Coated paper or paperboard

Abstract

The present disclosure relates to a method to reduce or remedy problems associated with pinholes in coatings, particularly in coatings for paper or paperboard based on carboxymethylcellulose. The present invention provides improved barrier properties, particularly oxygen barrier properties. The disclosure further relates to paper or paperboard coated with such coating compositions. The coated paper or paperboard according to the present invention may also be provided with additional coatings and / or be metallized.

Description

[0001] COATED PAPER OR PAPERBOARD

[0002] Technical field

[0003] The present invention relates to a method to reduce or remedy problems associated with pinholes in coatings, particularly in coatings for paper or paperboard based on carboxymethylcellulose. The coated paper or paperboard according to the present invention provides improved barrier properties, particularly oxygen barrier properties. The coated paper or paperboard according to the present invention may also be provided with additional coatings and / or be metallized.

[0004] Effective gas, aroma, and / or moisture barriers are required for example in the packaging industry for shielding sensitive products. Particularly, oxygen-sensitive products require an oxygen barrier to extend their shelf-life. Oxygen-sensitive products include many food products, but also pharmaceutical products and electronic industry products. Known packaging materials with oxygen barrier properties may be comprised of one or several polymer films or of a fiber-based substrate, e.g. paper or board, coated with one or several layers of an oxygen barrier polymer, usually as part of a multilayer coating structure.

[0005] Barriers are normally created by coating the fiber-based substrate with a composition which gives the substrate barrier properties. Different coatings can be applied depending on the required barrier properties. The most commonly used materials when forming a barrier on a fiber-based substrate, are polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamides (PA), ethylene vinyl alcohol (EVOH) or ethylene vinyl acetate (EVA). EVOH is normally used in order to create oxygen barriers and PE or PET is normally used in order to create a liquid and / or vapor barrier. The polymers are normally either laminated or extrusion coated to the fiber-based substrate. However, a polymer layer that gives a product barrier properties normally needs to be relatively thick and it is thus quite costly to produce such barrier. The most common way to approach reduction of oxygen transmission (OTR) through a paper or paperboard is to use multiple polymer layers. In this way, one layer can provide low OTR, whereas other layers can provide water repellency and / or low water vapor transmission rates.

[0006] Another commonly used barrier material is aluminum. Aluminum layers are typically used in order to improve the oxygen and light barrier of paper or paperboard products. The aluminum layers are thin, typically around 7-9 pm. Aluminum provides excellent barrier properties, but it significantly increases the carbon dioxide load of the product unless the aluminum can efficiently be recycled.

[0007] A problem with the most common polymer- and aluminum layers is also that they decrease the biodegradability and the possibility to efficiently recycle the packaging materials.

[0008] There is a demand from producers, converters and end users to avoid the use of synthetic polymer and aluminum layers in paper or paperboard products in order to decrease use of non-renewable materials to improve the biological degradability of the paper or paperboard products.

[0009] It is known in the art to provide nano-coatings that can be organic or inorganic, such as ceramic or metal nano-coatings. The nano-coatings are very thin, such as from about 0.1 nm to about 100 nm in thickness. For example, metallized surfaces using a very small amount of metal or metal oxides, such as aluminum or TiC>2, AI2O3, MgO or ZnO. For example, atomic layer deposition (ALD), dynamic compound deposition (DCD), chemical vapor deposition (CVD), such as plasma CVD, physical vapor deposition (PVD) and metal plasma-deposition are techniques suitable to provide a small amount of metal on a surface. However, it remains essential that the packaging material, when provided with a nano-coating such as being metallized, can maintain barrier properties and is sufficiently crackresistant. Typically, a pre-coating of for example a crosslinked polymer is required to enable the use of a nano-coating. One issue with film-forming polymers such as latex and thermoplastic fossil-based polymers is that the packaging material obtained is typically not considered as a monomaterial and issues may arise with recycling. A further problem with many film-forming polymers is that the film-forming polymers are usually provided in the form of aqueous solutions or dispersions. The water content of the solutions or dispersions may in some cases disrupt the paper substrate. Hydrophilic cellulose materials typically provide barrier properties to oxygen, but are sensitive to water and water vapour.

[0010] Therefore, a substrate adapted such that a very small amount of nano-coating can be applied is desirable.

[0011] One way of providing a barrier film or coating for fiber-based substrates, taking into account biodegradability and recyclability of the fiber-based products, is to provide coatings comprising carboxymethylcellulose (CMC), which may for example be applied as water-based dispersion coatings. Water-based dispersion coatings in general are prone to pinholes, which may arise during the application process. The raw materials are typically mixed in high-speed agitation by ultra- high-speed dispersers and pumped through pipes to an application station. This leads to formation of a micro-stable foam. When the coating is dried, pinholes may arise which act as defects and easily allow the migration of oxygen and water vapor into the package, defeating the purpose of the coating. In addition, bubble formation in coating compositions may lead to formation of pinholes. Similarly, so- called “sinkholes” may arise due to deficiencies in the underlying fiber-based layers, such that the coating composition cannot be evenly provided on the surface of the fiber-based layer. A further problem with pinholes is that the presence thereof essentially precludes the application of a nano-coating.

[0012] One solution to the problem with pinholes is the addition of defoamer or antifoaming agent to the coating formulation. Additionally, methods such as modifying coating processes to reduce the amount of turbulence are known to improve the quality of coating. Finally, the viscosity can be decreased to let the entrapped air release easily. However, these methods introduce new problems. The use of defoamer, for example, alters the chemistry of coating and adds cost to the product. Modifying coating processes may work in small scale trials but adapting this method on a large-scale industrial setup is complicated and may not lead to desired results. Decreasing viscosity leads to reduced coat weight and added costs incurred by removal of excess water from the applied coating by energy intensive drying.

[0013] Therefore, there exists a need for improved processes for providing fiber-based substrates, such as paper or paperboard, with improved barrier properties, ideally also having properties such that a very small amount of nano-coating can be applied. of the invention

[0014] It is an object of the present disclosure to provide a coated paper or paperboard that alleviates at least some of the problems caused by pinholes in prior art barrier films and coatings.

[0015] The present invention is based on the realization that pinholes in coatings, particularly in coatings comprising CMC, typically have a diameter of about 10-50 pm. Surprisingly, it has been found that a polyhydroxyalkanoate (PHA) dispersionbased coating comprising PHA particles, wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to 15013320:2009, is in a range of from 2 to 10 pm, preferably in a range of from 2 to 5 pm, can be forced into the pinholes in the wet state. In this context, Dso means that 50% of the particles measured have a diameter smaller than this value. Subsequent drying results in homogeneous pinhole free coating, with improved barrier properties. When a CMC-based first coating is used, in combination with a PHA dispersion coating according to the present invention, it is possible to achieve a packaging material which is entirely bio-based and biodegradable.

[0016] In the context of the present invention the term PHA refers to a polyhydroxyalkanoate polymer. Preferably, the PHA used in the preparation of the PHA dispersion used according to the present invention is a PHA copolymer such as poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH).

[0017] Thus, the present invention is directed to a coated paper or paperboard, wherein the paper or paperboard has been coated with a first coating and with a PHA dispersion coating provided on the first coating, wherein the first coating comprises at least 40 wt-% CMC, based on the total dry weight of the first coating and wherein the PHA in the PHA dispersion coating is present in the form of particles, wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to ISO13320:2009, is in a range of from 2 to 10 pm, preferably in a range of from 2 to 5 pm.

[0018] The aqueous PHA dispersion coating composition comprises 80-100 % by weight of PHA, wherein the PHA is present in the form of particles, wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to ISO13320:2009, is in a range of from 2 to 10 pm, based on the total dry weight of the aqueous PHA dispersion coating composition.

[0019] The aqueous medium used in the dispersion may be water or an aqueous solution, or it may comprise a mixture of water or an aqueous solution with an organic solvent. In a preferred embodiment, the aqueous medium is water based, i.e., it is comprised of more than 50 % by weight of water. In a more preferred embodiment, the aqueous medium is water. The viscosity of the dispersion is preferably in a range of from 200 to 1000 mPa s at 23°C and 50 RPM using a spindle LV-03 from Brookfield rheometer.

[0020] The aqueous PHA dispersion coating composition can be a dilute or a high solids composition. However, a high solids composition is typically preferred.

[0021] The terms “total solids content” or “solids content” as used herein refer to the total fraction of dissolved solids plus suspended and settleable solids in an aqueous solution or suspension. The high solids content combined with low viscosity reduces transportation costs of the coating composition and makes it possible to prepare coatings with a high basis weight in a single coating step. A high solid content also reduces the amount of water to be removed when the coating is dried, which reduces the drying energy requirement of the coating process. Preferably, the total solids content of the coating composition is at least 10 % by weight, more preferably at least 20 %, or at least 30 % by weight based on the total weight of the coating composition.

[0022] The total solids content of the PHA dispersion coating composition is typically 60 % by weight or less, preferably 50 % by weight or less, based on the total weight of the coating composition.

[0023] In some embodiments the aqueous PHA dispersion coating composition has a total solids content in the range 10-90 % by weight, preferably in the range of 20- 80 % by weight, more preferably in the range of 30-70 % by weight, based on the total weight of the coating composition.

[0024] The formulation of the aqueous PHA dispersion coating composition may vary depending on the intended use of the coating and the coated paper or paperboard. In some embodiments, the total solids content of the aqueous PHA dispersion coating composition consists solely of PHA. In other embodiments, the coating composition may further include a wide range of ingredients in varying quantities to improve the end performance of the product or processing of the coating.

[0025] In some embodiments, a plasticizer is added to the aqueous PHA dispersion coating composition to increase the elasticity of the resulting coatings and make them less brittle. The plasticizer typically also reduces the melting point of the polymer, thereby facilitating film forming at a low temperature. This allows the formed coatings to better withstand bending without losing their barrier properties. Preferably, the plasticizers are or comprise monosaccharides, disaccharides or polysaccharides, such as sugar alcohols, such as sorbitol and maltitol. In one embodiment, the plasticizer is a mixture of polysaccharides, such as a mixture comprising sorbitol and maltitol. In some embodiments, the aqueous PHA dispersion coating composition comprises 1-10 % by weight of a plasticizer based on the total dry weight of the aqueous PHA dispersion coating composition.

[0026] The aqueous PHA dispersion coating composition may comprise fillers. The amount of fillers is preferably less than 8 wt-% based on the total dry weight of the aqueous PHA dispersion coating composition. More preferably, the amount of fillers is less than 5 wt-% based on the total dry weight of the aqueous PHA dispersion coating composition. More preferably, the amount of fillers is less than 3 wt-% or less than 2 wt-% or less than 1 wt-% or less than 0.1 wt-% based on the total dry weight of the aqueous PHA dispersion coating composition.

[0027] The PHA dispersion coating composition is preferably provided in a formulation suitable for application to paper or paperboard using conventional paper coating equipment and techniques, such blade coaters and bar coaters. Accordingly, the coating composition may include 0.1-20% by weight of various additives, based on the total dry weight of the aqueous PHA dispersion coating composition, to impart suitable coating characteristics. Such coating additives may include, but are not limited to, a dispersing agent (e.g. a surfactant), a lubricant (e.g. a stearate), a rheology modifier, an insolubilizer, a humectant, a barrier chemical, and a pH adjusting agent (e.g. NaOH).

[0028] The aqueous PHA dispersion coating composition can be applied to a paper or paperboard and dried to form a coating. Thus, the coating preferably comprises 80-100 % by weight of PHA, wherein the PHA is present in the form of particles, wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to ISO13320:2009, is in a range of from 2 to 10 pm, based on the total dry weight of the coating.

[0029] The coating is preferably dry or substantially dry. The coating preferably has a total solids content above 90 % by weight, preferably above 95 % by weight.

[0030] The PHA coating obtained according to the present invention may generally have a basis weight of 25 g / m2or less. Typically, a basis weight of at least 10 g / m2is required to obtain acceptable barrier properties. In some embodiments, the basis weight of the coating is in the range of 1-25 g / m2, preferably in the range of 15-25 g / m2. In one embodiment, the coating according to the present invention is provided in one layer. The PHA dispersion coating according to the present invention is provided on a first coating having pinholes. The first coating is a coating based on or comprising CMC. Preferably, the first coating comprises at least 40 wt-% CMC, more preferably at least 45 wt-% CMC, based on the total dry weight of the first coating. The coating comprising CMC preferably has a basis weight of 5-20 g / m2, such as 10-18 g / m2.

[0031] The PHA coating according to the invention may generally have a thickness of 100 pm or less. In particular, the coating may have a thickness of 50 pm or less, or more specifically the coating may have a thickness of 15 pm or less or 10 pm or less. In some embodiments, the thickness of the coating is in the range of 1-15 pm, preferably in the range of 5-10 pm.

[0032] The coating is preferably prepared from an aqueous PHA dispersion coating composition as described above.

[0033] In some embodiments, the coating has an oxygen transmission rate (OTR), measured according to the standard ASTM D-3985 at 50% relative humidity and 23 °C, of less than 20 cc / m2 / 24h / atm, preferably less than 10 cc / m2 / 24h / atm, and more preferably less than 5 cc / m2 / 24h / atm and most preferably less than 2 cc / m2 / 24h / atm or less than 1 cc / m2 / 24h / atm or less than 0.1 cc / m2 / 24h / atm or less than 0.01 cc / m2 / 24h / atm.

[0034] In some embodiments, the coating has a KIT value above 8, preferably above 10 or above 11 , as measured according to standard TAPPI T559.

[0035] The fiber-based substrate is a paper or paperboard. More specifically, the PHA coating is provided on a first coating comprising CMC.

[0036] Paper generally refers to a material manufactured in sheets or rolls from the pulp of wood or other fibrous substances comprising cellulose fibers, used for e.g. writing, drawing, or printing on, or as packaging material. Paper can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end-use requirements.

[0037] Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example as flat substrates, trays, boxes and / or other types of packaging. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end-use requirements.

[0038] The aqueous PHA dispersion coating composition can either be added on the papermaking machine (on-machine coating) or on a separate machine (off- machine coating). A variety of paper coating equipment and techniques may be used for applying the coating composition, for example blade coaters, air knife coaters, and cast coaters. The coating composition can be applied to one side or both sides of the paper or paperboard.

[0039] The coated paper or paperboard according to the present invention is suitable for being repulped. In some embodiments, the coated paper or paperboard has a repulpability characterized by a reject rate (as determined according to the PTS RH 021 / 97 test method) below 20%, preferably below 10%, more preferably below 5%, and most preferably below 1%.

[0040] One aspect of the present invention is a method for the preparation of a coated paper or paperboard, comprising the steps of: a) providing a paper or paperboard; b) providing a first coating on the paper or paperboard, wherein the first coating comprises at least 40 wt-% CMC, based on the total dry weight of the first coating; c) providing a coating on the first coating, wherein said coating provided on the first coating comprising at least 80% by weight of PHA, wherein the PHA is present in the form of particles, wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to ISO13320:2009, is in a range of from 2 to 10 m, based on the total dry weight of the coating provided on the first coating; and d) drying the coatings to obtain the coated paper or paperboard.

[0041] In some embodiments the aqueous PHA dispersion coating composition has a total solids content in the range 10-90 % by weight, preferably in the range of 20- 80 % by weight, more preferably in the range of 30-70 % by weight, based on the total weight of the coating composition.

[0042] In the drying step d), the total solids content of the coating is increased by evaporation of water. The resulting coating preferably has a total solids content above 90 % by weight.

[0043] The term coating as used herein refers generally to a finishing operation in which the surface of a substrate, e.g. a fiber-based substrate such as a paper or paperboard, is covered with a composition to impart a desired finish or texture to the substrate or to improve its printability or other properties, such as optical or barrier properties.

[0044] Generally, while the products, polymers, materials, layers and processes are described in terms of “comprising” various components or steps, the products, polymers, materials, layers and processes can also “consist essentially of” or “consist of” the various components and steps.

[0045] The coated fiber-based product according to the present invention may be provided with additional coating layers, depending on the desired properties. In particular, it has been found that the coated fiber-based product according to the present invention is suitable for being vacuum coated. Thus, a further coating, which is a vacuum coating, may be provided. Thus, a very small amount of metal, such as aluminum may be provided on the PHA-based coating, by vacuum coating. For example, atomic layer deposition (ALD), dynamic compound deposition (DCD), chemical vapor deposition (CVD), physical vapor deposition (PVD) and metal plasma-deposition are techniques useful for providing barrier properties using only very small amounts of metal. While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

[0046] EXAMPLES

[0047] The coated samples were tested for total coat weight, scanning electron microscopy (SEM) and oxygen transmission rate (OTR) as follows:

[0048] - The coat weight was measured using the oven drying method by comparing the dry weight of the base board and the coated sample.

[0049] - The oxygen transfer rate (OTR) was measured in accordance to ASTM D3985 at a temperature of 23°C and at 50 % relative humidity (RH) using a Mocon Ox-Tran device 2 / 22.

[0050] - SEM samples were 20 pm gold spurted before analysis. The device used is a quanta 250 from SEI. The acceleration energy used was 5kV with a spot size or 4 in secondary electron detection mode.

[0051] The particles in PHA dispersion coatings were measured using a Malvern Mastersizer 3000 and according to international standard ISO 13320:2009.

[0052] Dispersions preparation:

[0053] A first CMC dispersion was prepared 24 h before at 11 wt% by mixing CMC dried powder with water for 2 hours at 40°C using a I KA overhead stirrer 2000 RPM. 24 hours later all components that make the final CMC dispersion were mixed together by adding filler and plasticizer to the already prepared CMC under stirring. Once all components were added the dispersion was set under high shear mixing (2000 RPM) using a disermill discovery 100 for 30 min at room temperature.

[0054] The PHA dispersion was prepared at a maximum of 3 days before coating to avoid bacterial development. First the PHA powder was added to water using an I KA ultra Turrax T 25 D until complete homogenization. Later on fillers and plasticizers were added until an homogenic dispersion is obtained. Finally, the whole dispersion was degassed under vacuum using a speedmixer DAC 1200-VAC from FlackTek.

[0055] Sample preparation:

[0056] All samples were coated in a same manner. The paperboard used is a commercial product from StoraEnso used for liquid packaging: Natura RFA CLC / F with a grammage of 270 gsm.

[0057] The base CMC coating (Finnfix, supplied by Nouryon) was coated using a laboratory coating machine using rod coating method using a TQC Sheen AB4120 laboratory applicator device. A total of 2 layers of CMC dispersion were applied. A first layer was applied on the board using a coating speed of 150 mm / s then dried in oven at 105° for 2 minutes then the second coat was applied and dried in the same manner as the first one.

[0058] The PHA dispersion was further applied in a 2 layers system using a TQC Sheen AB4120 laboratory applicator device and oven dried for 2 min at temperatures between 152-155°C. The same rod for both layer was used in order to reach a target coat weight of 10 gsm per layer. Coating speed used was as well 150 mm / s. Table 1: Comparison OTR for CMC coated samples only and CMC / PHA coated samples from same CMC base coating. instrument detection limit Figure 1a is a SEM picture of a PHA dispersion coated sample after drying in the oven during film formation.

[0059] Figure 1b is a SEM picture of PHA dispersion coated sample after drying and complete film formation.

[0060] Figure 1c is a SEM picture of CMC coated board, illustrating pinhole sizes in a range of from 16 to 27 pm.

Claims

CLAIMS1. A coated paper or paperboard, wherein the paper or paperboard has been coated with a first coating and with a PHA dispersion coating provided on the first coating, wherein the first coating comprises at least 40 wt-% CMC, based on the total dry weight of the first coating and wherein the PHA in the PHA dispersion coating is present in the form of particles, wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to 15013320:2009, is in a range of from 2 to 10 pm.

2. A coated paper or paperboard according to claim 1 , wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to 15013320:2009, is in a range of from 2 to 5 pm.

3. A coated paper or paperboard according to claim 1 or 2, wherein the basis weight of the PHA dispersion coating is in the range of 1-25 g / m2.

4. A coated paper or paperboard according to any one of claims 1-3, wherein the basis weight of the first coating is 5-20 g / m2.

5. A coated paper or paperboard according to any one of claims 1-4, wherein the oxygen transmission rate, measured according to the standard ASTM D-3985 at 50% relative humidity and 23 °C, is less than 2 cc / m2 / 24h / atm.

6. A coated paper or paperboard according to any one of claims 1-5, wherein said coated paper or paperboard has a repulpability characterized by a reject rate (as determined according to the PTS RH 021 / 97 test method) below 20%, preferably below 10%, more preferably below 5%, and most preferably below 1 %.

7. A coated paper or paperboard according to any one of claims 1-6, wherein the coated paper or paperboard has been provided with a further coating, which is a vacuum coating.

8. A method for the preparation of a coated paper or paperboard, comprising the steps of: a) providing a paper or paperboard; b) providing a first coating on the paper or paperboard, wherein the first coating comprises at least 40 wt-% CMC, based on the total dry weight of the first coating; and c) providing a coating on the first coating, wherein said coating provided on the first coating comprising at least 80% by weight of PHA, wherein the PHA is present in the form of particles, wherein the Dso of the PHA particles, determined using a Malvern Mastersizer 3000 using light scattering principle according to ISO13320:2009, is in a range of from 2 to 10 pm, based on the total dry weight of the coating provided on the first coating; d) drying the coatings to obtain the coated paper or paperboard.

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