CA1071085A

CA1071085A - Corrugated board adhesive

Info

Publication number: CA1071085A
Application number: CA262,365A
Authority: CA
Inventors: Kenneth H. Eaton; David Debnar
Original assignee: DURAL PRODUCTS Ltd
Current assignee: DURAL PRODUCTS Ltd
Priority date: 1976-09-30
Filing date: 1976-09-30
Publication date: 1980-02-05

Abstract

ABSTRACT OF THE DISCLOSURE

A method is provided for producing a water-resistant combined corrugated board employing a cold cure adhesive based on an aqueous dispersion of a water insoluble synthetic homo-polymer of vinyl acetate containing a cross-linkable material to which a cross-linking agent is added prior to use.

Description

~0'7~085 This invention relates to an improved adhesive formulation for corrugated board, more especially the invention is concerned with an adhesive formulation for the production of water-resistant corrugated board, a method of producing such a corrugated board and improved corrugated boards.
Corrugated paper board is widely used in the manufac-ture of shipping containers in view of its light weight coupled with strength and its cushioning effect. Such corrugated boards comprise a fluted or wavy sheet of paper glued to one or more sheets of paper called liners or facings.
Depending on the number of wavy or corrugated sheets and the number of liners the board may be a single-faced corrugated board (one corrugated sheet glue~ to a single liner), a single-wall corrugated board (one corrugated sheet sandwiched between two liners), double-wall corrugated board (two corrugated sheets sandwiched between three liners~, triple-wall corrugated board (three corrugated sheets sandwiched between four liners) and so on The corrugated sheet and the liner are glued at the areas of contact between the flutes of the corrugated sheet and the liner. As the glue there has conventionally been employed a starch-based adhesive formulation which requires heat to set or gel it. In the conventional manufacture of corrugated board the starch adhesive is applied to the flutes of a continuously fed corrugated sheet, and a pre-heated liner is pressed against the flutes, the pressure being maintained for a time to permit adequate setting of the starch adhesive.
The liner must be pre-heated to a temperature such that when it contacts the adhesive coated flutes, the starch adhesive is heated to its gelatinization or setting point which is of the order of 60 to 64C~ In order to introduce the required heat to the starch adhesive it is conventional to pre-heat the liner -1- ~ ' .

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, 107~085 by feeding it over a pre-heater which is at a temperature of 185 to 200C.
Thus the conventional method of manufacturing corrugated board employing starch adhesives requires expensive heating operations to pre-heat the liners. Furthermore due to the insulating properties of the paper, several pairs of pressure rolls and continuous heat are necessary to gel the starch and maintain the corrugated sheet and liner in contact while adequate setting is achieved.
10This necessity for an adequate setting time dictates the production speed and the maximum production speed decreases with increase in the number of layers of liner and corrugated sheet.
Furthermore corrugated boards manufactured with conventional starch adhesives have no wet strength and will disintegrate if exposed to water or even high humidity such as may be found in the summer months in North America. This lack -of wet strength presents particular problems when the corrugated board is employed in shipping containers for packed frozen foods, and may require the employment of expensive inner linings for the container.
Attempts have been made to produce a water-resistant corrugated board by adding a cross-linkable compound such as urea-formaldehyde or resorcinol formaldehyde to the conventional starch adhesive with varying degrees of water resistance being the net result, however, totally waterproof board has not been realized with existing starch systems.
The present invention provides a curable synthetic resin adhesive formulation for the manufacture of a water-resistant or water-proof corrugated board. Furthenmore the adhesive formulation of the invention does not require heat to effect the curing although some heating may be advantageous, and the pre-heating of the liner can be completely omitted - -, .:

1C~7~L085 with the consequent economy in energy costs.
m us the present invention provides the long sought a~ter water-resistant corrugated board and at the same time pro-vides economies of manufacture by avoiding the necessi-ty of ex-pen~iive pre-heating of the liner and post-heating of multi-wall board.
According to the invention there is provided a method of making a water-resistant combined corrugated paper board comprising a corrugated paper sheet adhered to a paper liner compri~ing i) providing a corrugated paper sheet having a plurality of flutes and a paper liner, ii) applying to flutes on one side of said sheet an adhesive composition comprising a mixture of an aqueous dispersion of a particulate water-insoluble homopolymer of vinyl acetate containing a water-soluble cross-linkable material, effective when cross-linked to provide water-proofing, and a cross-linking agent, iii) pressing said liner against said flutes, and iv) allowing said composition to set with cross-linking of said cross-linkable material to form a water-proof bond.
In preparing the corrugated paper sheet, the sheet is suitably passed between a pair of corrugated rolls, it is necessary to heat these corrugated~rolls-in order to ensure proper flexibility of the paper sheet. If the adhesive formu-lation of the invention is applied to the flutes of the cor-rugated sheet immediately after the corrugations are formed - and while the flutes still retain heat from the corrugated rolls, the curing of the adhe~ive formulation is so speeded up that it can be considered as being substantially instantaneous.
According to a particular embodiment of the invention there is provided a method for the continuou~ production of a water-resistant combined corrugated paper board comprising a corrugated sheet adhered to a liner comprising, a) continuously feeding a first paper sheet through a corrugating means to ~ 3 -~o7~085 produce a corrugated sheet havi~g a plurality of flutes, b) applying to flutes on a first side of said sheet a curable adhesive composition comprising an aqueous dispersion of a homo-polymer of vinyl acetate containing a water-soluble polyvinyl a~cohol and a curing agent effective to cross-link said polyvinyl alcohol, c) immediately applying a continuously fed second paper sheet against the adhesive coated flutes and pressing said second sheet against said adhesive coated flutes, and d) allowing said adhesive composition to set with cross-linking of said polyvinyl alcohol to form a board with a water-proof bond.
According to another aspect of the invention there is provided a water-resistant combined corrugated paper board comprising a corrugated paper sheet having a plurality of flutes and having on at lea~t one side a paper liner adhered at said flute~ by a cured adhesive composition comprising a major amount of a water-insoluble ~omopolymer of vinyl acetate and a cross-linked water-proof material.
There i~ further disclosed an adhesive composition com~
pri-qing an aqueous dispersion of a particulate water-insoluble homopolymer of vinyl acetate containing a cross-linkable water-soluble material, effective when cross-linked to provide water-proofing, and a cross-linking agent for isaid material.
According to a still further aspect of the invention there i~ provided an adhesive formulation comprising firi~t and second ~eparately packaged components said first component compri3ing a tacky aqueous dispersion of a particulate water-in~oluble homopolymer of vinyl acetate containing a water-oluble cross-linkable material, effective when cro~s-linked to provide water-proofing said second component comprising a cro~s-linking agent for ~aid cross-linkable material in said first component.

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Still ~urther ~aid invention provides a method o~
making a combined corrugated paper board comprising a corrugated paper sheet adhered to a paper liner compri~ing, i) providing a corrugated paper sheet having a plurality of flutes and a paper liner, ii) applying to flutes on one side o~ said sheet an adhe~ive composition having quick grab and comprising an aqueous dispersion of a particulate water-insoluble homopolymer of vinyl acetate, and iii) pressing said liner against the coated flutes to form a bond between said coated flutes and said liner.
Adhesive Formulation The adhesive formulation of the invention comprises a two component system. A first component comprises an aqueous dispersion of a solid water-insoluble homopolymer of vinyl acetate, which contains a cross-linkable material effective when croas-linked to provide water-proofing. The dispersion should display tackine~s or adhesiveness with good grab pro-pertie~ and should set readily by di~sipation of the water in the disper3ion. The second component compri~es a cross-linking agent or curing system effective to cure or cross-link the cross-linkable material at room temperature~.
The first and second components are supplied separately packaged to the corrugated board manufacturer and are mixed just prior to use.
The mixture of the fir~t and second components hac, of course, a limited shelf life and this is the reason for separately packaging the components and mixing them just prior to use. ~evertheless the aqueous mixture ha~ an adequate stability and remains fluid and useable for up to forty-eight hours after mixing. However, when the mixture is applied to a surface in a thin film, such as in the method of the invention, it cures rapidly.

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! The dispersed particulate vinyl acetate homopolymer ~ui.tably haq a particle size of about 0.5 to about 1.5 microns and preferably about 0.8 to about 1.0 microns. The polymer shGuld be one which in a high solids concentration in the aqueous dispersion produces a tacky composition having quick grab, and which when the water of the dispersion is dissipated provides the back-bone of the bond between the flutes of the corrugated ~heet and the liner, Satisfactory results are obtained when the solids concentration in the dispersion i~ from 35% to 6~/ol by weight of the dispersion and preferably from 45% to 58%~ more preferably 55% to 57%.
As the cross-linkable material there may be employed any cross-linkable material which yield~ hydroxyl, acid o~ other active ~ights and which can be cro~s-linked to provide water-proofing by chemical mean~. An e~pecially preferred cross-linkable material is a water-soluble polyvinyl alcohol which can be readily cured or cross~inked to a water-resistant material by a variety of inorganic and organic curing l~i :, .

~07~085 or cross-linking agents at room temperature, for example borax, boric acid, sodium hydroxide and inorganic salts, for example soclium carbonate, sodium sulphate, ammonium sulp~late, potassium sulphate, aluminium chloride, chromium nitrate and urea formal-dehyde; these cross-linking agents are conveniently employed in an aqueous solution.
The water soluble polyvinyl alcohols are manufactured by hydrolysis of polyvinyl acetate, by judicious selection of conditions for both polymerization of vinyl acetate to produce polyvinyl acetate and of hydrolysis, a range of polyvinyl alcohol products is available. By way of example, there may be mentioned the COVOL (trademark) series of polyvinyl alcohols.
This series includes polymers in which the percentage hydrolysis i8 from 85% to 100%, generally the greater the degree of hydrolysis the greater is the gelling or tack and the greater is the water resistance obtained by curing. On the other hand the grades with a lower degree of hydrolysis are more water soluble.
Generally these polyvinyl alcohols are available as white granular powders having a specific gravity of 1.28 to 1.31 and a bulk density of 33 to 44 with a % elongation ,~ (unplasticized) of up to 300.
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107~085 The polyvinyl alcohol is suitably employed in an amount of 1 to 5% and preferably 1.5 to 3.5%, by weight, based on the weight of the composition.
A particular advantage of polyvinyl alcohol is that it al~ist~ the vinyl acetate homopolymer in rendering the composition tacky. Further the polyvinyl alcohol acts as a disper~ion stabilizer preventing agglomeration of the di~persed vinyl acetate homopolymer particles.
The disper~ion may also contain a surfactant and a buffer, for example, ammonium hydroxide or sodium bicarbonate to maintain the pH acid when the di~persion is to be cross-linked.
The manufacture of the aqueous dispersion is further described by reference to the preferred aqueou~ dispersion containing polyvinyl acetate as the polymer and polyvinyl alcohol a~ the cross-linkable material. The dispersion i8 obtained by the emulsion homopolymerization of vinyl acetate monomer in an aqueous medium containing the polyvinyl alcohol in the presence of a polymerization catalyst for example a peroxide or persulphate, for example potassium persulphate.
A surfactant and a buffer are deqirably employed in the emulsion polymerization. This type of polymerization of vinyl acetate is of course well known and the aqueous dispersions of poly-vinyl acetate obtained are widely employed for example in latex water paints and textile fini~hing. Such di~per~ionq are oftan referred to a~ emul~ions, although they comprise a solid phase disper~ed in a liquid phase.

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107~085 The emulsion polymerization suitably employs by weight from 35 to 60% of vinyl acetate, from 1 to 5% of water-soluble polyvinyl alcohol, from 35 to 50% of water, from 0.1 to 2% of a surfactant and from 0.1 to 2% of a buffer, to a total of 100%.
The resulting dispersion has a viscosity of 1000 to 3000 cps and preferably at least 2000 cps, and a pH of 4 to 6, preferably about 4.5. In this particular system an acid pH is necessary for the cross-linking.
The second component of the adhesive formulation com-prises a cross-linking agent or curing system for the cross-linkable material. Suitable cross-linking agents for the pre-ferred polyvinyl alcohol cross-linkable material have been indicated above, and these are suitably employed in the form of aqueous solutions. The addition of the aqueous solution of the cross-linking agent, results in a lowering of the viscosity of the mixture to about 300 to 1200 cps, preferably 800 to 1200 cps. The pH may also be lowered to about 3.5 to 4.
It is also within the scope of this invention to omit the second component containing the cross-linking agent. In this case the corrugated board obtained will not be water-resistant but clearly for some uses this may not be especially important. In this embodiment the economic advantages of avoiding the pre-heating of the liners are still obtained.
Combined Corruqated Board Manufacture Combined corrugated board can be manufactured accord-ing to the method of the invention employing conventional pro-duction line assemblieq but omitting the necessity for pre-heating of the liner sheets and the necessity for extensive heating and pressure systems to set the adhesive.

The adhesive formulation employed is essentially a cold-set adhesive, in the method the adhesive is applied in a _ g _ , - , . . , . : : . . : -,. . . .

thin film to the flutes on one side of a corrugated sheet and the liner sheet is immediately applied to the coated flutes.
When the adhesive is applied in a thin film the water in the adhesive dissipates rapidly by absorption into the paper or along the channels created by the flutes and the hardening of the adhesive is almost instantaneous, for this reason it is essential that the liner sheet be immediately applied to the adhesive coated flutes.
The liner is not preheated as in the conventional method employing a starch adhesive and in fact such heat might be detrimental since excessive heat plasticizes the thermo-plastic polymer, weakening the initial grab strength.
The sheet to be corrugated is, however, pretreated by conventional procedures, firsk with live steam in a steam shower to stretch the fibres of the paper and prevent brittle flute formation in the subsequent corrugation.
The corrugating rolls and the pressure roll are also conveniently maintained at an elevated temperature suitably in the range of 200 to 350F, preferably 200-230F and more -preferably about 210F. This heat too is desirable for non-brittle flute formation. This heat is not necessary for the cross-linking or curing of the adhesive formulation of the invention, however, the heat retained by the flutes does in- ~-crease the rate of cure as might be expected.
The method of the invention permits a thinner adhesive coating thickness of as little as 4 mils as compared with 6 mils conventionally required with starch adhesives.
Further the rapid cure and the lack of prolonged heat and pressure steps for setting the adhesive, and the lack of any need for pre-heating the liner permit higher production speeds.

Employing the method of the invention the corrugated board can .
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~07~85 be produced at production line speeds of 60 to 600 ft/min; in con1rast in the conventional starch adhesive method speeds as high as 600 ft/min can only be achieved with light weight double faced corrugated board, For double wall corrugated board the maximum speed is significantly lower being a maximum of 270 to 375 ft/min. In the case or triple wall corrugated board maximum speeds are of the order of 135 ft/min.
This undesirable decrease in production speed results from the heating systems required. When producing double and triple wall board the medium liners are led over pre-heaters heated to 185 to 200C and the heat supplied to the liners must be high enough that the gelatinization point of the starch adhesive of 60 to 64C is reache~, since only under these conditions is good bond ensured.
The present invention employing a cold-set adhesive thus permits smaller coating thicknesses of adhesive and faster production speeds while at the same time producing a superior water resistant product.
The invention is illustrated in a particular embodi-ment by reference to the accompanying drawing in which FIGURE 1 is a schematic representation of a production line for producing a combined corrugated board.
With further reference to Figure 1 a production line for combined corrugated board comprises supply rolls 10 and 12 of continuous paper sheet 14 and 16, a corrugator 18, an adhesive applicator 20 and a pressure roll 22.
The corrugator 18 comprises a pair of co-operating corrugating rolls 24 and 26.
The adhesive applicator 20 comprises a bath 28 for adhesive 30, a pick-up roll 32 and an applicator roll 34.
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A steam applicator 36 is located between supply roll 10 and corrugating roll 24, The production line illustrates the manufacture of a combined corrugated board 38 composed of a corrugated sheet 40 having the flutes 42 and 44 on opposite sides, the flutes 42 being adhered to a liner 46. The corrugated board 38 can be collected on a roll or can pass directly to a second adhesive applicator (not sh~tn) similar to adhesive applicator 20 where adhesive can be applied to the exposed flutes 44 and a second liner sheet applied to the flutes 44. The resulting single-wall corrugated board can then if desired be passed through similar operations to produce double-wall or triple-wall corrugated board in a manner known per se employing apparatus such as that illustrated in Figure 1 for the manufacture of a ~ :-single-faced corrugated board.
In operation paper sheet 14 is continuously fed from supply roll 10 over a steam applicator 36 which directs steam onto the sheet 14; the steam treated sheet 14 is passed between the corrugating rolls 24 and 26 of the corrugator 18 to ::
: 20 form a corrugated sheet 40 having flutes 42 and 44 on opposite sides.
Adhesive 30 in the bath 28 is picked-up on a rotating pick-up roll 32 and passed to a rotating applicator roll 34.
The applicator roll 34 applies the adhesive in a thin coat to the flutes 42 of the corrugated sheet 40.
A continuous paper sheet 16 is fed from supply roll 12 to pressure roll 22. The corrugated sheet 40 with the ad-hesive on flutes 42 i9 fed with the sheet 16 between the cor-rugating roll 26 and the pressure roll 22 to secure the cor-rugated sheet 40 to the sheet 16 to form corrugated board 38.
During the operation the corrugating rolls 24 and 26 and also the pressure roll 22 are maintained at an elevated temperature, ~his in conjunction with the steam shower from steam applicator 36 aids the corrugating step and ensures adequate flexibility in the sheet 14 being corrugated.
The pressure roll 22 is not intentionally heated, in conventional apparatus the pressure roll 22 forms part of an assembly with corrugator 18 and is heated automatically by steam lines when corrugating rolls 24 and 26 are heated.
Alteration of the existing assemblies to prevent heating of the pressure roll 22 while feasible involves unnecessary expense.
It will be understood that the heating of the corrugating rolls 24 and 26 and the employment of steam applicator 36 are conventional procedures in the manufacture of corrugated board.
In this specification the reference to the adhesive bond being water-proof and the reference to water-proofing is intended to define a material which is substantially completely resistant to attack by water and which can withstand long ex-posure to water. The expression water-resistance is employed with reference to the corrugated boards. The boards of this invention clearly have increased resistance to water in view of their water-proof bond. However, the boards are not them-selves water-proof since the paper itself may disintegrate if subjected to prolonged exposure to water such as by immersion in water. In normal use the boards will not be immersed in water, although they will be exposed to water.
, Conventional corrugated boards employing starch adhesive are often described as water-resistant. It will be appreciated that there are different levels of water resistance .... . ' ....................... ' :. : ,, ' ' ~ '' . . '. :

107~085 and that the water-resistant boards of this invention which have a water-proof bond are markedly superior in their water-resistance to the conventional boards employing starch adhesives.
Example i) Manufacture of adhesive formulation There was introduced into a polymerization vessel having a capacity of 10,000 lbs, the following ingredients:
Vinyl acetate monomer 51 lbs Polyvinyl alcohol (COVOL-trademark~ 3 lbs Water 45 lbs Surfactant (di~alkyd-sulpho-succinate) 0.8 lbs Buffer (Sodium bicarbonate)0.1 lbs Potassium persulphate (polymerization catalyst) 0.1 lbs The ingredients were mixed in the vessel under emul-sion polymerization conditions with a mixing rotor rotating at 90 rpm, for 8 hours at a temperature of 75 - 90C.
The resulting creamy white aqueous dispersion was characterized by a solids content of 56-58%, a pH of 4 5 - 4 7 and a viscosity of 2,500 cps. The dispersion at room tempera-ture was extremely aggressive in tack, displayed good grab characteristics between paper sheets and was found to be incompatible with starch.
A solid film formed by pouring a portion of the aqueous dispersion onto a flat surface and allowing it to dry was found to be transparent and relatively brittle with poor resistance to water.

~07~085 Separately there was prepared a curing system com-prising G-1650 (trademark of Dural Products Limited for an aqueous dispersion of inorganic metal-salt) and G-1710 (trade-mark of Dural Products Limited for an aqueous organic aldehyde containing polymer).
The aqueous dispersion and the curing system were packaged separately as a two component system for shipping and storage prior to use.
ii) Manufacture of a water-proof combined corruqated board A water-proof combined corrugated board was manu-factured employing the production line schematically illustrated in Figure 1 and the two component adhesive formulation described in i) above.
The paper sheet 14 comprising a 26 lb. semichemical medium paper was fed from supply roll 10 at a speed of 250 ft/min and steam at 300F was directed at the sheet 14 from the steam applicator 36. The sheet 14 was fed through the corrugating rolls 24 and 26 which were rotated at 250 linear ft/min, which rolls were maintained at a temperature of 210F by circulating heated water internally therein.
The aqueous dispersion and the curing system were mixed and introduced into bath 28 to produce the adhesive formulation 30, There was employed 5% by weight of G-1650 (trademark) and 1%, by weight, of G-1710 (trademark) based on the weight of the dispersion.
The adhesive 30 formed by mixing the aqueous dispersion and the curing system had a viscosity of 800 to 1,200 cps and a pH of 3.5 to 4.0 and displayed a potlife of 48 hours at room temperature.
The adhesive 30 was applied to the flutes of the corrugated sheet in an amount of 2 lba dry adhesive per 1000 .. . : . - . -~o7~085 sq, ft. in the manner described and a 26 lb, kraft liner sheet 16 was immediately applied to the coated flutes, the corrugated sheet and the liner sheet being pressed together between corrug-ating roll 26 and pressure roll 22, The cross-linking was substantially instantaneous and there was recovered a combined corrugated board having a strong bond between the corrugated sheet and the liner, In order to demonstrate the resistance to water of the combined corrugated board thus obtained a first sample there-of was completely immersed in water at room temperature for 30days, The sample was removed from the water and dried and subjected to manual force in an attempt to separate the cor-rugated sheet from the liner. T~e test showed that 100% of the bond strength was retained, in the test the paper tore but there was no collapse of the bond, In a second test a second sample was boiled in water for 30 minutes. In the subsequent manual strength test the paper failed but the bond was maintained.
In a further test a film of the adhesive was formed on a flat surface and allowed to cross-link and dry to a solid film. The film did not dissolve when treated with water.
A further production run was made to demonstrate the increase in production speeds when employing the method of the invention as compared to the conventional method employing a starch adhesive, for the manufacture of heavy weight corrugated board.

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~o7~085 Paper qualities:
B flute 26 lbs medium 26 lbs C ~lute 42 lbs medium 26 lbs Backer 42 lbs In the method of the invention the cold-set adhesive was applied without pre-heating of the facer sheets.
Starch vs Cold-set Starch Cold-set (invention) Speed 220 ft/min 500 ft/min Adhesive spread 6 mils 4 mils The increase in produc~ion speed with the method of the invention was 2.27 times or 127%, higher production speeds were found to be possible with the method of the invention, although maximum speeds could not be realized due to limitations of take-off crews and paper'quality~ Further thinner adhesive -coating applications were possible, however, 4 mils was as low as the normal machine setting in the plant available could be adjusted.
~ 20 It will be understood that "paper" in this specifi-; cation refers to sheets or webs fabricated from vegetable or synthetic fibres or mixtures. Especially there may be mentioned products produced by chemically processing cellulosic fibres.
In particular, there may be mentioned papers manufactured from ; soft woods (coniferous trees), for example spruce, hemlock and pine as well as hard woods, for example poplar and oak, further a wide variety of sources have been used for specialty papers, for example, bamboo. Papers have further been manufactured from synthetic fibres and mixtures of synthetic fibres with cellulosic fibres. The present invention can be employed with :

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~o7~085 all such papers, although primarily it will be employed with conventional papers used for corrugated board manufacture.

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Claims

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

1. A method of making a water resistant combined corrugated paper board comprising a corrugated paper sheet adhered to a paper liner comprising i) providing a corrugated paper sheet having a plurality of flutes and a paper liner, ii) applying to flutes on one side of said sheet an adhesive composition comprising a mixture of an aqueous dispersion of a particulate water-insoluble synthetic homopolymer of vinyl acetate containing a water soluble cross-linkable material, effective when cross-linked to provide water-proofing, and a cross-linking agent, iii) pressing said liner against said flutes, and iv) allowing said composition to set with cross-linking of said cross-linkable material to form a water-proof bond.

2. A method according to claim 1, wherein said dis-persion is obtained by the emulsion homopolymerization in an aqueous medium of vinyl acetate in the presence of water soluble polyvinyl alcohol, such that said cross-linkable material is polyvinyl alcohol.

3. A method for the continuous production of a water resistant combined corrugated paper board comprising a corrugated sheet adhered to a liner comprising a) continuously feeding a first paper sheet through a corrugating means to produce a corrugated sheet having a plurality of flutes, b) applying to flutes on a first side of said sheet a curable adhesive composition comprising an aqueous dispersion of a homopolymer of vinyl acetate containing a water soluble polyvinyl alcohol and a curing agent effective to cross-link said polyvinyl alcohol, c) immediately applying a continuously fed second paper sheet against the adhesive coated flutes and pressing said second sheet against said adhesive coated flutes, and d) allowing said adhesive composition to set with cross-linking of said polyvinyl alcohol to form a board with a water-proof bond.

4. A method according to claim 3, wherein said aqueous dispersion and said curing agent are admixed prior to said applying in step b).

5. A method according to claim 4, wherein said homo-polymer has a particle size of 0.5 to 1.5 microns, and said dispersion contains, based on the total weight of said dispersion from 35 to 60% of vinyl acetate homopolymer, from 1 to 5% of polyvinyl alcohol and 35 to 50% of water to a total of 100% said dispersion having a viscosity of 2,000 to 3,000 cps and a pH of 4 to 6.

6. A method according to claim 3, 4 or 5, wherein following step d) said adhesive composition is applied to exposed flutes of a second side of said corrugated sheet whereafter a third paper sheet is immediately fed against the adhesive coated flutes of said second side and pressing said third sheet against said second side to form a second liner.

7. A method according to claim 3, 4 or 5, wherein prior to step a) said first paper is treated with steam, and wherein said corrugating means is heated to avoid brittle flute formation, said flutes of said corrugated sheet retaining heat from said corrugating means when said composition is applied in step b).

8. A water resistant combined corrugated paper board comprising a corrugated paper sheet having a plurality of flutes and having on at least one side a paper liner adhered at said flutes by a cured adhesive composition comprising a major amount of a water-insoluble synthetic homopolymer of vinyl acetate and a cross-linked water-proof material.

9. A board according to claim 8, wherein said cross-linked material is a cross-linked polyvinyl alcohol.

10. A board according to claim 8, wherein said cured adhesive is derived from a dispersion containing, based on the total weight of said dispersion, from 35 to 60% of vinyl acetate homopolymer, from 1 to 5% of polyvinyl alcohol and 35 to 50% of water to a total of 100%, said dispersion having a viscosity of 2000 to 3000 cps and a pH of 4 to 6, and said homopolymer having a particle size of 0.5 to 1.5 microns.

11. A method of making a water resistant combined corrugated paper board comprising i) providing a corrugated paper sheet having a plurality of flutes and a paper liner, ii) applying to flutes on one side of said sheet a cold-set adhesive composition consisting essentially of a mixture of I) an aqueous dispersion obtained by the emulsion homopolymerization in water of vinyl acetate monomer to vinyl acetate homopolymer in the presence of (a) a water soluble, cross-linkable polyvinyl alcohol in an amount effective to stabilize the vinyl acetate homopolymer and prevent agglomeration of dispersed particles of said homopolymer, (b) a polymerization catalyst, (c) a surfactant, and (d) a buffer, II) a curing agent in an aqueous medium effective to cross-link said polyvinyl alcohol, iii) pressing said liner against said flutes, and iv) allowing said composition to set with cross-linking of said polyvinyl alcohol to form a water-proof board, said water-proof board exhibiting fiber tear and 100% bond retention on immersion in water at room temperature for 30 days followed by drying and application of manual force to separate the corrugated sheet from the liner, and exhibiting paper failure and bond retention after boiling in water for 30 minutes.

12. A method according to claim 11, wherein said poly-vinyl alcohol is present in an amount of 1 to 5%, said surfactant is present in an amount of 0.1 to 2% and said buffer is present in an amount of 0.1 to 2% by weight, based on the weight of the dispersion.

13. A method according to claim 12, wherein said homo-polymer is present in said dispersion in an amount of 45 to 58%
by weight and has a particle size of 0.5 to 1.5 microns: said dispersion having a viscosity of 1000 to 3000 cps and a pH of 4 to 6, and said adhesive composition having a viscosity of about 300 to 1200 cps and a pH of about 3.5 to 4.

14. A method for the continuous production of a water resistant combined corrugated paper board comprising a corrugated sheet adhered to a liner comprising i) continuously feeding a first paper sheet through a corrugating means to produce a corrugated sheet having a plurality of flutes, ii) applying to flutes on a first side of said sheet, as a thin film, a cold-set adhesive composition consisting essentially of a mixture of:
(I) an aqueous dispersion obtained by the emulsion homopolymerization in water of vinyl acetate monomer to vinyl acetate homopolymer having a particle size of 0.5 to 1.5 microns in the presence of:
(a) 1 to 5%, by weight based on the weight of the dispersion, of a water-soluble, cross-linkable polyvinyl alcohol, effective to stabilize the vinyl acetate homopolymer and prevent agglomeration of dispersed particles of said homopolymer, (b) a polymerization catalyst, (c) 0.1 to 2%, by weight based on the weight of the dispersion of a surfactant, and (d) 0.1 to 2%, by weight, based on the weight of the dispersion of a buffer;

said dispersion containing 35 to 60%, by weight of homopolymer the balance being water and having a viscosity of 2000 to 3000 cps and a pH of 4 to 6, and (II) a curing agent in an aqueous medium, effective to cross-link said polyvinyl alcohol at room temperature, said adhesive composition having a viscosity of 800 to 1200 cps and a pH of 3.5 to 4 and having a pot life of about 48 hours at room temperature, iii) allowing the water in said thin film to rapidly dissipate and immediately applying a continuously fed second paper sheet against the adhesive coated flutes and pressing said second sheet against said adhesive coated flutes, and iv) allowing the adhesive composition to set with cross-linking of said polyvinyl alcohol to form a board with a water-proof bond;
said bond exhibiting fiber tear and 100% bond retention on immersion in water at room temperature for 30 days followed by drying and application of manual force to separate the corrugated sheet from the liner, and exhibiting paper failure and bond retention after boiling in water for 30 minutes.

15. A method according to claim 14, wherein following step (d) said adhesive composition is applied to exposed flutes of a second side of said corrugated sheet whereafter a third paper sheet is immediately fed against the adhesive coated flutes of said second side and pressing said third sheet against said second side to form a second liner.

16. A method according to claim 14, wherein prior to step (a) said first paper sheet is treated with steam, and wherein said corrugating means is heated to avoid brittle flute formation, said flutes and said corrugated sheet retaining heat from said corrugating means when said com-position is applied in step (b).

17. A method according to claim 15, wherein said second and third paper sheets are applied without preheating to the flutes of said corrugated sheet, and said corrugated sheet and second and third paper sheets are fed at speeds of 60 to 600 ft./min.

18. A method according to claim 17, wherein said speed is 375 to 600 ft./min.

19. A water-resistant combined corrugated paper board comprising a corrugated paper sheet having a plurality of flutes and having on at least one side a paper liner adhered at said flutes by a water-proof cured adhesive composition consisting essentially of a major amount of vinyl acetate homopolymer, and a cross-linked, polyvinyl alcohol which is water-soluble in its uncross-linked state and minor amounts of a polymerization catalyst, surfactant and buffer derived from the aqueous emulsion homopolymerization of said homo-polymer; said bond exhibiting fiber tear and 100% bond retention on immersion in water at room temperature for 30 days followed by drying and application of manual force to separate the corrugated sheet from the liner, and exhibiting paper failure and bond retention after boiling in water for 30 minutes.