WO1999038894A1

WO1999038894A1 - A process for producing monovinylidene aromatic polymers having a reduced volatiles content

Info

Publication number: WO1999038894A1
Application number: PCT/US1998/026632
Authority: WO
Inventors: Duane B. Priddy
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1998-01-30
Filing date: 1998-12-15
Publication date: 1999-08-05
Anticipated expiration: 2000-07-30
Also published as: AU1634799A

Abstract

The present invention is a process for producing a monovinylidene aromatic polymer having a reduced volatiles content comprising: a) polymerizing a vinyl aromatic monomer in the presence of benzocyclobutene (BCB) solvent at a temperature less than 200 °C to form a mixture of monovinylidene aromatic polymer, vinyl aromatic monomer and benzocyclobutene solvent, b) devolatilizing the mixture at a temperature of at least 200 °C, but less than 230 °C to vaporize the monomer and benzocyclobutene, and c) maintaining or heating the mixture at a temperature of from 230 to 260 °C such that the residual benzocyclobutene reacts with the residual vinyl aromatic monomer to form a polymeric nonvolatile material.

Description

A PROCESS FOR PRODUCING MONOVINYLIDENE AROMATIC POLYMERS HAVING A REDUCED VOLATILES CONTENT BACKGROUND OF THE INVENTION

This invention relates to the production of monovinylidene aromatic polymers. This invention particularly relates to the production of monovinylidene aromatic polymers having a reduced volatiles content.

Monovinylidene aromatic polymers, such as polystyrene, have been produced by continuous bulk polymerization processes utilizing solvents such as ethylbenzene, xylene or toluene. However, these solvents remain as residues in the monovinylidene aromatic polymer, even 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 and solvent within the monovinylidene aromatic polymer. For example, US-A-5, 468,429 discloses a flashing 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. Although steam stripping can be effective in lowering residual monomer and solvent levels, this additional processing step increases the cost of production.

Therefore, there remains a need to produce monovinylidene aromatic polymers having a reduced volatiles content, using existing processes without the need for steam stripping or other additional volatiles reduction steps. SUMMARY OF THE INVENTION

The present invention is a process for producing a monovinylidene aromatic polymer having a reduced volatiles content comprising: a) polymerizing a vinyl aromatic monomer in the presence of benzocyclobutene

(BCB) solvent at a temperature less than 200°C to form a mixture of monovinylidene aromatic polymer, vinyl aromatic monomer and benzocyclobutene solvent, b) devolatilizing the mixture at a temperature of at least 200°C, but less than 230°C to vaporize the monomer and benzocyclobutene, and c) maintaining or heating the mixture at a temperature of from 230 to 260°C such that the residual benzocyclobutene reacts with the residual vinyl aromatic monomer to form a polymeric nonvolatile material.

Traces of residual monomer and BCB solvent left in the monovinylidene aromatic polymer after devolatilization react with each other at high temperatures in the molten polymer to form polymeric non-volatile materials, thus eliminating taste

-1 - and odor problems caused by residual monomers and solvents. Additionally, the

BCB solvent has a lower chain transfer constant than traditional solvents such as ethylbenzene, allowing for the production of high molecular weight polymer at faster rates. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a process to produce a monovinylidene aromatic polymer having a reduced volatiles content. Monovinylidene aromatic polymers are produced by polymerizing vinyl aromatic monomers. Vinyl aromatic monomers suitable for use according to the present invention include, but are not limited to, those vinyl aromatic monomers known for use in polymerization processes, such as those described in US-A-4,666,987, US-A-4,572,819 and US-A-4,585,825, which are herein incorporated by reference. Preferably, the monomer is of the formula:

R'

I

AT C=CH₂

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 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,.

The solvent used in the present invention is benzocyclobutene, also known as bicyclo[4.2.0]ocat-1 ,3,5-triene which is represented by the following structure: x^^y

Benzocyclobutene can be used in similar amounts as those solvents traditionally used in vinyl aromatic monomer polymerizations. Typically, the BCB solvent is present in amounts of from 1 to 20 weight percent based on the total weight of the polymerization mixture, preferably from 2 to 15 weight percent, and more preferably from 5 to 12 weight percent. The vinyl aromatic monomer and benzocyclobutene solvent are typically mixed prior to polymerization, however, the benzocyclobutene solvent can also be added in later stages of the polymerization. The polymerization can also be conducted in the presence of an initiator.

Suitable initiators include any initiator capable of accelerating the polymerization of the vinyl aromatic monomer. Representative initiators include peroxide initiators such as peresters, for example tertiary butyl peroxybenzoate and tertiary butyl peroxyacetate, dibenzoyl peroxide, dilauroyl peroxide, 1 ,1 -bis tertiarybutyl peroxycyclohexane, 1 ,1 -bis tertiarybutylperoxy-3,3,5-trimethyl cyclohexane, and di- cumyl peroxide. Photochemical initiation techniques can be employed if desired. Preferred initiators include dibenzoyl peroxide, tertiarybutylperoxy benzoate, 1 ,1 -bistertiarybutylperoxy cyclohexane and tertiarybutylperoxy acetate.

Initiators may optionally be employed in a range of concentrations dependent on a variety of factors including the specific initiators employed and the conditions at which the mass polymerization is conducted. Specifically, initiators may be employed in amounts from 0 to 1000, preferably from 100 to 800, parts by weight per million parts by weight of vinyl aromatic monomer.

Other additives can also be used in the polymerization process including lubricants, oxidation inhibitors, plasticizers, flame retarding agents, photo stabilizers, chain transfer agents, coloring agents, fiber reinforcing agents, and fillers.

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. The polymerization is typically conducted at temperatures from 80, preferably from 90, more preferably from 100, and most preferably from 110 to 200°C, preferably to 190°C, more preferably to 180°C and most preferably to 170°C.

The polymerization is followed by a devolatilization step which removes unreacted monomer and solvent from the monovinylidene aromatic polymer. Devolatilization is typically conducted at temperatures above 200°C which causes most of the monomer and solvent to vaporize while heating the molten polymer. In general, the devolatilization temperature is a temperature at which most of the BCB solvent will vaporize, without reacting with the vinyl aromatic monomer. Typically, such temperatures are less than 230°C. At temperatures above 230°C, the reactivity of BCB is too high and it cannot vaporize out of the polymer before reaction occurs. Preferably, approximately 98 percent or more of the BCB solvent is vaporized from the polymer during devolatilization. The devolatilization is generally conducted at temperatures of from 200°C, preferably from 205°C, more preferably from 210°C, and most preferably from 215°C to less than 230°C, preferably to 225°C, more preferably to 220°C and most preferably to 215°C. The unreacted styrene monomer and BCB solvent are recoverable from the devolatilization process and can be recycled into any stage of the polymerization process. After devolatilization, the molten polymer typically contains from 100 to 5000 ppm BCB solvent based on monovinylidene aromatic polymer, preferably from 200 to 2500 ppm, more preferably from 500 to 2000 ppm and most preferably from 800 to 1500 ppm. The amount of unreacted monomer is typically from 200 to 2000, preferably below 1000 ppm, more preferably less than 500 ppm and most preferably less than 300 ppm based on monovinylidene aromatic polymer.

The devolatilizated molten polymer containing residual amounts of monomer and BCB solvent, is then exposed to temperatures at which the BCB is reactive. The residual BCB remaining in the polymer reacts with itself and styrene at temperatures of at least 230°C. Ideally, devolatilization occurs below 230°C and the molten polymer is then heated at a temperature of at least 230°C for a few minutes before it is pelletized. This can be easily accomplished by heating a polymer transfer line after devolatilization to a temperature of at least 230 °C as the molten polymer is being pumped to a die where it is extruded into strands and cut into granules. Such temperatures are typically from 230°C, preferably from 235°C, more preferably from 240°C, and most preferably from 245°C to 260°C, preferably to 255°C, more preferably to 250°C and most preferably to 245°C. The residual BCB solvent reacts

-4- with the unreacted vinyl aromatic monomer to form a polymeric nonvolatile material which remains in the monovinylidene aromatic polymer.

The amount of residual monomer and solvent remaining in the final polymer is dependent upon the level remaining after devolatilization. Generally, the final polymer will contain from 100 to 300 ppm of combined residual monomer and solvent. Typically, the final polymer will contain less than 300 ppm, preferably less than 250 ppm, more preferably less than 200 ppm and most preferably less than 150 ppm based on the weight of the monovinylidene aromatic polymer.

Claims

CLAIMS:

1. A process for producing a monovinylidene aromatic polymer having a reduced volatiles content comprising: a) polymerizing a vinyl aromatic monomer in the presence of benzocyclobutene (BCB) solvent at a temperature less than 200°C to form a mixture of monovinylidene aromatic polymer, vinyl aromatic monomer and benzocyclobutene solvent, b) devolatilizing the mixture at a temperature of at least 200, but less than 230°C, to vaporize the unreacted monomer and benzocyclobutene solvent, and c) exposing the mixture to a temperature of from 230 to 260°C such that the residual benzocyclobutene reacts with the residual vinyl aromatic monomer to form a polymeric nonvolatile material.

2. The process of Claim 1 wherein the polymerization is conducted at a temperature of from 100 to 170°C.

3. The process of Claim 1 wherein the devolatilization is conducted at a temperature of from 205 to 225°C.

4. The process of Claim 1 wherein step c) is conducted at a temperature of from 235 to 255°C.

5. The process of Claim 1 wherein the vinyl aromatic monomer is styrene.

6. The process of Claim 1 wherein the devolatilized mixture of step b) contains less than 2000 ppm residual benzocyclobutene solvent and less than 2000 ppm residual vinyl aromatic monomer.

7. The process of Claim 1 wherein the monovinylidene aromatic polymer has a reduced volatiles content of from 100 to 300 ppm based on the weight of the monovinylidene aromatic polymer.

8. The monovinylidene aromatic polymer produced by the process of

Claim 1.

9. The monovinylidene aromatic polymer of Claim 8 wherein the polymer is polystyrene.

10. The monovinylidene aromatic polymer of Claim 8 wherein the polymer is a high impact polystyrene.

11. The monovinylidene aromatic polymer of Claim 8 wherein the polymer is an acrylonitrile-butadiene-styrene polymer.