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Solvent Retention in Packaging

Solvent-based gravure and flexo inks used in packaging always retain small amounts of solvent in the final package. When measured and controlled, this is not a problem. However, excess solvent retention will cause the package to have an odor and can spoil the taste of food products.

The human senses of taste and smell are very acute, and residual levels of solvent in the 1-100 parts per million (ppm) range often can be detected readily and found objectionable. Even though there is always a physical barrier between the ink or other coating and the packaged product, the residual solvents can migrate partially into the product headspace or into the product itself.

Having established that solvent retains can be a problem, it is necessary to employ quality measures to guide manufacturing operations. Over the years, the major consumer product companies have come up with limits on acceptable solvent retention.

This standard will have an overall upper cumulative limit for all detected solvents. It also would have limits within this total for individual solvents. Each individual solvent would have a limit based on its tendency to be an odor or taste offender. The limit for ethyl acetate, for example, may be one-tenth of that for ethyl alcohol.

The specifications vary by consumer company, the product being packaged, and the package construction. It is based on what is found acceptable in each instance. This level of acceptability is first established by experienced taste and odor panels, which are run in parallel with gas chromatography (GC) readings to quantify the retention levels of the various solvents. Product supplied by the converter then must fall within these specifications.

The table below is an example showing some typical GC reading numbers for a flexo lamination print. Sample and specification readings are shown in two commonly used units. These are milligrams per ream (3,000 sq ft) and milligrams per square meter. The ppm numbers for the retained solvent are calculated from the basis weight of the substrate or finished construction. In the converting operation, samples are taken at the startup to confirm the press setup is capable of producing acceptable GC readings. This normally is not an issue since the press settings would have been established on a qualifying run. In practice, many problems can occur and for many different reasons. Here are some examples:

  • The most obvious problems come from changes on the press, such as line speed and dryer settings, or a change of reducing solvent in the ink or coating. Any and all of these adjustments may have been made to correct a print issue.
  • Rotogravure print generally has higher retains due to thicker ink films requiring more solvent to be removed.
  • A design change in which more ink coverage, heavier ink laydown, and more colors are used will result in higher retains. It should be noted that retained solvent increases exponentially with any of these design changes.
  • Use of metallic inks can cause higher retains since thicker ink films are required, and the metallic platelets in the ink present a physical barrier to solvent evaporation.
  • Use of a primer or lacquer will add to the retains.
  • For solvent adhesive lamination, readings off the press may be acceptable but must be rechecked after lamination. Where acceptable readings have been achieved with a specific ink and adhesive combination, a change of adhesive would require new qualification tests.

Corrective action for high GC converted product is expensive. Sometimes a further pass through the press can avoid discarding out-of-spec material. It should be noted inks other than solvent based can be sources of odor. For instance, in water-based ink, the solubilizing amine can be retained and cause problems not detectable by the GC test. In many oil-based inks for litho printing, the chemical drying process itself generates odorous side products, and special low-odor products are selected for odor-sensitive applications.

So, everything shipped within GC specs is acceptable for odor? Not always. Along with the GC test, an odor test by a human should be performed since there is the possibility that an off odor could be generated by a contaminant in manufacturing or in any of the input raw materials.

Solvent Spec mg/rm Sample mg/rm Spec mg/m2 Sample mg/m2
Ethyl Acetate 200 139 0.7 0.5
Methyl Ethyl Ketone 100 56 0.4 0.2
Ethyl Alcohol 1,500 836 5.4 3.0
Toluene 100 56 0.4 0.2
Total 1,900 1,087 6.9 3.9

Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; djvargent@sbcglobal.net.


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