ABSTRACT
A signifi cant impact on the properties of linear polyurethanes, which is somewhat underestimated, comes from the molecular weight and its distribution. And these factors are in turn affected by a number of process parameters and by the issues connected with the process equipment and organisation of process operations in time. The critical part of the PU production processes-alike in any step-growth polymerisation-is the feeding operation of the isocyanate and polyol components. The volumes of feeds should be closely monitored to reach the required degree of polymerisation. In order to obtain linear PUs with possibly high molecular weights, it is advantageous to run the process with the equimolar ratio of functional groups. In case of two-stage processes, that recommendation refers to the last stage of chain extension. That is accomplished in practice by the use of precise metering pumps. There are theoretical grounds for that. As provided for by the Carothers equation (68), which describes the relation between the number average degree Pn of non-stoichiometric step-growth polyaddition, conversion α of functional groups, and initial ratio r of functional groups, if the polymer with the maximum-under given conditions-molecular weight (i.e. degree of polymerisation Pn) is to be produced, complete conversion is required for the defi cient monomer:
On the other hand, huge excess of one of monomers (r > 2), which is the case in the production of isocyanate prepolymers, assures complete conversion of functional groups in the other reacting substance (polyol). At the same time, however, formation of products with higher molecular weights is limited, hence usually Pn is < 3 in such cases. The non-stoichiometric polyaddition or polycondensation processes usually yield polymers with high polydispersity-resulting inter alia from side reactions.
