Odor & Exempt Solvents

Published: May 31, 2005, By Dr. Richard M. Podhajny, Ph.D. Contributing Editor

Material Science

Requirements for low odor in flexible packaging change as new materials are introduced in printing and laminating operations. Retained solvent is the primary odor concern of the printer and converter, and tight odor standards are maintained for printed and laminated structures.

Although much of the flexible packaging industry has moved to water-based and radiation-curable technology, a significant portion still uses solvent-based inks and coatings.

Use of strong odiferous solvents such as toluene and methyl ethyl ketone has been reduced significantly. Most of the solvent systems use low-molecular-weight alcohols and esters, although we still use isopropyl acetate and n-propyl acetate in package printing and laminating applications.

Use of water-based systems reduces VOCs by replacing the bulk of the solvent with water, although some small levels of solvents typically are present. Odor in such systems is less dependant on residual solvent but more commonly on the retained ammonia, amines, or in some cases, residual acrylic monomers.

Alternative systems to solvent-based are water-based, radiation-curable, and exempt solvents that are negligibly photochemically reactive. Exempt solvents for inks and coatings include acetone, methyl acetate, volatile methyl siloxanes, p-chlorobenzotrifluoride (PCBTF), methylene chloride, and recently added by the US EPA, t-butyl acetate (TBAc).

Few of these solvents have been used successfully to produce compliant VOC inks and coatings, since their performance features have their own limitations. However, the introduction of non-HAP as well as VOC-exempt t-butyl acetate solvent offers the potential to reformulate some solvent-based inks and coatings.

TBAc is a good solvent for many ink resins and has an evaporation rate in the range of toluene and methyl ethyl ketone.

Low-VOC coatings using TBAc have been formulated for applications including urethane automotive refinishing coatings, nitrocellulose wood coatings, baking alkyd enamels, PSAs, industrial cleaners, as well as exempt solvents in flexo and gravure printing inks.

Use of these exempt solvents in flexo and gravure printing inks, adhesives, and coatings needs to be tested carefully as they can bring undesirable properties to the end products.

For example, the more promising exempt solvents, such as TBAc, have considerable odor which, if retained in a printed or laminated structure, can create offensive odors in the flexible package. Simply, you can’t have your cake and eat it too! TBAc is a great solvent and a slow evaporating one, but it has a strong and pungent odor.

It remains to be seen if this solvent can be incorporated into flexible packaging printing and converting without having to deal with another odor concern.

Incineration of conventional solvents used in the printing industry produces nitrous oxides (NOx), another precursor to photochemically reactive components in our ozone layer.

Use of exempt solvents actually may be more beneficial to the health of our ozone layer than using VOC solvents controlled by incineration. As a result, we may see more solvents added to the exempt VOC list if their photochemical activity proves negligible.

TBAc is one of the first solvents listed by EPA that provides low toxicity and negligible environmental impact. Unlike the other exempt solvents, this one has broader appeal due to its solvent power and drying characteristics.

It remains to be seen how well we can formulate inks and coatings using some of these exempt VOC solvents, but as with water-based and radiation-curable technology, they offer ink manufacturers, printers, and converters new options for meeting solvent emission compliance regulations.

It is necessary to establish the specific retained solvent limits of these exempt solvents to assure odor thresholds are well below detectable limits to the consumer. Every solvent has its unique odor characteristic, and its objectionable levels will be subjective. Odor panels will need to establish these acceptable levels.