Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING CELLULOSE ACETATE
The invention relates to a process for the production of cellulose acetate
having a DS between 1.5 and 2.9.
Cellulose acetate with a degree of substitution (DS) between 1.5 and 2.9 is
an important product for various industrial applications. Degree of
substitution DS designates the number of hydroxyl groups which are esteri-
fied on average with acetyl groups per anhydroglucose unit of the cellulose.
In the known processes for the production of cellulose acetate with a DS
between 1.5 and 2.9 in a first process stage cellulose is always reacted to
triacetate and the cellulose triacetate is subsequently hydrolyzed to the
desired degree of substitution. If acetylation is performed with an
acetylating agent deficiency or acetylation is prematurely broken off, no
homogeneous, partly acetylated product is obtained and instead there is
essentially a mixture of cellulose triacetate and unreacted cellulose. The
reason for this is the structure of cellulose, which on the one hand
contains amorphous fractions and on the other crystalline fractions.
Whereas the amorphous fractions are readily accessible to the acetylating
agent, the crystalline fractions are difficultly accessible, which leads to
an inhomogeneous reaction sequence. An indicator for a homogeneous substi-
tuent distribution in a partly acetylated cellulose acetate is its solu-
bility in acetone. For the same average acetyl group content, inhomogen-
eous cellulose acetates are insoluble in acetone.
Moreover, in the known technical processes, use is made of a more or less
large excess of acetylating agent, generally acetic anhydride, in order to
aid the reaction sequence. Both the need to pass through the triacetylated
stage and also the necessary use of an acetic anhydride excess in compari-
son with the stoichiometrically necessary quantity give rise to an
increased acetylating agent consumption, which in the case of 2,5-cellulose
acetate can be up to 31%. As for many industrial applications a 2,5-
cellulose acetate can be replaced by a cellulose acetate with a DS ~2,
30further acetylating agent savings could result from a process not requiring
the detour via cellulose triacetate.
Apart from cellulose, commercial pulps always contain a certain proportion
of other wood constituents, such as lignin and in particular hemicelluloses.
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Hemicelluloses or their acetylated forms impair both the processing, partic
ularly the solubility, filterability and spinability, and the product
characteristics of the industrially produced cellulose acetates. Admit-
tedly processes are known with the aid of which the influence of hemi-
celluloses is largely repressed, but they involve further acetic anhydride
consumption. Hemicelluloses are decomposed to monosaccharides and oligo-
saccharides and discharged during acetate washing. In addition, these
processes (high temperature hydrolysis) are relatively energy-consuming and
tedious, involving high investment costs. It is therefore advantageous to
carry out the separation of hemicelluloses and other wood constituents,
such as lignin and fats, by a stage consuming no acetylating agent.
The commercially available pulps suitable for acetylation belong to the
high-quality pulps and contain only relatively little hemicellulose (2 to 3
wt.%). There is a need for a process in which use can be made of less
high-quality pulps, i.e. pulps with a higher hemicellulose content. This
would represent a considerable cost saving, because pulp prices are rela-
tively high and cellulose acetate consists of approximately 60% cellulose.
The problem of the invention is consequently to provide a process for the
production of cellulose acetate with a DS between 1.5 and 2.9, which
requires the use of smaller acetylating agent quantities than the known
processes and in particular requires no detour via cellulose triacetate.
It is intended to supply cellulose acetates with a substantially homogen-
eous substituent distribution. The process must also permit the use of
less high-quality pulps, in that the admixtures such as hemicelluloses,
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AMENDED SHEET
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further processing of cellulose acetate-disturbing admixtures, such as hemi-
celluloses, lignin, fats, etc., in process stages consuming no acetylating
agent.
According to the invention this problem is solved in that in a first stage
the cellulose is impregnated with excess base, the base is removed again to
a molar ratio of base:anhydroglucose units (AHG) of less than 2.5 and,
accompanied by base catalysis, the cellulose is reacted with a first
acetylating agent to cellulose acetate With a DS of less than 1 and in a
second stage, accompanied by acid catalysis, with a second acetylating
agent to cellulose acetate with a DS of 1.5 to 2.9.
Bases catalyzing the first stage of the process according to the invention
are either liquid or in the form of a solution, preferably aqueous solution.
The bases or their solutions are able to swell the cellulose, i.e. they are
able to penetrate the crystalline areas of the cellulose and in this way
activate the latter. By acetylating to a DS of less than 1 the activation
state is preserved. It is assumed that the acetyl groups present in low
substituent density act as "spacers", which prevent a recrystallization of
the cellulose chains, if the base is consumed by the reaction or removed
in any other way.
The base for the catalysis of the first stage of the process according to
the invention is preferably caustic soda solution, particularly aqueous
caustic soda solution, preferably with a concentration of at least 20%.
In a preferred embodiment, in the first stage, the cellulose is impregnated
with an excess of base and the base is removed to a molar ratio of base:AHG
(anhydroglucose units) of less than 1 and subsequently the product obtained
is reacted with the first acetylating agent to a cellulose acetate with a
DS of less than 1.
"Excess of base" means that the base or its solution must at least com-
pletely wet the surface of the cellulose. Conventionally a molar ratio
Amended sheet
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3a
20 as
facilitates the homogeneous penetration of the cellulose with base. The
base is then largely removed again in order to minimize the formation of
byproducts from the reaction of the base with the first acetylating agent.
Preferably the base is removed again to the base quantity stoichiometric-
ally necessary for the formation of the cellulose acetate with DS < 1. Pref-
erence is given to residual base quantities of approximately 2 to 5 wt.%
base (solvent-free calculation basis), based on the cellulose. On removing
the base excess simultaneously undesired admixtures of cellulose, such as
lignin, hemicelluloses, fats, etc. are removed. It is e.g. known that
hemicelluloses are soluble in caustic soda solution, particularly in the
30 concentration range preferred in the process according to the invention and
can be separated from insoluble pulp by squeezing or pres,'sing out (cf.
K. Gotze, Chemiefasern nach dem Viskoseverfahren, 3rd edition, Springer-
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Verlag, Berlin, Heidelberg, New York, 1967). It is therefore a mayor
advantage of the process according to the invention that hemicelluloses are
separated in the first process stage and therefore cannot have a disturbing
effect during the processing of the products. It is also advantageous that
as a result it is possible to use less refined and therefore less costly
pulps with a higher hemicellulose content. The undesired admixtures can be
separated from the base in a suitable way and the base can be reused. The
base can be recycled by discharging the cellulose admixtures.
The removal of excess base can take place by the mechanical pressing out of
the impregnated cellulose. Alternatively or preferably in addition to
pressing out, the impregnated cellulose can be rinsed with a water-miscible,
polar, organic solvent. The solvent is preferably chosen from among mono-
hydric, dihydric or trihydric C1-C4 alcohols and amines. The monohydric
alcohol is preferably methanol or ethanol and the dihydric alcohol is
ethylene glycol.
As a result of this measure it is easily possible to reduce the base con-
tent of the cellulose to the extent stoichiometrically necessary for react-
ing with acetylating agent to the cellulose acetate with a DS of less than
1. Surprisingly, on washing out with the water-miscible, polar, organic
solvent, the amorphous, reactive state of the alkali cellulose is main-
tained. By mixing with the first acetylating agent, particularly acetic
anhydride, at ambient temperature a partly acetylated cellulose acetate
with a DS of less than 1 is obtained. If use is made of an alcohol, prefer-
ably a monohydric alcohol for washing out, it can be advantageously
recovered by distillation. The residual alcohol still present in the
alkali cellulose following the rinsing stage does not react with the
acetylating agent.
The cellulose acetate with a DS of less than 1 obtained in the first stage
of the process according to the invention can be stored following washing
with Water and drying. Alternatively washing can also take place with
glacial acetic acid. Washing permits the removal of residues of organic
solvent, first acetylating agent and particularly the reaction product
formed from the catalyst base and the first acetylating agent, e.g. sodium
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acetate when using caustic soda solution.
The cellulose acetate obtained in the first process stage has a degree of
substitution of less than 1, preferably less than 0.6 and in particular
between 0.1 and 0.5. It has a fibrous structure and according to the
X-ray diffraction spectrum is completely decrystallized. It has a uniform
reactivity and is consequently a suitable substrate for the second process
stage, in which the cellulose acetate with the desired DS is now directly
obtained, i.e. without the detour via triacetate.
In the second stage of the process according to the invention, the cellu-
lose acetate with a DS of less than about 1 obtained in the first stage,
accompanied by the addition of the corresponding second acetylating agent
quantity and accompanied by acid catalysis is directly reacted to cellulose
acetate with the desired degree of substitution in the range 1.5 to 2.9,
particularly 1.8 to 2.75 and in particularly preferred manner approximately
2.5. Preferably acetic anhydride is used as the second acetylating agent
and preferably sulphuric acid is used as the catalyst.
The process according to the invention has numerous advantages. Thus,
mercerization technology, i.e. the treatment of cellulose with base is well-
used, proven and cost-effective in operation. Corresponding plants are
used to a considerable extent throughout the world in viscose or cellulose
ether production. Excess caustic soda solution can be recovered without
significant cost. The setting of the degree of polymerization (DP) by con-
tact with caustic soda solution is well known and proven from the viscose
industry.
During mercerization the undesired admixtures of cellulose from the wood,
such as hemicelluloses, lignin, fats, etc. are separated from the cellulose,
which makes it possible to use much less refined pulps. Thus, it is in
part possible to move pulp purification from pulp factories to the further
processor.
To achieve the desired degree of substitution compared with the prior art a
much smaller acetylating agent quantity is required. In the first stage
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an amorphous cellulose acetate with a low DS is produced and its further
reaction to cellulose acetate with the desired DS takes place without the
problems arising in the known heterogeneous acetylation process as a result
of the crystalline cellulose fraction, such as inhomogeneous substituent
distribution and low product uniformity. Acetylation takes place at ambi-
ent temperature or slightly elevated temperatures. The products according
to the invention are characterized by a substantially homogeneous substi-
tuent distribution, which leads to an excellent solubility of the inventive
products in acetone. Thus, apart from 2,5-cellulose acetates, also pro-
ducts produced according to the invention with higher degrees of polymeriz-
ation are soluble in acetone.
The invention will now be illustrated by the following examples:
Example 1
100 g of cellulose with a DP of 750 are suspended in 500 g of 22 wt.% caus-
tic soda solution at 22°C in a 1 litre three-neck flask. After 15 min
the
mixture is pressed out in a laboratory chamber press at 150 bar. The press
cake (250 g) is suspended in 750 g of methanol at ambient temperature and
the alkali cellulose is filtered off. 150 g of methanol-containing alkali
cellulose are then added in a thermostatable reaction vessel at 20°C to
150 g of acetic anhydride and reacted. After 10 min excess anhydride is
filtered off and the decrystallized cellulose acetate is washed With water
and dried, giving 103 g of cellulose acetate with a DS of 0.25.
For cellulose-2,5-acetate production, in a thermostatable reaction vessel
100 g of dried 0,25-CA are suspended at 60°C in 1.5 litre of glacial
acetic
acid (99%). After 15 min 105 g of acetic anhydride are added and stirring
takes place for a further 5 min at 60°C. After adding 15 ml of a 1 wt.%
sulphuric acid solution in glacial acetic acid stirring takes place for
45 min at 60°C and the reaction mixture homogenizes. Following the pre-
cipitation of 2,5-CA in water, washing the product several times and drying
at 100°C, 150 g of cellulose-2,5-acetate are obtained.
The product is completely soluble in 1350 g of acetone. The solution is
CA 02283980 1999-09-14
filtered and can be processed to cellulose-2,5-acetate products.
Examples 2 to 4
The cellulose-2,5-acetate is prepared in accordance with example 1, except
for the fact that the 0,25-CA is not washed with water and dried, but is
instead washed with acetic anhydride (example 2) or acetic acid (example 3)
or is reacted directly without washing (example 4) to 2,5-CA.
In each case the products obtained are completely soluble in acetone.
