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In this article, we will discuss important considerations to take when choosing a polymer liner for environmental applications, the drawbacks of PVC, and the benefits of using strengthened polymers such as high-density polyethylene (HDPE).
Polymers, or plastics, are considered among the greatest inventions of our generation. Its introduction marked the first time that humans were not reliant on nature’s metal, stone, wood, and bone for production. Plastics were the perfect complement to our industrialized society, enabling a new level of manufacturing that supported the spread of material wealth across the globe (1). Unfortunately, the rapid proliferation of plastics led to unexpected uses.
The material’s low cost and ease of manufacturing, for instance, meant that plastics were often used in disposable products like grocery bags, straws, milk cartons, and six-pack rings. And unlike the disposable product’s short-term use, once dumped in nature it can take several hundred years to decompose—even based on optimistic estimates (2). The widespread use and dumping of disposable plastic products have led to an environmental crisis headlined by rapidly filling landfills and the Great Pacific Garbage Patch (3).
But the same materials that are being used to unintentionally pollute the Earth have the ability to help protect it. For decades, polymer liners made with polyvinyl chloride (PVC) and polyethylene (PE) have been used to separate landfill contents from the surrounding ecology. And environmental non-profits such as the Ocean Cleanup are designing autonomous systems made with plastic to help collect and recycle plastic waste (4).
Polymers liners for environmental applications
Modern landfills have two design phases: before its creation and before its closure. In both phases, engineers turn to polymer liners as a way to help prevent the contents of the landfill from leaking out into the surrounding environment. The liner’s chief purpose is to create a very-low permeability barrier against waste fluids or leachate. In order for the liner to do its job effectively, it has to maintain performance throughout a long service life spanning multiple decades.
Therefore, the best liners in environmental applications provide very-low permeability, high resistance to chemical degradation, high durability, and flexibility. Most of these properties are determined by the polymer’s molecular structure and the liner’s thickness. The polymer’s molecular structure determines the polymer’s tensile strength, density, and permeability. The thickness of the liner affects overall strength, weight, flexibility, and cost, with the liner’s permeability being directly proportional to its thickness.
For each landfill design phase, choosing a polymer and the liner’s thickness should be among the first steps (5).
The drawbacks of using PVC liners in environmental applications
Some polymers are better suited for environmental applications than others. While popular for its ease of use, PVC liners are not naturally flexible and rely on additives like phthalates for its flexibility. PVC can also contain traces of bisphenol A (BPA) and can release dioxins. Phthalates, BPA, and dioxins are listed by the National Institutes of Health (NIH) toxins known to cause adverse health effects and are suspected endocrine disruptors (6). In addition to being harmful, PVC additives can leach out over time causing embrittlement of the liner.
It is also important to note that the permeability of a PVC is about two orders of magnitude higher than an equivalent thickness of HDPE, which means any contaminants in the landfill are more likely to seep through the PVC liner over time, potentially causing soil and/or groundwater contamination. In place of PVC liners, engineers can use HDPE liners. HDPE liners provide many of the same benefits of PVC but is non-toxic and is made with a polymer that is very easy to recycle (7).
Using HDPE liners in environmental applications
HDPE is a unique configuration of polyethylene that exhibits minimal branching of its polymer chains and is therefore more rigid and has a higher density when compared with linear low-density polyethylene (LLDPE). HDPE liners will also confer a number of benefits regardless of its thickness, such as its high resistance to corrosion and long-term ultraviolet (UV) resistance.
Additionally, the chemical resistance of HDPE is among the best of commonly used lining materials and far better than PVC, especially against acids, bases, and hydrocarbons. The low temperature characteristics of HDPE are also better. While PVC can become brittle at -40°C, it requires a temperature closer to -90°C to have the same effect on HDPE.
- 1. https://www.sciencehistory.org/the-history-and-future-of-plastics
- 2. https://www.nationalgeographic.com/magazine/2018/06/plastic-planet-waste-pollution-trash-crisis/.
- 3. https://en.wikipedia.org/wiki/Great_Pacific_garbage_patch
- 4. https://theoceancleanup.com/milestones/system001/
- 5. Topliff, CR, “Uses and Installation of HDPE Liner to Reduce Soil Erosion and Prevent Water Loss.” West Texas A&M. (2018). Accessed on June 2019. https://wtamu-ir.tdl.org/handle/11310/153.
- 6. https://toxtown.nlm.nih.gov/chemicals-and-contaminants/polyvinyl-chloride-pvc
- 7. https://www.azocleantech.com/article.aspx?ArticleID=255