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Sunday, March 15, 2009

Lube Notes: Petroleum and Synthetic Oil Base Stocks and Additives

In the preceding Lube Notes, we covered basic lubrication, oil functions, additives and base stocks. Now, it’s time to construct finished lubricating oils. From what we have learned, it may seem like the only thing we need to do is pick a base stock oil, mix in some additives and presto, we have lubricating oil. If only it was that simple. Of course, it’s not.
Previously, we looked at the refining of petroleum and classifications for synthetic and petroleum oils. The base stock with which we choose to start will obviously have a direct bearing on the quality of the finished product. If cost is no concern, then all finished oils would be made using one of the synthetic base oils since they result in the best lubricating oils. However, cost is an important factor and will always be a consideration in choosing base stock oils. Most oils are manufactured by a reverse process where the final performance requirements dictate the quality or lack of quality of the ingredients. If the manufacturer is making an oil to meet the minimum performance criteria for the current classification, then no money will be spent on anything more than an adequate base stock. On the other hand, if the manufacturer is producing a high performance oil, then he will spend what is reasonably necessary to produce the final product’s higher level of performance.

Selecting a Base Stock

Considering that 95% of modern motor oils are multi-grade oils, the base stock oil will be a determining factor in the amount of Viscosity Improvers (VI) added to achieve the grade range of the finished oil. The lower the quality of the base stock, the more viscosity improver required in order to make the oil multi-grade (i.e., 5W-30, 10W-40, etc). VI additives are long chain, high molecular weight polymers that may serve some additional functions such as pour point depressants or dispersants. They are expensive and under extreme stress may suffer mechanical sheer. In most cases it makes sense to use better quality base stock oils in order to use less viscosity improver additives. Some synthetic base stock oils (Poly Alpha Olefins – PAOs and Esters) have such high natural Viscosity Indices that little if any additives are used to achieve the multi-grade finished product. These synthetics can pass the 5W viscosity test (winter) and the 30 viscosity (operational viscosity at 210*F) without the aid of viscosity improver additives.
Today, most petroleum base stock oils come from Group II (see table A). Group II oils are a definite improvement over Group I oils and current petroleum based oils are much better than those of 20 years ago. Synthetics are a little more varied than petroleum base stocks and are split between Group III hydro-cracked petroleum (addressed in the Summer ‘08 issue), PAO’s and Esters. Today, most synthetics for motor oils are made using Group III base stocks. Mobil and Amsoil continue to use Group IV PAOs (Amsoil blends Ester with PAO) with Redline oils coming primarily from Group V Esters.

Not all oils are created equally. The above chart shows the performance of seven motor oils in two catagories: oil evaporation expressed in percentage and pour point expressed in degrees fahrenheit. The two better performers (green) relative to the others are synthetics; the second from the left, the poorest performer listed, is a synthetic blend – as is the oil furthest to the right.

Selecting Additives

As detailed in the Spring ‘08 Lube Notes, oils cannot meet the requirements of the finished product without the aid of additives tailored to those requirements. Additives are chemical compounds designed to enhance specific properties of the finished lubricating oils. They can either add something new to the lubricating oil or enhance an existing property. All additives are not created equal. The quality of additives varies over a very wide range and you get what you pay for. Additives are selected to support the final product and vary for motor oils, gear oils and transmission oils. These different applications require the oils to function in specific ways to provide lubrication for the specific application.

API Classification

It has been said, “If you don’t know where you’re going, you probably won’t get there.” When formulating lubricating oils, this is more a law than a quaint phrase. It is imperative to know the required functions and performance of the finished oil prior to starting the design. The API (American Petroleum Institute) forms a committee to collect the requirements from the Original Equipment Manufacturers such as General Motors. Then the OEMs and the major oil companies hammer out the specific lubricating properties for a proposed classification. A series of performance tests are selected or developed and certified by the ASTM (American Society for Testing Materials) for qualifying prospective oils that meet the classification standards. Following this, the oil manufacturing company can begin to develop formulations of finished lubricating oils for testing to insure they meet the latest API classifications.


Blending the Finished Oils

Blending lubricating oils is part science and part art. Experience is as important as hard chemistry. The results one might expect for a given chemical equation are not always exactly what you experience with the finished product under real-world evaluation. Field trials are sometimes the only reliable tests to determine how a particular formula performs. Because of this, most companies begin in the lab with expected concentrations of additives and perform the required certification tests. The results of the lab tests help make adjustments to the additives and maybe the base stock as well. The level of quality demanded by the manufacturer will determine how much time is spent trying to achieve the best balance with regard to additives. The quality and type of base stock oil will affect how much of a particular additive must be used to overcome a weakness in the base stock oil. For example, petroleum oils are particularly susceptible to oxidation and require oxygen stabilizers whereas synthetic PAOs resist oxidation naturally and require minimal oxygen stabilizers.
As you can see by the upper half of the graphic on the opposite page (figure one), synthetics are much less volatile than petroleum oils. Uniform molecular structure reduces the evaporation of light weight molecules, enhancing the synthetic oil's ability to endure elevated temperatures without thickening. On the other hand, the purity of the synthetic base oils, – contaminates such as wax are eliminated – allows for significantly lower pour points and, in turn, easier starting in cold weather conditions (lower half, figure one).
Once the correct blend of additives and base stock oils is determined and all lab tests and selected field trials are complete, mass production begins. Oils are blended in various size tanks and mixed to insure uniform concentrations. The quality employed by the blending team is directly reflected in the consistency of the finished product. Samples are drawn and compared to required chemistry to insure the blend is acceptable and that the containers are filled with oil meeting the specifications on the label. Quality varies within the industry. Every year, spot testing around the country reveals oils that do not meet the standards proclaimed on the label of the container.
It is a shame these reports of failed spot checks are not published for the public to see. Companies are sometimes fined, but it would be nearly impossible to find any paper trail of these fines. Remember, you usually get what you pay for.
You may have heard the statement, “Oil is Oil, there’s really no difference. It doesn’t wear out, it just gets dirty.” Hopefully, the last four Lube Notes have helped you to see this statement as a silly oversimplification of the actual complexity of lubricating oils. Next time, I will shift gears and compare petroleum-based oils with synthetic based oils. We will investigate when it makes sense to use one or the other of these oils in your diesel engine.

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