Compostable material for a container, a container made thereof and a method for manufacturing compostable material for a container

Abstract

The invention relates to compostable packaging material comprising a fibrous sheet and a coextruded coating of two or more layers of compostable polymer as well as a method for manufacturing and using such material.

Description

The present invention relates to compostable packaging material, a method for manufacturing the same and a drinking cup. Background The fibre-based materials for containers such as packing paper or board are usually coated with a polymeric barrier layer(s) providing water vapor, grease and / or gas barrier. Such polymer-coated paper or board is also used for disposable tableware, such as drinking cups. Polyolefins, such as low-density polyethylene (LDPE), are widely used for coating due to their good heat-sealability and water vapor barrier properties. A disadvantage of the usual polyolefins is, however, that they may remain undegraded in the nature. Summary of the invention The aim of the invention was to find an improved compostable polymer coating for fibrous material for containers without compromising the sealing and barrier properties, adhesion to the fibrous substrate and running speed of the paper or board extrusion coating machinery. The first aspect of the invention is a compostable packaging material. Characteristic to said packaging material is given in claim 1. The second aspect of the invention is a method of manufacturing a compostable material. The characteristic steps of said method are given in claim 8. Further aspects of the invention are a disposable drinking cup, packaging materials for frozen and chill-food, fruits and vegetables, fish and meat, dried food and cereals and confectionery. Detailed description Outermost polymer layer as used herein means the outermost coating layer from the fibrous sheet. Innermost polymer layer means the coating layer closest to the fibrous sheet, in direct contact of said sheet. Inner or outer polymer layer as used here may refer to the middle layer i.e. the layer between the outermost and innermost layers of a coextruded 3-layer coating. Polybutylene succinate (PBS) and polyhydroxyalkanoate (PHA) are commercially available products made from renewable, non-fossil raw materials. PHA is produced from renewable raw materials. PBS may be produced in whole or in part from renewable raw materials, depending on the quality of renewable raw materials. This invention refers to PHA based polymers or its derivatives such as polyhydroxybutyrate (PHB), poly-3-hy-droxybutyrate (P3HB), poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers. E.g. polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA). The poly-3-hydroxybutyr-ate (P3HB) form of PHB is a common type of polyhydroxyalkanoate, other polymers including e.g. poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers. This invention is based on surprising finding that PHA coextruded in combination with PBS allows producing a coated fibrous sheet with sufficient barrier properties, sufficient heat sealing, good adhesion and runnability of the coating machine. In case of a disposable drinking cup the polymer coating is lying at least on the inner liquid-contact side of the cup or package. However, a container may be provided with a coating on both sides of the fibrous sheet. The coatings may be similar to each other or different. The fibrous sheet may be paper or board, paper-board as well as cardboard. The container as used herein may be for example a package for food or a disposable tableware such as a disposable drinking cup or plate, packaging materials for frozen and chill-food, fruits and vegetables, fish and meat, dried food and cereals and confectionery. In this connection the term compostable (or biodegradable) means materials that will decompose in natural aerobic (composting) and anaerobic (landfill) environments. Preferably the material described here is decomposed in home composting conditions (i.e. without active aeration, temperature control and / or mixing of the biomass). PLA is certified industrially compostable e.g. to Australian and European Standards (AS4736 and EN13432) whereas PBS is certified also home compostable. Biodegradation of polymers, in this connection used for coating, occurs when microorganisms metabolize the polymer to either assimilable compounds or to humus-like materials that are not harmful to the environment. The polymers may be derived from renewable raw materials, or petroleum-based plastics (polymers) which contain additives. PBS is biodegradable and compostable and may be of be derived from renewable raw materials or fossil (petroleum) origin. PHA originates from renewable material. Fibrous cellulosic material is known to decompose in a home compost. In this connection the term “adhesion” means adhesion of the coating to raw fibrous sheet (paper or board) constituting the fibrous sheet. The aim is that an attempt to detach the coating results in breaking within the fibrous substrate layer, instead of the coating peeling off. The term “heat-sealability” means that the polymer coating in softened or melted condition may be attached to an opposite surface of a material, which may be the same or another polymer or e.g. raw fibrous material (fibrous sheet). The present invention relates to a compostable packaging material comprising (a) a fibrous sheet and (b) two or more layers of polymeric coating coextruded onto at least one side of said fibrous sheet, wherein the innermost polymer coating layer in direct contact with fibrous sheet comprises polybutylene succinate (PBS) and in a coextruded 2-layer coating the outermost coating layer comprises polyhydroxyalkanoate (PHA) or in a coextruded 3-layer coating the outer coating layer being the middle layer comprises PHA and the outermost layer comprises PBS. In this context the expressions the innermost polymer coating layer or the innermost coating layer refer to the layer in direct contact with the fibrous sheet. Respectively, the expressions the outermost polymer coating layer or the outermost coating layer refer to the layer located as the outermost layer from the fibrous sheet. Both layers are applied by coextrusion. It has been found that the plastic layer (polymer layer) against the fibrous sheet must be PBS in order to ensure sufficient adhesion between the fibrous sheet and the coextruded coating. Sufficient adhesion to the fibrous sheet is not obtained with PHA as the innermost layer. The coating weight of PBS as the innermost coating layer is 2 (gsm) g / m2or more. This is required for sufficient adhesion. The total coating weight of polymer coating on one side of the fibrous substrate may be 10-30 (gsm) g / m2, such as 18-25 g / m2. This is required for sufficient barrier properties and enhances runnability and heat-sealability. In a multilayer coating weight the amount of polymer per layer may be 2-28 (gsm) g / m2. In this connection the expression multilayer coating refers to a coating comprising two or three coextruded polymer coating layers. With PHA as the outermost layer, the drawback is sticking to the chill-roll. The roll temperature must be raised above 50 °C to prevent sticking. In order to avoid the need to raise the temperature of the chill-roll, a 3-layer structure is produced with PBS on the surface. In this case, the temperature of the chill-roll can be considered typical for extrusion coating (10-20 °C). According to one embodiment the packaging material comprises a structure: a. fibrous sheet (such as board); 10-40 % PBS as the innermost layer; 20-70 % PHA as a middle coating layer and 10-60 % PBS as the outermost coating layer; or b. 10-60 % PBS as the outermost coating layer; 20-70 % PHA as a middle coating layer; 10-40 % PBS as the innermost layer; fibrous sheet (such as board) 10-40 % PBS as the innermost layer; 20-70 % PHA as a middle coating layer and 10-60 % PBS as the outermost coating layer, wherein the percentages refer to the percentual weight of said layer over the whole polymer coating. According to one embodiment the packaging material comprises a structure: a. fibrous sheet (such as board); 10-70 % PBS as the innermost layer; 90 - 30 % PHA as the outermost coating layer or b. 90 - 30 % PHA as the outermost coating layer; 10-70 % PBS as the innermost layer; fibrous sheet (such as board); 10-70 % PBS as the innermost layer; 90 - 30 % PHA as the outermost coating layer wherein the percentages refer to the percentual weight of said layer over the whole polymer coating. In one embodiment the amounts are 20 - 40 % PBS and 80 - 60 % PHA. In one embodiment the layers on the opposing sides of the fibrous sheet are different from each other. Increasing the amount of PBS is the innermost coating layer improves the adhesion of the coating. However, PBS used in high amounts in relation to PHA layer may result in a soft coating and compromise the barrier properties provided by PHA coating layer. A fibrous sheet may have a grammage (square weight) of 30 - 380 g / m2. For flexible package the packaging material may be 30- 130 g / m2. A coating here described may be produced by coextruding onto a fibrous substrate innermost, optionally a one middle and outermost coating layer. The present invention relates also to a method of manufacturing a compostable material for container described here. The method comprises coextrusion onto a fibrous sheet two or more layers of polymeric coating layers onto at least one side of said fibrous sheet. The polymers may be coextruded as an outer coating layer or inner coating or both inner and outer coating of said fibrous sheet, depending on the desired use of the packaging material. A coating is coextruded into direct contact with the fibrous sheet. The structure of the coating and the packaging material are discussed in more detail in connection of the description the compostable packaging material. The present invention further relates to a drinking cup of a packaging material manufactured as described here or of a packaging material described here. One of the advantages of PBS over PLA is its runnability properties. The runnability of PLA as a monolayer is poor due to the instability of the molten polymer layer. PLA is characterized by so-called the edge weaving phenomenon, i.e. the edges of the polymer layer do not remain stable, but the edges sway in the transverse direction, causing uneven coating and spots on the edges where there is no polymer at all. This phenomenon is emphasized while the running speed increases. Thus, the processing window of PLA is narrow, which has an effect on the runnability of the coextrusion of (PLA / PHA) structure as the edge weaving phenomenon of PLA? is visible also in the coextruded structure. This compromises the runnability of the PLA / PHA structure when compared to runnability of pure PHA. One advantage of the coating and method described here is improved runnability of the coating machinery. A sufficient amount of coextruded PHA provides stiffness to the coating thereby reducing the risk of angel hair formation in the die cutting process, and good barrier properties, whereas PBS improves the adhesion properties of the coating layer, heat-sealability and runnability in extrusion process. One further advantage with the present method is that it is technically easier to use only one polymer in the same screw instead of a mixture of polymers. Then there is no need to invest in polymer feeders, as each polymer to be fed must have its own pipeline, including polymer suction (plastic suction) equipment and gravimetric dispensers. The challenge of polymer mixtures may be the rheological differences of the polymer to be mixed. This can be seen as an inhomogeneous molten layer and thus also a polymer coating. In addition, the polymers to be mixed are not necessarily chemically compatible. As a summary, remarkable advantages of the structure are at least: ■ PBS as the innermost coating, in direct contact with the fibrous sheet provides a good adhesion, ■ PHA in the middle coating layer or on the outermost coating layer improves water vapor barrier properties and reduces tendency for angel hair formation in die cutting process PBS as the outermost coating layer eliminates the problem of sticking to the chill-roll and improves the runnability of the coextrusion coating due to better draw down properties. In addition, PBS's motor load is significantly lower than PHA's, which reduces the risk that motor load becomes the productivity bottleneck. The runnability of PBS is good, i.e. its melt strength (draw-down) and the stability of the edges of the melt layer are better than pure PHA, which enables the PBS / PHA structure to be coextrusion coated at a machine speed of 200 m / min or more. Preferably the machine speed is 250 m / min or more, most preferably 300 m / min or more. The high machine speed improves the economy of the manufacturing process. The coextruded layer of PHA increases the ability of the coating to withstand the vapour pressure generated within the fibrous substrate e.g. by the hot drink. Good adhesion provided by PBS as the innermost coating layer prevents the coating from loosening from the substrate layer and opening pathways to liquid penetration. A compostable polymer is extruded into direct contact with the fibrous sheet to be used as a packaging material. There is no need for a primer between the fibrous substrate and the coating of the invention. This simplifies the manufacturing process and reduces raw material costs. In the multilayer coextruded coating PBS layer is preferably the innermost layer to improve adhesion of the coating layer to the underlying fibrous substrate. The coatings on the opposite sides of the fibrous substrate may be similar or differ from each other, for instance a multilayer coating on one side and a monolayer coating on the opposite side. The total amount of polymer coating on one side of the fibrous substrate may be in the range of 10-30 (gsm) g / m2, typically about 18-25 g / m2. In a multilayer coating the amount of polymer per layer may be 2-28 g / m2. A preferred embodiment of the invention is a fibrous board-based packaging material comprising coextruded innermost, middle and outermost coating layers, the innermost and the outermost layer comprising or consisting of PBS and the middle layer comprising or consisting of PHA. The percentual weights are discussed above in connection of the packaging material. The innermost coating layer would provide superior adhesion in extrusion to the fibrous substrate and the outermost coating layer would enhance runnability of the coating machinery. Middle layer containing PHA supports the polymer layers during extrusion process and provides sufficient moisture / vapour barrier properties. Multilayered structure allows optimizing the raw material without compromising the extrudability or properties of the resulting coating. The invention further provides improved containers made of the packaging material as described above. Disposable drinking cups for hot drinks, especially hot coffee, are a prime example of such containers. Examples Advantages of the structure were proven. PBS against fiber provides adhesion, PHA in the middle or on the outermost layer improves water vapor barrier properties, PBS on the surface eliminates the problem of sticking to the chill- roll. If PHA is on the surface, the roll temperature must be > 50 °C, so that the coating does not stick to the roll and there is still a risk of sticking. PBS improves the runnability of the coextrusion coating due to better draw down properties. It is possible to run PBS as mono extrusion at 350 m / min (20 gsm) without edge weaving and PHA at 180 m / min. In addition, PBS's motor load is significantly lower than PHA's, which reduces the risk that motor load became the productivity bottleneck. Adhesion to the board substrate was studied as will be explained below. Half of A4 sized samples were cut and tested in machine direction. The corner of the sample was dipped (if necessary) to strong ethanol to enhance separation of the polymer coating at the beginning. The polymer coating was pulled apart from the substrate for and the force needed to separate the polymer coating from the substrate was evaluated. Evaluation: ■ Adhesion 0: The polymer coating is not attached to the fibre substrate at all, and the materials remain completely intact. ■ Adhesion 1: The polymer coating separates easily, and the materials stay completely intact ■ Adhesion 2: It is clearly necessary to pull off the polymer coating with some force and the materials break easily (no fibres are attached to the coating) ■ Adhesion 3: Some force is needed to pull off the layers and the materials break slightly. ■ Adhesion 4: It is difficult to separate the polymer coating intact from the fibre substrate and the materials break almost completely (useful adhesion level for many applications such as cups) ■ Adhesion 5: It is impossible to separate the polymer coating from the fibre-based substrate without fibre tearing, they tear immediately (the fibre based substrate breaks throughout the area of coating). Both samples were extruded using the same extruder, screw 6”, width 2500 mm. The melting temperatures of the plastics (polymers) were in accordance with the suppliers' recommendations i.e. 185 °C for PHA and 270 °C for PBS. Motor load: max 630 A. screw 6”, 28D PHA: max rpm 30 1 / min with 190 kg / h (185 °C), motor load as a bottle neck PBS: max rpm 55 1 / min with 410 kg / h (270 °C), motor load as a bottle neck WVTR (23C / 50 RH), using ISO 2528:1995 ■ PHA: 33 g / m2 / day (20 gsm) ■ PBS: 80 g / m2 / day (20 gsm) ■ PBS (3 gsm) / PHA (17 gsm): 40 g / m2 / day Results: ■ Uncoated bleached cup board with 215 gsm (g / m2) was coated with PHA 25 gsm (g / m2). Resulted adhesion level was 2-3 ■ Uncoated bleached cup board was coated with 215 gsm (g / m2) was coated with PBS 25 gsm (g / m2). Resulted adhesion level was 4-5 3-layer structure having PBS as the innermost layer showed the same adhesion levels in the experiments. Comparison of PLA and PBS as the innermost layer A comparison of adhesion properties of PLA and PBS using procedure disclosed above was made. The structure of the polymer coated fibrous sheet and adhesion between the fibrous sheet (BLMF or Cupforma Natura) and the extruded or coextruded polymers is show below. BLMF 60 gsm / PLA 20 gsm (adhesion 3) BLMF 60 gsm / PBS 20 gsm (adhesion 5) Cupforma Natura 195 gsm / PLA 5 gsm / PHA20 gsm (adhesion 3) Cupforma Natura 195 gsm / PBS 5 gsm / PHA20 gsm (adhesion 5) It is shown that PBS results in remarkably better adhesion to the fibrous sheet when compared to PLA. Heat-sealability and grease resistance In Table 1, grease resistance according to ASTM F119 (1982) and water vapour barrier properties (23 C, 50 % RH) of the materials produced are shown. Table 1. Properties of biopolymers when extruded on paper and board. Polymer weight (g / m2) Material PHA PBS Grease resistance ASTM-F119, WVTR 23°C, 50% RH 70 MF / 18 BioPBS - 20 >72 h 87 90 BLMF / 4 BioPBS / 20 PHA 20 4 >72 h 34 170 Cupforma Natura / 23 BioPBS - 23 >72 h 70 Cupforma Natura 184 / 3 BioPBS / 17 PHA 17 3 >72 h 38

Claims

1. A compostable packaging material comprisinga) a fibrous sheet andb) two or more layers of polymeric coating coextruded onto at least one side of said fibrous sheet,wherein the innermost polymer coating layer in direct contact with fibrous sheet comprises polybutylene succinate (PBS) and in a coextruded 2-layer coating the outermost coating layer comprises polyhydroxyalkanoate (PHA) or in a coextruded 3-layer coating the outer coating layer being the middle layer comprises PHA and the outermost layer comprises PBS.

2. The compostable packaging material of claim 1, wherein the coating weight of PBS is the innermost coating layer is at least 2 (gsm) g / m2.

3. The compostable packaging material of claim 1 or 2, wherein the total coating weight of polymer coating on one side of the fibrous substrate is 10-30 (gsm) g / m2, such as 18-25 g / m2.

4. The compostable packaging material of claim 3, wherein in multilayer coating weight the amount of polymer per layer is 2-28 (gsm) g / m25. The compostable packaging material of any of claims 1 to 4, wherein the fibrous sheet has square weight of 20 - 380 g / m2.

6. The compostable packaging material of any of the preceding claims, wherein the packaging material comprises a structure:a. fibrous sheet (such as board); 10-40 % PBS as the innermost layer; PHA 20-70 % PHA as a middle coating layer and 10-60 % PBS as the outermost coating layer orb. 10-60 % PBS as the outermost coating layer; 10-70 % PHA as a middle coating layer; 10-40 % PBS as the innermost layer; fibrous sheet (such as board) 10-40 % PBS as the innermost layer; 20-70 % PHA as a middle coating layer and 10-60 % PBS as the outermost coating layer wherein the percentages refer to the percentual weight of said layer over the whole polymer coating.

7. The compostable packaging material of any of claims 1-5, wherein the packaging material comprises a structure:a. fibrous sheet (such as board); 10-70 % PBS as the innermost layer; 90 -30 % PHA as the outermost coating layer orb. 90 - 30 % PHA as the outermost coating layer; 10-70 % PBS as the innermost layer; fibrous sheet (such as board); 10-70 % PBS as the innermost layer; 90 - 30 % PHA as the outermost coating layerwherein the percentages refer to the percentual weight of said layer over the whole 5 polymer coating.

8. A method of manufacturing a compostable material for container according to any of the preceding claims, comprising coextrusion onto a fibrous sheet two or more layers of polymeric coating layers onto at least one side of said fibrous sheet.

9. The method of claim 8, characterized in that said polymers are coextruded as10 an outer coating layer or inner coating or both inner and outer coating of said fibrous sheet.

10. A drinking cup of a packaging material manufactured by the method of any one of claims 8 or 9, or of a packaging material of any of claims 1 to 7.

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