Adhesion of Dimer-Based Polyurethanes to Various Substrates
Dimerized fatty acids are expected to have a high affinity for low-energy surfaces, since the long alkane chains show a structure similar to polyethylene.Polyurethane adhesives were prepared from a variety of polyether and polyesters diols and pure MDI in an NCO:OH ratio of 2.5.These materials were tested as adhesives for two polyethylene substrates and for steel after full cure.It should be realized that these tests were performed to quantify the difference in adhesion of the PU resins to polyethylene for the various polyesters and polyether diols,and neither adhesion promoters nor blends of polyester and polyether diols were used to optimize adhesion.
Figure 6 shows the adhesion of dimer-based polyurethanes on untreated polyethylene in comparison with systems based on adipate ester and PTMEG, and Figure 7 shows the adhesion on steel. The graphs show that introduction of dimerized fatty acids into PU adhesives results in a substantial improvement of the adhesion to polyethylene. Surprisingly, the adhesion on polar substrates such as steel is similar to or better than the adhesion of polyurethanes based on other polyols. This could be due to phase separation processes that take place in the adhesive during cure; the segments with most affinity for the substrate will move toward it, thus increasing the bond strength regardless of the polarity of the substrate.
Though the coating industry has benefited from solvent-containing polyurethane coating , a change to polyurethanes for low-solvent to solvent-free dispersions is necessary. These polyurethanes are used as single binders or in combination with acrylates or styrene acrylates in various coatings and adhesives applications, including anti-corrosion coatings; coatings for wood, plastic and textiles; printing ink;and adhesives.
Polyurethane dispersions have acquired an importance in these areas of application in spite of their comparatively high raw material price. Following are benefits of polyurethane dispersions.
- Good environmental compatibility, being aqueous (in some instances they contain small quantities of solvent such as N-methylpyrrolidone)
- Can be processed as one-component systems (in some cases chemical cross linking may be possible or necessary)
- High mechanical resistance, abrasion resistance, flexibility, yellowing resistance (dependent on raw materials),adhesion and impact resistance.
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