Kar Tean Tan

On the Origins of Sudden Adhesion Loss at a Critical Relative Humidity: Examination of Bulk and Interfacial Contributions

The origins for abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces have been explored using a model poly(methyl methacrylate) (PMMA) film on glass. The interfacial and bulk water concentrations within the polymer film as a function of D 2O partial pressure were quantified using neutron reflectivity. Adhesion strength of these PMMA/SiO 2 interfaces under the same conditions was quantified using a shaft loaded blister test. A drop in adhesion strength was observed at a critical RH, and at this same RH, a discontinuity in the bulk moisture concentration occurred. The moisture concentration near the interface was higher than that in the bulk PMMA, and at the critical RH, the breadth of the interfacial water concentration distribution as a function of distance from the SiO 2/PMMA interface increased dramatically. We propose a mechanism for loss of adhesion at a critical RH based upon the interplay between bulk swelling induced stress and weakening of the interfacial bond by moisture accumulation at the PMMA/SiO 2 interface.

Fundamentals of Adhesion Failure for a Model Adhesive (PMMA/Glass) Joint in Humid Environments

The origins for the abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces were explored using a poly(methyl methacrylate) (PMMA) film on silicon oxide as a model system. The interfacial and bulk water concentrations within the polymer film were quantified as a function of D2O partial pressure using neutron reflectivity. The adhesive fracture energies of these PMMA/SiO2 interfaces at the same conditions were determined using a shaft-loaded blister test. Discontinuities in the adhesive fracture energy, bulk moisture solubility, and the width of the interfacial moisture excess near the interface were observed at the critical RH. A mechanism based on the coupling of bulk swelling-induced stresses with the decreased cohesive strength due to moisture accumulation at the interface is proposed and is consistent with all experimental observations.

Durability of Building Joint Sealants

Predicting the service life of building joint sealants exposed to service environments in less than real time has been a need of the sealant community for many decades. Despite extensive research efforts to design laboratory accelerated tests to duplicate the failure modes occurring in field exposures, little success has been achieved using conventional durability methodologies. In response to this urgent need, we have designed a laboratory-based test methodology that used a systematic approach to study, both independently and in combination, the major environmental factors that cause aging in building joint sealants. Changes in modulus, stiffness, and stress relaxation behavior were assessed. Field exposure was conducted in Gaithersburg, MD, using a thermally-driven exposure device with capabilities for monitoring changes in the sealant load and displacement. The results of both field and laboratory exposures are presented and discussed.

Design, fabrication, and implementation of thermally driven outdoor testing devices for building joint sealants.

The paper describes the development, implementation, and testing of two thermally driven outdoor exposure instruments. These devices are unique in their ability to impose field generated thermally induced strain on sealant specimens while monitoring their resulting load and displacement. The instruments combine a fixed wood and steel supporting frame with a moving polyvinyl chloride frame, and employ differences in the coefficients of thermal expansion between the supporting frame and moving frame to induce strain on the sealant specimens. Two different kinds of instruments have been fabricated, winter/tension and winter/compression designs. In the winter/tension design, the thermally induced dimensional change is directly transferred to the specimens; while in the winter/compression design, the samples are loaded in an opposite direction with the dimensional change. Both designs are instrumented to monitor load and displacement and are built so that the strain on the specimen does not exceed ±25% over the range of temperatures expected in Gaithersburg, MD. Additionally, a weather station is colocated with the device to record environmental conditions in 1 min intervals. This combination of weather information with mechanical property data enables a direct link between environmental conditions and the corresponding sealant response. The reliability and effectiveness of these instruments are demonstrated with a typical sealant material. The results show that the instruments work according to the design criteria and provide a meaningful quantitative platform to monitor the mechanical response of sealant exposed to outdoor weathering.

Effect of Strain on the Modulus of Sealants Exposed to the Outdoors

The effects of applied strain on sealants exposed to outdoor weathering were examined for two sealant formulations, Sealants A and C. Both static and dynamic strain was applied to the sealants during the summer in a Gaithersburg, MD outdoor location. Both sealants exhibited a reversible change in equilibrium distance. Stress relaxation studies on all samples revealed that, for Sealant A, two mechanisms affected modulus change; exposure without applied strain increased the modulus while additionally applied strain decreased the modulus. Only one mechanism that decreased the modulus was found for Sealant C. A 7 % dynamic strain and a 25 % static strain were observed to produce equivalent modulus changes in both systems.

A Test Method for Monitoring Modulus Changes during Durability Tests on Building Joint Sealants

The durability of building joint sealants is generally assessed using a descriptive methodology involving visual inspection of exposed specimens for defects. It is widely known that this methodology has inherent limitations, including that the results are qualitative. A new test method is proposed that provides more fundamental and quantitative information about changes occurring in a sealant during durability testing. This test method utilizes a stress relaxation experiment to evaluate the non-linear viscoelastic behavior of sealants. In particular, changes in the time dependence of the apparent modulus can be observed and related to molecular changes in the sealant. Such changes often precede the formation of cracks and theultimate failure of the sealant. This paper compares results obtained from the new test method and the currently used descriptive methodology.

A Systematic Approach to the Study of Accelerated Weathering of Building Joint Sealants

An accurate service life prediction model is needed for building joint sealants in order to greatly reduce the time to market of a new product and reduce the risk of introducing a poorly performing product into the marketplace. A stepping stone to the success of this effort is the precise control of environmental variables in a laboratory accelerated test apparatus in order to produce reliable weathering data that can be used to generate a predictive model. This contribution reports a systematic study, using a novel laboratory test apparatus, investigating the individual and synergistic impacts of four environmental factors (cyclic movement, temperature, relative humidity, and ultraviolet radiation) on the durability of a sealant system. The apparatus used is unique because it not only allows the precise control of environmental factors but also permits in situ characterization tests so that the specimens need not be removed from the apparatus chamber. Graphical and quantitative statistical approaches have been used to analyze the data. The study shows that the critical role of each individual factor, as well as synergism among the different factors, can be readily quantified, and modes of degradation possibly can be identified.

Effects of Strain on the Long Term Rheology of Filled Viscoelastic Solids

Filled viscoelastic solids, or building joint sealants, are essential components of modern construction. They serve in the weatherproofing of buildings and structures by preventing moisture intrusion. The most important property is the ability to respond to imposed strain caused by temperature or moisture changes in the structural building components while maintaining the critical moisture/energy barrier properties. These materials are typically warranted for greater than 40 years. During this long service life, chemical and molecular changes within the sealant are accelerated by the weather and give rise to changes in the rheological and adhesive properties. While, ultra‐violet light, temperature, and humidity are critical to influencing the sealant changes within the environment, recent studies have shown that imposed strain on several different formulations of sealant during environmental exposure has a significant affect on the measured rheological properties. Cyclic strain on the sealant in the prese...