Patent Application
20130012695
The invention relates to the manufacture of fibres using cellulose nano-fibrils, in particular cellulose nano-fibrils extracted from cellulose material such as wood pulp.
Cellulose is a straight-chain polymer of anhydroglucose with β1-4 bonds. A great variety of natural materials comprise a high concentration of cellulose. Cellulose fibres in natural form comprise such material as cotton and hemp. Synthetic cellulose fibres comprise products such as rayon (or viscose) and a high strength fibre such as lyocell (marketed under the name TENCEL™).
Natural cellulose exists in either an amorphous or crystalline form. During the manufacture of synthetic cellulose fibres the cellulose is first transformed into amorphous cellulose. As the strength of the cellulose fibres is dependent upon the presence and the orientation of cellulose crystals, the cellulose material can then be re-crystallised during the coagulation process to form a material provided with a given proportion of crystallised cellulose. Such fibres still contain a high amount of amorphous cellulose. It would therefore be highly desirable to design a process to obtain cellulose-based fibres having a high content of crystallised cellulose.
Advantages of using cellulose for the manufacture of fibres includes its low cost, wide availability, biodegradability, biocompatibility, low toxicity, dimensional stability, high tensile strength, lightweight, durability, high hygroscopicity and easiness as to surface derivatization.
The crystallised form of cellulose which can be found in wood, together with other cellulose based material of natural origin, comprises high strength crystalline cellulose aggregates which contribute to the stiffness and strength of the natural material and are known as nano-fibres or nano-fibrils. These crystalline nano-fibrils have a high strength to weight ratio which is approximately twice that of Kevlar but, at present, the full strength potential is inaccessible unless these fibrils can be fused into much larger crystalline units. These nano-fibrils, when isolated from the plant or wood cell can have a high aspect ratio and can form lyotropic suspensions under the right conditions.
Song, W., Windle, A. (2005) “Isotropic-nematic phase transition of dispersions of multiwall carbon nanotube” published in Macromolecules, 38, 6181-6188 described the spinning of continuous fibres from a liquid crystal suspension of carbon nanotubes which readily form a nematic phase (long range orientational order along a single axis). The nematic structure permits good inter-particle bonding within the fibre. However natural cellulose nano-fibrils, once extracted from their natural material, generally form a chiral nematic phase (a periodically twisted nematic structure) when the concentration of nano-fibrils is above about 5-8% and would therefore prevent the nano-fibrils from completely orienting along the main axis of a spun fibre. Twists in the nano-fibril structure will lead to inherent defects in the fibre structure.
In the article “Effect of trace electrolyte on liquid crystal type of cellulose micro crystals”, Longmuir; (Letter); 17(15); 4493-4496, (2001), Araki, J. and Kuga, S. demonstrated that bacterial cellulose can form a nematic phase in a static suspension after around 7 days. However, this approach would not be practical for the manufacture of fibres on an industrial basis and is specifically related to bacterial cellulose which is difficult and costly to obtain.
Kimura et al (2005) “Magnetic alignment of the chiral nematic phase of a cellulose microfibril suspension” Langmuir 21, 2034-2037 reported the unwinding of the chiral twist in a cellulose nano-fibril suspension using a rotating magnetic field (5 T for 15 hours) to form a nematic like alignment. This process would not however be usable in practice to form a usable fibre on an industrial level.
Work by Qizhou et al (2006) “Transient rheological behaviour of lyotropic (acetyl)(ethyl) cellulose/m-cresol solutions, Cellulose 13:213-223, indicated that when shear forces are high enough, the cellulose nano-fibrils in suspension will orient along the shear direction. The chiral nematic structure changes to a flow-aligned nematic-like phase. However, it was noted that chiral nematic domains remain dispersed within the suspension. No mention was made relating to practical applications of the phenomena such as the formation of continuous fibres.
Work by Batchelor, G. (1971) “The stress generated in a non-dilute suspension of elongated particles in pure straining motion”, Journal of Fluid Mechanics, 46, 813-829, explored the use of extensional rheology to align a suspension of rod-like particles (in this case, glass fibres). It was shown that an increase in concentration, but especially an increase in aspect ratio of the rod-like particles results in an increase in elongational viscosity. No mention was made of the potential for unwinding chiral nematic structures present in liquid crystal suspensions. British patent GB1322723, filed in 1969 describes the manufacture of fibres using “fibrils”. The patent focuses primarily on inorganic fibrils such as silica and asbestos but a mention is made of microcrystalline cellulose as a possible, albeit hypothetical, alternative.
Microcrystalline cellulose is a much coarser particle size than the cellulose nano-fibrils. It typically consists of incompletely hydrolyzed cellulose taking the form of aggregates of nano-fibrils which do not readily form lyotropic suspensions. Microcrystalline cellulose is also usually manufactured using hydrochloric acid resulting in no surface charge on the nano-fibrils.
GB 1322723 generally describes that fibres can be spun from suspension which contains fibrils. However the suspensions used in GB 1322723 have a solids content of 3% or less. Such solids content is too low for any draw down to take place. Indeed, GB 1322723 teaches to add a substantial amount of thickener to the suspensions. It should be noted that the use of a thickener would prevent the formation of a lyotropic suspension and interfere with the interfibril hydrogen bonding that is desirable for achieving high fibre strength.
Also a 1-3% suspension of cellulose nano-fibrils, particularly one containing a thickener, would form an isotropic phase. GB 1322723 does not deals with the problems associated with using concentrated suspension of fibrils, and in particular using suspensions of fibrils which are lyotropic.
It is now provided a method which can be used to manufacture highly crystallised cellulose fibres using, in particular, naturally occurring crystallised cellulose.
The present invention is directed to a method for the manufacture of cellulose based fibres, in particular continuous fibres, of cellulose nano-fibrils being aligned along the main axis of the fibre, from a lyotropic suspension of cellulose nano-fibrils, said nano-fibril alignment being achieved through extension of the extruded fibre from a die, spinneret or needle, wherein said fibre is dried under extension and the aligned nano-fibrils aggregate to form a continuous structure and wherein the suspension of nano-fibrils, which has a solids content of at least 7% wt, is homogenised using at least one mechanical distributive mixing process, such as roll milling, prior to its extrusion.
Alternatively or additionally the suspension of non-fibrils may be heated prior to its extrusion.
Mixing is generally induced by mechanical action or by forced shear or elongational flow of the medium. Two types of mixing are generally discerned, namely dispersive mixing and distributive mixing. Dispersive mixing is defined as the breakup of agglomerates or lumps to a desired ultimate grain size of the solid particulates, or of the domain size (drops/Ic domains). On the other hand distributive mixing is defined as providing spatial uniformity of the components present in the medium.
The issue here is to impart both distributive and dispersive mixing to the suspension. This leads to a final suspension which is free from large-scale liquid crystal domains. Typically this means that liquid crystal domains cannot be visually observed in the suspension. Both parts of the mixing are important, so typically also distributive mixing contributes. The distributive mixing is beneficial as the lyotropic suspensions are often provided by a preceding centrifugation step leading to an inhomogeneous distribution of the particles in the medium (heavy/large particles at the bottom, light/small particles at the top), so distributive mixing is used for increasing the homogeneity of the spatial distribution of the particles in the medium.
The distributive mixing action as mentioned above is to provide an increased homogeneity of the particle sizes suspended in the medium, particularly in order to avoid large lc agglomerates so large-scale liquid crystal domains.
Generally speaking the aim of the mechanical, dispersive and distributive mixing process is to achieve a high degree of homogenisation.
The proposed mechanical mixing process also has the effect of reduction in standard deviation in zeta potential. Indeed it can be shown that the particularly stable process can be run in the standard deviation of the zeta potential is below 2 mV (for an average Zeta potential in the range of −35 to −27 mV), preferably below 1 mV.
So expressed differently, the mixing process leads to a low variation in the solids content. Typically the variation in the solids content is in the range of 1 to 0.01% preferably in the range of 0.25 to 0.05% (as determined with subsamples of 2 g each).
The mixing is typically induced by high shear or elongational flow of the medium. It takes place under pressure, typically in the range of 0.1 to 2 n/mm2, more preferably in the range of 0.5 to 1 n/mm2. The above-mentioned mechanical dispersive mixing process is preferentially carried out using a suspension with a solids content above 10% wt, preferably in the range of 20-40% wt.
The invention is further directed to a cellulose-based fibre which contains crystallised cellulose to a high degree and may be obtained by the method of the invention. According to a much preferred embodiment of the invention the fibre comprises a highly aligned or continuous micro-structure which provides said fibre with high strength.
It is highly preferred that the cellulose nano-fibrils used in the invention be extracted from a cellulose rich material.
All natural cellulose-based material which contains nano-fibrils, such as wood pulp or cotton, can be considered as starting material for this invention. Wood pulp is preferred as being cost effective but other cellulose-rich material can be used such as chitin, hemp or bacterial cellulose. Various sources of cellulose nano-fibrils, including industrial pulps from both hardwood and softwood have been tested satisfactorily. Also, microcrystalline cellulose (MCC) can be considered as a possible source of nano-fibrils provided it is subsequently broken down into individual cellulose nano-fibrils through an appropriate mechanical or acid hydrolysis process.
Various types of nano-fibrils can therefore be isolated and used in the process of the invention. Nano-fibrils with an aspect ratio (ratio of the longer dimension to the shorter dimension of the nano-fibril) superior to 7 and preferably ranging from 10 to 50 are particularly preferred.
A nano-fibril for use in a method according to the present invention is typically characterised in that it has a length in the range of 70 to 1000 nm. Preferentially the nano-fibrils are of type I cellulose.
Extraction of the nano-fibrils may most typically involve the hydrolysis of the cellulose source which is preferably ground to a fine powder or suspension.
Most typically the extraction process involves hydrolysis with an acid such as sulphuric acid. Sulphuric acid is particularly suitable since, during the hydrolysis process, charged sulphate groups are deposited on the surface of the nano-fibrils. The surface charge on the surface of the nano-fibrils creates repulsive forces between the fibres, which prevents them from hydrogen bonding together (aggregating) in suspension. As a result they can slide freely amongst each other. It is this repulsive force combined with the aspect ratio of the nano-fibrils, which leads to the highly desirable formation of a chiral nematic liquid crystal phase at a high enough concentration. The pitch of this chiral nematic liquid crystal phase is determined by fibril characteristics including aspect ratio, polydispersity and level of surface charge.
Alternative methods of nano-fibril extraction (like the use of hydrochloric acid) could be used but a surface charge would have to be applied to the nano-fibrils to favour their spinning into a continuous fibre. If the surface charge is insufficient to keep the nano-fibrils apart during the initial part of the spinning process, (before drying), the nano-fibrils may aggregate together and eventually prevent the flow of the suspension during spinning. Surface charge can be added by functionalising the cellulose with suitable groups such as sulphate esters, with the aim of reaching a Zeta-potential in the preferred ranges as defined further below. Once the hydrolysis has taken place, at least one nano-fibril fractionation step is preferably carried out, for example by centrifugation, to remove fibrilar debris and water to produce a concentrated cellulose gel or suspension.
In order to remove as much of amorphous cellulose and/or fibrilar debris as possible, subsequent washing steps may optionally take place. These washing steps may be carried out with a suitable organic solvent but is advantageously carried out with water, preferably with de-ionised water, and are followed by a separating step, usually carried out by centrifugation, to remove fibrilar debris and water as water removal is required to concentrate the nano fibrils. Three successive washing and subsequent centrifugation steps have provided suitable results.
Alternatively or additionally the nano-fibrils can be separated using phase behaviour of the suspension. At a critical concentration, typically around 5 to 8% cellulose, a biphasic region is obtained, one being isotropic, the other being anisotropic. These phases separate according to aspect ratio. The higher aspect ratio of the fibres forms the anisotropic phase and can be separated from the amorphous cellulose and/or fibrilar debris. The relative proportion of these two phases depends upon the concentration, the level of surface charge and the ionic content of the suspension. This method alleviates and/or suppresses the need for centrifugation and/or washing steps to be carried out. This method of fractionation is therefore simpler and more cost effective and is therefore preferred.
According to a particular embodiment of the invention it has been found advantageous to adjust the Zeta potential of the suspension using, for example, dialysis. Zeta potential can range from −60 mV to −20 mV but is advantageously adjusted to range from −40 mV to −20 mV, preferably from −35 mV to −27 mV and even more preferably from −34 mV to −30 mV. These ranges, and in particular this last range, is particularly suitable for nano-fibrils having an aspect ratio ranging from 10 to 50.
To do so the hydrolysed cellulose suspension mixed with deionised water can be dialysed against deionised water using, for example, Visking dialysis tubing with a molecular weight cut-off ranging preferably from 12,000 to 14,000 Daltons. The dialysis is used to increase and stabilise the Zeta potential of the suspension from around −60 to −50 mV to preferably between −34 mV and −30 mV (see
This step is particularly advantageous when sulphuric acid has been used for carrying out the hydrolysis.
The zeta potential was determined using a Malvern Zetasizer Nano ZS system. A Zeta potential higher than −30 mV often results is an unstable suspension at high concentration with aggregation of nano-fibrils taking place which can lead to an interruption in the flow of the suspension during spinning. A Zeta potential below −35 mV often leads to poor cohesion in the fibre during spinning, even at high solids concentrations of above 40%.
Industrially scalable technology such as spiral wound hollow fibre tangential flow filtration can be used to reduce dialysis times significantly. Such a technology can also be used to at least partially remove fibrilar debris and amorphous polysaccharides if the pore size is increased in the dialysis membranes from 12000-14000 Daltons up to a maximum of 300 000 Daltons.
As an alternative approach to increasing zeta potential, the suspensions can be taken out of dialysis at an earlier time (e.g. 3 days) and subsequently treated with heat (to remove some of the sulphate groups) or a counterion (such as calcium chloride) added to the suspension, typically in the range of 0.0065 to 0.0075 molar concentration, to reduce the zeta potential to the required level.
With respect to the heat treatment, suspensions can be submitted to temperatures ranging from 70-100° C., such as 90° C., over a suitable period of time. Such period may vary, for example, from 3 to 10, preferably 4 to 8, days for material treated at 90° C.
The nano-fibril suspension may comprise an organic solvent. However it is preferred that said suspension be water-based. Thus, the solvent or liquid phase of the suspension may be at least 90% wt water, preferably at least 95% wt, and even preferably 98% wt water.
To obtain the most suitable cellulose suspension for the spinning step the homogenised cellulose suspension can then re-centrifuged to produce the concentrated, high viscosity suspension particularly suitable for spinning.
An effective procedure involves 8000 RCF (relative centrifugal force) for 14 hours, followed by 11000 RCF for a further 14 hours. Alternative approaches such as partial spray drying or other methods of controlled evaporation to concentrate the gel could also be considered.
The cellulose suspension to be used in the spinning of the fibre is a lyotropic suspension (i.e. a chiral nematic liquid crystal phase). Once the chiral twist from such a cellulose suspension has been unwound, it permits the formation of a highly aligned microstructure, which is desirable to obtain high strength fibres.
Desirably, a 100% anisotropic chiral nematic suspension is used. Such suspensions are obtainable by suspension of the nano-fibrils. For cotton based cellulose nano-fibrils a cellulose concentration of 10% is a suitable minimum concentration. This may be lower for nano-fibrils with higher aspect ratio such as bacterial cellulose. However, in practice the preferred solids content for spinning is above 20%. In that case, it is believed that most, if not all, sources of nano-fibrils would be 100% anisotropic chiral nematic suspensions.
Conditions such as low levels of surface charge (for example above −30 mV) or overdosage of a counterion such as CaCl2 should be avoided as it can lead to undesirable aggregation of the nano-fibrils.
In the process of the invention, the viscosity of the suspension required for spinning (i.e. its concentration of solids and nano-fibril aspect ratio) may vary depending upon several factors. For example it may depend upon the distance between the extrusion point and the point at which the chiral structure of the fibre is unwound and then dried. A larger distance means that the wet strength, and therefore the viscosity, of the suspension have to be increased. The level of concentrated solids may range from 10 to 60% wt. However it is preferable to use suspensions having a high viscosity and a solid content percentage chosen from 20-50% wt, and more preferably of about 25-40% wt, and most preferably 25-35% wt. The viscosity of the suspension can be higher than 5000 poise. At these preferred concentrations the use of thickeners is not desirable. In any case the minimum concentration of solids should be above the level at which a bi-phasic region (where isotropic and anisotropic phases are present simultaneously, in different layers) occurs. This would normally be above 4% wt. but more typically above 6-10% wt. depending on the aspect ratio of nano-fibrils and the ionic strength of the solution.
In the case of centrifugation, this process produces a gradation of solids contents, with the first material to be concentrated being the larger sized nano-fibrils. By the end of the concentration process the final gel is usually heterogeneous although fibres using gels prepared in this manner can be spun. However, the heterogeneous nature of the gel may cause problems in the spinning process which can lead to blockage of the spinning die and subsequent fibre breakage. This is why subsequent to centrifugation preferentially a mixing process having a distributive mixing effect is used.
Thus, the cellulose suspension is advantageously homogenised before spinning using a dispersive mixing process to create a more uniform size distribution. Typically particle length ranges from 70-1000 nm.
Thus, according to one embodiment of the invention, homogenisation is carried out using mechanical mixing. The term mechanical mixing encompasses the use of dispersive mechanical homogenizers, such as roll mills and twin screw extruders.
The suspension used in the method of the invention may be homogenised using a classical paddle mixer. However, this method is only effective for suspension having a fairly low concentration of solids (i.e. lower that 5% wt).
However for suspensions having a high concentration of solids (i.e. typically in the range of 10-50% by weight, preferably in the range of 20-40% by weight) as the ones much preferred in the method of the invention, classical methods used for pumping and mixing are not optimal. This is due to the unexpected “shear yielding” (alternatively referred to as “shear banding”) characteristics of the suspensions at concentrations of above 5% solids concentration. This material will not mix easily or pump cleanly (i.e. without leaving large amounts of stagnant material sitting in the process).
Thus, it has been found that mechanical distributive and dispersive homogenization techniques, and in particular roll milling, ensures that the solids content and nano-fibril size distribution of the suspensions is as uniform as possible, to ensure uniformity of flow and minimize fibre breakage during spinning. This is of particular importance in an industrial process. Homogenization in this context means that a mixing process is used with a significant distributive mixing contribution.
According to a much preferred embodiment, roll milling is used to carry out suitable homogenization. Roll milling is carried out using a 2, or preferably a 3 roll mill. The roll gap/nip between rollers can be varied depending upon the viscosity of the suspension and the feed rate of the device. Typically, gaps ranging from 1 to 50 microns can be used. However, a final gap of less than 10 microns is preferred and 5 microns or less is more preferable.
For example, a 3-roll mill sold by Exakt Technologies (“Triple Roller Mill Exakt 80E Electronic”) was found particularly suitable. This particular 3-roll mill is a standard batch production machine, commonly used to mix paints and pigments and is industrially scalable. It basically creates a high shear stress and high tensile stress to material trying to flow between two rotating rollers (see
Other types of homogenizers involving the use of pressure, such as homogenizing valve technologies or a twin screw extruder, can also be used, provided the conditions in order to break down the large scale liquid crystal agglomerates are provided, which typically are high turbulence and shear, combined with compression, acceleration, pressure drop, and impact. Also the above mentioned homogenisation techniques can be combined in order to achieve the desired degree of homogenisation.
Spinning the Suspension into a Fibre
Accordingly, a particularly preferred embodiment of the method of the invention is carried out with a cellulose suspension in a chiral nematic phase and the spinning characteristics are defined such as to unwind the chiral nematic structure into a nematic phase to allow the subsequent formation at an industrial level of a continuous fibre in which the nano-fibrils aggregate together into larger crystalline structures.
To spin the cellulose suspension into fibres, the cellulose suspension of nano fibrils is first forced through a needle, a die or a spinneret. The fibre passes through an air gap to a take up roller where it is stretched and the nano-fibrils are forced into alignment under the extensional forces whilst the fibre dries. The level of extensional alignment is due to the velocity of the take up roller being higher than the velocity of the fibre as it exits the die. The ratio of these two velocities is referred to as the draw down ratio (DDR). The alignment of said nano-fibres is advantageously improved by the use of a hyperbolic dye designed to match the rheological properties of the suspension. The design of such dies is well documented in the public domain. For example
If the fibre is stretched and drawn down sufficiently then inter-fibril bonding will be sufficient to form a large crystalline unit. By large crystalline unit it is meant crystallised aggregates ranging from 0.5 microns in diameter, preferably up to the diameter of the fibre. The preferred size of fibres will be in the range of 1 to 10 microns. Although fibres of up to 500 microns or larger could be spun, it is unlikely that the size of the crystalline unit would exceed 5-10 microns. It is anticipated that fibres in the region of 1 to 10 microns would exhibit larger crystalline units and fewer crystalline defects and therefore higher strength. Larger crystalline structures are formed as draw down is increased and stronger fibres will result from the use of higher draw down ratios (DDR).
Preferably DDR are chosen to be superior to 1.2, advantageously 2. More advantageously the DDR is above 3. A draw down ratio chosen in the range of 2 to 20 is preferred to obtain fibres having large crystalline units (above 1 micron). Draw down ratios above this may be required to achieve larger aggregation. Draw down ratios of over 5000 may be used if smaller diameter fibres are required from large initial fibre diameters such as a reduction from 240 microns to 1 micron. However, such large draw down ratios are not necessarily required to achieve the aggregation that is required.
It is desirable that most of the water or solvent contained in the newly formed fibres as extruded through the die should be removed during, spinning. The removal of the liquid phase—or drying—can take a number of forms such as heat or microwave drying. The preferred approach uses heat to directly remove the liquid phase. For example the fibre can be spun onto a heated drum to achieve drying or can be dried using a flow of hot air, or radiant heat, applied to the fibre after its extrusion and, preferably, before it reaches the drum or take up wheel.
An alternative approach would be to pass the wet fibre through a coagulation bath to remove the majority of the water after which it could then be dried further through heating. Such bath could be made using concentrated solution of zinc chloride or calcium chloride.
According to a preferred embodiment, the process is carried out without any coagulation bath and using water as the carrying medium.
During the drying step the spun fibre is stretched and the chiral nematic structure within the suspension is unwound so that the nano-fibrils are oriented along the axis of the fibre in a nematic phase. As the fibre begins to dry, the nano-fibrils move more closely together and hydrogen bonds are formed to create larger crystalline units within the fibre, maintaining the nematic formation in the solid state.
It should be noted that according to a preferred embodiment of the invention the only additives to the suspension in addition to water are counter ions directed to control the surface charge of the fibres such as sulphate group.
The fibre according to the invention preferably contains at least 90% wt, advantageously at least 95% and more preferably above 99% of crystallised cellulose. According a variant of the invention the fibre is constituted of crystallised cellulose. A standard analytical method involving the use of, for example, Solid State NMR or X-Ray diffraction could be used to determine the relative proportion of crystalline and amorphous material.
According to a preferred embodiment of the invention, only trace amounts of amorphous cellulose (less than about 1% wt) are present at the surface or in the core of the fibre.
According to another preferred embodiment the fibre comprises micro-crystals which are highly aligned in the axial direction of the fibre. By “highly aligned” it is meant that above 95%, preferably more than 99%, of the micro crystals are aligned within the axial direction. Levels of alignment can be determined through assessment of electron microscopy images. It is further preferred that the fibre be made of such (a) micro crystal(s).
It is further preferred that the fibre according to the present invention is of high tensile strength, above at least 20 cN/tex, but more preferably in the range of 50 to 200 cN/tex.
According to the invention, the fibre may have a linear mass density, as calculated according to industry standards for industrial synthetic fibres such as Kevlar and carbon fibre, ranging from 0.02 to 20 Tex. Typically such fibres may have an linear mass density of around 1000 to 1600 kg/m3. The typical linear mass density of the fibres produced according to the invention is around 1500 kg/m3
According to a further embodiment the fibre is obtained using the method of the invention described within the present specification.
According to a particularly preferred embodiment of the invention, the process does not involve the use of organic solvents at least during the spinning step. This feature is particularly advantageous as the absence of organic solvent is not only economically profitable but also environmentally friendly. Thus, according to a feature of the invention, the whole process can be water-based, as the suspension used for spinning the fibre can be substantially water based. By “substantially water based” it is meant that at least 90% by weight of the solvent use in the suspension is water. The use of a water-based suspension during the spinning process is particularly desirable because of its low toxicity, low cost, ease of handling and benefits to the environment.
In order that the invention be more readily understood and put into practical effect, reference will now be made to the accompanying figures which illustrate some aspects of some embodiments of the invention.
a and 17b: is a picture of spin line rheometer used in example 4
The source of cellulose nano fibrils used in the example has been filter paper, and more particularly Whatman no 4 cellulose filter paper. Of course experimental conditions may vary for different sources of cellulose nano-fibrils.
The filter paper is cut into small pieces and then ball-milled to a powder that can pass a size 20 mesh (0.841 mm).
The powder obtained from ball milling is hydrolysed using sulphuric acid as follows:
Cellulose powder at a concentration of 10% (w/w) is hydrolysed using 52.5% sulphuric acid at a temperature of 46° C. for 75 minutes with constant stirring (using a hotplate/magnetic stirrer). After the hydrolysis period ends the reaction is quenched by adding excess de-ionised water equal to 10 times the hydrolysis volume.
The hydrolysis suspension is concentrated by centrifugation at a relative centrifugal force (RCF) value of 17,000 for 1 hour. The concentrated cellulose is then washed 3 additional times and re-diluted after each wash using deionised water followed by centrifugation (RCF value −17,000) for 1 hour. The following example illustrates the benefits of washing and repeated centrifugation resulting in fractionation with the subsequent removal of fibrilar debris.
Pictures of the concentrated suspension in one hand and the wash water have been obtained using Field Emission Gun-Scanning Emission Microscope (FEG-SEM) to show the impact of centrifugation on fractionation of the nano-fibril suspensions. Following hydrolysis and extraction three additional washes were carried out. All images reproduced in this study are shown at 25000× magnification.
The standard hydrolysis process was used on ball milled (Whatman N.4) filter paper (52.5% sulphuric acid concentration, 46° C. and 75 min). After hydrolysis of 30 grams of ball milled filter paper the diluted nano-fibril suspension was separated into 6 500 ml bottles, which were placed in the centrifuge. The first wash runs for one hour at 9000 rpm. (17000 G). After this time two different phases were obtained, an acidic solution product from hydrolysis (wash water) and a concentrated cellulose gel pellet (20% cellulose).
The gel pellet was dispersed in 250 ml of de-ionized water for further cleaning in this and subsequent washes. The solution was spun in the centrifuge for one hour and the cellulose gel pellet and wash water re-evaluated.
After the second wash there does not appear to be much difference in the structure of the nano-fibrils in the cellulose gel (
After the 3rd wash the cellulose nano-fibrils are easier to discriminate and the image of the gel (
At the end of the fourth centrifugation, the cellulose suspension is diluted again with deionised water then dialysed against deionised water using Visking dialysis tubing with a molecular weight cut-off of 12,000 to 14,000 Daltons.
The dialysis is used to increase the Zeta potential of the suspension from around −60 mV to −50 mV to preferably between −33 mV and −30 mV. In running deionised water the dialysis process can take around 2-3 weeks under ambient pressure.
Data is the average of at least 3 readings with standard deviation shown as error bars on the graphs. The zeta potential data were consistent between batches, indicating that after 1 day of dialysis a relatively stable but short lived equilibrium is achieved at a zeta potential between −50 mV and −40 mV, albeit with some variance as shown by the standard deviations. After 5 to 10 days (dependent on batch) the zeta value increases with an apparent linear trend until reaching about −30 mV after about 2 to 3 weeks of dialysis.
Industrially scalable technology such as spiral wound hollow fibre tangential flow filtration can be used to reduce dialysis times significantly from days to a few hours. As an alternative approach to speeding up the process the suspensions can be taken out of dialysis at an earlier time (e.g. 3 days) and subsequently treated with heat (to remove some of the sulphate groups) or a counterion such as calcium chloride to reduce zeta potential to the required level.
Dialysis is particularly advantageous when sulphuric acid has been used for carrying out the hydrolysis. A Zeta potential higher than −27 mV, normally higher than −30 mV, results is an unstable suspension at high concentration with aggregation of nano-fibrils taking place which can lead to an interruption in the flow of the suspension during spinning. A Zeta potential below −35 mV normally leads to poor cohesion in the wet fibre (prior to drying) during spinning, even at high concentrations. The low cohesion means the wet fibre flows like a low viscosity fluid, which cannot be subjected to tension and drawn down prior to drying. A process which is particularly advantageous in unwinding the chiral twist since if the fibre is fully dried under tension before the chiral twist is unwound, the fibre will shrink longitudinally resulting in fibre breakage. Once the nano-fibrils are aligned with the axis of the fibre, the shrinkage will take place laterally reducing fibre diameter and increase fibre coherence and strength. The nano-fibrils will also be able to slip between each other more easily facilitating the draw down process.
After dialysis, the cellulose preparations are sonicated using a Hielscher UP200S ultrasonic processor with a S14 Tip for 20 minutes (in two 10 minute bursts to avoid overheating) to disperse any aggregates. The dispersed suspension is then re-centrifuged to produce the concentrated, high viscosity suspension required for spinning.
In the first example of spinning the cellulose nano-fibril gel was concentrated to 20% solids using the centrifuge. In the second example the concentration was increased to 40% to increase wet gel strength.
The first spinning example involved the use of the apparatus (10) shown in
As better shown in
When a fibre is spun without draw down,
Under the faster drawing conditions, good fibril alignment was observed with the better draw down ratio.
To remove the irregularities associated with the drying by contact with the drum a different spinning facility is used in the subsequent example.
An alternative approach in which the fibre exiting from the die is allowed to dry without contact with the sort of drum that we used is given in a second spinning process described in example 4 herein below.
The second spinning example involves the use of a Spin line rheometer (32) which is shown in
The key differences between this spinning process and the one of the previous example are the following:
In the example given, once the nano-fibril suspension had been concentrated to around 40% solids (by centrifuging the cellulose suspension for 24 hours at 11000 rpm) it was decanted into a syringe which was then centrifuged at 5000 rpm for 10-20 minutes to remove air pockets. The gel was then injected into the Rheometer bore as a single plug to prevent further air cavities being formed. Air pockets in the gel may lead to a break in fibre during spinning and should be avoided. The DDR used in this example was fairly low at around 1.5 and an even better alignment should result from higher DDR.
For illustrative purposes,
Alternative method to reduce the zeta potential and effect of roll mill homogenisation.
The zeta potential of the suspensions used for spinning should advantageously be from of −35 to −27 mV. Above −27 mV the lyotropic suspension can be unstable. After standard dialysis treatment of three days, the zeta potential of the suspensions is typically below −40 mV (see
This example shows that heat treating the suspension at 90° C. prior to final concentration in the centrifuge is an alternative to the use of extended dialysis time and the use of calcium chloride (e.g. example 2).
Five batches of cellulose nano-fibril suspension were prepared from five 250 gram, industrially produced batches of Eucalyptus based 92 alpha cellulose pulp typically used as the cellulose source in the manufacture of viscose. The initial preparation including ball milling, hydrolysis and subsequent washing was the same as that described in Example 1. After washing, the five batches of suspensions, at 2% solids content, were placed in 15 mm diameter Visking dialysis tubing with a molecular weight cut-off of 12000 to 14000 Daltons. The suspensions were then dialysed for three days against continuously flowing deionised water.
At the end of the dialysis time, each batch of nano-fibrils was measured for Zeta potential using a Malvern Zetasizer Nano ZS system. Each batch was placed in an oven at 90° C. for between four and eight days. The different batches had different starting zeta potential values of between −50 mV and −40 mV and had to be exposed for different periods to heat treatment to increase Zeta potential to the target range of −34 to −30 mV. Every day, the zeta potential of each batch was measured (5 replicate measurements per batch) until they reached the target level of −34 to −30 mV. The suspensions were then concentrated in a centrifuge (14 hours at 8000 RCF and a subsequent 14 hours at 11000 RCF) to achieve a target of 30% solids content.
Table 1 shows the average zeta potential levels along with standard deviations. In all cases the average zeta potential was within the same range where we were able to spin fibres
To homogenize the suspension of Batch 1 before spinning, a “Triple Roller Mill Exakt 80E Electronic” was used. This Batch of suspension was milled using a 15 microns setting for the first nip and a 5 microns setting for the second nip. The resulting suspensions were re-passed through the roll mill 5 times until good homogenisation was achieved.
All five batches of concentrated gel (1 mixed and 4 unmixed) were then tested to determine if it was possible to spin fibre from them. In all cases we observed good fibre coherence during spinning. However, in all but one case (batch no 1 treated with the roll mill), spinning of the fibres was not consistent due to die blockage and fibre breakage. Die blockage was thought to be due to the heterogeneous nature of the gel. This hypothesis is supported by batch no 1 that was mixed with the roll mill. This mixing procedure visibly breaks down large scale liquid crystal domains (1 mm to 1 cm) within the suspension and significantly improves the consistency of the Zeta potential of the concentrated suspension and allows spinning of over 100 metres of fibre without die blockage and fibre breakage. Table 1 shows a significant reduction in standard deviation in zeta potential in the final mixed gel indicating good mixing at the micro scale. This was found impossible to achieve with a conventional mixing processes such as a paddle mixer or hand mixing with a spatula.
A 250 gram batch of an industrial, Eucalyptus based 92 alpha cellulose pulp was ball milled, hydrolysed and washed according the method described in Example 1. After washing the suspensions the suspension at 2% solids content, was placed in 15 mm diameter Visking dialysis tubing with a molecular weight cut-off of 12000 to 14000 Daltons. The suspensions were then dialysed for three days against continuously flowing deionised water.
After three days the suspension reached a zeta potential of −45 mV. 0.0075 molar CaCl2 was then added to the suspension until it reached a zeta potential of −32 mV. After CaCl2 addition the suspension was then concentrated in a centrifuge for 14 hours at 8000 RCF followed by a further 14 hours at 11000 RCF.
After concentration the suspension produced 200 mls of cellulose nano-fibrils at an average of 22% solids content. Solids content was determined from five subsamples (2 grams each) of material from the batch and evaluated for solids content.
The concentrated suspension was then mixed using the same 3 roll mill described in example 5 using a 15 microns setting for the first nip and a 5 microns setting for the second nip. The concentrated suspension was processed, through the mill a total of 10 times. Increased concentrations of solids are due to evaporation.
At zero, 2,4,6,8 and 10 cycles the solids content and variation in solids content (an indication of uniformity) was measured by taking five 2 gram samples for solids content determination.
Table 2 shows how the solids content increased from an average of 22.7% with no mixing to around 25% after 2 cycles and then remained relatively stable after 4, 6, 8 and 10 subsequent cycles. Most interestingly the standard deviation in solids content of the suspension which was 1.38% with no mixing reduced to 0.03% after 10 cycles indicating a significant improvement in the uniformity of the material. This improvement in uniformity was reflected in a significant reduction in die blockage and fibre breakage allowing the spinning of over 100 m of fibre without breakage.
The results indicate that a roll mill (or similar process capable of offering good distributive mixing) is effective for the preparation of suspensions and lead to uniform spinning conditions.
Other modifications would be apparent to the persons skilled in the art and are deemed to fall within the broad scope and ambit of the invention. In particular the DDR can be increased to improve alignment of nano-fibrils even further and reduce fibre diameter. This will assist in minimising defects within the fibre and increase aggregation of aligned nano-fibrils into larger aggregates. Also hyperbolic dies can be designed taking account of the rheology of the cellulose suspension to be spun. The design of such dies is well documented in the public domain as a mechanism for aligning other liquid crystal solutions such as that used in Lyocell.
For a power law fluid flowing through a hyperbolic die with slip at the interface, essentially constant extensional flow rate is obtained. The hyperbolic profile such as the one shown in
Using the following values:
Die exit angles (radians):
Die exit radius: rexit:=50 micron
Die flow rate:
Power law index (in shear flow): n:=0.5
We can calculate the extension rate in the die:
The function to describe the profile is:
The “Length to Diameter ratio” (L/D) is where L is measured from the exit of the die to the 45 degree entry point angle:
The length of the die is: L45:=2·rexit·LtoD45=5.577·mm
The diameter of the entry point is: r(L45)·2=0.612·mm
The total extensional strain on the material passing through the die is
| Number | Date | Country | Kind |
|---|---|---|---|
| 1006136.4 | Apr 2010 | GB | national |
| 1006201.6 | Apr 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP11/55680 | 4/12/2011 | WO | 00 | 9/19/2012 |
