Rheological and mechanical properties of recycled painted automotive bumper materials
Creators: name: Quazi, Rauha
Publisher: name: RMIT University
Year: 2024

description: The use of plastics in the automotive industries has increased significantly in the past few years. Concerns for the environment and the management of waste is encouraging the recycling and proper use of scrap material. Among others, polypropylene (PP) based bumpers are targeted for recycling because of their large volume and relatively simple material composition. Most of these bumpers are coated with polyurethane based paint which becomes dispersed into the matrix during reprocessing influencing the material properties. This seemed to be an important issue to investigate as it was shown by some other workers that the lowering of properties by the presence of paint particles was more significant than the material deterioration itself. So, with the complete recycling process, it is important to understand how the dispersed phase influences the processing and later the behaviour of the matrix so that in future investigations this knowledge can be utilised in further development of the recycled material. This has been the main theme of the project with the following major objectives: (i) to remove paint particles from the painted PP bumpers by melt filtration technique (ii) to determine the rheological and mechanical properties of the rubber toughened PP based composites dispersed with paint particles and glass microspheres and (iii) to understand the effect of dispersed phase on these properties by studying the morphology of the composites.<br><br>Both commercially recycled material and real system blends prepared in the laboratory were studied to observe the effect of dispersed paint on the polymer matrix. Firstly the following materials were characterised using tensile, impact and rheological tests: virgin bumper bar grade material (PPIEPDMlta1c) ; two batches of material injection moulded from the virgin grade and painted with different levels of paint layers as found on car bumpers (unfiltered and melt filtered) and recycled painted bumpers (unfiltered and melt filtered). Extrusion melt filtration was chosen to remove the paint particles from the pp phase and the quantitative measurement of the dispersed phase was done by digital image analysis. Result of this analysis showed that only about 50% paint was removed following filtration. Little variation was observed in viscosity values for all the samples with unfiltered batches showing slightly higher and filtered samples showing slightly lower viscosity compared to the virgin material. The increased value for the first case was attributed for the paint particles acting as the particulate phase in the matrix. On the other hand, lower viscosity was caused by deterioration of material with chain scission of PP.<br><br>The elongation and impact strength of the paint dispersed batches decreased drastically compared to the virgin material as the paint particles acted as crack initiators and propagators. For the blends prepared, well characterised paint particles were dispersed into the PP/rubber/talc and PP/rubber matrix to be used as the "real" system. Talc itself is used as a filler in the commercial grades to impart greater stiffness, so composites without talc were worth examining to compare the effect of paint over talc. Also "model" filled systems with glass micro spheres were studied to observe the relative importance of particle shape and filler concentration. The choice to use glass as a model filler was primarily based on its spherical shape, non-deformable nature and relatively small surface area. The glass filled systems served as a comparison and helped in the interpretation of the rheological and mechanical properties of the paint dispersed in the rubber toughened talc filled and unfilled PP. The rheological study over the processing temperature range showed that the presence of small fraction of the dispersed phase (upto 1.9 vol% for glass beads and 2.7 vol% for paint particles) had insignificant effect on viscosity and at these levels the effect of particle size, particle size distribution and particle shape can be neglected. Only with increased filler fraction (5 vol %) a trend in increased viscosity was observed.<br><br>From mechanical testing it was observed that the retention of tensile yield strength was nearly 100% but elongation at break dropped significantly compared to the base matrices. This reduction was explained by poor interphase between filler and matrix. The modulus values for glass bead filled PP/rubber matrix showed an increasing tendency for high rigidity of the filler itself whereas for paint filled matrix it dropped below the base matrix value. For PP/rubber/talc matrix the scenario was different where a decrease in modulus was observed for all the composites. This was explained by the much high rigidity of talc particles over glass beads and paint particles. Though small sized talc particles acted as reinforcing fillers when added to PP/rubber system, the inclusion of second dispersed phase could not overshadow its effect on matrix thus failing as a weak structure. The drop in modulus for paint<br>dispersed matrices was more significant indicating that irregular shaped paint particles had a smaller degree of interaction with the base matrix compared to spherical glass particles.<br><br>A sharp decrease in impact strength observed for all the composites again explained the poor adhesion between matrix and dispersed phase which was also attributed from the photomicrographs of the fractured specimens. A separated microstructure was revealed for the composites which emphasised the explanation that the inclusions reduced the ability of the elastomer phase to absorb applied energy by reducing the distance between particles. A decrease in fracture energy and toughness for all the composites was also explained by the same.<br><br>Models were proposed to predict crack propagation through glass bead and paint particle dispersed in the PP/rubber/talc matrices. For glass beads, crack propagation through the composite was attracted to the equator of particles so fractured surfaces showed clear hemispherical holes and top surfaces of debonded particles. For irregular paint dispersed composites, crack propagated through the interphase of filler and matrix thus debonding the paint particles.<br><br>The composites under study were also related in terms of their rheology, morphology and mechanical properties. A separated microstructure of the composites as observed from the micrographs leads to the explanation of the slight increase in viscosity of filled composites over unfilled ones. Also poor interphase between filler and matrix leads to decreased property values in mechanical tests compared to base matrices.<br><br>