Monovinylidene aromatic polymers, such as polystyrene, can be produced by continuous bulk polymerization processes which polymerize styrene monomer to approximately 80 percent conversion, leaving residual monomer in the polymer produced. The residual monomer and other volatiles are substantially removed by devolatilizing the polymer under vacuum at high temperature. However, some residual monomer remains after devolatilization, which can lead to taste and odor problems in some applications. To overcome this problem, steam stripping has been traditionally used to further lower the amount of residual monomer within the monovinylidene aromatic polymer. For example, US-A-5,468,429 discloses a flash and extrusion process utilizing acoustic treatments to increase devolatilization efficiency. Additionally, US-A-5,380,822 and US-A-5,350,813 disclose processes of stripping by injecting water or an organic fluid into the melt. These processes are costly to operate due to the condensation, separation and recycling of the stripping agent.

JP-H4-72331 by Washio et al. discloses a method of irradiating powder particles with between 2.5 and 30 Mrad of electrons to reduce the level of unreacted monomer.

However, e-beam irradiation increases the yellowness of the polymer.

Therefore, there remains a need to develop a process for further reducing residual monomer in monovinylidene aromatic polymers which is more efficient and cost effective which does not significantly alter the color of the polymer.

The present invention is a process for reducing the level of residual monomer in a monovinylidene aromatic polymer comprising exposing the monovinylidene aromatic polymer to e-beam irradiation at a strength of from 2 to 7 megarads (Mrad), at a temperature of from 50 to 125°C.

This process decreases the amount of unreacted monomer without significantly yellowing the polymer when compared to other e-beam processes at ambient temperatures.

Figure I is a graph depicting the effect of E-beam treatment of polystyrene at 25 and 85°C.

Figure II is a graph depicting the effect of E-beam temperature on the residual styrene monomer level.

Monovinylidene aromatic polymers suitable for the process of the present invention are those produced by polymerizing a vinyl aromatic monomer. Vinyl aromatic monomers include, but are not limited to those described in US-A-4,666,987, US-A-4,572,819 and US-A-4,585,825. Preferably, the monomer is of the formula: wherein R is hydrogen or methyl, Ar is an aromatic ring structure having from 1 to 3 aromatic rings with or without alkyl, halo, or haloalkyl substitution, wherein any alkyl group contains 1 to 6 carbon atoms and haloalkyl refers to a halo substituted alkyl group.

Preferably, Ar is phenyl or alkylphenyl, wherein alkylphenyl refers to an alkyl substituted phenyl group, with phenyl being most preferred. Typical vinyl aromatic monomers which can be used include: styrene, alpha-methylstyrene, all isomers of vinyl toluene, especially paravinyltoluene, all isomers of ethyl styrene, propyl styrene, vinyl biphenyl, vinyl naphthalene, and vinyl anthracene, and mixtures thereof. The vinyl aromatic monomers may also be combined with other copolymerizable monomers. Examples of such monomers include, but are not limited to acrylic monomers such as acrylonitrile, methacrylonitrile, methacrylic acid, methyl methacrylate, acrylic acid, and methyl acrylate; maleimide, phenylmaleimide, and maleic anhydride. In addition, the polymerization of the vinyl aromatic monomer may be conducted in the presence of predissolved elastomer to prepare impact modified, or grafted rubber containing products, examples of which are described in US-A-3,123,655, US-A-3,346,520, US-A-3,639,522, and US-A-4,409,369.

Polymerization processes and process conditions for the polymerization of vinyl aromatic monomers are well known in the art. Although any polymerization process can be used, typical processes are continuous bulk or solution polymerizations as described in US-A-2,727,884 and US-A-3,639,372.

Devolatilization following polymerization, typically reduces the monomer level in the polymer from 20 percent to less than 1000 ppm. The monovinylidene aromatic polymers produced, typically contain contaminant levels of from 500 to 1000 ppm of residual monomer. After devolatilization, the molten polymer is typically pumped through a die, extruded into strands and cut into granules. Irradiation is best carried out on the strands or granules, but can also be conducted on molded polymer as well.

The process of the present invention reduces the amount of residual monomer by irradiating the monovinylidene aromatic polymer with electron beam (e-beam) irradiation

at an elevated temperature. Surprisingly, this method increases the level of residual monomer reaction in the polymer and does not have the substantial yellowing affect as e-beam irradiation at room temperatures. Methods of e-beam irradiation of polymers are discussed in JP 04072331 by Washio et al.; US-A-4,585,808, US-A-4,537,734; EP- 018,058; ACS Polymer. Preprints. 28 (1), 301 (1987), and Radiation Physical Chemistry.

14,333 (1979).

The intensity of the e-beam irradiation is not particularly critical as similar results have been obtained using differing intensities. The intensity of the e-beam is typically from 0.5 to 1.5 millielectron volts (MeV), preferably from 0.6 to 1.4, more preferably from 0.6 to 1.2 and most preferably from 0.7 to 1.0 MeV.

Surprisingly, the level of monomer residuals is drastically reduced when the e-beam irradiation is between 2 and 7 Mrad at a temperature between 50 and 125°C.

The e-beam irradiation is typically conducted at a dosage of from 2, preferably from 2.5, more preferably from 3 and most preferably from 3.5 to 7, more preferably to 6, and most preferably to 5 Mrad. The temperature is typically from 50, preferably from 60, more preferably from 70 and most preferably from 75 to 125, generally to 120, typically to 115, preferably to 110, more preferably to 105, and most preferably to 100°C.

The amount of residual monomer left in the polymer after e-beam irradiation will depend upon the amount of residuals in the polymer prior to e-beam treatment. Upon irradiation, the amount of residual monomer in the monovinylidene aromatic polymer will typically be reduced by approximately 50 percent or more and contain less than 500 ppm residual monomer, preferably less than 400 ppm, more preferably less than 300 ppm and most preferably less than 200 ppm based on monovinylidene aromatic polymer.

The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted. Amounts are in weight parts per million (ppm) unless otherwise indicated.

EXAMPLE 1 STYRONTM 685D granules, having a weight average molecular weight (Mw) of 305,000 and initial residual styrene content of 500 ppm, are spread on a table and passed through an e-beam housed in a concrete vault. The beam intensity is 0.9 millielectron volts (MeV) and the atmosphere over the granules during the irradiation is ambient air. The temperature of the granules is controlled by use of an infrared heater placed above the table. TABLE I shows the results after irradiation.

Residual styrene is measured using head space gas chromatography which involves dissolving the polystyrene sample in orthodichlorobenzene in a sealed vial and heating to 80 °C. The vapors in the head space inside the vial are sampled and injected into a gas chromatograph. The amount of styrene in the headspace is measured and compared against polystyrene calibration standards to determine the level of residual styrene monomer in the polymer sample.

TABLE I 25 °C 85 °C Dose (Mrad) (ppm Stvrene) (ppm Stvrene) 0* 500 500 1* 410 350 2 380 270 3 290 120 4 230 97 5 190 120 6 150 120 7 150 120 8* 97 120 9* 81 130 in* 66 110

*Comparative Examples EXAMPLE2: STYRON 680, having a weight average molecular weight (Mw) of 190,000 and initial level of residual styrene 650 ppm of residual styrene, is injection molded into 0.8 cm. x 0.05 cm. plaques. The plaques are subjected to e-beam irradiation at an intensity of 1.25 MeV and dosages of 1-4 Mrad in ambient air inside a concrete vault. The temperature of the plaques are controlled by placing them on a hot plate. At temperatures greater than 100 °C (polystyrene softening point), the plaques are kept from flowing by containing them inside of a titanium ring around the perimeter of the discs. The residual styrene level in the treated discs are determined using head space gas chromatography.

TABLE II 1 Mrad* 2 Mrad 3 Mrad 4 Mrad Temp (°C) ppm styrene ppm styrene ppm stvrene ppm stvrene 25* 640 625 605 590 55 630 550 420 325 75 620 470 360 270 85 560 350 200 40 100 500 270 190 120 125 470 260 300 310 150* 450 310 400 410 175* 520 430 450 420 200* 470 460 390 430

*Comparative Examples The data in the TABLE II clearly shows that the optimum temperature range for treatment is between 50 and 125 °C.

EXAMPLE 3 Three grades of STYRONTM were irradiated as in Example 1 to reduce the residual styrene monomer content to a 100 ppm level (8 Mrad at 25°C and 4 Mrad at 85°C). Color measurements (Yellowness index (Yl)) are then determined using ASTM E308. Results are listed in TABLE 111.

TABLE ICI Yellowness Index STYRONTM Control* 25°C* 85°C 666-3.86 5. 28 1. 01 685-3.55 10. 32 3. 48 484-0.09 9. 27 3. 5 *Comparative Examples

The Yellowness index greatly increases when irradiated at 25°C. Irradiation at 85°C can obtain the same residual monomer level but with significantly better Yellowness index.