DMA is a method frequently used to characterize the specific properties of materials and is one of the most useful tools for determining the mechanical properties of polymeric and other time-dependent or viscoelastic materials. It is often implemented in assessing the correct degree of cure in structural resins and adhesives, and ultimately product quality. Unlike thermal methods, such as differential scanning calorimetry (DSC), the parameters measured are the same as those specified by engineers, for example modulus and damping factor. These give a realistic view of factors such as the time before a component can be handled at a given temperature and an upper use temperature where mechanical properties fall below specified levels.
In the study presented here the effects of differing levels of relative humidity and temperature on the curing behavior of silicone bathroom sealant are described. One-part silicone resins are commonly used as sealing caulks, e.g. for baths and windows. When exposed to atmospheric moisture curing starts and acetic acid is given off as a by-product of the curing reaction. Such resins have excellent adhesive and water repellent properties. They are also flexible and will not dry out, thereby maintaining a good waterproof seal for many years before further attention is required. If surfaces are wet when the resin is applied, then a rapid skinning of the surface can occur, which is an impediment to adhesion.
For many applications it is desirable to have an understanding of the rate of polymerization and cross-linking reaction that occurs. It is this reaction that provides a stable elastomer, good for providing a long-life seal.
Since the silicone material from the tube has the consistency of a thick liquid or paste, it is readily tested in compression geometry. The compression plates (Figure 1) are set to a constant gap of 1 mm in this case, using the 10 mm diameter plates. The static force adjustment mode was set to ratio compression with the default ratio (static / dynamic = 1.5). A dynamic displacement amplitude of 20 mm was requested at a frequency of 1 Hz. All tests were made under conditions of constant temperature and relative humidity of 10, 25 and 35 °C and 25, 50 and 60% RH, respectively. The TT DMA humidity chamber was used with the water recirculator used to provide a stable isotherm.
Figure 1: Compression clamp
Results and conclusion
Figure 2 shows the storage modulus E’ versus time at different humidities. The effect of the humidity on the cure process is readily evident from the storage modulus E’ versus time plots. Such experiments allow a thorough understanding of the cure process for these resins.