Polymer Blends Processing

Polymer processing: is an engineering specialty concerned with the operation carried out on polymeric materials or systems to increase their utility. This field can be divided into three categories:
(i) Modification (including mixing).
(ii) Forming, the processes that fix the shape of the products.
(iii) Finishing, painting, joining.
Morphology development
Polymer-polymer melt blends usually start in the solid state as millimeter sized pellets, powder, or flakes, and are then heated and sheared inside an intensive mixing device such as an extruder or batch mixer. These millimeter size domains melt and break up into micrometer or nanometer sized domains during the initial softening stages.
After this initial stage, depending on the composition of the two components, a processing phase inversion may occur, i.e., the minor phase (the component in lower concentration), softens first and becomes the matrix phase initially. After the major component softens, once it has sufficient melt concentration, the phases invert and the minor phase becomes the dispersed phase. If the major phase softens first and initially forms the matrix, no phase inversion will be seen.

Figure 1 Important mechanisms in morphology development during polymer blending of nylon/ethylene-propylene rubber blends. Initially a sheet morphology is formed, and the dispersed phase domain size decreases sharply (note logarithmic scale for y axis). A processing phase inversion may occur before breakup and coalescence mechanisms take over, leading to a stable drop size. The drop size can change by an order of magnitude because of compatibilization via reactive blending or by adding premade copolymer.



In the final stage of polymer blending, a balance is established between drop breakup and coalescence so that a stable morphology is formed and the final morphology depended on polymer rheological properties and processing parameters. Stabilization of the morphology during and after processing can be achieved using compatiblizers. Two main types of compatibilizers used in polymer blends are premade block copolymers and compolymers formed through in situ reaction between functionalized polymers. Of the two, the latter is practiced more in industry.
Initial morphology
It was found that the dispersed phase breaks up by stretching into sheets and these sheets subsequently broke up into smaller domains. Figure (2) is a summary of work by Scott, Macosko, Sundararaj, and coworkers showing how a pellet deforms into a sheet and subsequently breaks up into droplets.