Organic Compounds and Solvents
Barbara Kanegsberg
At the farmer’s market, “organic” refers to plants and
animals raised in a particular manner, without pesticides and herbicides.
However, an organic compound is not an assemblage of field-grown, free-range
atoms. An organic compound simply refers to any compound that contains carbon.
Carbon is essential to life and to many poisons and industrial toxins. Protein
and fats are carbon-based, as are most industrial compounds of interest including
cutting oils, lubricants, and solder fluxes. Many cleaning agents, even aqueous
cleaning agents, contain organic compounds.
When organic compounds or blends of organic compounds are used in cleaning, they are referred to as solvents. Those who are new to precision cleaning may find this concept of solvency to be a bit strange. After all, water is a great solvent for sugar. However, by general unspoken agreement, only organic chemicals are termed “solvents” in the context of cleaning. Using organic compounds are a reality of industrial life. Please be aware that even if a product is said to be solvent-free, it may contain organic compounds.
Atoms and molecules
Please keep in mind that an atom can be thought of as the simplest building
blocks (aside from sub-atomic particles, quarks, neutrinos, and all that ….).
In normal adult conversation, however, we need only deal with atoms joining
into molecules or bonding. Bonding is, to say the least, a bit stronger
than sharing a beer; changes in chemical bonds involve chemical reactions.
Carbon
can bind with up to four other atoms to form molecules. Typically, we are
concerned with carbon atoms bound to relatively few varieties of atoms,
hydrogen, oxygen, nitrogen, fluorine, chlorine, other carbon atoms, and
occasionally
silicon.
Solvency
Solvents operate on the principle that like dissolves like; or molecules
with similar structures tend to “hang out” together. So, for
example, we would like to choose solvents that are hydrocarbons (molecules
containing hydrogen and carbon) to remove hydrocarbon-based oils and lubricants.
Formulators, people who blend molecules together, look at three solvency
characteristics (or variables or parameters), specifically non-polar
(oily), polar (non-oily, almost more water-like), and hydrogen bonding
(a specialized,
useful type of non-oily characteristic). I myself tend to think of
these three characteristics sort of like the primary colors (red,
yellow, blue).
We can then choose just the right shade or blend or color saturation
to achieve just the right amount of solvency. It is important to
select the appropriate
type and degree of solvency. If the solvent is too weak, the soil
may not be removed rapidly enough; if it is too strong, the substrate
(the
object
you are trying to clean) may itself be damaged.
Some gentle solvents and blends
Sometimes, we want a very benign, gentle (some might say wimpy) solvent.
Molecules made out of fluorine bonded to carbon (fluorocarbons)
are often desirable specifically because they are not very reactive (i.e.
they
don’t
do very much) and because the fluorine suppresses fires. In precision cleaning
applications (where the part itself was very clean, but perhaps particles
had to be removed), very often the substrate itself (the object we are trying
to clean) is delicate and readily damaged; perhaps all we want to do is remove
particles. In such situations, classically, perfluorocarbons were used. “Per” is
a prefix meaning throughout or completely. The “per” in perfluorocarbons
means that the compound is fully fluorinated – no hydrogen getting
in the way. (So, if you have Perchloroethylene – you got it! Just chlorine
and carbon, no hydrogen.) Anyway, we liked the perfluorocarbons because they
were wonderful for high precision applications (they didn’t dissolve
much). Unfortunately, because they have a long atmosphere lifetime, perfluorocarbons
are thought to contribute to global warming. The manufacturers, proactively,
have developed more environmentally-benign hydrofluorocarbons and hydrofluoroethers.
Hydrofluoro means there is hydrogen and fluorine along with the carbon.
The ether means there is oxygen; the oxygen does not add aggressiveness,
in this
case.
More aggressive solvents and blends
Oxygen, depending on how it is assembled into the carbon molecule,
makes the molecule a bit more polar (water is very polar). Hydrofluoroethers
are very mild solvents. Alcohols contain oxygen, and many of
them behave in a
less “fat like” way than hydrocarbons. Acetone (the active ingredient
in classic nail polish remover) is a molecule called a ketone (the “one,” is
a dead giveaway, like methyl ethyl ketone). Ketones contain oxygen combined
in a double bond with carbon. That means the carbon can only bond with
two additional atoms. Many ketones are very powerful solvents because they
have
strong properties of polar, non-polar, and hydrogen bonding. Therefore,
ketones can be very useful in removing soils.
Chlorinated solvents like methylene chloride contain chlorine (the “chlor” is rather a dead giveaway for chlorine); brominated solvents like n-propyl bromide contain (you’ve got it!) bromine. These solvents tend to be aggressive. Some do not have a flashpoint. Trans-1,2,-dichloroethylene (known as trans); does have a flashpoint; but it can be combined with some of the gentle solvents to form a very stable, constant-boiling blend called an azeotrope.
Diversity; Chemical handling
A world with only aggressive solvents would be like an orchestra
with only trombones and drums. Industry needs a variety of
solvents, not
just aggressive
ones. It is possible to have too much of a good thing. Plastics,
elastomers, and other parts of the product can be dissolved
by aggressive solvents.
The preceding is only an introduction to chemicals and solvency. With all chemicals, we have to consider the worker safety profile and the environmental regulatory outlook. All solvents have to be managed carefully, whether or not they are under current regulatory scrutiny.
