Contents 6 Extrusion - Processing

The first task was to produce carriers from different recipes and in different shapes as shown schematically in Fig. 8. The raw materials diatomaceous earth, water and various binders are mixed to a paste, which is subsequently extruded through a shaped nozzle and cut off to wet pellets. The wet pellets are finally dried and heated in a furnace in an oxidising atmosphere (calcination). The nozzle geometry determines the cross section of the pellet (cf. Fig. 3) and the pellet length is controlled by adjusting the cut-off device. Important parameters in the extrusion process are the dry matter content and the viscosity of the paste. The pore volume distribution of the carriers is measured by Hg porosimetry, in which the penetration of Hg into the pores of the carrier is measured as a function of applied pressure, and the surface area is measured by the BET method, which is based on adsorption of nitrogen on the carrier surface [1]. 

Davis RD, Bur AJ, McBearty M et al. (2004) Dielectric spectroscopy during extrusion processing of polymer nanocomposites a high-throughput processing/characterization method to measure layered silicate content and exfoliation. Polymer 45 6487-6493... 

The attributes of the extrudate, such as density, thickness, and moisture content, are controlled by variables due to the formulation, extrusion process, and equipment design. The properties of the wet feed material and its ability... 

A patent has been recently issued on extrusion encapsulation to Miller and Mutka ( ). This patent contains no new formulation or process innovations but rather a refinement of the existing art (1, 3), A flow chart of the overall process is presented in Figure 1 with a view of the extrusion process presented in Figure 2. The carbohydrate matrix consists of a high DE maltodextrin and sugar, e.g., 6025 g of 20 DE maltodextrin solution (70% solids) plus 4125 g sugar (Example 1 in patent). This carbohydrate matrix is heated to promote solubility and then cooked to 120 to 124 C to reduce moisture content. They found a... 

Costabilizers. In most cases the alkyltin stabilizers are particularly efficient heat stabilizers for PVC without the addition of costabilizers. Many of the traditional coadditives, such as antioxidants, epoxy compounds, and phosphites, used with the mixed metal stabilizer systems, afford only minimal benefits when used with the alkyltin mercaptides. Mercaptans are quite effective costabilizers for some of the alkyltin mercaptides, particulady those based on mercaptoethyl ester technology (23). Combinations of mercaptan and alkyltin mercaptide are currently the most efficient stabilizers for PVC extrusion processes. The level of tin metal in the stabilizer composition can be reduced by up to 50% while maintaining equivalent performance. Figure 2 shows the two-roll mill performance of some methyltin stabilizers in a PVC pipe formulation as a function of the tin content and the mercaptide groups at 200°C. The test formulation contains 100 parts of PVC (Fikentscher K = 65), 1.2 parts of paraffin wax, 0.6 parts of calcium stearate, and 0.4 parts of methyltin-based stabilizers. 

We now follow the modeling approach outlined for the melt extrusion process. We assume steady state conditions and a given mass flow rate then, starting from the hopper, where initial conditions are known, calculations are made in finite steps, ending up at the die, with extrudate pressure, mean temperature, and solids content. If the flow rate at these conditions does not match that of the die, or if the calculations break down for some reason (e.g., insufficient solids conveying), calculations are repeated at a new mass flow rate. 

Khanal, R.C. Howard, L.R. Brownmiller, C.R. Prior, R.L. 2009a. Influence of extrusion processing on procyanidin composition and total anthocyanin contents of blueberry pomace. J. Food Sci. 74 H52-H58. 

As described previously, the process, especially if long dies are used in the extrusion process will result in compaction of the system. If the spheronization of the extrudate follows that described earlier, there will be further consolidation of the ma.ss. In fact. Chapman (52) used evidence of an increase in density of the pellets as part of his proof of the mechanism of spheronization. The liquid content of formulations is quite high, often 35-40%, hence the rate of removal of this and the potential influence of shrinkage can all influence the final structure. In some formulations, not all the liquid is removed from the pellets, which again can influence the final density. The density of the materials incorporated into the formulation will also influence the density of the pellets. Hence, it is not always possible to predict the final density of the pellets. Because of the influence of the material density, it is preferable to express the property as porosity."... 

Ziegler catalysts do not usually produce LCB, and it is this difference from Phillips catalysts that usually distinguishes which applications are suitable for each resin type. Film and injection molding are usually made from polymers of low LCB content, whereas blow molding, pipe, and other extrusion processes often benefit from the presence of LCB. Because... 

Figure 31.1 The processing of PET leads to loss of viscosity, while the addition of clays increases the viscosity at low contents. Extrusion screw speed is 50 rpm. Mold injection temperature is 7°C. The tests were made in a rheometer with parallel plates of 25 mm diameter. Source Reproduced with permission from Sanchez-Solis A, Romero I, Estrada MR, Calderas F, Manero O. Polym Eng Sci 2004 44 1094 [61]. Copyright 2004 John Wiley and Sons, Inc.

Several applications can be found in literature regarding the use of NIR for the paediction of the main physical and rheological parameters of pasta and bread. De Temmerman et al. in 2007 proposed near-infrared (NIR) reflectance spectroscopy for in-line determination of moisture concentrations in semolina pasta immediately after the extrusion process. Several pasta samples with different moisture concentrations were extruded while the reflectance spectra between 308 and 1704 ran were measured. An adequate prediction model was developed based on the Partial Least Squares (PLS) method using leave-one-out cross-validation. Good results were obtained with R2 = 0,956 and very low level of RMSECV. This creates opportunities for measuring the moisture content with a low-cost sensor. 

The lower the body moisture content the higher the plastic body attrition forces it is thus easily understandable that, whatever the other parameters of the extrusion process described above, that the water contained in the ceramic mass must provide the body with enough cohesion to equilibrate the attrition forces with the extrusion walls and the extrusion mouth. Should this not be so, the result will be fissures crossways to the direction of extrusion that are concentrated at the edges of the formed body (herring bone pattern). 

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