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FLEXO Magazine : May 2008
TECHNOLOGIES & TECHNIQUES TABLE 1: "GENERATIONS" OF SOLVENTLESS ADHESIVES Generation 1 Component or Isocyanate Characteristics 2 Component 1st lK MDI · Moisture Curing · Needs Porous Substrates 2nd 2K MDI . Low Viscosity . Broad Adhesion Profile . Some Isocyanate Monomer 3rd 2K TDI · Low/No Isocyanate Monomer · Higher Viscosity 4th 2K Aliphatic · UV Stable . Low/No Isocyanate Monomer . Also called 3rd Generation Aliphatic The one-part systems are based on urethane prepolymers that are endcapped with isocyanates. These isocyanate groups on the prepolymer rely on moisture from one of the sub- strates (typically paper) to cure the adhesive and create the bond strength. The two-part systems also contain a urethane prepolymer that is endcapped with isocyanate (as one com- ponent), however this is mixed with the second component (a polyol) before applying to the film. The isocyanate and polyol react with each other to form a crosslinked adhesive network that builds the bond strength. These two-component systems do not rely on moisture for crosslinking. One-Component (lK) Urethane prepolymer + H 2 0 = crosslinked adhesive network Two-Component (2K) Urethane prepolymer + polyol = crosslinked adhesive network When the one-component and two-component systems are applied, the adhesives are much lower in mo- lecular weight than solvent-based and water-based adhesives and have very little peel strength or shear strength. The lamination must be rolled up and re- main under tension as the adhesive reacts and the molecular weight increases. These laminations can take from one to multiple days before they can be processed further. During this period the adhesives are in continuous contact with the inks and without the proper choice of the ink and the adhesive, a poor perform- ing lamination results. This may be observed with poor adhe- sion to the inks, splitting of the ink caused by the adhesive soaking in and softening it, delamination of the films and other possible symptoms. lIb/ream (1.6g/m2). The weight ratios of the prepolymer to the polyol were modi- fied to maintain a 1.4 fold excess of iso- cyanate functionality to polyol function- ality. The laminations remained on rolls under tension until they were tested. Peel strengths were measured on a Thwing- Albert Peel Tester at 12in./minute. Values are given in g/lineal inch. RESULTS AND DISCUSSION Traditionally, the inks that have been used in flexible packaging have been solvent-based due to the higher performance of the solvent systems. But as environmental concerns increase due to the solvent emissions and the cost of solvent continues to rise , many new water-based ink technologies have been developed that rival these solvent systems. One crucial element for the success of using water-based inks in flexible packaging has been the interaction of these inks with the solventless adhesives. If water-based inks can- not be used with what has become the standard adhesive technology in the industry, then their use will be limited. We have studied the interaction of various solventless technology systems and their performance on a variety of water-based flexographic inks. I: The Effect of Prepolymer Composition. In order to inves- tigate the effect of prepolymer composition on the adhesive characteristics of a two-component solventless system with wa- ter-based flexographic inks, a variety of prepolymers were test- ed with one control polyol. Polyol C is a general-purpose polyol specifically formulated for good performance and appearance. A summary of the prepolymers tested is listed in Table 2. Prepolymer 1 2 3 4 EXPERIMENT Samples were mixed in the desired weight ratios and ap- plied to the primary web at a coatweight of approximately - Generation Composition NCO (%) Mix ratio (wtfwt) 2 Polyester/MDI 18 (0.85/1.0) 2 Polyester/MDI 15 (1.0/1.0) 3 Ultra low TDI 7.5 (2.0/1.0) 4 Aliphatic 22 (0.7 /1.0) The second-generation isocyanate system consisted of one polyester-based (Prepolymer 1) and one polyether-based (Prepolymer 2) MDI prepolymer. Both of these prepolmers have relatively high isocyanate monomer levels (18 percent and 15 percent NCO, respectively). The third-generation product (Prepolymer 3) is a stripped (ultra-low monomer) TDI system (<0.1 percent free monomer) with a polyester backbone. Finally, a 4th generation (Prepolymer 4) aliphatic isocyanate system was tested. These 4 prepolymers give us a wide range of chemistries to see how this affects the performance with water-based inks. MAY 2008 www.flexography.org FLEXO