Metallization of cross-linked polyurethane resins by reduction of polymer-incorporated metal ion

Polyurethane (PU) resins having different degree of cross-linking were prepared by the reaction of 4,4′-methylenebis(phenyl isocyanate) (MDI) with various molar ratio of two kind of pentaerythritol ethoxylates (PEE15, ratio of ethyleneoxy (EO) unit/hydroxyl (OH) unit=15/4; PEE3, EO/OH=3/4) in the presence of cobalt(II) chloride at 100 °C for 48 h. Metallization behavior of the CoCl2-containing PU resins by reduction with aqueous sodium tetrahydroborate solution was investigated by means of infra-red (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). It was confirmed by the measurement of degree of swelling and glass transition temperature that the PU resin having a higher molar ratio of PEE15/PEE3 has a lower degree of cross-linking. When the PU films were reduced at 20 °C, the PU resins with PEE15/PEE3=10/0, 8/2, and 6/4 afforded the films with a metallic luster, whose surface resistance increased in this order. The formation of cobalt metal on both the sides of the PU film was revealed from the results of the XPS and EPMA measurements. The PU resin having a lower degree of cross-linking could be more easily metallized by this reduction treatment.