Water assisted devolatilization

Abstract: A polymer melt may be devolatilized to less than 500, preferably less than 150 ppm of residual volatile material by injecting into the melt an amount of water greater than the amount of residual volatile material in the melt, typically not more than about 10 weight % and passing the melt through a flash chamber devolatilizer at a pressure of 8 mm of Hg or less at a temperature of from 200 to 350°C.

Description: The present invention relates to the devolatilization of melts of one or more polymers. More particularly the present invention relates to devolatilization of polymers of vinyl aromatic monomers or blends of polymers of vinyl aromatic monomers and polyphenylene oxide. US-A-3,773,740, issued November 20, 1973 in the name of T.T. Szabo, assigned to Union Carbide disclosed the devolatilization of polymers in a flash chamber. Typically the polymers are polymers containing at least one vinyl aromatic monomer. The patent teaches that from 0.50 to 2.75 weight % of water may be injected into a melt of the polymer. The pressure on the polymer melt is suddenly lowered to from about 2.666 x 10 3 to 5.332 x 10 3 Pa (20 to 40 mm of Hg (torr)). The water in the polymer melt is flashed to help remove residual vinyl aromatic monomers to about 0.30 weight % or about 3,000 parts per million (ppm). US-A-4,195,169, issued March 25, 1980 assigned to The Dow Chemical Company discloses devolatilizing polymers of styrene and acrylic acid or methacrylic acid by contacting the polymer melt with a compound of the formula ROH wherein R may be hydrogen or an alkyl radical. The devolatilization process does not increase the gel content in the resulting polymer (i.e. there are no insolubles in the resulting polymer). The polymers of the present invention are of a different polymeric character in that they do not contain any free carboxylic acid. The document EP-A-0 494 543 discloses that water injected to a polymer melt at a temperature between 260 and 300°C and pressures between 3400 and 5500 kPa aid in a subsequent vacuum devolatilization zone in the removal of low-molecular weight volatiles. The document US-A-43 83 972 discloses a cooperating devolatilizer, condenser and vacuum source in series whereby a gas injector is located between said devolatilizer and said condenser. Currently, the producers of polymers of vinyl aromatic monomers are seeking to produce polymers or blends of such polymers which contain less than about 200 ppm of monomers, oligomers and solvent. The Union Carbide Patent does not teach one how to reduce monomer, oligomer and solvent levels to those required today. A simple approach might be to merely further reduce the pressure within the devolatilizer. However, at pressures of about 10 torr and less, the water injected into the polymer melt cannot be cooled to sufficiently low temperatures to condense, because it will freeze at such low pressures in the condenser system between the devolatilizer and the vacuum source. Vapour pressure tables of water show at pressures of less than 4.579 mm of Hg water has to be cooled to less than 0°C to condense. Accordingly, if the pressure in the condenser is less than about 5 mm Hg it is very difficult to keep the system operational. Operating under very closely controlled procedures and by selecting lots of polymer from a particular batch about the lowest levels of residual monomer, oligomer and solvent that can be obtained are in the range of greater than 175 ppm, typically from 200 to 175 ppm. Applicants have discovered that one method for overcoming the drawback of the Union Carbide technology is to maintain the pressure within the devolatilizer lower than the pressure within the condenser. One method for achieving this pressure differential is to have a fluid ejector between the devolatilizer and the vacuum source. It is possible to eject a fluid such as steam at this point so as to maintain the very low vacuum required in the devolatilizer yet have a higher pressure in the condenser so that water won't freeze in the condenser. As a result it is possible to operate a water assisted devolatilizer at pressures below 6.665·10 2 Pa (5 torr) and to consistently obtain polymers having levels of monomers, dimers, trimers and solvents of less than 150, preferably less than 100 ppm. US-A-5,102,591, issued April 7, 1992 discloses a process to devolatilize a polymer blend of styrene and polyphenylene oxide by passing the blend through an extrusion devolatilizer. That is an extruder equipped with vacuum ports. In such a process the polymer or polymer blend does not descend vertically through a flash chamber. Rather, the melt is passed horizontally in the barrel of an extruder beneath a vacuum port. Additionally, the reference teaches a two stage devolatilization. That is, first the polyphenylene oxide is devolatilized then the polystyrene is added to the polyphenylene oxide and the blend is then devolatilized. US-A-5,145,728 discloses reducing the residual monomer and oligomer content of polystyrene by blending with it a block copolymer of styrene and butadiene, typically such as those sold under the trade mark K RESIN. The reference does not contemplate passing the polymer melt through a flash chamber devolatilizer. Rather, the polymer is devolatilized conventionally, then extrusion blended with the block copolymer. In the example at columns 4 and 5, the starting polymer is devolatilized using a screw extruder and water. Interestingly, the residual monomer and solvent level was not reduced below 150 ppm. The present disclosure does not contemplate such a process and does not contemplate a blend comprising block copolymers of a block of one or more C 8-12 vinyl aromatic monomers which are unsubstituted or substituted by a C 1-4 alkyl radical and one or more blocks of one or more C 4-6 conjugated diolefins. The process of the present invention has an advantage over the extrusion processes as there is a shorter history of shear under high temperature. Each time a polymer blend is passed through an extruder there is some degradation of polymer, particularly a reduction of molecular weight. Furthermore, extrusion processes are relatively expensive. The present invention provides: 1. a process for reducing the amount of residual monomer, optionally dimer, trimer, and solvent to less than 500 ppm in: (A) a polymer composition that is a polymer or polymer blend, comprising: (i) from 100 to 30 weight % of one or more monomers selected from C 8-12 vinyl aromatic monomers which are unsubstituted or substituted by a C 1-4 alkyl radical; and (ii) from 0 to 70 weight % of one or more monomers selected from C 1-4 alkyl esters of acrylic or methacrylic acid, acrylonitrile and methacrylonitrile; which polymers may be grafted on to from 0 to 40 weight % of one or more rubbery polymers selected from: (iii) polymers consisting of one or more C 4-6 conjugated diolefin monomers; and (iv) polymers consisting of from 20 to 80 weight % of one or more C 8-12 vinyl aromatic monomers and from 80 to 20 weight % of one or more C 4-6 conjugated diolefins; or (B) a polymer composition that is a blend of polymer (A) or polymer blend (A) and polyphenylene oxide, which polymer or polymer blend contains less than 1 weight % of residual monomer, dimer, trimer and solvents, the process comprises: (i) heating and maintaining said polymer or polymer blend at a temperature from 200° to 270°C; (ii) injecting into said polymer or polymer blend an amount of water greater than the amount of residual monomer, dimer, trimer, and solvent but less than 1 weight %, said injection being at temperatures from 200° to 270°C and pressures from 3.45 x 10 3 to 10.34 x 10 3 kPa (500 to 1500 psi); (iii) passing said polymer or polymer blend through a flash chamber devolatilizer (3,5) maintained at a temperature from 200° to 270°C and a pressure of less than 6.665 x 10 2 Pa (5 torr) to flash off a vapour phase comprising water, monomer, and solvent; (iv) withdrawing said vapour phase from said flash chamber devolatilizer and passing it to a condenser maintained at a reduced pressure greater than 6.665·10 2 Pa (5 torr) by ejecting a gas inert to said vapour phase into said vapour phase intermediate said flash chamber and said condenser; and (v) condensing the water in said vapour phase. Preferably, said polymer or polymer blend is maintained at a temperature from 225° to 235°C. Preferably, that pressure within said flash chamber devolatilizer (3,5) is maintained below 4.0 x 10 2 Pa (3 torr). Preferably, said gas inert to said vapour phase is selected from steam or other inert gases. According to the invention, said polymer or polymer blend is deposited on a distributer tray (7) within said flash chamber devolatilizer (3,5). Said polymer may be polystyrene. Said polymer blend may be a blend of polystyrene and polyphenylene oxide. Preferably, the amount of residual monomer ( and optionally dimer, trimer) and solvent is reduced to less than 150 ppm. Figure 1 is a schematic diagram of a cooperating arrangement of devolatilizer, gas ejector, condenser and vacuum source which may be used in accordance with the present invention. by way of one example. One embodiment of the present invention will now be briefly described in conjunction with the drawing , by way of one example. In the bulk or solution polymerization of a number of monomers containing one or more vinyl aromatic monomers, the monomers are fed to one or more reactors where they are polymerized to at least about 65% conversion. The polymer leaves the reactor, in the case of a tower process as illustrated by US-A-3,658,946 issued April 25, 1972, assigned to BASF and US-A-3,660,535, issued May 2, 1972, assigned to the Dow Chemical Company, or the last reactor 1, in the case of a Monsanto type process as illustrated by U.S.-A-3,903,202, issued September 2, 1975, assigned to Monsanto, and travels through a preheater 2. The preheater heats the polymer melt to a temperature of from about 200 to 270°C to increase the vapour pressure of the volatiles and reduce the viscosity of the melt to permit it to foam. An additional problem which must be considered is the cooling of the polymer melt due to the latent heat of vaporization of the volatiles as the melt travels through the devolatilizer. The heated polymer melt passes into the first stage 3 of a two-stage falling strand devolatilizer. The devolatilizer is operated at temperatures from 200 to 350, preferably from 210 to 255, most preferably from 225 to 235°C. Typically the pressure in the first stage of the devolatilizer will be from 1.333 x 10 2 to 6.0 x 10 3 , preferably less than 2.666 x 10 2 Pa (10 to 45, preferably less than 20 mm Hg). The polymer melt descends from the preheater and is deposited on the bottom of the vessel. As the polymer melt descends through the devolatilizer, volatiles within the melt are flashed off. The polymer melt at the bottom of the first stage 3 of the two-stage reactor should have a residual content of monomer, dimers, trimers and solvent of less than 2, preferably less than 1, most preferably less than 0.5 weight %. The melt is then collected and pumped to the second stage 5 of the two-stage devolatilizer. Between the first and second stage of the two-stage devolatilizer is a water injector unit 6. As noted above, the figure is a schematic diagram. The devolatilizer could comprise more than two stages. Water injector 6, is operated at temperatures from 200 to 275°C, preferably at temperatures approximate those in the devolatilizer, and at pressures from 3.45 x 10 3 to 10.34 x 10 3 kPa (500 to 1500 psi). The water injector is operated to provide less than 10, more typically less than 5, most typically less than 2, preferably less than 1, most preferably from 0.45 to 0.74, most desirably from 0.05 to 0.60 weight % of water into the polymer melt. Generally, for the injection of water to be useful in the reduction of volatile material in a polymer melt, the amount of water injected into the melt has to be greater than the amount of residual volatile materials in the melt. From a practical point of view, the amount of water injected into the melt should be in accordance with the above teaching. Preferably the water injector contains a mixing means such as a static mixer. The mixture of polymer melt and water then pass through the second stage 5 of the devolatilizer. The devolatilizer is operated at temperatures from 200 to 270, preferably from 210 to 255, most preferably from 225 to 235°C. The second stage 5 of the devolatilizer should be operated so that the polymer melt is exposed to a pressure of less than 6.665 x 10 2 , preferably less than 4.0 x 10 2 Pa (5, preferably less than 3 mm Hg or torr) . To increase the residence time of the polymer melt in the second stage 5 of the devolatilizer, a distributor tray 7 may be inside the devolatilizer. However, as noted above, a distributor tray is preferred, but not essential, in accordance with the present invention. To provide suitable appropriate residence times within the stages of the devolatilizer, a distributor tray 7 may be inside the devolatilizer. Various distributor designs have been described in US-A-4,934,433, issued June 19, 1990, US-A-5,069,750, issued December 3, 1991, and US-A-51 18 388. As the polymer descends through the last stage 5 of the falling strand devolatilizer typically in the form of a strand, it is deposited and usually held on the distributor tray in the form of a foam. The water in the polymer and residual monomer (and the optional dimers, trimers ) and solvent are flashed off. The polymer melt then descends to the bottom of the second stage 5 of the devolatilizer, or more generally, of the last stage of the devolatilizer, as the case may be. The melt is then pumped to a strand forming die 8, and the strands typically pass through a cooling water bath 9 into a pelletizer 10. The water vapour and volatile monomer(s), dimers, trimers, solvent(s) and any non-condensable gases are withdrawn overhead from the devolatilizer. In the figure, for the sake of convenience, only one common line 11 is shown. From a practical point of view, there may be several lines. It is most important that in the vapour line coming from the devolatilizer being operated at pressures of less than 6.665 x 10 2 Pa (5 mm Hg) and the condenser 13, and between the devolatilizer and the condenser, there be a gas ejector 12, preferably useful with steam or other inert gases. The rate of gas ejection should be such that the pressure in the last stage of the devolatilizer can be maintained below 6.665 x 10 2 Pa (5 mm Hg) and the pressure in the condenser can be maintained to permit the condensation of water without freezing. Downstream of the condenser is a vacuum source 14, which should be of sufficient size so as to be capable of maintaining the vacuum throughout the devolatilizer and condenser system. E.g., the polymer melt is a melt of one or more polymers selected from : polystyrene, high impact polystyrene (HIPS), styrene acrylonitrile polymers (SAN), acrylonitrile butadiene styrene polymers (ABS), styrene methyl methacrylate polymers, and butadiene styrene methyl methacrylate polymers (MBS), and one or more of the aforesaid polymers, most preferably high impact polystyrene blended with polyphenylene oxide. E.g., the polymers which may be treated in accordance with the present invention comprise: (i) from 100 to 30, preferably from 100 to 50, most preferably from 100 to 70 weight % of one or more monomers selected from : C 8-12 vinyl aromatic monomers which are unsubstituted or substituted by a C 1-4 alkyl radical; and (ii) from 0 to 70, preferably 0 to 50, most preferably from 0 to 30 weight % of one more monomers selected from : C 1-4 alkyl esters of acrylic or methacrylic acid; acrylonitrile and methacrylonitrile; which polymers may be grafted on to from 0 to 40, preferably from 0 to 20, weight % of one or more rubbery polymers selected from : one or more C 4-6 conjugated diolefin monomers and polymers comprising from 20 to 80, preferably from 40 to 60, weight % of one or more C 8-12 vinyl aromatic monomers and from 80 to 20, preferably from 60 to 40 weight % of one or more C 4-6 conjugated diolefins. Some vinyl aromatic monomers are styrene, α-methyl styrene, and p-methyl styrene. Some esters of acrylic or methacrylic acid are methyl methacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, and butyl acrylate. Some conjugated diolefin monomers are butadiene and isoprene. Most preferably the polymer melt is polystyrene or high impact polystyrene (HIPS). The present invention has been described in terms of the devolatilization of a polymer melt of polystyrene. However, the present invention may also be used in association with melts of other polymers such as acrylonitrile butadiene styrene polymers (ABS), styrene acrylonitrile polymers (SAN), and polymer blends. The present invention is particularly useful where polymers are solution blended. That is, miscible solutions of two polymers are mixed and the solvent(s) is/are removed. In such cases, it is often desirable to remove the solvent(s) to as low a level as possible. One commercially available blend in which the present invention may be useful is a blend of polyphenylene oxide (trademark) and polystyrene or a blend of polyphenylene oxide and high impact polystyrene. E.g., the weight ratio of polystyrene to polyphenylene oxide is from 90:10 to 10:90, preferably from 70:30 to 10:90. Other applications of the present invention will be apparent to those skilled in the art. The present invention will now be illustrated by the following examples in which, unless otherwise specified, parts is parts by weight (i.e. grams) and % is weight %. Sample Preparation Samples of commercially available crystal polystyrene pellets were mixed with various amounts of a specially prepared rubber modified polystyrene which contained about 1.2 weight % residual styrene monomer. The mixture of pellets was passed through an extruder to achieve a homogeneous melt blend. The extruder provided motive pressure to pass the melt blend through a static mixing device. Between the extruder and the mixing device, various amounts of water were injected into the melt. The melt was pumped into the devolatilizer onto a distributor. The polymer coming out of the devolatilizer was analyzed for residual monomer. The conditions and results of the experiments are set forth in Table 1. Vacuum Pa (mm/Hg) Water Rate (Weight/Percent) Melt Temperature °C (°F) Residual Styrene (PPM) Total Residual Volatiles (PPM) Atmospheric 0.0 246 (475) 1130 1240 2.666 x 10 2 (2.0) 0.0 241 (466) 290 310 2.666 x 10 2 (2.0) 1.0 239 (463) 80 80 1.333 x 10 2 (1.0) 1.0 246 (475) 70 70 1.333 x 10 2 (1.0) 1.0 245 (473) 40 44 2.666 x 10 2 (2.0) 1.0 230 (447) 110 110 2.666 x 10 2 (2.0) 1.0 228 (443) 100 110 These results show that residual monomer and volatiles may be consistently reduced to less than 150 ppm using the present invention. EXAMPLE 2 Four further runs were carried out in the same manner as Example 1. The conditions and the results of the runs are set forth in Table 2. Run Vacuum Pa (Torr) Water Rate (Weight/Percent) Melt Temperature °C (°F) Residual Styrene (PPM) Total Residual Monomers (PPM) 1 1.333 x 10 2 (1.0) 1.7 232 (450) 100 110 2 8.6 x 10 2 (6.5) 1.7 236 (457) 120 130 3 1.46 x 10 3 (11.0) 1.6 233 (452) 170 190 4 1.73 x 10 3 (13.0) 1.7 235 (455) 210 230 The experiments show that at higher levels of vacuum better results are obtained. If the devolatilization can be carried out at pressures below about 8 torr, excellent results can be obtained. However, when operating at these pressures, during condensation, care should be taken to avoid freezing the water injected into the melt.

Claims: A process for reducing the amount of residual monomer, optionally dimer, trimer, and solvent to less than 500 ppm in: (A) a polymer composition that is a polymer or polymer blend, comprising: (i) from 100 to 30 weight % of one or more monomers selected from C 8-12 vinyl aromatic monomers which are unsubstituted or substituted by a C 1-4 alkyl radical; and (ii) from 0 to 70 weight % of one or more monomers selected from C 1-4 alkyl esters of acrylic or methacrylic acid, acrylonitrile and methacrylonitrile; which polymers may be grafted on to from 0 to 40 weight % of one or more rubbery polymers selected from: (iii) polymers consisting of one or more C 4-6 conjugated diolefin monomers; and (iv) polymers consisting of from 20 to 80 weight % of one or more C 8-12 vinyl aromatic monomers and from 80 to 20 weight % of one or more C 4-6 conjugated diolefins; or (B) a polymer composition that is a blend of polymer (A) or polymer blend (A) and polyphenylene oxide, which polymer or polymer blend contains less than 1 weight % of residual monomer, dimer, trimer and solvents, the process comprises: (i) heating and maintaining said polymer or polymer blend at a temperature from 200° to 270°C; (ii) injecting into said polymer or polymer blend an amount of water greater than the amount of residual monomer, dimer, trimer, and solvent but less than 1 weight %, said injection being at temperatures from 200° to 270°C and pressures from 3.45 x 10 3 to 10.34 x 10 3 kPa (500 to 1500 psi); (iii) passing said polymer or polymer blend through a flash chamber devolatilizer (3,5) maintained at a temperature from 200° to 270°C and a pressure of less than 6.665 x 10 2 Pa (5 torr) to flash off a vapour phase comprising water, monomer, and solvent; (iv) withdrawing said vapour phase from said flash chamber devolatilizer and passing it to a condenser maintained at a reduced pressure greater than 6.665 x 10 2 Pa (5 torr) by ejecting a gas inert to said vapour phase into said vapour phase intermediate said flash chamber and said condenser; and (v) condensing the water in said vapour phase. A process according to claim 1, characterised in that said polymer or polymer blend is maintained at a temperature from from 225° to 235°C. A process according to claim 1 or 2, characterised in that said pressure within said flash chamber devolatilizer (3,5) is maintained below 4.0 x 10 2 Pa (3 torr). A process according to any one of claims 1 to 3, characterised in that said gas inert to said vapour phase is selected from steam or other inert gases. A process according to any one of claims 1 to 4, characterised in that said polymer or polymer blend is deposited on a distributer tray (7) within said flash chamber devolatilizer (3,5). A process according to any one of claims 1 to 5, characterised in that said polymer is polystyrene. A process according to any one of claims 1 to 5, characterised in that said polymer blend is a blend of polystyrene and polyphenylene oxide. A process according to any one of claims 1 to 7, characterised in that the amount of residual monomer, optionally dimer, trimer, and solvent is reduced to less than 150 ppm.

Inventors: SKILBECK JOHN PAUL, SKILBECK, JOHN PAUL
Applicants: NOVA CHEM INT SA, NOVA CHEMICALS (INTERNATIONAL) S.A.
Application year: 1993
Main classification: B01D3
IPC codes: C08F6, B01D3