Our customer obtained large glass parts from China for an outside application. Within a few months, the glass turned yellow and sometimes cracked. A borosilicate glass composition had been specified which was not UV-sensitive. Freshly fractured surfaces of the specified and the yellowed glass were quantitatively analyzed by XPS to perform an elemental composition glass analysis for all elements except hydrogen. The supplied glass had only one-ninth the boron concentration of the specified glass. The atomic concentration of sodium was 3 times higher, that of barium was doubled, and there was additional calcium in the Chinese-supplied glass. The yellowed glass also had potassium, chlorine, and zinc concentrations not found in the clear glass. The substituted glass did not have a small concentration of fluorine found in the specified glass and carbon found in it had carbon-oxygen bonding not found in the specified glass.
The specified borosilicate glass was found by TMA to have a lower coefficient of thermal expansion (CTE) than the substituted glass, which prevented the cracking found sometimes in the Chinese substituted glass parts. The cracking was due to a mismatch in the thermal expansion of the Chinese substituted glass with the structural materials that held the glass in place.