ACETYLATION OF LIGNOCELLULOSIC MATERIALS AND APPLICATIONS THEREOF

Abstract: The invention relates to various methods for the acetylation of lignocellulosic fibres such as jute, hemp, wood fibre, sisal, tow, straw and the like, and involves the mixing of the lignocellulosic material acetic anhydride in such quantities as to effect a 20 % weight gain of the material on completion of the acetylation reaction, and also adding energy to the mixture to catalyse the acetylation reaction. The energy may be ultrasonic, thermal, or in the form of microwave radiation.

Description: ACETYLATION OF LIGNOCELLULOSIC MATERIALS AND APPLICATIONS THEREOF This invention is concerned with the acetylation of lignocellulosic materials of all types including but not limited to jute, hemp, wood fibre, sisal, cotton, coir, wheat straw, hemp, tow, and flax, both seed and straw, and mixtures of same. For simplicity of description, these materials to be treated, which are mainly fibrous, will be referred to hereinafter simply as "the fibres" . In the process of the invention, the fibres may be in loose form or be in the form of a fabric or other structure which may additionally contain felt, or rope and bundled or wound on rolls. Acetylation of the fibres is effected where the propensity of the fibres to absorb water is undesirable and it is desired to minimise bio-degradation thereof. Although this invention is concerned with a general acetylation process for lignocellulosic materials, it is of particular use on materials in fibrous form which are used by being incorporated into composites where the hydrophobic qualities of such fibres are desirable. Such composites may be used in the pipe lining industry, or alternatively used in sheet moulding. More specifically, the invention has particular reference to materials for use in those lining systems which are referred to as 'cured in place' lining systems wherein a tubular structure of resin impregnable material is impregnated with resin and is then placed in position on the pipeline or passageway surface. The resin is subsequently cured so that the tube becomes a hard lining pipe. A detailed description of the pipe lining technology is given in U.S. Patent No. 4009063 and in U.S. Patent No. 4064211. However, it is not the intention of the invention to be limited to such application. It is already widely known that many naturally occurring fibrous materials such as wood, jute, hemp and the like may absorb water, and in many cases, may expand as water is absorbed. Furthermore, it is also known that acetylation of such materials may increase their resistance to water, and depending on the extent of the acetylation process, render them almost totally hydrophobic. Many methods of acetylation of have been proposed, though as a result of research carried out by us, none, with the exception of the acetylation of cotton to produce rayon has been successfully commercialised, and it is believed that this failure to reach commercialisation has been due to the expense of setting up the necessary plant, and the difficulty of handling the acetylating agent, which is almost exclusively acetic anhydride-a volatile and toxic chemical. Examples of proposed methods are described in European Patent No. 0213252, International Patent Application No. PCT/GB95/00371 , European Patent Application No. 0650998, and International Patent Application No. PCT/SE93/00712. These specifications describe methods of acetylation wherein particular significance is attributed to the means of removal of the excess acetylating agent. For example, in European Patent No. 0213252, the material to be acetylated is placed into a basket which is immersed in an acetylating agent which impregnates the fibres, and the acetylating agent is then drained before other steps in the process are executed. In International Application No. PCT/GB95/00371 , after the fibres have been exposed to the acetylating agent under specified conditions, the fibres are then stripped of most of the remaining acetylating agent and other acetylation reaction products by a hot inert gas. European Patent Application No. 0650998 discloses the use of a superheated chemical vapour to remove most of the excess acetylating agent and acetylation reaction products. The acetylating agent common to all processes disclosed in the above patent specifications is acetic anhydride, which has a particularly acrid vapour and is also an expensive chemical, and thus there is a requirement for recovery of the chemical and post-acetylation treatment of the acetylated fibres to remove any acrid vapours therefrom. Furthermore, the duration of the acetylation process is described above is lengthy. Although large batches of the fibres may be processed to increase the overall throughput of such processes, there is still a requirement that the fibres be brought into contact with an acetylating agent for a predetermined period of time and at certain conditions. Times may range from between an hour and ten hours, and if the acetylation conditions are particularly extreme, the duration of the process necessitates the construction of specialised acetylating vessels, which increases the cost of the process significantly. It is an object of the present invention to provide a simple method and means for the acetylation and treatment of the fibres which will be cost effective, which will lend itself to economic production, and and is quick compared to existing acetylation methods. It is a further object of the present invention, at least in one aspect, to provide an acetylation process which does not require that the acetylating agent be recovered either prior to or after the acetylation process is undergone. It is also a yet further object of the present invention to simplify the acetylation process and increase the efficiency of the process. It is a yet further object of the invention to provide a versatile and flexible acetylation system within which a number of different acetylation processes can be effected. According to the present invention, there is provided a process for the acetylation of lignocellulosic materials comprising the steps of adding a first reactant, being one of a number of or a mixture of lignocellulosic materials to a containment means, adding a second reactant, being an acetylating agent, to said containment means, characterised in that the amount of the acetylating agent added is sufficient to provide the required degree of acetylation of the lignocellulosic materials without substantial residue of the acetylating agent on completion of the reaction, and in that the reaction is catalysed by the addition of energy to the reactants. The requirement for additional process steps to remove or recover any residual acetylating agent is thus obviated. In a first aspect of the invention, the reactants are preferably added to the containment means in the required proportions, and the volume of the containment means is fixed, thermal energy being applied to the containment means and thus to the reactants, the thermal energy causing evaporation of the acetylating agent in the containment means and thus increasing the pressure therein. It is preferable that the thermal energy is supplied for a predetermined period of time after which the fixed volume containment means may be cooled. The said heating of the fixed volume containment means is preferably continued for a period of time between two and ten hours, the said time period being dependent on the temperature at which said heating is carried out, the size of the containment means and remoteness of the fibre mass from the source of heat. The acetylation reaction is mildly exothermic and thus where thermal energy is supplied from without the containment means, the heat of reaction aids uniform heating of the mass of fibres within the containment means. In a second aspect of the invention, there is provided a process for the acetylation of lignocellulosic materials comprising the steps of adding a first reactant, being one of a number of or a mixture of lignocellulosic materials to a containment means, adding a second reactant, being an acetylating agent, to said containment means, characterised in that the reaction is catalysed by the addition of ultrasonic energy to the reactants. The reactants are preferably mixed together, and further preferably periodically or continuously disturbed to ensure that localised acetylation does not occur. It is preferred that the means capable of transmitting ultrasonic energy is an ultrasonic probe. It is further preferred that the probe be disposed substantially centrally within the containment means such that it is surrounded by the reactants. The disturbance of the reactants is preferably effected by an actuator, the direction of motion of which alternates periodically. In a third aspect of the present invention, it is preferable that the volume of the containment means is fixed, and that a vacuum is drawn therein prior to the introduction of the acetylating agent. Preferably, thermal energy is supplied to the containment means and thus to the reactants to catalyse the acetylation reaction. By providing a vacuum in the chamber, any extraneous airborne particles, such as water vapour, are removed, and cannot impair the acetylation reaction. Acetylation preferably takes place after the vacuum is isolated from the chamber, but the pre-application of the vacuum enhances and accelerates the acetylation reaction, making it more efficient and therefore less expensive. Preferably, the containment means is heated prior to or during the drawing of the vacuum whilst the fibres are contained therein, so that the fibres are dried and brought up to a temperature suitable for the acetylation to take place. The heating may be supplied by circulation of hot oil around a heating jacket which encases the containment means. The fibres are preferably preheated prior to placement in the containment means, by being placed in an oven, and when the fibres are placed in the containment means, they are preferably compressed and held in a compressed state, for example by being held in wire basket. The method is preferably a batch process in that a batch of the fibres is acetylated and then is removed from the containment means and the process is repeated, although the invention may be carried out on a continuous basis. The batch process does provide economies in plant investment. When the fibres are jute, and the acetylating agent is acetic anhydride, the ratio of fibres to acetic anhydride for the desired level of acetylation of the fibres is preferably in the order, by weight, of 2.5 to 1. The acetic anhydride is heated to approximately to 110° C and then introduced into the containment means which is then again sealed. A constant temperature is preferably maintained in the containment means during the period of acetylation, which is in the order of 2 hours. It is further preferable that at the end of the period of acetylation, a vacuum is again applied to the chamber, to remove any airborne particles such as water vapour, acetic acid vapour, and unused anhydride. The products so drawn off are passed to a solvent recovery plant whereat any residual or recoverable acetic anhydride can be recovered for reuse. In a fourth aspect of the invention, it is preferable that the catalysing energy supplied to the reactants is in the form of microwave radiation. It is preferable that the containment means is sealed such that a pressure develops in the said containment means, either as a result of the heating of the acetylating agent and fibres, caused by the microwave radiation, or by other means. It is preferred that the temperature and pressure during the reaction be monitored throughout the period of acetylation. In a different embodiment of this aspect of the invention, it is preferable that the fibres are acetylated by continuously passing the reactants through a chamber within which the microwaves radiate. In this embodiment, the containment means is preferably a tube through which the reactants may be passed. It is further preferable that the reactants be urged by screw feed means through the tube, a portion of which is disposed in the microwave chamber, thus subjecting its contents to microwave radiation, reactants being urged into a first open end of the tube and acetylated fibres emerging from the alternate open end of the tube. It is preferable that pressure develops in the said tube, either as a result of the heating of the volatile acetylating agent and its subsequent evaporation caused by the microwave radiation, or by other means. It is further preferred that the microwave radiation is applied to the reactants intermittently. It is further preferable that the heating caused by microwave radiation in the chamber causes evaporation of any residual acetylating agent after acetylation has taken place, thus obviating the requirement to dry the acetylated fibres. It is yet further preferable that the microwave chamber be provided with temperature and pressure sensors, and/or an heating element and/or pressurising means to further control the temperature and pressure of the reactants within the tube. In a yet further embodiment of the fourth aspect of the invention, the reactants are preferably added to the containment means which is subsequently sealed and located within a chamber within which microwaves radiate. The tube, and thus the reactants, are subjected to microwave radiation for a predetermined period of time, after which the tube may be removed, and a different tube filled with reactants located in the said chamber. Acetylation of fibres in this manner can thus be effected on a batch basis. It is preferable that a pressure develops in the said tube, either as a result of the heating of the volatile acetylating agent and its subsequent evaporation and fibres caused by the microwave radiation, or by other means. In a yet further embodiment of the fourth aspect of the invention, the fibres are preferably joined in the form of a web of material, and said web being passed continuously through a chamber through which microwaves radiate. Preferably, the web of lignocellulosic material includes a felt or woven fabric layer. It is preferable that the acetylating agent is added to said chamber. It is further preferable that the web of fibres is subsequently passed through a drying chamber to remove any residual acetylating agent and noxious vapours which may result from the acetylation reaction. Preferably, the drying of the web is effected by a hot inert gas, which is further preferably passed through a condenser to recover any residual acetylating agent and/or acetylation rection products removed from the web of fibres. More specifically, the chamber is preferably adapted to receive a web of fibre which passes therethrough at a predetermined rate, acetylating medium being introduced thereinto proximate to entry of the web into the chamber and spent acetylating agent being removed proximate to the exit of the web from the chamber, the quantity of acetylating medium introduced being dependant on the rate of travel of the web through the chamber and the degree of acetylation of the fibres required. Preferably fibre drying means are provided prior to the entry of the fibres into the chamber, or after the exit of the fibre from the chamber, or in both such locations. It is further preferable that driven rollers feed the web into and extract the web from the chamber. Preferably, the web of material is rolled off a supply roller at the commencement of the process and onto a collection roller at the termination of the process. Preferably, supply roller, drive rollers, chamber, heating means, and collection roller are all provided within encasement means. In a yet further embodiment of the fourth aspect of the invention, the containment means containing the first reactant is preferably translated at a predetermined rate through a chamber through which the microwaves radiate, the second reactant preferably being released internally of the containment means at a rate dependant on both the rate of translation of the containment means through said chamber and the required degree of acetylation of the fibres. Preferably, the containment means takes the form of an open ended tube. Further preferably the acetylating agent is released internally of the said tube through a stationary probe. Alternatively, the chamber is preferably provided with tube means passing therethrough, the reactants being driven through said tube and thus through said chamber by screw feed means. Preferably, the acetylating agent is introduced at a location along the screw feed means prior to the entry of the fibres into the chamber, such that some mixing of the fibres and acetylating agent occurs on account of the action of the screw feed means. Preferably the screw feed means is associated with hopper feed means such that a large bundle of fibres may be loaded thereinto. In a fifth aspect of the invention, there is provided a process for the acetylation of lignocellulosic materials comprising the steps of substantially immersing the lignocellulosic material in an acetylating agent within containment means, applying ultrasonic energy to the material and acetylating agent, and recovering the acetylating agent simultaneously with and after the process. It is preferable that the ultrasonic energy is applied to the reactants at 150°C, the application of ultrasonic energy thereto being discontinued if the temperature increases above this level. It is preferable that the ultrasonic energy be applied to said containment means, and thus to the lignocellulosic material and acetylating agent, through an oil, which effectively transmits the ultrasonic energy. It is further preferred that the oil is heated to a suitable temperature and thus also transfers heat to the containment means, and ultimately to the lignocellulosic material and acetylating agent. The preferred method of recovery of the acetylating agent during the reaction is forced condensation, as the acetylating agent is volatile and evaporates during the acetylation process. In a sixth aspect of the invention, there is provided a process for the acetylation of lignocellulosic materials comprising the steps mixing lignocellulosic material with an acetylating agent, adding said mixture to containment means in which is disposed means capable of transmitting ultrasonic energy, characterised in that the said mixture is periodically or continuously disturbed to ensure that localised acetylation does not occur. The mixture is preferably in the form of a slurry. It is preferred that the means capable of transmitting ultrasonic energy is an ultrasonic probe. It is further preferred that the probe be disposed substantially centrally within the containment means such that it is surrounded by the mixture. The disturbance of the mixture is preferably effected by an actuator, the direction of motion of which alternate periodically. In the case where the mixture is in the form of a slurry, the actuator preferably effects a "push-pull" type motion on said slurry. In all aspects of the invention, the preferred acetylating medium is acetic anhydride. Again in all aspects of the invention, it is preferred that the temperature of the reactants does not rise above 200°C. Also in all aspects of the invention, the preferred degree of acetylation of the fibres is in the region of 16% to 20%, this figure being given as the percentage increase in weight of the fibres after the acetylation thereof has been effected. In the aspects of the invention which involve the use of microwave radiation, the preferred frequency of the microwave radiation is 0.98GHz, although the frequency of microwave radiation may be 2.45 GHz. It is further preferred in all aspects of the invention that the first reactant be jute. All aspects of the invention have the advantage that the acetylation process may be effected both on a small and large scale, and in the first aspect of the invention, the apparatus requirements are minimised. A further advantage of the invention is the increased efficiency of the acetylation process. In all aspects, the invention greatly reduces processing time, and thereby, we believe, makes the acetylation process attractive commercially. As stated herein, one use in which we are particularly interested is in the field of composites comprising fibres and curable synthetic resins and in particular where such composites are in the form of tubes for the lining of pipelines and passageways. The tubular composites which are used for this application, as disclosed in the abovementioned US patents, comprise synthetic fibre felts mainly of polyester, and a comparable resin. The use of fibres treated according to the invention, to replace some or all of the polyester fibres represents a considerable saving in cost of the laminate and also provides an environmental improvement. Additionally, the fibres treated according to the invention can be used in the extrudate described in International Patent Application No GB96/01133 which relates to the provision of an extruded composite to provide a lining tube for pipelines and passageways. Recent experimentation in the technological field of lining materials for pipelines and passageways are suggested that a material or substance which does not have expensive polyester felt as a constituent and which may be extruded into pipe or tube form and stored for later use may be of great value to the industry. In many cases, the resistance to extrusion of the materials and substances concerned is very great, and either machines capable of generating large torques are required to effect the extrusion, which increases the overall expense of the pipe or tube, or extrusion is not possible. The present invention has further advantage in that the force required to extrude such a material or substance is greatly reduced if acetylated jute fibres constitute a major proportion of said material or substance. With reference to 'cured in place' lining systems, acetylated fibres produced according to the invention are mixed with resin and subsequently extruded into tube form or rolled into sheets which are then formed into tubes. The use of acetylated jute fibres instead of polyester felt has the additional advantages of enhancing the physical characteristics of the lining material after curing of the resin impregnated therein has taken place due to the improved surfactant qualities of such fibres which result from the acetylation process, and the improved compatibility with the thermosetting resins which are commonly used in the formation of such materials. In the aspects of the invention which use microwave radiation to catalyse the acetylation reaction, the time taken to acetylate the fibres is further reduced in comparison with other acetylation processes, and may be as little as a few minutes. This obviously renders the acetylation process extremely commercially attractive. It is to be mentioned that further experiments may be needed to determine the best parameters for the different material fibres, but up to now good results have been obtained with jute fibres. In the aspect of the invention which covers the continuous acetylation of a web of fibres, the continuity of the process allows a large volume of fibres to be acetylated, which further increases the commercial attractiveness of the process. It should be pointed out, however, that although the present invention, which discloses several aspects, requires that the amount of acetylating agent which is used is matched to the quantity of fibres to be treated, in some cases, less acetylating agent than is required may be used because it has been found that the desirable properties of acetylated fibres can still be attained even if optimum acetylation is not achieved. It should also be noted that the acetylation reaction may occur under pressures of the order of 5 to 6 PSI. Specific embodiments of all aspects of the invention are now described by way of example with reference to the accompanying drawings wherein:- Figures la, lb, show perspective views of a container and lid therefore; Figure 2 shows a plant suitable for carrying out the process according to a second aspect of the present invention; Figure 3 shows microwave acetylation apparatus according to a fourth aspect of the invention; Figures 4 and 5 show microwave acetylation apparatus according to an alternative embodiment of a fourth aspect of the invention; Figure 6 shows a schematic perspective view of the apparatus required for continuous acetylation of a web of fibres; Figure 7 is a sectional elevation of a test rig in which a quantity of lignocellulosic fibres are being acetylated under controlled condition; Figure 8 is a sectional elevation of a first machine for carrying out an acetylation treatment on the fibres; Figure 9 is a sectional elevation of a second type of machine for carrying out an acetylation treatment on the fibres; Figure 10 shows a sectional view of apparatus for acetylation according to a yet further alternative embodiment of the fourth aspect of the invention; Figure 11 shows a schematic chemical equation of the reaction between acetic anhydride and fibrous material; Figure 12 shows modified apparatus for microwave acetylation of fibres; Figure 13 shows a schematic view of a yet further modified apparatus for microwave acetylation including hopper means; Figure 14 shows yet further modified acetylation apparatus for the acetylation of the web of fibre; Figure 15 shows a schematic diagram of apparatus for use in the fifth aspect of the invention, and Figure 16 shows a perspective view of apparatus for use in the sixth aspect of the invention. Referring initially to Figs, la, lb, according to an embodiment of the first aspect of the invention, a steel container 2R with upper edges 3aR, 3bR, 3cR, 3dR is substantially filled with jute fibres 4R, and a measured volume of liquid acetic anhydride is then added. Ideally, the volume of acetic anhydride which is added should be marginally greater than that required to produce a 20% increase in weight of the jute fibres after acetylation. (This percentage is based on the incremental weight of the jute fibres before and after the acetylation process as compared to the total weight of the fibres after the process.) The container 2R is provided with a number of threaded holes, indicated generally by reference numeral 5R, in the upper edges 3aR, 3bR, 3cR, 3dR which are adapted to receive the threaded portions of bolts 8R (only two of which have been shown for the purposes of clarity). Said bolts 8R secure a lid 6R with apertures 7R to the said upper edges of the container 2R, the contents of which are thus sealed off from the atmosphere. The lid 6R is optionally provided on its upper surface 6aR with a pressure gauge with an outlet on the lower surface 6bR to indicate changes in the pressure inside the container during the acetylation process. Jute fibres are often packed in bales which have been compressed prior to shipment as fibres generally occupy a very large volume unless compressed. The lid 6R is therefore also optionally provided with compression means (not shown) to compact the jute within the container, and ensure one or both of the following: a) the volume occupied by a certain mass of fibres is reduced to a minimum, and thus the maximum mass of fibre may be processed in a given volume of the container, and b) the compacted fibre mass is substantially immersed in the liquid acetic anhydride added previously. Some compression of the fibres within the container may also occur before the lid is bolted in place, and although the mass of fibres within the container may be immersed in the liquid acetic anhydride, due to the compression, the volume occupied by the fibres contains a sufficient mass of fibres to react completely with the added liquid and leave little or no residue. On securing the lid 6R by bolting to the upper edges 3R of the container 2R in the conventional manner, the said container is placed in an oven (not shown) previously heated to a temperature of between 80°C and 150°C. As the temperature of the contents of the container 2R rise, the liquid acetic anhydride, which is a naturally volatile liquid, vaporises and the pressure within the container thus also increases. Slight imperfections in the seal formed between the lid and the upper edges of the container may allow small quantities of the said vapours within the container to escape as the pressure increases, and it is for this reason that a slight excess of the liquid acetic anhydride is added to the jute fibres before the container is closed. The steady state gas pressure inside the container when heated to a temperature of 110°C is 2 bar, and it is the combination of this pressure and temperature which effects efficient acetylation of the jute fibres. The container is subjected to said heating for a period of approximately 3 hours after which it is allowed to cool naturally over a period of approximately 10 hours. It is preferable that the lid 6R remains securely attached to the container 2R during this cooling period, although this is not essential. On completion of the cooling phase of the process, the lid 6R may be removed, preferably within a fume cupboard as the vapours of the acetylation reaction products within the container may still be quite acrid, and the container left open to the air. Jute fibres which have been acetylated in this way show excellent hydrophobic properties. Referring now to Figure 2, reference 10T indicates a treatment chamber in which the fibres are acetylated. The chamber 10T has a heating jacket 12T providing a space through which hot oil or other heating medium can be circulated, by means of a pump 14T, oil heater 16T and appropriate piping 18T. The chamber also has a door by which the fibres to be treated can be charged into the chamber and such door closes the chamber in a fluid tight fashion. Attached to the top of the chamber and in communication therewith is a pressure gauge 20T, and a connection pipe 22T from the chamber leads through a control valve 24T to a controllable source of vacuum. The chamber is shown as being suspended by a weighing device 26T by which the chamber 10T and its contents can be weighed. It should be mentioned that the weighing device is only diagrammatically represented, and in fact the weighing device may be an electronic weighing device. At the top end at least, and preferably all around the jacket, is heat insulation material 28T. The plant also includes a tank 30T for the acetic anhydride. Said tank 30T is connected hydraulically with the chamber 10T by means of a connection pipe 32T, which contains an isolation valve 34T. Tank 30T is surrounded by a heater 35T of any suitable type, preferably electric, and it also has a pressure gauge 36T and it is adapted to be weighed by a weighing device 38T, which is preferably the same as device 26T. Operation of the plant is more or less as indicated herein, in that, in particular for the treatment of jute fibres, the fibres 39T are dried in an oven and weighed. They are then fairly tightly packed into a wire basket 40T and basket and fibres are placed in the chamber through the door, which is then sealed closed. The chamber 10T is heated by circulation of oil heated by the heater 16T until the temperature of the chamber is in the order of 130°C. At this time the valves 24T and 34T are closed, so that no vacuum is applied to the chamber and a measured quantity of acetic anhydride corresponding to the quantity of fibres in the chamber 10T in the tank 30T, is isolated from the chamber 10T. Next, the valve 24T is opened and a vacuum is applied to the chamber 10T, to remove any undesirable airborne materials. In the meantime, the acetic anhydride is heated by heater 35T to a temperature in the region of 110°C, which increases the pressure in the tank 30T. The vacuum to the chamber is turned off and the valve 34T is opened to allow the release of the anhydride into the chamber 10T, and when the pressures in the chamber 10T and the tank 30T have equalised, the valve 34T is closed, and as near a constant temperature is maintained in the chamber 10T as possible, for a treatment period of about two hours, during which time the acetylation of the fibres takes place. The temperature and pressure in the chamber are monitored during this period. Subsequently, the vapours in the chamber are vacuumed off and passed to a solvent recovery plant where any unused or recoverable acetic anhydride is recovered for reuse. When the vapours have been so removed, the temperature of the chamber 10T is maintained to dry the acetylated fibres. The above process provides acetylation in the order of 16% to 20% which is the desired range, and the degree of acetylation can be ascertained by taking the weight of the fibres before and after treatment, the degree of acetylation being the increase in weight of the fibres as a result of the treatment. The desired range of acetylation arises in that if there is too little acetylation, the fibres will not perform the desired function of repelling water and will not show enough increase in strength, and if any greater acetylation is effected, the fibres may tend to degrade. The processing time of the above method is much less than previously proposed methods, thus making it commercially very attractive. A specific embodiment of the fourth aspect of the invention is now described by way of example. A quantity of dry jute fibres is weighed prior to being subjected to the process. The said jute fibres are then deposited in a tube and a quantity of liquid acetic anhydride in excess of the amount required to acetylate the fibres is poured into the tube. It is to be understood that although a tube is mentioned here, any suitable receptacle may be used. The tube and contents are then placed in a microwave oven which is usually closed to inhibit the escape of harmful radiation, and subjected to microwaves of a desired intensity. The energy transmitted to the fibres and surrounding liquid acetic anhydride by the microwaves causes rapid heating thereof, and the temperature of the said fibres and anhydride in this example increases to 180°C in three minutes. This phase of the process is hereinafter referred to as the rapid heating phase. For the purposes of measurement and control of the acetylation process, the microwave oven may be fitted internally with pressure and temperature sensors. In this example, the microwave radiation is applied only intermittently after the initial rapid heating of the fibres and anhydride, in order that the temperature fluctuates only slightly from 180°C. This phase of the process is hereinafter referred to as the acetylating phase, but this does not exclude the possibility of partial acetylation of the fibres during the rapid heating phase of the process. In all aspects of the invention which involve microwave radiation, the microwaves may be introduced into the containment means by a number of microwave guides which are disposed at angular intervals around said containment means. This facilitates the application of microwaves through each of said guides in a rotating sequence, thus precluding the emergence of hot spots within said containment means and also evenly irradiating the reactants. The wave guides may be fed from individual microwave generators or by means of gated wave guides, whichever is more suitable in the circumstances. Tests have shown that the required degree of acetylation (as stated above) can be obtained by maintaining the temperature at 180°C during the acetylating phase for as little as 2Vz minutes. Further experiments have showed that times for the acetylating phase of 5 and 10 minutes also give excellent results. On completion of the acetylating phase, the fibres and remaining unreacted acetic anhydride are allowed to cool, and the fibres are subsequently dried and may be reweighed to ascertain the degree of acetylation achieved by the process. Referring now to Figure 3 in which is shown apparatus 10U which includes an extruder 12U having a barrel 13U with an helical screw 14U therein driven by a motor 16U and which is provided with a hopper 18U for receiving both the fibres to be acetylated and the acetic anhydride. It is also foreseeable that the fibres and the acetic anhydride may be mixed together in the desired quantities prior to deposition in the hopper 18U, or that only the fibres may be deposited in the hopper 18U, the acetic anhydride being added at a different point along the length of the extruder 12U. In the latter circumstance, mixing of the fibres and acetylating medium is expedited by the rotation of the helical screw 14U within the extruder 12U. In the case where both fibres and acetic anhydride are deposited in the hopper 18U, rotation of the helical screw 14U within the barrel 13U urges the fibres and the acetic anhydride towards the end of the extruder and into a pipe 20U which passes through a microwave chamber 22U and protrudes therefrom, opening into a receptacle 24U for the acetylated fibres. Although not shown here, vapour removal means and fibre drying means may be provided on the receptacle to ensure that the fibres treated by the process are free from any residual acetic anhydride and noxious vapours of acetylation reaction products. The microwave chamber 22U is powered through terminals 23U and microwave radiation shown generally by dotted lines 25U flows between an emitter 26U and a receptor 28U. The pipe 20U is provided internally with a pressure sensor 30U and a temperature sensor 32U to measure these properties of the fibres and acetic anhydride thus enabling an operator of the apparatus to control the process. In use, a mixture of fibres and acetylating medium is urged towards the end of the extruder where it is compressed as it passes into pipe 20U by constrictions 19aU, 19bU. The compressed mixture passes through the pipe 20U and is subjected to microwave radiation 25U in the chamber 22U. The said microwave radiation may be applied continuously or intermittently depending both on the temperature and pressure of the fibre/acetic anhydride mixture in the pipe 20U and the volumetric flow rate of the mixture through the said pipe. The intermittency of the microwave radiation is ideally controlled such that the temperature of the fibre/acetic anhydride mixture rises only intermittently to a value in the range 180°-200°C, as the fibres may become scorched if the temperature is maintained at such a value. Although in practice a quantity of the acetic anhydride marginally in excess of that required to produce a 20% degree of acetylation is added, it is intended that sufficient heat remains in the acetylated fibres to evaporate any residual acetic anhydride on completion of the acetylation process. This vapour is then removed either from the chamber 22U from the receptacle 24U by conventional means. The pressurisation of the mixture as a result of both the evaporation of the volatile acetic anhydride within the pipe 20U as it is heated and the compaction of the mixture as it is urged into and along the pipe 20U ideally does not exceed 4 atmospheres. Referring now to Figures 4 and 5, microwave acetylation apparatus 2V comprises a Pyrex tube 4V mounted at its ends 5aV, bV in a steel cylindrical container 6V and supported along its length by steel dividers 8V. The ends of the tube 5aV, bV are provided internally with locking tabs 10V as shown in Figure 5 in order that caps 12V provided with corresponding locking tabs (not shown) may be placed over the ends of the tube 4V and rotated to form a substantially airtight seal at both ends of the said tube. The cylindrical vessel 6V is provided with mountings 14V which receive the caps 12V thus ensuring the tube is disposed centrally within the cylindrical vessel and held securely therein. The mountings 14V of the said cylindrical vessel are attached at the ends thereof and protrude therefrom in order that the entire apparatus may be supported by clamps 16V as shown in Figure 5. The cylindrical vessel is further provided with microwave inlet windows 18V through which microwave radiation is allowed to pass from a magnetron (not shown) rated at 5kW, or alternatively from two magnetrons rated at 2.5kW. This facilitates continuous microwave radiation during the acetylation process in the event that one of said magnetrons fails, which is likely as such devices usually have limited life, although obviously a reduction in the efficiency of the acetylation process will result in such circumstances. The following description relates to the acetylation process as carried out in accordance with a particular embodiment of the fourth aspect of the invention wherein four Pyrex tubes of the type shown in Figure 4 are provided in order that the different steps involved in the process may be effected simultaneously. It will be appreciated however that the various steps of the process may be completed in the same Pyrex tube consecutively, and in this case only one tube is required. In the first step of the process, the tube is filled with jute fibres by conventional means, for example by screw feed or by a plunger mechanism, and capped as herein before described. The tube is then mounted in the cylindrical vessel as shown in Figur 4 where air is evacuated from the tube, also by conventional means. During the evacuation, low to moderate intensity microwaves may be caused to radiate through the tube, and thus the fibres, hence warming the fibres and aiding the drying thereof. This may take approximately 5 minutes It is not required that the drying of the fibres in the tube takes place with the tube mounted in the cylindrical vessel, or that the warming of the fibres is effected by microwave radiation, but where the fibres are so warmed, the tube must be mounted in microwave containment means to shield operators of the apparatus from harmful radiation. The second step of the process is as follows. The drawing of the vacuum inside the tube is then ceased, and if not already in position within the cylindrical vessel, the tube is so positioned. Although the vacuum is no longer being drawn, the caps seal the tube sufficiently to ensure that a negative pressure remains inside the tube. This aids the introduction of liquid acetic anhydride through a sealed inlet (not shown) in one of the caps. It is preferred that a quantity of acetic anhydride is added which is double that required to effect the desired degree of acetylation of the fibres (20%) . Microwaves are again caused to flow through the tube, heating the fibres to a temperature of approximately 180°C, the frequency, intensity, and possibly the intermittency of the microwaves being such that the temperature of the fibres remains substantially constant for approximately five minutes. It is during this step of the process that acetylation of the fibres occurs. Also, due to the volatility of the acetic anhydride (i.e. it boils at 140°C), and the heat created by the microwaves and the reaction in the tube, the pressure increases, which further catalyses the acetylation reaction. In order to avoid the occurrence of "hot spots" within the mass of fibres in the tube, a mode strirrer is used to disperse the concentrated microwave radiations from the magnetrons, such that the fibres and acetic anhydride vapour receive uniform quantities of radiation, and the fibres are thus uniformly acetylated. The vaporisation of the acetic anhydride further aids the transfer of heat away from such hot spots and throughout the tube. The magnetrons are also provided with circulators which comprise ferrite blocks to absorb any stray or reflected microwaves. Such circulators are cooled with water. For the third step of the process, a vacuum is again drawn in the tube and the fibres may be heated, either by microwave radiation or otherwise but at a much lower temperature than that of the acetylation reaction to avoid continuing or reversing the original reaction. Noxious acetic anhydride vapours and any acetylation reaction products are thus removed from the fibres. The third step, like the first, may be carried out with the tube located within or without the cylindrical vessel, but if the various steps of the process are to be completed simultaneously, the tube is located in the cylindrical vessel only in step two of the process, and removed thereafter to allow the next tube to be subjected to step two of the process. The final step of the process is to remove the caps from the tube and to expel the fibre from the tube by conventional means as described above, the empty tube being thus made available for use in the first step of the process. The fibres expelled from the tube may optionally be further dried in an oven. The method of rotation of the four tubes used in the process has not been described, but it will be appreciated that rotation mechanisms exist for a variety of different industrial applications, and it is assumed that such may be adapted for use in this invention. Referring now to Figure 6, acetylation apparatus is shown generally at 2W and comprises a metal encasement cabinet 4W within which the various individual pieces of apparatus required for the process are contained. These include a feed roller 6W and a collection roller 8W both of which are mounted on trolleys 10W, 12W for ease of removal from the encasement cabinet 4W, pairs of alignment rollers 14W, 16W, 18W, at least one of which also doubles as drive rollers, a microwave chamber 20W, and a drying chamber 22W. The microwave chamber 20W is provided on its outer surface with a pair of wave guides 24W, 26W which direct microwaves of a frequency of 0.915GHz into said chamber, and is also provided with inlet and outlet pipes 28W, 30W respectively by which the acetylating medium enters and leaves the chamber. Narrow slots 32W, 34W are provided at opposite ends of the microwave chamber 20W to allow a web of material passing from the feed roller 6W and through the pair of alignment rollers 14W into and out of said chamber. Narrow slots 36W, 38W are provided in either side of the drying chamber 22W for the same purpose. The drying chamber 22W is provided with a gas inlet opening 44W in its lower portion, and with a gas exhaust opening 46W in its upper section through which a hot inert gas passes when the apparatus is operating in a direction shown by arrow 48W. Alignment and/or drive rollers 16W, 18W ensure that the web of material is correctly aligned with the slots 36W, 38W as it passes therethrough, and that the web emerging from the drying chamber is correctly aligned and fed onto the collection roller 8W. The encasement cabinet 4W is provided with doors 40W, 42W to enable operators of the apparatus to access the various components within said cabinet, and also to allow the feed and collection rollers 6W, 8W to be removed and replaced easily. The material of the web to be acetylated is generally flax or jute which has been needled together into a web and subsequently carded in order that acetylation of the web is as uniform as possible across the width of said web. In operation, hot vaporised acetylating medium is passed into the microwave chamber 20W via inlet pipe 28W as the web of material is being drawn therethrough. The material is also subjected to microwave radiation from the wave-guides 24W, 26W. It will be appreciated that by adjusting the factors of acetylating medium temperature and flow rate, microwave radiation intensity and frequency, and velocity of the web through the chamber, the required degree of acetylation may be achieved. During the process, the encasement cabinet 4W is filled with an inert gas (e.g. Nitrogen) at a pressure slightly above that within the microwave chamber 20W such that escape of the acetylating medium through the slots 32W, 34W is minimised and therefore risk from the vaporised acetic anhydride to operators of the machinery and of pollution of the atmosphere thereby when the doors 40W, 42W are opened is minimised. This slight pressure differential is of similar advantage with regard to the escape of hot inert gas laden with residual acetic anhydride from the drying chamber 22W, although it is the intention of the invention that the vast majority of such gas will be forced through the exhaust port 46W by the flow. The gas from the exhaust port may be passed through a condenser to recover any residual acetic anhydride, which may be used in a subsequent process. The dried acetylated web of material is drawn then rolled onto the collection roll 8W, which is easily removed from the cabinet 4W through door 42W on completion of the process. It can be easily understood that acetylation process times may be radically reduced using the method according to this aspect of the invention, and that large volumes of fabrics and other materials may be acetylated in this manner. Referring now to Figures 7, 8 and 9, a test rig is shown in Figure 7, and this comprises a sealed chamber 10X in which is contained a test vessel 12X. The test vessel 12X comprises a glass sleeve 14X forming the body, closed by means of end caps 16X, 18X. The interior of the body 12X is therefore airtight sealed by the end caps 16X and 18X and contains a charge 20X of the lignocellulosic fibres and acetic anhydride for the acetylation of the fibres. End cap 16X has apertures to enable the connection of the interior of the body by a temperature probe 22X and a pressure sensing point 24X, these being connected respectively to temperature gauge 26X and pressure gauge 28X whereby the temperature and pressure of the interior of the body can be detected. The fibre and acetic anhydride are heated by means of microwaves which are charged into the chamber 10X which in itself is sealed, by a microwave generating apparatus 30X. Under test conditions, the temperature at the centre of the body was raised to 180°C, and the pressure inside the body was between 5 and 6 p.s.i. The microwave energy frequency selected for the treatment was the frequency available for microwave appliances, namely 2.5 GHz, and a 5 kW supply was used although only in the order of Vi kW/h was actually applied. During the testing, it was noticed that the anhydride condenses on the inner surface of the glass sleeve 12X, which being transparent to the microwaves remains relatively cold. This in fact provides that there is a temperature gradient from the centre to the outside of the body as indicated by reference 32X which means that the mass of the fibres and anhydride is hottest at the centre of the body, and coldest at the outside. This can lead to differential acetylation of the fibres depending on their location in the body, which is undesirable. Furthermore, it is the case that the higher the microwave frequency, the smaller must be any apertures in the system to prevent escape of the microwaves, but if a lower frequency is used for example in the order of 0.9 GHz, then much larger apertures in the equipment can be permitted. It is desirable therefore that the treatment be carried out using microwaves at a frequency in the order of 0.9 GHz. In order to address the problem of condensation, it is a further feature of this invention that an insulating sleeve be positioned around the inside of the sleeve or other chamber in which treatment takes place, in order to insulate the cold surface from the heated charge of fibres and anhydride. A suitable surface for such a sleeve may be a plastics material such as nylon, and in particular nylon 66. In the arrangement shown in Figure 8, the treatment chamber 40X is a body with a central bore lined by means of the insulator sleeve referred to above, such sleeve being indicated by reference 42X. At one end the bore communicates with a charging chamber 44X in which is slidably contained a piston 46X. The piston 46X can be swept through the chamber 44X by means of a power source not shown, to push a charge of fibres in the chamber 44X into the bore in the body 40X. The body 40X contains the microwave source, and when the charge of fibres is in the body 40X, it is subjected to microwave energy in much the same way as described in relation to Figure 7. Acetic anhydride can be added to the charge of fibres in the chamber 44X through an injection pipe 48X for other suitable means, and the fibres themselves are charged into the chamber 44X through a hopper feed arrangement 50X. When a charge of fibres is pushed by the piston 46X into the body 40X, the previously treated charge is ejected into an ejection sleeve 52X from which the fibres can be discharged as indicated by arrow 54X or arrow 56X. Sleeve 52X also contains a reaction piston 58X which seals the end of the bore in the body 40X as shown, but is pushed backwards when a new charge of fibres is inserted into the body 40X from the chamber 44X. Suitable sealing arrangements may be provided, or alternatively the entire apparatus can be contained in a containment vessel thereby preventing escape of any acetic anhydride vapour into the atmosphere. In the arrangement of Figure 9, instead of a piston 46X, a feed auger 60X is used, and there is no reaction piston at the outlet end, but rather simply a discharge elbow 62X through which the discharged fibres are pushed to be ejected as indicated by arrow 64X. The auger 60X is driven at appropriate speed, and thereby there may be a continuous feed of fibres and acetic anhydride into the equipment which will provide for greater efficiencies. All parts previously described in relation to 8 carry similar reference numerals and perform similar functions. In addition, the equipment in Figure 9 may be provided with vacuum extractor pipes 66X, 68X by which any vaporised acetic anhydride may be withdrawn, condensed and recirculated. The fibres soaked in the anhydride to be acetylated will be held in the bore of the body 40X for the appropriate length of time and at the appropriate temperature and pressure in order to satisfy the requirements for effective acetylation. These specific improvements and embodiments of the present invention provide for more effective processing. Any suitable fibres can be processed in accordance with this aspect of the invention, but it is the case that flax fibres which have been decorticated provide excellent results. Decortication removes foreign matter from the raw flax fibres. Referring to Figures 11 through 14, microwave acetylation apparatus 2Y comprises a chamber designated generally at 4Y within which is disposed a glass tube 6Y containing a fibre bundle 8Y and an amount of an acetic anhydride. The glass tube 6Y is provided with end plates 10Y which effectively seal the tube from the surrounding atmosphere and the atmosphere within the chamber 4Y thus creating a closed reaction container. The chamber is also provided with a pair of wave guides 12Y, which are in turn provided with microwave emitters 14Y which disperse microwave radiation within the chamber 4Y. To effect the process, lignocellulosic fibres are bundled within the tube 6Y, and optionally compressed therein to increase the weight of fibre within said tube, and an amount of acetic anhydride which is sufficient to effect the required degree of acetylation of the fibres is also added. The ends of the tube 6Y are then sealed with plates 10Y and the entire tube assembly is mounted within the chamber 4Y as shown in Figure 11. Microwave radiation is subsequently transmitted from the emitters through the glass tube and therefore through the fibre and acetic anhydride mixture in the direction of the arrows 16Y. The effect of said microwave radiation is to heat both the acetic anhydride and the fibres within the sealed glass tube 6Y. Such heating both catalyses the acetylation reaction, and vaporises the acetic anhydride, which in turn increases the pressure within the tube which also catalyses the acetylation reaction. After the mixture of fibres and acetic anhydride has been subjected to microwave radiation for only a few minutes, the entire quantity of acetic anhydride initially added will have been utilised, and the fibres will have been acetylated to the required degree. It is a feature of the invention that only a predetermined amount of acetic anhydride is added to the reaction vessel, because firstly a 20% increase in the weight of the fibres after the reaction is sufficient to render them suitably hydrophobic, and secondly, there is no need for additional process plant to recover unused acetic anhydride. The tube is then removed from the chamber, and the acetylated fibres are removed from the tube and optionally dried. Alternatively, the tube may remain in the chamber, the fibres being urged therefrom by conventional means. The chemical reaction of acetylation is schematically represented in Figure 12. Acetic anhydride 20Y is added to a fibrous compound designated generally at 22Y which will be in the form of a long polymeric chain, or other complex structure whose description is outside the scope of this patent application. Said structure will generally be provided with hydroxyl groups, one of which is shown at 24Y, and it is with these groups that the acetic anhydride reacts. The products of the reaction are acetylated fibre 26Y and acetic acid 28Y. On completion of the process, the acetic acid 28Y will be vaporised, and if recovered may be hydrolysed to provide water 30Y and acetic anhydride 32Y which may be reused in the acetylation process. Referring now to Figure 13 which demonstrates a different embodiment of the invention, the chamber 4Y is provided with capped extensions 30Y, 32Y within and between which an open ended glass tube 6Y is allowed to translate. Said open ended glass tube 6Y also contains fibres 8Y to be acetylated, and is initially positioned substantially entirely within the capped extension 30Y. The said capped extension is further provided with a stationary probe 32Y which introduces acetic anhydride inside the glass tube 6Y at a predetermined rate. In operation, the open ended glass tube is urged to move in the direction shown by arrow 34Y by means not shown in the figure, acetic anhydride liquid being introduced to the fibres contained within said glass tube as it moves from capped extension 30Y through the chamber 4Y and into capped extension 32Y. The stationary probe 32Y allows for a uniform dispersion of acetic anhydride throughout the fibres as they pass therearound and thereover, thus ensuring a uniform acetylation of said fibres. The description provided in relation to Figure 11 regarding microwave radiation and removal of the acetylated fibres from the tube applies equally to Figures 13, 14 and 15, and is therefore not repeated. Referring to Figure 14, a hopper feed system 40Y and screw feeder 42Y and associated power unit 44Y are provided proximate to an inlet extension 46Y of the chamber 4Y. Although not shown in Figure 14, it is to be assumed that the chamber 4Y is provided with microwave radiation means as in previous figures. In operation, dry fibres 48Y are deposited in the hopper feed system 40Y and allowed to fall under gravity towards the screw feeder 42Y at the bottom of said hopper feed system 40Y. Although not shown in this figure, it is possible that additional means may be used to urge the fibres towards said screw feeder. The chamber 4Y is provided in this instance with a stationary, fixed, and open ended glass tube 6Y within which a portion of the screw feeder 42Y is allowed to rotate under the action of power unit 44Y. The flights 43Y of the screw feeder 42Y act so as to urge fibres falling from the hopper system thereonto towards the extension inlet 46Y and subsequently into and through glass tube 6Y. Acetic anhydride is introduced at a location 50Y well before microwave radiation acts on the fibre and said anhydride. The rotation of the screw feeder 42Y and associated flights 43Y within the glass tube 6Y at this location ensures thorough mixing of the anhydride and the fibres prior to the acetylation process. The speed of rotation of the screw feeder 42Y, the flow rate of acetic anhydride into the inlet extension 46Y, and the intensity of the microwave radiation within the chamber 4Y are all dependent on the required degree of acetylation of the fibres. As these three parameters can be varied independently, it will be appreciated that an extremely flexible acetylation process results. After only a few minutes in the chamber, acetylated fibre emerges from an outlet extension 52Y completely free of excess acetic anhydride. Figure 15 demonstrates a yet further embodiment of the invention wherein a web of fibre 60Y of a thickness 62Y, which may be of the order or 200 mm but need not necessarily be so depending on the size of the acetylation apparatus, is fed through the chamber 4Y, entering through an inlet extension 46Y, and exiting through an outlet extension 52Y of said chamber. An open ended glass tube is adapted to seat snugly within said inlet and outlet extensions 46Y, 52Y, and is positioned with its ends substantially parallel to the ends of said inlet and outlet extension. Sealing means 64Y, 66Y are provided to seal the ends of the glass tube and prevent the escape of noxious acetic anhydride vapour from said glass tube. Said sealing means are adapted to brush against the translating web of fibre 60Y such that the sealing means 64Y will be deflected inwardly of the glass tube and sealing means 66Y will be deflected outwardly on account of the direction of travel of the web 60Y. As the location of the introduction of acetic anhydride (not shown in this figure) is likely to be in the vicinity of inlet extension 46Y, and noxious acetic acid vapours are likely to be removed, possibly under vacuum, at a location in the vicinity of outlet extension 66Y, the direction of deflection of the sealing means is convenient, as unwanted escape of vaporised acetic anhydride (on account of the heating effect of the microwave radiation) is prevented. Figure 16 shows a schematic diagram of apparatus used in the sixth aspect of the invention, and Figure 17 shows a perspective view of a cylindrical vessel adapted to transmit ultrasonic energy to materials passing therethrough, also in accordance with a sixth aspect of the invention. Referring to Figures 16 and 17, according to the sixth aspect of the invention there is provided apparatus 10Z. In this embodiment, it is to be assumed that the amounts of lignocellulosic material which are to be acetylated are small, and henceforth readily available laboratory apparatus may be used. However, the second aspect of the invention is not to be considered as limited to a process for the acetylation of small quantities of lignocellulosic material, as the required laboratory apparatus described hereunder is also available on an industrial scale, and thus scaling of the process may be effected as required. It should also be noted that support members for the apparatus shown in Figure 16 have not been shown in the interests of clarity. A flask 12Z, substantially filled with jute fibres immersed in liquid acetic anhydride indicated generally at 14Z, is suspended by conventional means such as a retort stand (not shown) with a portion of its body in an oil 16Z the said oil is contained in a tank 18Z. An ultrasonic probe 20Z supplied with power through wires 22Z is also disposed in said oil. (The medium through which the ultrasonic energy is transmitted greatly effects the efficiency of the transmission and it is widely known that oil provides a more effective transmission medium than air.) The flask 12Z is sealed with a bung 24Z provided with an aperture through which an outlet pipe 26Z is inserted, thus allowing vapour to escape from the flask whilst sealing the contents of the flask from the surrounding atmosphere. Outlet pipe 26Z passes through a condenser represented schematically at 28Z which is provided with a fluid inlet 30Z and a fluid outlet 32Z, and emerges from said condenser above a collecting beaker 34Z into which condensed liquid is deposited. Typically, the oil 16Z is initially heated before commencement of the process to a temperature of 100°C by conventional means, and both flask 12Z and ultrasonic probe 20Z are then dipped into the hot oil. Activation of the ultrasonic probe 20Z allows the transmission of ultrasonic energy to the flask, and more importantly to the jute fibres and liquid acetic anhydride 14Z, through the oil which simultaneously transmits heat to the flask 12Z and its contents. A fraction of the ultrasonic energy is also transmitted to the oil itself with a resultant steady increase in temperature. Ideally, the ultrasonic probe is deactivated intermittently, and preferably when the temperature of the oil reaches 106°C, to allow said oil temperature to return to 100°C, after which transmission of the ultrasonic energy may recommence. The acetylation of the fibres is thus catalysed by both heat and ultrasonic energy which increases the efficiency of the process substantially. The increase in temperature of the contents 14Z of the flask 12Z causes some evaporation of the already volatile liquid acetic anhydride, the vapours of which are allowed to escape through the outlet pipe 26Z and be transferred to the condenser 28Z. Cold water is typically forced through the condenser at fluid inlet 30Z thereof and discharges at fluid outlet 32Z to cool the outlet pipe 26Z and hence any vapours from the flask 12Z contained therein. The cooling of said vapours causes them to condense along the walls of outlet pipe 26Z and the effects of gravity on the resulting liquid carry said liquid towards the beaker 34Z where liquid acetic anhydride 36Z is recovered. Referring now to Figure 17, there is provided a cylindrical vessel 40Z with ends 42Z, 44Z which may be either open or closed depending on the particular embodiment of the invention implemented, as described hereunder. An ultrasonic probe 46Z is located within the cylindrical vessel 40Z along its access and is fixed in position by a support member 48Z. Power is supplied to the probe 46Z by cables which pass through said support members. In use, a slurry of jute fibres liquid acetic anhydride (not shown) is inserted or is forced into the cylindrical vessel 40Z and surrounds the probe 46Z. The acetylation reaction is catalysed by the transmission of ultrasonic energy from the probe to the slurry, and a highly efficient process results, especially as the liquid acetic anhydride compared of the slurry enhances the transmission of the ultrasonic energy to the jute fibres. In one embodiment, a continuous acetylation process may be effected if the slurry is caused to flow through the cylindrical vessel. In this case, the rate of flow of the slurry is determined by the acetylation reaction. In another embodiment, the ends 42Z, 44Z of the cylindrical vessel 40Z are closed, and a fixed amount of slurry within the said vessel is excited by a means located either inside or outside the said vessel to ensure uniform acetylation of the jute fibres in the slurry. In either or both of the above embodiments, heat may be applied to the slurry before or during the acetylation process to further catalyse the reaction. The most preferred reaction temperature is 100°C. In a yet further embodiment, the ultrasonic probe 46Z and associated port 48Z are removed from the cylindrical vessel 40Z, and an alternative ultrasonic probe (not shown) disposed either on the internal or external wall of the cylindrical vessel transmits ultrasonic energy into the slurry externally of the volume it occupies. The slurry may be disturbed or excited as described above to ensure uniform acetylation of the jute fibres therein, or may be caused to flow through the cylindrical vessel as previously described. Where acetylated jute fibres are subsequently mixed with a soft curable resin to form an extrudable mass which is then extruded into pipe form, the average fibre length in bundled jute bails (approximately 2 inches) ensure both that the viscosity of the extrudable mass is low enough to be extruded, and that the strength of the resulting extrudate is acceptable.

Claims: CLAIMS 1. A method for the acetylation of lignocellulosic materials comprising the steps of adding a first reactant, being one of a number of or a mixture of lignocellulosic materials to a containment means, adding a second reactant, being an acetylating agent, to said containment means, characterised in that the amount of the acetylating agent added is sufficient to provide the required degree of acetylation of the lignocellulosic materials without substantial residue of the acetylating agent on completion of the reaction, and in that the reaction is catalysed by the addition of energy to the reactants. 2. A method according to claim 1 characterised in that the containment means is cooled after the addition of the catalysing energy. 3. A method according to claim 1 characterised in that a substantially constant temperature is maintained in the containment means during the period of acetylation. 4. A method according to claim 1 characterised in that the temperature of the reactants does not exceed 200°C. 5. A method according to claim 1 characterised in that the degree of acetylation of the fibres on completion of the reaction is in the region of 16%-20%. 6. A method according to claim 1 characterised in that the acetylating medium is acetic anhydride. 7. A method according to claim 1 characterised in that the lignocellulosic material is jute fibres. 8. A method according to claim 1 characterised in that the lignocellulosic material is heated prior to placement in the containment means to dry same. 9. A method according to claim 1 characterised in that the acetylating agent is pre-heated prior to introduction into the containment means. 10. A method according to claim 1 characterised in that the containment means is heated whilst the fibres are contained therein. 11. A method according to claim 10 characterised in that the heating is effected by the circulation of hot oil around a heating jacket which encases the containment means. 12. A method according to claim 1 characterised in that the lignocellulosic material is held in a compressed state within the containment means. 13. A method according to claim 1 characterised in that the addition of the catalysing energy causes some evaporation of the acetylating agent, thus increasing the pressure within the containment means. 14. A method according to claim 1 characterised in that the containment means is provided with temperature and pressure sensors, and/or an heating element and/or pressurising means. 15. A method according to claim 1 characterised in that the temperature of reaction is sufficient to cause evaporation of the reaction products other than the acetylated lignocellulosic material and any unused acetylating agent. 16. A method according to any of the preceding claims characterised in that the volume of the containment means is fixed. 17. A method according to claim 16 characterised in that a vacuum is drawn in the containment means prior to the introduction of the acetylating agent, or on completion of the acetylation reaction, or at both such times. 18. A method according to claim 17 characterised in that the acetylation reaction occurs after the vacuum has been isolated from the containment means. 19. A method according to any of claims 16- 18 characterised in that the acetylation of lignocellulosic fibres is effected as a batch process in that a batch of the fibres is acetylated and is then removed from the containment means and the process is repeated. 20. A method according to any of the preceding claims characterised in that the catalysing energy added to the reactants is thermal energy. 21. A method for the acetylation of lignocellulosic materials comprising the steps of adding a first reactant, being one of a number of or a mixture of lignocellulosic materials to a containment means, adding a second reactant, being an acetylating agent, to said containment means, characterised in that the acetylation reaction is catalysed by the addition of microwave energy to the reactants. 22. A method according to any of claims 1-19 characterised in that the catalysing energy supplied to the reactants is in the form of microwave radiation. 23. A method according to claim 21 or 22 characterised in that the microwave radiation is applied intermittently. 24. A method according to any of claims 21-23 characterised in that the temperature and pressure are monitored during the acetylation reaction. 25. A method according to any of claims 21-23 characterised in that the acetylation of the lignocellulosic fibres is effected substantially continuously by passing the reactants through the containment means, a portion of which is located within a chamber within which microwaves radiate. 26. A method according to claim 25 characterised in that the containment means is in the form of a tube. 27. A method according to claim 26 characterised in that the reactants are urged through the tube, and thus through the chamber by screw feed means, the reactants being urged into a first open end of the tube, and acetylated lignocellulosic materials emerging from the alternate end thereof. 28. A method according to claim 27 characterised in that the acetylating agent is introduced at a location along the screw feed means prior to the entry of the lignocellulosic material into the chamber such that some mixing of the lignocellulosic material and the acetylating agent occurs on account of the action of the screw feed means. 29. A method according to either of claims 27 or 28 characterised in that the screw feed means is associated with hopper feed means such that a large bundle of lignocellulosic fibres may be loaded thereinto. 30. A method according to any of claims 21-29 characterised in that the containment means containing the reactants is translated at a predetermined rate through a chamber within which microwaves radiate, the acetylating agent being released internally of said containment means proximate to the entry thereof into the said chamber at a rate dependent on both the rate of translation of the containment means through the said chamber and the required degree of acetylation of the lignocellulosic materials. 31. A method according to claim 30 characterised in that the containment means is in the form of a tube. 32. A method according to claim 31 characterised in that the acetylating agent is released internally of the tube through a stationary probe. 33. A method of according to any of claims 21-29 characterised in that a fixed volume containment means containing the reactants is located within a chamber within which microwaves radiate. 34. A method according to claim 33 characterised in that the containment means, and thus the reactants are subjected to microwave radiation for a predetermined period of time, after which the containment means is removed, and a different containment means containing more reactants located in said chamber such that a batch process results. 35. A method according to any of claims 21-24 characterised in that the lignocellulosic material is in the form of a web which is passed substantially continuously through a chamber within which microwaves radiate. 36. A method according to claim 35 characterised in that the acetylating agent is introduced into the chamber at a location proximate to the entry of the web into said chamber, and spent acetylating agent and other vaporous and liquid acetylation reaction products removed at a location proximate to the exit of said web from said chamber, the quantity of acetylating agent introduced depending on the rate of travel of the web through the chamber and the degree of acetylation of the fibres required. 37. A method according to either claim 35 or 36 characterised in that lignocellulosic fibre drying means are provided prior to the entry of the web into the chamber , or after the exit of the fibre from the chamber, or in both such locations. 38. A method according to claim 37 characterised in that the drying of the web is effected by a hot inert gas. 39. A method according to the claim 38 characterised in that the hot inert gas which dries the acetylated web is passed through a condenser to recover any residual acetylating agent and/or acetylation reaction products. 40. A method according to any of claims 35-39 characterised in that driven rollers feed the web into and extract the web from the chamber. 41. A method according to any of claims 35-40 characterised in that the web is rolled off a supply roller at the commencement of travel of the web, and rolled onto a collection roller at the termination of travel of the web. 42. A method according to claim 41 characterised in that supply roller, drive rollers, chamber, drying means, and collection roller are all provided within encasement means. 43. A method according to claim 42 characterised in that said encasement means is charged at a pressure marginally greater than atmospheric pressure with an inert gas. 44. A method according to any of claims 21-43 characterised in that the frequency of the microwave radiation is 0.98GHz, or alternatively is 2.45GHz. 45. A method for the acetylation of lignocellulosic materials comprising the steps of substantially immersing the lignocellulosic material in an acetylating agent within containment means, applying ultrasonic energy to the material and acetylating agent, and recovering the acetylating agent simultaneously with and/or after the process. 46. A method according to claim 45 characterised in that the ultrasonic energy is applied to the reactants at 150°C, the application of ultrasonic energy thereto being discontinued if the temperature increases above this level. 47. A method according to either of claims 45 or 46 characterised in that the ultrasonic energy is applied to said containment means, and thus to the lignocellulosic material and acetylating agent, through an oil, which effectively transmits the ultrasonic energy. 48. A method according to claim 47 characterised in that the oil is heated to approximately 100°C and thus also transfers heat to the containment means, and ultimately to the lignocellulosic material and acetylating agent. 49. A method according to any of claims 45-48 characterised in that the method of recovery of the acetylating agent during the reaction is forced condensation. 50. A method for the acetylation of lignocellulosic materials comprising the steps mixing lignocellulosic material with an acetylating agent, adding said mixture to containment means in which is disposed means capable of transmitting ultrasonic energy, characterised in that the said mixture is periodically or continuously disturbed to ensure that localised acetylation does not occur. 51. A method according to claim 50 characterised in that the mixture is in the form of a slurry. 52. A method according to either of claims 50 or 51 characterised in that the means capable of transmitting ultrasonic energy is an ultrasonic probe. 53. A method according to claim 52 characterised in that the probe is disposed substantially centrally within the containment means such that it is surrounded by the mixture. 54. A method according to any of claims 50-53 characterised in that the disturbance of the mixture is effected by an actuator, the direction of motion of which alternates periodically. 55. A method according to claim 54 characterised in that the actuator effects a "push-pull" type motion on the mixture.

Inventors: MURRAY JOHN EDWARD, MURRAY, JOHN, EDWARD
Applicants: MURRAY JOHN EDWARD, SOUND PIPE LTD, MURRAY, JOHN, EDWARD, SOUND PIPE LTD.
Application year: 1997
Main classification: C08H5
IPC codes: C08B3, C08H5