Technical field

The present document relates to a method for producing a high solids content and highly redispersible microfibrillated cellulose (MFC). Background

Nanocellulose or microfibrillated cellulose (MFC) is a division of renewable materials, which are constituted of a cellulosic structure typically having a diameter of 5-60 nm and a length of several micrometers. The MFC is typically produced by delamination of wood pulp by mechanical pressure before and/or after chemical or enzymatic treatment (Klemm et al. 2011).

Various applications for MFC have been proposed and reviewed such as composite materials, rheological agents, non-caloric food additives, and transparent films (Klemm et al. 2011 , and Duran et al. 2012).

However the production process for the MFC, i.e. the delamination of wood pulps, usually results in a MFC suspension having a solid contents or concentration of about 2% MFC and the remaining part being water. A final production cost of MFC has been estimated to below 2000 EUR/tonne, and the low dry contents together with the estimated product price, forces the development towards a more feasible logistic procedure, e.g. by increasing the dry contents.

When increasing the dry contents of MFC suspensions, irreversible agglomeration phenomenon have been recorded (Klemm et al. 2011). This means that the redispersion of MFC after drying is difficult, due to this agglomeration process known as "hornification" or stiffening of cellulosic materials upon drying. This material cannot be used in rheological

applications nor be dispersed for composite applications. These irreversible agglomeration has also been studied and recorded for wood pulps in general (Fernandes Diniz et al. 2004).

The main strategy to prevent agglomeration or hornification of MFC has been the introduction of a steric barrier or electrostatic groups to block the hydrogen bonding of the cellulose chains. Some of the most commonly used additives for this purpose are poly-hydroxy-functionalized admixtures, carbohydrates or carbohydrate related compounds (e.g. glycosides, CMC, starches, oligosaccharides etc.). However rather large quantities of these additives must be used to prevent hornification (Klemm et al. 2011).

US 4481076 discloses one such method for producing a redispersable microfibrillated cellulose by addition of a compound capable of inhibiting hydrogen bonding, such as polyhydroxy compounds, gums, starches etc. US4481076 also discloses that e.g. sucrose is used as the additive and should be added in an amount of 0.5 to 2 times the weight of the MFC.

JP19840306 discloses another such method, in which a water soluble substance is mixed with the MFC at a ratio of 10% or more vis-a-vis the MFC. As the water soluble substance a sucrose, dextrin etc. may be used.

Another approach for preventing hornification is to add functional groups to the MFC, also with the aim of preventing hydrogen bonding. One such method is carboxymethylation (Klemm et al. 2011).

However, all of these methods are based on the addition of rather large quantities of co-additives. Even if some of these additives are suitable for use in food applications, this might still restrict the use of MFC in such

applications, and also in composite material applications, and the cost of the final product etc.

There is thus a need for an improved way of producing a MFC having a high solids content, with preserved material properties without using notable amounts of co-additives.

Summary

It is an object of the present disclosure, to provide an improved or alternative high solids content microfibrillated cellulose suspension, which eliminates or alleviates at least some of the disadvantages of the prior art methods.

The invention is defined by the appended independent claims.

Embodiments are set forth in the appended dependent claims and in the following description and drawings.

According to a first aspect, there is provided a method for producing a high solid contents and highly redisperisble microfibrillated cellulose, the method comprises the steps of providing a microfibrillated cellulose

suspension, having a solid contents of about 1-6 %, introducing a solvent into said suspension, said solvent having polar and non-polar properties, and removing said solvent from said suspension by gradually increasing the temperature gradient.

By "polar and non-polar properties" is meant a solvent being less polar than, i.e. water which is the main solvent for the original MFC suspension.

By "microfibrillated cellulose (MFC) suspension" is meant a MFC suspension produced by any conventional means.

By "redispersible" is meant a product that is such that when dispersed into water at ambient temperature and at initial solid content 90 % of initial viscosity is reached, where initial means before treatment.

This method may provide for a way of producing a high solids content

MFC, without deteriorating the material properties, and producing a MFC suspension that has an excellent redispersing ability, without adding notable amounts of co-additives. This method may thus allow for MFC to be used in the same way as never-dried MFC, but with even better properties than the never-dried starting material.

The method is based on the assumption that MFC arrange in ordered structures upon drying (Matthews et al. 2006 and Li et al. 2011). Molecular dynamics and molecular mechanics simulations indicate that water at cellulose crystal surfaces are far from a homogenous solvent. By

incorporating a solvent having both polar and non-polar properties the multiple solvent properties of water at the cellulose surface could be decreased, i.e. a solvent system exchange is performed at the cellulose surface. When decreasing the distribution of the different water "types" in the MFC suspension, the solvent can thereafter be removed by gradually increasing the temperature gradient, the removal acts as an increasing gradient.

The solvent mixture thus surrounding the MFC fibrils is unstrained by forming the more energetically favourable water solvent layers, which allows for a more readily dispersable MFC, thus providing not only a high solid contents MFC suspension, but also a MFC suspension having excellent rheological properties, and uses in for instance oil drilling and food

applications.

SEM analysis of the MFC suspension obtained by the method shows that the solvent exchange and gradient removal appears to not only prevent irreversible agglomeration and hornification but also open up the MFC structure, and that the MFC is actually delamination compared to never-dried MFC. The inventive MFC may also find applications such as acting as a reinforcing material in paper etc.

Also by increasing the solids content in the MFC suspension a more economical transportation may be feasible.

According to an embodiment of the first aspect of the solution, the method may further comprises decreasing the pressure gradient. Thus the ration between the pressure and temperature, i.e. gradually increasing the temperature while gradually decreasing the pressure, may provide for a more efficent solvent exchange.

According to another embodiment the solvent may be incorporated into the microfibrillated suspension by filtration or centrifugation.

Thus the suspension may be filtered before the gradient removal of the solvent system. This may also allow for a slightly higher MFC concentration in the suspension than that of an original or never-dried MFC suspension.

The MFC suspension may even further have been passed through a grinder, such as a Mausko grinder before the filtration.

The solvent may be removed by evaporation and the evaporation may be performed under reduced pressure. The solvent may further be removed by means of a rotary evaporator.

Using a rotary evaporator may increase the efficiency of the method.

According to another embodiment of the first aspect the polar and non- polar solvent may comprise any one of ethanol, methanol, acetone, tertbutanol or mixtures thereof.

According to one alternative the polar and non-polar solvent may be ethanol.

By using ethanol as the polar and non-polar solvent there is provided a way of producing a high solids MFC which is suitable for food applications etc., as ethanol is generally regarded as a "safe" solvent for these types of applications.

According to one alternative the polar and non-polar solvent may be added such that the concentration of solvent in a redispersed microfibrillated cellulose is less than about 5%, or less than about 4%, or less than about 3%, or less than about 2%, or less than about 1%.

This allows for the MFC suspension produced by the method to be used in various food applications as there is no need for any further additive chemicals besides the polar and non-polar solvent. The amount of solvent left in the MFC suspension after the gradient solvent removal is thus very low, and may be even further reduced by washing.

According to the first aspect the temperature may gradually be increased from 45°C to 80°C, or from 50°C to 75°C, or from 55°C to 70°C and the pressure may be reduced from 400 mbar to 100 mbar, or from 400 mbar to 75 mbar, or from 400 mbar to 50 mbar.

According to the first aspect a microfibrillated cellulose suspension having a solids content of more than 50%, or more or more than 55%, or more than 60%, or more than 70% may be produced and a microfibrillated cellulose having a pellicle size of about 2 to 5 mm may be produced.

By producing such a high solids content MFC considerable savings in transport and storage of the MFC may be achieved. Also as the properties of the high solids contents MFC, i.e. its redispersability being essentially unaffected, there is provided a method of producing an low cost MFC for many important applications.

According to a second aspect there is provide a high solids content and highly redispersable microfibrillated cellulose produced according to the method of the first aspect of the invention. The solid contents may be at least 50%, or at least 60%, or at least 65 %, or at least 70%.

According to one embodiment of the second aspect a redispersed microfibrillated cellulose suspension having a concentration of 1 ,20 % may have a viscosity at 50 rmp of about 2500 mPas.

According to a third aspect there is provided a high solids content and highly redispersable microfibrillated cellulose obtainable by the method of the first aspect.

According to a fourth aspect there is provided the use of the

microfibrillated cellulose according to the third aspect for rheology

applications as any one of an emulsion agent, a suspension stabilizing agent, or fluid suspension agent.The rheology application may be any one of a oil drilling application, food application, pharmaceutical application and cosmetic application.

According to a fifth aspect there is provided the use of the

microfibrillated cellulose according to the third aspect as any one of a reinforcing agent for pulp and/or paper materials, a barrier agent, a reinforcing agent for coating materials, and a composite material in paper or polymer composites. According to a sixth aspect there is provided the use of the

microfibrillated cellulose according to the third aspect as absorption agent in any one of a sanitary application, a wound dressing, and a coating agent.

According to a seventh aspect there is provided the use of the microfibrillated cellulose according to the third aspecet as a pharmaceutical or nutraceutical excipient.

Brief Description of the Drawings

Embodiments of the present solution will now be described, by way of example, with reference to the accompanying schematic drawings.

Fig. 1 is an overview of the MFC materials studied.

Fig. 2 is a graph showing viscosity for different treatments of the invention and the starting material.

Fig. 3 is a graph showing a comparison of the viscosity for different

MFCs and different redispersion methods.

Fig. 4a is a SEM photograph showing a 1000 times magnified surface of the ultrasonicated MFC "Fine" material.

Fig. 4b is a SEM photograph showing a 1000 times magnified surface of the ultrasonicated MFC according to the invention.

Fig. 4c is a SEM photograph showing a 1000 times magnified surface of the stirred MFC according to the invention.

Fig. 4d is a SEM photograph showing a 1000 times magnified surface of the stirred MFC "UF" material.

Fig. 5 is a flow chart of an example according to one embodiment of the present invention.

Fig. 6 is a flow chart if an example according to an alternative embodiment of the present invention.

Description of Embodiments

Fig 1 illustrate the materials used in the experiments and

measurements.

The main measurement techniques for characterization of

microfibrillated cellulose (MFC) are scanning electron microscope (SEM), through which information about a surface topography, composition and other properties may be obtained, and viscosity measurements at different rotational speeds. The starting material for the inventive method was so called never- dried MFC suspension, i.e. an MFC that has not previously been dried. The never-dried MFC was also used for the comparative tests.

The never-dried MFC suspension may be produced by any

conventional means, such as acid hydrolysis of cellulosic materials, e.g. disclosed in WO 2009021687 A1 , or MFC suspension produced by enzymatic hydrolysis of Kraft pulp cellulose, e.g. disclosed in WO20 004300 A1 , acid hydrolysis followed by high pressure homogenization, e.g. disclosed in US20100279019, or by any other means known to the skilled person. The concentration of MFC in such suspensions is usually about 1-6 % and the remaining part is water, even though there are methods for producing suspensions having a higher solids content, these are usually energy consuming or not possible to perform on an industrial scale.

In the experiments the never-dried MFC suspension was passed three times through a friction grinding machine, such as a Masuko grinder, to produce ultra-fine particles, called "Fine" in the figures.

Another fraction was passed eight times through the grinder and thus produced an even finer MFC fraction, called "UF" in the figures.

The samples were dispersed either only by stirring or by sonication (or ultrasonication) and stirring. In Fig. 1 the material and dispersion methods are presented.

By sonication or ultrasonication is meant that sound (usually

ultrasound) energy is utilized to agitate particles in the sample.

According to the inventive method a solvent, having both polar and non-polar properties or a mixture of such solvents is incorporated into the never-dried MFC suspension.

The polar and non-polar solvent, and may thus be a solvent such as ethanol, methanol, acetone, tertbutanol or mixtures thereof, or any other solvent which has both polar and non-polar properties. These type of solvents are all less polar than water, which is the main solvent in the original MFC suspension, i.e. in the never-dried MFC suspension.

The polar and non-polar solvents may be according to alternative embodiments be incorporated by means of centrifugation or filtration.

According to one embodiment ethanol was incorporated into the MFC suspension that had been passed three timed through a grinder by filtration (see Fig. 5), so called "MFC invention". The solids content in the MFC suspension after filtration was about 5 %. Through the incorporation of ethanol, the distribution of different water "types" in the MFC suspension is decreased and a "solvent exchange system" is thus created, where the polar and non-polar solvent takes the place of water at the surface of the MFC fibrils. The incorporation on the polar and non-polar solvent thus works as a "degradient" for water.

After the incorporation the polar and non-polar solvent, i.e. the ethanol, was removed by gradually increasing the temperature gradient.

According to one embodiment the temperature was gradually increased from 55°C to 70°C, while the pressure gradient was gradually decreased from 400 mbar to 50 mbar by means of a rotary evaporator or any other equipment which removes liquid phase, i.e. solvent and water in any form).

The temperature range within which the temperature gradient is increased may of course be set depending on the type of solvent exchange system used, but is usually in the range of 45°C to 100°C.

The pressure gradient range may also be set depending on the solvent exchange system used, an may usually be in the range of 1 bar to 5.5 mbar for a conventional rotary evaporator.

The removal of the polar and non-polar system by the increasing temperature thus acts as a gradient for water.

By the method according to the present disclosure pellicles of 2-5 mm in diameter and having a solid contents of about 60% may be achieved.

Preferably a solid contents of more than 50% is achieved, or even mote preferably of more than 60 %, or even more preferred of more than 70%.

The redispersion of the MFC works opposite to the gradient process. The solvent mixture surrounding the MFC fibrils is unstrained by forming the more energetically favourable water solvent layers.

The MFC pellicles thus formed by the inventive method may readily be dispersed in water.

As an example the pellicles formed were dispersed in deionized water to a final concentration of 0,36-1 ,2 %. The dispersion was performed overnight (approx. 12-14 h) and the rate of redispersion could be enhanced by mechanically crushing the pellicles.

As an alternative method the never-dried MFC suspension could be subjected to a gradually increasing temperature gradient, for the removal of the original water solvent system only, if only a small increase in the MFC concentration or solids content is sufficient.

Example

According to one example, as shown in Fig. 6, 211.1 g of ethanol was added to a suspension of MFC Fine (111.15 g) having a dry solid content of 3.23%.

The mixture was stirred and polar and non-polar solvent and MFC suspension having a dry solid content of 1.11% was obtained.

This suspension was filtrated and a 235.5 g of ethanol was

subsequently added to the solid residue, and a suspension of a dry solid content of 1.3% was thereby achieved. This suspension was stirred and thereafter the temperature gradient was gradually increased, and at the same time the pressure gradient was decreased according to the Table 1 below in order to evaporate the polar and non-polar solvent. After the gradient solvent exchange process a MFC suspension having a dry solids content of 74.41 % was achieved.

Experimental results

The samples shown in Fig. 1 were analysed with SEM and/or viscosity measurements.

Fig. 2 shows the results from the viscosity measurements for the stirred MFC invention 1.20% concentration, the stirred and sonicated MFC Fine, 1.18% concentration and the stirred MFC Fine 1.18% concentration are shown.

Viscosities were measured with a Brookfield viscometer with a V-74 spindle, and the values from the measurement are given in Table 2.

The viscosity of the sonicated MFC invention could not be measured due to a too severe dilution. This measurement shows that the MFC subjected to the present method has higher viscosity values than the corresponding starting material, i.e. the never-dried MFC Fine. Ultrasonication increases the viscosity of MFC Fine, but is not close to the non-sonicated MFC produced according to the present method.

Fig. 3 shows the MFC produced according to the invention "stirred MFC invention 0.85% concentration", compared with the "stirred MFC UF 0.82% concentration" and the "stirred and sonicated 2.5 min MFC Fine 0.70% concentration". From this measurement it can surprisingly be concluded that the MFC produced according to the present method reminds more of the finer MFC UF than its starting material. This indicates that a further delamination has taken place during the treatment method of the invention.

Table 2. Viscosit values for Fi s 2 and 3.

In Figs 4a-4d the SEM photographs are shown of a 1000 times order magnification of the MFC surface.

In Fig. 4a the ultrasonicated, 2,5 min, MFC Fine 0.70% concentration is shown.

In Fig. 4b the ultrasonicated, 0,5 min, MFC produced according to the present method, 0.36% concentration is shown.

In Fig. 4c the stirred MFC produced according to the present method, 0.85% concentration is shown.

In Fig. 4d the stirred MFC UF, 0.8% concentration is shown. The SEM analysis supports the viscosity results in that no irreversible agglomeration phenomenon has taken place during as a cause of the treatment method according to the invention. The SEM analysis rather shows that a delamination of the fibrils in the MFC produced according to the invention has taken place. The SEM pictures also show an increase in the fineness of the particles, when comparing the ultrasonicated MFC Fine with the

ultrasonicated MFC produced according to the inventive method the difference is clear.

These measurements supports the use of the MFC produced according to the inventive method for rheology applications such as oil drilling filler (Laukkanen et al. 2011).

Other rheology areas include food related areas, since no co-additives except the polar and non-polar solvent is used. The amount remaining in the MFC suspension when subjected to the treatment method of the invention is lower than 1 %, and may be even further reduced or purified by washing the MFC.

The SEM pictures further support that the method according to the invention not only prevents irreversible agglomeration of the MFC, but also opens up the MFC structure even more, i.e. causes an increased

delamination.

Based on the viscosity measurements and SEM analysis the reinforcing effect in different materials could be greater by using the MFC produced according to the inventive method than for starting materials of never-dried MFC (such as the Fine).

Sehaqui et al. 2011 have studied how to reinforce paper by addition of MFC. By adding MFC to paper macro fibers, either the mechanical properties can be enhanced or the amount of needed macro fibers can be lowered. This results in higher end value and lower production costs respectively.

References

Fernandes Diniz, J.M.B., Gil, M.H, Castro, J.A.A.M, (2004) Hornification - its origin and interpretation in wood pulps, Wood Sci Technol., 37, 489-494.

Duran, N., Lemes, A.P., Seabra, A.B, Review of Cellulose Nonocrystals Patents: Preparation, Composites and General Applications, (2012), Recent patents on Nanotechnology, 6, 16-28. Klemm, D, Kramer, F. Moritz, S., Lindstrom, T., Ankerfors, M., Gray, D. and Dorris, A. (2011) Nanocelluloses: A New Family of Nature-Based Materials, Angew. Chem. Int. Ed., 50, 5438-5466.

Laukkanen, A., Teirfolk, J-E., Salmela, J., Lille, M. (201 1) Agent and composition for oilfiled applications, WO201 1/089323.

Sehaqui, H., Allais, M., Zhou, Q., and Berglund, L, (210) Wood cellulose biocomposites with fibrous structures at micro- and nanoscale, Composite Science and Technology, 71 , 382-387.