Analytical characterization of polyurethanes
Advances in instrumentation coupled with computerization and information technology have greatly advanced our understanding of urethane chemistry and polyurethane materials. This increased knowledge base has resulted in greater predictability in polyurethane use and increased proliferation of polyurethane materials across the spectrum of applications. Modern methods of analysis have permitted nearly every kind of time- and frequency-resolved analysis. Morphology can be characterized down to angstrom levels allowing confident distinction between subtle differences in phase structure and mixing between materials. Prediction of properties, making of materials, and proper application of polyurethanes now require that a practitioner be familiar with the operation of numerous analytical instruments. Even more crucial is the understanding of the data produced by analyses and the ability to perform root cause analyses of material or application failure based on that data. This chapter will provide an introduction to selected experimental techniques and more in-depth discussion of the connection between data and its use. Although information is almost always useful, gathering information can be wasteful if the measurements do not apply directly to the problems to be solved. This chapter will also provide guidance on the most efficient methods of determining the root cause of material or application failure.
Before a material can be tested or applied, it must be made, and the incorrect making of a polyurethane is one of the most common causes for disappointing performance. The properties of a polyurethane are critically dependent on the proper ratio of reactants being joined, and determining the correct ratios requires accurate measurement of reactant functionalities. As discussed in Chapters 2 and 3, there are numerous unintended side reactions that can occur in the preparation of polyols and isocyanates that can significantly affect the amount of hydroxyl or isocyanate functionality in a given weight of material. Incorrect measurement of reactant functionalities can have many negative effects on final physical properties, primarily by resulting in reduced final polymer molecular weight or an incomplete network structure.
It is nearly impossible to employ perfect stoichiometric ratios when making a polyurethane; however, it is possible to be close enough that a high final molecular weight polymer network is formed that molecular weight is not the limiting factor driving polyurethane performance. The first and most critical initial step to making high-performance polyurethane materials is accurate measurement of the hydroxyl and isocyanate functionality per unit mass. ASTM is an organization that provides industry-defined, accepted, and explicit methods for performing these analyses. They provide a basis for confidence that all manufacturers of polyurethane building blocks are producing materials that can be used to make a desired material using normal laboratory techniques. The ASTM also provides guidance on alternative ways of obtaining the same piece of information using different analyses that can substitute if a particular technique is not accessible or if independent methods of measurement of the same quantity are desired. Furthermore, ASTM methods provide a gauge of measurement reliability and thus a means of determining if a measurement is being made with the proper level of precision to meet an industrial level of expectation.