Discussions and Summary
Sample #3 started to show sign of chalking around 3000 h in the QUV chamber; its elongation value also decreased sharply between 5000 and 10 000 h in the QUV chamber. Samples 1, 2, 6, and 7, which had 33 % lower concentration of tin and aminosilane, started to show signs of chalking around 10 000 h during QUV; their elongation value decreased sharply between 10 000 and 17 000 h exposure in QUV.
It is unclear why all the samples had a sharp increase on tensile strength at 2000 h in QUV and the tensile strength dropped back at 5000 h. However, the effect does not appear to be related to the tin or aminosilane concentrations.
Based on the observations of the material chalking phenomena, as well as the measurements of the materials’ mechanical properties, it is quite obvious that the concentration of tin in the formulation has the most pronounced impact on the long-term durability. The higher the concentration of the tin in the material, the faster the chalking. When chalking occurs, the elongation is reduced significantly.
The concentration of aminosilane in the product also has some impacts on durability, but not as significant as tin does. Sample #5, having low tin level, did not chalk after 17 000 h in QUV, despite the fact that the aminosilane level was high (however, samples with low tin level did not cure properly). Sample #8, having high tin level, chalked slower than sample #3 because the aminosilane level was lower in sample #8. Therefore a reduction of the aminosilane level can also improve the durability.
A coating made with the same formulation as sample #1 and a regular coating sample (as sample #3) were applied on the field for adhesion evaluation. The feedback indicated there was no substantial difference on the adhesion; the adhesion performance of both coatings was acceptable.
This study suggested that it is feasible to reduce current tin and aminosilane amounts by about 33 % and to improve coating’s durability by more than 50 %. The change does not significantly impact the material’s properties. Because of similar formulation/chemistry, these conclusions may also apply to some silicone sealants and adhesives.