Melt devolatilisation

Volatile removal may also be necessary in polymer compositions containing moisture, for example, with hygroscopic polymers and additives, such as wood flour, or where residual quantities of monomers or reactive by-products must be removed. Generally, in these situations, amounts of volatiles extracted are small, typically less than 3% by weight. 

Polymer melt devolatilisation has been the subject of extensive theoretical analysis [19], resulting in important practical implications for the preparation of filled polymer composites, with optimum particle wet-out and freedom from volatile matter. 

With polymer melts, devolatilisation is normally diffusion controlled. Within the bulk of the polymer melt, diffusion rate depends on a volatile concentration gradient existing 

Through diffusional mass transport, volatiles are moved to the interface between the melt and surrounding environment. Differences in volatile concentration at this surface and in the surrounding medium, determine the rate of volatile removal by convective mass transport. Normally, however, mass transport through the melt is the ratedetermining step. 

Under some circumstances, creation of bubbles in the melt phase can help convect volatiles at increased rates, with subsequent release into the environment. This may be achieved by injection of a carrier substance, such as water, into polymer causing foaming and diffusion of volatiles to the melt-cell interface [20]. 

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Since the presence of moisture can result in porosity in processed material, this can be removed by pre-drying the additive before processing, downstream melt devolatilisation, for example during extrusion of profile, or by use of low moisture content filler, available from some manufacturers. 

The particular features of the nylons should also be taken into account in extrusion. Dry granules must be used unless a devolatilising extruder is employed. Because of the sharp melting point it is found appropriate to use a screw with a very short compression zone. Polymers of the lowest melt viscosity are to be avoided since they are difficult to handle. Provision should be made to initiate cooling immediately the extrudate leaves the die. 

McKenaa, T. Design model of a wiped film evaporator, applications to the devolatilisation of polymer melts. Chemical Engineering Science 50(3), 453-467 (1995)... 

In a single-screw extruder, a screw is rotated inside a cylindrical barrel. The feed may be in the form of a pre-blend or dry blend, pellets or in the form of viscous melt in melt extrusion. The extruder acts as a pumping and mixing device. The extruder performs the additional functions of conveying and melting and also performs compounding and devolatilisation functions. 

In many compounding operations it is necessary to remove volatile components from melts or solution in order to avoid voids and sinks, thereby improving product quality, reducing waste, improving economy and eliminating health hazards. Devolatilisation is one of the important operations in the compounding and processing of polymers. 

This technique, also called extruder degassing, is applied to remove residual hydrocarbon components in those processes in which the product is extruded starting from the melted form, for instance solution LLDPE and high pressure LDPE. This technique requires an extended extruder since the polymer needs to be compressed, vacuum devolatilised and then compressed again for the final pelletising step. The hydrocarbon vapours coming from the vacuum dome are processed in a vacuum/scmbber system. The non-condensable substances, mainly ethylene, are sent to a flare, due to the risk of undesirable contamination with oxygen. 

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