The Worst Factor for Modulus Decrease

We have identified temperature, cyclic movement, and UV radiation as statisti­cally significant factors for modulus decrease over all robustness factor settings, but determining which factor is the most important among these three is also of

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.

Plot Character = UV (2 Settings: 0,1)

Temperature (3) & RH (6) & Movement (4) Combinations (==> 30)

Plot Character = UV (2 Settings: 0,1)

Temperature (3) & RH (6) & Movement (4) Combinations (==> 30)

FIG. 12—Block plot of (a) modulus ratio and (b) mean modulus ratio targeting UV across 30 distinct combinations of RH, cyclic movement, and temperature. The UV set­tings of “0” and “1” denote with and without UV exposure, respectively. The importance of UV in modulus decrease is evident from the consistently large block height and local arrangement ofUV across the various combinations of robustness factors.

interest. This determination is done by deciding which plot has the consistently largest block heights, along with consistent arrangement within blocks showing the target factor located at the bottom of each bar. An examination of all block plots leads to the identification of cyclic movement as the most important factor for modulus decrease, followed by UV and temperature; RH is the least impor­tant factor. This ranking is consistent with that obtained from the dex plots. In a study of various latex and solvent-borne acrylic sealant products, Karparti [37] also discovered that cyclic movement was the major aging factor during outdoor exposure, and that outdoor weathering without cyclic movement alone had a negligible effect. This systematic laboratory approach clearly reveals the importance of cyclic movement in the degradation of sealants in a quantitative manner.

Although the results here suggest that cyclic movement is the most impor­tant factor in degrading the sealants, followed by UV, this should not be misin­terpreted as meaning that cyclic movement causes the degradation of sealants in the absence of UV. This is not true for the sealant in this study, based on the information in the next section, and it would not be true for most sealants cur­rently in use. For most polymeric materials, UV is required in order to initiate degradation because it has sufficient energy to break many types of polymeric chemical bonds, which leads to secondary reactions promoted by other environ­mental factors. For applications of building sealants in which movement has a significant effect on deterioration, the addition of cyclic movement to the expo­sure test is essential in order to promote the type of failure produced during in­service application of the sealant as a secondary reaction following initiation by radiation.