It is important to recognize the fact that the discoloring issue can occur on the surface of not only PVC but also many different plastics such as polyolefin (PE/PP) and other polymeric materials. It is widely accepted in the industry that the over-oxidation of phenolic antioxidants contributes most significantly the discoloration. Although not being in CGT’s PVC formulation, phenolic antioxidants are used extensively in automotive polymers to provide heat and UV stability, for example, in polyurethane foams with which the PVC coverstock material is laminated. It is not possible or practical to eliminate the use of these additives. Oxidation of phenolic antioxidants in the PUR foam (such as BHT) can occur immediately during the lamination process, and the flame lamination process also provides vaporized channels to migrate the traces of the phenolic antioxidants and other contributing additives (such as amines) onto the vinyl surface. Phenolic antioxidants on the coverstock surface will be further oxidized via a reaction with atmospheric pollutants (mainly nitrogen oxides, NOx) to form to nitrated phenolic compounds (often called as “quinones”). Quinones containing phenolic hydroxyl groups are colored, varying from yellow to pink depending on the content of phenolic hydroxyl groups. Pinking or yellowing often forms during storage over days or weeks if the quinone has a high phenolic content, especially in a basic or alkaline environment when the surface pH value is higher than 7. Nitrogen oxides (NOx) are combustion products which can be found in poorly ventilated areas that use diesel gas/propane powered appliances/equipment (i.e., forklift trucks), cigarette smoke, space heaters or anything else that produces fossil fuel exhaust. These conditions are very typical in production plant environments or closed storage areas, especially during a shutdown and winter period, and can be greatly reduced by making sure the ventilation and air flow is adequate.
Several key factors will affect the pinking discoloration of PVC automotive coverstock materials:
- PVC raw materials: discoloring often occurs with white or light color PVC coverstock materials which use titanium dioxide TiO2 pigments. The quality and concentration of TiO2 used in a PVC formulation is more critical than any other raw materials used on the vinyl’s sensitivity to pinking or yellowing. At CGT, we ensure the grade and quantity of the TiO2 used in our PVC formulation are of the highest automotive quality which will not contribute to the discoloring issue. As for phenolic antioxidant, CGT does not add any to our vinyl formulation.
- Non-vinyl components such as PUR foam/adhesives and the lamination process: PUR foam/adhesives quality (with BHT or amine antioxidants) and lamination process (excessive burning and poor ventilation) will accelerate the migration of phenolic antioxidants on the vinyl surface.
- Atmospheric pollutants (NOx): NOx consist mainly of nitric oxide (NO) and nitrogen dioxide (NO2). Nitrogen oxide is readily converted to nitrogen dioxide in air. NO2 is a strong oxide which can promote the oxidation of phenolic antioxidants and generate colored species even at very low concentrations. NO2 concentrations in winter often exceed summer levels by 1.5-2 times with increased use of heating, lower ventilation rates, and higher outdoor concentrations.