Sunday, 31 January 2021

HDPE Plastic Bag Degradation - The Experiment

The increased amount of plastic bags in our environment lead to discussions about whether or not plastic bags degrade in the short or long term (for example, the 450 year plastic bottle degradation live stream) and how much they harm our environment.  Therefore, in the beginning of this year I started a plastic bag degradation experiment, using a piece of simple high density polyethylene (HDPE) bag and sea water.

The motivation – find out when bag degradation starts

For that I took a standard HDPE plastic bag and cut a 240x160x0.1 mm part out. Then I filled an empty marmalade glass with sea water right from the beaches of Sesimbra, Portugal (California Beach; close to our holiday rental). Then the glass filled with sea water and the HDPE bag was placed in a storage room without the influence of sunlight.

Every year around January, I will check the degradation progress. Let us see how the bag looks in one year and in 30 years.
HDPE Plastic Bag Degradation - The Experiment (up: the HDPE bag sample; down left: identification stamp on the bag; down right: glass with bag sample and sea water; start: January 2021;

It is a simple experiment and there are several scientific studies dealing with this topic. One of them is from Mr. Telmo Ojeda [1] who investigated the degradability of linear polyolefins under natural weathering conditions. Four different polymers were used for the study: high density polyethylene (HDPE), linear low density polyethylene (LLDPE), isotactic polypropylene (PP), oxo-biodegradable HDPE/LLDPE blend (containing a pro-oxidant additive to accelerate degradation).

They found out that it took less than a year for polyolefin films, which have low or no content of antioxidants to degrade by natural weathering. Mechanical properties got lost due to the decrease in molar mass caused by oxidative degradation. There are differences in terms of degradation speed of the investigated polymers. HDPE and LLDPE films showed a more slow degradation, however this degradation was significantly in a timeframe of few months. Rapid degradation could be found with PP and oxo-bio HDPE/LLDPE blend. Since pro-oxidant additives are present in the oxo-bio HDPE/LLDPE blend, acceleration of degradation was accepted and could be proven.  The PP film contained primary (sterically hindered phenols) and secondary (phosphite) antioxidant additives, which slow down the degradation process. However, the antioxidant additives, applied in low concentration, could not prevent the rapid photo-oxidative degradation.  Secondary antioxidant additives showed little influence to delay the abiotic degradation (photodegradation and hydrolysis) in HDPE and LLDPE.

The second study I selected was conducted by Napper and Thompson [2]. The researchers investigated the degradation of biodegradable, oxo-biodegradable, compostable, and HDPE bags over 3 years.

All the materials were exposed to three different environments: open-air, buried in soil, and immersed in sea water. The sea water has a tremendous impact on the compostable bag, which disappeared within three months. The same bag material was still present after 27 month of exposure in soil. However the mechanical strength was so much reduced that it could not hold weight without cracking. Interesting result was that all bag materials decompose into fragments after nine month exposure to open-air.

Governments in different European countries react and made already laws to ban plastic bags. In Austria, for example, plastic bags for shopping are forbidden since 2020. In Austria, the plastic bag consumption sums up to 7000 to 8000 tons per year which represents around 1% of overall waste. Looking at the per head consumption of plastic bags per year, it has a CO2 equivalent of a 15 km passenger car ride [3].

Fact is that plastic bags should not end up in our oceans nor in our environment in general. We have a littering problem and not plastic problem. I hope this conception is changing over the years ahead of us, since plastics are among the most environmentally friendly materials out there [4,5].

I will update you in 2022 on the degradation progress of the HDPE plastic bag immersed in sea water.

Thanks for reading and #FindOutAboutPlastics! 


Herwig Juster

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[1] Ojeda, Degradability of linear polyolefins under natural weathering, 2011

[2] I. E. Napper & R. C. Thompson, Environmental Deterioration of Biodegradable, Oxo-biodegradable, Compostable, and Conventional Plastic Carrier Bags in the Sea, Soil, and Open-Air Over a 3-Year Period,(2019)



[5] Chris DeArmitt - The Plastics Paradox:


  1. nice little project Herwig! I would assume that the exposure to sunlight/UV also would accelerate the degradation.

    1. Yes, indeed - thanks for your feedback Plastics Expert Witness! best regards, Herwig