TECHNICAL FIELD

The present invention relates to a process for the production of a nanocellulose material from a cellulosic precursor material.

PRIOR ART

Cellulose is a material that is widely available from renewable sources such as plant material. Cellulose is present as a fiber in the primary cell wall of green plants, where it is usually found in a mixture with hemicellulose, lignin, pectin and other substances. The cellulose fiber itself consists of crystalline and amorphous regions, and the crystalline regions are known as cellulose nanofibers (CNF) and nanocrystalline cellulose (NCC), which can both be separated from the amorphous regions, and exhibit mechanical properties that make them highly suitable for reinforcing use in material applications and other applications where CNF or CNC gels are of use such as in cosmetics, for example as gels.

However, the production of either cellulose nanofibers (CNF) or nanocrystalline cellulose (NCC) from cellulosic material such as wood pulp is technically demanding and energy- intensive, which is why there is a constant desire in the field of producing either cellulose nanofibers (CNF) or nanocrystalline cellulose (NCC) in both an uncomplicated and an energy-efficient manner which does involve a minimum of hazardous chemicals. EP 2 712 364 proposes that the swelling of the cellulosic precursor material in an aqueous solution of morpholine, piperidine or mixtures thereof can reduce the energy consumption by reducing the number of microfluidisation steps required to release the nanocellulose material from the cellulosic precursor by relying on the swelling agent property of aqueous morpholine and/or piperidine. However, the obtained aqueous suspension of nanocellulose must be further processed in order to yield a re-dispersible nanocellulose powder, which can be technically challenging. For instance, in the case where the nanocellulose obtained through said process is to be dried using supercritical fluids, the morpholine and piperidine must be removed beforehand because they tend to chemically react with the most commonly used supercritical fluids such as carbon dioxide and must be replaced by another inert processing fluid. In addition, morpholine and piperidine are hazardous substances which must be handled with care and which must be thoroughly removed from the nanocellulose material before commercialization, especially for pharmaceutical or food applications of nanocellulose.

In Green Chem., 2015, 17, 3401-3406, Sirvid et al. describe a pre-treatment of wood cellulose with choline chloride/urea as swelling agent before microfluidisation to release the nanofibrillated cellulose (NFC) from the pre-treated wood cellulose. However, before microfluidisation, the pre-treated wood cellulose is first washed with deionized water in order to remove the deep eutectic solvent after the pre-treatment and only then is the thus obtained aqueous suspension of pre-treated wood cellulose microfluidized. The thus obtained aqueous suspension of nanofibrillated cellulose (NFC) is then freeze-dried to prepare samples for further analysis. The removal of water from the nanocellulose by freeze-drying however yields nanocellulose that cannot easily be re-dispersed and having inferior rheological properties.

It is thus desirable to provide a simplified process for the manufacture of nanocellulose in which the overall energy can be reduced preferably without resorting to hazardous chemicals.

SUMMARY OF THE INVENTION

The present invention provides for a process in which the energy consumption of the overall process for the production of a non-derivatized or derivatized nanocellulose material from a cellulosic fibrous material as raw material can be reduced and in which the process liquids used allow safer processing of the raw material and furthermore allows subsequent spray-drying with commonly used supercritical fluids without chemical reaction between the supercritical fluids and the liquid to be removed. The process yields a nanocellulose material in the form of a solid such as for example solid particulate material that can easily be re-dispersed in aqueous solutions to yield a homogenous dispersion of nanocellulose material, thereby forming for example a liquid or gel and which disperion is nearly identical with a freshly (i.e. a never-dried) prepared nanocellulose dispersion in terms of rheological properties. This can be achieved by using a swelling agent that is a low-transition-temperature mixture (LTTM) such as to form for example a deep eutectic solvent which is soluble in a processing solvent and where the processing solvent is non- solubilizing for both the cellulosic fibrous material that is used as raw material and the derivatized or non-derivatized nanocellulose material.

It is an object of the present invention to provide a process for the production of a non- derivatized or derivatized nanocellulose material from a cellulosic fibrous material, comprising the steps of:

a. providing a suspension of cellulosic fibrous material in a continuous phase of a non-aqueous process liquid comprising a swelling agent and a processing solvent;

b. allowing the cellulosic fibrous material to swell such as to form a suspension of swollen cellulosic fibrous material in a continuous phase of non-aqueous process liquid; c. optionally refining said suspension of swollen cellulosic fibrous material in a continuous phase of non-aqueous process liquid to increase the fineness of the swollen cellulosic fibrous material;

d. removing the swelling agent from the process liquid such as to form a suspension of cellulosic fibrous material in a continuous phase of processing solvent;

e. subjecting the swollen and optionally refined cellulosic fibrous material to high- shear comminution such as to release of the non-derivatized nanocellulose material from the swollen and optionally refined cellulosic fibrous material and such as to form a dispersion of non-derivatized nanocellulose material in a continuous phase of a processing solvent;

f. contacting the dispersion of non-derivatized nanocellulose material in a continuous phase of a processing solvent with a supercritical fluid such as to remove the processing solvent and isolate the non-derivatized nanocellulose material, wherein the supercritical fluid is preferably supercritical carbon dioxide or ammonia;

characterized in that the swelling agent is a low-transition-temperature mixture (LTTM) and in particular a deep eutectic solvent and said low-transition-temperature mixture and in particular said deep eutectic solvent is soluble in the processing solvent and wherein the processing solvent is non-solubilizing for the cellulosic fibrous material and the non- derivatized nanocellulose material.

It is another object of the present invention to provide the use of a non-aqueous process liquid comprising a swelling agent and a processing solvent in a process for the production of a derivatized or non-derivatized nanocellulose material according to the above, characterized in that the swelling agent is a low-transition-temperature mixture (LTTM) and in particular a deep eutectic solvent and said low-transition-temperature _^ mixture (LTTM) and in particular said deep eutectic solvent is soluble in the processing solvent and wherein the processing solvent is non-solubilizing for the cellulosic fibrous material and the non-derivatized nanocellulose material.

It is yet another object of the present invention to provide a derivatized or non-derivatized nanocellulose material obtained by a process according the above, having an aspect ratio of at least 100 and wherein the derivatized or non-derivatized nanocellulose material is preferably in the form of a solid such as for example solid particulate material like a powder.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

Fig. 1 shows a graph in which the shear stress is shown in dependency of the shear rate for two samples of re-dispersed nanocellulose at consistency of 1% by weight. The filled symbols represent the re-dispersed nanocellulose spray dried from a suspension of nanocellulose in ethyl lactate according to the present invention and the open symbols represent the re-dispersed nanocellulose when spray dried from an aqueous suspension of nanoncellulose.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the context of the present application, the term "low transition temperature mixture" or "LTTM" refers to a liquid mixture of at least one hydrogen bond donor (HBD) and one hydrogen bond acceptor (HBA) counterpart that results in the formation of liquid mixture showing an unusually low freezing/melting point or glass transition point.

In the context of the present application, the term "deep eutectic solvents" or "DES" refers in particular to liquid mixtures of quaternary ammonium salts such as for example quaternary ammonium halide salts or guanidinium salts such as for example guanidinium halide salts, as hydrogen bond acceptor (HBA) in combination with one or more hydrogen bond donors (HBD) such as for example urea showing an unusually low freezing/melting point.

Exemplary hydrogen bond acceptors (HBA) are quaternary ammonium salts which are suitable for the formation of a deep eutectic solvent in combination with one or more hydrogen bond donors (HBD). Suitable quaternary ammonium salts are in particular quaternary ammonium halide salts such as choline chloride, (2- hydroxyethyl)dimethylethylammonium chloride, trimethylglycine and 2- (chlorocarbonyloxy)-N,N,N-trimethylethanaminium chloride or N-benzyl-2-hydroxy-N,N- dimethylethanaminium chloride.

Suitable hydrogen bond donors (HBD) that may be used for the formation of a deep eutectic solvent with quaternary ammonium halide salts are urea and derivatives thereof such as 1 -methyl urea, 1 ,3-dimethyl urea or 1,1 -dimethyl urea; thiourea and derivatives thereof, amides such as benzamide or acetamide and derivatives thereof; polyols such as glycerol, ethylene glycerol or propylene glycol, benzoic acid and derivatives thereof, dicarboxylic acids such as malonic acid, adipic acid, oxalic acid, succininc acid or citric acid, a-hydroxy carboxylic acid alkyl esters and derivatives thereof such as ethyl lactate. In the context of the present invention, low transition temperature mixtures and deep eutectic solvents which are liquids at room temperature, i.e. which have a freezing/melting points below 25°C, are preferred for reasons of energy consumption of the process, since no energy is needed in this case to melt and keep the low transition temperature mixtures and deep eutectic solvents in a liquid state. It is however possible to use low transition temperature mixtures and deep eutectic solvents having a freezing/melting points of between 25°C and 95°C, and preferably between 25°C and 50°C.

It is an object of the present invention to provide a process for the production of a non- derivatized or derivatized nanocellulose material from a cellulosic fibrous material, comprising the steps of:

a. providing a suspension of cellulosic fibrous material in a continuous phase of a non-aqueous process liquid comprising a swelling agent and a processing solvent;

b. allowing the cellulosic fibrous material to swell such as to form a suspension of swollen cellulosic fibrous material in a continuous phase of non-aqueous process liquid; c. optionally refining said suspension of swollen cellulosic fibrous material in a continuous phase of non-aqueous process liquid to increase the fineness of the swollen cellulosic fibrous material;

d. removing the swelling agent from the process liquid such as to form a suspension of cellulosic fibrous material in a continuous phase of processing solvent;

e. subjecting the swollen and optionally refined cellulosic fibrous material to high- shear comminution such as to release of the non-derivatized nanocellulose material from the swollen and optionally refined cellulosic fibrous material and such as to form a dispersion of non-derivatized nanocellulose material in a continuous phase of a processing solvent;

f. contacting the dispersion of non-derivatized nanocellulose material in a continuous phase of a processing solvent with a supercritical fluid such as to remove the processing solvent and isolate the non-derivatized nanocellulose material, wherein the supercritical fluid is preferably supercritical carbon dioxide or ammonia;

characterized in that the swelling agent is a low-transition-temperature mixture (LTTM) and in particular a deep eutectic solvent and said low-transition-temperature mixture and in particular said deep eutectic solvent is soluble in the processing solvent and wherein the processing solvent is non-solubilizing for the cellulosic fibrous material and the non- derivatized nanocellulose material.

In a preferred embodiment of the process according to the present invention, the swelling agent removed from the process liquid in step d. is used, i.e. recycled, to form the non- aqueous process liquid suspension of cellulosic fibrous material in step a..

In a preferred embodiment of the process according to the present invention, the processing solvent removed in step f. is used, i.e. recycled, to form the non-aqueous process liquid suspension of cellulosic fibrous material in step a..

In a more preferred embodiment of the process according to the present invention, the swelling agent removed in step d. and the processing solvent removed in step f. are used, i.e. recycled, to form the non-aqueous process liquid suspension of cellulosic fibrous material in step a.. This way, the process can be carried out more efficiently and preferably as a closed circuit with respect to swelling agent and processing solvent. Because in the present invention, the processing solvent is chosen such as to not chemically react with the supercritical fluid such as carbon dioxide and ammonia, the mixture of processing solvent and supercritical fluid that is removed can be easily separated by returning the mixture to a pressure and temperature at which the supercritical fluid returns to a gas state, thereby boiling off, and at which the processing solvent returns to a liquid state. Thus, also the fluid in a gas state can be used, i.e. recycled, to provide the supercritical fluid in step f.

In a preferred embodiment of the process according to the present invention, the low- transition-temperature mixture (LTTM) and in particular the deep eutectic solvent is a binary low-transition-temperature mixture (LTTM) and in particular a binary deep eutectic solvent, and preferably is a binary deep eutectic solvent of a quaternary ammonium salt with a hydrogen bond donor, more preferably of a quaternary ammonium halide salt such as a choline halide with a hydrogen bond donor chosen from urea or ethanolamine. In a more preferred embodiment of the process according to the present invention, the swelling agent is a binary deep eutectic solvent of choline chloride and urea or choline chloride and ethanolamine, wherein the choline chloride and urea or ethanolamine are present in a molar ratio of 1 :2. In a preferred embodiment of the process according to the present invention, the swelling agent is a ternary deep eutectic solvent, and preferably is a ternary deep eutectic solvent of a quaternary ammonium salt such as choline halide with a binary hydrogen bond donor chosen from glycerol/DBN or glycerol/DBU in a molar ratio of 1 :2:6

In a preferred embodiment of the process according to the present invention, the cellulosic fibrous material has a cellulose content of at least 90% by weight or is bleached chemical pulp preferably having a lignin content of less than 5% by weight and preferably has a lignin content of less than 1% by weight.

In a preferred embodiment the nanocellulose material obtained from the process according to the present invention may be in solid particulate form. This allows for simpler storage and dosing of the nanocellulose material.

In a preferred embodiment, the process according to the present invention yields a non- derivatized nanocellulose material from a cellulosic fibrous material or a derivatized nanocellulose material, depending on the chemical nature of the swelling agent and/or the processing solvent. It is however preferred that the chemical nature of the swelling agent and/or the processing solvent are such as to yield a non-derivatized nanocellulose material.

In a preferred embodiment of the process according to the present invention, the nonaqueous process liquid comprises of, or consists of, from 50 to 95 weight percent, preferably of from 75 to 95 weight percent of swelling agent and/or from 5 to 50 weight percent, preferably of from 5 to 25 weight percent of processing solvent, based on the weight of the non-aqueous process liquid. By including 50 or more weight percent of swelling agent in the non-aqueous process liquid, the swelling of the cellulosic fibrous material can be increased whereas the addition of the processing solvent helps to reduce the viscosity of the non-aqueous process liquid which allows for better refining in the ensuing refining step. Therefore, in a preferred embodiment of the process according to the present invention, the processing solvent has a viscosity at 25°C that is inferior to the viscosity of the swelling agent and wherein preferably the processing solvent has a viscosity of less than 500 mPa s at 25°C. In a preferred embodiment of the process according to the present invention, the cellulosic fibrous material is allowed to swell in the non-aqueous process liquid for no more than four hours at 50°C.

In the process according to the present invention, a suspension of cellulosic fibrous material is provided in a continuous phase of a non-aqueous process liquid comprising a swelling agent and a processing solvent. The swelling agent and a processing solvent are preferably freely miscible and the non-aqueous process liquid forms a continuous phase of liquid in which the solid cellulosic fibrous material is suspended. The suspension of cellulosic fibrous material can be provided by for example combining either the swelling agent, the processing solvent or the non-aqueous process liquid with the cellulosic fibrous material in a vessel. The cellulosic fibrous material may for example be bleached hardwood sulphite pulp in sheet form which is first pulverized in a knife mill to a particle size of less than 1 mm and then inserted together with the non-aqueous process liquid in a heated hydropulper in which the suspension of cellulosic fibrous material in a continuous phase of a non-aqueous process liquid is formed through agitation.

In a preferred embodiment of the process according to the present invention, the suspension of cellulosic fibrous material in a continuous phase of a non-aqueous process liquid comprises of from 1 weight percent of cellulosic fibrous material to 6 weight percent of cellulosic fibrous material and/or the suspension of swollen cellulosic fibrous material in a continuous phase of a process solvent comprises of from 1 weight percent of cellulosic fibrous material to 4 weight percent of cellulosic fibrous material and preferably comprises of from 0.1 weight percent of cellulosic fibrous material to 2 weight percent of cellulosic fibrous material.

In the process according to the present invention, the cellulosic fibrous material is allowed to swell such as to form a suspension of swollen cellulosic fibrous material in a continuous phase of non-aqueous process liquid. The time required to achieve a certain degree of swelling may vary depending on the swelling agent. The amount of swelling may be monitored by for example by visual inspection of a sample in vial and comparing the sample to a reference sample. As the cellulose material swells, the volume of the cellulose increases and the height of the swollen cellulose in a vial after gravitational setting increases and can be compared to a reference sample. One such example may be found in the specification of EP 2 712 364 where a swelling index is computed. In a preferred embodiment of the process according to the present invention, the suspension of cellulosic fibrous material in non-aqueous process liquid comprising a swelling agent and a processing solvent can be agitated in order to reduce the time needed to achieve a certain degree of swelling. For instance, such agitation may be achieved in a hydropulper.

In the process according to the present invention, suspension of swollen cellulosic fibrous material in a continuous phase of non-aqueous process liquid can optionally be refined to increase the fineness of the swollen cellulosic fibrous material. Increasing the fineness of the swollen cellulosic fibrous material ensures a more robust operation of the

microfluidizer in the ensuing process step, since in some cases the particle size of the swollen cellulosic fibrous is such that the processing module of the microfluidizer may become clogged after a certain time. In addition, most refiners such as disc refiners are optimized for accepting cellulose material having a particles size that is similar to the particle size of the swollen cellulosic fibrous material and thus, the increase in fineness of the swollen cellulosic fibrous material is particularly energy efficient. Suitable refiners are for example refiners having disk-shaped, cylindrical or conical refiner elements.

In the process according to the present invention, the swelling agent from the process liquid is removed such as to form a suspension of cellulosic fibrous material in a continuous phase of processing solvent. The removal of the swelling agent from the process liquid is removed before further processing of the cellulosic fibrous material can preferably be achieved by washing the suspension of swollen cellulosic fibrous material in a continuous phase of non-aqueous process liquid with processing solvent until the swelling agent is removed and a suspension of cellulosic fibrous material in a continuous phase of processing solvent is formed. The processing solvent can wash away the swelling agent because the swelling agent is chosen such as to be miscible with the swelling agent. It is understood that the processing solvent is processing solvent essentially free of water, i.e. a dry processing solvent, as water would be detrimental to obtaining an easily re- dispersable nanocellulose material. _n

In a preferred embodiment of the process according to the present invention, the processing solvent is chosen from esters of a hydroxyalkanaoic acids and from lower mono- or polyhydric alcohols such as propylene glycol. In the case where the processing solvent is chosen from esters of a hydroxyalkanaoic acids, it is preferably chosen from esters of a- hydroxycarboxylic acids such as for example esters of lactic acid. An example of an ester of lactic acid is ethyl lactate, preferably food-grade or pharmaceutically acceptable ethyl lactate. In the process according to the present invention, the swollen and optionally refined cellulosic fibrous material is subjected to high-shear comminution such as to release of the non-derivatized or derivatized nanocellulose material from the swollen and optionally refined cellulosic fibrous material and such as to form a dispersion of non-derivatized or derivatized nanocellulose material in a continuous phase of a processing solvent. A suitable apparatus for subjecting the swollen and optionally refined cellulosic fibrous material to high-shear comminution can be a microfluidizer such as Microfluidizer Processor M-110-EH equipped with a 200 μιη ceramic processor module arranged in series with a 100 μηι diamond interaction chamber and operating at 25 000 psi, available from the Idex Corp.

In the process according to the present invention, the dispersion of non-derivatized nanocellulose material in a continuous phase of a processing solvent is contacted with a supercritical fluid such as to remove the processing solvent and isolate the non-derivatized nanocellulose material, wherein the supercritical fluid is preferably supercritical carbon dioxide or ammonia. As an example, the dispersion of non-derivatized nanocellulose material in a continuous phase of a processing solvent is contacted with a supercritical fluid such as to remove the processing solvent by spray-drying the dispersion of non- derivatized nanocellulose material in a continuous phase of a processing solvent with supercritical carbon dioxide or ammonia in suitable spray-drying apparatus. The processing solvent is chosen such that the processing solvent it is not only miscible with the swelling agent but it is also miscible with the supercritical fluid, and in particular with supercritical carbon dioxide or supercritical ammonia. In a preferred embodiment of the process according to the present invention the dispersion of derivatized or non-derivatized nanocellulose material in a continuous phase of a processing solvent is contacted with a supercritical fluid to remove the processing solvent in a spray drying apparatus.

In a preferred embodiment of the process according to the present invention the derivatized or non-derivatized nanocellulose material is cellulose nanofiber (CNF).

It is another object of the present invention to provide a use of a non-aqueous process liquid comprising a swelling agent and a processing solvent in a process for the production of a derivatized or non-derivatized nanocellulose material according to the above, characterized in that the swelling agent is a low-transition-temperature mixture (LTTM) and in particular a deep eutectic solvent and said low-transition-temperature mixture (LTTM) and in particular said deep eutectic solvent is soluble in the processing solvent and wherein the processing solvent is non-solubilizing for the cellulosic fibrous material and the non-derivatized nanocellulose material.

It is yet another object of the present invention to provide a derivatized or non-derivatized nanocellulose material obtained by a process according the above, having an aspect ratio of at least 100 and wherein the non-derivatized nanocellulose material is preferably in the form of a powder.

EXAMPLES

12.9 kg of urea and 15.1 kg of choline chloride were mixed in a rotary drum blender and then transferred to a vessel equipped with a heating jacket and a mixing impeller. The mixture of urea and choline was then stirred at 20 rpm and heated to a temperature of 70 °C, during which a single liquid swelling agent was obtained. To this, 7.0 kg of ethyl lactate as processing liquid were added gradually while continuously stirring.

To the thus obtained non-aqueous processing liquid, 750 g bleached hardwood sulphite dissolving pulp was added and the mixture was transferred into a hydropulper operating at 50 °C where the mixture of cellulose material and non-aqueous processing liquid was agitated for 4 hours in order to swell the cellulose material.

The resultant suspension of swollen cellulose material in non-aqueous processing liquid was then transferred into the holding vessel of a laboratory disc refiner and recirculated continuously. The calculated cumulative refining energy was equivalent to 800 kWh/tonne of cellulose and the processing gap was 100 microns.

To an aliquot of the resultant swollen and refined swollen cellulose material (500 g) was added ethyl lactate (250 g), with stirring until visually homogeneous. This mixture was then filtered under reduced pressure in a large sintered filter funnel (diameter 300 mm), before washing the filter cake carefully with further aliquots of ethyl lactate (5 x 25 Og).

A sample of the above filter cake was resuspended in the processing solvent ethyl lactate (1000 g) using a laboratory rotor-stator mixer to give a final cellulose solids content of 1% by weight. This suspension of cellulose material in processing solvent was then passed twice at 10000 psi through a M-l 10-EH Microfluidizer Processor (Idex Corp) fitted with a 200 micron ceramic auxiliary processing module. The sample was then given a further 3 passes through the 200 micron ceramic auxiliary processing module arranged in series with a 100 micron diamond interaction chamber at 25000 psi.

The resultant suspension of nanocellulose in ethyl lactate was dried in a benchtop spray drying apparatus (Feyecon Development and Implimentation BV) using supercritical carbon dioxide. The nanocellulose suspension and pressurized carbon dioxide were metered separately into a chamber, prior to being conveyed through a capillary into an enclosed chamber, where the solid nanocellulose was collected.

The nanocellulose used as comparative was prepared by first treating bleached softwood pulp in a mixture of urea/choline chloride (2:1 mol), washing with water and re-suspending in water, high shear processing as above and finally also spray drying directly from the water suspension.

The respectively obtained nanocellulose powders were re-dispersed into de-ionized water at a consistency of 1% by weight by mixing with a laboratory rotor-stator (Ultra Turrax IKA) for 15 mins at 12000 rpm. Viscometric data were collected using a TA Instruments AR-G2 rotational rheometer, fitted with a serrated concentric cylinder measuring geometry. A decelerating stress ramp experiment was performed.

The nanocellulose of the present invention was seen to produce a homogeneous structured gel on re-dispersion, whilst the nanocellulose obtained from spray drying the aqueous suspension was a mobile, low viscosity fluid which exhibited phase separation on standing. As can be seen from the viscometric data presented in Fig. 1, the nanocellulose of the present invention (filled symbols) displays enhanced low shear viscosity arid apparent yield stress when compared to the nanocellulose obtained from spray drying the aqueous suspension (open symbols).

LIST OF REFERENCE SIGNS none