S-glass, R-Glass & Te-Glass

S-glass (the 'S' indicates high-strength) is 10-15% stronger than E-glass, but is otherwise similar in its properties to E-glass or R-glass. It is naturally boron free. Heat resistance is better than that of E-glass, a factor which can be of importance in some military applications, but is not exploitable when the material is used as a reinforcement for a polymer composite.

S-glass fibres have modified silicate network giving increases in mechanical properties summarised as:

Ref ASM Vol 21 Handbook.

S-glass is often the first choice for many structural applications because of its mechanical properties and temperature capabilities. However, their higher melt temperatures require more process energy making these fibres more expensive than E-glass.