Studies On The Stress Induced Effects On Polypropylene-iron-waste Tire Rubber During Pan-Milling And Their Composites

In this paper, the microstructure changes of polypropylene induced by a complex combination of shearing, compression, stretching and friction actions during single pan-milling or co-milling with iron, UHMWPE and waste tire rubber chips (WTR) were studied and the stress induced effects were revealed. The structure and morphological development of iron particulates and WTR during pan-milling and co-milling with polypropylene were also characterized. The properties of PP/Fe, PP/WTR and PP/WTR/Fe composites prepared through pan-milling techniques were investigated. The main conclusions are as follows:(1) By co-panmilling PP with bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate, used as radical scavenger, under ambient condition, the direct evidence of mechanically formed free radicals produced during pan-milling of PP was elucidated. The main chain scission owing to mechanical stress and the formation mechanism of free nitroxy radicals were illustrated.The crystalline structure changes of PP induced by pan-milling were characterized by X-ray diffraction and Raman spectroscopy. XRD analyses show that when PP was co-milled with UHMWPE, crystal transformation of PP occurred, its crystallinity and crystallite size decreased, whereas, only slight reduction of crystallinity and crystallite size were observed and no crystal transformation was found when PP was co-milled with WTR chips, and amorphization of PP was strongly enhanced by co-milling PP with iron. Raman spectra of PP during pan-milling indicates that the amorphization of PP was resulted from molecularmotion, chain deformation and deterioration of packing regularity caused by deformation under stress. The mechanism of the changes of PP caused by pan-milling was explored.(2) The size reduction of iron particulates is caused by stretching, compression, developing of microcracks and fragmentation during pan-milling. TEM shows that a part of the iron particles gets down to 40~120nm in dimension after 25 cycles of milling is offered. The particle size distribution analysis (PSD) shows that the PP/iron co-powder pan-milled for 30 cycles has a mean diameter of 9.6 um with multi-peak distribution. After 25 cycles of milling, the grain size of iron gets down to 22nm as calculated by Scherrer equation. The PSD suggests that the size reduction of PP is more effective when PP was co-milled with iron particulate. The experimental results indicate that PP and iron formed interfacial action during pan-milling. Based on the results of particle size analysis and SEM observation as well as microstructural characterization of PP/Fe co-powders, a pulverizing mechanism is proposed.The mechanical properties of PP/Fe composites indicate that Izod impact strength increased from 2.86 kJ/m2 to 4.52kJ/m2, 68.7% higher after 25 cycles of milling. The dynamic mechanical thermal analysis suggests that the loss modulus of PP/Fe composites increased with cycles of milling. By establishing the relationship between loss modulus and pan-milling, a empirical formula describing the pulverizing-dispersing property of polymer/filler blends brought about by pan-milling is suggested.Based on the SEM and EDS analysis, it is reasonable to assume that pan-milling can improve the dispersion of Fe particles in the PP matrix. The experimental results suggest that pan-milling offering a new route to the preparation of polymer/metal composites with well-dispersed metal fillers.(3) In present paper, pan mill type equipment is developed to make fine or ultra-fine waste rubber powder and polypropylene/waste tire rubber composite powder at ambient temperature. The BET specific surface area analysis showed thatABSTRACTthe specific surface area of waste tire rubber chips increased with cycles of pan-milling, and reached 7.23 m2/g from 0.0073m2/g after 25 cycles of milling. The analysis results of PSD and SEM suggest that the size of WTR powders gets down to several micrometer from tens of millimeter with wider distribution. SEM reveals that the shape of WTR powder is irregula...