`Glass fiber humidity degradation in fiberglass manufacturing`

Freshly pulled E-glass (aluminum borosilicate): 600–700 ksi tensile. One hour later: 500 ksi. Next day: 400 ksi. Month later: brittle. Hence epoxy-coat within 24 hours for fiberglass. Critical flaw size from Ashby fracture-toughness plot: ~10⁻⁴ mm, i.e. tens of angstroms — only modest surface etching is needed.

Glasses rated one will almost never show weathering effects. Those rated two will occasionally be troublesome, and those rated three require more careful consideration of durability. How do we prove this? Well, if I pull a glass fiber — a brand-new glass fiber, aluminum borosilicate glass — and I'm going to make it into fiberglass with the best strength, you can buy this from Corning, it's called E-glass, but they have different letters and different compositions. I had better coat that with the epoxy within 24 hours, because the humidity in the air is going to degrade the strength. When I pull it and measure the strength right away as a fresh new surface of that fiber, it may have six or seven hundred thousand pounds per square inch tensile strength. An hour later it may be five hundred thousand. The next day, four hundred thousand. A month later it's going to be — it is a piece of glass — but it'll be so brittle that you want to use it as a strengthening element in fiberglass. So when you make fiberglass, you epoxy-coat it real quick to protect it from the moisture in the air. On the ligands on the end of this you've got these silica tetrahedra, and then you've got sodium, and the sodium wants to form sodium hydroxides, it'll form silicon hydroxides. Those things will actually form little etched pits on the glass, microscopic — you can't see them.