But what, the non-chemists among us might be thinking, do we mean by an alpha olefin, let alone a polyalphaolefin or even an olefin? An olefin in organic chemistry – the branch of chemistry that looks at carbon compounds – is an unsaturated hydrocarbon molecule that contains at least one carbon-to-carbon double bond. Alpha olefin means the double bond in the linear alpha olefin feedstock is solely in the alpha position, between C-1 and C-2 in, for example, a C-10 normal (linear) paraffinic chain.

The LAO feedstock is usually made from polymerizing (forming multiple molecules into larger molecules that contain repeating structural units) ethylene gas – the output of an ethane or steam cracker – hence the LAO produced in this way is always even-numbered in carbon number terms since ethylene molecules have two carbon atoms. Once we have that C-10 feed, we can then relatively easily polymerize that into the base stock boiling range (C-20 to C-40) by dimerization or trimerization, etc., using a variety of catalysts, but all essentially Lewis acids.

Other C-number feeds have been and are used for PAO too, such as C-12 and C-14 – again oligomers of these feeds giving us grades in the base stock boiling range.

So why does LAO make such good base stocks? Because it gives us a base stock with the minimum number of branches, which is good for oxidative stability. In fact, PAO is unsurpassed as a hydrocarbon base stock with maximal inherent oxidation resistance because of this quality. If we try chasing pour points to anything like what PAO has naturally, with API Group III for example, the amount of branching we have to put in detracts from the inhibited oxidative stability and hence renders the exercise counterproductive.

Most are aware that PAO is one of the costlier hydrocarbon base stocks. One reason for this is that base stock manufacturers are in competition for limited availabilities of LAO with manufacturers of household washing detergents, where the LAO feed provides an ideal biodegradable detergent feed.

Internal olefins are available and have been used for base stock manufacturing as poly-internal olefins but are less well defined and so more variable both in manufacture and performance. In fact, it is the molecular consistency of PAO that has allowed the almost free interchange of PAO slates for lubricants carrying API or ACEA claims only.

PAOs are known for their outstanding ultra-low temperature performance, especially cold crank viscosity and mini-rotary viscosity. This comes about despite the fact that the base stocks are not even dewaxed during manufacture. There is no need to dewax a PAO, since at the molecular level, PAOs have little tendency to self-associate and pack into wax crystals, compared with the longer, less symmetrical molecules of, say, Group III stocks.

Some of the earlier version PAOs had only a moderately high viscosity index – sometimes not quite 120 for a 4 centistoke fluid. But this does not matter for very low temperature properties, since the performance is all important, with even a relatively low V.I. PAO still having outstanding low-temperature characteristics, essentially divorced from V.I.

The more modern and higher V.I. versions of PAO are made using a more up to date type of polymerization catalyst – so-called metallocene catalysts, rather than traditional Ziegler-Natta catalysts. But even metallocene catalysts have been around for olefin polymerization for more than 30 years. The essential difference between metallocene chemistry and the traditional catalyst type is that metallocene catalysis is homogeneous – in that it is essentially soluble in the reaction medium – versus the older type, which was heterogeneous, using various kinds of support.

The principal benefit of metallocene chemistry being soluble is that it allows detailed stereo-chemical control in the synthesis, which then allows the optimization of the PAOs V.I. through getting more of the right shaped molecules. To reiterate, PAO does not really need the highest V.I. to get exceptional very low-temperature performance, but it is an advantage to have high V.I. when it comes to ensuring good control of oil film thickness at higher usage temperatures.

PAO polymerization is not limited to the distillate base oils boiling range with polymerization to 40 and 100 cSt (kinematic viscosity at 100 degrees Celsius) being easily achievable and thus providing synthetic brightstock options.

Biodegradation of base oils is an important attribute, and again PAO has an advantage. Biodegradation is a complex issue that I have dealt with in previous articles in this publication, but it is assisted in hydrocarbon base stocks through minimal hydrocarbon chain branching. The LAO sub-unit is essentially an unbranched n-paraffinic olefin, which is an ideal candidate for biodegradability performance. In fact, some hydrocarbon biobased oils do use some LAO feedstock to boost both biodegradation performance and production volumes, since many biolubricant feedstocks are inherently highly branched.

Whether PAO is worth the cost difference in the finished lubricant compared with a Group III-based version is down to the lubricant marketer to discuss with their supply chain managers, since both base stocks are usually capable of meeting the latest industry and original equipment manufacturer specifications. Where this is not the case, formulators will sometimes use the minimum PAO treat in a blend along with Group III to keep the cost of goods under control.