US20250084590A1

US20250084590A1 - Coated papers with a semi-crystalline coating layer as packaging material

Info

Publication number: US20250084590A1
Application number: US18/729,805
Authority: US
Inventors: Kerstin Bartels; Simon Trosien; Markus Biesalski; Markus Wildberger
Original assignee: Technische Universitaet Darmstadt; Koehler Innovation and Technology GmbH
Current assignee: Technische Universitaet Darmstadt; Koehler Innovation and Technology GmbH
Priority date: 2022-01-17
Filing date: 2023-01-17
Publication date: 2025-03-13
Also published as: DE102022200463A1; EP4466403A1; CN118891414A; KR20240140105A; JP2025504861A; WO2023134835A1

Abstract

The application relates to a coated paper comprising a base paper and at least one semi-crystalline coating layer with amorphous regions and crystalline regions, applied directly or indirectly to the base paper, wherein the amorphous regions contain one or more natural polymers and/or one or more derivatives of natural polymers; wherein the crystalline regions comprise one or more crystallizable organic compounds; and wherein the permeability of at least one gas through the coated paper is reduced compared to the base paper. The application further relates to the coating used for producing the coated paper, the production method, as well as packaging for food.

Description

FIELD OF THE INVENTION

The invention relates to a coated paper with high barrier performance for use as packaging material for food.

BACKGROUND OF THE INVENTION

Packaging accounts for a large proportion of global plastic waste pollution, which is why the search for alternatives made from biodegradable materials is being advanced.
In particular, packaging food presents a challenge because good barrier performances against oxygen, water vapor, and microorganisms are needed.
Packaging materials for food often consist of plastics such as polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyethylene (PE), and polypropylene (PP), as these offer good barrier properties as well as low weight and high mechanical stability.
Paper-based packaging materials have many advantages over plastic materials, such as renewability, recyclability, and compostability. However, their application is limited due to often poor barrier properties and high sensitivity to moisture. To improve the barrier properties, paper-based packaging material can be laminated with aluminum or petroleum-based polymers like PE, EVOH, and PVC derivatives. However, these coatings complicate waste sorting and thus recycling, and reduce compostability. Therefore, the use of barrier layers based on naturally bio-based polymers or replacing conventional metal or plastic-based layers is highly desirable from an ecological perspective.
Examples of natural polymers tested for packaging applications include chitosan, hemicelluloses, microfibrillated cellulose, and starch. However, many natural polymers are hydrophilic, and films made from these materials are often hygroscopic, leading to a partial loss of their barrier properties at high humidity.
There is great interest in the forestry and agricultural industries in further processing by-products and waste products, such as lignin.
For example, lignin has been used in paper production. WO 2021/191097 A1 describes a method for producing paper. This method includes a wet phase and a dry phase. The wet phase involves making a fiber slurry with fibers in water, where the fibers are selected from the group of lignocellulose, hemicellulose, and cellulose. In the wet phase, an additive comprising enzymatically oxidized lignin is added. This is intended to improve the moisture resistance of the paper, particularly its compressive strength in the presence of moisture. It is also proposed to add a fatty acid, such as stearic acid, to the enzymatically oxidized lignin.
Although lignin, as a hydrophobic biopolymer, is suitable for use as a barrier layer on paper, research has so far been limited. A coating layer based solely on lignin does not exhibit sufficient barrier properties. Therefore, various attempts have been made in the state of the art to improve the properties of lignin so that it remains sufficiently tight even in moist environments, such as food packaging.
U.S. Pat. No. 9,902,815 B2, as well as the scientific publication by the same authors, Hult et al. 2013, describe methods for the esterification of lignin with fatty acids, particularly lignin esterified with a mixture of tall oil fatty acids. The main components of this mixture are unsaturated fatty acids such as oleic acid, linoleic acid, and linolenic acid, which were reacted with lignin to various degrees of esterification.
In addition to the esterification of lignin and tall oil, Hult et al. also implemented softwood and hardwood lignin with palmitic acid and lauric acid. It was shown that lignin esterified with the longer-chain palmitic acid had better barrier properties against water vapor than lignin esterified with lauric acid. The best barrier performance was achieved with a layer of hardwood lignin esterified with palmitic acid.
DE 10 2017 108 577 A1 relates to coatings comprising at least one polymer and at least one crystallizable material, as well as methods for their production. The polymer should have a viscosity of at most 10¹²mPa s at the melting temperature of the crystallizable material. This results in layers that are superhydrophobic and regenerable. However, these layers exhibit very low gas barrier properties.

SUMMARY OF THE INVENTION

The present invention is based, among other things, on the surprising finding that the barrier effect of a coating layer based on a natural polymer, such as lignin or lignin stearate, can be significantly increased by inducing semi-crystallinity, that is, having both crystalline and amorphous regions. This is achieved by adding a crystallizable organic compound, such as stearic acid, to the coating composition. A coated paper produced with this coating layer according to the invention has sufficient barrier effect for use in the food industry and is still biodegradable and recyclable.
Accordingly, the present invention relates to a coated paper comprising a base paper and at least one semi-crystalline coating layer with amorphous regions and crystalline regions, applied directly or indirectly to the base paper:

- wherein the amorphous regions contain one or more natural polymers and/or one or more derivatives of natural polymers
- wherein the crystalline regions comprise one or more crystallizable organic compounds, and
- wherein the permeability of the coated paper for at least one gas is reduced compared to the base paper.

The induced semi-crystallinity according to the invention was achieved, for example, with a coating composition containing a fatty acid as a crystallizable organic compound in combination with the derivative of a natural polymer, particularly lignin. Consequently, in a second aspect, the invention relates to a coating composition for coating papers, containing at least one solvent, at least one crystallizable organic compound, and at least one natural polymer and/or derivative of a natural polymer,

- wherein the crystallizable organic compound is selected from fatty acids, hydroxy fatty acids, or dicarboxylic acids or their esters, amides, or salts;
- wherein the natural polymer is selected from polysaccharides such as alginate, agar-agar, cutin, suberin, lignin, cellulose, chitosan, and starch, hydrocarbons such as rubber or balata, proteins such as collagen, keratin, fibroin, nucleic acids, lipids, polylactide (PLA), polyhydroxybutyrate (PHB), and polyhydroxyalkanoate (PHA); and
- wherein the solvent is selected from water, tetrahydrofuran (THF), toluene, ethyl acetate, and alcohols.

The essential crystallinity of the coating layer in the coated paper is particularly dependent on the method used to produce the coated paper. Consequently, in a third aspect, the invention relates to a method for producing a coated paper with a base paper and a semi-crystalline coating layer, comprising the steps of:

- a) producing a coating according to the second aspect by melt dispersion, high-pressure dispersion, or spray drying and subsequent mechanical dispersion of the components;
- b) providing a base paper;
- c) applying the coating composition to the base paper; and
- d) curing the coating composition to form the semi-crystalline coating layer.

In a fourth aspect, the invention relates to a packaging for food comprising the coated paper according to the first aspect.

FIGURES

FIG. 1 shows an elugram of Kraft lignin, indicating the number-average molar mass Mn and the weight-average molar mass Mw.

FIG. 2 shows a ³¹P-NMR spectrum of Kraft lignin. Signal A: internal standard, signal group B: aliphatic and phenolic hydroxy groups, signal C: carboxy group.

FIG. 3 shows a ³¹P-NMR spectrum of the lignin ester. Signal A: internal standard, signal group B: hydroxy groups, signal group C: carboxy groups.

FIG. 4 shows an IR spectrum of lignin stearate (bottom) compared with the starting material Kraft lignin (top) and stearic acid (middle).

FIG. 5 shows a ¹H-NMR spectrum of lignin stearate with the solvent CDCl₃.

FIG. 6 shows the results of determining the water vapor transmission rate (WVTR) of papers coated with different (inventive) coating compositions depending on the proportion of crystallizing wax/crystallizing fatty acid.

FIG. 7 shows in A) a scanning electron microscope image of the surface of the lignin stearate-stearic acid coating with EHT=10 kV, WD=6.3 mm, and a magnification of 100,000×. WD stands for “working distance” and refers to the distance between the objective lens of the scanning electron microscope and the sample being examined. EHT stands for “high tension” and refers to the high voltage used to accelerate the electrons in the scanning electron microscope. In B), an enlargement of a 4 μL water droplet on the lignin stearate-stearic acid coating is shown, taken with Dataphysics OCA35 including a tiltable stage under constant temperature and humidity (23° C., 50% relative humidity). Software: SCA 4.5.2 Build 1052. The contact angle is 103°.

FIG. 8 shows in A) a scanning electron microscope image of the surface of the AKD-CSE3 coating with EHT=10 kV, WD=6.44 mm, and a magnification of 100,000×. In B), an enlargement of a 4 μL water droplet on the AKD-CSE3 coating is shown, taken with Dataphysics OCA35 including a tiltable stage under constant temperature and humidity (23° C., 50% relative humidity). Software: SCA 4.5.2 Build 1052. The contact angle is 159°+3°.

FIG. 9 shows a diagram of the roll-off angle of water droplets on the surface of a lignin stearate-stearic acid coating as a function of the droplet volume.

FIG. 10 shows a diagram of the water vapor transmission rate (WVTR) in grams per square meter per day of coated papers with different lignin stearate-fatty acid coatings depending on the fatty acid content. The fatty acids are stearic acid and suberic acid.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The term “coating” in the context of the present invention and in accordance with the general understanding in the field of paper technology refers to coating agents containing or consisting of binders, additives, and optionally pigments or matrix pigments, which are applied (“coated”) onto the paper surface using special coating devices for surface refinement or modification of a base paper. Papers produced in this way are referred to as “coated papers”.
In the context of the present invention, a “coated paper” is understood to be a base paper that includes one or more layers applied by coating, i.e., coating layers. The layers of such a coated paper substrate can be functional layers and structure-forming layers (such as smoothing layers to level the surface).
The term “coating” is used in the invention as a generic term for all coatable coating compounds, preparations, and/or solutions in the paper industry for the treatment, modification, or refinement of a paper surface. A “coating layer” refers to the coating applied and cured on the base paper.
“Paper” is a flat material that essentially consists of fibers of plant origin and is formed by draining a fiber suspension on a screen. The resulting fiber web is compressed and dried. In the context of this invention, the flat materials “cardboard” and “paperboard”, which are produced in the same way, are also subsumed under paper. The distinction between paper, cardboard, and paperboard is made solely based on the basis weight, with paperboard having a grammage greater than 600 g/m², cardboard having a grammage greater than 150 and less than or equal to 600 g/m², and paper having a grammage of less than or equal to 150 g/m².
The synonymous terms “semi-crystalline” and “partially crystalline” generally refer to a solid, and particularly a layer, that contains both crystalline and amorphous regions (domains). A semi-crystalline layer typically contains a multitude of individual amorphous and crystalline regions.
The term “crystallinity” is synonymous with “degree of crystallinity” or “crystallization degree” and refers to the proportion of a partially crystalline solid that is crystalline. The most common methods for determining the degree of crystallization in polymers are density measurement, differential scanning calorimetry (DSC), X-ray diffraction (XRD), IR spectroscopy, or NMR spectroscopy. The measured value depends on the method used. According to the invention, the crystallinity of the coating is determined by XRD measurements.
The term “natural polymer”, synonymous with “biogenic biopolymer”, refers to a polymer synthesized in the cell of a living organism. Thus, the natural polymer is also biodegradable. The natural polymer is specifically a natural polymer within the meaning of Directive (EU) 2019/904 of the European Parliament and of the Council of 5 Jun. 2019 on the reduction of the impact of certain plastic products on the environment (see Article 3 No. 1 exclusion in the definition of plastic).
The term “derivatives of natural polymers” refers to polymers that are obtained by further processing biopolymers, according to the invention. These are also known as chemically modified polymers. Examples of derivatives of natural polymers are lignin derivatives, such as lignin esters, cellulose derivatives, and starch derivatives.
A “crystallizable organic compound” according to the invention refers to those organic compounds that can arrange themselves in a regular substance-specific form around a crystallization nucleus and form a seed crystal or crystal.
As used in this invention, the term “melting temperature” (T_m) refers to the temperature at which a substance melts, that is, transitions from the solid to the liquid state. The melting temperature for polymers and crystallizable materials can be determined by differential scanning calorimetry (DSC) according to DIN EN ISO 11357-3:2013. Preferably, a heating or cooling rate of 10 K/min is used.
The “glass transition temperature” (T_g) is the temperature at which polymers or plastics (but only wholly or partially amorphous polymers) transition from a liquid or rubbery, flexible state to a glassy or hard-elastic, brittle state; it is therefore also called the “softening temperature”. At this temperature, the polymer has a viscosity of 10¹²Pa*s. For polymers that do not have a melting temperature, the glass transition temperature takes the place of the melting temperature. Glass transition temperatures can be determined, for example, by differential scanning calorimetry (DSC) according to DIN EN ISO 11357-2:2014.
In the context of this invention, the “contact angle” of a liquid droplet on a surface is understood to be the angle formed by the intersection line between the droplet base and the surface with the horizontal. It is measured in degrees and depends on various factors, such as the surface tension of the liquid and the properties of the surface.
The “roll-off angle” is understood in this invention to be the inclination angle of a surface at which a droplet rolls off. It is typically used to characterize superhydrophobic surfaces with a very high contact angle, where the droplet is nearly spherical. At smaller contact angles, a droplet can also move off the surface, but it is usually deformed first and then slides over the surface. At a roll-off angle of 180°, the water droplet does not roll off but adheres to the coating layer, even if the droplet is hanging downwards.
In the context of this invention, “superhydrophobic” surfaces are defined as surfaces with contact angles of 145° or more with respect to water, preferably 150° or more with respect to water. At such high contact angles, typically only about 2 to 3% of the water droplet surface is in contact with the superhydrophobic surface; thus, it has extremely low wettability. Additionally, superhydrophobic surfaces are characterized by a roll-off angle of less than 10°.

Coated Paper and Coating

The present invention relates to a coated paper comprising a base paper and at least one semi-crystalline coating layer with amorphous and crystalline regions applied directly or indirectly onto the base paper:

- wherein the amorphous regions contain one or more natural polymers and/or one or more derivatives of natural polymers;
- wherein the crystalline regions comprise one or more crystallizable organic compounds; and,
- wherein the permeability of the coated paper for at least one gas is reduced compared to the base paper.

According to one embodiment of the coated paper, the permeability for at least one gas is lower with the same total application amount than the permeability of a coated paper with the same base paper and having each a coating layer of the natural polymer or its derivative and a coating layer of the crystallizable organic compound.
By means of the coating layer, the permeability of the coated paper for at least one gas is reduced compared to the base paper. This gas can be oxygen (O₂), nitrogen (N₂), carbon dioxide (CO₂), methane (CH₄), hydrogen (H₂), water vapor, or a mixture thereof, such as air. In particular, the water vapor transmission rate (WVTR) is reduced.
The inventors have found that the barrier performance or permeability, particularly the WVTR, in the system according to the invention is directly dependent on the crystallinity. Without being bound to theory, this effect is based on the embedding of crystallites formed from the crystallizable component in an amorphous matrix of natural polymer. The crystallites are impermeable to gases, especially polar molecules such as water in the vapor phase, due to their high packing density. For permeation through the thin semi-crystalline coating layer, the gases must take a longer path “around the crystallites,” which directly reduces the permeation coefficient and thus makes the semi-crystalline layer act as a water vapor barrier. This theory behind the effect is also supported by the computer-based model for diffusion through semi-crystalline and filled polymers described by Müller-Plathe (see Müller-Plathe Habilitation Thesis ETH Zurich 1993, page 67 et seq.).
Pure layers of natural polymer, such as lignin, exhibit significantly higher permeability, as do pure layers of crystallizable organic compounds like stearic acid, which have open areas due to free volumes between the crystallite structures through which gases can permeate.
Thus, the permeability, particularly the WVTR, for each system according to the invention can be adjusted by selecting the degree of crystallinity. Below a crystallinity of 10%, a semi-crystalline system generally has a barely measurable effect on the WVTR of the coated paper compared to a comparable non-crystalline layer. According to one embodiment, the crystallinity of the semi-crystalline coating layer ranges from 10% to 90%. A crystallinity above 90% typically does not lead to a change in barrier performance but can result in various structural disadvantages, such as disruption of the film's integrity. The crystallinity can be, for example, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68%, 70%, 72%, 74%, 76%, 78%, 80%, 82%, 84%, 86%, 88%, or 90%. According to a preferred embodiment, the crystallinity ranges from 10% to 40%. Above 40%, only a slight reduction in gas permeability can be achieved. According to a particularly preferred embodiment, the crystallinity ranges from 15% to 25%
The coating used to form the semi-crystalline coating layer preferably does not contain crystalline elements, and the crystallizable organic compound is preferably present in a non-crystalline form in the coating. Under certain circumstances, such as in aqueous solvents, it may be that the fatty acid molecules are present in a crystalline form to a small extent. This can occur, particularly when the fatty acid melts in the presence of water, is then dispersed, and cools. This state is referred to as essentially non-crystalline in the present context. According to one embodiment, the crystallizable organic compound is essentially in a non-crystalline form in the coating. Thus, it is not a coating comprising crystalline fillers in amorphous binders. Such a coating layer is referred to as a granular crystalline coating layer to distinguish it from the semi-crystalline coating layer according to the invention. The semi-crystallinity according to the invention is therefore not granular crystallinity. The crystallinity preferably develops only upon the application of the coating layer. According to one embodiment, the crystalline regions form during the application of the coating to the base paper. In another embodiment, the crystalline regions form during the curing of the coating. According to one embodiment, the crystalline regions form both during the application of the coating to the base paper and during the curing of the coating.
With the coating according to the invention, a coated paper with high barrier performance, particularly a very low WVTR, can be achieved. According to one embodiment, the WVTR with a coating weight of 10±1 g·m⁻²is no more than 40 g m⁻²d⁻¹.
This WVTR is significantly lower than comparable systems in the prior art. Although Hult et al. 2013 also describes a WVTR of 40 g m⁻²d⁻¹for a layer with lignin palmitic acid ester, this was measured under standard conditions, i.e., at 23° C. and 50% humidity. According to the invention, the WVTR is measured at 38° C. and over 90% humidity. A non-crystalline layer of lignin palmitic acid ester would have a WVTR of over 200 g m⁻²d⁻¹under the selected tropical conditions of 38° C. and over 90% humidity.
The WVTR of the coated paper according to the invention can, for example, be 40 g. m⁻²·d⁻¹, 38 g·m⁻²·d⁻¹, 36 g·m⁻²·d⁻¹, 34 g·m⁻²·d⁻¹, 32 g·m⁻²·d⁻¹, 30 g·m⁻²·d⁻¹, 28 g·m⁻²·d⁻¹, 26 g·m⁻²·d⁻¹, 24 g·m⁻²·d⁻¹, 22 g·m⁻²·d⁻¹, 20 g·m⁻²·d⁻¹, 18 g·m⁻²·d⁻¹, 16 g·m⁻²·d⁻¹, 14 g·m⁻²·d⁻¹, 12 g·m⁻²·d⁻¹, 10 g·m⁻²·d⁻¹, 8 g·m⁻²·d⁻¹, 6 g·m⁻²·d⁻¹, 4 g·m⁻²·d⁻¹, 2 g·m⁻²·d⁻¹, 1 g·m⁻²·d⁻¹. By selecting appropriate components of the coating and adjusting the crystallinity, a WVTR of no more than 20 g·m⁻²·d⁻¹, even no more than 10 g·m⁻²·d⁻¹can be achieved.
Preferably, the melting temperature (T_m) of at least one crystallizable organic compound is lower than the glass transition temperature (T_g) of at least one natural polymer and/or at least one derivative of a natural polymer.
This is important to prevent the formation of nanostructured surface structures that make the surface superhydrophobic but gas-permeable, particularly to water vapor. As shown in Example 7, superhydrophobic surfaces known from the prior art, comprising a polymer and a crystallizable organic compound where the melting temperature of the crystallizable organic compound is higher than the glass transition temperature of the polymers, have only a low water vapor barrier performance with a WVTR above 400 g·m⁻²−d−1. Accordingly, as shown in Example 9, in a system according to the invention with a derivative of a natural polymer, namely lignin stearate, and a fatty acid, the use of a fatty acid such as suberic acid with a melting temperature higher than the glass transition temperature of lignin stearate leads to significantly worse results in water vapor barrier performance compared to using a fatty acid like stearic acid with a melting temperature lower than the glass transition temperature of lignin stearate.
Accordingly, the melting temperature (T_m) of at least one crystallizable organic compound should be at least 1° C. lower than the glass transition temperature (T_g) of at least one natural polymer and/or the derivative of a natural polymer. For example, the melting temperature (T_m) of the crystallizable organic compound can be 1° C., 2° C., 3° C., 4° C., 5° C., 7° C., 10° C., 12° C., 15° C., 17° C., 20° C., 22° C., 25° C., 30° C., 35° C., 40° C., 45° C., 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., or 80° C. lower than the glass transition temperature (T_g) of at least one natural polymer and/or the derivative of a natural polymer. To ensure that a superhydrophobic surface does not partially form, the difference should be more than 1° C. According to one embodiment, the melting temperature (T_m) of at least one crystallizable organic compound is at least 5° C. lower than the glass transition temperature (T_g) of at least one natural polymer and/or the derivative of a natural polymer. According to another embodiment, the melting temperature (T_m) of at least one crystallizable organic compound is at least 10° C. lower than the glass transition temperature (T_g) of at least one natural polymer and/or the derivative of a natural polymer. According to another embodiment, the melting temperature (T_m) of at least one crystallizable organic compound is at least 20° C. lower than the glass transition temperature (T_g) of at least one natural polymer and/or the derivative of a natural polymer. According to another embodiment, the melting temperature (T_m) of at least one crystallizable organic compound is at least 30° C. lower than the glass transition temperature (T_g) of at least one natural polymer and/or the derivative of a natural polymer. The melting temperature of stearic acid is approximately 40° C. below the glass transition temperature (T_g) of lignin stearate. According to another embodiment, the melting temperature (T_m) of at least one crystallizable organic compound is at least 50° C. lower than the glass transition temperature (T_g) of at least one natural polymer and/or the derivative of a natural polymer.
As a result, the surface of the coating layer is not superhydrophobic. The coating layer thus has a contact angle with water of no more than 150°. The contact angle can be, for example, 80°, 85°, 90°, 95°, 100°, 105°, 110°, 115°, 120°, 125°, 130°, 135°, 140°, or 145°. Surfaces with angles greater than 145° are still considered superhydrophobic. Consequently, the contact angle is preferably no more than 145°. According to one embodiment, the contact angle is no more than 130°. According to another embodiment, the contact angle is no more than 115°.
Furthermore, the coating layer preferably has a roll-off angle of more than 10° with respect to a water droplet with a volume of 4 μL. The roll-off angle can be, for example, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, 85°, 90°, 95°, 100°, 105°, 110°, 115°, 120°, 125°, 130°, 135°, 140°, 145°, 150°, 155°, 160°, 165°, 170°, 175°, or 180°. According to one embodiment, the roll-off angle is more than 20°. According to another embodiment, the roll-off angle is more than 40°. According to another embodiment, the roll-off angle is more than 60°.
According to one embodiment, the proportion of crystallizable organic compounds relative to the total mass of the coating layer ranges from 1 to 60 wt. %. Below 1 wt. %, no measurable crystallinity can be generated, and the entire coating layer remains amorphous. When using more than 60% crystallizable organic compound, the film becomes too inhomogeneous, and holes can form, limiting the barrier performance. For example, the crystalline organic compound can constitute 1 wt. %, 2 wt. %, 4 wt. %, 5 wt. %, 6 wt. %, 8 wt. %, 10 wt. %, 12 wt. %, 14 wt. %, 16 wt. %, 18 wt. %, 20 wt. %, 22 wt. %, 24 wt. %, 26 wt. %, 28 wt. %, 30 wt. %, 32 wt. %, 34 wt. %, 36 wt. %, 38 wt. %, 40 wt. %, 42 wt. %, 44 wt. %, 46 wt. %, 48 wt. %, 50 wt. %, 52 wt. %, 54 wt. %, 56 wt. %, 58 wt. %, or 60 wt. % of the coating layer. The inventors have experimentally shown that the barrier performance does not continuously increase with increasing crystallinity. Rather, for the crystallizable organic compound stearic acid, a minimum WVTR is achieved at about 30 wt. % relative to the total mass of the coating layer. Above 50 wt. %, no increase in barrier performance is typically expected. Below 3 wt. %, only a slight crystallinity can be achieved, which hardly affects the barrier performance. According to one embodiment, the proportion of the crystalline organic compound ranges from 3 to 50 wt. %. According to another embodiment, the proportion of the crystalline organic compound ranges from 5 to 40 wt. %. According to yet another embodiment, the proportion of the crystalline organic compound ranges from 25 to 35 wt. %
The crystallizable organic compounds are preferably at least partially chain-like hydrocarbons, which are preferably branched. The aggregation of similar chains of such chain-like hydrocarbons allows for the formation of crystals or crystallites.
Crystallizable organic compounds that can be used according to the invention include, for example, fatty acids, fatty acid amides, fatty acid esters, salts of fatty acids, hydroxy fatty acids, hydroxy fatty acid amides, hydroxy fatty acid esters, salts of hydroxy fatty acids, or dicarboxylic acids, as well as their dicarboxylic acid esters, dicarboxylic acid amides, or salts of dicarboxylic acids. Examples of dicarboxylic acids that can be used according to the invention are tetradecanedioic acid, hexadecanedioic acid, octadecanedioic acid, icosanedioic acid, or docosanedioic acid. The crystallizable organic compound is preferably not starch. The crystallizable organic compound is preferably not suberic acid, especially when the natural polymer is lignin or lignin stearate.
The fatty acid used as the crystallizable organic compound can be a saturated or unsaturated fatty acid with 12 to 40 carbon atoms. Examples of saturated fatty acids include lauric acid, myristic acid, palmitic acid, margaric acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, lacceric acid, and geddic acid. Examples of unsaturated fatty acids include myristoleic acid, palmitoleic acid, margaroleic acid, petroselinic acid, oleic acid (OA), elaidic acid, vaccenic acid, gadoleic acid, gondoic acid, cetoleic acid, erucic acid, and nervonic acid. Examples of polyunsaturated fatty acids include linoleic acid (LA), alpha-linolenic acid (ALA), gamma-linolenic acid (GLA), calendic acid, punicic acid, alpha-eleostearic acid, beta-eleostearic acid, stearidonic acid, arachidonic acid, eicosapentaenoic acid (timnodonic acid, EPA), docosadienoic acid, docosatetraenoic acid (adrenic acid, ADA), docosapentaenoic acid (clupanodonic acid), docosahexaenoic acid (cervonic acid, DHA), and tetracosahexaenoic acid (nisinic acid).
According to one embodiment, the fatty acids used as crystallizable organic compounds have 16 to 18 carbon atoms and 0 or 1 double bond. According to one embodiment, the fatty acid is selected from margaric acid, stearic acid, palmitic acid, linoleic acid, α-linolenic acid, and γ-linolenic acid. According to one embodiment, the crystallizable organic compound is stearic acid or its amide or salt.
Fatty acid salts according to the invention include chromium (III) chloride complexes with fatty acids, as well as aluminum, calcium, sodium, potassium, and ammonium salts. Preferred fatty acid salts are monovalent salts from sodium, potassium, or ammonium ions.
The at least one crystallizable organic compound can be present as a fatty acid mixture or wax in the coating. Suitable waxes according to the invention include, among others, carnauba wax, candelilla wax, beeswax, and Japan wax.
According to one embodiment, the fatty acid mixture is a mixture of stearic acid, palmitic acid, oleic acid, linoleic acid, and/or linolenic acid. Further examples of fatty acid mixtures are a mixture of stearic acid, palmitic acid, and oleic acid; a mixture of stearic acid, linoleic acid, and linolenic acid; a mixture of stearic acid, palmitic acid, and linoleic acid; a mixture of stearic acid, palmitic acid, and linolenic acid; a mixture of stearic acid, oleic acid, and linoleic acid; a mixture of stearic acid, oleic acid, and linolenic acid; and a mixture of stearic acid, linoleic acid, and linolenic acid.
According to the invention, the proportion of natural polymers or their derivatives relative to the total mass of the coating layer can range from 40 to 99 wt. %. A single polymer or its derivative can be used, as well as mixtures of natural polymers, mixtures of derivatives of natural polymers, or mixtures of natural polymers and derivatives of natural polymers. Without being bound to theory, it is believed that the natural polymers or their derivatives form the amorphous regions and are also involved in the formation of the crystalline regions. It is assumed that the natural polymers form the amorphous matrix in which the crystalline regions are embedded. The proportion of natural polymers depends on the proportion of crystallizable organic compounds and the presence of any other components. For example, the natural polymers or their derivatives can constitute 40 wt. %, 42 wt. %, 44 wt. %, 46 wt. %, 48 wt. %, 50 wt. %, 52 wt. %, 54 wt. %, 56 wt. %, 58 wt. %, 60 wt. %, 62 wt. %, 64 wt. %, 66 wt. %, 68 wt. %, 70 wt. %, 72 wt. %, 74 wt. %, 76 wt. %, 78 wt. %, 80 wt. %, 82 wt. %, 84 wt. %, 86 wt. %, 88 wt. %, 90 wt. %, 92 wt. %, 94 wt. %, 96 wt. %, 98 wt. %, or 99 wt. %. According to one embodiment, the proportion of natural polymers ranges from 50 to 95 wt. %. According to another embodiment, the proportion of natural polymers ranges from 60 to 90 wt. %. According to yet another embodiment, the proportion of natural polymers ranges from 65 to 75 wt. %.
Natural polymers that can be used according to the invention include hydrocarbons such as rubber or balata, proteins such as collagen, keratin, fibroin, nucleic acids, polysaccharides such as alginate, agar-agar, cutin, suberin, lignin, cellulose, chitosan, and starch, lipids, polylactide (PLA), polyhydroxybutyrate (PHB), and polyhydroxyalkanoate (PHA). Derivatives that can be used according to the invention include cellulose derivatives such as methylcellulose (MC) and hydroxypropylmethyl cellulose (HPMC), methylhydroxyethylcellulose (MHEC), starch derivatives such as methylated starch, ethylated starch, hydroxyethyl starch, hydroxypropyl starch, carboxymethyl starch, starch formate, starch acetate, starch propionate, or starch butyrate, suberin derivatives, cutin derivatives, or lignin derivatives. These natural polymers and their derivatives are believed to be suitable for forming an amorphous matrix around a crystallizable organic compound. The crystallizable organic compound is preferably not identical to the natural polymer or its derivative. Therefore, the crystalline and amorphous regions of the coating layer according to the invention are not merely different states of a single substance.
Fatty acids are considered absolutely safe for the human body and are listed as E 570 on the ingredient list of foods, making them suitable for food use (“dual-use substance”). This is an advantage for use in food packaging materials, such as the coated paper according to the invention, as there may be a transfer of substances from the packaging to the food due to (prolonged) contact.
According to one embodiment, the natural polymer is suberin. Suberin is a plant biopolymer deposited in cell walls. Suberized cells are found in both secondary closure tissues and underground plant organs. The name “suberin” is derived from the cork oak (Quercus suber). Suberin can be divided into two different domains: a polyphenolic domain and a polyaliphatic domain. In the polyaliphatic fraction, dicarboxylic acids, hydroxy acids, long-chain fatty acids, and hydroxycinnamic acids have been found. Current research also suggests that glycerol is a very prominent monomer in the compound. The phenolic portion shows a similarity to lignin, although the content of monolignols is significantly lower than in lignin. Due to the ester bonds in suberin, the model of suberin's chemical structure appears to resemble the model of lignin-fatty acid esters' chemical structure, suggesting that the results shown here for lignin-fatty acid esters may be transferable to suberin.
According to one embodiment, the natural polymer is lignin. Lignin is a high-molecular-weight, aromatic substance that fills the spaces between cell membranes in woody plants, contributing to the formation of wood. Lignin can be regarded as a high-molecular-weight (MR approx. 5000 to 10000) derivative of phenylpropane, composed of structures based on coumaryl alcohol, coniferyl alcohol, or sinapyl alcohol, depending on the type of wood. The lignin of different wood or plant species (grasses, deciduous, or coniferous trees) varies in the percentage of these alcohols. The components interconnect in various forms (ether and C—C bonds), creating a three-dimensional network. Besides the variability of each lignin molecule, lignin from different wood or plant species differs in the proportion of these alcohols or their derived phenyl residues. Coniferous wood lignin predominantly contains coniferyl units (about 90%), which have a guaiacyl residue (3-methoxy-4-hydroxyphenyl residue) and is therefore called G-lignin. Deciduous wood lignin contains varying amounts of guaiacyl residues and sinapyl elements, which include a syringyl residue (3,5-dimethoxy-4-hydroxyphenyl residue). The syringyl content can range from 5 to 65%, resulting in GS-lignin. Lignin in partially lignified grasses and other monocots is characterized by a high content of about 15 to 35% coumaryl elements, forming para-hydroxyphenylpropane and, along with an equal amount of syringyl and 50 to 70% guaiacyl units, constituting HGS-lignins.
According to the invention, the lignin can be derived from coniferous woods, deciduous woods, grass plants, or annual plants. According to one embodiment, the lignin is derived from coniferous woods. In another embodiment, the lignin is derived from deciduous woods.
Various methods for extracting lignin are known to the skilled person, including the Kraft process, the sulfite process, the soda-anthraquinone process, the GRANIT process, the Alcell™ process, and the Organocell process. According to one embodiment, the lignin is derived from coniferous woods using the Kraft process. These methods are summarized in Nitz et al. 2001. According to one embodiment, the lignin is derived from deciduous woods using the Kraft process.
According to one embodiment, the derivative of a natural polymer is an ester of a natural polymer. Suitable esters of natural polymers according to the invention include cellulose esters, starch esters, cutin esters, suberin esters, and lignin esters. According to one embodiment, the ester is an ester of lignin and one or more fatty acids, hydroxy fatty acids, or dicarboxylic acids.
The acid used for the esterification of lignin preferably has a similar chain length and degree of branching as the acid used as the crystallizable organic compound. The chain length of the two acids should differ by no more than 8 carbon atoms. Preferably, the chain length differs by no more than 5 carbon atoms. Particularly preferably, the difference in chain length is no more than 3 carbon atoms. Additionally, the number of double bonds, i.e., the degree of saturation, between the acid used for lignin esterification and the acid used as the crystallizable organic compound should match.
According to one embodiment, the coating or the coating layer contains an ester of a natural polymer, wherein the fatty acid is identical to at least one of the fatty acids used as the crystallizable organic compound. Selecting identical fatty acids provides the best conditions for the formation of crystals in the semi-crystalline coating layer. According to one embodiment, the coating or the coating layer contains a lignin fatty acid ester, wherein the fatty acid is identical to at least one of the fatty acids used as the crystallizable organic compound. Another advantage of using identical fatty acids is that any unreacted excess fatty acid from the esterification process can be used, thus saving a processing step. According to one embodiment, the crystallizable organic compound is stearic acid, and the derivative of a natural polymer is lignin stearate.
The synthesis of the ester of the natural polymer can be carried out according to the Schotten-Baumann esterification principle (see Example 2). Suitable reaction conditions are known to the skilled person and can be found in Example 2. For example, lignin stearate can be obtained by reacting stearoyl chloride with Kraft lignin.
The lignin stearate used in the production of the coated paper according to the invention can have an esterification number of at least 40%. The esterification number can be, for example, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%. According to one embodiment, the esterification number is at least 90%.
Furthermore, the lignin stearate preferably has a glass transition temperature (T_g) in the range of 70 to 140° C. Above 140° C., there could be problems in technical applications, as processing (melt dispersion, drying, and film formation) might become problematic. Below approximately 70° C., the matrix may remain too soft after drying, potentially causing “sticking” when rolling up the paper webs and detaching the coating when unrolling the webs. The temperature can be, for example, 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., 100° C., 105° C., 110° C., 120° C., 125° C., 130° C., 135° C., or 140° C. More preferably, the lignin stearate has a glass transition temperature (T_g) in the range of 120 to 140° C.
The coating layer can contain one or more additional components depending on the application and requirements, such as a protective colloid or a filler. Protective colloids are particularly suitable for keeping particles stable in aqueous dispersion, thus enabling a homogeneous coating of the paper. Suitable protective colloids include polyvinyl alcohol, polysorbates, PEG alkoxylates, and sorbitan fatty acid esters (SPAN). Suitable fillers are particularly pigments such as nanoclay. Nanoclay is used as an accelerator for crystallization.
Both the natural polymer or its derivative and the crystallized organic compound used can contain impurities resulting from their extraction or production. Examples of impurities include unesterified lignin polymer and polysaccharides such as celluloses and xylans. Preferably, these impurities make up only a very small part of the coating and thus the resulting coating layer. According to one embodiment, the proportion of impurities in the coating layer is below 1 wt. % of the total mass of the coating layer.
According to one embodiment, apart from unavoidable impurities, the coating does not contain any synthetic polymer that is petroleum-based and not biodegradable.
Due to the materials present in the coating layer, the coated paper is biodegradable. “Biodegradability” refers to the ability of organic chemicals to be decomposed biologically, i.e., by living organisms or their enzymes. Ideally, this chemical metabolism proceeds completely to mineralization, but it can also stop at degradation-resistant transformation products. The OECD guidelines for testing chemicals, which are also used in the context of chemical approval, are generally recognized. The tests in the OECD test series 301 (A-F) demonstrate rapid and complete biodegradation (ready biodegradability) under aerobic conditions. Different test methods are available for well or poorly soluble as well as volatile substances. “Biodegradable” or “biologically degradable” within the meaning of the present invention refers to papers that exhibit biodegradability measured according to OECD 301 F of at least 40% or measured according to OECD 302 C (MITI-II test) of at least 20%, thus exhibiting inherent or basic biodegradability. This corresponds to the limit for OECD 302 C according to the “Revised Introduction to the OECD Guidelines for Testing of Chemicals, Section 3, Part 1, dated 23 Mar. 2006”. From a threshold of at least 60% measured according to OECD 301 F, microcapsule walls are also referred to as rapidly biodegradable.
According to one embodiment, the coated paper exhibits easy biodegradability according to OECD 301.
Furthermore, the coated paper is recyclable. Paper recycling refers to the dissolution and processing of waste paper, used cardboard, and paperboard in paper industry facilities with the goal of producing new paper, cardboard, and paperboard. To a lesser extent, waste paper is first converted into recycled paper pulp, which is then used to manufacture new paper. Deinking, the process of removing printing ink from printed waste paper, is a key process in paper recycling. The recyclability evaluation can be carried out using the INGEDE Method 11. The coated paper according to the invention achieves a deinkability score of over 50 using the INGEDE Method 11. Preferably, the deinkability score is over 70.
With the components used in the inventive coating layer, the coated paper can be approved for direct or indirect food contact. It is particularly suitable for approval according to the guidelines of the European Food Safety Authority.
Any type of paper can be used as the base paper for the coated paper, including cardboard, paperboard, or regular paper. Papers with low basis weight are often in demand for food packaging because they are flexible and material-efficient. The inventive coating layer significantly enhances the barrier performance, especially for such papers.
According to one embodiment, the base paper has a basis weight of less than 150 g. m⁻². The basis weight can be, for example, 150 g·m⁻², 145 g·m⁻², 140 g·m², 135 g·m⁻², 130 g·m⁻², 125 g·m⁻², 120 g·m⁻², 115 g·m⁻², 110 g·m⁻², 105 g·m⁻², 100 g·m⁻², 95 g·m 2, 90 g·m⁻², 85 g·m⁻², 80 g·m⁻², 75 g·m⁻², 70 g·m⁻², 65 g·m⁻², 60 g·m⁻², 55 g·m⁻², 50 g m⁻², 45 g. m⁻², 40 g·m⁻², 35 g·m⁻²oder 30 g·m⁻². According to one embodiment, the basis weight is below 100 g·m⁻². According to another embodiment, the basis weight is below 80 g. m⁻². According to another embodiment, the basis weight ranges from 50 to 80 g·m⁻².
The coated paper can contain additional layers besides the semi-crystalline coating layer. According to one embodiment, the coated paper contains an additional layer selected from a coating, an ink, a sealing medium, and an adhesive.
The additional layer can be arranged on the semi-crystalline coating layer, between the base paper and the semi-crystalline coating layer, or on the side of the base paper opposite the semi-crystalline coating layer.
The semi-crystalline coating layer can be applied directly to the base paper. In this case, the semi-crystalline coating layer is in direct contact with the base paper. Indirect application means that one or more layers are positioned between the coating and the base paper.
Additional layers can further reduce the permeability of the coated paper for at least one gas compared to the base paper or form barriers for liquids or viscous substances such as fats, oils, and hydrocarbons.
An additional layer can particularly:

- a) comprise at least one hydrophobic polymer, e.g., based on a polyacrylate, a styrene/butadiene copolymer, and/or a polyolefin,
- b) comprise at least one hydrophilic polymer, e.g., based on a polyvinyl alcohol,
- c) comprise at least one inorganic pigment, e.g., a platelet-shaped pigment such as a layered silicate like kaolin,
- d) comprise at least one inorganic pigment and a binder,
- e) comprise amorphous and crystalline regions, e.g., the coating composition according to the invention,
- f) contain or consist of substances selected from the group of lipophilic substances, paraffins, especially hard paraffins, waxes, particularly microcrystalline waxes, waxes based on vegetable oils or fats, waxes based on animal oils or fats, vegetable waxes, animal waxes, low molecular weight polyolefins, polyterpenes, and their mixtures,
- g) reduce or prevent the migration of substances, particularly hydrophobic substances, e.g., substances according to the preceding point f), to prevent or reduce the migration of substances from underlying layers to a food product, especially a fatty food product.
- h) comprise or consist of at least one metal, e.g., aluminum, gold, and/or a metal oxide, e.g., aluminum oxide, in particular, be a metallized layer,
- i) be at least heat- or cold-sealable,
- j) comprise at least one adhesive,
- k) comprise or consist of at least one thermoplastic material, particularly as a heat-sealable material.

The base paper can be a single- or double-sided coated base paper or an uncoated base paper.
In coated base paper, the surface is refined with a binder-containing coating. The coating material used can have starch, starch derivatives, chalk, kaolin, casein, or plastic dispersion as its main component. This gives the base paper a more closed, smoother, and more stable surface.
However, uncoated base paper can also be surface-treated and contain up to 5 g/m²of pigments.
For use as packaging in the food sector, the paper requires a certain tensile strength or breaking strength. According to one embodiment, the coated paper has a tensile strength in the fiber direction ranging from 3.0 bis 6.0 kN·m⁻¹. The tensile strength in the fiber direction can be, for example, 3.0 kN·m⁻¹, 3.2 kN·m⁻¹, 3.4 kN·m⁻¹, 3.5 kN·m⁻¹, 3.6 kN·m⁻¹, 3.8 kN·m⁻¹, 4.0 kN·m⁻¹, 4.2 kN·m⁻¹, 4.4 kN·m⁻¹, 4.5 kN·m⁻¹, 4.6 kN·m⁻¹, 4.8 kN·m⁻¹, 5.0 kN·m⁻¹, 5.2 kN·m⁻¹, 5.4 kN·m⁻¹, 5.5 kN·m⁻¹, 5.6 kN·m⁻¹, 5.8 kN·m⁻¹, 6.0 kN·m⁻¹. According to one embodiment, the tensile strength in the fiber direction ranges from 3.5 bis 5.5 kN m⁻¹. According to another embodiment, the tensile strength in the fiber direction ranges from 4,0 bis 5.0 kN m⁻¹.
The induced semi-crystallinity according to the invention was exemplified with a coating containing a fatty acid as a crystallizable organic compound in combination with a derivative of a natural polymer, particularly lignin. Consequently, according to a second aspect, the invention relates to a coating for coating papers, containing at least one solvent, at least one crystallizable organic compound, and at least one natural polymer and/or derivative of a natural polymer.
As described regarding the coating layer in the coated paper according to the first aspect, the crystallizable organic compound is preferably selected from fatty acids, hydroxy acids, or dicarboxylic acids or their esters, amides, or salts. Furthermore, the crystallizable organic compound in the coating is present in a non-crystalline form. Since the coating is the starting material for the formation of the coating layer, the definitions of the crystallizable organic compound in the first aspect of the invention apply here as well.
The natural polymer is particularly selected from hydrocarbons such as rubber or balata, proteins such as collagen, keratin, fibroin, nucleic acids, polysaccharides such as alginate, agar-agar, cutin, suberin, lignin, cellulose, chitosan, and starch, lipids, polylactide (PLA), polyhydroxybutyrate (PHB), and polyhydroxyalkanoate (PHA). The definitions of the natural polymer or the derivative in the first aspect of the invention also apply here.
Unlike the coating layer, the coating additionally contains at least one solvent. Suitable solvents are water, tetrahydrofuran (THF), toluene, ethyl acetate, and alcohols such as ethanol or isopropanol. Water is the preferred solvent.
According to one embodiment, at least one fatty acid in the ester of the natural polymer, particularly in the lignin ester, is identical to at least one fatty acid used as the crystallizable organic compound. According to one embodiment, the crystallizable organic compound is stearic acid and the derivative of a natural polymer is lignin stearate.
Due to the solvent content, the proportion of natural polymers and/or their derivatives in the coating differs from the proportion in the coating layer. According to one embodiment, the proportion of natural polymers and/or their derivatives relative to the total mass of the coating is in the range of 6 to 30 wt. %. The proportion can be, for example, 6 wt. %, 8 wt. %, 10 wt. %, 12 wt. %, 14 wt. %, 16 wt. %, 18 wt. %, 20 wt. %, 22 wt. %, 24 wt. %, 26 wt. %, 28 wt. %, or 30 wt. %. According to one embodiment, the proportion is in the range of 8 to 25 wt. %. According to another embodiment, the proportion is in the range of 15 to 25 wt. %.
According to one embodiment, the proportion of crystallizable organic compounds relative to the total mass of the coating is in the range of 2 to 15 wt. %. The proportion can be, for example, 2 wt. %, 4 wt. %, 6 wt. %, 8 wt. %, 10 wt. %, 12 wt. %, 13 wt. %, 14 wt. %, or 15 wt. %. According to one embodiment, the proportion is in the range of 4 to 12 wt. %. According to another embodiment, the proportion is in the range of 5 to 8 wt. %.
According to one embodiment, the proportion of the solvent relative to the total mass of the coating is in the range of 60 to 95 wt. %. The proportion can be, for example, 60 wt. %, 62 wt. %, 64 wt. %, 66 wt. %, 68 wt. %, 70 wt. %, 72 wt. %, 74 wt. %, 76 wt. %, 78 wt. %, 80 wt. %, 82 wt. %, 84 wt. %, 86 wt. %, 88 wt. %, 90 wt. %, 92 wt. %, 94 wt. %, or 95 wt. %. According to one embodiment, the proportion is in the range of 70 to 90 wt. %. In another embodiment, the proportion is in the range of 75 to 85 wt. %.
At a dry content of 10 wt. %, i.e., a solvent content of 90 wt. %, the proportion of polymer is preferably in the range of 6.5 wt. % to 7.5 wt. % and the proportion of crystallizable organic compound in the range of 2.5 wt. % to 3.5 wt. %. At a dry content of 15 wt. %, i.e., a solvent content of 85 wt. %, the proportion of polymer is preferably in the range of 9.8 wt. % to 11.3 wt. % and the proportion of crystallizable organic compound in the range of 3.8 wt. % to 5.3 wt. %. At a dry content of 25 wt. %, i.e., a solvent content of 75 wt. %, the proportion of polymer is preferably in the range of 16.3 wt. % to 18.8 wt. % and the proportion of crystallizable organic compound in the range of 6.3 wt. % to 8.8 wt. %. At a dry content of 30 wt. %, i.e., a solvent content of 70 wt. %, the proportion of polymer is preferably in the range of 16.3 wt. % to 18.8 wt. % and the proportion of crystallizable organic compound in the range of 6.3 wt. % to 8.8 wt. %. At a dry content of 40 wt. %, i.e., a solvent content of 60 wt. %, the proportion of polymer is preferably in the range of 26.0 wt. % to 30.0 wt. % and the proportion of crystallizable organic compound in the range of 10.0 wt. % to 14.0 wt. %.

Manufacturing Process

The essential crystallinity of the semi-crystalline coating layer in the coated paper is particularly dependent on the process used for producing the coated paper. Consequently, according to a third aspect, the invention relates to a process for producing coated paper with a base paper and a semi-crystalline coating layer, comprising the steps of:

- a) preparing a coating according to the second aspect by melt dispersion, high-pressure dispersion, or spray drying followed by mechanical dispersion of the components;
- b) providing a base paper;
- c) providing a base paper
- d) curing the coating to form the semi-crystalline coating layer.

According to one embodiment, the application of the coating to the base paper is preferably carried out using a curtain coater or a doctor blade coater.
As explained regarding the crystallinity of the coating layer of the coated paper according to the first aspect of the invention, the crystallinity of the semi-crystalline coating layer can be adjusted by the proportion of crystallizable organic compounds relative to the total mass of the coating using the process according to the invention.
In addition, the curing temperature, curing time, and curing pressure influence the crystallinity.
According to one embodiment of the process, the curing temperature is in the range of 20 to 300° C. According to one embodiment of the process, the curing temperature is in the range of 120 to 140° C.
According to one embodiment of the process, the curing time is in the range of 10 seconds to 15 minutes. According to one embodiment of the process, the curing time is in the range of 1 to 3 minutes.
According to one embodiment of the process, the curing pressure is in the range of 0.2 bar to 3 bar. According to one embodiment of the process, the curing pressure is in the range of 0.9 bar to 1.1 bar.
Since the coated paper is produced by the described process, the invention also relates to coated paper produced by the process according to the second aspect.

Packaging

According to a fourth aspect, the invention relates to packaging for food comprising the coated paper according to the first aspect.
This can include, for example, packaging for dried foods, foods sold cold that require further preparation, packaging containing food portions for more than one person, or packaging with food portions for one person where more than one unit is sold.
Examples of suitable packaging include stand-up pouch packaging, tubular bag packaging, or wrapping paper. According to one embodiment, the packaging is a tubular bag packaging.

EXAMPLES

Example 1—Characterization of Lignin

The main focus of lignin characterization is on the molecular weight of the lignin and the number of hydroxyl groups.

Molecular Weight

The determination of the molecular weight is carried out using GPC.


Device Parameters - GPC 4

Flow rate	1	mL/min
RI detector temperature	35°	C.
Sample injection volume	100	μL
Internal standard injection	20	μL
volume
System test injection volume	20	μL
Standards injection volume	20	μL

Guard column	PSS MCX	5 μm 8 × 50 mm
Main column	PSS MCX	1000 Å und MCX 100000 Å
	5 μm 8 × 300 mm
Eluent/solvent buffer	NaCl 0.1 mol/L + NaOH 0.1 mol/L
Internal standard	Ethylenglycol (1 μL in 1 mL Eluent)
System test/plate number	2-3 μL Ethylenglcol in 1 mL Eluent

Standards

Poly(styrene sulfonic acid)

1 mg in 1 mL eluent

sodium salt single standard
from PSS

FIG. 1 shows an example of a Kraft lignin used.

- Number average molecular weight Mn: 1000 g*m⁻²*d⁻¹
- Weight average molecular weight Mw: 5000 g*m⁻²*d⁻¹

Number of Hydroxyl Groups

The number of hydroxyl groups can be determined by titration and by ³¹P-NMR spectroscopy.

Titration:

The OH number of Kraft lignin (KL) is determined according to DIN EN ISO 4629-1.

³¹P-NMR-Spektroskopie

The phosphorylation of the lignin derivatives is carried out according to the procedure of Granata et al. For this, 25 mg of the dry lignin is dissolved in 150 μL of DMF. After dissolving, 100 μL of pyridine, 200 μL of a solution of the internal standard endo-N-hydroxy-5-norbornene-2,3-dicarboximide (25.0 mmol*L⁻¹in pyridin/CDCl ₃1,6:1) and 50 μL of a solution of chromium (III) acetylacetonate (32.64 mmol/L, in pyridin/CDCl ₃1,6:1) are added. The solution is purged with argon for a few minutes and then, under an argon atmosphere, 150 μL of the phosphitylation reagent 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (0.944 mmol) and 300 μL of CDCl₃are added. The resulting solution is transferred to an argon-purged NMR tube.
From the integrals of the signals, the amount of hydroxyl groups can be calculated. For the Kraft lignin shown in FIG. 2 , a hydroxyl group content of 6.18 mmol/g and a carboxyl group content of 0.5 mmol/g are obtained.

Example 2—Preparation of Esterified Lignins

The synthesis of lignin stearate was carried out following the principle of a Schotten-Baumann esterification using stearoyl chloride with Kraft lignin.
Lignin was dissolved in dry 1,4-dioxane (50 g/L) (N₂atmosphere). After approximately one hour, stearoyl chloride (2.8 g per 1 g lignin) and pyridine (0.25 g per 1 g lignin) were added. The reaction was stirred for 6 hours at 80° C.
The reaction solution was then added to 3 L of water and stirred for 2 hours. The precipitate was filtered (Po.2), washed with water, and dried in a vacuum oven at 40° C.
The solid was added to 1 L of ethanol and heated to reflux for 6 hours. After cooling to room temperature, the mixture was centrifuged (4,700 rpm, 10 min). The supernatant was decanted, and the solid was added to 200 mL of ethanol, stirred at 60° C. for approximately 1 hour, allowed to cool to room temperature, and centrifuged. This step was repeated 2 to 3 times.
The solid was dried in a vacuum oven at 40° C. To remove any insoluble residues, the solid was dissolved in a small amount of THF, the solution was centrifuged, and the decanted solution was evaporated in a rotary evaporator to obtain the product.
The synthesis process was repeated using stearamide from Croda, candelilla wax, and carnauba wax instead of stearic acid.

Example 3—Characterization of Lignin Esters

The most important factor for characterizing the lignin esters is the degree of esterification. This was determined using ³¹P-NMR spectroscopy and titration.

³¹P-NMR Spectroscopy

The degree of esterification can be calculated from the molar amounts of hydroxyl groups in the lignin ester (nLE) and the used Kraft lignin (nKL). The total molar amount of hydroxyl groups nKL (—OH and —COOH) in the used Kraft lignin is 6.68 mmol per gram of lignin (see Example 1). The molar amount of hydroxyl groups in the lignin ester can be determined analogously. The obtained spectrum is shown in FIG. 3 .
The molar amount of hydroxyl groups (—OH) is: 1.10 mmol*g⁻¹
The molar amount of carboxyl groups (—COOH) is: 0.09 mmol*g⁻¹
The total molar amount nu of hydroxyl groups (—OH and —COOH) is: 1.19 mmol*g⁻¹
The degree of esterification E is calculated as follows:
$E = 1 - (\frac{n_{LE}}{n_{KL}})$ $E = 1 - (\frac{1, 19}{6, 68}) = 0, 82 = 8 2 %$
Titration according to DIN EN ISO 4629-1:2016

Softening Temperature

The softening temperature of the lignin ester is determined optically. A small amount of powdered lignin ester is placed on a watch glass and placed in a convection oven. The oven temperature is increased in 10° C. increments, and the temperature at which the powder melts into a drop is recorded.
The softening temperature of lignin stearate is approximately 130° C.

IR-Spektroskopie

FIG. 4 shows the IR spectrum of lignin stearate compared with Kraft lignin and stearic acid. The successful conversion can be identified by the signal at 1750 cm{circumflex over ( )} 1, which represents the C═O stretching vibration of an ester. In comparison, the C═O stretching vibration of the acid is located at 1700 cm{circumflex over ( )}-1.

¹H-NMR Spectroscopy

FIG. 5 shows the 1H-NMR spectrum of lignin stearate.

Example 4—Preparation of Coatings

A) Coating Based on Organic Solvents

The solids content of a coating was between 10 and 25 weight percent. 1 g of a mixture of lignin stearate and stearic acid (e.g., 7:3) was weighed into a 5 mL container with a lid and dissolved in 3 mL of a mixture of tetrahydrofuran (THF) and ethyl acetate (EE) (1:2).
The preparation was repeated with stearamide, carnauba wax, and candelilla wax instead of stearic acid. Depending on the wax used, the solvent mixture was varied to ensure solubility and subsequent wettability on the paper to be coated. The coatings from lignin stearate/carnauba wax and lignin stearate/candelilla wax were prepared in a mixture of THF/EE in a ratio of 2:1.
B) Aqueous-Based Coating with Spray-Dried Lignin
The solids content of a coating was between 10 and 25 weight percent. 3.75 g of a spray-dried lignin stearate-stearic acid mixture (1:1) was dispersed in 15 mL of a solution of 1% SPAN-60 in water using an Ultrathorax. 5 mL of the dispersion was placed in a 25 mL Speedmixer cup. A hole was punched in the lid of the cup. The dispersion was degassed in the Speedmixer at 800 rpm and a pressure of 30 mbar for four minutes.
The preparation was repeated with stearamide, carnauba wax, and candelilla wax instead of stearic acid.

Example 5—Preparation of Coated Papers

An RK K303 Multicoater was used to coat the papers (see Table 1).
The base paper was placed on waste paper so that the standard K-bar to be clamped rested on the base paper. The corresponding bar was clamped into the Multicoater. A weight was placed on the paper behind the bar. The desired coating speed was set (10-20 m/min for organic medium, 3 m/min for aqueous dispersion). The coating (approximately 3-5 mL) was applied with a pipette in front of the bar across the width of the paper, and the coating process was started. The bar moved over the paper, distributing the coating evenly.
After coating, the paper was transferred to a cardboard carrier, fixed at the corners to prevent rolling, and dried in an oven at 130° C. for 10 minutes (for organic medium: solvent evaporation at room temperature, melting of the coating at 130° C. for approximately 3 minutes).

TABLE 1

Coating Area: max. 350 × 440 mm
Substrate Size max. 350 × 580 mm
Waste Paper Rolls: 375 mm Breite × 150 mm

	Wet Film
Standard K bar	Thickness	Wire Diameter mm

K303 bar No. 0	4	0.05
K303 bar No. 1	6	0.08
K303 bar No. 2	12	0.16
K303 bar No. 3	24	0.31
K303 bar No. 4	40	0.51
K303 bar No. 5	50	0.64
K303 bar No. 6	60	0.76
K303 bar No. 7	80	1
K303 bar No. 8	100	1.27
K303 bar No. 9	120	1.5

Example 6—Determination of Water Vapor Transmission Rate of Coated Papers

The determination of the water vapor transmission rate (WVTR) is carried out based on DIN 53122-1 as a gravimetric method.
Round test specimens with a diameter of 6.3 cm are punched out from the papers to be tested. A desiccant (e.g., silica gel beads) is placed into a measuring vessel. A sealing ring is placed on the edge of the vessel, followed by the test specimen with the water vapor barrier side facing the desiccant, another sealing ring, and a metal ring placed on top of the test specimen. The vessel is sealed with a lid that has a hole with a diameter of 5.7 cm. The scheme of such a measurement setup in cross-section can be seen in the figure.
The prepared measuring vessel is placed in a climate chamber maintained at a temperature of 38° C. and a relative humidity of >90%. After conditioning in the chamber for approximately 16 hours, the measuring vessel is weighed. Repeated weighings are performed at intervals of 2-3 hours until at least three measurement results are obtained. Between weighings, the measuring vessel is stored in the climate chamber.
From the mass gain, the water vapor transmission rate can be calculated in grams per square meter per day (g*m⁻²*d⁻¹). At least two test specimens are measured for each barrier to be examined.
The results for the coatings with

- a) Stearic acid
- b) Candelilla wax
- c) Carnauba wax
- d) Stearamide
- are shown in FIG. 6 .

Comparison to the measurement method by Hult et al. (measured: coating from aqueous dispersion with 20% dry content (spray-dried lignin) in 1% SPAN-60 solution)
Tropical: 36.2±20.0 g*m⁻²d⁻¹(two measurements: 50.4 and 22.1 g*m⁻²d⁻¹; the first value is likely affected by defects.)
According to Hult: 2.0±0.1 g*m⁻²d⁻¹(two measurements: 2.0 und 1.9 g*m⁻²d⁻¹).
Paper coated with pure lignin stearate has a WVTR of 368,0±10,6 g·m⁻²·d⁻¹indicating almost no barrier performance against water vapor.
When the proportion of stearic acid is increased, a WVTR of 17,1±1,0 g·m⁻²·day⁻¹can be achieved at a content of 30 wt. %. With further increases in the stearic acid content, the WVTR increases again, and the barrier performance deteriorates.
By using the composite of lignin stearate and stearic acid with a 30 wt. % stearic acid content, a very good barrier performance against water vapor under tropical conditions can be achieved.
Low WVTR values were also achieved with the waxes b) candelilla wax, c) carnauba wax, and d) stearamide. For candelilla wax, the minimum WVTR was also achieved at a 30 wt. % content. For carnauba wax and stearamide, the minimum WVTR was achieved at 50 wt. %.

Example 7 (Comparison)—Investigation of the Surface Structure and WVTR of a Non-Inventive Superhydrophobic Layer According to DE 10 2017 108 577

To demonstrate that the inventive layers structurally differ from the superhydrophobic layers according to DE 10 2017 108 577, a base paper was coated with a superhydrophobic layer as described in Example 1 of DE 10 2017 108 577. A CCK coated paper made from hardwood and softwood pulps with a total basis weight of 63 g/m²was used as the base paper. This was first coated with an aqueous dispersion of alkyl ketene dimer (AKD) (Basoplast 2030 LC, BASF) of the polymer CSE3. The basis weight of this application was 10 g/m². The polymer CSE3 is a fully substituted cellulose ester (DS: 3) of stearic acid.
After complete drying, the substrate was heated in an oven for 5 minutes at 120° C. and then cooled under laboratory conditions (22±3° C./35% relative humidity, RH).
The melting temperature of AKD determined by differential scanning calorimetry (DSC) was −60° C., and that of the polymer CSE3 was −55° C.
After complete cooling of the coated paper, nanostructured, superhydrophobic surfaces were obtained due to the crystallization of the wax, as seen in the scanning electron microscope image (see FIG. 7A). The contact angle was determined with a Dataphysics OCA35 including a tiltable table under constant temperature and humidity (23° C., 50% relative humidity). Using the images and the software SCA 4.5.2 Build 1052, the contact angle was calculated. No magnification is provided. The contact angle of a 4 μL water drop was 159±3° (see FIG. 7B).

Example 8—Investigation of the Surface Structure and Hydrophobicity of the Inventive Papers

A coating based on a mixture of lignin stearate and stearic acid (7:3, i.e., 30% stearic acid content) was prepared as described in Example 3. For this, 1 g of the mixture was weighed into a 5 mL container with a lid and dissolved in 3 mL of a mixture of THF and EE (1:2).
This coating was applied to CCK coated paper made from hardwood and softwood pulps with a total basis weight of 63 g/m²as described in Example 3 (coating weight: 5 g/m²). After coating, the paper was transferred to a cardboard carrier, fixed at the corners to prevent rolling, and dried in an oven at 130° C. for 10 minutes.
The glass transition temperature (T_g) of lignin stearate is about 130° C. The melting point of stearic acid is 69° C., which is significantly below the T_gof lignin stearate. Therefore, the formation of a superhydrophobic layer was not expected. This was confirmed by the scanning electron microscope image of the lignin stearate-stearic acid layer in FIG. 8A, which shows a surface structure significantly different from the superhydrophobic layer.
The contact angle of a 4 μL water drop was 103.2±1.4° (see FIG. 8 B). As expected due to the low contact angle, the roll-off angle could not be easily determined. The roll-off angle (RoA) of a 4 μL water drop was not measurable because the device can only reach a 70° tilt, and no rolling was observed at this tilt. Therefore, various drop volumes were applied, and the roll-off angles were determined. The results are shown in the diagram in FIG. 9 .
By fitting a curve, the theoretical roll-off angle of a 4 μL water drop on the lignin stearate-stearic acid layer was calculated (see Table 2). The calculated roll-off angle for the 4 μL drop exceeds 180°. Thus, no rolling occurs.

TABLE 2

Parameters of the Curve Fitting According to FIG. 9

	Model	Allometric1
	Equation	y = a*x{circumflex over ( )}b
	Reduced Chi-Square	2.72953
	Adjusted R-Square	0.94634

		Value	Standard Error

B	a	1333.21158	538.88074
B	b	−0.91034	0.10777

Example 9—Comparison of WVTR of Inventive Coated Papers

To test the influence of the melting temperature ratio of the crystallizable organic compounds on the WVTR, coatings were made from either stearic acid or suberic acid as the crystallizable organic compound and lignin stearate as the natural polymer, as described in Example 8. The proportion of fatty acids was varied. For the preparation of the suberin-containing coatings, a mixture of THF and methanol (1:1) was used.
Stearic acid has a melting temperature T_m˜69° C., significantly below the glass transition temperature of lignin stearate T_g˜130° C. In contrast, the melting temperature T_mof suberic acid is T_m˜140° C., above the glass transition temperature of lignin stearate.
The coatings were prepared as described in Example 8 but with varying amounts of stearic acid or suberic acid. The fatty acid content was 20 wt. %, 30 wt. %, 40 wt. %, 50 wt. %, or 60 wt. %.
These coatings were applied to the corresponding base paper as described in Example 8. After completion, the WVTR of the coated papers was determined as described in Example 6. The result is shown in FIG. 10 .
From the diagram, it can be seen that the suberic acid-lignin stearate coatings have significantly higher WVTR values with a minimum of about 200 g·m⁻²·d⁻¹at a 30 wt. % content of suberic acid. The values of the coatings with stearic acid were significantly lower, with a minimum of 24 g·m⁻²·d⁻¹also at a 30 wt. % content.
For further advantageous embodiments of the inventive device, reference is made to the general part of the description and the appended claims to avoid repetition.
Finally, it should be expressly pointed out that the embodiments of the inventive device described above serve merely to discuss the claimed teaching but do not limit the invention to these embodiments.

Claims

1. A coated paper comprising a base paper and at least one semi-crystalline coating layer with amorphous regions and crystalline regions, applied directly or indirectly to the base paper:

wherein the amorphous regions contain one or more natural polymers and/or one or more derivatives of natural polymers;

wherein the crystalline regions comprise one or more crystallizable organic compounds, and

wherein the permeability of the coated paper for at least one gas is reduced compared to the base paper.

2. The coated paper according to claim 1, wherein the at least one crystallizable organic compound has a melting temperature T_mthat is lower than the glass transition temperature T_gof the at least one natural polymer and/or its derivative.

3. The coated paper according to claim 1, wherein the permeability of at least one gas is lower than the permeability of a coated paper with the same base paper and each a coating layer of the natural polymer or its derivative and a coating layer of the crystallizable organic compound.

4. The coated paper according to claim 1, wherein the coating layer has a contact angle with water of no more than 150° and/or wherein the coating layer has a roll-off angle with a 4 μL water drop of more than 10°.

5. The coated paper according to claim 1, wherein the crystallinity of the semi-crystalline coating layer is in the range of 10% to 90%.

6. The coated paper according to claim 1, with a water vapor transmission rate (WVTR) of no more than 40 g m⁻²d⁻¹measured at 38° C. and over 90% humidity, and with a coating weight of 10±1 g·m⁻².

7. The coated paper according to claim 1, wherein the proportion of crystallizable organic compounds based on the total mass of the coating layer is in the range of 1 to 60 wt.

8. The coated paper according to claim 1, wherein the proportion of natural polymers or their derivatives based on the total mass of the coating layer is in the range of 40 to 99 wt. %.

9. The coated paper according to claim 1, wherein at least one crystallizable organic compound is selected from fatty acids, hydroxy fatty acids, or dicarboxylic acids or their esters, amides, or salts.

10. The coated paper according to claim 1, wherein the fatty acids are saturated or unsaturated fatty acids with 12 to 40 carbon atoms.

11. The coated paper according to claim 10, wherein the at least one crystallizable organic compound is present as a fatty acid mixture or wax in the coating.

12. The coated paper according to claim 1, wherein the natural polymers are selected from the group consisting of proteins, peptides, nucleic acids, polysaccharides, lipids, polyhydroxyalkanoates (PHA), cutin, suberin, lignin, cellulose, chitosan, and starch.

13. The coated paper according to claim 12, wherein the lignin is derived from conifers, hardwoods, grass plants, or annual plants, and extracted using a method selected from the Kraft process, sulfite process, soda-anthraquinone process, GRANIT process, Alcell™ process, and Organocell process.

14. The coated paper according to claim 12, wherein the derivative of a natural polymer is an ester of a natural polymer.

15. The coated paper according to claim 10, wherein at least one fatty acid in the lignin ester is identical to at least one fatty acid used as a crystallizable organic compound.

16. The coated paper according to claim 10, wherein the crystallizable organic compound is stearic acid and the derivative of a natural polymer is lignin stearate.

17. The coated paper according to claim 1, wherein the coated paper has at least one of the following characteristics:

the coated paper is biodegradable;

the coated paper is recyclable; or

the coated paper can be approved for direct or indirect food contact.

18. A coating for coating papers comprising at least one solvent, at least one crystallizable organic compound, and at least one natural polymer and/or derivative of a natural polymer;

wherein the crystallizable organic compound is selected from fatty acids, hydroxy fatty acids, or dicarboxylic acids or their esters, amides, or salts;

wherein the natural polymer and the derivative of a natural polymer are selected from the group consisting of proteins, peptides, nucleic acids, polysaccharides, lipids, polyhydroxyalkanoates (PHA), cutin, suberin, lignin, cellulose, chitosan, and starch; and

wherein the solvent is selected from water, tetrahydrofuran (THF), toluene, ethyl acetate, and alcohols.

19. The coating according to claim 18, with at least one of the following features:

the crystallizable organic compound is not in crystalline form;

the proportion of natural polymers and/or their derivatives relative to the total mass of the coating is in the range of 6 to 30 wt. %;

the proportion of crystallizable organic compounds relative to the total mass of the coating is in the range of 2 to 15 wt. %, or

the proportion of the solvent relative to the total mass of the coating is in the range of 60 to 90 wt. %.

20. A method for producing a coated paper with a base paper and a semi-crystalline coating layer, comprising the steps:

a) producing a coating according to claim 18 by melt dispersion, high-pressure dispersion, or spray drying and subsequent mechanical dispersion of the components;

b) providing a base paper;

c) applying the coating to the base paper; and

d) curing the coating to form the semi-crystalline coating layer.

21. A coated paper produced by the method according to claim 20.

22. A packaging for food comprising the coated paper according to claim 1.