Plastic yellowing and aging are challenging obstacles to long-term use. Often this leads to a premature failure during the useful service life.
For the plastic industry, this can translate into the following:
- risk of financial loss
- operational downtime.
So, can you prevent the yellowing of plastics and slow down its degradation? Good news is there are ways to reduce often-damaging effects during processing, use or storage.
Whether you are a manufacturer, converter, or OEM, preventing plastic yellowing could be tricky.
This useful guide can enable you to understand the root causes and save different types of plastics. Plastics are chemically different and that affects their behavior in use. Accordingly, knowledge of distinct measures is needed to prolong their service life.
First, let us explore the root cause of yellowing and its impact on plastic’s useability.
Why Plastic Is Yellowing?
Plastic yellowing often occurs when polymer chains degrade. This happens either in the presence of oxygen and UV light (photooxidation) or in sufficient heat (thermal oxidation). As a result, “yellow bodies” are created that impart yellow coloration.
Impurities in the plastics may themselves absorb light and give a yellowish aspect. They can also accelerate the degradation process. Likewise, an inadequate amount of additives or fillers can discolor plastic during processing or storage. Common interactions that lead to discoloration are the following:
- Thermal
- UV radiation
- Pollution contact
- Mechanical
- Hydrolytic
- Chemical
- Biological
Yellowing is a visual indicator that plastic is losing its properties and might become brittle. Depending on the type, plastics change color at different rates because of varying reasons.
Do All Plastics Turn Yellow?
In the majority of plastics, yellowing is spontaneous and cannot be stopped. The time to have a certain degree of yellowing or degradation relates to the type of plastic and its application.
Commodity Plastics like polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are at increased risk. Without additives, they are inherently unstable and can even degrade during processing.
In the presence of UV light, oxygen in the atmosphere produces carbonyl groups. This changes the color to yellow-brown and the plastic begins to flake away until it becomes useless.
Gas fading in polyethylene films and articles is a common problem. Atmospheric pollutants (such as NOx) can react with antioxidants in them and impart a pinkish or yellowish tinge. Unlike conventional discoloration, gas fading develops slowly over time during storage.
High-Performance Plastics are more expensive and are better at aging than standard plastics. Common examples include:
- Polyesters
- Polycarbonates
- Polyurethane
- Polyamides
Polyesters, like polyethylene terephthalate (PET) and polycarbonates (PC) turn yellow in outdoor applications. Bottlers and compounders often experience reduced transparency in clear products. Such plastics are sensitive to the UV radiation of the sun and artificial sources. This sensitivity double folds in the presence of heat, moisture, and oxygen. Same holds for white or light-colored nylon (a polyamide) fabrics. Yellowing can happen everywhere, such as during finishing, storing, and even at home. Such phenomena are known to manufacturers as phenolic or warehouse yellowing. Oxides of nitrogen in the air can react with phenolic antioxidants in packaging material. This results in yellowable phenols leading to discoloring or stains on textiles.
“White” textiles constitute around 20% of all global production. Therefore, manufacturers require correct precautions to ensure no financial loss due to rejections and consumer complaints.
In this regard, recycled PET plastic degrades in sunlight at a faster pace. Unlike virgin PET, the level of yellowing is significantly higher. This limits the use of recycled PET in bottles and clear products.
Plastics More Sensitive to Yellowing
The susceptibility of plastic to yellow or degradation is often related to its structure. Polymers and epoxies having aromatic groups are more prone to UV damage. Aromatic polyurethanes (PU) are a good example in this regard. They turn yellow by free radical pathways occurring in the backbone of the polymer. Over time, light or heat-induced oxidation forms yellow-colored quinones.
Most common reasons for PU discoloration are:
- High-temperature exposure during manufacturing or in the end-use
- Exposure to pollutants such as NOx in the air
- Ultraviolet light
- Butylated hydroxytoluene (BHT), a commonly used antioxidant
An Ounce of Prevention
Benjamin Franklin famously advised: “An ounce of prevention is worth a pound of cure”. Identifying the cause paves the way for preventive action. Luckily, there are several ways to prevent the damaging effects. For example, during moulding, the plastics can undergo thermo-oxidation. This requires the addition of antioxidants to prevent such degradation.
Plastics for outdoor uses should be thoroughly processed (end-capping of polymer chains). Moreover, UV absorbers and hindered amine stabilizers must be added. This improves weatherability and
protection from UV light-induced oxidation.
Clear plastics like PET and PC require special care in this regard. Adding
light stabilizers and yellowing whitening chemicals frequently solves the yellowing issue. Gas fading can be reduced by limiting the interaction of NOx pollutants with PE articles. Processing additives can also limit this issue in warehousing applications.
Preventing the degradation of common plastics require good knowledge of different factors. Plastic properties, processing conditions, and application environment are among the few. An understanding of the interaction of these factors is crucial before making a balanced formulation. Irrespective of the plastic type, you have to be careful in the selection of the additives. Wrong stabilizers or antioxidants can react with fillers or pigments, resulting in coloration or damage.
Check out all commercially available light stabilizers/UV absorbers
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