Two of the more interesting developments in the world of ballpoint pens include space pens and erasable pens.
Space Pens, or pressurized pens, are a technological novelty. Take, for example, the Fisher Space Pen. A space pen's ink reservoir is pressurized (~40 lb/sq. in.), and the ink is a special viscoelastic ink (like thick rubber cement). The ballpoint must rotate in order for the thick ink to liquefy, allowing it to write smoothly and dependably on most surfaces, even under water. Ordinary ballpoint pens rely on gravity to feed the ink and have an opening in the top of the ink cartridge to allow air to replace the ink as it is used. There is no hole in space pens, eliminating evaporated or wasted ink as well as leakage from the rear of the ink reservoir. In addition, a space pen can last up to 100 years, compared with the average two-year shelf life of a standard ballpoint pen.
Since the 1960s, when the "Space Race" began, space pens have been used by the U.S. astronauts on all manned space flights, including lunar trips, and were also used by many of the Russian cosmonauts on the Soyuz space flights and the MIR space station.
Erasable pens were tremendously popular when they were introduced in the early 1980s. They combine the readability of brightly colored or black ink with the eraser functionality of a pencil. While the pens are still manufactured under names like Gillette Eraser Mate, they aren't as commonly used as they were before. Patents US2966418 and US4097290, among others, describe these pens in detail.
What makes erasable ballpoint pens so different from traditional ballpoint pens is the "ink" -- instead of being made from oils and dyes, it is made of a liquid rubber cement. As you write, the ballpoint rolls on the paper and dispenses the rubber cement ink (the resulting mark is known as a trace). Modern erasable pens work by allowing a ballpoint pen to leave a definite and intense black or colored trace which looks like an ink trace, but is capable of being easily erased shortly after writing (usually up to 10 hours). After that time, the trace will harden and become non-erasable.
Erasable ink generally consists of 15 percent to 45 percent (by weight) natural rubber that is dissolved in a series of volatile organic solvents with varying boiling points.
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