Effect of amorphous/recycled polypropylene ratio on thermo-mechanical properties of blends for lubricant applications

Abstract

This work focuses on the development of gel-like formulations for use as lubricating greases. These dispersions were formulated using mineral oil and recycled/amorphous polypropylene blends. In particular, the influence of the polypropylene concentration ratio (Wi) on the rheology, thermal and some lubricant performance properties of polymer/oil blends were studied. The rheological responses of these systems were quite different, influenced mainly by the concentration of amorphous polypropylene. At intermediate temperatures (25 °C, 50 °C) and high recycled polypropylene concentrations (W_i > 0.5), the evolution of small-amplitude oscillatory shear (SAOS) functions was very similar to those found with other commercial lubricating greases. Samples with lower concentrations of amorphous polypropylene (12.5%) present maximum ${{\text{G}}_{\text{N}}}^{\mathrm{°}}$ values in the temperature range studied. An Arrhenius-type equation can be used to quantify the thermal dependence of the linear viscoelastic functions of these gel-like dispersions. In general, sample formulations studied exhibited values of the friction coefficient comparable to those found for traditional lithium lubricating greases.

Introduction

Nowadays, especially in the developed world, there is great interest in the use of recycled resources for industrial applications for their environmental benefits. In general terms, there has been a marked tendency to increase the use of recycled materials as a result of government legislation and increasing public concern for the environment. According to statistics from the Association of Plastics Manufactures in Europe, in 2010, polypropylene represented 19% of thermoplastic consumption in Europe. In terms of market share, this is the largest of the “big five” plastic types (polypropylene, polyethylene, polyvinyl chloride, polystyrene, polyethylene terephthalate) in Europe [1]. Isotactic polypropylene can be produced from low cost petrochemical raw materials, making it an inexpensive thermoplastic, relative to others, while showing good processability and excellent chemical and physical properties [2]. However, due to its chemically stabilized state during its life and high volume-to-weight ratio, it is one of the most visible forms of waste in landfills [3]. One industrial application for this waste material is as a modifying agent for lubricating greases, since the resultant blends show similar or improved performance compared to virgin polymers [4], [5]. Lubricating greases are semi-solid colloidal dispersions of a thickening agent (usually a metal soap) in a liquid lubricant (mineral or synthetic oil). Greases are the preferred lubricants in hard-to-reach places involving mechanical abrasion or dynamic systems. Many important functional properties, such as their ability to flow under external forces, mechanical stability under shearing, thermal susceptibility, dripping and spattering, etc., depend on the nature of their components and the presence of some performance additives [6], [7].

Polymers with ability for swelling and gel formation in organic media could find a wide application in industry as thickening agents. The main advantage of these gel-like dispersions is that they do not require extensive manufacturing expertise for production, in contrast to traditional soap-based lubricating greases, which require a particular and rather sophisticated technology with several processing stages [8]. Previous studies [9], [10], have explored an approach to the development of new polyolefin/oil blends using high-density polyethylenes (HDPEs) and polypropylenes (PPs) as potentially applicable lubricating greases. From the results, it may be concluded that linear viscoelasticity functions increase when using polyolefins instead of traditional lithium thickeners, although a lower density of entanglements is also found, yielding higher values for the loss tangent. This behaviour is especially observed in HDPE/oil blends, which also show different frequency dependence of SAOS functions. In contrast, viscoelastic moduli for PP/oil blends are similar to those obtained with traditional lithium lubricating greases [10]. However, in spite of the promising results, the use of recycled polypropylene imposes some limitations on the final product, relating mainly to the maximum working temperature that can be achieved, poor mechanical stability and issues relating to sample expelling at high shear rates [9]. This work, explores the possibility of using blends of recycled/amorphous polypropylene as the thickener agent to formulate gel-like dispersions in mineral oils, potentially applicable as lubricating greases. The combined effect that both polymers, used as thickener agents, exert on the rheology, thermal properties and some lubricant performance properties of the resulting gel-like dispersions in mineral oil was also evaluated.