Glenn Gordon

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.

Vinyl ether-modified poly(hydrogen silsesquioxanes) as dielectric materials

Vinyl ether-modified poly(hydrogen silsesquioxanes) or PHSQ were prepared via a platinum-catalyzed hydrosilylation reaction of PHSQ with an alkyl vinyl ether (VE) in toluene. The product formed in a near quantitative yield and its composition was characterized by multinuclear magnetic resonance spectroscopy. Multi-detector size exclusion chromatography revealed that relative to the PHSQ starting material, the PHSQ–VEs increased in molecular weight and radius of gyration, and the relationship between intrinsic viscosity and molecular weight suggested a branched structure. Thermal analyses indicated a cure onset around 100 °C; an onset of thermal decomposition at ca. 230 °C; and mass loss completed by 550 °C. Evolved gas analysis from thermogravimetric experiments revealed the initial elimination of the ethylene linkage, followed by cleavage of the carbon–carbon bonds. The materials prepared by pyrolysis at 425 °C were porous. Nitrogen porosimetry measured an increase in microporosity—from 0.187 to 0.295 cm3 g−1 (<5 nm)—when the VE content was increased from 10 to 50 wt%. The PHSQ–VEs were spin-coated onto silicon wafers and cured either at 400, 425, or 450 °C. The dielectric constant of the spin-coated films ranged from 2.3 to 3.0, and the modulus was between 2.2 and 12.9 GPa depending on material composition.

PSA Release Force Profiles from Silicone Liners: Probing Viscoelastic Contributions from Release System Components

Abstract Release force profiles of an acrylic- and rubber-based pressure sensitive adhesive (PSA) from silicone release coatings containing different levels of a high-release additive (HRA) were measured. The profiles of release force differed dramatically for the two different adhesive types. The general trends of either increasing or decreasing release force profiles with peel rate were predominantly attributed to the relative ability of the adhesive component to dissipate and store energy (i.e., tan δ) over the operating frequency range. The addition of HRA enhanced the dissipative character (G″ and tan δ increased) of the release coating which resulted in higher release forces. An empirical model based on the viscoelastic properties of the adhesive and release coating was proposed to describe release force profiles and initial estimates for the fitting parameters were determined. The release model was shown to predict successfully the impact of adhesive thickness on the release force profile using an acrylic PSA which was not used for the model development. Some evidence was also obtained for the validity in omitting the contributions of the elastic backing components from the model.

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.