What is Polyethylene?
Polyethylene (PE) is the world’s most used plastic. In its basic form, it is primarily used for packaging. More specifically, it’s a thermoplastic polymer, commonly found in Low-density (LDPE) or High-density (HDPE) forms. HDPE is the moderately stiffer of the two, with a greater tensile strength, and greater resistance to damage. LDPE is soft and quite pliable and is often used in plastic films like cling wrap or lightweight products like grocery bags.
Manufacturing Polyethylenes
Polyethylene can be produced using a variety of chemical processes and catalysts, but it all starts with naphtha, or petroleum, a component of crude oil which is separated during the refining process. When the separated petroleum is heated, it releases ethylene gas, which is used in the production of antifreeze, alcohol, mustard gas, and other organic compounds in addition to polyethylene. Since ethylene originates in crude oil, which is derived from an organic source, ethylene is classified as an organic chemical.
It may be surprising to note that fruits actually emit ethylene gas as part of the ripening process, so the agricultural industry regularly uses it to promote ripening with fruits that are picked while still green. High level ethylene producers include certain varieties of apples, bananas, melons, pears and peaches.
In the production of polyethylene, a catalyst is introduced to ethylene gas inside a reactor. The production of plastic is a sophisticated industry, and there are a variety of reactor types used to produce a wide range of physical properties in the resulting polymer by actively managing heat, use of additives and incorporating several additional types of organic molecules (monomers). Monomers are organic molecules that are selected for their ability to form chemical bonds between other molecules. Use of monomers and comonomers enable individual ethylene molecules to form long polymer chains and even 3-D networks.
Ethylene, on its own, is a stable molecule with a double bond between carbon atoms. In order to create polymers, the manufacturing process requires that the strong double bond be broken, permitting new molecular configurations to be created. This is where catalysts come in. Once the double bond is broken, the atoms will reform into long chains of 3 carbon atom molecules. These chains could presumably build endlessly unless additional hydrogen atoms are introduced into the mix. When a single hydrogen atom is available to occupy the covalent bond, no more chain links can be added, ending the reaction.
Management of individual molecules and their connections isn’t possible, so manufacturers manage the formation of polymer chains by varying the amount of hydrogen and comonomers added in the reactor. Determining the composition of the mixture is a matter of playing probabilities. When plenty of hydrogen is available, for example, growing chains are more likely to encounter a reaction-ending hydrogen atom, resulting in a prevalence of shorter chains.
The length of the polymer chain isn’t the only variable when manufacturing polyethylene. When the double bond in ethylene molecules is broken, allowing new atoms and molecules to connect, there are still several different molecular shapes that can form, from highly randomized connections to neatly alternating connections along the sides of the chain. Each type of distribution produces a different set of physical properties in the resulting product, such as density, stiffness, flexibility, strength, etc. Manufacturers encourage specific chain structures by manipulating heat, pressure, and other environmental factors. Low heat and pressure produce denser chains which yield stiff HDPE, for example. At the other end of the spectrum, high heat and pressure disrupts orderly chain formation, yielding an elastic product.
In theory, the variety of polyethylene products could be nearly infinite. Predictably, there’s a wide variety of polyethylene geosynthetics to choose from, possibly with significantly different properties, especially when additives are incorporated after the polymerization step. For example, many geosynthetics include carbon black, antioxidants, and heat stabilizers. These additives help prevent the type of degradation from UV rays that are common in other plastics.
Partially Crystalline Solids
As we discussed above, the properties of polyethylene materials depend on how their constituent polymers are organized in the chain. When polyethylene polymers line up in orderly (straight) structures, they’re in a crystalline formation. More randomly arranged areas are called amorphous regions. All types of polyethylene are, in fact, partially crystalline solids, meaning they have a combination of both amorphous and crystalline regions. As you might imagine, materials with a high percentage of highly ordered crystalline structures are strong in a structural sense, but they’re also stiff and may be brittle. In contrast, products with a higher percentage of amorphous regions are lightweight and quite flexible but are typically easily deformed. Because the properties of the PE sheets are so dependent on the arrangement of polymers, the degree of crystallization is carefully managed during manufacture.
HDPE has a relatively organized (read more crystallized) polymer chain, with long lines of orderly, closely aligned polymers. Being orderly and closely packed, the resulting material is heavier and stiffer than LDPE, but is also much less vulnerable to damage. LDPE’s polymers are more disorganized (amorphous), so LDPE is extremely flexible but also not as strong.
Many manufacturers combine HDPE and LDPE plastics in a single product to harness the advantages of both types while eliminating the disadvantages. Check out the section below on composite geomembranes for specifics.
