Cellulose acetate with high moldability and process for production thereof

Cellulose acetate having a high moldability and low solution viscosity in spite of having a high average degree of polymerization is obtained. The 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 a cellulose acetate having a molecular weight distribution Mw/Mn of 1 to 1.7. As the washing solvent, those swell or partially dissolve the cellulose acetate, for example, those which dissolve 0.1 to 30% by weight of cellulose acetate can be used. This solvent includes, for example, a solvent having a solubility parameter .delta. of 7 to 12.5 (ketones, ethers, organic acid, esters, etc.).

FIELD OF THE INVENTION 
The present invention relates to cellulose acetate, particularly cellulose 
triacetate, which is highly moldable to a film and the like, and to a 
process for the production thereof. 
BACKGROUND ART 
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 according to its degree of acetylation. One is cellulose 
triacetate (hereinafter referred to as CTA) having 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 for many years in the field of plastics, fibers and films (for 
example, photographic film, etc.). Further, cellulose acetate has 
attracted attention from the viewpoint of the global environment, because 
it possesses a biodegradability and the like. 
A molded product of cellulose acetate, such as CTA, can be prepared 
generally by the 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 its processing is required and high speed molding, high speed spinning 
and 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 be molded 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 the 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, p. 46-47, 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 these properties become more remarkable as the acetylation 
degree increases. The physical properties of certain polymeric material 
greatly depend on its crystallizability. That is, those having a high 
crystallizability are imparted with strength while flexibility, for 
example, elongation is impaired, resulting in a brittle product. Of 
course, CTA is not an exception, and has a high crystallizability due to 
its homogeneous structure. That is, for a cellulose acetate, the higher 
the degree of acetylation becomes, the higher the crystallizability will 
be. Moreover, crystals are generally formed because lower molecular weight 
materials act as nuclei. Accordingly, when CTA or CDA is used, the molded 
product is generally imparted with a flexibility by adding a plasticizer. 
For example, phthalate plasticizers, such as diethyl phthalate, may often 
be used for acetate plastics used as a grip of a screwdriver or the like. 
In addition, a cellulose acetate, particularly CTA, has a 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 these 
physical defects, a plasticizer is also used in this case. Adding 
ingredients such as a plasticizer is accompanied, not only with a 
decreased yield of the final product due to bleedout during molding, but 
also with economical disadvantages. Thus, the development of materials for 
producing a film having excellent physical properties while maintaining 
the characteristics of CTA by addition of a small amount of a plasticizer 
has been expected. 
Accordingly, an object of the present invention is to provide a cellulose 
acetate having a low solution viscosity and excellent moldability, in 
spite of its high molecular weight and degree of polymerization. 
Another object of the present invention is to provide a cellulose acetate 
having a high solubility in a solvent and high moldability, in spite of 
having a high average molecular weight and high average degree of 
substitution. 
Still another object of the present invention is to provide a cellulose 
acetate useful in producing molded products having a high moisture 
resistance and dimensional accuracy by a molding process at a high 
processing speed using a solution of a cellulose acetate having a low 
solution viscosity. 
Still another object of the present invention is to provide a process for 
producing a cellulose acetate having the excellent properties described 
above. 
DISCLOSURE OF THE INVENTION 
The present inventors have studied intensively to attain the above objects. 
As a result, we have found that: 
(1) treatment of a cellulose acetate, such as CTA, with a solvent to elute 
components having a low molecular weight, results in a narrow range of 
Mw/Mn, which is an index of the distribution of the molecular weight of 
the cellulose acetate; 
(2) removal of the low molecular weight components results in significantly 
reduced viscosity of the concentrated solution within a certain range of 
Mw/Mn, in spite of having a high molecular weight, thereby improving the 
moldability; 
(3) reduction of the crystallizability of the material, that is, removal of 
low molecular weight materials which serve as seeds in the formation of 
crystals, results in improved physical properties, particularly the film 
strength of the molded product and improved flexibility, and the present 
invention has been completed on the basis of such findings. 
That is, cellulose acetate of the present invention has a molecular weight 
distribution Mw/Mn of 1 to 1.7, according to gel permeation 
chromatography, and is highly moldable. This cellulose acetate includes, 
for example, cellulose acetate having an average degree of acetylation of 
52 to 62.5%, molecular weight distribution Mw/Mn of about 1.2 to 1.7, 
cellulose acetate comprising an acetone extract of less than 5%, with a 
viscosity average degree of polymerization (DP) of not less than 290, and 
the concentrated solution viscosity (.eta.), according to the falling ball 
viscosity method for viscosity average degree of polymerization (DP), is 
expressed by the following formula (1): 
EQU 2.814.times.ln(DP)-11.753&lt;ln(.eta.)&lt;7.28.times.ln(DP)-37.059,(1) 
particularly, CTA. 
This cellulose acetate can be obtained by washing cellulose acetate with a 
solvent and eluting low molecular weight components of the cellulose 
acetate. Washing solvents include solvents that swell or particularly 
dissolve cellulose acetate, for example, those which dissolve 0.1 to 30% 
by weight of the cellulose acetate when the cellulose acetate is dispersed 
at the temperature of 25.degree. C. and a solids concentration of 5% by 
weight. Examples of the washing solvent include, for example, ketones, 
ethers, organic acids and esters. Most of these solvents have a solubility 
parameter .delta. of about 7 to 12.5. The present invention also 
encompasses the following aspects. 
The present invention includes cellulose acetate wherein the average degree 
of acetylation is not less than 59%, acetone extract is not more than 5%, 
and concentrated solution viscosity (.eta.) for viscosity average degree 
of polymerization (DP) according to the falling ball viscosity test is 
expressed by the following formula (2): 
EQU 2.814.times.ln(DP)-11.753&lt;ln(.eta.)&lt;6.29.times.ln(DP)-31.469(2) 
(wherein, DP is a positive integer of not less than 290). 
Further, the present invention includes a process for producing a cellulose 
acetate which is characterized by having an average degree of acetylation 
of not less than 59%, acetone extract of not more than 5% and concentrated 
solution viscosity (.eta.) for viscosity average degree of polymerization 
(DP) according to the falling ball viscosity test expressed by the 
following formula (2): 
EQU 2.814.times.ln(DP)-11.753&lt;ln(.eta.)&lt;6.29.times.ln(DP)-31.469(2) 
(wherein, DP is a positive integer of not less than 290), which comprises 
washing the cellulose acetate obtained according to an ordinary method 
with one or more solvents selected from ketones, acetate esters and 
cellosolves. 
Preferred cellulose acetates contain not less than 70% of those having a 
particle size of 20 mesh path. 
The present invention will now be explained in detail. 
A cellulose acetate is preferably an acetate ester of (cellulose acetate), 
however, it may include mixed acid esters of other organic acids, for 
example, an ester of an aliphatic organic acid having about 3 or 4 carbon 
atoms (e.g., cellulose acetate propionate, cellulose acetate butyrate), 
cellulose acetate phthalate, etc.; mixed acid esters of inorganic acids 
(e.g., cellulose acetate nitrate), so long as it contains an acetate ester 
as a main ingredient. 
Cellulose acetate of the present invention is characterized by having a 
narrow molecular weight distribution Mw/Mn (Mw: weight-average molecular 
weight; Mn: number-average molecular weight) according to gel permeation 
chromatography. That is, the Mw/Mn of 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, about 1.4 to 1.65, and is often about 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 
by casting and the like (particularly, the moldability at high speed) is 
impaired. When the lower limit of the Mw/Mn becomes closer to 1.0, the 
solution viscosity decreases but the strength of the molded product is 
impaired. Accordingly, the 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.times.10.sup.4 to 100.times.10.sup.4, preferably, 
5.times.10.sup.4 to 75.times.10.sup.4, more preferably, about 
10.times.10.sup.4 to 50.times.10.sup.4. 
Further, the cellulose acetate of the present invention has a low 
concentrated solution viscosity in spite of having a high molecular weight 
and degree of polymerization relative to ordinary cellulose acetates 
because the low molecular weight components are removed. Although the 
reason for this is not clear, it is considered that the higher molecular 
weight materials control the apparent viscosity in a 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 the cellulose acetate can be selected within the range 
so as not to impair the mechanical properties and the like of the molded 
products. The viscosity-average degree of polymerization (DP) of cellulose 
acetate (especially, CTA) is, for example, preferably not less than 290 
(e.g., 290 to 400), more preferably, about 250 to 350 (e.g. 300 to 350). 
For a cellulose acetate having a viscosity-average degree of 
polymerization of less than 100, the mechanical properties of the molded 
product is likely to be impaired. 
The degree of acetylation of the cellulose acetate (percentage of the 
bonded acetate) can be selected from the range of from 52 to 62.5%. The 
preferred degree of acetylation of the 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 dimensional stability is likely to be impaired. Accordingly, 
particularly preferred cellulose acetates include those having a high 
degree of acetylation, for example, CDA and CTA, particularly, CTA. 
Among such cellulose acetates, CTA having the following properties has a 
high moldability, because it has a high solubility in a solvent and its 
solution viscosity can be reduced in spite of having a high hygroscopicity 
and dimensional stability, as well as a high degree of acetylation. 
The distribution of molecular weight (Mw/Mn) is 1.3 to 1.65, particularly 
1.4 to 1.65. 
The weight-average molecular weight (Mw,.times.10.sup.4) is 5 to 100, 
particularly 10 to 50. 
The degree of acetylation is 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 the 
viscosity of the cellulose acetate solution can be selected according to 
the application. The solution viscosity of the cellulose acetate may be an 
index of its high speed moldability, particularly in a casting method and 
a spinning method. That is, since cellulose acetate having a low solution 
viscosity enables cast coating and spinning at high speeds and forms a 
smooth surface in a short time (that is, has a high leveling), even high 
speed molding results in a high moldability and improved production of a 
molded product. The solution viscosity of cellulose acetate can be 
selected from a range which does not impair its moldability at a 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 about 30 to 65 seconds, according to the falling 
ball viscosity method 1 described below. 
Falling Ball Viscosity Method 1 
A 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 a cellulose acetate having 15% by weight of solids 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.sup.3 (23.degree. C.)) is allowed to fall 
therein, and the time (sec.) required for the ball to fall a distance of 
10 cm between two gage marks was determined as the viscosity. 
Further, the cellulose acetate of the present invention is characterized by 
containing an 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 (.eta.) for the viscosity-average degree 
of polymerization (DP) according to the falling ball viscosity method 2 
expressed by the following formula (1): 
EQU 2.814.times.ln(DP)-11.753&lt;ln(.eta.)&lt;7.28.times.ln(DP)-37.059(1). 
The above formula (1) in the present invention is obtained from experiments 
performed by the present inventors. For cellulose acetate having a 
viscosity-average degree of polymerization of not less than 290, the 
viscosity of the concentrated solution increases exponentially as the 
degree of polymerization increases, while the 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): 
EQU 2.814.times.ln(DP)-11.753&lt;ln(.eta.)&lt;6.29ln(DP)-31.469 (2). 
The method for determination of the viscosity of a concentrated solution 
(.eta.) according to the falling ball viscosity method 2 is as follows: 
Falling Ball Viscosity Method 2 
A 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.degree. 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 the 
viscosity. 
In the present invention, CTA obtained by an ordinary method is washed with 
ketones, acetate esters, cellosolves, or the like, to remove low molecular 
weight materials, inhibit crystallization and provide a material for a 
film having excellent physical properties. 
The characteristics of the CTA obtained by this washing is that after the 
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 the CTA so that the amount of the low molecular weight CTA after 
re-extraction should be no more than 5%. 
The improved properties of, for example, the films are considered to be 
attributable to the removal of the low molecular weight materials in the 
product by washing, thereby preventing the formation of unnecessary 
crystals and resulting in increased non-crystal portions in the film, to 
afford flexibility and additional transparency to the film. 
The cellulose acetate can be produced by an ordinary method, for example, 
the sulfuric acid catalyst method, the acetic acid method, the methylene 
chloride method and the like. The cellulose acetate is generally obtained 
from cellulose acetate after an activation treatment with acetic acid or 
the like by preparing cellulose triacetate with acetic anhydride using a 
sulfuric acid catalyst and adjusting the degree of acetylation by 
saponification (hydrolysis). The Mw/Mn of cellulose acetate obtained 
according to such a method is generally about 1.8 to 3.0. 
According to the present method, the cellulose acetate is washed with a 
solvent to produce a cellulose acetate having a narrow molecular weight 
distribution. As these solvents, those which do not completely dissolve 
but swell or partially dissolve the cellulose acetate may be used. 
Solvents which swell or partially dissolve the cellulose acetate may be 
those capable of dissolving and eluting low molecular weight components, 
and the percentage of the component soluble in a solvent for the cellulose 
acetate is not particularly limited so long as the high molecular weight 
component can be separated. In order to remove the low molecular weight 
component of the cellulose acetate and efficiently obtain the 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 the cellulose acetate when the cellulose 
acetate is dispersed at ambient temperature (25.degree. C.) at a solids 
concentration of 5% by weight. The solvents capable of eluting the low 
molecular weight component are generally those which dissolve about 1 to 
20% by weight of the cellulose acetate. When the component soluble in a 
solvent for the cellulose acetate is less than 0.1% by weight, the low 
molecular weight component 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 the cellulose acetate on an 
industrial scale. 
This washing solvent can be selected depending on the type of cellulose 
acetate. For selection of the washing solvent, the solubility parameter 
.delta. can be referenced (for example, H. Burrell; Off. Dig., 29, 1069 
(1957)). The solubility parameter .delta. may be obtained, as described, 
for example, in J. H. Hildebrand, R. L. Scott; "Solubility of 
Non-electrolytes" Chap. 20, Reinhold (1950), according to the following 
formula: 
EQU .delta.=(E/V).sup.0.5 
(wherein E represents molar heat of evaporation (cal) and V represents 
molecular volume (cc)). 
Washing solvents include, for example, ketones such as acetone (10.0, 
hereinafter the data of solubility parameter .delta. 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; carbitols 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 the 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 have 
high solvating properties. For these good solvents, the solubility of the 
cellulose acetate can be controlled by altering the percentage of the 
components in the solvent mixture or by the addition of another solvent. 
Among the above solvents, water may be used with hydrophilic solvents, 
particularly a water-soluble solvent such as acetone, acetic acid, etc. 
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 efficient elution of the low molecular weight 
components, at least one solvent selected from polar solvents other than 
halogenated hydrocarbon, for example, ketones, ethers, organic acids, 
esters, cellosolves and carbitols may be 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 and 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) and which are selected from ketones, ethers, 
organic acids and esters. 
When the cellulose acetate is CDA and CTA, particularly CTA, which has a 
high degree of acetylation, the 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) and ethers (e.g., 
tetrahydrofuran). 
When a solvent mixture containing poor solvents (e.g., water and/or 
alcohols) are used as a washing solvent, the greater the amount of the 
poor solvent, 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 the 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 the cellulose acetate, the cellulose acetate 
may be in various forms, for example, powder, granules, fibers, flakes, 
etc. 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 wet 
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 of from 30.degree. C. to the boiling point of 
the solvent (for example, about 40 to 90.degree. 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 about 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 about 50 to 5,000 
parts by weight, preferably 100 to 2,500 parts by weight based on 100 
parts by weight of the cellulose acetate. The cellulose acetate subjected 
to the washing treatment with a solvent is generally separated by 
filtration, centrifugation or the like, and dried. 
Using the 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. The cellulose acetate of the present invention may be used 
in various forms depending on the types of the molding methods (for 
example, powder, pellets, etc.). However, it possesses a high solubility 
in a solvent as well as a low solution viscosity. Therefore, it is often 
used as a cellulose acetate solution (dope). Representative molding 
methods using a cellulose acetate solution include, for example, a process 
for production of a film or sheet (photographic film, etc.) by a casting 
method including a spinning method, a process for the production of fibers 
by spinning. Further, the cellulose acetate of the present invention, 
which has a low solution viscosity, can be utilized in other applications, 
for example, plastics, lacquer, electrical insulating material and the 
like. The cellulose acetate solution may be prepared using a good solvent, 
depending on the type of the cellulose acetate, and this 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 another cellulose ester (for example, an ester of an organic acid 
such as cellulose propionate, cellulose butyrate; an ester of an inorganic 
acid such as cellulose nitrate, cellulose sulfate, cellulose phosphate and 
the like). In addition to the above solvents, to the cellulose acetate, 
optional additives may be added, for example, an ester type plasticizer 
(e.g., triacetin, triethylene glycol diacetate, triethylene glycol 
dipropionate, dibutyl phthalate, dimethoxyethyl phthalate, triethyl 
citrate ester, etc.), inorganic fine particles (e.g., kaolin, talc, 
diatomaceous earth, quartz, calcium carbonate, barium sulfate, titanium 
oxide, alumina, etc.), a thermostabilizer (e.g., a salt of alkaline earth 
metal such as calcium, magnesium, etc.), coloring agents, etc. 
Since the cellulose acetate of the present invention has a narrow molecular 
weight distribution Mw/Mn and the 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 high moldability in spite 
of having a high average degree of polymerization and high average degree 
of substitution. Accordingly, it is useful to use a cellulose acetate 
solution with a low solution viscosity to obtain molded products having a 
high moisture resistance and dimensional accuracy by a molding process at 
a high processing speed. 
According to the present method, a cellulose acetate having the excellent 
properties as described above can be produced by a simple operation like 
washing with a solvent.

EXAMPLES 
The present invention will be illustrated in detail referring to the 
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. The molecular weight, molecular weight distribution, degree of 
acetylation, viscosity-average degree of polymerization, physical 
properties of the film and acetone extract were determined as follows: 
(1) Molecular weight and Molecular weight distribution 
Measurement was carried out using a high speed liquid chromatography system 
"GPC-LALLS" having a gel filtration column connected to a detector for 
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 .mu.l 
Standard sample: methyl polymethacrylate (Mw=188,200) 
Temperature: 23.degree. C. 
(2) Degree of acetylation (%) 
The degree of acetylation can be determined by the saponification method. 
That is, a dried cellulose acetate, such as CTA, was 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 was carried out at 25.degree. C. for 2 hours. 
Phenolphthalein was added as an indicator and an excess of sodium 
hydroxide was titrated with 1N sulfuric acid (concentration factor: F). In 
the same method as described above, a blank test was carried out. The 
degree of acetylation was calculated according to the following formula: 
EQU Degree of acetylation (%)=(6.005.times.(B-A).times.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 blank test, F is a concentration factor of 1N sulfuric acid, 
W is the weight of the sample). 
(3) Method for measuring and calculating viscosityaverage degree of 
polymerization (DP) 
Oven-dried cellulose acetate (about 0.2 g, precisely weighed) was dissolved 
in a solution of methylene chloride: ethanol=9:1 (100 ml). The time (sec.) 
required for the solution to drop was measured by an Ostwald viscometer at 
25.degree. C. The degree of polymerization was obtained according to the 
following formula: 
EQU .eta..sub.ret =T/T.sub.0 
EQU .eta.!=(ln.eta..sub.ret)/c 
EQU DP=.eta.!/km 
T: time (sec.) required for the sample solution to drop 
T.sub.0 : time (sec.) required for the solvent alone to drop 
c: concentration (g/l) 
km: 6.times.10.sup.-4 
(4) Process for preparing film 
A film used for the measurement of mechanical strength was prepared by 
dissolving a given amount of cellulose acetate and a plasticizer in a 
solvent, which was then filtered and cast on a glass sheet so that the 
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 tensile strength 
Film cut into 10 cm length (initial length of the sample, 5 cm) was 
stretched at a stretching speed of 20 mm/min according to ISO1184-1983, 
with the initial sample length being 5 cm, and the tensile strength was 
obtained from the load at the breaking point. 
(ii) Measurement of 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 tearing strength 
The load required for tearing was determined according to ISO6383/2-1983 
using film cut into 5.times.6.4 cm pieces. 
(iv) Measurement of 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 acetone extract 
The 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 
ovendried, 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), 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.degree. 
C. for 120 minutes. After neutralization with magnesium acetate, CTA was 
obtained by saponification and aging at 63.degree. C. for 30 minutes. 
The obtained CTA was divided into four equal pieces, and to each piece was 
added ten times by weight of methyl acetate (Example 1), acetone (Example 
2), tetrahydrofuran (Example 3), and 80% aqueous acetic acid (Example 4), 
which were then 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). 
Comparative Examples 1 to 4 
According to an ordinary method altering esterification and saponification 
conditions, cellulose acetates having 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.degree. C. for 100 to 110 minutes, neutralized with magnesium 
acetate, then saponified and matured at 60 to 63.degree. C. for 20 to 40 
minutes, to prepare four samples of cellulose acetate. 
Comparative Example 5 
CTA obtained in Example 1 was washed using a poor solvent for low molecular 
weight components. That is, using 10 times by weight of aqueous acetic 
acid solution (acetic acid/water=5/5 (by weight ratio)) based on the CTA 
obtained in Example 1, a washing treatment was carried out in the same 
manner as described in Example 1, followed by filtration and drying, 
providing the 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 the 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 
the viscosity-average degree of polymerization and the 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) 
are shown in FIG. 1. 
TABLE 1 
______________________________________ 
Viscosity of 
Mn .times. Acetylation 
Conc. solution 
10.sup.4 
Mw .times. 10.sup.4 
Mw/Mn Degree (%) 
(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. 
9.9 19.5 2.13 61.1 58.2 
Com. Ex. 
9.1 20.0 2.02 60.8 77.3 
2 
Com. Ex. 
8.9 19.9 2.24 61.0 75.5 
3 
Com. Ex. 
10.5 21.2 2.36 60.9 99.7 
4 
Com. Ex. 
8.7 18.7 2.16 60.9 45.5 
5 
______________________________________ 
TABLE 2 
__________________________________________________________________________ 
Viscosity- 
Relation Between Degree of 
average 
polymerization and Falling Ball 
Physical Properties (MD/TD) 
Acetone Degree of 
Viscosity Tensile Tearing 
Tensile 
Extract polymerization 
Lower Upper 
Lower 
Strength 
Bending 
Strength 
Elongation 
(%) (DP) Limit 
In(.eta.) 
Limit (2) 
Limit (1) 
(kg/mm.sup.2) 
Strength 
(gf) 
(%) 
__________________________________________________________________________ 
Example 1 
0.2 343 4.67 
4.67 
5.25 5.44 13.0/10.2 
140/120 
26/26 
53/40 
Example 2 
0.4 332 4.58 
4.66 
5.05 5.20 12.7/12.6 
120/150 
25/24 
48/53 
Example 3 
1.0 314 4.43 
4.58 
4.69 4.80 12.5/12.3 
130/140 
24/24 
48/45 
Example 4 
0.3 329 4.56 
4.78 
4.99 5.14 12.8/12.0 
140/130 
25/23 
50/48 
Com. Ex. 1 
11 314 4.43 
4.71 
4.69 4.80 10.9/11.0 
110/105 
20/21 
44/45 
Com. Ex. 2 
13 318 4.46 
4.99 
4.77 4.89 11.8/10.5 
120/100 
22/2l 
46/40 
Com. Ex. 3 
12 318 4.46 
4.97 
4.77 4.89 12.0/12.1 
115/120 
20/20 
47/44 
Com. Ex. 4 
10 335 5.61 
5.25 
5.10 5.27 12.3/11.8 
100/104 
19/20 
47/47 
Com. Ex. 5 
17 298 4.28 
4.46 
4.37 4.42 11.0/11.3 
125/123 
17/17 
42.41 
__________________________________________________________________________ 
*MD: Direction of casting the film 
TD: Direction perpendicular to that of casting the film 
As is obvious from Tables 1 and 2 and FIG. 1, the cellulose acetate 
obtained in the Examples had a lower solution viscosity compared with the 
cellulose acetate of Comparative Example having 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 by weight) at room temperature for 30 minutes, drained 
and dried to obtain a cellulose acetate (component 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, the component extracted and removed 
in acetone was 12% by weight based on the weight of the starting material, 
the properties of which were as follows: degree of acetylation, 60.9%, 
viscosity-average degree of polymerization, 196. The film properties of 
the resulting sample without a 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 the concentrated solution 
of this sample was expressed as follows: ln(.eta.)=4.66, i.e., 
4.50&lt;ln(.eta.)&lt;4.85&lt;4.98, which satisfied formulae (1) and (2). 
Comparative Example 6 
The 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 the concentrated solution was as follows: ln(.eta.)=4.31, i.e., 
4.29&lt;ln(.eta.)&lt;4.39&lt;4.44, which satisfied formulae (1) and (2). The 
molecular weight distribution was 2.20. The film properties are given in 
Table 1. 
Comparative Example 7 
Cellulose acetate having 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 concentrated solution was 
as follows: ln(.eta.)=4.74, i.e., 4.43&lt;4.69&lt;ln(.eta.)&lt;4.80, which did not 
satisfy formula (2) but did satisfy formula (1). The molecular weight 
distribution was 2.07. The film properties are given in Table 1. 
Comparative Example 8 
Cellulose acetate having 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 the concentrated solution 
was as follows: ln(.eta.)=4.68, i.e., 4.21&lt;4.22&lt;4.24&lt;ln(.eta.), which did 
not satisfy formulae (1) and (2). The molecular weight distribution was 
2.11. The film properties are given in Table 3. 
TABLE 3 
______________________________________ 
Ex- Com. Com. Com. 
ample 5 Ex. 6 Ex. 7 Ex. 8 
______________________________________ 
Gen- Average degree of 
60.9 60.9 60.8 61.7 
eral acetylation (%) 
Prop- 
Viscosity average 
322 299 314 291 
er- degree of 
ties polymerization 
Phys- 
Tensile Strength 
11.8/ 12.0/11.5 
12.2/12.2 
10.9/11.3 
ical (kg/mm.sup.2) 
11.8 
Prop- 
Bending Strength 
130/160 120/130 
140/110 
110/110 
er- (time) 
ties Tearing Strength 
25/24 22/23 22/23 16/17 
(MD/ (gf) 
TD) Tensile Elongation 
51/47 43/45 47/40 41/42 
(%) 
______________________________________ 
*MD: Direction of casting the film 
TD: Direction perpendicular to that of casting the film