Troubleshooting Tips: Pinking Discoloration in Polyolefins

Pinking discoloration is a well-known phenomenon that occurs in polymers such as HDPE, LLDPE and polyolefin elastomers. This discoloration is often caused by over-oxidation of hindered phenol type anti-oxidants which are commonly used in polyolefin's. This over-oxidation results in the formation of a colored species, called quinones, which tend to vary in color from yellow to red. One of the main causes of pinking is related to exposure to oxidative gases such as NO_xand SO₂. These gases are formed by the burning of natural gas, from the exhaust of fork-lift trucks, from welding gases, from corona treatment and from gas powered heaters. Pinking can also be caused by a chemical interaction between the phenolic anti-oxidant and other chemical species. Leaching of phenols or SO₂from some types of cardboard packaging can also cause pinking.

Polymers are melt processed at elevated temperatures in the presence of oxygen and this can cause auto-oxidation of the hindered phenol anti-oxidant. Not only can this cause discoloration, but this over-oxidized anti-oxidant is then more susceptible to pinking from NO_xgases. Adding secondary anti-oxidants to the polymer may help to minimize this over-oxidation.

Pinking can also be caused by interactions with pigments. The presence of titanium +4 in titanium dioxide based white masterbatch colorants can trigger over-oxidation of the hindered phenol anti-oxidant. The presence of iron Fe+3, found as an impurity in titanium dioxide, can also cause a red color to form. It has been shown that basic conditions (high OH^-) promote the formation of quinone compounds in the presence of metals such as titanium. Basic conditions can also be found with certain additives such as hindered amine light stabilizers (HALS) and certain catalyst deactivators that may also be in the polymer formulation.

Pinking can be reversed by exposure to strong UV light. The UV light source can be sunlight, florescent lights or UV lights. Exposure to the UV light reverses the reaction that formed the colored species causing the color to return to normal. The color reversal can take for a few hours to several hours depending on the intensity of the UV light.

Pinking can be prevented or eliminated by:

• Eliminating the exposure to oxidative gases. Improving ventilation can also help with this.
• Packaging parts in plastic bags inside of cardboard boxes may help.
• Use high purity white colorants.
• Use polyolefin grades without HALS.
• Ensure that processing conditions are within recommended temperatures and that residence time in the extruder barrel is not excessive.
• Consider adding secondary anti-oxidants to the formulation to better protect the polymer during processing.