SE429238B - SET TO REDUCE MONOMER CONTENT IN EXPANDABLE TERMOPLASTES - Google Patents

Description

. 20. 25. 7806665-1 2 A serious problem in all polymerization themics is the residual monomer content.

Moiaoxnererrua are more or less toxic and since the polymerization can never be carried out to a 100% conversion, both the obtained polymer and the process water are contaminated by unreacted monomers. This problem is particularly accentuated in the polymerization of expandable beads, as these also contain a third phase, the propellant, in which monomers can be dissolved.

Large amounts of acrylonitrile in copolymers, such as polyvirxyliderzyl fluoride-alcrylahitrile and styrene-alaylrlitrile, drastically limit their hydrophilization areas and market, as free acrylic fitril in the process water and the remaining acrylonitriles on the part of the particles.

In recent years, a large number of methods have been proposed for the purification of polymers from residual monomer, so-called stripping, especially in polyvirzyl doride telmology.

According to the most common methods, elevated temperatures are used to cause diffusion of the monomers from the polymer phase. The elevated temperature increases the mobility of the monomene molecules and makes the polymer softer and both of these factors increase the stripping speed.

The most commonly used method today for removing vinyl chloride from polyvinyl chloride is to treat the resulting polymer and process water with steam at temperatures in the range of 80-12500 and at ambient pressure. In some methods, the stripping is carried out at reduced pressure. Such a method is described, for example, in West German patent application 25 21 780.

It is not possible to use these methods to remove monomers from expandable beads, as these are not sensitive to heat and pressure. At higher temperatures, as well as at lower pressures, the beads expand due to the propellant. Another problem is that monomers in expandable beads, and especially acrylonitrile, are more water soluble than vinyl chloride and thus more difficult to remove from the aqueous phase. In addition, some of the monomers commonly used to make expandable beads have a high boiling point, which makes it difficult to remove them from the aqueous phase by raising the temperature. Almylnitrile thus has a boiling point of 77 ° C, styrene 1145 ° C and v fi flyliaemdøria 32%, while vimnnoride has a boiling point of -1U ° c.

The high concentration of propellant inside the beads and the high temperature of the monomer stripping give a considerable pressure inside the beads and they expand in the unfilled or gas-filled part of the stripping apparatus. _ .. ._ ___.-......_..__ --_ - _ ~ -e 7806665-1 3 The pre-expanded beads are a problem and lead to increased production costs. The pre-expanded beads must be separated from the non-expanded beads by filtration or otherwise and the pre-expanded beads increase the amount of waste.

Attempts have been made to avoid pre-expansion of the beads by applying a high pressure during the stripping, eg 500-1500 kPa with an inert gas, eg nitrogen.

However, this does not eliminate the pre-expanded beads. The gas does not in fact give a higher external pressure to the beads. An equilibrium between the gas inside the beads and the gas in the "free volume" of the stripping apparatus is established. The beads continue to expand due to the "free volume" and the high hydrocarbon pressure inside the beads.

The present invention provides a stripping process for expandable beads, by which pre-expansion of the beads is significantly reduced. The process also reduces the monomer content in the process water.

According to the invention, the monomer content of expandable plastic beads prepared by polymerizing a ethylenically unsaturated monomer or a mixture of ethylenically unsaturated monomers in aqueous suspension in the presence of a propellant is reduced by heating the resulting slurry of expandable beads and water, in the presence of a water-soluble initiator, to a temperature in the range of 65-15000 in a vessel, which is completely filled with liquid medium under such a pressure that the liquid medium cannot flow out of the vessel, until a substantial part of the monomer content is reduced .

By using a vessel which is completely filled with liquid medium during the heat treatment of the expandable beads, the liquid applies a pressure to the surface of the beads, which prevents them from expanding during the stripping process. The residual monomers in the expandable beads are in the iron phase with the monomers in the aqueous phase. Then. The monons in the aqueous phase are eliminated by the water-soluble initiator, it is assumed that more monomer diffuses from the beads and the propellant into the aqueous phase and the process functions as a stripping process. The diffusion is relatively fast due to the relatively high temperature.

The vessel used is preferably the polymerization autoclave, but any suitable vessel can be used. By connecting, for example, the polymerization autoclave to an external liquid pressure device, such as a water line or an expansion vessel, which is partially filled with liquid medium and which is provided with suitable means for applying pressure to the water surface, it can be automated. 15. 20. 25. 30. 55- 7806665-1 The lalaven is easily filled during the heat treatment of the bead slurry. The vessel is suitably kept filled by adding water. However, any suitable liquid can be used such as, for example, the stripping is carried out in a vessel separate from the autoclave, the process water from the polymerization antoclave for the expandable beads. If a vessel other than the autoclave is used for the stripping process, it is of course within the scope of the invention to add additional liquid medium, eg water, to fill the vessel, or to subject part of the slurry rig of the beads and water stripping process obtained after the polymerization. .

The temperature range for the treatment of the slurry can be varied within wide limits and higher temperatures provide a more efficient evaporation. The temperature should exceed 6500, the upper limit is largely economically conditioned and it can be set at 15000. Good results are obtained when the temperature exceeds the glass transition temperature of the polymer in question. Suitably the range is in the range 70-1200 and preferably 75-9500. The rate of stripping also depends on the time that the slurry was subjected to the veneering process and at, for example, a temperature of 7500, a suitable time for alnr-yl] -litrile / polyalidenyl fluoride beads for up to 8 hours. At higher temperatures the time can be reduced and at an evaporation temperature of 95 ° C a time of one hour gives sufficient reduction of the monomer content in the beads and the process water.

The pressure required on the filled vessel is a function of both temperature and type of fuel and can be easily determined by simple tests. A fuel with a low boiling point of course requires a higher pressure, as the bead otherwise expands and the pressure in the vessel for a part of the liquid medium rises. to the expansion center. The pressure required for, for example, polystyrene beads with pentane as propellant is in the range 500-2000 kPa at temperatures of 75-9500, while at isobutane, which has a boiling point of -12 ° C, a pressure is required in the range 1000- 2500 kPa at the same temperatures.

Examples of suitable water-soluble initiators for ethylenically unsaturated monomers include well-known inorganic free radical catalysts such as hydrogen peroxide and potassium or ammonium acid persulfates, or organic free radical catalysts such as hydroperoxides, cyclohexanone peroxide or methyl isobutyl ketone. It is also within the scope of the invention to add a mixture of water-soluble initiators or a combination of water-soluble and monomer-soluble initiators such as peroxides, eg lauryl peroxide, peroxide / carbonates, eg cetyl peroxydilearic carbonate, or azo compounds. The amount of water-soluble initiator 10. 15. 20. 5 may be in the range of 0.01 to 5 percent, based on the amount of monomer charged, and is preferably in the range of 0.1 to 2 percent.

Suitable liquid volatile fuels are: petroleum urnets, pentane, isopentane, neo-pentane, hexane, heptane, cyclopentane, cyclohexane, isobutylene, n-butane and isobutane. The propellant can be used in known manner in amounts of 2 to 95, preferably 5 to H0% by weight of the monomers.

The process according to the invention is applicable to all ethylenically unsaturated monomers or mixtures of such monomers, which can form polymer beads containing propellant. Examples of such monomers are styrene, vinylides, chloride, acrylic esters, methacrylic esters, almyl nitrile and metal acrylonitrile.

The process is preferably used for expandable beads of styrene or styrene and up to HO% of its weight of copolymerizable ethylenically unsaturated monomers, especially copolymerization of styrene with up to 40% by weight of acrylonitrile, based on styrene, or copolymerization of vinylidene fluoride percentage and up to 140%. on vinylides fl chloride, acrylonitrile or vinyl chloride-id, especially copolymerization of 65-90% by weight of virylene chloride and 35-10% by weight of acrylonitrile. The beads are preferably prepared by copolymerization, a co-donor being alcrylrayitrile or methacrylonitrile.

The characteristic of the process is the use of a vessel which is filled with the reaction medium during the heat treatment process. It is thus obvious that all known polymerization recipes for the above monomers can be applied.

It is of course possible to keep the autoclave filled during the polymerization or the final part thereof by connecting the autoclave to an external pressure source or an expansion vessel, which is partially filled with water and which has flexible means for applying a pressure to the water surface. According to one embodiment of the invention, polymerization of the beads is stopped after 70 percent conversion, preferably 95 percent, after which the water-soluble initiator is charged, the autoclave is filled and the temperature is raised according to the invention during the final polymerization.

Example 1 (Iron-based tests) The polymerization test was carried out in a 13 l autoclave equipped with a stirrer and a jacket for heating and cooling the reaction medium. The autoclave was connected l0. 15. 25. 30. 7806665-1 .b, _ with an outer tank, which is also used for charging the monomer mixture to the reactor.

The following recipe was used for evaluation of the system: Vinylidene chloride / acrylonitrile in a ratio of 70:30 100.0 parts Fuel, pentane 10.0 parts Suspending needle, polyvinyl alcohol 1.0 parts Suspending agent, colloidal silicon diol type in 0.5 parts Initiator, lauryl peroacid 1.0 parts Water 1400.0 parts The water, 8.0 l, was charged together with the suspending agents to the autoclave and this was evacuated for 900 seconds. Vinylidene chloride, acrylonitrile, pentane and initiator (total 2.9 L) were charged to the autoclave from the outer tank, after which the connection between the autoclave and this was broken. The temperature was raised to 60 ° C, where it was kept below 12 degrees. under these conditions, the pressure rose to the 500 kPs. The autoclave was cycled from 6,000 to about 2,500. The resulting bead suspension contained unexpanded beads and the amount of waste collected on a DIN.60 sieve was about 15 grams per 1000 grams of monomer charged.

The amounts of residual monomer in the beads were: acrylonitrile 3200 mg / kg and vinylidene chloride 29000 mg / kg. The amount of acrylonitrile in the process water was 11110 mg / kg.

Example 2 (Comparative Experiment) The polymerization was carried out by the same recipe and according to the same procedure as in Example 1. However, 1.0 part of potassium persulfate in aqueous solution was charged to the autoclave after 12 hours of polymerization and the temperature was raised to 7500 and kept there for 1 hour. tilmnar. The amount of waste was more than 100 g per 1000 g of monomer charged.

The amount of residual monomers in the beads was: acrylyl lithrile 50 .mu.g / kg and vinylidene chloride 1500 Ing / kg and the amount of acrylonitrile in the process water was 30 mg / kg. As can be seen from this example, the monomer content in the beads and the process water was significantly reduced, while the amount of waste was unacceptably high.

Example 3 The polymerization was carried out with the same recipe and according to the same procedure as in 10. 15. 20. 25.

STAY. 7806665-1 Example 1. However, 1.0 part of potassium persulfate in aqueous solution was added to the autoclave after 12 hours of polymerization. The autoclave was connected to the outer tank and kept completely filled with water from the tank at a pressure of 850 kPa. The temperature was raised to 75 ° C and maintained there for 11 hours.

The amount of waste was 15 g per 1000 g of monomer charged. The amount of residual monomer in the beads was: acrylic 50 tril 50 mg / kg and vinylidene chloride 11150 ng / kg. The mean length of the process water was 25 mg / kg.

By keeping the autoclave completely filled with water according to the procedure, the monomer content in the beads and the process water could be considerably reduced and the amount of pre-expanded beads kept at an Example i! .

A series of experiments were performed with different monomer systems. The experiments were performed in a 13 l autoclave equipped with a stirrer and a jacket for cooling and heating the reaction medium. The autoclave was connected to an outer tank which was also used to charge monomer to the autoclave. The following recipe was used, where all lengths are given in parts by weight: Experiment I Experiment II Experiment III Styrene 100 - 90 Acrylonitrile - - 10 Methyl methacrylate - 100 - Suspension element, polyvinyl alcohol 1.0 1.0 1.0 Suspending agent, colloid of 0.5 0.5 0.5 silica type Fuel, pentane 10 10 10 Initiator, lauryl peroxide 1.0 1.0 1.0 Water 1100 400 400 Water, 8.0 l, was charged together with the suspension to the autoclave and this was evacuated for 900 seconds. . Monorner resp. monomer mixture, fuel and initiator (total 2.9 l) were charged to the autoclave from the outer tank, after which the connection between the tank and the autoclave was broken. The temperature was raised to 5500 and held there for 12 hours. Thereafter, the autoclave was cooled to about 2500 and the resulting bead suspension was examined for residual monomer content. The results are shown in the following table. 7806665-1 The experiment was repeated with the difference that after 12 hours of polymerization, 1.0 part of potassium persulfate solution was added to the autoclave. The autoclave was connected to the outer tank and kept completely filled with water from the tank at a pressure of 850 kPa. The temperature was raised to 75 ° C and kept there for 6 hours. The residual monomer content of the treated beads was then examined.

The result is shown in the table.

Residual monomer content Ing / kg polymer without reduction with reduction acc. Invention Experiment I 50 000 500 Experiment II 20,000 600 Experiment III Styrene 110,000 300 acrylonitrile 5,000 9

Claims (5)

Claims 1. Methods for reducing the content of expandable thermoplastic beads prepared by polymerizing an ethylenically unsaturated monomer or mixing mixtures in aqueous suspension in the presence of a propellant, characterized in that a reflux of the expandable beads and water, in the presence of a water-soluble initiator of ethylenically unsaturated monomers, is heated to a temperature in the range of 65-150 ° C in a vessel which is completely filled with liquid medium under such a pressure that it the liquid medium cannot flow out of the vessel until a substantial part of the monorene content has been reduced. 2. A method according to claim 1, characterized in that the vessel is a polymerization autoclave. 3. A method according to claim 1 or 2, characterized in that the temperature is in the range 70-12000. ll. 4. A method according to claims 1 to 3, characterized in that the liquid medium is water. 5. A method according to claim 5, characterized in that one of the monomers is acrylonitrile. SUMMARY The invention relates to a method of reducing the monomer content of expandable thermoplastic beads, which are prepared by polymerizing an ethylenically unsaturated monomer or a mixture of such monomers in aqueous suspension in the presence of a propellant. The residual monomers in both the beads and the process water are reduced by treating the slurry of beads and water obtained after the polymerization in a vessel at a temperature exceeding 6,500 in the presence of a water-soluble initiator. To avoid expansion of the beads, the vessel is kept completely filled with a liquid, preferably water, during the heat treatment. The process is preferably used for expandable beads of styrene, copolymer of styrene or copolymer of vinylidene chloride and acrylonitrile.