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FLEXO Magazine : May 2008
TECHNOLOGIES & TECHNIQUES ø z - ø c CJ :: III ..... III - >< III ..... II. Using Solvent less Adhesives with Water-Based Inks in Flexible Packaging By Ronald R. Davies, Ph.D. T he food packaging industry has seen steady growth in flexible packaging in the recent past at the expense of rigid packaging. Flexible packaging consists of multiple layers of film bonded together with adhesives. This lamina- tion provides the barrier, sealant and appearance properties of the package. The majority of these laminations are produced with a reverse-printed ink on the inward facing portion of the outermost film. This ink layer then contacts the adhesive used to bond this film and the next film together. The adhesion between the ink and the adhesive are critical for the integrity of the whole package. The types of adhesives used to bond the various layers of the laminate together are solvent-based, solventless, water- based and radiation-curable (UVjEB). Solventless adhesives, which are based on two-part urethane chemistry, are generally the most cost-effective choice for making flexible packaging laminates. Solvent-based, water-based and radiation-curable inks have all been used to prepare exceptionally attractive packages. The majority of the printing today is done with solvent-based inks, but as environmental concerns increase, alternatives to solvent are desired. It has been generally assumed that solventless adhesives cannot be used in combination with water-based inks. This has been a limitation for converters desiring to laminate us- ing solventless adhesives. This paper will compare different solventless adhesives and their performance when applied on flexographic water-based inks. The results with various prepolymer and polyol combi- nations typically used in flexible packaging adhesives will be discussed. In particular, peel adhesion and ink decaling will be covered. While not every solventless adhesive works on every water- based adhesive, these experiments demonstrated that combi- nations of solventless adhesives and water-base inks do exist, which give bond strength and appearance that meet or exceed industry requirements. Contrary to what has been generally - accepted, water-based inks can be used in conjunction with solventless adhesives. Solventless adhesives have been shown to perform well on water-based inks. Regardless of type, all adhesives are applied in a continu- ous layer, typically to the outer film (the primary web) and mated to another film (the secondary web). With water-based and solvent-based adhesives the adhesive is dried first, then nipped to the secondary film. With solventless, no drying is needed before it is nipped to the secondary film. In most flex- ible packaging applications, inks have been reverse printed onto the primary web. This leads to direct contact between the adhesive and the inks. A schematic of a cross-section of a typi- cal two-ply structure is shown in Figure 1. FIGURE 1. CROSS-SECTION OF A TYPICAL TWO-PLY STRUCTURE. Because the adhesive is in contact with the reverse printed inks, the interaction between the inks and the adhesive is critical to the overall performance of the package. SOLVENTLESS ADHESIVES Globally, solventless adhesives are the fastest growing adhesive technology in film laminating. The application process involves metering a curable adhesive onto a multiple application roll con- figuration that applies the adhesive to a primary substrate, which is then mated to a secondary substrate via a heated nip. The chemistry of solventless adhesives is urethane-based with one-part (1K) and two-part (2K) systems available. The various types are commonly differentiated by what "generation" they are (Table 1). MAY 2008 www.f I exog ra p hy.o rg FLEXO