Method for coating a paper substrate

Abstract

The present invention relates to a method for coating a paper substrate comprising: i) providing a paper substrate; ii) applying to the paper substrate an aqueous barrier coating composition comprising: a) from 10 to 50 wt% of a modified cutin extract (as dry matter), wherein said modified cutin extract is obtained by thermal treatment of 5 a raw cutin extract at a temperature comprised between 40 and 120 °C; b) at least one divalent or trivalent fatty acid metal salt, wherein the weight ratio, based on the dry content, of the modified cutin extract to the at least one divalent or trivalent fatty acid metal salt is from 10:1 to 1.1:1.

Description

[0001] M ETHOD FOR COATING A PAPER SUBSTRATE

[0002] TECHNICAL FIELD

[0003] The present invention relates to a method for coating a paper substrate to provide said paper substrate with improved barrier properties.

[0004] STATE OF THE ART

[0005] In the paper and packaging industry, barrier coating compositions are used for improving barrier properties (such as water and / or oil resistance) of paper substrates. Traditionally, barrier coating compositions are based on film-forming polymers derived from fossil resources.

[0006] In the recent years there is a growing interest in replacing the synthetic polymers traditionally employed in the barrier coating compositions without adversely affecting the barrier properties of the coated paper substrate. This may be achieved by introducing compounds derived from renewable resources.

[0007] In this context, compounds extracted from plant matter and their derivatives may represent an attracting alternative.

[0008] Cutin is a support biopolyester involved in waterproofing the leaves and fruits of higher plants. Cutin is the main component (between 40% and 85 wt%) of the plant cuticle, the continuous and lipidic extracellular membrane that covers the aerial parts of leaves, fruits and non-lignified stems of plant.

[0009] From a chemical point of view, cutin is a polymeric network of polyhydroxylated C16and Cis fatty acids cross-linked by ester bonds. However, when extracted from plant matter (for example through the methods described in WC2O15 / O28299 or WO2016 / 187581), cutin undergoes depolymerization. Therefore, a raw cutin extract is mainly composed of hydroxy fatty acids and fatty esters, as well as their oligomers. G. Tedeschi et al., ACS Sustainable Chem. Eng., 2018, 6, 11, 14955-14966 describe a procedure for the fabrication of composite free-standing films inspired by plant cuticles, consisting of the blending of sodium alginate, unsaturated and polyhydroxylated fatty acids derived from the agro-waste of tomato peeling industrial process, beeswax in water and ethanol, and subsequent thermal treatment.

[0010] WO2023 / 187159 describes a method for coating a paper substrate by using a modified cutin extract, wherein said modified cutin extract is obtained by thermal treatment of a raw cutin extract at a temperature comprised between 40 and 120 °C. However, a need still exists in the art to further improve the barrier properties of a paper substrate, particularly water resistance, by using a modified cutin extract.

[0011] SUMMARY OF THE INVENTION

[0012] It is therefore an object of the present invention a method for coating a paper substrate comprising: i) providing a paper substrate; ii) applying to the paper substrate an aqueous barrier coating composition comprising, based on the total weight of composition: a) from 10 to 50 wt% (% by weight) of a modified cutin extract (as dry matter), wherein said modified cutin extract is obtained by thermal treatment of a raw cutin extract at a temperature comprised between 40 and 120 °C; b) at least one divalent or trivalent fatty acid metal salt, wherein the weight ratio, based on the dry content, of the modified cutin extract to the at least one divalent or trivalent fatty acid metal salt is from 10:1 to 1:1.

[0013] With the expression "modified cutin extract", we refer to the modification of a raw cutin extract through a thermal treatment. Without wishing to be bound by any theory, it is believed that the thermal treatment allows the regeneration of the cutin biopolyester polymeric network starting from the hydroxyl fatty acids that are the main components of the raw cutin extract.

[0014] DETAILED DESCRIPTION OF THE INVENTION

[0015] Preferably, the method for coating a paper substrate comprises: i) providing a paper substrate; ii) applying to the paper substrate an aqueous barrier coating composition comprising, based on the total weight of composition: a) from 10 to 50 wt% of a modified cutin extract (as dry matter), wherein said modified cutin extract is obtained by thermal treatment of a raw cutin extract at a temperature comprised between 60 and 100 °C, preferably between 80 and 100 °C; b) at least one divalent or trivalent fatty acid metal salt, wherein the weight ratio, based on the dry content, of the modified cutin extract to the at least one divalent or trivalent fatty acid metal salt is from 9:1 to 1.5:1.

[0016] The composition of the invention comprises, based on the total weight of composition, from 10 to 50 wt%, preferably from 20 to 40 wt% of a) a modified cutin extract (as dry matter), wherein said modified cutin extract is obtained by thermal treatment of a raw cutin extract at a temperature comprised between 40 and 120 °C, preferably between 60 and 100 °C, more preferably between 80 and 100 °C.

[0017] The raw cutin extract suitable for preparing the modified cutin extract of the present invention can be obtained by any extraction method known in the art for extracting cutin from plant matter.

[0018] As used herein, "plant matter" refers to any portion of a plant that contains cutin including, for example, fruits (in the botanical sense, fruit peels and juice sacs), leaves, stems, barks, seeds, flowers, or any other portion of the plant.

[0019] According to the invention, plant matter can include agricultural by-products such as, for example, tomato peels, grape skins, apple peels, pepper peels, lemon peels, lemon leaves, lime peels, lime leaves, orange peels, orange leaves, orange fruit, clementine leaves, clementine fruit, mandarin leaves, mandarin fruit, pea seeds, grapefruit peels, grapefruit leaves, grapefruit seeds, papaya peels, cherry fruits, cranberry skins, grass clippings.

[0020] The raw cutin extract of the present invention is obtained preferably from tomato, more preferably from waste tomato peels.

[0021] According to the invention, the raw cutin extract obtained from waste tomato peels is a complex mixture of long-chain co-hydroxy acids with typically a 16- or 18-carbon skeleton and their oligomers, whose main component is 10,16- dihydroxyhexadecanoic acid.

[0022] According to the invention, the raw cutin extract is water-based and typically has a dry content ranging from 30 to 95 wt%, preferably from 50 to 80 wt%. In addition, the raw cutin extract typically has a pH ranging from 3 to 6. The raw cutin extract has a Brookfield® viscosity (measured on a 31 wt% aqueous solution, at 25 °C and at pH 9) comprised between 10 and 500 mPa*s, preferably between 50 and 200 mPa*s.

[0023] The process for preparing the modified cutin extract of the present invention comprises a thermal treatment at a temperature comprised between 40 and 120 °C, preferably between 80°C and 100 °C.

[0024] Advantageously, the process for preparing the modified cutin extract of the present invention comprises a thermal treatment at a temperature comprised between 60°C. which is the approximate melting temperature of dry raw cutin extract, and 100°C.

[0025] As used herein, "thermal treatment" refers to the application of heat to a raw cutin extract. Heat may be applied by conduction (for example, a heating coil), by convection (for example, heat transfer through a fluid, such as water or air), and / or by radiation (for example, heat transfer using electromagnetic waves).

[0026] Solvents selected among water, organic solvents or mixtures thereof can be added in the process for preparing the modified cutin extract of the present invention. Preferably the solvent which can be added is water.

[0027] The solvent used in the process may be removed by distillation under reduced pressure.

[0028] The thermal treatment for preparing the modified cutin extract of the present invention is conducted at atmospheric pressure or under reduced pressure (typically between 0.001 to 0.6 bar). Preferably, the thermal treatment is conducted at atmospheric pressure.

[0029] The thermal treatment for preparing the modified cutin extract of the present invention may be carried out in the presence of a catalyst. Non-limiting examples of the catalysts that may be used include, for example, titanium tetraisopropoxide, dibutyltin oxide, tin octanoate, aluminum isopropoxide, zirconium acetylacetonate, zirconium tetra butoxide, tin(l l) n-octanoate, tin(ll) 2-ethylhexanoate, tin(ll) laurate, dibutyltin dichloride, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin dimaleate, dioctyltin diacetate, antimony triethoxide or boronic acid derivatives, such as pyridineboronic acid. Preferably, the catalyst is zirconium acetylacetonate.

[0030] The thermal treatment for preparing the modified cutin extract of the present invention can be carried out under acidic (i.e., at a pH ranging from 2 to 7, preferably from 3 to 7) or alkaline conditions (i.e., at a pH ranging from 7 to 12, preferably from 7 to 11). Preferably, the thermal treatment is carried out under acidic conditions.

[0031] When the thermal treatment is carried out under acidic conditions, the thermal treatment is started immediately, as the raw cutin extract typically already has an acidic pH, ranging from 3 to 6. After the end of the thermal treatment, the obtained modified cutin extract is treated with a base until a pH ranging between 7 and 11 is reached. The base is preferably ammonia, an inorganic base or an amine. More preferably, the base is ammonia.

[0032] When the thermal treatment is carried out under alkaline conditions, before starting the thermal treatment the raw cutin extract is treated with a base until a pH ranging between 7 and 11 is reached. The base is preferably ammonia, an inorganic base or an amine. More preferably, the base is ammonia.

[0033] The process for preparing the modified cutin extract of the present invention comprises a thermal treatment that may last from 2 to 72 hours. The process for preparing the modified cutin extract of the present invention comprises a thermal treatment that preferably lasts from 2 to 24 hours, more preferably from 3 to 15 hours.

[0034] The modified cutin extract of the present invention is water-based and typically has a dry content ranging from 10 to 60 wt%, preferably from 20 and 40 wt%, and pH comprised between 6 and 11, preferably between 7 and 10. The modified cutin extract typically has a Brookfield® viscosity (measured on a 31 wt% aqueous solution, at 25 °C and at pH 9) comprised between 100 and 3000 mPa*s, preferably between 300 and 1500 mPa*s.

[0035] The composition of the invention further comprises b) at least one divalent or trivalent fatty acid metal salt.

[0036] In the fatty acid metal salt, the metal is selected from, calcium, zinc, magnesium, beryllium, barium, strontium, aluminum, iron, gallium, or indium. Preferably the metal is selected from calcium, magnesium, or zinc.

[0037] In the fatty acid metal salt, the fatty acid is a linear or branched, saturated or unsaturated monocarboxylic acid having from 4 to 40 carbon atoms, preferably from 10 to 28 carbon atoms, more preferably from 14 to 20 carbon atoms. Suitable fatty acids include, but are not limited to, stearic acid, oleic acid, palmitic acid, linoleic acid, linolenic acid, arachidonic acid, icosapentaenoic acid, docosahexaenoic acid, montanic acid, myristic acid, or lauric acid. Preferably the fatty acid is stearic acid.

[0038] The divalent or trivalent fatty acid metal salt is preferably a divalent fatty acid metal salt. The divalent or trivalent fatty acid metal salt is preferably selected from calcium stearate, magnesium stearate, or zinc stearate. More preferably, the divalent or trivalent fatty acid metal salt is calcium stearate.

[0039] According to the invention, the fatty acid metal salt may be in the form of an aqueous dispersion or a powder.

[0040] According to one preferred embodiment, the fatty acid metal salt is mixed with modified cutin extract after that the preparation of the modified cutin extract has been already carried out. According to another embodiment, the fatty acid metal salt is added during the preparation of the modified cutin extract.

[0041] The weight ratio, based on the dry content, of the modified cutin extract to the at least one divalent or trivalent fatty acid metal salt is from 10:1 to 1.1:1, preferably from 9:1 to 1.5:1, more preferably from 9:1 to 2:1.

[0042] The aqueous barrier coating composition of the invention has a solids content from 10 to 60 wt%.

[0043] The aqueous barrier coating composition of the invention can further contain customary additives in the field of paper coating, such as pigments, binders, waxes, thickeners, antiblocking agents, dyes, flow control agents or defoamers.

[0044] Suitable pigments include, for example, metal salt pigments such as, for example, calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate and calcium carbonate. Calcium carbonate may be natural ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), lime or chalk. Further suitable pigments include, for example, silica, alumina, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earths, talc or silicon dioxide. Suitable binders include both synthetic and natural binders. Suitable synthetic binders include, but are not limited to, (meth)acrylic (co)polymers, ethylene- (meth)acrylic copolymers, styrene-(meth)acrylic copolymers and polyvinyl alcohol. Suitable natural binders include, but are not limited to, carboxymethyl cellulose, starch and starch derivatives.

[0045] Suitable starch includes natural starch, such as potato, wheat, maize, rice or tapioca starch.

[0046] Suitable starch derivatives include, but are not limited to, chemically modified starch, such as for example hydroxyethyl starch, hydroxypropyl starch or phosphate starch. Preferred starch and starch derivatives include degraded starch or degraded starch derivatives having a molecular weight Mn of 500 to 30,000 Da, more preferably from 500 to 20,000 Da. Said degraded starch and starch derivatives are obtained from the degradation of natural starch or chemically modified starch. Said degradation may be oxidative, thermal, acidic or enzymatic degradation.

[0047] Suitable waxes include, but are not limited to, polyethylene, polypropylene, paraffins, amides, waxes of vegetable of origin, carnauba waxes, or mixtures thereof.

[0048] The aqueous barrier coating composition of the invention are usually applied to the paper substrate at a coat weight from 0.5 to 40 g / m2, preferably from 1 to 25 g / m2.

[0049] Suitable paper substrates typically have a grammage from 25 to 800 g / m2, preferably from 30 to 700 g / m2, more preferably from 40 to 500 g / m2.

[0050] The application of the aqueous barrier coating composition of the invention to the paper substrate can be carried out for example by roller coating, spray coating, curtain coating, blade coating, immersion coating, gravure roll coating, reverse direct gravure coating, rod coating, soft-tip blade coating, jet coating and / or combinations thereof.

[0051] The present invention is further illustrated by the following examples.

[0052] EXAMPLES

[0053] The raw cutin extract used in the examples is obtained from waste tomato peels and has a Brookfield® viscosity (measured on a 31 wt% aqueous solution, at 25 °C and at pH 9) of 100 mPa*s.

[0054] The modified cutin extract used in the examples was prepared according to Example 2 of WO2023 / 187159: the obtained modified cutin extract has a dry content of 32 wt%.

[0055] A 50 wt% aqueous dispersion of calcium stearate was used.

[0056] Various aqueous barrier coating compositions (Examples 1-10) were prepared by simply mixing each component under stirring. Each composition has a solids content of 32 wt%. For each composition, the amount of each component, based on the dry content, is reported as % by weight in Table 1.

[0057] Table 1

[0058] *Comparative

[0059] Applicative tests

[0060] The obtained aqueous barrier coating compositions (Examples 1-10) were applied at a coat weight of 5.0 g / m2on a paper substrate having a grammage of 80 g / m2.

[0061] Water resistance and oil resistance of the coated paper substrates were evaluated.

[0062] Liquid water resistance was tested using the Cobb method, as described by TAPPI Method T 441-om. The test time was 600 s. This method determines the amount of liquid water absorbed by paper or paperboard in a specific time under standardized conditions. Oil absorption capacity was tested using the KIT method, according to TAPPI Test Method UM 557. In this test numbered (from 1 to 16) solutions of increasing hydrophobicity are applied onto the paper substrate. The highest numbered solution that does not stain the surface is reported as result of the KIT test. Table 2 reports the results of the Cobb test and KIT test for aqueous barrier coating compositions previously prepared (Examples 1-10).

[0063] Table 2

[0064] *Comparative nd = coating film was completely removed, when the coated paper substrate was contacted with water

[0065] The results reported in Table 2 show that the compositions of the invention (Examples

[0066] 5-8) improved both water and oil resistance properties of the paper substrate, when compared with a raw cutin extract (Comparative Example 1), calcium stearate (Comparative Example 2), or a mixture of a raw cutin extract with calcium stearate

[0067] (Comparative Example 3).

[0068] When compared with a modified cutin extract (Comparative Example 4), the compositions of the invention (Examples 5-8) showed a significant improvement in terms of water resistance, while maintaining the same level of oil resistance.

[0069] When the amount of calcium stearate is too high (Comparative Examples 9 and 10), both water and oil resistance of the paper substrate are adversely affected.

Claims

CLAIMS1. A method for coating a paper substrate comprising: i) providing a paper substrate; ii) applying to the paper substrate an aqueous barrier coating composition comprising, based on the total weight of composition: a) from 10 to 50 wt% of a modified cutin extract (as dry matter), wherein said modified cutin extract is obtained by thermal treatment of a raw cutin extract at a temperature comprised between 40 and 120 °C; b) at least one divalent or trivalent fatty acid metal salt, wherein the weight ratio, based on the dry content, of the modified cutin extract to the at least one divalent or trivalent fatty acid metal salt is from 10:1 to 1.1:1.

2. The method for coating a paper substrate according to claim 1, wherein the weight ratio, based on the dry content, of the modified cutin extract to the at least one divalent or trivalent fatty acid metal salt is from 9:1 to 1.5:1.

3. The method for coating a paper substrate according to claim 1, wherein the weight ratio, based on the dry content, of the modified cutin extract to the at least one divalent or trivalent fatty acid metal salt is from 9:1 to 2:1.

4. The method for coating a paper substrate according to claim 1, wherein the modified cutin extract is obtained by thermal treatment of a raw cutin extract at a temperature comprised between 60 and 100 °C.

5. The method for coating a paper substrate according to claim 1, wherein the at least one divalent or trivalent fatty acid metal salt is a divalent fatty acid metal salt.

6. The method for coating a paper substrate according to claim 1 or 2, wherein the at least one divalent or trivalent fatty acid metal salt is selected from calcium stearate, magnesium stearate, or zinc stearate.

7. The method for coating a paper substrate according to claim 1, wherein the at least one divalent or trivalent fatty acid metal salt is calcium stearate.

8. The method for coating a paper substrate according to claim 1, wherein the aqueous barrier coating composition comprises a) from 20 to 40 wt% of a modified cutin extract (as dry matter).

9. The method for coating a paper substrate according to claim 1, wherein the aqueous barrier coating composition has a solids content from 10 to 60 wt%.

10. The method for coating a paper substrate according to Claim 1, wherein the aqueous barrier coating composition further comprises pigments, binders, waxes, thickeners, antiblocking agents, dyes, flow control agents or defoamers.

11. The method for coating a paper substrate according to claim 1, wherein the fatty acid is the fatty acid is a linear or branched, saturated or unsaturated monocarboxylic acid having from 14 to 20 carbon atoms.

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