Aqueous adhesive systems: giving credit where it is due.
- Published: August 01, 1996, By Bentley, David J., Jr.
The Div. of the History of Chemistry and the Office of Public Outreach of the American Chemical Society recently designated Kem-Tone wall paint as a National Historic Chemical Landmark. Initially marketed 55 years ago, this product from Sherwin-Williams was the first paint supplied as an aqueous system. Its ease of use revolutionized home redecorating by providing simple application and cleanup.
Recent improvements in laminating adhesives used for the flexible packaging industry show that developmental efforts with aqueous systems have achieved significant advancements. A two-component flexible packaging adhesive is available that complies with the composition requirements of Food and Drug Administration regulation 21 CFR 177.1390, laminate structures, for use at temperatures of 250 deg and above. With a high degree of adhesion to many substrates, the adhesive provides the heat, chemical, and water resistance needed for boil-in-bag and retortable applications. It is a polyurethane dispersion cured with an isocyanate.
Aqueous systems for flexible packaging laminations have existed for about 20 years. The first products were acrylic copolymers with a pressure-sensitive nature. Their performance allowed use only for low-demand applications such as snack food packaging.
With the advent of epoxy and amine curing systems with advanced acrylic copolymers, performance improved enough to extend the use of the aqueous systems to more demanding applications such as laminations for meat and cheese packaging. Now the PU dispersion and isocyanate adhesive system has advanced aqueous technology to include the most demanding heat-, moisture-, and chemical-resistance properties.
The development of the aqueous flexible packaging laminating adhesives parallels the history of solvent adhesives. Lamination of flexible substrates for packaging applications began approximately the same time as the introduction of Kem-Tone. Initial adhesives were formulated cellulonic products dissolved in solvent. Formulated thermoplastic rubbers soon followed. Both adhesive systems found use mainly in snack food packaging or similar applications. The introduction of two-component PU systems and the isocyanate cured polyester materials allowed extension to more demanding boil-in-bag and condiment packaging applications.
As the popularity of flexible packaging laminating adhesives in solvent declined due to environmental concerns, aqueous systems were considered. Earlier development programs in solvent adhesives followed a progression from easiest to hardest end use. Researchers in the aqueous field used the same approach. That is why the first adhesive was the p-s acrylic copolymer and the latest development was the more complex PU dispersion cured with isocyanate.
Why did the development of a high-performance aqueous adhesive take so long? Consider the paradox familiar to those with a chemistry background: What container would hold a universal solvent? The ketone, ester, and aromatic hydrocarbon solvents used in flexible packaging laminating adhesives were more universal in their solvency than water. They easily dissolved the polymers, resins, plasticizers, and other additives necessary to formulate a high-demand adhesive product. Water dissolves few, if any, of these ingredients and is less universal in its dissolving power than solvents for the typical ingredients used in adhesive formulating.
Different techniques were therefore necessary to develop aqueous adhesives for flexible packaging laminating. Essentially, researchers had to prepare the systems as dispersions or emulsions in water. This is more difficult than dissolving an ingredient in solvent. In addition, making an emulsion or dispersion usually involves incorporation of certain additives that interfere with final adhesion. Making flexible packaging laminating adhesives in water meant starting anew. This explains the progression from easy-to-prepare systems for less demanding applications to the more difficult products for difficult applications involving specific resistance properties.
Polymer chemists and formulators working for adhesive companies deserve credit for their development of aqueous adhesives for high-performance flexible packaging applications. Perhaps their work will be recognized someday with a designation such as that given Kem-Tone.
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.
