Accelerated Weathering Test—Cohesion/Adhesion Properties After Exposure to Artificial Light Through Glass and Water
The purpose of this test is to evaluate the sensitivity of the structural sealant to artificial weathering, combining the effect of exposure to UV/visible radiation and water, by measuring the residual mechanical strength of the structural sealant.
As required by ISO/FDIS 28278-2, in this test initial tensile strength values of 10 test specimens are determined after a conditioning period of 28 days at (23 ± 2)°C and (50 ± 5)% RH. After this conditioning, the tensile test values of an additional 10 test specimens are determined after an immersion in demineralized water at a controlled temperature of (45 ± 1)°C with simultaneous exposure to UV light. The test specimens are immersed such that the upper glass surfaces are flush with the water level. During the total time of immersion (1008 h), the test specimens are exposed to the radiation from OSRAM Vitalux lamps. The intensity of the radiation on the upper side of the test specimen shall be (50 ± 5) W/m2 for the wavelength range from 300 to 400 nm. The ISO/FDIS
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28278-2 standard suggests that test method and requirements of ASTM C1184 shall be applied to structural sealant evaluation.
For the artificial weathering condition, the authors took a more long term approach along with alternative testing equipments. Xenon-arc radiation, regarded as the artificial source most similar to solar radiation, is often used for its realistic effects in evaluating the long term life cycle of building materials. The xenon-arc exposure cycle consisted of 102 min light only at 0.55 W/(m2 nm) at 340 nm followed by 18 min of light plus water spraying. The reason for simultaneous water spraying with radiation is to simulate a harsher accelerated weathering condition in the equipment. Based on the correlation factor provided by one of the technical institute members to join the evaluation, 5000 h of exposure to xenon-arc radiation at 0.55 W/(m2 nm) at 340 nm corresponds to an accumulated UV (300-400 nm) exposure of 1108 MJ/m2, converted to actual exposure in the field, this is equal to 4 years of outdoor exposure in Seosan, Korea .
A xenon artificial weathering machine made by ATLAS laboratory in USA, the model Ci4000, was used for this part of the study. The test was started in April 2009 and finished in February 2010.
The weathering evaluation guidance in ISO/FDIS 28278-1 states that the stress at the breaking point after weathering must be at over 75 % of the initial tensile strength (DXmean > 0.75) and the type of failure shall be at least 90 % cohesive (rupture >90 % cohesive). After weathering, the samples B-3, B-1, B-4, and C-1-a displayed adhesive failure.
As seen in Fig. 8, several products could not meet the value required by ISO/ FDIS 28278-1 for the tensile strength specification after weathering compared to their initial value. In the case of ASTM C1184, the requirement is set at >345 kPa after 5000 h artificial light exposure. Based on the mechanical strength itself, B-4, B-3, and C-1-a had lower tensile values than that required by ISO/FDIS 28278-1
FIG. 8—The tensile strength at breaking point of various sealants before and after various periods of xenon-arc/water spray weathering.
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but still they could meet the minimum requirement proposed by ASTM C1184 after weathering. For example, B-3 had the lowest value for tensile strength after 5000 h xenon exposure (397 kPa), but could still fulfill the minimum requirement set by ASTM C1184. Considering the results of the evaluation, it is necessary that the structural performance assessment of structural silicone sealants consider both their adhesion durability as well as mechanical strength. For example, although sealant B-3 showed adhesive failure mode, it had 397 kPa of tensile strength after 5000 h weathering. If there is no adhesion durability on the structural joint, it is impossible to justify the integrity of structural glazing for long term use.