The present invention relates to a sheet-like product and to a method for its manufacture according to preambles of the enclosed independent claims.

Cellulosic fibre based materials, such as paper and board, are conventionally used for packaging various goods, including foodstuffs and beverages, as well as for disposable articles, such as cups, tableware and the like. The use of cellulosic fibre based materials is increasing, as there is a general desire to decrease or completely replace fossil fuel based plastics in packaging applications. The need to find sustainable and renewable packaging solutions is expanding the use of paper and board even to new areas, as the businesses and consumers become more aware about sustainability and environmental impact of the product packaging.

It is common to surface-treat paper and board in order to improve their barrier properties and to make them more suitable for various packaging applications. Coating layers and/or surface size layers are applied on the surface of the paper and board in order to produce protective barrier layers, for example, against grease, water, water vapour, oxygen, aroma and/or mineral oil. Many of the conventional barrier layers are applied to the surface by extrusion or glue lamination, and they usually comprise one or more polymers which form a protective film on the surface of the paper or board, and prevents the interaction between the packaged goods and the environment.

Polymers, which are used for surface treatment of paper and board, have different degrees of crystallinity. Crystalline polymers, i.e. polymers with a high degree of crystallinity, are generally known to be harder and more brittle than semi-crystalline or non-crystalline amorphous polymers. Surface layers comprising hard and brittle polymers easily break during folding, creasing and other converting steps, when the coated cellulosic substrate is formed into a final product. The damage caused to the surface layer during such converting steps will make the surface layer leak and reduce its barrier properties, as the layer will be easily penetrated by the grease, water, oxygen, etc. The higher degree of crystallinity usually also increases the film forming and melting temperature of the polymer, which makes their processability more demanding. For these reasons, it would appear that the use of polymers with a low degree of crystallinity would be advantageous in surface treatment of paper, board or the like. A low degree of crystallinity is, however, known to reduce the barrier properties of the final coating or surface sizing layer. Furthermore, use of amorphous polymers can make surface layers tacky, which easily leads to contamination of the processing equipment, such as leading rolls, fabrics and other machine elements. Amorphous polymers with low melting temperatures may also increase undesired blocking tendency of the coated paper or board, i.e. they cause unwanted sticking of the material to itself during winding and storage.

There is an increasing trend and desire to use biobased and/or biodegradable components in manufacture of various products. This trend applies also to materials which are used for packaging of goods, especially with growing desired to reduce petroleum-based materials and the awareness of microplastics and their harmful effects to living organisms. Unfortunately, many of the suitable biobased and/or biodegradable polymers show a low degree of crystallinity, increasing the blocking tendency of the coated paper.

There is a need to increase the use of biobased and/or biodegradable polymers as well as overcome the problems associated with the different crystallisation degrees of the used polymers. On one hand, it is desired that the polymer will provide the surface layer with desired barrier properties, on the other hand it is desired that the polymer will not cause process problems, such as contamination of the equipment.

The object of the present invention is to minimize or even eliminate the problems existing in the prior art.

One non-limiting object of the present invention is to provide a sheet-like product which has good barrier properties, and which is easy to produce and process, even when a biobased and/or biodegradable polymer is used for providing at least one coating layer. A further non-limiting object of the present invention is to provide a method for manufacturing a coated sheet-like product without problems associated with surface layer tackiness.

These objects are attained with the invention having the characteristics presented below in the characterising part of the independent claims. Some preferable embodiments are disclosed in the dependent claims.

The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, even if this is not always separately mentioned.

A typical sheet-like product according to the present invention, such as paper, board or the like, comprises

- a substrate comprising cellulosic fibres and having a first large surface, wherein at least a first coating layer and a second coating layer are applied on the first large surface of the substrate by curtain coating to form a barrier coating, the second coating layer forming the surface of the sheet-like product,

- the first coating layer comprising a first polymer having a first crystallization degree and a first melting point, and

- the second coating layer comprising a second polymer having a second crystallization degree and a second melting point, wherein the first polymer is a biobased and/or biodegradable polymer, and wherein the second crystallization degree and/or the second melting point of the second polymer is higher than the first crystallization degree and/or the first melting point of the first polymer.

A typical method according to the present invention for making a sheet-like product according to the invention comprises

- forming a barrier coating comprising at least a first coating layer and a second coating layer on a first large surface of a substrate comprising cellulosic fibres, such as paper, board or the like, by applying the first coating layer and the second coating layer simultaneously by curtain coating on the first large surface of the substrate, without an intermediate drying between the first coating layer and the second coating layer, wherein the first coating layer comprises a biobased and/or biodegradable first polymer which is a biobased and/or biodegradable polymer having a first crystallization degree and a first melting point, and the second coating layer comprises a second polymer having a second crystallization degree and a second melting point, which second crystallization degree and/or the second melting point is higher than the first crystallization degree and/or the first melting point of the first polymer, the second coating layer forming the surface of the sheet-like product, and

- subjecting the formed barrier coating to impingement drying with hot air for increasing the temperature of the second coating layer over the second melting point.

Now it has been surprisingly found that it is possible to circumvent the problems associated with tackiness of the biobased and/or biodegradable polymers and to obtain good barrier properties by simultaneous application of two or more coating layers by curtain coating, where the second coating layer, which forms the surface of the coated product, has a higher crystallization degree and/or melting point temperature than the biobased and/or biodegradable first polymer of the coating layer underneath. It was unexpectedly realised that the first polymer, which is a biobased and/or biodegradable polymer with a lower crystallization degree and/or melting point, can be protected by a second coating layer, while it provides the first layer with the desired elasticity for the barrier coating. In this way it is possible to form a barrier coating that has good barrier properties, is not brittle, and does not contaminate the machine parts. At the same time it is also possible to increase the amount of biobased/biodegradable constituents in the barrier coating and make the barrier coating more sustainable.

In the present context the term “biobased polymer” denotes a polymer which is produced from renewable nature derived raw materials directly or using mass- balance approach, excluding petroleum based raw materials. Biobased polymers comprise polymers derived from polysaccharides, such as sugar-based biopolymers, starch-based biopolymers, and cellulose-based biopolymers; polymers derived from proteins or lipids, as well as polymers obtained from microbiologically produced material, such as polyesters. Biobased polymers suitable for use in the present invention are preferable biodegradable.

In the present context the term “biodegradable polymer” encompasses both biobased polymers and petroleum-based polymers as long as they are degradable by biological activity, e.g. by microorganisms, such as bacteria, fungi, algae, and/or enzymes, with the degradation accompanied by a lowering of the molar mass of the original polymer. The degradation products are non-toxic and environmentally acceptable, usually carbon dioxide, water and inorganic compounds. Preferably biodegradable polymers in the present context are compostable.

In the present context, the crystallization degree is understood as the ratio between crystalline and amorphous parts of the polymer. For the present purposes, crystallization degree can be determined by using differential scanning calorimetry (DCS), according to standard ISO 11357-1 :2016.

The first polymer may be selected from biobased and/or biodegradable polyesters, such as polybutylene succinate, poly(butylene succinate-co-adipate), polyhydroxyalkanoates, polycaprolactones, polylactides, any of their mixtures and their copolymers. The first polymer may also be a copolymer of two or more said polyesters, or a mixture of two or more said polyesters. According to one preferable embodiment, the first polymer may be a polyhydroxyalkanoate, such as polyhydroxybutyrate, polyhydroxyvalerate, polyhydroxyhexanoate or any of their copolymers.

The first polymer may have a melting point in a range of 50 - 120 °C, preferably 60 - 110 °C. The melting point can be determined by using differential scanning calorimetry (DCS), according to standard ISO 11357-1 :2016. The melting point of the first polymer provides optimal flexibility for the barrier layer and minimises the risk of leakage through cracking of the barrier layer.

The first polymer may have a glass transition temperature T _g <0 °C, for example in a range from -40 °C to 0 °C. The glass transition temperature of the first polymer is lower than the first melting point temperature. The glass transition temperature can be determined by using differential scanning calorimetry (DCS), according to standard ISO 11357-1 :2016.

The second polymer may be a thermoplastic elastomer.

The second polymer may preferably be selected from a group comprising polyesters, such as polybutylene succinate, poly(butylene succinate-co-adipate), polyhydroxyalkanoates, polylactide or polycaprolactone; polyethyleneterephtalate; polyolefins, such as polyethylene or polypropylene; styrene acrylate copolymers; styrene butadiene copolymers; ethylene acrylic and methacrylic acid copolymers; poly(glycolic acid); butyl vinyl alcohol, polyvinyl alcohol; ethylene vinyl acetate and chitosane. According to one preferable embodiment, the second polymer may be selected from biobased and/or biodegradable polyesters, such as polybutylene succinate, poly(butylene succinate-co-adipate), polyhydroxyalkanoate, polycaprolactone or polylactide. The second polymer may also be a copolymer of two or more said polyesters, or a mixture of two or more said polyesters. According to one preferable embodiment the second polymer may be a polyhydroxyalkanoate, such as polyhydroxybutyrate, polyhydroxyvalerate, polyhydroxyhexanoate or any of their copolymers.

The second polymer may have a melting point in a range of 120 - 180 °C, preferably 80 - 170 °C or 80 - 140 °C. The melting point of the second polymer provides the coating with required flexibility while minimising the risk of blocking. The melting point of the second polymer also enables secure heat sealing of the sheet-like product, e.g. for manufacture of packages, hot and/or cold cups and the like.

The second polymer may have a glass transition temperature T _g >20 °C, for example in a range from 20 °C to 100 °C , preferably from 20 °C to 40 °C. The glass transition temperature of the second polymer is lower than the second melting point temperature. The glass transition temperature can be determined by using differential scanning calorimetry (DCS), according to standard ISO 11357-1 :2016. The first polymer and the second polymer may be different from each other. For example, the second polymer may be a non-biodegradable and/or non-biobased polymer.

According to one embodiment, the first polymer and second polymer are both biobased and/or biodegradable polymers. In this manner it is possible to maximize the amount of biobased and/or biodegradable polymer the barrier coating. According to one preferable embodiment, the first polymer and second polymer may both be biobased and/or biodegradable polymers and they may be formed from same structural monomer units, as long as the second crystallization degree and/or the second melting point of the second polymer are higher than the first crystallization degree and/or the first melting point of the first polymer. Use of first and second polymer formed from same structural monomers units decreases the risk of repulsion and delamination between the different coating layers.

According to one preferable embodiment of the invention the crystallization degree of the second polymer is higher than the crystallization degree of the first polymer.

According to one embodiment of the invention the second coating layer may comprise a crystallization promoter. The crystallization promoter may be finely- divided inorganic particle, such as mica or talc, or an organic salt, such as sodium benzoate. The crystallization promoter may provide effective nucleation and increase crystallization speed in the second coating layer during drying. Furthermore, use of crystallization promoter may generally improve the crystallization process in the second coating layer and reduce the operating challenges. Especially, when the second coating layer comprises a biobased and/or biodegradable polymer, the coating layer preferably comprises a crystallization promoter.

The first coating layer and/or the second coating layer may further comprise inorganic mineral particles. The inorganic mineral particles may be selected from kaolin, talc, mica, calcium carbonate or any mixture thereof. Inorganic mineral particles may improve the barrier properties achieved. The first coating layer and/or the second coating layer may comprise <50 weight-%, preferably <25 weight-%, of inorganic mineral particles. For example, the first coating layer and/or the second coating layer may comprise inorganic mineral particles in amount of 5 - 50 weight- %, preferably 10 - 25 weight-%, calculated from the dry weight of the coating layer. The first coating layer and/or second coating layer comprising inorganic mineral particles may be in form of an aqueous dispersion having solids content of 20 - 65 weight-%, preferably 30 - 60 weight-%, at the time of application. Preferably, the amount of inorganic mineral particles in the first coating layer is higher than the amount of inorganic mineral particles in the second coating layer. According to one preferable embodiment, the second coating layer is free of inorganic mineral particles.

According to the present invention it is thus possible to form barrier coating with at least a first coating layer and a second coating layer that provides a barrier layer for grease, water, water vapour transmission, oxygen transmission, aroma and/or mineral oil. The sheet-like product is especially suitable for use in food and beverage packaging.

In the present context the substrate for the sheet-like product comprises cellulosic fibres. The cellulosic fibres may be fibres obtained by chemical or mechanical pulping, or recycled fibres, or any mixture thereof. The substrate may be paper, board or the like. At the time of application of the first and the second coating layer, the substrate is usually in form of a moving cellulosic web, having a first large surface and a second large surface, which are parallel with each other. The substrate may have a basis weight or grammage from 40 g/m ² to 550 g/m ² , preferably from 80 g/m ² to 450 g/m ² . According to one preferable embodiment the substrate may have a basis weight or grammage from 175 g/m ² to 400 g/m ² , more preferably from 175 g/m ² to 350 g/m ² . According to another preferable embodiment the substrate may have a basis weight from 40 g/m ² to 200 g/m ² , preferably from 60 g/m ² to 120 g/m ² .

According to the present invention at least a first coating layer and a second coating layer are applied simultaneously by curtain coating on the first large surface of the substrate, without an intermediate drying between the first coating layer and the second coating layer. The first and the second coating layer can be applied as aqueous dispersions or aqueous emulsions comprising the respective polymer. The coating layers are applied simultaneously from different, separate, slots. The laminar flows of the coating compositions from the different coating slots enable immiscibility of the applied coating layers.

The first coating layer and the second coating layer are subjected to impingement drying with hot air for increasing the temperature of the second coating layer over the second melting point. The impingement drying with hot air ensures that the water is removed from the coating layers by heat transfer, which takes place from the outside inwards and the outer surface of second coating layer will be in direct contact with the hot drying air. This enables the formation of a temperature gradient through the coating layer. The second polymer in the second coating layer preferably rapidly reaches its melting point temperature for film forming. When the temperature of the second coating layer reaches the melting point of the second polymer, the temperature of the first layer may have at least partially increased over the melting point of the first polymer, as it is lower than the melting point of the second polymer. The more slowly crystallizing first polymer in the first coating layer will eventually reach its film forming temperature, without disturbing the process by its tackiness, since the first coating layer is protected by the second coating layer with the faster crystallizing second polymer, providing a non-tacky outer surface for the applied barrier coating.

According to one preferable embodiment the temperature of the hot air in impingement drying may be at least 300 °C, preferably at least 400 °C, even more preferably at least 425 °C. High air temperatures in the impingement drying ensure fast crystallization of the second layer and minimize the risks for process problems caused by tacky coating surface.

The temperature of the hot air in impingement drying may be adjusted according to the travel speed of the web to be coated. The higher the travel speed, the higher the temperature of the hot air should be in order to guarantee that the temperature of the second coating layer is increased over the second melting point.

As noted above, the second coating layer forms the surface of the sheet-like product. In this manner it is possible to avoid any problems that might be caused by tackiness of the first polymer in the first coating layer. According to one preferable embodiment the crystallization degree of the second polymer is higher than the crystallization degree of the first polymer. In this manner it is possible to ensure that the surface of coated web does not contaminate process equipment during further processing.

The sheet-like product may comprise two or more first coating layers. It is possible to apply more than one first coating layers simultaneously by curtain coating. The first polymers in the two or more first coating layers may be different from each other, or identical to each other, but each first polymer has first crystallization degree and/or first melting point, which is lower than the second crystallization degree and/or the second melting point of the second polymer in the second coating layer.

The first coating layer preferably has a higher coat weight than the second coating layer. According to one embodiment the first coating layer may have a coat weight >3 g/m ² , preferably >5 g/m ² . The first coating layer may have the coat weight in a range of 3 - 15 g/m ² , preferably 5 - 12 g/m ² , more preferably 5 - 10 g/m ² . It is possible to apply a relatively thick first coating layer, as it is protected by the second coating layer. The second coating layer may have a coat weight <6 g/m ² , preferably <5 g/m ² or <3 g/m ² . The second coating layer may have the coat weight in a range of 0.5 - 6 g/m ² , preferably 1 - 5 g/m ² , more preferably 1 - 3 g/m ² . The coat weight of the second coating layer is preferably as thin as possible to enable a fast film forming and crystallization, while still enabling a formation of a uniform coating layer on top of the first coating layer.

The sheet-like product may comprise at least one precoat layer arranged between the surface of the substrate and the first coating layer of the barrier coating. This means that at least one precoat layer is applied on the surface of the substrate before forming the barrier coating. The precoat layer(s) reduce(s) the penetration of the first coating material into the cellulosic fibre web by sealing or closing the web surface and improves the coating hold-out. The precoat layer(s) may be applied by any conventional coating technique used in manufacture of paper and board. According to one preferable embodiment the precoat layer(s) may be applied by using a size press, a metered size press, a rod or blade coater. After the application the precoat layer(s) is/are preferably dried before the application of the first and the second coating layer. The drying may be performed by using an air dryer, infrared dryer, cylinder dryers or any combination of these.

According to one embodiment of the invention the precoat layer or one of the precoat layers may comprise a film forming natural or synthetic polymer, such as starch; a cellulose derivative, such as carboxymethyl cellulose or hydroxyethyl cellulose; and/or polyvinyl alcohol. The precoat layer may be thin, while it still effectively closes the surface of the substrate. The coat weight of the precoat layer may be <5 g/m ² , preferably <4 g/m ² or <3 g/m ² . For example, the coat weight of the precoat layer may be in a range of 0.3 - 15.0 g/m ² , preferably 0.3 - 5.0 g/m ² , more preferably 0.5 - 4 g/m ² , even more preferably 0.5 - 3.0 g/m ² or 1 .0 - 2.0 g/m ² .

According to another embodiment the precoat layer or one of the precoat layers may comprise inorganic mineral particles. The inorganic mineral particles may be selected from calcium carbonates, clay, talc, mica, kaolin or titanium dioxide. More preferably, the inorganic mineral filler particles may be selected from kaolin, talc, mica, calcium carbonates, such as ground calcium carbonate or precipitated calcium carbonate, or any mixture thereof. The precoat layer comprises further a binder, which binds the inorganic mineral particles to the precoat layer and to the substrate surface. The binder may be a synthetic polymer latex, such as styrene acrylate latex, styrene butadiene latex or polyvinyl acetate latex; polyvinyl alcohol; starch; or carboxymethyl cellulose. The precoat layer may have a coat weight <30 g/m ² , preferably <20 g/m ² , more preferably <15 g/m ² . For example, the precoat layer comprising inorganic mineral particles may have a coat weight in a range of 4 - 30 g/m ² , even more preferably 4 - 20 g/m ² , more preferably 5 - 15 g/m ² . According to one embodiment the substrate is subjected to calendering after application of precoat layer. Calendering after application of the precoat improves the sealing and closure of the substrate surface by mechanically pressing the precoat layer at least partially within the substrate. Calendering reduces the porosity and improve the smoothness of the precoated substrate surface. Any suitable calendering unit may be used. Typical calendering units suitable for this purpose are heated or unheated, single or multinip, calenders with hard or soft rolls, shoe nip or metal belt calenders.

According to one embodiment, one or more intermediate layers are arranged between the first coating layer and the second coating layer. The optional intermediate layers are applied simultaneously with the first coating layer and the second coating layer by curtain coating without any intermediate drying. The intermediate layer(s) may provide the formed coating with further functional properties, and/or improve its barrier properties.

According to another embodiment the second coating layer is arranged directly on the first coating layer without any intermediate layers.

It is possible to arrange at least one cooling unit after the impingement drying. The cooling unit cools the coated web before winding into the jumbo reel. An appropriate reaction time should, however, be allowed between the impingement drying and the cooling unit, in order to allow desired film forming and crystallization in the coated layers, especially in the first coating layer. The cooling unit reduces the blocking risk during winding and the following storage.

The method according to present invention is suitable for both on-line and off-line application.

Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.