Microhardness injection-moulded polymers

These include cold drawn, high pressure oriented chain-extended, solid slate extruded, die-drawn, and injection moulded polymers. Correlation of hardness to macroscopic properties is also examined. In summary, microhardness is shown to be a useful complementary technique of polymer characterization providing information on microscopic mechanical properties. 

Figure 7.2 schematically shows the geometry of an injection-moulded polymer bar in which z is the injection direction. Indentations were made on the yz plane. In all cases an indentation anisotropy arises because the microhardness is maximum when the indentation diagonal is parallel to the injection direction (Hj) and minimum when the diagonal is normal to it (Hy). The large value corresponds to... 

In summarizing, it can be concluded that the microhardness of elongational flow injection moulded PE is influenced by a local double mechanical contribution (a) a plastic deformation of crystal lamellae under the indenter, and (b) an elastic recovery of shish-fibrils parallel to the injection direction after load removal. Further, the Shish-crystals are preferentially formed when high orientation occurs, i.e. at zones near the centre of the mould and at an optimum processing temperature Tp around 145-150 °C. Below this temperature overall orientation decreases due to a wall-sliding mechanism of the mbber-like molten polymer. 

Figure 7.10 shows the microhardnesses of several injection moulded starch samples processed at different temperatures with different initial water content. There is an increase in H with the injection moulding temperature in the range 80-110 °C, from 120 MPa to 140 MPa (Balta Calleja etal, 1999). These microhardness values are notably higher than those found for conventional injection moulded thermoplastic polymers like PE (50-60 MPa) (Rueda et al 1989 Balt Calleja et al., 1995). 

---