Polyethylene twin-screw extruders

One of the earliest published studies on extraction in twin-screw extruders was conducted by Todd (1974). In this work devolatilization was conducted under vacuum using two different polymeric systems, polystyrene in one and polyethylene in the other. In the case of polystyrene, styrene was not used as the volatUe component so as to avoid problems associated with further polymerization or depolymerization instead, use was made of mixtures of thiophene and toluene or ethylbenzene. Todd found good agreement between the measured exit concentrations of the volatile component and the predicted values using Pe = 40 in the solution to Eq. (38) (see Fig. 15). The value of 5 in Eq. (39) was not reported and it is not known whether a value was chosen to provide a fit with the data or whether it was known a priori. In any event, what is clear is that the exit concentration varies with IVwhich suggests that mass transfer is occur-... 

Several studies have been performed to evaluate the mixing capabilities of twin screw extruders. Noteworthy are two studies performed by Lim and White [12,13] that evaluated the morphology development in a 30.7 mm diameter screw co-rotating [28] and a 34 mm diameter screw counter-rotating [3] intermeshing twin screw extruder. In both studies they dry-mixed 75/25 blend of polyethylene and polyamide 6 pellets that were fed into the hopper at 15 kg/h. Small samples were taken along the axis of the extruder and evaluated using optical and electron microscopy. 

Composite formulations were prepared as follows The straw samples as received from INEEL were ground to 0.69 mm in a hammer mill and oven dried to 1.1% moisture. The dried straw samples were then blended with various amounts of high-density polyethylene (HDPE), lubricants, and maleated polyethylene blends (MAPE) (see Table 2). The mixed formulations were then extruded with a 35-mm Cincinnati Milacron Model CMT 35 counterrotating conical twin screw extruder (Cincinnati Milacron, Batavia, OH), which produced a 9.525 x 38.1 mm2 solid cross-section. Flexural strength, density, and water sorption were measured for the extruded samples according to ASTM Standard Methods (13,14). 

U.S. Pat. No. 5,938,994 [112] describes a WPC material produced as feedstock pellets in a twin screw extruder, and comprising wood flour (about 20-80%) and polyethylene (80-20%) as a preferred plastic. 

A different approach was used by the Ferruzy Company, the main difference being the use of high boiling-point plasticizer instead of water for the destructuration of starch. In this technology, starch was plasticized together with polymers such as polyethylene-vinyl alcohol (EVOH), EAA, poly-e-caprolactone, with small amounts of moisture, in a twin-screw extruder [49], to produce an intimate mixture between starch and the synthetic polymer. The commercial trade name of this product family is Mater-Bi . 

Different polymer blends like PE (polyethylene)/PS (polystyrene) [10-11] and PMMA (polymethylmethacrylate)/PS [12-13] have been produced using supercritical C02-assisted extrusion. Fully intermeshing twin-screw extruders have been used in these studies. A decreased shear thinning behavior on dissolution of supercritical CO2 into blends was observed. The obtained reduction in viscosity ratio resulted in a finer dispersion of the minor phase, which is desirable to create a good polymer blend. The effect of supercritical CO2 on the dispersion of the minor phase for a PMMA/PS blend can be seen clearly in Fig. 12.5. 

Figure 6.27 Morphology development of polyethylene/polyamide-6/SEBS-g-MA reactive blends system in an intermeshing co-rotating twin-screw extruder [74].

With good temperature control and the low shear, these extruders are well suited for compounding and for extrusion of rigid poly(vinyl chloride). Typically, high-speed (200- to 500-r/min ) extruders are employed for compounding, whereas low-speed (10- to 40-r/min ) machines are used for profile extrusion. Conical twin-screw extruders with their tapering screws (Fig. 5.31 ) are utilized almost exclusively for chlorinated polyethylene and rigid poly(vinyl chloride). 

Ganzeveld (4) studied the grafting of maleic anhydride on high-density polyethylene in a counterrotating 40 mm twin-screw extruder. The polymer (Stamylan 7359, DSM) was tumble mixed with maleic anhydride (Nourymix MA-901 and 903, AKZO) and fed with a controlled feeder to the extruder. The initiator (di-tert butyl peroxide) was fed separately. The wall temperature ranged from 120 to 210°C. No devolatilization was used in this study, but the samples obtained were dried in a vacuum oven for 2 h to remove the unreacted maleic anhydride. The amount of maleic anhydride grafted was determined by titration. 

The functionalization of EPDM with MA is carried out in a Berstorff 25 mm corotating twin screw extruder. EPDM (Keltan 740 DSM), Perkadox 14 (bis(tert. butyl peroxy isopropyl) benzene AKZO half life times 5 min at 175 °C 10 sec at 210 °C [4]) and Nourymix(50/50 wt% masterbatch of MA on polyethylene AKZO) were mixed and starved fed to the first section of the extruder. A nitrogen atmosphere was maintained over the reaction mixture throughout the extruder. In the last sections free maleic anhydride and other volatile components were removed by applying vacuum on the last vent. Standard extrusion conditions were throughput rate 1 kg/h residence time 5 minutes rotor speed 150 rpm Mass temperatures in the die 220-260 OC. The MA-content in the rubber was determined with potentiometric titration and infrared analysis. 

Fig. 10. Variation of an. average drop diameter during compounding in an intermesliing, co-rotating twin screw extruder for 5 vol% polyethylene dispersed in polystyrene, extruded at three screw speeds, N = 150,200, and 250 rpm, at a throughput 0 = 5 kg/hr. The points are experimental, the curves computed from model-2.

Compound mixing was performed on different compounding equipment. For EVA organoclay-based nanocomposites, a laboratory twin-roll mill and an internal mixer heated to 145 °C were used. A corotating twin screw extruder from Leistritz, Germany, with a 27-mm screw diameter and an aspect ration of 40 L/D was used to generate polyethylene nanocomposites. The mass temperature was 190 °C at the extruder die. 

Nano composites linear low-density polyethylene/Na" -montmorillonite (LL-DPE/MMT) were prepared by components blending in melt using Hake twin-screw extruder at temperature 473 K [4]. 

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