20
MARINE TECHNICIAN TODAY
|
FALL 2014
I
n the preceding articles, we covered
the principles of lubrication and
the basics of motor oil formulation,
including oil functions, additives and
base stocks. Base stocks were touched
on briefly, and now it’s time to take a
closer look at the properties of base
stocks to better understand differing
performance qualities in 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.
When refining crude oil into usable petroleum, base stock groupings
are established to rate the oil’s physical characteristics. As you can
see in Table A, there are five groups and the physical and chemical
properties measured are Viscosity Index, Saturates, Sulphur and
Other. The Viscosity Index (VI) measures how responsive the oil
is to changes in temperature; a high VI reflects a slow response to
changes in temperature. Saturates are defined as the hydrocarbon
alkanes; in other words, this is the actual petroleum used for the
base stock; the groups are separated by percentage. Sulphur
content is measured with concordant limits established for the
groups; no sulphur would be the best. The category of “Other” is
reserved for synthetic base stocks that exceed the requirements
of the grouping process for petroleum base stocks. Synthetics are
pure hydrocarbons with very high VI and zero percent sulphur, so
they are in separate groupings.
Selecting a Base Stock
Selecting a base stock 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
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,
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.
Selecting Additives
As detailed in the previous volume of Marine Technician Today,
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
BASE STOCKS AND THE FINISHED PRODUCT
By DanWatson
,
MANAGEMENT TIPS
API BASE OIL CLASSIFICATION GROUPS (Table A)
Group
Viscosity Index
Saturates
Sulphur
Other
I
80-119
>90%
and/or
>0.03%
II
80-119
>90%
and
>0.03%
III
>120
>90%
and
>0.03%
IV
PAO
V
Esters and
all others
