Dimensionally stable adhesive containing enzymatically modified starch

Description

[0001] The present invention relates to a dimensionally stable adhesive composition containing enzymatically modified starch and to the use of the adhesive composition as a glue stick.

[0002] Glue sticks are commonly known as cylinders filled with semi-solid adhesive, the contents of which can be pushed out and retracted by turning a screw-type mechanism at the end of the stick. The first glue stick was introduced to the market in 1969, and since then, extensive efforts have been made to replace the petroleum-based adhesives initially used with more sustainable alternatives.

[0003] DE 1811466 describes a glue stick consisting of a shaping framework substance formed from a gel of alkali or ammonium salts of aliphatic carboxylic acids with 8 to 36 carbon atoms in water and / or water-miscible organic solvents, and known water-soluble or water-dispersible adhesives as the adhesive component.

[0004] WO 03 / 029376 relates to a dimensionally stable, soft-abradable glue stick made of an aqueous preparation of a synthetic polymer and a soap gel as a framework-forming component, wherein the aqueous preparation of the synthetic polymer contains an at least largely solvent-free aqueous polyacrylate dispersion or a solid alkali-soluble polyacrylate, both of which have free acid groups.

[0005] WO 2014 / 019001 discloses a glue stick comprising an aqueous preparation of an acrylic acid ester copolymer as the adhesive component and a soap gel as the framework-forming substance, wherein sodium salts of C 12 to C 22 fatty acids, in particular C 14 to C 18 fatty acids, are contained as the gel structure-forming soap and wherein the acrylic acid ester has an acrylic acid content of 10-25%.

[0006] The disadvantage of the glue sticks described in the prior art is that they continue to rely on petrochemical components. Another approach to creating glue sticks based on renewable raw materials is the use of starch.

[0007] DE 19908560 describes a dimensionally stable, softly abradable glue stick, consisting of an aqueous preparation of starch ethers and sucrose as the adhesive component and a soap gel as the shaping framework structure.

[0008] EP 2455436 discloses a highly homogeneous adhesive composition comprising enzymatically branched starch having a viscosity stability index SI of less than or equal to 1.3, determined by the quotient of the viscosity after 14 days and the viscosity after 2 hours after preparation of the composition.

[0009] WO 2012 / 110594 relates to a glue stick containing an aqueous preparation of at least one hydroxyalkylcarboxymethyl starch having a viscosity of less than 2,000,000 mPas, determined as a 40% by weight aqueous solution using a Brookfield viscometer RTV at 20 °C, spindle 7, 20 rpm, and soap, as well as its production and use for the surface bonding of substrates, in particular for bonding paper, cardboard, wood and / or plastic.

[0010] WO 2014 / 200344 describes an aqueous adhesive composition comprising highly branched starch (HBS), obtained by reacting a starch or a starch derivative with a glycogen branching enzyme (EC 2.4.1.18), and a carboxylmethyl (CM) polysaccharide derivative. US2012 / 121873A1 discloses glue sticks made of enzymatically modified starch and kaolin and proposes the use of fatty acid esters for hydrophobization and sugars, urea, and nitrate for moisture regulation.

[0011] The starch-based glue sticks available in the prior art also have the disadvantage that they still contain components that originate from fossil resources and cannot be obtained at all or only partially from renewable, plant-based raw materials. Therefore, there is still a need for sustainable compositions that can be used as glue sticks. It is therefore the object of the present invention to provide a dimensionally stable adhesive in which the conventionally used chemically modified starches are replaced by alternatively processed starches, while ensuring that such adhesives exhibit high adhesive strength and dimensional stability as well as good abrasion resistance.

[0012] It has surprisingly been found that this object is achieved by a water-based, dimensionally stable adhesive composition as defined in claim 1. Preferred embodiments are described in the subclaims.

[0013] A first subject of the present invention is therefore a water-based, dimensionally stable adhesive composition containing, in each case based on the total weight of the adhesive composition, a) at least 10% by weight of solubilised and enzymatically modified starch with a dynamic viscosity of less than 500,000 mPas, measured according to Brookfield, obtainable by modifying native starch using a glycogen branching enzyme (EC 2.4.1.18); b) at least 20% by weight of sucrose; c) at least 3% by weight of builder substance selected from C 12 to C 22 fatty acids and / or their salts, wherein the total proportion of starches, in particular digested and enzymatically modified starches, and sucrose is below 65% by weight based on the total weight of the adhesive and the weight ratio of starches, in particular digested and enzymatically modified starches, to sucrose is less than 0.70.

[0014] The dynamic viscosity of the starch can be measured as a 40 wt.% solution in deionized water (x <1 µScm -1< ) ​​at a temperature of 20 °C according to Brookfield at a speed of 20 rpm with spindle 7 at a dynamic viscosity above 50,000 mPas, with spindle 6 at a dynamic viscosity up to 50,000 mPas and spindle 4 at a dynamic viscosity below 10,000 mPas using a Brookfield Viscometer DV2T extra.

[0015] For the purposes of the present invention, solubilized starch is water-soluble starch obtainable from the native starch granule by gelatinization. Gelatinization is an irreversible, usually thermally induced process in which the starch granule is swollen in the presence of water, the starch passes into the aqueous phase, and a polymeric solution and / or dispersion is formed. Starch solubilized can be chemically modified or degraded and also enzymatically modified, in particular rearranged. In the context of the present invention, a starch is considered sufficiently solubilized if its dynamic viscosity as a 40 wt. % solution in deionized water at 20°C, measured according to Brookfield at a speed of 20 rpm with spindle 7 after the gelatinization process, is less than 500,000 mPas.In the context of the present invention, the starch portion of the adhesive composition according to the invention comprises any polysaccharide based on α-D-glucose with amylose and amylopectin components. Therefore, in addition to enzymatically modified, digested starch portions, the starch portion also includes chemically modified and / or chemically degraded digested starch, insofar as these are present as the aforementioned polysaccharides.

[0016] To obtain the starch used in the adhesive composition of the invention, the starch can be digested and then converted with a glycogen branching enzyme (EC 2.4.1.18). Normal starch consists of two components: the nearly linear amylose with α-1,4 glycosidic bonds and the α-1,6 branched amylopectin. Branching enzymes are enzymes capable of converting the α-1,4 glycosidic bonds present in amylopectin and amylose into α-1,6 bonds, thereby creating new branching sites. Upon incubation with digested starch, the amylose and / or long side chains of the amylopectin are transferred to form new α-1,6 glycosidic bonds. This leads to a shortening of the average side chain length and a significant reduction in the interaction capacity of the branched molecules.The branching enzyme-modified starch combines a number of functional properties such as low viscosity at high concentrations, no retrogradation and a transparent solution with further functional properties such as sufficiently high wet tack.

[0017] Accordingly, the invention provides the use of a digested and enzymatically modified starch having a dynamic viscosity of less than 500,000 mPas measured according to Brookfield, which is obtained by treating starch or starch derivatives with glycogen branching enzyme (EC 2.4.1.18). The degree of branching can vary and depends on the intended application. Typically, the digested and enzymatically modified starch has a molecular degree of branching of at least 4%, which is therefore preferred. In a preferred embodiment, the digested and enzymatically modified starch as a component of the adhesive composition according to the invention has a molecular degree of branching of at least 5.0%. This provides a highly stable product. Preferably, the degree of branching in this context is at least 5.5%, and is, for example, in the range from about 5.5 to about 6.5%.The degree of molecular branching, as used herein, refers to the relative amount of α-1,6-glycosidic bonds over the total of α-1,6- and α-1,4-glycosidic bonds ((α, -1,6 / (α- 1,6 + α-1,4) * 100%) and can be determined by methods known in the art. Typically, and therefore preferably, the degree of branching in the context of the present invention does not exceed 8.0%, most preferably not exceeding 7.5%.

[0018] Preferably, the starch or starch derivative is first digested before being contacted with the branching enzyme. Starch gelatinization, as mentioned above, is a process in which the intermolecular bonds of the starch molecules are broken in the presence of water and heat, allowing the hydrogen bonding sites (the hydroxyl hydrogen and the oxygen) to absorb more water. This irreversibly dissolves the starch granule. The penetration of water increases the randomness in the overall granule structure and reduces the number and size of the crystalline domains. The gelatinized starch can be adjusted to the desired pH by adding acid or base. Once the desired temperature is reached, the branching enzyme is added, and the solution is held at the desired temperature for the desired time.Alternatively, the branching enzyme can be added to a starch suspension at room temperature and, with stirring, the slurry can be heated to the desired temperature and maintained at that temperature for the desired time.

[0019] The branching enzyme may be derived from any suitable microbial source, preferably it is a thermostable glycogen branching enzyme obtained from a mesophilic or thermophilic organism. Preferably, the glycogen branching enzyme is Aquifex aeolicus, Anaerobranca gottschalkii or Rhodothermus obamensis.

[0020] Within the scope of the present invention, it was surprisingly found that no dimensionally stable adhesive compositions can be obtained from compositions as conventionally described for glue sticks when using digested and modified starch, so that a limitation of the total proportion of starches and sucrose is indicated. Thus, within the scope of the present invention, it has been found that in order to achieve particularly advantageous properties, in particular with regard to dimensional stability and abrasion resistance, the total proportion of starches in the adhesive composition according to the invention should be kept within a strictly defined range. Therefore, an embodiment is preferred in which the total proportion of starches, in particular digested and enzymatically modified starches, and sucrose is below 60% by weight, but preferably above 40% by weight, particularly preferably above 45% by weight.-%, each based on the total weight of the adhesive.

[0021] A glue stick is particularly characterized by a combination of the properties of abrasion resistance, dimensional stability, and compressive strength, which must be coordinated with one another in addition to sufficient adhesive strength. Surprisingly, it was found that the desired combination of properties can be achieved particularly when the adhesive contains starch and sucrose in a weight ratio of less than 0.70. By adjusting this weight ratio, the properties of the adhesive according to the invention could be further improved, with one embodiment being preferred in which the weight ratio of starch, in particular digested and enzymatically modified starch, to sucrose is less than 0.60 and preferably greater than 0.30.At a weight ratio greater than 0.70, the required compressive and abrasion resistance of the adhesive could not be achieved. In the context of the present invention, compressive strength is defined as the maximum load measured when the adhesive in stick form collapses under compressive load parallel to the longitudinal axis. At a weight ratio below 0.30, a decrease in adhesive strength was observed.

[0022] The starch contained in the adhesive according to the invention is characterized by a dynamic viscosity of less than 500,000 mPas. In a preferred embodiment, the dynamic viscosity of the digested and enzymatically modified starch is below 200,000 mPas, preferably below 100,000 mPas, particularly preferably below 50,000 mPas, but is preferably at least 10,000 mPas, the dynamic viscosity being determined as described above. At a dynamic viscosity below 10,000 mPas, in addition to the longer processing at elevated temperature required for low viscosities, an undesired color change of the adhesive was also observed in some cases, so that a dynamic viscosity of at least 10,000 mPas is preferred for most applications. In contrast, it was surprisingly found that at a dynamic viscosity below 200.000 mPas suppresses gel formation and recrystallization in the adhesive and produces a homogeneous and storage-stable adhesive. By further reducing the dynamic viscosity to below 100,000 mPas, preferably below 50,000 mPas, the pumpability and filling properties of the adhesive in the liquid and heated state could be significantly improved. The viscosity range of 10,000 to 50,000 mPas was identified as a good compromise between process time and energy consumption and the provision of sufficiently degraded starches for sufficient adhesive strength and color stability of the adhesive according to the invention. Furthermore, this viscosity range ensures that in the production of cylindrical adhesives, as required for supply as glue sticks, less waste due to incorrect filling of the stick tubes and overall less downtime during filling is achieved.

[0023] The starting starch for the digested and enzymatically modified starch used in the adhesive composition according to the invention is preferably a starch of natural origin. This natural starch can be any conventional tuber, cereal, or legume starch, e.g., pea starch, corn starch, including waxy maize starch, potato starch, including waxy potato starch, amaranth starch, rice starch, including waxy rice starch, wheat starch, including waxy wheat starch, barley starch, including waxy barley starch, tapioca starch, or sago starch. In a preferred embodiment, the digested and enzymatically modified starch is obtainable from cereals, tubers, roots, and / or legumes, preferably obtainable from tubers, particularly preferably from sweet potatoes and / or potatoes, especially preferably from potatoes.

[0024] Starches of natural origin generally have an amylose content of 20% to 40%, depending on the plant species from which they are obtained. This amylose content is also preferred within the scope of the present invention. However, there are also amylopectin-rich starches that have a significantly increased amylopectin content, or products that contain an increased amylose content. Starches with a high amylopectin content can also be digested and enzymatically degraded within the scope of the present invention for adhesive compositions according to the invention; starches with amylopectin contents above 80% can also be used. Amylopectin is already a branched starch, and the inventive enzymatic modification with a branching enzyme can further increase this already natively high branching.

[0025] Since starch is naturally heterogeneous, in addition to the starch molecules within the preferred range according to the invention with the above-mentioned properties, other starch molecules can also be present in the enzymatically branched starches produced in accordance with the invention for the adhesive composition. In preferred embodiments, the proportion of enzymatically modified starch, based on the total starch content, is at least 65 wt. %, preferably at least 80 wt. %, particularly preferably at least 90 wt.

[0026] The adhesive according to the invention is characterized in that it is obtained mainly from natural raw materials. Therefore, an embodiment is preferred in which the proportion of digested and chemically modified starches, in particular hydroxyalkylcarboxymethyl-modified starches, based on the total starch content, is preferably less than 20% by weight, more preferably less than 10% by weight, most preferably less than 5% by weight. The chemically modified starches are in particular those which arise as products of the condensation between the hydroxyl groups of the anhydroglucose units (AGE) of starch molecules and alcoholic hydroxyl groups of other compounds. In a particular embodiment, however, at least 0.1% by weight, preferably at least 1.0% by weight, but preferably less than 5.0% by weight, more preferably less than 3.0% by weight, of the chemically modified starches, based on the total starch content, can be-% of hydroxyalkylcarboxymethyl-modified starches, each based on the total weight of the adhesive, may be advantageous for adjusting the rheological properties during liquid filling of the adhesives according to the invention in the industrial production of glue sticks.

[0027] The backbone structure of the adhesive according to the invention is formed from C 12 - to C 22 -fatty acids and / or salts thereof. In a preferred embodiment, the adhesive according to the invention contains at least 4% by weight, preferably at least 5% by weight, but preferably not more than 10% by weight, of builder substances selected from C 12 - to C 22 -fatty acids and / or salts thereof, based in each case on the total weight of the adhesive. The backbone structure is preferably selected from C 14 - to C 18 -fatty acids and / or salts thereof. The salts of the backbone structure are particularly preferably the sodium, potassium, or calcium salts of the corresponding fatty acids or mixtures of these salts.

[0028] The adhesive of the invention is water-based. A water-based adhesive in the context of the present invention is an adhesive that contains at least 5% by weight of water, based on the total weight of the adhesive. In a preferred embodiment, the adhesive of the invention contains at least 10% by weight, preferably at least 15% by weight, more preferably at least 20% by weight, and especially preferably at least 25% by weight of water, based on the total weight of the adhesive.

[0029] The adhesive composition according to the invention preferably has a pH of at least 9.0, particularly preferably at least 9.5, but preferably not higher than 11.0. The pH of the adhesive composition can be adjusted using conventional alkaline substances such as sodium hydroxide and, as a property of the adhesive composition, is determined directly in the liquefied composition at 80°C using a pH-sensitive glass electrode after calibration with standard buffers.

[0030] In a particularly preferred embodiment, the adhesive composition according to the invention contains the following components: a) 10-25% by weight of solubilised and enzymatically modified starch with a dynamic viscosity of less than 500,000 mPas measured according to Brookfield, obtainable by modifying native starch using a glycogen branching enzyme (EC 2.4.1.18); b) 20-45% by weight of sucrose; c) 3-10% by weight of builder substance selected from C 12 to C 22 fatty acids and / or their salts; d) at least 20% by weight of water; e) less than 5% by weight of solubilised and chemically modified starch; and optionally f1) at least 0.1% by weight, preferably at least 1.0% by weight, but preferably less than 3.0% by weight of hydroxyalkylcarboxymethyl-modified starch and preferably less than 3% by weight, particularly preferably less than 1% by weight, of those digested and chemically modified starches which are not hydroxyalkylcarboxymethyl-modified starch; or f2) less than 3% by weight, preferably less than 1% by weight.-% of solubilized and chemically modified starches. where the information in wt.% refers to the total weight of the adhesive.

[0031] The adhesive composition of the invention is particularly suitable for use as a glue stick. Therefore, the present invention further provides a glue stick formed from the adhesive composition of the invention.

[0032] A further object of the present invention is the use of the glue stick according to the invention for the surface bonding of substrates, in particular for bonding paper, cardboard and / or plastic to one another.

[0033] The present invention is explained in more detail with reference to the following examples, which are in no way to be understood as a limitation of the inventive concept.

[0034] Table 1 summarizes exemplary formulations, with the data given as wt.% of the total weight of the composition. The resulting adhesives were measured, and the results are recorded in Table 1. Table 1: Comparison example Example 1 Example 2 Example 3 Example 4 enzymatically modified starch 20 18 16 16 14 Viscosity of enzymatically modified starch (solution 40%) 15000-25000 mPas Fatty acid C 14 1,9 1,9 1,9 1,9 1,9 Fatty acid C 16 3,72 3,72 3,72 3,72 3,72 Sucrose 25 30 35 40 40 Sodium hydroxide (50% solution) 0,2-1,0 0,2-1,1 0,2-1,2 0,2-1,3 0,2-1,4 Water rest rest rest rest rest Total starch and sucrose content 45 48 51 56 54 Weight ratio starch : sucrose 0,80 0,60 0,46 0,40 0,35 Abrasion resistance unacceptable satisfactory good good good Compressive strength / N 33 65 90 101 96 Adhesive strength on paper N / A very good (paper tear) very good (paper tear) very good (paper tear) very good (paper tear)

[0035] The 10 kN Pro Line testing device from Zwick Roell is used to measure the compressive strength.

[0036] The 30 mm long adhesive strip, cut directly above the piston, is inserted between two holding pieces. These are stainless steel discs with a thickness of approximately 10 mm, each with a circular 3 mm recess adapted to the respective pin diameter. The pin, equipped with the holding pieces, is placed centrally on the test table of the compressive strength tester. The height of the measuring head is adjusted to the height of the test piece. The measuring head is then moved against the pin to be tested at a feed rate of 120 mm / min. The highest compressive force achieved determines the compressive strength in N. For a diameter of 21 mm, the compressive strength of a glue stick should be between 60 and 120 N.

[0037] As can be seen from the data provided, the adhesive composition according to the invention is characterized in particular by a combination of high compressive strength, good abrasion resistance and high adhesive strength.

Claims

1. A water-based, dimensionally stable adhesive composition containing, in each case based on the total weight of the adhesive composition, a) at least 10 wt.% broken down and enzymatically modified starch having a dynamic viscosity of less than 500,000 mPas measured according to Brookfield and obtained by modifying native starch by means of a glycogen branching enzyme (EC 2.4.1.18); b) at least 20 wt. % sucrose; c) at least 3 wt.% builder selected from C12 to C22 fatty acids and / or the salts thereof, wherein the total proportion of starches, in particular broken down and enzymatically modified starches, and sucrose is below 65 wt.% based on the total weight of the adhesive composition, and the weight ratio of starches, in particular broken down and enzymatically modified starches, to sucrose is less than 0.70.

2. The adhesive composition according to claim 1, characterized in that the total proportion of starches, in particular broken down and enzymatically modified starches, and sucrose is below 60 wt.%, but preferably above 40 wt.%, particularly preferably above 45 wt.%, based on the total weight of the adhesive composition.

3. The adhesive composition according to one or both of the preceding claims, characterized in that the weight ratio of starch, in particular broken down and enzymatically modified starch, to sucrose is less than 0.60 and preferably greater than 0.30.

4. The adhesive composition according to one or more of the preceding claims, characterized in that the dynamic viscosity of the broken down and enzymatically modified starch is below 200,000 mPas, preferably below 100,000 mPas, particularly preferably below 50,000 mPas, but is preferably at least 10,000 mPas.

5. The adhesive composition according to one or more of the preceding claims, characterized in that the broken down and enzymatically modified starch is obtained from cereals, tubers, roots and / or legumes, preferably obtained from tubers, particularly preferably from sweet potatoes and / or potatoes, especially preferably from potatoes.

6. The adhesive composition according to one or more of the preceding claims, characterized in that the proportion of enzymatically broken down starches is, based on the total starch proportion, at least 65 wt.%, preferably at least 80 wt.%, particularly preferably at least 90 wt.%.

7. The adhesive composition according to one or more of the preceding claims, characterized in that the proportion of broken down and chemically modified starches, in particular hydroxyalkyl-carboxymethyl-modified starches is preferably less than 20 wt.%, particularly preferably less than 10 wt.%, very particularly preferably less than 5 wt.%, based on the total starch proportion.

8. The adhesive composition according to one or more of the preceding claims, characterized in that said adhesive composition contains at least 4 wt.%, preferably at least 5 wt.%, but preferably no more than 10 wt.% builders selected from C12 to C22 fatty acids and salts thereof, based in each case on the total weight of the adhesive composition.

9. The adhesive composition according to one or more of the preceding claims, characterized in that the salts of the C12 to C22 fatty acids are selected from sodium, potassium and / or calcium salts.

10. The adhesive composition according to one or more of the preceding claims, characterized in that said adhesive composition contains at least 10 wt.%, preferably at least 15 wt.%, particularly preferably at least 20 wt.%, especially preferably at least 25 wt.% water, based on the total weight of the adhesive composition.

11. The adhesive composition according to one or more of the preceding claims, characterized in that the adhesive composition has a pH of at least 9.0, preferably of at least 9.5, but preferably no higher than 11.0.

12. The adhesive composition according to one or more of the preceding claims, containing a) 10-25 wt.% broken down and enzymatically modified starch having a dynamic viscosity of less than 500,000 mPas measured according to Brookfield and obtained by modifying native starch by means of a glycogen branching enzyme (EC 2.4.1.18); b) 20-45 wt.% sucrose; c) 3-10 wt.% builder selected from C12 to C22 fatty acids and / or the salts thereof; d) at least 20 wt.% water; and e) less than 5 wt.% broken down and chemically modified starch; and, where applicable, f1) at least 0.1 wt.%, preferably at least 1.0 wt.%, but preferably less than 3.0 wt.% hydroxyalkyl-carboxymethyl-modified starch and preferably less than 3 wt.%, particularly preferably less than 1 wt.% broken down and chemically modified starches that do not represent a hydroxyalkyl-carboxymethyl-modified starch; or f2) less than 3 wt.%, preferably less than 1 wt.%, broken down and chemically modified starches, wherein the wt.% specifications are based on the total weight of the adhesive composition in each case.

13. A glue stick formed from an adhesive composition according to at least one of claims 1 to 12.

14. The use of a glue stick according to claim 13 for surface-to-surface bonding of substrates, in particular for bonding paper, cardboard and / or plastics material to one another.