WO1998022526A1

WO1998022526A1 - Method for drying water-containing styrol polymerizates

Info

Publication number: WO1998022526A1
Application number: PCT/EP1997/006116
Authority: WO
Inventors: Dietrich Scherzer; Klaus Hahn; Karl-Heinz Batscheider; Michael Witt; Bernhard Wagner; Wolfram Husemann
Original assignee: Basf Aktiengesellschaft
Priority date: 1996-11-18
Filing date: 1997-11-05
Publication date: 1998-05-28
Also published as: DE19647599A1; AU5318098A

Abstract

The invention relates to a method for drying water-containing styrol polymerizates, in which the particles are treated with an aqueous salt solution.

Description

Process for drying water-containing styrene polymers

description

The invention relates to a method for drying water-containing styrene polymer particles, in particular expandable styrene polymer particles.

Foams based on styrene polymers have been known for a long time and have been described many times in the literature. Of particular importance here are the so-called polystyrene particle foams, which are produced by foaming polystyrene particles containing blowing agents and then welding these foamed particles into blocks, moldings and other foamed objects.

The expandable polystyrene particles containing blowing agents are usually produced by suspension polymerization. The styrene, which may also contain other olefinically unsaturated comonomers or polymers which are soluble in styrene, is suspended and polymerized in an aqueous solution. The blowing agent is usually added in the course of the polymerization, but it is also possible to add the blowing agent to the polystyrene particles in a subsequent process step.

Naturally, polystyrene particles produced in this way, but also plastic particles produced by other processes, contain more or less water, which can be both dissolved and in the form of very fine droplets in the polymer. Especially in the case of expandable styrene polymers which have been prepared by dissolving polystyrene recyclate in monomeric styrene and polymerizing this solution, the internal water content is very high, often in excess of 5% by weight.

The presence of this "inner water" is very unfavorable in any case, since it cannot be removed with quick-acting drying methods such as centrifuging.

Polystyrene particles that have only dried superficially become moist again on the surface due to the migration of water from the inside, which considerably impairs the free-flowing properties required for perfect processing. In addition, such moist particles have a corrosive effect on numerous metals in packaging and machines. The further processing of polystyrene particles containing water, including for example to foam or injection molded parts, is adversely affected by the internal water. For these reasons in particular, it is a technical requirement to dry the particles to such an extent that they no longer contain any significant amounts of water.

Plastic particles, including expandable polystyrene particles, are usually dried using warm air to remove the internal water. Warm air and reduced pressure are often used in combination. In order to prevent undesirable expansion of polystyrene beads containing blowing agent during drying, the air temperature during drying must not be higher than 70 ° C. Nevertheless, the expandable polystyrene beads deteriorate even at lower temperatures because the water also removes blowing agents. This reduction in the blowing agent content also has an effect on the foam structure and thus on the processing properties. In addition, the pentane usually used as a blowing agent has to be processed by complex processes, e.g. Adsorption on activated carbon, from which the dryer exhaust air is removed.

DE-A-16 04 339 describes a process for drying expandable polystyrene particles with air at temperatures of 10 to 35 ° C. At these low temperatures, foaming of the beads is prevented and the loss of blowing agent is minimized, but the drying time required is relatively long. Drying times of more than 12 hours are not uncommon, especially with high internal water contents, such as occur in the manufacture of polystyrene beads containing recycled material. This severely limits the capacity of the production facilities.

US Pat. No. 4,303,783 describes a method for producing clear, transparent polystyrene beads for reflector applications. The beads produced by polymerization in aqueous suspension are only dried by evaporating the water (conventionally), but small, air-filled cells form on the surface of the beads, which apparently cause opacity. Then the polystyrene beads are treated according to the requirements above their glass temperature at 110 to 130 ° C. for 2 to 20 minutes with a salt solution, the air cells disappearing due to the melting of the polystyrene without water being able to diffuse in.

The object of the invention was to develop a process for drying styrene polymer particles, in particular expandable styrene polymers, which reduces the internal water content of the particles in a short time, without it in the case of blowing medium-containing polystyrene particles leads to loss of blowing agent or foaming of the particles.

The object of the invention was surprisingly achieved in that the styrene polymer particles were treated at from 20 to 105 ° C. with an aqueous salt solution and then separated from the salt solution.

Expandable polystyrene particles containing blowing agent, which were produced by the suspension polymerization process, can be dried particularly advantageously by the process according to the invention.

In the case of polystyrene particles, treatment with the brine is expediently carried out immediately after the suspension polymerization after the particles have been separated from the suspension medium.

The concentration of the brine should be between 10% by weight and the saturation concentration and in particular 15 to 30% by weight. If the concentration is below 10% by weight, the time for drying is too long. If the concentration is too high, problems due to salt excretion can occur. With increasing salt concentration and at a higher temperature, the pearls dry faster.

Is preferably carried out at temperatures between 25 and 100 ° C, in particular between 30 and 60 ° C. At temperatures above the glass transition temperature of the polymer, with blowing agent-containing polystyrene above about 70 ° C, the particles soften and there is a risk of them converging. It is therefore expedient in this case to add suspension stabilizers, for example calcium phosphate, polyvinylpyrrolidone, polyvinyl alcohol or methyl cellulose in amounts of 0.05 to 0.5% by weight, based on the polymer. The optimal duration of drying depends on the internal water content of the polystyrene particles, the concentration of brine and the drying temperature. It can easily be determined by preliminary tests. The contact between polystyrene particles and brine should last longer than 1 min. With an internal water content of 1 to 2% by weight, a salt concentration of 15 to 30% by weight and a treatment temperature between 30 and 60 ° C., the preferred treatment time is 30 to 600 minutes. To carry out the method according to the invention, it is essential that the surface of the water-containing plastic particles is at least temporarily completely covered by the salt solution.

This can be done in different ways. This allows the particles to enter the vessels with the saline solution. It is necessary to keep the polymer particles and the brine in motion. This variant of the drying process according to the invention is therefore expediently carried out discontinuously in stirred tanks. Especially with styrene polymers containing blowing agents and when working at elevated temperatures above 50 ° C, it is necessary to operate at elevated pressure, i.e. to work in closed vessels. The pressure can rise to 20 bar or even higher.

However, it is also possible to spray the particles with the salt solution one or more times. This variant is particularly advantageous if the particles are present as a solid bed, for example in a vertical tube, and the salt solution is continuously sprayed on from above. Due to the action of gravity, the salt solution seeps downwards while the particles are wetted. The salt solution can be collected at the bottom and, if necessary after concentration, used again to spray the particles. This method can also be carried out in such a way that only so much salt solution is removed that the air between the particles is completely displaced and the particles are thus completely covered in front of the salt solution. Here, the particles and the salt solution can also be led to one another in cocurrent or countercurrent. This guidance of pearls and salt solution in cocurrent or countercurrent can take place both in vertical and in horizontal drying apparatus, for example pipes, but boiler cascades are also conceivable.

In principle, all aqueous salt solutions can be used as brine. Salts of inorganic acids, in particular chlorides, preferably alkali chlorides, are particularly advantageous. Due to the good availability, sodium chloride solution is expediently used.

After drying, the plastic particles can be freed of the brine in a known manner, for example by means of screen spinning, and can be worked up normally. The brine can be reused. It is only necessary to concentrate them if necessary. Particular advantages of the present invention Trocknungsver ^¬ drive as set forth in blowing agent-containing expandable styrene polymer, usually an internal water content of between 1 and 2 wt - have.%. Expandable polystyrene particles produced in particular using recycled polystyrene, which mostly have very high internal water contents, can be dried gently and advantageously using the process according to the invention.

The polystyrene particles dried by the process according to the invention are produced by the known suspension polymerization process. For this purpose, the monomers used, mostly styrene, optionally in a mixture with other olefinic comonomers and / or with styrene-soluble polymers, are converted into an aqueous emulsion and thus polymerized. As

The usual systems, for example molecular colloids or so-called Pickering salts, for example magnesium pyrophosphate, can be used as suspension ^stabilizers . The blowing agent is usually added in the course of the polymerization. However, it is also possible to polymerize without addition of blowing agent and, if necessary, to add the blowing agent in a later process step. Following the polymerization, the polymer particles are separated from the suspension medium in a customary manner, for example by means of a centrifugal screen, and dried without further pretreatment by the process according to the invention.

After drying, the polystyrene particles are separated from the salt solution, optionally washed with water and, as usual, worked up by surface drying, separation into sieve fraction and surface coating.

Styrene polymers containing blowing agents can be processed directly into foams. Blowing agent-free polymers can be processed into moldings, for example in the injection molding process, or converted into expandable styrene polymers by impregnation with blowing agents by customary processes.

To produce polystyrene particle foams and foam moldings, the expandable polystyrene particles are pre-foamed in the usual way using steam and foamed in non-gas-tight molds and welded into molded parts. Further information on the raw materials used and the manufacturing process can be found, for example, in the "Plastic Handbook", Volume 5 "Polystyrene", edited by R. Vieweg and G. Daumiller, Carl Hanser Verlag, Munich, 1969.

With the method according to the invention it is possible to reduce the internal water ^content of particulate, in particular of styrene polymers produced by means of suspension polymerization, in an elegant manner to values below 0.7, preferably below 0.4% by weight. The process according to the invention can be used advantageously, in particular in the case of expandable styrene polymers, since there are no blowing agent losses and no foaming of the particles. In addition, the otherwise customary storage of the beads for several days for the purpose of conditioning can be omitted.

Particular advantages result from the drying of styrene polymers which have been prepared by dissolving up to 60% by weight, preferably 10 to 50% by weight, of recycled polystyrene in monomeric styrene and subsequent suspension polymerization in the aqueous phase. Such polymers usually have very high internal water contents, some of which are 5% by weight and above. Drying such products by conventional drying processes is so time-consuming that there are limitations in production capacity. On the other hand, with such a lengthy drying process, blowing agent losses and the associated loss of quality in the processing properties cannot be avoided.

These disadvantages could be completely overcome.

The invention is illustrated by the following examples:

Examples 1 to 8

20 l of salt solution were placed in a closed, heatable stirred tank under a nitrogen pressure of 2 bar. 10 kg of polystyrene beads, prepared by suspension polymerization of a solution of 15% by weight polystyrene recyclate in monomeric styrene, were added, the container closed and heated to the drying temperature with stirring and kept at this temperature.

The container was then cooled to 30 ° C. and the polystyrene beads were separated off by sieving. The saline solution was then restored by adding an appropriate amount of saline brought to the initial concentration and used to dry the next batch of polystyrene beads.

The salt concentrations, drying temperatures and drying times as well as the water content of the polystyrene beads are given in the table. Examples 1 to 6 used as polystyrene beads were those with a recycled content of 15%, based on the styrene polymer, and an internal water content of 6.5% by weight.

.%, And a pentane content of 6.1 wt - - In Examples 7 and 8, a freshly prepared without expandable polystyrene recyclate (EPS ^® F 14 from BASF AG) having an internal water content of 1.4 weight was.% Is used.

Table 1

nb - not determined *% by weight

** based on the weight of the polystyrene beads

K ⁺ means table salt

M ⁺⁺ means magnesium sulfate

Examples 9 to 11 (comparison)

The Styropor ^® F 14 used in Examples 7 and 8 was dried with air. The results are shown in Table 2.

Table 2

Examples 12 and 13

In a 2 liter glass tube with an inner diameter of 10 cm, the underside of which was provided with a glass frit, 1 kg of freshly produced, centrifugally moist Styrofoam F14 from BASF AG with an internal water content of approximately 1.0% by weight was placed. About a 50 to 40 ° C thermostatted metering vessel was a 30 wt. -% aqueous sodium chloride solution at a constant flow rate of 40 ml / min from above onto the bead bed.

After the times given in the table, a sample of EPS beads was taken, washed with deionized water, the surface water removed and the internal water content determined by the Karl Fischer method.

The results are summarized in the table.

table

* based on the polystyrene beads

Claims

claims

1. A method for drying water-containing styrene polymer particles, characterized in that the particles are treated with an aqueous salt solution at a temperature between 20 and 105 ° C and then separated from the salt solution.

2. The method according to claim 1, characterized in that blowing agent - medium-containing styrene polymer particles are dried.

3. The method according to claim 1, characterized in that it is carried out discontinuously in a stirred vessel, optionally under increased pressure.

4. The method according to claim 1, characterized in that it is carried out in a vessel continuously flowed through by the salt solution.

5. The method according to claim 1, characterized in that the aqueous salt solution at least 10 wt. -% is.

6. The method according to claim 1, characterized in that the aqueous salt solution is the solution of an inorganic salt.

7. The method according to claim 1, characterized in that the aqueous salt solution is sodium chloride solution.

8. The method according to claim 1, characterized in that the

Drying is carried out at a temperature of 25 to 100 ° C.