CELLULOSE ACETATE HAVING HIGH MOLDABILITY AND PROCESS FOR PREPARING THE SAME

Abstract: Cellulose acetate of high moldability with low solution viscosity in spite of high average degree of polymerization is obtained. Low molecular weight components of cellulose acetate (e.g., CTA having average degree of acetylation of 59.0 to 62.5 %) are eluted with a washing solvent to produce cellulose acetate having molecular weight distribution Mw/Mn of 1 to 1.7. As washing solvent, those swell or partially dissolve cellulose acetate, for example, those dissolve 0.1 to 30 % by weight of cellulose acetate can be used. Such solvent includes, for example, a solvent having solubility parameter δ of 7 to 12.5 (ketones, ethers, organic acid, esters, etc.).

Description: The present invention relates to cellulose acetate (particularly cellulose triacetate) highly moldable, for example, to films, and to a process for the production thereof. Generally, cellulose acetate is a semi-synthetic polymer obtained from cellulose as a starting material, by esterification with acetic anhydride. Currently commercially available cellulose acetate can be roughly divided into two groups, mainly according to the degree of acetylation. One is cellulose triacetate (hereinafter referred to as CTA) with a degree of acetylation not less than 59%, and the other is cellulose diacetate, which ranges widely and that with a degree of acetylation of about 50 to 59% is referred to as cellulose diacetate (CDA). In other words, it is cellulose acetate soluble in acetone. Cellulose acetate, especially CTA, which has excellent physical properties, particularly good processability and high optical properties, has been utilized through many years in the field of plastics, fibers, films (for example, photographic films). Further, cellulose acetate has attracted attention from the viewpoint of global environment, because it possesses, for example, biodegradability. The molded product of cellulose acetate such as CTA can be prepared generally by fluidization of a solution of cellulose acetate dissolved in a solvent to a desired form, followed by removal of the solvent, for example, by evaporation (see, for example, JP-B 45-9074, JP-B 49-4554, JP-B 49-5614). On the other hand, as the uses of cellulose acetate increase, speeding up of the processing technique is required, and high speed molding, high speed spinning, high speed processing of the molded product have been tried. For example, in a process for producing a film, it is proposed to cast a solution of cellulose acetate at a high speed to mold into a film. In order to improve the moldability corresponding to such speeding up, it is proposed to reduce the viscosity of a concentrated solution of cellulose acetate. In order to reduce the viscosity of such a concentrated solution, in general, the average degree of polymerization of cellulose acetate has been reduced. However, when cellulose acetate with a low average degree of polymerization is used, the mechanical strength of the molded product will be impaired ("Effect of the degree of polymerization of CTA on the physico-mechanical properties of thread", Krim Volokna, 1985, No.3, p46-47, etc.). Particularly, it is difficult to reduce the solution viscosity for cellulose acetate with a high degree of acetylation, especially CTA, while retaining a high degree of polymerization. Further, a molded product of cellulose acetate is generally rigid and brittle, and such properties become more remarkable as the acetylation degree increases. Physical properties of a certain polymeric material greatly depend on its crystallizability. That is, those having high crystallizability are imparted with strength while flexibility, for example, elongation, is impaired, resulting in brittle product. Of course, CTA is not an exception, and has high crystallizability due to its homogeneity of the structure. That is, for cellulose acetate, the higher the degree of acetylation becomes, the higher the crystallizability will be. Moreover, crystals are generally formed, lower molecular weight materials being nuclei. Accordingly, when CTA or CDA is used, the molded product is generally imparted with flexibility by adding a plasticizer. For example, phthalate plasticizers, such as diethyl phthalate, may be often used for acetate plastics, e.g. used for a grip of a screwdriver. In addition, cellulose acetate, particularly CTA, has utility as a raw material for various films due to its excellent transparency. However, it has defects, for example, the film is rigid and brittle. To overcome such physical defects, a plasticizer is also used in this case. Adding ingredients, such as a plasticizer, is accompanied not only with decreased yield of the final product due to bleedout during molding, but also with economical disadvantages. Thus, the development of materials for producing films having excellent physical properties while maintaining characteristics of CTA by addition of a small amount of a plasticizer has been expected. "Chemical Abstracts, Vol. 99, No. 24, 12.12.1983, Columbus, Ohio, US; Abstract No. 196 801" discloses two samples of cellulose triacetate having a molecular weight distribution M w /M n of 1.30 to 1.6. US-A-2 024 666 discloses a process of fractionally extracting a cellulose acetate which comprises subjecting it to extraction by a liquid selected from the group consisting of acetone-water and acetic add-water. This document further discloses a process of removing degraded products by extraction from an acetone-soluble organic acid ester of cellulose in which the acetyl value of the extracted portion has substantially the same acetyl value as that of the bulk of the ester, which comprises subjecting such ester to extraction by a liquid which is a solvent for the degraded portion of the ester but a nonsolvent for the bulk of the ester. FR-A-1 049 975 discloses treating in a Soxhlet apparatus technical cellulose acetate with ethyl acetate or other organic solvents which do not dissolve the acetate at ordinary temperature but which by extraction at higher temperature eliminate the fraction of the acetate having a low molecular weight. Accordingly, the object of the present invention is to provide cellulose acetate with low solution viscosity and excellent moldability, in spite of its high molecular weight and degree of polymerization and with high solubility in a solvent and high moldability, in spite of its high average molecular weight and high average degree of substitution, which is useful in producing molded products with high moisture resistance and dimensional accuracy by a molding process at high processing speed using a solution of cellulose acetate with low solution viscosity. Another object of the present invention is to provide a process for producing cellulose acetate having excellent properties as described above. The present inventors have studied intensively to attain the above objects. As the results, we have found that: (1) treatment of cellulose acetate, such as CTA, with a solvent to elute components with low molecular weight results in narrow range of Mw/Mn which is an index of the distribution of the molecular weight of cellulose acetate; (2) removal of low molecular weight components results in a significantly reduced viscosity of the concentrated solution within a certain range of Mw/Mn in spite of a high molecular weight, thereby improving the moldability; (3) reduction of crystallizability of the material, that is, removal of low molecular weight materials to be seeds in the formation of crystals, results in improved physical properties, particularly film strength of the molded product and improved flexibility, and the present invention has been completed on the basis of such findings. Thus, the present invention provides a cellulose acetate with high moldability having a molecular weight distribution Mw/Mn of 1 to 1.7 according to. gel permeation chromatography, an acetone extract of not more than 5 % and a viscosity average degree of polymerization (DP) of not less than 290, the viscosity of the concentrated solution (η) for the viscosity average degree of polymerization (DP) according to the falling ball test being expressed by the following formula (1): 2.814 x In(DP)-11.753 ≤ In (η) ≤7.28 x In(DP)-37.059 Such cellulose acetate includes, for example, cellulose acetate having average degree of acetylation of 52 to 62.5%, and a molecular weight distribution Mw/Mn of about 1.2 to 1.7, particularly, CTA. Such cellulose acetate can be obtained by washing cellulose acetate with a solvent eluting low molecular weight components from the cellulose acetate. Washing solvents include solvents that swell or partially dissolve cellulose acetate, for example, those which dissolve 0.1 to 30 % by weight of cellulose acetate when cellulose acetate is dispersed at the temperature of 25°C at solid concentration of 5 % by weight. Examples of the washing solvents include, for example, ketones, ethers, organic acids and esters. Most of such solvents have a solubility parameter δ of about 7 to 12.5. The present invention also encompasses the following aspects. The present invention includes cellulose acetate wherein average degree of acetylation is not less than 59%, the amount of acetone extracts is not more than 5%, and the concentrated solution viscosity (η) for the viscosity average degree of polymerization (DP) according to falling ball viscosity test is expressed by the following formula (2): 2.814 × ln(DP)-11.753≦ In(η) ≦ 6.29 × In (DP)-31.469 (wherein, DP is an positive integer of not less than 290). Further, the present invention includes a process for producing cellulose acetate which is characterized by having an average degree of acetylation of not less than 59%, acetone extracts of not more than 5% and a concentrated solution viscosity (η) for the viscosity average degree of polymerization (DP) according to falling ball viscosity test expressed by the following formula (2): 2.814 × ln(DP)-11.753 ≦ ln(η) ≦ 6.29 × ln(DP)-31.469 (wherein, DP is a positive integer of not less than 290), which comprises washing cellulose acetate obtained according to an ordinary method with one or more selected from ketones, acetate esters and cellosolves. Preferred cellulose acetate contains not less than 70 % of those having a particle size of 20 mesh path. Especially, the present invention provides a process for producing the cellulose acetate according to the present invention, which comprises washing the cellulose acetate with a solvent eluting low molecular weight components from the cellulose acetate, wherein the washing solvent is at least one selected from organic adds and ethers; or which comprises washing the cellulose acetate with a solvent eluting low molecular weight components from the cellulose acetate, wherein the washing solvent dissolves 0.1 to 30 % by weight of cellulose acetate when the cellulose acetate is dispersed at 25°C at a solid concentration of 5% by weight; or which comprises washing cellulose triacetate having an average degree of acetylation of 59.0 to 62.5 % with a solvent selected from ketones, ethers, organic acids and esters and having a solubility parameter δ of 8.5 to 11.5. Preferred embodiments of said process are set forth in the subclaims. The present invention will be illustrated in detail. Cellulose acetate is preferably an acetate ester of cellulose (cellulose acetate), however, it includes mixed acid esters with other organic acids [for example, esters with aliphatic organic acids having 3 or 4 carbon atoms (e.g., cellulose acetate propionate, cellulose acetate butyrate), cellulose acetate phthalate]; mixed acid esters with inorganic acids (e.g., cellulose acetate nitrate), as far as they contain the acetate ester as main ingredient. The cellulose acetate of the present invention is characterized by a narrow molecular weight distribution Mw/Mn (Mw: weight-average molecular weight; Mn: number-average molecular weight) according to gel permeation chromatography. That is, Mw/Mn of the cellulose acetate of the present invention is 1 to 1.7 (e.g., 1.2 to 1.7), preferably, 1.3 to 1.65, more preferably, 1.4 to 1.65, often 1.3 to 1.6. When the molecular weight distribution Mw/Mn exceeds the above maximum value, the viscosity of the cellulose acetate solution increases and the moldability e.g. by casting (particularly, the moldability at high speed) is impaired. When the lower limit of Mw/Mn becomes closer to 1.0, the solution viscosity decreases but the strength of the molded product is impaired. Accordingly, Mw/Mn is preferably more than 1. The weight-average molecular weight of cellulose acetate is not particularly limited and selected according to the application. For example, it is 1 x 10 4 to 100 x 10 4 , preferably, 5 × 10 4 to 75 × 10 4 , more preferably, 10 × 10 4 to 50 × 10 4 . Further, the cellulose acetate of the present invention has a low concentrated solution viscosity in spite of high molecular weight and degree of polymerization relative to the ordinary cellulose acetates because low molecular weight components are removed. The reason is not clear, but it is considered that the higher molecular materials control the apparent viscosity in high molecular weight solution. In the present invention, it is assumed that the amount of the high molecular weight materials does not change while the average molecular weight becomes higher. Accordingly, the average degree of polymerization of cellulose acetate can be selected within the range so as not to impair e.g. the mechanical properties of the molded products. The viscosity-average degree of polymerization (DP) of cellulose acetate (especially, CTA) is not less than 290 (e.g., 290 to 400), preferably, 250 to 350 (e.g. 300 to 350). For cellulose acetate with a viscosity-average degree of polymerization of less than 100, the mechanical properties of the molded product are likely to be impaired. The degree of acetylation of the cellulose acetate (percentage of the bonded acetate) can be selected from the range from 52 to 62.5%. The preferred degree of acetylation of cellulose acetate is not less than 59% (e.g., 59.0 to 62.5%), particularly 59 to 62% (e.g., 60 to 61.5%). When the degree of acetylation is low, the hygroscopicity is likely to increase and the dimensional stability is likely to be impaired. Accordingly, particularly preferred cellulose acetate includes those with a high degree of acetylation, for example, CDA and CTA, particularly, CTA. Among such cellulose acetates, CTA having the following properties has high moldability, because it has a high solubility in a solvent and the solution viscosity can be reduced in spite of high hygroscopicity and dimensional stability as well as high degree of acetylation. Distribution of molecular weight (Mw/Mn): 1.3 to 1.65, particularly 1.4 to 1.65 Weight-average molecular weight (Mw, x 10 4 ): 5 to 100, particularly 10 to 50 Degree of acetylation: 59.0 to 62.5%, particularly 59 to 62% (e.g., 60 to 62%). As mentioned above, since the cellulose acetate of the present invention has a high solubility in a solvent even with a high degree of acetylation, the cellulose acetate content in the solution and viscosity of cellulose acetate solution can be selected according to the application. The solution viscosity of cellulose acetate may be an index of high speed moldability, particularly in a casting method and a spinning method. That is, since cellulose acetate with a low solution viscosity enables cast coating and spinning at high speed, and smoothens the surface in a short time (that is, high leveling), even high speed molding results in high moldability and improved production of a molded product. The solution viscosity of cellulose acetate can be selected from the range so as not impair the moldability at high speed, for example, the viscosity of a 15% solution containing 13 % by weight of cellulose acetate and 2 % by weight of triphenyl phosphate is 20 to 70 seconds, preferably 30 to 65 seconds according to the falling ball viscosity method (1) described below. Falling ball viscosity method (1) Cellulose acetate, such as CTA, (42.7 parts by weight) with triphenyl phosphate (6.8 parts by weight, 16 % by weight based on cellulose acetate) is dissolved in a mixed solvent [n-butanol/methanol/dichloromethane = 3/15/82 (weight ratio)] (280.5 parts by weight) to prepare a solution of cellulose acetate with 15 % by weight of solid content including triphenyl phosphate. This solution is injected in a glass tube (diameter, 2.53 cm; length, 31.9 cm; distance between two gage marks, 9.93 cm). Then a steel ball (diameter, 3.20 mm; made of stainless steel; specific gravity, 7.87 g/cm 3 (23°C)) is allowed to fall, and the time (sec.) required for the ball to fall for a distance of 10 cm between two gage marks was determined as viscosity. Further, the cellulose acetate of the present invention is characterized by containing acetone extract of not more than 5 %, having a viscosity-average degree of polymerization (DP) of not less than 290, and a concentrated solution viscosity (η) for the viscosity-average degree of polymerization (DP) according to falling ball viscosity method (1) expressed by the following formula (1): 2.814 × In(DP)-11.753 ≦ In(η) ≦ 7.28 × In(DP)-37.059 The above formula (1) in the present invention is obtained from the experiments performed by the present inventors. For cellulose acetate with a viscosity-average degree of polymerization of not less than 290, the viscosity of a concentrated solution increases exponentially as the degree of polymerization increases, while cellulose acetate of the present invention behaves in a different way. Thus, we calculated the formula (1) by plotting the viscosity-average degree of polymerization against the concentrated solution viscosity. It is particularly preferred to satisfy the following formula (2): 2.814 × in(DP)-11.753 ≦ In(η) ≦ 6.29 × In(DP)-31.469 The method for the determination of the viscosity of a concentrated solution (η) according to falling ball viscosity method (2) is as follows: Falling ball viscosity method (2) Cellulose acetate was dissolved in methylene chloride : methanol = 8:2 (weight ratio) to obtain a solution having a cellulose acetate concentration of 15 % by weight, and the solution was filled in a viscosity tube with an inner diameter of 2.6 cm. After the temperature was adjusted to 25°C, a steel ball (diameter, 3.15 mm; 0.135 g) was allowed to fall through the solution and the time (sec.) required for the ball to fall for a distance of 10 cm between two gage marks was determined as viscosity. In the present invention, CTA obtained by an ordinary method is washed with e.g. ketones, acetate esters or cellosolves to remove low molecular weight materials, to inhibit crystallization and to provide a material for a film with excellent physical properties. The characteristics of the CTA obtained by such washing is that after CTA obtained according to an ordinary method is washed once, the amount of the low molecular weight CTA existing in the washings is about 10 to 15 %, whilst cellulose acetate after washing is reduced to not more than 5 % upon rewashing and extraction. In other words, it is required to wash and extract so that the amount of the low molecular weight CTA after re-extraction should be not more than 5%. The improved properties of, for example, films are considered to be attributable to removal of the low molecular weight materials in the product by washing, thereby preventing formation of unnecessary crystals, resulting in increased non-crystal portions in the film to afford flexibility and additional transparency to the film. Cellulose acetate can be produced by an ordinary method, for example, the sulfuric acid catalyst method, the acetic acid method or the methylene chloride method. Cellulose acetate is generally obtained from cellulose acetate after activation treatment with e.g. acetic acid by preparing triacetate with acetic anhydride using a sulfuric acid catalyst and adjusting the degree of acetylation by saponification (hydrolysis). Mw/Mn of cellulose acetate obtained according to such method is generally 1.8 to 3.0. According to the present method, cellulose acetate is washed with a solvent to produce cellulose acetate with a narrow molecular weight distribution. As said solvents, those which do not completely dissolve but swell or partially dissolve cellulose acetate may be used. Solvents which swell or partially dissolve cellulose acetate may be those capable of dissolving and eluting low molecular weight components, and the percentage of the components soluble in a solvent of cellulose acetate is not particularly limited as long as the high molecular weight component can be separated. In order to remove low molecular weight component from the cellulose acetate and efficiently obtain a high molecular weight component, it is preferable to use a solvent which dissolves 0.1 to 30 % by weight, preferably 1 to 25 % by weight, more preferably 5 to 15 % by weight of cellulose acetate when the cellulose acetate is dispersed at ambient temperature (25°C) at a solid concentration of 5 % by weight. The solvents capable of eluting low molecular weight components are generally those which dissolve 1 to 20% by weight of cellulose acetate. When the amount of components soluble in a solvent of cellulose acetate is less than 0.1% by weight, low molecular weight components cannot be eluted by repeated washing operations, and when it is more than 30 % by weight, it is not economical and it is difficult to efficiently produce cellulose acetate in an industrial scale. Such washing solvent can be selected depending on the type of cellulose acetate. For the selection of such a washing solvent, solubility parameter δ can be referenced (for example, H. Burrell; Off. Dig., 29 , 1069 (1957)). Such solubility parameter δ may be obtained, as described, for example, in J. H. Hildebrand, R. L. Scott; "Solubility of Non-electrolytes" Chap. 20, Rein hold (1950), according to the following formula: δ = (E/V) 0.3 (wherein E represents the molar heat of evaporation (cal) and V represents the molecular volume (cc)). Washing solvents include, for example, ketones, such as acetone (10.0, hereinafter the data of solubility parameter δ is simply given in parentheses), methyl ethyl ketone (9.3), diethyl ketone (8.8), methyl isobutyl ketone (8.4), diisopropyl ketone (8.0), diisobutyl ketone (7.8); ethers, such as dibutyl ether (7.1), dioxane (9.9), tetrahydrofuran (10.2); organic acids, such as formic acid, acetic acid, propionic acid, butyric acid, lactic acid; esters, such as methyl acetate (9.6), ethyl acetate (9.1), isopropyl acetate (8.4), butyl acetate (8.5), amyl acetate (8.5), cellosolve acetate (8.7), methyl propionate, ethyl propionate, ethyl lactate; cellosolves, such as methyl cellosolve (9.9), ethyl cellosolve, isopropyl cellosolve, propyl cellosolve, butyl cellosolve (8.9), methyl cellosolve acetate, cellosolve acetate; carbitol, such as methyl carbitol, ethyl carbitol (9.6), propyl carbitol, butyl carbitol (8.9); halogenated hydrocarbons, such as chloroform (9.3), dichloromethane (10.2), dichloroethane (9.5), carbon tetrachloride; nitro compounds, such as nitromethane (12.7), nitroethane (11.1), nitropropane; aprotic polar solvents, such as acetonitrile (11.9), N,N-dimethylformamide (12.1), N,N-diethylformamide (10.6), dimethylacetamide (10.8), diethylacetamide (9.9), dimethylsulfoxide; and a solvent mixture thereof. In addition, in order to control the solubility of cellulose acetate, the above solvents can be used as a solvent mixture with other solvents, such as water; alcohols, such as methanol (14.5), ethanol (12.7), n-propanol (11.9), isopropanol, n-butanol (11.4), isobutanol, diacetone alcohol, cyclohexanol (11.4); aliphatic hydrocarbons, such as pentane (7.0), hexane (7.3), heptane (7.4), octane (7.2); alicyclic hydrocarbons, such as cyclohexane (8.2), methyl cyclohexane (7.8); aromatic hydrocarbons, such as benzene (9.2), toluene (8.9), xylene (8.8), ethyl benzene (8.8). Dichloromethane alone or a solvent mixture of dichloromethane and ethanol (9:1 by weight ratio) is a good solvent for cellulose acetate and highly soluble. For such good solvents, the solubility of cellulose acetate can be controlled by altering the percentage of the components in the solvent mixture or by addition of an other solvent. Among the above solvents, water may be used with hydrophilic solvents, particularly water-soluble solvents, such as acetone or acetic acid. Preferred washing solvents include those having solubility parameters of 7 to 12.5, preferably 8 to 12 (e.g., 8.5 to 11.5), more preferably 9 to 11 (e.g., 9 to 10.5). For an efficient elution of the low molecular weight components, at least one solvent selected from polar solvents other than halogenated hydrocarbons, for example, ketones, ethers, organic acids, esters, cellosolves, carbitols, may be often used. In particular, to enhance the elution efficiency of the low molecular weight components, for example, ketones, such as acetone; ethers, such as tetrahydrofuran; organic acids, such as acetic acid, esters, such as methyl acetate, are preferred independently of the above mentioned solubility parameter. Particularly preferred solvents include those having a solubility parameter of 8.5 to 11.5 (preferably 9 to 11) which are selected from ketones, ethers, organic acids and esters. When the cellulose acetate is CDA or CTA, particularly CTA, which has high degree of acetylation, washing solvents are preferably ketones (e.g., acetone), esters (e.g., methyl acetate), organic acids having about 2 to 4 carbon atoms (e.g., acetic acid), ethers (e.g., tetrahydrofuran). When a solvent mixture containing poor solvents (e.g., water and/or alcohols) is used as a washing solvent, the more the poor solvent exists, the more the elution efficiency of the low molecular weight components will be impaired. Accordingly, the ratio of the poor solvent to the low molecular weight components can be selected within the range wherein the low molecular weight components in cellulose acetate can be eluted, and is, for example, not more than 40% by weight (e.g., 5 to 35 % by weight), preferably not more than 30 % by weight (e.g., 10 to 30% by weight) of the total amount of.the washing solvent. For the washing treatment of cellulose acetate, the cellulose acetate may be in various forms, for example, powder, granules, fiberst or flakes. The washing treatment of cellulose acetate includes the ordinary methods, for example, a method wherein cellulose acetate is impregnated or dispersed in the above solvent, a method wherein cellulose acetate is wetted or immersed with the above solvent, then a solvent is optionally added and the solvent is separated by, for example, centrifugation. The washing treatment may be optionally carried out with warming or heating, for example, at the temperature from 30°C to the boiling point of the solvent (for example, 40 to 90°C) to enhance the elution efficiency of the low molecular weight components. For washing, 1 to 50 % by weight, preferably 3 to 30 % by weight, more preferably 5 to 20 % by weight (e.g., 5 to 15 % by weight) of cellulose acetate is generally washed out The amount of the above solvent used is not particularly limited, but selected from a wide range. For example, it is generally 50 to 5000 parts by weight, preferably 100 to 2500 parts by weight, based on 100 parts by weight of cellulose acetate. The cellulose acetate subjected to a washing treatment with a solvent is generally separated by filtration, centrifugation or the like, and dried. Using thus obtained cellulose acetate as a material for molding enhances the productivity of the molded product without decreasing the average molecular weight which has an effect on the strength of the molded product. Cellulose acetate of the present invention may be used in various forms depending on the types of the molding methods (for example, powder or pellets). However, it possesses high solubility in a solvent as well as low solution viscosity. Therefore, it is often used as a cellulose acetate solution (dope). A representative molding method using a cellulose acetate solution includes, for example, a process for the production of a film or sheet (e.g. a photographic film) by a casting method including a spinning method and a process for the production of fibers by spinning. Further, cellulose acetate of the present invention, which has low solution viscosity, can be utilized in other applications, for example, plastics, lacquer and electrical insulating materials. A cellulose acetate solution may be prepared using a good solvent depending on the types of the cellulose acetate, and such a good solvent is properly selected from the above solvents (for example, a halogenated hydrocarbon, such as dichloromethane). For molding, the cellulose acetate of the present invention can be used with an other cellulose ester (for example, an ester with an organic acid, such as cellulose propionate, cellulose butyrate; or an ester with an inorganic acid, such as cellulose nitrate, cellulose sulfate, cellulose phosphate). In addition to the above solvents, to cellulose acetate may be optionally added various additives, for example, ester type plasticizers (e.g., triacetin, triethylene glycol diacetate, triethylene glycol dipropionate, dibutyl phthalate, dimethoxyethyl phthalate, triethyl citrate ester), inorganic fine particles (e.g., kaolin, talc, diatomaceous earth, quartz, calcium carbonate, barium sulfate, titanium oxide, alumina), thermostabilizers (e.g., a salt of an alkaline earth metal, such as calcium, magnesium), coloring agents. Since the cellulose acetate of the present invention has a narrow molecular weight distribution Mw/Mn and low molecular weight components are removed, the solution viscosity can be reduced and the moldability can be improved without reducing the degree of polymerization. The cellulose acetate has a high solubility in a solvent and a high moldability in spite of a high average degree of polymerization and a high average degree of substitution. Accordingly, it is useful to use a cellulose acetate solution with a low solution viscosity to obtain molded products with high moisture resistance and dimensional accuracy by a molding process at high processing speed. According to the present method, cellulose acetate with excellent properties as described above can be produced with a simple operation like washing with a solvent. [Brief Description of the Drawings] Fig. 1 is a graph showing the relationship between the weight-average molecular weight and the falling ball viscosity of cellulose acetate obtained in Examples 1 to 4 and Comparative Examples 1 to 5. [Example] The present invention will be illustrated in detail referring to Examples and Comparative Examples. In the following Examples and Comparative Examples, the viscosity of the concentrated solution was determined according to the "falling ball viscosity method" described above. Molecular weight, molecular weight distribution, degree of acetylation, viscosity-average degree of polymerization, physical properties of a film, and the acetone extract were determined as follows: (1) Molecular weight and Molecular weight distribution Measurement was carried out using the high speed liquid chromatography system "GPC-LALLS" having a gel filtration column connected to a detector for the refractive index, light scattering. The measuring conditions are as follows: Solvent: methylene chloride Column: GMHx1 (manufactured by Toso) Sample concentration: 0.1 %(w/v) Flow rate: 1 ml/min. Sample injection: 300 µl Standard sample: methyl polymethacrylate (Mw=188,200) Temperature: 23°C (2) Degree of acetylation(%) The degree of acetylation can be determined by the saponification method. That is, dried cellulose acetate, such as CTA, is precisely weighed and dissolved in a solvent mixture of acetone and dimethylsulfoxide (4:1 by volume ratio), to which was added a given amount of 1N aqueous sodium hydroxide, and saponification is carried out at 25°C for 2 hours. Phenolphthalein is added as an indicator and the excess of sodium hydroxide is titrated with 1N sulfuric acid (concentration factor: F). In the same method as described above, a blank test is carried out. The degree of acetylation is calculated according to the following formula: Degree of acetylation (%) = (6.005 x (B-A) × F)/W (wherein A is the volume (ml) of 1N sulfuric acid required for titration of the sample, B is the volume (ml) of 1N sulfuric acid required for titration of the blank test, F is the concentration factor of 1N sulfuric acid, W is weight of the sample). (3) Method for measuring and calculating the viscosity-average degree of polymerization (DP) Oven-dried cellulose acetate (about 0.2 g, precisely weighed) is dissolved in a solution of methylene chloride : ethanol = 9:1 (100 ml). The time (sec.) required for the solution to drop is measured by an Ostwald viscometer at 25°C. The degree of polymerization is obtained according to the following formula: η ret = T/T 0 [η] = (In η ret )/c DP = [η]/km T: the time (sec.) required for the sample solution to drop To: the time (sec.) required for the solvent alone to drop c: concentration (g/1) km: 6 x 10 -4 (4) Process for preparing a film A film used for the measurement of the mechanical strength was prepared by dissolving a given amount of cellulose acetate and a plasticizer in a solvent, which was filtered and cast on a glass sheet so that clearance and casting speed would be constant, followed by drying. (5) Physical properties of the film The physical properties of the film were measured as (i) tensile strength, (ii) bending strength, (iii) tearing strength, (iv) tensile elongation. Each evaluation method is shown below. (i) Measurement of the tensile strength A film cut into 10 cm length (initial length of the sample, 5 cm) was stretched at stretching speed of 20 mm/min according to IS01184-1983, the initial sample length being 5 cm, and the tensile strength was obtained from the load at the breaking point. (ii) Measurement of the bending strength According to ISO8776-1988, the reciprocating number of bending required for the film (cut into 12 cm length) to be broken was determined. (iii) Measurement of the tearing strength The load required for tearing was determined according to ISO6383/2-1983 using a film cut into 5 x 6.4 cm pieces. (iv) Measurement of the tensile elongation The tensile elongation was determined from the elongation of the film at the breaking point, by stretching the film cut into 10 cm length (initial length, 5 cm; 20 mm/min) according to ISO1184-1983. (6) Determination of the acetone extract Extraction of cellulose acetate (2.g) was carried out for 8 hours using a Soxhlet extractor and acetone as a solvent. The extract residue was oven-dried, and weighed to calculate the acetone extract. Examples 1 to 4 Cellulose (100 parts by weight, with a moisture content of 5%) was activated by pretreatment with acetic acid (36 parts by weight) and then esterified using sulfuric acid (7.8 parts by weight), acetic anhydride (260 parts by weight) and acetic acid (400 parts by weight) at 36°C for 120 minutes. After neutralization with magnesium acetate, CTA. was obtained by saponification and aging at 63°C for 30 minutes. The obtained CTA was divided into four equal parts. Each ten times the weight of methyl acetate (Example 1), acetone (Example 2), tetrahydrofuran (Example 3), and 80% aqueous acetic acid (Example 4) were added to the CTA, respectively. The samples were stirred at room temperature for 120 minutes, filtered and dried to obtain purified CTA. The amount of CTA washed out was 25 % by weight for methyl acetate (Example 1), 15 % by weight for acetone (Example 2), 8 % by weight for tetrahydrofuran (Example 3) and 12 % by weight for aqueous acetic acid (Example 4), respectively. Comparative Examples 1 to 4 According to an ordinary method, altering esterification conditions and saponification conditions, cellulose acetates with four different molecular weights were prepared. That is, cellulose (100 parts by weight, with a moisture content of 5 %) was activated by pretreatment with acetic acid (36 parts by weight), then esterified using sulfuric acid (7.8 parts by weight), acetic anhydride (260 parts by weight) and acetic acid (400 parts by weight) at 36 to 40°C for 100 to 110 minutes, neutralized with magnesium acetate, then saponified and matured at 60 to 63°C for 20 to 40 minutes, to prepare four samples of cellulose acetate. Comparative Example 5 The CTA obtained in Example 1 was washed using a poor solvent for the low molecular weight components. That is, using 10 times the weight of an aqueous acetic acid solution (acetic acid/water = 5/5 (by weight ratio)) based on CTA obtained in Example 1, washing treatment was carried out in the same manner as described in Example 1, followed by filtration and drying, providing cellulose acetate. The degree of acetylation, molecular weight (Mn, Mw), molecular weight distribution and viscosity of the concentrated solution (falling ball viscosity method (1) of cellulose acetate obtained in the above Examples and Comparative Examples are given in Table 1, and the acetone extract, viscosity-average degree of polymerization (DP), relation between viscosity-average degree of polymerization and viscosity of the concentrated solution (falling ball viscosity method(2) and properties of the film are given in Table 2. The relation between the weight-average molecular weight and the viscosity (falling ball viscosity measurement (1)) is given in Fig. 1. Mnx10 4 Mwx10 4 Mw/Mn Acetylation Degree (%) Viscosity of Conc. solution (sec) Example 1 14.2 21.5 1.51 60.5 56.1 Example 2 13.0 20.8 1.60 60.9 55.2 Example 3 12.5 19.7 1.58 61.0 50.9 Example 4 13.6 20.6 1.52 60.9 62.3 Com.Ex. 1 9.9 19.5 2.13 61.1 58.2 Com.Ex. 2 9.1 20.0 2.02 60.8 77.3 Com.Ex. 3 8.9 19.9 2.24 61.0 75.5 Com.Ex. 4 10.5 21.2 2.36 60.9 99.7 Com.Ex. 5 8.7 18.7 2.16 60.9 45.5 As obvious from Tables 1 and 2 and Fig. 1, cellulose acetate obtained in the Examples had a lower solution viscosity compared with cellulose acetate of the Comparative Example with the same molecular weight. Example 5 Cellulose acetate obtained by an ordinary method (degree of acetylation, 60.9 %; viscosity-average degree of polymerization, 299) was stirred in acetone (10 times weight) at room temperature for 30 minutes, drained and dried to obtain cellulose acetate (components soluble in acetone, 0.4 %). The properties were as follows: average degree of acetylation, 60.9 %; viscosity-average degree of polymerization, 322; molecular weight distribution, 1.59. On the other hand, components extracted and removed in acetone existed in 12 % by weight based on the weight of the starting material, properties of which were as follows: degree of acetylation, 60.9 %, viscosity-average degree of polymerization, 196. Film properties of the resulting sample without low degree of polymerization material are given in Table 3. The relation between the viscosity-average degree of polymerization and the falling ball viscosity of a concentrated solution of this sample was expressed as follows: In(η)=4.66, i.e., 4.50 < In(η) < 4.85 <4.98, which satisfied the formulae (1) and (2). Comparative Example 6 Cellulose acetate synthesized in Example 1 before washing with acetone (acetone soluble matter, 12 %) was used. The relation between the viscosity-average degree of polymerization and the falling ball viscosity of a concentrated solution was as follows: In(η)=4.31, i.e., 4.29 < In(η) < 4.39 < 4.44, which satisfied the formulae (1) and (2). The molecular weight distribution was 2.20. The film properties are given in Table 3. Comparative Example 7 Cellulose acetate with an average degree of acetylation of 60.8 and a viscosity average degree of polymerization of 314 was obtained according to an ordinary method. This sample contained 13% of acetone soluble matter. The relation between the viscosity average degree of polymerization and the falling ball viscosity of a concentrated solution was as follows: In(η)=4.74, i.e., 4.43 < 4.69 < In(η)<4.80, which did not satisfy the formula (2) but (1). The molecular weight distribution was 2.07. The film properties are given in Table 3. Comparative Example 8 Cellulose acetate with an average degree of acetylation of 61.7 and a viscosity average degree of polymerization of 291 was obtained according to an ordinary method. This sample contained 12 % of acetone soluble matter. The relation between the viscosity average degree of polymerization and the falling ball viscosity of a concentrated solution was as follows: In(η)= 4.68, i.e., 4.21 <4.22<4.24< In(η), which did not satisfy the formulae (1) and (2). The molecular weight distribution was 2.11. The film properties are given in Table 3. Example 5 Com.Ex. 6 Com.Ex. 7 Com.Ex. 8 General Properties Average degree of acetylation (%) 60.9 60.9 60.8 61.7 Viscosity average degree of polymerization 322 299 314 291 Physical Properties (MD/TD) Tensile Strength (kg/mm 2 ) 11.8/11.8 12.0/11.5 12.2/12.2 10.9/11.3 Bending Strength(time) 130/160 120/130 140/110 110/110 Tearing Strength (gf) 25/24 22/23 22/23 16/17 Tensile Elongation (%) 51/47 43/45 47/40 41/42 * MD: Direction of casting of the film TD: Direction perpendicular to that of casting of the film

Claims: Cellulose acetate with high moldability having a molecular weight distribution Mw/Mn of 1 to 1.7 according to gel permeation chromatography, an acetone extract of not more than 5 % and a viscosity average degree of polymerisation (DP) of not less than 290, the viscosity of the concentrated solution (η) for the viscosity average degree of polymerisation (DP) according to the falling ball test being expressed by the following formula (1): 2.814 x In(DP)-11.753 ≤ In (η) ≤ 7.28 x In(DP)-37.059 The cellulose acetate according to claim 1, which has an average degree of acetylation of 52 to 62.5 % and a molecular weight distribution Mw/Mn of 1.2 to 1.7. The cellulose acetate according to claim 1, which has an average degree of acetylation of 59.0 to 62.5 % and a molecular weight distribution Mw/Mn of 1.4 to 1.65. The cellulose acetate according to claim 1, wherein the viscosity of the concentrated solution (η) for the viscosity average degree of polymerization (DP) according to the falling ball test is expressed by the following formula (2): 2.814 x ln(DP)-11.753 ≤ In(η) ≤ 6.29 x In(DP)-31.469 The cellulose acetate according to claim 1 or 4, wherein the viscosity degree of polymerisation (DP) is 290 to 400. A process for producing the cellulose acetate according to any one of claims 1 to 5, which comprises washing the cellulose acetate with a solvent eluting low molecular weight components from the cellulose acetate, wherein the washing solvent is at least one selected from organic acids and ethers. The process for producing cellulose acetate according to claim 6, wherein the washing solvent swells or partially dissolves cellulose acetate. A process for producing the cellulose acetate according to any one of Claim 1 to 5, which comprises washing the cellulose acetate with a solvent eluting low molecular weight components from the cellulose acetate, wherein the washing solvent dissolves 0.1 to 30 % by weight of cellulose acetate when the cellulose acetate is dispersed at 25°C at a solid concentration of 5 % by weight. The process for producing cellulose acetate according to Claims 8, wherein 1 to 50 % by weight of cellulose acetate is washed out by washing. The process for producing cellulose acetate according to claim 8 or 9, wherein the washing solvent is at least one selected from ketones, ethers, organic acids and esters. The process for producing cellulose acetate according to claim 10, wherein the washing solvent is acetone or methyl acetate. A process for producing the cellulose acetate according to any one of claims 1 to 5, which comprises washing cellulose triacetate having an average degree of acetylation of 59.0 to 62.5 % with a solvent selected from ketones, ethers, organic acids and esters and having a solubility parameters of 8.5 to 11.5. The process for producing cellulose acetate according to claim 12, wherein the solvent further contains water or an alcohol and can elute low molecular weight components from the cellulose acetate.

Inventors: KIYOSE ATSUNOBU, MURAKAMI KO, OKANO TOSHINORI, SHIMAMOTO SHU, TANIGUCHI HIROKI, KIYOSE, ATSUNOBU, MURAKAMI, KO, OKANO, TOSHINORI, SHIMAMOTO, SHU, TANIGUCHI, HIROKI
Applicants: DAICEL CHEM, DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1996
Main classification: C08B3
IPC codes: C08B3, C08L1