Melt devolatilization extrusion process
View Patent ↗Thermally sensitive polymers containing polymerizable carbon-carbon unsaturation and/or aliphatically bound halogen are devolatilized in a devolatilizing extruder. The thermally sensitive polymer is blended with a second polymer, which does not contain polymerizable carbon-carbon unsaturation or more than 5% by weight aliphatically bound halogen, and which has a molecular weight of from 25,000 to 175,000. The blend is then devolatilized in the extruder to produce a devolatilized polymer blend. Thermal degradation of the thermally sensitive polymer is minimized in this process.
1. A melt devolatilization extrusion process, comprising
a) feeding a heat-plasticized polymer mixture that includes
component 1): at least one first thermoplastic polymer containing polymerizable carbon-carbon unsaturation, at least 10% by weight of aliphatically bound halogen, or both;
component 2): at least one second thermoplastic organic polymer, which is compatible with component 1) at the relative amounts thereof that are present, wherein the second organic polymer is substantially devoid of polymerizable carbon-carbon unsaturation, contains less than 5% by weight of halogen, and has a weight average molecular weight of from about 25,000 to about 175,000 as measured by GPC against a polystyrene standard; and
component 3): from 1 to 60% by weight, based on the total weight of the polymer mixture of at least one volatile compound;
into a devolatilizing extruder while at a temperature above the boiling temperature of the at least one volatile compound;
b) separating the heat-plasticized polymer mixture in the devolatilizing extruder into a devolatilized, heat-plasticized polymer blend containing not more than 3,000 ppm of volatile compounds and a separate vapor phase containing the at least one volatile compound; and
c) discharging the vapor phase through at least one vent in the devolatilizing extruder barrel and discharging the devolatilized polymer blend from the devolatilizing extruder through an outlet downstream of the vent(s), wherein:
A) the devolatilized polymer blend discharged from the extruder outlet has a melt shear viscosity at 180° C. of no greater than 500 Pa·s at a shear rate of 100 s −1 and of no greater than 300 Pa·s at a shear rate of 1000 s −1 as measured by capillary rheometry; and
B) during steps a), b) and c), the polymer mixture and the devolatilized polymer blend are maintained at a temperature of no greater than 100° C. above the glass transition temperature of the first thermoplastic polymer.
2. The process of claim 1 , wherein the devolatilizing extruder contains an extruder barrel that includes at least one internal rotating screw, an inlet for introducing the polymer mixture into the extruder barrel, at least two vents downstream of said inlet for removal of gasses from the extruder barrel, and an outlet located at one end of the extruder barrel for discharge of the devolatilized polymer blend.
3. The process of claim 2 , wherein a stripping agent is introduced into the barrel of the devolatilizing extruder upstream of at least one of the vents, and the stripping agent is removed from the devolatilizing extruder through at least one of the vents.
4. The process of claim 3 , wherein the stripping agent is water, air, nitrogen, argon or carbon dioxide.
5. The process of claim 2 , wherein the internal rotating screw contains one or more reverse elements located upstream of at least one vent.
6. The process of claim 5 , wherein a melt seal is produced upstream of at least one vent, and the pressure at each melt seal is no greater than about 1.0 MPa.
7. The process of claim 5 , wherein the specific energy input is from 200 to 400 Joules per gram of the heat plasticized polymer mixture fed into the devolatilizing extruder.
8. The process of claim 1 , wherein component 1) includes a thermoplastic polymer containing at least 25% by weight aliphatically bound halogen.
9. The process of claim 1 , wherein component 2) has a glass transition temperature of from 80 to 180° C.
10. The process of claim 9 , wherein component 2) is a polymer or copolymer of a vinyl aromatic monomer.
11. The process of claim 1 , wherein the heat-plasticized polymer mixture further contains at least one thermal stabilizer.
12. The process of claim 1 , wherein component 1) and at least part of component 3) are a solution of a brominated butadiene-styrene block copolymer in a solvent from a bromination reaction.
13. The process of claim 1 , wherein the devolatilized polymer blend obtained from the extruder is solidified.
14. The process of claim 13 , wherein the solidified polymer blend is combined with a bulk polymer and melt processed.
15. The process of claim 14 wherein the melt processing is a foaming extrusion process.
16. The process of claim 14 , wherein the bulk polymer has a molecular weight of at least 200,000.
17. The process of claim 16 wherein the bulk polymer is a homopolymer of styrene or a copolymer of styrene with ethylene, propylene, acrylic acid, maleic anhydride, acrylonitrile or a mixture of any two or more thereof.
18. The process of claim 14 , wherein the bulk polymer is dissolved in a monomer or monomers and the monomer or monomers is suspension polymerized to form polymer beads, the polymer beads are infused with a blowing agent to form expandable polymer beads, and the expanded polymer beads are thermally expanded to form a foamed article.