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FLEXO Magazine : May 2008
TECHNOLOGIES & TECHNIQUES TABLE 3: SUMMARY OF THE PERFORMANCE OF DIFFERENT PREPOLYMERS AND POLYOL C Prepolymer Generation Composition Peel Strength Peel strength Peel strength Peel strength (28 days) (g/in.) (4 days) (g/in.) (7 days) (g/in.) 1 2 Polyester IMDI 180 170 140 180 2 2 Polyester IMDI 200 210 160 190 3 3 Ultra 10wTDI 280 280 220 230 4 4 Aliphatic 170 180 140 170 Prepolymers of different chemical compositions were blended with Polyol C and OPP was laminated to LDPE using the resul- tant mixture. The OPP had previously been printed with a water- based flexographic ink that was predominantly red. The data in Table 3 summarizes the results of this experiment. It is interesting to note that the failure mode on all of these samples was a splitting of the ink. The stronger laminations had less splitting, meaning that they dove in and weakened the ink less than the weaker bonded systems. II: The Effect of Polyol Composition. In a similar manner, one prepolymer (Prepolymer 1) was mixed with a variety of polyols, of different compositions, to investigate how the choice of polyol can affect the performance on water-based flexographic inks. Polyol A is a polyester-based polyol, Polyol B is polyether-based and the Polyol C is a hybrid system. These 3 polyols provide a wide range of chemical composition and can show us trends in how these compositions affect the performance with a specific ink. The polyol characteristics along with the peel strengths of the resultant laminations are summarized in Table 4. This data clearly shows that the polyester polyol (Polyol A) pro- vides the strongest bond to the red water-based flexographic ink. The peel value is twice that of the laminations made from the other two polyols. The polyether (Polyol B) and the hybrid (Polyol C) gave similar results, however, these lower values might be ac- ceptable for many application. III: The Effect of the Color. In many instances, different per- formances will be seen with different colors of ink. This is often the case with both solvent-based and water-based inks. To test this, Prepolymer 1 and Polyol A were used to laminate PET to LDPE. The PET had been previously printed with a water-based flexographic ink job that contained areas of different colors. A comparison of the adhesive characteristics for the different col- ors can be seen in Table 5. These values range from around 500g/in. for the clear, blue and white to about 300g/in for the brown and red. The differ- ences (other than color) between these are not known, but vari- ous factors can affect the interaction of the ink and the adhesive. Co-solvents, which are typically low-boiling alcohols, are added to inks and will react with the isocyanate of the prepolymer. This would change the overall structure of the adhesive network. Another factor that can affect the bond strength is surfactants and wetting agents. These are added to stabilize the pigments and can be different with different colors. They can have an ef- fect on how the adhesive wets out on the ink and could therefore change the bond strength. CONCLUSIONS As the desire to move away from solvent-based inks increases, companies are looking for ways to replace them with water-based inks. In flexible pack- aging, there is also a desire to move toward solventless technology since this is the efficient and most cost effective means to laminate films. In order for water-based inks to be successful in flex- ible packaging applications, they must be compatible with solventless adhesives. With the right choice of solventless adhesives, a flexible packaging lamination can be cre- ated with water-based flexographic inks that has properties that are comparable to those of solvent-based inks. . Ink color Peel (g/in.) 2 290 300 480 490 540 Brown Red Clear Blue White ABOUT THE AUTHOR: Ronald R. Davies, Ph.D is a senior section leader in the R8(p laboratory at HB Fuller, where his group develops solventless and water-based adhesives for flexible packaging. He has been working on adhesive development at HB Fuller for 12 years, with .five years focused on the needs of the flexible packaging industry. Davies has a B.S. in Chemistry and a B.S. in Mathematics from the University of Wisconsin- Madison. He worked on water-based and radiation-curable coatings at PPG Industries for four years before returning to graduate school at the University of Minnesota where he earned a Ph.D. in Organic Chemistry. Polyol Polyol Composition Mix ratio (wt/wt) Peel Strrength Peel strength Peel strength Peel strength (28 days) (4 days) (g/in.) (7 days) (g/in.) (g/in.) A Polyester (1. 2 /1.0) 300 340 320 340 B Polyether (1.4/1.0) 180 170 130 200 C Hybrid (0.85/1.0) 180 170 140 180 - FLEXO MAY 2008 www.f I exog ra p hy.o rg