缅北强奸

The helmet that Neil Armstrong wore when landing on the moon in 1969 and coloured trumpet mouthpieces are made of the same plastic, Lexan. In 1953 Daniel Fox at General Electric was looking for better insulating materials for wires when he mixed together a couple of ingredients that he hoped would yield a polymer with suitable properties. What he got was a goop that hardened to an extent that he couldn鈥檛 even remove his stirring rod. Banging the curious new material against a hard surface had no effect; it just couldn鈥檛 be broken. Lexan, as the new material came to be known, would never be used as wire insulation but it would find a host of other uses. It was as clear as glass, but virtually unbreakable! Electric meter covers were traditionally made of glass and often made inviting targets for rambunctious rock-throwing teenagers. But Lexan meter covers solved this problem. Lexan was also ideal for astronauts鈥� helmets, automobile headlight assemblies and panels for greenhouses. Uses mushroomed. Football and hockey helmets, protective visors, laptop housings, iPods, cell phones, water bottles, bulletproof shields, skateboards, airplane canopies, compact discs and DVDs joined the list of items made from Lexan. And then along came plastic mouthpieces for trumpets and trombones to serve as an alternative to brass pieces. The plastic doesn鈥檛 change temperature as easily as brass, providing more comfort for musicians鈥� lips. It is also cheaper and comes in a variety of attractive colours. Chemically, Lexan is a 鈥減olycarbonate,鈥� made by linking together molecules of phosgene and bisphenol A in an alternating fashion. Since polymerization never goes to 100% completion, there are always trace amounts of bisphenol A left over in Lexan. Some of this can escape from the plastic to the concern of environmentalists. They worry that bisphenol A has estrogenic properties and can have a disruptive effect on human hormones. This concern is not totally unreasonable since some experiments have shown that animals exposed to tiny amounts of bisphenol A experience physiological effects. No such effects have ever been found in humans. In any case, as with any such issue, it comes down to a risk-benefit evaluation. Life as we know it, with our computers, iPods and plethora of sports equipment, would be drastically altered if Lexan were removed from the market. Police would be deprived of their shields, athletes of their helmets, fighter pilots of their protective canopies. Real lives would be lost for the sake of protecting ourselves from the theoretical risk attributed to trace amounts of bisphenol A leaching out from Lexan products.