Multiple extrusion

An extrusion is comprised of a basket (cylinder) to contain the mix and a hydraulically actuated piston to press the mass through multiple extrusion dies. The baskets are jacketed for temperature control and contain internal screens to retain large agglomerates. Since the casting powder strands are unperforated, the dies are simple cylindrical barrels with conical lead-in sections. Extrusion rates are normally controlled by orifices in the hydraulic supply line however, constant-rate positive displacement devices are also used. As they extrude, the strands are collected separately in suitable containers. 

All six candidate compounds provide better process stability than BHT when evaluated alone after 5 extrusions. In conjunction with phenolic AO-1, all six compounds exhibit superior synergism and provide better process stability and color than ]) and (8) during multiple extrusion. 

Polypropylene. The six organophosphorus compounds (Figure 1, 0.05 weight %) were each formulated together with AO-1 (0.10%) in Hercules general purpose Profax 6501 polypropylene and were subjected to multiple extrusion in two series, one at 500 F (260°C) and another at 550°F (288°C). Comparison of the test compounds in the presence of AO-1 were made at each temperature by measuring the melt flow rate and color development after the first, third and fifth extrusions (6,7). The results after the fifth extrusion are shown in Table I. 

High Density Polyethylene (HDPE). The preferred method of evaluating HDPE was found to be the time to cross link at 220 0 using the Brabender apparatus, as is made evident by the rapid increase in torque. Determining melt viscosity by multiple extrusion showed only small differences compared to the base polymer. 

PEN (100-0) / PET (0-100) SSE at 275-315°C / effect of multiple extrusion passes / NMR / DSC / effect of residual titanium catalyst levels Stewart et al., 1993a... 

Standard test methods to analyze the thermal stability of polymers cover multiple extrusions and accelerated heataging. Multiple extrusions evaluate the melt-processing stability of a polymer or polymer formulation and determine... 

Figure 11.8 Stabilizer efficiency shown through multiple extrusion experiment.

The stability of PVC formulations is tested, for instance, according to ISO 305 by static heat tests, where test specimens are aged at constant temperature and mainly color changes (e.g.. Yellowness Index) are recorded. Alternatively, to assess the performance of heat stabilizers, the time until the dehydrochlorination reaction takes place can be measured, for example, through pH change (ISO 182). Combined thermal and shear stress is analyzed via dynamic tests such as two-roll mill processing or multiple extrusion. 

The external diffusion process is based upon the fabrication of a Nb-Cu-Ta composite in a manner analogous to the processing used for NbTi. The surface of a wire strand is plated or dip-coated with Sn which is then diffused and reacted to form NbaSn. The cost of this process is lower than that of current bronze processes, since the use of a Cu matrix removes the need for repeated anneals during drawing, and multiple extrusions are not required. Because the amount of Sn is not restricted to 13 wt.% of the matrix, a high overall critical current density can be achieved without the need for thin reaction layers and very fine filaments (--2 pm in diameter). Thus, a single extrusion with several hundred filaments, compared with thousands for the bronze process, is all that is required to obtain a high critical current density in an external diffusion conductor. 

Recycling/regrind performance Multiple extrusion passes determine whether haze or yellowness index inaeases as the clarifier experiences repeated melt processing... 

Gonzales, V. A., Velazquez, G. N., Sanchez, J. L. A. (1998). Polypropylene chain scissions and molecular weight changes in multiple extrusions. Polym. Degrad. Stab., 60, 33-42... 

Canevarolo SV. Chain scission distribution function for polypropylene degradation during multiple extrusions. Polym Degrad Stab 2000 70 (l) 71-76. 

Choudhury A, Mukheijee M, Adhikari B. Thermal stabihty and degradation of the post-use reclaim milk pouches during multiple extrusion cycles. Thermochim Acta 2005 430 (l-2) 87-94. 

The measurement of OIT based on TGA was used for monitoring the re-stabilization of post-use LDPE samples, subjected to multiple extrusion cycles. This method has abilities and hmitations as well which are discussed in literature (Kyriakou et al. 1999). The use of a re-stabilization system improved the oxidative stability of LDPE. A linear calibration curve correlating OIT values to the amount of re-stabilization system was obtained. Nevertheless, hmitations concerning quantitative determinations appeared to exist, as a change in the behavior of re-stabilization system during subsequent re-melting cycles was observed. 

As with many mechanical systems, this process improved with each new machine that was built, and it is a good example of the benefit of close cooperation between the adhesive manufacturer, the equipment supplier, and the applicator. Another good example of automation is the application of paste epoxies to bond the lens to the reflector of halogen auto headlamps. The degree of automation is not quite as complete since most of these machines still use human operators to load and unload lenses on the adhesive application system. The primary objective in this case is to apply a very uniform bead of adhesive around the perimeter of the lens. In this process, a two-dimensional curve-following robot carries multiple extrusion nozzles around several lenses placed on a turntable. Lenses are then manually placed on the lamp reflector. The assembly is then carried on a chain conveyor into the curing oven. 

Lamination of polymer films, both styrene-based and other polymer-types, to styrene-based materials can be carried out during the extrusion process for protection or decorative purposes. For example, an acrylic film can be laminated to ABS sheet during extrusion for protection in outdoor applications. Multiple extrusion of styrene-based plastics with one or more other plastics has grown rapidly in the last 10 years. 

Figure 7-53. This Navistar International truck fascia is of multiple-extrusion blow-molded parts made from Himont s HiFax polypropylene. The fascia parts, which range from 8 to 17 lbs. and come in lengths from 24 in. to 6 ft., are molded on 50-lb. accumulator head capacity machines. The ability to mold the PP in an integral color eliminates the need to paint. 

Moss, S., and Zweifel, H. (1989) Degradation and Stabilization of High Density Polyethylene during Multiple Extrusions. Polym. Degradat. Stabil., 25,... 

Environmental stress crack resistance (ESCR) is critical to many blow molded bottle applications. Changes in MW which may affect rheology may also affect ESCR, therefore it was a logical step to observe changes in ESCR with multiple extrusion passes. 

This response provided the most dramatic effect from multiple extrusion passes. The virgin homopolymer, the mixed color PCR and the virgin copolymer decreased in ESCR, and the natural PCR showed little change The magnitude of the decrease for the homopolymer and mixed color PCR was about 20%, with the virgin copolymer dropping by over 60%. 

Table 2. ESCR response to multiple extrusion passes...

Zahavich, A.T.P, Latto, B., Takacs, E. and Vlachopoulos, J. (1997) The effect of multiple extrusion passes during the recycling of high density polyethylene, Advances in Polymer Techology 16, 11-24. 

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