| MadSci Network: Chemistry |
Henry, The density of plastics is usually measured in terms of grams per cubic centimeter, or g/cc. Since water has a density of 1.0 g/cc, any material with a higher density than 1.0 will be heavier than water and will sink. If you placed some HDPE or PP in water you would notice that they both float so their density must be lower than water, or some value less than 1.0 g/cc. This tells us a general range, but not which is denser than the other. To measure densities less than 1.0, we can mix water with another liquid [like alcohol] in certain ratios to make a series of solutions with different densities. By placing the specimen in each solution, we can observe when it either floats, sinks or is suspended. When we find it suspended, the density of the sample is exactly equal to the density of the solution. By this method we learn that HDPE has a density of around 0.96 g/cc and PP is around 0.91. I say "around" because many factors such as tacticity, crystallinity, additives etc. also play a role in density. The short answer to your question is yes. To learn more about plastics check out http://www.4spe.org This is the home page of the Society of Plastics Engineers. Ken
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