Molecular Degradation and Screw Design

In the extrusion of medical devices, molecular degradation is often a critical issue. This degradation of the MW can be strongly influenced by the geometry of the extruder screw. Unfortunately, little information on this problem is available in the open literature. Paakinaho et al. [127] published results of a study on the MW reduction along the length of a single screw extruder for three polylactic acid (PLA) resins see Fig. 8.111. 

Molecular weight changes by screw speed, screw length, and atmosphere 

The effect of screw geometry on MW is shown in Fig. 8.113. In this study, a 0.75-inch extruder was used with a thermoplastic elastomer (TPE). Three screw geometries were tested a harrier screw, a CRD mixing screw, and a Maddock mixing screw. 

These studies were conducted by a large medical device company in the development of implantable devices. The MW results are critical because the polymer is used for implantable CRM lead wire insulation, which is susceptible to hydrolytic and metal ion degradation in the body. 

In this particular application, the molecular weight of the extruded product had to be above a certain minimum level. This turned out to be a significant challenge because the early extrusion trials did not achieve the minimum MW level in the TPE. As a result, a special screw was designed for this application to minimize the stresses that the polymer melt is exposed to in the extrusion process—this is the CRD mixing screw. 

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