ADHESIVE AND METHOD FOR ITS DRYING AND / OR CURING

Description

[0001] The invention relates to the fields of physics and chemistry and concerns a process for drying and / or curing adhesive, as it can be used, for example, in the paper industry, in particular in the packaging industry (paper, cardboard and carton).

[0002] The packaging market is a steadily growing market in which paper products play a central role. These products are increasingly fulfilling the claim of being an ecologically sound alternative to plastics. They could underscore this claim in the future if the adhesives widely used in paper products, which are often indispensable, were also based on renewable raw materials.

[0003] Paper is a flat material consisting primarily of fibers, usually of plant origin, and is formed by dewatering a fiber suspension on a screen. The resulting fiber fleece is compressed and dried (Wikipedia, keyword "paper").

[0004] Since the development of the packaging industry, there have been efforts and desires to use paste in the form of aqueous emulsions to bond paper and cardboard as quickly as possible. Numerous paste products are known from the state of the art.

[0005] From an ecological, occupational hygiene, and health perspective, there is no better adhesive than paste. Paste is used for adhering water vapor-permeable wallpaper, as well as in bookbinding, corrugated board production, and for the manufacture of papier-mâché, etc. In these applications, the drying and curing time of the parts to be bonded is not as crucial as it is in the packaging industry, for example, where most production processes are based on fast assembly lines. Here, the drying and curing time of the paste plays a crucial role when bonding the parts. Compared to the modern plastic hot melt adhesives used in this production, which often only require 5-6 seconds to bond, pastes have a drying and curing time of several hours under similar bonding conditions and are therefore not suitable for such applications.

[0006] Therefore, pastes have not yet made it onto the market for these assembly-line-manufactured products in the packaging industry. In all other criteria for use (cost, environmental impact, health), pastes would be optimal and more suitable than plastic adhesives for paper and cardboard.

[0007] DE 20 2012 011 582 U1 discloses a method for bonding using hot-melt adhesives, in which assemblies made of wood, plastic, metal, glass, textiles, cardboard, etc., are joined together in a few seconds by heating with an electric current. Thermal heat sources for heating the adhesive surface, such as induction heat, glowing hot materials, flames, liquids, and microwaves, are specified for heat transfer.

[0008] DE 10 2005 044 462 A1 states that during the production of laminate, the bonding of joints and the release of plasticizers are achieved under the influence of energy in the form of thermal energy, ultrasonic energy, high-frequency energy or UV energy.

[0009] DE 693 28 350 T2 describes how a hard sheet is pressed from cellulose pulp and cured using a microwave. The drying time for the sheet is 3 to 5 minutes at a microwave power of 2 to 5 kW per square foot and a frequency of 2.5 GHz.

[0010] Furthermore, DE 693 00 499 T2 and WO 99 / 36624 disclose the production of paper cushions from waste paper pulp or embossed paper wallpaper with water-based glue. Here, too, the materials can be dried using heat energy, such as a microwave.

[0011] According to EP 1 073 796 B1, a paste for faster bonding of parts of the embossed wallpaper is produced by irradiation via microwaves (infrared) or other radiation.

[0012] From WO 2015 / 084372 A1, an adhesive is known which consists of water, polyol - which can be starch -, a second compound - which can be melamine or urea - and a crosslinker - which can be citric acid.

[0013] According to WO 2017 / 072186 A1, an aqueous crosslinkable adhesive is known which consists of a carbohydrate compound and a first and second crosslinker, wherein the crosslinkers are different from each other and the second crosslinker is capable of undergoing radical polymerization.

[0014] Furthermore, EP 3 034 555 A1 discloses a crosslinkable resin which is an aqueous dispersion of components a) to c), wherein a) a water-insoluble natural starch, b) a non-polymeric polycarbocyclic compound which may be, for example, the anhydride of citric acid and c) a polyol which may be an alkylene glycol or a sugar alcohol.

[0015] A disadvantage of the state-of-the-art solutions is that no process for bonding paper products using paste is yet known, in the broadest sense, for use in high-speed assembly line production. The drying and curing times required for paste are still insufficient for this purpose.

[0016] The invention is based on the object of providing a simple and cost-effective method for drying and / or curing a bio-based adhesive which has drying and curing times of ≤ 20 s.

[0017] This object is achieved by the invention defined in the claims. Advantageous embodiments are the subject of the dependent claims.

[0018] In the method according to the invention for drying and / or curing adhesive on the surfaces to be bonded which are in contact with the adhesive, an adhesive which essentially contains at least 30 to 70% by mass of starch, 1 to 15% by mass of at least one dicarboxylic acid and a total water content of 70 to 98% is exposed to the action of electromagnetic waves with wavelengths of 100 mm to 100 cm, wherein the action of the electromagnetic waves is realized with a power of 800 to 1600 watts within a time of 20 to 5 s, wherein within the powers and times the exposure time is the shorter, the higher the power.

[0019] Advantageously, an adhesive containing 40 to 60 wt% starch, 1 to 5 wt% of at least one dicarboxylic acid and 70 to 98 wt% water is exposed to the action of the electromagnetic waves.

[0020] Also advantageously, water, even more advantageously drinking water, distilled water or purified water from the paper industry, is added to the adhesive.

[0021] Furthermore, the effect of electromagnetic waves is advantageously realized by means of microwaves.

[0022] And also advantageously, electromagnetic waves with a power of 880 to 1600 W are applied, with these acting for 10 to 5 s.

[0023] It is also advantageous if the starch and the dicarboxylic acids are mixed before or during the preparation of the adhesive without the addition of water and are subjected to a temperature increase, advantageously up to 200 °C and within at least 30 minutes.

[0024] With the solution according to the invention, it is possible for the first time to provide a simple and cost-effective method for drying and / or curing a bio-based adhesive which has drying and curing times of ≤ 20 s.

[0025] This is achieved by a process in which an adhesive consisting advantageously of 40 wt% potato starch and 10 wt% citric acid is mixed and heated with stirring to a temperature of 155 °C and held there for 1 h and stirred, and further consisting of 50 wt% water, which has been added and the adhesive has been stirred at a temperature of 130 °C for 2 hours and cooled to room temperature (20 °C).

[0026] This is further advantageously also achieved by a process in which an adhesive consisting of 47 wt.% wheat starch or thermoplastic starch or synthetic acetyl starch and 5 wt.% citric acid and / or adipic acid is mixed and heated with stirring to a temperature of 155 °C and held there and stirred for 1 h, and further consisting of 48 wt.% water which has been added and the adhesive has been stirred at a temperature of 130 °C for 2 hours and cooled.

[0027] The starches mentioned below are potato starch, wheat starch, thermoplastic starch or synthetic acetyl starch.

[0028] The dicarboxylic acids mentioned below are citric acid and / or adipic acid.

[0029] Furthermore, it is particularly advantageous if the starch and the dicarboxylic acids are mixed before the production of the adhesive, but also during the production process of the adhesive as a whole, without the addition of water and the mixture is exposed to an increase in temperature.

[0030] The temperature increase may only be carried out to a temperature below the decomposition temperature of the starch and the dicarboxylic acids. The mixture is preferably heated to a temperature of up to 200 °C, and even more preferably to 155 °C. The heating phase can last up to 30 minutes, and / or the holding time at the maximum temperature can be up to 30 minutes.

[0031] By increasing the temperature of the mixture, a "caking" or "melting" of starch and dicarboxylic acids occurs, which has proven to be very advantageous for the further process of adhesive production but also for the properties of the adhesive.

[0032] This mixing and temperature increase without water also makes it possible to achieve short drying and curing times when bonding the adhesive.

[0033] The starch present has advantageously been subjected to a temperature increase before or during production and can advantageously be used as thermoplastic starch and / or biologically produced starch and / or synthetically produced starch. Biologically produced starch is potato starch. If biologically produced starch is used according to the invention, this is particularly advantageous for the adhesive, as all components are biologically produced and thus no environmentally harmful components are used. All such biological components are, of course, biodegradable and renewable raw materials.

[0034] Citric acid or adipic acid or mixtures thereof can be used as dicarboxylic acids.

[0035] Drinking water can be used as water, but distilled water or purified wastewater from the paper industry can also be used.

[0036] The bio-based adhesive that can be dried and / or cured using the process according to the invention is particularly advantageous for use in the paper industry. Within the scope of the present invention, the adhesives that can be dried and / or cured using the process according to the invention are intended to be used in the paper industry, which is a branch of the basic materials industry and deals with the production of paper, cardboard, and paperboard (Wikipedia, keyword "paper industry").

[0037] However, the use of the adhesive that can be dried and / or cured using the process according to the invention is also possible in other industries, in particular where rapid drying and / or curing of the adhesive is of interest.

[0038] The adhesive that can be dried and / or cured using the process according to the invention can use one or a mixture of dicarboxylic acids. This has proven to be beneficial, as the dicarboxylic acids serve as preservatives, modifiers, and / or plasticizers for the starch.

[0039] The adhesive can be produced in various ways. It is advantageous if the starch and the dicarboxylic acids are mixed without the addition of water before or during the adhesive production process and subjected to a temperature increase, advantageously up to 200 °C and within at least 30 minutes. However, it is also possible to produce the adhesive by adding the starch to water and simultaneously or subsequently homogenizing the mixture, advantageously by stirring or using ultrasound. The one or more dicarboxylic acids can be added and homogenized simultaneously with the starch or afterward.

[0040] By increasing the temperature during the homogenization of the adhesive, the time required for homogenization can be significantly shortened. While sufficient homogenization of the adhesive at room temperature requires up to several days, homogenization at higher temperatures usually only requires a few hours. When increasing the temperature, care must be taken to ensure that the temperatures are not used at or above the decomposition temperatures of the components of the adhesive according to the invention.

[0041] For example, the homogenization of biological starch in water can be carried out at temperatures up to 155 °C and sufficient homogenization takes 1 to 2 hours.

[0042] After the adhesive has been produced, it is in a liquid to viscous form.

[0043] After cooling to room temperature, the adhesive can be immediately processed and cured or stored.

[0044] If the adhesive contains a high water content, the adhesive is a suspension that can be processed easily and without problems.

[0045] If the adhesive has a lower water content, it is more viscous and a suspension of adhesive and water can be created by adding water.

[0046] A suspension is a heterogeneous mixture of a liquid and finely distributed solids (particles) (Wikipedia, keyword suspension).

[0047] The suspension of the adhesive and water has a total water content of 70 to 98%, whereby the total water content of the adhesive is advantageously between 70 to 94%, more advantageously between 90 to 92%.

[0048] This suspension can then be used as an "adhesive" in the paper industry or in other applications where rapid drying and / or curing of the adhesive is important.

[0049] The adhesive, in the form of a suspension, is then applied in a more or less thin layer to one of the surfaces to be bonded, advantageously paper, cardboard, or carton surfaces, and brought into contact with the other surface to be bonded. According to the invention, the surfaces in contact with the adhesive are then at least partially exposed to electromagnetic waves with wavelengths of 100 mm to 100 cm.

[0050] Such electromagnetic waves can advantageously be microwaves. A further advantageous wavelength range for the electromagnetic waves used is the range from 10 cm to 100 cm.

[0051] According to the invention, the electromagnetic waves are applied with a power of 800 to 1600 watts within a time of 20 to 5 seconds.

[0052] According to the invention, it is important that the exposure time of the electromagnetic waves to the surfaces to be bonded with the adhesive is shorter, the higher the power of the electromagnetic waves is.

[0053] With advantageous electromagnetic wave powers of 880 to 1600 W, only 10 to 5 seconds are required to achieve a good to very good bond. The good to very good bonding effect of the adhesive is achieved when it is as completely dry and / or cured as possible.

[0054] Optimal curing times of ≤ 10 s could be achieved.

[0055] Also of particular importance for the inventive solution is that it has been discovered that the total water content of the adhesive in suspension form is between 70 and 98%, with higher water contents leading to a good to very good bonding effect at the same electromagnetic wave power. A total water content of 90 to 94% is particularly advantageous.

[0056] Particularly short curing times for achieving good to excellent bonding properties were achieved at a total water content of 92%. Above a total water content of 94%, longer curing times are required for the same electromagnetic wave power.

[0057] In addition, it has been found according to the invention that in order to achieve maximum adhesive effect, the adhesive with a total water content of 92% can be treated with the shortest exposure times of the electromagnetic waves at all power levels in the range according to the invention and cures within the shortest drying and curing times of ≤ 10 to 5 s.

[0058] It was not to be expected that ever shorter times for achieving a good to very good adhesive effect with the same power of the electromagnetic waves could be achieved with increasing water content of the adhesive in the form of the suspension up to a total water content of 92%, and that these times for this adhesive effect would become longer again with an increasing total water content above 92%.

[0059] However, drying and curing times of ≤ 20 s are achieved throughout the entire range of the invention.

[0060] The heating of materials and liquids using electromagnetic waves in a microwave has been known for decades.

[0061] For example, microwaves are used for bonding wood components, textiles, leather or plastics, whereby the adhesives are native or synthetic hot melt adhesives.

[0062] The adhesive is not a hot melt adhesive, as it does not need to be melted beforehand to achieve the adhesive effect, i.e. to cure it.

[0063] The inventive use of electromagnetic waves for drying and / or curing the adhesives within such short times of ≤ 20 s is not known from prior art solutions.

[0064] The solution according to the invention is particularly advantageous in the paper industry, since paper, cardboard and carton are processed here and very short times for drying and / or curing of the adhesive must be realized within an assembly line production.

[0065] One such application example could be the fastest possible bonding of paper and cardboard for the production of folding boxes. Folding boxes can fulfill all essential packaging functions, including a marketing function. Folding boxes have a market volume of approximately EUR 9 billion in Germany, and the trend is rising.

[0066] Therefore, the use of the adhesive according to the invention could bring significant advantages here.

[0067] The invention is explained in more detail below using several exemplary embodiments. Example 1

[0068] 40 g of Agrana potato starch is mixed with 10 g of citric acid and heated to a temperature of 155 °C while stirring. The mixture is then stirred for 1 hour. 50 g of water is then added and stirred again at 130 °C for 2 hours. The adhesive is then cooled to room temperature (20 °C).

[0069] These starting materials result in an adhesive composition of 40 wt.% starch, 10 wt.% citric acid and 50 wt.% water.

[0070] Varying amounts of water are added to this adhesive to create a practical suspension. The resulting adhesives have total water contents of 72%, 86%, and 92% by mass and produce light-brown adhesives. These are liquid at temperatures of 100-130°C and have a honey-like consistency at room temperature of approximately 20°C.

[0071] These adhesives are applied thinly to cardboard bases and the cardboard part to be joined is placed on the adhesive point and immediately placed in the microwave at a frequency of 2.45 GHz, corresponding to a wavelength of 12.22 cm, and power levels of 800 and 1600 watts.

[0072] With a water content of 72% by mass and a power of 1600 W, a curing time of 15 s is achieved.

[0073] With a water content of 86% w / w and a power of 800 W, the curing time is 10 s.

[0074] With a water content of 92 wt.% and a power of 1600 W, a curing time of 5 s is achieved.

[0075] Afterwards, we immediately checked whether the cardboard pieces were firmly glued together. All adhesives had dried and cured, and the cardboard pieces were all firmly glued together. Example 2

[0076] 10 g of Agrana wheat starch is mixed with 1 g of citric acid and heated to a temperature of 155 °C while stirring. The mixture is then heated and stirred for 1 hour. 10 g of water is then added and stirred again at 130 °C for 2 hours. This adhesive is liquid at 100-130 °C and has a honey-like consistency at room temperature of approximately 20 °C.

[0077] The starting materials result in an adhesive composition of 47 wt% starch, 5 wt% citric acid and 48 wt% water.

[0078] This adhesive is then diluted with water to produce a practical suspension to adjust total water contents of 72 wt%, 86 wt% and 92 wt%.

[0079] These adhesives in the form of suspensions are applied thinly to cardboard bases and the cardboard part to be joined is applied to the adhesive point and immediately placed in the microwave at a frequency of 2.45 GHz, corresponding to a wavelength of 12.22 cm, and powers of 800 and 1600 watts.

[0080] With a water content of 72% by mass and a power of 1600 W, a curing time of 20 s is achieved.

[0081] With a water content of 86% w / w and a power of 800 W, the curing time is 12 s.

[0082] With a water content of 92 wt.% and a power of 1600 W, a curing time of 8 s is achieved.

[0083] Afterwards, we immediately checked whether the cardboard pieces were firmly glued together. All adhesives had dried and cured, and the cardboard pieces were all firmly glued together. Example 3

[0084] 30 g of SOLANYL C1201 as a thermoplastic starch is mixed with 1 g of adipic acid and heated to a temperature of 155 °C while stirring. The mixture is then heated and stirred for 1 hour. 10 g of water is then added and the mixture is stirred again at 130 °C for 2 hours. This adhesive is liquid at 100-130 °C and has a honey-like consistency at room temperature of approximately 20 °C.

[0085] The starting materials result in an adhesive composition of 47 wt% starch, 5 wt% adipic acid and 48 wt% water.

[0086] This adhesive is then diluted with water to produce a practical suspension to adjust total water contents of 72 wt%, 86 wt% and 92 wt%.

[0087] These adhesives in the form of suspensions are applied thinly to cardboard bases and the cardboard part to be joined is applied to the adhesive point and immediately placed in the microwave at a frequency of 2.45 GHz, corresponding to a wavelength of 12.22 cm, and powers of 800 and 1600 watts.

[0088] With a water content of 72% by mass and a power of 1600 W, a curing time of 20 s is achieved.

[0089] With a water content of 86% w / w and a power of 800 W, the curing time is 12 s.

[0090] With a water content of 92 wt.% and a power of 1600 W, a curing time of 8 s is achieved.

[0091] Afterwards, we immediately checked whether the cardboard pieces were firmly glued together. All adhesives had dried and cured, and the cardboard pieces were all firmly glued together. Example 4

[0092] 20 g of synthetic acetyl starch is mixed with 0.5 g of citric acid and 0.5 g of adipic acid and heated to a temperature of 155 °C while stirring. The mixture is then stirred for 1 hour. Then, 10 g of water is added and the mixture is stirred again at 130 °C for 2 hours. This adhesive is liquid at 100-130 °C and has a honey-like consistency at room temperature of approximately 20 °C.

[0093] The starting materials result in an adhesive composition of 47 wt% starch, 5 wt% citric acid and adipic acid and 48 wt% water.

[0094] This adhesive is then diluted with water to produce a practical suspension to adjust total water contents of 72 wt%, 86 wt% and 92 wt%.

[0095] These adhesives in the form of suspensions are applied thinly to cardboard bases and the cardboard part to be joined is applied to the adhesive point and immediately placed in the microwave at a frequency of 2.45 GHz, corresponding to a wavelength of 12.22 cm, and powers of 800 and 1600 watts.

[0096] With a water content of 72% by mass and a power of 1600 W, a curing time of 20 s is achieved.

[0097] With a water content of 86% w / w and a power of 800 W, the curing time is 12 s.

[0098] With a water content of 92 wt.% and a power of 1600 W, a curing time of 8 s is achieved.

[0099] Afterwards, we immediately checked whether the cardboard pieces were firmly glued together. All adhesives had dried and cured, and the cardboard pieces were all firmly glued together.

Claims

1. Method for drying and / or curing adhesive on the surfaces to be bonded that are in contact with the adhesive, wherein an adhesive substantially containing at least 30% to 70% by mass of starch, 1% to 15% by mass of at least one dicarboxylic acid and a total water content of 70% to 98% is exposed to the activity of electromagnetic waves having wavelengths from 100 mm to 100 cm, where the activity of the electromagnetic waves is realized with a power of 800 to 1600 watts within a time of 20 to 5 s, and where within the powers and times, the higher the power, the shorter the activity time.

2. Method according to Claim 1, wherein an adhesive containing 40% to 60% by mass of starch, 1% to 5% by mass of at least one dicarboxylic acid and 70% to 98% by mass of water is exposed to the activity of the electromagnetic waves.

3. Method according to Claim 1, wherein water, advantageously drinking water, distilled water or purified water from the paper industry is added to the adhesive.

4. Method according to Claim 1, wherein the activity of electromagnetic waves is realized by means of microwaves.

5. Method according to Claim 1, wherein electromagnetic waves with a power of 880 to 1600 W are employed and they act for 10 to 5 s.

6. Method according to Claim 1, wherein the starch and the dicarboxylic acids before or during the production of the adhesive have been mixed without addition of water and subjected to a temperature increase, advantageously up to 200°C and within at least 30 min.