Introduction to Polymer Science and Technology

Polymer processing

Dispersive mixing breaks up large particles and disperses them as smaller particles throughout the melt, appropriate for dispersion of pigments, fillers and liquid additives into polymers, and for blending polymers. Distributive mixing facilitates uniform distribution of additives throughout the melt, and is accomplished by breaking and recombining the melt stream. It is a low shear process and therefore appropriate for the incorporation of fibres and flakes into polymers, where the mechanical degradation of these additives is not desirable.

Figure 3.24Conveyer elements and kneading disks are assembled on the shafts of twin screws

Most twin screws have constant diameters and, therefore, form a parallel arrangement but some are conical in shape with decreasing screw diameter towards the screw end and form a conical combination. They are also distinguished with respect to their operational modes: intermeshing co-rotating (fully wiping) or counter-rotating (fully calendering) and non-intermeshing/tangential counter-rotating. Strong (1996, p269) describes differences as, "The self-wiping nature of the co-rotating screws is much more complete than in the counter-rotating, thus in the co-rotating case there is less likelihood that material will become stagnant. Mixing is better in co-rotating systems; therefore, they are popular for compounding where good mixing is essential. In counter-rotating screws the material is brought to the junction of the two screws and builds up a material bank, most of which gets carried forward to the end of the screws, hence pumping is more efficient in counter-rotating screw systems than in co-rotating systems."

The screws are modular, allowing different elements and screw geometries to be assembled to accomplish required mixing. A conveying element is characterized by the pitch of the flights, and is expressed as a ratio of the pitch length (P) to the screw diameter (or the inner diameter of the barrel) (D) and ranges from 0.5 to 2. The conveying rate increases with P/D. Conveying elements with low P/D values (restrictive elements) or elements with reverse flights are used at the end of the melting zone to work the material, generating higher shear heat to enhance melting.

Other screw elements that are used in twin-screw extruders include kneading and mixing elements. These elements (Figures 3.25 and 3.26) often supplied as individual pieces provide flexibility in configuring a screw. Kneading blocks perform both distributive and dispersive mixing depending on the number of elements in a block of a given length and the widths of the individual elements (disks): higher numbers increase distributive mixing and so do the narrow disks but the wider disks provide better dispersive mixing. These topics are covered in greater detail by Giles et al. 2005 and Chung 2000.

conveyorelementSs,

kneading blockof disks

Figure 3.25Examples of A block of kneading elements placed between conveyer elements

Date: 2015-12-11; view: 865