Coagulation of Polyurethanes

The polymer most often used for the coagulation process is polyurethane. Nearly all leather substitutes sold in the world consist partially or totally of polyurethanes. Therefore a whole chapter is dedicated to the coagulation of polyurethanes. In 1951 a coagulation process for a polyurethane was published for the first time [1]. Since 1954 [36] additional process steps have been developed to produce microporous polyurethanes [32]. The structure of the soft segments of the polyurethanes influences the coagulation of the polyurethanes remarkably. Soft segments with a higher hydrophilic property result in polyurethanes with a better coagulation behavior. The structure of the hard segments is of minor importance [33]. Only linear polymers have good solubility in organic solvents. Crosslinked polymers are normally insoluble in solvents; due to a certain decomposition crosslinked polyurethanes can be dissolved in heated DMF because hot DMF is able to split branched polymer chains which results in a linearity of the polymer-chains. The NCO/OH ratio determines the linearity of the polyurethane chain. A ratio NCO/OH < 1 results in easily soluble OH-terminated polyurethanes [35], NCO/OH > 1 results in crosslinked, poorly soluble ones (details see Chap. 25.1). The following principally different process types are known: