Lube-Tech
Over the past 80 years, the versatility of synthetic ester chemistry has led to the adoption of many classes of esters in the lubricant industry and include, for example, mono- and di-carboxylic acid esters, neo-polyol esters, complex esters and aromatic esters(2). The past 10-15 years has seen the development and advancement of estolides and high-oleic containing vegetable oils leading to the emergence of new bio-lubricant products.
Polyalkylene glycols (PAG) are also an option when formulating bio-lubricants. Many PAGs below a molecular weight of about 1500g/mol. (approx. ISO VG-68) offer high levels of biodegradability. It is known that they perform well in equipment when the thermal stresses are high. A significant advantage of PAGs versus many esters is their superior hydrolytic stability making them a preferred choice where water ingress into the lubricant is a concern. However, most PAGs today are derived from petrochemical feedstocks such as ethylene oxide, 1,2-propylene oxide and 1,2-butylene oxide(3), and this restricts their use where environmental regulations stipulate a high renewable carbon content. PAGs with much higher molecular weights often have low levels of biodegradability. Table 1 describes typical environmental aspects of many classes of base fluids that historically are used in formulating bio-lubricants.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.144 page 2
performance is to build the molecule using lower molecular weight blocks that can biodegrade in less than 28 days and linking them together with ester bonds. This “Designed for Sustainability” approach has led to the development of hybrid-esters that combine the functional performance advantages of both synthetic esters and PAGs together with the environmental profiles of natural esters. Starting with materials that are known to be non-toxic, non-bioaccumulating, and biodegradable and linking them together with a secondary ester bond that is significantly more stable to hydrolysis than a traditional neo-polyol ester (NPE) allows for the first base oils truly designed for a total lifecycle(4). The new family of hybrid-base oils developed using this approach are being described as secondary polyol esters™ (SPE). Below are some of their functional performance properties as base oils for environmentally acceptable lubricants. Unique features are highlighted which may provide advantages for the creation of future generations of bio-lubricants. More recently, formulations have been developed containing SPEs to demonstrate their suitability for use in equipment and especially in equipment operating in environmentally sensitive areas.
Chemistry of SPEs
Table 1: Typical biodegradability ranges and bio-based (renewable carbon) content.
A recent approach to developing higher viscosity base oils that maintain biodegradability without sacrificing
The versatility of ester chemistry is well known. For example, top-tier esters such as saturated neo-polyol esters are derived by reacting polyols such neo-pentyl glycol, trimethylol propane and pentaerythritol with saturated acids, and these esters are commonly used in applications such as refrigeration oils, compressor fluids and aviation turbine lubricants. They can be described as primary polyol esters since the esterification occurs on a primary hydroxyl of the polyol (Figure 1). Many saturated NPEs have excellent thermo-oxidative stability, good low temperature properties and are slower to hydrolyze than natural esters. Nonetheless, this hydrolysis is a limitation on their use in applications where there is a risk of water ingress into the lubricant.
LUBE MAGAZINE NO.173 FEBRUARY 2023 27
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