Ubiquitous in every day life and accounting for the bulk of material used to create milk bottles, laundry detergent bottles, and margarine tubs, it may surprise some readers to learn that HDPE plastic actually begins life as thick black crude oil pumped out of the ground.
HDPE plastics are produced from a component of crude oil, naphtha, which is cracked (heated at very high temperatures) in order to extract the ethylene gas which is released when the chemical structure of naphtha deteriorates. The ethylene gas is then polymerized, a process which involves the free gas particles joining together to produce long chains of ethylene molecules.
There are many different types of polyethylene. HDPE is a particular type of polyethylene with a very dense linear structure which gives it an increased tensile and molecular strength. HDPE is normally defined as being polyethylene with minimum density of 0.941 grams per cubed centimeter. Within the category of HDPE plastics, molecular weights range from 100,000 to 500,000, which is where the great variance in HDPE grades arises. Common grades of HDPE plastics include blow molding grade HDPE plastics, injection molding grade HDPE plastics, film grade HDPE plastics, and pipe grade HDPE plastics. Each of these grades have slightly different physical properties, the most notably important one being MFI, or melt flow index. Melt flow index describes how quickly the HDPE plastic flows when in liquid form, applications such as pipe making and blow molding require a highly viscous slow flowing HDPE plastic, whereas injection molding requires a quickly flowing HDPE resin.
HDPE plastics were first invented in the 1950’s, though it took some time to discover ways to reliably produce large amounts of homogeneous HDPE, as the processes under which HDPE is produced must be tightly controlled in order to produce the different grades of HDPE.
Ethylene naturally polymerizes at high temperatures and pressures, but it is not commercially viable to operate facilities under these conditions. So instead, catalysts are used to lower the temperature at which polymerization will take place, and also direct the type of polymer which results. HDPE’s largely linear structure with little molecular branching is commonly produced by Zeigler-Natta catalysts, or chromium catalysts.
HDPE lends itself well to recycling, and a great deal of reprocessed HDPE, recycled HDPE, and regrind HDPE is available on the market. The bulk of recycled HDPE plastics are post consumer scrap reclaimed from kerbside recycling programs. Millions of tons of HDPE plastic milk bottles and other household vessels are collected and recycled every year. For the most part, recycled HDPE plastics are equally as useful as virgin HDPE plastics, though they may not be used in food contact applications. There is some risk that contaminants found in the scrap HDPE from which recycled HDPE is made may pass through the various cleaning and filtration steps and degrade and contaminate the end product. If one is dealing with a reputable supplier, and the specific application is not overly intensive, recycled HDPE is an economically sound alternative to virgin HDPE plastic.
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