Fusion of particle and bead polymers

Equation (6.8) fails when the layer thickness approaches the diameter of any solid particles in the melt. The total shear strain received by polymer, passing down an extruder screw of length L equal to 40 times the diameter, 

There are various methods of assessing the state of fusion of PVC. Intact particle boundaries, coated with stabilisers, etc. are very weak. Consequently, immersing the PVC in a liquid such as methylene dichloride causes the particles to separate, giving rise to a white powdery surface (Fig. 6.20). The wall of the pipe has been chamfered to expose the different layers. Fusion is only complete at the inner and outer surfaces, that experienced the highest levels of shear in passing through the extrusion die. A pipe with this level of fusion would have inferior mechanical properties. The fusion of PVC particles is referred to as gelation. As PVC is mixed in an internal mixer, viscous dissipation causes a steady rise in temperature. Samples are removed at various 

UHMWPE powder has a size of about 200 pm (Fig. 6.21) with sub-particles of size 1 pm visible on their surfaces. 

During ram extrusion (Section 5.4.4), the molten particles are forced into contact with each other. Polyethylene chains diffuse across the particle boundaries, and, with time, a strong bond develops. The UHMWPE however retains a memory of its particle structure, especially when calcium stearate was used prior to the late 1990s—the stearate coats the particles before processing, and it makes the interfaces more visible when micro-tomed slices are examined in an optical microscope (Fig. 6.22a). There are parallels with PVC particle fusion, but it is not possible to use shear flow to disrupt the UHMWPE boundaries. 

The chapter starts by analysing viscoelastic models, to show how the different phenomena are connected. This is followed by engineering design calculations of the creep deflections of viscoelastic structures, using modifications of methods for elastic materials. Finally, vibration damping and energy losses in cyclic loading are considered. 

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