High-concentration starch adhesive

Abstract: The present invention relates to a high-concentration starch adhesive prepared by treating a starch slurry at a temperature of 100°C or above to form a gelatinzied solution and suspending an ungelatinized starch in the gelatinized solution. According to the present invention, the viscosity of the starch adhesive can be kept stable over a long period of time; the problems such as warping due to the expansion and contraction, by the absorption of water, of the material to be adhered, can be solved; and the quantity of heat necessitated in the adhesion step can be greatly reduced.

Description: The present invention relates to a high-concentration starch adhesive having a high concentration of starch and having a viscosity stable over a long period of time and a process for preparing it. In the paper-converting industry, an adhesive prepared by dispersing starch in water, an alcohol or the like and gelatinizing it by heat, steam or the like is usually used. It is difficult to prepare the starch adhesive having a high concentration, because its viscosity becomes high at a low temperature. To obtain a high-concentration starch adhesive, the gelatinized solution must be kept warm or a modified starch or dextrin is necessary to be used as a carrier. To keep the starch warm requires suitable equipments, and the use of the processed starch or dextrin poses problems such as high costs and a decrease of the adhesive power. A laminating adhesive having a relatively high concentration used in the production of corrugated boards is prepared by the Stein-hall method (hereinafter referred to as SH method) wherein a carrier part comprising a starch paste completely gelatinized by an alkali and by heat is mixed with a main starch part which is an ungelatinized starch. The carrier used for maintaining the main starch part, preventing the gelation and increasing the fluidity is prepared by adding sodium hydroxide to raw starch, processed starch or high-amylose starch, the amount of sodium hydroxide being 10 to 16% by weight based on the starch, and dissolving the starch under heating at 50 to 60°C. However, a carrier concentration higher than 15% can not be obtained, because the viscosity is high, even in the presence of sodium hydroxide. Further, although a stable paste can be obtained in the initial stage of the preparation, the main starch is gradually swollen with sodium hydroxide contained in the carrier which increases the viscosity. Therefore, the main starch concentration is limited. Under these circumstances, the maximum total starch concentration of the adhesives prepared by the SH method is 25%. Such a low-concentration adhesive containing a high water content reduces the adhesion speed and retards the drying of the adhesive layer in the adhesion step. Thus, the material to be adhered, such as paper is swollen by the absorption of water, which causes problems, such as warping. Another problem of the low-concentration adhesive is that a lot of energy is necessary for the evaporation of water. Various methods were proposed for stabilizing the viscosity of the adhesives prepared by the SH method and increasing the concentration thereof. For example, a method wherein raw starch is mixed with an acid-treated starch, oxidized starch or dextrin to form a carrier part [Japanese Patent Publication for Opposition Purpose (hereinafter referred to as 'J.P. KOKOKU') No.55-35076), a method wherein an etherified starch or starch oxidized with hypochlorite is used for preparing the carrier part (J.P. KOKOKU Nos.54-1575 and 56-43268), and a method wherein starch is oxidized with perborate to form a low-viscosity carrier [Japanese Patent Unexamined Published Application (hereinafter referred to as 'J.P. KOKAI') No.56-129278]. In these methods, the viscosity of the carrier part is reduced by the oxidation or the like to increase the concentration of the adhesive. However, even when sodium hydroxide is used in these methods, a concentration higher than 30% cannot be obtained. In addition, the adhesive power is reduced by the oxidation of the carrier starch. In another method wherein a high-amylose starch is used as a carrier starch, the increase of the relative amount of amylose in the high-amylose starch is limited in view of breeding and, in addition, when the relative amount of amylose is increased, the paste is seriously gelled at a low temperature. Under these circumstances, a slightly oxidized starch or a low-viscosity high-amylose starch is frequently used as a high-concentration carrier. However, even in such a case, the addition of a strong alkali such as NaOH or KOH in a large amount is indispensable for inhibiting the gelation of the paste, to keep its fluidity and increase the main starch keeping capacity. When such an alkali is used, the ungelatinized main starch added to the carrier part is swollen with the alkali used in a large amount, to make the viscosity of the adhesive unstable. A method wherein a modified starch is formed under high pressure is disclosed in J.P. KOKAI No.51-151345. However, the product has a high viscosity even at a high temperature and it cannot be used as an adhesive at ambient temperature. DD 234 428 discloses a mixture of starch and emulgator (such as a fatty acid monoester) and water which when heated to a temperature of 90 to 120°C does not gelatinize but forms a starch which is dispersible in boiling water. It is essential for the process disclosed in this document that the starch does not gelatinize so that the temperature treatment of the starch mixture must not be above 120°C and for more than 20 minutes. Summary of Invention The object of the present invention is, therefore, to provide a high-concentration starch adhesive having the following advantages: the adhesion step can be completed rapidly; the problems of the quality due to the swelling and drying of the material to be adhered caused by water can be solved; the quantity of heat necessitated in the adhesion step can be reduced; and the viscosity thereof after mixing with the main starch remains unchanged while time passes. This object is achieved by a high-concentration starch adhesive obtainable by treating a starch slurry which comprises a fatty acid monoester or diester of a polyethylene glycol having the following general formula wherein n represents a numeral of 10 to 200, and R and R' each represent a saturated or unsaturated aliphatic group having 7 to 31 carbon atoms or an aldehyde selected from formaldehyde, acetaldehyde, propionaldehyde, butylaldehyde, isobutylaldehyde, glyoxal, acrolein, crotonaldehyde, propiolaldehyde and benzylaldehyde, at a temperature of 100°C or higher until a gelatinized solution is formed, and then suspending an ungelatinized starch in said gelatinised solution. The inventors have further found that the starch slurry per se treated at 100°C or above is also useful as a high-concentration starch adhesive. The invention also provides a process for producing such starch adhesives. The starch used in the present invention may be any of those usually used in the SH method. They include, for example, corn, wheat, rice, potato, sweet potato, tapioca and sago starches as well as oxidized materials thereof, cationic starches, etherified starches and esterified ones. Water, an alcohol or a mixture of them is used for preparing the slurry of the starch. The alcohol is, for example, ethanol or methanol. The carrier concentration in the present invention can be the maximum concentration as long as the starch can be dispersed in a slurry-forming liquid such as water and as long as it can be transferred to the form of a liquid. It is, for example, 30 to 60%. The gelatinization temperature is 100°C or above, preferably 150°C or above but lower than 210°C. At a temperature lower than 150°C, the viscosity of the starch is lowered, whereas at 210°C or above, excess decomposition occurs. Usually, the starch is gelatinized with vapor of water or the like at 100 to 180°C by means of a jet cooker. The vapor is not immediately liquefied and it does not reduce the viscosity of the starch in such a case. The gelatinization time is usually 5 to 150 min. The amount of the fatty acid ester of polyethylene glycol used is preferably 0.1 to 5% by weight based on the starch. When it is less than 0.1% by weight, the stable and gelatinized carrier having a high concentration cannot be easily obtained and, on the contrary, more than 5% by weight of the ester is economically disadvantageous. From the viewpoint of the stabilization effect and economical factors, the amount of the aldehyde to be used is preferably 0.1 to 10% by weight based on the starch. When the above various additives are used in the present invention, the viscosity is lowered and the time required for the heat treatment is shorter than the time required when they are not used. The fatty acid monoester or diester of polyethylene glycol and the aldehyde are preferably added to the unheated starch slurry to be used as a starting material for the carrier. The lowering of the viscosity can be accelerated by adding 0.01 to 2% by weight, based on the starch, of an acid such as sulfuric acid, hydrochloric acid, phosphoric acid, metaphosphoric acid or boric acid or an oxidizing agent such as a persulfate or perborate, to the starch slurry. However, when the slurry is treated with the acid or oxidizing agent, the starch is apt to be aged, which usually leads to a deterioration of the affinity towards the paper or a decrease of the adhesive power. When the above-described fatty acid ester of polyethylene glycol or the aldehyde is added to the starch slurry, the gelatinized starch will have a high viscosity stability, a high fluidity and a high power of maintaining the main starch keeping capacity at a temperature widely ranging from low to high temperature. The concentration of the gelatinized solution (carrier) thus obtained can be increased to 60% and the viscosity thereof becomes variable over a wide range of 100 to 100,000 cps by varying the treatment temperature and time. Even when the concentration is increased to 60%, the viscosity is changeable depending on the temperature change and it does not gel or lose its fluidity even at room temperature. The high-concentration gelatinized solution (carrier) can be used as an adhesive as it is. Further, the high-concentration starch adhesive can be prepared by mixing the carrier with the medium wherein the main starch is suspended, or by directly throwing the starch powder into the carrier. When the carrier per se is used as an adhesive, the viscosity thereof must be reduced by heating for a relatively long period of time. In this case, usually the concentration is 10 to 50%, gelatinization temperature is 100 to 210°C and gelatinization time is 5 to 150 min. and, preferably, the concentration is 30 to 40%, the gelatinization temperature is 150 to 210°C and the gelatinization time is 30 to 150 min. The viscosity is desirably 500 to 2500 cps. Borax, boric acid or the like can be added to the adhesive to increase green bonding or a small amount of an alkali such as sodium hydroxide can be added thereto in order to improve the paper-wetting properties or to lower the gelatinization temperature of the main starch. The following examples will further illustrate the present invention. Reference Example 1 40 parts of corn starch was mixed with 60 parts of water to form a starch suspension, which was passed through a reaction tube having a diameter of 12.7 mm and a length of 3760 mm, which tube was heated at 180°C, by means of a pump having an outlet pressure of 15 kgf/cm² and a feeding rate of 6.5 mℓ/min. for 73 min., to obtain a carrier having a concentration of 40% and a Brookfield viscosity of 2150 cps (30°C). On the other hand, 20 parts of corn starch was mixed with 30 parts of water to form a main starch having a concentration of 40%. 50 parts of the carrier prepared as described above was added thereto under stirring to obtain Starch Adhesion A having a total starch concentration of 40%. Example 1 40 parts of corn starch was mixed with 60 parts of water. 0.5%, based on the solid starch, of polyethylene glycol/stearic acid diester was added to the mixture to form a starch suspension, which was passed through a reaction tube having a diameter of 12.7 mm and a length of 3760 mm, which tube was heated at 180°C, by means of a pump having an outlet pressure of 12 kgf/cm² and a feeding rate of 40 mℓ/min. for 11.9 min., to obtain a carrier having a concentration of 40% and a Brookfield viscosity of 9740 cps (30°C). On the other hand, 26.7 parts of corn starch was mixed with 40 parts of water to form a main starch having a concentration of 40%. 33.3 parts of the carrier prepared as described above was added thereto under stirring to obtain Starch Adhesive C having a total starch concentration of 40%. Example 2 30 parts of corn starch was mixed with 70 parts of water. 1.0%, based on the solid starch, of glyoxal was added to the mixture to form a starch suspension, which was passed through a reaction tube having a diameter of 12.7 mm and a length of 3760 mm, which tube was heated at 180°C, by means of of a pump having an outlet pressure of 12 kgf/cm² and a feeding rate of 62.5 mℓ/min. for 7.6 min. to obtain a carrier having a concentration of 30% and a Brookfield viscosity of 25,700 cps (30°C). On the other hand, 33.3 parts of corn starch was mixed with 43.7 parts of water and 0.8 part of borax, to form a main. 22.2 parts of the carrier prepared as described above was added thereto under stirring to obtain Starch Adhesive D having a total starch concentration of 40%. Starch Adhesives C and D were used for double-face laminating corrugated boards. The results are shown in Table 1. Example 1 C Example 2 D SH method Starch concentration (%) 40 40 22 Brookfield viscosity (cps) 153 523 350 Viscosity stability good good bad Adhesive added (wet) (g/m²) 12.5 12.0 22.7 Adhesive strength (kgf) (double face side) 27.1 25.8 25.6 Green bonding O ○ O ○ ○ Warping good good bad Drying energy (kcal/m²) 2.23 2.24 2.47 Note 1) Combination: K220×SCP125×K220 A flute Note 2) Adhesive strength: JIS Z 0402 Note 3) Green bonding: determined from the double-face shearing adhesive power by peeling the boards, 20 sec. after the lamination. Examples 3 to 7 The same procedure as that of Example 1 was repeated except that a monoester or diester shown in the following Table 2 was used as a fatty acid monoester or diester of polyethylene glycol represented by the following formula and that the conditions shown in Table 2 were employed, and the properties of the paste thus formed were examined: For comparison, a starch adhesive was prepared by the conventional SH method and the properties thereof are also shown in Table 2. It is apparent from Table 2 that the adhesives of the present invention have a higher concentration and more excellent viscosity stability than those of the adhesive prepared by the SH method. Examples 8 to 11 The procedure of Example 2 was repeated except that an aldehyde shown in Table 3 was used and that the conditions shown in Table 3 were employed, and the properties of the paste thus formed were examined. For comparison, a starch adhesive was prepared by the conventional SH method and the properties thereof were also determined. The results are shown in Table 3. It is apparent from Table 3 that the adhesives of the present invention have a more excellent viscosity stability than that prepared by the conventional SH method, although its concentration is higher than that of the latter. Examples 12 and 13 and Comparative Example 1 These examples and comparative example relate to a starch adhesive comprising only a carrier. Starch suspensions having a starch concentration shown in Table 4 were prepared as Examples 12 and 13 and Comparative Example 1. Then, the same procedure as that of Example 1 was repeated in Example 12 and the same procedure as that of Example 2 was repeated in Example 13 except that the gelatinization temperature was changed to 200°C to form carriers. On the other hand, a carrier was prepared in Comparative Example 1 in the same manner as that of Reference Example 1 except that the dissolution was conducted under atmospheric pressure. The properties of the carrier adhesives thus obtained are shown in Table 4. By peeling the layers immediately after the lamination, it was revealed that the paper layers were deeply and firmly adhered in Examples 12 and 13. In contrast, the sample in the comparative example had a high water content and the adhesion was insufficient. After air-drying for two days, warping was observed in Comparative Example 1. Example 14 30 parts of corn starch were mixed with 70 parts of water. 0.1% of a stearic monoester (the alkyl group has 17 carbon atoms) were added to the mixture and were passed through a reaction tube having a diameter of 12.7 mm and a length of 3760 mm, which tube was heated to 130°C, by means of a pump having an outlet pressure of 10 kgf/cm² and a feeding rate of 1.1 mℓ/min. for 433 min. to obtain a carrier having a concentration of 30% and a Brookfield viscosity of 87,500 cps (30°C). On the other hand, 40 parts of corn starch were mixed with 60 parts of water to form the main starch having a concentration of 40%. 20 parts of the carrier prepared as described above was added thereto under stirring to obtain Starch Adhesive E having a total starch concentration of 32%. Starch Adhesive E was used for double-face laminating corrugated boards. The results are shown in Table 5. Example 14 E Starch concentration (%) 32 Brookfield viscosity (cps) 720 Viscosity stability good Coating amount (wet) (g/m²) 12.0 Adhesive strength (kgf) (double face side) 25.7 Initial adhesive power O ○ Warping none Drying energy (kcal/m²) 2.31 Note 1) Paper structure: K220×SCP125×K220 A flute Note 2) Adhesive strength: JIS Z 0402 Note 3) Initial adhesive power: determined from the double-face shearing adhesive power by peeling the boards, 20 sec. after the lamination. A paste having a high concentration and a low viscosity has been prepared by treatment with an oxidizing agent in the prior art. In this process, however, the molecule of the starch dissolved in water is gradually cut from its end and, therefore, the product has a quite low molecular weight and a very low adhesive power. Another reason is that the pyranose ring is cut. Although the mechanism has not been theoretically elucidated, the adhesive power can be maintained strong in the present invention because the starch molecule is cut at any desired portion by only heat at high temperature in a short period of time and because the glycoside bond is cut by the thermal decomposition. According to the present invention, the carrier having a high concentration is prepared by gelatinizing a starch at high temperature of 100°C or above and it is mixed with the main starch to form the adhesive having a viscosity which can be kept stable over a long period of time. The adhesive thus formed can be stored and reused conveniently. The present adhesion helps to achieve a rapid processing of paper or preparation of a corrugated board. Another advantage of the invention is that the quality problems, such as warping due to the expansion and contraction of the material to be adhered, caused by the absorption of water, can be avoided,and the quantity of heat required for the adhesion step can be greatly reduced.

Claims: A high-concentration starch adhesive obtainable by treating a starch slurry which comprises a fatty acid monoester or diester of a polyethylene glycol having the following general formula wherein n represents a numeral of 10 to 200, and R and R' each represent a saturated or unsaturated aliphatic group having 7 to 31 carbon atoms or an aldehyde selected from formaldehyde, acetaldehyde, propionaldehyde, butylaldehyde, isobutylaldehyde, glyoxal, acrolein, crotonaldehyde, propiolaldehyde and benzylaldehyde, at a temperature of 100°C or higher until a gelatinized solution is formed, and then suspending an ungelatinized starch in said gelatinized solution. The adhesive of claim 1, wherein said temperature is 150 to 200°C. The adhesive of claim 1, wherein the starch concentration of said gelatinised solution is 30 to 60% by weight. The adhesive of claim 1, wherein said fatty acid monoester is contained in an amount of 0.1 to 5% by weight based on the total weight of starch. The adhesive of claim 1, wherein said aldehyde is contained in an amount of 0.1 to 10% by weight based on the total weight of starch. A high-concentration starch adhesive obtainable by treating a starch slurry which comprises a fatty acid monoester or diester of a polyethylene glycol having the following general formula wherein n represents a numeral of 10 to 200, and R and R' each represent a saturated or unsaturated aliphatic group having 7 to 31 carbon atoms or an aldehyde selected from formaldehyde, acetaldehyde, propionaldehyde, butylaldehyde, isobutylaldehyde, glyoxal, acrolein, crotonaldehyde, propiolaldehyde and benzylaldehyde, at a temperature of 100°C or above until a gelatinized solution is formed. The adhesive of claim 6, wherein said temperature is 150 to 200°C. The adhesive of claim 6, wherein the starch concentration of said gelatinized solution is 30 to 60% by weight. The adhesive of claim 6, wherein said fatty acid monoester or diester is contained in an amount of 0.1 to 5% by weight based on the total weight of starch. The adhesive of claim 6, wherein said aldehyde is contained in an amount of 0.1 to 10% by weight based on the total weight of starch. A process for producing a high-concentration starch adhesive, comprising treating a starch slurry which comprises a fatty acid monoester or diester of a polyethylene glycol having the following general formula wherein n represents a numeral of 10 to 200, and R and R' each represent a saturated or unsaturated aliphatic group having 7 to 31 carbon atoms or an aldehyde selected from formaldehyde, acetaldehyde, propionaldehyde, butylaldehyde, isobutylaldehyde, glyoxal, acrolein, crotonaldehyde, propiolaldehyde and benzylaldehyde, at a temperature of 100°C or higher until a gelatinized solution is formed and then suspending an ungelatinized starch in said gelatinized solution. The process according to claim 11 wherein the temperature is 150 to 200°C. The process according to claim 11 wherein the starch concentration of said gelatinized solution is 30 to 60% by weight. A process for producing a high-concentration starch adhesive, comprising treating a starch slurry which comprises a fatty acid monoester or diester of a polyethylene glycol having the following general formula wherein n represents a numeral of 10 to 200, and R and R' each represent a saturated or unsaturated aliphatic group having 7 to 31 carbon atoms or an aldehyde selected from formaldehyde, acetaldehyde, propionaldehyde, butylaldehyde, isobutylaldehyde, glyoxal, acrolein, crotonaldehyde, propiolaldehyde and benzylaldehyde, at a temperature of 100°C or above until a gelatinized solution is formed. The process according to claim 14 wherein the temperature is 150 to 200°C. The process according to claim 14 wherein the starch concentration of said gelatinized solution is 30 to 60% by weight.

Inventors: KARUBE NAGAHISA, NAGAI SHIGEKI, SATO HARUO, KARUBE, NAGAHISA, NAGAI, SHIGEKI, SATO, HARUO
Applicants: HONSHU PAPER CO LTD, HONSHU PAPER CO., LTD.
Application year: 1989
Main classification: C09J103
IPC codes: C09J103