In general, natural cellulosic materials are dissolved physicochemically to prepare the cellulose solutions (cellulose dope) or are converted chemically into reducing sugars or alcohols. However, in the above physicochemical and chemical treatments, it is required to enhance interfacial contact and affinity between the cellulosic materials and the used chemical substances in order to accelerate processing and to increase the yield by conversion.

The present invention relates to a novel technology which can induce effective changes in the structure and the morphology of polymeric cellulosic materials of various origins such as cellulose, hemicellulose and the like, in order to improve the physicochemical processability and the affinity towards chemical reagents.

The process of the present invention can be applied to the renewable natural resources or the wastes such as wood, pulp, saw dust, paper, rice straw, corn stalk, corn cob or the like, which contain cellulose or hemicellulose.

The present invention involves a technology which destroys the crystalline structure of the cellulosic materials and reduces the size of the crystalline particles (or crystalline fibers) to enhance their processability and chemical affinity, by chain explosion with supercritical carbon dioxide repeated several times for the dried or partially dried cellulosic materials after chemical pretreatment.

BACKGROUND ART Conventionally, in order to decrease the crystallinity of cellulosic materials, several treating methods are used. Examples of such method include the use of an acid or base, the successive use of an organic solvent and ice water, or the application of microwave or ultrasonic wave.

The use of an acid or base is one of the popular treatments, but this method includes problems such as process corrosion and environmental pollution by spent acid or base. Further, since the recovery of the

processing media used in such processes are very difficult due to the nature of the process, and therefore the capital investment required is increased enormously, making this method undesirable in process economics.

When an organic solvent or a complex-forming chemical substance is used, there are several problems including a process corrosion due to a highly corrosive property of the materials used: environmental and toxicological problems, or the absence of recovering method.

The application of external energy such as microwave or ultrasonic wave also requires large energy consumption and increases equipment cost. However, it is known to be difficult to reproduce the effects of this method consistently.

Meanwhile, the chain explosions for polymers using nitrogen, steam or ammonia were proposed. However, these methods have some drawbacks of corrosiveness, high energy cost, low explosion efficiency, and great loss of molecular weight or the like.

Carbon dioxide can easily reach its supercritical state from ambient atmospheric condition and, at its supercritical condition, it exhibits excellent penetration into the polymeric chains. Since the density of the supercritical fluids can be sensitively adjusted by controlling the process variables such as temperature and pressure, it has been employed as the explosion media for the pretreatment of starch and some cellulosic materials.

However, it is experimentally observed that when the above simple explosion with supercritical carbon dioxide is applied to cellulosic materials having high molecular weight and high crystallinity, that is, to the cellulosic materials of very high a-fraction, the explosion effects are very limited.

The present invention provides a method of changing the structure of cellulosic materials, including those of high a-fraction, to enhance the affinity thereof to the processing solvents or the reactants without using expensive and/or corrosive chemicals and with less loss of molecular weight.

The present invention also provides the methods of producing cellulose solutions or the feedstock of reducing sugars or alcohols.

DISCLOSURE OF THE INVENTION The method of changing the micro-structure of cellulosic materials according to the present invention comprises pretreating cellulosic materials with aqueous caustic soda solutions or ammonia solutions, drying them

totally or partially, making supercritical carbon dioxide under 200 atm"500 atm and 35'C"200'C penetrate into cellulose chains, and subjecting them to an explosion treatment repeatedly.

The resulting materials can be used as the raw materials of the cellulose solutions or the feedstock of reducing sugars or alcohols.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a comparative graph M hich shows the XRD patterns of untreated sample of a-cellulose and samples of a-cellulose treated by various implementations of chain explosion.

Figure 2 (a) is a SEM photograph showing the crystalline surface structure of a-cellulose subjected to a simple chain explosion with no chemical pretreatment.

Figure 2 (b) is a SEM photograph showing the crystalline surface structure of a-cellulose subjected to triple chain explosions with no chemical pretreatment.

Figure 3 (a) is a SE-NI photograph showing the crystalline surface structure of a-cellulose subjected to one chain explosion after chemical pretreatment with caustic soda solution.

Figure 3 (b) is a SEM photograph showing the crystalline surface structure of a-cellulose subjected to three chain explosions after chemical pretreatment with caustic soda solution.

Figure 4 (a) is a SEM photograph showing the crystalline surface structure of a-cellulose subjected to one chain explosion after chemical pretreatment with ammonia solution.

Figure 4 (b) is a SELI photograph showing the destroyed crystalline structure of a-cellulose subjected to three chain explosions after chemical pretreatment with ammonia solution.

BEST MODE OF CARRYING OUT TEE INVENTION The method of changing the micro-structure of cellulosic materials according to the present invention comprises pretreating cellulosic materials with aqueous caustic soda solution or ammonia solution, drving them totally or partially, making supercritical carbon dioxide under 200 atm"500 atm and 35 t ~ 200 t penetrate into cellulose chains, and subjecting them to an explosion treatment repeatedly.

The resulting materials can be used as the raw materials of the

cellulose solutions or as the feedstock of reducing sugars, or alcohols.

Polymeric chains of cellulose experience the long-range interactio ns due to their interchain and intrachain hydrogen bonding forces. The crystalline regions of cellulose formed by such interaction are very rigid and resistant to the physicochemical attacks of other chemical substances, and thus they exhibit poor processability and very low reactivity. In order to improve-these processability and reactivity of the cellulosic fibers, the chain explosions by steam, ammonia or the like are conventionally applied to these crystalline materials.

Recently, a method was reported in which Avicelt, one of the microcrystalline cellulosic materials, is subjected to an chain explosion to modifv the structure of cellulosic cy-stallites and the resulting materials were able to be converted into a glucose solution in ahighyield.

Cellulosic materials are usuallv classifie into a-, -and y- fractions according to their solubilities in a 17.5% NaOH solution -Cellulose has the typical molecular structure and the crystalline form of pure cellulose and its chain structure is venu rigid. Its crystallinity is much higher and the disordered crystalline and amorphous parts are less than those of other fractions of cellulose. Therefore, cellulosic materials of high a-fraction show very poor processability and low reactivity due to their inherent crystalline structure.

Natural cellulose has the cellulose-I morphological structure. In general, this structure is usually transformed bv mercerization into the new structure of cellulose which is then subjected to the physical processes or to the chemical rections.

The present invention enables the microscopic structure of chemically pretreated cellulosic materials to be effectively changed by the chain explosion using carbon dioxide at the supercritical conditions. The effects of such chain explosion can be compared and confirmed by analyzing X-ray diffraction (XRD) patterns and observing photographs from scanning electron microscopy (SEM).

Mercerized celluloses of cellulose-n structure can be prepared by dipping a-celluloses for about 2 hours in a 18 wt% NaOH solution of an amount corresponding to 3 ~ 5 times weight of the used cellulose. The above treated sample is washed on a filter medium to prepare a wet

sample, which may be dehydrated further to prepare a dry sample, by using driers such as vacuum dryer.

As to ammonia-pretreated samples, the above mercerized cellulose is washed on a filter medium more than three times with a 35 wt% ammonia water of an amount corresponding to a half weight of the-nsed dry cellulose to prepare samples in wet or dry state.

Chain explosion system may comprise a fluid-supplying section of supercritical carbon dioxide, a section for maintaining constant temperature and pressure before explosion, and an explosion section such as a stirred vessel or the converging-diverging nozzles wherein the chain explosions take place. The chain explosion system is designed to protect itself from the mechanical impact caused by a sudden pressure release and the high pressure for supercritical carbon dioxide. These systems may be operated batchwise, in a semi-continuous mode, or continuously.

Sudden expansion that leads to chain explosion can be implemented by opening valves attached to the high-pressure vessel or by making the supercritical fluid carrying fine powders of the cellulosic materials flow through the converging-diverging nozzles.

It is necessary to expose the cellulose substrate to the supercritical carbon dioxide for longer than penetration time before explosion so that the explosion medium can deeply penetrate into the inner part of the substrate.

The above-mentioned cellulosic materials that have been chemically pretreated can be subjected to one explosion treatment or to multiple explosion treatments repeatedly in the chain explosion system of a batch, semi-continuous or continuous type.

The efficiency of chain explosion is approximately proportioned to the density change of the explosion medium before and after the explosion.

Therefore, it is preferable to make the initial conditions of the supercritical state in order to maximize the density difference. This prefers higher pressure above critical pressure and slightly higher temperature above critical temperature.

When a wet sample is subjected to the chain explosion, the explosion trajectory can be traced on the pressure-temperature phase diagram of carbon dioxide and water, and in most cases, the explosion path meets the solid-liquid phase boundary line of water wherein

water freezes.

This means that, the swelling water penetrated inside near the crystalline surface freezes during the chain explosion. These formed ice particles can play a role as the wedges that destroy and change the crystalline structure of the cellulosic materials. This effect of structural change becomes more remarkable when the sample is subjected to multiple chain explosions.

It is also possible to use a"modifier", which added in a small amount to the explosion medium and can improve the efficiency of the chain explosion. Examples of such modifier include water, alcohols, amines, fatty acids or the likes. By using such modifiers, it is possible to make the structural change of the substrate more drastically and to adjust the explosion behavior.

Since the chain explosion treatment according to the present invention can be applied to cellulosic materials of high a-fraction, the present invention can be more effectively applied to other cellulosic materials of low a-fraction such as rice straw, saw dust, pulp, paper, wood or the like to change the morphological structure of the crystallites constituting cellulose.

The change in the crystalline structure can be compared and visualized by the analysis of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The quantitative comparison of the dissolution rates and solubilities to a specific solvent may be used as the indirect methods.

By utilizing the chain explosion treatments according to the practicing process of the present invention, the reaction rates and yields can be increased noticeably in the reactions of cellulosic materials for reducing sugars and alcohols.

EXAMPLES The present invention will next be described in detail by way of examples, which should not be construed as limiting the invention.

Example 1 a-Cellulose and 18 wt% NaOH solution were mixed in the weight ratio of about 1: 5 and treated for 2 hours at room temperature. The mercerized sample thus obtained was dried used as the substrate for chain explosion.

The dry substrate was exposed to the medium of supercritical carbon

dioxide (99.9%) at 50'C and 350 atm for minutes and then subjected to the chain explosion. The chain explosion was repeated three times.

The resulting sample was mixed with a solvent mixture containing 4.5wt% of lithium chloride in dimethylacetamide and then, was dissolved by stirring at 25 *C. After 24 hours, the solvent was removed and the amount of the dissolved cellulose was measured. These experimental results are shown in Table 1.

Example 2 The same procedure as Example 1 was conducted, except that the sample treated with NaOH solution in Example 1 was washed with 35 wt% ammonia water.

The results of the experiments are shown in Table 1.

Table 1 Sample Ratio of No. Experimental Condition Dissolved Amount Remark 1Untreated-cellulose1.000 Control I 2 Pre-treated with 18 wt% NaOH solution 1.100 Dry sample and one chain explosion 3 Three chain explosions for Sample No.2 Sample 4 Washed Sample No. 2 with 35 wt% NH40H1.109Dry Sample solution and one chain explosion 5 Three chain explosions of Sample No. 4 1.946 Wet Sample Example 3 The same procedure as Example 1 was conducted, except that supercritical carbon dioxide further contained water, ethanol, benzylethyla- mine, dimethylamine or monochloroacetic acid.

Example 4 The same procedure as Example 1 was conducted, except that a-cell ulose was replaced with hemicellulose, that is, wood, pulp, saw dust, paper, rice straw, corn stalk, corn cob.

Example 5 The same procedure as Example 1 was conducted, except that the chain explosion was repeated twice instead of 3 times.

Example 6 The same procedure as Example 1 was conducted, except that it was conducted at 100 *C and 400 atm instead of 50 C and 350 atm.

Example 7 The same procedure as Example 1 was conducted, except that it was conducted at 150'C and 450 atm instead of 50 C and 350 atm.

INDUSTRIAL APPLICABILITY The present invention enables efficient change of crystalline structure even in the cellulosic materials of high a-fraction without using expensive and/or corrosive chemicals and with less loss of molecular weight. The present invention can be applied to increase the solubilities of the cellulosic materials in their solutions as described in the above Examples and to increase the yield of glucose (or reducing sugars) in their hydrolysis reactions. When alcohol is produced by fermentation of glucose, the yield of alcohol is known to be proportional to the concentration of glucose in the broth. Therefore, when applied to a process for alcohols as above, the present invention can produce alcohols in a higher yield than the conventional methods.