Extrusion generalities

Formulations for extrusion generally include light and heat stabilizers, lubricants, which facilitate molding, and colorants. These materials are generally purchased in a compounded ready to use cube form, in order to minimize irregularities in blending, etc. 

During extrusion, general browning typified by carameli-zation, Maillard, and oxidative decomposition reactions are paramount in flavor compound formation. Temperature and shear conditions occurring during extrusion can provide the chemical and physical means whereby complex starch and protein can be partially degraded to provide reactants that can then participate in browning. 

The simulation of extrusion generally includes consideration of the melting of the polymer in the barrel, flow of the melt in the die, and the cooling of the extruded shape. There are several simulation packages on the market, employing different viscosity models to describe the flow characteristics of the polymer melt. The material properties needed for simulation of extrusion process are listed in Table 11.7. 

Initiation or chain start when an initiator (in reactive extrusion generally a peroxide) decomposes and initiates a radical on a monomer molecule. 

The pore size distributions obtained by the mercury intrusion and liquid extrusion techniques are expected to be different. Mercury intrusion allows access to the pores from both sides of the mat and the entire pore volume is likely to be sampled. For pores that have large surface openings, the liquid extrusion generally tends to imderestimate the pore volumes relative to those measured by intrusion porosimetry. Liquid extrusion measurements yield the pressure needed to push the liquid past the most constricted part (or the throat ) of the pore. The pore volume of the channel is estimated based on the throat diameter. This also introduces directionality to the liquid extrusion measurement For a sample where porosity includes converging or diverging channels, the pore volumes (and pore dimensions) obtained from the liquid extrusion method will depend on the direction of the gas flow into the membrane. However, this is not expected to be a serious source of error in routinely characterizing nanofiber mats. 

Extrusion. In general, extmsion is the process of forcing a polymer melt through a die (104,105). Typical extmsion appHcations include initial resin pelletization after manufacture and production of film, sheet, pipe, tubing, and insulated wire. The HDPE extmsion temperature is around 150°C, the pressure 40—50 MPa (5800—7250 psi). An extmsion production line usually consists of an extmder (mono- or twin-screw) with a die at the end, a cooling and shaping device, a pulling device (a roUer), and a cutter. 

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

As plastics can have quite different viscosities, they will tend to behave differently during extrusion. Fig. 4.3 shows some typical outputs possible with different plastics in extruders with a variety of barrel diameters. This diagram is to provide a general idea of the ranking of materials - actual outputs may vary 25% from those shown, depending on temperatures, screw speeds, etc. 

Another event which may occur is hydrolysis. This is a chemical reaction between the plastic and water. It occurs extremely slowly at room temperature but can be significant at moulding temperatures. Hydrolysis causes degradation, reduction in properties (such as impact strength) and it is irreversible. Table 4.3 indicates the sensitivity of plastics to moisture. Note that generally extrusion requires a lower moisture content than injection moulding to produce good quality products. 

Pultrusion is also used to make structural shapes from composite materials. The incoming material is generally unidirectional and must be pulled through the pultrusion die because the uncured composite material is entirely too flexible to push (as in extrusion processes). The incoming material can be preshaped by various guides and rollers as in Figure 1-18. 

Additives such as antioxidants and photostabilizers of low-molecular weight face two major problems (1) they may evaporate during high temperature moulding and extrusion process or (2) they may migrate to the surface of the plastic and get extracted. There are, in general, three ways of overcoming these problems. 

In the two typical shaft seal manufacturing processes shown in Fig. 2, the extrusion process is only required if the compression molding process is being used. There are basically two types of extruders in general use, the cold feed and the hot feed. In both cases their role is to produce the preforms for use with compression molding. The critical issues with the preform are the shape, the dimensions, and the weight. 

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