Investigation of High Temperature Stability of Tackifiers By: Erik Willett, Daniel Vargo Functional Products Inc. Macedonia, OH Abstract
Tackifiers are vital component of lubricant oils and greases. These products prevent fling-off due to the centrifugal force of high-speed parts, eliminate drip under the constant effect of gravity, and improve the feed of lubricant through mechanical systems. This feat is achieved by very high molecular weight polymers which impart an elastic-like behavior, often called ‘stringiness’ or ‘tackiness’, to oil and grease. The generation of heat and resulting oxidative damage under high performance/heavy duty applications is a key concern for establishing long-lasting stringiness in lubricants. The premiere tackifier chemistry, polyisobutylene (PIB), has a known vulnerability to heat. Base oil purity of lubricants and their additives can accelerate or slow the loss of tack in oil at high temperature. Tack in Group III/IV base oils shows excellent thermal stability which implicates certain components of Group I/II oils. Olefin copolymer tackifiers show greater thermal stability than PIB chemistry but are much less effective at adding tack to oil. This article presents a series of thermal studies on PIB and olefin copolymer tackifiers using Group III oil spiked with model compounds from different Group I/II impurities. It is shown that mono- or polycyclic aromatics have a complex relationship depending on the molecular weight of the tackifier. Sulfur demonstrates a beneficial effect while nitrogen accelerated degradation. Further testing led to the finding of a novel ‘tack preservative’ which extended the thermal stability of PIB tackifiers beyond the stability of competing olefin copolymers. A technical discussion of tackifier science and the underlying thermodynamics of thermal degradation is included.
I. Introduction
Tackifiers provide a unique form of non-Newtonian behavior separate from viscosity that aids a lubricant in remaining on a surface under shear. This is observed as tackiness or stringiness in that the modified lubricant will produce fine filaments of oil due to the tackifier additive. High molecular weight polymers (Mn > 200k, Mv >1M) pre-dissolved into oil are the active ingredient in these additives. Mn and Mv are conventions used to describe the non-uniform distribution of polymer chain lengths in a sample as either a population-weighted or sizeweighted average, respectively.1 Large polymers are known for their ability to impart many new properties to their solutions in oil or solvent.2 Background information about the chemistry and physics of polymers is given in Appendix 1. A tackifier product largely consists of two components: the base oil and a dissolved polymer of high molecular weight. Olefin copolymers (OCP) or polyisobutylene (PIB) at Mn>100K (Mv>1M) are typical current state-of-the-art polymers. Their structures are shown below in Figure 1. The tackifier is added to lubricants at treat rates of less than 1wt% to provide adherence between the oil and surfaces. The tack of a lubricant may be characterized by methods as simple as working the oil between one’s fingers to more quantitative methods like a ductless siphon.3 Ductless siphon, pictured in Schematic 1, is a reproducible method for quantitatively measuring the amount of tack in an oil sample. However, tack tests are performed at room temperature and do not encompass the range of conditions the tackifier product will experience during its service life.
Figure 1: Chemical structure of PIB and OCP polymers - 44 VOLUME 81, NUMBER 6
