Accelerated Weathering Tests and Their Correlation

In instances when an actual service life is specified by the customer, the predicted service life has to be estimated from accelerated weathering data. To arrive at such an estimate, a correlation between acceler­ated weathering results, which can be obtained in a reasonable span of time, and service life data are necessary.

Good and statistically reliable correlations between accelerated weathering data and outdoor service life are difficult to obtain and are hampered by the fact that this data are only valid for the particular sealant chemistry and exposure conditions used [1,2,9]. In other cases, the correlation cannot be verified, due to the very long actual outdoor exposure times needed for high weatherability sealants. Therefore, the practical value of these procedures is limited, and formulators will often have to find their own methods to estimate sealant service life.

Formulators often use rules of thumb to gauge outdoor performance, which equate a specific time of accelerated weathering with one year of service life, even though limitations and poor correlation of this approach are known [9]. These rules are not always based on systematic studies but rather on the formu – lator’s practical experience. They are a practical compromise used in daily formulation work and can vary from formulator to formulator.

Accelerated weathering has its justification for relative ranking of a series of materials [2,9]. Accel­erated weathering performance is also specified in many national and international standards for sealants, and as opposed to outdoor exposure, it can be performed under defined conditions and can be reproduced

[1] . It has to be kept in mind, however, that no accelerated weathering test can be expected to give precisely the same results as an outdoor exposure test, nor can it be expected to predict the exact perfor­mance throughout the actual service life of a sealant.