Trivex (1.53) vs Polycarbonate (1.59) vs MR-8 (1.60): Tensile Strength, Drilling & Clarity

Understanding these materials requires inspecting their molecular polymers:

• Polycarbonate ($n = 1.59$): A thermoplastic resin. Pellets are melted at 300°C and injection-molded under massive pressure. This creates significant frozen internal mechanical stresses (birefringence), which degrades optical purity.
• Trivex ($n = 1.53$): A thermoset quasi-prepolymer combining polyurethane chemistry with nitrogen-enriched crosslinking. Cured slowly in thermal molds, it is 100% free of internal molding stress.
• Mitsui MR-8 ($n = 1.60$): A patented polythiourethane polymer. Sulfur atoms incorporated into the urethane backbone deliver high refractive index without sacrificing polymer crosslink density.

In rimless spectacles, holes are drilled directly through the lens to anchor metal bridge and temple screws. Under daily flexing and torque:

$$\text{Tensile Strength: } \text{Trivex / MR-8 } (> 80\ \text{kgf}) \quad \gg \quad \text{Polycarbonate } (\approx 40\ \text{kgf})$$

Polycarbonate lenses drilled for rimless frames frequently develop spiderweb micro-cracks (crazing) around the drill holes within 6 months. Furthermore, exposure to common solvents (such as alcohol lens wipes or perfume) causes instant stress-cracking in polycarbonate. Trivex and MR-8 are virtually impervious to chemicals and withstand over 80 kg of pull force without fracture.

1. Chromatic Abbe Value: Polycarbonate has a dismal Abbe of 30, causing rainbow fringing. Trivex boasts an outstanding Abbe of 45, and MR-8 achieves a crisp 41.
2. Specific Gravity (Weight): Trivex is the lightest lens material on Earth with a specific gravity of just $1.11\ \text{g/cm}^3$ (polycarbonate is 1.20, CR-39 is 1.32, 1.67 is 1.37). For mild prescriptions, Trivex floats effortlessly on the nose bridge.