The dependence of impact properties on talc content and molecular weight (M) of polypropylene (PP) were investigated for talc-filled polypropylene (f-PP). The following results were obtained.
(1) For impact, a fracture mode transition from ductile to brittle exists, regardless of the presence of filler. The impact strength (I_S) of PP increases rapidly with increasing temperatures above the ductile-brittle transition temperature (T_db).
(2) The I_S of PP depends strongly on molecular weight, but in f-PP, this dependence is decreased with increasing talc content at temperatures above T_db.
(3) The T_db of f-PP increases (-20→0°C) with increasing talc content (25→55wt%). The T_db of PP is almost constant for the molecular weight range of 2.70×10⁴∼6.71×10⁴ despite the different flow properties (MFI=ca. 1∼26g/10min).
(4) The dependence of I_S on talc content for f-PP at temperatures above T_db is mainly controlled by fracture strain ε rather than fracture stress σ.
(5) Increasing the molecular weight of PP is an effective way to increase the impact strength I_S. The use of talc filler shifts T_db to higher temperatures. These results provide the basic concepts for material design for impact strength.

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This paper describes a convenient method to determine the viscoelastic properties of plastics based on short time relaxation tests after constant strain rate uniaxial compression. This method does not require special testing equipment, can be performed on ordinary material testing machines in the factory, and is based on an extension of the Testing Method for Compressive Properties of Plastics (JIS K 7208). In this paper, we analyze stress relaxation curves obtained from the tests mentioned above and discuss a simple method based on a 6-parameter viscoelastic model (3 parallel Maxwell elements). This model is based on an approximation of stress relaxation to a three story structure and an introduction of two differential coefficients at the initiation of stress relaxation. Furthermore, for wide application to a range of viscoelastic behavior, concrete reducible equations between two equivalent 6-parameter viscoelastic models are shown.

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