PROCESS FOR PRODUCING FINE SILK FIBROIN POWDER

Abstract: A process is provided for preparing crystalline fine powder of silk fibroin from a silk substance in an industrially advantageous fashion. The process for the preparation of fine powder of the silk fibroin is such that, after the silk substance is brought into an alkaline aqueous solution at a temperature of 95 °C or higher in order to cause a strength of said silk substance to deteriorate, the resulting silk substance is subjected to treatment with alkali and dried, and the dry silk substance is pulverized into finely divided powder.

Description: The present invention relates to a process for preparing fine powder of silk fibroin from a silk substance. Fine powder of silk fibroin is useful as an additive to food products, beverages and cosmetic products. Previously fine powder of silk fibroin has been prepared by dissolving a silk substance in an aqueous solution containing a neutral salt, removing the neutral salt from the solution to yield an aqueous fibroin solution, adding a precipitating agent to the resulting aqueous solution to cause the silk fibroin to precipitate therein, and separating the precipitate from the solution and drying the precipitate or lyophilizing the aqueous solution, thereby yielding fine powder of silk fibroin. This process, however, cannot be said to be satisfactory from the point of view of industrial application because it requires the silk substance to be dissolved in water forming an aqueous solution. On the other hand, a silk substance cannot be pulverized into finely divided crystalline powder with high efficiency by mechanically dividing the silk substance directly into fine powder because the silk substance is so high in mechanical strength that it is very hard to divide it into fine powder. It is an object of the present invention to provide a process for preparing fine powder of crystalline silk fibroin from a silk substance, which can offer industrial advantages. SU 1826999 - A describes a process for producing powder from natural silk in which the silk is contacted for 60 to 80 minutes at 105 to 120°C in a 5.7% aqueous carbonate solution . The resulting solution is decanted and degraded fibre is ball milled to a powder. The present invention provides a process for the preparation of fine powder of silk fibroin, in which a silk substance is contacted with an alkaline aqueous solution at a temperature of 95°C or higher to degrade the strength of the silk substance, the resulting silk substance is then subjected to dealkalinization and drying, and the resulting dried silk substance is thereafter finely pulverized, characterised in that the contacting of the silk substance with the alkaline aqueous solution is carried out by filling the silk substance in a bag made of a cloth-like material having reticulate fine gaps and immersing the silk substance-containing bag in the alkaline aqueous solution. The silk substance to be used for the present invention may include, for example, cocoons, cocoon thread, silk yarn, silk cloth and waste. The process according to the present invention is preferably carried out first by treating a silk substance by bringing it into contact with an alkaline aqueous solution at a temperature of 95 °C or higher, preferably approximately 100 °C. As an alkaline substance for the alkaline aqueous solution, there may be employed sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide or the like. The extent of alkalinity of the alkaline aqueous solution may be pH 11.5 or higher, preferably pH 12 or higher. The treatment of the silk substance with the alkaline aqueous solution may be carried out by immersing the silk substance in the alkaline aqueous solution. The period of time (treatment time) during which the silk substance is kept in contact with the alkaline aqueous solution is a period of time long enough to cause the strength of the silk substance, whichever threads produced by sericulture or wild silkworms, to deteriorate to such a suitable extent to which the silk substance can be pulverized into finely divided powder. For silk threads, for example, it is such that they may come in contact with the alkaline aqueous solution until their tensile strength becomes approximately 0.05 g/d or less. Generally, the treatment time may be in the range of from 1 to 4 hours; however, additional 2 or 3 hours or longer hours are required when the raw silk thread is large in fineness or when attachment of siren can be recognized thereon or when a silk substance such as wild silk thread is difficult to deteriorate. The specific period of time required for allowing the silk substance to come in contact with the alkaline aqueous solution, however, can be determined readily by a simple preliminary experiment for investigating the relationship between the readiness of pulverizing the silk substance with its strength deteriorated after the alkali treatment and the period of time during which the silk substance comes in contact with the alkaline aqueous solution. The silk substance treated with the alkaline aqueous solution, after separation from the alkaline aqueous solution, is washed to remove alkaline substance attached thereto with water followed by drying, Then, the silk substance is pulverized into finely divided powder in a manner as will be described hereinafter. Although the pulverization of the silk substance into finely divided powder can be conducted by a general pulverization technique, it is preferred to divide it into fine powder by a two-step process in which the first step comprises dividing the silk substance into coarse powder and the second step comprises dividing it into fine powder. The pulverization of the silk substance into coarsely divided powder may be effected so as to yield coarsely divided powder having an average particle size ranging from approximately 15 to 30 microns, and the pulverization thereof into finely divided powder may preferably be effected so as to yield finely divided powder having an average particle size ranging from approximately 3 to 6 microns. When the silk substance is to be treated with the alkaline aqueous solution in accordance with the process of the present invention, the silk substance is placed in a bag made of a cloth material having a fine gap reticulated structure and it is treated by immersing the bag in an alkaline aqueous solution. Components dissolved from the silk substance into the alkaline aqueous solution upon treatment therewith and very small silk particles can be removed readily from the bag toward the outside by allowing them to pass through the fine gaps of the cloth material forming the bag. More specifically, the components of the silk substance and the fine silk substances can be removed from the bag to the outside through the fine gaps of the cloth material forming the bag, for example, by shaking the bag in the alkaline aqueous solution at plural times up and down or in right-and left directions, by dipping the bag into the alkaline aqueous solution and then pulling it above the solution, or the like. As the cloth material constituting the bag, there may be used a woven or non-woven cloth made of alkali-resist fibers. As the alkali-resist fibers, there may be mentioned cotton fibers, synthetic resin fibers and so on. The dimension of the fine gaps may appropriately be determined in accordance with the dimension of the silk substance to be removed and it may generally be set to be small enough to allow only the fine silk substances having the size of 1 micron or smaller to pass wherethrough in the alkaline aqueous solution. The treatment of the silk substances in the bag in the manner as described offers the advantage that the silk substances can be washed with water after treatment while they are kept filled in the bag. By subjecting the silk substances to treatment with the alkaline aqueous solution and to the treatment after the treatment using the bag, it further offers the advantage that the silk substances treated can be collected with high efficiency. The present invention can provide a highly efficient and industrially advantageous production of fine powder of silk fibroin from the silk substances. The fine powder of silk fibroin to be produced by the present invention is in a crystalline state and it has properties as high as silk thread has and is superior in, moisture diffusion property and moisture permeability. The method according to the present invention is simple in treatment operation because it does not require a step for dissolving the silk substances thoroughly. Further, the fine powder of silk fibroin produced by the present invention is superior in crystalline properties and as high in quality as silk thread because non-crystalline portions of the silk substances are removed upon treatment with the alkaline aqueous solution. Therefore, the method according to the present invention is superior as a process for preparing crystalline fine powder of silk fibroin on an industrial scale. The present invention will now be described in more detail by way of a Comparative Example and Examples according to the invention. Comparative Example Using raw silk yarn having approximately 30 d (deniers) as a sample, 1 part by weight of the raw silk yarn was immersed in an alkaline aqueous solution consisting of 1 part by weight of sodium carbonate and 30 parts by weight of water and then boiled. In this treatment, the sample raw silk yarn lost its weight as time went by and the weight of the silk yarn reduced by approximately 10% by the treatment for 40 minutes and by approximately 16% by the treatment for 1 hour. Further, in the previous treatment, the tensile strength of the raw silk yarn deteriorated as time went by and it reduced to approximately 0.04 grams per denier (g/d) by the treatment for 1 hour. The tensile strength of the raw silk yarn in this case was measured with a measurement instrument "Tensilon Model UTM-II". The raw silk yarn treated in the manner as described hereinabove was then washed with water and subjected to dewatering four times in order to remove alkali, followed by drying at room temperature. Then, the dried silk yarn was pulverized into coarsely divided powder having an average particle size of approximately 20 microns and then into finely divided powder. In this case, the pulverization into coarsely divided powder was conducted with a usual mill ("Sample Mill", manufactured by Fuji Denki Kogyo K.K.) and the pulverization into finely divided powder was conducted with a fine mill ("Current Jet", manufactured by Nisshin Engineering Co., Ltd.). It was observed that particle sizes of the fine powder of the raw silk yarn pulverized in the manner as described hereinabove (fine powder of silk fibroin) were distributed in the range of from 1 to 20 microns and their average particle size was approximately 5 microns. The fine powder of the silk fibroin was then measured by a polarization microscope and it was found to have a birefringence in the range of from 4.0 x 10 -3 to 5.0 x 10 -3 . This birefringence is substantially equal to that of silk yarn fibroin having a birefringence of 5.2 x 10-3. Therefore, it can be thought that the orientation of fibroin molecules in finely powdery form is equal to that of the silk yarn fibroin. Further, this is supported by a X-ray diffraction photograph of the fine powder of the silk fibroin and it was found to show a beta (b) type that was equal to that of silk yarn. Furthermore, as a result of the thermal observation of the finely divided powder of the silk fibroin by a DSC instrument, no exothermic peak caused to otherwise appear due to crystallization was recognized in the DSC curve for the fine powder of the silk fibroin and it was thus confirmed that the fine powder had a crystallinity as high as silk yarn fibroin. In addition, it was found from the hygroscopic and moisture diffusion curves that the fine powder of the silk fibroin had a moisture content lower by about 1% than silk yarn and it was confirmed that it had substantially the same hygroscopic and moisture diffusion properties as high as the silk yarn has. From the foregoing description, it can be seen that the fine powder of the silk fibroin had the same configuration and functions as the silk yarn As described hereinabove, the fine powder of the silk fibroin was produced from raw silk yarn at a yield rate of approximately 70% by the treatment with alkali for about 1 hour. Example 1 The procedures of the Comparative Example were followed in substantially the same manner with the exception that raw silk yarn was filled in a bag and subjected to treatment with alkali and washing with water. As the bag, there was employed a bag made from cotton cloth woven by warp and weft, each having yarn number count 50, at a warp density of 25 threads per cm and a weft density of 20 threads per cm in a thickness of 0.25 cm and a weight of 120 grams per meter. By using this bag, substances dissolved during the alkali treatment and superfine powder having approximately 1 micron or smaller were removed through the cotton cloth into the bag. In this Example, the raw silk yarn used as a raw material provided fine powder of silk fibroin at a yield rate as high as 70% to 60% by the alkali treatment for about 2 to 3 hours. Example 2 The procedures of the Comparative Example were followed in substantially the same manner with the exception that silk cloth was employed in place of the raw silk yarn. This Example provided fine powder of silk fibroin at a yield rate as high as 75% by the alkali treatment for 1 hour.

Claims: A process for the preparation of fine powder of silk fibroin, in which a silk substance is contacted with an alkaline aqueous solution at a temperature of 95°C or higher to degrade the strength of the silk substance, the resulting silk substance is then subjected to dealkalinization and drying, and the resulting dried silk substance is thereafter finely pulverized, characterised in that the contacting of the silk substance with the alkaline aqueous solution is carried out by filling the silk substance in a bag made of a cloth-like material having reticulate fine gaps and immersing the silk substance-containing bag in the alkaline aqueous solution. A process as claimed in Claim 1 in which the cloth-like reticulation has a size of 1 µ m or less. A process as claimed in either of the preceding claims in which the pulverization of the dried silk substance is carried out by first coarsely grinding the dried silk substance into an average particle size ranging from 15 to 30 µ m, followed by fine grinding into an average particle size ranging from 3 to 6 µ m.

Inventors: TSUBOUCHI KOZO, TSUBOUCHI, KOZO
Applicants: NAT INST OF AGROBIO SCIENCES, NATIONAL INSTITUTE OF AGROBIOLOGICAL SCIENCES
Application year: 1996
Main classification: C08J3
IPC codes: C08J3