Method for producing dispersion-coated paperboard having at least two PHA layers having different crystallinity and coated paperboard

Abstract

A dispersion coated paperboard (1) for food packaging applications, the coated paperboard comprising a paper or paperboard substrate (2) having a first side (2a) and a second side (2b). The paper coated paperboard (1) further comprises a first layer (3) coated with a PHA dispersion, the first layer (3) being coated on at least one side (2a, 2b) of the paperboard substrate (2); and a second layer (4) coated with a PHA dispersion, said second layer (4) being coated on at least one of said first layers (3) wherein the crystallinity of the second layer (4) is higher than the crystallinity of the first layer (3).

Description

Technical Field

[0001] The present invention relates to a dispersion-coated cardboard for food packaging applications, the coated cardboard comprising a paper or cardboard substrate having a first side and a second side.

[0002] The present invention also relates to a method for manufacturing a dispersion-coated cardboard for food packaging. Background Art

[0003] Cardboards for food packaging applications are typically dispersion-coated or extrusion-coated or laminated with a plastic film to provide barrier properties to the cardboard. Unfortunately, many polymers are non-sustainable and / or non-compostable, which makes them less attractive, especially in home or industrial recycling and composting.

[0004] Many dispersion barriers based on, for example, styrene / acrylates, styrene / butadiene, polyvinylidene chloride (PVDC) or similar emulsions have been used as dispersion coatings for paper and cardboard. These emulsions are designed to have physical properties that ensure good film formation when applied offline or online in paper or cardboard manufacturing. On the other hand, a low or reduced film-forming temperature or melting temperature (Tm) means a higher risk of white spot deposits (via broke) on the paper machine, but also means a greater tendency to stick and self-adhere.

[0005] PHA is a thermoplastic bio-based polymer synthesized by bacterial fermentation. PHA can be used in packaging applications and is considered to decompose in all types of environments, but especially in bacteria-rich environments such as compost.

[0006] It is known to coat cardboard for food packaging with PHA coatings.

[0007] The disadvantage of these PHA dispersion-coated cardboards is that they lack thermoformability and good barrier properties, especially after being formed into a 3D shape.

[0008] Dispersion-coated cardboards with PHA are more sensitive to cracking, etc., due to less coating weight and different coverage rates, as well as consolidation and solidification of the dispersion on the cardboard. In addition, PHA grades with a lower Tm have a greater tendency to stick during thermoforming.

[0009] Therefore, there is a need to solve the problem of combining new features of PHA to obtain a dispersion-coated multi-layer barrier having:

[0010] - Improved WVTR and good water resistance (no pinholes, low COBB 600)

[0011] - High grease resistance: KIT>6

[0012] - Suitable for thermoforming or disposable items requiring heat sealing.

[0013] - A rigid cardboard laminate that is home and industrially compostable.

[0014] - A method of manufacturing such a coating, either online, offline, or on a converting line.

[0015] Object of the Invention

[0016] One object of the present invention is to provide a coated cardboard that has good barrier properties and is recyclable and reusable.

[0017] Another object is to provide a coated cardboard with improved recyclability, particularly pre - consumer and post - consumer recyclability.

[0018] Yet another object is to provide a coated cardboard that solves or at least reduces the above - mentioned problems. Summary of the Invention

[0019] According to the present invention, the coated cardboard further comprises:

[0020] - A first layer coated with a PHA dispersion on at least one side of a cardboard substrate; and

[0021] - A second layer coated with a PHA dispersion on at least one of the first layers, wherein the crystallinity of the second layer is higher than the crystallinity of the first layer.

[0022] The present invention also discloses a method of manufacturing a dispersion - coated cardboard for food packaging, wherein the method comprises the following steps:

[0023] - Providing a paper or cardboard substrate having a first side and a second side;

[0024] - Dispersion - coating at least one side of the cardboard with a PHA dispersion to form a first layer; and

[0025] - Dispersion - coating at least one of the first layers with a PHA dispersion to form a second layer, wherein the crystallinity of the second dispersion - coated layer is higher than the crystallinity of the first dispersion - coated layer.

[0026] A great benefit of the present invention is that the two - layer structure with a higher crystallinity in the outer layer than in the inner layer provides good thermoformability and good barrier properties, particularly after being formed into a 3D shape. The two - layer structure of the present invention provides for maintaining barrier properties after a 3D forming operation. There is typically a risk that a normal barrier coating will develop defects when subjected to high temperatures (such as in thermoforming, deep - drawing, or heat - sealing operations). In thermoforming or deep - drawing, for example, the temperature of the male die and / or the female die is higher than 80°C, for example, between 120°C and 300°C. The present invention shows that the claimed invention solves these problems.

[0027] Definition

[0028] Polyhydroxyalkanoates (PHA)

[0029] In the context of this patent application, PHA or polyhydroxyalkanoates shall mean a family of polyesters that have various structures and are synthesized by a wide range of natural and genetically engineered bacteria and genetically engineered plant crops. PHAs can be synthesized by 30% of the bacteria living in soil under a wide range of environmental conditions and media. Bacteria produce PHAs through the fermentation of sugars or lipids for the purpose of storing carbon and energy. Examples of bacterial strains that can produce PHAs include Alcaligenes eutrophus, Alcaligenes latus, Azotobacter, Aeromonas, Comamonas, Pseudomonads, and other genetically engineered organisms, such as genetically engineered microorganisms like Pseudomonas, Ralstonia, and Escherichia coli. Generally, PHAs are formed by the enzymatic polymerization of one or more monomer units within living bacterial or plant cells. More than 100 different types of monomers have been identified and incorporated into PHA polymers, including 3-hydroxybutyric acid and 3-hydroxyvaleric acid. PHAs can be classified as homopolymers, such as the well-known polyhydroxybutyrate (PHB), or copolymers, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Additionally, depending on the size of the carbon chain, they are further classified as short-chain length (SCL), medium-chain length (MCL), or long-chain length (LCL) PHAs. Since they constitute a broad family of biodegradable polymers, PHAs exhibit very versatile properties that can be beneficial for many different industrial applications, including cosmetics, biomedicine, and packaging, to name just a few. Detailed Description

[0030] Figure 1 A first embodiment is shown, in which one side of the paper of the cardboard substrate is coated with a first dispersion-coated layer and a second dispersion-coated layer. The second layer has a higher crystallinity than the first layer.

[0031] Figure 2 A second embodiment is shown, in which both sides of the paper of the cardboard substrate are coated with a first dispersion-coated layer, and one of the first layers is coated with a second dispersion-coated layer. The second layer has a higher crystallinity than the first layer.

[0032] Figure 3A third embodiment is shown, in which both sides of the paper of the cardboard substrate are coated with a first dispersion-coated layer, and in which each said first layer is coated with a second dispersion-coated layer. The second layer has a higher crystallinity than the first layer.

[0033] Hereinafter, reference will be made to Figures 1-3 to further describe the present invention. Note that Figures 1-3 the accompanying drawings in

[0034] The present invention relates to a dispersion-coated cardboard 1 for food packaging applications. The coated cardboard comprises a paper or cardboard substrate 2 having a first side 2a and a second side 2b.

[0035] Those skilled in the art recognize that many different types of substrates 2 are possible, but the preferred substrate 2 comprises less than 30 wt% of high-yield fibers, preferably 0 - 20 wt%, and most preferably 1 - 15 wt%, as this gives better compostability of the final product. High-yield fibers are pulps having a KAPPA value above 70, preferably above 75, and most preferably above 80. Those skilled in the art recognize that lower KAPPA values are also possible, for example if the pulp is oxygen-delignified and unbleached.

[0036] The preferred substrate 2 is a multi-layer sheet of paper or cardboard, such as SBS, FBB, LPB, kraftliner, and multi-layer sheet wrapping paper. Due to cost, stiffness, and compostability, the sides 2a, 2b of the substrate to be coated are preferably unbleached.

[0037] The dispersion-coated cardboard 1 further comprises a first layer 3 of PHA dispersion-coated on at least one side 2a, 2b of the cardboard substrate 2. Figure 1 A first embodiment is disclosed, in which only the first side 2a of the substrate 2 is coated with the first layer 3, while the second side 2b is not coated. Figure 2 and Figure 3 Second and third embodiments are disclosed, in which both sides 2a, 2b of the substrate are coated with the first layer 3.

[0038] The PHA of the first dispersion-coated layer 3 is preferably medium-chain length polyhydroxyalkanoate (mcl-PHA). Mcl-PHA, like PHBH, has shown better properties, including the ability to form a film when applied by dispersion coating. In addition, it is not easy to obtain a proper dispersion of short-chain length PHA (scl-PHA), and the processability will be difficult because it will require a higher temperature for drying / curing (properly melting the polymer to form a continuous film).

[0039] Those skilled in the art recognize that many of the mcl-PHAs in the first layer 3 may be selected from poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB4HB), poly(3-hydroxyoctanoate-co-3-hydroxyhexanoate) (PHOHH), poly(3-hydroxyoctanoate-co-3-hydroxydecanoate) (PHOHD), and poly(3-hydroxyoctanoate-co-3-hydroxydodecanoate) (PHDHDD) or mixtures thereof. Preferably, the mcl-PHA in the first layer 3 is PHBH. Alternatively and exceptionally, some short-chain-length polyhydroxyalkanoates (scl-PHAs) may also be used, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) PHBV with a high V content (V content > 5%).

[0040] In a preferred embodiment, according to ASTM E794-06 (2018), the crystallinity of the first dispersion-coated layer 3 is 10 - 45%, preferably 10 - 30%. This crystallinity range has shown better adhesion to the substrate 2 and better tolerance to interfacial stress when formed into a 3D package.

[0041] According to ASTM E794-06 (2018), the melting temperature (Tm) of the first dispersion-coated layer 3 is preferably 100 °C to 145 °C. The Tm in the first layer 3 should not be too high, because then adhesion is affected and delamination may occur after conversion into a package. PHAs with lower crystallinity generally have a lower Tm than PHAs with higher crystallinity, and thus are equal to or lower than 145 °C. However, the Tm cannot be too low, because it will become sticky and not suitable for, for example, hot filling.

[0042] The application amount of the first layer 3 is preferably 7 - 20 gsm, and more preferably 8 - 15 gsm.

[0043] Finally, the cardboard 1 also includes a second PHA dispersion-coated layer 4 coated on at least one of the first layers 3. Figure 1 A first embodiment is disclosed, in which the first layer 3 is coated with the second layer 4. Figure 2 A second embodiment is disclosed, in which one of the two first layers 3 is coated with the second layer 4. Figure 3 A third embodiment is disclosed, in which both of the two first layers 3 are coated with the second layer 4.

[0044] The PHA of the dispersion-coated second layer 4 is preferably a medium-chain-length polyhydroxyalkanoate (mcl-PHA).

[0045] Mcl-PHA, such as PHBH, has shown better properties, including the ability to form a film when applied by dispersion coating. In addition, it is not easy to obtain a proper dispersion of short-chain length PHA (scl-PHA), and the processability will be difficult because it will require a higher temperature for drying / curing (appropriately melting the polymer to form a continuous film).

[0046] Those skilled in the art recognize that many mcl-PHAs in the second layer 4 may be selected from poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB4HB), poly(3-hydroxyoctanoate-co-3-hydroxyhexanoate) (PHOHH), poly(3-hydroxyoctanoate-co-3-hydroxydecanoate) (PHOHD), and poly(3-hydroxyoctanoate-co-3-hydroxydodecanoate) (PHDHDD) or mixtures thereof. Preferably, the mcl-PHA in the second layer 4 is PHBH. Alternatively and exceptionally, some scl-PHA may also be used, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) PHBV with a high V content (V content > 5%).

[0047] In a preferred embodiment, according to ASTM E794-06 (2018), the crystallinity of the second dispersion-coated layer 4 is 30 - 70%, preferably 30 - 50%.

[0048] According to ASTM E794-06 (2018), the melting temperature (TM) of the second dispersion-coated layer 4 is between 120 °C and 160 °C. Advantageously, the second layer 4 has these high melting temperatures so as not to cause any problems in thermoforming or 3D forming. It also provides better heat resistance, less risk of deposits or thermal stickiness.

[0049] The application amount of the second layer 4 is preferably 3 - 20 gsm, and more preferably 5 - 15 gsm.

[0050] According to the present invention, the crystallinity of the second dispersion-coated layer 4 is higher than that of the first layer 3. The crystallinity in the second layer 4 is preferably at least 10% higher than that of the first layer 3, and more preferably 20% higher than that of the first layer.

[0051] The dispersion coating formulations for the first layer 3 and the second layer 4 contain PHA and additives, such as:

[0052] 0 - 30 wt% (based on the amount of PHA) of fillers, such as clay, calcium carbonate, talc, kaolinite, montmorillonite, bentonite, silica, chitin, titanium dioxide, nanoclay, nanocellulose, or mixtures thereof.

[0053] 0 - 5 wt% of a dispersant and / or a humectant, such as glycerol, sorbitol, mannitol, xylitol, ethylene glycol, fatty acids, monosaccharides, urea, hemicellulose, etc.

[0054] 0 - 30 wt% of a rheology modifier and / or a water retention agent, such as PVOH, PVOH / Ac, EVOH, PVAc, cellulose derivatives, polysaccharides, proteins, alginates or various derivatives and / or mixtures thereof.

[0055] - 0 - 30 wt% of a nucleating agent, such as talc, mica, boron nitride, crystalline nanocellulose, sodium benzoate, calcium carbonate, silica, ionomers, clays, acetals, titanium oxide, dibenzylidene sorbitol, benzophenone, benzaldehyde acetal, lithium benzoate, sodium benzoate, potassium benzoate, thymine, sodium organophosphate)

[0056] 0 - 5 wt% of a wetting agent and / or an antifoaming agent, such as surfactants or surface - active polymers, such as polysorbates, aromatic poly(ethylene oxide), sorbitan derivatives, block copolymers of poly(ethylene oxide) and poly(propylene oxide), poly(alkylene glycol ethers), alkyl sulfates, alkyl phosphates, stearate saponins, polyether siloxanes, silicones, stearates, diols, vegetable oils).

[0057] As described above, the total amount of additives in the first layer 3 and the second layer 4 is less than 30 wt% in each layer, preferably less than 25 wt%, and most preferably less than 20 wt%.

[0058] The content of PHA in each layer (i.e., the first layer 3 and the second layer 4) is at least 70 wt% PHA.

[0059] The solid content of PHA in the dispersion (i.e., the dispersion for the first layer 3 and the second layer 4) > 20 wt%, preferably > 35 wt%, and most preferably 45 - 60 wt%.

[0060] The dispersion coating can be applied by using roll coating, spray coating, curtain coating, knife coating, slot coating, dip coating, gravure roll coating, reverse direct gravure coating, bar coating, soft - tip knife coating and / or combinations thereof. Preferred coating methods are knife coating and bar coating.

[0061] Surprisingly, it has been shown that the dispersion - coated cardboard of the present invention having a dispersion - coated first layer 3 and a dispersion - coated second layer 4 (wherein the second layer has a higher crystallinity than the first layer 3) provides good thermoformability and good barrier properties, especially after being formed into a 3D shape.

[0062] According to ASTM E794 - 06 (2018), the total crystallinity of the first layer 3 and the second layer 4 of the cardboard is higher than 45%, preferably higher than 50%.

[0063] The melting temperature Tm and the crystallinity were both determined according to ASTM E794 - 06 (2018) and measured by differential scanning calorimetry (DSC).

[0064] According to the standard EN13676:2001, the number of pinholes in the coated cardboard of the present invention is less than 10 pinholes / m 2 , preferably less than 5 pinholes / m 2 .

[0065] Experimental and test results

[0066] Dispersion barrier coatings were prepared on a base substrate using two different PHA grades of coatings.

[0067] The first grade, PHA1, is PHBH provided in dry form and contains 20 wt% stabilizer to obtain a stable dispersion when mixed in water. The melting point of the PHA is 130 °C, and the average particle size D50 of the PHA dispersion is 1.8 μm.

[0068] The second grade PHA, PHA2, is also PHBH but is provided as a wet dispersion. The melting point is 145 °C, and the average particle size (D50) of the PHA dispersion is 2.9 μm.

[0069] PHA1 is of the PHBH type with a crystallinity below 30%, and PHA2 is of the PHBH type with a crystallinity above 30%, as determined according to ASTM E794 - 06 (2018).

[0070] The PHA samples were applied to the cardboard using a rod coater and then dried to ensure film formation. The base substrate is cardboard grade, Natura RFA CLC / F 260 mN, 270 gsm.

[0071] The first PHA1 coating and the second PHA2 coating have approximately the same coating weight, about 10 gsm. Thus, a total of approximately 20 gsm.

[0072] DSC (Differential Scanning Calorimeter) tests were carried out by performing a temperature scan from - 20 °C to + 200 °C at a rate of 10 min / °C. The coating was removed from the paper substrate before analysis and tested in the following mode: heat - cool - heat - cool - heat. The crystallinity and melting behavior were determined from the calorimetric curve.

[0073] Experiment 1 - Comparison (double coating with PHA1)

[0074] In the case of intermediate drying, the cardboard substrate was double-coated with PHA1. The samples coated with the double-dispersion showed improved water vapor barrier, while the heat sealability of this grade was poor. Further DSC analysis of the samples showed that the melting of the samples started at very high temperatures, which confirmed its poor heat-sealing properties at lower temperatures (150 °C), pressure (500 N), and short residence times (2 s).

[0075] Experiment 2 - Comparison (with a precoat and topcoat of PHA2)

[0076] In this case, as described above, the PHA grade PHA2 with a higher crystallinity was double-coated on the board. This grade and the resulting coating produced pinholes and were of poor quality, and thus were not suitable as a WVTR barrier.

[0077] Experiment 3 - Double coating with PHA1 in the precoat and PHA2 in the topcoat

[0078] In this case, PHA1 (lower crystallinity) was used in the precoat, and PHA2 (higher crystallinity) was used in the topcoat. This gave very good heat-sealing properties as well as no pinholes and a very good water vapor barrier.

[0079] Experiment 4 - Double coating with PHA2 in the precoat and PHA1 in the topcoat

[0080] In this case, PHA2 (higher crystallinity) was used in the precoat, and PHA1 (lower crystallinity) was used in the topcoat. This gave good heat-sealing properties as well as no pinholes, but an unsatisfactory water vapor barrier, especially when the WVTR was measured under tropical conditions.

[0081] Experiment 5 - Double coating with a 50-50 blend of two PHA grades in the precoat and topcoat

[0082] The precoat and topcoat were prepared with a 50-50 blend of the two PHAs. In this case, the WVTR properties were good, but at the same level as in Example 3. The crystallinity of this sample was at a very high level.

[0083]

[0084]

[0085] Normalized with respect to the coating thickness

[0086] Above, the present invention has been described in some specific embodiments. However, those skilled in the art recognize that other embodiments and variations are possible within the scope of the appended claims.

Claims

1. A dispersion-coated cardboard (1) for food packaging applications, said coated cardboard comprising a paper or cardboard substrate (2) having a first side (2a) and a second side (2b), characterized in that, The coated cardboard (1) further comprises: - a first layer (3) coated with a PHA dispersion, which is coated on at least one side (2a, 2b) of the cardboard substrate (2); and - a second layer (4) coated with a PHA dispersion, which is coated on at least one of the first layers (3), wherein the crystallinity of the second layer (4) is higher than that of the first layer (3).

2. The dispersion-coated cardboard according to claim 1, wherein the PHA dispersion is a medium-chain-length polyhydroxyalkanoate (mcl-PHA).

3. The dispersion-coated cardboard according to any one of claims 1-2, wherein the PHA is selected from poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB4HB), poly(3-hydroxyoctanoate-co-3-hydroxyhexanoate) (PHOHH), poly(3-hydroxyoctanoate-co-3-hydroxydecanoate) (PHOHD), and poly(3-hydroxyoctanoate-co-3-hydroxydodecanoate) (PHDHDD) or a mixture thereof.

4. The dispersion-coated cardboard according to claim 1, wherein the PHA is poly(3-hydroxybutyrate-co-3-hydroxyvalerate) PHBV, a short-chain-length (scl-PHA) with a high V content.

5. The dispersion-coated cardboard according to any one of claims 1-4, wherein the crystallinity in the second layer (4) is at least 10% higher than that in the first layer (3), preferably 20% higher than that in the first layer (3).

6. The dispersion-coated cardboard according to any one of claims 1-5, wherein according to ASTM E794-06 (2018), the total crystallinity of the first and second layers (3, 4) is higher than 45%, preferably higher than 50%.

7. The dispersion-coated cardboard according to any one of claims 1-6, wherein according to ASTM E794-06 (2018), the crystallinity of the dispersion-coated first layer (3) is 10-45%, preferably 10-30%.

8. The dispersion-coated cardboard according to any one of claims 1-7, wherein according to ASTM E794-06 (2018), the crystallinity of the dispersion-coated second layer (4) is 30-70%, preferably 30-50%.

9. The dispersion-coated cardboard according to any one of claims 1-8, wherein according to ASTM E794-06 (2018), the melting temperature of the first dispersion-coated layer is between 100 °C and 140 °C, while the melting temperature of the second dispersion-coated layer is between 120 °C and 160 °C.

10. The dispersion-coated cardboard according to any one of claims 1-9, wherein the number of pinholes is less than 10 pinholes / m according to standard EN 13676:2001 2 , preferably less than 5 pinholes / m 2 .

11. The dispersion-coated cardboard according to any one of claims 1-10, wherein the first layer (3) is applied in an amount of 7-20 gsm, preferably 8-15 gsm, and the second layer (4) is applied in an amount of 3-20 gsm, preferably 5-15 gsm.

12. The dispersion-coated cardboard according to any one of claims 1 - 11, wherein the paper or cardboard substrate (2) comprises less than 30 wt% of high-yield fibers, preferably 0 - 20 wt% and most preferably 1 - 15 wt%.

13. The dispersion-coated cardboard according to any one of claims 1 - 12, wherein the high-yield fibers are pulp having a Kappa number of 70 or more, preferably 75 or more, and most preferably 80 or more.

14. A method for manufacturing a dispersion-coated cardboard (1) for food packaging, wherein the method comprises the following steps: - providing a paper or cardboard substrate (2) having a first side (2a) and a second side (2b); - dispersion-coating at least one side (2a, 2b) of the cardboard with a PHA dispersion to form a first layer (3); and - dispersion-coating at least one of the first layers (3) with a PHA dispersion to form a second layer (4), wherein the crystallinity of the second dispersion-coated layer (4) is higher than the crystallinity of the first dispersion-coated layer (3).

15. The method according to claim 14, wherein the PHA dispersion is a medium-chain-length polyhydroxyalkanoate (mcl-PHA).

16. The method according to any one of claims 14 - 15, wherein the PHA is selected from poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB4HB), poly(3-hydroxyoctanoate-co-3-hydroxyhexanoate) (PHOHH), poly(3-hydroxyoctanoate-co-3-hydroxydecanoate) (PHOHD), and poly(3-hydroxyoctanoate-co-3-hydroxydodecanoate) (PHDHDD), or a mixture thereof.

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