Vented extruders

Many types of equipment have been developed to improve the evaporation of solvent in order to provide energy savings. The most widely used techniques for devolatilization are the falling strand devolatilizer (FSD), the thin-film evaporator and the vented extruder [113]. 

Two-stage and multiple-stage vented extruders are commonly used to remove volatile components from molten resin streams prior to downstream processing. The vent ports can be open to the atmosphere, or they can be attached to elaborate vacuum systems. For very specialized systems, stripping agents such as water, nitrogen, and carbon dioxide can be added upstream of the vent, mixed into the resin stream, and then devolatilized in the vent area. This technique can be employed to remove difficult components or components at a higher rate from the resin. 

Figure 13.3 Schematic of a vent flow diverter in a two-stage vented extruder...

For example, if a two-stage vented extruder has a screw that was designed for resins with an Mi range of 2 to 20 dg/min, and plant personnel are asked to manufacture a resin with an MI of 100 dg/min, the extruder may not process the resin acceptably. For this case and flood feeding, the extruder is likely to have resin flowing into the vent because the first stage can deliver more resin than the second stage can pump. One practical solution to this problem is to operate the slide valve in a partially open manner such that it controls the rate of the extrusion process. This will allow the extruder to operate at a lower specific rate and allow operation without the flow of resin into the vent. 

EJ (Propellants). American sol vent-extruded propellants developed during WWII at the Allegheny Ballistics Laboratory and at the Radford Development Department of the Hercules Powder Co. They oontd K oer-chlorate-carbon black filler in a double-base propint matrix... 

A. Tukachinsky, Y. Talmon, and Z. Tadmor Foam-enhanced Devolatilization of Polystyrene Melt in a Vented Extruder, AIChE J., 40, 670-675 (1994). 

HPM and Johnson Plastics Machinery report that they can use vented extruders, and vented screw plasticators on injection molding machines, to eliminate the pre-drying step required by many plastics, and thus reduce processing costs 5-20% ( y. 

The generation of catalytic amounts of carbodiimides from diisocyanates is also applied in the manufacturing of polyamides from MDI and dicarboxylic acids and/or dicar-boxylic acid terminated prepolymers. These thermoplastic polyamides are produced continuously by reaction polymerization using a vented extruder to remove the carbon dioxide byproduct. Thermoplastic polyamides are manufactured today by the Dow Chemical Co., which purchased the Chemical Division of Upjohn in 1980. 

Carbodiimides are also used as catalysts in the formation of polyamides from dicarboxylic acids and diisocyanates. The carbodiimide catalyst is generated in situ from the diisocyanate using dimethylphospholene oxide as the catalyst. In this manner segmented thermoplastic poly(ether amides) and poly(ester amides) are obtained from the acid terminated monomers and diisocyanates by reaction polymerization processes. This reaction is best conducted in a vented extruder because carbon dioxide is the byproduct. 

I became involved in carbodiimide chemistry in my research work on isocyanates at the former Donald S. Gilmore Research Laboratories of the Upjohn Company in North Haven, CT. Carbodiimides are readily synthesized from isocyanates using a phospholene oxide catalyst. This reaction can be conducted without a solvent, and the byproduct is carbon dioxide. We used this reaction in the manufacture of a liquid version of MDI (4,4 -diisocyanatodiphenylmethane), which today is sold in huge quantities worldwide. By reacting MDI with dicarboxylic acids in a vented extruder we manufactured a family of thermoplastic polyamide elastomers, which are sold today by the Dow Chemical Company. Also, N-sulfonylcarbodiimides were synthesized for the first time in our laboratories. They are the precursors of the antidiabetic sulfonamides, such as Upjohn s Tolbutamide (Orinase). Because of the close relationship of isocyanates with carbodiimides we studied many linear and cyclic carbodiimide reactions, especially their cycloaddition reactions. 

The OPS extrusion process typically uses a two-stage vented extruder. The most common size OPS lines use either a 200 or 220 mm diameter extruder. The length-to-diameter (L/D) ratios range from about 24 to 32, the higher ratio extruder... 

E-7. A sol vent-extruded composite proplnt developed in USA during WWIL It was described as Unit No 593 in conf Propellant Manual SPIA/M2(1962)... 

Devolatilization. Following polymerization of many polymers, removal of residual monomer or solvents is necessary. Ultimately this removal becomes controlled by diffusion from the polymer no matter what type of process is employed (27). Many processes use a steam-stripping operation for this purpose, and others employ devolatilization in a vented extruder. Removal of residual vinyl chloride monomer from poly(vinyl chloride) was an issue of much concern following the discovery that this monomer is a carcinogen. 

Antioxidants slow down the plastic degradation under high tanperature, attrition, and so on, hence, minimize the VOC formation and the respective decrease of density. Moisture in the ingredients also leads to a decrease of the final density of the material, hence, cellulose fiber should be dried, if necessary. Last, but not the least, vented extruders remove VOCs and steam from hot melt and greatly increase density of the final product. 

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