Technical field

[0001 ] The present invention relates generally to a method for producing thermoplastic composites comprising nano-cellulose, specifically thermoplastic composites comprising 0.1-75% nano-cellulose by weight. The invention furthermore relates to products formed by the thermoplastic composite and articles produced from said thermoplastic composite products.

Background art

[0002] Dispersion of nano-cellulose in polymers and products derived from these polymers are known. However, the dispersion is often difficult, for example due to the tendency of the nano-cellulose to agglomerate. It is therefore difficult to get nano-cellulose homogenously dispersed in a polymer, such as a thermoplastic. Dispersions of nano-cellulose can be concentrated by various means, including reverse osmosis and distillation of water. The viscosity increases significantly upon concentration. Unfortunately, nano-cellulose has a tendency to re-agglomerate rather quickly, therefore incorporating as much nano-cellulose as possible in plastics is difficult. Avoiding re-agglomeration and achieving a homogenous distribution in the polymer will furthermore be a compromise between performance and costs.

[0003] Maleic anhydride (MALA) has traditionally been used as additive, e.g. at 1-6 %, typically 2-3 % wt., for increasing the adhesion between plastic and wooden material. MALA has a range of negative properties, including toxicity, smell, cost and corrosive stress for process equipment.

[0004] The following disclosures use surface-active ingredients, known as surfactants, to enable mixing of thermoplastics and cellulose in general: US 2008 0241 262 (nano-shells), US 2016 0303 102 (drug formulations), WO 2010 072 665 (Nano-fibers for textiles produced by electrospinning), US 2011 0250 256 (Genic Co, Seoul, Korea, Nano-web porous films, including gelling agent). [0005] One possible way of mixing thermoplastics with cellulose is disclosed in US 810 5682 (Gang Sun et al, Univ, of California, Oakland, 2007). It describes the mixing of certain cellulose derivatives, such as cellulose acetate butyrate (CAB), with microfibers based on thermoplastics in an extrusion device, using solvents including water. It is disclosed that the matrix (CAB) and thermoplastic are “thermodynamically immiscible”, however it is clear to those skilled in the art that any alkyl-modification of sugar-based chemicals renders those more hydrophobic, easing blending in an extruder.

[0006] US 2013 013 3848 (Akzo Nobel, Heijnesson-Hulten et al.) discloses cellulosic sheet products (paper) comprising thermoplastic microspheres.

[0007] A planetary extruder is a type of extruder generally comprising a central extruder spindle surrounded by a plurality of planetary spindles around its circumference. The planetary spindles are sometimes arranged with threads or inclined tracks, having an angle of approximately 45°. When the central spindle turns, the planetary spindles turn with it, but in the opposite direction. The barrel, in which the spindles are commonly arranged, may be arranged with tracks on its inner surface to mesh with the planetary spindles. An example of a planetary extruder is disclosed in US Patent US4268176A. Another version of a planetary extruder is disclosed in US Patent US9061442B2, both of which are incorporated herein by reference. In the latter, a plurality of screws, also referred to as degassing screws, surround a central shaft. Both of these two extruders display how a plurality of screws may provide both a high level of kneading, as well as efficient degassing.

Definitions

[0008] Nano-cellulose: Nano-cellulose shall be understood as cellulosic fibres, in particular cellulosic fibres obtained by defibring of lignocellulosic raw-material, optionally bleached, with typical fibre lengths of 10 nanometres up to 100 micrometres, wherein a size distribution of non-agglomerated fibres may peak at approximately 50-500 nanometres. The term may be understood as for example nanocrystalline cellulose (NCC) or microfibrillar cellulose (MFC).

[0009] Thermoplastics: Thermoplastics shall be considered as the group of polymers which melt at a temperature in the range 60 - 200°C, comprising polyolefins, including polyethylene (PE), polypropylene (PP), polyesters including polyethylene terephthalate (PET), polylactic acid (PLA) and other plastics which can be processed in compounding devices such as twin-screw compounding machines or in extruders, specifically planetary extruders.

[0010] Compounding device: As will be described, in the method of the present disclosure, an aqueous dispersion is brought into contact with a hydrophobic polymer with the intention to transfer ingredients from the aqueous phase to the polymer. This requires intensive mixing and removal of water. Therefore, a compounding device shall denote any intensive mixer, such as kneaders, batch mixers, continuous mixers, extruders capable of mixing materials and standard twin-screw compounders which are able to mix polymer blends under pressure, and which allow venting and removal of gases such as water vapour in a controlled manner, e.g. by pressure relief valves.

Summary of invention

[0011 ] An object of the present invention is to overcome at least some of the difficulties outlined above. This is done by providing a method for producing a thermoplastic composite comprising 0.1-75 % by weight nano-cellulose, comprising the following steps: providing nano-cellulose as a waterborne dispersion wherein said dispersion comprises 0.2-30% by weight nano-cellulose; providing molten thermoplastic in a compounding device which allows for both mixing and controlled venting of gas through a venting system of the compounding device; adding said nano-cellulose dispersion to the compounding device and mixing said nano-cellulose dispersion and the thermoplastic in said compounding device to form the thermoplastic composite, wherein the mixing is performed at a temperature of 60-200° C and at a pressure of 1 bar or higher, preferably 10 bar or higher; removing water in the form of water vapour by controlled venting through the venting system of the compounding device; and forming the composite into a solidified composite product, such as, but not limited to, an extrusion product, a pellet or a film; wherein the water removal from the nano-cellulose dispersion during mixing is at least 90% by weight, preferably at least 95%.

[0012] The affinity of cellulose to plastics is the driving force for the incorporation of cellulose into plastics. However, since the nano-cellulose is highly prone to agglomeration into larger clusters of cellulose, it is provided in a water dispersion at low concentrations of 0.2-30% by weight nano-cellulose. Thus, the dispersion comprises a lot of water which must be removed. The water presents difficulties when striving for a homogenous dispersion of nano-cellulose in a thermoplastic due to the hydrophobic nature of the polymer. Therefore, controlling the temperature, pressure, mixing and venting is vital for achieving a homogenous dispersion. Furthermore, nano-cellulose may replace MALA in plastic / wood composites, i.e. adhesion between macroscopic, micrometre to millimetre size wooden material and plastic is improved by incorporation of nanocellulose.

[0013] In an embodiment of the present disclosure, the nano-cellulose is cellulosic fibres, preferably cellulosic fibres obtained by defibring of lignocellulosic raw-material, and with fibre lengths of 10 nanometres to 100 micrometres.

[0014] In an embodiment of the present disclosure, the nano-cellulose is one of nanocrystalline cellulose (NCC) or microfibrillar cellulose (MFC).

[0015] In an embodiment of the present disclosure, the nano-cellulose dispersion further comprises an alcohol, preferably below 50% by weight.

[0016] In an embodiment of the present disclosure, the nano-cellulose dispersion is added continuously to the compounding device.

[0017] In an embodiment of the present disclosure, the nano-cellulose dispersion is added to the compounding device by use of a pressure pump.

[0018] In an embodiment of the present disclosure, the thermoplastic is an organic polymer with a molecular weight above 10 000 g/mol. [0019] In an embodiment of the present disclosure, the thermoplastic is based on monomers comprising ethylene, and furthermore being selected from: polyester, polyolefins, high density polyethylene, propylene, butene, isoprene, butadiene, terephthalic acid, lactic acid (PLA), glycols, thermoplastic elastomers including SBS, SIS and SEBS or a mixture thereof.

[0020] In an embodiment of the present disclosure, the method further comprises the step of, after forming of the composite, drying the composite to a residual water content of below 10% by weight, preferably below 5% and more preferably below 1 % by treatment in a drying device.

[0021 ] In an embodiment of the present disclosure, the nano-cellulose dispersion further comprises carbon nano tubes (CNT) in an amount such that the content of CNT in a final composite composition ranges from 0.1 % by weight to 10% by weight, and wherein the CNT may be dispersed in the nano-cellulose dispersion by one of: dispersion by ultrasound separately in water and thereafter mixed with nano-cellulose water dispersion; or dispersed directly in the nano- cellulose dispersion.

[0022] The CNT gives the composite many advantageous properties e.g. electrical conductivity or microwave absorbency. Furthermore, mixing the CNT/cellulose dispersion with thermoplastics results in a faster phase transfer of CNT/cellulose into the thermoplastic compared with the phase transfer rate of only nano-cellulose. This is for example due to the increased hydrophobicity of the CNT/cellulose complex.

[0023] In an embodiment of the present disclosure, the compounding device is a twin-screw compounder.

[0024] In an embodiment of the present disclosure, the compounding device is an extruder comprising a screw assembly comprising more than two extruder screws. [0025] In an embodiment of the present disclosure, the more than two extruder screws are arranged surrounding a central axis of the extruder, wherein the central axis is one of a central screw or a central drive shaft.

[0026] In an embodiment of the present disclosure, the compounding device is a planetary extruder.

[0027] The planetary extruder is a compounding device which provides a large polymer surface available for the nano-cellulose dispersion and thereby improves the dispersion of nano-cellulose in the polymer. The planetary extruder is highly effective in shearing both molten plastics and agglomerated nano-cellulose. This is partly due to the threaded surfaces of the central and the planetary spindles. This way, the extruder constantly provides much more new, available surface than many other extruder types. New surface is important for the adsorption and incorporation of nano- cellulose in the polymer. The planetary extruder is furthermore equally efficient at lower (e.g. 0.5%) and higher (e.g. 25%) concentration of nano-cellulose in the dispersion.

[0028] In another aspect of the present disclosure there is provided a composite product comprising 0.1-75 % by weight nano-cellulose and a thermoplastic, produced by the method according to the present disclosure.

[0029] In an embodiment of the present disclosure, the thermoplastic is produced from recycled material, preferably with a content of a polymer from a single source of at least 90 %, such as high-density polyethylene from plastic bottles, low-density polyethylene films from agricultural use, or PLA from food packaging from fast food restaurants.

[0030] In yet another aspect of the present disclosure there is provided a composite article produced from a composite product according to claim 8 or 9, wherein the composite article is an extruded or injection-moulded article. Description of embodiments

[0031] In the following, a detailed description of a method to produce a thermoplastic composite comprising a thermoplastic polymer and 0.1-75% by weight nano-cellulose is provided. The method results in thermoplastics comprising well dispersed nano-cellulose. The nano-cellulose may give the thermoplastic improved properties such as increased impact resistance, increased tensile strength, and improved barrier properties. Improved barrier properties may for example be provided by producing a film made of the thermoplastic composite, wherein the nano-cellulose may prevent or lessen diffusion of air through the film. The disclosed method is energy-efficient, economic and scalable.

[0032] The method generally comprises mixing of a liquid or molten thermoplastic and nano-cellulose provided as water-borne dispersion in a compounding device. Upon mixing e.g. in a kneader or compounding device or during extrusion, nano-cellulose transfers to the organic polymer phase, and water is vaporized and removed through a venting system of the compounding device. The waterborne nano-cellulose dispersion comprises 0.2-30% by weight, preferably 0.2-10% by weight, more preferably 0.3-5% by weight nano-cellulose and the water removal from the nano-cellulose dispersion during mixing is at least 90% by weight, preferably at least 95%.

[0033] In one embodiment the thermoplastic is an organic polymer with a molecular weight above 10 000 g/mol. In one embodiment the thermoplastic is based on monomers comprising ethylene. The thermoplastic may for example be selected from: polyester, polyolefins, high density polyethylene, propylene, butene, isoprene, butadiene, terephthalic acid, lactic acid (PLA), glycols, thermoplastic elastomers including SBS, SIS and SEBS or a mixture thereof. The thermoplastic may for example be, but is not limited to, polyethylene (PE) and polypropylene (PP) in the case of polyolefins, or polyethylene terephthalate (PET), polylactic acid (PLA) or similar in the case of polyesters. In one embodiment, the thermoplastic is produced from recycled material, preferably with a content of a polymer from a single source of at least 90 %, wherein the single source may be for example high- density polyethylene from plastic bottles, low-density polyethylene films from agricultural use, or PLA from food packaging.

[0034] For low-performance products, simple one-screw extruders may be used as the compounding device. For high-performance goods with higher demands on strength, barrier properties or adhesion to various substrates, more advanced compounding devices may be used such as twin-screw extruders or planetary extruders.

[0035] In this disclosure it is considered that materials of very different viscosities are mixed. One technical problem to be solved by the invention is therefore to provide a large surface of hydrophobic plastic for the polar phase of nano-cellulose, in order to facilitate the efficient transfer of nano-cellulose to the organic polymer phase without the nano-cellulose re-agglomerating to micrometre or millimetre size particles during the removal of water. For some applications, one-screw or twin-screw extruders provide a sufficiently large polymer surface for the nano-cellulose dispersion.

[0036] In one embodiment, an extruder is used having a screw assembly comprising more than two screws in order to provide an even larger contact surface between the polymer and the nano-cellulose dispersion. The screw assembly comprising more than two screws should be understood as more than two screws arranged in the same section of the extruder, being arranged essentially in parallel. The extruder may furthermore comprise a plurality of sections along its length, each comprising a screw assembly comprising one or a plurality of screws. In this embodiment, the extruder comprising more than two screws enables both efficient mixing with little mechanical stress on the wooden or cellulosic fibres, as well as removal of water as steam through one or more venting zones. The extruder has a modular structure, with adjustable compounding and degassing sections, and can therefore be adapted to the compound at hand and is suitable for various combinations of thermoplastic and nano-cellulose. Low concentrations of cellulose in water require more intensive or longer agitation times. Various degassing stations are advisable. As the volume of material decreases significantly upon water removal, it is practical to produce master batches with different residual water levels, and the final dry product is produced from master batches. In one embodiment the extruder comprising more than two screws is a planetary extruder.

[0037] The waterborne nano-cellulose dispersion comprises 0.2-30% by weight, preferably 0.2-10% by weight, more preferably 0.3-5% by weight nano-cellulose. In one embodiment, the nano-cellulose dispersion is added to the compounding device containing the molten thermoplastic by means of a pressure pump.

[0038] In one embodiment of the present disclosure, the nano-cellulose dispersion further comprises up to 50% of an alcohol, preferably ethanol. However, other alcohols are possible such as isopropanol or methanol. The alcohol is also vaporized during mixing in the compounding device and let off through the venting system of the compounding device.

[0039] In order to vaporize and remove the water, and possibly alcohol, the temperature during mixing of the composite in the compounding device is preferably 60-200°C, more preferably 100-200°C. Furthermore, a pressure inside the compounding device is kept at above 1 bar, preferably above 10 bar. To achieve this, the venting system of the compounding device allows for controlled venting in order to both release water vapor, and possibly alcohol, as well as keeping the desired pressure in the compounding device. The venting system furthermore provides that the water/alcohol is not removed too quickly, in order to make sure that the nano-cellulose is sufficiently dispersed in the thermoplastic before vaporizing.

[0040] In one embodiment, additives are added to the compounding device to be mixed with the composite. Additives may for example be, but are not limited to, functional additives such as carbon nano tubes (CNT) for imparting e.g. electrical conductivity or microwave absorbency, antioxidants, colorants, or fillers including wood fibres and mineral fillers such as silica, titanium dioxide, or other fillers, preferably with small particle sizes. [0041] In one embodiment, polar particles such as fine silica or titanium dioxide can be added, e.g. at concentrations of 0.2-3% wt. of the final composite product. This is beneficial for the preservation of the nano-size distribution, i.e. for the prevention of agglomeration and subsequent presence of agglomerated cellulose particles in the final products.

[0042] In one embodiment, wooden materials including medium-density fibres (MDF) or wooden waste materials are added, in weight ratios of thermoplastic I wooden material between 5/1 to 1/5. Thereby, materials for economically competitive composites can be obtained, alternatively pellets and sticks for use as combustible materials are prepared. These articles are useful for cooking food in outdoor activities or in developing countries where food is prepared over open fire. The a.m. products can replace firewood, and are even preferred to firewood due to higher energy density, clean combustion profile and due to the fact that recycling of plastic waste, in particular low density polyethylene is enabled. In this embodiment, one of the main advantages of the nano-cellulose is to increase adhesion between the plastic and the wooden material, thereby significantly reducing, or even eliminating the need to add adhesion increasing agents such as MALA. In this embodiment, the content of nano-cellulose is typically below 1 % by weight. Furthermore, residual water from the nano-cellulose dispersion may be used as blowing agent to afford porous products with higher surface, e.g. for easier use as combustible fuel.

[0043] In one embodiment, carbon nano tubes (CNT) are added to the composite. The CNT are dispersed in water using ultrasound through known procedures. The CNT water dispersion is added to the nano-cellulose water dispersion, and the mixture is further dispersed in order to afford complexes of CNT and nano-cellulose. CNT may also be directly dispersed with ultrasound in the nano-cellulose water dispersion; however, increased temperatures are required in that case. Mixing said CNT/cellulose dispersion with thermoplastics results in a faster phase transfer of CNT/cellulose into thermoplastics compared with the phase transfer rate of pure nano-cellulose. This is for example due to the increased hydrophobicity of the CNT/cellulose complex. Products resulting from the CNT enhanced composite are characterized by increased electrical conductivity and microwave absorbance. The weight ratio CNT I nano-cellulose vary depending on intended use, e.g. between 5:95 to 95:5, but a 30:70 to 70:30 weight ratio is preferred.

[0044] Mixing of the composite is continued until the desired homogeneity is reached and/or the desired amount of nano-cellulose is dispersed in the polymer. Homogeneity may for example be measured by visual measuring, using a suitable visual aid, of an average distance between the individual dispersed nano-cellulose fibres. In one embodiments the desired amount of nano-cellulose is achieved by performing the method of the present disclosure one time. In another embodiment the method of the present disclosure is repeated a plurality of times to achieve the desired amount of nano-cellulose in the polymer.

[0045] After the desired homogeneity is reached and/or the desired amount of nano-cellulose is dispersed in the polymer, a thermoplastic composite product is formed having improved mechanical and other properties, including barrier properties. For example, such products may be typical extrusion products, or pellets for example for use in injection moulding machines, or films. The product may be used on its own, or further processed into a composite article. Forming into an article may be performed for example by extrusion, pressure-moulding, injection-moulding, blow-moulding, rotation-moulding or other suitable method. The article may for example be articles for cooking or heating, extruded articles such as tubes or profiles for fencing and terraces and construction, injection- moulded or pressure-moulded articles for furniture and car interior purposes, films for packaging coatings, adhesives, sealants, or other end-uses.

[0046] In one embodiment, the mixed raw materials or products can be further dried to a residual water content of below 10% by weight, preferably below 5% and more preferably below 1 % by treatment in a drying device for example by vacuum.

[0047] Preferred embodiments of a method for producing thermoplastic composites comprising nano-cellulose and its derived products have been disclosed above. However, a person skilled in the art realises that this can be varied within the scope of the appended claims without departing from the inventive idea.

[0048] All the described alternative embodiments above or parts of an embodiment can be freely combined or employed separately from each other without departing from the inventive idea as long as the combination is not contradictory.