JPS6129245B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing volatile substances from thermoplastic resins. In recent years, thermoplastic resins have been used in an extremely wide range of fields, including automobile parts, home appliances, toys, food containers, and the like. In the production of this thermoplastic resin, if volatile substances such as unreacted monomers and solvents remain in the product, the inherent mechanical properties such as strength of the resin, thermal properties such as heat resistance, or chemical resistance may deteriorate. reduce the chemical properties of In addition, these volatile substances may be released into the atmosphere during resin molding, worsening the working environment, and when used in food containers, the volatile substances in the resin may migrate into the food containing it. There is also. For this reason, polystyrene resins used in food containers and the like are required to contain volatile substances of 500 ppm or less. In the case of styrene-acrylonitrile resin (AS resin), the toxicity of acrylonitrile is suspected, and legislation has been enacted in the United States to regulate the extractable acrylonitrile content in AS resin. A vented extruder is generally used to remove volatile substances from the resin.
However, in many cases, the target cannot be achieved by exhausting air only from one vent hole, so a multi-stage vent system with two or more vent holes is used. However, the multi-stage vent extruder requires a longer screw in the extruder, which causes the resin to remain in the extruder for a longer time, resulting in deterioration of the field, color, and transparency of the resin. For this reason, attempts have been made to remove volatile substances by injecting water into various parts of the extruder. However, if water is injected near the resin supply port of the extruder, the resin will foam due to the vaporization of the water, significantly reducing the extrusion capacity and making it impossible to maintain a constant supply of resin to the extruder.
Furthermore, since water and resin become a non-uniform mixture of resin, vent-up is likely to occur, making regular extruder operation impossible. Injecting water into the screw part of the extruder is also considered, but it is difficult to satisfy the conditions of not reducing the extruder's performance or causing vent-up, and the area where water and resin are mixed is also considered. Because of the short duration, the effect of water injection was not sufficiently apparent. Therefore, the present inventors conducted extensive research into a method for removing volatile substances that did not have the above-mentioned drawbacks, and found that the molten thermoplastic resin and water were mixed using an element with a passageway capable of stirring, and then directly subjected to vent-type extrusion. The present invention was completed based on the discovery that it is possible to remove volatile substances without the disadvantages seen in conventional methods by supplying the volatile substances to a machine and exhausting them through a vent hole. Thermoplastic resins used in the present invention include vinyl aromatic compounds such as styrene and α-methylstyrene, halogenated vinyl compounds such as vinyl chloride and vinylidene chloride, diene compounds such as butadiene and isoprene, acrylonitrile, methyl methacrylate, etc. acrylic compounds or olefins such as ethylene and propylene, or copolymers of these, or copolymers of these monomers and other copolymerizable monomers, such as general-purpose polystyrene, resistant High impact polystyrene, ABS resin, polymethyl methacrylate, polyacrylonitrile, AS resin, polybutadiene, polyisoprene, styrene acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyvinyl chloride, polyvinylidene chloride,
Examples include polyethylene, polypropylene, etc., and it is particularly preferably applicable to resins in a molten state that have been polymerized by bulk polymerization or solution polymerization. Further, the water to be mixed with the resin need not contain any substances that adversely affect the quality of the polymer or the materials used in the extruder, such as corrosion, and may contain additives or solvents necessary for the resin. It is also possible to use a surfactant or the like as a mixing aid with the resin. The amount of water to be injected is usually 0.1 to 10% by weight (based on the resin). Examples of commercially available elements having passages capable of stirring used in the present invention include a static mixing unit (manufactured by Sluzer) and a static mixer (manufactured by Kenix).
Hymixer (manufactured by Toray Industries, Inc.) is available, and the number of elements required varies depending on the shape of the element, type of resin, temperature, viscosity, molecular weight, composition of the injection liquid, amount of injection, shape of the injection port, etc. However, in general, the greater the viscosity of the resin or the greater the amount of injection liquid, the greater the number of elements required. When injecting 1.0% by weight (based on resin) of water into AS resin, 12 units of static mixing unit BKM manufactured by Sluzer are required. This element needs to be sealed and connected to the extruder. The extruder used in the present invention is a single-screw or multi-screw extruder having one or more vent holes. The mixed vapor of water, unreacted monomer, and solvent is exhausted from the vent hole of the extruder under atmospheric pressure or reduced pressure, but preferably under reduced pressure of 300 mmHg absolute or less. By applying the method of the present invention to the granulation process using a vented extruder for thermoplastic resin, it is possible to eliminate the unreacted material remaining in the resin without reducing the extruder's capacity due to venting, etc., and without deteriorating the quality of the product, such as the color of the product. It becomes possible to remove volatile substances such as monomers and solvents. The method of the present invention will be explained with reference to FIG. A molten thermoplastic resin containing at least 90% by weight of resin, e.g., polymerized by bulk polymerization or solution polymerization and from which most unreacted monomers and solvent have been removed using a flash evaporator thin film evaporator, is transferred to a relay tank. 1, a polymerization tank, a heating tank, etc., by a gear pump 2, an extrusion device, etc., and fed into an element 3 having a passage capable of stirring.
There is a water inlet 7 before the resin is supplied to the element, from which water is injected into the resin passage, and the water and resin are mixed within the element. This mixture of water and resin is led to a vent-type extruder 4, and a mixed vapor of water, unreacted monomer, and solvent is exhausted from a vent hole 5. The molten resin passageway is maintained at a temperature at which the resin can flow by a heating device. The method of the present invention will be explained below using examples. Examples 1 to 4, Comparative Examples 1 to 5 Molten AS at 230°C polymerized by bulk polymerization
The method of the invention is applied to the resin. This AS resin contains 24% by weight of acrylonitrile in the polymer, has a melt index of 2.1 g/10 minutes, and contains 4642 ppm of unreacted monomer and solvent (of which 316 ppm is acrylonitrile monomer) based on the polymer. ) Contains. This AS resin is led out from a relay tank 1 shown in FIG. 1 using a gear pump 2, passed through a static mixing element 3, and then led to a vent type extruder 4. This extruder has a single screw with a screw diameter D of 25 mm, and where L is the screw length, L/D is 30. Water is injected from the inlet 7 using a plunger pump 8. The injected water is thoroughly mixed with the resin in the static mixing element and sent to the vented extruder. The water, unreacted monomer, and solvent injected through the vent hole 5 of this vented extruder are exhausted under atmospheric pressure or under reduced pressure using a vacuum pump 6, and the resin is extruded from the extruder head as a strand 9. be done. The static mixing element used here is the BKM type manufactured by Sluzer, and each element has a diameter of 51.2 mm and a length of 68 mm, consisting of 16 elements. Table 1 shows the results when 10.50% by weight (based on the polymer) of water was injected using this device. It has been found that most of the unreacted monomers and solvent in the resin can be removed by injecting water. In particular, it is possible to effectively reduce acrylonitrile monomer, which is particularly toxic. No decrease in extruder performance was observed due to water injection. As a comparative example, Table 1 also shows the results when no water was injected and when 1.0% by weight (relative to polymer) was injected from the injection port 10 near the resin supply port. Injection of the extruder greatly reduced the capacity of the extruder, and vent-ups frequently occurred, making steady extrusion impossible. It was also found that the effect of removing unreacted monomers and solvents was inferior to that of Examples. Examples 5 and 6, Comparative Examples 6 and 7 The method of the present invention is applied to high impact polysterene resin (HIPS) melted by a heating device. this
HIPS contains 2304 ppm (of polymer) unreacted monomer and solvent with a melt index of 0.36 g/10 min. This HIPS is heated and melted at 240°C and supplied to the apparatus shown in Figure 1. Same as Example 1
Table 2 shows the results when water was injected into HIPS, mixed using a static mixing element, and then extruded using a vented extruder. Here, water was injected in an amount of 1.0% by weight based on the resin, and exhaust at the vent was carried out in the atmosphere and under reduced pressure. Table 2 also shows the results when HIPS was extruded without injecting water as a comparative example.

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【table】 [Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the apparatus used to carry out the method of the invention. 1: Relay tank, 2: Gear pump, 3: Element having a passage with stirring ability, 4: Vent type extruder, 5: Extruder vent hole, 6: Vacuum pump, 7:
Water inlet (for the present invention), 8: plunger pump, 9: resin strand, 10: water inlet (for comparison).

Claims (1)

[Claims] 1 When processing molten thermoplastic resin in a vented extruder, it is recommended that water be mixed with the molten resin in advance using an element with a passage capable of stirring, then supplied as is to the vented extruder, and then exhausted from the vent hole. Features a method for removing volatile substances from thermoplastic resins.