Specially Designed Extruders

In 1976, Meluch and Campbell at General Motors obtained a patent10 for a steam hydrolysis depolymerization of PUR. High-pressure steam hydrolyzes flexible PUR foams rapidly at temperatures of 232-316°C to form diamines and polyols. The diamines are distilled and extracted from the steam and the polyols are obtained from the hydrolysis residue. In 1977, Bayer AG obtained a patent for a continuous PUR hydrolysis process using a specially designed extruder.11... 

The validity of the theoretical assumptions was evaluated by comparing the two models predictions with the experimentally measured drop diameter at different axial positions in the twin screw extruder. Experimentally, after the extrusion reached steady state, the screw rotation was stopped and the molten blend was quenched within a specially designed extruder barrel. It was estimated that the PS/PE blends were quenched within 7-10 sec. The second model predicted well the morphology evolution of non-compatibUized blends of PS in PE, and their inverse of PE in PS (see Figure 9.16 in Chapter 9). 

Recently, technologies on reactive extrusion have been developed with commercial significance. The graft copolymer can be prepared by interchain reactions which occur in specially designed extruders. This method has been adapted in toughening nylons, as will be discussed in Section IV-B. 

An alternative process is bulk-phase halogenation (74-76). Dry butyl rubber is fed into a specially designed extruder reactor and contacted in the melt phase with chlorine or bromine vapor. By-product halogen acids are vented directly, avoiding the need for a separate neutralization step. Halogenated rubbers comparable in composition and properties to commercial products can be obtained. 

For most applications, PBT is not used in the pure form which comes out from the polycondensation equipment, but as compounded materials produced by mixing and homogenizing PBT melt with specific additives in specially designed extruders. Sometimes, only a small amount of additive is combined with the PBT. In other cases, high loadings of a variety of ingredients can lower PBT content to below 30 %. Characteristic for all those cases is that the PBT is still the continuous phase. Like many crystalline polymers, the low melt viscosity of PBT and its ability to mix with many types of filler and with other polymers make it very amenable to the formation of composites and blends. 

There are many ways in which foams can be processed and used as slabs, blocks, boards, sheets, molded shapes, sprayed coatings, extruded profiles, foamed in place in existing cavities, in which the liquid material is poured and allowed to foam, and as structural foams (Chapter 6, STRUCTURAL FOAM). Conventional equipment such as extruders, injection, or compression machines is used. However specially designed machines are available to just produce foamed products. 

The modular principle is also frequently used in the design of the extruder screw. The screw comprises individual screw elements, which are threaded onto a shaft. The elements are positively fitted to the shaft to transmit the required torque. They are also attached together and axially clamped. The elements of a screw module are typically made of nitrided steel. Special designs are required for wear protection and corrosion protection depending on the respective requirements ... 

The gearbox has two main tasks reducing the motor speed to the operating speed of the extruder and distributing the torque to the two closely adjacent output shafts. These tasks call for a specially designed gearbox. 

In 1967 Per-Anders Persson of Nitro Nobel AB in Sweden (now part of Dyno Nobel Europe) invented a non-electric initiation system, designated Nonel that has revolutionized this aspect of the explosives Industry. The Nonel system consists of an extruded hollow plastic tube that contains an internal coating of a mixture of powdered molecular explosive and aluminum. The plastic tube is inserted into and attached to a specially designed detonator or blasting cap. The Nonel tubing can be initiated... 

A twin-screw extruder is used to reduce residual monomers from ca 50 to 0.6%, at 170 °C and 3 kPa widi a residence time of 2 ruin (94). In another design, a heated casing encloses die vented devolatilization chamber, which encloses a rotating shaft widi specially designed blades (99,100). These continuously regenerate a large surface area to facilitate die efficient vaporization of monomers. The devolatilization equipment used for die production of polystyrene and ABS is generally suitable for SAN production. 

Extruded structural foams are produced with conventional extruders and a specially designed die with an inner, fixed torpedo located at the center of its opening. The extrudate from this die is hollow. The outer layer of the extrudate cools and solidifies to form solid skin the remaining extrudate expands toward the interior of the profile. The Celuka process, developed by Ugine-Kuhlmann [93], is widely used commercially. 

A tubular reactor of special design internal screw mixer has been developed at Wroclaw University of Technology [56, 57]. The melted plastics from a screw extruder are... 

The flow sheet of this process is shown in Figure 26.2. Prepared plastic fluff is sent to a bag filter set on the top deck (Figure 26.1) by a pneumatic conveyer after treatment by a second magnetic separator and an aluminum separator. Then, the plastic fluff is sent to two series of dehydrochlorinators, specially designed, reciprocally rotating twin-screw-type extruders, through a vertical screw conveyer. This equipment is composed of a melter of plastic fluff aud circulator of waste plastic melt. Each is a horizontal type... 

As part of the polymerization scenario described in Subsect. 1.2.2, the compounding extruder is to be designed by a company specializing on extruder design and construction, supported by simulations. As the necessary knowledge is often not present in chemical companies, aspects of cross-organizational cooperation come into play, which will be treated later in Subsect. 4.1.7. 

If the material to be extruded is very moist and/or sticky the cutting device would lump the strands into globs of wet material and the discharge plate with wiper bar would further aggravate this situation. A special design is available in which the main shaft is driven from the top. Then, the wet strands fall directly into a dryer or on a belt, thus minimizing aggregation (Fig. 8.60). 

PP/EVAl blends with lamellar morphology were produced either in a single screw extruder with a specially designed die [Lohfink and Kamal,... 

Reactive compatibihzation in a specially designed twin-screw extruder was carried out compounding maleated polypropylene, PP-MA (0-0.14 wt% MA, with PA-6. During the reaction a di-block copolymer was formed at the interface. As the copolymer content increased from zero to 20 wt%, the number average diameter of PA-6 drops decreased from the initial value d = 20, to d = 0.14 um at 20 wt% copolymer. The concentration dependence of shear viscosity also changed with compatibilization from negative deviation from the log additivity rule, NDB, to positive deviation, PDB [Nishio et al., 1992]. 

Single-screw extruders have been widely used for blend preparation however, they do not offer sufficiently high stress levels to improve mixing thus, special designs of screws have been developed such as those with mixing heads or barrier zones that increase residence time and enhance blend mixing. 

To really control the mixture and dispersion of additives, or to do reactive extrusion, a twin-screw extruder is necessary. Twin-screw extruders are available in several different configurations, but for the work of compounding the most often used is a closely intermeshing co-rotating extruder, with a specially designed screw for the particular task. In order to accomplish this with the least amount of machining. 

---