Designing For Degradation In Chemistry

Kate Onissiphorou

by Kate Onissiphorou

29th June 2022

Designing materials for easy degradation is one of the twelve principles of green chemistry. It aims to purposely design and make products that break down into non-harmful constituents that do not persist in the environment or get magnified in the food chain.

Many of the synthetic polymer chemicals we use on a daily basis, such as those found in plastic, rubber, and electronic waste, are not environmentally friendly. This is because they don’t degrade easily after they’re used.  

Degradable materials are only meant to last within the useful lifetime of a product made from those materials. For example, biodegradable plastic bags are designed to be gradually decomposed by bacteria after they’re thrown away or once they get wet.

Unlike petroleum-based plastics, biodegradable plastic bags don’t take hundreds of years to decompose. Crucially, because they won’t become microplastics that enter the food chain, they don’t pose a threat to marine organisms.  

What does degradation mean in chemistry? 

Degradation is simply the breakdown of substances (materials) into simpler constituent substances. Organic molecules usually degrade in several stages, starting with the removal of a functional group.

In nature, complex organic compounds, which include living tissues and organisms, degrade when their useful function ends. For example, the cells in a multicellular organism regularly undergo apoptosis or self-destruction after a certain period of time. Graphic showing apoptosis, or programmed cell death, in three stages

Apoptosis, or cell death, is necessary to ensure that tissues and organs are functioning efficiently. Damaged cells are consumed by their own enzymes to give way to new cells. The process also prevents cancer from developing.

Similarly, macro-organic compounds also break down once they’ve fulfilled their purpose. Energy-carrying molecules like carbohydrates, for instance, split into simpler constituent molecules and elements during metabolism as energy is released.

Chemists can make some materials much easier to degrade into innocuous, more fundamental substances once they’ve served their intended purpose. There are several ways they can do this, one of which is to use alternative feedstocks or polymers. This is true for biodegradable plastics.

Person pulling a plastic bag made of plant based cassava to test for strengthSome of the biodegradable plastics currently on the market include:

  • Starch-based plastics
  • Soy-based plastics
  • Cellulose-based plastics
  • Lignin-based plastics

These types of plastics are made from polymers derived from naturally-occurring materials like cellulose, starch, polyhydroxyalkanoates, polylactide, polycaprolactone, and collagen.

What is an example of chemical degradation? 

Some chemicals degrade when they’re exposed to direct sunlight, such as in the case of lignin, which is found in tree stems. When the tree bark is removed, the stem reacts with UV light from the sun and causes discolouration.

Materials derived from lignin can undergo photodegradation, also known as photooxidative degradation.

What causes chemical degradation?

Chemicals can degrade in several ways, depending on their composition and molecular structure. Various factors can trigger the chemical degradation process, some of which are outlined below.

  • Heat at certain temperatures, some chemicals (particularly organic ones) can become unstable and start to break down. Many organic compounds, such as the vitamins found in fruit and vegetables, start to degrade and lose their potency at temperatures above 48 °C
  • Acids and bases strong inorganic acids and bases act as catalysts and can easily break down organic compounds. For example, esters undergo hydrolysis in the presence of a strong acidic solution, as shown below. A graphic showing esters undergo hydrolysis in the presence of a strong acidic solution
  • Sunlight some organic compounds undergo photodegradation as a result of the ionising radiation from the sun’s ultraviolet rays. 
  • Enzymes organic tissues start to decompose when they die. The cells themselves produce digestive enzymes that destroy and break down the cells.
  • Bacteria and fungi the decomposition of organic tissues is mainly facilitated by decomposers. They secrete enzymes that break down the organic molecules in dead tissues.
  • Water or moisture many organic substances either dissolve easily in water or degrade when they absorb moisture. If an organic substance is dissolved in water, it may react with the water itself or become more reactive with other substances. 
  • Oxygen some organic compounds, especially medicines, are sensitive to oxygen. These compounds can be easily oxidised when they’re exposed to the air.

How degradation is related to sustainable chemistry

Often used interchangeably with green chemistry, sustainable chemistry is a broad approach to environmentally-friendly practices in chemistry.

It primarily focuses on solving the environmental problems brought about by the manufacture and use of chemical products. Sustainable chemistry aims to improve the efficiency of using natural resources to meet the societal and economic needs for chemical products. 

Degradation can help to achieve the goals of sustainable chemistry by eliminating or at least reducing the impact of chemical products on the environment.

How degradation can be designed for green chemistry

Designing for degradation is the tenth principle of green chemistry.

Illustration of a chemical beaker made of leaves in a green meadowEveryday items such as plastics, soaps/detergents and electronic products (including batteries), can be designed to degrade into non-toxic components.

For example, two similar molecules or isomers of sodium dodecyl benzene sulfonate have different rates of degradation. The linear isomer is biodegradable and non-toxic while the branched isomer is harder to degrade.

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