JPS5839430A - Method of eliminating volatile substance - Google Patents

Abstract

PURPOSE:To eliminate included volatile substances without lowering the extrusion ability of an extruder or causing the clogging or the like, by premixing a melted thermoplastic resin and water, feeding the mixture to a vent extruder, and exhausting a gas mixture through a vent port. CONSTITUTION:The melted thermoplastic resin is led from a polymerizing tank 1 by a gear pump 2, and water in a quantity of about 0.1-10wt% of the resin is added from a water inlet 7. Then the mixture is introduced into an element 3 with a passage having an ability for mixing such as a static mixing unit, a high mixer or the like so that the resin and water may be homogeneously mixed. Then the mixture of resin and water is introduced into a vent extruder 4 where a gas mixture including water and the volatile substances such as unreacted monomers, solvents, etc. contained in the resin is evacuated via the vent port 5 to be eliminated, and the resin is extruded from the extruder head to obtain a resin strand 9.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing volatile substances from thermoplastic resins.

Thermoplastic resins have become extremely popular in recent years! It is used in automobile parts, home appliances, toys, food containers, etc. In the production of this thermoplastic resin, if volatile substances such as unreacted monomers and solvents remain in the product, the resin's inherent mechanical properties such as strength, thermal properties such as heat resistance, and chemical resistance may be affected. Decreases isokinetic chemistry. 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. In polystyrene resin used for food containers, etc., volatile substances contained in the resin can be reduced by 500%.
It is desired to reduce the amount to 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 content of re-extracted acrylonitrile in As resin.

A vented extruder is generally used to remove volatile substances from the resin.

However, in many cases, exhaust gas from only seven vent holes does not reach the 11 target, 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 resin decomposition and poor color and transparency.

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, making it impossible to maintain a constant supply of resin to the extruder, and causing uneven distribution of water and resin. Since the resin is mixed with the resin, vent-up is also likely to occur, making regular extruder operation impossible. Injecting water into part of the extruder screen will reduce the extruder capacity.
) It was difficult to satisfy the condition of not causing vent-up, and the area where water and resin were mixed was also short, so the effect of water injection was not sufficiently manifested.

Therefore, the present inventors conducted extensive research into a method for removing volatile substances that does not have the above-mentioned drawbacks, and finally found that they mixed molten thermoplastic resin and water using an element with a passageway capable of stirring, and then vented the mixture as it was. The present invention was completed based on the discovery that volatile substances can be removed without the disadvantages seen in conventional methods by feeding the mixture into a type extruder and exhausting the gas through a vent hole.

The thermoplastic resin used in the present invention includes styrene,
From vinyl aromatized metals such as α-methylstyrene, vinyl halides such as vinyl chloride and vinylidene chloride, diene compounds such as butadiene and isoprene, acrylic compounds such as acrylonitrile and methyl methacrylate, or olefins such as ethylene and propylene. Polymers or copolymers thereof or monomers thereof and others (11Jl, polymerization lIr1llt 7c rlt M
Body Ka rIi, r ;! :+ II; lr O(k −
rb, 11. h.

For example, general purpose polystyrene, high impact polystyrene, AB
S resin, polymethyl methacrylate, polyacrylonitrile, As resin, polybutadiene, polyisoprene,
Examples include styrene-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyvinyl chloride, polyvinylidene chloride, polyethylene, polypropylene, etc., and are particularly preferably applied 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 may contain any necessary additives or solvents in the resin as long as it does not contain any substances that have an adverse effect on the quality of the polymer or the materials used in the extruder, such as corrosion. It is also possible to use a surfactant or the like as a mixing aid with the resin. The amount of water injected is usually 0. / to 10% by weight (based on the resin) is suitable.

Examples of commercially available elements having passages capable of stirring used in the present invention include static mixing units (manufactured by Sluzer), static mixers (manufactured by Kenix), and Hei mixers (manufactured by Toshi). The number of elements required depends on the shape of the element, type of resin, temperature, viscosity, molecular weight, composition of the injection liquid,
Although it varies depending on the amount of injection, the shape of the injection port, etc., the number of elements required generally increases as the viscosity of the resin increases and as the amount of injection liquid increases. When injecting 10% by weight (based on resin) of water into As resin, 12 static mixing units (BKM type manufactured by Sluzer) are required. This element needs to be sealed and connected to the extruder.

One extruder used in the present invention is a -shaft or multi-shaft extruder having one or more bait holes.

Exhaust of the mixed vapor of water, unreacted monomer, and solvent from the vent hole of the extruder is carried out under atmospheric pressure or reduced pressure, but it is preferably carried out under reduced pressure of 300 μm absolute or less.

By applying the method of the present invention to the granulation process of thermoplastic resin using a vented extruder, it is possible to maintain the granulation rate in the resin without reducing the extruder's capacity due to venting, etc., and without degrading the quality of the product, such as the color of the product. It becomes possible to remove volatile substances such as unreacted monomers and solvents.

The method of the present invention will be explained with reference to FIG. at least 90
A molten thermoplastic resin containing a resin content of - weight, for example, a resin polymerized by bulk polymerization or solution polymerization and from which most of the unreacted monomers and solvent are removed using a flash evaporator and a thin film evaporator, is used in a relay tank.・is extracted from a polymerization tank, heating tank, etc. using a gear pump or extrusion device,
It is fed into an element 3 having a passage capable of stirring.

A water injection lower is located 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 introduced into a vented extruder, and the mixed vapor of water, unreacted monomer, and solvent is exhausted from the vent hole S. 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 7-q, Comparative Examples/-S The method of the present invention is applied to As resin at 23[theta]C that has been polymerized and melted by bulk polymerization. This As resin contains 24% by weight of acrylonitrile in the polymer, and the melt index is! , / 11/10 minutes, and the amount of unreacted monomer and solvent is 12 ppm based on the polymer (among which, acrylonitrile monomer is 3/1).

ppm) Contains.

This As resin is led out from the relay tank shown in FIG. 7 using a gear pump, passed through the static mixing element 3, and then led to the vent type extruder 1c. This extruder is a single screw with a screw diameter of 2. ! −
1111, and if the screw length is L, L off is 30. Water is injected through the injection lower, and the water is thoroughly mixed with the resin inside the static mixing element and sent to the vented extruder. Vent hole S of this vent type extruder
The injected water, unreacted monomers, and solvent are evacuated under atmospheric pressure or under reduced pressure using a vacuum pump 6, and the resin is extruded as a strand 9 from the extruder head. The static mixing element used here is a BKM type manufactured by Sluzer, and has a diameter of 5/21III and a length of BGM.

Using this device, 70. Table 1 shows the results when 0% by weight (based on polymer) was injected. 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 suspected of being 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 70% by weight (based on polymer) was injected from the injection port 10 near the resin supply port. Due to injection, the capacity of the extruder was greatly reduced, and vent-up occurred frequently, making steady extrusion impossible. It was also found that the effect of removing unreacted monomers and solvents was inferior to that of Examples.

Example,! -, A Comparative Examples 6 and 7 Impact-resistant polystyrene resin (
The method of the present invention is applied to HIPS). It contains 230'lppm (relative to polymer) of this reaction monomer and solvent.

This HI PS is heated and melted to 2'lθ°C and supplied to the apparatus shown in FIG. HIPS in the same manner as Example/
The second result is when water is injected into the tank, mixed with a static mixing element, and then extruded with a vented extruder.
Shown in the table. Here, water was injected in an amount of 10% by weight based on the resin, and exhaust at the vent was carried out in the atmosphere and under reduced pressure. As a comparative example, the results when HIPS was extruded without injecting water are also shown in Table 2.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the apparatus used to carry out the method of the invention. /: Relay tank 2: Gear pump 3: Element q having a passage with stirring ability: Vented extruder S: Extruder vent hole B: Vacuum pump 7: Water inlet (for the present invention) g: Plunger pump q: Resin strand 10: Water inlet (for comparison) Patent applicant Mitsui Toatsu Chemical Co., Ltd.

Claims (1)

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