PRODUCTION PROCESS AND ADHESIVES FOR CORRUGATED BOARD

Abstract: The subject matter of the invention is a process for production of a corrugated board using an aqueous adhesive which contains polyvinyl alcohol and clay, or polyvinyl alcohol, clay and a water-soluble boron compound. The concentration of the polyvinyl alcohol in the adhesive solution and the viscosity of the adhesive are as defined in the patent claim. The adhesive is applied having a tempera­ ture of 60 to 95°C. The process of the invention is excellent in energy saving. The corrugated board can be manufactured at high speed without heating after application of the adhesive, which is easy to handle and has improved adhesion property.

Description: The present invention relates to a heat energy saving and high speed production process for a corrugated board, and to adhesives used for said process. More particularly, the present invention relates to a process of manufacturing a corrugated board having an excellent effect on heat energy saving characterized in that said corrugated board is manufactured at high production speed without heating after application of a water-based adhesive having temperature of 60 - 95 °C, said adhesive containing polyvinyl alcohol and clay, or polyvinyl alcohol, clay and a water-soluble boron compound, which is easy to handle and has improved adhesion. The manufacture of corrugated board essentially consists of combining sheet linerboards with a corrugated medium by means of adhesives. However, too much heat energy is consumed in this operation because a water-based starch glue is used, gelation and concentration of which occur only through heating and drying. In the manufacture of corrugated boards, a single-faced board is first made on a single facer. That is, a fed sheet medium is corrugated while a starch adhesive is applied onto the tips of the flutes of said corrugated medium with an applicator roll, and a sheet linerboard is bonded by means of heat and pressure, on said medium to which the starch adhesive has been applied. Then, the single-faced board manufactured on said single facer is transported to a glue machine, wherein a starch adhesive is applied onto the flute tips of said single-faced board. The board is then taken between the canvas and the heat tables of a double facer heated to about 170°C. After said single-faced board is combined with a double-faced linerboard for a single-wall corrugated board, or with a double-faced linerboard and another single-faced board for a double-wall corrugated board, weight rolls firmly press the combined board to canvas-covered heat tables to gelatinize and dry the adhesive. Thereafter, the resulting single-wall board or double-wall board is slit and scored on a slitter scorer, is cut into a given length on a rotary cutoff, and is converted into single-wall corrugated blanks or double-wall corrugated blanks. The adhesion on the double facer is carried out by heating the starch adhesive applied onto the flute tips of the single-faced board through the double-faced linerboard in case of a single-wall board or through the double-faced linerboard and another single-faced board in case of a double-wall board. Further, since the single-faced board having formed flutes is combined with the double-faced linerboard on the double facer, high pressure can not be applied on the combined board to avoid flute damage. Therefore, heat transfer efficiency in the heating table is extremely low. Therefore, much heat energy is consumed in the double facer resulting in a severe problem. In order to overcome the above defect, production processes for corrugated board which employ adhesives requiring no heating have been studied these past years (See Japanese Patent Laid open Publication Nos. 32570/1981, 70074/1981 and 105952/1981). Furthermore, various studies have been carried out for polyvinyl alcohol type adhesives for paper (See Japanese Patent Publication No. 22579/1961, Japanese Patent Laid Open Publication No. 3336/1972 and USP 3,135,648 etc.). However, as disclosed in the above-mentioned Japanese Patent Laid Open Publication Nos. 32570/1981 and 70074/1981, the adhesives which can be applied to paper at a high temperature but which gelatinize upon cooling have the property that the adhesive applied at a high temperature shows increasing viscosity and hardening upon cooling. Therefore, at low speed, there is the defect that the adhesive applied onto the corrugated medium is hardened before it is combined with the linerboard, and that good adhesion cannot be achieved Some set-back type starch adhesives containing a high content of amylose have an excellent setting property, but these adhesives have the defect that once the adhesive is hardened upon cooling, it can not become fluid even if it is heated again. Then, such adhesives are very difficult to handle, e.g. as regards conveying through pipes, applying and storing and can not be used in practical application. On the other hand, Japanese Patent Laid Open Publication No 105952/1981 discloses a method wherein an adhesive is applied either onto the corrugating medium or the linerboard, and gelatinizing agents are coated onto the other. Thereafter, the two parts are contacted and adhesion is achieved by the reaction of the adhesive and the gelatinizing agents. However, this method has the defect that the process is complicated. Polyvinyl alcohol type adhesives disclosed in Japanese Patent Publication No. 22579/1961, Japanese Patent Laid Open Publication No. 3336/1972 and USP 3,135,648 are insufficient in green bond, and corrugated boards can not be manufactured at high speed by a method which does not need heating in heat tables after application of the adhesive. Also, heat energy saving could not be accomplished. Recently, corrugated board production is generally done at a speed of 100 m/min or more. Further, under the viewpoint of productivity, corrugated boards are slit and scored on a sitter scorer and cut lengthwise on a rotary cutoff direct after the double facer. This means that high stress is applied at an earlier stage of adhesion. Therefore, high initial setting ability and high green bond strength are required as characteristics of adhesives. Thus, the development of adhesives for corrugated board is very difficult. When the production speed is over 100 m/min, the period of time between adhesive application on a glue machine and the slitting/scoring on a slitter scorer is only 4 - 10 seconds. Therefore, in such a short period of time, a green bond strength which can stand the stress from a slitter scorer or a rotary cutoff is required. In this point, adhesives for corrugated board are different from those for laminated paperboard or paper tube. Furthermore, it is required that the amount of adhesive applied is controlled under the remarkably high shear rate where the adhesive has excellent fluidity. On the other hand, the production speed of the corrugater is changed frequently and combining is often carried out at low production speed because paper change and/or order change must be carried out. That is, on high production speed, it is required that an adhesive has a remarkably high setting ability and green bond. On low production speed, the adhesive must have long open-time because it takes long time from application of the adhesive in the glue machine to entering into canvas-covered tables. Thus, there has not been developed a process of manufacturing a corrugated board in which much heat energy can be saved and said corrugated board can be manufactured at high speed without heating after application of a water-based adhesive. The present inventors have intensively studied an improved process for production of corrugated boards. As the result, it has been found that by using an adhesive containing a specific polyvinyl alcohol and by applying the adhesive, the temperature of which is 60 to 95 °C, onto the flute tips of the corrugated medium and combining it with a lingerboard, corrugated boards can be manufactured at low production speed as well as at high production speed without heating after application of the adhesive. The present invention provides a process for production of a corrugated board by using a water-based adhesive which consists essentially of polyvinyl alcohol (hereinafter, referred to "PVA") and clay, the ratio of the clay to the PVA being in the range of 200 to 600 parts by weight (hereinafter, referred to "parts")of clay to 100 parts by weight of PVA, the solid content of the adhesive being in the range of 25 to 60 % by weight (hereinafter, referred to "%") which process is characterized in that (a) the concentration of polyvinyl alcohol in the adhesive solution used (excluding solids other than polyvinyl alcohol from adhesive) is in the range defined by the following formulas: Y ≧ -0.0014 x + 9.4 (i) Y ≦ -0.0061 x + 26.3 (ii) 200 ≦ X ≦ 3500 (iii) wherein Y is defined by the following formula: P = Polyvinyl alcohol content in the adhesive (by weight) W = Water content in the adhesive (by weight). and X is the degree of polymerization of the polyvinyl alcohol, (b) the viscosity of the adhesive as measured with a Brookfield viscometer is from 200 to 6000 mPa.s at 85 °C, and the viscosity increase of the adhesive is in the range of 1.5 to 8 times when the temperature of the adhesive is decreased from 85 to 30 °C, (c) the clay is kaolinite, halloysite, pyrophyllite and/or sericite and the pH of the adhesive is not higher than 6, (d) the adhesive is applied with a temperature of 60 to 95°C onto the flute tips of the corrugated medium, and the corrugated medium coated with the adhesive utilized for the production of a corrugated board. Thereby, corrugated boards can be satisfactorily produced in a high production speed without heating after application of the adhesive. Figure 1 shows the relationship between the degree of polymerization and the PVA concentration in the solution excluding solids other than PVA from the adhesive used in the present invention, wherein X represents the degree of polymerization of PVA and Y represents the concentration of PVA as defined by following formula : P = Polyvinyl alcohol content in the adhesive (by weight) W = Water content in the adhesive (by weight). Figure 2 shows the relationship between a Hercules viscosity at 60 - 95 °C under the shear rate of 3 x 10 ⁴ sec⁻¹ and the viscosity measured by a Brookfield viscometer, wherein A represents Hercules viscosity and B represents the viscosity measured by a Brookfield viscometer. According to the present invention, when the amount of clay is less than 200 parts to 100 parts of PVA, green bond and initial setting ability are extremely bad. On the other hand, if it exceeds 600 parts, it is inappropriate because it induces excessively low final adhesion strength and high solid content of the adhesive to give excessively high adhesive viscosity. Therefore, it is preferably in the range of 200 to 600 parts based on 100 parts of PVA, more preferably 200 to 450 parts. The solid content of the water-based adhesive is preferably in the range of 25 to 60 %. When the solid content of the adhesive is less than 25 %, green bond is too weak. On the other hand, if it exceeds 60 %, handling on a corrugator is bad and final adhesion strength is too weak. Further, the viscosity of the adhesive also affects adhesion. It is preferable that the viscosity of the adhesive at 85 °C as determined by a Brookfield viscometer is in the range of 200 to 6000 mPa.s more preferably 300 to 3000 mPa.s. Furthermore, it is more preferable that the viscosity of the applied adhesive, which has a temperature of 60 to 95 °C, is in the range of 300 to 4000 mPa.s. The higher the viscosity, the better its green bond is, but final adhesion strength becomes worse. On the other hand, the lower the viscosity is, the higher is the final adhesion strength, but its green bond is insufficient. It is desirable that the degree of polymerisation of PVA used in the present invention is in the range of 200 to 3500. When the degree of polymerization is less than 200, final adhesion strength is extremely low. On the other hand, when the degree of polymerisation is greater than 3500, final adhesion strength becames lower too, because viscosity of PVA solution itself is extremely high and PVA can not be incorporated into the adhesive in a high content to satisfy initial setting ability. It is finally preferable that the degree of polymerization is in the range of 200 to 3500, more preferably 300 to 2500. Moreover, in order to produce a corrugated board at high production speed without heating after applying the adhesive of 60 to 95 °C, it has been found that the relation of PVA concentration in the solution excluding solids other than PVA from adhesive to the degree of polymerization of PVA is considerably important. That is, within the degree of polymerization of 200 to 3500, PVA having relative low degree of polymerization may be used in a wide range of PVA concentration. However, when the PVA concentration of the adhesive is excessively high, the adhesive has remarkably high water retention and a less green bond. On the other hand, when PVA concentration is excessively low, the tack of adhesive and initial setting ability becomes worse and final adhesion strength becomes lower. In order to maintain the quality of the corrugated board, the PVA concentration must be relatively high. Contrarily, when the degree of polymerization is relatively high, viscosity of PVA solution itself becomes high. Then, higher PVA concentration induces extremely high viscosity of adhesive, higher water retention and less initial setting ability. Therefore, it is required that the PVA concentration is relatively low. However, when the PVA concentration becomes excessively low, the tack of adhesive is insufficient and the initial setting ability is inferior. PVA having a relative high degree of polymerization may be used only in a narrow range of PVA concentration in the adhesive. Such PVA requires that the PVA concentration must be lower than that of PVA having low degree of polymerization to decrease the viscosity of adhesive and the solid content of adhesive must be maintained within a somewhat high range by a high content of clay. However, when the adhesive has an excessively high content of clay and an excessively low content of PVA, it naturally induces a remarkably low final adhesion strength. Thus, it has been found out that the relation between the PVA concentration and the degree of polymerization of PVA must be controlled within a very limited range in order to produce corrugated boards at high production speed without heating after application of an adhesive having a temperature of 60 to 95 °C. As the results of an intensive study, it has been found that an adhesive satisfying the following formulas (i), (ii) and (iii) is preferable. Y ≧ -0.0014 X + 9.4 (i) Y ≦ -0.0061 X + 26.3 (ii) 200 ≦ X ≦ 3500 (iii) wherein Y is a PVA concentration in the aqueous solution excluding the solids other than PVA from the adhesive defined as follows: P = Polyvinyl alcohol content in the adhesive (by weight) W = Water content in the adhesive (by weight). and X is the degree of polymerization (viscosity-average degree of polymerization defined in JIS K 6726). That is, the area with the oblique lines in FIG. 1 defines the suitable area. Then, although the degree of hydrolysis of PVA is not limited specifically, it is preferably in the range of not less than 70 mol% , more preferably, not less than 80 mol%. The degree of hydrolysis of PVA is measured by the method defined in JIS K 6726. There can be used various kinds of PVA such as anion modified PVA, cation modified PVA, acrylamide modified PVA, lactone modified PVA, other various modified PVA produced by copolymerisation, after-modified PVA (e.g. acetal modified PVA, and modified PVA by other method in combination with the above non-modified PVA. Such modified PVA can be used alone or in combination of two or more. The clay used in the present invention is kaolinite, halloysite, pyrophyllite and/or sericite. The clay has preferably an average particle size of not more than 2 µm for an excellent initial setting ability. However, if the viscosity of an aqueous slurry being prepared by adding the clay into water is very high, such clay has less processability, can not give a relative high solid content of adhesive and shows less improved green bond. It is preferable that the viscosity of the aqueous slurry containing the clay as measured with a Brookfield viscometer is not more than 1000 mPa.s ( 25 °C) in 25 to 40 % of an aqueous dispersion, when the clay is used alone or in combination with an anionic polymer or metal salt thereof, soluble glass, metal salt of phosphorous compound, and/or nonion surfactant. By the way, when the corrugated medium to which the adhesive has been applied is adhered to the linerboard, the adhering mechanism is as follows. The water of the adhesive is absorbed by the corrugated medium and the linerboard. As a result, the adhesive is concentrated and tackified. This mechanism mainly contributes to the initial setting ability and green bond of the adhesive in this invention. Further, the viscosity of the adhesive also has a considerable effect on the green bond, so that it is better that the viscosity of the adhesive is higher. On the other hand, as to final adhesion strength, it is deemed that impregnation of the adhesive into considerable depth of the paper layer of the corrugated medium and the linerboard is required so that the adhesive has preferably a lower viscosity. As mentioned above, in order to produce a corrugated board, it is required that sufficient adhesion strength is obtained within only several seconds between adhesive application to the single-faced board and slitting and scoring. Then, although remarkably excellent green bond is the most desired characteristics for adhesion of the corrugated board, and it is also required that the adhesive has a high final adhesion strength for securing the desired strength of the final products. Then, as mentioned above, not only the value of the viscosity of adhesive is essential, but the adhesive temperature at the time of application is essential. That is, when the temperature of the adhesive is too low at the time of application both green bond and final adhesion strength can not be satisfied at the same time. As the results of our various studies with respect to the temperature of the applied adhesive, it has been found that when the temperature of the applied adhesive is 60 to 95°C, both green bond and final adhesion strength are satisfactory at the same time. That is, when the temperature of the applied adhesive of the present invention is less than 60 °C, both green bond and final adhesion strength can not be satisfied at the same time and a corrugated board can not be produced at a high production speed, even though the viscosity of the adhesive is high. When the temperature of the adhesive at the time of application is not less than 60°C, there can be obtained sufficient green hood and the combination with the linerboard is achieved at a high production speed of not less than 140 m/min. There can be also obtained a corrugated board having sufficient final adhesion strength. The higher temperature is at the time of application, the better is the adhesion. On the other hand, a too high temperature induces an excessive evaporation of water from the adhesive, which makes the control of the concentration of adhesive difficult, and gives film of the adhesive on the surface together with less workability. As the result , the upper limit of adhesive temperature at the time of application is 95°C. It is finally preferably that the temperature of adhesive at the time of application is in the range of 60 to 95 °C, more preferably 70 to 90 °C. The initial setting ability and green bond of the adhesive in the present invention is improved extremely because by increasing the temperature of said adhesive the surface energy of said adhesive is reduced, the water of said adhesive penetrates easily into the corrugated medium and the linerboard, and tack and concentration of said adhesive are increased rapidly. The solids content of the adhesive used in the present invention seems to affect significantly upon the adhesion, and a higher solids content of the adhesive brings a greater green bond if the water retention of the adhesive is not excessively high. If the adhesive gelatinizes by decreasing of the adhesive temperature to about the room temperature, it induces less processability of the adhesive during conveying through a pipe or storage. Further, it also induces less adhesion because if in case of low production speed, the adhesive applied onto a single faced corrugated board has long open time especially in a double facer part the temperature of the applied adhesive is decreased. Then, the adhesive gelatinizes or the viscosity of the adhesive is extremely increased before the contact to a liner to be laminated. Therefore, such defect is not induced if the viscosity of the adhesive is increased only by 1.5 to 8 times when the temperature of the adhesive decreases from 85 to 30 °C. In order to adjust the increase of the viscosity of the adhesive in the range of 1.5 to 8 times when the temperature of adhesive decreases from 85 to 30 °C, one or more of the aforementioned clays are used; i.e. kaolinite, halloysite, pyrophyllite and/or sericite. As mentioned below, when a water-soluble boron compound is used, the increase of the viscosity of the adhesive can be adjusted in the range of 1.5 to 8 times under specific conditions such as using of the aforementioned specific clays or by operation under neutral to acidic condition. An adhesive, which contains a PVA and a considerable amount of gelatinizing agents such as Congo red or resorcinol or contains a PVA, a considerable amount of a water-soluble boron compound and an alkali filler, can not show an increase of viscosity within the range of 1.5 to 8 times. Moreover, at present the corrugating machine is operated at a high production speed of 100 m/min or more, and the control of application amount of adhesive in a glue machine is carried out under a considerable high shear rate of not less than 1 x 10 ⁴ sec⁻¹ so that the fluid characteristics of the adhesive under high shear rate seem to affect upon the adhesion significantly. As the result of our studies, it has been found that the viscosity (A) at 60 to 95 °C of the adhesive as measured with a Hercules viscometer under the shear rate of 3 x 10 ⁴ sec⁻¹ and the viscosity (B) at the same temperature as measured with a Brookfield viscometer should satisfy the following formula (iv) A / √ B ≦ 6.3 (iv) and the viscosity of the adhesive as measured with a Brookfield viscometer should be in the range of 300 to 3000 mPa.s at 85 °C. Although there is no lower limit of A / √ B , it is usually not less than 1. This relation is shown in FIG. 2, which illustrates that the adhesion of an adhesive having a lower ratio (formula iv) of the viscosity under high shear rate to that under a lower shear rate is greater than that of an adhesive having a higher ratio. In this connection, Japanese Patent Laid Open Publication No. 3336/1972 exemplifies an adhesive showing pseudoplasticity under a shear rate of 1 x 10² to 1 x 10⁴ sec⁻¹, which relates to a specific PVA (the viscosity of 4 % aqueous solution is in the range of 9 to 16 mPa.s) at a temperature of not more than 50 °C. On the other hand, the present invention relates to an adhesive employing PVA having a wide range of degree of polymerization of 200 - 3,500. Further, with respect to the viscosity measured by a Brookfield viscometer at 85 °C being 300 - 3,000 mPa.s, it has been found that the adhesives, which have a ratio of viscosity at a high shear rate of 3 x 10⁴ sec⁻¹ (Hercules viscosity) to the square root of the viscosity measured at a low shear rate which is lower than the specific value, are especially excellent as the adhesives for the production of the corrugated board. Furthermore, we have found that the adhesive temperature of the time of application , as mentioned above, considerably affects initial setting ability. That is, when the temperature is not higher than 50 °C, the initial setting ability at a high production speed is bad. It has been also found that the adhesives, which have a lower ratio of Hercules viscosity at a high shear rate to the viscosity at a low shear rate with the increasing of the temperature,are excellent in adhesion property. As illustrated above, the water-based adhesive containing PVA and clay as main ingredients can be used for production of a corrugated board in a heat-saving and high-speed production, however, water-soluble boron compound can be also used in combination with PVA and clay. As the water-soluble boron compound, boric acid, borate such as borax, and borate ester of polyhydric alcohol such as glycerol and ethylene glycol can be used, and among them boric acid is suitable. It is preferred to use the water-soluble boron compound in an amount of not more than 15 parts, based on 100 parts of PVA. The lower bound is not specially limited, however, it is desirably used in an amount of not less than 0.1 parts. The more is used, the better green bond is provided because of increased tackiness of adhesives. In this case, however, it is not desirable that the adhesives tend to be gelatinized at a low temperature. Moreover, by decreasing of the adhesive temperature from 85 to 30 °C, the ratio of increase of the viscosity becomes more than 8 times to induce lower processability. It is more preferable to use a relatively greater amount of water-soluble boron compound with a PVA having a low degree of polymerization. On the other hand, it is preferable to use a lower amount of such compound with a PVA having a high degree of polymerization. It is preferred that the viscosity of the solution excluding solids other than PVA and the water-soluble boron compound from adhesive measured at 60 - 95 °C by a Brookfield viscometer is in the range of 100 - 1,000 mPa.s. The addition of the following oxides into the adhesives improves the adhesion, however, in this case the viscosity is decreased. Therefore, the viscosity of the solution excluding solids other than PVA and the water-soluble boron compound from adhesive measured at 60 - 95 °C by a Brookfield viscometer is suitably in the range of 150 - 1,500 mPa.s. When a relatively greater amount of water-soluble boron compound is used, attention should be paid on the pH of the adhesive. That is, when the adhesive is alkaline, the bonding of PVA with the water-soluble boron compound is facilitated and the adhesive is easily gelatinized at a low temperature. Then, the ratio of increase of viscosity can not be in the range of 1.5 - 8 times, when the temperature of adhesive decreases from 85 to 30 °C, which results in less handling of adhesives and less improved adhesion. Therefore, the pH of the adhesives is not higher than 6. The adhesives for the production of a corrugated board having such properties may include those containing only PVA and clay, or PVA, clay and a water-soluble boron compound. In order to improve its fluidity and adhesion property, however, it is more preferably to add to them metal salts of phosphoric acids, such as sodium polyphosphate or sodium hexamethaphosphate; dispersing agents for clay such as water glass; anionic polymeric compounds and metal salts thereof such as polyacrylic acid and salts thereof, sodium alginate, isobutene - maleic anhydride copolymer; nonionic surface active agents such as higher alcohol added with ethylene oxide, copolymers of ethylene oxide and propylene oxide; or guar gum. Among the above said fluid additives, sodium hexamethaphosphate, sodium polyacrylate or sodium alginate is preferred. It is preferred to add them to the adhesive in an amount of 0.01 - 2 parts, based on 100 parts of clay. With more than 2 parts, the initial setting ability becomes bad. On the other hand, with less than 0.01 parts, no effect is achieved. Moreover, it is effective for improvement of adhesion property to add peroxides such as sodium periodate, periodic acid, potassium periodate, sodium persulfate, potassium persulfate or ammonium persulfate. Among them, periodic acid, sodium periodate and sodium persulfate are preferred. The addition of peroxide will decrease the viscosity to facilitate handling and also improve tackiness of adhesives as well as initial setting ability. However, incorporation of an excessive amount of peroxide excessively decreases the viscosity and the initial setting ability becomes bad. Therefore it is preferred to incorporate an amount of 0.01 - 10 parts of peroxide based on 100 parts of PVA. Addition of less than 0.01 parts of peroxide has little effect. Further, the addition of saccharose, colloidal silica, calcium carbonate, carbon black, white carbon or silica anhydride improve tackiness and adhesion property. It is preferred to incorporate an amount of 0.1 - 20 parts, based on 100 parts of clay, more preferably 0.5 - 15 parts. The amount of less than 0.1 parts has little effect, and an amount of more than 20 parts induces an adverse effect on fluidity, and such amount cannot be used. Even if the addition of large amount of white carbon or calcium carbonate results in bad adhesion, the addition of small amount of clay improves adhesion property. As mentioned above, it is prefered that the degree of polymerization of PVA is 200 - 3,500, more preferably 300 - 2,500. It is prefered that the degree of hydrolysis of PVA is not less than 70 mol %, more preferably not less than 80 mol%. The PVA can be used in combination with one or more grades thereof. As the results of investigations about combination of various PVA grades, it has been found that the adhesion having a specific combination of more than two grades of PVA shows a further improved adhesion property compared to that of adhesion employing only one grade of PVA, even if they have the same average degree of polymerization degree and the same average degree of hydrolysis. That is, regarding the degree of polymerization, a mixed PVA which contains 10 - 90 parts of PVA having a degree of polymerization of 200 - 800, preferably 300 - 600, and 90 - 10 parts of PVA having a degree of polymerization of 1,100 - 3,500, preferably 1300 - 3500, more preferably 1,700 - 2,500 shows further improved adhesion compared to that obtained from only one grade of PVA having the same average degree of polymerization (said PVA is polymerised adjusting the desired degree of polymerization and hydrolysis per se), while both of mixed PVA and one grade of PVA have almost the same viscosity. When either a PVA having a degree of polymerization of 200 - 800 or a PVA having a degree of polymerization of 1100 - 3500 is used in an amount of less than 10 parts based on 100 parts of the whole amount of PVA, the adhesive shows no adhesion different from that obtained by using only one grade of PVA. Although the reason is not clear, it can probably be attributed to the fact that the degree of polymerization of the mixed consisting of two or more grades of PVA having a different degree of polymerization is widely distributed compared with that of only one grade of PVA. That is, the mixed PVA contains a PVA having a relatively low degree of polymerization and a PVA having a relatively high degree of polymerization compared to the only one grade of PVA, even if they have the same average degree of the polymerization. In the adhesive containing such mixed PVA, the PVA having the relatively low degree of polymerization facilitates the penetration of an adhesive solution into the paper layer and improves the adhesive property because the clay is well dispersed by that PVA, while the PVA having the relatively high degree of polymerization contributes to improve tackiness which significantly affect a green bond of the adhesive. As to degree of hydrolysis, it has been found that the adhesive containing mixed PVA which consists of 10 - 90 parts of PVA (a) having a degree of hydrolysis of 72 - 100 mol% and 90 - 10 parts of PVA (b) having a degree of hydrolysis lower by 2 - 30 mol% than that of PVA (a) is superior in adhesion property to the adhesive containing the PVA which consists of only one grade of PVA (PVA prepared adjusting the desired degree of hydrolysis per se) having same degree of hydrolysis. Although the reason for further improvement of adhesion property is not clear, it can probably be attributed to the the fact that the degree of hydrolysis of mixed PVA consisting of two or more grades of PVA having a different degree of hydrolysis is widely distributed compared with that of only one grade of PVA. That is, the mixed PVA contains a PVA having a relatively low degree of hydrolysis and a PVA having a relatively high degree of hydrolysis compared with that of the only one grade of PVA, even if they have the same average degree of hydrolysis. Generally, the PVA having the relatively low degree of hydrolysis shows relatively lower surface energy when dissolved in water and can easily penetrate into the paper layer, while the PVA having the relatively high degree of hydrolysis contributes to the green bond owing to its excellent wet tack or the adhesive containing that PVA, so that an adhesive containing such mixed PVA is improved in both green bond strength and final adhesion strength. However, it should be noted that the adhesive containing the PVA which contains a greater amount of PVA having a higher degree of hydrolysis and a higher degree of polymerization shows higher viscosity, so that said adhesive may hardly be adjusted to high solid content. While, it should be noted that the adhesive containing the PVA which contains a greater amount of PVA having a lower degree of hydrolysis and a lower degree of polymerization especially shows less wet tack. When using more than two grades of PVA, there may be many possibilities for the combination of PVA having different degrees of polymerization and/or hydrolysis, such as a combination of more than two grades of PVA having the same degree of polymerization but different degree of hydrolysis, a combination of those having the same degree of hydrolysis but different degree of polymerization, a combination of PVA having lower degree of polymerization and higher degree of hydrolysis with PVA having higher degree of polymerization and lower degree of hydrolysis, and vice versa. The adhesive employing the PVA of above combination in certain ratio shows an further improved adhesion property compared with those employing only one grade of PVA. Among them, the adhesive containing a combination of PVA which comprises completely hydrolyzed PVA having a lower degree of polymerization and partially hydrolyzed PVA having a higher degree of polymerization, and containing boric acid shows an remarkably excellent adhesion property. Although the PVA which is hydrolyzed in the conventional methanol solution is effective, PVA which is hydrolyzed in methyl acetate or other solvent, or a mixture of methanol and other solvent can be also used. The adhesives of the present invention can be used for paper, such as laminating paper; board and corrugated board, paper tube, paper bag and the like, in addition to a corrugated board application. In the above usage, it is not necessary to conduct an adhesion in a short time as in case of a corrugated board. However, recently, an improvement in initial setting ability has been required with the increase of production speed. Therefore, the present invention can employ advantageously such a high production speed. In order to prepare the present adhesive, the mixture of PVA, clay, water-soluble boron compound or other additives previously prepared may be firstly poured into water with stirring to prepare a slurry, or a metal salt of a phosphoric acid, a dispersant for clay such as water-glass, additives such as an anionic polymeric compound and a metal salt thereof and a nonionic surface-active agent, clay, PVA,a water-soluble boron compound or other additives may be poured into water one after the other with stirring to prepare a slurry. The process for preparing the adhesive may be conducted in a batch process or in a continuous process, and it may be prepared by heating and dissolving by any heating process which comprises, for example directly blowing a steam vapor into the slurry solution in a jet-cooker or a preparation bath, or indirectly heating the preparation bath or pipe. Peroxide may be added either before heating and dissolving, or after heating at high temperature. However, it is preferably to add it into the slurry solution just before heating for efficiency. The process is excellent in heat energy saving and can be carried out at high production speed . The production can be carried out by pre-heating of the linerboard or the single faced corrugated boards, or by heating of the heat tables. According to the process for production of a corrugated board of the present invention, it becomes possible to produce at high speed of 100 m/min or more, and if desired, 120 m/min or more. It is also possible to produce at remarkably high speed of up to 140 m/min. The process for production of a corrugated board of the present invention has the great advantage that it can produce corrugated boards within a wide range of production speed, from low to high, without heating after application of the adhesive and laminating, and it also is excellent in heat energy saving. Moreover, the process for production of a corrugated board of the present invention effectively reduces warpage of the corrugated board sheets. The present invention is further illustrated by the following table tests and examples in detail but is not limited thereto. A. PVA and clay used are as follows: (1) PVA (2) Clay I: UG-90 (ENGELHARD) Average Particle Size, 0.3 µm, Kaolinite II: ASP-400 (ENGELHARD) Average Particle Size, 4.8 µm, Kaolinite III: KANATANI-CLAY NSF (KANATANI KOGYO) Particle Size less than 2 µm, 99%, Halloysite B. Evaluation of green bond and initial setting ability, evaluation of suitability for laminating at a high or low production speed, measurement of adhesive strength and Viscosity were carried out as follows: (1) Evaluation of green bond and initial setting ability according to table test An applicator roll controlled to a given temperature was rotated in an adhesive controlled at a given temperature. The amount of adhesives on the roll was controlled with a doctor blade, thereafter the flute tips of the single faced corrugated board were lightly pressed onto the roller to apply adhesive thereon. Then, immediately, it was laminated to a liner, and thereafter pinched lightly for 3 - 10 seconds. Then, the resulting products were subjected to forced peeling test and the condition of adhesion was observed (o ○: Excellent, o: Good, Δ: Not Bad, x: bad, xx: Very Bad). As single faced corrugated board and linerboard, 220 g/m² of JIG B-Grade linerboard and 125 g/m² of JIS B-Grade corrugating medium were used, respectively. (2) Evaluation with Corrugator The corrugator used is 1600 mm wide and has a maximum speed of 140 m/min. The conventional heating on the double facer was not carried out. A linerboard (JIS B-Grade; 220 g/m²) and a corrugating medium (JIS B-Grade; 125 g/m²) were mainly used. The adhesive of the present invention, which was prepared as mentioned above, was supplied to a glue machine of which glue unit was held to 60 - 95 °C, was transferred and applied onto the flute tips of a single faced corrugated board with an application roll to adhere said single-faced board to the linerboard, the resulting corrugated board was passed through a slitter-scorer with corrugator to slit and score, and further to a rotary cutoff and was cut on the rotary cutoff. (a) Evaluation of adhesive suitability for production at a high speed with a corrugator Adhesive suitability was evaluated by Highest Combining Speed representing the highest speed that corrugated board is combined at without any adhesion failure in slit and/or scored parts of the corrugated blank. For finding Highest Combining Speed,the corrugated board made on the corrugator was immediately delaminated at the end of a rotary cutoff, and was checked whether there was said adhesion failure. (b) Evaluation of adhesive suitability for production at a low speed with a corrugator The corrugator was operated at the lowest speed of 25 m/min and the adhering condition was examined at the end of a rotary cutoff on the corrugator. Low Speed Evaluation means the evaluation of adhering condition of corrugated board manufactured at 25 m/min. (3) Method for adhesive strength Adhesive strength was measured with single adhesion separating test specified in JIS Z-0402. (4) Measurement of Viscosity (a) Brookfield viscosity (mPa.s) Brookfield viscometer, No. 1 - No. 4 rotor, 30 - 60 rpm. (b) Viscosity under high shear rate (mPa.s) Hercules high speed viscometer (produced by KUMAGAI RIKI KOGYO Co., LTD.) Bob B; highest rotating speed, 8800 rpm; sweep time 10 sec ; Hercules viscosity (η) is defined by the following equation: T: torque (mN/m) S: shape factor = 0.0004 N: rotation = 7,200 rpm (shear rate; 3 x 10⁴ sec⁻¹) The terms "Parts" and "%" used herein mean parts by weight and weight %, respectively. Examples 1 to 29 The mixtures having the formula shown in Table 1 was heated and stirred for one hour and dissolved to prepare an adhesive. The green bond and initial setting ability and the result of the adhesive evaluation test with a corrugator of these adhesives are shown in Table 2. In the adhesive evaluation test, "A" flute was used and the application amount represents the amount of solid content of an adhesive applied onto single-face side of corrugated board. As is clear from Examples 1 - 29, it has been found that an excellent adhesion can be obtained by applying said 60 - 95 °C adhesive having a ratio of PVA concentration against polymerisation degree thereof within an area shown by the oblique lines of the Figure 1, and having a solid content of 25 - 60 % and a viscosity being within a range of 200 to 6,000 mPa.s at 85 °C, and the viscosity of the adhesive being increased not more than 8 times when the adhesive is cooled from 85 °C to 30 °C. As is clear from Comparative Example 2, an adhesive having excessively high viscosity at 85 °C has a inferior adhesion. As is clear from Comparative Example 6, an adhesive easily to be gelatinized is inferior in laminating ability at a low production speed. Further, the temperature of the adhesive at the time of application greatly effects the green bond, initial setting ability and suitability for combining at a high production speed. That is, a temperature not lower than 60 °C is suitable (see Examples 1 - 29), and when the temperature of adhesive at the tile of application is below 60 °C (see Comparative Examples 3 and 4), the green bond and initial setting ability and suitability for combining at a high production speed were insufficient. As is clear from Examples 7 - 29, the adhesives, which have a viscosity of an aqueous PVA-boric acid solution at temperature of 60 - 95 °C in a range of 100 to 1,500 mPa.s , shows a good adhesion property. As is clear from Examples 10 - 20, the adhesion is further improved by adding an anionic polymer compound, a phosphoric acid, peroxide, colloidal silica (Snowtex® C, NISSAN CHEMICAL CO., LTD.), white carbon (Silton® R-2, Mizusawa Kagaku kk), Carbon black (#44, Mitsubishi Kasei KK), calcium carbonate, silica anhydride (AEROSIL® 380, AEROSIL NIPPON) or guar gum (MAYPROGUM® SN-20, SANSHO KK). In Example 19, an adhesive is prepared in the same manner and compounding ratio as described in Example 10 except that the mixture of PVA (A) and (F) was substituted for PVA (C), wherein the average degree of polymerization of the PVA mixture was adjusted to the same as that of PVA (C). The adhesive of Example 19 shows almost the same viscosity as that of example 10, but it also shows a further improved adhesion. Example 21 or 23 contained one grade of PVA (K) or PVA (L) respectively, and Example 22 contained two grade of PVA (D) and (E). The average decree of polymerization and the average degree of hydrolysis of mixed PVA which comprises PVA (D) and (E) is nearly equal to those of PVA (K). Example 24 comprises three grades of PVA (A), (I) and (M). Example 25 comprises three grades of PVA (A), (I) and (F). The average degree of polymerization and the average degree of hydrolysis of mixed PVA which comprises PVA (A), (I) and (M), or PVA (A), (I) and (F), is nearly equal to those of PVA (L). In the comparison of example 21 with 22, or 23 with 24 and 25, it has been found that the adhesive comprising two grades of PVA or more shows a further improved adhesion compared to that of one grade of PVA. Further, in the comparison of examples 24 and 25, it is obvious that the adhesive of example 24 using completely hydrolyzed PVA having low degree of polymerization and partially hydrolyzed PVA having high degree of polymerization, shows a better adhesion property. As is obvious from Examples 24, 25, 26, 27, and 29, each adhesive, which contains two grades or more of PVA having different degrees of polymerization and hydrolysis in a specified ratio, shows a remarkably excellent adhesion property. The viscosity of a 40% slurry solution of clay (I) at 25 °C measured with a Brookfield viscometer was 10 mPa.s and that of a 32 % slurry solution of clay (III) was 800 mPa.s. Example 30 25 Parts of PVA (D), 72.3 parts of clay (I), 2.5 parts of boric acid, and 0.2 parts of sodium alginate were dissolved in water to give an adhesive having a solid concentration of 27.8 %. The viscosity of adhesive measured by Brookfield viscometer was 1,960 mPa.s at 65 °C and 820 centipoises at 90 °C. Hercules viscosity of the adhesive under high shear rate of 3 x 10⁴ sec⁻¹ was 250 mPa.s at 65 °C and 155 mPa.s at 90 °C. The ratio at the same temperature, (Hercules Viscosity)/√ (Brookfield Viscosity) , was 5.6 at 65 °C and 5.4 at 90 °C (see Fig. 2). A adhesive evaluation test was carried out by using the adhesive at 65 °C. The adhesive had a good green bond and a good combining ability at high production speed, and also had a high final adhesion strength value of 21.7 kg/40cm² at an application amount of 6.0 g/m² (solid base). Example 31 24 Parts of PVA (D), 73.6 parts of clay (III), and 2.4 parts of boric acid were dissolved in water to give an adhesive having a solid concentration of 27.6 %. The viscosity of adhesive measured by Brookfield viscometer was 1,550 mPa.s at 65 °C and 360 mPa.s at 90 °C. Hercules viscosity of the adhesive under high shear rate of 3 x 10⁴ sec⁻¹ was 280 mPa.s at 65 °C and 145 mPa.s at 90 °C. The ratio at the same temperature, (Hercules Viscosity)/√ (Brookfield Viscosity) , was 7.1 at 65 °C and 7.6 at 90 °C, that is, the ratio at 90 °C is higher than that at 65 °C (see Fig. 2). An adhesive evaluation test was carried out at 65 °C by using the adhesive. Green bond and combining ability at high production speed of this adhesive is inferior to that of Example 30. Example 32 24 Parts of PVA (C), 73.2 parts of clay (I), 2.4 parts of boric acid, 0.1 parts of sodium polyacrylate and 0.3 parts of sodium persulfate were dissolved in water to give an adhesive having a solids concentration of 35.5 %. The viscosity of the adhesive measured by a Brookfield viscometer was 850 mPa.s at 90 °C, while Hercules viscosity of the adhesive at high shear rate (3 x 10⁴ sec⁻¹) was 156 mPa.s at 90 °C. The adhesive had a ratio of 5.4, [((Hercules Viscosity)/√ (Brookfield Viscosity) = 156/√ 850 ] (see Fig. 2). A adhesive evaluation test was carried out at 90 °C by using the adhesive. The adhesive had a good green bond and a good suitability for combining at a production speed of not less than 140 m/min, and also had a high adhesion strength of 21.5 kg/40cm² at an application amount of 6.5 g/m² (solid base). Example 33 24 Parts of PVA (B), 73.3 parts of UG-90 (average particle size: 0.3 µm), 2.4 parts of boric acid and 0.2 parts of sodium polyacrylate were dissolved in water to give an adhesive having a solids concentration of 37.0 %. The viscosity measured by a Brookfield viscometer was 980 centipoises at 85 °C and 3,510 mPa.s at 30 °C. Further, the adhesive having a solids concentration of 42.4% prepared in the same manner as described above except that ASP-400 (average particle size: 4.8 µm) was used instead of the said UG-90 had a viscosity measured by a Brookfield viscometer of 1,000 mPa.s at 85 °C and 4,300 mPa.s at 30 °C. In the comparison of the green bond of both adhesives, both adhesives had a good green bond, however, that of the adhesive using UG-90 was better. Moreover, the adhesive using white carbon instead of clay had a bad green bond.

Claims: A process for production of a corrugated board using an adhesive which consists essentially of polyvinyl alcohol and clay, the ratio of clay to polyvinyl alcohol being in the range of 200 to 600 parts by weight of clay to 100 parts by weight of polyvinyl alcohol, the solid content of the adhesive being 25 to 60 % by weight characterized in that (a) the concentration of the polyvinyl alcohol in the adhesive solution used excluding solids other polyvinyl alcohol from adhesive is in the range defined by the following formulas: Y ≧ -0.0014 X + 9.4 (i) Y ≦ -0.0061 X + 26.3 (ii) 200 ≦ X ≦ 3500 (iii) wherein Y is defined by the following formula: P = Polyvinyl alcohol content in the adhesive (by weight) W = Water content in the adhesive (by weight). and X is the degree of polymerization of the polyvinyl alcohol, (b) the viscosity of the adhesive as measured with a Brookfield viscometer is from 200 to 6000 mPa.s at 85 °C, and the viscosity increase of the adhesive is in the range of 1.5 to 8 times when the temperature of the adhesive is decreased from 85 to 30 °C, (c) the clay is kaolinite, halloysite, pyrophyllite and/or sericite, and the pH of the adhesive is not higher than 6, (d) the adhesive is applied with a temperature of 60 to 95 °C onto the flute tips of the corrugated medium, and the corrugated medium coated with the adhesive is utilized for the production of a corrugated board. A process according to claim 1, wherein the adhesive used additionally contains a water-soluble boron compound, the ratio of the water-soluble boron compound to the polyvinyl alcohol being not more than 15 parts by weight of boron compound to 100 parts by weight of polyvinyl alcohol. A process according to claims 1 and 2, wherein the viscosity (A) of the adhesive as measured at 60 to 95 °C with a Hercules viscometer under the shear rate of 3 x 10 ⁴ sec⁻¹ and the viscosity (B) of the adhesive as measured at the same temperature with a Brookfield viscometer are related by the following formula (iv): 1 ≦ A / √ B ≦ 6.3 (iv) and the viscosity of the adhesive as measured with a Brookfield viscometer is in the range of 300 to 3000 mPa.s at 85 °C. A process according to claims 1 and 2, wherein the clay has an average particle size of not more than 2 µm. A process according to claims 1 and 2, wherein the viscosity of the adhesive as measured with a Brookfield viscometer is from 300 to 4000 mPa.s at 60 to 95 °C. A process according to claims 1 and 2, wherein the polyvinyl alcohol used comprises 10 to 90 parts by weight of a polyvinyl alcohol having a degree of polymerization of 200 to 800 and 90 to 10 parts by weight of a polyvinyl alcohol having a degree of polymerization of 1100 to 3500. A process according to claims 1 and 2, wherein the polyvinyl alcohol used comprises 10 to 90 parts by weight of a polyvinyl alcohol (a) having a degree of hydrolysis of 72 to 100 mol% and 90 to 10 parts by weight of a polyvinyl alcohol having a degree of hydrolysis lower by 2 to 30 mol% than that of (a). A process according to claims 1 and 2, wherein a metal salt of phosphoric acid, sodium polyacrylate, sodium alginate and/or guar gum are added to the adhesive in an amount of 0.01 to 2 parts by weight to 100 parts by weight of clay. A process according to claims 1 and 2, wherein a peroxide is added to the adhesive in an amount of 0.01 to 10 parts by weight to 100 parts by weight of polyvinyl alcohol. A process according to claims 1 and 2, wherein saccharose, colloidal silica, calcium carbonate, carbon black, white carbon, and/or silica anhydride are added to the adhesive in an amount of 0.1 to 20 parts by weight to 100 parts by weight of clay. A process according to claim 2, wherein the viscosity as measured with a Brookfield viscometer of the solution excluding solids other than polyvinyl alcohol and water-soluble boron compound from adhesive is 100 to 1500 mPa.s at 60 to 95 °C. An adhesive which consists essentially of polyvinyl alcohol, clay and a water-soluble boron compound, the ratio of clay to polyvinyl alcohol being in the range of 200 to 600 parts by weight of clay to 100 parts by weight of polyvinyl alcohol, the ratio of the water-soluble boron compound to the polyvinyl alcohol being not more than 15 parts by weight of boron compound to 100 parts by weight or polyvinyl alcohol, the solid content of the adhesive being 25 to 60 % by weight characterized in that (a) the concentration of the polyvinyl alcohol in the adhesive solution excluding solids other than polyvinyl alcohol from adhesive is in the range defined by the following formulas: Y ≧ -0.0014 x + 9.4 (i) Y ≦ -0.0061 X + 26.3 (ii) 200 ≦ X ≦ 3500 (iii) wherein Y is defined by the following formula: P = Polyvinyl alcohol content in the adhesive (by weight) W = Water content in the adhesive (by weight). and X is the degree of polymerization of the polyvinyl alcohol, (b) the viscosity of the adhesive as measure with a Brookfield viscometer is from 200 to 6000 mPa.s at 85 °C, and the viscosity increase of the adhesive is in the range of 1.5 to 8 times when the temperature of the adhesive is decreased from 85 to 30 °C, and (c) the polyvinyl alcohol contain 10 to 90 parts by weight of a polyvinyl alcohol having a degree of polymerization of 200 to 800, and 90 to 10 parts by weight of a polyvinyl alcohol having a degree of polymerization of 1100 to 3500, (d) the clay is kaolinite, halloysite, pyrophyllite and/or serisite, and the pH of the adhesive is not higher than 6. The adhesive according to claim 12, wherein the polyvinyl alcohol comprises 10 to 90 parts by weight of a polyvinyl alcohol (a)having a degree of hydrolysis of 72 to 100 mol% and 90 to 10 parts by weight of a polyvinyl alcohol having a degree of hydrolysis lower by 2 to 30 mol% than that of (a). An adhesive which consists essentially of polyvinyl alcohol, clay and a water-soluble baron compound, the ratio of clay to polyvinyl alcohol being in the range of 200 to 600 parts by weight of clay to 100 parts by weight of polyvinyl alcohol, the ratio of the water-soluble baron compound to the polyvinyl alcohol being not more than 15 parts by weight of baron compound to 100 parts by weight of polyvinyl alcohol, the solid content of the adhesive being 25 to 60 % by weight characterized in that (a) the concentration of the polyvinyl alcohol in the adhesive solution excluding solids other than polyvinyl alcohol from adhesive is in the range defined by the following formulas: Y ≧ -0.0014 X + 9.4 (i) Y ≦ -0.0061 X + 26.3 (ii) 200 ≦ X ≦ 3500 (iii) wherein Y is defined by the following formula: P = Polyvinyl alcohol content in the adhesive (by weight) W = Water content in the adhesive (by weight). and X is the degree of polymerization of the polyvinyl alcohol, (b) the viscosity of the adhesive as measured with a Brookfield viscometer is from 200 to 6000 mPa.s at 85 °C, and the viscosity increase of the adhesive is in the range of 1.5 to 8 times when the temperature of the adhesive is decreased from 85 to 30 °C,and (c) the polyvinyl alcohol comprises 10 to 90 parts by weight of a polyvinyl alcohol (a) having a degree of hydrolysis of 72 to 100 mol% and 90 to 10 parts by weight of a polyvinyl alcohol having a degree of hydrolysis lower by 2 to 30 mol% than that of (a), (d) the clay is kaolinite, halloysite, pyrophyllite and/or serisite, and the pH of the adhesive is not higher than 6. The adhesive according to claims 12 to 14, wherein the viscosity (A) of the adhesive at 60 to 95 °C as measured with a Hercules viscometer under the shear rate of 3 x 10 ⁴ sec⁻¹ and the viscosity (B) of the adhesive as measured at the same temperature with a Brookfield viscometer are related by the following formula (iv): 1 ≦ A / √ B ≦ 6.3 (iv) and the viscosity of the adhesive as measured with a Brookfield viscometer is in the range of 300 to 3000 mPa.s at 85 °C.

Inventors: KOHNO, HIROJI, KOYAKUMARU, TAKATOSHI, OKAYA, TAKUJI, ONISHI, HIROYUKI, SHIRAKAWA, KINICHI, TOKUNO, MASATERU
Applicants: KURARAY CO., LTD., RENGO CO., LTD.
Application year: 1985
Main classification: C08J3
IPC codes: B32B27, C09J129, C09J5, B32B3, B31F1, B31F5