Relative humidity, polymer weathering

The oriented film specimens were mounted on aluminum frames and exposed on an Atlas Weather-Ometer, Model 65WR. An 18 minute spray cycle together with an 102 minute cycle at 55% relative humidity and approximately 65°C was used. At regular intervals, the test specimens were removed from exposure and their tensile strength measured on an Instron Model 1102. A decrease in tensile strength, expressed as tenacity, over the tensile strength of the same formulation before exposure, is a measure of the deterioration of the physical properties of the polymer. "Failure" in this test is defined as a loss of 50% or more of the initial sample tenacity. 

Evaluation. For screening stabilities at Interfaces, polymer/mirror samples were exposed in an Atlas Weather-Ometer at 70°C and 50% relative humidity, using continuous radiation... 

Outdoor weathering is undoubtedly the most valuable test of practical polymer stability because of the extreme complexity of the natural conditions. Indeed, though photodegradation is initiated by light, ambient temperature, humidity, rainfall and atmospheric contaminants all have an influence on the course of the degradation, and the interrelationship of these factors is not yet fully understood. Several chemical reactions, the relative importance of which are dependent on the weathering conditions, usually contribute to the failure of the sample. 

Adhesives exposed outdoors are affected primarily by heat and humidity. Thermal cycling, ultraviolet radiation, and cold are relatively minor factors. Structural adhesives, when exposed to weather, rapidly lose strength during the first six months to a year. After two or three years, the rate of decline usually levels off, depending upon the climate zone, polymer, adhesive, and stress level. The following are important considerations when designing an adhesive joint for outdoor service ... 

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