Mixing of Polymer Melts

Example 5.22 Preparation of Polymer Blends from the Melt 

Safety precautions Before this experiment is carried out. Sect. 2.2.5 must be read as well as the material safety data sheets (MSDS) for all chemicals and products used. 

In practice, polymer blends are often produced by mechanical mixing in the melt. Most of the blend combinations are immiscible and the product is demixed but often in very small dimensions so that often scanning or transmission electron microscopy (SEM, TEM) have to be used to analyse the morphology in detail. 

In the laboratory simple tests for the miscibility of polymer melts can be carried out, in which the turbidity of the system is an indicator for the immiscibUity  

Two polymer solutions (5 wt%) are successively dripped into a common precipitation bath under vigorous stirring. The precipitate is a powder blend that consists of small grains of the components. 

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Viscoelastic Measurement. A number of methods measure the various quantities that describe viscoelastic behavior. Some requite expensive commercial rheometers, others depend on custom-made research instmments, and a few requite only simple devices. Even quaHtative observations can be useful in the case of polymer melts, paints, and resins, where elasticity may indicate an inferior batch or unusable formulation. Eor example, the extmsion sweU of a material from a syringe can be observed with a microscope. The Weissenberg effect is seen in the separation of a cone and plate during viscosity measurements or the climbing of a resin up the stirrer shaft during polymerization or mixing. 

Due to the laminar flow of polymer melts in an extruder, mixing is entirely the result of kinematic distributing processes related to the extruder operating characteristics. This can be described in terms of the ratio of drag and pressure flows. The actual homogenization process (distribution process) depends entirely on the intensity and direction of these flow portions and not on the viscosity of the melt. 

Among the technological innovations of the twentieth century, the compounding of polymeric materials stands out for its flexibility and diversity. Literally thousands of material combinations suitable for a myriad of end-use applications are the result of compounding. For purposes of this chapter, compounding can be defined as the combination and melt-mixing of polymers and value-adding components to create more useful materials. ... 

Polyesters, polyetherimide, melamine-formaldehyde, polyurethanes, polyurethane-ureas and polyamides (nylons) are examples of condensation polymers prepared by REX. " Because a small molecule is produced in condensation reactions, vent ports are employed. Between the ports are melt sealing screw sections, to prevent back mixing of volatilizing melt. Sealing screw sections are constructed by a right handed-left handed sequence of screw elements. Another chemistry feature of step-growth reactions is their sensitivity to errors in stoichiometric feed proportions. This poses problems with solid feed materials, which are normally converted to liquid form for more... 

Powder samples of 10u average diameter were prepared by air jet micronization of polymeric or composite chunks. Composite materials were prepared by melt mixing of polymer and carbon black. Carbon black was added to the surface by milling the polymer powder and carbon black with glass beads for an hour, and subsequently heating the particles to tack the carbon black to the particle surface. Static resistivity measurements were performed on packed powders in a procedure similar to ASTM D257. 

The significant improvement in flow properties of resin IV vs. V is evident from the data. First, the torque from Brabender mixing indicates that resin IV is an easier-flowing material (Table II). The lower torque values for IV indicate the necessity of a lower-energy input to mix the polymer melt. This lower rotational force therefore indicates the polymer melt has a lower melt viscosity. Secondly, injection-molding conditions demonstrate the improved processability of resin IV (Table III) in comparison with resin V. At the same injection cylinder temperatures, the injection pressure for the tensile bar and Izod/heat distortion bar molds is lowered by 300 psi and 250 psi, respectively. 

In practice, composites are mostly prepared by mechanical mixing. Discontinuous and continuous melt mixing reactors (kneading chambers, extruders) are used. Principally, also laboratory equipment like metal stirrers can be used to mix fillers with a polymer melt but the typical high viscosity of polymer melts hampers a homogeneous dispersion of the filler particles in the melt. 

Polymer alloys play an important role in the industrial production of high-impact polymers (see Chapter 6). They can be produced by the mixing of solutions of appropriate polymers or by mixing of the melts. With fibers, a polymer fiber is swollen with an appropriate second monomer and the second monomer is polymerized. Conjugate fibers (see Chapter 12) have a partial polymer alloy character. 

A brief coverage of the topic of blown film coextrusion is provided in this chapter. Coextmsion is the process of feeding a single die with two or more different polymer melt streams. Within the die, the various flow streams are combined to form a single-ply film comprised of the individual layers (Fig. 5.1). Because of the high viscosity of polymer melts, the individual layers tend not to mix but to retain their positions within the combined flow stream. In some cases, over seven layers of polymer are extruded into a film. For each type of polymer layer in the final structure, a different extruder is connected to the die (Fig. 5.2). 

Equipment for Blend Preparation by Melt Mixing of Polymers 113... 

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