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MECHANICAL ANALYSIS

The mechanical analysis of polyurethanes is a very broad topic, and it is difficult to cover in depth without a companion discussion associated with 2-phase composite theory and the theory of block copolymers, which are covered in Chapter 4. However, most measurements associated with the performance of a specific polyurethane in a particular application begin with some kind of mechanical analysis. This is obviously true when one considers that a polyurethane foam must function under compression, an elastomer under compression or tension, an adhesive under tension or shear, a coating under complex wear processes and chemical insult, and so on. Certainly, techniques such as X-ray scattering or spectroscopy may provide in-depth understanding of how or why a polyurethane structure exists. They do not, however, provide a predictive measure of how that particular structure will perform in a lap shear test, how long the structure will elongate under a particular load, or how much permanent set the structure will exhibit when sat upon in July in a humid or dry environment. For this information, there are few substitutes for well-thought-out tests of mechanical performance. The fact is that whether for foams, adhesives, elastomers, coatings, sealants, or any other imaginable application, the specific mechanical tests are often very unique to the industry or even customer. The ASTM has numerous tests with specific methods for testing. Many tests such as those covered in Section 5.1 are well used and truly applied across industry and academia. Mechanical testing is handled differently because of regional differences in application requirements (i.e., Northern Europe vs. South America) or because of historical testing protocols that predate standardized methods. Some tests simply reflect the biases of material scientists represented within a certain company. In this section, there will not be an attempt to cover all the tests that exist. This could probably fill a book in itself and would represent a moving target as test requirements can change rapidly. Rather, this section covers mechanical testing of materials in large strain (displacement) modes such as occur in tensile, tear, and elongation tests and in small strain modes such as that occurs in DMA. These tests each provide very useful and different information on material properties. In addition, DMA can be employed to provide information on reaction kinetics, rheology, frequency, and temperature-dependent properties of materials—but only at relatively low strains.

 
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