Rheological Modification of Lubricating Greases with Recycled Polymers from Different Plastics Waste

There is a growing interest on the development of new materials based on recycled polymers from plastics waste, since the use of such plastics represents a low-cost source of raw material. The purpose of the present work is to evaluate, from a comparative point of view, the effect that different waste and virgin polymers, used as viscosity modifier additives, exert on the rheological properties of standard lithium lubricating greases. Grease formulations containing diverse polymers, differing in nature and/or origin, were manufactured and rheologically characterized. Particularly, the influences of the type of polymer, the molecular weight and the presence of carbon black, used as filler in recycled polymers, have been evaluated. Small-amplitude oscillatory shear (SAOS) and viscous flow measurements were carried out, as well as calorimetric and thermogravimetric analysis. In general, recycled polymers induce a more important rheological modification than virgin polymers of the same nature. Thus, the addition of some recycled polymers such as HDPE, LDPE, PP, and EVA copolymer to lithium lubricating grease significantly increases the values of the rheological parameters analyzed. The crystallinity degree, mainly dependent on the nature of the polymer, and the carbon black content of recycled polymers have been pointed out as the most highly influencing parameters on the rheology of the lubricating greases studied. However, an apparent loss in mechanical stability for lubricating greases containing recycled polymers has been found when they were submitted to a severe mechanical treatment.