Counter-Rotating Tangential twin screw extruders perform a variety of processing tasks exceedingly well, but perhaps what CRT extruders "do best" is to separate volatile substances from thermoplastic materials by evaporation. The design of the extruder to be used will depend on the properties and quantities involved. A single one-diameter vent under vacuum and properly placed relative to the screws will in many cases reduce three or four percent of a low boiling-point solvent to a satisfactory level. On the other hand, a feed stream consisting of twenty percent polymer and eighty percent volative hydrocarbons may require a multi-vented extruder with large and small vents upstream and downstream of the feed port. If each vent is isolated from the others by appropriate screw design, regulated vent pressures, above or below atmospheric. can be used to control the rate of devolitalization in each vent. So-called mechanical filters—which are in essence small close-clearance extruders—may be installed in the vents to scour solids from the vapor streams.
The use of so elaborate equipment as a CRT extruder to remove relatively large proportions of solvent may appear at first glance not be cost effective. But there are polymer solutions and suspensions that at some point in the devolatilization process are very difficult to handle in any other way. And by this means, volatiles can be reduced to a few parts per million and even to "trace" levels that cannot be measured by normal analytical procedures. Pressures below one millimeter of mercury have been maintained at the vents of CRT research extruders, but the results therefrom did not appear to this investigator to be worth the extra effort.
The following volative substances (and there are others) in the quantities shown have been removed from thermoplastic materials in pilot plant CRT extruders:
Acetone 40-80%
Acrylonitrile .04%
Ammonia 2%
Benzene 65-85%
Butanol 5-10%
Carbon tetrachloride 5-87%
Chloronaphthlene 4%
Cyclohexane 5-92%
Dimethyl Formamide 31%
Ethanol 5%
Ethyl acetate 20.50%
Ethyl benzene .12-57%
Formaldehyde with trioxane 5.15% incl
Freon 50%
Heptane 5-93%
Hexane 20-90%
"High boiling" 3-50%
Water 2-20%
Hydrocarbons-misc. 0.5-35%
Isopropanol with monomer 90% incl.
Light ends" 50°/c
Methanol 92%
Methytene chloride with monochlorobenzene 50-85% incl.
Methyl isobutyl ketone with
menomer 12°/c incl.
Methyl n-butyl ketone .35%
Monomers 37Oppm-75%
"Odor" trace-1°4
Pentane 10-75%
Petroleum distillate 10%
Phenol 2%
Styrene with Toluene 8% mcI
Tetrahydrofurari with cyclohexane 80% incl.
Toluene 40-80%
Vinyl acetate
Listed below are some of the materials that have been devolatilized in pilot plant CRT extruders:
Acrylonitrile butadiene styrene
Acetal
Cellulose acetate
Filter cake
Nylon
Petroleum pitch
Phenoxy resin
Polyamide
Polybutadiene
Polybutylene
Polycarbonate
Polyethylene
Polyformaldehyde
Polymethylmethacrylate
Polyoctenamer
Polystyrene
Polysulfone
Polypropylene
Polyvinyl acetate
Polyvinyl chloride
Polyurethane
Rubbers-various
Styrene acrylonitrile
Polyvinyl alcohol
Polymer gum
- James Branegan
See also
Extruder controls
Extrusion of thermoplastic foams
Mean residence time characterization in corotating twin screw extruders
Old vs new extruders
Polymer devolatilization
Twin screw extruders design and operating characteristics