Proposal of Wai-Kuen (Connie) Wong of
Oxidation Degradation Mechanisms of Corrugated High Density Polyethylene Pipes
Oxidation degradation is a major factor in governing the service lifetime of high density polyethylene (HDPE) pipe. To minimize the oxidation degradation (in soil and exposed to ambient environment), antioxidants (AO) and carbon black (CB) are added to preserve the engineering properties of the pipe. The service life of AOs is essential to the longevity of the pipe, and thus AOs depletion mechanisms should be thoroughly evaluated. The current method to predict the lifetime of AOs utilizing thermal acceleration aging coupling with oxidation induction time (OIT) test to assess the AO depletion rate requires a long testing time which may not be suitable for evaluating new formulations. In addition, the AO depletion behavior varies with incubation media, such as air or water. This research study is to understand the AO depletion mechanisms and to model the migration and depletion of AOs by diffusion.
Two groups of materials are included in this study. Group-A is a set of controlled samples with known type and amount of antioxidants and Group-B is a commercially available corrugated HDPE pipe. Samples in Group-A consist of five AO formulations based on different concentrations of two AOs (Irganox® 1010 and Irgafos® 168) and different concentrations of three types of CBs. A total of 20 formulations are included in this study. Group-B is a 36-inch diameter corrugated HDPE pipe (QC36) which contains a proprietary antioxidant and CB formulation. Samples are subjected to two incubation conditions: forced air ovens and water bath at temperatures from 65 to 85oC. The research is divided into three stages: i) establishing AO depletion rates and depletion profiles; ii) identifying the dominant effect on the AO depletion in air and water; iii) developing a diffusion model.
(i) AO depletion rates and profiles:
Using OIT test to measure the global AO depletion of the incubated pipe samples is essential in the lifetime prediction of AOs; however it does not provide the information regarding the depletion mechanisms. The depletion of AOs is governed by their mobility within the polymer and the migration out of the polymer. For example, our test results indicated that one of the CBs used in this study interacted with AOs in the polymer leading to a significantly faster AO depletion rate than the corresponding sample without CB. In order to understand the mobility of AOs inside the polymer, the changing of AO depletion profiles across the thickness of the sample is monitored with incubation time. The AO profiles will be applied to Stage (iii) of the study for setting appropriate boundary conditions of the Fick’s 2nd law in air and water incubation environments.
(ii) Dominant effect on the AO depletion:
The depletion of AOs is a complex mechanism, involving physical leaching, evaporation, and/or chemical reaction. Depending on the incubation media, the dominating effect on the AO depletion will be identified. AO evaporation can be assessed using nitrogen gas by placing samples inside a container with N2 gas, capped, and maintained slightly above atmospheric pressure, not exceeding 5 psi, to ensure a pure N2 environment at temperatures of 85oC and 65oC. For leaching and hydrolysis reaction of AOs, samples are immersed in the de-aired water containing oxygen of 1 mg/l ± 0.5 inside the pressure cell. The head space is filled with N2. The cells are placed in forced air ovens at 85oC and 65oC. The de-aired water is replaced at predetermined intervals. The retrieved de-aired water is then analyzed by HPLC for AOs or AO related compounds to distinguish physical leaching and hydrolysis mechanisms.
(iii) Diffusion Model:
The diffusion model will be developed based on Fick’s 2nd Law. The numerical modeling method solving the differential equation and assumptions being applied to the model are still in the researching stage. The approach to solve the AO depletion modeling is estimating the diffusion coefficient by using the concentration profile obtained from stage 1 and by combining the parameters of the evaporation, the consumption, and the leaching of AO in stage 2. The outcome of the model is to predict the service lifetime of the corrugated HDPE pipes.
The study will provide a greater understand of AO depletion mechanisms in corrugated HDPE pipes and HDPE materials in both air and water environments.