CA1235119A

CA1235119A - Porous spherical cellulose acetate particles

Info

Publication number: CA1235119A
Application number: CA000472623A
Authority: CA
Inventors: Kazuhiro Yamazaki; Manabu Uchida; Etsuo Ohtake
Original assignee: Daicel Chemical Industries Ltd
Current assignee: Daicel Corp
Priority date: 1984-01-24
Filing date: 1985-01-23
Publication date: 1988-04-12
Also published as: GB2152936A; GB2152936B; DE3502329A1; GB8501769D0; FR2558475A1; FR2558475B1

Abstract

Abstract of the Disclosure Cellulose acetate is formed into porous, spherical particles, having an acetylation degree of 49 to 60 %, a particle diameter of 0.05 to 10 mm, a sphericity of 0.7 or larger, a pore volume of 0.4 cc/g or larger and a collapsing strength of 9 kg or higher.

Description

~35~

POROUS SPHERICAL CELLULOSE ACETATE PARTICLES - I.

The invention relates to particles of cellulose acetate and then a process for preparing the same. Cellulose acetate which the invention especially concerns has a combined acetic acid, or an acetvlation degree, of 49 to 60 I, which corresponds to an esterification degree of 2.0 to 2.8.
Cellulose acetate is known to have moldability and volubility in a common organic solvent such as acetone, acetic acid and ethyl acetate. For this reason it has been used in the form ox chips, flakes, powder, fiber, film or mound articles.
While moldings of cellulose acetate are excellent in properties including transparency, doublet, touch, impact strength, etc., this material itself exhibits properties such as absorptivity, liquid retentivity, and easy nettability of its surface.
Therefore, if particles that are both porous and spherical can be obtained from it, these particles ;

3L;~3~19 will be useful as enzyme carriers, chromatographic packings, slow-releasing adsorbent for perfuming agents, deodorants, medicines or agricultural chemicals, and ion exchangers. The reason why spherical particles are advantageous is that they have good flu-idiot as compared with particles of other shapes. For example, spherical particles carrying an enzyme give a good efficiency of contact with a reaction solution when they are used in a packed bed, a fluidized bed, or an agitation tank. In this case, it is desirable that disintegration or deformation of particles does not occur even when the height of the packed bed is increased or when a pressure or an impact of the blade of the agitation tank is exerted on the particles. The disintegration or deformation of the particles in a packed bed results in the formation of unevenly dense portions and causes an uneven flow of a reaction solution and a lowered reaction efficiency. Therefore, the particles must be those which are tough enough to withstand compression or impact.
On the other hand, the particles must have a large pore volume because the amount of an enzyme or a drug fixed or adsorbed thereon is proportional to the pore volume within the particles. Particles of a large pore volume necessarily have a small specific

(2) ~2~5~L9

3 5702 198 gravity. In general, the requirement that particles should have a small specific gravity or density is contradictory to the require-mint that they should have sufficient toughness and, therefore, none of the prior arts has succeeded in meeting both of these no-quirements.
As a result of extensive studies, the inventors of the present invention have succeeded in producing porous spherical par-tides having a good sphericity, a toughness, and a large pore vow fume by using cellulose acetate as a starting material.
According to one aspect of the invention, cellulose ace-late is formed into porous, spherical particles, having an acutely-lion degree of 49 to 60%, a particle diameter of 0.05 to 10 mm, a sphericity of 0.7 or larger, a pore volume of 0.4 cc/g or larger and a collapsing strength of 9 kg or higher.
According to another aspect of the invention there is pro-voided a process for producing porous spherical cellulose acetate particles comprising forming a dope by dissolving a cellulose ace-late of a degree of esterification of 2.0 to 2.8 in an acetic acid/
water (80/20 to 90/10, by weight) solvent so that the concentration of the cellulose acetate may be 25 + 7 White; feeding said dope through a supply pipe to a coagulation bath comprising an acetic acid/water (30/70 to 42/58, by weight) solvent and having a tempera-lure of 40 + 5C; operating an agitator having a rotor passing near a discharge port on the end of said supply pipe; effecting precipi-station and granulation while adding water so that the acetic acid to water ratio of the coagulation bath may be maintained at a . , pa ~235119 5702-198 substantially constant value during said operation; centrifugally separating precipitated particles from the solution; washing these particles with water of 40 to 90C; and drying the particles.
In a preferable embodiment, a sphericity is 0.8 or greater, a pore volume is 0.65 cc/g or greater and a degree of esterification is 2.0 to 2.8.

- pa -Jo o o ~235 In addition, the invention includes an improvement that the surface of the above defined particles has been saponified. This surface-saponified particles have the surface portion consisting essentially of cellulose and the core portion consisting essentially of cellulose acetate, having an acetylation degree of 48 to 59 %, a particle diameter of Owe to 10 mm, a sphericity of 0.7 or greater, a pore volume of 0.4 cc/g or greater and a collapsing strength of 10 kg or higher.
In a more preferable embodiment of the surface-saponified particles, an acetylation degree of 50 to 58 and a particle diameter is 0.5 to 10 mm.

The process for producing spherical particles according to the present invention is the so-called "precipitation process". This process consists in using a solution of a high-molecular substance as a dope; adding said dope to a bath comprising a medium Sue in which the high-molecular substance is not soluble but the solvent of said solution is soluble, that is, a coagulation bath; and applying thereto a shear-in force to form particles while precipitating the high-molecular substance by solvent rival.
In the present invention, acetic acid/water mix-lures are used as the-solvent for a cellulose acetate solution, that is, a dope, and the medium for a kiwi adulation bath. The solvent for the dope has a come position of an acetic acid to water ratio of 80/20 to 90/10 (by weight: the same applies hereinafter), and the medium for the coagulation bath has a come position of an acetic acid to water ratio of 30/70 to 42/58.
The dope is prepared by dissolving a cellulose acetate of a degree of esterification of 2.0 to 2.8 in the above solvent for a dope so that the concentra-lion of the cellulose acetate may be 25 7 % (by weight; the same applies hereinafter), preferably about 25 %. It is preferable that the viscosity of the dope is about 2,000 poises at 40c. A supply pipe leads from a dope tank to a coagulation bath, and a nozzle on one end of the supply pipe is submerged in the coagulation bath. The coagulation bath is provided ~235119 with an agitator having blades thereon. Each ox the blades has a cutting edge on the front side to pass near the holes of the nozzle and cut a rod-like continuous body , of the dope which is extruded from the holes of the nozzle and is coagulating. The cut fine pieces form spherical particles while they are precipitating and coagulating.
Since the acetic acid concentration of the acetic acidjwater solution brought in by the dope during continued performance of the precipitation and co-adulation is higher than the initial acetic acid con-citation of the coagulation bath, it is necessary to maintain the acetic acid to water ratio of the coagulation bath constant by adding such an amount of water as to counterbalance the acetic acid brought in. This addition of water makes it possible to keep the concentration gradient of particle medium constant-to thereby afford uniform particles continuously.
The size of particles obtained by this process depends on a nozzle hole diameter, a feed rate of a dope, an agitator speed, etc., and the sphericity is improved when the feed rate and the agitator speed are set such that the cut length of the rotor may be nearly equal to the nozzle hole diameter. Particles having a particle diameter in the range of 0.05 to 10 mm are practical. Those having a particle diameter O (3 '7 ~L235~19 of less than 0.05 mm show a poor efficiency ox pro-diction, while those having a particle diameter en-ceding 10 mm can difficultly acquire an excellent sphericity or a pore volume.
In the process of the present invention, it is also possible to use an aged dope formed in the step of producing a cellulose acetate having a degree of esterification of 2.0 to 2.8 directly as the dope for the precipitation step. Namely, while this cellulose is one which is obtained by saponifying and aging cellulose triacetate by treatment with an acetic acid/water solvent, the reaction solution at the time of completion of the aging is called aged dope, because it is in a state in which a cellulose acetate of a degree of esterification of 2.0 to 2.8 is dissolved in a highly concentrated aqueous acetic acid solution, and has a viscosity of as high as about 1,000 to 3,000 poises. The separating and recovering step of cellulose acetate from the aged dope is the so-called "precipi-station step". It is to be noted that the aged dope usually contains an inorganic salt, e.g., magnesium ,.
sulfate, which is a product of neutralization of a catalyst (usually, sulfuric acid) used in the acutely-lion reaction. If such a water-soluble salt is present in a large amount, the size of the internal pores O (I 8 ~35119 will tend to increase in the precipitation step, thus reducing the pore volume. Therefore, it is preferable that the salt content of the aged dope when it is sent to the precipitation step is 1.5 % or below. The salt content depends on the conditions of the acutely-lion step, that is, on the amount of a catalyst used, so that it is necessary to choose conditions include in a smaller amount of catalyst used. Such a process for the acetvlation under conditions of a smaller amount of a catalyst is described, for example, in Japanese Patent Laid-Open No. 59801/1981.
The particles according to the process of the present invention are thought to be formed in such a manner that a-tough shell portion is formed during a relatively early period of the precipitation step and then a thick aqueous acetic acid solution held within the particles is replaced with a relatively thin aqueous acetic acid solution in the coagulation bath, during which time the internal pores are formed.
Namely, it may be presumed that the tough shell is formed on an interface of a relatively large concentra-lion gradient and, thereafter, the replacement of the solvents at a relatively smaller concentration dip-furriness proceeds within the particles, and passages left after the highly concentrated aqueous acetic o o 9 ~235:119 acid solution has passed from internal pores.
Although, as can be seen from the above, the structure of a particle is established almost come pletely in the precipitation stew, the particles still contain a large amount of acetic acid within them.
These particles are centrifugally separated from the solution, subjected to a washing treatment in a water bath and then dried. The acetic acid remaining within the particles is replaced with water in the washing step, so that the structure of a particle is effected somewhat by the conditions of the washing step. Namely, it is preferable that the washing is carried out with water of 40 to 90~C. Particles of a larger pore volume can be obtained if the above treatment is carried out at a higher temperature.
The spherical particles of the present invention are extremely tough and have a large pore volume, so that they are highly suited for use in packings, car-fiefs, adsorbent, etc. Further, because of the tough-news of their shells, the particles can be used also in the production of sistered bodies.
When the particles must have surfaces which are more hydrophilic than those of the cellulose acetate, or when the particles are used in contact with a medium which dissolves, swells, or plasticizes the cellulose Lo acetate, they are used in the form of regenerated cellulose particles which can be obtained by saponify-in the cellulose acetate particles with an alkali.
This saponification may be effected to such an extent that only the surface are saponified or that the par-tides are completely saponified. It is also possible to obtain hollow particles of regenerated cellulose by saponifying only the subrace and treating the par-tides with a solvent which dissolves cellulose acetate.

The surface-saponified particles of the invention is prepared by soaking the porous,, spherical particles of cellulose acetate as disclosed above with an aqueous solution of sodium hydroxide having a concentration of 0.2 to 2 wt. % for 1 to 6 minutes at a ratio of liquid to solid in the range between 3 and 10.

The comparison between the particles before the surface saponification of the present invention and those after the surface saponification of the present invention has revealed that the particle diameter and the sphericity are somewhat reduced and the pore volume is also reduced, but that the collapsing strength is somewhat increased in some cases, or decreased in 1~3 other cases. It was also found that a content of combined acetic acid representative of the composition was kept at 95 or above, suggesting that only a small portion of each particle was saponified.
It is doubtless that the saponification occurs on only the very surfaces of particles in view of the fact that the surfaces or the particles cite_ the saponification treatment are modified and a hick content of combined acetic acid is maintained as mentioned above.
The reason why the saponification cannot be attained except on the surfaces through the particles before saponification are porous and have excellent impreg-notability may be that the aqueous alkali solution which is used in the present invention imparts moderate sealability to the surfaces of the particles to be treated, and this prevents the alkali from infiltrate in into the insides of the particles. It is presumed that this surface-swollen layer is reconverted into a layer which is porous yet dense, when the treated particles are washed with water and dried.
The surface-saponified porous spherical cellulose acetate particles of the present invention have the same applications as those of the porous spherical cellulose acetate particles before the surface supine-o (I 1 " 123~
ligation, and nave a particularly excellent effect of impregnation and release of polar substances.
Although the present invention relates to surface-saponified porous spherical cellulose acetate particles and to a process for producing the same, it is also possible to produce surface-saponified porous spherical particles of lower fatty acid cellulose esters, such as cellulose preappoint and cellulose bitterroot, as substitutes 'or cellulose acetate, by a similar and to use these in similar applications.

The present invention will be described with reference to an example, but it should be noted that the present invention is by no means limited thereto.

Employ 1 A ripening dope containing a cellulose diacetate concentration of 25 %, a magnesium sulfate concentration of 0.9 % and a viscosity at 40c of 2,100 poises was added to a coagulation bath of a composition of an acetic acid to water ratio of 40/60 to effect precipitation and granulation. The apparatus was operated by using a nozzle diameter of 3 mm, a revolution speed of a three-blade agitator of 1,000 rum, and a coagulation bath temperature of 40C to obtain spherical precipi-toted particles. The particles were centrifugally separated, washed with water of 60C, and dried.
The physical properties of the particles were measured according to the following methods.
Sphericity:
20 particles were taken up at random and the largest and smallest diameters of each particle were measured with a micrometer.
The sphericity was determined according to the following equation.
sphericity = smallest diameter/largest donator -Pore volume:
A mercury porosimeter (a product of Carlo Era) was used.
The volume of mercury intruded into the pores at a pressure in the range of 0 to 1,000 (kg/ cm2 G) corresponded to a volume of pores in the range of 75 to 75,000 (A). The pore volume is represented in terms of cc per one gram of the sample.
Collapsing strength:
A Monsanto tablet hardness tester (a product of Iowa Medical Machine Manufacturing Co., Ltd.) was used. An average of the measured values of 10 particles was calculated.
The determined values of the produced spherical particles are as follows:
sphere diameter 3.5 4.0 mm sphericity 0.82 pore volume 0.81 cc/g collapsing strength 11.1 kg.

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Content of combined acetic acid About 5 g of a powered sample was dried in a dryer at 100 to 105C for 2 hours, and weighed accurately. 50 cc of purified acetone was added to this sample and the mixture was dissolved completely. 50 cc of a 0.2 N aqueous Noah solution and 50 cc of a 0.2 N aqueous HAL solution were added thereto in sequence. The resulting solution was titrated with a 0.2 N
aqueous Noah solution by using phenolphthalein as an indicator.
The degree of acetylation was calculated according to the following equation combined acetic acid (%) = _(____B)_X___X_l Al X 100 weight (g) of sample wherein A: volume (cc) of 0.2N aqueous Noah solution B: volume (cc) of 0.2N aqueous Noah solution added in a blank test F: factor of 0.2N aqueous Noah solution In the invention a combined acetic acid or a content of combined acetic acid which is determined in the above shown manner is called also as an acetylation degree. This may be calculated to an esterification degree.

(14) o O 1 5~.23~ 9 Example 2 1,000 g ox porous spherical particles comprise in cellulose acetate of a degree of acetylation of 54.5 % (a product of Dozily Chemical Industries, Ltd.), and having an average particle diameter of 3.6 mm, a sphericity of 0.87, a collapsing strength of 15 kg, and a pore volume of Owe cc/g were immersed in 500 cc OX
a 1% aqueous solution of sodium hydroxide at the room temperature for 3 minutes. After centrifugally separating the solution, the particles were washed with warm water of 40C until the washing became neutral, and then dried in a dryer at 100 to 110C for 3 hours to obtain sur_ace-saponified porous spherical particles. These particles had an average degree of acetylation of 53.4 %, an average particle diameter of 3.2 mm, a sphericity of 0.8Z, a pore volume of 0.43 cc/g, and a collapsing strength of 17 kg.

Example 3 100 g of porous spherical particles comprising cellulose acetate of a degree of acetylation of 55.2 % (a product of Dozily Chemical Industries, Ltd.), and having an average particle diameter of So mm, a sphericity of 0.90, a collapsing strength of 14 kg, and a pore volume of 0.75 cc/g were immersed in 500 cc of a 0.5% aqueous solution of sodium hydroxide and treated in the same manner as that in Example 2 to obtain surface-saponified porous spherical particles. These particles had an average content of combined acetic acid of 54.5%, an average particle diameter of 5.1 mm, asphericity of 0.85, a pore volume of 0.51 cc/g, and a collapsing strength of 15 kg.
Application Example 2 g of bergamot oil (lonelily acetate content of 40~) was added to 10 g of the surface-saponified spherical particles obtained in Example 2 and the mixture was agitated. After about 10 minutes, the liquid was completely impregnated, and the surfaces of the particles became dry and non sticky. Separately, the spherical particles before surface saponification used in Example 2 were impregnated with a perfume by a similar treatment. The surfaces of the particles were sticky and tended to stick to each other.
On the contrary, when the surface-saponified spherical particles which were obtained in Example 2 and impregnated with a perfume were stored at 80 C, their surfaces did not become sticky.
The evaporation rate of the perfume in an open air at a room temperature was determined. 10 days were necessary for 50% evaporation, and 45 days were necessary for 90% evaporation.
This suggested an excellent effect of slow releasing.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Porous, spherical particles of cellulose acetate consis-ting essentially of a cellulose acetate having an acetylation degree of 49 to 60%, a particle diameter of 0.05 to 10 mm, a sphericity of 0.7 or larger, a pore volume of 0.4 cc/g or larger and a collapsing strength of 9 kg or higher.

2. Porous, spherical particles according to Claim 1, wherein the sphericity is at least 0.8, the pore volume is at least 0.65 cc/g and the degree of esterification is 2.0 to 2.8.

3. Porous, spherical particles according to Claim 1, wherein the surface of the particles has been saponified.

4. Porous, spherical particles according to Claim 2, wherein the surface of the particles has been saponified.

5. Porous, spherical particles according to Claim 3 or 4, wherein the surface of the particles consists essentially of cel-lulose and the core of the particles consists essentially of cel-lulose acetate having an acetylation degree of 48 to 59%, a particle diameter of 0.05 to 10 mm, a sphericity of 0.7 or greater, a pore volume of 0.4 cc/g or greater and a collapsing strength of 10 kg or higher.

6. Porous, spherical particles according to Claim 3 or 4, wherein the surface of the particles consists essentially of cel-lulose and the core of the particles consists essentially of cel-lulose acetate having an acetylation degree of 50 to 58%, a particle diameter is 0.5 to 10 mm, a sphericity of 0.7 or greater, a pore volume of 0.4 cc/g or greater and a collapsing strength of 10 kg or higher.

7. A process for producing porous spherical cellulose acetate particles comprising forming a dope by dissolving a cellulose ace-tate of a degree of esterification of 2.0 to 2.8 in an acetic acid/
water (80/20 to 90/10, by weight) solvent so that the concentration of the cellulose acetate may be 25 ? 7 wt. %; feeding said dope through a supply pipe to a coagulation bath comprising an acetic acid/water (30/70 to 42/58, by weight) solvent and having a tem-perature of 40 ? 5°C; operating an agitator having a rotor passing near a discharge port on the end of said supply pipe; effecting pre-cipitation and granulation while adding water so that the acetic acid to water ratio of the coagulation bath may be maintained at a substantially constant value during said operation; centrifugally separating precipitated particles from the solution; washing these particles with water of 40 to 90°C; and drying the particles.

8. A process for preparing surface-saponified, porous, sphe-rical particles of cellulose acetate, which comprises soaking porous, spherical particles according to Claim 1 or 2 in an aqueous sodium hydroxide solution having a concentration of 0.2 to 2 wt. % for 1 to 6 minutes at a ratio of liquid to solid in the range between 3 and 10.