Have you ever noticed how plastic furniture changes its colour when exposed to the sunlight? I am living in Spain, so I can observe this effect everyday. The plastic furniture turns from a nice bright colour into a faint one, but as you surely know, not only the color changes, but also other properties, such as the durability, the gloss and sometimes the shape of the exposed part.
The process which damages our garden plastic furniture is well-known and is called UV degradation or photodegradation. This process is very unwelcome and there exists several chemical substances which are dedicated to prevent or postpone it.
What is photodegradetion?
Photodegradation is an irreversible chemical process where the sunlight is a key factor. The process starts with photons, which have a large amount of energy and come from the sunlight. When they get close to the plastic part, their energy can be transferred to the molecules or atoms within the polymer.
There are two additional factors which helps degradation:
– Absorbency of chemical groups – most of the chemical groups used in the polymers, have a big absorbency within the UV spectrum (diagram below). This means that if your plastic chair contains some chemical groups the process can go faster.
– Radiation energy from the light wave – when the light wave reaches the earth surface the amount of radiation energy depends on the geographic location. This means that in Spain the same plastic chair will degradate faster than in Norway.
Many natural and synthetic polymers are affected by the ultraviolet light, infrared radiation or visible light. However, researchers confirm that the major part of the photodegradative mechanism is correlated to the absorption of the ultraviolet light (ranges 280nm-400nm).
What happens when the photon transfer energy to the polymer?
The degradation mechanism will vary from polymer to polymer and as I mentioned before this depends also on the chemical groups absorbency to UV. The process can look as one of the below reactions:
When the carbonyl group is in the main chain, the above reactions take place. They cause segmentation of the main chain which results in a decrease of the molecular weight.
If the carbonyl group is in the side-chain, only the second reaction can happen. In both cases the radicals obtained in the first process may initiate a further oxidation called photooxidation.
The most reactive parts of the polymer chain are the chemical groups which are called chromophores. The complexity of the following reactions is very high and still are not defined.
Our polymer chain is broken and is loosing the desired properties. These are the results of the explained process we can unfortunately observe in our gardens.
In a next post we will find out what are the consequences of the photodegradation and how to avoid them.