Lesson 2 - Classification and Properties
In this lesson we will discuss what materials of construction are and why a better understanding of them is important to us. We examine the general classification of materials and highlight some of the most important ones in each relevant class. We will review the properties of materials and go into greater detail on these where required. To conclude, we focus on three things that can impact materials greatly: corrosion, degradation and welding.
Polymer Degradation
Polymers can degrade just like metals can corrode. We will look at these processes in more detail so we can understand the limitations of polymers and the impact of degradation on lifespan and functionality.
In general, polymer degradation is not visible; it can happen throughout the material. Degradation can take a number of forms:
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Weathering is the long term breakdown of a polymer.
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Swelling and dissolution is where the polymer absorbs moisture or is dissolved in a solvent.
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Scission is the rupturing of polymer bonds.
You may have come across a few old beer crates that have shown sign of weathering. There are a number of mechanisms at work here. The main ones we will look at are thermal degradation, light (UV) degradation and hydrolysis.
Thermal degradation
Certain polymers, such as polymethylmethacrylate, break down at high temperatures. Oxidation can also occur at these higher temperatures if oxygen is present. The polymers undergo scission, and components can react with each other. A vast range of breakdown products can be produced, and these can be toxic.
Light degradation
Ultraviolet light reacts with carbon bonds in the polymer to cause free radicals. These can then further react with oxygen. Polypropylene and low-density polyethylene are particularly susceptible to UV degradation.
Hydrolysis
When certain polymers are formed, water molecules are expelled as a by-product. Some polymers are susceptible to absorbing moisture, and in the presence of an acid these polymers can breakdown again. This process is called hydrolysis (meaning splitting with water). The figure below gives a generic representation of the process.
Figure: Typical hydrolysis of a polymer
For some interesting reading on how biodegradable plastics really are visit the following link:
https://www.smithsonianmag.com/smart-news/do-biodegradable-plastic-bags-actually-biodegrade-180972074/