WO2024084004A1

WO2024084004A1 - Method for paper finishing, composition for a paper coating, paper and use of the paper

Info

Publication number: WO2024084004A1
Application number: PCT/EP2023/079181
Authority: WO
Inventors: Oliver Vogt; Andreas Geissler; Stephan Felix SCHWAB; Markus BIESALSKI; Eddie Koenders
Original assignee: Technische Universität Darmstadt
Priority date: 2022-10-21
Filing date: 2023-10-19
Publication date: 2024-04-25
Also published as: DE102022127915A1

Abstract

The invention relates to a method for paper finishing, a composition for a paper coating, paper and the use of the paper. The method is distinguished in that thin, homogeneous, visually attractive and flexible paper coatings can be realized. The composition for a paper coating is optimized in conjunction with the method.

Description

Process for paper finishing, composition for a paper coating, paper and use of the paper

The invention relates to processes for paper finishing, a composition for a paper coating, paper and the use of the paper. The process is characterized in that thin, homogeneous, visually appealing and flexible paper coatings can be produced. The composition for a paper coating is optimized in conjunction with the processes.

State of the art

The principle of sustainability is becoming increasingly important within society, which makes paper particularly attractive as a biological, recyclable material based primarily on renewable raw materials. Paper products are already of great importance in our daily lives. With global production of ~ 423 million tonnes (2021), the average global annual consumption is ~ 54 kg of paper per capita (Paper Performance Report for 2023). With a value more than 4 times higher, Germany is even one of the world leaders in paper consumption. It is foreseeable that paper products will penetrate even more fields of application in the future or be used as a substitute for energy-intensive, non-recyclable or ecologically questionable materials. The variety of uses of paper can be impressively demonstrated by the approximately 3000 types of paper that are optimized for a wide variety of application scenarios. The specific properties of these paper products are usually generated by special fibers, additives and coatings. The finishing of paper products, which includes functionalization with additives as well as coatings, has the purpose of implementing properties that a pure fiber fabric, a so-called raw paper, does not have. The functions addressed by these processes can be extremely diverse. In addition to purely optical and haptic properties, such as whiteness, gloss and smoothness of the surface, as well as mechanical properties, e.g. tensile and splitting strength, the interaction with other media can be influenced, particularly via coatings. This plays a role in compatibility with downstream process steps, including printability and bondability, in terms of barrier properties or also in the resistance of paper products, e.g. to water, temperature, light and chemicals. But antimicrobial properties or the influence on fire properties can also be influenced.

Classic systems for finishing paper products using paper coatings contain, in addition to mineral components, in almost all cases synthetic compounds of petrochemical origin. These primarily function as binders to bind inorganic pigments in to fix it to the substrate in a closed film. Globally, the paper industry uses around 3 million tonnes of synthetic compounds every year, including copolymers such as styrene-butadiene and styrene-acrylates. This corresponds to almost 1% of the paper mass produced.

Even though the functionality of these coatings has proven itself, the use of petrochemical raw materials for paper finishing is proving to be ineffective and not very future-proof. In addition to the high consumption of valuable resources, the role of these materials in recycling and biological degradation is increasingly being questioned critically. While the paper fiber itself is characterized by its high recyclability, polymer additives and binders from coatings act as contaminants in the recycling process and are not recycled. Strictly speaking, these polymers must therefore be classified as "single-use plastics".

If the paper products are not subjected to regulated recycling, it should be ensured that the materials released into the environment can be broken down without leaving residues. This requirement is undoubtedly met for cellulose, the main component of paper, but only to a limited extent for other components. In the worst case, after enzymatic decomposition of the fibers, the other polymer components remain unchanged and often finely fragmented, which would make paper a source of microplastics. Many paper users are not aware of this, which leads to a rather careless handling of paper products. This phenomenon is omnipresent in the form of littering. In order to reduce the entry of synthetic and persistent polymers into soils, water bodies and organisms as much as possible, the problem of microplastics and corresponding strategies for dealing with them in the future have been discussed and researched in depth in politics and the media for some time.

Almost all approaches to avoiding petrochemical ingredients in paper focus on the use of bio-based alternatives. Biopolymers are either used as a mixture or chemically modified in such a way that they can be used to replace classic functional chemicals. This strategy is fundamentally justified and can help to reduce the use of fossil raw materials. Depending on the biopolymers used, however, some of these materials are not available in the quantities required by the paper industry, compete with use as food, or have been chemically modified to such an extent that the original advantage of biodegradability has been lost. Taking into account currently applicable regulations and those being planned (the EU's SUPD (Single-Use Plastics Directive) and its PPWR (Packaging and Packaging Waste Regulation), which have been incorporated into national law), this strategy for the use of bio-based alternatives has been made even more difficult, as chemical and enzymatic treatments of natural substances also change the designation of the plastic. and result in bans and limitations on use, regardless of whether this really affects the recyclability and biocompatibility of the material or whether it is only a degradation reaction or a temporary dissolution process.

EP 3426730 B1 discloses heat-treated kaolin pigments, processes for their preparation and applications for paper (coatings) and other coatings. EP 3426730 B1 teaches that the dispersants used in the process should be free of alkali and alkaline earth metals, in particular free of sodium.

EP 0572037 B1 discloses coating pigments for cellulose-containing printing media, in particular for paper and cardboard, which contain at least one swellable layered silicate. Minerals from the smectite group are used as swellable layered silicates, such as bentonite, montmorillonite, hectorite, saponite or nontronite. The smectite group is classified as three-layer clay minerals. These minerals are characterized by non-integer layer charges (X ~ 0.25 to 0.6). This charge of the layers is a basic requirement for the extraordinarily high water retention and thus the swelling capacity of the disclosed representatives of the smectite group. EP 0572037 B1 teaches that the European bentonites with a higher content of exchangeable alkaline earth ions, in particular calcium ions, must be activated with targeted activation using suitable alkali compounds.

EP 0585411 B1 discloses pigments that can be used as coating and filler pigments in the paper industry or in the production of paints and varnishes, including organic and petrochemical binders. EP 0585411 B1 discloses composite particles of titanium dioxide and calcined kaolin, which are mixed and bonded to form uniform particles using cationic polyelectrolytes. These composite pigments can be used in a conventional coating color formulation to coat paper.

It is therefore an object of the invention to provide a process for paper finishing that is environmentally friendly, energetically efficient and sustainable in terms of the raw materials used. In this context, a further object of the invention is to provide a corresponding composition for a paper coating that meets these requirements. Furthermore, it is also an object, under these additional conditions, to provide a paper that is characterized by a thin, smooth, visually appealing, well-adhering and/or flexible coating.

Summary of the invention

In a first aspect, the invention relates to a method for paper finishing comprising the steps: - Providing paper,

- Providing a composition for a paper coating, the composition comprising a metakaolin and a water glass, and optionally additives, the composition preferably being according to this disclosure,

- Applying the composition to the paper, and

- Curing the composition on the paper, whereby the composition applied to the paper is protected from drying out.

In a second aspect, the invention relates to a composition for a paper coating comprising: a) a metakaolin with the following components (in wt. %): 50 to 65 SiO2, 25 to 45 Al2O3, 0 to 5 Fe2O3, 0 to 5 TiO2, and b) a water glass consisting of an aqueous solution of alkali silicate, selected from sodium silicate, potassium silicate and/or lithium silicate, with a solids content of between 10 and 50 wt. %, preferably between 20 and 40 wt. %, based on the mass of the solution, wherein the alkali silicate has a molar modulus, i.e. a SiO2/R2O ratio, of between 1.7 and 3.8, preferably between 2.0 and 2.5, wherein R = Li, K, and/or Na, wherein the mass fraction of the metakaolin in the composition is between 15 and 35 wt. %, and wherein the mass fraction of the water glass in the composition is between 10 to 50 wt.%, preferably between 20 and 40 wt.%.

In a third aspect, the invention relates to a paper which is surface-modified with a paper coating, wherein the paper coating comprises the composition according to this disclosure, and/or wherein the paper coating has resulted from the curing of the composition according to this disclosure.

In a fourth aspect, the invention relates to a use of the paper for packaging or magazines, for decorative purposes as well as for construction materials, paper furniture, in interior design, and/or lightweight construction.

Geopolymers

The term geopolymers refers to inorganic structures that can be produced on the basis of an aluminosilicate source, e.g. fly ash or metakaolin, and an alkaline activator such as water glass. During the production of geopolymers, a cross-linking reaction takes place between water glass and the aluminosilicate source, whereby certain chemical requirements for the water glass and the aluminosilicate source as well as their quantitative composition must be met.

The setting behavior and properties of a geopolymer glue depend essentially on the molar silicon to aluminum ratio of the aluminosilicate source. This ratio is particularly advantageous for highly reactive metakaolin, which is obtained by calcining kaolin at 600 to 750 °C. Geopolymers made from metakaolin usually harden at room temperature, while fly ash-based geopolymers require higher temperatures.

In addition to the material advantages associated with the use of geopolymers as coating materials, such as high fire resistance and chemical resistance as well as excellent mechanical properties, there are numerous reasons that make the use of such a system particularly attractive for paper products.

By using geopolymers, no additional organic substances are introduced into the paper. In addition to the (hoped-for) SUPD compliance, this also offers potential for reducing organic contaminants in recycling, improving process stability, increasing product quality and reducing the chemical oxygen demand in wastewater. Coating with a geopolymer reactive system also makes it possible to save drying energy in the production process.

The inventors have discovered that special geopolymer formulations are suitable for applying thin, smooth, well-adhering, flexible, if required glossy and color-variable layers to paper substrates, which are at least equal to conventional paper coatings, but at the same time do not contain any organic components.

The metakaolin required for the synthesis of the geopolymer, or its starting material kaolinite, is a well-known raw material within the paper industry. Around 40% of the kaolinite mined globally is used as a filler and coating pigment in the paper industry, especially when the focus is on the gloss and smoothness of the products.

The crucial difference of a geopolymer finishing system is that the metakaolin is not cross-linked by softening organic binders, but is reacted by an alkaline activator solution, which then forms a matrix on the paper that hardens with moderate heat input. Alkali silicate solutions, so-called water glasses, are used as activator solutions. Only through complex optimization of the geopolymer recipe and adjustments in the application and Drying procedure makes paper finishing possible within the scope of the invention described here.

Through systematic research, the inventors have discovered that purely inorganic paper finishing systems based on special geopolymer formulations solve the task of addressing the previously mentioned disadvantages of petrochemical raw materials in paper finishing as well as the political and social problems associated with the use of bio-based alternatives.

The selection of the metakaolin is mainly based on the degree of whiteness, which is of particular interest for the appearance of the coated paper, and on the particle size, which has a considerable influence on the liquid requirement of the formulation.

The established coating technologies of the paper industry, such as blade coating, doctor blade application or the cast coating process, are suitable for applying the geopolymer formulation to paper substrates. It should only be noted that the rheology and reactive character of the geopolymers differs considerably from conventional coating colors and therefore certain adjustments and precautions must be taken.

Mineral components

Since mineral components in the form of fillers and pigments are an integral part of paper anyway, this inorganic fraction, which remains after incineration and is therefore referred to as ash, is not subject to any quantitative regulations. The inorganic load cannot actually be biodegraded and, provided it does not contain any heavy metals, is completely uncritical for the biodegradation of the fiber fraction or for the surrounding ecosystem. During recycling, the inorganic material, i.e. usually chalk, kaolin, talcum and titanium dioxide, is broken down together with the secondary fibers and processed into new paper materials.

Description of the invention

Paper finishing processes

In a first aspect, the invention relates to a method for paper finishing comprising the steps:

- Providing paper, - Providing a composition for a paper coating, the composition comprising a metakaolin and a water glass, and optionally additives, the composition preferably being according to this disclosure,

- Applying the composition to the paper, and

By using purely inorganic coating color formulations, petrochemical binders are completely replaced in paper coating and high-quality, SlIPD (Single-Use Plastic Directive)-compliant materials with an improved property profile are produced. The so-called "geopolymers" produced are created by the polycondensation of water glass, an alkaline activator, with metakaolin. To date, these aluminosilicate networks have been used as high-performance building materials due to their high mechanical, chemical and thermal stability.

By applying the composition to the paper in a controlled manner and then curing the composition on the paper in accordance with an adapted or optimized geopolymer recipe, it was possible to produce very well-adhering, thin, flexible and, depending on requirements, bright or glossy layers on the paper. The nanoporous structure of the resulting coating also enables excellent printability using high-speed inkjet processes, whereby despite the high gloss, smudge resistance is achieved after less than 0.3 seconds.

In order to ensure the desired surface quality and gloss of the coating, the still wet geopolymer coatings must be protected from drying out and hardened under moist conditions. Since water is part of the geopolymerization reaction, it must not escape excessively quickly, as would be the case with classic paper drying. It is therefore necessary to protect the surface accordingly. This can also have a positive effect on the quality of the surface at the same time.

In one embodiment, the method comprises the step of moistening the paper.

Pre-moistening the paper ensures that it is no longer able to extract significant amounts of water from the geopolymer formulation, which means that re-moistening can be kept to a minimum. Pre-moistening or moistening causes the paper to swell and expand. It is advantageous if the paper is already moist before the composition is applied, because then geopolymerization, ie the hardening of the composition, is not accompanied by a change in the paper. There is then no There are no waves between the forming geopolymer layer and the paper, but the interface remains flat.

Since very rapid coating color immobilization - as with conventional dispersion-based coating colors - promotes the brittleness of the geopolymer coating, the papers can be pre-moistened on a laboratory scale using a size press and water and conditioned in closed containers for 12 hours. This significantly improves the coating quality because the dimensional changes in the paper and the coating can be brought into line during the drying process. The "floe breakage effect" that occurs as a result of drying shrinkage deformations can be avoided or at least reduced in this way. For industrial coating processes, this procedure also offers the potential to save considerable amounts of drying energy, since the freshly produced papers can be coated directly in the wet state and do not have to be dried first.

In a separate aspect, the invention relates to a process for paper production and paper finishing comprising the steps:

- Providing paper by means of a process customary in the state of the art, whereby the final thermal drying is omitted,

- Applying the composition to the paper, and

The separate aspect for a process for paper production and paper finishing combines the paper production processes known in the prior art with the paper finishing according to this disclosure. The person skilled in the art knows the processes and steps for paper production that are usual in the prior art, ie the provision of a pulp suspension, the sheet formation by means of a screen, the subsequent mechanical and hydrodynamic dewatering processes and the final thermal drying. The disclosed process for paper production and paper finishing is advantageous because in particular the drying step in paper production can be omitted for the time being and the paper produced can be subjected to finishing directly. This process can save considerable amounts of drying energy. In addition, The paper does not need to be moistened separately for paper finishing because it leaves the shortened production process moistened. The disclosed combined process for paper production and paper finishing is therefore very efficient in terms of both energy and process economy.

The embodiments described below apply equally to the disclosed process for paper finishing and the combined process for paper production and paper finishing. The compositions described in this disclosure are equally applicable and valid for both processes.

In one embodiment of the method, the step of moistening the paper comprises adjusting the fiber saturation point by impregnating the paper with water in a size press and then storing it in a closed container.

This ensures uniform and maximum swelling and stretching of the paper fibres, which enables the resulting product to be flat.

In one embodiment of the method, the composition is applied to the paper by means of blade coating, doctor blade application, a high-speed inkjet process or a cast coating process. In one embodiment of the method, the composition is applied to the paper by means of a flexographic process or screen printing process.

Cast coating processes are advantageous if gloss and smoothness of the coating are desired. Flexographic or screen printing processes are advantageous if a spatially resolved application is to be achieved.

In one embodiment of the process, the curing of the composition takes place at a temperature between 10 and 80 °C, preferably between 20 to 80 °C, more preferably between 60 to 80 °C.

The time for curing and the temperature depend on the molar modulus of the water glass used, with low molar moduli being associated with higher reactivity and faster curing (see Table 4). Curing of the geopolymer at room temperature is possible for water glasses with a molar modulus of 1.7 and a molar modulus of 2.8 and takes 6 hours and 12 to 24 hours respectively. Increasing the temperature to 60 to 80 °C leads to accelerated curing and a more even and better covering line with the same flexibility.

In one embodiment of the method, the composition is prepared by prior

Degassed using an absolute pressure of 50 mbar or less. Any Bubbles in the composition caused by stirring, rapid mixing and/or blending are thus advantageously removed.

In one embodiment of the method, the composition applied to the paper is protected from drying out by one of the following measures:

- covering the composition applied to the paper with an airtight and/or waterproof film, the airtight and/or waterproof film being made of PE, PP or PET,

- Storing the composition applied to the paper in a climate-controlled cabinet,

- Rewetting of the composition applied to the paper by spraying with water using spray nozzles.

Covering the composition applied to the paper with an airtight and/or waterproof film is a suitable measure on a laboratory scale. The use of a polypropylene (PP) film works well. The use of a PET film (Hostaphan) works very well and is preferred. Polyethylene (PE) film is also possible, but only works to a limited extent, especially when heated, e.g. to temperatures between 60 and 80 °C. In the case of a PE film, the softening temperature of the selected polyethylene must be above the temperature required for curing.

Storing the composition applied to the paper in a climate-controlled cabinet is advantageous and is equivalent to an incubator in which the temperature and humidity can be adjusted. In particular, the curing temperature can be adapted and adjusted to the selected water glass, i.e. in particular the molar modulus of the water glass.

On an industrial scale, the method of choice is to remoisten the composition applied to the paper by spraying it with water (or misting it with water vapor) using spray nozzles. The system for spraying water using spray nozzles can be scaled and implemented accordingly depending on the throughput.

Composition for a paper line

In a further aspect, the invention relates to a composition for a paper coating comprising: a) a metakaolin with the following components (in % by weight): 50 to 65 SiC>2, 25 to 45 Al2O3, 0 to 5 Fe2Ü3, 0 to 5 TiC>2, and b) a water glass consisting of an aqueous solution of alkali silicate, selected from sodium silicate, potassium silicate and/or lithium silicate, with a solids content between 10 and 50 wt.%, preferably between 20 and 40 wt.%, based on the mass of the solution, wherein the alkali silicate has a molar modulus, ie a SiO2/R2O ratio, between 1.7 and 3.8, preferably between 2.0 and 2.5, where R = Li, K, and/or Na, wherein the mass fraction of metakaolin in the composition is between 15 and 35 wt.%, and wherein the mass fraction of water glass in the composition is between 10 and 50 wt.%, preferably between 20 and 40 wt.%.

In a complex optimization project, the inventors established the requirements for the chemical composition, both quantitatively and qualitatively, for a paper coating. Both the molar modulus of the alkali silicate used turned out to be critical, as did - in combination with this - the coordinated mass proportions of metakaolin and water glass in the composition. The specified ranges for the molar modulus and the mass proportions of metakaolin and water glass within the composition lead to good results.

For all metakaolins tested, it was shown that a good compromise can be achieved between processability, i.e. mixing and application, and curing times and flexibility with regard to low brittleness in the set state.

In one embodiment, the mass fraction of metakaolin in the composition can be increased to up to 50% by weight, which advantageously reduces the risk of drying shrinkage cracks and increases the opacity of the coating. However, the viscosity increases significantly at a mass fraction of up to 50% by weight, so that the process must be adapted accordingly. In one embodiment, the mass fraction of metakaolin in the composition can be up to 40% by weight, which generally results in highly suitable and spreadable compositions. Depending on whether other mineral additives are used, the metakaolin content can also be reduced to 15% by weight.

The expert is able to determine the rheological properties and the reactivity of the composition and/or deliberately adjust them. The character of the geopolymers differs significantly from conventional coating colors, so that certain adjustments and precautions must be taken within the process. While conventional coating colors can be formulated and applied up to a maximum dynamic viscosity of approx. 1500 mPas, geopolymer-based coating colors can have a significantly higher viscosity. This requires reduced application speeds or the application of larger layer thicknesses. This prevents the shear rate in the fluid from increasing too much, which could lead to film tears or damage to the substrate or excessive stress on the coating unit. At the same time, geopolymers have pronounced shear-thinning properties, which can be advantageous for their behavior in the coating gap. In order to maintain their processability, ie, for example, their conveyability and flowability, the geopolymers must be kept in continuous motion to counteract an excessive increase in viscosity.

While conventional coating colors are fixed to the substrate by thermally softening binder polymers, geopolymers are reactive systems that can react with themselves as well as with hydrophilic, organic, mineral and metallic surfaces. Adhesion to technical surfaces of mixing, conveying and coating units must be avoided, so cleaning intervals must be observed and hydrophobic coatings must be used on the technical surfaces. As soon as the geopolymer has hardened, it can only be removed with the loss of the top metal or mineral layer.

In one embodiment, the composition for a paper coating comprises a) a metakaolin with the following components (in wt. %): 50 to 65 SiO2, 25 to 45 Al2O3, 0 to 0.5 Fe2Oa, 0.2 to 3 TiO2, preferably 1 to 2 TiO2, and b) a water glass consisting of an aqueous solution of alkali silicate, selected from sodium silicate, potassium silicate and/or lithium silicate, with a solids content between 10 and 50 wt. %, preferably between 20 and 40 wt. %, based on the mass of the solution, wherein the alkali silicate has a molar modulus, i.e. a SiO2/R2O ratio, between 1.7 and 3.8, preferably between 2.0 and 2.5, wherein R = Li, K, and/or Na, wherein the mass fraction of the metakaolin in the composition is between 15 and 35 wt. %, and wherein the mass fraction of the Water glass in the composition is between 10 and 50 wt.%, preferably between 20 and 40 wt.%.

For a white paper coating or a white paper, a Fe2O3 content of 0 to 0.5 wt.% is preferred, which can be increased by the use of 0.2 to 3 TiO2, preferably 1 to 2 TiO2 wt.%.

In one embodiment, the composition for a paper coating comprises a) a metakaolin with the following components (in wt. %): 50 to 65 SiO2, 25 to 45 Al2O3, 1 to 5 Fe2O3, 0 to 1 TiO2, and b) a water glass consisting of an aqueous solution of alkali silicate, selected from sodium silicate, potassium silicate and/or lithium silicate, with a solids content between 10 and 50 wt. %, preferably between 20 and 40 wt. %, based on the mass of the solution, wherein the alkali silicate has a molar modulus, ie a SiCh/F^O ratio, between 1.7 and 3.8, preferably between 2.0 and 2.5, where R = Li, K, and/or Na, where the mass fraction of metakaolin in the composition is between 15 and 35 wt.%, and where the mass fraction of water glass in the composition is between 10 and 50 wt.%, preferably between 20 and 40 wt.%.

For a paper coating with a desired color, such as a wood-like grain, a Fe2O3 content of 1 to 5 wt.% is preferred. In addition, the composition can have a TiO2 content of 0 to 1 wt.%.

In one embodiment, the composition for a paper coating comprises additives, fillers, e.g. talc and CaCO3, and/or pigments, e.g. TiO2.

CaCOs is an inexpensive filler. The properties, especially the flexibility and whiteness of the coatings, can be further improved by adding pigments, calcium carbonate and/or titanium oxide. In this context, the geopolymer formulation proves to be compatible with all mineral fillers and pigments commonly used in the paper industry, whereby the solid-to-liquid ratio should be maintained and the additive is added as a partial replacement for the metakaolin.

However, the amount of additives, fillers and/or pigments used must be limited and coordinated in such a way that the setting process is not unduly delayed or even stopped. The use of titanium dioxide must be limited to a maximum of 20% by weight and that of Neuburg Siliceous Earth to a maximum of 15% by weight so that the geopolymers can still harden.

Further embodiments

In one embodiment, concerning both the process and the composition, the alkali silicate has a molar modulus, i.e. a SiO2/R2O ratio, between 2.0 and 2.5, or between 1.9 and 2.7, or between 1.8 and 2.9.

In one embodiment, concerning both the method and the composition, the alkali silicate is potassium silicate having a molar modulus, i.e. a SiO2/K2O ratio, between 2.0 and 2.5, or between 1.9 and 2.7, or between 1.8 and 2.9.

In one embodiment, concerning both the method and the composition, the water glass consists of an aqueous solution of sodium silicate, having a solids content of between 10 and 30 wt.% based on the mass of the solution, wherein the Sodium silicate has a molar modulus, ie a SiC>2/Na2O ratio, between 1.7 and 3.8, or between 1.8 and 2.9, or between 1.9 and 2.7, preferably between 2.0 and 2.5.

In one embodiment, relating to both the method and the composition, the water glass consists of an aqueous solution of lithium silicate, having a solids content between 10 and 50 wt.%, preferably 20 and 40 wt.%, based on the mass of the solution, wherein the lithium silicate has a molar modulus, i.e. a SiO2/Li2O ratio, between 1.7 and 3.8, or between 1.8 and 2.9, or between 1.9 and 2.7, preferably between 2.0 and 2.5.

In one embodiment, relating to both the method and the composition, the water glass consists of an aqueous solution of potassium silicate, having a solids content between 10 and 50 wt.%, preferably 20 and 40 wt.%, based on the mass of the solution, wherein the potassium silicate has a molar modulus, i.e. a SiO2/K2O ratio, between 1.7 and 3.8, or between 1.8 and 2.9, or between 1.9 and 2.7, preferably between 2.0 and 2.5.

Paper

In one aspect, the invention relates to a paper which is surface-modified with a paper coating, wherein the paper coating comprises the composition according to this disclosure, and/or wherein the paper coating has resulted from the curing of the composition according to this disclosure.

The resulting papers are characterized by a thin, smooth, visually appealing, well-adhering and/or flexible coating. In addition, the papers are environmentally friendly and sustainable in terms of the raw materials used due to the absence of synthetic organic substances. Furthermore, the resulting papers have low CO2 emissions per kilogram of paper produced compared to conventional paper finishing processes.

Use

In one aspect, the invention relates to a use of the paper for packaging or magazines, for decorative purposes as well as for construction materials, paper furniture, in interior design, and/or lightweight construction.

Due to their visually appealing, printable and abrasion-resistant coating, the papers produced are very well suited for packaging or magazines, for decorative purposes as well as for construction materials, paper furniture, interior design and/or lightweight construction. Geopolymers themselves are very fireproof and give the coated papers increased fire resistance and can be used, for example, in exhibition stand construction, where fire protection must be guaranteed. The coated papers are also suitable for use in the mobility sector, eg in trains and airplanes.

Definitions, materials and examples

Metakaolins

Metakaolins are tempered clays which, in addition to kaolinite as the main aluminosilicate source or as the main mineral, can also contain smectite/montmorillonite and illite. Metakaolins with the following general composition (in % by weight) can be used in the coating color formulations described here: 50 to 65 SiO2, 25 to 45 Al2O3, 0 to 5 Fe2O3, 0.2 to 2 TiO2. The expert knows the mineral kaolinite as a frequently occurring layered silicate from the kaolinite-serpentine group with the crystal chemical composition Al4[(OH)s|Si4Oio]. The structure and stoichiometric composition of kaolinite changes through thermal treatment (or tempering), e.g. by calcination in air and at atmospheric pressure. At 550 to 600 °C, dehydration begins, leading to amorphous meta-kaolinite with the molecular formula AhSi2O7 or AhO3-2SiO2. With continued thermal treatment up to 900 °C, dehydroxylation takes place.

Metakaolins may also contain the following additional components (in wt.%): 0.5 to 5 CaO, 0.2 to 4 MgO, 0.2 to 1 K2O, and 0.2 to 3 Na2O.

The grain sizes of the metakaolins are preferably between 0 and 50 pm, in particular between 1 and 5 pm. The whiteness of the metakaolins should be as high as possible for paper applications and should be at least 85 to 95%.

The following metakaolins, with their stated compositions, were used and investigated in the context of the invention:

1. Calcined kaolin DG 80 (PRECHEL GmbH): 52 ± 2 wt.% SiO2, 45 ± 2 wt.% AI2O3, < 0.8 wt.% Fe2O3, < 1.3 wt.% TiO2,

2. MetaStar® 501 ,

3. K-1100 (KAOPOZZ™): 54 ± 2 wt% SiO ₂ , 43 ± 2 wt% AI2O3, 1.3 wt% Fe ₂ O ₃ ,

1.2 wt.% TiO2, 0.7 wt.% Na2O + K2O, 0.8 wt.% CaO + MgO, 4. Antec MM: 52 ± 1 wt.% SiO2, 45 ± 1 wt.% Al2O3, < 0.5 wt.% Fe2O3, < 0.8 wt.% TiO2, < 0.2 wt.% Na2O, < 0.1 wt.% K2O, < 0.5 wt.% CaO, < 0.2 wt.% MgO,

5. PowerPozz® white (Newchem GmbH): 54 - 56 wt.% SiO2, 40 - 42 wt.% AI2O3, < 1.4 wt.% Fe2O3, < 0.4 wt.% K2O,

Table 1: Material properties of the metakaolins Antec MM, K-1200, K-1100, Metastar 501 and PowerPozz.

The pozzolan activity describes the reactivity with calcium hydroxide.

The amorphous fraction was determined by XRD analysis.

Water glass

In the context of this application, water glass refers exclusively to aqueous solutions of amorphous, water-soluble sodium, potassium and lithium silicates, with a solids content of between 10 and 50% by weight based on the mass of the solution. Water glass is also understood by the expert as a trivial term for water-soluble alkali silicates, with water glasses based on potassium silicate being the most suitable.

Water glasses were used as alkaline activators, which together with the aluminosilicate source, e.g. fly ash or metakaolin, form the inorganic structures of the geopolymer.

A reaction occurs between the metakaolin used and the alkaline activator solution, which forms a hardening matrix. This reaction was observed at moderate Heat input tests were carried out at temperatures above 23 °C, at temperatures above 40 °C, and at temperatures between 60 and 80 °C.

A characteristic parameter for the alkali silicates used is the molar modulus. The molar modulus corresponds to the molar ratio of SiC>2 to R2O, where R corresponds to one of the three alkali metals, i.e. sodium (Na), potassium (K) or lithium (Li). The smaller the molar modulus, the more reactive the alkali silicate used is.

Geosil 14517 is an aqueous solution of potassium silicate with a solids content of 45.0 wt.% based on the mass of the solution, which, due to its composition, leads to stable bonds with high strength, especially in combination with alkaline-activated fillers. Geosil 14517 has a molar modulus of 1.7 and is classified as very reactive.

Betol K28-T is a potassium silicate with a solids content of 28.0% by weight based on the mass of the solution. Betol K28-T reacts with mineral substrates by silicification. The good binding power and high temperature resistance are advantageous in the formulation of fireproof and acid-resistant adhesives. The molar modulus of Betol K28-T is 3.8 and has comparatively long drying and reaction times.

A 1:1 wt.%/wt.% mixture of Geosil 14517 and Betol K28-T was used as activator solution.

Experiment 1 : Production of a glossy coating

Taking into account the factors of gloss, flexibility, whiteness and printability, the following composition, shown in Table 2, was developed.

Table 2: Composition of a geopolymer formulation for producing a glossy paper coating

The OA solution (optical brightener) used was CALCOFLUOR White, a fluorescent blue dye, at a concentration of 5 mmol. In a separate experiment, pure water was used instead of the OA solution.

The components (Table 2) were mixed in a speed mixer (Hauschild DAC 400) to form a degassed geopolymer glue. The device was operated for 4 minutes at 2000 rpm and 30 mbar. This leads to a homogeneous mixture of the coating formulation, after which the actual surface application was carried out using a film drawing frame 60 (Byk, with a film width of 60 mm). A matt opacity test card (from Byk) was used as the substrate and coated with a wet film thickness of 90 pm. Pre-moistening was not carried out with this paper substrate. The still wet coating was covered with a PET film (Hostaphan®) and the laminate produced in this way was stored for one day at 60 °C. After the geopolymerization reaction had taken place under these conditions, the film was removed and the glossy paper coating exposed. The geopolymer-coated paper produced in this way is suitable for printing using high-speed inkjet processes due to the nanoporosity of the geopolymer.

Experiment 2: Production of a matte coating

The components according to Table 3 were mixed in a speed mixer for 4 minutes at 2000 rpm and 30 mbar to form a degassed geopolymer glue. After the coating formulation was homogeneously mixed, the actual surface application was carried out using a film drawing frame. In this case, a carrier board (Luhne Messtechnik) in accordance with ISO 5269-2:2004 was used as the substrate and coated with a wet film thickness of 60 μm. This paper also did not require prior moistening. After coating, the sample was covered with PET film (Hostaphan®) and stored under surface pressure at 23 °C and approx. 40% relative humidity. In this case, the film was already applied to the immobilized coating, so that a matt, printable coating was achieved after a reaction time of less than one day. These samples were stored for up to 30 days without any change in quality being observed.

Table 3: Composition of a geopolymer formulation for the production of a matt paper coating

Experiment 3: Reaction conditions or setting conditions

The reaction conditions or setting conditions depend on the molar modulus of the water glass used, with low molar moduli being associated with higher reactivity. For water glasses with different molar moduli, it was determined which reaction time must be observed depending on the temperature in order to ensure sufficient setting of the geopolymer coating (Table 4). Table 4: Curing times for water glasses with different molar moduli depending on temperature

Claims

Expectations

1. Process for paper finishing comprising the steps:

- Providing paper,

- providing a composition for a paper coating, the composition comprising a metakaolin and a water glass, and optionally additives, the composition preferably being according to any one of claims 8 to 11,

- Applying the composition to the paper, and

2. Method according to claim 1 comprising the step of moistening the paper.

3. A method according to claim 2, wherein the step of moistening the paper comprises adjusting the fiber saturation point by impregnating the paper with water in a size press and then storing it in a closed container.

4. The method according to any one of claims 1 to 3, wherein the composition is applied to the paper by blade coating, doctor blade application, a high-speed inkjet process or by a cast coating process.

5. A process according to any one of claims 1 to 4, wherein the curing of the composition takes place at a temperature between 10 and 80 °C, preferably between 20 to 80 °C, more preferably between 60 to 80 °C.

6. A process according to any one of claims 1 to 5, wherein the composition is degassed by prior application of an absolute pressure of 50 mbar or less.

7. A method according to any one of claims 1 to 6, wherein the composition applied to the paper is protected from drying out by one of the following measures:

- covering the composition applied to the paper with an airtight and/or waterproof film, the airtight and/or waterproof film being made of PE, PP or PET, - Storing the composition applied to the paper in a climate-controlled cabinet,

- Rewetting of the composition applied to the paper by spraying with water using spray nozzles. Composition for a paper coating comprising: a) a metakaolin with the following components (in wt. %): 50 to 65 SiC>2, 25 to 45 AI2O3, 0 to 5 Fe2C>3, 0 to 5 TiC>2, and b) a water glass consisting of an aqueous solution of alkali silicate, selected from sodium silicate, potassium silicate and/or lithium silicate, with a solids content between 10 and 50 wt. %, preferably between 20 and 40 wt. %, based on the mass of the solution, wherein the alkali silicate has a molar modulus, ie a SiO2/R2O ratio, between 1.7 and 3.8, preferably between 2.0 and 2.5, wherein R = Li, K, and/or Na, wherein the mass fraction of the metakaolin in the composition is between 15 and 35 wt. %, and wherein the mass fraction of the water glass in the composition is between 10 and 50 wt. %, preferably between 20 and 40 wt.%. Composition for a paper coating according to claim 8 comprising: a) a metakaolin with the following components (in wt. %): 50 to 65 SiO2, 25 to 45 Al2O3, 0 to 0.5 Fe2O3, 0.2 to 3 TiO2, preferably 1 to 2 TiO2, and b) a water glass consisting of an aqueous solution of alkali silicate, selected from sodium silicate, potassium silicate and/or lithium silicate, with a solids content between 10 and 50 wt. %, preferably between 20 and 40 wt. %, based on the mass of the solution, wherein the alkali silicate has a molar modulus, ie a SiO2/R2O ratio, between 1.7 and 3.8, preferably between 2.0 and 2.5, wherein R = Li, K, and/or Na, wherein the mass fraction of the metakaolin in the composition is between 15 and 35 wt. %, and wherein the mass fraction of the water glass in the composition is between 10 and 50 wt. %, preferably between 20 and 40 wt. %. Composition for a paper coating according to claim 8 comprising: a) a metakaolin with the following components (in wt. %): 50 to 65 SiO2, 25 to 45 Al2O3, 1 to 5 Fe2O3, 0 to 1 TiO2, and b) a water glass consisting of an aqueous solution of alkali silicate, selected from sodium silicate, potassium silicate and/or lithium silicate, with a solids content between 10 and 50 wt.%, preferably between 20 and 40 wt.%, based on the mass of the

Solution, wherein the alkali silicate has a molar modulus, i.e. a SiO2/R2O ratio, of between 1.7 and 3.8, preferably between 2.0 and 2.5, where R = Li, K, and/or Na, wherein the mass fraction of metakaolin in the composition is between 15 and 35 wt. %, and wherein the mass fraction of water glass in the composition is between 10 and 50 wt. %, preferably between 20 and 40 wt. %. Composition for a paper coating according to one of claims 8 to 10, further comprising additives, fillers, e.g. talc and CaCOs, and/or pigments, e.g. TiO2. Paper which is surface-modified with a paper coating, wherein the paper coating comprises the composition according to one of claims 8 to 11, and/or wherein the paper coating has resulted from the curing of the composition according to one of claims 8 to 11. Use of the paper according to claim 12 for packaging or magazines. Use of the paper according to claim 12 for decorative purposes. Use of the paper according to claim 12 for construction materials, paper furniture, in interior design, and/or lightweight construction.