Patent Application
20160222592
The invention relates to a packaging material comprising a starch-based barrier coating, to a method for producing a packaging material comprising a starch-based barrier coating, to a starch-based coating compound, to the use thereof for surface-sealing of packaging materials, and to a device for producing the multi-layer packaging material according to the invention.
In 2009, the Cantonal Laboratory of Zürich pointed out the problem of the migration of mineral oils from cardboard packaging to so-called “dry” food in scientific publications. This problem arises in particular in the case of packaging made of recycling cardboard. The inks from the newspaper production, but also the inks for printing the cardboard packaging, were identified as main sources for the mineral oil contamination. However, due to the fact that the migration occurs mainly via the gas phase, it cannot be ruled out that mineral oil residues from secondary packaging or from adjacent packaging reaches into the food when using virgin fiber-based cardboard as well.
119 samples of dry food from various packaging types were examined with regard to their mineral oil content (mineral oil saturated hydrocarbons (MOSH)) in a publication by A. Vollmer et al. from the year 2011 (Eur Food Res Technol (2011) 232:175-182). In addition to these saturated aliphatic and cyclic hydrocarbons (MOSH), the aromatic hydrocarbons (mineral oil aromatic hydrocarbons (MOAH)) in the form of mixtures of predominantly alkylated aromatic hydrocarbons play an important role as relevant contaminants. Food without additional repackaging absorb up to 70% of the MOSH and a large portion of the MOAH from the packaging via the gas phase (migration). The mineral oil contaminants thus far exceed the maximum permitted levels. According to this study, additional inner packaging made of paper or polyethylene (PE) was not able to limit the mineral oil migration. However, the authors came to the conclusion that inner packaging made of polypropylene (PE), acrylate-coated PE, polyethylene terephthalate (PET) or films comprising an aluminum coating effectively prevented the MOSH migration into the dry food at least for a period of 3 months.
In its Opinion No. 008/2010 of Dec. 9, 2009, the German Federal Institute for Risk Assessment (Deutsches Bundesinstitut für Risikobewertung (BfR)) came to similar conclusions. The elimination of direct contact of recycling paper and cardboard with dry food comprising a large specific surface by using inner bags (e.g. PET films) comprising a barrier effect are proposed as short-term courses of action in the publication, which can be obtained under http://www.bfr.bund.de/de/fragen_und_antworten_zu_mineraloel_uebergaengen_aus_verpackungsmaterialien_auf_lebensmittel-50470.html.
Such additional inner packaging, however, are not desired for economic reasons on the one hand, because it increases the price of the packaging, and, on the other hand, because it is non-environmental, because it increases the volume of waste.
The paper industry thus attempts to use further approaches to deal with the problem of the mineral oil contamination in the packaging sector. On the one hand, attempts are made to eliminate the sources for the contamination, in that fresh cardboard material and mineral oil-free inks are used. The contaminations can be prevented in this way. Foregoing the use of recycling paper, however, is extremely disadvantageous from an environmental and economic aspect.
Cleaning the old paper prior to producing the cardboard would also be a suitable way to prevent the mineral oil contamination of the food. To adhere to the migration limits, it will most likely be necessary to remove up to 99% of the mineral oil from the old paper.
However, the suitable large-scale methods for this still do not exist and it is furthermore already foreseeable that this is an extremely cost- and resource-intensive approach.
Further approaches lie in the use of multi-layer laminates and in the application of barrier layers to the inside of cardboard packaging, so as to eliminate the migration of the mineral oil contamination via the gas phase. It gives the manufacturers of paper and cardboard packaging, in particular of cartonboard and corrugated cardboard liners the opportunity to quickly react to the existing demand for new packaging solutions, without having to forego the use of recycling paper, which is extremely worthwhile ecologically.
It is proposed in WO13076241A2 to use aqueous polyvinyl acetate-based dispersions to produce a coating on film-shaped substrates to reduce the diffusion of oil-containing substances from packaging in food and medical products.
However, the polyvinyl acetate-based barrier layer is not soluble in water and thus interferes with the recycling of a cardboard, which is treated in this manner, in recycling processes, as they are currently common. In addition, the use of a synthetic coating, which has a poor biodegradability, tarnishes the look of the cardboard packaging as a packaging solution, which is extremely worthwhile ecologically. Due to the fact that such coatings are also comparatively expensive, there is still a need for cost-efficient ecologically worthwhile coating means comprising a good barrier effect.
Starch-based coatings are also known in the paper industry. In the coating industry, for example, they are used in coating colors to provide an increased stability to cardboard, or to improve the paper characteristics, such as, e.g., to reduce dust and to smooth surfaces for a better printability.
When being cooked in solution, starch already develops a high viscosity in the case of small portions of solids. For example, a 10% starch solution can have a viscosity of >5,000 mPas at 40° C. and is thus unusable in common coating processes. On principle, partially degraded starches comprising average molecular weights of Mw<<1,000,000 g/mol are used so as to keep the viscosity of the starch solution and thus of the coating compound within manageable limits (for example a 50% starch solution comprising a viscosity of <3,000 mPas at 40° C.), in the case of higher portions of solids.
Starch coatings from such degraded or partially degraded starches are relatively brittle and break easily in response to the production of the cardboard packaging. Such coatings do not withstand in particular the mechanical stresses, in response to folding and creasing of the cardboard packaging, so that undesired surface defects are created in the coating, through which, in turn, mineral oil residues can migrate into the packaged food. Such compositions are thus unusable as barrier coating, because the coating must be flexible, so as to be capable of being creased and folded, for example when processing the coated packaging material to form folding boxes.
Methods for coating paper and cardboard are generally known to the person of skill in the art. A generally common device for coating, in particular paper and cardboard, as it corresponds to the prior art, is illustrated schematically in
The coating compound can be applied to the substrate surface, e.g., by means of applying by doctor, blade coating, casting, rolling, spraying, printing or other methods, which are suitable to apply liquid compounds. A common embodiment of a coating aggregate 4 is illustrated in an exemplary manner in
The demands on the mechanical characteristics and on the barrier effect of the coating thus conflict with the demands by the coating process. At present, this problem has not yet been solved satisfactorily.
It is thus an object of the invention to provide improved compositions for the coating of packaging materials, which can be used in an economically worthwhile manner, which can be applied to planar/extensive/flat/sheet substrates in an efficient and cost-effective manner, and which thereby simultaneously have an excellent barrier characteristic.
It is furthermore an object of the present invention to provide for an improved method for coating paper and a device, which is suitable for doing so.
These objects are solved by means of the features of the independent claims. Preferred embodiments are reflected in the dependent claims.
According to a first aspect, the present invention relates to a multi-layer packaging material, comprising a planar/extensive/flat/sheet substrate and at least one starch-based barrier layer applied thereto.
The invention furthermore relates to a coating compound, which is suitable to produce the packaging material according to the invention, and which is preferably used for the production of the packaging material.
A further aspect of the invention relates to a method for producing this packaging material as well as to a device, which is suitable to carry out this method.
The invention further relates to the use of a starch-based coating compound for surface-sealing packaging materials, to prevent or to reduce the migration of lipophilic contaminants, which are contained in the packaging material, for example, into the packaged good. The coating compounds according to the invention are preferably used for this.
Lipophilic contaminants in terms of the present invention comprise mineral oil contaminants, which originate from residues of printing inks in recycled paper, for example. The contaminants comprise linear, cycled and aromatic hydrocarbons, in particular MOSH and/or MOAH.
The method for producing a packaging material comprising a starch-based barrier coating or barrier layer, respectively, differs significantly from known methods in that a coating compound comprising a highly-molecular starch in the form of a suspension of starch particles is applied to a substrate, which is to be coated, and that the suspension on the substrate is gelatinized “in situ” on the substrate with the help of hot steam, preferably of water vapor, which can include further integral parts, if applicable, and is thus solidified to form a barrier coating. In a preferred embodiment, this has the result that, in the finished state, the coating, which is located on the substrate, has particles of more or less highly destructured starch granules, which are connected/bonded to one another, and thus clearly differs from starch coatings, which are produced from dissolved starch.
As natural polymer from re-growing raw material sources, starch has the crucial advantage that the price is low and that the environmental balance is positive as compared to the synthetic products. A further advantage is that starch has already established itself in the paper industry and does not represent an interference factor in the existing recycling processes.
According to the present invention, a coating compound in the form of a low-viscous suspension of granular starch comprising long-chained or highly molecular starch molecules, respectively, is used, which can be applied efficiently with a high solids content to a suitable substrate or to a carrier layer, respectively, of a packaging material, preferably to paper or cardboard, and which is subsequently gelatinized and formed into a film by means of heat, preferably by means of steam, more preferably water vapor. The application of the starch compound in the form of a suspension thereby makes it possible to combine a high solids content in the application compound with a relatively low viscosity, as it is required for coating, while simultaneously using starch types, which have preferably not been degraded, thus long-chained starch types. Surprisingly, it turned out that long-chained starch types, in particular in the form of a suspension, can be used in a worthwhile manner for coating and that the use of these long-chained starch types in the barrier coating furthermore leads to significantly improved mechanical characteristics of the coating. The average molecular weight Mw of the starches used according to the invention should be at least 500,000 g/mol, preferably at least 1,000,000 g/mol. The abbreviation Mw in the present patent application thereby always refers to the weight average of the molecular weight.
The coating compound according to the invention represents a suspension, the solids portion of which includes starch as main component. It is in accordance with the invention that these suspended starch particles are agglutinated, that is, gelatinized, to form a cohesive barrier coating, which significantly limits or completely prevents the migration of lipophilic contaminants, in particular of MOSH (Mineral Oil Saturated Hydrocarbons) and MOAH (Mineral Oil Aromatic Hydrocarbons), into the packaged product by means of the external impact of heat and moisture immediately after applying to the substrate (“in situ”). The barrier effect is crucially dependent on the fact that a barrier coating, which is virtually free from surface defects (e.g. pinholes), is obtained across the entire surface, and that the barrier coating is flexible and not sticky, so that the paper can be processed by means of the creasing and folding techniques, which are common in the packaging industry, without the barrier coating being damaged or impacted in a noteworthy manner thereby. This is the only way to ensure that the packaged food is protected sufficiently against the mineral components passing through, even in the case of longer storage periods.
Starch is the main component of the coating compound according to the invention. In terms of the invention, starch, the average molecular weight Mw of which is at least 500,000 g/mol, preferably at least 1,000,000 g/mol and more preferably at least 2,000,000 g/mol (determined via GPC-MALLS), is used in the case of at least one coating process, thus in the case of at least one layer of the barrier coating or barrier layer, respectively. The average molecular weights are determined by means of GPC-MALLS (gel permeations chromatography by means of multi-angle laser light scattering) after pressure cooking the starch at 150° C. in a closed autoclave. The “in situ” gelatinization according to the invention in combination with the use of aqueous coating compounds comprising a high content of highly-molecular starches allows the use of coating compounds, which can be processed well, which have a low viscosity, and which have a relatively low water content, so that the water quantity applied to reach the desired layer thicknesses/starch quantities can be dried off subsequently by means of the systems, which are available and which are common in the paper industry, without having to make large investments for more powerful drying sections. Further advantages, which are associated with the reduction of the water quantity, which is applied to the paper substrate to be coated, will be explained below. It is an enormous advantage not only with regard to the additional system costs, when the quantity of water, which must be dried off, can be kept as low as possible.
The lower the molecular weight, the more brittle the starch becomes, even in the case of higher plasticizer contents. On the other hand, the viscosity of starch in solution increases massively with the molecular weight of the starch. The viscosity in solution is dominated primarily by the largest starch macromolecules. As a result, a 10% solution of desired large starch macromolecules can already result in a viscosity of >5,000 mPas. However, a lower viscosity is desired for applying the starch compound to the paper. Solutions comprising a water content of at least 90% would be necessary to apply a sufficient quantity of suitable high-molecular starch macromolecules to the paper. To obtain the same layer thickness with a 10% solution as with a 50% solution, roughly 10-times the amount of water is required. Water, which must dried off at high costs. To obtain a 50% pourable starch solution, the starch must be degraded very strongly and the mechanical characteristics thereof are then far different than the required characteristics.
The approach according to the invention of not applying the starch to a packaging material, which is to be coated, for example a paper, as a solution, but to use at least a part of the starch in the aqueous coating compound in the form of non-gelatinized, suspended starch granules, provides a way out of this situation. When using such granular starch, however, it must be ensured that the starch is at least partially released from the granules on the substrate, that is, that the starch is gelatinized, so that the large starch macromolecules contained therein can take effect. To be able to gelatinize the starch granules, sufficient water must be present, and the granules must be heated up to the gelatinizing temperature. Water thereby diffuses into the granules and a phase conversion in the starch granule then occurs under the influence of the water, whereby the partially crystalline structure converts into an amorphous structure. Due to the fact that the starch granules absorb a lot of water hereby, even a highly fluid compound solidifies thereby. In response to a planar application, a film comprising a certain stability is obtained. The suitable barrier film results from subsequent drying.
Native starches are present in the form of starch granules. These granules are birefringent under the polarization microscope. The person of skill in the art is well aware that starch granules can be gelatinized, for example in aqueous solutions. In response to gelatinizing, the starch granules absorb water and thereby swell considerably. Gelatinized starch granules also burst and disintegrate easily, for example under shear, into fragments, which can then also dissolve completely, until a genuine molecular solution is created. The transition from non-swollen starch via gelatinized starch granules to dissolved starch can be subdivided into the following stages:
Stage 1: the crystallinity of the starch is at most partially destroyed, under the polarization microscope
stage 1.1: maximally 5% of the granules are no longer birefringent
stage 1.2: 5-10% of the granules are no longer birefringent
stage 1.3: 10-20% of the granules are no longer birefringent
stage 1.4: 20-30% of the granules are no longer birefringent
stage 1.5: 30-40% of the granules are no longer birefringent
Stage 2: the crystallinity of the starch is substantially destroyed, under the polarization microscope
stage 2.1: 40-50% of the granules are no longer birefringent
stage 2.2: 50-60% of the granules are no longer birefringent
stage 2.3: 60-80% of the granules are no longer birefringent
stage 2.4: 80-100% of the granules are no longer birefringent
Stage 3: maximally 5% of the granules are birefringent
stage 3.1: and 1-10% of the granules have burst
stage 3.2: and 10-20% of the granules have burst
stage 3.3: and 20-30% of the granules have burst
stage 3.4: and 30-50% of the granules have burst
stage 3.5: and 50-70% of the granules have burst
stage 3.6: and 70-100% of the granules have burst
Burst starch granules are characterized in that the starch granules have tears on the surface and/or that the previously relatively smooth surface has been deformed significantly (e.g. wrinkled surface). In addition to starch particles, which are still present as whole granules, starch particles, which have disintegrated into fragments, can also be present. However, the starch granules as well as the fragments can still be recognized as entities.
Stage 4: no birefringence whatsoever is observed, the starch granules are substantially destroyed.
Stage 4.1: fragments of starch granules are still present, the majority of the starch is present in dissolved form
Stage 4.2: the starch is present in completely dissolved form
In the context with the present patent application, a granular starch is identified as a starch, which is destructured to stage 1.5 at the most. Preferably, the starch is destructured up to stage 1.4 at most, more preferably up to stage 1.3, more preferably up to stage 1.2, and most preferably up to stage 1.1 at most. Starches comprising one of these destructuring stages can also be identified as non-gelatinized starches.
In terms of the present invention, starches of destructuring stages 2 and 3 are considered to be gelatinized starch or gelatinized starch particles. Gelatinized starches thus have a destructuring degree in the range of at least stage 2.1 to stage 3.6 at the most. In ascending order, stages 2.2, 2.3 and 2.4 in each case also represent preferred lower limits. In descending order, stages 3.5, 3.4, 3.3, 3.2 and 3.1 in each case also represent preferred upper limits.
A dissolved starch is destructured to at least stage 4.1. According to a particularly preferred embodiment of the present invention, the dissolved starch is present as genuine molecular solution.
The destructuring degree of the starch particles can be determined easily under the polarization microscope, for example with a 200-fold magnification, by means of the above graduations.
Unless specified otherwise, it applies for the present patent application that all of the plasticizer portions are based on the sum of starch plus plasticizer. The portion of the thickening agent(s) is based on the sum of starch plus thickening agent. The portion of the additive(s) is based on the dry mixture or composition, respectively. Unless specified otherwise, the information % by weight always refers to proportion by weight per proportion by weight.
After subtracting an optionally comprised filler component, the aqueous coating compound according to the invention comprises the following portions of components:
In accordance with an embodiment according to the invention, the coating compound consists of the components a) to d) comprising the above-mentioned portions and one or a plurality of the thickening agents defined below comprising the specified portions.
In accordance with a further embodiment according to the invention, the coating compound consists of the components a) to d).
The coating compound can be used directly to coat the substrate.
Viscosity and pH-Value of the Coating Compounds
The lower limit of the viscosity of the coating compounds according to the invention in mPas, measured at 40° C. by means of a Brookfield viscometer at a rotational speed of 100 rpm, is 50, preferably 70, more preferably 100, most preferably 150.
The upper limit of the viscosity in mPas is 5,000, preferably 3,000, more preferably 2,500, more preferably 2,200, more preferably 2,000, more preferably 1,800, more preferably 1,600, most preferably 1,500.
If the viscosity is too low, an application weight, which is too low, and thus a barrier coating, which is too thin, is obtained when using a starch-based coating compound. If the viscosity is too high, an application weight, which is too high, is obtained, or a controlled, even application is impacted disproportionately when using a starch-based coating compound.
The pH-value of the coating compound according to the invention is preferably >4, preferably >5, preferably >6, more preferably >6.5, most preferably >6.7 and, on the other hand, preferably <10, preferably <9, preferably <8.5.
According to a further aspect, the present invention relates to the use of the above-explained coating compound for the surface sealing of packaging materials, to prevent or to reduce the migration of lipophilic contaminants, such as mineral oil residues, in particular MOSH and/or MOAH contained in the packaging material.
In particular, the coating compound according to the invention is used for surface sealing the inner sides of packaging.
After subtracting an optionally comprised filler component (which will be defined in more detail below), the barrier coating or barrier layer, respectively, according to the invention comprises the following components:
In the barrier coating, which is applied to a substrate, >20% by weight, preferably >30% by weight, more preferably >50% by weight, more preferably >70% by weight, more preferably >90% by weight, more preferably >95% by weight, most preferably >99% by weight of the starch is present in the form of gelatinized starch particles. The gelatinized starch particles contribute to the advantageous mechanical characteristics, such as the flexibility of the coating, in particular if macromolecules, which move easily and which are chosen from the thickening agents, are located between the particles.
The gelatinized starch particles of the barrier layer follow from the granular starch, which is used in the coating compound. In other words, the gelatinized starch or the gelatinized starch particles, respectively, of the barrier layer have a higher destructuring degree than the granular starch, which is used in the coating compound.
A starch, in the case of which at least 40%, preferably at least 50%, more preferably at least 60%, more preferably at least 80% of the starch granules are no longer birefringent, are/is identified as gelatinized starch particles or gelatinized starch. Gelatinized starch particles or gelatinized starch also comprises starch in a state, in which maximally 5% of the starch granules are still birefringent, and wherein at least 1%, preferably at least 10%, more preferably at least 20%, more preferably at least 30%, more preferably at least 50%, more preferably at least 70% of the starch granules have burst.
To provide for the required mechanical characteristics of the barrier coating, it is first and foremost the fraction of the largest macromolecules, which is relevant. >20% by weight, preferably >30% by weight, more preferably >40% by weight, more preferably >50% by weight, most preferably >60% by weight of the starch of the barrier coating are to thus have a molar mass Mw of >1,000,000 g/mol.
In a preferred embodiment >20% by weight, preferably >30% by weight, more preferably >40% by weight, more preferably >50% by weight, most preferably >60% by weight of the starch of the barrier coating are to have a molar mass Mw of >2,000,000 g/mol.
In a further preferred embodiment, >20% by weight, preferably >30% by weight, more preferably >40% by weight, more preferably >50% by weight, most preferably >60% by weight of the starch of the barrier coating are to have a molar mass Mw of >3,000,000 g/mol.
In a further preferred embodiment, >20% by weight, preferably >30% by weight, more preferably >40% by weight, more preferably >50% by weight, most preferably >60% by weight of the starch of the barrier coating are to have a molar mass Mw of >5,000,000 g/mol.
In a further preferred embodiment, >20% by weight, preferably >30% by weight, more preferably >40% by weight, more preferably >50% by weight, most preferably >60% by weight of the starch of the barrier coating are to have a molar mass Mw of >10,000,000 g/mol.
As already mentioned above, the average molecular weight Mw of the used starches is at least 500,000 g/mol, preferably at least 1,000,000 g/mol. It turned out to be advantageous, when this weight average of the molecular weight distribution Mw of the starch of the barrier coating in g/mol is >500,000, preferably >1,000,000, more preferably >2,000,000, more preferably >2,500,000, more preferably >3,000,000, more preferably >4,000,000, more preferably >5,000,000, more preferably >7,000,000, particularly preferably >10,000,000.
On principle, the upper limit of the average molecular weight Mw of the used starches, in particular of the granular starch, is not limited, except for by the natural conditions. Preferably, the upper limit is 50 million g/mol.
Due to the fact that in the case of the production methods according to the present invention, the used starch molecules can virtually not be degraded with regard to the molecular weight thereof, the above-mentioned information relating to the molecular weights of the starch in the barrier coatings can also be transferred to the coating compounds. Here, the information relates in particular to the used granular starch, which can be found as gelatinized starch particles in the coating.
The barrier effect of the barrier coating increases as the surface weight increases. The surface of papers, pasteboard and cardboard is not really smooth, but has a more or less pronounced roughness—a type of mountain and valley landscape. To obtain a good barrier effect, the valleys must initially be filled and the layer should then also still cover the highest mountain peaks. Due to the fact that rougher papers are also used in the packaging industry for price reasons, and due to the fact that a good barrier layer is to also be obtained for these papers, it is necessary that thicker barrier coatings can be applied. A barrier effect, which is as good as possible, and rougher papers thus require a larger application quantity or a larger surface weight, respectively. However, the material and process costs are also increased with this, in particular because thick layers can only be obtained by repeatedly applying coating compounds, wherein the applied layer must be dried in-between in each case. In addition, the flexibility of the barrier coating is reduced as the thickness increases.
The upper limit of the surface weight (dry compound) of the barrier in g/m2 is thus preferably 80, preferably 70, more preferably 60, more preferably 55, more preferably 50, more preferably 45, more preferably 40, more preferably 35, more preferably 30, most preferably 25.
On the other hand, the lower limit for the surface weight of the barrier in g/m2 is 3, preferably 5, more preferably 7, more preferably 9, most preferably 10.
A starch, in the case of which at least 60%, preferably at least 70%, more preferably at least 80%, more preferably at least 90%, more preferably at least 95%, most preferably approximately 100% of the starch granules are birefringent, is identified as granular starch. In response to the gelatinization, the starch granules lose their birefringent character, which is due to the partially crystalline structure of the starch granules. Birefringent is thus synonymous with non-gelatinized. Suspended in water, birefringent starch granules only absorb water to a limited extent, hardly swell, and act like solid particles. They create only little viscosity. The starch granules absorb water only after being heated to the gelatinizing temperature and swell strongly, and then create a highly distinct viscosity.
The upper limit for the portion of the granular starch in the coating compound in % by weight, after deducting an optionally comprised filler component, is 75 at the most, preferably 70 at the most, preferably 65 at the most, more preferably 60 at the most, more preferably 56 at the most, more preferably 53 at the most, more preferably 49 at the most, most preferably 47 at the most.
After deducting an optionally comprised filler component, the lower limit for the portion of the granular starch in the coating compound in % by weight is at least 10, preferably at least 15, preferably at least 20, more preferably at least 25, more preferably at least 28, more preferably at least 31, most preferably at least 34.
The preferred weight average of the molecular weight distribution Mw of the granular starch in g/mol is >500,000, preferably >1,000,000, preferably ≧2,000,000, preferably >2,500,000, preferably >3,000,000, preferably >4,000,000, preferably >5,000,000, preferably >7,000,000, particularly preferably >10,000,000.
With regard to the origin and the preparation, any granular starches or mixtures thereof can on principle be used as granular starch. For example, they can be used in the native state, as well as in the physical and/or chemically/enzymatically modified state.
With regard to the origin, root starches, such as, for example, potato starches or tapioca starches are preferred, because, as compared to starches of a different origin, they have low gelatinizing temperatures, and because the solidification or gelification, respectively, of the coating compound to a barrier coating is thus already possible at low temperatures. Tapioca starch is particularly preferred. Tapioca starch is colorless and flavorless and genetically modified alternatives of tapioca starches are not yet known. Pea starch is further preferred, because it turned out that it has particularly good film-forming characteristics.
In a preferred embodiment, the granular starch is used in the native, that is, in the non-modified state. Usable characteristics can be obtained herewith at low costs.
In a further preferred embodiment, substituted granular starches, such as starch esters and starch ethers, are used, such as, for example, hydroxypropylated or acetylated starches. These modifications lead to a particularly high expansibility of the barrier coating, which is an important advantage in response to the creasing and folding of the barrier coating. In the alternative, oxidized starches are used. Hydroxypropylated starches are particularly preferred.
In a further preferred embodiment, cross-linked granular starches are used, in particular cross-linked starch esters or cross-linked starch ethers, respectively, for example starch phosphates and starch adipates. Preferably, the cross-linking is pronounced lightly. Such starches are commercially available. By increasing the molecular weight, which is associated with the cross-linking, improved mechanical characteristics are obtained. Cross-linked hydroxypropylated starches, in particular lightly cross-linked hydroxypropylated starches are particularly preferred. In terms of a simpler processing, non-cross-linked hydroxypropylated starches are preferred.
According to a further embodiment of the present invention, the granular starch is a non-cross-linked starch or a mixture of non-cross-linked starches.
In a preferred embodiment, substituted granular tapioca starch is used, in particular hydroxypropylated tapioca starch. Preferably, cross-linked substituted tapioca starch, such as, for example, hydroxypropylated starch phosphate.
In a further preferred embodiment, substituted granular pea starch is used, in particular hydroxypropylated pea starch. Preferably, cross-linked substituted pea starch, such as, for example, hydroxypropylated starch phosphate
The amylose content of the granular starch(es) in % by weight is preferably <60, more preferably <50, more preferably <40, more preferably <37, more preferably <35. It turned out that high amylose contents can lead to a reduced expansibility of the barrier coating.
The amylose content of the granular starch(es) in % by weight is preferably >=0, more preferably >0.5, more preferably >0.7, more preferably >1, more preferably >2.5, most preferably >5. Amylose contents, which are too low, can lead to a reduced expansibility of the barrier coating.
According to a preferred embodiment, so-called “waxy” starches are not used in the coatings and the coating compounds according to the invention.
Granular starches comprising a dextrose equivalent (DE) of <3, more preferably <1, most preferably <0.7, more preferably <0.5, more preferably <0.2, more preferably <0.1, particularly preferably <0.05 are further preferred. The dextrose equivalent of a polysaccharide mixture identifies the percentage of reducing sugar in the dry substance. It corresponds to the quantity of glucose (=dextrose), which had the same reducing power per 100 g of dry substance. The DE value is a measure for how far the polymer degradation has occurred. In the case of high DE values, inferior mechanical characteristics are obtained. The dextrose equivalent is determined according to the ISO standard 5377.
According to preferred embodiments of the invention, granular starches are used, which are approved for applications for contact with food.
The coating compound can optionally also comprise dissolved starch. In the case of aqueous systems, the dissolved form of starch is typically obtained by means of the following measures, which are well-known to the person of skill in the art: cooking the starch, in particular by means of a jet cooker, heating to a temperature above the gelatinizing temperature, dissolving extruded, amorphous starch, using pre-gelatinized starch. As defined above, dissolved starch can be present in the form of a genuine molecular solution, but fragments of destructured starch granules can also be present. Gelatinized starch granules disintegrate easily into fragments under shear and can then dissolve completely. The gelatinized starch granules also disintegrate into fragments by means of cooking them for a longer period of time, and a genuine molecular solution is finally created. A molecular solution is obtained very quickly by means of the jet cooker.
Dissolved starch can be used just as the thickening agents mentioned below, to increase the viscosity of the coating compound and to modify the characteristics of the barrier coating. According to the present invention, the use thereof is optional.
With regard to suitable starch and preferred types, substantially the same statements as were made with regard to the granular starch, apply for the dissolved starches. Limitations relating to the molecular weight are exceptions. Dissolved starch can also have a smaller molecular weight than granular starch. According to an embodiment, short-chained starch comprising a molecular weight Mw of less than 500,000 g/mol can also be used as dissolved starch.
After deducting an optionally comprised filler component, the portion of the dissolved starch in the coating compound in % by weight is preferably <50, more preferably <40, more preferably <30, more preferably <20, more preferably <15, more preferably <10, more preferably <7, most preferably <5.
According to one embodiment, the lower limit of the portion of the dissolved starch in the coating compound in % by weight, after deducting an optionally comprised filler component, is 0. The lower limit can also be 0.5% by weight, more preferably 1% by weight, more preferably 2% by weight, most preferably 3% by weight, in each case after deducting an optionally comprised filter component.
Water is important for adjusting the viscosity of the coating compound and for the gelatinization after converting the coating compound into a barrier coating. The higher the water content of the coating compound, the lower the viscosity thereof, and the lower the required gelatinizing temperature. On the other hand, a high water content makes drying more difficult, because more water must then be removed from the barrier coating.
The upper limit for the water content of the coating compound in % by weight, after deducting an optionally comprised filler component, is 90, preferably 85, more preferably 80, more preferably 75, more preferably 72, more preferably 69, most preferably 66.
The lower limit for the water content of the coating compound in % by weight, after deducting an optionally comprised filler component, is 25, preferably 30, more preferably 35, more preferably 40, more preferably 44, more preferably 47, more preferably 51, most preferably 53.
A portion of the water of the coating compound can be bound in the solidified barrier layer. The remaining water is lost in response to drying the packaging material as well as in response to the subsequent storing. The maximum water content of the barrier layer in the finished packaging material directly after the production is maximally 25% by weight, preferably maximally 20% by weight, more preferably maximally 15% by weight, more preferably maximally 10% by weight, more preferably maximally 7% by weight, more preferably maximally 5% by weight, more preferably maximally 3% by weight.
In response to the coating of paper, the person of skill in the art tends to refer to solids content instead of to water content, wherein it goes without saying that the two characteristics are directly interlinked. The solids content defines the sum of all of the components, including possible fillers, except for water.
The solids content of a sample can be determined by means of simply considering the difference of the sample prior to and after drying. For this purpose, the samples are dried, for example, in a standard drying cabinet for 10-60 minutes at 130° C. To determine the solids content of the coating compound on the substrate, thus in the packaging material, a piece of coated substrate is used as sample.
The lower limit for the solids content of the coating compound in % by weight, after deducting an optionally comprised filler component, is 10, preferably 15, more preferably 20, more preferably 25, more preferably 28, most preferably 34.
The upper limit for the solids content of the coating compound in % by weight, after deducting an optionally comprised filler component, is 75, preferably 70, more preferably 65, more preferably 60, more preferably 56, more preferably 53, more preferably 49, most preferably 47.
On principle, all of the plasticizers listed in the prior art for starch as well as any mixtures thereof are possible as plasticizer. A small plasticizer content leads to a brittleness of the barrier layer in the case of low humidity, while a high plasticizer content leads to adhesives and to a soft material of little expansion in the case of high humidity.
Plasticizers can be used individually or in mixtures of different plasticizers. Preferably, polyols are used, such as, for example, glycerin, sorbitol, maltitol, erythritol, xylitol, mannitol, galactitol, tagatose, lactitol, maltulose, isomalt, maltol etc., but also various sugars, such as saccharose/sucrose, maltose, trehalose, lactose, lactulose, galactose, fructose etc., as well as mono- and oligosaccharide. Glycerin is particularly preferred as plasticizer. Water is also a plasticizer for starch, but is not counted as plasticizer here and will be considered separately.
The upper limit for the plasticizer content of the coating compound in % by weight, based on starch (granular starch plus dissolved starch) plus plasticizer, is 70, preferably 60, more preferably 55, more preferably 50, more preferably 46, most preferably 42.
In one embodiment, the lower limit for the plasticizer content of the coating compound in % by weight, based on starch (granular starch plus dissolved starch) plus plasticizer, is 0. In such embodiments, the coating compound according to the invention, except for unavoidable contaminants, thus does not comprise a plasticizer. According to preferred embodiments, the lower limit in % by weight is 5, preferably 10, more preferably 15, more preferably 20, more preferably 25, more preferably 28, more preferably 31, more preferably 32.5, most preferably 33.5, in each case based on starch (granular starch plus dissolved starch) plus plasticizer.
The limits for the plasticizer content of the barrier coating correspond to the limits for the plasticizer content of the coating compound.
In preferred embodiments, plasticizers comprising a maximum melting temperature of 150° C. (for the anhydrous plasticizer), preferably 125° C., particularly preferably 110° C., more preferably 95° C., most preferably 70° C. are used. The portion of these plasticizers in the total plasticizer content in % by weight is >50, preferably >70, particularly preferably >80, particularly preferably >90. As the melting temperature of the plasticizer decreases, the plasticizing effect thereof increases.
In a preferred embodiment, plasticizers comprising a molar mass in g/mol of >90, preferably >120, preferably >140, preferably >150, most preferably >160 are used. The portion of the plasticizers, which fulfill this condition, in the total plasticizer content in % by weight is >10, preferably >20, particularly preferably >30, most preferably >40. The ability of the plasticizer to migrate decreases as the molar mass thereof increases, the plasticizer then has a reduced tendency to migrate from the barrier coating into the paper. The barrier coating loses flexibility as a result of the migration of the plasticizers.
In preferred embodiments, a combination of at least 2 plasticizers is used, preferably of at least 3 plasticizers, wherein at least 5%, preferably at least 10%, most preferably at least 15% of the individual plasticizers are represented in the combination. 2 isomeric plasticizers are hereby considered to be different plasticizers. The tendency of the individual plasticizers to crystallize can be reduced by the combination of plasticizers. The plasticizing effect disappears in response to crystallization.
A thickening agent or a mixture of thickening agents can be added to the coating compound, so as to adjust the viscosity of the coating compound to a desired value, and so as to obtain the homogeneity of the coating compound, in particular so as to prevent the sedimentation of the granular starch and so as to thus simplify the production process of the barrier coating. In addition, thickening agents are used advantageously for modifying the mechanical characteristics of the barrier coating, in particular to increase the flexibility thereof.
On principle, all hydrophilic substances and mixtures thereof, which increase the viscosity, are possible as thickening agents, in particular hydrophilic polymers and of those, preferably those from plant-based sources. Preferably, this polymer contains polar groups, such as, for example, hydroxyl groups, carboxyl groups or also ionic groups, such as carboxylate- or sulfonate groups.
Hydrocolloids and rubbers, such as galactomannans, such as guar rubber or locust bean gum; cellulose derivatives, in particular cellulose ethers; pectins, in particular rhamnogalakturonanes and protopectins; dextrans; xanthan; zymosan; hydrocolloids of seaweed, such as alginates, agar-agar, agarose, carrageen and carrageenans; furcellaran; hydrocolloids of lichens, such as lichenins and isolichenins, or hydrocolloids as exudates of woods, such as tragant (astragalus rubber), karaya rubber, rubber arabicum, kutira rubber; inulin; latex; chitin; chitosan; gellan; collagen; gelatin; casein as well as any combinations thereof, are examples for thickening agents.
Dissolved starch can be used for the same functionality as the thickening agents, but is not counted as thickening agent in the context of the present invention and will be considered separately.
In preferred embodiments, the maximum portion of thickening agent of the coating compound, as well as in the barrier coating, in % by weight, based on starch plus thickening agent, is 50, more preferably 40, more preferably 30, more preferably 20, more preferably 10, more preferably 5, more preferably 2.5, most preferably 1.5.
If present, the minimum portion of thickening agent of the coating compound, as well as in the barrier coating, in % by weight, based on the starch plus thickening agent, is 0.01, preferably 0.05, more preferably 0.1, more preferably 0.3, more preferably 0.6, more preferably 0.8, most preferably 1.0. Due to the fact that the thickening agent represents an optional component, the minimum portion can also be 0% by weight.
According to preferred embodiments, xanthan is used as thickening agent, because xanthan can prevent the sedimentation of suspended starch particles in a particularly effective manner. A maximum portion of <2.5% by weight of xanthan as thickening agent of the coating compound and of the barrier coating turned out to be particularly advantageous in further embodiments. The minimum portion of xanthan in % by weight is 0, preferably 0.01, more preferably 0.05, more preferably 0.1. Xanthan can in particular be used in combination with one or a plurality of any of the above-mentioned granular starches. In particular, any combination with the other components, thus dissolved starch, plasticizers, fillers, and additives are also comprised according to the invention.
In a further preferred embodiment, water-soluble cellulose derivatives are used, such as, for example, methylcellulose, ethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, hydroxyethylmethylcellulose or hydroxypropylmethylcellulose.
Preferably, polyvinyl alcohol (PVA) is furthermore used, because PVA does not only create viscosity and stabilizes the coating compound, but also improves the mechanical characteristics of the barrier coating, in particular the flexibility thereof. Preferably, PVA comprising hydrolysis degree of >70%, more preferably of >75% more preferably >80%, most preferably >85% is used. A hydrolysis degree of <99%, more preferably of <98%, most preferably <96% is furthermore preferred. According to DIN 53015, a 4% solution of the PVA at 20° C. preferably has a viscosity in mPas of >3, preferably >5, more preferably >7, more preferably >10, most preferably >15.
In a preferred embodiment, the maximum portion of PVA in % by weight, based on the dry coating compound after deducting the optionally comprised filler component, is 40, more preferably 30, more preferably 20, more preferably 15, more preferably 10, more preferably 7, more preferably 5, particularly preferably 4. The minimum portion of PVA in % by weight is 0, preferably 0.1, more preferably 0.3, more preferably 0.6, more preferably 1, more preferably 1.5, more preferably 2, most preferably 3. PVA can in particular be used in combination with one or a plurality of any of the above-mentioned starches. In particular, any combination with the other components, thus dissolved starch, plasticizers, fillers and additives are also comprised according to the invention.
The above-mentioned ranges for xanthan and PVA are to obviously be understood such that, if one component or both components is/are present, they form a portion of the thickening agent or that the thickening agent includes only xanthan and/or PVA. The value ranges for xanthan and PVA are thus not to be understood as additive to the above-mentioned general value ranges for the thickening agent.
The limits for the portion of thickening agent of the barrier coating correspond to the limits for the portion of thickening agent of the coating compound.
All components, which are virtually insoluble in water or which are present in the coating compound as well as in the barrier coating in the form of particles, such as, for example, pigments, glass particles, soot particles, mineral particles, such as titanium dioxide, talc, carbonates, are identified as filler component of the coating compound or of the barrier coating, respectively. It is important to note here that starch particles and also the birefringent starch granules are never counted as being fillers in the context of the present invention. For the most part, the filler component is mathematically deducted in the formulations, because it is not significant for most of the functional components of the formulation, whether or not a filler component is present. Unless otherwise specified and unless obvious otherwise, the % by weight information relating to the composition of the barrier coating in each case refer to the portions without the filler component.
The portion of the filler component in % by weight in the case of the dry barrier coating is <70, preferably <50, more preferably <30, more preferably <20, more preferably <10, more preferably <5, most preferably <3. The filler component must also be heated in response to the gelatinization and thus requires a higher energy input in this step. The flexibility of the barrier coating is mainly reduced in the case of higher portions of filler.
The limits for the portion of filler of the barrier coating correspond to the limits for the portion of filler in the coating compound. The portion of the filler(s) is not considered when calculating the 100% by weight of the components of the coating compound. In fact, the filler is added additionally to the 100% by weight of the other components when formulating the weight. However, the filler is considered when determining the solids content/the surface weight of the barrier layer.
If present, the minimum portion of the filler component in % by in the case of the dry barrier layer is 0.1, more preferably 0.2, more preferably 0.5, more preferably 0.8, particularly preferably 1.0. Due to the fact that the filler component represents an optional component, the minimum portion can also be 0% by weight.
For example, the following additives can be used as further components of the coating compound: surface-active agents, such as, for example, ionic or non-ionic tensides, wetting agents, antifoam agents, stabilizers, dyes, further polymers in addition to those already mentioned, biozides, pH-regulators, thixotropic agents.
The portion of the additive or additives, respectively, based on the dry coating compound, is 0 to maximally 5% by weight. The portion is preferably <3, more preferably <2, more preferably <1, most preferably <0.7. If present, the minimum portion of the additive or additives, respectively, is preferably 0.1% by weight.
The limits for the additive content of the barrier coating correspond to the limits for the additive content of the coating compound
The coating compound according to the invention and thus the barrier layer can further optionally contain lecithin and/or fatty acids.
The lecithin is preferably soy lecithin. Lecithin reduces the water sensitivity of the barrier layer, in particular the ductility is increased in response to low humidity.
The lower limit of the lecithin portion in the coating compound or the barrier layer in % by weight, based on the starch, is 0, preferably 0.01, more preferably 0.05, more preferably 0.1. The upper limit of the lecithin portion in the coating compound or the barrier layer in % by weight, based on the starch, is 10, preferably 7, more preferably 5, more preferably 4, more preferably 3.
Edible fatty acids are preferred in the case of the fatty acids. Stearic acid is particularly preferred. Fatty acids reduce the water sensitivity of the barrier layer, in particular the adhesiveness is increased in the case of high humidity.
The lower limit of the fatty acid portion in the coating compound or the barrier layer in % by weight, based on the starch, is 0, preferably 0.01, more preferably 0.05, more preferably 0.1. The upper limit of the fatty acid portion in the coating compound or the barrier layer in % by weight, based on the starch, is 10, preferably 7, more preferably 5, more preferably 4, more preferably 3.
The following compositions of the coating compound turned out to be particularly suitable for providing a barrier layer with regard to the mechanical characteristics and/or the carrier effect of the barrier layers obtained therefrom as well as with regard to the processability of the coating compounds:
Preferably, the starch is a hydroxypropylated pea starch, comprising a molecular weight in million g/mol in the range of preferably 1 to 20, preferably of 2.5 to 10. The plasticizer content in % by weight, based on starch and plasticizer, is in the range of 0-45%, preferably of 15 to 37%, the starch content, based on the formulation without an optionally comprised filler, is in the range of 25 to 65% by weight, preferably of 30 to 50% by weight. Preferably, the coating compound contains a portion of PVA of 1 to 30% by weight, preferably 1 to 20% by weight, more preferably 1 to 10%. Preferably, the barrier layer is applied in two passes, wherein a surface weight (dry) of 3, preferably 4, more preferably 5 to 15 g/m2, is applied for each pass. Preferably, the plasticizer is glycerin.
A hydroxypropylated tapioca starch or a hydroxypropylated potato starch can also be used instead of the hydroxypropylated pea starch.
According to a further preferred embodiment of the coating compound, the starch is a hydroxypropylated tapioca starch comprising a molecular weight in million g/mol in the range of preferably 1 to 20, preferably of 2.5 to 10. The plasticizer portion in % by weight, based on starch and plasticizer, is in the range of 0-45%, preferably of 15 to 37%, the starch portion, based on the formulation without an optionally comprised filler, is in the range of 25 to 65% by weight, preferably of 30 to 50% by weight. The coating compound preferably contains a portion of PVA of 1 to 30% by weight, preferably 1 to 20% by weight, more preferably 1 to 1 to 10%. The barrier layer is preferably applied in two passes, wherein a surface weight (dry) of 3, preferably 4, more preferably 5 to 15 g/m2 is applied for each pass. Preferably, the plasticizer is glycerin.
According to a further preferred embodiment of the coating compound, the starch is a hydroxypropylated potato starch comprising a molecular weight in million g/mol in the range of preferably 1 to 20, preferably of 2.5 to 10. The plasticizer content in % by weight, based on starch and plasticizer, is in the range of 0-45%, preferably of 15 to 37%, the starch content, based on the formulation without an optionally comprised filler, is in the range of 25 to 65% by weight, preferably of 30 to 50% by weight. The coating compound preferably contains a portion of PVA of 1 to 30% by weight, preferably 1 to 20% by weight, more preferably 1 to 10%. The barrier layer is preferably applied in two passes, wherein a surface weight (dry) of 3, preferably 4, more preferably 5 to 15 g/m2 is applied for each pass. Preferably, the plasticizer is glycerin.
The packaging material according to the invention has several layers and comprises a planar substrate as carrier layer and at least one barrier layer, which is applied to the planar substrate, and which is composed and structured as explained above.
According to a preferred embodiment, the finished, dried barrier layer is solid and non-tacky.
The packaging material according to the invention can be obtained by means of the method according to the invention, which will be described in detail below.
The barrier coating is applied to a planar substrate, which is suitable as packaging material. Papers are preferred packaging materials. Papers, which are possible as substrate for the barrier coating, have a surface weight in g/m2 of preferably <800, more preferably <600, more preferably <500, more preferably <400, most preferably <380. On the other hand, the surface weight thereof in g/m2 is preferably >30, more preferably >50, more preferably >70, more preferably >90, more preferably >110, most preferably >120. In summary, paper is understood to be paper in a narrower sense, as well as cardboard and pasteboard. When reference is made hereinafter to paper, pasteboard or cardboard is to also always be comprised. Particularly preferably, papers, which are used as food packaging, in particular in the form of folding boxes, are used as substrate for the barrier coating.
The barrier coating is applied to the rear side of the papers, which follows from the fact that a barrier is to be obtained against the interior of the packaging. The rear sides of papers, which are suitable for packaging, are typically rough, while the front sides, which are mostly provided for printing, are smooth. However, papers comprising rather smooth rear sides are preferably used, because smooth surfaces can be refined significantly more easily with good barrier coatings. A smoother rear side can be obtained, for example, by means of a precoat. The precoat refers to a layer, which is applied to the paper substrate before the barrier layer. The precoat can already be applied at the paper manufacturer.
On the front side of the substrate, thus on the outside of the finished packaging, the packaging materials according to the invention can have further coatings, which are known to the person of skill in the art, or can be printed. In any event, the barrier layer does not form the outside of the finished packaging, but is arranged in such a manner that it establishes a border between the planar substrate, thus the carrier material, and the interior of the packaging and thus the packaged good contained in the packaging. If desired, the packaging material according to the invention can comprise further layers.
According to a further aspect, the present invention also relates to packaging, in particular folding packaging, which comprise the packaging material according to the invention or which can be made therefrom, respectively. The starch-based barrier layer does not form the outside of the packaging hereby.
The method according to the invention for producing the multi-layer packaging material comprises the following steps:
According to the invention, the coating compound is a suspension with granular starch suspended therein. This suspension is gelatinized and dried on the substrate in situ. All of the coating compositions or coating compounds, respectively, according to the invention, which are explained in the present application, can preferably be used or utilized, respectively, in the method according to the invention.
Methods for applying liquids, such as suspensions, to planar substrates are generally known. The coating can take place in different ways: e.g., the barrier coating can be coated, printed, cast, sprayed, rolled or applied in a planar and even manner in a different way. The layer thickness required for forming an effective barrier can thereby be applied in one or a plurality of passes.
In a preferred embodiment, the barrier or coating compound, respectively, is applied by coating or casting. Known and suitable coating methods are the blade coating, doctor coating and size press, for example. A particularly suitable casting method is the curtain coating, wherein particularly good barrier coatings can be obtained with comparatively small application quantities.
Preferably, the barrier compound is produced/applied in more than one coat. Particularly preferably, the coating is produced in 2 coating passes, between which the substrate web does not necessarily need to be rolled up again. Surprisingly, a better barrier is obtained, when a barrier layer of a certain surface weight of, e.g., 20 g/m2 is applied in 2 passes, wherein, e.g., 10 g/m2 are applied in each case, instead of in one pass, in which 20 g/m2 are applied.
In the case of a smooth paper, a sufficient barrier coating can be obtained in 1 or 2 coating passes. In the case of a rough paper, 2 to 4 coating passes might be necessary.
The upper limit for the surface weight (dry compound) in g/m2 for an individual application is preferably 30, preferably 25, more preferably 20, more preferably 18, more preferably 16, more preferably 15, most preferably 14. The thinner the application, the less the blister formation and pinholes can be expected and the easier the application compound can be dried.
The lower limit for the surface weight (dry compound) in g/m2 for an individual application is preferably 3, preferably 4, more preferably 5, more preferably 6, more preferably 7. The thicker the application, the better the coverage of the paper and the more effective the barrier.
In a preferred embodiment, a first barrier layer or a first application for the barrier layer, respectively, is obtained in-line with a paper machine. That is, the first application is obtained directly following the production of the paper on the fresh paper as rear side coating, without the paper web having been rolled up first. The second application is then carried out at a different location, after the paper web comprising the first application has been rolled up.
In a preferred embodiment, a precoat, which, on the one hand, serves the purpose of preventing the water from the coating compound of the following barrier layers from penetrating into the paper, the pasteboard or the cardboard, and, on the other hand, to smooth the surface, which is to be coated can be applied to the paper, the pasteboard or the cardboard, or can have been applied ahead of time, for example by the paper manufacturer. In particular in the case of paper, which is highly absorbent, such a precoat offers significant advantages, because the water of the coating compound is available better through this for the gelatinization of the granular starch. Preferably, the precoat also has characteristics, which reduce the migration of aliphatic and aromatic hydrocarbons.
In the case of a precoat, compounds can be applied, which are used in the paper industry to improve the surfaces, in particular to reduce the surface roughness, to improve the printability and the machine operation. A mineral precoat, for example a carbonate precoat, can thus be used. On the other hand, a starch-based compound can also be applied in the case of a precoat.
In a particularly preferred embodiment, this precoat is produced by means of a precoat compound of dissolved starch. With reference to the starches and starch types, which are suitable for this, the statements made above to describe the granular starch apply accordingly. A starch-based precoat differs from the barrier layer according to the invention in that the precoat does not have all of the features of the barrier layer according to the invention, thus for example no gelatinized starch particles within the specified quantity range.
The preferred weight average of the molecular weight distribution Mw of the dissolved starch used in the precoat in g/mol is >500,000, preferably >1,000,000, preferably >2,000,000, preferably >2,500,000, particularly preferably >3,000,000, most preferably >4,000,000.
The above statements in the paragraphs “solids content of the coating compound” and “plasticizer” also apply with regard to solids content and plasticizer portion of the precoat compound from dissolved starch.
The upper limit for the surface weight of the precoat in g/m2 is 30, preferably 25, more preferably 20, most preferably 15.
The lower limit for the surface weight of the precoat is 1, preferably 2, more preferably 3 and most preferably 5 g/m2.
One or a plurality, preferably 2, 3 or 4 layers of the coating compound according to the invention can then be applied to the precoat as explained in detail above.
In the case of the heavier papers, as they are preferably used as substrate for the barrier coating, the speed, at which the paper web, which is to be coated, is moved at the paper manufacturer, is approximately 100-1,000 m/min, which corresponds to a speed of 1.7 to 17 m/s. Most of the methods lie in the range of 200-800 m/min. In response to the further processing of papers, such as, for example, in the case of a coater, lower speeds are also used in parts. The barrier coating according to the invention can be produced in a continuous process at the speeds, which are standard in the industry.
It is advantageous, when the coating compound is applied to the substrate, which is to be coated, preferably the paper, at an increased temperature. The higher this temperature, the lower the temperature increase, which is required in response to the subsequent gelatinization, and the quicker the gelatinization can be reached.
In a preferred embodiment, the temperature in ° C. of the coating compound in response to application to the substrate, preferably paper, is >20, more preferably >30, more preferably >35, more preferably >40, most preferably >45.
If the temperature of the coating compound is too high, it will already solidify prior to the application as a result of the gelatinization, and can then no longer be used for a coating. The upper limit of the application temperature depends on the composition of the coating compound, but clearly follows for the person of skill in the art from the above-mentioned demands on the viscosity and thus the coatability of the coating compound.
When the coating compound is heated up slowly and when the viscosity thereof is measured, the viscosity of the coating compound decreases continuously, until a viscosity minimum is reached at the temperature Tv, whereby the viscosity initially increases slowly, then very quickly. The viscosity minimum thereby characterizes the ideal temperature of the coating compound. The application is made easier by means of the low viscosity or a solids content, which is as high as possible, can still be processed, respectively. On the other hand, only a minimum temperature increase is then still required for gelatinizing the coating compound. The viscosity minimum of the coating compound depends primarily on the starch, which is used. For native starches, Tv in the case of potato starch and tapioca starch is 60° C., in the case of corn starch it is 75° C., in the case of wheat starch it is 80° C., in the case of waxy corn starch and in the case of pea starch, it is 65° C. If the starches are substituted, such as, for example hydroxypropylated, the specified temperature are reduced by 10° C.
Preferably, the temperature of the coating compound in response to application in ° C. in a range of Tv comprising the following upper limits is: around <11, more preferably <7, more preferably <5, most preferably <3 above Tv, and the following lower limit: <20, more preferably <15, more preferably <11, more preferably <7, more preferably <5, most preferably <3 below Tv.
An advantageous effect can be obtained, when the coating compound is applied to a preheated paper, wherein the paper can be heated by means of infrared heaters, for example. The heat from the paper can then also be used to heat up the applied coating compound.
In a preferred embodiment, the paper is thus heated up, so that the side facing the coating has a temperature in ° C. of >30, more preferably >40, more preferably >50, more preferably >60, more preferably >70, most preferably >80 immediately prior to applying the coating.
A particularly advantageous effect can be obtained, when the side of the paper, which faces the coating, is heated up directly prior to the application of the coating to such an extent that at least the lowermost, that is, the portions of the granular starch of the applied coating compound directly facing the paper, already gelatinize at least partially as a result of the heat from the paper and thus solidify. The penetration of water from the coating compound into the paper is made more difficult through this, because the water is bound much stronger in the gelatinized layer than in the applied suspension. During the production process, the gelatinized portions at the boundary between applied coating compound and paper surface act as boundary layer against a penetration of water into the paper. The subsequent drying is significantly facilitated through this, the process can be accelerated, and the blister formation is counteracted.
In a particularly preferred embodiment, the surface of the paper facing the coating is heated to a temperature above Tv in ° C. of >5, more preferably of >10, more preferably of >15, more preferably of >20, more preferably of >25, more preferably of >30, most preferably of >35.
In response to the in situ gelatinization, the coating compound applied to the paper substrate is heated up on the paper. Preferably, the coating compound is heated to more than 50° C., more preferably to at least 60° C., more preferably to at least 70° C., more preferably to at least 80° C., more preferably to at least 90° C., more preferably to at least 100° C.
It turned out that the IR radiant heaters, which are common in the drying devices of the paper industry, are not sufficient for the in situ gelatinization of the coating compound according to the invention, because the heat energy is introduced too slowly and because at least a part of the water, which is required for the gelatinization, evaporates, before a sufficient gelatinization can be obtained. This effects particularly the uppermost layer of the barrier coating, which dries out quickest. Due to the common heating aggregates, a suitable barrier coating can thus not be embodied.
According to the present invention, this problem in response to the in situ gelatinization of the starch compound is solved in that, upstream of the generally common dryer, means are arranged, with the help of which steam, preferably water vapor, is applied to the coated surface. A high energy is released quickly as a result of the condensation of the steam on the coated surface, and is distributed to the coating compound as a result of the positive heat conductivity of water. It turned out that suitable steam can in particular be applied by means of a steam shower. In the paper industry, steam showers are used for re-wetting paper webs. In terms of the present invention, a corresponding design can be used, if it fulfills the following criteria.
In a preferred embodiment, the steam, which is supplied to the steam shower, has a temperature in ° C. of >105, more preferably >110, more preferably >115, more preferably >120, most preferably >125. The upper limit of the steam temperature in ° C. is preferably <200, preferably <180, more preferably <170, even more preferably <160, most preferably <150.
In a preferred embodiment, the steam, which escapes from the steam shower, has a temperature in ° C. of >100, more preferably >101, more preferably >103, most preferably >105, so that the steam condensates only when it has reached the substrate.
In a preferred embodiment, the steam, which is emitted from the steam shower, has a temperature in ° C. of <150, more preferably <140, more preferably <130, more preferably <125, more preferably <120, most preferably <115. If the temperature is too high, the steam might not cool down sufficiently so as to condensate on the substrate. Preferably, this steam is dry saturated, that is, it is close to the condensation limit, but does not contain a condensate.
In a preferred embodiment a steam quantity in g/m2 of >0.02, more preferably >0.04, more preferably >0.06, more preferably >0.07, more preferably >0.08, most preferably >0.09 is deposited or condensed, respectively, on the substrate for each g/m2 of the coating compound. As the steam quantity increases, the gelatinization is accelerated and the gelatinization degree increases. Water, which is applied to the coating compound prior to the application of the steam, e.g. by means of a spray device, is also counted to belong to the coating compound here.
In a preferred embodiment, a steam quantity in g/m2 of <50, more preferably <25, more preferably <10, more preferably <5, more preferably <3, more preferably <1, more preferably <0.7, most preferably <0.5 for each g/m2 of the coating compound. As the steam quantity decreases, the quantity of water condensate, which must be dried off subsequently, is reduced and the drying is thus accelerated and simplified. Water, which is applied to the coating compound prior to the application of the steam, is also counted as belonging to the coating compound here.
The steam is to thereby be distributed across the entire coating surface as evenly as possible. The technical requirements for the design of a steam chest, which fulfills these conditions, are known to the person of skill in the art.
Thanks to the above-mentioned teaching relating to compositions and method parameters, it was a surprise that it was possible to obtain a complete gelatinization and thus a solidification of the coating compound on the substrate within a few hundredths of a second, thus an in situ gelatinization, wherein a low-viscous aqueous liquid is virtually converted abruptly into an elastic solid, and the water of the coating compound is bound therein.
In that the in situ gelatinization of the starch was made possible within such a short time period by means of applying a suitable water vapor, the problem that water from the coating compound substantially penetrates into the porous paper, was also solved or was at least significantly eased at the same time. On the one hand, such water, which is absorbed by the paper, is missing in response to the gelatinization and, on the other hand, represents a problem in response to the drying of the barrier coating. A comparatively long and efficient drying section would be required to dry off such water, which is located on the bottom, and, even in the case of relatively gentle and slow drying, this water, below the barrier coating, which has already solidified, easily forms steam bubbles, which break open the barrier coating by the formation of bubbles/blisters and which thus substantially destroy the barrier coating.
It turned out that particularly good results can be obtained, if the coating compound on the paper is sprayed with water prior to the application of steam, which can occur, for example, by means of a spray bar, which includes a row of spray nozzles, via which the required water is distributed finely and evenly on the coated web. A water film, which is as even as possible and which facilitates the subsequent gelatinization and which leads to particularly good barrier coatings, is to thereby be created on the coating compound, most likely in that the uppermost layer of the coating compound is protected against drying out too soon.
In a preferred embodiment, a water quantity in g/m2 of <10, more preferably of von <7, more preferably of <5, more preferably of <3, most preferably <1 is thereby sprayed on for each m2 of substrate. The finer the water droplets, which are sprayed on, the less water is required.
In a preferred embodiment, the temperature of the water, which is sprayed on, in ° C. is >30, more preferably >40, more preferably >50, more preferably >60, more preferably >70, most preferably >80.
As the temperature of the water, which is sprayed on, increases, the coating compound is heated up and a quicker gelatinization is realized subsequently. In the case of higher temperatures, the uppermost layer of the coating compound is at least partially gelatinized and thus solidified as a result of the warm or hot water, which is sprayed on. The water, which is sprayed on, then forms a particularly effective protective film against drying out, because it can barely penetrate into the coating compound. Such a water film on the surface can then be dried very easily and quickly by means of the common drying methods, which are used after the steam application. In particular, blister formation can be prevented through this, even in the case of quick drying.
The paper, which is coated with the coating compound, can be dried by means of the drying methods, which are common in the paper industry. Mainly infrared heaters and hot air hoods are used thereby. It is common thereby for that side of the paper to be treated with IR or hot air, on which a layer, which is to be dried, has been applied. In a preferred embodiment, however, at least a part of the drying method is carried out in such a manner that the other side of the paper (to which no layer, which is to be dried, has been applied) is treated with IR or hot air.
In a further preferred embodiment, both sides are simultaneously treated with IR or hot air in response to at least a part of the drying process.
The advantage of these preferred methods is that the formation of blisters and pinholes can be suppressed more easily and that better barriers are thus available.
The invention will be described in more detail below by means of exemplary embodiments in connection with the drawing:
A paper having a known surface is dried in a circulating air oven at 130° C. for 15 min and is then weighed. The surface weight of the untreated paper can be obtained in g/m2 from the weight and the known surface.
A coated paper having a known surface is dried in the same manner and is weighed, and the surface weight of the coated is thus obtained in g/m2. The surface weight of the coating is obtained in g/m2 from the difference of the two surface weights.
Mw is understood to be the weight average of the molecular weight distribution.
If the starch is present in the form of powder, the starch is suspended in water with a concentration of 3% of dry substance. This suspension is then heated up in a mini autoclave to 150° C. while stirring and is held there for 20 min. The solution obtained in this manner is then cooled down to approximately 60° C., is thinned to 0.3%, and is filtered with a 0.005 mm membrane filter. The filtered solution is then measured with GPC-MALLS (gel permeations chromatography using multi-angle laser light scattering).
If the starch is to be analyzed with regard to its molecular weight in a barrier layer on a paper, the following 2 methods can be used to obtain the starch solution.
1) The starch is scraped off the paper by means of a scalpel or it is abraded using a fine abrasive paper. The powdery material obtained thereby can be transferred into a solution, which is present in the form of powder, in the same manner as the one described above for starch.
2) In the alternative, the barrier layer can be analyzed together with the paper. For this purpose, the material is cut into pieces of approx. 2*2 mm and is suspended in the autoclave and is stirred at room temperature overnight. The same process as described above for starches in the form of powder is then carried out. In response to the filtration, however, a coarse filter is used first, so as to filter out the insoluble paper components. Due to the fact that common papers can already include starch even without a barrier layer, a reference measurement is made, if necessary, with the uncoated paper or a reference measurement is made with the paper, from which the barrier layer was removed mechanically, respectively. An assessment can then be made from the reference measurement, which components are to be attributed to the barrier and which components are to be attributed to the paper during the GPC-MALLS analysis.
An Alliance 2695 separation module from Waters, a DRI detector 2414 from Waters, a MALLS detector Dawn-HELEOS from Wyatt Technologie comprising a wavelength of 658 nm and a K5 flow-through cell were used for the measurements. A SUPREMA gel column set was used for the GPC column, exclusion limits S30000 with 10E8-10E6, S1000 with 2E6-5E4, S100 with 1E5-1E3. Eluent: DMSO with 0.09 m NaNO3. Temperature: 70° C. Evaluation: Astra software 5.5.0.18. A refractive index increment do/dc of 0.068 was used for the calculation.
Recovering Starch Particles from the Coating of the Packaging Material
The produced barriers contain gelatinized starch particles. These starch particles can be separated from the soluble components (these are in particular plasticizers, soluble starch, thickening agents, if applicable), for example by dissolving the barrier at 70° C. for 30 min at a stirring speed of less than 60 revolutions per minute and the quantitative portion thereof at the barrier can thus be measured.
In a preferred embodiment, the minimal portion in % by weight of the starch in the barrier, which can be recovered after dissolving the barrier at 70° C. for 30 min and a stirring speed of less than 60 revolutions per minute, is 30, preferably 40, more preferably 50, more preferably 55, more preferably 60, more preferably 65, particularly preferably 70%.
In a further preferred embodiment, the portion of the compound is determined, which can be recovered after dissolving the barrier at 70° C. for 30 min and a stirring speed of less than 60 revolutions per minute, and based on the compound of the barrier, is determined. The determination according to this definition is simpler than the determination according to recovery method No. 1, because it can also be applied, when the composition of the barrier is not known exactly. The minimum portion in % by weight of the compound, which can be recovered, is 25, preferably 35, more preferably 40, more preferably 45, most preferably 50.
A suspension consisting of 37% by weight (percent by weight based on the dry weight is mentioned hereinafter in each case, unless specified otherwise) of a hydroxypropylated pea starch (Mw=19,000,000 g/mol), 18% by weight of glycerin (99.5%) and 45% water is homogenized at room temperature in a mixer comprising an anchor stirrer, and is provided in a coating device, in which a sedimentation of the particles is prevented by continuously circulating the compound. The suspension has a viscosity of 200 mPas at 24° C., a measured solids content of 52% and a pH of 7.7. The rear side of a standard folded box cardboard comprising a surface weight of 230 g/m2 and a roll width of 60 cm serves as substrate. The starch compound was applied via a combo blade coating system by means of a profiled roll doctor (profile C35, 0.8 bar doctor pressure) at a machine speed of 150 m/min. Immediately after the application, water was sprayed onto the coated surface in a planar manner at a flow rate of 12 g/m2 by means of a spray bar and the starch coating was gelatinized immediately afterwards with saturated steam, which was brought to the surface by means of steam shower. The steam thereby had a temperature of approx. 120° C. and was operated such that the flow rate was approx. 30 g/m2 of steam. After the steam chest, the paper web was dried by means of a gas-IR heater and subsequent hot air drying according to the settings, which are common in the paper finishing. The drying system consisted of 4 gas IR radiant heaters, followed by 3 drying hoods comprising hot air. The output and temperature of the drying units was adjusted such that the paper surface at the measuring points between the drying elements showed a temperature, which was as high as possible, but not more than 110° C.
It was then possible to roll up the cardboard and it did not stick. The surface weight of the dry coating was determined by means of differential weighting to be 10 g/m2.
A quick test with spray oil showed a significantly reduced wetting of the cardboard surface with oil as compared to the uncoated cardboard surface.
As example 1, but two coatings à 10 g/m2 were applied. A test with spray oil showed an even more reduced wetting of the coated cardboard surface. The barrier effect as compared to MOH (mineral oil hydrocarbons) was determined with the help of a migration measurement with less than 35 C atoms through the coated cardboard to be >90%.
With regard to the migration measurement, it is to be noted that a standard measuring method does not exist at this time, but the results of different measuring methods match surprisingly well. The percentages refer to the ratio, by which the quantity of MOSH and MOAH was reduced as compared to the uncoated raw cardboard. A description for a method can be found in: K. Fiselier, K. Grob in ‘Packaging Technology and Science’, 2012 Vol. 25, issue 5, p. 285-301).
At 30-35° C., a suspension was mixed from 34.9% by weight of starch (hydroxypropylated tapioca starch, Mw=19,000,000 g/mol), 13.7% by weight of glycerin (99.8%), 3.5% by weight of polyvinyl alcohol (comprising a viscosity of 8 mPas, hydrolysis degree 88%) and 47.9% by weight of water with a total viscosity of 1670 mPas at 34° C. The suspension was placed into a combo blade coating device, where it was circulated so as to ensure the homogeneity.
The suspension was spread onto a standard folded box cardboard comprising a surface weight of 350 g/m2 at a web speed of 120 m/min by means of a 20 mm smooth doctor at a doctor pressure of 0.6 bar. Directly after spreading, approximately 18 g/m2 of water was applied in a planar manner by means of a water spray bar. With the help of a steam shower, 117° C. hot, saturated steam with a flow rate of approx. 35 g/m2 was blown onto the 60 cm wide paper web immediately afterwards. After the steam shower, the web surface had a temperature of approx. 90° C. The coating was then moved through a drying system comprising 4 gas IR radiant heaters, followed by 3 drying hoods comprising hot air. The output and temperature of the drying units was adjusted in such a manner that the paper surface displayed temperatures, which were as high as possible, but not more than 110° C., at the measuring points between the drying elements.
It was then possible to roll up the cardboard and it did not stick. The surface weight of the dry coating was determined by means of differential weighting to be 13 g/m2.
A cover coat with the same suspension was applied once again under the same conditions to the roll, which had already been coated, only increasing the doctor pressure to 1.5 bar. The applied surface weight was measured to be 8.5 g/m2.
The total coating had a surface weight of 21 g/m2.
Two samples of the coated cardboard were in each case subjected to a migration test for MOSH/MOAH, wherein one sample was first grooved in the center)(1×90°. The test showed that the barrier effect in the case of the grooved sample was only reduced by approx. 1.5% as compared to the non-grooved cardboard.
Prior to the coating with a suspension of particulate starch, a precoat was produced with dissolved starch. The starch solution had the following composition in % by weight: 17.8% starch (slightly degraded hydroxypropylated pea starch, Mw=4,500,000 g/mol), 9.5% glycerin (99.8%), 72.7% water. The mixture was mixed cold and was cooked in the jet cooker, so that a clear solution was created. Said clear solution had a viscosity of 1180 mPas (Brookefield viscometer) at 55° C.
The precoat was applied on the combo blade by means of a roll doctor (20 mm smooth doctor, 0.8 bar doctor pressure) at a machine speed of 350 m/min to a folded box cardboard weighing 230 g/m2. The coating weight after drying was determined to be 9.6 g/m2.
A cover coat was applied to this precoat with a suspension consisting of 34.5% by weight of starch (HP tapioca starch, Mw=19,000,000 g/mol), 18% glycerin (99.8%), 3.5% PVA (with a viscosity of 8 mPas, hydrolysis degree of 88%) and 44% of water. At 29° C., the suspension had a viscosity of 1510 mPas and a pH-value of 7.0. It was applied by means of a 20 mm smooth doctor at 2.5 bar contact pressure and a machine speed of 150 m/min. Approx. 7 g/m2 of water were sprayed on by means of a water bar. The adjustments of the steam chest were the same as in Example 3. Immediately after the steam chest, the coated web surface had a temperature of 89° C. After drying, the coating weight was determined to be 8.6 g/m2.
A starch solution comprising the following composition in % by weight was produced by means of cooking in a jet cooker: 25.7% starch (degraded, hydroxypropylated pea starch, Mw=170,000 g/mol), 11.8% glycerin (99.8%), 62.5% water. The solution had a Brookefield viscosity of 990 mPas at 33° C. and a pH of 7.9. A coat of 14 g/m2 was applied by means of a profiled roll doctor (C40) at a machine speed of 200 m/min and was dried.
A cover coat comprising a surface weight of 9.5 g/m2 was also applied to this precoat by a curtain coater at 170 m/min. The starch solution used for this purpose had the following composition in % by weight: 24.4% starch (degraded, hydroxypropylated pea starch, Mw=170,000 g/mol), 11% glycerin (99.8%), 1% polyvinyl alcohol (comprising a viscosity of 40 mPas, hydrolysis degree of 98%), 63.6% water. The solution had a Brookefield viscosity of 920 mPas at 45° C. and a pH von 7.9.
In the test, the coating showed only a slight barrier effect as compared to hydrocarbons. In SEM pictures of the surface, the surface showed a good coverage with the barrier layer, but several tears in the layer. The tear formation was attributed to the high degree of degradation of the used starch.
A suspension consisting of 34.5% by weight of starch (HP tapioca starch, Mw=1.9*107 g/mol), 18% glycerin (99.8%), 3.5% PVA (comprising a viscosity of 8 mPas, hydrolysis degree of 88%) and 44% of water was mixed. At 32° C., the suspension had a viscosity of 1810 mPas and a pH of 7.0. The suspension was applied to a standard folded box cardboard comprising a surface weight of 250 g/m2 in the combo blade coater by means of a smooth doctor 20 mm at 1.5 bar doctor pressure. A steam chest was not used, the web was guided through the drying system immediately after the coater and was dried with the following settings:
An insignificant improvement of the barrier effect as compared to the raw cardboard was shown in the spray test. SEM pictures of the coated surfaced showed that the granular structure of the starch was still visible and that the starch compound did not form a film.
| Number | Date | Country | Kind |
|---|---|---|---|
| 01556/13 | Sep 2013 | CH | national |
| 01603/13 | Sep 2013 | CH | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/069162 | 9/9/2014 | WO | 00 |
