Coating composition, method for preparing and use thereof

Abstract

The disclosure relates to a coating composition comprising lactide-grafted cellulose, to a method for producing a coating composition comprising lactide-grafted cellulose and to the use thereof. More particularly the disclosure relates to providing a coating composition comprising lactide-grafted cellulose, wherein the amount of polylactide is between 40 wt-% and 90 wt-% of the dry weight of the lactide-grafted cellulose and the lactide-grafted cellulose has a degree of polymerization over 3 and a degree of substitution between 0.5 and 3. The method for producing the coating composition comprises grafting cellulose with polylactide by ring-opening graft polymerization using a catalyst.

Description

[0001] COATING COMPOSITION, METHOD FOR PREPARING AND USE THEREOF

[0002] FIELD OF THE DISCLOSURE

[0003] The present disclosure relates to a coating composition comprising lactide-grafted cellulose, to a method for producing such a coating composition comprising lactide- grafted cellulose and to the use thereof. More particularly the disclosure relates to providing a coating composition comprising lactide-grafted cellulose, wherein the amount of polylactide is between 40 wt-% and 90 wt-% of the dry weight of the lactide-grafted cellulose and the lactide-grafted cellulose has a degree of polymerization over 3 and a degree of substitution between 0.5 and 3. The method for producing the coating composition comprises grafting cellulose with polylactide by ring-opening graft polymerization using a catalyst.

[0004] BACKGROUND OF THE DISCLOSURE

[0005] Coating technology is a fast-growing technology area, where the surface of a solid material is improved or restored by using a covering that is applied to the surface of an object, usually referred to as a substrate. Using a thin layer of substance on the surface of the substrate can resist contact forces and provide protection against material wear, or can, for example in packings, provide protection against, for example, water, water vapour, grease, oil, and gases such as oxygen. Coatings can also be used as sealants. Numerous materials, including metal, ceramic, polymers, and composite coatings, are rigorously used to tune the properties of different substrates.

[0006] Polymeric coating materials are essential part of contemporary fiber products. Coatings that are currently being employed for cellulosic products are mainly based on fossil feedstocks and generally do not biodegrade in a reasonable timescale. For example, polyethylene, which is the most common coating material for cellulosic products, is fossil and non-biodegradable. Thereto biodegradable polymers currently on the market are often expensive and lack required functionality. The bio-based alternatives for example often have limited composting abilities and are classified as plastic products.

[0007] Although the interest in replacement of for example fossil-based plastic materials has risen spectacularly, there is still a pressing need to replace fossil materials in paper coatings and coatings in general with renewable alternatives.

[0008] BRIEF DESCRIPTION OF THE DISCLOSURE

[0009] An object of the present disclosure is to provide a coating composition with improved properties, especially what comes to, biodegradability, thermal processability, water vapor transmission and / or O2 transmission, so as to solve the urgent need to find alternative materials to make renewable thermoplastic materials and water-insoluble polymers for coating applications.

[0010] The object of the disclosure is achieved by providing a coating composition and a method for preparing a coating composition comprising a lactide-grafted cellulose and use thereof for coating, which are characterized by what is stated in the independent claims. The preferred embodiments of the disclosure are disclosed in the dependent claims.

[0011] The disclosure is based on the idea of providing lactide-grafted cellulose with thermoplastic behaviour, where cellulose is grafted with polylactide. It has surprisingly been noticed that by varying the length of the grafts, i.e. the amount of polylactide of the dry weight of the lactide-grafted cellulose, coatings with improved properties as well as glass transition temperatures (Tg) in the 30-100 °C range or even below 30 °C can be achieved for the grafted materials according to the disclosure.

[0012] An advantage of the method and coating composition of the disclosure and use thereof is that faster biodegradation, especially compared to the corresponding polylactide homopolymer is achieved. Especially disposable objects are of interest as the coating composition of the disclosure degrades without contributing to the microplastic problem.

[0013] Further advantages are achieved when the coating composition is produced from renewable resources. Preferably the coating composition comprises only non-fossil additives and also the polylactide is from renewable resources.

[0014] When organic catalysts are used instead of metal-based catalyst, residual metals of the catalysts are avoided and the coating composition is more suitable for example for biomedical and food applications, such as food packing.

[0015] The coating composition of the disclosure is typically essentially non-toxic and typically free of microplastics.

[0016] Moreover, since glass transition temperatures (Tg) in the 30 - 100 °C range or even below 30 °C can be achieved for the grafted materials of the disclosure, the processability is easier than for traditional cellulose esters, which typically have glass transition temperatures over 100 °C. Due to the improved processability, coating is easier with the coating composition of the disclosure. The glass transition temperatures of the lactide-grafted cellulose of the disclosure are preferably below 100 °C, typically between 40 °C and 70 °C. Further advantages are improved water vapor and gas barrier properties of coated substrates, such as coated paper or coated paperboard.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the following the disclosure will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

[0019] Figure 1 A - C shows DP, DS and Tgfor three different celluloses grafted with PLA and the different amounts of PLA in the material (w-%);

[0020] Figure 2 shows the weight average molecular weight (g / mol) for three different celluloses grafted with PLA and the different amounts of PLA in the material (w-%);

[0021] Figure 3 shows the oxygen transmission rate (OTR) expressed as cm3 / m2d for different coating thicknesses (pm) of commercial poly(lactic acid) (PLA) reference and lactide- grafted cellulose of the disclosure;

[0022] Figure 4 shows water vapor transmission rate expressed as g / m2d for different coating thicknesses (pm) of commercial poly(lactic acid) (PLA) reference and lactide-grafted cellulose of the disclosure;

[0023] Figure 5 shows water absorption expressed as g / m2for different coating thicknesses (pm) of commercial poly(lactic acid) (PLA) reference and lactide-grafted cellulose of the disclosure;

[0024] Figure 6 shows biodegradability of three different celluloses, prepolymers of the disclosure and a commercial poly(lactic acid) (PLA) reference; and

[0025] Figure 7 shows cold seal quality after heat sealing of a commercial poly(lactic acid) (PLA) reference and lactide-grafted cellulose of the disclosure.

[0026] DETAILED DESCRIPTION OF THE DISCLOSURE

[0027] The disclosure relates to a coating composition wherein the coating composition comprises 50 - 100 wt-% lactide-grafted cellulose of the total dry weight of the coating composition. The lactide-grafted cellulose of the coating composition comprises between 40 wt-% and 90 wt-% polylactide of the dry weight of the lactide-grafted cellulose, and the lactide-grafted cellulose has a degree of polymerization (DP) over 3 and a degree of substitution (DS) between 0.5 and 3.

[0028] The disclosure also relates to a method for preparing a coating composition, characterized in that the method comprises a first step of grafting cellulose with polylactide (PLA) by ring-opening graft polymerization using a catalyst, wherein a prepolymer having a DP below 3 is obtained, a second step of grafting the prepolymer with polylactide (PLA) by ring-opening graft polymerization using a catalyst, obtaining lactide-grafted cellulose, wherein the amount of polylactide is between 40 wt-% and 90 wt-% of the dry weight of the lactide-grafted cellulose, and the method further comprises providing a coating composition comprising 50 - 100 wt-% of the lactide-grafted cellulose.

[0029] In embodiments of the disclosure the prepolymer obtained in the first step of the method is used as a macroinitiator in the second step of the method. Typically, the prepolymer comprises polylactide grafts with 2 - 3 repeating units and has a DP below 3. The lactide- grafted cellulose obtained in the second step typically comprises polylactide grafts with over 10 repeating units and has a degree of polymerization (DP) over 3 and a degree of substitution (DS) between 0.5 and 3.

[0030] The disclosure further relates to a coating obtained by using the coating composition of the disclosure and to the use of the coating composition of the disclosure for coating of a substrate. Typically, the substrate is used for packaging purposes and / or the substrate is in sheet, film, or web format, preferably chosen from any of paper, paperboard, plastic films, biopolymer films such as cellulose films or cellulose-based films, or combinations thereof.

[0031] The term “prepolymer” as used herein refers to the macroinitiator, with a DP below 3, which is obtained after an initial grafting with polylactide by ring-opening graft polymerization using a catalyst, i.e. by functionalization of cellulose with lactide. The prepolymer is used as a macroinitiator to synthesize lactide-grafted cellulose with longer polylactide grafts and a DP over 3. Generally, a prepolymer is a substance which represents an intermediate stage in polymerization.

[0032] The term “homogenous solvent system” as used herein refers to a homogenous ringopening graft polymerization where the grafting is carried out homogenously, i.e. essentially all the cellulose is dissolved and there is essentially only one phase.

[0033] The term “lactide-grafted cellulose” as used herein refers to the product obtained after chain extensions with lactide by ring-opening graft polymerization using a catalyst, where the prepolymer obtained in an initial grafting is used as a macroinitiator to synthesize lactide-grafted cellulose with longer polylactide grafts and a DP over 3 and a DS between 0.5 and 3.

[0034] The term “degree of polymerization” (DP) is the number of lactide units in one chain grafted from the cellulose backbone. The term “degree of substitution” (DS) is the average number of hydroxyl groups per repeating unit that were replaced by a given substituent, which is defined by the amount of hydroxyl groups in the repeating unit that can be chemically modified.

[0035] Polylactide (PLA) is a thermoplastic polyester. It is also known as polylactic acid or poly(lactic acid) The monomer is typically made from fermented plant starch such as from corn, cassava, sugarcane or sugar beet pulp. Several industrial routes afford usable (i.e. high molecular weight) PLA. Two main monomers are used: lactic acid, and the cyclic di-ester, lactide. The most common route to PLA is the ring-opening polymerization of lactide with various metal catalysts (typically tin octoate) in solution or as a suspension. The metal-catalysed reaction tends to cause racemization of the PLA, reducing its stereoregularity compared to the starting material (usually corn starch). Due to the chiral nature of lactic acid, several distinct forms of polylactide exist. Poly-L-lactide (PLLA) is the product resulting from polymerization of L,L-lactide (also known as L- lactide) and progress in biotechnology has resulted in the development of commercial production of the D enantiomer form. Polymerization of a racemic mixture of L- and D- lactides usually leads to the synthesis of poly-DL-lactide (PDLLA), which is amorphous. Use of stereospecific catalysts can lead to heterotactic PLA which has been found to show crystallinity. The degree of crystallinity, and hence many important properties, is largely controlled by the ratio of D to L enantiomers used, and to a lesser extent on the type of catalyst used. During biodegradation, the PLA is hydrolysed via random non- enzymatic hydrolysis of the ester groups in the polymer backbone into oligomers and lactic acid, which are then metabolized by micro-organisms. Hence the biodegradation of PLA is often a slow process.

[0036] Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands of 0(1 — >4) linked D-glucose units. Due to the hydroxyl groups in cellulose, it forms a fibrillated network that gives cellulose its robust mechanical properties, but also makes it challenging as the intra- and intermolecular hydrogen bonding prevents the dissolution of cellulose in common organic solvents and water. Several types of cellulose are known, and these forms are distinguished according to the location of hydrogen bonds between and within strands. Natural cellulose is cellulose I, with structures la and I0. Cellulose produced by bacteria and algae is enriched in la while cellulose of higher plants consists mainly of I0. Cellulose in regenerated cellulose fibres is cellulose IL With various chemical treatments it is possible to produce the structures cellulose III and cellulose IV. Many properties of cellulose depend on its chain length or degree of polymerization, the number of glucose units that make up one polymer molecule. Cellulose from wood pulp has typical chain lengths between 300 and 1700 units; cotton and other plant fibres as well as bacterial cellulose have chain lengths ranging from 800 to 10,000 units. Plant-derived cellulose is usually found in a mixture with hemicellulose, lignin, pectin and other substances, while bacterial cellulose is quite pure, has a much higher water content and higher tensile strength due to higher chain lengths. In the embodiments of the disclosure the weight average molecular weight Mwof the cellulose is essentially below 1000 kDa (1 000 kg / mol), preferably below 500 kDa, typically between 60 and 500 kDa.

[0037] In embodiments of the disclosure the ring-opening graft polymerization catalyst is typically; an organic nucleophilic catalysts, preferably chosen from one of 4- dimethylaminopyridine (DMAP) and N-heterocyclic carbenes (NHCs); an organic base, preferably chosen from 8-diazabicyclo[5.4.0]undec-7-ene (DBll), 1 ,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2- bis[tris(dimethylamino)phosphoranyl-idenamino]-2A5,4A5-catenadi(phosphazene) (t- BUP4); an acid catalyst, preferably trifluoromethane-sulfonic acid (TfOH); a mono- and multinuclear metal complex containing both main group metals (e.g. Al, Mg, Ca) and transition metals (e.g. Zn, Ti, Co) as well as heterometallic complexes; an organic metal salt, preferably tin(ll) octanoate and aluminum isopropoxide; or an enzymatic catalyst. Typically, the ring-opening graft polymerization catalyst is an organic base, preferably chosen from 8-diazabicyclo[5.4.0]undec-7-ene (DBll), 1 ,5,7-triazabicyclo[4.4.0]dec-5- ene (TBD) and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino) phosphoranyl-idenamino]-2A5,4A5-catenadi(phosphazene) (t-BuP4). Organic catalysts are preferred compared to more traditional metal-based catalysts due to their suitability for a range of reaction conditions, solvents, and monomers. Furthermore, organic catalysts are typically easy to remove from the resultant polymers Typically, the molar ratio of catalyst:polylactide is between 1 :100 to 1 :5, preferably between 1 :60 to 1 :5. More preferably the ring-opening graft polymerization catalyst is an organic base and the molar ratio of catalyst:polylactide is between 1 :100 and 1 :20, preferably between 1 :60 and 1 :20, more preferably the ring-opening graft polymerization catalyst is chosen from 8- diazabicyclo[5.4.0]undec-7-ene (DBll), 1 ,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 1- tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranyl-idenamino]- 2A5,4A5-catenadi(phosphazene) (t-BuP4) and the molar ratio of catalyst:polylactide is between 1 :60 and 1 :20. In the method of the disclosure the catalyst used in the first and second step can be different or the same, preferably the catalyst in the first step is the same as the catalyst in the second step. The ring-opening graft polymerization is typically done in a homogenous solvent system. In embodiments of the disclosure the coating composition of the disclosure typically comprises 50 - 100 wt-% lactide-grafted cellulose, preferably more than 60 wt-% lactide- grafted cellulose, more preferably more than 90 wt-% lactide-grafted cellulose and most preferably at least 95 wt-% lactide-grafted cellulose of the total dry weight of the coating composition, the rest being additives. The additives are preferably chosen from one or more of any of plasticizers, dispersants, surfactants, antistatic agents, antioxidants, foaming or antifoaming agents, waxes, fillers, pigments, neutralizing agents, thickeners, compatibilizers, brighteners, rheology modifiers, preservatives, biocides, crosslinking agents, LIV stabilizers, coefficient of friction modifiers, release agents, flow additives, pH adjusting agent and / or combinations thereof. Typically, all additives are non-toxic. The additive(s) are typically added in an amount of between 0.5 and 50 wt-% of the total dry weight of the composition, preferably the amount of additive(s) is between 1 and 50 wt- %, more preferably the amount of additive(s) is between 5 and 50 wt-% and most preferably the amount of additive(s) is between 2 and 10 wt-% of the total dry weight of the composition, including the amount of additives being between two of the following values; 0 wt-%, 0.5 wt-%, 1 wt-%, 2 wt-%, 3 wt-%, 4 wt-%, 5 wt-%, 6 wt-%, 7 wt-%, 8 wt- %, 9 wt-%, 10 wt-%, 12 wt-%, 15 wt-%, 20 wt-%, 25 wt-%, 30 wt-%, 35 wt-%, 40 wt-%, 45 wt-% and 50 wt-% of the total dry weight of the coating composition. Preferably, all additives are non-fossil.

[0038] According to an embodiment of the disclosure the coating composition consists of lactide-grafted cellulose of the disclosure and one or more additives, preferably chosen from one or more of any of plasticizers, dispersants, surfactants, antistatic agents, antioxidants, foaming or antifoaming agents, waxes, fillers, pigments, neutralizing agents, thickeners, compatibilizers, brighteners, rheology modifiers, preservatives, biocides, crosslinking agents, LIV stabilizers, coefficient of friction modifiers, release agents, flow additives, pH adjusting agent and / or combinations thereof.

[0039] In embodiments of the disclosure the coating is typically applied by any printing or coating method, including but not limited to spraying, brush coating, blade or rod coating, roll application using single or multiple rolls, jet or fountain coating, gravure coating, gravure printing, flexographic printing, ink-jet printing, dipping, die or curtain coating, short dwell coating, air knife coating, cast coating, size press application, coating during the calendering process, foam coating, extrusion coating, powder coating and combinations thereof. Typically, the difference between printing methods and coatings methods are that in printing, the application roller is in contact with the substrate, whereas during coating procedures the process can be contactless or with contact. Typically, the thickness of the coating is from 3 to 30 pm, preferably from 5 to 20 pm.

[0040] In embodiments of the disclosure the substrate is typically material in sheet, film or web format preferably chosen from any of paper, paperboard, plastic films, biopolymer films such as cellulose films or cellulose-based films combinations thereof and / or material in container format or any intermediate products prior to processing into container format, preferably chosen from articles made of fibers and cellulose having a three-dimensional structure, more preferably chosen from any of boxes, plates, bowls, cups, bottles, cans, lids, trays or intermediate products thereof. Paper and paperboard typically refer to different cellulosic materials, such as printing paper, packaging and industrial papers, greaseproof paper, glassine, parchment paper, linerboard, corrugating medium, recycled paperboard, bleached paperboard, writing paper, etc. Cellulose films are made from cellulose. For example, cellophane is a polymeric cellulose film made from the cellulose from wood, cotton, hemp, or other sources. Naturally obtained and synthetic biopolymers possess advantages by avoiding use of fossil carbon.

[0041] In embodiments of the disclosure the coating composition of the disclosure is biodegradable compared to a coating composition comprising polylactide homopolymer. The biodegradability can be assessed by measuring the conversion of the materials to CO2 in soil.

[0042] In embodiments of the disclosure the composition of the disclosure is typically used as adhesives, sealants, primers, inks, overprint varnishes, release coatings, sizing agents, and / or barrier coatings. The disclosure also relates to an adhesive, sealant, primer, ink, overprint varnish, release coating, sizing agent, and / or barrier coating comprising a coating composition comprising a lactide-grafted cellulose, wherein the amount of polylactide is between 40 wt-% and 90 wt-%, preferably between 45 wt-% and 90 wt-%, more preferably between 55 wt-% and 90 wt-% of the dry weight of the lactide-grafted cellulose. The disclosure also relates to the use of the coating composition of the disclosure, comprising lactide-grafted cellulose, as a barrier, for example for water vapor and / or O2.

[0043] EXAMPLES

[0044] Analysis methods used in the examples

[0045] Size exclusion chromatography was used to determine the molecular weights of the cellulose precursors and derivatives. The samples were dissolved in DMAc / 8% LiCI by ethyl isocyanate derivatization assisted dissolution and filtered (0.45 pm) before measurement. The eluograms were obtained using two PLgel MiniMixed GPC columns, a precolumn, and a Waters 2414 Refractive index detector with DMAc / 0.8 % LiCI as eluent (0.36 mL / min, T = 80 °C). The molecular weight distributions were calculated against pullulan standards (6 100 - 708 000 g / mol) using Waters Empower 3 software.

[0046] A Mettler Toledo Differential Scanning Calorimeter, Model DSC820 System STARe SW 12.00 was used to determine glass transition temperatures (Tg, midpoint of the transition), melting temperatures (Tm), and crystallization temperatures (Tc), using 40 pl sealed aluminum crucibles under nitrogen flow (50 ml / min). 5-10 mg samples were enclosed in the crucibles by cold pressing and the lids were punctured to release pressure during heating. The following temperature profile was used: (1) isothermal phase (2 min) at 0°C, (2) heating from 0 °C to 240 °C, (3) isothermal phase (2 min) at 240 °C, (4) cooling from 240 °C to -50 °C, (5) isothermal phase (2 min) at -50 °C, (6) heating from -50 °C to 240 °C, (7) isothermal phase (2 min) at 240 °C, (8) cooling from 240 °C to 25 °C. Selected samples were re-examined after keeping the samples at 25 °C for one week to detect possible crystallization in the samples.

[0047] Water vapor transmission rate (WVTR) expressed as g / m2d was measured according to modified standard ASTM F1249-13 “Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor”. Measurements were carried out with MOCON PERMATRAN-W 3 / 34 G WVTR Analyzer. Circular pieces with a diameter of 7 cm were cut and masked on both sides with an adhesive back Al foil. Test area was 5 cm2. Thickness was measured from the center of each masked sample. Temperature was 23 °C and relative humidity of the test gas 85%. Coated side faced the test gas. Test mode was convergence by cycle with 6 cycles and 1%. Re-zeroing was carried out after each 6 individual test cycles.

[0048] Oxygen transmission rate (OTR) expressed as cm3 / m2d was measured according to modified standard ASTM D3985-17 “Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor”. Measurements were carried out with SYSTECH 8101 OxySense® Oxygen Transmission Rate Analyzer. Circular pieces with a diameter of 7 cm were cut and masked on both sides with an adhesive back Al foil. Test area was 5 cm2. Thickness was measured from the center of each masked sample. Temperature was 23 °C and both carrier and test gas (100% O2) had relative humidity of 0%. Test was stopped automatically when the last 8 measurements were within 5%. Coated side faced the carrier gas. Re-zeroing was carried out after each 6 individual test cycles.

[0049] Water absorption expressed as g / m2for a specific time was measured according to modified standards ISO 535:2014 Paper and Board - Determination of Water Absorptiveness - Cobb Method and TAPPI T 441 Cobb Test. Samples providing a test area of 25 cm2were cut. Thickness of each sample was measured from five equally separated locations from the sample. Testing was carried out with a Cobb Sizing Tester (Gurley, Model 4180-B Genuine). Test time was 30 min.

[0050] Heat sealing performance was assessed by assessing cold seal quality with a Kopp hot bar sealing device (Labormaster HCT 3000). Both upper and lower sealing bars were heated to the same temperature. Sealing force (250 N) and sealing time (2 seconds) were kept constant. Sample was cut into 2 cm wide strips. Heat sealing was carried out coating against coating with two sets of strips for each temperature setting. Sealed strips were allowed to cool and then separated manually. Quality of the seals was evaluated visually using a 5-step ranking from 1 to 5 (no seal, weak seal with sound, < 50% sealed area damaged, > 50% sealed area damaged, and complete damage in board), respectively. Complete damage in board indicates excellent cold seal performance.

[0051] The biodegradability of materials with 400 pm particle size was evaluated in soil conditions using the ISO 17665 (2012). Soil was obtained from an agricultural field located in Helsinki, Finland. The soil was sieved to a 2 mm particle size, and the soil moisture content was adjusted to 76%. Organic matter content of the soil was 9 % (dry weight), pH 6.4, and the C / N ratio 19. For each replicate, 500 mg of material was mixed with 75 g of soil (dry weight) in a 1000 mL glass bottle sealed with septum and screw cap with a hole. Three replicate bottles were prepared for each tested material, for blank containing only soil and for microcrystalline cellulose with particle size 20 pm (Sigma- Aldrich / Merck KGaA, Darmstadt, Germany) that was used as a reference compound. The carbon content of the materials was evaluated using a Flash 2000 EA CHNS-0 (Thermo Fisher Scientific Oy, Espoo, Finland) elemental analyzer. Same analysis was also used to measure the nitrogen and carbon content of the soil.

[0052] As an exception to the standard, CO2 concentrations in the head space volumes of the bottles were measured at regular intervals by inserting a needle through the septum of the bottles and directly measuring the CO2 concentration using a Servoflex MiniFoodPack 5200 infrared analyzer (Servomex, Crowborough, UK). Bottles were then aerated to remove excess CO2 before continuing the experiment. Net CO2 production from the test materials was calculated by subtracting the average amount of CO2 produced in the blank soils (no sample) from the amount of CO2 produced in the test material bottles. The biodegradation percentages were calculated from the ratio between the net CO2 production and the theoretical CO2 production calculated based on the elemental carbon content of the material. The degree of substitution (DS, equation 1), the degree of polymerization (DP, equation 2), molar substitution (MS, equation 3), and the percentage of PLA weight in the material (w-%, equation 4) for celluloses grafted with PLA were determined from1H-NMR spectra. pg > terminal lactyl units > b> anhydroglucose units 3A — (b+b')

[0053] Example 1 Preparing polylactide-qrafted cellulose

[0054] The polymerizations of the PLA grafts were performed in two steps. First short grafts were polymerized in homogenous solution of cellulose in DMAc / LiCI at 80 °C, resulting in cellulose esters with on an average one lactide unit per anhydroglucose unit (AGU). Even a low degree of substitution influences the solubility of the cellulose, resulting in cellulose esters that were soluble in DMSO. Increasing the amount of LA to the number of AGU repeating units during the polymerization in DMAc / LiCI was not observed to increase neither the DP nor DS.

[0055] In the second step, the polymerization was conducted in DMSO and the monomer to hydroxyl group (LA / OH) ratio was used to control DP (see Table 1 ). LA / AGU ratios of 7 / 1 and 12 / 1 resulted in the PLA chains growing from the existing lactide functionalities, as the DS was unaffected in the materials after the polymerization. Increasing the LA / AGU ratio to 20 / 1 , increased both the DP and DS, resulting in materials consisting of 87 - 90 wt-% PLA.

[0056] Materials

[0057] Acetone (VWR, 99 %), chromium(lll)acetylacetate (CrAA, 97%, Aldrich), 1 ,8- diazabicyclo[5.4.0]undec-7-ene (DBU, 98%, Millipore), N,N-dimethylacetamide (DMAC, 99 %, Acros Organics), dimethylsulfoxide (DMSO, VWR, 99.8 %), ethanol (94 %, Altia), L-lactide (99%, NatureWorks), lithium chloride (99%, Acros Organics), polylactide (Ingeo 4060D, NatureWorks, MW -190 000 g / mol?), and toluene (VWR, 99 %) were used as received. Commercial softwood sulphite pulp was obtained from Domsjd Fabriker AB (Sweden) and used as received after freeze drying. Hydrolysis of the pulp was conducted as described by Willberg-Keyrilainen et al (“The Effect of Cellulose Molar Mass on the Properties of Palmitate Esters.”, Carbohydr Polym 2016, 151 , 988-995).

[0058] Functionalization of cellulose with L-lactide

[0059] Cellulose (10.0 g, 61.7 mmol repeat units) was dispersed in DMAC (340 mL) and heated to 130 °C. After three hours of stirring the dispersion was cooled to 90 °C and LiCI (31.9 g, 0,8 mol) was added. The dispersion was left to cool to room temperature overnight under stirring, during which the cellulose dissolved. The solution was then heated to 80°C and L-lactide (44.5 g, 0.31 mol) was added. After the L-lactide has dissolved, DBU (9.4 g, 61.6 mmol) was added to the solution. After three hours, the product was precipitated in water, filtered, washed with water, and dried in vacuo at 40 °C. The crude product was then dissolved in DMSO and precipitated in toluene, filtered, washed with toluene, and dried in vacuo at 40 °C. 20.0 g of product was obtained. Celluloses with different molecular weights were used in the grafting process; commercial sulphite pulp named C (Mw = 413 kg / mol, C), and two hydrolysed pulps named HC1 and HC2 (Mw = 127, HC1 and 68 kg / mol, HC2). Degrees of substitution and polymerization was determined by 1 H-NMR.

[0060] The results are shown in Table 1 (C-PLA-1 , HC1-PLA-1 and HC2-PLA-1) and in Figures 1 and 2 (~30 w-% LA (%)).

[0061] Chain extensions with L-lactide

[0062] Cellulose functionalized with L-lactide (2.0 g, 8.9 mmol repeat units) was dissolved in DMSO (60 mL). L-lactide (25.6 g, 0.18 mol) and DBU (1.4 g, 8.9 mmol) was added, and the solution was heated to 80°C. After three hours, the product was precipitated in ethanol, filtered, washed with ethanol, and dried in vacuo. The crude product was dissolved in acetone and precipitated once more in ethanol, filtered, washed with ethanol, and dried in vacuo at 60 °C. 11.0 g of product was obtained. The degrees of substitution and polymerization were determined from either 1 H or 13C-NMR spectra.

[0063] The results are shown in Table 1 (C-PLA-2, C-PLA-3 and C-PLA-4, HC1-PLA-2, HC1- PLA-3 and HC1-PLA-4 and HC2-PLA-2, HC2-PLA-3 and HC2-PLA-4) and in Figures 1 and 2 (~45 w-% LA (%), ~55 w-% LA (%) and ~90 w-% LA (%)). Table 1 a1.8-diazabicyclo(5.4.0)undec-7-ene (DBU) as catalyst.bDetermined from 1 H-NMR in DMSO-d6 + 5% D2O.cThe average number of LA per AGU (=DPxDS)dDetermined from MS

[0064] As can be seen from the results, the molecular weight of the cellulose starting material had no effect on the functionalization with LA. The differences observed were mostly in the viscosity of the cellulose solutions and dissolution behaviour of the materials. The materials made from the hydrolysed celluloses resulted in less viscous solutions and dissolved faster in DMSO than the commercial sulphite pulp. During the hydrolysis of cellulose with ozone, Willberg-Keyrilainen et al. observed a decrease in the viscosity from 490 ml / g to 110 ml / g with the molecular weight decreasing from 520 kg / mol to 58 kg / mol.

[0065] The materials are mostly amorphous with glass transition temperature (Tg) range of 30- 100 °C. The Tgcan be tuned by the choice of monomer and length of the grafts. A further larger scale lactide-grafted cellulose was produced according to the process described above. The features of the lactide-grafted cellulose after functionalization and chain extension are shown in Table 2.

[0066] Table 2

[0067] Example 2

[0068] Commercial poly(lactic acid) (PLA) reference (Ingeo 4060D, NatureWorks, MW -190 000 g / mol?) and lactide-grafted cellulose of Table 2 (HC1-PLA-6), were dissolved in acetone (concentration 15% and 23%, respectively, -200 mL).

[0069] The coating was applied onto the pigment coated top side of a folding boxboard from MetsaBoard (Prime FBB Bright 210 g / m2).

[0070] The coating was carried out with an ERICHSEN laboratory sheet coater at a speed of 50 mm / s. Grooved metering rods and metering gaps were used to adjust the coat weight.

[0071] Coated samples (with a coating thickness between 5 and 20 pm) were first dried in an air-circulating oven for 5 min with a temperature setting of 45 °C and then allowed to dry at room temperature at least for 24 h.

[0072] The oxygen transmission rate (OTR) expressed as cm3 / m2d, water vapor transmission rate expressed as g / m2d and water absorption expressed as g / m2were measured as described above.

[0073] The results are shown in Figure 3, Figure 4 and Figure 5. As can be seen from the results the lactide-grafted cellulose is a better barrier for both water vapor and O2 than commercial PLA.

[0074] The biodegradability was measured as described above. The biodegradability of samples C-PLA-1 and HC2-PLA-1 was investigated by measuring the conversion of the materials to CO2 in soil, to get an understanding of how the molecular mass of the cellulose starting material affects the biodegradation rate of the materials. The results are shown in Figure 6. As can be seen from the results the biodegradability of the lactide-grafted cellulose samples is much better than the biodegradability of the homopolymer PLA. The molecular mass of the cellulose starting material affects the biodegradation rate of the materials, but the difference to the biodegradability of the homopolymer PLA is still significant.

[0075] Example 3

[0076] Paperboard samples coated as presented above in Example 2 with a commercial poly(lactic acid) (PLA) reference and a lactide-grafted cellulose of Table 2 (HC1-PLA-6) dissolved in acetone, were tested. The coating thickness was 8-9 pm in both cases. Heat sealing performance was assessed by assessing cold seal quality as described above. The results are shown in Figure 7. As can be seen from the results the seal quality of the lactide-grafted cellulose was at least similar as that of the commercial PLA coated in the same manner.

Claims

CLAIMS1 . A coating composition, characterized in that the coating composition comprises 50 - 100 wt-% lactide-grafted cellulose of the total dry weight of the coating composition, wherein the lactide-grafted cellulose comprises between 40 wt-% and 90 wt-% polylactide of the dry weight of the lactide-grafted cellulose, wherein the lactide-grafted cellulose has a degree of polymerization (DP) over 3, and wherein the lactide-grafted cellulose has a degree of substitution (DS) between 0.5 and 3.

2. The coating composition according to claim 1 , characterized in that the coating composition further comprises one or more additive(s), preferably chosen from any of plasticizers, dispersants, surfactants, antistatic agents, antioxidants, foaming or antifoaming agents, waxes, fillers, pigments, neutralizing agents, thickeners, compatibilizers, brighteners, rheology modifiers, preservatives, biocides, crosslinking agents, UV stabilizers, coefficient of friction modifiers, release agents, flow additives, pH adjusting agent and / or combinations thereof.

3. The coating composition according to any of the preceding claims, characterized in that the coating composition comprises more than 60 wt-% lactide-grafted cellulose, preferably more than 90 wt-% lactide-grafted cellulose and more preferably at least 95 wt-% lactide-grafted cellulose of the total dry weight of the coating composition.

4. The coating composition according to any of the preceding claims, characterized in that the coating composition consists of lactide-grafted cellulose and optionally one or more additives.

5. The coating composition according to claim 4, characterized in that the amount of additive(s) is between 0.5 wt-% and 50 wt-%, preferably between 1 and 50 wt-%, more preferably between 5 and 50 wt-% and most preferably between 2 and 10 wt- % of the total dry weight of the coating composition.

6. A method for preparing a coating composition, characterized in that the method comprisesa first step of grafting cellulose with polylactide by ring-opening graft polymerization using a catalyst, wherein a prepolymer with a DP below 3 is obtained, a second step of grafting the prepolymer with polylactide grafts by ring-opening graft polymerization using a catalyst, obtaining lactide-grafted cellulose with a DP above 3, wherein the amount of polylactide is between 40 wt-% and 90 wt-% of the dry weight of the lactide-grafted cellulose, and providing a coating composition comprising 50 - 100 wt-% of the lactide-grafted cellulose.

7. The method according to claim 6, characterized in that the coating composition comprises more than 60 wt-% lactide-grafted cellulose, preferably more than 90 wt- % lactide-grafted cellulose of the total dry weight of the coating composition.

8. The method according to any of claims 6 - 7, characterized in that the coating composition is produced from renewable resources.

9. The method according to any of claims 6 - 8, characterized in that the ring-opening graft polymerization is done in a homogenous solvent system.

10. The method according to any of claims 6 - 9, characterized in that the ring-opening graft polymerization catalyst is an organic nucleophilic catalysts, preferably chosen from one of 4-dimethylaminopyridine (DMAP) and N-heterocyclic carbenes (NHCs); an organic base, preferably chosen from 1 ,8-diazabicyclo[5.4.0]undec-7-ene (DBll), 1 ,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 1-tert-butyl-4,4,4-tris(dimethylamino)- 2,2-bis[tris(dimethylamino)phosphoranyl-idenamino]-2A5,4A5-catenadi(phosphazene) (t-BuP4); an acid catalyst, preferably trifluoromethane-sulfonic acid (TfOH); a mono- and multinuclear metal complex containing both main group metals and transition metals as well as heterometallic complexes; an organic metal salt, preferably chosen from tin(ll) octanoate and aluminium isopropoxide; or an enzymatic catalyst.11 . The method according to any of claims 6 - 9, characterized in that the ring-opening graft polymerization catalyst is an organic base, preferably chosen from 8- diazabicyclo[5.4.0]undec-7-ene(DBU), 1 ,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranyl- idenamino]-2A5,4A5-catenadi(phosphazene) (t-BuP4).

12. The method according to any of claims 6 - 11 , characterized in that the molar ratio of catalyst:polylactide is between 1 :100 and 1 :5, preferably between 1 :60 and 1 :20.

13. The composition according to any of claims 1 - 5 or the method according to any of claims 6 - 12, characterized in that the lactide-grafted cellulose comprises between 40 wt-% and 90 wt-% polylactide, preferably between 45 wt-% and 90 wt-% polylactide, more preferably between 55 wt-% and 90 wt-% polylactide of the dry weight of the lactide-grafted cellulose.

14. The method according to any of claims 6 - 13, characterized in that the method further comprises adding one or more additive(s), preferably chosen from any of plasticizers, dispersants, surfactants, antistatic agents, antioxidants, foaming or antifoaming agents, waxes, fillers, pigments, neutralizing agents, thickeners, compatibilizers, brighteners, rheology modifiers, preservatives, biocides, crosslinking agents, LIV stabilizers, coefficient of friction modifiers, release agents, flow additives, pH adjusting agent and / or combinations thereof as part of the coating composition.

15. The method according to claim 14, characterized in that the amount of additive(s) is between 0.5 and 50 wt-%, preferably the amount of additive(s) is between 1 and 50 wt-%, more preferably the amount of additive(s) is between 5 and 50 wt-% and most preferably the amount of additive(s) is between 2 and 10 wt-% of the total dry weight of the composition.

16. A coating obtained by using the coating composition according to any of claims 1 - 5 or the coating composition obtained by the method of any of claims 6 to 15.

17. Use of the coating composition of any of claims 1 - 5 or the coating composition obtained by the method of any of claims 6 to 15 for coating of a substrate.

18. The use according to claim 17, characterized in that the substrate is used for packaging purposes.

19. The use according to any of claims 17 - 18 characterized in that the substrate is in sheet, film or web format, preferably chosen from any of paper, paperboard, plastic films, biopolymer films such as cellulose films or cellulose-based films, or combinations thereof.

20. The use according to any of claims 17 - 19 characterized in that the substrate is material in container format or an intermediate product prior to processing intocontainer format, preferably chosen from articles made of fibers and cellulose having a three-dimensional structure, more preferably chosen from any of boxes, plates, bowls, cups, bottles, cans, lids, trays or intermediate products thereof.

21. The use according to any of claims 17 - 20 characterized in that the coating composition is biodegradable compared to a coating composition comprising polylactide homopolymer.

22. Use of the coating of claim 16, the coating composition of any of claims 1 - 5 or the coating composition obtained by the method of any of claims 6 to 15 as adhesives, sealants, primers, inks, overprint varnishes, release coatings, sizing agents, and / or barrier coatings.

23. Use of the coating of claim 16, the coating composition of any of claims 1 - 5 or the coating composition obtained by the method of any of claims 6 to 15 as a barrier.

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