Patent Application
20250230607
The present invention relates to a barrier paper, a method for producing such a barrier paper, the use of the barrier paper as a packaging material and a package comprising the barrier paper.
Packaging generally refers to the covering or (partial or complete) wrapping of an object, more particularly for its protection or for better handling. Accordingly, a packaging material comprises the material that forms such packaging.
Packaging materials can be composed of paper, plastics, and/or metals, for example. The present invention deals with 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 leakage of the packaged goods. For this purpose, the packaging material should fulfil different criteria depending on the packaged goods and packaging process. That is, in addition to so-called barrier properties against water, grease, oxygen or mineral oil, for example, suitable packaging materials should also meet mechanical and process-specific requirements. Depending on the packaging system, a packaging material should have sufficient tear resistance, an appropriate coefficient of friction (friction value) and flexibility; it should be either sealable, more particularly heat-sealable, and ultrasonic sealable or compatible with a cold-seal adhesive, as well as printable from the outside and should not lose its protective effect during the entire conversion and packaging process.
Depending on its composition and grammage, paper can meet many mechanical requirements, but due to its physical properties and porous structure, it requires an additional coating that provides it with heat sealability or barriers, for example.
Known paper-based, coated packaging materials often include compounds such as polyvinylidene chloride (containing halogen), or are composite materials of paper and plastic films, have a tear resistance that could be improved, which can lead to running problems on packaging systems, and/or are often not recyclable due to an excessively high coating content, adhesive components, optical inhomogeneities, or the formation of so-called adhesive impurities (stickies) via the paper fibre stream, and/or are not sufficiently resistant to buckling due to their metallic surface coating, and/or are not effective in humid climates due to their respective polymer. In addition, coatings via vacuum deposition typically require a particularly smooth and chemically suitable surface, which is typically created using a precoat.
Polyvinyl alcohols are widely known as linear water-soluble, biodegradable barrier coatings, also for paper. Such coatings present good barriers against oil, grease, oxygen, solvents and other non-polar gases, liquids, or solids. Due to their hydrophilicity, however, polyvinyl alcohols are highly permeable to polar compounds such as water. This can also influence the barrier effect against non-polar migrants, since polyvinyl alcohols absorb moisture very well, swell, and thus create pathways through the barrier coating at the molecular level.
There have been many attempts to achieve barrier deterioration at relative humidities above 50 or 60% by adding pigments, such as disclosed in WO 2010/129032, for example, or else by cross-linking the polymer chains, such as disclosed in WO 2020/109401 or U.S. Pat. No. 6,444,750, for example. However, it is not only these microscopic defects that are detrimental to the use of polyvinyl alcohols as barriers; the occurrence of macroscopic defects due to mechanical stress in packaging and converting systems also and above all impair the range of use of polyvinyl alcohols.
Polyvinyl alcohol is understood to be a fully saponified polyvinyl acetate, which is a (thermoplastic) plastic of the following formula (I), which is usually synthesized by means of radical polymerization of vinyl acetate.
The ester groups in polyvinyl acetate are relatively easy to saponify under alkaline conditions, which converts the polymer into polyvinyl alcohol, thereby making it hydrophilic and water-sensitive.
A partially saponified polyvinyl acetate is also known as 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 copolymers of polyvinyl alcohol are also of great technical importance. Copolymers are all polymers that consist predominantly (>50%) of vinyl alcohol or vinyl acetate units, regardless of the number of different monomers used in the synthesis.
Such polyvinyl alcohol copolymers preferably comprise polyethylene vinyl alcohols.
In a narrower sense, saponification here means the hydrolysis of an ester by the aqueous solution of a hydroxide, e.g. by sodium hydroxide, or by special enzymes (esterases). In contrast to acidic ester hydrolysis (the reverse reaction of esterification), they are irreversible, as the proton required for esterification is missing from the carboxylic acid. The products of the reaction are the alcohol and the salt of the acid (carboxylation) from which the ester was made up. In a wider sense, any hydrolysis of an ester can be described as saponification.
Moreover, metallized barrier papers having a low water vapour transmission rate (WVTR) of less than 5 g/m2/d at 38° C. and at 90% relative humidity are already known from the document WO 2021/023661 A1, for example.
From document WO 2021/251449 A1, a paper substrate for metal vapour deposition comprising a base paper and, on the base paper, a layer containing a polyvinyl alcohol resin is known.
Preferably, the paper substrate comprises a further layer containing a water vapour barrier resin as a filling layer, this layer preferably containing a flat pigment. Examples of the metal vapour deposition layer include aluminum, tin, nickel, copper, gold, platinum, silver, cobalt, chromium, and the like; aluminum is preferred due to its high light shielding property and low cost.
WO 2021/260043 A1 also describes metallizable barrier papers.
The object of the present invention is to eliminate the disadvantages of the known materials and to provide a material which is suitable as a packaging material, in particular for moisture- and oxidation-sensitive and fatty food products, and which 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:
Furthermore, it is the object of the present invention to further improve the applicational properties of metallized barrier papers, to find a method for the production of improved barrier papers, and to indicate their use.
This object is achieved by a barrier paper according to claim 1 with the following features:
A barrier paper comprising at least
Surprisingly, it has been found that the inventive structure of such barrier papers was able to decisively improve the applicational properties thereof. Especially it was possible to achieve at least equivalent properties as an environmentally friendly and sustainable replacement for plastic packaging for consumer goods and food, while retaining the excellent recycling properties of paper packaging, especially in waste paper cycles. In addition to the arrangement of the coating colour layer S1, its properties are of particular importance, both for the subsequent barrier layer B1 and for the properties of the barrier paper as a flexible packaging material.
It has been shown that barrier papers according to the invention can be used in common packaging machines for plastic packaging without any problems and that common packaging, such as heat-sealed tubular bags for sensitive foodstuffs such as chips and chocolate or ultrasonically sealed inner liners for tobacco products, can be formed and also sealed tightly enough.
In the following, preferred embodiments of the barrier paper according to the invention are mentioned:
Preferred is a barrier paper, wherein the coating colour layer S1 has at least one of the following properties:
The grease resistance of a barrier paper according to the invention according to the palm kernel grease test has in particular at least one of the following values listed under a) to f) (number of penetrations with a diameter <1 mm/number of penetrations with a diameter >1 mm):
Barrier paper wherein the coating colour layer S1 comprises or consists of at least one water-soluble polymer and/or a water-dispersible polymer.
Preferred is a barrier paper, wherein the at least one water-soluble polymer and/or the at least one water-dispersible polymer is selected from the group of:
In the context of the present disclosure and unless otherwise stated, the onset temperature is determined by DSC according to DIN EN ISO 11357-1:2010-03 as the intersection of the extrapolated baseline and the inflection tangent at the beginning of the melting or crystallization peak.
In particular in the case of polymers selected from this group, the barrier layer B1 can thus be applied directly to the coating colour layer, that is, e.g., without an intermediate layer, without an adhesion promoter or pretreatment of the coating colour layer, e.g. by means of corona or plasma treatment.
In addition, due to the special suitability of polyvinyl alcohols, it is possible to dispense with a precoat and still easily and reliably apply a barrier layer B1, in particular a metallization.
Preferred is a barrier paper, wherein the at least one water-soluble polymer and/or the at least one water-dispersible polymer comprise an at least partially saponified polyvinyl alcohol and/or an at least partially saponified polyvinyl alcohol copolymer, each with an onset temperature of less than 210° C. as determined by DSC.
Preferred is a barrier paper, wherein the at least one water-soluble polymer and/or the at least one water-dispersible polymer comprise a partially saponified polyvinyl alcohol with a degree of saponification of 30% to 95%, an average molecular weight of more than 0 and less than 100,000 g/mol, and with an onset temperature of less than 200° C. as determined by DSC.
Preferred is a barrier paper, wherein the at least one water-soluble polymer and/or the at least one water-dispersible polymer comprise a partially saponified polyvinyl alcohol with a degree of saponification of greater than 95% to 100%, an average molecular weight of more than 70,000 g/mol, and with an onset temperature of less than 200° C. as determined by DSC.
Preferred is a barrier paper, wherein the at least one polymer comprises 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 more than 60,000 g/mol, and with an onset temperature of less than 210° C. as determined by DSC.
Preferred is a barrier paper, wherein the partially saponified polyvinyl alcohol and/or the partially saponified polyvinyl alcohol copolymer have a viscosity of less than 30 mPas at a dry content of 4%, particularly preferably less than 20 mPas, and most particularly preferably less than 15 mPas.
Preferred is a barrier paper, wherein the at least one polymer comprises a mixture of water-soluble polymers and/or water-dispersible polymers previously mentioned as preferred.
Preferred is a barrier paper, wherein the coating colour layer S1 has a surface tension in the range of 25 to 80 mN/m, in particular 25 to 75 mN/m. Particularly preferred ranges are 45 to 70 mN/m. Preferably, the polar part of the surface tension is in the range of 55 to 80%.
These surface tensions (surface energies) are particularly advantageous for the direct application of barrier layers B1, particularly during metallization (wetting, nucleation, growth). Conventional coatings, e.g. based on BOPP or PET, must first be treated, e.g. using Corona plasma, to achieve a surface tension of at least 25 mN/m, preferably at least 35 mN/m.
Polyvinyl alcohols are suitable as suitable polymer for the coating colour layer S1 due to their high surface tension (surface energy) of about 50 to 70 mN/m, which in particular also includes a high polar part of about 70 to 80%, particularly so that thin metal layers, which have a significantly lower and predominantly disperse surface tension. For example, we determined a surface tension of 27.6 mN/m with a polar part of only 3.6% for a coating colour layer S1 based on polyvinyl alcohol onto which Al was vapour-deposited using physical vapour deposition. Pure aluminum has a very high surface tension of 1200 mN/m. In reality, aluminum is coated with an oxide layer, resulting in a surface tension of approx. 40 mN/m. As a result, the metal layer (barrier layer B1) not only adheres via weak forces as on the mostly non-polar plastics (polyolefins, polyesters, acrylic copolymers, etc.), but is more firmly anchored, which results in a high composite strength, which also proves to be an advantage when folding the barrier paper, since the metal layer (barrier layer B1) does not detach during folding, even with the more demanding outside fold.
In addition, there are other preferred water-soluble, and thus polar, polymers that possess a certain oxygen barrier and are particularly suitable for the coating colour layer S1, but also for the coating B2. Those that can be mentioned are native and modified starches, partially degraded starches, modified soluble celluloses, nanocelluloses and other polymeric carbohydrates such as chitosan, pullulan, curdlan, xanthan gum, hemicelluloses, pectins, dextran, gum acacia, karaya, guar gum, gellan gum, etc. Since many carbohydrates tend to be brittle, it is advisable to add smaller molecules that act as plasticizers in hydrophilic polymers. Examples include sugar alcohols such as glycerol and sorbitol, poly(ethylene)glycols. Other natural polymers that can be used are proteins such as casein, corn zein, keratin, collagen, gelatine, whey protein, wheat gluten, rapeseed protein, soy protein, kafirin, oat avenin, rice bran protein, lupin protein, cotton proteins or peanut proteins. Non-water-soluble polymers can also be used, especially those that do not form an oxygen barrier, but do form a barrier against fats and mineral oil vapours. These also have a high, predominantly polar surface energy.
To all these polymers mentioned, pigments may also be added that improve the barrier effect. However, care must be taken here to ensure that these do not significantly impair the smoothness of the respective layer. This can be achieved by selecting platelet-shaped pigments with a high form factor (aspect ratio), whose thickness is less than that of the respective layer and whose proportion does not exceed 50% (w/w) of the layer. Examples of such pigments are kaolins, natural and synthetic phyllosilicates (mica), talc, precipitated Ca-carbonates.
The coating colour layer S1 can be applied to one side of the base paper or to both sides. Several coating colour layers S1 can be arranged on one side of the base paper, wherein in particular the composition or layer thickness or rather the basis weight can be the same or different. The coating B2 and the coating colour layer S1 can be the same or different, wherein in particular the composition or layer thickness or rather the basis weight can be the same or different
Preferred is a barrier paper, wherein the barrier layer B1 comprises or consists of metals, in particular Al, Cu, Sn, Zn, Ag, Au, Ti, In, Si, metal alloys, metal oxides, in particular Al2O3, SiO2, mixed oxides or a combination thereof.
A preferred barrier paper is one in which the barrier layer B1 is applied by means of vacuum deposition or other known or conventional processes.
Preferred is a barrier paper, wherein the barrier layer B1 has at least one of the following features:
The barrier layer B1 can be applied to one side of the base paper or to both sides. Several barrier layers B1 can be arranged on one side of the base paper, wherein in particular the composition or layer thickness can be the same or different.
Preferred is a barrier paper, wherein the coating B2 comprises or consists of at least one polymer, in particular a polymer selected from the group of
Preferred is a barrier paper, wherein the coating B2 has at least one of the following properties:
Preferred is a barrier paper, wherein the barrier paper has at least one of the following features:
Preferred is a barrier paper, wherein the barrier paper has at least one of the following features:
Buckling resistance of at least one barrier property, in particular in the area of the inside fold or the outside fold, means that the at least one barrier property deviates from the barrier property of the barrier paper by less than a maximum of 100%, preferably a maximum of 75%, particularly preferably a maximum of 50%, and very particularly preferably a maximum of 25%, in the fold or in the area of the fold. For example, if the measured value for a barrier paper according to the invention of the WVTR=1.5 g/m2/d (at 38° C. and 90% relative humidity), the measured value for WVTR in the area of the inside fold (i.e. the inside fold is in the measuring surface) or directly at the inside fold is in the range from 0 to 3 g/m2/d (less than a maximum of 100%).
Preferred is a barrier paper, wherein the coating colour layer S1, the barrier coating B1, and the barrier coating B2 are removable in the waste paper cycle.
Preferred is a barrier paper, wherein the barrier paper after reprocessing according to INGEDE method 11 achieves the following scores according to the Assessment of Printed Product Recyclability, Deinkability Score:
Preferred is a barrier paper, wherein a precoat comprising at least one inorganic pigment and a polymeric binder is present between the base paper and the coating colour layer S1, wherein the inorganic pigment is preferably platelet-shaped and preferably comprises a talc, a precipitated calcium carbonate, a silicate, preferably a phyllosilicate or kaolin, and/or the polymeric binder comprises a polymeric binder based on a polyacrylate.
Preferably, the precoat comprises at least one polymeric binder and at least one organic and/or one inorganic pigment, and optionally further coating colour components in the usual quantities.
Pigments: phyllosilicates.
Polymeric binders: styrene acrylate latex, butadiene latex.
Rheology modifiers such as acrylate-based thickeners and/or zirconium-based crosslinkers.
Preferred is a barrier paper, in which the basis weight of the coating colour layer S1 and the coating B2 is in the range from 4 to 20 g/m2, preferably from 8 to 15 g/m2, based on the dried end product (air dry).
Preferred is a barrier paper, wherein the barrier paper is free of halogen-containing compounds except for unavoidable traces.
Preferred is a base paper, wherein the base paper has a basis weight of 20 to 120 g/m2, preferably 40 to 100 g/m2.
Preferred is a barrier paper, wherein the base paper has a long fibre content of 10 to 80% and a short fibre content of 20 to 90% (w/w), a long fibre being a fibre with a fibre length of 2.6 to 4.4 mm and a short fibre being a fibre with a fibre length of 0.7 to 2.2 mm.
Preferred is a barrier paper, wherein the base paper comprises or consists of 100% virgin fibre pulp and up to 100% recycled fibres, or comprises or consists of blends of both.
Preferred is a barrier paper, wherein the base paper comprises up to 90% recycled fibres.
Preferred is a the base paper preferably has a starch precoat or starch-containing precoat on one or both sides, in particular to improve printability.
The present invention further relates to a method for producing a barrier paper, characterized in that an aqueous suspension comprising the starting materials of the coating colour layer S1 is indirectly or directly applied to the base paper, wherein the aqueous coating suspension has a solids content of 5 to 50 wt %, preferably 10 to 30 wt %, and is applied by a curtain coating process, preferably by a double curtain coating process with an operating speed of the coating plant of at least 200 m/min.
Preferably, after the usual drying of the coating colour layer S1, the barrier paper is remoistened with a dampening unit or a steam blow box in order to achieve a moisture content of between 2.5 and 7%, preferably between 3.5 and 5%, particularly preferably between 4 and 4.5%, based on the total mass. This is ideal for barrier coating B1, especially if metallization is involved, in order to maintain the flexibility of the coating colour layer S1 and the base paper, but also to prevent too much moisture from entering a coating chamber (e.g. vacuum chamber).
Preferably, after the usual remoistening of the coating colour layer S1, the barrier paper is remoistened with a dampening unit or a steam blow box in order to achieve a moisture content of between 2.5 and 7%, preferably between 3.5 and 5%, particularly preferably between 4 and 4.5%, based on the total mass.
Preferably, after the usual remoistening of the coating colour layer S1, the surface of the coating colour layer S1 is smoothed/satinized to a smoothness value of at least 200 Bekk seconds, in particular 500 Bekk seconds, using one or more roller smoothing unit(s), glazing calenders or shoe calender(s).
Preferably, after the usual smoothing of the coating colour layer S1, the barrier layer B1 is applied directly or indirectly to the coating colour layer S1 using conventional methods.
Subsequently, the coating B2 can be applied directly or indirectly to the barrier layer B1 using conventional methods and, if necessary, dried using conventional methods.
Following the coating B2, a full-surface or partial sealing lacquer, in particular a heat-sealing lacquer or cold-sealing lacquer, can be applied directly or indirectly to the coating B2 and/or to the opposite outer side of the barrier paper by means of conventional processes and dried directly or indirectly by means of conventional processes.
Following the coating B2, a full-surface or partial print can be applied directly or indirectly to the coating B2 or to the opposite outer side by means of conventional processes to make the packaging recognizable and to provide information, and dried directly or indirectly by means of conventional processes.
The present invention also relates to the use of the barrier paper according to the invention or the barrier paper produced according to the method as packaging material for food, consumer goods, tobacco products or as a component of packaging material, in particular packaging material based on cardboard or paperboard, in particular as packaging material for food, in particular for sausage, cheese, coffee, muesli bars, chocolate, chocolate-containing products, or chips.
The present invention also relates to a package comprising a barrier paper according to the invention or a barrier paper produced according to the method, wherein the package is preferably a cold-sealed package, a heat-sealed package, an ultrasonically sealed package, in particular a tubular bag package.
In a preferred embodiment, the coated side (inner side) of the packaging faces inwards (inner side of the packaging) and the outer side faces outwards. This makes it possible to package moist and/or greasy goods in particular. In this case, the outer side is printed and the inner side is sealed with the inner side (A:A sealing) and/or the inner side is sealed with the outer side (A:B sealing).
In particular, there is the embodiment of a packaging in which the coated side (inner side) faces to the outside (outer side of the packaging) and the outer side faces to the inside. This makes it possible to protect particularly moisture-sensitive packaged goods from drying out. In this case, the inside is printed and the outside is sealed with the outside (A:A sealing) and/or the outside is sealed with the inside (A:B sealing).
The quality of the barrier paper according to the invention, in particular with regard to its buckling resistance, can be as determined by dynamic mechanical thermal analysis (DMTA), among other methods.
Dynamic mechanical thermal analysis (DMTA) is a method for measuring the viscoelastic properties of a polymer or a material system. By applying cyclic sinusoidal stress/strain forces to induce deformation without destruction, a phase angle can be calculated by shifting the sinusoidal response of the material. The phase angle of a viscoelastic material is defined as a value greater than =0° (a purely elastic material) and less than =90° (a purely viscous material). The tan value is therefore a value that represents the ratio between the modulus of elasticity (G′) and the modulus of viscosity (G″)—given by the following equation:
The tan value (a quantified loss attenuation effect), the modulus of elasticity (G′) and the modulus of viscosity (G″) of packaging materials that retain their barrier properties after buckling or folding can be compared with packaging materials that lose their intended barrier properties. We obtained a tan for a material that retains its barrier properties after buckling or folding and a tan for a material that loses its barrier properties after folding. The tan (loss factor) is within the acceptable range for viscoelastic materials (tan=0-1.0).
The barrier paper according to the invention is further preferably characterized in that the values for the loss factors tan Delta measured as a function of the temperature by means of the dynamic mechanical thermal analysis (DMTA) of the barrier paper pass through a maximum at the temperature Tg and a first inflection point at the temperature Tw with increasing temperature.
Preferably, the temperature Tg is lower here than the temperature Tw.
Barrier papers which have values for the loss factor tan Delta measured as a function of the temperature a maximum at the temperature Tg and a first inflection point at the temperature Tw, preferably wherein Tg is lower than Tw, with increasing temperature are characterized by the fact that they retain the barrier properties in an advantageous manner after buckling.
Preferably, the amount of the difference between tan delta at the temperature Tg and tan delta at the temperature Tw is >0.013, preferably >0.014 and particularly preferably >0.015.
Especially with such values for the difference between tan delta at temperature Tg and tan delta at temperature Tw, the barrier properties are advantageously maintained after buckling.
As explained above, the onset temperature is determined using 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 the inflection tangent are determined from the temperature-dependent heat flow signal. For 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 usually used to calibrate the temperature of a DSC.
The barrier paper according to the invention is characterized in particular by the fact that it is particularly suitable as a packaging material for moisture- and oxidation-sensitive as well as greasy objects, in particular foodstuffs, and can be used to produce pouches by means of heat-sealing or cold-sealing application, wherein a water-based cold-sealing adhesive can be used for the cold-sealing application. Furthermore, no layers or barrier layers based on halogen-containing compounds need to be present.
Partially saponified polyvinyl alcohols have the advantage over fully saponified polyvinyl alcohols (PVOH) or polyethylene vinyl alcohols (EVOH) that they have a significantly lower optimum sealing temperature in the heat-sealing process. This has no negative effect on the sealed-seam strength. Furthermore, partially saponified polyvinyl alcohols have a slightly lower viscosity, with otherwise the same concentration. A high viscosity is rather disadvantageous, as 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 therefore requires a coating system with a higher drying capacity, but can also be difficult to realize 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 is slowed down. Furthermore, the accumulation of gaseous water in the coating is more likely to occur, 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.
Particularly in the case of water and moisture-sensitive coating colour layers S1, e.g. based on polyvinyl alcohols, these must be protected from damp environments. This can be achieved by vacuum deposition of metals with minimal application. Vacuum deposition also requires a high degree of smoothness or minimal roughness.
Due to its minimal thickness, the vacuum deposition layer must in turn be protected against scratching, abrasion, oxidation and breaking. Moreover, an additional sealing layer is required for certain packaging production processes in order to produce pouches or similar packaging forms. This is achieved by applying a coating B2 to the barrier layer B1 (e.g. vacuum deposition layer). Additionally, the coating B2 may further improve the oxygen or water vapour barrier.
The barrier paper according to the invention is also characterized by an improved oxygen barrier, an improved grease barrier, an improved mineral oil barrier and an improved water vapour barrier.
The barrier paper according to the invention further has an improved buckling resistance, without impairing the barrier effect, and additionally is characterized by a high tear resistance.
Additionally, the barrier paper according to the invention is recyclable via the waste paper cycle.
If the barrier paper according to the invention is used as a packaging material for food, it is characterized in particular by the fact that it does not affect and/or change the taste of the packaged food.
In addition, the barrier paper according to the invention is heat-sealable and exhibits improved cold-sealing properties (compatible with the sealing medium, such as, e.g., a water-based cold-sealing adhesive), wherein the sealing seams each exhibit a sufficient moisture resistance.
Further, the barrier paper according to the invention is easy to print on the uncoated side (outside) and on the coated side (inside).
Finally, the barrier paper according to the invention can be produced relatively easily and with low coating weights, and can be recycled via the waste paper cycle.
In the following, the term “comprise” can also mean “consisting of”.
Substances are described as “hydrophobic” if they cannot be mixed with water or can only be wetted by water using surfactants. Substances are described as “hydrophilic” if they can be mixed with water or wetted by water without the use of surfactants. Hydrophobic polymers are also known as non-polar polymers and hydrophilic polymers are also known as polar polymers.
Hydrophobicity or hydrophilicity can be defined using the log P value, for example. The n-octanol/water partition coefficient Kow (spellings such as octanol/water partition coefficient are also common and correct) is a dimensionless partition coefficient known to one skilled in the art, which 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 log P value is the decadic logarithm of the n-octanol-water partition coefficient Kow. The following applies:
Kow is greater than one if a substance is more soluble in fat-like solvents such as n-octanol and less than one if it is more soluble in water. Accordingly, log P is positive for hydrophobic/lipophilic and negative for hydrophilic/lipophobic substances.
The low permeability of polyvinyl alcohols to oxygen, mineral oil, grease, and other non-polar migrants is due to their relatively high hydrophilicity.
In addition, polymers containing ethylene, such as (saponified) polyethylene vinyl alcohols, have a lower water vapour permeability, which is due to the ethylene content and the associated lower hydrophilicity.
In principle, the base paper used for the coated paper according to the invention is not limited.
However, it is preferred that the base paper has a basis weight of 20 to 120 g/m2, preferably 40 to 100 g/m2.
Furthermore, it is preferred that the base paper has a composition with a long fibre content of 10 to 80% (w/w), preferably 20 to 50% (w/w), and a short fibre content of 20 to 90% (w/w), preferably 50 to 80% (w/w).
Long fibres are meant to be fibres with a fibre length of 2.6 to 4.4 mm, and short fibres are meant to be fibres with a fibre length of 0.7 to 2.2 mm.
Additionally, 0% to 20%, preferably 0% to 5% of fillers, wherein the value 0% is preferably excluded, GCC (ground calcium carbonate), which is 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, as well as the usual auxiliary agents, such as retention agents and/or sizing agents, may be present.
The advantage of such a base paper is, on the one hand, its high flexibility and low stiffness 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, bursting strength, and tear propagation.
Common packaging systems include vertical and horizontal form-fill-seal machines for the production of stand-up pouches, flowpacks, pillow packs and more; machines that bring together two webs of the same or different materials and join them by heat sealing, cold sealing or ultrasonic sealing, e.g. also tray sealers, chamber belt machines (also with vacuum), pouch filling and sealing machines, thermoforming packaging machines, linear filling machines that apply lids by heat sealing for sealing, wrapping machines with a final heat sealing step, blister packaging machines, and X-fold packaging machines.
The barrier paper according to the invention is further preferably characterized in that a precoat comprising at least one inorganic pigment and a polymeric binder is present between the base paper and the coating colour layer S1.
The inorganic pigment is preferably platelet-shaped and comprises in particular a talc, precipitated calcium carbonate or silicates, preferably phyllosilicates and most preferably a kaolin.
As suitable polymeric binders, acrylate-based or styrene/butadiene-based binders can be named in particular. 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 cross-linked starches, so-called biolatices) and polymer-starch hybrid latices are also possible.
The polymeric binder preferably comprises a polymeric binder based on a polyacrylate.
All in all, the precoat can be a hydrophobic precoat.
In another embodiment, the precoat all in all is hydrophilic.
The precoat preferably contains 1 to 70% (w/w), preferably 5 to 50% (w/w), of a polymeric binder. The quantity refers to the dried precoat in the final product.
The precoat also preferably contains 50 to 95% (w/w), preferably 80 to 90% (w/w), of an inorganic pigment. The quantity refers to the dried precoat in the final product.
Additionally, the precoat may contain additives such as thickeners, e.g. acrylate-based thickeners, surfactants, and/or rheology modifiers. The use of crosslinkers is also conceivable. Preferably, the precoat contains a zirconium-based crosslinker and is itself crosslinked with formaldehyde.
These additives are preferably each present in an amount of 0 to 2% (w/w), preferably of more than 0 to 2% (w/w), wherein the value 0% is preferably excluded. The quantity refers to the dried precoat in the final product.
The application amount of the precoat is preferably 1 to 10 g/m2 and particularly preferably 2 to 6 g/m2. The quantity refers to the dried precoat in the final product.
If such a precoat (also called primer) is applied, this has the advantage that the paper surface is sealed and the further coating colour layer 1 applied to it only migrates slightly into the paper, and thus generates sufficient layer adhesion.
Furthermore, this precoat reduces the average roughness depth of the base paper and offers an advantageous “holdout”, which is characterized by an application covering the whole area and a defined surface energy, so that a coated coating colour layer 1 can form optimally. In addition, the precoat imparts ply adhesion between the base paper and the coating colour layer 1, which can be important for later sealing applications.
The coating colour layer 1 applied to the precoat preferably comprises an at least partially saponified polyvinyl alcohol and/or an at least partially saponified polyvinyl alcohol copolymer. The formulation used for the processing process preferably has an amount of 10 to 100, particularly preferably 50 to 99.8% (w/w) polymer.
The coating colour layer 1 may further contain additives, such as thickeners, e.g. acrylate-based thickeners; surfactants, e.g. sulfosuccinates; extensional rheology auxiliaries, e.g. polyacrylamides, carboxymethylcellulose, polyvinyl alcohols; and/or crosslinking agents, such as aldehydes and polyvalent aldehydes, zirconates, polyvalent epoxides, epichlorohydrin resins and/or hydrazides.
These additives are preferably each contained in an amount of 0.1 to 1% (w/w), based on the total weight of the coating colour layer 1.
In one embodiment, the barrier paper according to the invention is further preferably characterized in that the at least partially saponified polyvinyl alcohol and/or the at least partially saponified polyvinyl alcohol copolymer has an average molecular weight of less than 100,000 g/mol.
In another embodiment, the barrier paper according to the invention is further preferably characterized in that the at least partially saponified polyvinyl alcohol and/or the at least partially saponified polyvinyl alcohol copolymer has an average molecular weight of more than 30,000 g/mol or of more than 40,000 g/mol or of more than 50,000 g/mol or of more than 60,000 g/mol or of more than 70,000 g/mol.
In one embodiment, the barrier paper according to the invention is further 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 100%.
In another embodiment, the barrier paper according to the invention is further 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 barrier paper according to the invention is further 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 of 30% to 95%.
In another embodiment, the barrier paper according to the invention is further 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 barrier paper according to the invention is further 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 barrier paper according to the invention is further preferably characterized in that the at least partially saponified polyvinyl alcohol and/or the at least partially saponified polyvinyl alcohol copolymer has an onset temperature of less than 200° C. as determined by DSC.
The barrier paper according to the invention is further preferably characterized in that the at least one polymer comprises a partially saponified polyvinyl alcohol with a degree of saponification of less than 95% or of 30% to 95%, an average molecular weight of more than 0 and less than 100,000 g/mol, and with an onset temperature of less than 200° C. as determined by DSC.
In another embodiment, the barrier paper according to the invention is further preferably characterized in that the at least one polymer comprise a partially saponified 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 of less than 200° C. as determined by DSC.
In another embodiment, the barrier paper according to the invention is further preferably characterized in that the at least one polymer comprises 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 more than 60,000 g/mol, and an onset temperature of less than 210° C. as determined by DSC.
It has been shown that a partially saponified polyvinyl alcohol copolymer such as polyethylene vinyl alcohol is generally more flexible than a polyvinyl alcohol.
In another embodiment, the barrier paper according to the invention is preferably characterized in that the at least one polymer comprises a mixture of a partially saponified polyvinyl alcohol with a degree of saponification of 1% to 95%, an average molecular weight of more than 0 and less than 100,000 g/mol and an onset temperature of less than 200° C. as determined by DSC, a partially saponified 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 of less than 200° C. as determined by DSC, 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 more than 60,000 g/mol and an onset temperature of less than 210° C. as determined by DSC.
The degree of saponification was determined in accordance with DIN EN ISO 3681 as follows:
To polyvinyl alcohol (PVOH) (1 g), distilled water (70 mL) and neutralized ethanol (30 mL) are added and heated under reflux until completely dissolved. After cooling, the solution is neutralized with potassium hydroxide solution (0.1 M). In the case of partially saponified PVOH types, more potassium hydroxide solution (50 mL, 0.1 M) is added and heated under reflux (60 min). In the case of fully saponified PVOH types, a reduced quantity of additional potassium hydroxide solution (25 mL, 0.1 M) is used with a likewise reduced reflux time (30 min) in order to prevent the absorption of carbon dioxide in the excess lye. The excess lye is then re-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 using Eq. 1:
The mean molecular weights were as determined by size exclusion chromatography (GPC) under the following conditions:
The samples were dissolved in solvent (5 mg/mL) at a temperature of 80° C. for three hours and injected via autosampler.
To determine the calibration curve, several PMMA standards with different molecular weights were measured. By measuring the intrinsic viscosity, the calibration curve was converted into a universal calibration curve.
The barrier paper according to the invention is further preferably characterized in that the polyvinyl alcohol has a viscosity of less than 30 mPas or less than 20 mPas, particularly preferably less than 15 mPas at a dry content of 4%.
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 case of application and therefore less energy has to be used for drying; additionally, higher process speeds can be achieved. In addition to a financial advantage, this is also reflected in the use of a larger drying window on coating systems.
As already mentioned above, the quality of the barrier paper according to the invention, in particular with regard to its buckling resistance, can be as determined by dynamic mechanical thermal analysis (DMTA).
Dynamic mechanical thermal analysis (DMTA) can also be used to determine the glass transition temperature (Tg)—the temperature at which the polymer chains change from a solid, frozen state to a moving state, but before the polymer chains slide past each other, which corresponds to the melting point (Tm). It is advantageous if a viscoelastic state is present in a packaging material in order to maintain a barrier property after creasing or folding; therefore, a Tg below the operating and usage temperature is ideal. The Tg is determined in the DSC by the temperature at which tanning reaches a maximum. Preferred polymers have a glass transition temperature Tg corresponding to the tan maximum.
The barrier paper according to the invention preferably has a breaking force of >80 N/15 mm, preferably >90 N/15 mm in the running direction, and >40 N/15 mm, preferably >50 N/15 mm transverse to the running direction of the paper.
The barrier paper according to the invention also preferably has a dynamic coefficient of friction (CoF) 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 barrier paper according to the invention is further preferably characterized in that the basis weight of the individual layers is from 4 to 20 g/m2, preferably from 4 to 15 g/m2, based on the dried end product (air dry).
The barrier paper according to the invention is further preferably characterized in that the coating B2 can be wetted with conventional water-based cold seal adhesives. For this purpose, the surface energy of the coating B2 is >35 mN/m>40 mN/m, preferably >50 mN/m, particularly preferably >55 mN/m.
Such a wettability of the coating B2 has the advantage that no coating-free areas form during the application and drying process of cold seal adhesives, and that sufficient adhesion is also ensured in the case of application.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has an oxygen transmission rate (OTR) 10 cm3/m2/d (23° C., 0% relative humidity) of less than 10, preferably less than 5.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has an oxygen transmission rate (OTR) 10 cm3/m2/d (23° C., 50% relative humidity) of less than 10, preferably less than 5.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has an oxygen transmission rate (OTR) 10 cm3/m2/d (23° C., 70% relative humidity) of less than 20, preferably less than 15.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has an oxygen transmission rate (OTR) 10 cm3/m2/d (23° C., 80% relative humidity) of less than 50, preferably less than 40.
The barrier paper according to the invention preferably also does not lose this oxygen barrier due to the mechanical stress of 180° folding with a roller that exerts a load of 330 g/cm on the resulting fold and where the coating can be on the inside (inside fold) or the outside (outside fold).
The oxygen transmission rate (OTR) is determined in accordance with ISO 15105-2.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a grease barrier in accordance with test condition I according to DIN 53116.
The barrier paper according to the invention preferably also does not lose this grease barrier due to the mechanical stress of 180° folding with a roller that exerts a load of 330 g/cm on the resulting fold and where the coating can be on the inside (inside fold) or the outside (outside fold).
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a mineral oil barrier (hexane) of <10 g/m2/d.
The barrier paper according to the invention preferably also does not lose this mineral oil barrier (hexane) due to the mechanical stress of 180° folding with a roller that exerts a load of 330 g/cm on the resulting fold and where the coating can be on the inside (inside fold) or the outside (outside fold).
The mineral oil barrier is as determined by filling hexane into a beaker (solvent-resistant), sealing it tightly with the coated paper and monitoring the weight loss over time.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a water vapour permeability g/m2/d (23° C., 50% relative humidity) of less than 10, preferably less than 5.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a water vapour permeability g/m2/d (23° C., 85% relative humidity) of less than 10, preferably less than 5.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a water vapour permeability g/m2/d (38° C., 50% relative humidity) of less than 10, preferably less than 5.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a water vapour permeability g/m2/d (38° C., 70% relative humidity) of less than 20, preferably less than 10.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a water vapour permeability g/m2/d (38° C., 90% relative humidity) of less than 20, preferably less than 10.
The barrier paper according to the invention is further preferably characterized in that the barrier paper has a water vapour barrier. This remains even if the coating comes into contact with grease, which is not the case for all water vapour barriers.
The barrier paper according to the invention is further preferably characterized in that it is resistant to buckling without fold and in the inside fold as well as in the outside fold.
The coated paper according to the invention is characterized by recyclability via the waste paper cycle. Preferably conditionally recyclable preferably recyclable by assessment according to PTS PTS-RH 021.
The coated paper according to the invention is characterized by recyclability via the waste paper cycle. Preferably by an assessment according to UNI 11743 Level C preferred Level B, particularly preferred Level A, very particularly preferred Level A+.
The coated paper according to the invention is characterized by biodegradability in the marine environment according to ASTM D7081 The barrier paper according to the invention is sensorially harmless and achieves at least a value of ≤2 (according to standard DIN1230-1:2010-02).
The barrier paper according to the invention can be heat-sealed and preferably produces a sealed-seam strength (cold tack) of >3.5 N/15 mm, particularly preferably >5.0 N/15 mm, at the optimum sealing temperature, wherein the sealed-seam strength for the barrier paper was determined as follows:
The barrier paper was sealed at 3.3 bars for 0.3 s in the temperature range from 100° C. to 230° C. transverse to the paper running direction and the sealed-seam strength (cold tack) was determined in accordance with DIN 55529 (2012).
“Heat sealing” is preferably understood to mean the joining of two layers of the barrier paper by means of localized heat and/or pressure. In further embodiments, the coated side of the paper can also be joined to an opposite side of the paper that cannot be heat-sealed by itself, to an opposite side of the paper that can be heat-sealed, or to another paper by heat-sealing.
Due to its compatibility with common cold sealing media, the barrier paper according to the invention is also cold sealable. Cold sealing is generally understood to mean that a cold seal adhesive is applied to the section of a flat packaging material to be sealed using a pressure process. A cold seal adhesive has the property that it only develops an adhesive effect under and after increased pressure between the sealing jaws of a packaging machine and is otherwise not sticky or only sticky to a limited extent.
Ultrasonic sealing means focusing the energy of the ultrasound on the area to be welded and the heat generated by friction, particularly for heat sealing. The usual sealing time is between 100 and 200 milliseconds. In contrast to heat sealing, e.g. using sealing jaws, the heat required with ultrasonic technology is only generated within the materials to be welded. The tools remain cold and help to dissipate the heat.
The heat-sealed barrier paper, the cold-sealed barrier paper and also the ultrasonically sealed barrier paper are characterized by a high moisture resistance of the sealed seam.
The barrier paper according to the invention is also tear-resistant.
The barrier paper according to the invention is further characterized by the fact that the taste of food packaged therein is not affected.
The barrier paper according to the invention can be obtained economically using known manufacturing processes.
It is preferred, however, to obtain the barrier paper according to the invention by a process in which an aqueous suspension comprising the starting materials of the coating colour layer S1 is applied to the base paper, wherein the aqueous application suspension has a solids content of 5 to 50 wt %, preferably of 10 to 30 wt %, and is applied by means of a curtain coating process, preferably by means of a double curtain coating process with an operating speed of the coating system of at least 200 m/min.
This process is more particularly advantageous from an economic point of view and due to the even application over the paper web.
If the solids content falls below a value of around 10 wt %, the economic efficiency deteriorates because a large amount of water has to be removed in a short time by gentle drying, which has a detrimental effect on the coating speed. If, on the other hand, the value of 50% (w/w) is exceeded, the only effect is an increased technical effort to ensure the stability of the coating colour curtain during the coating process and the drying of the applied film, since the machine has to run very quickly again in this case.
In the curtain coating process, a free-falling curtain of a coating dispersion is formed. The coating dispersion in the form of a thin film (curtain) is “poured” onto a substrate by free fall in order to apply the coating dispersion to the substrate. DE 10 196 052 T1 discloses the use of the curtain coating process in the production of information recording materials, wherein multilayer recording layers are realized by applying the curtain, which consists of several coating dispersion films, to substrates.
In a preferred embodiment of the process according to the invention, the aqueous, deaerated application suspension has a viscosity of about 100 to about 800 mPas (Brookfield, 100 rpm, 20° C.). If the value drops below about 100 mPas or exceeds about 800 mPas, this leads to poor runnability of the coating mass on the coating unit. Particularly preferably, the viscosity of the aqueous, deaerated application suspension is about 200 to about 500 mPas.
In a preferred embodiment, the surface tension of the aqueous application suspension can be adjusted to about 25 to about 70 mN/m, preferably to about 35 to about 60 mN/m (measured in accordance with the standard for bubble pressure tensiometry (ASTM D 3825-90), as described below), in order to optimize the process. Better control over the coating process can be achieved by determining the dynamic surface tension of the coating colour and adjusting it by selecting the appropriate surfactant and determining the required amount of surfactant.
It has been shown that, compared to dispersions, especially those with a small particle size and therefore a high particle surface area, polyvinyl alcohol solutions require significantly less surfactant 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 formed via a capillary in a liquid 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 σ:
When a gas bubble is produced at the tip of a capillary in a liquid, the curvature first increases and then decreases again, resulting in the occurrence of a pressure maximum of. The greatest curvature and thus the greatest pressure occur when the radius of curvature corresponds to the capillary radius.
Pressure characteristics for the bladder pressure measurement, position of the pressure maximum:
The radius of the capillary is determined using a reference measurement carried out with a liquid with a known surface tension, usually water. Once the radius is known, the surface tension can be calculated from the maximum pressure, pmax. Since the capillary is immersed in the liquid, the hydrostatic pressure p0 resulting from the immersion depth and the density of the liquid must be subtracted from the measured pressure (this is done automatically with modern instruments). This results in the following formula for the bubble pressure process:
The measured value corresponds to the surface tension at a certain surface age, the time from the start of bubble formation to the occurrence of the pressure maximum. By varying the speed at which the bubbles are produced, the dependence of the surface tension on the surface age can be acquired, resulting in a curve in which the surface tension is plotted against time.
This dependency plays an important role in the use of surfactants, as the equilibrium value of the interfacial tension is not even reached in many processes due to the sometimes low diffusion and adsorption rates of surfactants.
The individual coatings can be formed on-line on a paper machine with a coating unit or in a separate coating process off-line on a coating machine.
In other embodiments, the individual layers can also be applied to the base paper using the following processes:
The coating colour layer S1 can be applied to the base paper and/or to existing precoats using a printing process.
The coating colour layer S1 can be applied to the base paper and/or to existing precoats by 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. The disadvantages are lower application speeds, higher energy consumption and a higher minimum application weight.
The coating colour layer S1 can be applied by laminating or lining paper, e.g. in the form of plastic films, on the base paper and/or on existing precoats.
The coating colour layer S1 and the precoat can also be applied one after the other over a number of application steps.
The present invention further relates to a barrier paper obtainable by the process described above.
Due to its comparatively high polarity, the coating colour layer S1 is also suitable for applying further barriers in the form of aqueous polymer solutions or dispersions by melt extrusion or lamination with films.
The coating colour layer S1 can be smoothed to achieve a Bekk smoothness of at least 200 Bekk seconds, in particular 500 Bekk seconds.
A further layer is applied to the metallized layer by means of vacuum deposition.
If necessary, the coating colour layer S1 can be improved with respect to all barriers by applying a barrier layer B1, in particular an ultra-thin metal layer, metal oxide layer, or other inorganic compound by vapour deposition or, in particular, vacuum vapour 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 coating colour layer S1, in particular that of the comprehensive polymers, favours the ply adhesion of polar materials such as metal oxides and other inorganic oxides as well as polarizable electrically conductive materials such as metals. The barrier papers obtained in this way can in turn be made sealable again by applying a heat-sealing layer or a cold-sealing adhesive.
A further coating B2 is applied to the vacuum-deposited barrier layer B1 using gravure printing (also curtain coating or other printing processes). A non-contact application has the advantage that the surface of the material to be coated cannot be destroyed or adversely affected.
The present invention also relates to the use of a barrier paper as described above or a barrier paper obtainable by the method described above as a packaging material.
The present invention finally also relates to the use of a barrier paper as described above or a barrier paper obtainable by the process described above as a packaging material for food, in particular for greasy, moisture-sensitive, and oxidation-sensitive food.
The present invention further preferably relates to the use of a barrier paper as described above or a barrier paper obtainable by the method described above as a packaging material for e.g. cereal bars, chocolate, meat, sausage, cheese, chocolate-containing products, butter and margarine, snacks, chips, e.g. potato or lentil chips, products baked in vegetable oil such as chips, salty snacks, biscuits, crackers, protein bars, sports and fitness bars, nutritional supplements, soup cubes, cosmetic products such as powder and make-up, coffee and tea, pet food, tobacco products (e.g. as inner liners), yogurt (as lids).
In a further, preferred embodiment of the invention, the barrier paper according to the invention is applied to cardboard or paperboard, in particular by lining, laminating, or bonding.
Finally, the present invention also relates to the use of a composite in which a barrier paper according to the invention is applied to cardboard or paperboard, in particular by lining, laminating, or bonding, as a packaging material for food, in particular for fatty and oxidation-sensitive food.
This allows to produce packaging materials in an easy and economical way that offer the advantages of both material components, such as the increased strength and rigidity of cardboard or paperboard compared to coated paper and the advantages of barrier paper described above. The application may be done using starch or aqueous dispersion adhesives, for example.
Preferably, the barrier paper can therefore be a component of packaging materials based on cardboard or paperboard.
With these packaging materials according to the invention, heavier foodstuff in particular can be packaged safely and presented to the customer in the shop in an appealing way in the form of stand-up packaging.
These packaging materials preferably have a mass fraction of greater than 95% (w/w) of the uniform material type paper, cardboard, or paperboard. Here, a further advantage of the present invention is that these packaging materials according to the invention are not composite packaging according to § 3 (5) of the Packaging Act 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 a packaging comprising a barrier 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 may be a cold-sealed packaging. A cold-sealed package is preferably suitable for packaging food such as chocolate, chocolate-containing products, bars, for example muesli bars, and/or other confectionery. This is due on the one hand 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 more quickly, since heating a heat-sealing medium is comparatively time-consuming.
The packaging can also be a heat-sealed packaging. A heat-sealed packaging is preferably suitable for use as secondary packaging or for containers via dosing and filling scales.
The packaging may also be a cold-sealed packaging
The packaging may further be primary or secondary packaging, in particular a heat-sealed or cold-sealed tubular bag packaging, flow wrap, stand-up pouches, wrapping paper, 3- and 4-side seal pouches, lids.
The barrier paper can also be used turned so that the inside faces outwards and the outside faces inwards. This ensures the use in damp environments very well. In this case, additional partial or full-surface sealing lacquer must be applied or the packaging must be ultrasonically sealed. In this case, the B2 coating is coated or printed.
The coating B2 can be opaque to prevent the barrier layer B1 from shining through. Alternatively, the B2 coating can also be printed with an opaque layer.
Preferably, the barrier paper according to the invention comprises at least one base paper having two sides (surfaces) with different roughness (rougher side r and smoother side g). Preferably, the rougher side (r) of the base paper is used for coating by means of precoat, coating colour layer S1, barrier layer B1 and barrier layer B2 (inner side). The less rough side (r) is preferably used for printing via gravure printing (outer side).
In a particular embodiment, the smoother side (g) can also be used for coating by means of precoat, coating colour layer S1, barrier layer B1 and barrier layer B2 (inner side),m and the rougher side (r) can be used for printing (outer side). This embodiment has a poorer printability, but due to its less rough surface it leads to an improvement of the barrier properties of the coating colour layer S1 or to a saving of material of the coating colour layer S1 while maintaining the same barrier properties (e.g. same property with lower layer thickness).
The invention is explained in detail below with reference to non-limiting examples and Figures.
Not shown: Printing on the barrier paper using standard printing processes, in particular by means of flexographic, gravure or digital printing processes.
The present invention is explained in more detail below in several examples. The examples serve only to illustrate the invention, but do not limit it.
The following coatings were applied to a base paper with a basis weight of 60 g/m2 or 70 g/m2 and with 40% long fibre and 60% short fibre content, using 100% virgin fibre pulp.
In all examples, the precoat contains 75.9% pigment (phyllosilicate), 22.8% latex (styrene-acrylate latex) and 1.3% rheology modifiers (0.2% acrylate-based thickener, 1.1% zirconium-based crosslinker).
The precoat was applied to the base paper using a doctor blade.
In examples 1 to 5, polyvinyl alcohols were used as polymers. In examples 6 to 8, partially saponified polyethylene vinyl alcohols were used as polymers.
The coating colour layer S1 of examples 1 to 5 comprises a pure polymer coating. Example 1′ comprises a polymer coating with 99.8% polyvinyl alcohol as polymer (example 1; degree of saponification: 87%; Mw: 50900) and 0.2% rheology modifiers (Na docusate).
The coating colour layer S1 of examples 9 and 10 and of comparative examples 1, 2, 3, and 5 corresponds to the coating colour layer S1 of example 1. The use of the coating colour layer S1 from example 1′ led to comparable results.
In Examples 9 and 10 and Comparative Examples 1, 2, 3 and 5, an approximately 40 nm to 80 nm thick barrier layer B1 was applied directly to the coating colour layer S1 or to the precoat by means of vacuum deposition of aluminum.
Embodiment by means of gravure printing and embodiment by means of curtain coating.
The following properties were examined:
The barrier papers obtained were examined. The results are shown in the following tables.
Table 1 shows the results for intermediate products that only have the coating colour layer S1, but have no barrier layer B1 and no coating B2 yet (examples Ex. 1 to Ex. 8).
Table 2 shows the results for barrier papers according to the invention, which in particular have a coating colour layer S1, a barrier layer B1 and a coating B2 (examples Ex. 9 and Ex. 10), as well as the comparative examples V1 to V5.
The partially saponified polyvinyl alcohols used have very low hexane and oxygen transmission rates. This is probably due to their comparatively high hydrophilicity.
The polyvinyl alcohols with a higher degree of saponification are characterized in the coating colour by a higher viscosity with the same dry content. This only makes sense from a chemical point of view, as the higher polarity means that each molecule interacts more strongly with the surrounding solvent (water).
This must be considered as rather disadvantageous, 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 realize in terms of application technology, depending on the desired application weight. In addition, the diffusion of water molecules and thus the drying process itself is slowed down. Furthermore, the accumulation of gaseous water in the coating is more likely to occur, leading to the formation of macroscopic coating defects.
The water vapour permeability of the polyethylene vinyl alcohols tested is lower than that of the polyvinyl alcohols, which is presumably due to the ethylene content and the associated lower hydrophilicity.
In general, fully saponified polyvinyl alcohols (PVOH) should be more brittle than partially saponified PVOH due to the larger number of hydrogen bonds they can form.
In particular, in order to maintain the essential or all barrier properties of the barrier papers even in the crease fold or crease area after buckling, the barrier papers according to all embodiments of the invention have the following characteristics:
Without being bound by this theory, the inventors considered the following:
The dynamic mechanical thermal analysis (DMTA) shows that the barrier papers according to the invention can maintain a barrier after buckling if the barrier papers have a significant viscoelastic component. A so-called “rubber-elastic-regime” state can be interpreted by quantifying the change in the tan delta value (tan δ at the glass transition temperature (Tg) to the first inflection point (Tw) of the tan delta curve with increasing temperature. A barrier material that retains its barrier properties after buckling, which surprisingly can also include a metal-coated layer, has a tan delta change of >0.013.
The dynamic mechanical thermal analysis (DMTA) also provides the glass transition temperature (Tg)—the temperature at which the macromolecule polymer chains transition from a solid, frozen state to a mobile state, but before the macromolecule polymer chains slide past each other, which is the melting point (Tm). It is advantageous if an elastic state is present in the barrier paper, in order to maintain a barrier after creasing or folding; therefore, a Tg below the operating and usage temperature is ideal. In the DSC, the Tg is as determined by the temperature at which the tan delta reaches a maximum. One of the examples has a glass transition temperature Tg=8.4° C. corresponding to the tan delta maximum of 0.099 and a tan delta change of 0.0185.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102022109277.1 | Apr 2022 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/059786 | 4/14/2023 | WO |
