METHOD FOR PREPARING ANTISTATIC EXPANDABLE POLYSTYRENE

Abstract: Quaternary ammonium salts which act as antistatic agents for styrene polymers are coated on the expandable polymer particles in the form of an organosol. The hydrocarbon medium from the organosol is removed under vacuum and the coated polymer particles are molded.

Description: The field of the invention is expandable styrene polymer particles for the preparation of molded articles. The present invention is particularly concerned with coating the particulate, expandable styrene polymers with organosols containing quaternary ammonium salts and using the coated expandable styrene polymers in making molded articles. Expandable styrene polymers, like most plastics, are insulators with low surface conductivity. As a result, they pick up and hold static electricity for extended periods of time. An effective antistatic treatment for expandable styrene polymers must provide a webb of ionic and/or hydrophilic pathways through which a charge can be dissipated. These pathways are most effective as a network throughout the molded part rather than as a molded part surface coating, as in the dipping procedure. Antistatic agents do not readily adhere to the expandable bead surface in most coating procedures and are easily removed from the beads during the expansion and molding processes. Due to the sensitive nature of suspension stability in polymerization and impregnation, antistat addition during these steps is not practical. U.S. Patent 4,495,224 teaches to prepare expandable styrene polymers having improved molding characteristics by coating the polymer particles with glycerin esters of long chain fatty acids. The coating is made by forming organosols with pulverulent glycerin esters in aliphatic hydrocarbons. BRIEF SUMMARY OF THE INVENTION It has now been found that excellent antistatic properties can be obtained with expandable styrene polymer particles by coating the particles with an organosol made from glyceryl esters and quaternary ammonium salts dispersed in a normal or cyclic aliphatic hydrocarbon or blends thereof, with or without an aromatic component. The organosol gives uniform coating to the particles and subsequent molding give parts having antistatic composition both on the surface and throughout the molding due to the internal coating on the particles which produces a network effect for electrostatic charge dissipation. According to the present invention, there is provided a method for preparing expandable styrene polymer particles with an adherent coating of quaternary ammonium salt comprising:- (a) adding to said expandable styrene polymer particles in a closed mixing vessel, with mixing, an organosol of at least one quaternary ammonium salt and at least one long chain fatty acid glyceryl ester in a hydrocarbon component selected from normal and cyclic aliphatic hydrocarbons andd blends thereof, with or without an aromatic component; (b) removing said hydrocarbon component by reducing the pressure in said mixing vessel; and (c) separating said expandable styrene polymer particles coated with said quaternary ammonium salt. In accordance with this invention, a variety of expandable thermoplastic homopolymers and copolymers can be rendered antistatic. The polymers may be derived from vinyl aromatic monomers including styrene, isopropylstyrene, alpha-methylstyrene, nuclear methylstyrene, chlorostyrene, tertbutylstyrene, as well as copolymers prepared by the copolymerization of a vinyl aromatic monomer with monomers such as butadiene, alkyl methacrylates, alkyl acrylates, acrylonitrile, and maleic anhydride wherein the vinyl aromatic monomer is present in at least 50% by weight of the copolymer. For the purpose of convenience, these polymers and copolymers are referred to herein as "styrene polymers". Especially useful are polystyrene, and copolymers of styrene with 5-30 weight percent of acrylonitrile, maleic anhydride or methyl acid maleate. The styrene polymers may be in the form of beads, granules, or other particles convenient for the expansion and molding operations. Especially useful are the beads formed by the suspension polymerization of the vinyl aromatic monomers alone or in combination with a minor amount of copolymerizable monomer. The styrene polymers can, of course, be produced by any of the known procedures. The preferred method of preparation is the suspension technique, which gives bead or pellet shaped product. The styrene polymer particles may be made expandable by impregnating the styrene polymer particles with a suitable blowing agent. The beads can be recovered from a polymerization process and resuspended in water for impregnation or the impregnation can be performed directly on the polymerized beads without separation. To render the polymer particles expandable, the blowing agent may be incorporated into the particles, as for example in U.S. Patent No. 2,983,692, by suspending the particles in water in a ratio of between 0.3 to 1 and 1.5 to 1 (polymer-to-water) with the aid of suspending agent systems such as tricalcium phosphate in combination with a surfactant to promote wetting. During the impregnation of the particles with blowing agent, other additives can also be incorporated, such as internal fast-cool agents, pigments and dyes, stabilizers, anti-lump agents, self-extinguishing agents, plasticizers, and the like. The impregnation is conventionally carried out at temperatures ranging from about 80° to 150°C. Increasing the temperature makes the impregnation proceed at a faster rate. The blowing agents are compounds which are gases or which form gases on heating. Preferred blowing agents include aliphatic hydrocarbons containing from 4-6 carbon atoms in the molecule, such as butanes, pentanes, hexanes, and the halogenated hydrocarbons which boil at a temperature below the softening point of the polymer. Mixtures of these agents may also be used, such as a mixture of 40-60% n-pentane and 60-40% trichlorofluoromethane. Usually from 3-20% of blowing agent per 100 parts of polymer is incorporated by the impregnation. The particular quaternary ammonium salt useful in the invention is not critical. The salt must be dispersible or soluble in the organosol paste and must be an antistat for the polymer to which it is applied. Examples of suitable salts are methyl tricaprylyl ammonium chloride (sold as Aliquat 336 by Henkel Corp.), dicoco alkyl dimethyl ammonium chlorides (sold as Arquad 2C-75 by Armak Chemicals), soya dimethyl ethyl ammonium ethylsulfate (sold as Larostat 264A by Jordan Chemical Co.), coco alkylethyl dimethyl ammonium ethylsulfate (sold as Larostat 377DPG by Jordan Chemical Co.), trialkyl alkylalkoxylated ammonium salts, and dialkyl bisalkoxylated ammonium salts (these latter two salts made and claimed in U.S. patents 4,622,345 and 4,628,068). The organosols may be made by starting with a mixture of (a) normal or cyclic aliphatic hydrocarbon or blends thereof, with or without an aromatic component, and (b) glyceryl esters of long chain fatty acids, preferably in an amount of 10% to 20% based on total organosol, and then adding quaternary ammonium salt, preferably in an amount of 2-40% based on total organosol. It is equally possible to use a commercially available mixture of the mono-, di-, and tri-fatty acid esters of glycerin e.g. in amounts to give 10-20% by weight of the glyceryl esters based on total organosol. There is no particular order of addition of the ingredients which must be followed. Fatty acids having from 10 to 18 carbon atoms in their chain have been found useful herein. All solid ingredients of the organosol should be small (particle size less than 100 µm) for ease of dispersion. The preferred hydrocarbon is pentane and may be any of the isomeric pentanes or mixtures of them, e.g. n- pentane, isopentane, cyclopentane, and mixtures thereof. The use of fatty acid soaps, such as zinc stearate, enhances the stability of the organosol and serves to plasticize the polymers and act as a flow agent. Fine silica can also be added to the organosol to act as a flow agent and help lubricate the beads during molding. The fine silica and/or the zinc stearate can also be dry blended onto the beads which have been coated by the organosol. In general, the flow agent should form 1000 to 4000 ppm of the organosol. The coating formed on the styrene polymer beads by this procedure is made up of long chain fatty acids in a volatile hydrocarbon base, which improves the adhesion of the antistatic agent to the beads. The following examples are meant to illustrate, but not limit the invention. All parts and percentages are by weight unless otherwise designated. EXAMPLE I To illustrate the range of quaternary ammonium compound which can be applied to the polymer beads a series of antistatic pastes were made up as follows: To 135 g of n-pentane in a bottle equipped with a magnetic stirrer was added 11.4 g of methyl tricaprylyl ammonium chloride and the mixture stirred until well dispersed. To the mixture was then added 7.5 g of glyceryl tristearate and 7.5 g of glyceryl monostearate and again stirred until well dispersed. The paste (10.75 g) was added to 250 g of polystyrene beads having a particle size of through 16 and on 35 mesh (1 mm to 420 µm) (U.S. Standard Sieve) and tumbled for 30 min to give beads having 3000 ppm of antistat. The beads were then tray dried for about 15 min. and then pre-expanded to a density of about 1.3 pcf (2,08·10⁻² g/cm³) in a Buccaneer pre-expander. The samples were then compression molded into samples for the ETS Static Decay Meter, and tested for rate of static decay of the sample. The control sample was prepared in the identical manner, but leaving out the quaternary compound. Other samples were prepared and treated using 3.8 g, 7.6 g, and 15.2 g of quaternary compound to give final polymer discs containing 1000, 2000. and 4000 ppm of antistat. The results of the testing are shown in Table I. It can be seen from the data that the antistats are effective from 1000 to 4000 ppm in the polymer. It should be noted, however, that at concentrations above 3000 ppm for this particular organosol pasted sample, a high amount of soft agglomerations occurred which could lead to impaired bead flow properties. EXAMPLE II To illustrate the use of different quaternary ammonium compounds, with and without a flow agent, various pastes were prepared as in Example I using the salt and concentration listed in Table II. Where flow agents were used, the flow agent was applied to the polymer beads by dry blending after application of the antistatic paste. Results are shown in Table II. EXAMPLE III To illustrate the adherence of the antistat to the surface of the particles 2200 ppm of coco alkylethyl dimethyl ammonium ethylsulfate was coated onto particles of polystyrene which had been impregnated prior to separation from the polymerization reactor by the method of this invention and by normal dryblend coating methods and then washed with water. The static results are shown in Table III. Similar results were obtained starting with beads prepared by separation from the polymerization reactor, screened and impregnated. It can be seen from the results that washing the beads coated by direct dryblending causes the antistat to disappear. On the other hand, washing the beads from the invention caused little or no decay of antistat properties.

Claims: A method for preparing expandable styrene polymer particles with an adherent coating of quaternary ammonium salt comprising:- (a) adding to said expandable styrene polymer particles in a closed mixing vessel, with mixing, an organosol of at least one quaternary ammonium salt and at least one long chain fatty acid glyceryl ester in a hydrocarbon component selected from normal and cyclic aliphatic hydrocarbons and blends thereof, with or without an aromatic component; (b) removing said hydrocarbon component by reducing the pressure in said mixing vessel; and (c) separating said expandable styrene polymer particles coated with said quaternary ammonium salt. The method of claim 1 wherein the organosol contains 2 to 40% by weight based on total organosol of quaternary ammonium salt, and 10% to 20% by weight of a mixture of mono-, di- and tri-fatty acid esters of glycerin, with the remainder being normal or cyclic aliphatic hydrocarbon or blend thereof, with or without an aromatic component. The method of claim 1 wherein the organosol contains 2 to 40% by weight based on total organosol of the quaternary ammonium salt, and 10% to 20% by weight of a mixture of mono-, di- and tri-stearates of glycerin, with the remainder being n-pentane, isopentane, cyclopentane, or a mixture thereof. The method of claim 1, claim 2 or claim 3 wherein the organosol also contains 1000 to 4000 ppm of a flow agent. The method of claim 4 wherein the flow agent is selected from zinc stearate and fine silica. The method of any one of claims 1 to 5 wherein the quaternary ammonium salt is selected from methyl tricaprylyl ammonium chloride, dicoco alkyl dimethyl ammonium chloride, soya dimethyl ethyl ammonium ethylsulfate, coco alkylethyl dimethyl ammonium ethylsulfate, isostearamidopropyl ethyl dimethyl ammonium ethylsulfate, trialkyl alkylalkoxylated ammonium salts, and dialkyl bisalkoxylated ammonium salts. Expandable styrene polymer particles having an adherent coating of one or more quaternary ammonium salts and one or more long chain fatty acid glycerides. The particles of claim 7 wherein said one or more long chain fatty acid glycerides are a mixture of mono-, di-, and tri-stearates of glycerin.

Inventors: STICKLEY, SUSAN GLESSNER
Applicants: ARCO CHEMICAL TECHNOLOGY, L.P.
Application year: 1988
Main classification: C08J9
IPC codes: C08K5, C08F12, C08J9