Polycarbonate Substrate for Large Capacity Optical Memory Disk

Polymethylmethacrylate (PMMA) was usually used for the substrate for recently developed 20 cm ϕ or 30 cm ϕ optical memory disks because of the excellent optical properties, especially low level of birefringence. But warping by heat and water absorption during usage and storage is the drawback for PMMA substrate from the viewpoint of durability of disks. Polycarbonate (PC) on the other hand is not liable to warp but has generally poorer moldability and higher birefringence. This is the reason why PC could not be used for the disk substrate larger than compact disk size (12 cmϕ).