WO2021259636A1

WO2021259636A1 - Aqueous adhesive mixture for gluing paper materials, and method for producing same

Info

Publication number: WO2021259636A1
Application number: PCT/EP2021/065405
Authority: WO
Inventors: Peter Bitomsky; Monika Sebald; Malte Kleemeier; Andreas Hartwig; Anatoli DAVYDOV
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung eingetragener Verein; Papiertechnische Stiftung (Pts)
Priority date: 2020-06-26
Filing date: 2021-06-09
Publication date: 2021-12-30
Also published as: EP4172285A1; DE102020116925A1

Abstract

The present invention relates to an aqueous adhesive mixture for gluing paper materials, which mixture comprises starch and silica sol. The invention also relates to a glued paper product glued using the aqueous adhesive mixture according to the invention, and to the use of an aqueous adhesive mixture according to the invention for gluing paper surfaces together. In addition, the invention relates to a method for producing a corresponding aqueous adhesive mixture.

Description

Aqueous adhesive mixture for gluing paper materials and a method for producing the same

The present invention relates to an aqueous adhesive mixture for bonding paper materials comprising starch and silica sol. It also relates to a bonded paper product, bonded with the aqueous adhesive mixture according to the invention, and the use of an aqueous adhesive mixture according to the invention for adhering paper surfaces. It also relates to a method for producing a corresponding aqueous adhesive mixture.

State of the art:

In paper processing and in particular in the production of corrugated cardboard, adhesive mixtures, in particular starch glue, are used, which connect the individual layers of paper. To cross-link the starch molecules in starch paste and control the rheological and adhesive properties, the addition of borax (sodium tetraborate) has so far been essential. Since 2010 boric acid and sodium borate are from the European Chemical Agency (ECHA) classified as CMR substances (carcinogenic, mutagenic and reprotoxic) "particularly worrying". In 2011, as part of REACH, the classification was tightened, summarizing all boron compounds as SVHC substances (Substances of Very High Concern) and requiring labeling from a content of 0.1%. From 2015, the “Health hazard” label will also be mandatory. On July 1, 2015, ECHA issued the recommendation to include borax in the list of substances according to Annex XIV of the REACH regulation. As a result, use from 2021 would only be possible with a special license.

The consequences for the corrugated cardboard industry would be far-reaching. When trying to adhere to the prescribed limit values, numerous corrugated cardboard manufacturers and processors accept poorer gluing properties and lower production speeds. This has a detrimental effect on the productivity and the quality of the packaging.

If the EU directive on the classification of substances containing boron is tightened up, or if the approval for the use of borax in corrugated cardboard adhesives is completely withdrawn, commercially available starch adhesives can no longer be used for the production of corrugated cardboard. Since boron compounds are irreplaceable in the production of starch glues according to the current state of the art, there is a risk that starch will lose one of the most important markets with the corrugated cardboard industry and will be completely replaced by petroleum-based adhesive systems. Manufacturing process of corrugated cardboard and corrugated cardboard adhesives

Corrugated cardboard is made up of one or more layers of corrugated paper that is glued to one layer or between several layers of another paper or cardboard. Depending on the number of layers of the corrugated paper and the thus glued cover and intermediate layers, there is a subdivision into one-sided, single-wall, two- and three-wall corrugated cardboard. Corrugated cardboard is manufactured in a corrugated cardboard plant (WPA).

The typical manufacturing process is characterized by three work steps:

Embossing of the wave (corrugation),

When the wave crests are glued together, the wave and ceiling are pressed together. In the first step, the paper is heated and moistened in the preheater. This gives the necessary elasticity so that the paper can be deformed. Then, under the effect of pressure and heat, this is passed between two corrugated rollers that mesh like a gear. The wave is shaped ("corrugated"). After the corrugation, vacuum or overpressure systems hold the corrugated paper web on the corrugating roller until it is fixed by gluing it to the top sheet. Glue based on starch is then applied to the tips of the corrugated peaks of the corrugated paper, whereby the corrugation is glued to a smooth cover sheet to form the one-sided corrugated cardboard.

If a second cover sheet is glued to the one-sided corrugated cardboard in a lamination plant, this creates the single-wall corrugated cardboard. If more single-sided corrugated cardboard made of corrugated and smooth paper web is added, the result is multi-corrugated cardboard.

In the heating and pulling section, the corrugated cardboard sheets are pulled over heating plates (up to 180 ° C). The heat required for gluing is supplied to the glue points and the paper. At the same time, excess moisture is also removed. Starch adhesives based on corn, potatoes, wheat and, in small quantities, also based on peas are used almost exclusively for gluing corrugated cardboard. Starch is a vegetable polysaccharide that consists of aD-glucose units. Depending on the fruit and provenance, it consists of different carbohydrate proportions of poorly soluble amylose and more easily soluble amylopectin. The key process in the manufacture of corrugated cardboard is the firm adhesive bond between the corrugated paper web and the liner within a few milliseconds, if possible. The starch adhesives most frequently used for this purpose are preferably manufactured using the so-called Stein-Hall process. To produce a Stein-Hall glue, in the first step approx. 11% by weight to 14% by weight of the total starch content is completely gelatinized in the presence of water and sodium hydroxide solution with the introduction of heat. The starch portion contained in the so-called “primary batch” is also referred to as primary or carrier starch and is used to adjust the viscosity and the water retention capacity. The starch dissolved in the paste also serves as an anti-settling agent for the remaining starch content, which is added in its native granular form in the subsequent step and is referred to as secondary starch. Finally, borax (sodium tetraborate) is added as an additive to the Stein-Hall glues in the state of the art. As a crosslinker, this influences the rheological Properties of the starch adhesives and is mainly used to control the setting speed (acceleration of glue crosslinking), stability and surface wettability. Further additives are plasticizers, defoamers and rheological auxiliaries as required. Fig. 1 shows schematically a flow chart for the production of a Stein-Hall glue with boron.

In general, the mixture contains 11-14% by weight carrier starch and 86-89% by weight secondary starch. The total solids content is about 25-27%. Sodium hydroxide is added to the primary batch in order to reduce the gelatinization point to approx. 53-55 ° C.

Table 1: Exemplary Stein-Hall recipe

Borax as a rheological additive in corrugated cardboard adhesives

In the prior art, boron is added to native starch in the secondary _{batch as borax (sodium tetraborate Na 2} ß ₄ 07 10 H2O) or as boric acid (H3BO3). The reaction between caustic soda and sodium tetraborate when the primary and secondary batches are mixed together produces sodium metaborate, which at high pH values (10-13) forms so-called borax bridges between neighboring starch molecules through a condensation reaction and hydrogen bonds. The three-dimensional cross-linking of the starch molecules by borax results in highly branched polymer chains, which increase the molecular weight of the starch. The significantly increased immobilization of the starch molecules leads to higher viscosity and improved adhesive and fluid properties. shafts of the adhesive. The effect of borax is particularly evident in the gelatinization point of the starch, when the high viscosity of the adhesive is achieved for the optimal combination of liner and corrugated paper - so-called "green bond". In addition to the improved film formation of the starch adhesive on the paper, borax has a positive influence on the water retention capacity of the adhesive. Its high viscosity counteracts the loss of water and primary strength in the adhesive zone (penetration into the fiber interior). In this way, enough starch remains for optimal gelatinization. In addition, borax also acts as a preservative.

Borax as a hazardous substance Boron compounds are extremely harmful to health. The acute effects include irritation to the mucous membranes, gastrointestinal disorders, effects on the central nervous system and skin damage after massive intoxication. Since borax is mainly absorbed through the respiratory tract, breathing and body protection measures are mandatory in glue kitchens. For these reasons, borax is undesirable in packaging that comes into direct contact with food. Borax is classified as a water-polluting substance (WGK 1: slightly water-polluting) and must not be discharged into the wastewater. Since experience shows that boron substances only degrade after 8 to 10 years, they also represent an indirect burden on the environment due to the long retention time in waste paper recycling. Alternative borax-free starch adhesives would make a decisive contribution to environmental relief. Their use could relieve the waste paper cycle and thus contribute to its self-cleaning.

Since 2010, boric acid and sodium borate have been classified by the European Chemical Agency (ECHA) as CMR substances (carcinogenic, mutagenic and reprotoxic) "of particular concern". In 2011, within the framework of REACH, the classification was tightened, summarizing all boron compounds as SVHC substances (Substances of Very High Concern) and requiring labeling from a content of 0.1%. When trying to adhere to the prescribed limit values, numerous corrugated cardboard manufacturers and processors accept poorer gluing properties and lower production speeds. This has a detrimental effect on the productivity and the quality of the packaging.

If the EU directive on the classification of substances containing boron is tightened further, or if the approval for the use of borax in corrugated cardboard adhesives is completely withdrawn, then commercially available starch adhesives for the manufacture of corrugated cardboard cannot be used be used more. Since boron compounds are irreplaceable in the production of starch glues according to the current state of the art, there is a risk that starch will lose one of the most important markets with the corrugated cardboard industry and will be completely replaced by petroleum-based adhesive systems. The patent WO 2011/089053 A1 describes the use of polyacrylic acid which is neutralized with sodium hydroxide solution and in this form forms a gel which is intended to replace borax as a theological additive in whole or in part. In the disclosure, the theological properties of borax-free formulations are described as a function of the shear rate, which are relevant for processing from the adhesive supply to the application. Together with the gel points, the rheological data obtained are compared with a classic Stein-Hall recipe based on borax, and processing advantages are highlighted. However, the temperature-dependent increase in viscosity, which is essential for heat sealing, is not shown. The quality of the bonded joints with regard to dry and wet strengths is also not discussed in detail. Since the further use of borax is kept open as an option within the scope of the claims, the concept as a full replacement is questioned.

In addition to sodium polyacrylate, document DE 10249838 A1 proposes further water-soluble polymer compounds with thickening properties such as carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carrageenan, guar gum, pectin, sodium alginate, carboxylated starch or polyvinyl alcohol. The same concept is followed here as in the idea described above. The basic viscosity of the adhesive is increased, whereby the rheology of pseudoplastic liquids is present, which dilute with the application of shear during the delivery of the adhesive and build up viscosity after application without shear. However, the resulting final viscosity does not come close to the viscosity when using borax, so that special application rollers are described in the context of the process in order to enable the concept to be implemented successfully.

As a substitute for borax, water glasses (sodium and potassium silicates) are proposed in document DE 2629103 A1, which are added in place of borax or in combination with borax and are intended to meet the rheological requirement profile. Due to their inorganic structure, these adhesives are characterized by their pronounced brittleness, so that use in corrugated cardboard bonding is out of the question. Water glasses were already in use as adhesives decades ago, but they did not hold up and were replaced by starch-based systems. Another approach is the substitution of borax by sodium aluminate or aluminum sodium dioxide described in WO 2013087530 A1 when gluing cardboard boxes. When added to the starch glue composition, after the formation of a water-soluble sodium starch aluminate, crosslinking with other starch molecules occurs. Despite the improvements in toxicity and industrial hygiene described as beneficial and a similar rheological effect as with the use of borax, the benefits of this substituent can be viewed as very limited: Due to its irritating and corrosive effect, its use requires personal protective equipment and suitable exhaust ventilation measures in production . A substitution test by the Institute for Occupational Safety and Health of the German Statutory Accident Insurance (IFA) gave sodium aluminates a “medium” rating for acute and chronic health hazards in terms of toxicological effects, which does not meet the requirements of corrugated cardboard manufacturers.

Another disadvantage of sodium aluminate in starch glue is the high amount required to achieve an adhesive effect similar to that of borax. The resulting strongly alkaline pH value means that the secondary starch gelatinizes at low temperatures and no native starch remains to spontaneously stick together. In addition, direct transfer to corrugated cardboard production is difficult due to other process-related conditions in core production. The background to this is the high production throughput, which requires adapted kinetics of the bond strength development for the gluing and for the drying and flatness of the corrugated cardboard.

Another approach to a solution is described in WO 2015104371 A1 in which a substitute mixture based on inorganic salts is selected for the substitution while the process and processing properties remain the same. The mixtures used are salts based on zirconium. According to the information, this approach replaces borax and products containing boron in common starch formulations and glue processing in the corrugated cardboard industry. An equally strong bond strength is propagated here. This additive appears to be suitable as a substitute in glue production and processing for the production of corrugated cardboard. However, the resulting corrugated cardboard tends to have limited wet strengths.

EP 0972812 A1 and DE 102006045384 A1 each describe adhesive formulations which include, inter alia, starch and silica sol, although the use of boron-containing substances in the adhesive formulations described is not limited or even completely excluded. In addition, the problem becomes the necessary reduction of boron contents in adhesive mixtures for bonding paper materials is not recognized or addressed in any of the documents EP 0972812 A1 and DE 102006045384 A1.

In the current state of the art for borax-free gluing of corrugated cardboard, none of the proposed methods has so far been able to establish itself. This is due to the fact that either the rheological and adhesive properties of the borax systems are not achieved or the substitutes shown are also critically assessed from a toxicological point of view. The influence on the rheology is usually limited to the fact that the adhesives are made structurally viscous. However, this only partially achieves the viscosity increase required to build up the “green bond”, so that the use of borax is kept open as an option and the resulting adhesives are not free from boron and its compounds.

Against this background, it was the object of the present invention to provide an adhesive mixture which is suitable for gluing paper materials and comprises a component which at least partially also provides the advantages of boron-containing compounds necessary for the production of corrugated cardboard gluing, so that the amount of boron can be reduced in the adhesive mixture while maintaining the adhesive and processing properties. It was preferred that boron-containing compounds can be completely dispensed with. In order to keep the expense of replacing the adhesive mixture for the manufacturers of paper products, in particular corrugated cardboard manufacturers, as low as possible, the existing machine configurations should preferably be able to remain in place when the adhesive mixture sought is used. It should therefore preferably be possible to adapt the adhesive mixtures to the requirements of current corrugated cardboard production plants so that the industrial use of the adhesive mixtures sought is ensured. With particular preference, one or more of the following or, with particular preference, all points should be possible:

Adjustment of the rheological, procedural and procedural requirements of the new adhesive approaches to the corrugated cardboard production - No mechanical modifications to adapt the corrugated cardboard production to the borax-free adhesives Unchanged production and quality security of the corrugated cardboard

As before, the corrugated cardboard can be recycled without restriction, manufactured with a new adhesive system

This object is achieved by an aqueous adhesive mixture for bonding paper materials, comprising starch and silica sol and comprising <0.5% by weight boron, based on the total weight of the anhydrous adhesive mixture after curing.

Paper materials in the context of the present invention are preferably corrugated cardboard base papers based on cellulose, which can be divided into liner papers and corrugated papers. Depending on the mechanical strength, liner papers can be divided into Kraftliner, Testliner and Schrenz. The latter in particular have a more or less high proportion of waste paper. Corrugated papers are mainly processed for the corrugation and are characterized by their high rigidity. For processing corrugated paper, corrugated pulp and semi-cellulose are available. The basis weights vary based on the type of paper between 50 and 450 g / m ² . Starch in the sense of the present invention corresponds to the definition of starch described above. Preferred starch to be used according to the invention comes from wheat, corn, potatoes, rice, peas and mixtures thereof. Particularly preferred starch comes from wheat, corn, potatoes and their mixtures.

For the purposes of the present invention, silica sols are colloidal dispersions of polysilicic acid particles with 10-60% by weight, preferably 10-30% by weight, of the particles in water. The primary particles present in this state as non-agglomerated are preferably anionically stabilized against agglomeration in the pH range from 1-14, in particular in the pH range> 8. They have mean particle sizes in the range from 1 to 200 nm, preferably 1 to 50 nm, more preferably 1 to 15 nm. The particle sizes can preferably be distributed monomodally or bimodally, more preferably monomodally.

In case of doubt, the mean particle size is determined by means of electromicroscopy, the largest diameter appearing in the electron microscope being the diameter to be evaluated and the numerical mean being determined from at least 100 particles.

The particle size can be determined in solution by means of dynamic light scattering (DLS). The median (D50 value) of the particle size distribution is given as the particle size. Alternatively, the particle size can be determined by transmission electron microscopy. For this purpose, at least 100 particles are measured and a particle size distribution is formed.

In contrast to aqueous dispersions of solid silicas, all manufacturing steps in the manufacture of silica sols preferably take place in the liquid phase. For example, starting from an aqueous alkaline waterglass solution, the particle growth takes place after dealkalization by ion exchange to the required particle size with subsequent particle stabilization (preferred). The polysilicic acid / silicon dioxide particles are preferably predominantly approximately spherical (ie the ratio of the largest to the smallest diameter of a sphere is a maximum of 2, preferably a maximum of 1.5).

In the context of the present invention, it is possible that “silica sol” comprises electrostatically stabilized polysilicic acid / silicon dioxide particles, it being preferred that all corresponding particles are electrostatically stabilized.

It is also possible for “silica sol” in the sense of the present invention to include surface-modified polysilicic acid / silicon dioxide particles, it also being possible for all of the polysilicic acid / silicon dioxide particles to be modified accordingly.

An optional surface modification with aluminum is preferred, which can be done, for example, by adding aluminum salts. Alternatively, the surface can preferably be organically modified by reaction with silanes. By modifying silanes, it is possible to sterically stabilize the particles in addition to the preferably electrostatic stabilization.

Surprisingly, it has been found that metering into silica sol in the context of the present invention represents the provision of a substance which imparts properties similar to or in some cases even more favorable properties in terms of paper bonding than boron-containing compounds to the aqueous adhesive mixture. Although boron is not necessarily excluded as a component of the aqueous adhesive mixture for the purposes of the present invention, it is possible to reduce the boron content considerably or even to do without it, which is of course desirable in the context of the concerns described above about the boron-containing compounds. Accordingly, preference is given to an aqueous adhesive mixture according to the invention, comprising boron in the range from> 0% by weight to <0.05% by weight, or 0% by weight, based in each case on the total weight of the anhydrous adhesive mixture after curing. An aqueous adhesive mixture according to the invention is particularly preferred, comprising boron in the range from> 0% by weight to <0.005% by weight or 0% by weight, based in each case on the total weight of the anhydrous adhesive mixture after curing. An aqueous adhesive mixture according to the invention, comprising no boron, is very particularly preferred.

The effect of borax is based on the cross-linking of free hydroxyl groups of the starch with the formation of boric acid starch esters and / or hydrogen bonds, which result in the required jump in viscosity and the formation of the “green bond”. Alternatively, crosslinking reactions based on silica sols are conceivable, which are present as colloids in an aqueous medium and are stabilized against agglomeration and gelation by electrostatic charging of their particle surfaces. Stabilization is usually carried out by adding alkali, which generates a negative electrical charge in the same direction for all the silica sol particles. If the stabilization is lost, the irreversible agglomeration takes place through the transition from the uncrosslinked sol to the spatially crosslinked gel state. The stability is influenced, among other things, by the particle size and the concentration of the silicon dioxide particles. If the solids concentration increases during the processing of a correspondingly modified paper adhesive in the corrugated cardboard plant due to evaporation, evaporation or water penetration into the paper substrate, the distance between the silica sol particles is reduced until the electrostatic repulsion from the van, which acts at a very short distance Waals attraction is overcompensated and gelation occurs spontaneously, which results in the required sudden increase in viscosity. In their own experiments, the inventors have found that long-term stabilizations of the aqueous adhesive mixture according to the invention are in many cases sensitive to the addition of polyvalent ions. However, it is also possible to use aqueous adhesive mixtures according to the invention without further cations being present, that is to say those in which all cations present originate only from the silica sol component and optionally added sodium hydroxide solution.

Accordingly, an aqueous adhesive mixture is preferred according to the invention, comprising (i) no cations in addition to those of the silica sol and sodium hydroxide solution or (ii) divalent and / or polyvalent cations. As an alternative or in addition to the addition of salt, various silica sol qualities are of interest. It is therefore possible to achieve the desired properties by selecting suitable silica sols. It is basically the case that the larger the specific surface area of the silica sol particles or the smaller the particles, the greater their reactivity. Accordingly, an aqueous adhesive mixture according to the invention is preferred, the silica sol having an average particle size of 1 nm-200 nm, preferably 1 nm-50 nm, more preferably 1 nm -15 nm and / or the specific surface area of the particles in the silica sol being 25 m ² / g-1500 m ² / g, preferably 100 m ² / g-1500 m ² / g, more preferably 200 m ² / g -1000 m ² / g. Of course, the desired properties can also be influenced by the fact that a suitable amount of silica sol is present; accordingly, it is preferred according to the invention that the proportion of silicon dioxide-containing particles from the silica sol in the adhesive mixture according to the invention based on the total weight of the anhydrous adhesive mixture after curing is 1 wt. -% to 15% by weight, preferably 1% by weight to 5% by weight.

In principle, the specific surface area of the particles in the silica sol is determined using the BET method in case of doubt.

In general, the rate of gelation and the strength of the resulting gel depend on the type, concentration, electrolyte concentration and type of electrolyte. The increase in viscosity occurs very quickly after an incubation period (processing time), which speaks in favor of the surprisingly adequate replacement of borax. The alkaline or acidic, anionic or the acidic, cationic types of silica sol, in contrast to borax, do not require labeling according to the Hazardous Substances Ordinance and the corresponding EC guidelines and are toxicologically harmless. Silica sols belong to the class of silicic acids. Other representatives of this class are water glasses, silica gels and precipitated and pyrogenic silicas. All types mentioned can be traced back to monosilicic acid (orthosilicic acid) Si (OH) ₄ and describe a more or less high degree of condensation with the elimination of water to form meta- and polysilicic acids. At the end of this chain is the silica anhydride, known as quartz S1O2. In addition to the silica sols described, the likewise nanoparticulate pyrogenic silicas and the water-soluble water glasses, as alkali salts of low molecular weight silicas, have a low degree of condensation and were in some cases considered to be potential substitutes for borax, while highly condensed silicas in the form of silica gels or precipitated silicas were out of the question for the purposes of the idea.

Consequently, in DE 2629103 A1, water glasses (sodium and potassium silicates) were proposed as an adequate substitute for borax, which were added in place of borax or in combination with borax and should meet the property profile required for the production of corrugated cardboard. However, due to their pronounced brittleness, these adhesives are not used in corrugated cardboard bonding. Water glasses were already in use as adhesives decades ago, but they did not hold up and were replaced by starch-based systems. It was all the more surprising that the inventors' extensive experiments have now shown that silica sols, in particular the preferred aqueous adhesive mixtures described above, can impart similar and in some cases even better properties in terms of bonding paper materials than is the case with boron compounds .

An aqueous adhesive mixture according to the invention is preferred, the starch in both granular and colloidally dispersed form.

It has been found in the context of the invention that this form of starch interacts particularly well with the silica sols to be used according to the invention in order to produce the desired properties in the sense of paper bonding.

According to the invention, an aqueous adhesive mixture according to the invention is preferred, the adhesive mixture being a Stein-Hall glue.

A Stein-Hall glue is produced as described above. In the context of the present invention, it goes without saying that boron is not an essential component of a Stein-Hall glue.

Stein-Hall glues have been used successfully in paper bonding for a long time, especially in the manufacture of corrugated cardboard.

Part of the invention is also a bonded paper product, bonded with an adhesive mixture according to the invention.

These paper products can have a reduced, preferably no, boron content. A paper product according to the invention which comprises a corrugated cardboard is particularly preferred, the corrugated cardboard having been glued with the adhesive mixture according to the invention. In case of doubt, “corrugated cardboard” is to be understood in the sense of the description given above. Part of the invention is also the use of an aqueous adhesive mixture according to the invention for gluing paper surfaces together, in particular for producing corrugated cardboard.

Furthermore, part of the invention is a method for producing an aqueous adhesive mixture according to the invention, comprising the steps a) providing starch, b) providing silica sol, c) optionally providing further constituents and d) dispersing the provided constituents, although this is of course preferred is that the components mentioned are provided in a preferred variant described above.

Part of the invention is also a method for producing a bonded paper product according to the invention, comprising the steps of a) providing an aqueous adhesive mixture according to the invention, b) providing the paper parts to be glued together and c) gluing the paper parts to be glued together with the aqueous adhesive mixture.

In general, within the scope of the invention, through the definition and testing of alternative cross-linking systems, innovative even borax-free Stein-Hall glues could be transferred into industrial practice, taking into account the typical running times of corrugated cardboard machines and economic aspects. The suitability of the new development is based on the range of services of established borax-containing adhesive systems.

The goal of an alternative to borax in corrugated cardboard glue was achieved, among other things, by combining colloidal silica sols in the presence of polyvalent ions in the form of aqueous salt solutions. The use of aqueous salt solutions as a co-additive is a sufficient but not necessary condition in all cases. This means that when using colloidal silica sols according to the invention, the aim can also be achieved without the addition of polyvalent ions in the form of aqueous salt solutions. The successful testing of this system was demonstrated using the example of a silica sol and an aqueous calcium hydroxide solution in the context of pilot plant and operational tests and was proof of its practical suitability. The adhesive mixtures according to the invention are preferably based on the conventional composition and the usual production process for Stein-Hall glues. Essentially, the addition of borax has been replaced by the addition of silica sol and salt solution. The processing and the running properties of the adhesive mixtures according to the invention were inconspicuous in industrial corrugated cardboard plants. The borax-free bonding had no negative impact on the strength and wet strength properties of the corrugated cardboard produced. In addition, the achieved bond strength shaft / ceiling was at least equivalent to the borax-containing reference. The unrestricted recyclability of the packaging material could also be confirmed with the complete replacement of borax. Measured against the performance spectrum of borax-containing glue systems, the expectations of the new additive system are more than fulfilled. The adhesive mixtures according to the invention have the following advantages:

The adhesive mixtures according to the invention allow high processing speeds.

A wet strength of the corrugated cardboard produced that is comparable to that of borax could be demonstrated. - The stacking compressive strengths of the corrugated crates examined are on the level of their borax-containing counterparts.

There is no evidence of aging effects on the strength properties.

The unlimited recyclability of the borax-free corrugated cardboard was confirmed. - The processing properties of the boxes produced are good.

All relevant food and chemical approvals can be achieved without restrictions. Examples:

The presented types of silica sols differ in their particle size distribution and in their specific surface area. With Levasil 200A / 30, there is also a surface-modified variant in which surface-bound silicon has been partially replaced by aluminum.

Table 2: Characterization of the investigated silica sols

Examples 1 to 7: Brabender viscograph measurements of aqueous wheat starch suspensions To investigate the gelatinization and the rheological behavior of a wheat starch / water mixture in the presence of sodium hydroxide solution and various crosslinking additives, subject-specific Brabender viscograph measurements were carried out. The individual mixtures are heated from 30 ° C. to 85 ° C. at a constant heating rate of 1.5 K min- ^1. The temperature is held at 85 ° C. for 5 minutes and then cooled to a final temperature of 45 ° C. ^{at a cooling rate of 1.5 K min -1.} The rotational speed is 75 rev min ^-1. The compositions of the mixtures are shown in Table 3. In addition to the silica sols mentioned, mixtures with typical amounts of borax as a reference and a commercially available crosslinker under the name "Gilunal A", which is recommended in starch adhesives for gluing corrugated cardboard, are also examined for comparison.

Table 3: Composition of aqueous wheat starch suspensions

The influence of the different additives can be compared on the basis of the Brabender curves obtained.

Fig. 2 shows the Brabender viscograph measurements with silica sols in comparison with the established borax system and Gilunal A.

The curve for the mixture from Example 2 shows the measurement profile of a typical starch glue with the addition of borax. The falling curve from a measuring time of 60 minutes is due to the already complete gelation of the glue, so that the measuring device is no longer able to measure further rheological effects. The measurements show that both types of silica sol in different proportions (Examples 4 to 7), in contrast to Gilunal A (Example 3), have a pronounced crosslinking effect which, due to a significant increase in viscosity, decreases as a result of the onset of gelation during the cooling phase a measuring time of 50 minutes. This behavior is comparable to the effect of borax (example 2).

Table 4: Influence of various crosslinking additives on gelatinization behavior

Despite barely visible differences in the crosslinking mechanism of the silica sol examples, the different gelatinization temperatures (see Table 4) show that an increased silica sol addition (7%) seems to slow down the swelling process of the starch granules (delayed gelatinization, visible through higher temperatures at the start and maximum of gelatinization ). This means that the optimum amount of silica sol added is maximized.

Examples 8 to 10: Production of corrugated cardboard under operating conditions In the production of single-wall corrugated cardboard, two borax-containing Stein-Hall glues (Examples 8 and 9) were produced and compared with a borax-free alternative (Example 10) under operating conditions.

Table 3: Compositions of Stein-Hall glues of Examples 8 to 10 The glue preparation proceeds according to the following scheme. The primary starch is added to the primary water heated to 30 ° C and mixed. The agitator is set to approx. 1000 rpm. The sodium hydroxide solution is then stirred in, the speed of the stirrer being increased to 1500 rpm and shearing for 15 minutes. For Examples 8 and 9, the secondary water, the secondary starch and borax are added one after the other. In a departure from this, the preparation of example 10 takes place in that instead of borax, first the silica sol (Levasil CT24PL) and finally calcium hydroxide are added in the form of an aqueous dispersion. The salt is dissolved or dispersed in water beforehand by removing 14 times the amount of secondary water based on the weight of the salt itself.

The stirrer speed is increased to 1500 rpm - max. 2000 rpm - and the Stein-Hall approach is sheared for a further 15 minutes to complete. With regard to solids content, viscosity and gel point, the examples are in the “target range”.

To produce the single-wall cardboard with a B- ^{flute, corrugated paper with 90 g / m 2} from Parenco is used for the flute, inner and outer ceiling. Three tests were carried out at 100, 200 and 250 m / min each.

Experiment A: Connection of shaft and inner ceiling with example 8 as well as connection of the outer ceiling with example 9 (exclusively borax-containing gluing)

In the one-sided machine (single facer), the paper web formed by corrugated rollers is glued to the inner ceiling using Example 8 (containing borax), so that one-sided corrugated cardboard is produced. In the laminating plant (double facer), the corrugated tips of the one-sided corrugated cardboard are glued with glue from Example 9 (containing borax) and the preheated outer cover is connected to the one-sided corrugated cardboard when it enters the dryer section.

Experiment B: Connection of shaft and inner ceiling with example 8 (containing borax) and connection of the outer ceiling with example 10 (borax-free)

In this experiment, too, the corrugation and inner ceiling are glued to the borax-containing example 8, while the outer ceiling is laminated to the one-sided corrugated cardboard with the borax-free alternative (example 10). Experiment C: Connection of the shaft with the inner and outer ceiling using example 10 (exclusively borax-free gluing)

In this experiment, the borax-free alternative (example 10) is used both for the one-sided corrugated cardboard and for the subsequent lamination. Corrugated cardboard samples are taken from all test set-ups and subjected to industry-standard tests for their strength.

1. Edge crush resistance (ECT)

The edge compression test according to DIN EN ISO 3037 or TAPPI T 811 provides information about the strength of the corrugated cardboard when the wave is upright. 2. Flat crush resistance (FCT)

Flat compression tests are tested in accordance with the DIN EN ISO 3035 or TAPPI T 825 standards. In the flat crush test, the single-wall corrugated cardboard sample is loaded perpendicular to its surface. The resistance that corrugated cardboard opposes to this force provides information about its behavior during further processing and use.

The pressure effect at which the wave no longer regresses is represented by the value FCT FmaxLokai.

3. Pin Adhesion Test (PAT)

In the Pin Adhesion Test, according to Tappi T 821, a glued strip of corrugated cardboard is separated on the floor and ceiling with the help of steel needles. The test result is the maximum force that occurs based on the total length of the bond.

4. Box Compression Test (BCT)

In order to determine the maximum load that unfilled corrugated cardboard packaging can withstand before it collapses, the BCT test (compression test) is carried out in accordance with DIN 55440-1. A closed, unfilled packaging is used for testing

(Corrugated box with the FEFCO / ESBO code 0201: 250 x 150 x 200 mm ³ ) initially air-conditioned and then in a compression testing machine between two metal plates placed. Then it is loaded by the advance of a pressure plate until the corrugated cardboard packaging fails. This enables statements to be made regarding the stability properties of the boxes.

ECT

FCT

FCTFmaxLocal

PAT: Bonding Single Facer (inner ceiling)

PAT: Bonding Double Facer (outer ceiling)

BCT

Table 4: Strength data of corrugated cardboard made with linen from examples 8 to 10

The ECT, FCT, PAT and BCT tests on the finished corrugated cardboard show only marginal differences in relation to the different production speeds. As a result, the borax-free example 10 with silica sol additive can be used in all machine groups of the corrugator without any loss of strength. The different corrugated cardboard samples were examined for their recyclability. The tests included the defibering behavior, the potential for sticky formation (fault-free sheet formation) and the properties of the recovered fiber material with regard to optical inhomogeneities. Based on the tests carried out and in accordance with the criteria of the PTS method PTS-RH: 021/97, the samples are rated as "recyclable". Consequently, the addition of silica sol (Example 10) did not have any adverse effects on the recyclability of corrugated cardboard.

Claims

Expectations:

1. Aqueous adhesive mixture for bonding paper materials, comprising starch and silica sol and comprising <0.5% by weight boron, based on the total weight of the anhydrous adhesive mixture after curing.

2. Aqueous adhesive mixture according to claim 1, comprising boron in the range of

> 0% by weight to <0.05% by weight or 0% by weight, based in each case on the total weight of the anhydrous adhesive mixture after curing.

3. Aqueous adhesive mixture according to claim 1 or 2, comprising boron in the range from> 0% by weight to <0.005% by weight or 0% by weight, based in each case on the total weight of the anhydrous adhesive mixture after curing.

4. Aqueous adhesive mixture according to one of the preceding claims, comprising no boron.

5. Aqueous adhesive mixture according to one of the preceding claims, comprising (i) no cations in addition to those of the silica sol and any sodium hydroxide solution present or (ii) divalent and / or polyvalent cations.

6. Aqueous adhesive mixture according to one of the preceding claims, wherein the silica sol has an average particle size of 1 nm-200 nm, preferably 1 nm-50 nm, more preferably 1 nm -15 nm.

7. Aqueous adhesive mixture according to one of the preceding claims, wherein the proportion of silicon dioxide-containing particles from the silica sol is based on the

Total weight of the anhydrous adhesive mixture after curing is 1% by weight to 15% by weight, preferably 1% by weight to 5% by weight.

8. Aqueous adhesive mixture according to one of the preceding claims, wherein the specific surface area of the particles in the silica sol is 25 m ² / g-1500 m ² / g, preferably 100 m ² / g-1500 m ² / g, more preferably 200 m ² / g is -1000 m ² / g.

9. Aqueous adhesive mixture according to one of the preceding claims, wherein the starch is present in both granular and colloidally dispersed form.

10. Aqueous adhesive mixture according to one of the preceding claims, wherein the adhesive mixture is a Stein-Hall glue.

11th Glued paper product glued with an adhesive mixture according to one of the preceding claims.

12. The bonded paper product of claim 11, wherein the product is corrugated cardboard.

13. Use of an aqueous adhesive mixture according to one of the claims

I to 10 for gluing paper surfaces together.

14. A method for producing an aqueous adhesive mixture according to any one of claims 1 to 10, comprising the steps: a) providing starch, b) providing silica sol, c) optionally providing further components and d) dispersing the provided components.

15. A method for producing a bonded paper product according to claim

I, I or 12, comprising the steps of a) providing an aqueous adhesive mixture according to one of claims 1 to

10, b) providing the paper parts to be glued together and c) gluing the paper parts to be glued together with the aqueous adhesive mixture.