FI125900B - Process for Preparation of Nanocellulose Composite - Google Patents

Description

Method of manufacturing a nanocellulose composite

The present invention concerns a method of manufacturing a nanocellulose composite according to the preamble of claim 1.

According to such a method a nanocellulose feedstock is milled in liquid phase to produce a nanocellulose containing dispersion. The nanocellulose is combined with a polymer component.

The present invention also concerns novel nanocellulose composites.

The current production method of nanocellulose is based on milling and fluidisation in water. The energy consumption in the milling is high, 10-20 times higher (50,000 to 100,000 kwh/ton) than in the traditional pulping processes.

The process results in an aqueous dispersion of 0.5-2 w-% nanocellulose in water. Production of higher consistencies is not possible due to limitations of the processing, resulting from the hydrogen bonding network formed by the nanocellulose hydroxyl groups and water. Evaporation of water is not, either, possible without agglomeration of the nanocellulose. If agglomerates are formed, re-dispersion effort equal to original milling is required.

As far as conventional manufacture of nanocellulose is concerned, reference is made to methods known per se, for example, as disclosed in US 2007/0207692, WO 2007/91942, JP 2004204380 and US 7,381,294.

JP 2009/292045 discloses a wet dispersing method which includes treating surface-modified cellulose fibres using 0.01-0.5-mm-average-diameter beads to give microfibrils with average fiber diameter from 2 nm to 1.0 pm. Thus, modifying cellulose fibers with acetic anhydride, adding a dispersing agent of cellulose acetate propionate to a MeCCfi solution containing the cellulose acetate fibers, and dispersing the cellulose acetate fibers in a mill gave a dispersion showing fiber average diameter 900 nm.

JP 2009/102630 discloses manufacturing microfibrous cellulose, involves ozonizing cellulose fiber, dispersing the fiber in water to obtain water-based suspension liquid of cellulose fiber, and pulverizing the fiber to obtain microfibrous cellulose with maximum fiber width of 1000 nm or less.

WO 2009081881discloses manufacture of fibre composites, involving disentangling fiber in pressure homogenizer which is made to eject 100 MPa or more pressure in pressure-reduced condition and/or ultrasonic wave with power density of 1 W/cm2 or more in frequency of 15 KHz/1 MHz and conjugating the fiber to matrix material; and disentanglement method of cellulose fiber, which involves irradiating ultrasonic wave to the dispersion liquid of cellulose fiber in which the average of maximum length which is obtained from the plant-derived raw material is 10 mu m to 10 cm.

There are further problems relating to the known technology. Thus, transportation of low consistency water dispersions is very inefficient. The high water content and also the hydroxyl functionalities of the cellulose prevent dispersion in thermoplastics and thermoset monomers (which typically react with water)

Suggested methods (freeze drying, filtering) for extraction of water and forming dry nanocomposite are inefficient and not scalable to industrial production.

Attempts have been made to disperse nanocellulose prepared in aqueous media milling into both thermoplastic and thermoset polymers. In thermoplastics, the differences in the polarity of the matrix (non-polar), fibre (highly polar) and water (highly polar) prevent dispersion and reinforcement or require expensive, inefficient surface modifications multiplying the price of the fibre material and resulting in a composition consisting essentially of the modification/coating - due to high specific surface area of nanocellulose.

In thermosets, some attempts in laboratory to disperse nanocellulose prepared by the standard aqueous milling have been published and protected. However, they require the drying off of the water (99% of composition) and result in agglomerates which need to be re-dispersed by expensive, slow and mostly inefficient methods such as ultrasound.

It is an aim of the present invention to provide composite structures of high nanocellulose contents.

The invention is based on the idea of producing the composite structures by milling the raw-material of the nanocellulose material in the presence of a monomer of a reactive thermoset polymer. Subsequently, the nanocellulose dispersion so obtained is hardened to form a resin, which optionally can be still further cured.

As a result, a nano-cellulosic composite is obtained wherein the nanocellulosic components are cross-bonded to the thermoset polymer components of the composite.

More specifically, the present method is mainly characterized by what is stated in the characterizing part of claim 1.

The present product is mainly characterized by what is stated in the characterizing part of claim 12.

Considerable advantages are obtained by the invention. Thus, the present composites are high performance composites with a biobased reinforcement, in particular a reinforcement based on a material obtained from a renewable source. These materials will provide a substitute or replacement of conventional, expensive, glass and carbon fibre composites which are based on non-renewable materials and which require labour intensive processing and manufacturing.

Further, the present method provides for manufacturing of the reinforcement directly in the would-be composite. Expensive and energy intensive drying of any aqueous dispersion, as conventionally called for, is avoided. Agglomeration of the nanocellulose during such a drying process can also be avoided. There is no need for manual hand lay-up processing related to long-fibre composites by using the present technology of nano-scale reinforcement of the composites. There will be formed a strong cross-linking of the cellulosic fibres to the polymer matrix, based on covalent bonds between the components.

Experiments performed show that in less polar solvents or other media the milling can be done at 20-50 times the consistency that is possible with water. The consistency can be further raised by centrifugal concentration to a level sufficient or even above the ideal reinforcement contents in the following be composite structure. Due to the low viscosity of the dispersions (in contrast with the water-nanocellulose dispoersions), they can be applied by liquid pumping methods such as those in resin transfer molding (RTM) processing.

Next embodiments are discussed more closely with reference to the manufacture of reinforced thermoset resins using pearl mills or basket mills.

As referred to in the above discussion, in the present invention, a suitable raw-material, such as macroscopic cellulose, for the target nanocellulose material, is combined at a predetermined ratio with a component of or a mixture of components of a thermoset resin. In the following the “component(s)” will also be referred to as “resin precursors”.

The feeds are milled together such as to achieve cross-linking at least essentially based on covalent bonds or similar chemical bonds. Some physical bonds may also be formed during the processing.

In an advantageous embodiment the conditions of the milling are non-aqueous. In particular, essentially water-free processing is considered a highly preferred embodiment. Water-free conditions will assist in avoiding the problems relating to drying of the nano-cellulosic material.

The cellulose raw-material and the resin precursor(s), for example, monomer(s), are subjected to milling in, for example, a pearl mill or basket mill or a similar mill capable of exerting milling or grinding forces for diminishing the size of the raw-material into nano-size scale.

In a pearl mill there are typically grinding pearls, typically ceramic beads, or sand particles which provide the grinding effect. Typically, the average diameter of the pearls or beads is about 0.01-0.5-mm.

In a basket mill there is atypically a hub with agitator pins. Said pins create a high-energy zone or a vortex within a basket, and when the material is fed into the basket it will pass through the vortex it will be subjected to the high-energy action.

The nanocellulose can be synthesised from various cellulosic materials, such as wood, agrofibres, such as annual plants, and food production side-streams.

The cellulose feedstock is preferably chemically pretreated and purified.

Thus, in one embodiment, the raw-material of the nanocellulose comprises macroscopic cellulose, for example cellulosic fibres, in particular cellulosic fibres obtained by defibering of lignocellulosic raw-material, optionally bleached.

Vegetal (annual or perennial) fibres instead of wood are another interesting material source; for example nanofibers separated from a residue from carrot juice production, are highly homogeneus nanofibers with better mechanical properties than wood nanofibers. The obtained nanocellulose is in the form of fibres with a diameter between 1.5-5 nm and length up to several micrometers.

In the present context, “nano-scale” stands for a smallest size of the particles amounting to about 1 to 950 nm, in particular about 10 to 900 nm, for example about 20 to 500 nm, typically about 25 to 200 nm. In particular, the raw-material is diminished such that at least 5 %, preferably at least 10 %, in particular at least 20 % of the particles exhibit a smallest dimension in the nano-scale range.

In one embodiment, "nanocellulose" refers to any cellulose fibers with an average (smallest) diameter (by weight) of 10 micrometer or less, preferably 1 micrometer or less, and most preferably 200 nm or less. The "cellulose fibers" can be any cellulosic entities having high aspect ratio (preferably 100 or more, in particular 1000 or more) and in the above-mentioned size category. These include, for example, products that are frequently called fine cellulose fibers, microfibrillated cellulose (MFC) fibers and cellulose nanofibers (NFC). Common to such cellulose fibers is that they have a high specific surface area, resulting in high contact area between fibers in the end product. As a result dispersed NFCs will form networks within polymer matrices.

The aspect ratio of the nanocellulose is typically greater than 5.

Typically milling is carried out in liquid phase, i.e. in the liquid phase formed by the component(s) of the thermoset resin. Thus, in one embodiment, the liquid phase resin precursor(s) - for example monomer(s) - of the thermoset are first added to the mixing space of the mill

In one embodiment, a liquid dispersion is formed having a consistency (solid matter content based on the total weight of the dispersion) of at least about 0.01 %, preferably about 0.1 to 50 %, in particular about 0.2 to 25 %, for example about 0.3 to 20 %, typically about 0.5 to 15 %.

Milling is continued until a sufficient reduction in dimension of the cellulose has been achieved.

Optionally, the liquid dispersion can be heated to lower the viscosity and to enhance the process. The heating can be carried out before addition of the raw-material, in practice the cellulose material, or during the addition or at intervals between additions of the raw-material.

Typically, temperatures in excess of room temperature can be used, for example at least 30 °C, in particular at least 35 °C, for example 40 to 100 °C, in particular about 45 to 90 °C, at ambient pressure. The heating is preferably carried out at a temperature below the boiling point of the liquid precursor at the prevailing pressure.

Typically, milling is carried out in ambient pressure in an open vessel. Naturally, by suitable modifying the milling equipment, operation at increased pressures of, for example 1.1 to 15 bar(a) is also possible.

Following the milling step, a second component of the thermoset along with further additive(s), if any, is added to the dispersion. The second component is typically a hardener which achieves the cross-linking of the monomer and, thus, setting of the thermo set. The addition of the second component of the thermoset along with further additive(s) is preferably carried out in a separate mixing step. This mixing can be carried out in the same equipment or in a separate mixing or blending unit. Mixing is continued to allow for the formation of a resin.

Finally, the resin thus obtained can be further treated and used in various fashions.

For example, the resin can be transferred to a mold. It can be impregnated to a fibre structure. It can be applied as a coating on a substrate surface.

As mentioned above, additional benefits of the present technology are obtained by replacement of hand-lay up processing in fibre composites with a 100 % liquid injection based system.

Further setting and curing of the material or composite can be achieved by applying heat.

Typically, temperatures in excess of room temperature are applied, for example at least 30 °C, in particular at least 50 °C, for example 60 to 400 °C, in particular about 100 to 350 °C.

The thermoset monomers used herein include for example polyols of polyester resins and epichorohydrin of epoxy resins.

For epoxy resins, the hardener is typically a polyfunctional curative. Basically, a molecule containing a reactive hydrogen may react with the epoxide groups of the epoxy resin. Hardeners for epoxy resins can be selected from the group consisting of amines, acids, acid anhydrides, phenols, alcohols and thiols.

As known in the art, there are some combinations of epoxy resins and hardeners which will cure at ambient temperature, but in practice many require additional heat, a typical hardening temperature being higher than to 100 °C, in particular higher than 150 °C, for example up to a maximum of about 200°C for most of the systems.

In the case of polyester resins, the polyol is typically a glycol, such as ethylene glycol, which is combined with an acid, such as phthalic acid or maleic acid. Hardening is achieved by creating free radicals at the unsaturated bonds of the material. The free radicals can be induced by adding a compound that easily decomposes into free radicals, such as an organic peroxide, for example benzoyl peroxide or methyl ethyl peroxide.

The resin can be photocurable.

Summarizing the above, nanocellulose will be produced by pearl milling of chemically pretreated and purified cellulose in the pre-polymer using polyester and epoxy resins. Both the polyester and epoxy resins readily react with the cellulose fibre surface hydroxyl groups, resulting in a cross-linked composite structure where the reinforcement fibre is integrally bound and its full potential may be exploited - without surface modification.

The present composites are useful for high-performance composite applications. The current, strong trend of automotive industry to reduce the weight of structures by replacing metals with FR composites. The drawback of present composites has been their high price and difficult recycling. With the present technology, these problems can be overcome.

Other applications include uses in the fields of, for example, aeronautics and aerospace, sports equipment, coatings and lacquers, glues and adhesives.

Claims (12)

A process for the preparation of nanocellulose composites comprising a nanocellulose material mixed with a polymer, characterized in that the process comprises: comprising a polymer precursor or a mixture of precursors, and - curing the dispersion. Process according to Claim 1, characterized in that the nanocellulose dispersion is combined with the curing component of the thermosetting resin. Process according to Claim 1 or 2, characterized in that the nanocellulose raw material comprises cellulose fibers, in particular cellulose fibers obtained from the lignocellulosic raw material, which are optionally bleached. Method according to any one of the preceding claims, characterized in that the feeds are co-milled to achieve crosslinking based on at least substantially covalent bonds or similar chemical bonds. Method according to one of the preceding claims, characterized in that the grinding is carried out under anhydrous, in particular substantially water-free, conditions. A method according to the preceding claim of Tonk, characterized in that the cellulosic raw material and the polymer precursor (s) are subjected to refining in a bead or basket mill or a similar refiner capable of providing refining or grinding forces to reduce the raw material to a nanoscale size. A method according to the preceding claim of Tonk, characterized in that a liquid dispersion is formed which has a consistency of at least about 0.01%, in particular about 0.1-50%. A method according to the preceding claim of Tonk, characterized in that the liquid dispersion can be heated to reduce the viscosity and enhance the process. Method according to one of the preceding claims, characterized in that the second component of the thermosetting resin is added together with any other additives, in particular to effect crosslinking of the monomer. A method according to claim 9, characterized in that the reinforced polymer is transferred to a mold, impregnated into a fibrous structure or applied to a substrate surface, or a combination of these operations is performed. The method according to the preceding claim, wherein the polymer precursor is a polyol of a polyester polymer or an epichlorohydrin of an epoxy polymer. A nanocellulose composite obtained by the method of Tonk as claimed in claims 1-11.