Method for producing cellulose acylate composition and cellulose acylate film Number:7,521,127 from the United States Patent and Trademark Office (PTO) owispatent A method for producing a cellulose acylate composition, which comprises filtering a solution in which cellulose acylate satisfying the following formulae 1 to 3 and having an M/S defined by following formula 4 of 0.5 to 2 is dissolved in a solvent through a filter having a retention particle size of 0.1 to 40 .mu.m, and mixing the filtered solution with a poor solvent to reprecipitate cellulose acylate: 1.5.ltoreq.A+B.ltoreq.3 Formula 1 0.ltoreq.A.ltoreq.1.8 Formula 2 1.2.ltoreq.B.ltoreq.3 Formula 3 where A is a substitution degree for an acetyl group of a hydrogen atom which constitutes a hydroxyl group of cellulose, and B is a substitution degree for an acyl group having 3 to 7 carbon atoms of a hydrogen atom which constitutes a hydroxyl group of cellulose, M/S={(a molar content of alkali metal)/2+(a molar content of alkali earth metal)/2}/(a molar content of sulfur). Formula 4<H composition, cellulose, Method, for, prod, 7521127.html, Patent, pto, 7521127

Title: Method for producing cellulose acylate composition and cellulose acylate film

Abstract: A method for producing a cellulose acylate composition, which comprises filtering a solution in which cellulose acylate satisfying the following formulae 1 to 3 and having an M/S defined by following formula 4 of 0.5 to 2 is dissolved in a solvent through a filter having a retention particle size of 0.1 to 40 .mu.m, and mixing the filtered solution with a poor solvent to reprecipitate cellulose acylate: 1.5.ltoreq.A+B.ltoreq.3 Formula 1 0.ltoreq.A.ltoreq.1.8 Formula 2 1.2.ltoreq.B.ltoreq.3 Formula 3 where A is a substitution degree for an acetyl group of a hydrogen atom which constitutes a hydroxyl group of cellulose, and B is a substitution degree for an acyl group having 3 to 7 carbon atoms of a hydrogen atom which constitutes a hydroxyl group of cellulose, M/S={(a molar content of alkali metal)/2+(a molar content of alkali earth metal)/2}/(a molar content of sulfur). Formula 4

Patent Number: 7,521,127 Issued on 04/21/2009 to Oya

Inventors: Oya; Toyohisa (Minami-ashigara, JP)
Assignee: Fujifilm Corporation (Minato-Ku, Tokyo, JP)

Appl. No.: 11/768,489

Filed: June 26, 2007

Foreign Application Priority Data


Jun 28, 2006 [JP]			2006-178686

Current U.S. Class: 428/532 ; 106/170.57; 428/1.2; 428/533; 428/536; 524/401

Current International Class: B32B 23/04 (20060101)

Field of Search: 428/532,533,536,1.2 106/170.57 524/401

References Cited [Referenced By]

U.S. Patent Documents


7226499	June 2007	Matsufuji et al.

Foreign Patent Documents


2000-352620	Dec., 2000	JP
2001-188128	Jul., 2001	JP
2003-213004	Jul., 2003	JP
2006-045500	Feb., 2006	JP

Primary Examiner: Kiliman; Leszek
Attorney, Agent or Firm: Buchanan Ingersoll & Rooney PC

Claims

What is claimed is:

1. A method for producing a cellulose acylate composition, which comprises filtering a solution in which cellulose acylate satisfying the following formulae 1 to 3 and having an M/S defined by following formula 4 of 0.5 to 2 is dissolved in a solvent through a filter having a retention particle size of 0.1 to 40 .mu.m, and mixing the filtered solution with a poor solvent to reprecipitate cellulose acylate: 1.5.ltoreq.A+B.ltoreq.3 Formula 1 0.ltoreq.A.ltoreq.1.8 Formula 2 1.2.ltoreq.B.ltoreq.3 Formula 3 where A is a substitution degree for an acetyl group of a hydrogen atom which constitutes a hydroxyl group of cellulose, and B is a substitution degree for an acyl group having 3 to 7 carbon atoms of a hydrogen atom which constitutes a hydroxyl group of cellulose, M/S={(a molar content of alkali metal)/2+(a molar content of alkali earth metal)/2}/(a molar content of sulfur). Formula 4

2. The method for producing the cellulose acylate composition according to claim 1, wherein the cellulose acylate used in the solution comprises 25 ppm or less of potassium and 25 ppm or less of sodium.

3. The method for producing the cellulose acylate composition according to claim 1, wherein the filter has a retention particle size of at least 2 to 20 .mu.m.

4. The method for producing the cellulose acylate composition according to claim 1, wherein a filter aid is used during the filtering.

5. The method for producing the cellulose acylate composition according to claim 1, wherein the reprecipitated cellulose acylate comprises 10 pieces or less of impurity particles having a particle size of 40 .mu.m or more per 100 g of the cellulose acylate.

6. The method for producing the cellulose acylate composition according to claim 1, wherein the reprecipitated cellulose acylate comprises 5 pieces or less of impurity particles having a particle size of 40 .mu.m or more per 100 g of the cellulose acylate.

7. The method for producing the cellulose acylate composition according to claim 1, wherein the cellulose acylate composition is in the form of a solution, a melt, a gel, a pellet, or a film.

8. The method for producing the cellulose acylate composition according to claim 1, wherein the cellulose acylate composition is in the form of a pellet or a film.

9. A cellulose acylate film produced using the method for producing the cellulose acylate composition according to claim 1.

10. The cellulose acylate film according to claim 9, wherein the amount of remaining organic solvent is 0.03% by mass or less.

11. The cellulose acylate film according to claim 9, which has an in-plane retardation (Re) satisfying the following formula i and a retardation in the thickness direction (Rth) satisfying the following formula ii: -500 nm.ltoreq.Re.ltoreq.500 nm Formula i -500 nm.ltoreq.Rth.ltoreq.500 nm. Formula ii

12. A cellulose acylate film, which is obtained by stretching the cellulose acylate film according to claim 9 in at least one direction by 0.1 to 500%.

13. A retardation film comprising the cellulose acylate film according to claim 9.

14. A polarizing plate comprising a polarizing layer and a protective film provided on at least one side of the polarizing layer, wherein the protective film is the cellulose acylate film according to claim 9.

15. An optical compensation film comprising an optically anisotropic layer formed by aligning a liquid crystalline compound on the cellulose acylate film according to claim 9.

16. An anti-reflection film comprising an anti-reflection layer on the cellulose acylate film according to claim 9.

17. An image display device comprising the cellulose acylate film according to claim 9.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cellulose acylate composition which contains a very small amount of fine impurity and is useful for an optical film, and a method for producing the same. Furthermore, the invention pertains to a high grade optical film, a retardation film, a polarizing plate, an optical compensation film, an anti-reflection film, and an image display device employing the above-mentioned cellulose acylate.

2. Description of the Related Art

Due to the transparency, toughness, and optical isotropy, cellulose acetate is increasingly finding its usefulness in a variety of applications, including the use in the support of photographic sensitive materials, as well as the use in optical films for image display devices including liquid crystal display devices and organic EL display devices. With regard to the optical film for liquid crystal display devices, methods of using cellulose acetate for polarizing plate protective films, or for retardation films for liquid crystal display devices of STN (Super Twisted Nematic) mode or the like by stretching the film to attain in-plane retardation (Re) and retardation in the thickness direction (Rth), are being implemented.

In recent years, there have been developed display devices of VA (Vertical Alignment) mode, OCB (Optical Compensated Bend) mode, or IPS (In-Plane Switching) mode, where higher values of retardation such as Re and Rth are required compared with the STN mode. Thus, an optical film material having property of manifesting various types of retardation according to the type of liquid crystal mode is on demand.

Stretchability of cellulose acetate is poor, and an area where retardation due to stretching and alignment of only polymer is realized is limited. Furthermore, since cellulose acetate is a relatively hydrophilic polymer, a change in retardation caused by humidity is significant.

In order to cope with such demand, a cellulose acylate film has been disclosed as a new material for optical film, which is produced by a solution casting method in which a solution of mixed esters of an acetyl group and a propionyl group of cellulose (mixed acylate of cellulose such as cellulose acetate propionate) is flow cast on a support, a portion of the solvent is evaporated, and then a cellulose acylate film is peeled off from the support (see JP-A-2001-188128). In addition, there is a process in which cellulose acetate butyrate and cellulose acetate propionate are used as mixed acylate of cellulose that has a melting temperature lower than that of cellulose acetate and then melt cast to form an optical film (see JP-A-2000-352620). The melt casting has the following advantages. Since an organic solvent is not used during the casting, a dissolution or dry process may be omitted unlike the solution casting, and a load to environment is low.

Mixed acylate of cellulose such as cellulose acetate propionate and cellulose acetate butyrate is an excellent material that increases retardation of cellulose acetate. Meanwhile, in the case of when a process where cellulose and acid anhydrides that are industrially available are reacted in the presence of an acid catalyst to produce cellulose acylate is used, reactivity is lower as compared to cellulose acetate, impurities including unreacted cellulose may easily remain. The impurities are observed as black point impurities or bright point impurities under a crossed Nicols condition. In the case of when cellulose acylate is used as an optical film, optical defects may be formed or light leakage may occur, thus it is required that the amount of impurities is set to be very small.

An activation process in which an acetic acid is added to raw material cellulose and a temperature is maintained at 40.degree. C. for 1 hour or more is disclosed as a process of reducing unreacted cellulose of mixed acylate of cellulose (see JP-A-2006-45500).

This process is useful to reduce the amount of unreacted substances while the degree of polymerization of cellulose acylate is maintained at a relatively high level. However, it is difficult to remove impurities other than unreacted cellulose, and when the required amount of unreacted substance is very small, it is necessary to perform a process of removing the impurities.

In the case of when cellulose acylate is used as a raw material of the optical film, if the casting process is the solution casting process, even though cellulose acylate containing a large amount of impurity is used, a cellulose acylate solution is prepared, filtered by using a filter having a small retention particle size, and cast to significantly reduce the amount of impurity of products (see JP-A-2003-213004).

Furthermore, a process of filtering melts of cellulose ester in the case of when the melt casting is performed is disclosed. In the process, desirable filtration precision is 5 .mu.m or less (see JP-A-2000-352620).

However, there is a problem in that replacement of a filtering material during the filtration of the melts is more difficult as compared to the filtration of the solution.

Therefore, in order to produce cellulose acylate useful to provide acceptable optical properties when the melt casting is performed, it is required that the amount of fine impurity contained in the raw material of cellulose acylate used to perform the melting is set to be very small to make the filtration of melts unnecessary. However, this is difficult to be achieved by using a known method, thus there is a need to provide a method of solving this problem.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for producing cellulose acylate composition which contains a very small amount of fine impurity and is useful for an optical film. It is another object of the invention to provide a high grade optical film, a retardation film, a polarizing plate, an optical compensation film, an anti-reflection film, and an image display device employing the above-mentioned cellulose acylate.

In order to obtain an acceptable optical film, it is required that the amount of impurities having a retention particle size of 40 .mu.m or more is set to 0.1 or less pieces of particles per 1 g of the cellulose acylate composition. However, it has been considered that it is industrially impossible to remove the impurity having a retention particle size of 40 .mu.m or more from the melts so that the amount of the impurity is in the above-mentioned range.

Accordingly, the present inventors have devotedly conducted researches, and as a result, the above-mentioned problems are solved by the following means.

The above-mentioned objects of the invention are concretely achieved by the following aspects of the invention. (1) A method for producing a cellulose acylate composition, which comprises filtering a solution in which cellulose acylate satisfying the following formulae 1 to 3 and having an M/S defined by following formula 4 of 0.5 to 2 is dissolved in a solvent through a filter having a retention particle size of 0.1 to 40 .mu.m, and mixing the filtered solution with a poor solvent to reprecipitate cellulose acylate: 1.5.ltoreq.A+B.ltoreq.3 Formula 1 0.ltoreq.A.ltoreq.1.8 Formula 2 1.2.ltoreq.B.ltoreq.3 Formula 3 where A is a substitution degree for an acetyl group of a hydrogen atom which constitutes a hydroxyl group of cellulose, and B is a substitution degree for an acyl group having 3 to 7 carbon atoms of a hydrogen atom which constitutes a hydroxyl group of cellulose, M/S={(a molar content of alkali metal)/2+(a molar content of alkali earth metal)/2}/(a molar content of sulfur). Formula 4 (2) The method for producing the cellulose acylate composition according to (1), wherein the cellulose acylate used in the solution comprises 25 ppm or less of potassium and 25 ppm or less of sodium. (3) The method for producing the cellulose acylate composition according to (1) or (2), wherein the filter has a retention particle size of at least 2 to 20 .mu.m. (4) The method for producing the cellulose acylate composition according to any one of (1) to (3), wherein a filter aid is used during the filtering. (5) The method for producing the cellulose acylate composition according to any one of (1) to (4), wherein the reprecipitated cellulose acylate comprises 10 pieces or less of impurity particles having a particle size of 40 .mu.m or more per 100 g of the cellulose acylate. (6) The method for producing the cellulose acylate composition according to any one of (1) to (4), wherein the reprecipitated cellulose acylate comprises 5 pieces or less of impurity particles having a particle size of 40 .mu.m or more per 100 g of the cellulose acylate. (7) The method for producing the cellulose acylate composition according to any one of (1) to (6), wherein the cellulose acylate composition is in the form of a solution, a melt, a gel, a pellet, or a film. (8) The method for producing the cellulose acylate composition according to any one of (1) to (6), wherein the cellulose acylate composition is in the form of a pellet or a film. (9) A cellulose acylate film produced by the method for producing the cellulose acylate composition according to any one of (1) to (8). (10) The cellulose acylate film according to (9), wherein the amount of remaining organic solvent is 0.03% by mass or less. (11) The cellulose acylate film according to (9) or (10), wherein an in-plane retardation (Re) satisfies formula i and a retardation in the thickness direction (Rth) satisfies formula ii: -500 nm.ltoreq.Re.ltoreq.500 nm Formula i -500 nm.ltoreq.Rth.ltoreq.500 nm Formula ii (12) The cellulose acylate film according to any one of (9) to (11), which is obtained by stretching the cellulose acylate film according to claim 9 in at least one direction by 0.1 to 500%. (13) A retardation film comprising the cellulose acylate film according to any one of (9) to (12). (14) A polarizing plate comprising a polarizing layer and a protective film provided on at least one side of the polarizing layer, wherein the protective film is the cellulose acylate film according to any one of (9) to (12) or the retardation film according to (13). (15) An optical compensation film comprising an optically anisotropic layer formed by aligning a liquid crystalline compound on the cellulose acylate film according to any one of (9) to (12) or the retardation film according to (13). (16) An anti-reflection film comprising an anti-reflection layer on the cellulose acylate film according to any one of (9) to (12) or the retardation film according to (13). (17) An image display device comprising one or more selected from the group consisting of the cellulose acylate film according to any one of (9) to (12), the retardation film according to (13), the polarizing plate according to (14), the optical compensation film according to (15), and the anti-reflection film according to (16).

A cellulose acylate composition according to a production method of the invention contains a very small amount of fine impurity and is useful for an optical film, particularly, an optical film using a melt casting process. Furthermore, a high grade optical film, a retardation film, a polarizing plate, an optical compensation film, an anti-reflection film, and an image display device employing the above-mentioned cellulose acylate are of good quality and have excellent optical properties.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a cellulose acylate composition and a method for producing the same according to an aspect of the invention will be described in detail. The explanation of the constitutional requirements described below is based on the representative embodiments of the invention, but the invention is not intended to be limited by those embodiments. In addition, the numerical value range expressed with symbol ". . . to . . . " in the present specification implies that the range includes the values described before and after the symbol ". . . to . . . " as the lower limit value and the upper limit value.

<Cellulose Acylate>

A detailed description will be given of cellulose acylate which is preferably used in the method according to the aspect of the invention.

(Basic Structure)

The .beta.-1,4-bonded glucose unit constituting the cellulose has free hydroxyl groups at the 2-position, 3-position, and 6-position. Cellulose acylate is a polymerized product (polymer) having part or all of the hydroxyl groups that are chemically-modified. In the aspect of the invention, the term "degree of substitution" indicates the sum of the ratios at which the hydroxyl groups of cellulose are substituted in respect to the 2-position, 3-position, and 6-position (for example, esterification of 100% means that the substitution degree is 1). Furthermore, a natural cellulose raw material may contain a polymer (hemicellulose) of sugars (for example, xylose, mannose, or the like) other than glucose or components other than cellulose, for example, lignin, according to the type of its original living things or purification process. However, in the aspect of the invention, a polymer that is produced by using the cellulose raw material containing them is described generically as cellulose acylate.

The cellulose acylate according to the aspect of the invention features in satisfying the following formulae 1 to 3: 1.5.ltoreq.A+B.ltoreq.3 Formula 1 0.ltoreq.A.ltoreq.1.8 Formula 2 1.2.ltoreq.B.ltoreq.3 Formula 3

In formulae, A represents the substitution degree for an acetyl group of a hydrogen atom that constitutes a hydroxyl group of cellulose, and B represents a substitution degrees for an acyl group having 3 to 7 carbon atoms of a hydrogen atom that constitutes a hydroxyl group of cellulose.

(Substituent Designated by B)

In the aspect of the invention, the substituent that is designated by B has preferably 3 to 6 carbon atoms, more preferably 3 or 4 carbon atoms carbon atoms, and particularly preferably 3 or 4 carbon atoms. If the number of carbon atoms is 7 or more, processability is lowered. If cellulose acylate is used for films, a glass transition temperature of the polymer is sometimes not preferable.

Preferable examples of the substituent that is designated by B may include a propionyl group, a butyryl group, a pentanoyl group, a heptanoyl group, a hexanoyl group, an isobutyryl group, a pivaloyl group, and the like. More preferred are a propionyl group, a butyryl group, a hexanoyl group, and most preferred are a propionyl group and a butyryl group.

It is preferable that the cellulose acylate according to the aspect of the invention satisfy the following formulae 4 to 6: 2.0.ltoreq.A+B.ltoreq.3 Formula 4 0.05.ltoreq.A.ltoreq.1.8 Formula 5 1.2.ltoreq.B.ltoreq.2.95 Formula 6

It is more preferable that the cellulose acylate according to the aspect of the invention satisfy the following Formulae 7 to 9: 2.5.ltoreq.A+B.ltoreq.2.99 Formula 7 0.1.ltoreq.A.ltoreq.1.7 Formula 8 1.2.ltoreq.B.ltoreq.2.9 Formula 9

It is most preferable that the cellulose acylate according to the aspect of the invention satisfy the following Formulae 10 to 12: 2.6.ltoreq.A+B.ltoreq.2.98 Formula 11 0.1.ltoreq.A.ltoreq.1.55 Formula 12 1.3.ltoreq.B.ltoreq.2.85 Formula 13

In the case of when A+B is less than 1.5, hydrophilicity of cellulose acylate excessively increases, resulting in increased humidity dependency of the cellulose acylate composition, and thus it is not desirable. In the case of when A+B is 1.5 or more, satisfactory character is obtained depending upon its use. In the case of when the cellulose acylate composition according to the aspect of the invention is an optical film, A+B is preferably 2.0 or more, more preferably 2.5 or more, and particularly preferably 2.6 or more.

A may have any value in the range of 0 to 1.8. In the case of when the cellulose acylate composition according to the aspect of the invention is a film, A is preferably 0.05 or more, more preferably 0.1 or more in order to obtain desirable optical properties. As to the upper limit thereof, A is preferably 1.8 or less and more preferably 1.7 or less, and particularly preferably 1.55 or less in views of cost, the shape of the film surface, and the like.

B may have any value in the range of 1.2 to 3. In the case of when the cellulose acylate composition according to the aspect of the invention is a film, B is more preferably 1.25 or more, and particularly preferably 1.3 or more in order to obtain desirable optical properties, mechanical properties, and castability. As to the upper limit thereof, B is preferably 2.9 or less, more preferably 2.85 or less in views of optical properties and processability.

Preferable examples of cellulose acylate that is used in the aspect of the invention may include cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate heptanoate, cellulose acetate hexanoate, and cellulose acetate pentanoate. More preferable are cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate heptanoate, and cellulose acetate hexanoate. Most preferable are cellulose acetate propionate and cellulose acetate butyrate.

<Method for Producing Cellulose Acylate>

(Raw Material and Pretreatment)

For the cellulose raw material, materials derived from hardwood pulp, softwood pulp, and cotton linter are preferably used. For the cellulose raw material, materials of high purity having a .alpha.-cellulose content of 92 to 99.9% by mass are preferably used.

When the cellulose raw material is in a sheet form or in a lump form, the material is preferably pulverized in advance, and the cellulose is preferably continued to be pulverized until a fluffy state, a feather state, or a power state is attained.

(Activation Process)

In the aspect of the invention, the cellulose raw material is preferably subjected to a pretreatment (activation) of coming into contact with an activating agent, prior to etherification. In the case of when etherification is performed, water or a sodium hydroxide aqueous solution is preferably used as an activating agent. In the case of when esterification is performed, a carboxylic acid is preferably used. The method of addition can be selected from methods of spraying, dropping, immersion, and the like, and the activation may be performed at a predetermined temperature for a predetermined time. Details of the activation treatment are disclosed in JP-A-2006-45500.

(Acylation Process)

In order to produce cellulose acylate according to the aspect of the invention, cellulose is preferably acylated in the presence of catalyst. Specifically, it is preferable to acylate the hydroxyl group of cellulose by adding a carboxylic acid anhydride to cellulose and reacting them in the presence of a Bronsted acid or Lewis acid as the catalyst. For the catalyst, a sulfuric acid can be favorably used.

The synthesis of cellulose acylate having the large substitution degree at 6-position is disclosed in JP-A-1999-5851, JP-A-2002-212338, JP-A-2002-338601, or the like.

(Acid Anhydride)

The acid anhydride of carboxylic acid is preferably an acid anhydride of a carboxylic acid having 2 to 7 carbon atoms, and examples thereof may include anhydrous acetic acid, propionic acid anhydride, butyric acid anhydride, 2-methylpropionic acid anhydride, valeric acid anhydride, 3-methylbutyric acid anhydride, 2-methylbutyric acid anhydride, 2,2-dimethylpropionic acid anhydride (pivalic acid anhydride), hexanoic acid anhydride, 2-methylvaleric acid anhydride, 3-methylvaleric acid anhydride, 4-methylvaleric acid anhydride, 2,2-dimethylbutyric acid anhydride, 2,3-dimethylbutyric acid anhydride, 3,3-dimethylbutyric acid anhydride, cyclopentanecarboxylic acid anhydride, heptanoic acid anhydride, cyclohexanecarboxylic acid anhydride, benzoic acid anhydride, and the like. More preferred ones are anhydrous acetic acid, propionic acid anhydride, butyric acid anhydride, valeric acid anhydride, hexanoic acid anhydride, and heptanoic acid anhydride, and particularly preferred ones are anhydrous acetic acid, propionic acid anhydride, and butyric acid anhydride.

For the method of obtaining a mixed acylate of cellulose, it is preferable to use these acid anhydrides in combination. The mixing ratio is preferably determined in accordance with the rate of substitution of the target mixed ester. The acid anhydride is usually added in an equivalent excess with respect to the cellulose. That is, the acid anhydride is preferably added in an amount of 1.1 to 50 equivalents, more preferably 1.2 to 30 equivalents, and particularly preferably 1.3 to 10 equivalents, with respect to the hydroxyl group of the cellulose.

For a mixed acylation method, mention may be made of a method of mixing or sequentially adding two types of carboxylic acid anhydrides and allowing them to react, a method of using a mixed acid anhydride of two types of carboxylic acids (for example, mixed acid anhydride of an acetic acid and a butyric acid), and a method of using acid anhydrides of a carboxylic acid and a different carboxylic acid (for example, acetic acid anhydride and butyric acid anhydride) as the starting materials to synthesize a mixed acid anhydride (for example, mixed acid anhydride of an acetic acid and a butyric acid) in the reaction system and reacting the mixed acid anhydride with cellulose.

(Catalyst)

For the catalyst for acylation used for the production of the cellulose acylate according to the aspect of the invention, it is preferable to use a Bronsted acid or a Lewis acid. The definitions for the Bronsted acid and the Lewis acid are described in, for example, "Encyclopedia of Physics and Chemistry", Vol. 5 (2000). Preferred examples of the Bronsted acid may include a sulfuric acid, a perchloric acid, a phosphoric acid, a methanesulfonic acid, a benzenesulfonic acid, a p-toluenesulfonic acid, and the like. Preferred examples of the Lewis acid may include zinc chloride, tin chloride, antimony chloride, magnesium chloride, and the like.

For the catalyst, the sulfuric acid or the perchloric acid is more preferred, and the sulfuric acid is particularly preferred. It is preferable to use the sulfuric acid and other catalysts in combination. A preferred amount of addition for the catalyst is 0.1 to 30% by mass, more preferably 1 to 15% by mass, and particularly preferably 3 to 12% by mass, with respect to the cellulose.

(Solvent)

In carrying out the acylation, a solvent may be added for the purpose of adjusting the viscosity, rate of reaction, stirrability, ratio of acyl substitution, or the like. For the solvent, dichloromethane, chloroform, carboxylic acid, acetone, ethyl methyl ketone, toluene, dimethylsulfoxide, sulfolane, or the like may be used, but preferably used are carboxylic acids. Examples of such carboxylic acid may include a carboxylic acid having 2 to 7 carbon atoms (for example, an acetic acid, a propionic acid, a butyric acid, a 2-methylpropionic acid, a valeric acid, a 3-methylbutyric acid, a 2-methylbutyric acid, a 2,2-dimethylpropionic acid (pivalic acid), a hexanoic acid, a 2-methylvaleric acid, a 3-methylvaleric acid, a 4-methylvaleric acid, a 2,2-dimethylbutyric acid, a 2,3-dimethylbutyric acid, a 3,3-dimethylbutyric acid, a cyclopentanecarboxylic acid), and the like, and more preferably, the acetic acid, the propionic acid, the butyric acid, and the like. These solvents may be used in mixtures.

(Conditions for Acylation)

In carrying out the acylation, an acid anhydride, a catalyst, and optionally a solvent may be mixed first, and then cellulose may be mixed with these, or alternatively, the acid anhydride, catalyst and solvent may be separately and sequentially mixed with cellulose. However, it is usually preferable to prepare a mixture of an acid anhydride and a catalyst, or a mixture of an acid anhydride, a catalyst, and a solvent, as the acylating agent, and then to react the mixture with cellulose. In order to suppress temperature elevation in the reactor due to the heat of reaction upon acylation, the acylating agent is preferably cooled in advance. The cooling temperature is preferably -50 to 20.degree. C., more preferably -35 to 10.degree. C., and particularly preferably -25 to 5.degree. C. The acylating agent may be added in the liquid state, or in the solid state by freezing the agent into a crystal, flake, or block form.

In addition, the acylating agent may be added all at once or may be added in portions, to the cellulose. Alternatively, cellulose may be added all at once or may be added in portions, to the acylating agent. In the case of when the acylating agent is added in portions, an acylating agent of identical composition may be used, or a plurality of acylating agents of different compositions may be used. For preferred methods, mention may be made of 1) a method of first adding a mixture of an acid anhydride and a solvent and then adding a catalyst, 2) a method of first adding a mixture of an acid anhydride, a solvent, and a portion of a catalyst, and then adding a mixture of the other potion of the catalyst and a solvent, 3) a method of first adding a mixture of an acid anhydride and a solvent, and then a mixture of a catalyst and the solvent, 4) a method of first adding a solvent, and then a mixture of an acid anhydride and a catalyst, or a mixture of an acid anhydride, a catalyst and a solvent, and the like.

The acylation of cellulose is an exothermic reaction, but for the method for producing cellulose acylate according to the aspect of the invention, it is preferable if the maximum reached temperature during acylation is 50.degree. C. or lower. When the reaction temperature is 50.degree. C. or lower, it is preferable because a situation where depolymerization proceeds, making it difficult to obtain cellulose acylate having degrees of polymerization appropriate for the uses of the invention, or the like does not occur. The maximum reached temperature upon the acylation is preferably 45.degree. C. or lower, more preferably 40.degree. C. or lower, even more preferably 35.degree. C. or lower, and particularly preferably 30.degree. C. or lower. The reaction temperature may be controlled by using a temperature adjusting device, or may be controlled by means of the initial temperature of the acylating agent. The reaction temperature may be also controlled by the heat of vaporization of liquid components in the reaction system, generated by pressure reduction in the reactor. In addition, since the exotherm of the acylation is significant at the beginning of the reaction, the temperature may be controlled by cooling the reaction system during the initiation of the reaction and then heating the system, or the like. The termination point of the acylation may be determined by such means as the light permeability, solution viscosity, a change in temperature of the reaction system, solubility of the reactants to an organic solvent, observation with a polarized microscope, and the like.

The minimum temperature of the reaction is preferably -50.degree. C. or more, more preferably -30.degree. C. or more, and particularly preferably -20.degree. C. or more. Time for acylation is preferably 0.5 to 24 hours, more preferably 1 to 12 hours, and particularly preferably 1.5 to 6 hours. For 0.5 hours, the reaction does not proceed sufficiently under normal reaction conditions, while the reaction time exceeding 24 hours is not preferable in the aspect of industrial production.

(Acyl Composition)

The degree of acylation preferably satisfies the following Formula B: 0<(MA/MB).ltoreq.2.0 Formula B

In Formula B, MA represents the total molar amount of the acetyl group contained in the reaction mixture for the acylation process. Specifically, MA is the summed molar amount of the acetyl group contained in the acylating agent, the acetyl group contained in the carboxylic acid used for the pretreatment process, and the acetyl group contained in the produced cellulose acylate. MB represents the total molar amount of the acyl group having 3 to 7 carbon atoms contained in the reaction mixture for the acylation process. Specifically, MB is the summed molar amount of the acyl group having 3 to 7 carbon atoms contained in the acylating agent, the acyl group having 3 to 7 carbon atoms contained in the carboxylic acid used for the pretreatment process, and the acyl group having 3 to 7 carbon atoms contained in the produced cellulose acylate.

As such, the total molar amount of the acetyl group and the total molar amount of the acyl group having 3 to 7 carbon atoms are determined by the compositions and amounts of the activating agent, acylating agent (acid anhydride and carboxylic acid) and solvent (carboxylic acid) used in the pretreatment process. According to the aspect of the invention, the amount of the acyl group of acid anhydride is calculated in terms of the constituting carboxylic acid. That is, it is calculated such that 1 mole of an acid anhydride is equivalent to 2 moles of an acyl group. Likewise, the number of moles of the acyl group in the produced cellulose acylate is calculated in terms of the carboxylic acid generated when all of the ester bonds are hydrolyzed. Although the amounts of the acid anhydride and carboxylic acid in the reaction mixture are sequentially changed as the acylation of cellulose proceeds, by performing the calculation, the total number of moles of all the acyl groups contained in the acid anhydride, carboxylic acid, and produced cellulose acylate in the reaction mixture is constant throughout the process of acylation, as long as no new acid anhydride or carboxylic acid is added to the reaction system.

The acylation process according to the aspect of the invention refers to the period from the initiation of acylation of the hydroxyl groups of cellulose to the point when substantially most of the hydroxyl groups of the cellulose are acylated (for example, the substitution degree for an acyl group is 2.0 or greater, preferably 2.5 or greater, more preferably 2.8 or greater, and particularly preferably 2.9 or greater), and does not include the phase where the acylation is substantially almost completed, and further addition of acid anhydride or carboxylic acid to the reaction system has virtually no effect on the acyl composition of the product cellulose acylate.

According to the aspect of the invention, MA/MB is preferably such that 0<(MA/MB).ltoreq.2.0, more preferably 0.001.ltoreq.(MA/MB).ltoreq.1.5, even more preferably 0.01.ltoreq.(MA/MB).ltoreq.1.0, and particularly preferably 0.05.ltoreq.(MA/MB).ltoreq.0.7. When MA/MB exceeds 2, the substitution degree for an acetyl group of the cellulose acylate becomes excessively high, and there may be problems such as deterioration of stretchability, increase of the melting temperature for melt casting, resulting in difficult casting, and the like.

(Reaction Terminating Agent)

According to the method for producing cellulose acylate used in the aspect of the invention, it is desirable to add a reaction terminating agent after the acylation reaction.

For the reaction terminating agent, any substance capable of decomposing acid anhydrides may be used, and preferred examples thereof may include water, alcohol (for example, ethanol, methanol, propanol, isopropyl alcohol, etc.) or compositions containing these, and the like. Further, the reaction terminating agent may contain a neutralizing agent to be described later. Upon addition of the reaction terminating agent, a large exotherm surpassing the cooling capacity of the reactor apparatus is generated, possibly causing a decrease in the degree of polymerization of cellulose acylate, precipitation of cellulose acylate into an undesired form, or the like. Thus, in order to avoid such inconveniences, it is preferable to add a mixture of water and a carboxylic acid such as acetic acid, propionic acid, butyric acid, or the like, rather than to directly add water or alcohol, and the acetic acid is particularly preferable as the carboxylic acid. The composition ratio of the carboxylic acid and water used may be at any arbitrary ratio, but it is preferable to have the content of water in the range of 5 to 80% by mass, more preferably 10 to 60% by mass, and particularly preferably 15 to 50% by mass.

The reaction terminating agent may be added to the acylation reactor, or the reactants may be added to the vessel of the reaction terminating agent. The reaction terminating agent is preferably added over a time period of 3 minutes to 3 hours. When the time for addition of the reaction terminating agent is 3 minutes or more, there does not occur a situation where the exotherm becomes excessively large, causing a decrease in the degree of polymerization, insufficient hydrolysis of the acid anhydride, reduced stability of the cellulose acylate, or the like, and thus it is desirable. When the time for addition of the reaction terminating agent is less than or equal to 3 hours, there does not occur a problem such as deterioration of industrial productivity or the like, and it is desirable. The time for addition of the reaction terminating agent is preferably 4 minutes to 2 hours, more preferably 5 minutes to 1 hour, and particularly preferably 10 minutes to 45 minutes. Upon addition of the reaction terminating agent, the reactor may be cooled or not cooled, but for the purpose of inhibiting depolymerization, it is also desirable to inhibit temperature increase by cooling the reactor. It is also preferable to have the reaction terminating agent cooled in advance.

(Neutralizing Agent)

During the process of acylation reaction termination or after the process of acylation reaction termination, a neutralizing agent or its solution may be added for the purpose of hydrolysis of the excessive anhydrous carboxylic acid remaining in the system, neutralization of part or all of the carboxylic acid and esterification catalyst, adjustment of the amount of remaining sulfate radicals and the amount of remaining metal, and the like.

Preferred examples of the neutralizing agent may include carbonates, hydrogen carbonates, organic acid salts (for example, acetates, propionates, butyrates, benzoates, phthalates, hydrogen phthalates, citrates, tartrates, etc.), phosphates, hydroxides, or oxides of ammonium, organic quaternary ammonium (for example, tetramethylammonium, tetraethylammonium, tetrabutylammonium, diisopropyldiethylammonium, etc.), alkali metals (preferably lithium, sodium, potassium, rubidium, and cesium, even more preferably lithium, sodium, and potassium, and particularly preferably sodium and potassium), elements of Group 2 (preferably beryllium, calcium, magnesium, strontium, barium, beryllium, calcium, and magnesium, and particularly preferably calcium and magnesium), metals of Groups 3 to 12 (for example, iron, chromium, nickel, copper, lead, zinc, molybdenum, niobium, titanium, etc.), or elements of Groups 13 to 15 (for example, aluminum, tin, antimony, etc.), and the like. These neutralizing agents may be used in mixtures, or may form mixed salts (for example, magnesium acetate propionate or potassium sodium tartrate). In the case of when the anion of such neutralizing agent is divalent or higher, the agent may form hydrogen salts (for example, sodium hydrogen carbonate, potassium hydrogen carbonate, sodium dihydrogen phosphate, magnesium hydrogen phosphate, etc.).

More preferred examples of the neutralizing agent include carbonates, hydrogen carbonates, organic acid salts, hydroxides, or oxides of ammonium, alkali metals, elements of Group 2, or elements of Group 13, or the like, and particularly preferred examples include carbonate, hydrogen carbonate, acetate, or hydroxide of sodium, potassium, magnesium, or calcium.

Preferred examples of the solvent for the neutralizing agent may include water, alcohols (for example, ethanol, methanol, propanol, isopropyl alcohol, etc.), organic acids (for example, acetic acid, propionic acid, butyric acid, etc.), ketones (for example, acetone, ethyl methyl ketone, etc.), polar solvents such as dimethylsulfoxide, and solvent mixtures thereof.

(Partial Hydrolysis)

The cellulose acylate thusly purchased has a substitution degree for an acyl group of nearly 3, but for the purpose of obtaining cellulose acylate of desired degree of substitution, a process of partially hydrolyzing the ester bonds by maintaining the cellulose acylate at 20 to 90.degree. C. for a few minutes to a few days in the presence of a small amount of catalyst (generally an acylation catalyst such as remaining sulfate) and water, in order to reduce the substitution degree for an acyl group of the cellulose acylate to a desired degree (so-called aging) is usually carried out. The amount of the sulfuric acid ester bound to the cellulose can be reduced in the process of partial hydrolysis by also allowing hydrolysis of the sulfuric acid ester of cellulose, and by adjusting the conditions for hydrolysis.

(Termination of Partial Hydrolysis)

It is preferable to terminate the partial hydrolysis at the time point of obtaining the desired cellulose acylate by completely neutralizing the catalyst remaining in the system using a neutralizing agent or its solution as described above.

In the case of when a sulfuric acid is used as the catalyst, the amount of the neutralizing agent added to the reaction mixture is preferably an excessive equivalent amount with respect to the sulfate radicals (free sulfuric acid or cellulose-bound sulfuric acid). According to the aspect of the invention, the neutralizing agent may be added in portions, but it is desirable to add the neutralizing agent after completion of the partial hydrolysis (aging) so that the amount of the neutralizing agent is an excessive equivalent amount with respect to the sulfate radicals. The sulfuric acid bound to the cellulose (cellulose sulfate) is a monovalent acid, but the equivalent of the neutralizing agent is calculated in terms of free sulfuric acid. Thereby, the equivalent of the neutralizing agent may be determined from the amount of the sulfuric acid added. A preferred amount of the neutralizing agent to be added is preferably 1.2 to 50 equivalents, more preferably 1.3 to 20 equivalents, and particularly preferably 1.5 to 10 equivalents, with respect to sulfate radicals.

It is also desirable to effectively remove the catalyst (for example, sulfuric acid ester) in the solution or bound to the cellulose by adding a neutralizing agent which produces a salt of low solubility to the reaction solution (for example, magnesium carbonate, magnesium acetate, etc.).

(Post-Heating Process)

The reaction mixture after the termination of the partial hydrolysis is preferably further maintained at 30 to 100.degree. C. for at least one hour (post-heating process). By carrying out this process, a cellulose acylate having good thermal stability may be purchased by reducing the amount of the sulfuric acid bound to cellulose acylate. As to the reason for the reduction of the amount of the sulfuric acid bound to cellulose acylate in this process, although details have not been clarified, it is believed that heating a cellulose acylate solution in the presence of base in excess leads to gradual de-esterification of the sulfuric acid ester which is more likely to undergo hydrolysis than acyl ester, and free sulfuric acid that is neutralized by the base drives the equilibrium to be lopsided to the production system and thus promotes the reaction.

For the post-heating process, the maintenance temperature is preferably 30 to 100.degree. C., more preferably 40 to 100.degree. C., even more preferably 50 to 90.degree. C., and particularly preferably 60 to 80.degree. C. When the temperature is set to 30.degree. C. or more, the effect of reducing the amount of bound sulfuric acid is easily purchased, while when the temperature is set to 100.degree. C. or less, the process is improved in the aspect of operability or safety. Additionally, for the post-heating process, the time for maintenance is preferably 1 to 100 hours, more preferably 2 to 100 hours, and particularly preferably 2 to 50 hours. When the time is set to 1 hour or more, the amount of bound sulfuric acid is efficiently reduced, while when the time is set to 100 hours or less, industrial productivity is improved. For the post-heating process, the reaction mixture is preferably stirred. Furthermore, the neutralizing agent may be further added during the post-heating process.

(Re-Precipitation)

The desired cellulose acylate may be purchased by mixing the cellulose acylate solution thusly purchased into a poor solvent such as water and/or an aqueous carboxylic acid solution (for example, acetic acid, propionic acid, butyric acid, etc.), or by mixing a poor solvent into the cellulose acylate solution to re-precipitate the cellulose acylate, and washing and stabilizing the purchased cellulose acylate. The re-precipitation may be carried out continuously or in a batch mode with definite amounts. It is also preferable to control the form or molecular weight distribution of the re-precipitated cellulose acylate by adjusting the concentration of the cellulose acylate solution and the composition of the poor solvent by means of the mode of substitution or degree of polymerization of the cellulose acylate.

Furthermore, for the purpose of purifying cellulose acylate by using a production method other than the method according to the aspect of the invention, improving the purification effect of cellulose acylate by using the production method according to the aspect of the invention, adjusting the molecular weight distribution or apparent density, or the like, the operation of conducting re-precipitation may be carried out once or several times, as needed, by re-dissolving the once re-precipitated cellulose acylate in a good solvent (for example, acetic acid, acetone, etc.), performing the filtration, and subjecting the solution to a poor solvent (for example, water, carboxylic acid (for example, acetic acid, propionic acid, butyric acid), etc.). In connection with this, it is preferable that the solvent and the poor solvent be filtered in advance to remove fine impurities therefrom.

(Washing)

The produced cellulose acylate is preferably subjected to washing. The washing solvent may be any one that has low dissolvability for cellulose acylate and is capable of removing impurities, but usually washing water such as water or warm water is used. The temperature of the washing water is preferably 20 to 100.degree. C., more preferably 30 to 95.degree. C., and particularly preferably 40 to 95.degree. C. The temperature during the washing process may be constant or may vary within an arbitrary temperature range; however, the invention preferably comprises a step of washing the cellulose acylate at preferably 40 to 95.degree. C., more preferably 50 to 95.degree. C., and particularly preferably 60 to 90.degree. C., for preferably 1 to 100 hours, more preferably 2 to 50 hours, and particularly preferably 3 to 10 hours. The above-described washing process at 40 to 95.degree. C. and another washing process in another temperature range may be combined.

The washing treatment may be carried out in a so-called batch mode where alternation of filtration and washing liquid is repeated, or may be carried out using a continuous washing apparatus. It is preferable to reuse the waste water generated in the re-precipitation and washing processes as the poor solvent for the re-precipitation process, or to recover the solvent such as carboxylic acid by means of distillation or the like and reuse the solvent.

The course of washing may be traced by any means, but preferred examples may include methods involving hydrogen ion concentration, ion chromatography, electric conductivity, ICP, elemental analysis, atomic absorption spectrum, and the like.

Such treatment allows removal of the catalyst (a sulfuric acid, a perchloric acid, a trifluoroacetic acid, a p-toluenesulfonic acid, a methanesulfonic acid, zinc chloride, etc.) of cellulose acylate, the neutralizing agent (for example, carbonate, acetate, hydroxide, or oxide of calcium, magnesium, iron, aluminum, or zinc, etc.), the reaction product between the neutralizing agent and the catalyst, the carboxylic acid (acetic acid, propionic acid, butyric acid, etc.), the reaction product between the neutralizing agent and carboxylic acid, and the like, and thus is effective in enhancing the stability of the cellulose acylate.

(Stabilization)

It is also preferable to treat the cellulose acylate, after the washing by warm water treatment, with an aqueous solution of weak alkali (for example, carbonate, hydrogen carbonate, hydroxide, oxide, or the like of sodium, potassium, calcium, magnesium, aluminum, or the like), in order to further enhance the stability or to reduce the odor of the carboxylic acid. In this connection, it is preferable that the solution of the used stabilizer be filtered to remove contained fine impurities therefrom.

The amount of remaining impurities may be controlled by the metal content in the water used (the amount of metal ions contained in the water used as washing water or the like as trace components), the amount of the washing liquid, the washing temperature, time, agitation method, the form of the washing vessel, or the composition or concentration of the stabilizer. According to the aspect of the invention, the conditions for the acylation, partial hydrolysis, neutralization, and washing are preferably set such that the amount of remaining sulfate radicals (in terms of the content of sulfur atoms) is 50 to 500 ppm. The amount of remaining alkali metal and the amount of Group 2 element also may be adjusted by the conditions of partial hydrolysis, neutralization, and washing.

According to the aspect of the invention, preferably, a ratio (M/S) of a molar content (M) of alkali metals (potassium, sodium, etc.) and elements of alkali earth mrtals (magnesium, calcium, etc.) to a molar content (S) of sulfur that are contained in cellulose acylate is defined by formula 4 and is 0.5 to 2. Furthermore, it is preferable that the ratio is 0.7 to 1.5. M/S={(a molar content of alkali metal)/2+(a molar content of alkali earth metal)/2}/(a molar content of sulfur) Formula 4

It is preferable that a potassium content be 25 ppm or less and a sodium content be 25 ppm or less of the invention.

In the case of when the cellulose acylate satisfies the above-mentioned relation, thermal stability may be made good. The lower limits of contents of potassium and sodium are not limited.

(Drying)

In order to adjust the water content in the cellulose acylate to a preferred amount in the aspect of the invention, it is desirable to dry the cellulose acylate. The method of drying is not particularly limited as long as the desired water content can be purchased, but it is preferable to carry out the drying efficiently by using the means such as heating, blow drying, pressure reduction, and agitation individually or in combination. The drying temperature is preferably 0 to 200.degree. C., more preferably 40 to 180.degree. C., and particularly preferably 50 to 160.degree. C. The cellulose acylate according to the aspect of the invention has a water content of preferably 2 wt % or less, more preferably 1 wt % or less, and particularly preferably 0.7 wt % or less.

(Filtration)

Cellulose acylate that is produced by using the above-mentioned production method usually contains unreacted cellulose or impurities provided from raw materials or reaction devices, and in the case when cellulose acylate is applied to an optical film, the impurities may incur problems.

In order to avoid the above-mentioned problems, in the aspect of the invention, cellulose acylate is dissolved in a solvent to prepare a solution, and the solution is filtered by using a filter having a retention particle size of 0.1 to 40 .mu.m and mixed with a poor solvent to re-precipitate cellulose acylate.

(Solvent)

Any solvent may be used in respect to the cellulose acylate as long as solubility of the cellulose acylate is high. Preferable examples of the solvent may include a carboxylic acid (formic acid, an acetic acid, a propionic acid, a butyric acid, and the like), ketone (aceton, ethyl methyl ketone, ethyl isobutyl ketone, and the like), ester (methyl acetate, ethyl acetate, acetic acid butyl, acetic acid isopropyl, and the like), a halogen-based solvent (dichloromethane, chloroform, dichloroethane, and the like), and the like. The above-mentioned solvents may be used singly or in a mixture form containing two or more solvents.

More preferable examples of the solvent include a carboxylic acid (an acetic acid, a propionic acid, a butyric acid, and the like), ketone (aceton, ethyl methyl ketone, ethyl isobutyl ketone, and the like), and the like. Particular preferable examples of the solvent include a carboxylic acid (an acetic acid, a propionic acid, and the like) and ketone (acetone and the like).

Preferable examples of the poor solvent that is used during the re-precipitation may include water, alcohols (methanol, ethanol, isopropyl alcohol, butanol, and the like), hydrocarbon-based solvents (pentane, hexane, heptane, toluene, and the like). The above-mentioned poor solvents may be used singly or in a mixture form containing two or more solvents. A mixture of the poor solvent and another solvent may be used without deviation from the scope of the invention.

More preferable examples of the poor solvent include water and alcohols (methanol, ethanol, isopropyl alcohol, and the like), and particular preferable examples of the poor solvent include water and alcohols (methanol and the like).

(Specific Procedure of the Filtration Process)

The retention particle diameter of the filter that is used to perform the filtration is preferably 1 to 30 .mu.m, more preferably 1 to 20 .mu.m, and particularly preferably 2 to 20 .mu.m. The retention particle diameter of the filter is set to 0.1 .mu.m or more to prevent filtration pressure from being significantly increased and to easily perform industrial production thereof. Furthermore, the retention particle diameter of the filter may be set to 40 .mu.m or less to easily remove the impurities and to improve optical performance of the purchased film. In addition, the filtration may be repeated twice or more, and the filters having different retention particle sizes may be used in combination.

The filtration may be performed at any temperature as long as the filtration is capable of being performed. However, it is possible to reduce the viscosity of the solution when the filtration temperature is preferably 30 to 100.degree. C., more preferably 35 to 80.degree. C., and particularly preferably 40 to 70.degree. C.

Furthermore, in respect to the filtration pressure, the filtration is performed at preferably 0.001 to 10 MPa, more preferably 0.001 to 5 MPa, and particularly preferably 0.01 to 1 MPa.

According the aspect of the invention, a solution where the carboxylic acid is added to the cellulose acylate may be filtered by using a filter having a retention particle size of 0.1 to 40 .mu.m at any step between the an early step of the acylation process and the re-precipatation process in order to efficiently remove or reduce unreacted substances, insoluble salts, and other impurities from the cellulose acylate in cellulose during the production of the cellulose acylate. The retention particle size may be purchased according to the method that is disclosed in JIS P 3801.

Particularly, according to the aspect of the invention, if the filtration is performed immediately before the re-precipitation process, this is preferable in views of stability of quality of products and a reduction in filtration viscosity. In the case of when the filtration is performed through two stages, it is preferable that rough filtration be performed by using a filter having a retention particle size of 20 to 40 .mu.m and additional filtration be performed by using a filter having a retention particle size of 0.1 to 20 .mu.m. Preliminary filtration may be performed by using a filter having a retention particle size of 40 to 100 .mu.m before the filtration according to the aspect of the invention.

In the production method according to the aspect of the invention, the material of the filter is not limited as long as the material is not negatively affected by the solvent. Preferable examples of the material of the filter may include a cellulose-based filter, a metal filter, a sintered ceramic filter, a Teflon filter (PTFE filter), a polyethersulfone filter, a polypropylene filter, a polyethylene filter, a glass fiber filter, or a mixture thereof.

As to the type of the material of the filter, the filter having an electric charge capture ability may be preferably used. The filter having the electric charge capture ability means the filter that has an ability capable of capturing and removing the electrically charged impurities. Generally, in the filter, electric charges are provided to a filtration material. Examples of the filter may include filters that are disclosed in JP-T-1992-504379 and JP-A-2000-212226.

Preferably, a dead-end filtration process in which sellite, layered clay minerals (preferably, talc, mica, kaolinite, and the like, and more preferably talc), and the like are mixed with the cellulose acylate solution and the resulting mixture is filtered is used.

In order to control the filtration pressure or handlability, it is preferable to perform dilution using a suitable solvent before the filtration.

The filtration may be selected from pressure filtration, vacuum filtration, or normal pressure filtration, but it is preferable to perform the pressure filtration.

(Amount of Impurity)

The amount of impurity that is contained in a cellulose acylate composition according to the aspect of the invention and has a particle size of 40 .mu.m or more is preferably 0.1 particles/g or less, more preferably 0.05 particles/g or less, and particularly preferably 0.01 particles/g or less.

The amount of the impurity may be measured by using a microscope and a light scattering type of particle detector. The shape of impurity is generally a needle or a particle, but is not limited thereto. In the aspect of the invention, in respect to the amount of impurity, the impurity contains unreacted cellulose, impurity added from the outside, gellated substances, and side products which are not compatible with the cellulose acylate.

(Degree of Polymerization)

In respect to the degree of polymerization of cellulose acylate that is used in the aspect of the invention, the number average degree of polymerization that is measured by using a GPC method is preferably 80 to 1000, more preferably 100 to 850, even more preferably 120 to 650, and particularly preferably 130 to 450. In this connection, the number average degree of polymerization that is measured by using the GPC method may be obtained by dividing a number average molecular weight by an average molecular weight of the repeating unit.

The average degree of polymerization may be measured by the intrinsic viscosity method of Uda, et al. (Kazuo Uda and Hideo Saito: Journal of the Society of Fiber Science and Technology, Japan, Vol. 18, No. 1, pp. 105 to 120, 1962), in addition to the molecular weight distribution measurement by gel permeation chromatography (GPC), or the like. These are described in detail in JP-A-1997-95538.

According to the aspect of the invention, the weight average degree of polymerization/number average degree of polymerization of the cellulose acylate is preferably 1.0 to 5, more preferably 1.3 to 4, and particularly preferably 1.5 to 3.5.

The average subst