January 7, 2021
Why Polyolefins Are a Popular and Sustainable Plastic Option
A mainstay of outdoor furniture, unique properties make these recyclable, clean plastics more sustainable than other synthetics.
Sometimes names are confusing. Polyolefin is a term used to refer to a class of polymers that includes two of the three highest-volume plastics on the planet, polypropylene (PP) and polyethylene (PE)—the other is PVC—yet neither has the word olefin in its name. In fact, the chemical term olefin is not even used much anymore, having been superseded by the word alkene, the name for the repeating units of carbon–carbon double bonds in these plastics. Still, the name olefin has stuck as a generic term.
Olefins are wonderfully simple compared with many other plastics as they are not much more than solid petroleum (admittedly a problem when it comes to fire retardancy) and have been called “clean” in that there are no additional problematic chemicals required in their production, such as styrenes, chlorine, or bisphenol A. Also, rare among plastics, olefins float on water, which helps distinguish them from other synthetic materials in recycling.
LOW-DENSITY AND LINEAR LOW-DENSITY POLYETHYLENE (LDPE, LLDPE)
Formulated in 1933, LDPE was the first polyethylene and has been used for all types of applications where a lightweight, low-cost, medium-strength, and heat-resistant plastic is needed. The “linear” version is a 1960 upgrade with enhanced strength and temperature resistance and is taking over many of the traditional applications for LDPE in pipes, flexible tubing, and geomembranes.
HIGH-DENSITY POLYETHYLENE (HDPE)
House wrap, synthetic wicker and rattan, chemical-resistant work surfaces, outdoor decking, and vertical paneling for bathrooms all use this durable, slightly slippery plastic. It has a milky transparency in very thin sections and recycles well but has limited performance with temperatures much above boiling water.
ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE (UHMWPE)
Too expensive for commodity use, UHMWPE, with trade names of Dyneema and Spectra, claims to be “15 times stronger than steel” and is used primarily in ropes to pull oil tankers but also in rigging and architectural mesh.
This olefin is used in building siding and some building wrap, as reinforcement in concrete, road paving, and other construction, and in carpets and rugs intended for the outdoors. It is also the go-to for roto-molded large outdoor furniture. Though it is not the most rigid of plastics (all olefins suffer from low stiffness), adding some glass fiber makes it a viable alternative to ABS and nylon in molded commercial furniture.
There has been a vibrant recycling economy for both HDPE and PP, with milk containers the most notable of household items that end up as new content. Different olefins can be recycled together, but that can affect the performance of the end product. Bio-based versions exist; Braskem in Brazil has already maxed out capacity for its sugarcane-based HDPE. A bio-based PP is in the works from Braskem and others, but no large-volume sources are available yet.
Andrew Dent is the EVP of materials research at Material ConneXion and chief material scientist at SANDOW.
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