EP0000572B1 - Process for preparing expandible polystyrene beads - Google Patents

Description

The invention relates to a process for the preparation of spherical, expandable styrene polymers with a narrow bead size distribution by polymerizing styrene which is absorbed in particulate styrene polymers.

Spherical, expandable styrene polymers are usually prepared by polymerizing styrene in aqueous suspension in the presence of a blowing agent. This creates a pearl size distribution that corresponds to a Gaussian distribution. From this spectrum of pearls, only selected fractions can usually be sold on the market, while others cannot be used directly for processing reasons. This is especially true for those particles whose size is less than about 0.4 to 0.5 mm. Attempts are being made to feed these inevitable fractions back into suspension polymerization by redissolving them in monomeric styrene. However, this generally results in products of poor quality.

Expandable styrene polymers can also be melted in an extruder under pressure at elevated temperatures, pressed out through nozzles and the resulting polystyrene strands can be cooled in a water bath and converted into cylindrical expandable granules of uniform size in a granulator. These extruder granulate particles can then be converted into an essentially uniform, spherical bead material in aqueous suspension at temperatures above their softening point and elevated pressures (DT-OS 25 34 833). However, one has to work at relatively high temperatures of around 125 ° C., which means that the styrene polymer particles obtained have a relatively high internal water content.

DT-OS 23 38 132 describes a process in which selected sieve fractions are suspended in aqueous phase before styrene polymers, monomeric styrene is added dropwise to this suspension at elevated temperature with stirring and this is polymerized in the presence of an organic peroxide and a polymerization retarder. With this time-consuming procedure it is achieved that the added styrene is absorbed in the polymer particles and polymerized there, whereby, depending on the proportions, differently enlarged particles with an essentially uniform size are obtained. The task of the polymerization retarder here is to suppress the polymerization of the monomeric styrene in favor of diffusion into the polymeric particles. If the polymerization were carried out without a polymerization retarder, then besides the desired uniformly large styrene polymer beads, fine powdered polystyrene would be obtained.

The object of the invention was to develop a simple process for producing spherical expandable styrene polymers of essentially uniform particle size. It should be assumed that particulate styrene polymers are enlarged by polymerizing monomeric styrene, the shape of the particles being converted into a spherical shape, provided that it is not spherical from the outset.

It has now been found that this object is achieved if the particulate styrene polymers in aqueous suspension are first impregnated with styrene at temperatures below 100 ° C. and then polymerized at elevated temperature in the presence of organic peroxides.

• A. treibmittelhaltiges Styrolpolymerisat einer Teilchengröße zwischen 0,1 und 5 mm wird in wäßriger Suspension bei Temperaturen H: 0 und 75°C 0,5 bis 5 Stunden lang mit 10 bis 100 Gew.% (bezogen auf das treibmittelhaltige Polymerisat) Styrol imprägniert,
• B. das in den Styrolpolymerisat-Teilchen absorbierte Styrol wird anschließend in der wäßrigen Suspension bei Temperaturen zwischen 90 und 140°C in Gegenwart von 0,01 bis 1 Gew.% (bezogen auf das Styrol) organischem Peroxid polymerisiert.

The invention accordingly relates to a process for the preparation of spherical, expandable styrene polymers by polymerizing stryol in the presence of particulate, blowing agent-containing styrene polymer in aqueous suspension with the following process steps:

• A. blowing agent-containing styrene polymer having a particle size between 0.1 and 5 mm is impregnated in aqueous suspension at temperatures H: 0 and 75 ° C. for 0.5 to 5 hours with 10 to 100% by weight (based on the blowing agent-containing polymer) of styrene,
• B. the styrene absorbed in the styrene polymer particles is then polymerized in the aqueous suspension at temperatures between 90 and 140 ° C in the presence of 0.01 to 1 wt.% (Based on the styrene) of organic peroxide.

Styrene polymers are to be understood as homopolymers or copolymers of styrene which can contain up to 50% by weight of comonomers in copolymerized form. Possible comonomers are: a-methylstyrene, ring-halogenated or ring-alkylated styrenes, acrylonitrile, methacrylonitrile, esters of acrylic and methacrylic acid with alcohols with 1 to 8 carbon atoms, maleic anhydride or even small amounts of compounds which contain two polymerizable double bonds, such as butadiene , Divinylbenzene or butanediol acrylate.

The styrene polymers can contain conventional additives, such as e.g. Dyes, especially lightfast pigment dyes, also plasticizers, stabilizers, other plastics, e.g. Polyethylene or polyisobutylene, fillers, cell regulators or flame retardants.

The styrene polymers used according to the invention contain, in homogeneous distribution, blowing agents or a blowing agent combination which the styrene polymer does not dissolve and whose boiling point is below the softening point of the corresponding styrene polymer, for example low-boiling aliphatic hydrocarbons, CnH2.1 + ₂ (with n = 3 to 6), cycloaliphatics or halogenated aliphatic hydrocarbons, preferably in amounts of 2 to 12% by weight on the polymer.

The particle size of the styrene polymer used can vary within wide limits between 0.1 and 5 mm. You can e.g. start from unsaleable or hard-to-sell fractions from a styrene suspension polymer with an average particle diameter between 0.2 and 0.5 mm. They are expediently separated into partial fractions with a substantially uniform particle size by sieving. You can also use regrind that was produced by grinding edge fractions with too large a particle diameter. This regrind is also expediently broken down into partial fractions with a uniform particle size by sieving. Finally, extruder granules can be used which were obtained from edge fractions by melting, extruding through perforated nozzles and granulating. Due to the method of manufacture, these granules have essentially uniform particle sizes. Cylindrical particles with a diameter between 0.2 and 2 mm and a length between 0.3 and 3 mm are preferably used here.

The blowing agent-containing styrene polymer particles are suspended in water, preferably with the addition of an inorganic or organic suspension stabilizer, with stirring. Stabilizers include in question: water-soluble or water-dispersible homo- or copolymers of vinyl pyrrolidone and acrylic acid, polyvinyl alcohols and cellulose ethers; also poorly soluble alkaline earth metal phosphates, carbonates or sulfates, optionally together with emulsifiers or surfactants. They are preferably used in amounts of 0.01 to 4% by weight, based on the styrene polymer.

10 to 100, preferably 50 to 90% by weight (based on the blowing agent-containing styrene polymer) of monomeric styrene are added to this suspension. The styrene can again be replaced by up to 50% of its weight by the above-mentioned ganant comonomers. The suspension is then kept at temperatures between 0 and 75 ° C. and in particular between 20 and 50 ° C. for 0.5 to 5 hours, preferably 1 to 3 hours. The styrene should not yet polymerize, even in the presence of conventional organic peroxides. However, it diffuses into the styrene polymer, so that it is impregnated with monomers. The diffusion with monomers into the polymer particles is particularly complete and uniform if the impregnation takes place under pressure up to 20 bar, preferably between 3 and 10 bar, and if the suspension, e.g. by stirring vigorously.

Following the impregnation step A, the absorbed styrene is polymerized in a polymerization step B by increasing the temperature to 90 to 140 ° C., preferably to 100 to 125 ° C. This polymerization is advantageously carried out directly in the suspension obtained in impregnation step A 'in the presence of the same suspension stabilizers and under the same pressure, likewise with stirring. The polymerization is triggered by organic peroxides in amounts of 0.01 to 1% by weight (based on the monomers). These peroxides can either only be added to the suspension in the polymerization stage, but they are preferably already present in the suspension during the impregnation step. They may already have been contained in the styrene polymer from the point of polymerization, or they may have been added to it during extrusion. However, it is also possible to add the peroxides, dissolved in styrene, to the suspension during the impregnation step.

In the course of the polymerization step, the volume of the styrene polymer particles increases in accordance with the ratio of polymer to monomer, and cylindrical or otherwise irregularly shaped particles are converted into spherical ones.

If in the case of round particles the marginal fractions are too small, the process according to the invention makes it easy to enlarge the particles in such a way that they can be used in practice for the production of foam moldings. You can also repeat process steps A + B two or more times if you want to achieve a correspondingly large increase in particle size.

When converting cylindrical extruder granules into spherical beads, the process according to the invention has the additional advantage that the treatment temperature can be reduced by about 10 to 15 ° C. because of the softening effect of the absorbed monomers on the polymer. As a result, the internal water content of the styrene polymer beads drops and they no longer need to be dried as much or not at all. In addition, it is possible to introduce soluble auxiliaries, such as amines or bromine compounds, into the styrene polymer in a targeted manner, which enables simple regulation of the cell structure of the foams subsequently produced.

The expandable styrene polymers produced according to the invention can be converted into foams in a customary manner by treatment with hot gases with the liberation of the blowing agents.

The parts and percentages given in the examples relate to the weight.

EXAMPLE 1

17 liters of water are placed in a closed 40 liter stirred pressure vessel. Then 110 g of trisodium phosphate and 13 kg of a spherical polystyrene, which contains 6.2% pentane as blowing agent, and have an average bead diameter of 0.5 mm are filled. The suspension is mixed with stirring with 330 ml of a 35% calcium chloride solution, then 15 ml of an arylalkyl sulfonate as an emulsifying aid and 680 ml of a 10% aqueous solution of polyvinylpyrrolidone are introduced. 3.25 kg of styrene, which contains 0.36% of tert-butyl perbenzoate, are added to this aqueous solution. After 1 bar of nitrogen has been injected, the mixture is stirred at 20 ° C. for 1 hour, then the temperature is raised to 90 ° C. in the course of 3 hours and to 115 ° C. for a further 2.5 hours, and is left there for 6 hours. After cooling, beads with an average diameter of 0.54 mm are obtained without fine powdery polystyrene being formed on the side. The beads are separated from the aqueous phase, washed, dried and surface-coated with 0.04% zinc stearate. The product tends to stick when pre-foaming with water vapor. When foaming in a molding machine at a pressure of 1.0 bar, minimum mold dwell times of 200 seconds are achieved.

EXAMPLE 2

Example 1 is repeated, but the expandable polystyrene beads are colored dark blue with a pigment. A product with an even color distribution in the pearl is obtained. White, uncolored polystyrene particles that would result from separate polymerization of the styrene are not formed.

EXAMPLE 3

17 liters of water in which 110 g of trisodium phosphate are dissolved are placed in a 40 liter stirred pressure vessel. For this purpose, 330 ml of a 35% calcium chloride solution are added dropwise within 15 minutes. Then 10.8 kg of a 6%. Filled with pentane polystyrene with an average pearl diameter of 0.2 mm. 5.42 kg of styrene, in which 19.5 g of tert-butyl perbenzoate are dissolved, are then added. After stirring for one hour at 20 ° C., 1.5 ml of emulsifier and 680 ml of a 10% polyvinylpyrrolidone solution are added. The mixture is then heated to 105 ° C. within 5 hours with stirring. At this temperature, 1,350 ml of pentane are pressed into the reaction mixture using nitrogen. The mixture is then heated to 115 ° C. and left at this temperature for a further 4 hours. After cooling, the beads obtained are worked up with an average diameter of 0.22 mm as described in Example 1. Foaming pearls with a very low bulk density of 10 g / l are obtained by pre-foaming with steam for 6 minutes.

EXAMPLE 4

Example 1 is repeated, using 100% monomeric styrene, based on expandable polystyrene, in which 0.36% of tert-butyl perbenzoate is dissolved. 1.5 l of pentane are also added. The correspondingly processed end product has a blowing agent content of 6.8% and an average pearl diameter of 0.6 mm. The very fine-celled material with 12 cells / mm shows the same foaming behavior as described in Example 1.

EXAMPLE 5

In the apparatus described in Example 1, 17 l of water are filled in, in which 110 g of trisodium phosphate are dissolved. 6.5 kg of a polystyrene granulate with a length of 0.5 mm and a diameter of 0.8 mm, which contains 5% pentane, are added. Then 330 ml of a 35% calcium chloride solution are added, then 15 ml of an emulsifier and 680 ml of a 10% aqueous solution of polyvinylpyrrolidone and finally 6.5 kg of styrene in which 0.36% of tert-butyl perbenzoate are dissolved. The mixture is stirred at 20 ° C. for 2 hours, pressurized with 5 bar of nitrogen, then the mixture is heated uniformly to 100 ° C. within 3 hours, after which 700 g of pentane are pressed in with nitrogen. The mixture is then heated to 115 ° C. within 2 hours. This temperature is maintained for 6 hours. The round beads obtained after working up have an average diameter of 1.0 mm with a narrow distribution width. The product coated with 0.04% zinc stearate produces foam beads with a bulk density of less than 12 g / l and a cell count of 10 to 12 cells / mm when pre-foaming.

EXAMPLE 6

Example 5 is repeated, but using blowing agent-containing polystyrene extruder granules to which 1% hexabromocyclododecane and 1,000 ppm n-hexylamine have been added during extrusion. The result is a pearl-shaped material with an average diameter of 1.0 mm and the distribution breadth remains narrow. When pre-foaming, a coarse-celled product with 6 to 8 cells / mm is obtained.

Claims (3)

1. A process for the production of spherical, expandable styrene polymers by polymerization of styrene in the presence of particulate styrene polymer, containing expanding agent, in aqueous suspension, characterized by the following process steps:

A. styrene polymer containing expanding agent and having a particle size between 0.1 and 5 mm is impregnated with 10 to 100% by weight (based on the polymer containing expanding agent) for 0.5 to 5 hours in aqueous suspension at temperatures between 0 and 75°C,

B. the styrene absorbed in the styrene polymer particles is then polymerized in the aqueous suspension at temperatures between 90 and 140°C in the presence of 0.01 to 1% by weight (based on the styrene) of an organic peroxide.

2. A process for the production of styrene polymers according to claim 1, characterized in that in step A spherical styrene polymer particles having a mean diameter between 0.2 and 0.5 mm are used, the diameter being increased in stage B as a result of the styrene polymerizing.

3. A process of the production of styrene polymers according to claim 1, characterized in that in step A cylindrical styrene polymer particles having a diameter between 0.2 and 2 mm and a length between 0.3 and 3 mm are used, the cylindrical shape being transformed into a spherical shape with an increase in volume as a result of the styrene polymerizing.