4. Conclusion

From a comparative study of the crystallization behavior of iPP and PA6 under pressure and high cooling rates, the following conclusions can be drawn:

IPP:

• An increase of solidification pressure is seen to determine a parallel decrease of density and alpha phase content, the greatest differences being located among ambient pressure and 8 MPa, i.e. at a pressure quite low with respect to the one attained in polymer processing. Microhardness results confirm the trend, with some differences in the high cooling rate region.

• For cooling rates above 20 C/s the effect of pressure could be considered negligible; in the rest of the explored cooling rate range (from 0.01 to 20 C/s) the effect of pressure on the final density is very remarkable.

• The observed decrease of the alpha phase content is mainly balanced by an increase of mesomorphic phase content, while the amorphous phase content remains nearly constant.

PA6:

• An increase of solidification pressure determines a larger density and a larger microhardness. The increase of density and MH is of the same order of magnitude than the decrease observed in iPP.

• A "critical cooling rate" around which the most significant structural changes occur was found to be around 10 C/s at all the investigated pressures.

• The increase of pressure determines an increase of alpha phase crystallinity leaving almost unaffected the gamma crystallinity.

• The increase of pressure is accompanied by a process of perfectioning of the crystalline domains, as witnessed by the increase of long period in the low cooling rate...