Process for separating low molecular weight compounds from an impure thermoplastic polymer or mixture
First Claim
1. A process for treating an impure thermoplastic polymer, including raw polymers and recycled polymers, during passage thereof through an extrusion apparatus to separate and remove low molecular weight compounds therefrom, the extrusion apparatus being comprised of a heated worm housing in which are defined, in the order recited, a heating zone, a first low pressure zone, an extraction zone, a first high pressure zone, a last low pressure zone, and a last high pressure zone, and an extruder head, the process comprising:
- a. heating the impure thermoplastic polymer by passing the impure thermoplastic polymer through the heating zone of the extrusion apparatus to provide a plasticized polymer;
b. introducing a separating agent having a critical pressure, Pc, into the plasticized polymer passing through the first high pressure zone of the extrusion apparatus at an introduction pressure, Pe, the low molecular weight compounds of the plasticized polymer being at least partially soluble in the separating agent;
c. extracting at least a part of the low molecular weight compounds from the plasticized polymer into the separation agent during passage of the plasticized polymer through the extraction zone;
d. removing the separating agent charged with low molecular weight compounds from the extrusion apparatus as the plasticized polymer passes through a low pressure zone, which low pressure zone is one of upstream or downstream of the first high pressure zone and is upstream of the last low pressure zone, and which has a residual pressure, P, which is lower than the introduction pressure, Pe, in the first high pressure zone and which satisfies the requirement 1 bar≧
P≧
Pc ;
e. collecting the separating agent charged with low molecular weight compounds as it is removed from the extrusion apparatus;
f. thermal vacuum treating the plasticized polymer as it passes through the last low pressure zone to vacuum degas the plasticized polymer at a pressure, Pv, for which Pv <
0.5 bar and thereby further remove separating agent and low molecular weight compounds from the plasticized polymer; and
g. passing the plasticized polymer through the last high pressure zone to the extruder head from which purified thermoplastic polymer is recovered.
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Abstract
A process for treating an impure thermoplastic polymer, including raw and recycled polymers, during extrusion to separate low molecular weight compounds therefrom, including: introducing a separating agent into plasticized impure thermoplastic polymer passing through a first high pressure zone, the low molecular weight compounds of the plasticized polymer being at least partially soluble in the separating agent; extracting at least a part of the low molecular weight compounds from the plasticized polymer into the separation agent during passage of the plasticized polymer through an extraction zone; removing the separating agent charged with low molecular weight compounds from the extruder as the plasticized polymer passes through a low pressure zone having a residual pressure, P, which is lower the introductory pressure, Pe, and for which 1 bar≧P≧Pc ; collecting the charged separating agent as it is removed from the extruder; thermal vacuum treating the plasticized polymer as it passes through a last low pressure zone to vacuum degas the plasticized polymer at a pressure, Pv, for which Pv <0.5 bar and thereby further remove separating agent and low molecular weight compounds from the plasticized polymer; and passing the plasticized polymer through a last high pressure zone to an extruder head. The combination of solvent extraction in the extruder with subsequent vacuum degassing results in removal of large amounts of low molecular weight compounds with a relatively small expenditure of energy since energy-intensive vacuum degassing is needed only for removal of residuals.
48 Citations
19 Claims
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1. A process for treating an impure thermoplastic polymer, including raw polymers and recycled polymers, during passage thereof through an extrusion apparatus to separate and remove low molecular weight compounds therefrom, the extrusion apparatus being comprised of a heated worm housing in which are defined, in the order recited, a heating zone, a first low pressure zone, an extraction zone, a first high pressure zone, a last low pressure zone, and a last high pressure zone, and an extruder head, the process comprising:
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a. heating the impure thermoplastic polymer by passing the impure thermoplastic polymer through the heating zone of the extrusion apparatus to provide a plasticized polymer; b. introducing a separating agent having a critical pressure, Pc, into the plasticized polymer passing through the first high pressure zone of the extrusion apparatus at an introduction pressure, Pe, the low molecular weight compounds of the plasticized polymer being at least partially soluble in the separating agent; c. extracting at least a part of the low molecular weight compounds from the plasticized polymer into the separation agent during passage of the plasticized polymer through the extraction zone; d. removing the separating agent charged with low molecular weight compounds from the extrusion apparatus as the plasticized polymer passes through a low pressure zone, which low pressure zone is one of upstream or downstream of the first high pressure zone and is upstream of the last low pressure zone, and which has a residual pressure, P, which is lower than the introduction pressure, Pe, in the first high pressure zone and which satisfies the requirement 1 bar≧
P≧
Pc ;e. collecting the separating agent charged with low molecular weight compounds as it is removed from the extrusion apparatus; f. thermal vacuum treating the plasticized polymer as it passes through the last low pressure zone to vacuum degas the plasticized polymer at a pressure, Pv, for which Pv <
0.5 bar and thereby further remove separating agent and low molecular weight compounds from the plasticized polymer; andg. passing the plasticized polymer through the last high pressure zone to the extruder head from which purified thermoplastic polymer is recovered. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)
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Specification