SPRAYING OF MICROFIBRILLATED CELLULOSE

Abstract

The present invention relates to a process for the spraying of microfibrillated cellulose (MFC), which has a comparatively high solids content, onto a surface, thus forming a stable and homogeneous film coating of MFC on said surface. Therein, the MFC of the comparatively high solids content is subjected to a pressure drop in a nozzle, which pressure drop exerts shear pressure onto the MFC, thus lowering the viscosity during the spraying and coating process.

Claims

1. A process for spraying microfibrillated cellulose onto a surface, said microfibrillated cellulose having a MFC solids content in range of 1% weight by weight (“w/w”)-50% w/w, wherein said process comprises at least the following steps: (i) providing microfibrillated cellulose (MFC) in a solvent, wherein solids content of the MFC is in a range of 1% weight by weight (“w/w”)-50% w/w and providing a surface; (ii) spraying said microfibrillated cellulose from step (i), through a nozzle, onto said surface, wherein said nozzle provides a pressure drop of from 2 bar to 200 bar, preferably of wherein said pressure drop is measured in regard to a volume segment upstream of the nozzle and a volume segment downstream the nozzle, wherein the temperature of said surface is from −25° C. to 200° C.

2. The process according to claim 1 wherein a coated film of the MFC from step (ii) is subjected to a subsequent drying step (iii), in which the film is dried by applying heat from 20° C. to 220° C.

3. The process according to claim 1, said process comprising, subsequent to step (ii) and/or subsequent to step (iii), an additional step (iv) of mechanically adjusting film thickness and/or film morphology of the film.

4. The process according to claim 1, wherein said nozzle is part of a system that comprises at least one feeder, at least one pump and at least one nozzle.

5. The process according to claim 1, wherein a cross-section of the exit of the nozzle, from which on the MFC expands toward the surface is essentially round or oblong.

6. The process according to claim 1, wherein a cross-section of the exit of the nozzle, from which on the MFC expands toward the surface is slit-like.

7. The process according to claim 1, wherein the nozzle is cone-shaped or tapered such that a cross-section of the nozzle continually decreases, perpendicular to said cross-section.

8. The process according to claim 1, wherein said microfibrillated cellulose comprises “fibrils” having a diameter in a range of 1 nm to 1 μm and a length in a range of 0.5 μm to 500 μm.

9. The process according to claim 1, wherein the solvent essentially consists of water.

10. The process according to claim 1, wherein MFC is sprayed onto said surface from a distance in a range of from 100 mm to 1000 mm.

11. The process according to claim 1, wherein providing the MFC in the solvent comprises providing a drying additive in the solvent with the MFC.

12. A coating, adhesive, (surface) size, paint, ink, de-icing coating, thixotropic coating, coating in scar and wound care, coating in a composite material, separation technology, film forming application, battery, flexible electronic, or textile comprising microfibrillated cellulose disposed on a surface prepared by a process according to claim 1.

13. The process according to claim 1, wherein the microfibrillated cellulose is prepared by a process, which comprises at least the following steps: (a) subjecting a cellulose pulp to at least one mechanical pretreatment step; (b) subjecting the mechanically pretreated cellulose pulp of step (a) to a homogenizing step, which results in fibrils and fibril bundles of reduced length and diameter vis-a-vis cellulose fibers present in the mechanically pretreated cellulose pulp of step (a), said step (b) resulting in microfibrillated cellulose; wherein the homogenizing step (b) involves compressing the cellulose pulp from step (a) and subjecting the cellulose pulp to a pressure drop.

14. The process of claim 5, wherein a largest length scale describing said cross-section is from 0.1 mm to 10 rum a smallest length scale describing said cross-section is from 0.1 mm to 10 mm.

15. The process of claim 6, wherein a width of the nozzle is at east 10 times a height of the nozzle.

16. The process of claim 9, wherein the solvent comprises at least 90% water.

17. The process of claim 1, wherein the solids content of the MFC is in a range of 4% w/w-15% w/w.

18. The process of claim 1, wherein the solids content of the MFC is in a range of 5% w/w-15% w/w.

19. The process of claim 1, wherein the pressure drop is of from 5 bar to 100 bar.

20. The process of claim 10, wherein the distance is in a range of from 200 mm to 7000 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0045] The invention is described in more detail in the following, with reference to the enclosed figures, which are only meant to be illustrative, wherein:

[0046] FIG. 1 shows microfibrillated cellulose at a MFC dry matter content of approx. 8% to 10%; the “paste”-like structure of MFC is apparent.

[0047] FIG. 2 shows a schematic representation of a unit for spraying, comprising a pump and a feeder.

[0048] FIG. 3 shows an exemplary embodiment of the nozzle (left panel); the right panel shows an example of a spray pattern on a surface

[0049] FIG. 4 shows a photograph of a non-woven web as a substrate (surface) as coated with a layer of MFC.

[0050] FIG. 5 shows photographs of various substrates (surfaces) coated with a layer of microfibrillated cellulose, respectively.

[0051] “Microfibrillated cellulose” (MFC) in accordance with the present invention is to be understood as relating to cellulose fibers that have been subjected to a mechanical treatment resulting in an increase of the specific surface and a reduction of the size of cellulose fibers, in terms of cross-section (diameter) and/or length, wherein said size reduction preferably leads to “fibrils” having a diameter in the nanometer range and a length in the micrometer range.

[0052] In cellulose, which is the starting product for producing microfibrillated cellulose (typically present as a “cellulose pulp”), no, or at least not a significant or not even a noticeable portion of individualized and “separated” cellulose “fibrils” can be found. The cellulose in wood fibres is an aggregation of fibrils. In cellulose (pulp), elementary fibrils are aggregated into microfibrils which are further aggregated into larger fibril bundles and finally into cellulosic fibres. The diameter of wood based fibres is typically in the range 10-50 μm (with the length of these fibres being even greater). When the cellulose fibres are microfibrillated, a heterogeneous mixture of “released” fibrils with cross-sectional dimensions and lengths from nm to μm may result. Fibrils and bundles of fibrils may co-exist in the resulting microfibrillated cellulose.

[0053] Microfibrillated cellulose consists of fibrils in constant interaction with each other in a three-dimensional network. The most important performance properties of MFC high viscosity at rest, shear thinning (thixotropic) behavior, water holding capacity are a result of the existence of this entangled network.

[0054] In the microfibrillated cellulose (‘MFC’) as described throughout the present disclosure, individual fibrils or fibril bundles can be identified and easily discerned by way of conventional optical microscopy, for example at a magnification of 40×, or by electron microscopy.

[0055] In accordance with the present invention, the term “suspension” is understood to mean a liquid, in which solid particles (here: fibers) are dispersed, as generally understood by the skilled person and as defined in the IUPAC “Gold Book”, [PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry); page 606].

[0056] In the present invention, the suspension of microfibrillated cellulose fibers in a solvent, has the consistence of a “paste” and shows non-Newtonian flow properties (see FIG. 1). Such a suspension/paste is sometimes also referred to as a “gel” (or “hydrogel” if the solvent is water).

[0057] Unless indicated otherwise, any parameter referred to in the present disclosure is measured at standard conditions, i.e. at room temperature (20° C.), ambient pressure (1 bar) and 50% ambient humidity. Unless indicated otherwise, any ratio given for an amount of component of the overall system is meant to be given in % weight relative to the overall weigh of the content of the system (i.e. excluding packaging).

[0058] No limitations exist in regard to the solvent, as long as the solvent is capable to keep the MFC fibers in suspension under conditions typical for storage and transport.

[0059] In embodiments of the invention, the solvent is a hydrophilic solvent, preferably a polar solvent, further preferably a protic solvent.

[0060] In a preferred embodiment, said at least one liquid is water, a water-compatible solvent or an organic solvent or any mixture of two or more of said liquids. Preferred liquids are protic liquids, i.e. liquids in which the molecules of the liquid have a dissociable hydrogen atom.

[0061] Preferred protic liquids are water, lower alcohols, ethylene glycol and oligo(ethylene glycols), and mixtures of said protic liquids. Therein, the term “lower alcohol” comprises alcohols having from one to 10 carbon atoms in the carbon backbone. Preferred alcohols are methanol, ethanol, the propanol isomers, butanol isomers, and mixtures of said alcohols. The term “oligo(ethylene glycol)” encompasses diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, and mixtures of said glycols. Further suitable liquids are e.g. dimethylsulphoxide and glycerol.

[0062] Preferred solvents are water or alcohol or any mixture of such solvents. In preferred embodiments the solvent essentially consists of water, i.e. comprises at least 90%, preferably at least 95%, further preferably at least 99% of water. “Water” can be distilled water, processed water or tab water as commonly used in industrial applications.

[0063] In a preferred embodiment, the liquid used in the method of the invention comprises water in combination with another liquid, preferably one or more of the aforementioned protic liquids.

[0064] In an alternate embodiment that is particularly preferred when the end use of the dried MFC is in the field of polymers, adhesives, coatings, gel coats or paints, the at least one liquid is or comprises an organic solvent, or at least one liquid is an organic solvent.

[0065] Depending on the liquid used, however, the addition of (an) additive(s), including, but not limited to surfactants, cellulose derivatives, salts, dispersing aids, preservatives, polysaccharides, proteins, drying additives, may be advantageous and therefore within the scope of the present invention.

[0066] As already indicated above, in principle, any type of microfibrillated cellulose (MFC) may be used in accordance with the present invention, as long as the fiber bundles as present in the original cellulose pulp are sufficiently disintegrated in the process of making MFC so that the average diameter of the resulting fibrils is in the nanometer-range and therefore more surface of the overall cellulose-based material has been created, vis-à-vis the surface available in the original cellulose material. MFC may be prepared according to any of the processes described in the art, including the prior art specifically cited in the “Background”-Section above.

Origin of the Cellulose Used to Prepare the MFC

[0067] In accordance with the present invention, there is no specific restriction in regard to the origin of the cellulose, and hence of the microfibrillated cellulose. In principle, the raw material for the cellulose microfibrils may be any cellulosic material, in particular wood, annual plants, cotton, flax, straw, ramie, bagasse (from sugar cane), suitable algae, jute, sugar beet, citrus fruits, waste from the food processing industry or energy crops or cellulose of bacterial origin or from animal origin, e.g. from tunicates.

[0068] In a preferred embodiment, wood-based materials are used as raw materials, either hardwood or softwood or both (in mixtures). Further preferably softwood is used as a raw material, either one kind or mixtures of different soft wood types. Bacterial microfibrillated cellulose is also preferred, due to its comparatively high purity.

Modified (Derivatized) and Non-Modified (Un-Derivatized) Cellulose/MFC

[0069] In Principle, the Microfibrillated Cellulose in Accordance with the Present Invention May be unmodified in respect to its functional groups or may be physically modified or chemically modified, or both.

[0070] Chemical modification of the surface of the cellulose microfibrils may be achieved by various possible reactions of the surface functional groups of the cellulose microfibrils and more particularly of the hydroxyl functional groups, preferably by: oxidation, silylation reactions, etherification reactions, condensations with isocyanates, alkoxylation reactions with alkylene oxides, or condensation or substitution reactions with glycidyl derivatives. Chemical modification may take place before or after the defibrillation step.

[0071] The cellulose microfibrils may, in principle, also be modified by a physical route, either by adsorption at the surface, or by spraying, or by coating, or by encapsulation of the microfibril. Preferred modified microfibrils can be obtained by physical adsorption of at least one compound. The MFC may also be modified by association with an amphiphilic compound (surfactant).

[0072] However, in preferred embodiments, the microfibrillated cellulose is not physically modified.

[0073] In a preferred embodiment of the present invention, the microfibrillated cellulose is prepared by a process, which comprises at least the following steps: [0074] (a) subjecting a cellulose pulp to at least one mechanical pretreatment step; [0075] (b) subjecting the mechanically pretreated cellulose pulp of step (a) to a homogenizing step, which results in fibrils and fibril bundles of reduced length and diameter vis-à-vis the cellulose fibers present in the mechanically pretreated cellulose pulp of step (a), said step (b) resulting in microfibrillated cellulose; [0076] wherein the homogenizing step (b) involves compressing the cellulose pulp from step (a) and subjecting the cellulose pulp to a pressure drop.

[0077] The mechanical pretreatment step preferably is or comprises a refining step. The purpose of the mechanical pretreatment is to “beat” the cellulose pulp in order to increase the accessibility of the cell walls, i.e. to increase the surface area.

[0078] Prior to the mechanical pretreatment step, or in addition to the mechanical pretreatment step, enzymatic (pre)treatment of the cellulose pulp is an optional additional step that may be preferred for some applications. In regard to enzymatic pretreatment in conjunction with microfibrillating cellulose, the respective content of WO 2007/091942 is incorporated herein by reference. Any other type of pretreatment, including chemical pretreatment is also within the scope of the present invention.

[0079] In the homogenizing step (b), which is to be conducted after the (mechanical) pretreatment step, the cellulose pulp slurry from step (a) is passed through a homogenizer at least once, preferably at least two times, as described, for example, in PCT/EP2015/001103, the respective content of which is hereby incorporated by reference.

EXAMPLES

Example 1

Preparation of Microfibrillated Cellulose

[0080] MFC in accordance with the present invention is commercially available and commercialized by Borregaard as “Exilva”, based on cellulose pulp from Norwegian spruce (softwood).

[0081] The MFC used for the spraying was present as a paste, having a solids content of 10%. The solvent was water.

[0082] The MFC was provided in two different qualities, named Exilva P and Exilva F. The differences between Exilva P and Exilva F are related mainly to the size of the aggregates of microfibrils and consequently to the 3D-network properties. Exilva “F” has higher Brookfield viscosity, surface area (water retention) and higher tensile strength than Exilva “P”.

Example 2

Preparing of an MFC Coating on a Non-Woven Substrate

[0083] 10% Exilva paste was fed into a progressive cavity pump with hopper and feeder (Netzsch) (see FIG. 2). This pump was feeding, in parallel, two smaller progressive cavity pumps with back-flow Each of the two smaller pumps was feeding directly to a full cone spraying nozzle, using pressurized air (Schilck, model 930/7-1).

[0084] Transportation from pump to nozzle: short as possible pipe and hose with large diameter (10 mm) and preferably no or few restrictions/bends.

[0085] Nozzle settings balancing flow-rate of paste with amount/pressure of atomizing air. Flow-rates of paste per nozzle: tested 200 to 1000 grams/min. Corresponding air-pressure: 3-6 bars.

[0086] Adjusted nozzle cap in outer position to get optimal and even spraying pattern.

[0087] The two nozzles were placed (fixed positions) vertically in the direction of the movement of the template. The nozzles were displaced to avoid interference of the droplets. The load of the paste to the template (Nonwoven felt) was tested from ca 5 to 50 g of dry Exilva/m.sup.2.

[0088] To evaluate the results, the coated surface (see FIG. 4) was dried at 180° C. for 60 sec in a convection oven and analyzed for air-permeability. The permeability decreased from ca 3500 to 100-300 L/m.sup.2*s.sup.−1.

Example 3

Preparing of MFC Coatings on Various Surfaces

[0089] 10% Exilva paste was fed into progressive cavity pump with hopper and feeder (Netzsch) (see FIG. 2). This pump was feeding one smaller progressive cavity pump with back-flow. This smaller pump was feeding directly to a full cone spraying nozzle, using pressurized air (Schilck, model 930/7-1)

[0090] Transportation from pump to nozzle: short as possible pipe and hose with large diameter (10 mm) and preferably no or few restrictions/bends.

[0091] Nozzle settings balancing flow-rate of paste with amount/pressure of atomizing air. Flow-rates of paste per nozzle: tested 200 to 1000 grams/min. Corresponding air-pressure: 3-6 bars.

[0092] The nozzle was placed (fixed position) vertically in the direction of the movement of the template, at a distance of approximately 30 cm.

[0093] The following surfaces were coated with microfibrillated cellulose, in accordance with the present invention: plexiglass, wood, painted chipboard, steel plate, rubber and cotton fabric. The coated surfaces were evaluated visually (see FIG. 5), and the coated layers were found even and free from agglomerations. The MFC coating adhered well to the different surfaces.