Recyclable Plastics: High Density Polyethylene (HDPE)
“Paper or plastic bag” is a common phrase known to many when buying groceries or other goods for either commercial or personal use. What actually makes up the plastic bags that we use to carry around items in? Where does it come from: How is it made and reused? The answer is High Density Polyethelene (HDPE). The common recycling symbol associated with this type of plastic container is the #2 in the recycle symbol. Other items that generally use HDPE are Milk and juice bottles, detergent bottles, and freezer bags to name a few of its uses in our everyday lives. When HDPE is recycled it is available in both profile lumber and sheet form, allowing for many popular outdoor products such as waste & recycling receptacles, decking, bike racks, benches, picnic tables, fencing and furniture.
HDPE is typically made from petroleum products and natural gas used to generate the ethylene used in the process. The process was originally proposed by Karl Ziegler using a catalyst of aluminum triethyl and titanium tetrachloride in the 1950’s, and further refined in the 1960’s by the Phillips Petroleum Corporation using a catalyst of Chromium oxide on silica, to generate the long straight chains associated with HDPE.(1) Advancements have been made in the realm of the catalyst to increase the uniformity of the HDPE chains and the rate at which they are made, by Brett Fors et al.(2) using a novel titanium controlled catalyst.
The chemistry involved with the creation of HDPE is a polymerization reaction. Ethylene(H2C=CH2) with heat and a catalyst, the catalyst would have an active site for the ethylene to attach to, the double bond would break in the interaction with the active site on the metal to form a new carbon-carbon bond, with another molecule of ethylene, increasing the carbon chain and density of the polymer. Methods to control the parameters of the polymer in general, such as length, branching density, and weight are influenced by the addition of pressure and hydrogen gas, which is able to terminate the polymerization process by “end-capping” the active site on the ethylene growth chain as seen in the figure below.
The chemistry synopsis of the making and uses of HDPE:
· Name is High Density Polyethylene and recycle code is #2
· Used in everyday items like milk jugs and detergent bottles
· can be recycled into furniture and collection bins
· To make the material you need ethylene, heat, and a catalyst
· To control the polymerization reaction, you can use pressure and hydrogen gas
· Process has been known since the 1950’s and continuously improved on since.
Test your knowledge with the interactive activity below on HDPE:
2) J. Am.Chem. Soc. 2020, 142, 3, 1443–1448
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