Extruder and process for preparing a mixture of polymer and diluent

1. A twin screw extruder for preparing a polymeric solution, the extruder comprising:

(a) an elongated housing having an inlet end, an outlet end, an extruder shaft length L and a pair of interconnecting bores disposed within said housing;

(b) a pair of elongated extruder shafts, each having an axis of rotation, said pair of elongated extruder shafts disposed within said pair of interconnecting bores and drivable in at least one direction of rotation;

(c) a plurality of extruder screw segments positioned along said pair of elongated extruder shafts in a fixed angular relationship therewith, said plurality of extruder screw segments selected to form multiple extruder stages, said multiple extruder stages comprising an inlet stage, a mixing stage, an outlet stage, and a dispersion stage,

(d) a material inlet adjacent said inlet end of said elongated barrel for introducing at least one polymer; and

(e) a first fluid inlet located within said dispersion stage for introducing a solvent;

wherein:

the inlet stage has a length Li of about 10% L≦Li≦about 20% L, the dispersion stage has a length Ld of about 15% L≦Ld≦about 25% L, the mixing stage has a length Lm of about 35% L ≦Lm≦about 45 L, and the outlet stage has a length Lo of about 20% L≦Lo≦about 30% L;

the inlet stage comprises a first forward full flight screw segment having a length of 0.75 D, a second forward full flight screw segment having a length of 1.00 D, and six third forward full flight screw segments, each having a length of 0.75 D;

the dispersion stage comprises eleven forward kneading segments, each having a length of 0.50 D, and two neutral kneading segments, each having a length of 0.50 D;

the mixing stage comprises a full flight screw segment, having a length of 0.75 D, a pair of neutral kneading segments, each having a length of 0.50 D, a forward kneading segment having a length of 050 D, a plurality of gear kneading segments, each having a length of 1.50 D, and a plurality of forward kneading segments; and

the outlet stage comprises five first forward full flight screw segments, each having a length of 1.5 D, and a second forward full flight screw segment having a length of 1.00 D and a pair of third full flight screw segments each having a length of 0.75 D.

2. The twin screw extruder of claim 1 , further comprising a second fluid inlet located within said mixing stage for introducing a solvent.

3. The twin screw extruder of claim 1 , wherein the axes of rotation are substantially parallel.

4. The twin screw extruder of claim 1 , wherein said elongated extruder shafts are co-rotating.

5. The twin screw extruder of claim 1 , wherein said elongated extruder shafts are counter-rotating.