Printech Archive
Re: VOC content/photochemical reactivity

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Date: Fri Feb 25 2000 - 13:34:09 CST


I agree with your opinion that VOCs and VOC content determination is one of
the more simpler, but confusing aspects of environmental regulation. The more
you look into it, the more complicated it becomes. As Dave Salman point out
in his response to your email, unless a chemical is placed on the list of
exempt compounds, it is considered a VOC. In order to get on the list of
exempt compounds, a formal petition needs to be prepared and submitted to
EPA. The petition needs to include the scientific data supporting the claim
for negligible photochemical reactivity.

In terms of determining VOC content, Method 24 is principal method for many
materials, principally inks and coatings. Method 24 allows for the
subtraction of water and exempt compounds from the final VOC content

While the above seems straight forward, the confusion begins in the
determination of the photochemical reactivity of any given chemical. EPA has
not published a formal protocol on this extremely essential subject. What
appears to be current policy is that the reactivity of the chemical in
question needs to be compared to that of ethane. Those chemicals whose
reactivity is greater than ethane are VOCs and those, whose reactivity is
less, can be placed on the exempt list.

Specifically, the reactivity is measured by how much ozone is produced on
either a molar or gram basis. The gram basis looks at the weight of ozone
produced per weight of test compound compared. The other measure is molar,
which is best thought as a concentration and not weight. EPA has changed its
position several times over the years on the on significance of the mole vs.
gram basis and it appears to be shifting back to the mole basis.

As background, acetone was added to the list of exempt compounds based on the
gram approach to measuring reactivity. When EPA delisted acetone and those
who follow this issue realized the significance of the shift in policy from a
molar to a weight basis, EPA was flooded with petitions, including the
petition submitted by the printing industry to delist ink oil. After the
acetone decision, EPA began to examine its revised policy and has decided
that a weight basis is far more difficult to defend and is not technically

In discussions with EPA on how photochemical reactivity is defined, acetone
should be viewed as an anomaly. While it seems to be certain that ethane will
be used as the benchmark for comparison, the measure of reactivity has gone
through some changes. Since the vast majority of chemical reactions are
examined on a molar basis, mole comparisons are more scientifically
acceptable. It is interesting to note that on a molar basis acetone produces
more ozone than ethane.

Therefore, in the methyl acetate decision, EPA stated that only those
compounds that produce less ozone than ethane on a molar basis would be
considered for delisting. The good aspect for methyl acetate is that is less
reactive than ethane on both a molar and gram basis. EPA's position in the
proposal to delist t-butyl acetate also further strengthens the molar
approach. However, EPA did solicit comment on how reactivity should be best

The standard approach to measuring photochemical reactivity is to use a smog
chamber, which simulates what would happen to the test compound as it is
released into the air. There are two active smog chambers in the US, one in
California and the other in North Carolina. Each smog chamber uses a
different approach to generate data. One uses a simpler urban air mix as the
matrix and UV lights, while the other uses a more complex urban air mix and
sunlight to drive the photochemical reactions. EPA generally likes to see
data from both chambers in evaluating the reactivity of a chemical.
Computer models complimented with actual research data have not evolved to a
point where a structure activity relationship can be easily predicted. There
is some research being conducted in this area, but it is not sufficient to
have definitive conclusion for the very near future. Some of the research has
also indicated that the reactivity of any given compound is greatly
influenced by the "environment" in which it is emitted. Several studies have
shown that the reactivity of ethane, which was once thought of being stable,
actually varies.

It is also interesting to note that some of the older literature on
photochemical reactivity indicates that hexane along with other chemicals is
not photochemically reactive. This conclusion was based on some early work
performed in the 1960's and 1970's. The concept of just regulating
photochemically reactive chemicals was first incorporated into regulation via
Rule 66, which was implemented in Los Angles. Other states quickly followed
suit and you still have today regulations on the books in certain states that
limit the emissions of photochemically reactive chemicals as well as VOCs.
These regulations, of course, add confusion to the entire topic of regulating
air pollution emissions.

The last point to be made in this overall discussion is the role of Method
24. Method 24 has served as the backbone of VOC content determination since
the early 1980's. However, it does have some limitations and has been
modified since its original release. The modifications have not kept pace
with evolutions in materials. More reactive cured (e.g., UV and EB) and
water-based materials are used today than in the 1980's and this has placed
demands on Method 24. While Method 24 has been modified, it needs to be
further modified. One good modification that occurred was to address VOC
content determination of reactive cured materials. The problem is that the
modification only accommodates "thick film" inks, paints, and coatings. The
method still can't be used for thin film materials, such as UV inks and
coatings used by printers. An ASTM subcommittee is currently working on a
modification, but until one is designed, approved, and adopted, it could be
years. In the interim EPA is working on some guidance.

The other major problem with Method 24 is that it can't tolerate materials
with high levels of water. While there are some precision adjustments
permitted, its precisions and reproducibility with water-based materials
leaves a lot to be desired. EPA has been working on this problem for several
years, but has yet to find an appropriate solution. EPA does have some
existing guidance on precision adjustments.

To a lesser extend, there have been problems reported with precision and
reproducibility with solvent-based materials, which were the materials that
the method was designed to measure. It seems that if the same sample is sent
to different labs, you can get different results. I only have a limited
amount of data on this topic and a thorough evaluation of this problem with
graphic arts materials needs to be performed.

I hope that the above answer has shed some light on your question. If you
have any more questions, let me know.

Gary Jones
Graphic Arts Technical Foundation
200 Deer Run Road
Sewickley, PA 15143
412/741-6860 x608 - Phone
412/741-2311 - Fax

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