EP4041548A1

EP4041548A1 - Coated paper

Info

Publication number: EP4041548A1
Application number: EP21794510.4A
Authority: EP
Inventors: Aljoscha FÖLL; Claus Jurisch; Christian Kind; Marius SCHULTE; Thomas SERRER; Andrew VOGT
Original assignee: Koehler Innovation and Technology GmbH
Current assignee: Koehler Innovation and Technology GmbH
Priority date: 2020-10-16
Filing date: 2021-10-14
Publication date: 2022-08-17
Also published as: DE102020127373A1; KR20220119681A; WO2022079178A1; CN114867605A; US20230039088A1; JP2023523674A

Abstract

The invention relates to a coated paper comprising a base paper and a barrier layer applied thereto, the barrier layer comprising at least one polymer, the polymer containing an at least partly saponified polyvinyl alcohol and/or an at least partly saponified polyvinyl alcohol copolymer, each of which has an onset temperature below 210°C ascertained by DSC.

Description

Coated paper

description

The present invention relates to a coated paper, a method for producing such a coated paper, the use of the coated paper as packaging material and a packaging comprising the coated paper.

Packaging generally refers to the cover or (partial or complete) covering of an object, in particular for its protection or for better handling. Thus, a packaging material includes the material that forms such a package.

Packaging materials can, for example, be based on paper, plastics and/or metals. The present invention relates to paper-based packaging materials.

The main requirements for packaging materials of any origin are to protect the packaged goods from external influences and to prevent the packaged goods from escaping. For this purpose, the packaging material should meet different criteria depending on the packaged goods and the packaging process. In addition to so-called barrier properties against, for example, water, grease, oxygen or mineral oil, suitable packaging materials should also meet mechanical and process-specific requirements. Depending on the packaging system, a packaging material should have sufficient tear strength, a suitable coefficient of friction (coefficient of friction) and flexibility, it should be either heat-sealable or compatible with a cold-seal adhesive, and it should be printable from the outside and should not lose its protective effect throughout the conversion and packaging process.

Depending on its composition and basis weight, paper can meet many mechanical requirements, but due to its physical properties and porous structure, it needs an additional coating that provides it with heat sealability or barriers, for example.

Known paper-based, coated packaging materials often contain compounds such as polyvinylidene chloride (halogen-containing), or are composites of paper and metal or plastic foils, have tear strength that needs improvement, which can lead to running problems on packaging systems, and/or are more adhesive due to an excessively high proportion of coating Components or formation of so-called stickies via the paper fiber flow often cannot be recycled.

Polyvinyl alcohols are well known as linear water-soluble, biodegradable barrier coatings, also for paper. Such coatings have good barriers against oil, grease, oxygen, solvents and other non-polar gases, liquids or solids. Due to their hydrophilicity, however, polyvinyl alcohols have very high permeabilities for polar compounds such as water. This can also affect the barrier effect against non-polar migrants, since polyvinyl alcohol absorbs moisture very well, swells and thus creates paths through the barrier coating at the molecular level.

There have been many attempts to reduce the barrier drop at relative humidities above 50 or 60% by adding pigments, for example as disclosed in WO 2010/129032, or by crosslinking the polymer chains, for example in WO 2020/109401 or US 6,444,750. However, not only these microscopic defects are disadvantageous for the use of polyvinyl alcohols as barriers, also and above all the occurrence of macroscopic defects due to the mechanical stress in packaging and converting plants impair the range of use of polyvinyl alcohols.

Polyvinyl alcohol is understood as meaning a completely saponified polyvinyl acetate, which is a (thermoplastic) plastic of the following formula (I), which is usually synthesized by radical polymerization of vinyl acetate.

The ester groups in polyvinyl acetate are relatively easy to saponify with alkaline water, converting the polymer into polyvinyl alcohol and making it hydrophilic and sensitive to water.

A partially saponified polyvinyl acetate is also referred to as a partially saponified polyvinyl alcohol.

In the present invention, the term partially saponified polyvinyl acetate can be used synonymously with the term partially saponified polyvinyl alcohol.

The degree of saponification indicates the proportion of ester groups that have been saponified and are now present as -OH groups. For example, a polyvinyl alcohol with a degree of saponification of 90% is a vinyl acetate polymer in which 90% of the ester groups originally present have been saponified. This polyvinyl alcohol therefore contains 90% OH groups and 10% ester groups. At a degree of saponification of 100%, only OH groups are present, since all the ester groups originally present have been saponified.

In addition to the pure homopolymer, many polyvinyl alcohol copolymers are also of great industrial importance. Copolymer is understood to mean all polymers which consist predominantly (>50%) of vinyl alcohol or vinyl acetate units, regardless of the number of different monomers used for the synthesis. Such polyvinyl alcohol copolymers preferably include polyethylene vinyl alcohols.

Under saponification is meant here in the narrower sense, the hydrolysis of an ester by the aqueous solution of a hydroxide, such as. B. by sodium hydroxide, or by special enzymes (esterases). In contrast to acidic ester hydrolysis (the reverse reaction of esterification), they are irreversible because the carboxylic acid lacks the proton required for esterification. The products of the reaction are the alcohol and the salt of the acid (carboxylation) that made up the ester. In a broader sense, any hydrolysis of an ester can be referred to as saponification.

The object of the present invention is to eliminate the disadvantages of the known materials and to provide a material that is suitable as a packaging material, in particular for oxidation-sensitive and greasy food, and can be used for the production of packaging such as tubular bags by means of heat or cold sealing processes. In addition, the material should not contain any barrier layers based on halogen-containing compounds. Furthermore, the material according to the invention should have one or more of the following properties compared to known packaging materials:

Improved oxygen barrier, improved grease barrier, improved mineral oil barrier, improved kink resistance/flexibility, recyclability via the waste paper cycle, improved heat sealing properties, improved cold sealing properties (compatible with the sealing medium, such as a water-based cold sealing adhesive), high tear strength, the taste of the package contents is not altered, an aroma seal has, the outside should be printable. The material should also have an improved water vapor barrier and the sealed seam should be as moisture-resistant as possible. The packaging material should also be suitable for metallization, particularly in the micrometer and nanometer range, in order to further optimize the barrier properties if necessary. Ultimately, the material should be able to be produced as economically as possible.

This object is achieved by a coated paper according to claim 1, ie by a coated paper comprising a base paper and a barrier layer applied thereto, the barrier layer being at least one polymer and is characterized in that the polymer comprises an at least partially hydrolyzed polyvinyl alcohol and/or an at least partially hydrolyzed polyvinyl alcohol copolymer, each with an onset temperature of less than 210° C. determined by means of DSC.

The onset temperature was determined by DSC as follows:

The (extrapolated) onset temperature (according to DIN EN ISO 11357-1:2010-03) is the intersection of the extrapolated baseline and the inflection tangent at the beginning of the melting or crystallization peak in a DSC measurement. The baseline and turning tangent are determined from the temperature dependent heat flow signal. In the case of pure and homogeneous materials, the initial temperature can be specified as the melting temperature. In contrast to the peak temperature, the onset temperature is less dependent on the heating rate and the sample mass. Furthermore, onset temperatures are commonly used for temperature calibration of a DSC.

A paper coated in this way is characterized in particular by the fact that it is particularly well suited as a packaging material for oxidation-sensitive and greasy objects, in particular food, and can be used to produce bags by means of heat or cold sealing applications, with a water-based cold sealing adhesive being used for the cold sealing application can. Furthermore, no barrier layers based on halogen-containing compounds have to be present.

Compared to fully saponified polyvinyl alcohols (PVOH) or polyethylene vinyl alcohols (EVOH), the partially saponified polyvinyl alcohols have the advantage that they have a significantly lower optimal sealing temperature in the heat sealing process. This does not negatively affect the strength of the sealed seam. Furthermore, partially hydrolyzed polyvinyl alcohols have a slightly lower viscosity, with otherwise the same concentration. A high viscosity tends to be seen as a disadvantage, since in this case the PVOH solution has to be diluted more, which is why a larger amount of water has to be dried in the coating process. This not only costs energy and thus requires a coating system with a higher drying capacity, but can also be difficult to implement in terms of application technology, depending on the desired application weight. In addition, the diffusion of water molecules at high viscosities and thus the drying itself slowed down. Furthermore, it is more likely that gaseous water will accumulate in the coating, leading to the formation of macroscopic coating defects.

Therefore, partially saponified polyvinyl alcohols or partially saponified polyvinyl alcohol copolymers, such as polyethylene vinyl alcohols, are preferred over the fully saponified variants.

The coated paper according to the invention is also distinguished by an improved oxygen barrier, an improved grease barrier, an improved mineral oil barrier and an improved water vapor barrier.

The coated paper according to the invention also has improved crease resistance without impairing the barrier effect and is also notable for high tear strength.

The coated paper according to the invention can also be recycled via the waste paper cycle.

If the coated paper according to the invention is used as a packaging material for food, it is distinguished in particular by the fact that it does not affect and/or change the taste of the packaged food.

The coated paper according to the invention is also heat-sealable and exhibits improved cold-sealing properties (compatible with the sealing medium, such as a water-based cold-seal adhesive), with the seal seams each having sufficient moisture resistance.

The coated paper according to the invention is also easy to print on the uncoated side (outside).

The coated paper according to the invention is also suitable for metallizations in the micrometer and nanometer range in order to further optimize the barrier properties if required. Finally, the coated paper according to the invention can be produced relatively easily and with low coating weights and recycled via the waste paper cycle.

In the following, the term “comprise” can also mean “consisting of”.

Substances are referred to as "hydrophobic" which cannot be mixed with water or can only be wetted by water using surfactants. Substances are referred to as "hydrophilic" which can be mixed with water or wetted by water without the use of surfactants to let. Hydrophobic polymers are also referred to as non-polar polymers and hydrophilic polymers as polar polymers.

Hydrophobicity or hydrophilicity can be defined, for example, via the logP value. The n-octanol-water partition coefficient K _ow (also spellings such as octanol/water partition coefficient are common and correct) is a dimensionless partition coefficient known to those skilled in the art that indicates the ratio of the concentrations of a chemical in a two-phase system of n-octanol and water and is therefore a measure of the hydrophobicity or hydrophilicity of a substance. The logP value is the common logarithm of the n-octanol-water partition coefficient K _ow . The following applies:

^SiSi

K _ow = P =^ ci and log P = log -^ = log cw ^c w _o ^s ' - c _w ^Sl with c ₀ ^Si = concentration of a chemical in the octanol-rich phase and c _w ^Si = concentration of a chemical in the water-rich phase .

Kow is greater than one when a substance is more soluble in fat-like solvents such as n-octanol, and less than one when it is more soluble in water. Correspondingly, log P is positive for hydrophobic/lipophilic and negative for hydrophilic/lipophobic substances.

The low permeability of polyvinyl alcohols to oxygen, mineral oil, fat and other non-polar migrants is due to their relatively high hydrophilicity. In addition, ethylene-containing polymers, such as (saponified) polyethylene vinyl alcohols, have a lower water vapor permeability, which can be attributed to the ethylene content and the associated lower hydrophilicity.

In principle, the base paper used in the coated paper according to the invention is not restricted.

However, it is preferred that the base paper has a basis weight of 20 to 120 g/m ² , preferably 40 to 100 g/m ² .

Furthermore, it is preferred that the paper has a composition with a long fiber content of 10 to 80%, preferably 20 to 50%, and a short fiber content of 20 to 90% by weight, preferably 50 to 80% by weight.

Long fibers are fibers with a fiber length of 2.6 to 4.4 mm and short fibers are fibers with a fiber length of 0.7 to 2.2 mm.

In addition, 0% to 20%, preferably 0% to 5%, preferably excluding the value 0%, of fillers such as GCC (ground calcium carbonate) known for example under the trade name Hydrocarb 60 or Hydroplex 60, PCC (precipitated calcium carbonate ), which is known, for example, under the trade name Precarb 105, natural kaolin and/or talc, and customary auxiliaries such as retention aids and/or sizing agents.

The advantage of such a base paper is, on the one hand, its high flexibility and, on the other hand, its good processability on existing packaging systems for flexible materials such as plastic films, the maintenance of high machine availability and the achievement of the necessary puncture resistance.

Common packaging systems are, for example, vertical and horizontal tubular bag machines (form-fill-seal) for the production of stand-up bags, flow packs, pillow packs, etc., machines that bring together two webs of the same or different materials and connect them by heat sealing, e.g. B. also traysealer, chamber belt machines (also with vacuum), bag filling and sealing machines, thermoforming packaging machines, linear filling machines, the attaching lids by heat sealing to close, overwrapping machines with final heat sealing step, blister packaging machines and x-fold packaging machines.

The coated paper according to the invention is also preferably characterized in that a primer comprising at least one inorganic pigment and a polymeric binder is present between the base paper and the barrier layer.

The inorganic pigment is preferably in the form of flakes and comprises in particular a talc, precipitated calcium carbonate or silicates, preferably sheet silicates and very particularly preferably a kaolin.

In particular, acrylate-based or styrene/butadiene-based binders should be mentioned as suitable polymeric binders. In principle, all polymers that can be used as binders for pigment coatings in the paper industry are suitable. Starch-based binders (solutions of modified starches, dispersions of crosslinked starches, so-called biolatices) and polymer-starch hybrid latices are also possible.

The polymeric binder preferably comprises a polymeric binder based on a polyacrylate.

The pre-coat may be an overall hydrophobic pre-coat.

In another embodiment, the precoat is generally hydrophilic.

The precoat preferably contains 1 to 70% by weight, preferably 5 to 50% by weight, of polymeric binder. The amount refers to the dried primer in the final product.

The precoat also contains preferably 50 to 95% by weight, preferably 80 to 90% by weight, of inorganic pigment. The amount refers to the dried primer in the final product.

In addition, the primer can contain additives such as thickeners, eg acrylate-based thickeners, surfactants and/or rheology modifiers. Also the use of Networking is conceivable. The primer preferably contains a zirconium-based crosslinking agent and is itself crosslinked with formaldehyde.

These additives are preferably each contained in an amount of 0 to 2% by weight, preferably greater than 0 to 2% by weight, with the value 0% preferably being excluded. The amount refers to the dried primer in the final product.

The amount of primer applied is preferably 1 to 10 g/m ² and particularly preferably 2 to 6 g/m ² . The amount refers to the dried primer in the final product.

If such an undercoat (also called a primer) is applied, this has the advantage that the paper surface is sealed and the other barrier layer coated on it migrates only slightly into the paper, thus creating sufficient layer adhesion. Furthermore, this primer reduces the average roughness of the base paper and offers an advantageous "holdout", which is characterized by an area-wide application and a defined surface energy, so that a coated barrier layer can form optimally Barrier layer, which can be important for later sealing applications.

The barrier layer applied to the primer comprises an at least partially hydrolyzed polyvinyl alcohol and/or an at least partially hydrolyzed polyvinyl alcohol copolymer. The formulation used for processing preferably has an amount of 10 to 100% by weight, particularly preferably 50 to 99.8% by weight, of polymer.

The barrier layer can also contain additives such as thickeners, e.g. acrylate-based thickeners, surfactants, e.g. sulfosuccinates, extensional rheology aids, e.g. polyacrylamides, carboxymethylcellulose, polyvinyl alcohols, and/or crosslinking agents such as aldehydes and polyhydric aldehydes, zirconates, polyhydric epoxides, epichlorohydrin resins and/or hydrazides.

These additives are preferably each contained in an amount of 0.1 to 1% by weight based on the total weight of the barrier layer. In one embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially hydrolyzed polyvinyl alcohol and/or the at least partially hydrolyzed polyvinyl alcohol copolymer has an average molecular weight of less than 100,000 g/mol.

In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially hydrolyzed polyvinyl alcohol and/or the at least partially hydrolyzed polyvinyl alcohol copolymer has an average molecular weight of greater than 30,000 g/mol or greater than 40,000 g/mol or greater than 50,000 g/mol or greater than 60,000 g/mol or greater than 70,000 g/mol.

In one embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially hydrolyzed polyvinyl alcohol and/or the at least partially hydrolyzed polyvinyl alcohol copolymer has a degree of hydrolysis of 30% to 100%.

In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially saponified polyvinyl alcohol and/or the at least partially saponified polyvinyl alcohol copolymer has a degree of saponification of 30% to less than 100%.

In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially saponified polyvinyl alcohol and/or the at least partially saponified polyvinyl alcohol copolymer has a degree of saponification of less than 95% or from 30% to 95%.

In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially saponified polyvinyl alcohol and/or the at least partially saponified polyvinyl alcohol copolymer has a degree of saponification of 95% to 100%.

In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially saponified polyvinyl alcohol and/or the at least partially saponified polyvinyl alcohol copolymer has a degree of saponification of 95% to less than 100%. In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least partially hydrolyzed polyvinyl alcohol and/or the at least partially hydrolyzed polyvinyl alcohol copolymer has an onset temperature, determined by DSC, of less than 200°C

The coated paper according to the invention is also preferably characterized in that the at least one polymer is a partially hydrolyzed polyvinyl alcohol with a degree of hydrolysis of less than 95% or from 30% to 95%, an average molecular weight of greater than 0 and less than 100,000 g/mol and a determined by DSC onset temperature less than 200°C.

In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least one polymer is a partially hydrolyzed polyvinyl alcohol with a degree of hydrolysis of 95% to 100%, an average molecular weight of greater than 70,000 g/mol and an onset temperature determined by DSC of less than 200°C.

In another embodiment, the coated paper according to the invention is also preferably characterized in that the at least one polymer is a partially hydrolyzed polyvinyl alcohol copolymer, preferably a partially hydrolyzed polyethylene vinyl alcohol, with a degree of hydrolysis of 95% to 100%, an average molecular weight of greater than 60,000 g/mol and an onset temperature of less than 210°C, as determined by DSC.

It has been found that a partially hydrolyzed polyvinyl alcohol copolymer such as polyethylene vinyl alcohol is generally more flexible than polyvinyl alcohol.

In another embodiment, the coated paper according to the invention is preferably characterized in that the at least one polymer is a mixture of a partially hydrolyzed polyvinyl alcohol with a degree of hydrolysis of 1% to 95%, an average molecular weight of greater than 0 and less than 100,000 g / mol and a means of DSC determined onset temperature of less than 200 ° C, a partially hydrolyzed polyvinyl alcohol with a degree of saponification of 95% to 100%, an average molecular weight of more than 70,000 g / mol and an onset temperature determined by DSC of less than 200 ° C and / or a partially saponified polyvinyl alcohol copolymer, preferably a partially saponified polyethylene-vinyl alcohol, with a degree of saponification of 95% to 100%, an average molecular weight of greater than 60,000 g/mol and an onset temperature of less than 210° C. determined by DSC.

The degree of saponification was determined based on DIN EN ISO 3681 as follows:

PVOH (1 g) is diluted with dist. Water (70 mL) and neutralized ethanol (30 mL) were added and refluxed until complete dissolution. After cooling, the solution is neutralized with potassium hydroxide solution (0.1 M). In the case of partially hydrolyzed PVOH types, more potassium hydroxide solution (50 mL, 0.1 M) is added and heated under reflux (60 min.). In the case of fully hydrolyzed PVOH types, a reduced amount of additional potassium hydroxide solution (25 mL, 0.1 M) is used with a likewise reduced reflux time (30 min.) to avoid the absorption of carbon dioxide in excess alkali. The excess base is then back-titrated with hydrochloric acid (0.1 M) against phenolphthalein as an indicator. A blind test is carried out in parallel.

The degree of saponification (%) can be calculated according to Eq. calculate 1:

(100 - 0.1535) x ester number

Degree of saponification (%) = - - - - x 100 (Eq. 1)

(100 — 0.0749) x ester number where the ester number according to Eq. 2 is determined:

(Consumption K0H _PV0H (mg) - Consumption K0H _blank (mg)) X 5.61

Ester number (mg KOH/g) = x 100 (eq. 2)

Weight otro (g)

The mean molecular weights were determined by size exclusion chromatography (GPC) under the following conditions:

eluent: DMSO/+ 0.1M LiCl; Precolumn: 10 pm, guard, ID 8.00 mm x 50.00 mm; Columns: 10 pm, 30 Å, ID 8.00 mm x 300.00 mm; 10 pm, 3000 Å, ID 8.00 mm x 300.00 mm; 10 pm, 3000 Å, ID 8.00 mm x 300.00 mm; Pump: 1260 HPLC pump; flow rate: 1.0 mL/min; Injector: 1260 autosampler; injection volume: 200 pL; sample concentration: 5.0 g/L; temperature: 80 °C; Detectors: SECcurity ² refractive index detector (RI) - WEG eta 1001 HT viscometer; Calculation: WinGPC UniChrom Version 8.33 The samples were dissolved in the solvent (5 mg/mL) at a temperature of 80 °C for three hours and injected using an autosampler.

A number of PMMA standards with different molecular weights were measured to determine the calibration curve. By measuring the intrinsic viscosity, the calibration curve was converted into a universal calibration curve.

The coated paper according to the invention is also preferably characterized in that the polyvinyl alcohol at a dry content of 4% has a viscosity of less than 30 mPa*s or less than 20 mPa*s, particularly preferably less than 15 mPa*s.

The viscosity is determined at 23° C. using a Brookfield viscometer at 100 rpm.

A viscosity in this range has the advantage that higher solids contents can be used in the application and therefore less energy has to be used for drying, and higher process speeds can also be achieved. In addition to a financial advantage, this is also reflected in the use of a larger drying window on coating systems.

The coated paper according to the invention preferably has a breaking strength of >80 N/15 mm, preferably >90 N/15 mm in the machine direction, and >40 N/15 mm, preferably >50 N/15 mm transversely to the paper machine direction.

The coated paper according to the invention also preferably has a dynamic coefficient of friction of <0.7, preferably <0.6, particularly preferably <0.5. This refers to the friction of the coated side on the coated side (coating against coating).

The coated paper according to the invention is also preferably characterized in that the basis weight of the barrier layer is 5 to 20 g/m ² , preferably 8 to 12 g/m ² , based on the dried end product (Lutro). The coated paper according to the invention is also preferably characterized in that the barrier layer can be wetted with common water-based cold-seal adhesives. For this purpose, the surface energy of the barrier coating is >40 mN/m, preferably >50 mN/m, particularly preferably >55 mN/m.

Such a wettability of the barrier layer has the advantage that no unlined areas form during the application and drying process of cold-seal adhesives and there is also sufficient adhesion for the application.

The coated paper according to the invention is further preferably characterized in that the coated paper has an oxygen permeability cm ³ /m ² /d (23°C, 0% relative humidity) of less than 10, preferably less than 5.

The coated paper according to the invention is further preferably characterized in that the coated paper has an oxygen permeability cm ³ /m ² /d (23°C, 50% relative humidity) of less than 10, preferably less than 5.

The coated paper according to the invention is further preferably characterized in that the coated paper has an oxygen permeability g/m ² /d (23° C., 70% relative humidity) of less than 20, preferably less than 10.

The coated paper according to the invention is further preferably characterized in that the coated paper has an oxygen permeability g/m ² /d (23° C., 80% relative humidity) of less than 25, preferably less than 15.

The oxygen permeability (or oxygen transmission rate - OTR) is determined according to ISO 15105-2.

The coated paper according to the invention is also preferably characterized in that the coated paper has a grease barrier corresponding to test condition I according to DIN 53116. The coated paper according to the invention also preferably does not lose this grease barrier due to the mechanical stress of 180° creases with a roller that exerts a load of 330 g/cm on the resulting crease and where the coating is on the inside (inner crease) or on the outside (outer crease). can.

The coated paper according to the invention is also preferably characterized in that the coated paper has a mineral oil barrier (hexane) of <10 g/m ² /d.

The mineral oil barrier is determined by placing hexane in a beaker (solvent resistant), sealing it tightly with the coated paper and monitoring the weight loss over time.

The coated paper according to the invention is also preferably characterized in that the coated paper has a water vapor barrier. This persists even when the coating comes into contact with grease, which is not the case for all water vapor barriers.

The coated paper according to the invention is also preferably characterized in that it is crease-resistant both when not creased and when it has an inner crease as well as an outer crease.

The coated paper according to the invention is distinguished by the fact that it can be recycled via the waste paper cycle.

The coated paper according to the invention is heat-sealable and, at the optimum sealing temperature, preferably produces a seal seam strength of >3.5 N/15mm, particularly preferably >5.0 N/15mm, the seal seam strength for the coated paper being determined as follows:

The coated paper was sealed at 3.3 bar for 0.3 seconds in the temperature range from 100°C to 230°C transverse to the direction of paper travel and the seal seam strength was determined according to DIN 55529 (2012). "Heat sealing" is preferably understood to mean the joining of two layers of coated paper by means of local heat and/or pressure. In other embodiments, the coated side of the paper can also be connected to a non-heat-sealable opposite side of the paper or to another paper by heat sealing will.

The coated paper according to the invention can also be cold-sealed due to its compatibility with common cold-sealing media. Cold sealing is generally understood to mean that a cold sealing adhesive is applied to the section of a flat packaging material to be sealed using a printing process. A cold-seal adhesive has the property that it develops an adhesive effect only under and after increased pressure between the sealing jaws of a packaging machine and is otherwise not tacky or only to a limited extent.

Both the heat-sealed paper and the cold-sealed paper are characterized by high moisture resistance of the sealed seam.

The coated paper according to the invention is also tear-resistant.

The coated paper according to the invention is also characterized in that the taste of the food packaged therein is not affected.

The coated paper according to the invention can also be metallized in the nanometer range, for example with Al2O3 or Al.

The coated paper according to the invention is also preferably characterized in that a further layer comprising metals, in particular aluminum, and/or metal oxides, in particular aluminum oxide and/or silicon oxide, is applied to the barrier layer.

The coated paper according to the invention can be obtained economically using known production methods.

However, it is preferred to obtain the coated paper according to the invention using a process in which an aqueous suspension comprising the starting materials of the barrier layer is applied to the base paper, the aqueous application suspension having a solids content of 5 to 50% by weight, preferably from 10 to 30% by weight, and applied with a curtain coating method (curtain coating), preferably with a double curtain coating method (double curtain coating) at an operating speed of the coater of at least 200 m/min.

This method is particularly advantageous from an economic point of view and because of the uniform application over the paper web.

If the value of the solids content falls below about 10% by weight, the economics deteriorate, since a large amount of water has to be removed in a short time by gentle drying, which has a disadvantageous effect on the coating speed. On the other hand, if the value of 50% by weight is exceeded, then this only leads to increased technical effort to ensure the stability of the coating color curtain during the coating process and the drying of the applied film, since the machine is very busy again in this case has to run fast.

In the curtain coating process, a freely falling curtain of a coating dispersion is formed. The coating dispersion, which is in the form of a thin film (curtain), is "cast" onto a substrate by free fall in order to apply the coating dispersion to the substrate. DE 10 196 052 TI discloses the use of the curtain coating coating process in the production of information recording materials , whereby multilayer recording layers are realized by applying the curtain consisting of several coating dispersion films onto substrates.

In a preferred embodiment of the method according to the invention, the aqueous, deaerated application suspension has a viscosity of about 100 to about 800 mPa*s (Brookfield, 100 rpm, 20° C.). If the value falls below about 100 mPa*s or the value of about 800 mPa*s is exceeded, this leads to poor runnability of the coating slip on the coating unit. The viscosity of the aqueous, deaerated application suspension is particularly preferably about 200 to about 500 mPa*s. In a preferred embodiment, to optimize the process, the surface tension of the aqueous application suspension can be reduced to about 25 to about 70 mN/m, preferably to about 35 to about 60 mN/m (measured based on the standard for bubble pressure tensiometry (ASTM D 3825-90) , as described below). Better control over the coating process is obtained by determining the dynamic surface tension of the coating color and adjusting it by selecting the appropriate surfactant and determining the required amount of surfactant.

It has been shown that polyvinyl alcohol solutions require significantly less surfactant than dispersions, especially those with a small particle size and thus a large particle surface, in order to generate an identical surface tension.

The dynamic surface tension is measured using a bubble pressure tensiometer. The maximum internal pressure of a gas bubble that is formed in a liquid via a capillary is measured. According to the Young-Laplace equation, the internal pressure p of a spherical gas bubble (Laplace pressure) depends on the radius of curvature r and the surface tension o:

When a gas bubble is created at the tip of a capillary in a liquid, the curvature first increases and then decreases again, resulting in a pressure maximum. The greatest curvature and thus the greatest pressure occurs when the radius of curvature corresponds to the capillary radius.

Pressure curve when measuring the bladder pressure, position of the maximum pressure:

The radius of the capillary is determined using a reference measurement, which is carried out using a liquid with a known surface tension, usually water. If the radius is then known, the surface tension can be calculated from the maximum pressure pmax. Since the capillary is immersed in the liquid, the hydrostatic pressure pO, which results from the immersion depth and the density of the liquid, must be subtracted from the measured pressure (takes place automatically with modern measuring instruments). This results in the following formula for the bubble pressure method:

The measured value corresponds to the surface tension at a specific surface age, the time from the start of bubble formation to the occurrence of the pressure maximum. By varying the generation speed of the bubbles, the dependence of the surface tension on the surface age can be recorded, resulting in a curve in which the surface tension is plotted against time.

This dependency plays an important role for the use of surfactants, since the equilibrium value of the interfacial tension is not reached in many processes due to the partially low diffusion and adsorption rates of surfactants.

The formation of the individual coatings can take place on-line on a paper machine with coating unit or off-line in a separate coating process on a coating machine.

In further embodiments, the individual layers can also be applied to the base paper using the following methods:

The barrier layer can be applied to the base paper and/or to existing primers by means of a printing process.

The barrier layer can be applied to the base paper and/or to existing primers by means of extrusion.

This technique has the advantage that significantly more material can be applied, but this is only of interest if the overall product does not need to be recyclable as paper. Disadvantages, on the other hand, are lower application speeds, higher energy consumption and a higher minimum application weight. The barrier layer can be applied to the base paper and/or to existing primers by means of lamination or lining of paper, for example in the form of plastic films.

The barrier layer and the primer can also be applied one after the other in several application steps.

The present invention also relates to a coated paper which can be obtained using the processes described above.

Due to its comparatively high polarity, the barrier layer is also suitable for applying further barriers in the form of aqueous polymer solutions or dispersions, by melt extrusion or lamination with films.

The barrier layer can be improved in terms of all barriers, if required, by applying an ultra-thin metal layer, metal oxide layer, or other inorganic compound by vapor phase deposition or, in particular, vacuum vapor phase deposition. Such thin barrier layers are only effective if they adhere to a very smooth, flexible substrate that does not contain any particles (e.g. inorganic pigments) and can form a closed layer on it. The comparatively high surface energy of the barrier layer, in particular that of the polymers comprising it, favors the layer adhesion of polar materials such as metal oxides and other inorganic oxides as well as polarizable electrically conductive materials such as metals. The papers obtained in this way can in turn be made sealable again by applying a hot-seal layer or a cold-seal adhesive.

The present invention also relates to the use of a coated paper as described above or a coated paper obtainable by the process described above as packaging material.

Finally, the present invention also relates to the use of a coated paper as described above or a coated paper obtainable by the method described above as a Packaging material for food, in particular for fatty and oxidation-sensitive food.

The present invention also preferably relates to the use of a coated paper as described above or a coated paper obtainable by the method described above as packaging material for e.g. muesli bars, chocolate, products containing chocolate or crisps.

In a further preferred embodiment of the invention, the paper coated according to the invention is applied to cardboard or paperboard, in particular by lining, laminating or gluing.

Finally, the present invention also relates to the use of a composite in which a coated paper according to the invention is applied to cardboard or cardboard, in particular by lining, laminating or gluing, as packaging material for food, in particular for fatty and oxidation-sensitive food.

In this way, packaging materials can be produced in a simple and economical manner which have the advantages of both material components, such as the increased strength and rigidity of cardboard or paperboard compared to coated paper and the described advantages of coated papers. The application can take place, for example, using starch or aqueous dispersion adhesives.

The coated paper can thus preferably be a component of packaging materials based on cardboard or paperboard.

With these packaging materials according to the invention, heavier foods in particular can be securely packaged and presented to the customer in an appealing manner in the form of upright packaging.

These packaging materials preferably have a mass fraction of more than 95% by weight of the uniform type of material paper, cardboard or paperboard. A further advantage of the present invention results here that these packaging materials according to the invention are not composite packaging according to §3 (5) Are packaging law and thus this embodiment of the present invention contributes significantly to reducing the impact of packaging waste on the environment.

The present invention also relates to packaging comprising a coated paper as described above or in a composite with cardboard or paperboard as described above.

The features of the use according to the invention apply analogously to the packaging according to the invention.

The packaging can be a cold-sealed packaging. A cold-sealed packaging is preferably suitable for packaging foods such as chocolate, products containing chocolate, bars, for example muesli bars, and/or other confectionery. On the one hand, this is due to the heat sensitivity of the chocolate and, on the other hand, to a possible higher machine speed. Packaging machines based on cold seals can be operated faster because it takes a comparatively long time to heat up a heat sealing medium.

The packaging can also be a heat-sealed packaging. A heat-sealed packaging is preferably suitable for serving as secondary packaging or packaging of containers via dosing and filling scales.

The packaging can also be cold-sealed packaging

The packaging can also be a tubular bag packaging, in particular a cold-sealed or cold-sealed tubular bag packaging.

The invention is explained in detail below using non-limiting examples. examples

The following coatings were applied to a 60 g/m ² base paper containing 40% long fibers and 60% short fibers.

Undercoat/ primer:

The primer contains 75.9% pigment (layered silicate), 22.8% latex (styrene butadiene latex) and 1.3% rheology modifiers (0.2% acrylate-based thickener, 1.1% zirconium-based crosslinker).

Barrier layer:

In Examples 1 to 5 and in Comparative Example 1, polyvinyl alcohols were used. In Examples 6 to 7, polyethylene vinyl alcohols were used.

The barrier layer of Examples 1 to 7 and Comparative Example 1 comprises a pure polymer coating. Example 1' comprises a polymer coat with 99.8% polyvinyl alcohol (example 1; degree of saponification: 87%; M _w : 50900) and 0.2% rheology modifiers (Na docusate).

For this purpose, the primer was applied using a blade. The barrier layer of Examples 1 to 7 and of Comparative Example 1 was applied using a doctor blade; in contrast to Example 1, the barrier layer of Example 1' was applied using a curtain coater.

The following properties were examined:

Coat weight: application weight of the barrier coating in g/m ² .

This is determined by weighing the difference between coated and uncoated paper.

Viscosity: The viscosity was determined with a Brookfield viscometer at 23° C. and a speed of 100 rpm, with a dry content of 4%.

WVTR: Water vapor transmission rate, determined according to ISO

15106-2. OTR: Oxygen transmission rate, determined according to DIN 15105-2

HVTR: Hexane vapor transmission rate. Here, n-hexane is filled into a beaker (solvent-resistant), tightly sealed with the test item, and the weight loss is monitored over time. In the case of creased samples, a crease of 180° is produced with a roller which exerts a load of 330 g/cm on the resulting crease and in which the coating can be on the inside (inner crease) or on the outside (outer crease).

Palm kernel fat test: analogous to DIN 53116. In the case of creased samples, a crease of 180° is produced with a roller which exerts a load of 330 g/cm on the creased crease and in which the coating can be on the inside (inner crease) or on the outside (outer crease). .

Display paper: Evaluation of the display paper mentioned in DIN 53116. Here, fat penetration points with a diameter (d) >/< 1 mm are counted.

Sample paper: Evaluation of the back of the sample paper mentioned in DIN 53116. This is not part of the standard, but was carried out for better differentiation.

Seal seam strength: The samples are sealed at 3.3 bar for 0.3 seconds in the temperature range from 100°C to 220°C transverse to the direction of paper travel and the seal seam strength is determined according to DIN 55529 (2012). The optimum sealing temperature and, for comparison, the sealing force at 150° C. (optimal sealing temperature of example 1) are given. DSC melting temperature/

-Onset: The DSC curves were recorded with a Mettler DSC 20 S in cold-welded aluminum crucibles and perforated lids. The heating rates were 10 K/min in the range between 30°C and 280°C. The melting temperatures were determined from the peak minima of the melting process.

Surface tension or energy: Contact angle measuring device OCA 20 (DataPhysics) with

Software SCA 20 Measuring principle: OWRK method (Owens, Wendt, Rabel, Kaelble) Measuring liquids used and origin of the entered material constants:

Water and diiodomethane (according to Buscher) and 1,5-pentanediol (according to Gebhardt)

The coated papers obtained were examined. The results are shown in the table below.

The partially hydrolyzed polyvinyl alcohols used have very low hexane and oxygen transmission rates. This is presumably due to their relatively high hydrophilicity.

The polyvinyl alcohols with a higher degree of saponification are characterized in the coating color by a higher viscosity with the same dry content. This only makes sense from a chemical point of view since each molecule interacts more strongly with the surrounding solvent (water) due to the higher polarity.

This is to be seen as a disadvantage, since a large amount of water has to be dried in the coating process at low dry contents. This not only costs energy, but can also be difficult to implement depending on the desired application weight. In addition, the diffusion of water molecules and thus the drying itself is slowed down. Furthermore, it is more likely that gaseous water will accumulate in the coating, leading to the formation of macroscopic coating defects.

The water vapor permeability of the polyethylene vinyl alcohols examined is lower than that of the polyvinyl alcohols, which is probably due to the ethylene content and the associated lower hydrophilicity.

In general, fully saponified PVOHs should be more brittle than partially saponified PVOHs due to the greater number of hydrogen bonds they can form. This also applies to comparative example 1 (see palm kernel fat test).

In example 2, this could not be proven to date. The reason for this is assumed to be the higher molecular weight. It has already been shown for polyvinyl alcohol fibers (e.g. Gotoh et al., Polymer Journal, Vol. 32, No. 12 (2000), pp. 1049-1051: "Molecular Weight Dependence of Tensile Properties in Poly(vinyl alcohol) Fibers") that different molecular weights among otherwise identical polyvinyl alcohols have an immense influence on their physical properties.Thus, the elongation at break of heavier polyvinyl alcohols is lower, but the force required for this is many times higher.

Claims

29 claims

1. Coated paper, comprising a base paper and a barrier layer applied thereto, wherein the barrier layer comprises at least one polymer, characterized in that the polymer is an at least partially hydrolyzed polyvinyl alcohol and/or an at least partially hydrolyzed polyvinyl alcohol copolymer, each with an onset determined by means of DSC temperature less than 210°C.

2. Coated paper according to claim 1, characterized in that a primer comprising at least one inorganic pigment and a polymeric binder is present between the base paper and the barrier layer.

3. Coated paper according to claim 2, characterized in that the inorganic pigment is platelet-shaped and preferably comprises a talc, a precipitated calcium carbonate, a silicate, preferably a sheet silicate or kaolin and/or that the polymeric binder comprises a polymeric binder based on a polyacrylate .

4. Coated paper according to any one of the preceding claims, characterized in that the at least one polymer is a partially hydrolyzed polyvinyl alcohol with a degree of hydrolysis of 30% to 95%, an average molecular weight of greater than 0 and less than 100,000 g/mol and with an onset determined by DSC -Temperature less than 200°C.

5. Coated paper according to any one of the preceding claims, characterized in that the at least one polymer is a partially saponified polyvinyl alcohol with a degree of saponification of greater than 95% to 100%, an average molecular weight of greater than 70,000 g/mol and with an onset determined by means of DSC temperature of less than 200°C.

6. Coated paper according to any one of the preceding claims, characterized in that the at least one polymer is a partially hydrolyzed polyvinyl alcohol copolymer, preferably a partially hydrolyzed polyethylene vinyl alcohol, with a degree of hydrolysis of 95% to 100%, a 30 average molecular weight greater than 60,000 g/mol and having an onset temperature determined by DSC of less than 210°C.

7. Coated paper according to any one of the preceding claims, characterized in that the at least one polymer comprises a mixture of the polymers defined in claims 4, 5 and/or 6.

8. Coated paper according to any one of the preceding claims, characterized in that the partially hydrolyzed polyvinyl alcohol and/or the partially hydrolyzed polyvinyl alcohol copolymer at a dry content of 4% has a viscosity of less than 30 mPa*s, particularly preferably less than 20 mPa*s and very particularly preferably less than 15 mPa*s.

9. Coated paper according to any one of the preceding claims, characterized in that the basis weight of the barrier layer is from 5 to 20 g/m ² , preferably from 8 to 12 g/m ² , based on the dried end product (Lutro).

10. Coated paper according to any one of the preceding claims, characterized in that the coated paper, in particular the barrier layer, is free from halogen-containing compounds.

11. Coated paper according to any one of the preceding claims, characterized in that the base paper has a basis weight of 20 to 120 g/m ² , preferably 40 to 100 g/m ² .

12. Coated paper according to any one of the preceding claims, characterized in that the base paper has a long fiber content of 10 to 80% and a short fiber content of 20 to 90% by weight, with a long fiber being a fiber with a fiber length of 2.6 to 4.4 mm and a short fiber is a fiber with a fiber length of 0.7 to 2.2 mm.

13. Coated paper according to any one of the preceding claims, characterized in that a further layer comprising metals, in particular aluminum, and/or metal oxides, in particular aluminum oxide and/or silicon oxide, is applied to the barrier layer.

14. Process for producing a coated paper according to any one of the preceding claims, characterized in that an aqueous suspension comprising the starting materials of the barrier layer is applied to the base paper, the aqueous application suspension preferably having a solids content of 5 to 50% by weight from 10 to 30% by weight, and is applied with a curtain coating process (curtain coating), preferably with a double curtain coating process (double curtain coating), at an operating speed of the coater of at least 200 m/min.

15. Use of a coated paper according to any one of claims 1 to 12 or a coated paper obtainable by the method according to claim 13 as packaging material or as a component of packaging material, in particular packaging material based on cardboard or cardboard, in particular as packaging material for food, in particular for Granola bars, chocolate, products containing chocolate, or chips.

16. Packaging comprising a coated paper according to any one of claims 1 to 12 or a coated paper obtainable by the method according to claim 13, wherein the packaging is preferably a cold-sealed packaging, a heat-sealed packaging, in particular a tubular bag packaging.

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