The Sweet Smell of Success in Coatings and Adhesives

After poor visual appearance and improper physical properties, objectionable or unacceptable odor probably ranks as the third most common cause for rejections of finished products in the converting industry.

Odor is somewhat subjective — what smells bad to some people may smell good to others. However, because most items made by converters are for food products or other consumer goods rather than industrial applications, odors of almost any sort are unacceptable.

Most adhesives and coatings use high-molecular-weight polymers as the principle active ingredient in their formulations. Other ingredients such as tackifiers also may be polymeric. Generally, polymers themselves do not have any characteristic odor. If they do, the cause often is due to the presence of some unreacted monomer. For example, small amounts of acrylic monomers can readily impart an odor to an acrylic polymer that most people will find offensive.

Some ingredients in adhesives and coatings are basically inert. Examples are fillers such as clay or pigments such as titanium dioxide or zinc oxide. These materials essentially have no odor. Other materials with relatively low molecular weight compared with polymers such as oils or plasticizers may have characteristic odors. When a formulator is selecting oils, plasticizers, or similar materials, he or she must consider their potential to impart odor to the final product.

The discussion so far has concentrated on the active ingredients in adhesives and coatings. Obviously, the manufacturers of these materials are the only people that can control the odor of a material as they supply it. Such manufacturers are, therefore, extremely careful when formulating and preparing adhesives and coatings to ensure odor-free products.

At the next step in the converting process, the coating and drying operation, a very common cause of odors is failure to dry materials containing solvent completely. Even very small quantities of solvents such as heptane, methyl ethyl ketone, and others retained through incomplete drying can readily impart an odor. Causes of retained solvent can be insufficient temperature during drying, excessive line speed, and high coating weights.

Contamination is a frequent cause of odor in adhesives and coatings applied to a substrate. This normally occurs inadvertantly in the facilities of the formulator or at the converting plant. Contamination can happen through the improper use of an ingredient or an ingredient that is itself contaminated.

An odor test is very easy to run for a coated or laminated product. Place a specific area of coating or lamination in a clean jar and seal it with a lid. In the case of a lamination, the lamination should be cut with a razor or scissors into strips to create additional edges for any odor-causing materials to escape from between the plies of the laminate.

Usually, the aging occurs in an oven at an elevated temperature to force any odor-causing ingredients into the atmosphere of the closed jar. This test requires individuals that are very adept at determining odors to evaluate the atmosphere of the jar when opened.

In some industries other than converting, the addition of masking agents or odor absorbers and neutralizers is possible. These usually are impractical in converting, however, since the use of these types of additives can interfere with the normal functions of an adhesive or coating. In addition, the odor of a masking agent can itself be unacceptable.

Odor in converted products is a potentially serious problem. Everyone in the chain of operations involving an adhesive or coating has a responsibility to minimize the circumstances than can cause odor. Each operation essentially must rely on the previous operation to supply an odor-free product.

If you want one key point to remember: Avoid contamination when using an adhesive or coating.

David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at e-mail: dbentley@unm.edu.


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