Pinking Discoloration in Polyolefins
Pinking discoloration is a well-known phenomenon that occurs in polymers such as HDPE, LLDPE and polyolefin elastomers.
Pinking discoloration is a well-known phenomenon that occurs in polymers such as HDPE, LLDPE, POE, PP and other polyolefins. This discoloration is associated with over-oxidation of a phenolic compound, most commonly an antioxidant additive. This discoloration is often caused by using an incorrect or inefficient amount of the antioxidant or from a chemical interaction (over-oxidation) that results in the formation of a colored species (quinones). These quinones can cause a color change from yellow to dark red. Because the amount of quinones formed is often small, they tend to show up as a pink color, rather than a red color. The color can also be influenced by the presence of other additives, especially ones that may be more alkaline.
Most polymers incorporate phenolic antioxidants to help prevent oxidation of the polymer during processing and/or end use. These antioxidants donate hydrogen atoms to quench free-radicals that are formed during oxidation, as well as to generate phenoxy groups which can help to scavenge free-radicals. These antioxidants are consumed during the initial processing of the material, during the life of the finished product as well as during any additional processing due to recycling. Depending on the heat history, melt temperature, and residence time in the machine barrel, this could cause over-oxidation of the antioxidant. The type of antioxidant, its concentration, and whether it is combined with a secondary phosphite antioxidant will determine how easily it may be over-oxidized.
Phenolic antioxidants can also be over-oxidized by reacting with free radicals that can be found in the atmosphere (NOx gases). These include gaseous combustion products that can be formed by fork-lift truck exhaust, gas or oil heaters, welding gases, corona treatment, cigarette smoke, and even air pollution.
Pinking can also be caused by interactions with certain pigments. The presence of titanium +4 in titanium dioxide based white colorants can trigger over-oxidation of the phenolic antioxidant. The presence of iron Fe+3, which can be an impurity in titanium dioxide can also cause a red color to form. It has also been shown that alkaline conditions (high OH-) promote the formation of quinone compounds in the presence of metals such as titanium. Alkaline conditions can also be found with some additives such as hindered amine light stabilizers (HALS) and certain catalyst deactivators that may also be in the polymer formulation.
The pinking effect can be reversed by exposure to strong UV light. The UV light source can be sunlight, fluorescence lights or UV lights. Exposure to UV light basically bleaches out the pink color by causing the quinone to go away. However, it doesn’t truly reverse the reaction that caused the pinking to form, it just changes the quinone to another chemical structure that doesn’t have as much color.
PINKING CAN BE PREVENTED OR ELIMINATED BY:
- Minimizing the exposure to oxidative gases (NOx).
- Improving building ventilation.
- Improving venting of corona discharge exhaust.
- Packaging parts in plastic bags inside of cardboard boxes.
- Use high purity white colorants.
- Use polyolefin grades without HALS.
- Minimize temperatures and residence time in the extruder barrel.
- Minimize the use of regrind.
- Consider adding secondary antioxidants to the formulation to better protect the polymer during processing.
- Switch to a non-phenolic antioxidant.