Loren Dale Lower

Estimating the Stresses in Linear Viscoelastic Sealants Subjected to Thermally-Driven Deformations

A framework for linear viscoelastic analysis of sealants is presented for analyzing stresses resulting from thermally driven deformations. Assuming that the strains induced within the sealant are proportional to the change in temperature from the strain-free state, the nominal stress state within the sealant can be estimated. The analysis method is used to estimate the stress states resulting from assumed diurnal temperature profiles for two representative Dow Corning silicone glazing sealants: a conventional elastomer and a crosslinked hot melt adhesive containing a high volume fraction of a silicate-based nanoparticle filler. The latter exhibits considerably more rate- and temperature-dependence than conventional silicones. The viscoelastic analysis allows for comparisons of stresses resulting in these two sealant systems, which are presented for several sinusoidal thermal profiles. However, the pronounced yielding behavior exhibited by the hot melt appears to limit the stress buildup, resulting in stress states that are significantly below those predicted using the linear viscoelastic model. Estimates of the yielding envelope for a representative thermal cycle profile are provided, based on experimental results reported elsewhere for the time and rate dependent yielding of the hot melt.

Using Rheology Test Methods to Assess Durability of Cured Elastomers Undergoing Cyclic Deformation

The current measurement test method to assess elastomeric sealant durability is ASTM C719. This method requires a minimum of five weeks of curing and conditioning before being subjected to ten movement cycles at room temperature and then ten movement cycles at variable temperatures. This method is a fine predictor of sealant movement capability for products used in moving joints in commercial construction applications. ASTM E1886 suggests that building assemblies be subjected to 9000 cycles of wind pressure. Sealant materials are typically used to anchor glazing assemblies into frames, and the choice of the correct sealant is critical to passing the test criteria specified in ASTM E1866. Rheological instruments have the capability to characterize the dynamic mechanical behavior of elastomeric materials undergoing oscillatory (cyclic) deformation under controlled test conditions and, therefore, provide a laboratory tool for assessing durability. Cyclic testing can be conducted under controlled strain (deformation) conditions at frequencies that simulate joint movement due either to thermal expansion differentials or seismic events, or under controlled stress (load) that model hurricane-force wind loads or design pressures. An immediate stress-softening response was observed from controlled-strain experiments at 15 % movement that was ascribed to the Mullins effect; however, three of the four cured silicone sealants exhibited a modest recovery over the remaining four days of cyclic testing. Under controlled-stress cycling at 0.138 MPa for 150 minutes at 0.5 Hz, the silicones exhibited ultimate deformations well below their rated movement capabilities. The results from both types of rheology test methods did not reveal outward signs of fatigue and suggest which elastomeric materials will perform better under the drastic cycling that occurs in ASTM E1866 and ASTM C719 testing.