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.