Properties of the extrudates

The tensile properties of the extruded blends and composites are presented in Table 2. Compared to the neat PP, a clear reinforcement was achieved after twin-screw blending. The reinforcing effect was even more pronounced with the higher take-up speed (H), evidently due to the extremely fibrillar morphology, as seen in Fig. 3. 

It is important for the process engineer to know the rheological properties of a material since the properties dominate the flow of the material in extrusion processes and also dominates the physical and mechanical properties of the extrudates. Therefore, it is also important to measure the properties utilizing a similar miniaturized extruder in the laboratory so that a process engineer knows the flow properties in the system by simulating the production line. Also, it is desirable to know the flow properties of a material to be processed in the range of shear rates of equipments to be used. 

Die swell is dependent upon the L/D ratio of the die. The phenomenon is a limiting factor in the drive to reduce moulding cycles, since the conditions which lead to excess swelling lead also to quality deficiencies in appearance, form and properties of the extrudate. In order to control the swelling the temperature of the melt can be increased, which causes a decrease in relaxation time. A long tapered die has also been found to reduce post-swelling. 

Results are presented of an investigation of the effect of extrusion conditions, such as melt temp., screw speed and draw-down, on process morphology of rigid PVC foams and the properties of the extruded profile, including density, cell structure and surface appearance. The optimum conditions for the best quality profile found in this study were 190 to 196C melt temp, and no greater than 20 rpm screw speed on a 63.5 mm single screw. 15 refs. 

It is evident that some knowledge of the properties of the extruded material must be known for these processes to work, such as the melting temperature of metals and plastics or the formulation of food dispersions which can be reliably formed into elongated stable structures. 

Initially, the focus has been on the bulk properties of the extruded mass, such as rheology and rheological change within the processed materials. This allows predictive performance of the raw material formulation and a primary means of specifying ingredient performance and process control, and will be examined further below. 

It has long been the aim to relate the properties of the extruded product or half-product to the state of the extrudate as it leaves the die. We have noted that this is often attempted via the apparent viscosity of the melt but must fail if mixes are regarded as simple power law fluids. Elasticity in melts is easily demonstrated as die swell, caused by elastic normal forces. This occurs with many cereal flours and protein/carbohydrate mixes (e.g., soya flour, grits and concentrates) and does not involve puffing, or gas expansion. It occurs immediately after the die and may relax before the structure is set. Nonetheless, die swell expansions of up to 200% have been recorded (Guy and Home 1998). 

Much of the published work on extrusion has attempted to correlate process conditions and formulation with final product properties. Correlations are almost always found, and may be systematic within the particular set of variables examined, but do not survive when further parameters are examined in other studies. This makes them of limited use in even qualitative explanation of product properties. Re-examination of published data, bearing in mind the effect of material states on the expansion process after the die, shows that there is a systematic explanation that can be related to the material properties of the extruded mass, during the dynamic formation of bubbles and cellular structures. However, because of the transformation of materials within the barrel, both at the microscopic and molecular level, it is unrealistic to expect test methods on raw materials alone to relate directly to product structures and... 

Technical constraints are often imposed on the design of the monolith geometry by the extrusion process, as well as by the mechanical properties of the extrudate the specific SCR application (e g., high-dust vs. low-dust) is also crucial for the definition of the catalyst geometrical features. Here, attention is paid to the influence that the monolith parameters (wall thickness, channel size, channel shape) have on both DeNOx reaction and SO2 oxidation in order to advance guidelines for optimization of the catalyst geometry. 

Determination of shear rate vs. shear stress curves by application of the ram extruder allow characterization of the rheological properties of the extruded material according to the basic type of curve, as expressed by Eqs. (8)-(ll). 

The influence of the butyl side groups on the tensile properties of the extrudates was even stronger than that of the methyl side groups. This is demonstrated in Table 3 for extrudates where the nominal draw ratios were 9 and 15. The strong decrease of the axial room temperature modulus with increasing size of the side groups went parallel with the increase of the amount of die swell. 

Medium pressure agglomeration in pellet mills can be easily simulated because even a single bore vith the correct diameter to length ratio and featuring all other details of the orifice (e.g., inlet chamfer, discharge cone, relief bore, etc. [B.97]) can be used to determine the extrusion characteristics of a particular feed material and the properties of the extrudate. 

The properties of the extruded copolymer are also anisotropic (Table II) ( ). 

All the results of DSC, elastic recovery, and x-ray measurements on the sample portions formed under characteristic deformation flow components indicate that the complex stress strain field in an extrusion die produces markedly different deformation flow profiles on crystalline-state extrusion of HDPE, but results in no significant effect on the morphology and properties of the extrudates prepared under the conditions used in this work. It is remembered, however, that an earlier electron microscopy study found marked morphological variations across the radius of HDPE extrudates prepared at higher temperatures (134 to 137°C) under the combined effects of chain orientation and pressure (4). The T, of the undeformed isotropic... 

Extruded catalysts were made by means of a Winkworth extruder. This could produce a range of shaped (tri-lobe, quadri-lobe, star shapes etc) and cylindrical extrudates at 10 kg h scale with 1-4 mm diameter. A wide variety of lubricants and binding additives were investigated to aid the extrusion process and improve the crush strength of the resulting extrudates. Finding additives that improved the physical properties of the extrudate but did not interfere with the FT chemistry proved to be a challenge but eventually suitable additives were found. 

The rheological properties of the paste control to a large extent the final properties of the extruded or injection moulded part. Different additives, such as dispersants and lubricants, are added to the powder-polymer mixture to promote deagglomeration and reduce the... 

Antimony trioxide, widely used in PVC formulations, gave a reasonably good fire performance but, as expected, increased the smoke emissions detected. It was not foimd to affect the thermal stability of the rigid PVC, and had no effect on either the colour or impact properties of the extruded PVC. 

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