A van der Wal’s research while affiliated with University of Twente and other places

Polypropylene-EPDM rubber blends with rubber content varying between 0 - 30 vol. %, were made on a twin screw extruder and their mechanical properties studied. These included the measurement of the notched Izod fracture energy and the notched tensile behavior as function of strain rate (0.003 - 300 s-1) and also as a function of temperature (-40° - 140°C). During high speed testing, the deformation zone of the sample, warm up, and temperature development were followed using an infrared camera. The structure of the deformation zone was studied by scanning electron microscopy. The blends were found to exhibit a sharp brittle-ductile transition and have high fracture energies in the ductile region. By increasing the rubber content, the brittle ductile transition shifted strongly towards the lower temperatures. The deformation process changed with temperature and test speed, the test speed effect possibly being due to a strong adiabatic heating in the fracture plane and therefore a thermal blunting mechanism is proposed.

The effect of the rubber particle size and rubber content on the fracture behaviour of polypropylene–EPR blends was studied at low and high test speeds. The particle size was varied by changing the molecular weight of the EPR phase, and ranged from about 0.5 to around 4.0 μm. The fracture behaviour was determined as a function of temperature by the notched Izod impact test (high test speed) and by a tensile test on notched Izod bars at 1 mm/s (low test speed). At high test speed the brittle–ductile transition temperature (Tbd) increases with increasing particle size. At low test speed the Tbd decreases slightly with increasing particle size. The weight average particle size gave a better correlation with the notched Izod results than the number average particle size. This suggests that the larger particles initiate the fracture more easily.

The effect of the rubber content on the deformation and impact behaviour of polypropylene–EPDM rubber blends is studied. The blends are made on a twin screw extruder. The rubber content ranged from 0 to 40 vol.%. The tensile modulus and the yield stress decrease linearly with increasing rubber content. The crystallinity of the PP phase as measured with differential scanning calorimetry did not change with rubber content. The fracture behaviour was studied with a notched Izod impact test and with an instrumented single edged notched tensile test at 1 m/s and 1 mm/s. The blends were studied in temperature range from −80 to 120°C. The brittle–ductile transition temperature (Tbd) decreases with increasing rubber content from 85°C for pure polypropylene to −50°C for a 40 vol.% blend, a shift of 135°C. The Tbd with the notched Izod test and the SEN at 1 m/s are very comparable. The Tbd for the SEN at 1 mm/s if compared to the 1 m/s are at a 30°C lower temperature. The brittle–ductile transition at the low test speed is gradual, while at high test speeds the transition is abrupt, discontinuous. A good criterion for the onset of ductility is the crack propagation displacement.

The deformation mechanism of polypropylene–EPDM rubber blends during fracture was studied by post-mortem SEM fractography. The deformation mechanism was determined for various blend morphologies and test conditions. Brittle fracture merely gives rise to voids, which are caused by voiding of the rubber particles. In the case of ductile fracture, voiding of rubber particles and strong shear yielding of the matrix takes place. In this yielding process these voids become elongated. As the fracture surface is approached the voids are more deformed. At high test speed, in ductile fracture, along the fracture surface a layer is formed without deformation. The thickness of this layer is 10–100μm. This layer without deformation indicates that during deformation relaxation of the matrix material in this layer has taken place. With the formation of the melt layer the impact energy increases. The relaxation layer has thus a blunting effect. If a blend with large EPDM particles (1.6μm) is deformed, in the rubber particles now several cavities are formed and these cavities are positioned near the interface with the matrix. It can be expected that it is an advantage to have several small cavities instead of one large cavity. The polypropylene matrix was found to deform by a shear yielding mechanism and multiple crazing was not observed.

Polypropylene–EPDM blends were prepared on a twin screw extruder with a rubber content 0–40 vol%. On these materials the yield strength and the notched tensile behaviour was studied as function of test speed (10−4–10 m/s). With an infrared temperature camera the heat development in the notched samples is studied as function of test speed. On fractured materials the structure of the deformation zone is studied in the middle of the sample, perpendicular to the fracture plane. The yield strength increases with the strain rate and at high rates this increase is stronger. The fracture energy shows a complex relationship with test speed. At low test speeds the fracture energy decreases rapidly with test speed. At intermediate test speeds however the fracture energy increases with increasing test speed. On the micrographs of the high speed deformed samples the formation of a melt zone was observed. The temperature rise in the notched samples starts at approximately 10−5 m/s, and increases almost linearly with the logarithm of the test speed. At 10 m/s a surface temperature increase to 90°C was observed.

The effect of matrix properties, i.e. crystallinity and molecular weight, on the impact behaviour of polypropylene–EPDM blends was studied. The blends were made on a twin-screw extruder. The impact strength was determined as a function of temperature, using a notched Izod impact test. The matrix crystallinity was varied by varying the matrix isotacticity, and ranged from 33 to 50 wt%.With increasing temperature the polymers show a sharp brittle–ductile transition. This brittle–ductile transition temperature (Tbd) shifts to higher temperatures with increasing crystallinity of the polypropylene. However, the balance of properties and the modulus–Tbd relationship were better with blends made with higher crystalline PP.The matrix molecular weight was decreased by treating a high molecular weight PP–EPDM (85/15 vol%) master blend with peroxide. In this way blends were obtained with a high MFI and a small rubber particle size. The matrix MFI of the blends thus obtained ranged from 2 to 30 dg min−1. With decreasing matrix molecular weight the Tbd increased. The peroxide treated blends exhibited a considerably lower Tbd than comparable blends made in the standard way with a similarly small particle size. Peroxide treatment of a master blend is an effective method of preparing blends with a high MFI, small particle size and good ductility.

The macroscopic cavitation and yield behaviour of nylon-6/rubber blends was studied. The type of rubber (poly(butadiene), ethylene propylene copolymer (EPDM) or polyethylene (LDPE), the rubber concentration and the rubber particle size was varied. The onset of cavitation was determined by measuring the intensity of the transmitted light from an incident laser beam. Both the yield stress and the cavitation stress appeared to increase with increasing strain rate and rubber modulus. No linear relation between the shear modulus and the cavitation stress was found. The data indicate that blends with a very small particle size have a relatively high cavitation stress. In all cases, a high cavitation stress of the elastomer resulted in a high yield stress of the blend.