Process for the preparation of a fatty acid ester of cellulose

Abstract: A fatty acid ester of cellulose excellent in transparency, filterability and spinnability in spite of its high content of mannose and xylose, wherein the total molar content of mannose and xylose accounts for at least 7% of that of glucose, mannose and xylose, and the amount of filtration satisfies a specified requirement. Such a fatty acid ester of cellulose is prepared by using a pulp having a low α-cellulose content as a raw material and a fatty acid corresponding to the ester as a diluent and adding an organic solvent in an amount of at least 10% by weight based on the diluent in any step during the preparation. In the preparation of cellulose diacetate, which is excellent in filterability and transparency, by the acetic acid process, the cellulose diacetate obtained by aging is dissolved in a solvent which can dissolve it and the resulting cellulose diacetate is recovered.

Description: The present invention relates to a process for the preparation of a fatty acid ester of cellulose. More particularly, the present invention relates to a process for the preparation of a fatty acid ester of cellulose excellent in transparency, filterability and spinnability from a low-grade dissolving pulp as the raw cellulose material. Among cellulose derivatives, fatty acid esters of cellulose, particularly cellulose acetate hold an important position industrially in virtue of their wide fields of applications including fibers for example for clothes, cigarette filter, plastics, film and coating materials. Although wood pulps are mainly used as the raw material for preparing fatty acid esters of cellulose. a highly purified high-grade dissolving pulp is particularly useful. This is because when the conventional, common process for preparing thereof is applied to a low-grade wood dissolving pulp, a large amount of insoluble matters in a solvent, i.e., the so-called insoluble residues are formed to impair the spinnability and the transparency as a plastic remarkably, so that filtration is required to secure these qualities. The standard process for the preparation of cellulose acetate, which is a representative fatty acid ester of cellulose, basically comprises (i) a pretreatment step of disaggregating and disintegrating a starting pulp having a relatively high α-cellulose content, spraying the resulting pulp with acetic acid and mixing, (2) an acetylation (esterification) step of reacting the pretreated pulp obtained in (1) with a mixed acid comprising acetic anhydride, acetic acid and an acetylation catalyst (e.g., sulfuric acid), (3) an aging step, i.e., a hydrolysis step, of hydrolyzing the obtained cellulose acetate to a desired degree of acetylation, and (4) a post-treatment step of precipitating the hydrolyed cellulose acetate, recovering the precipitates and subjecting the precipitates to purification, stabilization and drying, successively. According to U.S. Patent No. 2827455 (patented on March 18, 1958; Assignee; Celanese Corporation of America), the esterification in step (2) is conducted with a mixture containing a lower aliphatic acid anhydride and a water-immiscible solvent for the ester being formed, such as methylene chloride, chloroform, ethylene chloride and tetrachloroethane. Improvement techniques for the above standard process from various standpoints have been reported in order to enable the preparation of a high-quality cellulose ester from a low-grade dissolving pulp which is available at a low cost. U.S. Patent No. 3767642 (patented on Oct. 23, 1973; Assignee: CELANESE Corp.) discloses that cellulose acetate excellent in transparency, filterability and spinnability is prepared by disintegrating a wood dissolving pulp having an α-cellulose content of 92 to 93% in a dilute aqueous solution of acetic acid to obtain a slurry, subjecting the slurry to a so-called slurry pretreatment wherein the removal of liquid and replacement of the liquid by acetic acid are repeated alternately, acetylating the resulting pulp according to the process of the prior art, neutralizing the sulfuric acid catalyst contained in the reaction system, and subjecting the reaction mixture to saponification and aging at 125 to 170°C. Further, U.S. Patent No. 4306060 (patented on Dec. 15, 1981; Assignee: DAICEL CHEMICAL Ind., Ltd.) discloses a process for preparing cellulose acetate excellent in transparency, filterability and spinnability which comprises disintegrating a high-grade dissolving pulp having a high α-cellulose content by a common dry process, acetylating the resulting pulp in the presence of a small amount of sulfuric acid as the catalyst at high temperature to complete the acetylation in a short time, completely neutralizing the sulfuric acid catalyst contained in the system, and subjecting the reaction mixture to saponification at 110 to 120°C. Further, it is also disclosed that this process enables the utilization of a low-grade dissolving pulp having a low α-cellulose content. The characteristic common to the above techniques is that the saponification is conducted at high temperature. Such high-temperature saponification promotes the destruction and/or deformation of a hemicellulose acetate which otherwise damages the characteristics of cellulose acetate, whereby the hemicellulose acetate is converted into a harmless substance. Attempts are disclosed to solve the above problem with attention being paid on the phenomenon that a low-grade wood dissolving pulp sheet is generally difficult to disintegrate owing to its high density, which leads to the poor dispersibility of the pulp in the reaction medium to result in the formation of an insoluble residue. More specifically, there is disclosed that a process wherein a pulp sheet is softened by wet disintegration to remove the heat generated in the disintegration as latent heat of evaporation, by which the thermal degradation of the pulp is inhibited to give cellulose as the raw material rich in reactivity, thus enabling the acetylation of the cellulose free from the gel formation due to unreacted matters and insufficient reaction. Further, there is disclosed that a process which comprises conducting the dry disintegration of a starting pulp on an improved disintegrator to thereby depress the deterioration of the reactivity of the pulp during disintegration to such an extent as not to have any effect on the acetylation, thus enabling the acetylation of cellulose free from the gel formation due to unreacted matters and insufficient reaction. However, when the above-mentioned techniques are applied to the acetylation of a pulp having an α-cellulose content as especially low as 85 to 93%, the effect is too insufficient to avoid the lowering in the filterability of the resulting cellulose acetate. Meanwhile, cellulose acetate is one of the organic acid esters of cellulose, the use of which is spread over many fields including the materials for clothes, cigarette filter tips, plastics, films, coating materials, medicines, foods, cosmetics and building materials and the production of which is the highest among cellulose derivatives, thus being industrially important. The so-called acetic acid process using acetic anhydride as an acetylating agent, acetic acid as a diluent and sulfuric acid as a catalyst is a representative one as the industrial process for the preparation of cellulose diacetate. This process comprises (1) the acetylation step for pretreating a starting pulp having a relatively high α-cellulose content as the raw material by disaggregation, disintegration and spraying with acetic acid to activate the starting pulp and thereafter treating the activated pulp with a mixed acid comprising acetic anhydride, acetic acid and sulfuric acid to give cellulose triacetate; (2) the aging step, i.e., the hydrolysis step, for hydrolyzing the obtained cellulose triacetate into cellulose diacetate having a desired degree of acetylation; and (3) the post-treatment step for separating the cellulose diacetate from the reaction mixture by precipitation, followed by purification, stabilization and drying. The term "cellulose diacetate" used in this specification refers to one having a degree of acetylation of 50 to 57%. Further, the term "degree of acetylation" refers to a value as determined and calculated according to the method stipulated in ASTM-D-871, i.e., a value obtained by calculating the acetyl group content of cellulose acetate as the amount of acetic acid. The cellulose diacetate prepared by the above process is again dissolved in a solvent such as acetone, molded and thereafter used as a commercial product. Improvement techniques have been proposed from various standpoints to prepare a high-quality cellulose diacetate from a low-grade wood dissolving pulp having a low α-cellulose content through the above-mentioned essential steps. Japanese Patent Publication-A No. 501/1987 (published on Jan. 6th, 1987) discloses that cellulose acetate very excellent in hue can be prepared from a starting pulp having a low α-cellulose content for the preparation of viscose and cellulose ether by preliminarily swelling the pulp with a solvent. Further, U.S. Patent No. 3846403 (patented on Nov. 5th, 1974; Assignee: CANADIAN CELLULOSE CO.) discloses that in the preparation of cellulose ester, cellulose ester having characteristics not inferior to those prepared from a purer cellulose source is obtained by reacting a pulp with an esterifying agent other than an objective one, i.e., a lower alkanoic acid, to form hemicellulose ester prior to the reaction with the objective lower alkanoic anhydride. According to this technique, a high-quality cellulose ester can be obtained even from a starting pulp having a hemicellulose content far higher than 5%, i.e., as high as 8%, 10% or even above. However, the technique described in the above Japanese Patent Publication-A No. 501/1987 is one for improving the hue of the cellulose acetate prepared from a low-grade wood dissolving pulp, so that it is silent upon the effect of improving other physical properties. Further, the technique has a defect that little effect is attained when a pulp having an α-cellulose content of 90% or below is used. On the other hand, U.S. Patent No. 3846403 discloses that even when the hemicellulose content of a starting pulp is 10% or above, the physical properties can be improved, and involves the description on the improvement in the filterability and the effect of reducing the weight of the insoluble residue with respect to cellulose triacetate, which is, however, different from the subject matter of the present invention, i.e., cellulose diacetate. Thus, it is unclear with respect to the properties of the cellulose diacetate. Generally, a high-grade wood dissolving pulp having a high α-cellulose content is used as the raw cellulose material. However, obtaining a high-grade one in quantity will become difficult because of the saving of resources on the global scale and the environmental pollution by pulp factories, so that the changeover to a low-grade wood dissolving pulp is believed to be unavoidable. When cellulose diacetate is prepared from a low-grade wood dissolving pulp by the process of the prior art, the obtained cellulose diacetate gives a large amount of insolubles when dissolved in a solvent, which causes various problems such as lowering in the transparency of the product, an increase in the yellowness and a remarkable lowering in the characteristics in the filtration prior to the spinning step. There is a correlation between the α-cellulose content of a starting pulp and the properties of cellulose diacetate prepared from the pulp, so that when the adaptability to acetylation of a pulp is discussed only from the standpoint of chemical factors, the use of a starting pulp having an α-cellulose content of 93% or below gives only a very poor cellulose diacetate which is practically unusable as a commercial product owing to a remarkable increase in the amount of insolubles in a solvent such as acetone. Various studies have been made on the above insolubles and abundant literature thereon can be found. Further, it is pointed out that the hemi-cellulose contained in a starting pulp may participate in the formation of the insolubles [see, e.g., Ueda et al., J. Japan Wood Research Soc., 34 (4), 346-353 (1988)]. In order to prepare cellulose diacetate containing little insoluble components, therefore, it is thought necessary to employ either the means (1) of using a highly-purified, high-grade wood dissolving pulp having a low hemicellulose content as the raw material or the means (2) of reducing or removing the insolubles in the cellulose diacetate prepared from a low-grade dissolving pulp. The former means, i.e., the use of a high-grade wood dissolving pulp as the raw material is actually industrially employed. However, the purification of a pulp by the removal of hemicellulose therefrom not only involves a lowering in the yield in preparing pulp from wood as the raw material to result in an increase in the production cost, but also is thought to be undesirable from the standpoint of the effective utilization of resources on the global scale. Further, even the content of a solvent insolubles in the cellulose acetate prepared from a high-purity pulp cannot be said to be enough low, but is expected to be further lowered for an improvement in the quality. The present inventors have recovered the insoluble residue which is present in the reaction mixture obtained by acetylating a low-grade wood dissolving pulp having a low α-cellulose content in a large amount and have made extensive studies on the residue. As a result of the studies, they have ascertained that the insoluble residue is an association comprising cellulose triacetate and glucomannan triacetate. Furthermmore, they have ascertained that this insoluble residue keeps the form of wood pulp fiber though it is considerably swollen in the acetylation system. On the basis of this information, the present inventors have studied the solubility of acetylated glucomannan contained in a low-grade wood dissolving pulp having a low α-cellulose content in various solvents to find that the amount of the insoluble residue can be reduced in such a state that glucomannan acetate is not removed from the reaction solution but remains therein by conducting the acetylation in a system wherein part of the acetic acid used as the diluent for the acetylation is replaced by an organic solvent, by adding an organic solvent as the third component to the reaction system in the course of the reaction and thereafter continuing the reaction, so that cellulose acetate excellent in filterability, transparency and spinnability can be prepared from a low-grade wood dissolving pulp having an α-cellulose content far lower than that of the pulp which was conventionally used according to the prior art. Further, it has also been found that this process can be applied to the preparation of other fatty acid esters of cellulose. The present invention has been accomplished on the basis of these findings. Thus, the present invention provides a process for the preparation of a fatty acid ester of cellulose comprising the steps of (a) optionally pretreating a starting pulp, (b) esterifying the pulp in order to prepare a cellulose triester, characterized in that the process further comprises the steps of (c) optionally aging or hydrolyzing the cellulose triester of step (b) in order to obtain a cellulose diester, (d) post-treatment, comprising precipitating and recovering the cellulose triester of step (b) or the cellulose diester of step (c), washing the recovered precipitate, and purifying and drying it, wherein a pulp having an α-cellulose content of 85 to 93% by weight is used as the raw material, a fatty acid corresponding to the ester is used as a diluent, and an organic solvent is added in an amount of at least 10% by weight based on the amount of the diluent at least in step (b). The process of the present invention provides a triester or diester of cellulose with a fatty acid, wherein the total molar content (as determined by the analysis of constituent saccharides) of mannose and xylose in the triester or diester accounts for at least 7%, preferably 7 to 30%, of that of glucose, mannose and xylose therein and the amount P 2 (g) of filtration achieved in the filtration under given conditions which will be described below for a period of 40 minutes starting with 20 minutes after the initiation of the filtration satisfies the following requirement (A) or (B): (A) for the triester of cellulose with a fatty acid P 2 > -0.14 η + 86; and (B) for the diester of cellulose with a fatty acid P 2 > -0.16 η + 40 [wherein η is the viscosity (centipoise, hereinafter referred to as "cP") of a 6% solution of the triester or diester in a prescribed solvent as determined by the method which will be described below]. The fatty acid ester of cellulose obtained according to the present invention has excellent filterability, though it contains xylose and mannose in large amounts as determined by the analysis of constituent saccharides of the fatty acid ester. Further scope and the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. The process according to the present invention will now be described. The preparation of cellulose acetate, specially cellulose triacetate or cellulose diacetate, will now be described as an example of the process according to the present invention. The wood dissolving pulp to be used as the raw cellulose material in the process for the preparation of cellulose acetate according to the present invention is a low-grade wood dissolving pulp having an α-cellulose content of 85 to 93% by weight. Such a pulp is generally available in sheet form and it is preferable that the sheet have a basis weight of 300 to 850 g/m 2 , a density of 0.40 to 0.50 g/cm 3 and a bursting strength of 50 to 350 kPa, though the sheet to be used is not limited to those having such characteristics. Although the pretreatment of the starting pulp prior to the reaction is not always necessary, it is a usual practice. For example, a pulp is treated on a disintegrator to obtain a fluffy pulp. This fluffy pulp is activated by pretreatment as follows: for example, 100 parts by weight of the fluffy pulp is sprayed with 20 to 100 parts by weight of acetic acid and the obtained mixture is stirred in a hermetically sealed container fitted with a stirrer at a temperature of 20 to 50°C for 0.5 to 2 hours (pretreatment step). Then, 200 to 400 parts by weight of acetic anhydride as an acetylating agent, 300 to 500 parts by weight of acetic acid and 0.5 to 5 parts by weight of an acidic catalyst are added to the resulting pulp, followed by mixing by stirring. The acetylation catalyst is not particularly limited but may be any one so far as it accelerates the acetylation. The catalyst includes sulfuric acid and perchloric acid, of which sulfuric acid is preferable. Although the order of the addition of these acids is not particularly limited, it is suitable that the catalyst is added in a state premixed with acetic acid or acetic anhydride, because decomposition occurs when the catalyst comes in contact with a pulp. The contents undergo heat buildup owing to reaction, so that the temperature of the contents is controlled so as to rise at a nearly constant ratio, by which the contents are brought finally to a temperature ranging from 50 to 85°C in 20 to 60 minutes. The resulting contents are maintained at that temperature for 3 to 20 minutes to acetylate cellulose, thus giving cellulose triacetate (primary cellulose acetate) (acetylation step). Then. in order to prepare cellulose diacetate (secondary cellulose acetate), the above cellulose triacetate is subjected to hydrolysis (saponification or aging). More specifically, the acetylation catalyst contained in the reaction mixture of the acetylation is neutralized and thereafter steam is blown into the system under pressurizing at 98 to 980 kPa (1 to 10 kg/cm 2 ) to bring the system to a temperature ranging from 125 to 170°C. The system is preferably maintained in this temperature range for 3 minutes to 6 hours, by which the acetyl groups of the cellulose triacetate are hydrolyzed to give cellulose diacetate having a desired degree of acetylation (aging or hydrolysis step). After the completion of the aging or hydrolysis step, the reaction mixture is thrown into dilute aqueous solution of acetic acid to form precipitates. Which are recovered, washed, purified and dried (post-treatment step). According to the present invention, an organic solvent is added to the acetylation step and may further be added in any step of the above pretreatment, aging and post-treatment. Alternatively, it may be added in a plurality of steps thereof. The amount of the organic solvent to be added is at least 10% by weight, preferably 10 to 90% by weight, still more preferably 30 to 70% by weight based on the amount of the diluent, such as acetic acid. The above-mentioned process is one conducted according to the acetic acid method which is a typical process for the preparation of cellulose acetate, and can be variously modified. As described above, one of the characteristics of the process according to the present invention is to use a fatty acid corresponding to the objective cellulose ester as a diluent for the reaction. The other characteristic of the process according to the present invention is to add an organic solvent as a third component in the reaction step of the acetic acid process. The specific method of the addition of the organic solvent is not particularly limited. Although the addition of the organic solvent may be conducted in the pretreatment-activation step in the acetic acid process, the addition in this step hinders the swelling of pulp fibers, delays the acetylation, or makes it difficult to dissolve cellulose acetate having a low degree of replacement as formed progressively by the acetylation in the reaction system efficiently. Accordingly, it is preferable to conduct the addition of the organic solvent for the first time as a third component in the acetylation step, still preferably in a step wherein the acetylation of a wood dissolving pulp proceeds considerably. Then, the acetylation is further continued to form a triester of cellulose with a fatty acid. According to such a process, the insoluble matter comprising a fatty acid ester of cellulose having a high degree of replacement and a fatty acid ester of glucomannan can be effectively dissociated and dissolved. Additionally, the organic solvent may be added in the aging or post-treatment step. The organic solvent to be used in the present invention as a third component may be any one, so far as it does not hinder the objective esterification. The organic solvent is preferably one exhibiting a high dissolving power for glucomannan acetate, which is specifically an aprotic compound which has one or two dipolar groups, such as e.g. a halogen-carbon linkage, a carbonyl group, a nitro group, a sulfur-oxygen linkage, and/or an ether linkage, and a melting point of 50°C or below and is compatible with the fatty acid used as the diluent at an arbitrary ratio. Preferred examples of the organic solvent to be used in the present invention include halogenated hydrocarbons such as dichloromethane (methylene chloride), chloroform, carbon tetrachloride, 1,1,2,2-tetrachloroethane, trichloroethylene and dichloroacetic acid; a solvent of a nitro compound such as nitromethane, nitroethane and 1-nitropropane; and ethyl cellosolve actate and cyclohexanone. When the organic solvent added to the system causes troubles, for example, when the system suffers from phase separation owing to the addition of water in the neutralization, aging or precipitation step, part or the whole of the organic solvent may be removed from the reaction system in a suitable step by heating or evacuation, when the melting point of the organic solvent is lower than that of the diluent. Further, when the organic solvent is added after the completion of the reaction, it may be removed by washing, heating, evacuation or the like. The fatty acid ester of cellulose prepared by the process of the present invention includes cellulose propionate, cellulose butyrate, cellulose acetate propionate and cellulose acetate butyrate in addition to the above-mentioned cellulose acetate. These esters can be each prepared in the same manner is that described above for the preparation of cellulose acetate except that one or more corresponding fatty acids are used instead of the acetic acid at a desired ratio. The triester or diester of cellulose with a fatty acid obtained according to the present invention will now be described. The method and conditions of the analysis of constituent saccharides are as follows: (1) method The analysis of constituent saccharides is conducted according to the alditol acetate method [see Borchadt, L.G.; Piper, C.V.: Tappi, 53 , 257 - 260 (1970)]. (2) conditions for measurement gas chromatograph: Shimadzu GC-7A Gas Chromatograph column: 10% Silar 10C column (2 m) injection temp.: 250°C column temp.: 230°C carrier gas: helium (40 ml/min) Each content was determined by averaging the values obtained by conducting the analysis thrice. The fatty acid ester of cellulose obtained according to the present invention is one wherein the total molar content of mannose and xylose in the ester accounts for at least 7% of that of glucose, mannose and xylose therin. The amount of filtration and the viscosity of 6% solution are determined by the following methods: (i) amount of filtration A solution of cellulose ester prepared by dissolving cellulose ester, e.g., cellulose acetate, in a given solvent in a given concentration is filtered through a given filter cloth (diameter: 15 mm, filtration area: 1.77 cm 2 ) at a given temperature under a constant pressure to determine the amount of filtration. The amount of filtration achieved in the filtration for a period of 20 minutes after the initiation of the filtration is represented by P 1 (g), and that achieved for a period of 40 minutes (from 20 minutes after the initiation of the filtration to 60 minutes after the initiation of the filtration) is represented by P 2 (g). The triester of cellulose with a fatty acid and the diester of cellulose with a fatty acid are different in solubility in a solvent, so that the conditions to be employed for the triester are different from those for the diester. The conditions for each ester are as specified in Table 1. The triester of cellulose with a fatty acid obtained according to the present invention has the amount P 2 (g) of filtration satisfying the above described requirement (A), and the diester of cellulose with a fatty acid obtained according to the present invention has the amount P 2 (g) of filtration satisfying the above described requirements (B). (ii) viscosity of 6% solution A triester of cellulose with a fatty acid or a diester of cellulose with a fatty acid is completely dissolved in the same solvent as that used in the determination of the amount of filtration in a concentration of 6%. The obtained solution is examined for falling time (t 1 , sec) by the use of an Ostwald viscometer at 25 = 0.1°C. Further, a standard liquid, of which the absolute viscosity η 2 (cP) and density D 2 (g/ml) at 25°C is known, is also examined for falling time (t 2 , sec) by the use of the same viscometer. The viscosity η 1 (cP) of the sample solution is calculated by the following formula: η 1 = [η 2 /(D 2 × t 2 ]) × D 1 × t 1 wherein η 2 /(D 2 × t 2 ) is a conversion factor inherent in the viscometer used; D 1 is the density of the sample solution to be examined and is 1.235 g/ml with respect to the triester of cellulose with a fatty acid and 0.823 g/ml with respect to the diester of cellulose with a fatty acid in the calculation. The P 2 value determined by the above method increases as the clogging of the filter with particulate impurities decrease, and serves as an indication of the filterability of cellulose ester. At the same time, the P 2 value is affected by the viscosity of the sample solution, so that a sample solution having a lower viscosity gives a larger P 2 value. The fatty acid ester of cellulose obtained according to the present invention having such viscosity and amount of filtration P 2 as to satisfy the above requirements (A) or (B) is a product which is excellent in filterability and has, needless to say, a suitable viscosity, though it contains xylose and mannose in large amounts as determined by the analysis of constituent saccharides. The fatty acid ester of cellulose obtained according to the present invention has excellent transparency, filterability and spinnability which could not be attained in the prior art, though it is prepared from a low-grade wood dissolving pulp having a low α-cellulose content. Thus, the present invention enables the high-level utilization of a low-grade wood dissolving pulp which was impossible according to the prior art. Further, the process according to the present invention can be carried out without any drastic change in the process for the preparation of a fatty acid ester of cellulose according to the prior art, thus being economically advantageous. Examples The present ivention will now be described in more detail with reference to the following Examples which should not be considered to limit the scope of the present invention. Unless otherwise stated, all parts and percentages in the Examples are by weight. Examples A-1 to A-10 and Comparative Example A-1 A dissolving pulp (α-cellulose content: 87.5%) prepared by the sulfite process was disintegrated with water in a household mixer and, after the replacement of the water by acetone, dried to give a pulp having a water content of 5%. The resulting pulp was thrown into a pretreatment machine, where 100 parts of acetic acid was uniformly sprayed on 100 parts of the pulp having a water content of 5%, followed by mixing for 30 minutes at 40°C to pretreat and activate the pulp. Separately, a mixture comprising 250 parts of acetic anhydride, 375 parts of acetic acid and 1.0 part of sulfuric acid was prepared, cooled to 12°C and placed in a kneader acetylator. The pretreated and activated pulp prepared above was thrown into the acetylator, followed by mixing by stirring. The contents of the acetylator underwent heat buildup owing to the reaction of acetic anhydride with the water entrained by the pulp and that of acetic anhydride with the cellulose, so that the temperature of the contents was controlled by external cooling so as to rise from about 16°C (initial) to 77°C in 60 minutes. 125 parts of an organic solvent (see Table A-1) was added into the machine and the resulting mixture was further maintained at 77°C for 12 minutes to conduct acetylation. The obtained reaction mixture was centrifuged (at 7000 rpm for 30 minutes) to recover insolubles and the insoluble residue content of the obtained cellulose triacetate was determined. The degree of acetylation of the cellulose triacetate and that of the insoluble residue were both about 61%. The insoluble residue content of the product prepared in each acetylation is given in Table A-1. In Comparative Example A-1, no organic solvent was added. As understood from the insoluble residue content given in Table A-1, the filterability of the cellulose triacetate can be improved by the addition of an organic solvent to give cellulose triacetate product having an excellent transparency. The total molar amounts of mannose and xylose as determined by the analyses of constituent saccharides, the amounts (P 2 ) of filtration and the viscosities (η) of a 6% solution with respect to the cellulose triacetates prepared in the Examples A-1, A-2, A-5 and A-6 and the Comparative Example A-1 are given in Table A-2. The cellulose triacetates prepared in the Examples A-1, A-2, A-5 and A-6 were products satisfying the requirement: P 2 > -0.14 η + 86. Comp. Ex. Example A-1 A-1 A-2 A-5 A-6 mannose + xylose (mole %) 9.2 9.3 9.1 8.9 8.8 P 2 (g) 34 48 45 49 48 η (cP) 340 320 325 335 332 . Examples A-11 to A-20 and Comparative Example A-2 A dissolving pulp (α-cellulose content: 87.5%) prepared by the sulfite process was disintegrated by the use of an attrition mill. The fluffy and disintegrated pulp was dried to a water content of 5%. The resulting pulp was thrown into a pretreatment machine, where 100 parts of acetic acid was uniformly sprayed on 100 parts of the pulp having a water content of 5%, followed by mixing at 40°C for 30 minutes to pretreat and activate the pulp. Separately, a mixture comprising 250 parts of acetic anhydride, 375 parts of acetic acid and 1.0 part of sulfuric acid was prepared, cooled to 12°C was placed in a kneader acetylator. The pretreated and activated pulp prepared above was thrown into the acetylator, followed by mixing by stirring. The contents of the acetylator underwent heat buildup owing to the reaction of acetic anhydride with the water entrained by the pulp and that of acetic anhydride with the cellulose, so that the temperature of the contents was controlled by external cooling so as to rise from about 16°C (initial) to 77°C in 60 minutes. 125 parts of an organic solvent (see Table A-3) was added into the acetylator and the resulting mixture was further maintained at 77°C for 12 minutes to conduct acetylation. In Comparative Example A-2, no organic solvent was added. Then, the organic solvent was removed from the reaction mixture by utilizing heat of reaction. 10 parts of a 20% aqueous solution of magnesium acetate was added to the resulting system, by which the sulfuric acid contained in the system was completely neutralized and the system was brought to a state containing excessive magnesium acetate. The completely neutralized reaction mixture was transferred into an autoclave and steam of a gauge pressure of 490 kPa (5 kg/cm 2 ) was blown into the autoclave in a hermetically sealed state to raise the temperature of the contents of the autoclave to 150°C in about 60 minutes. The contents were maintained at 150°C for 50 minutes and gradually flushed into the open air to lower the temperature thereof to 100°C. A dilute aqueous solution of acetic acid was added to the reaction mixture under vigorous stirring to separate flaky cellulose diacetate, which was recovered, fully washed with water and dried. The degree of acetylation of the flaky cellulose diacetate thus obtained was about 55% and the viscosity and amount of filtration thereof are as given in Table A-3. As apparent from these results. the obtained cellulose diacetate was a product excellent in filterability and spinnability. Further, the total molar contents of mannose and xylose in the cellulose diacetates prepared in the Examples A-11, A-12, A-15 and A-16 and the Comparative Example A-2 were 7.5, 7.2, 7.0, 7.1 and 7.3%, respectively, as determined by the analysis of constituent saccharides, and the amounts of filtration of the cellulose diacetates are as given in Table A-3. The cellulose diacetates prepared in the Examples A11, A-12, A-15 and A-16 are products satisfying the requirement: P 2 > -0.16 η + 40. The degree of acetylation of each cellulose acetate was determined by the method stipulated by ASTM D-871 (1970).

Claims: A process for the preparation of a fatty acid ester of cellulose, comprising the steps of (a) optionally pretreating a starting pulp, (b) esterifying the pulp in order to prepare a cellulose triester, characterized in that the process further comprises the steps of (c) optionally aging or hydrolyzing the cellulose triester of step (b) in order to obtain a cellulose diester, (d) post-treatment, comprising precipitating and recovering the cellulose triester of step (b) or the cellulose diester of step (c), washing the recovered precipitate, and purifying and drying it, wherein a pulp having an α-cellulose content of 85 to 93% by weight is used as the raw material, a fatty acid corresponding to the ester is used as a diluent, and an organic solvent is added in an amount of at least 10% by weight based on the amount of the diluent at least in step (b). The process of claim 1, wherein the organic solvent is a halogenous solvent and the amount thereof is 10 to 90% by weight based on the amount of the diluent. The process of claim 1, wherein the organic solvent is a solvent of a nitro compound and the amount thereof is 10 to 90% by weight based on the amount of the diluent. The process of claim 1, wherein the fatty acid ester of cellulose to be prepared is cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate propionate or cellulose acetate butyrate. The process of claim 1, wherein the fatty acid ester of cellulose to be prepared is cellulose acetate.

Inventors: ASAI TANEMI, HABU NAOTO, IWATA RYOUTA, KAINO YOSHIAKI, MATSUMURA HIROYUKI, SAKA SHIRO, SHIMAMOTO SHU, ASAI, TANEMI, HABU, NAOTO, IWATA, RYOUTA, KAINO, YOSHIAKI, MATSUMURA, HIROYUKI, SAKA, SHIRO, SHIMAMOTO, SHU
Applicants: DAICEL CHEM, DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1993
Main classification: C08B3
IPC codes: C08B3