What is plastic oxidation?

Corrosion in plastics

In polymers, aging is essentially considered synonymous with oxidation. But there are important differences between chemical aging, oxidation, and the less recognized physical aging. Let’s unravel this mystery.

In the late 1970s, the use of plastic water bottles became popular when the FDA approved the use of PET polyester as a suitable material for water bottles. The amorphous variety of this polymer is transparent and tough and possesses ideal barrier properties when used at the correct molecular weight grade and processed correctly.

When amorphous PET products were stored at elevated temperatures in unheated warehouses (at approximately 50°C or 120°F), they lost much of their molded product qualities within weeks. At the same time, tests showed that the material was getting stronger and stiffer.

At the time there were some well-understood mechanisms that could explain the appearance of brittle behavior in plastic materials. The most obvious was a reduction in molecular weight. During processing, this is normally caused by prolonged exposure to elevated temperatures while the material is in the molten state.

What acid dissolves plastic

As oxidation is a process that causes materials to deteriorate over time, its effects or the potential for these effects are not always evident when tests are performed on new products.

In high-speed PE film extrusion, some antioxidants will be consumed during the process, so the ability of the final product to perform in the application environment is a matter of how much protection the raw material had initially and how well it was preserved by the processor. (Photo: Hosokawa Alpine).

But like all materials they have their weaknesses, and one of them is their susceptibility to oxidation. Polypropylene (PP) is more vulnerable than polyethylene (PE) due to some important differences in its chemistry, but both materials rely on the incorporation of additives known as antioxidants to survive the elevated temperatures of the melting process and the demands of various application environments.

All polypropylenes melt between 150 and 165 C (302-329), depending on whether they are homopolymers or copolymers. The major focus in terms of elevated temperature performance is on retention of strength and stiffness as temperature increases. But exposure to elevated temperatures has a long-term effect that can affect the end user because the level and type of antioxidant incorporated into the material can limit the material’s ability to perform at low temperatures, even over the long term.

Table of chemical resistance of plastics

The main advantages of plastics compared to metals include chemical compatibility and resistance to chemicals and corrosion. By choosing the right polymer family, the user can achieve resistance in the harshest environmental conditions without the need for additional protection such as surface treatment, painting or cathodic protection. In our product portfolio you will find acid-resistant plastic materials, polymers resistant to highly alkaline media, including hot water and steam, and solvent-resistant plastics. Users can also choose a balanced chemical-resistant plastic suitable for a wide variety of applications. The table provides indications of the pH limits and substance categories that can be used as a general guide to the chemical resistance of plastics, and also shows where the various product families can be used under ambient temperature conditions and without mechanical load.

Chemical resistance of plastics

In Mexico, the most common degradable plastics are called oxodegradable plastics, which decompose after an oxidation process (due to the effect of temperature or solar radiation) and exposure to the action of microorganisms, making it impractical to deposit them in landfills, dumps or in the environment.

In order for oxodegradable plastics to obtain such characteristics, an additive is added to conventional polyethylene that reacts on contact with oxygen in the air. “In this way, the material’s molecules fragment and can be broken down by bacteria and fungi.

For these reasons, the researcher and her team decided to evaluate the different types of degradable plastics in different environments, for example, in compost, mixed with other waste, in marine environments, in anaerobic conditions, in soil and outdoors, to determine in which cases decomposition occurred and what factors triggered it.

Although there is no exact data, it is estimated that conventional plastic can remain longer in the environment, while degradable plastics disintegrate more quickly if they are handled properly. However, the accumulation of any type of plastic does represent a major ecological problem.