POLYVINYL ALCOHOL COMPOSITION USABLE IN THE PRODUCTION OF FAST SETTING STARCH BASED CORRUGATING ADHESIVE PRODUCING COMPOSITIONS AND USE THEREOF IN MANUFACTURE OF CORRUGATED BOARD

Abstract: Aqueous concentrates of partially hydrolysed polyvinyl alcohol are used in the production of basic boric acid-starch containing aqueous adhesives which find use in manufacture of corrugated board. The additional components of the adhesive are either present in the concentrate during storage and transport thereof or are themselves made up into concentrates for multiple tank make-up on site of the final adhesive composition.

Description: This invention relates to a starch based adhesive composition that can be used to form corrugated board. More particularly the invention relates to a starch based adhesive that undergoes rapid increase in viscosity under conditions of heat and pressure resulting in a fast-setting green bond. Corrugated paper board or liner board is commonly prepared by first forming a corrugated element or medium by passing a cellulosic sheet between corrugating rolls forming a substantially sinusoidal or serpentine cross-section in the sheet. An adhesive is commonly applied to the tips of the thus formed sinusoidal portion called flutes and a non-corrugated or Planar cellulosic line is applied against the adhesive coated flutes of the corrugated elements as the corrugated sheet passes between a corrugating roll and a pressure roll. The resulting paper product having the corrugating medium on one side and the planar liner on another side is called a single-faced element. The single-faced element may be used as is in certain applications as a liner or buffer material within a container. More commonly adhesive is applied to the flute tips of the single-faced element and a second liner sheet is subsequently applied to the adhesive liner in a "double facer" operation. The second liner sheet is exposed to conditions of heat and pressure during its contact with the adhesive. In a so-called "hot-plate" dryer section a continuously moving belt with a hot-plate or platen surface is used to partially remove water from the adhesive bond line and establish a cured adhesive bond at temperatures of about 300°-400°F (149-204°C). The adhesives which have been developed to form the corrugated board or paper board are generally starch base adhesives. The earliest corrugating board adhesive patent, U.S. Patent No. 2,102,937, discloses an adhesive composition comprising gelatinized starch, ungelatinized starch, alkali and borax in a water base. This adhesive has formed the basis for the manufacture of essentially all corrugated paper board to date. This adhesive is a carrier adhesive, wherein a portion of the starch forms a carrier gelatinized phase and the balance of the starch forms an ungelatinized phase. Under conditions of heat and pressure the ungelatinized starch rapidly gelatinizes, absorbs water and causes a rapid increase in viscosity and adhesivity of the starch-adhesive composition. Borax increases the level of viscosity as well during final gelatinization. More recently a number of patents have disclosed a corrugating board adhesive that can be characterized as a conventional starch-based adhesive containing finely divided solid particles of a substantially room temperature insoluble polyvinyl alcohol. The water insoluble polyvinyl alcohol disclosed in these patents becomes soluble at an elevated temperature. A substantially insoluble polyvinyl alcohol is used in these patents since the common belief of the corrugating industry is that soluble polyvinyl alcohol will gel in the presence of borax containing starch based adhesives. The use of insoluble polyvinyl alcohol permits the use of the polyvinyl alcohol in starch based adhesives since at storage or ambient temperatures the polyvinyl alcohol, in solid form, cannot interact with starch or borax. After application to the flute tips, at conditions of high temperature and pressure, the polyvinyl alcohol can rapidly dissolve and substantially increase the viscosity and bond strength of the bond line between the corrugating medium and the liner material. The speed at which corrugating machines can be run is limited by the rapidity of viscosity increase in the bond line between the liner and corrugated medium. As the viscosity of the adhesive rises under conditions of heat and pressure in the corrugating machine, a "green bond" is formed which holds the components together until the adhesive system forms the final heat-cured corrugated bond. The green bond relies solely on the viscosity of the product to maintain the integrity of the product during curing. Conventional starch based adhesives rely on the response of borax and ungelatinized starch under conditions of heat and pressure to increase the viscosity of the adhesive during green bond formation. More recent corrugating adhesives containing additionally insoluble polyvinyl alcohol utilize the increase in viscosity that results as the polyvinyl alcohol is solubilized at high temperature. However, both the conventional and the solid polyvinyl alcohol containing conventional adhesives increase viscosity at a rate that leaves substantial room for improvement. If the rate at which viscosity increase occurs in the bond line can be increased, then the corrugating machines can be run at a substantial increase in speed resulting in an increase in productivity. Accordingly a substantial need exists for a corrugating adhesive having a faster set or a substantial increase in the rate of viscosity development or in the rate of green bond formation during the manufacture of corrugated board. We have learned that problems relating to the incompatibility of soluble polyvinyl alcohol compositions can be solved and an adhesive having a substantially improved fast set or increase in the rate of viscosity increase can be achieved by adding to a corrugating adhesive comprising starch, alkali base, boric acid and water, a controlled amount of a partially hydrolysed soluble polyvinyl alcohol (see EP-A-0 162 682). We have found contrary to the teachings of the art, that the incompatibility of polyvinyl alcohol compositions can be controlled by carefully controlling the amount of polyvinyl alcohol in relation to boric acid in the final adhesive composition and by selecting a partially hydrolysed polyvinyl alcohol composition. Further a selection of a preferred lower molecular weight polyvinyl alcohol can improve the compatibility of the adhesive and the polyvinyl alcohol. As a result of further studies, we have been able to identify specific formulations enabling improvements in adhesive composition production to be achieved, leading in turn to improvements in the manufacture of corrugated board. Thus, according to one aspect of the invention, there is provided an aqueous polyvinyl alcohol composition comprising a major portion of water and from 0.01 weight% to 20 weight% of a cold water soluble partially hydrolyzed polyvinyl alcohol having a level of hydrolysis of about 85.5% to 91%, usable in the production of a basic starch containing aqueous adhesive further comprising water, from 0.1 part to 5 parts boric acid per 100 parts water, from 5 weight% to 50 weight% starch and from 0.1 weight% to 10 weight% of base. In a second aspect of the invention, there is provided an aqueous two-part starch adhesive premix for use in the production of a basic starch-containing aqueous adhesive composition employable in the production of corrugated board, said premix comprising: (a) an aqueous polyvinyl alcohol composition comprising a major portion of water and from 0.01 wt-% to 20 wt-% of a cold water soluble polyvinyl alcohol having a level of hydrolysis of about 85.5% to 91%; and (b) in a separate container, from 0.1 part to 5 parts boric acid per 100 parts water, from 5 wt-% to 50 wt-% starch, and from 0.1 wt-% to 10 wt-% of base, the remainder comprising water. Finally, according to a third aspect of the present invention, there is provided a method for the manufacture of corrugated board in which at least one liner is bonded to at least one fluted medium with an adhesive wherein the adhesive has been produced by incorporating water in an aqueous cold water soluble polyvinyl alcohol concentrate composition to produce a polyvinyl alcohol concentration in the range of 0.01 to 2.0 parts by weight per 100 parts by weight of adhesive, the polyvinyl alcohol having a degree of hydrolysis of 85.5% to 91%, which adhesive comprises 0.1 to 5 parts by weight of boric acid, 5 to 50 parts by weight of starch and 0.1 to 10 parts by weight of base each per 100 parts by weight of water. In the method for the manufacture of corrugated board according to this invention, it is sufficient to proceed via the steps of (a) applying an effective bonding amount of the aforesaid adhesive to the fluted medium to form a bond line; (b) contacting the fluted medium with the liner at the bond line; and (c) curing the bond line to bond the liner to the fluted medium. As in the case of EP-A-0 162 682, while we do not wish to be held to a theory of action of the final ready for use adhesive composition, we believe that the problems relating to phase separation and gelling in polyvinyl alcohol containing adhesive is directly related to an interaction between the hydroxyl groups of the starch or boric acid and hydroxyl groups of the polyvinyl alcohol. We have found that these harmful interactions can be prevented by controlling the available concentration of hydroxyl groups by controlling the concentration of polyvinyl alcohol in the solution and by selecting a partially hydrolysed polyvinyl alcohol wherein a small but important proportion of the hydroxyl groups of the polyvinyl alcohol molecule remain acetylated. We have further found that in formulating corrugating adhesives with the partially hydrolysed polyvinyl alcohol that there is a threshold concentration of the partially hydrolysed soluble polyvinyl alcohol above which substantial gelling or incompatibility results. However, below the threshold polyvinyl alcohol acts to substantially increase fast set of the green bond or viscosity increase during corrugating boric manufacture. Since the polyvinyl alcohol is in solution, the rate of green bond formation is increased by the presence of the alcohol, but is not limited by the rate of dissolution of a solid polyvinyl alcohol. Since green bond formation is increased currugating machine speeds can be increased proportionately. Still further we have found that the lower molecular weight polymers are more resistant to gel formation. The aqueous adhesive composition used in producing corrugating board according to the invention is to be distinguished from certain known partially hydrolysed polyvinyl alcohol-containing starch adhesives. Thus, US-A-4008116 discloses liquid adhesive solutions containing 12-25% of polyvinyl alcohol and starch with a polyvinyl alcohol:starch ratio of from 65-85:15-35. The composition of necessity contains a thickener and uses 80-88% hydrolysed polyvinyl alcohol. However, contemplated here is the use of a relatively high molecular weight polyvinyl alcohol, essentially a polyvinyl alcohol having a viscosity of (1.8-5.5 x 10⁻² Pa.s) as a 4 wt-% solution whose viscosity is measured at 20°C with a Brookfield viscometer. This is not surprising since the adhesive according to US-A-4008116 is intended for labelling of bottles where it is required that rapid adhesion takes place with low priority being given to green bond formation and ease of manipulation before onset of final bonding. Mention is also made of Chemical Abstracts, vol. 74, no. 2, page 6, no. 4886u where a polyvinyl alcohol starch-based adhesive is disclosed. Here there is no disclosure of any concentration of polyvinyl alcohol or of degree of hydrolysis of the polyvinyl alcohol. Moreover, there is failure to teach a specific concentration ratio of PVOH to boric acid compound which will resolve the gelling problems which are often found with polyvinyl alcohol starch-based adhesives. Briefly, the ready for use corrugated board adhesives are prepared by forming an aqueous emulsion to have an alkaline pH and contain a soluble partially hydrolysed PVOH, starch, a strong base and a boric acid compound and optionally a basically reacting crosslinkina resin, and wax. The corrugated paper board adhesive is preferably formed from a suspension of starch particles in water. The viscosity and solids content are important factors influencing the speed with which the paper board machine can be operated and also effects the quality of the finished paper board. The suspending paste or carrier medium must be sufficiently viscous to maintain a smooth suspension of the starch particles, but not so viscous as to interfere with the application of the adhesive to the paper board. The nature and proportion of dissolved solids in the carrier medium influences both pot life and the speed with which a given paper stock absorbs water from the applied adhesive. The water concent of the adhesive efects adhesive properties. A small degree of water absorption from the adhesive to the paper is desirable in forming strong bonds between the paper plies. Absorption of water beyond that required to give good bonding is undesirable since it robs the adhesive of water needed to complete the gelatinization of the suspended starch upon heating in the corrugator and leads to inferior bonding. The starting compositions used of the first two aspects of the present invention assist particularly in enabling the objectives of the preceding paragraph to be attained. In particular it has been found in the practice of the present invention that it is convenient to utilise polyvinyl alcohol containing concentrates in water which are diluted with appropriate compositions or merely with water depending upon what constituents of the final adhesive are present additional to the polyvinyl alcohol in the polyvinyl alcohol-containing concentrates. The adhesive compositions are produced by combining with an aqueous polyvinyl alcohol composition the remaining ingredients of the composition. The aqueous polyvinyl alcohol compositions may be concentrated solutions in water of the hydrolysed polyvinyl alcohol generally containing at least 18 wt-%. preferably from 18 to 45 wt-% and most preferably from 18 to 40 wt-% polyvinyl alcohol. However, according to the first two aspects of the invention, the "concentrates" may be somewhat more dilute, having concentrations of 0.01 to 20 wt-%, when separate addition of water may often be avoided. Thus, in the preparation of the aqueous adhesives of this invention we have found that the polyvinyl alcohol aqueous concentrates are typically diluted at plant site from higher concentrations to a concentration of 20 wt-% polyvinyl alcohol or less prior to charging to the adhesive manufacturing equipment if the polyvinyl alcohol is added to the adhesive composition in the presence of alkali and borax. If such relatively dilute polyvinyl alcohol aqueous concentrate composition is added to a two-tank system in which the polyvinyl alcohol is cooked into a starch suspension prior to blending the resulting composition into a separate emulsion comprising starch, borax and alkali no dilution is required since the tank containing the polyvinyl alcohol when finished will contain less than 20 wt-% polyvinyl alcohol and its contents will be ready for use, as such if the polyvinyl alcohol concentration is sufficiently low, or after dilution with water to form the final ready for use adhesive. The aqueous polyvinyl alcohol concentrate compositions of the first aspect of this invention are typically prepared by blending the desired polyvinyl alcohol in water and agitating the thus-formed composition until uniform. Preferably the aqueous composition can be heated during agitation to ensure complete dissolution of the polyvinyl alcohol in water. The liquid polyvinyl alcohol concentrates used in the practice of the invention can contain other components which can beneficially affect the properties of the aqueous adhesive. The concentrate compositions can contain other soluble or emulsifiable resin compositions including ketone resins, crosslinking resins, lignosulfonate compositions, KRATON rubbers, acrylic latexes, etc. Further, the concentrates can contain defoamers, preservatives, perfumes, dyes, antioxidants and other useful additive compositions. Polyvinyl Alcohol Polyvinyl alcohol (PVOH), a polyhydroxy polymer having a polymethylene backbone with pendant hydroxy groups, is a water soluble synthetic resin. It is produced by the hydrolysis of polyvinyl acetate. The theoretical monomer does not exist. Polyvinyl alcohol is one of the very few high molecular weight commercial polymers that is water soluble. It is commonly available as a dry solid and is available in granular or powdered form. PVOH grades include a "super" hydrolysed form (99.3% plus removal of the acetate group), a fully hydrolysed form (99%+ removal of the acetate group), a form of intermediate hydrolysis (about 98 to 91% removal of acetate group), and a partly hydrolysed (about 91% to 85% removal of the acetate group) polyvinyl alcohol. The properties of the resins vary according to the molecular weight of the parent polymer and the degree of hydroysis. Polyvinyl alcohols are commonly produced in nominal number average molecular weights that range from about 20,000 to 100,000. Commonly the molecular weight of commercial polyvinyl alcohol grades is reflected in the viscosity of a 4 wt-% solution measured in centipoise (cP) at 20° C. with a Brookfield viscometer. The viscosity of a 4% solution can range from 0.5 to 6.5 x 10⁻² Pa.s. Variation in film flexibility, water sensitivity, ease of solvation, viscosity, block resistance, adhesive strength, dispersing power can all be varied by adjusting molecular weight or degree of hydrolysis. Solutions of polyvinyl alcohol in water can be made with large quantities of lower alcoholic cosolvents and salt cosolutes. Polyvinyl alcohols can react with aldehydes to form acetals, can be reacted with acrylonitrile to form cyanoethyl groups, and can be reacted with ethylene or propylene oxide to form hydroxy alkylene groups. Polyvinyl alcohols can be readily crosslinked and can be borated to effect gelation Polyvinyl alcohol is made by first forming polyvinyl acetate or a vinyl acetate containing copolymer such as an ethylene vinyl acetate copolymers and removing the acetate groups using a base catalyzed alkanolysis. The production of polyvinyl acetate or a vinyl acetate copolymer can be done by conventional processes which controls the ultimate molecular weight. Catalyst selection temperatures, solvent selection and chain transfer agents can be used by persons skilled in the art to control molecular weight. The degree of hydrolysis is controlled by preventing the completion of the alkanolysis reaction. Polyvinyl alcohol is made in the United States by Air Products & Chemicals, Inc. under the tradename VINOL™, by duPont under the trade name ELVANOL™, and by Monsanto under the trade name GELVITOL™. The polyvinyl alcohol compositions for use in this invention comprise polyvinyl alcohol made from a homopolymer or interpolymer of vinyl acetate that is partially hydrolysed i.e. 85 to 91% of the hydroxyl groups are free of acetate. The partially hydrolysed polyvinyl alcohol is available in low, medium or high molecular weight variations characterized by the Brookfield viscosity of a 4 wt-% aqueous solution at 20°C. The preferred low molecular weight polyvinyl alcohol has a viscosity of from 0.4-1 x 10⁻² Pa.s, medium molecular weight composition has a viscosity of about 1.5-3 x 10⁻² Pa.s and a high molecular weight composition has a viscosity of about 3.5-6 x 10⁻² Pa.s. The preferred polyvinyl alcohol compositions have a reduced particle size adapting the composition for rapid dissolution. The particle size of the most preferred polyvinyl alcohol is such that 99+ % of the product will pass through a -80 Tyler Standard mesh screen (more than 1.77 x 10⁻⁴ m The most preferred polyvinyl alcohol composition is made from a low molecular weight homopolymer of vinyl acetate that is partially hydrolysed resulting in the removal of about 87% to 91% of the acetate groups from the vinyl acetate homopolymer. Starch The starch based adhesives used in producing the adhesive of the method of this invention are commonly characterised as being either a carrier adhesive, a no carrier adhesive, or a carrier/no carrier adhesive. The term carrier adhesive implies the presence of a thin base of gelatinized starch in which ungelatinized or nonhydrated starch particles are emulsified or suspended. The term no carrier adhesive implies the substantial absence of gelatinized or hydrated starch in the adhesive emulsion. The term carrier/no carrier adhesive implies that the distinction between gelatinized starch and ungelatinized starch is not clear in the adhesive composition. Generally carrier/no carrier adhesives are prepared in a way that a substantial amount of partly hydrated or partly gelatinized starch is formed. These terms are imprecise since during the preparation of the starch based adhesives some starch inherently becomes gelatinized during processing at any temperature. However substantial proportions of starch remain ungelatinized and nonhydrated during preparation which can gelatinize and interact with PVOH, and borax during green bond formation. It is generally preferred practice to use a starch composition wherein the starch is present in the form of ungelatinized starch particles suspended in a carrier phase of gelatinized starch in an aqueous emulsion wherein there are from 0.01 to 10 parts of gelatinized starch and from 0.01 to 40 parts of ungelatinized starch per 100 parts of the aqueous adhesive composition. Here and hereinafter all parts are parts by weight. The starch used in both the carrier phase and suspended starch phase is a commodity chemical produced from a number of root, stem or fruit, plant sources. Starch is a high molecular weight polymeric carbohydrate polysaccharide, most commonly composed of six carbon monosaccharide units joined by alpha-glycan linkages having an approximate molecular formula of (C₆H₁₀O₅) n , wherein n equals 10² to 10⁶. Starch occurs naturally in the form of white granules and comprises linear and branched polysaccharide polymers. Starch is commonly manufactured by first soaking and grinding the plant starch source, and separating the starch from waste matter. The starch is commonly reslurried and processed into a finished dried starch product. Commercial starch often has the form of a pearl starch, powdered starch, high amylose starch, precooked or gelatinized starch, etc. Preferably pearl starch is used in making both the carrier phase and the particulate starch suspended in the carrier. Base The adhesives used in the method of the invention contain a strong base which promotes the production of crosslinking species from the resin and acts to cure the adhesive. Esentially any strong base can be used; however preferred bases are alklai metal hydroxides used in such amount for the adhesive composition to have a pH of from 9 to 13, preferably 11 to 12. The most preferred strong bases include sodium and potassium hydroxide. Particularly preferred is the use of sodium hydroxide in a concentration of from 0.02 to 0.75 parts by weight per 100 parts by weight of the adhesive composition. The strong base also lowers the gel point of the starch, which increases the rate of hydration of the starch during curing of the glue line. Boric Acid Compound The adhesives used in this invention include a boric acid compound which cooperates with the gelatinized and nongelatinized starch during heating and curing by reacting with available hydroxyl groups of the starch, forming a starch borax complex which aids in gelling the carrier starch molecules and the suspended starch molecules. The cooperative reaction increases the tack of the adhesive and produces strong bonds. Any boric acid compound having free hydroxyl groups attached to the boron atom can be used, although commercial boric acid (orthoboric acid, H₃BO₃ and its hydrated forms H₃Bo₃.XH₂O) and borax (sodium tetraborate decahydrate, and other hydrate and anhydrous forms Na₂B₄O₇.XH₂O) (X being 0 to 10) are commonly used. For a discussion of boric acid compounds such as boron oxides, boric acids and borates, see Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Edition, Vol. 3, pp.608-652. The boric acid compound increases the tackiness, viscosity, and cohesive strength of the adhesive. As the adhesive cures in the glue line, a gel structure forms between the hydroxyl group of the polysaccharide, the basic acid compound and the PVOH. The exact mechanism of the crosslinking reaction between these compounds is unknown. Further, the borax buffers the caustic nature of the base used in the adhesive stabilizing the adhesive to the hydrolytic effect of the base. Crosslinking Resins Resins that can be used in the adhesives used in the method of this invention include resins that upon heating in basic media generate crosslinking species that react with and crosslink the hydroxyls in adjacent starch molecules. The crosslinking reagent tends to reduce the hydrophilic nature and the water solubility of the starch molecules by effectively removing the availability of hydroxyl groups to water and by introducing aliphatic alkylene-type crosslinking moieties. A preferred class of crosslinking resins comprise well known condensation products of the reaction of a ketone and an aldehyde compound. These resins are characterized as a polyether polymer but can contain a variety of other monomers such as urea, melamine, etc. In the presence of heat and base, they commonly decompose to produce species that effectively crosslink the starch molecules. Preferred resins are acetone-formaldehyde resins, acetone-urea-formaldehyde resins, acetone-melamine-formaldehyde resins comprising 15 to 30 wt-% acetone, about 5 to 50 wt-% formaldehyde and 0 to 15 wt-% of third monomer. One example of the preferred crosslinking resin embodiment of this invention comprises the Ketack series of resins manufactured by American Cyanamid . Wax The term wax is used very broadly and is applied to a wide variety of materials. The materials can have properties resembling certain well known waxes or can be used to provide physical properties similar to those associated with well known properties of wax such as sealing, polishing, candle making, etc. Many waxy substances have been found in nature and have been known to be useful for many years. Historically waxes include substances that are natural products. Chemically natural waxes are esters of fatty acids and monohydric fatty alcohols. Physically waxes are water repellant solids having a useful degree of plastic character. However, modern waxes include various synthetic substances that can replace natural waxes in many preparations. Natural waxes generally comprise an ester of a saturated fatty acid and a long chain monohydric alcohol. Long chain fatty acids often include acids having greater than 16 carbon atoms and most commonly about 26 carbon atoms. The aliphatic monohydric alcohol commonly has at least 14 and can range as high as 36 carbon atoms. Petroleum, mineral or other synthetic waxes often consist of saturated hydrocarbons having aliphatic or omen chain structures with relatively low branching or side chains. Particularly preferable waxes for the water resistant corrugated board adhesive are petroleum waxes, candelilla wax and beeswax. Particularly preferred petroleum waxes are micro-crystalline waxes, slack waxes and paraffin waxes. In somewhat greater detail, the fast set corrugating adhesives used in the method of this invention can be made by combining the ingredients in water and heating the mixture until a smooth uniform composition is formed. Preferably the adhesive is made by combining a starch slurry with a solution of the base in water heating the mixture to at least partially gelatinized the starch and then adding the remaining components under conditions of mixing to form the smooth corrugated adhesive. The adhesive can contain from 5 to 150 parts of starch and 0.1 to 10 parts of base per 100 parts of water. In "carrier" adhesives the carrier comprises about 1 to 25 parts of starch and the ungelatinized phase comprises 5 to 50 parts of starch. The adhesive can contain from 0.1 to 5 parts of boric acid per each 100 parts of water and optionally from 0.1 to 5 parts of basically reacting crosslinking resin per 100 parts of water and from 0.1 to 5 parts of wax in an emulsion form per each 100 parts of water. The fast setting adhesives produceable from the compositions of this invention contain from 0.01 to 2.0 parts of polyvinyl alcohol per each 100 parts of adhesive. Preferably the adhesive contains from 0.1 to 1.5 parts of polyvinyl alcohol and most preferably for reasons of rapid set and compatibility with the adhesive components from 0.25 to 1.25 parts of polyvinyl alcohol per 100 parts of adhesive. The ratio of polyvinyl alcohol to boric acid compound is important in the invention since the interaction between polyvinyl alcohol and boric acid can cause substantial gelling problems in the manufacture of the adhesive. Accordingly, polyvinyl alcohol is used in a ratio of less than 9 parts by weight of polyvinyl alcohol per each part of boric acid compound, expressed as H₃BO₃, preferably 1 to 8 parts of polyvinyl alcohol per part of boric acid compound, expressed as H₃BO₃, and most preferably 2 to 7 parts of polyvinyl alcohol per each part of boric acid compound, expressed as H₃BO₃, for reasons of obtaining a fast setting adhesive free of unacceptable component interactions. The following Examples illustrate the invention:- EXAMPLE 1 Into a 1200 milliliter stainless steel beaker equipped with an electrically driven mechanical stirrer were placed 61.2 grams of deionized water. Stirring was initiated and into the room temperature water were placed 36.66 grams of a 85.5-88.7% hydrolysed low molecular weight polyvinyl alcohol. Stirring was initiated until the polymer was dispersed in water and the beaker was heated in a 99°C (210°F) water bath to a temperature of 85°C (185°F). Into the stirred mixture wereas added 1.94 grams of Colloids 675 and 0.20 grams of preservative, Proxel GXL. The stirring was continued for 1/2 hour until the polymer had completely dissolved. EXAMPLE 2 Into a 15.4 dm³/ (4 U.S. gallon) stainless steel tank mixer having a pentagonal cross-section equipped with a mechanically driven propeller agitator along the bottom, were placed 622.5 grams of deionized water and 112.5 grams of starch (CLINTON 121B). The tank mixer was heated to 54.4°C (130°F) and into the heated starch slurry were added an aqueous solution of 27.0 grams of sodium hydroxide dissolved in 62.2 grams of deionized water over a period of 5 minutes. The resulting solution was mixed for 15 minutes until a gel formed. Into the gel were added 435.7 grams of cooling deionized water. The resulting solution was mixed for 5 minutes and was transferred to a 1200 milliliter stainless steel beaker. In the stainless steel tank mixer were placed 2129.5 grams of deionized water which was heated to a temperature of 32°C (90°F). Into the heated water were added 1075.0 grams of starch (CLINTON 121B) and 12.64 grams of borax. The mixture was agitated for 10 minutes and into the uniform mixture was added 61.5 grams of a 20 wt-% solution of 85.5-88.7% hydrolysed low molecular weight polyvinyl alcohol (VINOL 203S). After the tank solution became uniform the solution held in the stainless steel beaker was slowly added back to the tank over a 30-minute period. After the mixture in the tank had become uniform, the viscosity of the adhesive was 15.3 No.5 Zahn cup seconds. EXAMPLE 3 Example 2 is repeated exactly except that 30 grams of a 37.7 wt-% solution of the polyvinvyl alcohol is used. EXAMPLE 4 Into the tank mixer described in Example 2 were placed 1334.4 grams of deionized water which was heated to 54.4°C (130°F). Into the heated water was placed 120 grams of starch (CLINTON 121B) along with 22.4 grams of pellet sodium hydroxide. The mixer was operated for 15 minutes until the composition was uniform. Into the uniform composition were added 2045.6 grams of room temperature water. The composition was reheated to 38°C (100°F) and into the heated mixture were added 906.8 grams of starch (CLINTON 121B) and the composition was mixed until smooth. Into the uniform composition were added 13.6 grams of borax. The resulting mixture was agitated for 15 minutes and into the uniform composition were added 71.2 grams of a 18.4 wt-% aqueous solution of VINOL 203S (85.5-88.7% hydrolysed, low molecular weight), 0.1 wt-% Colloids 675 and 0.1 wt-% Proxel GXL preservative. The resulting adhesive had a gel point of 63.3°C (146°F) and a solids content of 24%.

Claims: An aqueous polyvinyl alcohol composition comprising a major portion of water and from 0.01 weight% to 20 weight% of a cold water soluble partially hydrolyzed polyvinyl alcohol having a level of hydrolysis of about 85.5% to 91%, usable in the production of a basic starch containing aqueous adhesive further comprising water, from 0.1 part to 5 parts boric acid per 100 parts water, from 5 weight% to 50 weight% starch and from 0.1 weight% to 10 weight% of base. The composition claimed in claim 1 wherein the aqueous polyvinyl alcohol composition additionally contains up to 50 wt-% of a starch. The composition claimed in claim 2, wherein the aqueous polyvinyl alcohol composition comprises 5 wt-% to 50 wt-% starch. An aqueous two-part starch adhesive premix for use in the production of a basic starch-containing aqueous adhesive composition employable in the production of corrugated board, said premix comprising: (a) an aqueous polyvinyl alcohol composition comprising a major portion of water and from 0.01 wt-% to 20 wt-% of a cold water soluble polyvinyl alcohol having a level of hydrolysis of about 85.5% to 91%; and (b) in a separate container, from 0.1 part to 5 parts boric acid per 100 parts water, from 5 wt-% to 50 wt-% starch, and from 0.1 wt-% to 10 wt-% of base, the remainder comprising water. A premix as claimed in claim 4, wherein the polyvinyl alcohol and the boric acid compound, expressed as H₃BO₃, are present in a weight ratio of less than 9:1. A premix as claimed in claim 4 or 5, wherein the boric acid compound is H₃BO₃.XH₂O or Na₂B₄O₇.XH₂O, wherein X is a number from 0 to 10. A premix as claimed in claim 4, 5 or 6, wherein the base is sodium hydroxide. A composition or premix as claimed in any preceding claim wherein the polyvinyl alcohol has a Brookfield viscosity as a 4 wt-% aqueous solution at 20°C which is from 0.4 - 1 x 10⁻² Pa.s. A composition or premix as claimed in any preceding claim the partially hydrolysed polyvinyl alcohol has a degree of hydrolysis of from 87 to 89%. A composition or premix as claimed in any preceding claim, wherein the aqueous polyvinyl alcohol composition has been produced from a concentrate produced in turn from a polyvinyl alcohol having a particle size of less than 80 Tyler standard mesh (more than 1.77 x 10⁻⁴ m mesh). A composition as claimed in claim 2 or 3 or any one of claims 8 to 10, or a premix as claimed in any one of claims 4 to 10, wherein the starch is employed in the form of an aqueous emulsion wherein ungelatinized starch particles are suspended in a carrier phase of gelatinized starch, the gelatinized starch and ungelatinized starch being present in a weight ratio of from 0.01 to 10:0.01 to 40. A method for the manufacture of corrugated board in which at least one liner is bonded to at least one fluted medium with an adhesive wherein the adhesive has been produced by incorporating water in an aqueous cold water soluble polyvinyl alcohol concentrate composition to produce a polyvinyl alcohol concentration in the range of 0.01 to 2.0 parts by weight per 100 parts by weight of adhesive, the polyvinyl alcohol having a degree of hydrolysis of 85.5% to 91%, which adhesive comprises 0.1 to 5 parts by weight of boric acid, 5 to 50 parts by weight of starch and 0.1 to 10 parts by weight of base each per 100 parts by weight of water. A method as claimed in claim 12, wherein the polyvinyl alcohol and boric acid compound, expressed as H₃BO₃, are present in the concentrate in a weight ratio of less than 9:1. A method as claimed in claim 12 or 13, wherein the polyvinyl alcohol is a low molecular weight polyvinyl alcohol wherein the Brookfield viscosity of a 4 wt-% aqueous solution at 20°C is from 0.4 - 1 x 10⁻² Pa.s. A method as claimed in claim 12, 13 or 14, wherein the partially hydrolysed polyvinyl alcohol has a degree of hydrolysis of from 87 to 89%. A method as claimed in any one of claims 12 to 15, wherein the concentrate has been produced from a polyvinyl alcohol having a particle size of less than 80 Tyler standard mesh (more than 1.77 x 10⁻⁴ m mesh). A method as claimed in any one of claims 12 to 16, wherein the starch employed in the production of the concentrate is in the form of an aqueous emulsion wherein ungelatinized starch particles are suspended in a carrier phase of gelatinized starch, the gelatinized starch and ungelatinized starch being present in a weight ratio of from 0.01 to 10:0.01 to 40. A method as claimed in any one of claims 12 to 17, wherein the boric acid compound is H₃BO₃.XH₂O or Na₂B₄O₇.XH₂O, wherein X is a number from 0 to 10. A method as claimed in any one of claims 12 to 18, where the adhesive has a pH of from 9 to 13, preferably from 11 to 12. A method as claimed in any one of claims 12 to 19, wherein the adhesive is rendered alkaline by means of sodium hydroxide present in a concentration from 0.02 to 0.75 wt-% of the aqueous adhesive composition produced. A method as claimed in any one of claims 12 to 19, wherein the concentration of polyvinyl alcohol in said concentrate composition is from 18-40 wt-%. A method as claimed in claim 12, wherein the concentrate composition is the product of bringing together the contents of separate containers to form a premix as defined in any of claims 4 to 11.

Inventors: KRANKKALA, PAUL L.
Applicants: H.B. FULLER LICENSING & FINANCING, INC.
Application year: 1987
Main classification: C09J103
IPC codes: C09J103, D21H27, B31F1, B32B29, C08L3, C08L29, C09J129, D21H19