- November 30, 2003, Dr. Richard M. Podhajny, Ph.D. Contributing Editor
Packaging products consist of substrates, and they can contain adhesives, primers, coatings, or inks. Converting processes involved in package manufacturing can involve corona treating, ozone treatment, coating, laminating, printing, and metallizing. Combinations of these materials and processes can lead to unwanted odors.
The source of an odor is not always obvious. Retained solvents still are the primary source of odors in packaging products. Secondary odors can be created by chemical reactions that produce volatile odoriferous products.
Among the chemical reactions that can cause odor are oxidation, decomposition of materials under excessive process conditions, and formation of odoriferous products within the package construction.
The presence of ketone and ester retained solvents is the most problematic. These volatile solvents are the most odoriferous and can be objectionable at small concentrations. Alcohols, alkanes, and glycols are considerably less objectionable. However, even among alcohols, some are more objectionable than others; e.g., butanol and isopropanol are more odoriferous than ethanol.
Secondary sources of odor are much more difficult to identify. For instance, the auto-oxidation of polyamide resins is a known source of secondary odor that can be catalyzed by some metals. This auto-oxidation process can produce objectionable odoriferous by-products not evident immediately. This process requires the absorption of oxygen and moisture.
Most packaging products are printed using flexographic or rotogravure products that use a soluble recipe that minimizes odor through an appropriate selection of materials that do not possess objectionable odor. Other printing processes — such as lithography — are less suitable, since the inks dry by an oxidative process that produces odoriferous by-products, such as volatile aldehydes.
The use of water-based, UV-curable, and solventless materials can bring new potential sources of odor to the package.
Water-based inks and coatings contain various chemicals, including odorous ammonia, amines, and low-molecular acrylic monomers. Ammonia is removed easily from the package through normal drying, but some amines are more difficult to remove. The presence of amines can create an undesirable odor, which can migrate within the package construction. Acrylic monomers usually are not a problem due to their low concentration, but in some cases they can be detected. The presence of these materials is not detected easily by gas chromatography unless the GC is configured to detect these materials.
Use of UV-curable materials can produce “acrylic” odors that can be detected in the packaging. In addition to acrylic odors, some odors can be due to the photoinitiators or their decomposition products.
Some odor problems can originate in the converting process itself. For example, extruders can reach high temperatures at which organic polymers can decompose into small molecules that can be detected readily. “Hot spots” in the extruder can form pockets where thermal decomposition is possible.
Corona treatment is a high-energy process of creating oxidation products, including ozone. High temperatures as well as high levels of UV are produced, and this can degrade the substrate. For example, anti-oxidants can be decomposed by the corona treatment and can lead to unwanted product discoloration and produce undesirable odors.
As a result of advances in new materials, it is now more difficult to identify specific sources of packaging odors. Conventional use of gas chromatography for retained solvents will not guarantee odor-free products. New workable standards for acceptable odor levels must be established and test methods identified.
No matter how sophisticated the technology becomes, in the end it is the consumer's criteria for acceptable odor that dominates the landscape.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; firstname.lastname@example.org