CHAPTER 2 CHARACTERIZATION OF ACRYLIC FOAM TAPE FOR BUILDING GLAZING APPLICATIONS
This article presents the results of testing conducted to characterize the performance of 3M™ VHB™ structural glazing tape in both shear and tension. Creep rupture testing results provided the failure time at a given static load and temperature, and ramp-to-fail testing results provided the ultimate load resistance at a given rate of strain and temperature. Parallel testing was conducted on three structural silicone sealants to compare performance. Using the time temperature superposition principle, master curves of VHB tape storage and loss moduli in shear and tension were developed with data from a dynamic mechanical analyzer (DMA). The thermal shift factors obtained from these constitutive tests were successfully applied to the creep rupture and ramp-to-fail data collected at 23°C, 40°C, and 60°C (73°F, 104°F, and 140°F), resulting in master curves of ramp-to-fail strength and creep rupture durability in shear and tension.
3M’s VHB structural glazing tape consists of a closed-cell acrylic foam core with an acrylic adhesive on both sides (3M Technical Guide 2007). The tape investigated in this study, designated G23F, was developed by 3M to attach curtain wall glazing panels to building facades. This function is commonly performed by structural silicone sealants, although a similar VHB structural glazing tape has been successfully used in this capacity in South America since 1990 (3M Technical Guide 2007). To date, VHB tape has been used as a structural glazing adhesive on buildings in countries including the USA, Germany, Brazil, Israel, India, and Thailand (Austin, 2008). The load resistance of VHB tape is highly dependent on the rate or duration of loading, providing higher resistance in response to dynamic loads but being quite limited when subjected to sustained loads. Because of this property, glazing installations utilizing VHB tape are designed so that the dead load of the glazing is primarily supported through other means. VHB tape provides resistance to the action of wind loads pulling or pushing on the glazing, while also retaining the compliance required to accommodate differential thermal expansion of the glazing components.
VHB tapes have been the subject of several third-party studies. Winwall Technology Pte Ltd performed tests including ASTM E283 air infiltration, ASTM E331 water penetration, and ASTM E330 maximum structural wind load, with a focus on very high loading over short durations of ten seconds and one minute (3M Technical Bulletin 2007). This study found that G23F VHB tape exceeded the requirements of these structural glazing ASTM standards.
Another third-party VHB tape study was performed at Michigan Technological University (Heitman 1990).This study examined five VHB tapes similar to G23F. Along with long-term shear and tensile creep rupture tests, the study investigated UV light resistance, water resistance, and tensile full-reversal cyclic fatigue. The study concluded that the controlling design factor for the tapes tested was sustained creep load endurance limits.
3M (Kremer 2005) had also investigated the long-term static load resistance of several VHB tapes similar to the G23F that is specifically investigated in this study. These creep rupture studies have established a short-term dynamic maximum design strength of 85 kPa (12 psi), and a long-term static maximum design strength of 1.7 kPa (0.25 psi) (3M Technical Guide 2007).
This study was designed to address the effect of a wide variety of loading rates (through ramp-to-fail testing) and durations of static load (through creep rupture testing) such as might be experienced by VHB tape used in the field and subject to wind loading. These durability tests were comparable to those performed by Kremer (2005) and Heitman (1990) on similar VHB tapes, although this study supplemented strength and durability test data with use of the time temperature superposition to combine results collected at several temperatures, effectively extending the time range of the experiments. Results could provide insights into the robustness of VHB tape for glazing applications and lead to an improved understanding for design purposes.
In addition to VHB tapes, three structural silicone sealants were tested to provide a direct comparison of these two methods for attaching glazing panels. The structural silicones tested were a one-component sealant (Dow Corning 995), a two-component sealant (Dow Corning 983), and another one-component sealant (Dow Corning 795). For simplicity, in tables and charts, these materials are labeled S1, S2, and S3, respectively.
Dynamic mechanical analysis (DMA) testing was performed to determine viscoelastic properties specific to VHB G23F tape and to develop shift factors for the generation of time temperature superposition (TTSP) master curves. These curves consist of elevated temperature tests shifted to simulate longer loading times, and low-temperature tests shifted to simulate shorter loading times.
Creep rupture tests were performed to examine the effect of several static loads on the time required to fail VHB tape and silicone sealant specimens. These tests were performed in large batches on a 72-station pneumatic test frame, which only records the time of complete failure. The creep rupture tests investigated the long-term response of the materials (minutes to months). Shift factors determined from DMA constitutive data produced smooth master curves of creep rupture data, which predicted results on a long-term time scale that would have otherwise been beyond the time scope of this research project.
Ramp-to-fail tests were performed with several constant strain rates and temperatures to examine their effects on tensile and shear strength of VHB tape specimens. These tests were performed individually on an Instron test frame, which records the resisting force as a specimen is loaded at a prescribed crosshead rate. These tests investigated the short-term response of the material (seconds to minutes), and strength master curves were again constructed using the shift factors determined from DMA constitutive data.
Dynamic mechanical analysis (DMA) testing was performed on VHB G23F tape (lot # 97278) using a TA Instruments 2980 Dynamic Mechanical Analyzer. The frequency sweep included 75, 31.2, 10, 3.2, and 1 Hz, which are evenly spaced on a logarithmic scale. The applied temperatures ranged from -50°C to 150°C (-59°F to 302°F), using 10°C (18°F) increments. In order to examine the effect of temperature history on the specimen, some test replicates were initiated at the low end of the temperature range and progressed to the higher temperature, and some test replicates were performed in the reverse manner. The applied strain amplitudes for various test geometries and temperature ranges are presented in Table 2.1. The selected amplitudes produced specimen dynamic resistances from 0.01 N to 12 N, which were between the lower limit of recording capability and the upper limit of loading capability of the apparatus. In order to prevent the dynamic resistance from dropping below or exceeding the allowable range, the amplitude often had to be modified as the specimens were heated or cooled. No inconsistencies or unusual jumps in recoded data were observed when the amplitudes were modified during testing.
CHAPTER 1 Introduction
CHAPTER 2 Characterization of Acrylic Foam Tape for Building Glazing Applications
Results and Discussion
CHAPTER 3 Postulating A Simple Damage Model for the Long-Term Durability of Structural Glazing Adhesive Subject to Sustained Wind Loading
Sources of Wind Data
Wind Speed and Adhesive Stress
Prediction of Failure Time Based on Adhesive Stress
Results and Discussion
CHAPTER 4 Conclusions
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