Indentation creep of heterogeneous blends of poly(ethylene terephthalate)/impact modifier

The indentation of a sphere into a flat support under a constant load provides information on the resistance of polymeric materials to pressure, which may be of basic importance in various applications. The existing theoretical background for indentation creep in an elastic–plastic regime is reviewed briefly. The indentation creep was measured over the interval 0.1–100 min; in addition, five auxiliary measurements lasting 0.4, 1, 4, 10 and 40 min were performed with the same load. After a recovery period 10 times longer than the creep time, the irreversible part of the indentation depth was read off. The acquired experimental data were fitted with a suitable function in order to calculate irreversible deformation for any time in the interval 0.1–100 min. An essential part (up to 50%) of the indentation depth was found to be irreversible; plastic deformation grew with the creep time somewhat faster than total deformation. Indentation mainly proceeded in short periods after the initiation of loading, while a further increase in penetration depth was much slower; however, no “break” was observed in the time dependencies of measured properties. The acting pressure was linearly decreasing with the logarithm of time due to the rising penetration depth and contact area in the course of individual measurements. The penetration depth/sphere radius ratio was in all cases smaller than 0.10. The logarithm of compliance, which was proportional to the fraction of impact modifiers, was for all samples a linear function of the logarithm of creep time. Although the slope of these dependencies was rather small, it increased with the modifier content, thus indicating the rising creep rate. The effects of two types of impact modifiers on the indentation creep of PET were virtually identical.