Effects of Carbon Black on the Depletion of Antioxidants in HDPE under Air and Water Conditions

The primarily function of carbon black (CB) is to protect the polymer from ultraviolet degradation. That said, conflicting results have been reported regarding the improvement of the long-term performance of the polymer by combining CB with antioxidants (AOs). This paper presents the effects of CB on the depletion of two types of AOs in high density polyethylene (HDPE). Carbon black with a particle size of 75 nm were blended with HDPE resin in the amount of 0 and 2.5 wt.%. Antioxidants, Irganox 1010 (I-1010) and Irgafos 168 (I-168), at different concentrations were then added to the samples with and without CB. Ten test formulations were generated. The interaction between the CB and AOs were accelerated by incubating samples in a forced air oven and in a water bath at a temperature of 85°C. The antioxidant depletion rate was monitored using the oxidative induction time (OIT) test. For unstabilized HDPE samples (with 0% of AO), the CB delayed the onset of oxidation. On the other hand, a strong interaction between the CB and AOs was found in samples that consisted of both, as indicated by a rapid decrease of the OIT value in both incubation environments. Furthermore, the depletion of AOs in CB samples was significantly faster in water than air environment, particularly in the initial 90 days. The hydrolysis/leaching of AOs as well as the enhanced CB/AO interaction due to ingress of moisture may be the factors for such an accelerated depletion rate.