JPH0444018B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention is directed to polyvinyl alcohol fibers (hereinafter referred to as polyvinyl alcohol fibers).
This is a method for producing hollow activated carbon fibers by carbonizing PVA fibers. Several types of activated carbon fibers are already well known. For example, natural fibers, recycled cellulose fibers, phenol fibers, PVA fibers, etc. are used as raw materials. When activated carbon is in the form of fibers, it has a significantly larger macroscopic contact area than the conventionally widely used granular or powdered adsorbents, and has a faster adsorption speed, no dust generation, and lower pressure loss. It has many advantages in terms of form. However, by making it hollow, the macroscopic contact area increases, and it exhibits unique physical properties that combine the adsorption properties unique to activated carbon and the permselectivity of hollow fiber walls. Hollow activated carbon fibers made from polyvinyl alcohol resin and having thin fiber walls that have the properties of semipermeable membranes are not known. The present invention aims to provide a novel material having such unique physical properties. [Prior art] A method for manufacturing carbon fiber using PVA fiber as a raw material is described in Japanese Patent Application Laid-open Nos. 49-24897, 50-35431, 50-52320, and 50-52321, and the gist is that the process involves dehydration. A method in which PVA fibers coated or impregnated with a catalyst are heat-treated at 180° to 300° in an oxidizing atmosphere to carbonize them, and if necessary, treated at higher temperatures to promote graphitization and improve mechanical properties. It is. All of these are intended to be carbon fibers as structural materials, and include the same elements in terms of carbonization as PVA fibers, but they are not hollow in shape and are not activated carbon. In addition, Japanese Patent Publication No. 54-3973 relates to a method for producing active fibers, in which 3 to 15% of a dehydration catalyst is added to the spinning dope, and the resulting PVA fibers are
After heat treatment at 340° to a yield of 65-85% and carbonization, heat treatment at 600° to 1000°C in an inert gas containing water vapor.
This is a method of activating to a yield of 10 to 35%.
(Page 1, column 1, lines 16-28 of the same publication, scope of claims). Furthermore, there is a close relationship between the method of adding the dehydration catalyst and the adsorption properties of the obtained activated carbon, and activated carbon fibers with high adsorption properties (iodine adsorption amount of 1600 to 1700 mg/g) can only be obtained when added to the spinning dope in advance. (Page 3, Table 1 of the same publication, and column 6, lines 5 to 8 of the same publication). [Problems to be Solved by the Invention] The above-mentioned activated carbon fibers are in the form of ordinary fibers and are not hollow. Further, with the extrusion molding technology of activated carbon powder, it is impossible to produce hollow fibers having a diameter of 10-odd microns or less, which is the object of the present invention. The present inventors studied the degree of penetration of the dehydrating agent, the heat treatment conditions, and the shape of the resulting carbide in the step of immersing PVA fibers in an aqueous dehydrating agent solution and then heat-treating and carbonizing them. As a result, it was found that there is a close relationship between the two, and that hollow carbides can be obtained under certain conditions. Furthermore, regarding the method of activating the carbide, the present invention was completed by using wet-spun or dry-spun PVA fiber as a raw material and devising other activation conditions to significantly increase adsorption. [Means to solve the problem] After applying a dehydrating agent only to the surface layer of the PVA fibers, heat-treat the fibers until they become dark brown or black to prevent them from melting, and then heat from 400° to 1000°C within 5 minutes. This method of manufacturing hollow activated carbon fibers is characterized by carbonization and activation at elevated temperatures. The present invention will be explained in detail below. There are two methods for producing PVA fiber: dry spinning and wet spinning.Dry spinning is a method in which a spinning solution containing an aqueous solution of polyvinyl alcohol is spun into the air, dried, and stretched to make yarn, and wet spinning is a method in which a spinning solution containing an aqueous solution of polyvinyl alcohol is spun into the air, dried, and stretched to make yarn.
This method involves spinning into a concentrated aqueous electrolyte solution such as Na ₂ SO ₄ or NaOH, washing with water, drying, and stretching to make yarn. The present invention is applicable to PVA fibers obtained by either method. In the case of wet-spun PVA fibers, yarns with fine fineness can be obtained and hollow parts can be easily created. In addition, dry-spun PVA fibers can yield thicker yarns than wet-spun PVA fibers, and the diameter of the hollow portion can be increased. PVA fibers can also be applied to PVA fibers with trace amounts of boric acid or MgSO ₄ added to improve their physical properties, or PVA fibers that have been crosslinked with a crosslinking agent such as aldehydes to improve water resistance. Further, fibers made of a copolymer containing vinyl alcohol as a main component and other monomers such as ethylene and vinyl chloride may also be used. Furthermore, in the present invention, polyvinyl chloride (PVC)
A so-called PVA fiber is produced by mixing an aqueous solution of PVA with an emulsion containing PVA as the main raw material and spinning it in a mirabilite bath in the same manner as in the case of wet spinning of ordinary PVA fiber.
PVA-PVC fibers can also be used as raw material fibers. The dehydrating agent used in the present invention is preferably a strongly acidic inorganic acid such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, metaphosphoric acid, etc., and also a Lewis acid such as ZnCl ₂ AlCl ₃
and _TiCl2 are also effective. In addition, if there is a problem due to the material being acidic, an ammonium salt can be used. This is thought to be due to thermal decomposition in the next heat treatment step, where ammonia is scattered and the generated inorganic acid acts as a dehydrating agent. Ammonium salts include ammonium sulfate, ammonium chloride, ammonium nitrate, ammonium phosphate, and phosphoric acid 1.
Diammonium hydrogen, monoammonium dihydrogen phosphate, ammonium metaphosphate, ammonium polyphosphate, etc. are effective. The dehydrating agent can be applied in the form of an aqueous solution to the surface layer of the PVA fibers by a dip method or a nip method using mangrol. Here, the surface layer refers to a region close to the surface, which roughly corresponds to the skin when separated into the skin and core. [Function] The present invention does not allow the dehydrating agent to penetrate uniformly to the center of the fibers, but instead allows the dehydrating agent to be applied only to the portion near the surface layer and quickly dries the fibers, thereby reducing the penetration of the dehydrating agent in conjunction with the infusibility in the subsequent carbonization process. The main part is to melt and remove the unfilled core portion (hereinafter referred to as the center layer) to form a hollow shape. Therefore, it is necessary that the dehydrating agent permeates the surface layer only to a certain depth and does not penetrate into the center. Any such attachment method may be used, but as a result of investigating the relationship between various immersion conditions and the formation of hollow shapes, we found that
40%, immersion temperature 40° to 80°C, immersion time 5 seconds to 2 minutes,
It has been found that it is suitable to use a method in which the amount of dehydrating agent deposited is 5 to 20% by weight, and rapid drying with warm air is performed immediately after deposition. That is, by the above method, 5 to 20% by weight of the dehydrating agent is attached only to the surface layer of the fibers, but if the dehydrating agent is less than 2%, the dehydrating reaction in the heat treatment process is insufficient, the fibers are easily deformed, and a good hollow shape is not achieved. In some cases, it may not be possible to obtain the desired results, and if more than 20% is deposited,
The dehydration reaction tends to proceed easily, and the inner diameter of the hollow portion tends to become smaller. PVA fibers need to be dried quickly after being coated with a dehydrating agent. The drying method is not particularly limited, but good results can be obtained by using a method that rapidly dries in 3 to 4 minutes using hot air at 120°C. If it is heated rapidly after drying, it may melt, so it is preferable to gradually heat it from 180°C to 300°C until it becomes dark brown or black. Next, it is necessary to raise the temperature from 400°C to 1000°C within 5 minutes to complete carbonization. At this time, the PVA molecule becomes a polyene structure due to a dehydration reaction and becomes infusible. The speed of the dehydration reaction due to heat treatment increases as the temperature increases, but since the softening point of PVA is 220° to 240°C, if this temperature range is reached while the dehydration reaction is insufficient, the PVA fibers will melt and shrink, resulting in significant deformation, which is not good. A hollow shape cannot be obtained. When the dehydration reaction progresses through heat treatment, PVA
The fiber changes from brown to dark brown or black, becomes infusible, and its softening point gradually increases. Therefore, in order to form a good surface layer condition, the heat treatment temperature must always be kept below the softening point depending on the progress of the dehydration reaction. Since rapid heating may cause melting and deformation, and the hollow portion may be blocked, a method of gradually heating from 180° to 300°C is preferable. On the other hand, since the center layer does not contain a dehydrating agent, it is not made infusible, and when the temperature reaches 220° to 240° C. or higher, it is thought to gradually melt and form a hollow portion. However, although the shape of the hollow part formed at this stage is still incomplete, the surface layer is further carbonized by raising the temperature from 400° to 1000°C within 5 minutes and performing high-temperature carbonization for a short time. As the melting progresses, the melting of the central layer also progresses, and the cross-sectional shape of the hollow portion also becomes a regular circle and becomes completely black. It is thought that if the temperature increase rate is lowered, the carbonized hollow portion becomes narrower before it is melted and removed due to insufficient melting in the center. Heat treatment and carbonization are performed in an inert gas, and the treatment time is not particularly limited, but
Good results can be obtained when heating to 300°C for 60 minutes or from 220° to 300°C for about 60 minutes. Carbon monoxide gas is also used in carbonization.
If the process is carried out in either hydrogen gas or nitrogen gas, or in a mixed gas, a regular hollow shape can be easily obtained. Note that the shape of the hollow portion can be either a continuous type or an independent type. As detailed above, the heat treatment and high-temperature carbonization steps, together with the dehydrating agent deposition step, play an important role in forming the hollow shape, which is the essential part of the present invention. Hollow activated carbon fibers are obtained by activating the hollow carbon fibers obtained in the carbonization process. Although the activation method is not particularly limited, in general, as the activation progresses, the adsorption properties improve, but the strength and yield decrease, exhibiting antinomian properties. In order to give properties that are relatively balanced between the two, it is necessary to maintain the inside of the furnace at a relatively high temperature of 800° to 1100°C with combustion gas of liquefied petroleum gas, and add carbonized PVA fibers to heat the furnace for 20 to 30 minutes. A rapid activation method is preferred, in which the activation yield is 40-60%. [Effect] The thickness of the hollow activated carbon fiber of the present invention is not limited as long as the fiber wall exhibits semipermeability, but usually the outer diameter is 5 to 20 μm and the thickness of the fiber wall is 4 to 10 μm. A few μ,
The volume of the hollow part is 10-70%. Therefore, the fiber wall exhibits unique physical properties that combine the adsorptive properties unique to activated carbon and the properties of a semipermeable membrane. Furthermore, the hollow portions can be continuous or independent; however, in order to bring out the characteristics of the activated carbon fibers of the present invention, activated carbon fibers having continuous pores are cut into bundles into appropriate lengths. It is preferable to use a method in which the fiber is sealed in a glass tube, and only the outside of the fiber walls at both ends are fixed to the inner wall of the glass tube with a thermosetting resin, so that the hollow part and the outside of the fiber are completely separated by the fiber wall. Benzene adsorption amount of hollow activated carbon fiber is 120~130
%, BET surface area reaches 1600-2500 m ² /g.
Granular activated carbon has a surface area of 1500 to 1700 even after activation.
m ² /g is the limit, the hollow activated carbon according to the present invention has extremely high performance and also has a high equilibrium adsorption amount. Furthermore, despite its large surface area, it also has excellent strength properties, which is thought to have been imparted during the high-temperature carbonization process. In addition, the pore size distribution peculiar to activated carbon shows a relatively broad distribution centered on 20 to 30 Å, which is similar to that of coconut shell carbon, a typical activated carbon, which shows a relatively sharp distribution centered on 7 to 10 Å. Considerable differences are recognized and attract attention. This is considered to depend on the physical properties of the PVA fiber. The hollow activated carbon fibers of the present invention are used for ozone decomposition, sugar solution decolorization and purification, adsorption base, and fields that require functions as semipermeable membranes, such as highly accurate adsorption separation of organic compounds, and nitrogen removal in the air. It can be used for the separation of oxygen, ethanol and water, etc. In the case of gas separation, two columns are used alternately, and continuous separation operation is also possible using the pressure swing method. [Example] Hereinafter, aspects of the present invention will be specifically described, but the present invention is not limited thereto. Example 1 PVA fiber for industrial materials (1800d/1000f, strength
10.5 g/d, elongation 7%), 70 g each of (NH ₄ ) ₂ SO ₄ and 70 g of (NH ₄ ) ₂ were dissolved in 1000 g of water, and the above PVA fiber was placed in a display mangle at 70°C for 10 seconds in this aqueous solution. The liquid was squeezed out and dried at 105°C for 3 minutes. The adhesion rate of the dehydrating agent was 7.3 wt%. This fiber is heated to 300℃
From 400℃ after heat treatment until it becomes blackish brown.
The temperature was raised to 900°C for 3 minutes in N ₂ and carbonization was carried out. Thereafter, activation was performed in steam at 1050°C until the activation yield was 50%. The treatment conditions and physical property values are shown in Table 1. The surface area is Sorptomatic 1800 manufactured by Carlo Erba in Italy.
It was measured by the BET method (Brunauer Emmett & Teller method), which is a conventional method for activated carbon. The shape of the hollow portion was observed in cross section using an optical microscope and a scanning electron microscope. In Example 1 above, when activated under the same conditions as above without performing heat treatment at 300°C or less until it turned blackish brown, the hollow part was narrow and the shape was extremely irregular. It became. Comparative example 1 The heating time from 400° to 900°C was 10 minutes, and
Same conditions as Example 1, but the inner diameter of the hollow part was 0.6μ,
With an internal void ratio of 0.2%, the space was extremely narrow or almost non-existent. It is thought that when the temperature was gradually raised, the melting state of the center where the dehydrating agent had not penetrated was poor, and the material was carbonized before being melted and removed. Furthermore, when the heating rate was further reduced and the heating time was increased to 20 minutes, no hollow portions were observed at all. Comparative Example 2 This was treated under substantially the same conditions as in the other examples in which the adhesion rates of the dehydrating agent (NH ₄ ) ₂ SO and (NH ₄ ) ₂ HPO ₄ were increased to 12% and 13%, respectively, but the hollow inner diameter
It had a diameter of 2.1μ and an internal void ratio of 3.2%, and the hollow part was narrow and had independent holes. Examples 2 to 5 Sulfuric acid, zinc chloride, or phosphoric acid was used as the dehydrating agent, and the high-temperature carbonization conditions were also varied. The treatment conditions and physical property values are shown in Table 1.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 shows Example 1, FIG. 2 shows Example 2, and FIG. 3 shows scanning electron micrographs of the cross sections of activated carbon fibers obtained in Comparative Example 1. (6000x)

Claims (1)

[Claims] 1. After attaching a dehydrating agent to the surface layer of polyvinyl alcohol fibers, heat treatment is performed until the fibers become dark brown or black so as not to melt, and then heated from 400°C to
A method for producing hollow activated fibers, which comprises raising the temperature to 1000°C within 5 minutes, carbonizing it, and then activating it. 2. The method for producing hollow activated carbon fibers according to claim 1, wherein the polyvinyl alcohol fibers are wet-spun or dry-spun polyvinyl alcohol fibers. 3 When attaching a dehydrating agent, the concentration of the dehydrating agent aqueous solution is 2 to 40% by weight, the immersion temperature is 10°C to 80°C, the immersion time is 5 seconds to 2 minutes, and the amount of dehydrating agent attached is 5 to 20% by weight.
A method for producing hollow activated carbon fibers according to claims 1 and 2. 4. The method for producing hollow activated carbon fibers according to claim 1, wherein the inert gas during carbonization is at least one of carbon monoxide gas, hydrogen gas, and nitrogen gas.