applied by conventional coating technlques at lower temperatures, i.e. in the range of 40~C.-80~C., than employed when molten homo-geneous compositions are used. These adhesive blends are viscous systems ~up to 20,000 cps at 70C.) and contain -extender as an additional component. Generally they are lower in viscosity than the molten homogeneous systems described above. Solutions of only polymer, i.e. poly-vinyl alcohol in the diluent (e.g. water) at about 20 percent or more polymer by weight would be excessively high in viscosity. In the heterogeneous adhesive blends of this invention the crystalline solvent functions as a ~iscosity depressant to overcome the high viscosity which occurs when it is not present. These heterogeneous sy~tems do not develop "~et" or ~Iquick tack" as rapidly a~ do the molten homogeneous compsotions, but do gi~e ~iber tearing, water reslstant bond3 when set. Full advan-tage of adhe~ive capabillty is realized when the visco~ity reducing di]uent, u~ually water, is removed, a~ter applica-tion o~ the adhesive, either by evaporation or heatlng as is the caæe with many existing solvent containing adhesive s~stems~ These systems also are con~iderably lower in cost than current polyvinyl alcohol adhe~ives.
Another embodiment of the pre~ent invention employ~
an especially pre~erred type o~ polyvinyl alcohol which has been heat treated at a temperature of 70C. to 190C. a~ a disperslon ln methanol and optionally water and 2-5 carbon monohydric alcohols, esters, ketones~ ethersJ hydrocarbons and chloro-hydrocarbons in which the polyvinyl alcohol ls insoluble, in order to increase its cry~tallinity as reflec -ted by a reductlon in cold water æolubillty. mi~ type o~ poly-vinyl alcohol has a cold water solubles content o~ less than 10 by weight at 25C. while maintaining its solubility in 80C-100C water. A more detailed de~crlption o~ such polyvinyl alcohol products and o~ the method of treatment employed to obta~n them can be ~ound in Bri~tol USP 3,654,247. Polyvinyl alcohol ~ultable for uæe in this embodiment can al~o be obtained by heat treatment at a temperature o~ at lea3t 50C as a dispersion in a treating liquid compri~ing a 2 to ?0 wei~lt percent solution o~ acetic acld in a ~olvent comprising methanol or methyl acetate or a mixture o~ methanol and methyl acetate. A more detailed de~cription of such polyvinyl alcohol product~ and of the method of treatment employed to obtain them can be ~ound in Bristol USP 3,487,~870 U3ing thi~ type of polyvlnyl alcohol heterogeneous adhe~ive ~y~tem~ can be prepared which are low vi~cosity (up to 2000 cps at 70C,), stable suspensions and can be stored in this state at room temperature for long periods oP tlme. These systems are suspen~ions of finely divided polyvinyl alcohol in a solution o~ the crystalline ~olvent (preferably urea) ~n the viscosity reducing diluent (preferably water). me finely divlded PVA u~ed ln this ~ystem wlll remain suspended for a con~iderable perlod of tlme after its initial dispersion during preparation.
Continued stable suspension durlng uæe can be maintained by a minimum of agitation~ such as mild stirring or during periods of flow or pumping. Use of hydrocolloid~ such as guar gum, aids su~pension and/or redispersion during periods of static s~orage, for exa~ple, overnight storaae. rme resultant suspen~ion may be applied Prom room temperature (about 20C.) to about 60C~ to cellulosic materials by conventional methods lncluding spraylng, roll coating, doctor blade or any other emulg~on coatlng technique~ Ap- -plied by these methods the adhesive compositlon adheres ~
firmly to the cellulosic material a~ a coating when air ~-dried and as such may be stored indefinitely unt~l its bonding capability is act~vated by Purther heating. In this form the adhesive compositionæ oP the present inven-tion are characterized a~ heat activatable systems.
Except for the requiremen~ oP employing the specially treated polyvinyl alcohol above referred to in the activa-table low viscosity, ~table suspen~ion embodiment of the adhesive compo~ition~ of the present in~ention all commer-clal grades o~ polyvlnyl alcohol te~ted were ~ound to be operable a~ blend component~. mese grades include both comple~ely hydrolyzed (i.e., about 99 percent or higher) and partially hydrol~zed (about 88 percent) types covering a degree of polymerization (DP) range of about 300-2000 (number _ g average ba~is)~ as well as ~ully hydrolyzed copolymers of polyvinyl alcohol (lncluding ones containing about 4 percent methyl methacrylate comonomer) o~ sim~lar DP range~ 1~e vlscosity measured at 20C using a 4 percent aqueou~ solution) of the polyvinyl alcohol tested ln the blends ranged ~rom about 4 to about 65 cps. Preferably the viscosity should be from about 10 -to about 65 cp~
The polyvlnyl alcohol pre~erred as a component~- ;
in the adhesives under discussion whether treated accord-;
ing to Bxistol USP 33654,247, or not is a 99 percent hydro-lyzed grade with a DP of 800 and a 4 percent aqueous vis-cosity (20C.) o* 12-14 cps~ The E/VOH copolymer~ suit-able in the blends o~ the present invention contain at least about 50 mole percent VOH unit~.
m e amount o~ polyvinyl alcohol or E/VOH copoly-mer in the blend~ o~ the present lnvention can range between about 2 and 35 percent by weight. In the molten homogeneou~ systems of the present invention about 10 to 35 percent by weight polyvinyl alcohsl should be present.
The use o~ such amounts of polymer provide strong bonds in the dr~ state. In the viscous heterogeneou3 embodiment o~
the present inventionJ about 5 to 25 percent by ~eight o~
pol~ner should be employed. Polymer levels below about 5 do not have ~u~lcient adhesive capability ~or satis~actory bonding whereas blends containing above about 25~ are too ViBCou~ for ~atls~actory handling and application o~ adhe-sives to ~ubstrates~ Low viscosity, stable suspension can be made according to the present invention with about 2 to 35 percent by weight polyvinyl alcohol. Levels o~ polymer below about 2~ do not re~ult in water resistant bonds whereas concentrations above about 35~ are not only economically unattractive but have a tendency to alminish ~uspension characteristics and result in a pasty consistency~
The aaheslves of the present invention ex~lbit excellent ambient water reisstance whlle remalnlng ~ully repulpable in hot water. Ambient temperature water re~is-tance was mea~ured by the ply separation test, ASTM-D 1028.
Accordingly, they are sultable, when used as a hot melt, where lower cost wax free ~ystems are called for~ They are suitable ~or bonding cellulosic substrates under wet ambient conditions. As such~ they can be used a~ corru-gatln~ adhesive~. When used in the form o~ a low viscos-ityJ stable suspen~ion they are suitable aæ low-cost post-activatable carton and case ~ealing adhe~ives. ln these s~stems ~iber tearing bonds can be relaized if the polym~r concentration in the adhesive blend is 2 percent or higher ~calculated on a dry weight basis~
High humidlty per~ormance J determined by aging bonded ~tructures at 90 percent relat;ive humld~ty for 1 2 weeks, wa~ ~ound to be excellent~ ~or ex~mple, fiber tearing bonds were observed ln te~ts with conditioned board sample prepared with the molten homogeoeous adhe-: ~i ve containing 20 percent of P~A (calculated on a dry weight basis). Bonded ætructure~ prepared with the low viscosity~ ~table su~pen910n type adhesi~e~ containing as little as 2 percent of the PVA described in the Bristol patent, yielded ~iber tearing bonds after submer~ion ln water for at least 24 hours.
The crystalline solvent ~or the polymer compon-ent mu~t satisfy the followlng requlrement~: (1) it must be crystalline a~ 40C., (2) it must ~unction as solvent ~or the polymer, i.e., dissolve at least 5 weight percent o~ the latter at or above the melting point o~ such Ls~

solvent, and (3) it must be miscible with the viscosity reducing diluent. The following compounds were ound to satisfy these conditions: (1) urea; solid alkyl substi-tuted ureas con~aining a total of up to 9 carbon atoms, such as methyl urea (m.p. 95C. - 98C.), ethyl urea (m.p. 92C.), l,l-diethyl urea (m.p. 112C.); thiourea;
biuret, and -caprolactam; (2) solid aliphatic amides con-taining up to 6 carbon atoms such as acetamide, propionam-ide, butyramide; (3) solid polyhydric compounds such as pentaerythritol, orbitol, mannitol and tris (hydroxymethyl) aminomethane; and (4j ammonlum carboxylate salts such as ammonium formate. In the above context "solid" means that the specified compound is in the solid~state at 20C.
Urea is the most preferred crystalline solvent for the polymer primarlly because of its less hygroscoplc nature and because its melting polnt (132C.) falls within~
the preferred range of 75C. - 175GC. -Caprolactam is also a preferred crystalline solvent. Thiourea ls~also prefer~
red although it has a tendency to corrode stainless steel~
equipment.
The crystaIline solvent for the polymer should ~ be present in an amount of about 10-80 percent by welght.
; Below this ranye the products tend to be too visoous and have poor "tack" while concentrations above 80 percent have a detrimental effect on the ambient temperature water resistance of the obtainable bonds.
The amount of crystalline solvent in the various ; embodiments of the present invention is as follows: about 40-80 percent by weight in the molten homogeneous blends, about 10-45 percent by weight in the viscous heterogeneous ~ 12 -blends and about 15-68 percent by weight in the low viscos-ity stable suspension.
In addition to the binder polymer and the crys~
talline solvent therefor, adhesive blends of the present invention contain a viscosity reducing diluent which is fully compatible regardless of the state of the adhesive blend, i.e. molten homogeneous form, viscous heterogeneous form or as a low vlscosity stable suspension. When used with molten homogeneous or viscous heterogeneous blends "viscosity reducing diluent" means that this component reduces the viscosity of the blend, i.e. that the viscos-ity of a molten blend containing the polymer, crystalline solvent for the polymer, and the viscosity reducinq dilu-ent, measured-at a temperature ~rom about 90C. to about 135C., is lower than ~he viscosity of a blend containing the same amount of polymer anù crystalline solvent ~7ithout any diluent being present, measured at the same tempera-ture. When used with low viscosity, he~erogeneous suspen-sions containing PVA described in Bristol USP 3,~97,987 the "viscosity reducing diluent" functions as a carrier to facilitate application of the polymer and crystalline sol-vent into the cellulosic substrate. In-contrast to the restricted number of compounds suitable as the crystalline solvent for the polymer, a relatively broad range of mate-rials can be used as the viscosity reducing diluent. A
primary requirement is that such diluents be relatively non-reactive in the blends. Anothe~r requirement is that the diluent be fully miscible in the blend although it does not have to be a solvent for the binder component by itself.
Thus the viscosity reducing diluent can be water, liquid ~ `
9~ ~:

polyh~dric compounds such as glycols having 2 to 9 carbon atoms, glycol ethers (wherein the acyl group contains 2 to 4 carbon atoms), lo~er molecular weight (up to a molecular ~eight of 1,000) polyalkylene oxides and their alkyl ethers (alkyl group containing 1 to 4 carbon atoms), polyalkylene oxide esters twherein the acyl group contains ~ to 4 car-bon atoms), liquid alkyl substituted ureas (having up to 9 carbon atoms in the alkyl group), such as tetramethyl urea and tetraethyl urea; liquid aliphatic amides contain-ing up to 6 carbon atoms, such as formamide and dimethyl sulfoxide. The term "liquidi' in this context means that ~ ~ -t~e named compounds are in the liquld state at 20C~
- Some solid diols also manifest a viscosity reducing~effect.
Water, ethylene glycol, propylene glycol, diethylene glycol, ~ ;~
dipropylene glycol, triethylene glycol, dimethyl sulfoxide, and formamide are preferred viscosity reducing~diluents.
These diluents should be present in an amount of about 5 to 80 percent by weight in the adhesive composi-tions of the present invention.
At viscosity reducing diluent concentrations of below about 5 percent, the melt is very gassy due to the evolution of ammonia resulting from the reaction of poly-vinyl alcohol and urea. At viscosity reducing diluent con-centrations of above about 80 percent, the adhesive does not develop green strength or fiber tearing properties as ; rapidly as desired.
The concentration of vis~osity reducing diluent has a signiicant effect on the properties and character-istics of the diferent types of adhesive blends which can be prepared by the present invention. With .5~

homoge~eous melts, viscosity reducing diluent concentra-tions of below 5 percent promotes the evolution of ammonia resulting from the reaction of polyvinyl alcohol and urea.
~t viscosity reducing diluent concentrations of above ab,o'ut 40 percent, the molten homogeneous adhesive blend does not develop green strength or fiber tearing properties as rapidly. , ' Molten homogeneous blends containing up to about 15 percent o-f water as the viscosity reducing diluent require handling temperatures of 115C.-135C. and need to be stored in tightly covered containers to prevent loss of water. If about 15-30 percent of water is present, the melt polnt of the blend is depressed to permit handllng at about 90C-110C. provided loss of water through evapora-tion is prevented. With melts~containing more than about 30 percent of water handling at about 100C. is simpler , because of improved stability and lower viscosities but '~
loss of water from these blends still should be minimized.
Also, homogeneous blends containing more than abou~ 30 ' percent of water generally do not devel~p green strength ;
and fiber tearing bond and water resistance characteristics ~ ~, as do the lo~er water content homogeneous blends~
With the viscous heterogeneous blends of this invention handling temperatures of about 40C. 80C, can be realized. These application temperatures are desirable because the loss of viscosity reducing diluent can be further minimized. With this type of adhesive composition diluent concentrations below about Z0 percent result in too high viscosity. On the other hand, viscosity reducing dilu-ent concentrations above about40 percentyield compositions 5~(~

having too low viscosity and such compositions do not have sufficient tackiness to unction a.s adhesives.
With the low viscosity stable suspension type o~
adhesive co~positions of this invention diluent concentra-tion can vary from about 30 to about 80 percent. Below 30 percent concentration of viscosity reducing diluent there is not sufficient dlluent (e.g. water~ present to adequa-tely dissolve the crystalline solvent or satisfactorily disperse the polymer binder, i.e. polyvinyl alcohol. At - 10 ab~ve about 80 percent water concentration the subsequently post heat activatable adhesive is too runny to stick satis- ;
factorily upon contact to the cellulosic substrate to which it is applied.
The homogeneous molten adhesive systems of the ~
present invention can be prepared several ways as illustra-ted by the lollowing. A dry blend consisting of, for example, 28 parts of Elvanol~ 90-50 (polyvinyl alcohol, degree of hydrolysis 99.0-99.8 percent, 4 percent aqueous viscosity 12-14 cps. at 20C., a~ailable from E. I: du Pont de Nemours and Company) and 72 parts of urea pulverized through a 40 mesh screen is added with stirring to 35 parts of water, heated to 70C.-85C. The addition is carried out at a sufficiently slow rate to maintain a minimum temperature of 70C. in the mixture and requires about 1/4 to 1/2 hour. The blend is held at 85C.-90C. for an additional 15 minutes to obtain a high level of homogeneity.
The xesulting product is stable. N\ adverse effects were noted on the blend after storage for two days at the application temperature of 90~C.-95C. provided that storage ~as in a covered container to prevent loss of water.

lS9~

A similar procedure can be employed to produce a molten homogeneous adhesive system wherein the viscosity reducing diluent i5 a glycol, (for example, propylene glycol) in-stead of water except that a blending temperature of 120C-130C. is used and the product is applied at 110C.-115C.
In the preparation of molten homogeneous adhesive systems of the present invention the polymer binder or binder and crystalline solvent components are added incrementally to the melt in order to prevent the formation of mixtures with excessive intermediate viscosities.
The molten homogeneous a~hesives of the present invention (e.g., bzsed on PVA, urea and water) provide the - unexpected and desirable advantage of yielding bonds which can be made highly water resistant. The preferred method of accomplishing this is by "heat setting". For example, after being heated briefly, corrugated board prepared ~ith the adhesive or this invention exhlbits an enormous improvement in ambient temperature water resistance as measured by the dead load shear test. This test consists of mounting a 2 cm X 15 cm section of double faced board with the flutes running horizontally in water at ambient temperature under 350 g. (buoyanc~ corrected) dead load shear. In order to apply this shear force to only five flute sections (the test zone) the front liner was slit in the flute direction immediately above the test zone; like~
wise the back liner was cut ~ust below the test zone. A
"pass" in the test corresponded to the specimen supporting the 350 g. load for at least 24 hours. Board produced with a PVA/urea/water adhesive melt without "heat setting"
ails in 1/2-2 minutes. With appropriate "heat setting"

.. . .

~115~3~

>85 percent of the specimens tested will pass the test.
A number of methods for accomplishiny "heat setting" of the bonded substrates have been developed. One method, which involves the direct application of heat to the bond through the face of the liner is illustrated in Example 8. Another involves contacting the sample with the surface of an infrared lamp until the temperature in the glue line reaches 21~F.; under these conditions 100 percent of the specimens passed the test. Still another procedure involves the use of radio-frequency energy. Employing a "radio frequency heater" (Fl'~C b~odel 67, sold by 1~. T `
Larose and Associa.es, Inc. of Cohoes, N.Y.), and operat-ing at 40 megacycles under the following condltions, bonds are produced that pass the test without exception.
VoltageExposure Time . . . _. , 100 Volts25 sec.
- 125 Volts~ 10 sec.
140 Volts5~sec.
150 Volts5 sec.
175 Volts2 secu 200 Volts-<2 sec.
The incorporation of a variety of wetting agents has also been found to improve the water resis-tance of bonds prepared with the present adhesives used as a hot melt. In the absence of such wetting agents or if a post heat treatment is omitted, bond failure in the dead load shear test on occasion\can be adhesive (iOe.
between the bond and the paper). Incorporation of an appropriate wetting agent facilitates wet-out of the substrate by the adhesive; as a result the cohesive st~ength of the adhesive, which is of a high order even under wa-ter immersion conditions, becomes limiting and -the incidence of survival of the bonds under water immer-sion conditions is increased significantly. Wetting agents found to be operable include sodium lauryl sulfate, polyethylene oxide-alkyl phenol adducts and their phos-phate esters (e.g., TRITON* X-305 and TRITON QS-9 available from Rohm & Haas), quaternary ammonium salts, various inorganic salts such as alkali phosphates, and sodium nitrilotriacetate. These materials are generally incorporated at about 1-5 percent level based on weight of adhesive. Use of a wetting agent has been found to reduce the severity of the post heat treatment required to produce bonds capable of surviving the ambient tempera-ture water resistance test when adhes:ive blend is applied -~
as a melt.
The incorporation of a latent curing agent is suggested when it is desired to obtain resistance to ~-boiling water. Accordingly, a curing system of titanium citrate and glycollic or citric acid can be used. About 2 percent titanium citrate and about 2 to 4 percent of either glycollic or citric acid (per weight of ; adhesive) is suitable. The acid reduces the pH of the blend to 3.5 - 4.5 where it exhibits normal melt flow behavior. Curing can be accomplished by either contact-ing the bonded substrates with ammonia (aqueous or gase-ous) or by heating them at a temperature above 160C. for ~-several minutes such that sufficient àmmonia evolves from the adhesive -to affect the curing reaction.
Another curing system developed for the molten * denotes trade mark 3L5~

adhesives of the present invention, wherein the liquid cosolvent is water, is based on resorcinol and formalde-hyde. The preferred curing agent consists of ~ parts of resorcinol, 2 parts of urea and 3 parts of 37 percent formalin, which are converted into a resole by heating for 15 minutes at 90C ~ 95C. This resole, when incorporated into the adhesive at a 2-10 percent level ~base~ on weight of adhesive), yielded bonds which after 14-21 days aging under ambient conditions developed suf-ficient water resistance to pass the dead load shear test.
The resole compositions based on resorcinol,urea and formalin can range in composition from 1:1:1 to

3 resorcinol/urea/formalin and can be employed over a concentration range af about 1-20 percent expressed in terms of the weight of the adhesive. In addition to aging at ambient temperature in a semidry state for~14-21 days, curing of such blends can be carried out by heating for several minutes at 120C - 150C.
The polyvinyl alcohol adhesive blends of the present invention containing urea as the crystallizable PVA solvent were found to be non-co~bustible, unable to support a flame in the vertical, relatively non-charring with little or no tendency to exhibit after-glow. However, at the levels employed, these materials do not reduce significantly the combustibility of cellulosic materials bonded by them unless they are used in conjunction with a known flame proofing agent such as ammonium sulfamate.
Viscous heterogeneous adhesive systems, suitable or use in the temperature range of 40C.-80C., can be prepared as follows. Fifty parts of urea are ad~ed to 70 par-ts of water and the solution is heated to 70C. Twenty parts of EIIVANOL~ 90-50 is added with stirring at a uni-form rate to the urea solution, maintaining the tempera-ture at 70C. Mixing is continued until the blend is uniform at 70C, at which time 93 parts of finely divided calcium carbonate is added. Mixing is continued until the blend temperature is 70C. The resulting heterogeneous ~
blend is stable and may be stored for several days in a ~ ;
covered container at 40-80C. with no loss in adhesive properties.
Low viscosity heterogeneous stable suspension type adhesive systems which can be post heat activated to obtain water resistant bonds may be prepared as follows.
Forty parts of urea are added to forty parts of water at 25C. resulting in a clear solution. Twenty parts of ELVANOL~ 90-50 treated as described in Bristol USP
3 497 487 are dispersed in the urea solution. One tenth of one percent guar gum (J~C,UAR* A40-F available from Stein, Hall and Company, Inc.) is then dissolved in the aqueous phase resulting in a suspension which remains stable for several days at room temperature.
The following examples are presented to illus-trate, but not to restrict, the present invention. Per-centages and parts are by weight unless otherwise stated.
Examples 1 to 85 illustrate the molten homogeneous blend embodiment of the present invention.
EXAMPLE l A mixture of 400 g. of urea and 400 g. of water ~as placed in a BRABENDER* mixer and was agitated. As soon as most of the urea dissolved, 200 g. of polyvinyl alcohol * denotes trade mark 3~ ~

(ELVANOL~ 90-50, degree of hydrolysis 99.0 - 99.8 percent,

4 percent aqueous viscosity 12-14 cps at 20C., avai]able from E. I. du Pont de Nemours and Company) was added as rapidly as possible, producing a thin slurry. IIeat was applied. As the temperature of the solution increased progressive thickening of the mixture was observed, its viscosity reaching a maximum at 40C. - 50C. As heating was continued, the mixture thinned out, clariEied and `
became quite fluid at a temperature of 80C. - 90C.
(Viscosity of about 2000 cps.); this operation required about 15-20 minutes. The mixture was placed in a polyethy-lene bag and chilled to below 5C. to induce gelation. The gelled product was chopped and stored in a moisture-tight container to prevent loss of water.

Using a Cowles dissolver, 1500 g. of urea and 200 g. of water was heated to 125C. - 135C. with agita-tion. To the molten mass was added 500 g. of polyvinyl alcohol (ELVANOL 90-50) at as fast a rate as possible.
Agitation was continued until the mixture was fully melted and had clarified. At this point it was poured onto TEFLO~ sheeting where it immediately crystallized. The resulting slab was broken up and pulverized. The finely divided product was either used in this form or was com-pression molded into cartridges for application by hot melt gun. The product was stored in a moisture-tight container.

A 0.5~1.0 mil film of the adhesive composition of Example 2 was prepared between MyLAR sheets by press-ing in a heated Pasadena press. The film was frozen, one ,~

5~

of the MYLAR~ shee-ts was lifted and a piece of kraft paper inserted. Mild heat (90C. - 95C.) and pressure (10-20 psig) was applied in the Pasadena press to bond the kraft ;~
paper and one side of the adhesive film. The structure was frozen again, the second sheet oE M~L~R~ was lifted, and a second piece of kraft paper inserted. The second sheet of kraft paper was then bonded in a Sentinel heat sealer using 1/2 second contac-t time, 70 psig pressure and ~
a bar temperature of 135C. - 175C. The laminate exhib- ~-ited strong, fiber tearing bonds after soaking in ambient temperature water but underwent rapid delamination in water heated to about 70C.
EXAM~LE 4 A 6" roll coater assembly equipped with a heated -and jacketed pan was used for appl~ing the melted adhesive to the applicator roll. This equipment was provided with a dual feed of kraft paper rolls such that a laminated structure could be produced. The experiment was carried out by first melting the adhesive composition of Example 1 in a separate jacketed reservoir and holding it at a temperature of about 85C. - 95C. Once the paper feed had been started, operating at 20 ft./min., the melted adhesive was pumped into the heated applicator pan at the ;~
rate at which it was applied to the paper. In this manner, ;~
a structure in which two sheets of kraft paper were bonded with 1-2 mil adhesive (dry basis) was produced. The;-~
adhesive could be applied smoothly without gaps and no ~ i undesirable adhesive build up occurred on the applicator ;
roll as the run proceeded. At the completion of the experiment the equipment was cleaned up easily by hot water A :

washing. The bonded structure exhibited excellent high humidity and ambient temperature water resistance. It was readily repulpable in hot water.

In this experimenk a 25" corrugator was used with the applicator roll heated to 85C. Approximately l pound of the blend of Example L was,melted and was placed on this roll where it was quickly ,spread into a continu-o~s coating on the roll by a doctor blade assembly. A
single faced corrugated,structure was passed over the roll at about lO0 ft./min. such that the tips of the flutes were wetted by the adhesive. ~The second facing was then applied and the resulting structure passed through several pressure rolls before being cut about 10-12 feet down the~ line. The adhesive exhibited excellent rheoloqy on the machine as long as its water content;was maintained. Adequate "green strength" had developed in the~structure~by the~time it reached the collector station and fiber tear was observed , after the structure had been allowed to age 3-5 minutes~
The corrugated structure exhibited little or no undesirable 'ribbing" (in direct contrast to structures prPpared with conventional corrugating adhesives).'~ Equipment cleanup ~t the end of the trial was re1atively simple due to the ease of removing adhesive build up from the app1icator roll , and also because of the ready solubility of the adhesive in hot water. Examination of the resulting corrugated ' structure indicated excellent performance for the adhesive.
The dry pin adhesion (corrugated flute to liner bond) proved to be superior to starch (e.g., 85-lO0 lbs. vs. 50-70 l~s.) at about l/2-2/3 the loading je.g., 1.5 lbs. vs. 2.0-3.0 - 2~ -~lQ~L5~

lbs. per 1000 ft.2 board). The corrugated board bonded ~'' with the adhesive of this Example exhibited excellent ;
ambient temperature water re,sistance in the ply separation test, fiber 'cear being observed after 10 days immersion.
In contrast starch bonded structures delaminated after 1-2 minutes immersion. The polyvinyl alcohol-urea bonded ~ ~' structure was readily repulpable in hot water.

A 23/68/9 polyvinyl alcohol (EL~A~IOL~ 90-50)/
urea/water blend, compression molded into 7/8'l x 1 1/2'l ;~
cylinders was applied using a conventional design hot melt gun. The material handled well over a 130C. - 150C. ~
temperature range exhibiting good ~uick tack on paper , boards forming fiber tearing bonds within about 1-3 seconds.
Upon cooling to below 100C - 110C. -the bonds exhibited 100 percent fiber tear.

The blend of Example 2, in pulverized form, was placed in all LTI DYNAPPLY* 45D case sealing unit equipped with an LTI G-type 4 nozzle head. The material was melted at a temperature of 2650F. - 300F. and then applied by pumping through the nozzle assembly. The adhesive was applied to kraft paper and its adhesion and ~uick tack .
performance were noted. The molten adhesive was observed to form fiber tearing bonds upon contact with the kraft. ' The glue line appeared to have an open time of 1-3 seconds produciny fiber tearing bonds in laminates almost as soon as the facing sheet was brought into contact with the adhesive-containing sheet. These bonds exhibited excellent ,~
ambient tem~erature water resistance but underwent * denotes trade mark , - 25 -", . .
,.. . .

delamination in water at 165 F. or higher.
_XAMPLE
A 2 cm x 12 cm section of corrugated board~
bonded with an 18/47/35 ELVAMOL~ 90-50/urea/water blend and air-dried for several days, was used in the dead load shear test for water resistance. This section of board was heated for 30 seconds at 300F. in a Pasadena press under llght pressure and was subjected to the dead load shear test for ambient temperature water resistance. The sample survived ~48 hours immersion under the test con-ditions. The results obtained by carrying out the heat setting under a variety of conditions are summarized in Table I.

~L~Q~9~t TABLE I
.
Heat Settinq Incidence of Survival Duration Tempërature(l) >24 hr.(2) .. . .
- ~ .
2 h~. 200F. 65 15 min. 200F. 75%
1 min. 300F. 100%
30 sec. 320F. 70%
1 sec. 480F. 65 1/2 sec. 570F. 35 1/2 sec. 680F. 70%

(1) Temperature of surace against which out~
side of corrugated board was pressed for the time indicated.
; (2) Based upon 6-8 samples having adhesive;
loadings ranging from 0.75-2.5 lb./
1000 sq. ft.~of corrugated board.
EXA~LES 9-24 Several blends containing various glycol and glycol ethers have been also examined.~ Both the melt characteristics and the bonding performance of these blends have been investigated. Results of this study are summarlzed in Table II. As~wil1 be noted, with the exception of glycerine, whIch yielded a~highly unstable melt, all of the other additives tested ylelded blends~
which exhibited good-to-excellent overall bonding performance. The only deficiency picked up in the testing ~ -was the pooxer water resistance ~of the resulting bonds 3~ in most cases. This deficiency is not expected to significantly reduce the utility of the blends of the present invention, e.g., in uses such as case sealing and book binding adhesives.

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EX~MPLES 25-61 ~ olten homoge.neous blends o the present inven-tion and some comparative blends are further illustrated by the compositions and the.~r characteristics summarized in Table III.

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EX~PLES 62-70 Molten homogeneous blend type adhesives based upon ELVANOL~ 90-50 PVA and/or EVAL* 80 E/VOH copolymer exhibiting excellent water resistance are illustrated in these examples.
A mixture of ELVANOL~ 90-50 and/or EVAL 80 (33/67 per weight E/VOH copolymer sold by Kurashiki), urea and propylene glycol was heated at about 135C. with stirring for about l/4 to 1/2 hour uniil a homogeneous melt ;~
was obtained. The latter was then applied as a thin coating to one side of a 2 x 3 cm section of liner. A bond was ~
produced by bringing a second section against this coating ;
and cooling the resulting lap ~oint to room temperature.
Finally the bonded structure was immersed in water, stored for 24 hours at ambient temperature and hand-pulled to test for delamination resistance as well as extent of ~ ~ -bond failure by fiber tear. As will be noted in Table IV
by observing the characteristics of compositions containing EVAL 80 alone or combined with ELVANOL~ 90-50j the presence of even relatively small amounts of EVAL 80 (e.g., 3-4%) conferred a high level of delamination resis-tance to the derived bonds even when a difficult-to-wet liner such as the resin sized product was used. Fiber tearing bonds were observed when the EVAL 80 level in the blend reached about 13%. Blends containing EVAL 80 are not fully repulpable and thus would be useful in the preparation of water-resistant corrugated board where repulpability was not re~uired.
Bonds between sections of the same three types of liners prepared with a 20/60/20 ELVANOL 90-50/urea/
* denotes trade mark ~ .

5i90 propylene ~lycol composition undexwent delamination in about 5 to 15 minutes in this test.

' : ' , : ~ ~' .
:
- - ' ~:
; :

' ' ..
.
'' Molten homogeneous blend type adhesives based upon ELVANOL~ T-25 and 71-30 PVA are compared with the same -type of adhesives based upon ELVANOL~ 90-50 PVA in these examples. ELVANOL~ T-25 PVA has a degree of hydro-lysis of at least about 99 percent, a 4 percent aqueous viscosity of 25-31 cps. at 20C., and is available from E. I. du Pont de Nemours and Company. ELVANOL~ 71-30 PVA
has a degree of hydrolysis of 99.0-99.8 percent, a 4 percent aqueous viscosity of 28-32, and is available Erom E.I. du Pont de Nemours and Company. The blend composi-tions and their characteristics are summarized in Table V.

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r, ~ r .J ~ l ~ : :

V~ h I o o o o o~ ~ ~ ~ o o o o u) u~ o ~ Ul O O It> O O

h E I.i a ~ ~ 3 v o v~ ~; c ~ a~
V ~ h ' h ~ C ~: o C ~ o C: ~ a ~ 3 ~ ~ ~ a '~ ~ ' u~ a ~ , ~ ~ A S

la h h h ~ h h h ~p ~ ~ o ~ S, "~ h h h I ~ 1 ~ ' _ ~ ~ ~ ~ V ~ V ~ ~ , E- X X ~ ; ~ Z~ ~ ~ ~ ~ ~ ~ - X 3 ~ X ~ ~ U
~,, ~ O O N N O N N O N N O N N O O O N N O N N ~

i O
L~ h ~ N o ~ ¦
L ~ U O V
E~ ., 3 s ~ N N .17 ~ U I ~
~r U ~I Lr~ o h ~ , ~ V I I h E~

O ^ ~ O O O ~ ,, ~ ~ 3 ¦
~o k ~ F 3 U
U--L~¦ r.J Ir~ O Lq ~ N h X ¦ .

V ~ ~ N N N ONli_~ ~ ~ ' h ¦ E~ 0 ~
O U ¦ o or o 10 oo o~ o )11 ~ I
., ~1: ol o ~D ~ o v\ o m ~ ~r ~ :
F. L3 ¦ O N 1`1 h L l ~ O O V~ 3 V
r~ ~r O
h 'D ~D V ~ ~ VJ ~ r` ~ J .

~ , ~n ~, V ~: ~ a ~, 5~ ~, ,, (~1 h v ' ~ o o u~ a c a O a ~ X a C~ X ~ a O ,d ~ ~a h V E~ E~ ~ ~ ~ O~d U ~ d h O
h ~ .n n sn 5 n .ù a a a a ,a a .~a n O d h h h ~ ,d ~1 h¦ O ~ ~~ ~ ,~,, ,, ,, ,, ,.,,, , ~ ,~ o ~ ~ 0 E~ ~a _ ¦ U U OU U O OU O O O O O O O E~ r~ a o U Xu N N ~1 N ~ r~ N 4 ~ N er ~I N b:~ V JJ h 5n O
N r I~1 ~1 ~I N~I N ~1 ~1 1~ ~ l t~ d O E 1.1) h a O~O ~ U ~a ~aO ~Oa ~Oa OO aO ~ ~- R V
h .IJ t ¦ LOq txq ~tq ~Xq tXl ~ ~Xq~ ~Oq ~q ~ ,q ~q ~ O CJ C a Id rl ~d ::- S ~ .~ ~ iJI C ~ ~1 X
F~ C ~ c 5 C5~ 5~ 5 C 5 5 5Cl 5 ~a a , E~ tq ~ql hI I a~ ~~ ~ ~ ~ h O ~d o I L
o ~ Lq o ~ r ra ~ ~r 4~ UN~ N r-i N N r-l O W L~q ~a 'aa Ld J- :

I ~ a." ~ .a ~ ~ a 3 ~ D O ~9 0 r ~ o r ~D o ~r x ~q t4 1 ~a JJ

tq o~ l r ~ o ~j N t~l æ ~ ~d Ld L1 ,a X a a Lq . ' ~ ~ ~ N j r~ L ~ ~ l ~ N ~1 O ~n j ~ I ~ V 1) 0 e m ~q tq JJ ~ ~ ~ t4 0 8 rl~

~n; 4 r~ ~Ll ~ 7 '~ S 'E~ & 3d ~Xq JJ
¦ ,I N r~ ~ 1~ C r-~COo c'o~ cO cO cO cO ~ N _ r U~

-- 3 8 -- `

I ~

. -'' ', A

Examples 86 to 89 lllustrate the viscous hetero-geneous blend embodiment of the present invention.

. . . : ~,, Sixty grams of urea and 53 grams of water were added to a 0125 liter stainless steel beaker heated in an electrical heating block and equipped with an air driven stirrer. When the temperature oE the solution was 75C, 20 grams of polyvinyl alcohol (Elvanol~ 90 50) was added ; ;
at a uniform rate and stirring continued until a clear solution was obtained. Viscosity of this solution was 850 centipoise at 70C. A comparable composition prepared without urea was a firm gel at 70C. Forty-our grams of pigment grade calcium carbonate was stirred into the solu-tion resulting in a heterogeneous blend containing 70 total solids. Viscosity of the blend was 1800 centipoise -at 70C, 2800 centipoise at 60C. and 7500 centipoise at 50C. measured with a Brookfield Viscometer, Model RVT, spindle #2 at 5 rpm. -EX~MP~E 8~
_ A thin coating (about 1-2 mil) of the adhesive blend of Example 86 was applied to the tip (0.3 cm depth) of a 2.5 x 5.0 centimeter kraft paper coupon. A bond was produced by bringing a second coupon against this coating and allowing the lap joint (0.8 cm2) to set at room tempera- `
ture. The bonded structure was immersed in water, under a 350 gram load (buoyancy-corrected), and stored for 24 hours at ambient temperature. After this exposure a fiber tear-ing bond was obser~ed at ambient temperature but the adhesive coating was repulpable in 70C water.

'~"

, L5~

Eighty one grams of ~-caprolactam and 91 grams of water were added to a 0.25 liter stainless steel beaker and heated as described in Example 86. When the solution was at 75C, thirty grams of polyvinyl alcohol (Elvanol~
90-50) was added and stirring continued until a clear solu-tion resulted. Viscosity of thissolution at 70C. was 276 centipoises. A comparable composition prepared without ~-caprolactam was a firm gel at 70C. One hundred and four grams of pigment grade calcium carbonate was stirred into this solution to make a tacky, heterogenous blend containing 70 percent total solids. Brookfield viscosity of this blend was 1200 centipoises at 55C.

A bonded structure was prepared with the adhesive ~ -blend of E~ample 88 by the process described in Example 87 with the exception of drying the bonded joint under an infrared heat lamp at a surface temperature of 100C. for -~
five minutes. The bonded structure was immersed in water as described in Example 87. After 24 hours a fiber tear-ing bond was observed at ambient temperature but the adhe-sive coating was repulpable in 70C. water.
Examples 90 to 93 illustrate the low viscosity, stable suspension embodiment of the present invention.
EXAMPLES 90 to_93 Post heat activatable PVA based adhesive suspen-sions were prepared by dissolving urea in water, adding guar gum as a suspending agent, then dispersing a heat treated grade of polyvinyl alcohol, prepared as described in Bristol USP 3,497,487, to the solution. Adhesive 5 9C~

compositions prepared by this method are summarized in Table VI. :
A thln coating of each of the suspension des-cribed in Table VI was applied to the tip (0.3 cm depth) of a 2.5 x 5.0 centimeter kraft paper coupon and set aside to dry. A bond was produced by bringing a second uncoated coupon against this dried coating and heating -~
the overlaped section in a Sentinel Heat Sealer for 7 seconds at 14 psi clamp pressure with a bar temperature of 190C. The resultant bonded structures were immersed in water as described in Example 87. After 24 hours exposure under these conditions a fiber tearing bond was observed but the adhesive coating was repulpable in 70C ~ :
water. :
TABLE VI
Blend Composition (wt. ratlo) "Jaguar" Solids Example PVA Urea Water ~ 40-F (%) .
gO lO 20 120 0.20 20 ~:~

91 20 40 26 0.06 70 92 l~ 70 320 0.30 20 93 lO 60 46 0.10 60 -41- : -.~ , .
-

Claims (45)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An adhesive composition consisting essen-tially of about 2 to 35 percent by weight of a polymer selected from the group consisting of polyvinyl alcohol and ethylene/vinyl alcohol copolymers containing at least 50 mol percent vinyl alcohol, about 10 to 80 percent by weight of at least one solvent for said polymer, which solvent is crystalline at 40°C. and is selected from the group consisting of urea, solid alkyl substituted ureas containing a total of up to 9 carbon atoms, thiourea, biuret, .epsilon.-caprolactam, solid aliphatic amides containing up to 6 carbon atoms, solid polyhydric compounds and ammonium carboxylate salts, about 5 to 80 percent by weight of at least one viscosity reducing diluent selected from the group consisting of water, liquid polyhydric compounds, liquid alkyl substituted ureas having up to 9 carbon atoms in the alkyl group, liquid aliphatic amides containing up to 6 carbon atoms, and dimethyl sulfoxide and 0 to about 60 percent by weight extender selected from the group consisting of starch, dextrin, clay, silica, carbon black, talc, calcium carbonates barium sulfate and vinyl polymer lactices, provided that when said adhesive composition is a suspension, said polymer is polyvinyl alcohol which has a cold water solubles content of less than 10 percent by weight at 25°C while being soluble is 80°C to 100°C water.

2. The composition of Claim 1 wherein said adhesive composition is a homogeneous system consisting essentially of about 10 to 35 percent by weight of a poly-mer selected from the group consisting of polyvinyl alco-hol and ethylene/vinyl alcohol copolymers containing at least 50 mol percent vinyl alcohol, about 40 to 80 percent by weight of at least one crystalline solvent for said polymer, and about 5 to 45 percent by weight of at least one viscosity reducing diluent.

3. The composition of Claim 2 wherein said crystalline solvent for the polymer is selected from the group consisting of urea, solid alkyl substituted ureas containing a total of up to 9 carbon atoms, thiourea, biuret, .epsilon.-caprolactam, solid aliphatic amides containing up to 6 carbon atoms, pentaerythritol, sorbitol, mannitol and tris(hydroxymethyl) aminomethane, and ammonium carboxylate salts.

4. The composition of Claim 3 wherein said crystalline solvent for the polymer is urea, or .epsilon.-capro-lactam.

5. The composition of Claim 4 wherein said crystalline solvent for the polymer is urea.

6. The composition of Claim 3 wherein said polyvinyl alcohol is at least 99 percent hydrolyzed and has a 4 percent aqueous viscosity at 20°C. of from about 10 to about 65 centipoises.

7. The composikion of Claim 6 wherein said polyvinyl alcohol has a 4 percent aqueous viscosity at 20°C of from about 12 to about 14 centipoises.

8. The composition of Claim 6 wherein said polyvinyl alcohol has a 4 percent aqueous viscosity at 20°C of from about 25 to about 31 centipoises.

9. The composition of Claim 6 wherein said polyvinyl alcohol has a degree of hydrolysis of from about 99.0 to about 99.8 percent and a 4 percent aqueous viscosity at 20°C of from about 28 to about 32 centipoises.

10. The composition of Claim 3 wherein said viscosity reducing diluent is selected from the group consisting of water, liquid glycols, liquid glycol ethers wherein the alkyl group contains l to 4 carbon atoms, liquid glycol esters wherein the acyl group contains 2 to 4 carbon atoms, liquid polyalkylene oxides having a molecu-lar weight of up to about 1000, their liquid alkyl ethers wherein the alkyl group contains 1 to 4 carbon atoms, their liquid esters wherein the acyl group contains 2 to 4 carbon atoms, liquid alkyl substituted ureas having up to 9 carbon atoms in the alkyl group, liquid aliphatic amides contain-ing up to 6 carbon atoms, and dimethyl sulfoxide.

11. The composition of Claim 10 wherein said viscosity reducing diluent is selected from the group consisting of water and liquid glycols having 2 to 9 carbon atoms.

12. The composition of Claim 10 wherein said viscosity reducing diluent is selected from the group consisting of water, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, dimethyl sulfoxide and formamide.

13. The composition of Claim 3 containing about 1 to 5 percent by weight, based upon adhesive solids, of a wetting agent selected from the group consisting of sodium lauryl sulfate, polyethylene oxide alkyl phenol adducts and their phosphate esters, quaternary ammonium salts, alkali phosphates and sodium nitrilotriacetate.

14. A curable homogeneous adhesive composition consisting essentially of the composition of Claim 3 and a curing agent selected from the group consisting of (a) about 2 percent titanium citrate and about 2 to 4 percent glycollic or citric acid, based upon the weight of the adhesive, and (b) about 1 to 20 percent, based upon the weight of the adhesive, of a resole prepared by heating for about 15 minutes at about 90°C. to 95°C. about 1 part by weight of resorcinol, about 1 part by weight of urea and about 1 to 3 parts by weight of 37 percent formalin, provided that curing agent (b) is employed only in hydro-melt adhesives wherein the viscosity reducing diluent is water.

15. The composition of Claim 3 containing in addition up to about 10 percent by weight of at least one extender selected from the group consisting essentially of starch, dextrin, clay, silica, carbon black, talc, calcium carbonate, barium sulfate and vinyl polymer latices.

16. A structure comprising at least two cellulosic substrates bonded together by means of the homogeneous adhesive composition of Claim 3.

17. The structure of Claim 16 wherein the crys-talline solvent for the polymer is urea or .epsilon.-caprolactam.

18. The composition of Claim 1 wherein said adhesive composition is a heterogeneous system consisting essentially of about 5 to 25 percent by weight of a poly-mer selected from the group consisting of polyvinyl alcohol and ethylene/vinyl alcohol copolymers containing at least 50 mol percent vinyl alcohol, about 10 to 45 percent by weight of at least one crystalline solvent for said polymer, and about 20 to 40 percent by weight of at least one viscosity reducing diluent, and about 5 to 60 percent by weight extender.

19. The composition of Claim 18 wherein said crystalline solvent for the polymer is selected from the group consisting of urea, solid alkyl substituted ureas containing a total of up to 9 carbon atoms, thiourea, biuret, .epsilon.-caprolactam, solid aliphatic amides containing up to 6 carbon atoms, pentaerythritol, sorbitol, mannitol and tris(hydroxymethyl) aminomethane, and ammonium carboxylate salts.

20. The composition of Claim 19 wherein said crystalline solvent for the polymer is urea, or .epsilon.-capro-lactam.

21. The composition of Claim 20 wherein said crystalline solvent for the polymer is urea.

22. The composition of Claim 19 wherein said polyvinyl alcohol is at least 99 percent hydrolyzed and has a 4 percent aqueous viscosity at 20°C. of from about 10 to about 65 centipoises.

23. The composition of Claim 20 wherein said polyvinyl alcohol has a 4 percent aqueous viscosity at 20°C. of from about 12 to about 14 centipoises.

24. The composition of Claim 20 wherein said polyvinyl alcohol has a 4 percent aqueous viscosity at 20°C. of from about 25 to about 31 centipoises.

25. The composition of Claim 20 wherein said polyvinyl alcohol has a degree of hydrolysis of from about 99.0 to about 99.8 percent and a 4 percent aqueous viscos-ity at 20°C. of from about 28 to about 32 centipoises.

26. The composition of Claim 19 wherein said viscosity reducing diluent is selected from the group consisting of water, liquid glycols, liquid glycol ethers wherein the alkyl group contains 1 to 4 carbon atoms, liquid glycol esters wherein the acyl group contains 2 to 4 carbon atoms, liquid polyalkylene oxides having a molecular weight of up to about 1000, their liquid alkyl ethers wherein the alkyl group contains 1 to 4 carbon atoms, their liquid esters wherein the acyl group contains 2 to 4 carbon atoms, liquid alkyl substituted ureas having up to 9 carbon atoms in the alkyl group, liquid aliphatic amides containing up to 6 carbon atoms, and dimethyl sulfoxide.

27. The composition of Claim 26 wherein said viscosity reducing diluent is selected from the group consisting of water, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, dimethyl sulfoxide, and formamide.

28. The composition of Claim 27 wherein said viscosity reducing diluent is water.

29. A structure comprising at least two cellu-losic substrates bonded together by means of the hetero-geneous adhesive composition of Claim 19.

30. The structure of Claim 29 wherein the crystalline solvent for the polymer is urea or -caprolactam.

31. The composition of Claim 1 wherein said adhesive composition is a suspension consisting essentially of about 2 to 35 percent by weight of polyvinyl alcohol, said polyvinyl alcohol having a cold water solubles content of less than 10 percent by weight at 25°C while being soluble in 80°C.-100°C. water; about 15 to 68 percent by weight of at least one crystalline solvent for said polymer, and about 30 to 80 percent by weight of at least one viscosity reducing diluent b

32. The composition of Claim 31 wherein said crystal-line solvent for the polymer is selected from the group consist-ing of urea, solid alkyl substituted ureas containing a total of up to 9 carbon atoms, thiourea, biuret, .epsilon.-caprolactam, solid aliphatic amides containing up to 6 carbon atoms, pentaerythritol sorbitol, mannitol and tris(hydroxymethyl) aminomethane, and ammonium carboxylate salts.

33. The composition of Claim 32 wherein said crys-talline solvent for the polymer is urea or .epsilon.-caprolactam.

34. The composition of Claim 33 wherein said crystalline solvent for the polymer is urea.

35. The composition of Claim 32 wherein said polyvinyl alcohol is at least 99 percent hydrolyzed and has a 4 percent aqueous viscosity at 20°C. of from about 10 to about 65 centipoises.

36. The composition of Claim 35 wherein said polyvinyl alcohol has a 4 percent aqueous viscosity at 20°C. of from about 12 to about 14 centipoises.

37. me composition of Claim 35 wherein said polyvinyl alcohol has a 4 percent aqueous viscosity at 20°C. of from about 25 to about 31 centipoises.

38. The composition of Claim 35 wherein said polyvinyl alcohol has a degree of hydrolysis of from about 99.0 to about 99.8 percent and a 4 percent aqueous viscosity at 20°C. of from about 28 to about 32 centi-poises.

39. The composition of Claim 32 wherein said viscosity reducing diluent is selected from the group con-sisting of water, liquid glycols, liquid glycol ethers wherein the alkyl group contains 1 to 4 carbon atoms, liquid glycol esters wherein the acyl group contains 2 to 4 carbon atoms, liquid polyalkylene oxides having a molecular weight of up to about 1000, their liquid alkyl ethers wherein the alkyl group contains 1 to 4 carbon atoms, their liquid esters wherein the acyl group contain 2 to 4 carbon atoms, liquid alkyl substituted ureas having up to 9 carbon atoms in the alkyl group, liquid aliphatic amides containing up to 6 carbon atoms, and dimethyl sulfoxide.

40. The composition of Claim 39 wherein said viscosity reducing diluent is selected from the group consisting of water, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, dimethyl sulfoxide, and formamide.

41. The composition of Claim 40 wherein said viscosity reducing diluent is water.

42. The composition of Claim 33 containing about 0.05 to 0.5 percent by weight guar gum.

43. me composition of Claim 41 containing about 0.1 to 0.3 percent by weight guar gum.

44. A structure comprising at least two cellu-losic substrates bonded together by means of the adhesive composition of Claim 32.

45. The structure of Claim 44 wherein the crystalline solvent for the polymer is urea or .epsilon.-capro-lactam.