The Physics of Cleaning, Part 4: Teas Diagram
(Teas and Sympathy for Solvent Selection)
Ed Kanegsberg
Cleaning is the process of removing soils, matter out of place. Many
cleaning processes work primarily by dissolving the soil. Understanding
how solvency
processes work can help determine the best cleaning agent to use in a
particular application.
The adage “like dissolves like” has been familiar to most of
us since our earliest childhood science classes. Like most adages, it contains
more than a germ of truth. Water-like molecules such as ethanol dissolve
readily in water. Oils and oil-like molecules tend to dissolve in one another.
However, to cite another adage, “oil and water don’t mix.”
While “like dissolves like” is a fairly good guideline, science
does not operate by sayings but rather by physical interactions. In this
installment of “The Physics of Cleaning,” I will introduce
the Teas diagram, a tool for indicating relative solvency.
In earlier installments of this series, I discussed the three classes
of forces that cause materials to stick to one another: polar,
hydrogen bonding,
and non-polar. Most compounds are characterized as having a mix
of the three forces rather than purely one type. All liquid and solid
compounds
have non-polar
forces; these are a consequence of the proximity of one molecule
to another. There are some compounds that are purely non-polar
but
there
are none
that are purely polar or purely hydrogen bonding. The ratios and
the absolute
strengths of these forces play critical roles in determining the
solvency of one substance in another.
A Simple Triangle
A convenient way to depict the relative strengths of the three
forces is by a Teas Diagram. If the relative strengths of the
forces are
known, a compound
can be plotted in an equilateral triangle. The three legs of
the triangle each represent the percentage of one of the three forces
for that compound.
For instance, a compound that is characterized as 35% polar,
15%
hydrogen bonding, and 50% non-polar would be plotted as shown
in Figure 1.
Figure 1. Teas Diagram (from J. Burke, “The Handbook for Critical Cleaning”,
CRC Press, 2001)
One way to look at a Teas Diagram is to consider the analogy
to a color triangle. Each color can be depicted as having
a fixed
ratio
of three
primary colors
(magenta, cyan, and yellow). So a Teas Diagram in effect
depicts the “color” or
style of the solvent.
How a triangle helps with cleaning
How can the Teas Diagram help you to optimize your cleaning
process? If you know the nature of the soil, it can help
you to pick an
appropriate solvent
to remove that soil. For instance, many manufacturing machining
processes involve oils used as lubricants or coolants.
Once the part has been
machined, the oil needs to be removed. Oils are largely
characterized by non-polar
forces. Therefore, a highly non-polar solvent (one that
would be depicted near the lower right-hand corner of the Teas
Diagram) would be a likely
candidate. Alkanes such as pentane and hexane (100% non-polar)
and mineral spirits (~90%
non-polar) are effective solvents for oil-based materials.
Water
(18% non-polar) is ineffective as a solvent for oils. However,
water-based cleaning agents
are used as part of many cleaning processes. We will provide
details in future installments of this series.
Suppose you need to change the solvent, but you do not
know the ratio of the polar, non-polar, and hydrogen
bonding forces
of the
soil.
If you know
that a particular solvent has proven to be effective
in the past, you may be able to select another solvent that
has
similar force
ratios.
For instance,
suppose you had been using HCFC-141b as a degreasing
solvent. Since production and use of this solvent was stopped a
few years ago
due to its effect
on the stratospheric ozone layer, other solvents that
plot near these on a Teas
Diagram would be logical replacement candidates. HCFC-141b
was itself an interim replacement for CFC-113 (Freon)
and 1-1-1-trichloroethane, that had
been phased out a decade ago. Similarly placed solvents
include halogenated
compounds such as perchloroethylene (PCE), trichloroethylene
(TCE)
and normal-propyl bromide (nPB).
Solvency clusters
A number of common cleaning solvents are depicted on
a Teas Diagram in Figure 2. Note that there are some
clusters.
Chlorinated
and
brominated solvents
form a cluster in the lower right, close to the purely
non-polar “yellow” corner.
Acetone, methyl acetate and Parachlorobenzotrifluoride (PCBTF) form another
cluster, somewhat closer to the polar “red” corner. Water is
not close to either of these clusters but is much closer to the hydrogen
bonding “blue” corner.
Using a Teas Diagram alone may be a good screen but does not give the full picture of whether or not a particular chemical will be useful for your particular application. For example, HFC 43-10 mee (used in the Vertrel products), is in the same cluster on the Teas diagram as other halogenated solvents, because it has the same solvency style. However, it is a very weak solvent; while the ratios are similar, you also have to consider the absolute numbers, the actual strengths of the forces. Hansen solubility parameters (another solubility tool that will be discussed in the next installment) provide those absolute numbers. In the final analysis, no amount of theoretical prediction will replace actual testing; these solvency tools can be used to limit the scope of the needed testing so that you do not need to do a doctoral thesis on your project.
In future installments, I will discuss some other
solvency tools and additional considerations that
must be made
to select the best
process.
