Helix Oxford 12 inch 30cm Shatter Resistant Ruler

Product Information

Plastic rulers were originally made from thermoplastics like Acrylic/Luvite/Perspex/Plexiglass (Polymethyl methacrylate, or PMMA) or Polystyrene (PS). They are easily injection moulded or extruded and can be made transparent. However both materials have the disadvantage of being brittle and when bent or hit hard enough they will violently shatter, leaving razor sharp edges behind (the same was true of old Tic Tac boxes, which were also made of PS). If you want a bulletproof clear plastic the most obvious choice would be Lexan (Polycarbonate, or PC). It is very strong and has a hard surface (it's used in bulletproof glass and safety glasses), but given enough shear load it will still shatter into shards (as anyone who has snapped a CD will know). To get rid of the tendency to shatter the plastic for a child safe ruler needs to be more flexible, so Polystyrene-butadiene-styrene (SBS) is used instead. It is a thermoplastic elastomer that is a copolymer of polystyrene (which gives it a hard surface and durability) and polybutadiene (which is a synthetic rubber that gives it flexibility, and which allows you to twang it off the edge of the desk). As it's still a thermoplastic it can still be injection moulded, but unlike acrylic or straight polystyrene it will gradually bend and tear under a shear load rather than fail catastrophically.