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Deterioration of Polyethylene Exposed to Chlorinated Species in Aqueous Phases: Test Methods, Antioxidants Consumption and Polymer Degradation
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents a study of antioxidant depletion in water containing chlorinated species (water containing 10 ppm either Cl2 or ClO2, buffered to pH = 6.8), the degradation products in the aqueous phase, and polyethylene pipe degradation scenarios. A low molecular weight hydrocarbon analogue (squalane) was used instead of solid polyethylene as the host material for the antioxidants, and the depletion of antioxidants has been studied. The phenolic antioxidant Irganox 1010 was consumed ca. 4 times faster in water containing 10 ppm ClO2 than in water containing 10 ppm Cl2. The different degradation products in extracts from the aqueous phase identified by infrared, liquid chromatography and mass spectrometry revealed the different degradation mechanisms between ClO2 (" cleavage) and Cl2 (hydrogen substitution). The squalane test shows no energy barrier between 30 and 70 °C, and the activation energy of the antioxidant in solid PE was found to be ca. 21 kJ mol-1. A linear relationship has been established between the time to reach antioxidant depletion in the polyethylene tape samples and the time to reach depletion in samples based on squalane containing the same antioxidants. The surface oxidation and surface embrittlement of PE tape on long time exposure have been studied by IR and SEM. Pressure testing on medium density PE pipes with a controlled pH aqueous media (6.8 ± 0.2) containing 4 ppm either ClO2 or at 90 °C showed that the stabilizers were rapidly consumed towards the inner pipe wall and the rate of consumption in ClO2 was 4 times greater than in Cl2 solution. The subsequent polymer degradation was an immediate surface reaction. It was confirmed by differential scanning calorimetry, infrared spectroscopy and size exclusion chromatography that, in the surface layer which came into contact with the oxidizing medium, the amorphous component of the polymer was heavily oxidized leaving a highly crystalline powder with many carboxylic acid chain ends in extended and once-folded chains.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , 61 p.
Series
Trita-CHE-Report, ISSN 1654-1081
Keyword [en]
Antioxidants, Polyethylene pipes, Chlorine dioxide, Degradation
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-116562ISBN: 978-91-7501-533-0 (print)OAI: oai:DiVA.org:kth-116562DiVA: diva2:590038
Public defence
2013-02-01, D3, Lindstedtsvägen 5, KTH, Stockholm, 14:02 (English)
Opponent
Supervisors
Funder
Formas
Note

QC 20130122

Available from: 2013-01-22 Created: 2013-01-21 Last updated: 2013-01-22Bibliographically approved
List of papers
1. A new method for assessing the efficiency of stabilizers in polyolefins exposed to chlorinated water media
Open this publication in new window or tab >>A new method for assessing the efficiency of stabilizers in polyolefins exposed to chlorinated water media
2009 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 28, no 6, 661-667 p.Article in journal (Refereed) Published
Abstract [en]

The chlorine used as disinfectant in tap water degrades most materials, including polyethylene. The most adequate (functional) test method, the pressure test, is complicated and expensive because the chlorinated aqueous media (Cl-2 or ClO2 in water) are unstable and they undergo reactions that are dependent on the pH. A new method which assesses the protection efficiency of phenolic antioxidants in polyolefins was developed. The method uses a liquid hydrocarbon analogue, squalane, in which antioxidants are dissolved. The organic phase was dispersed in the aqueous chlorinated phase (containing 10 ppm of either Cl-2 or ClO2; pH=6.8) at 70 degrees C by intense stirring. The depletion of antioxidant (Irganox 1010) was monitored by standard DSC determination of the oxidation induction time. It was shown that 300 min of exposure was sufficient to obtain useful data.

Keyword
Polyolefins, Antioxidants, Chlorinated water media, polyethylene pipes
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-18713 (URN)10.1016/j.polymertesting.2009.05.010 (DOI)000269295200014 ()2-s2.0-67650087904 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Deterioration of polyethylene pipes exposed to water containing chlorine dioxide
Open this publication in new window or tab >>Deterioration of polyethylene pipes exposed to water containing chlorine dioxide
Show others...
2011 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 96, no 5, 790-797 p.Article in journal (Refereed) Published
Abstract [en]

Chlorine species used as disinfectants in tap water have a deteriorating effect on many materials including polyethylene. There are only very few scientific reports on the effect on polyethylene pipes of water containing chlorine dioxide. Medium-density polyethylene pipes stabilized with hindered phenol and phosphite antioxidants were pressure tested with water containing 4 ppm chlorine dioxide at 90 degrees C and pH = 6.8 as internal medium. The stabilizers were rapidly consumed towards the inner pipe wall; the rate of consumption was four times greater than in chlorinated water (4 ppm, pH = 6.8) at the same temperature. The depletion of stabilizer occurred far into the pipe wall. A supplementary study on a polymer analogue (squalane) containing the same stabilizer package showed that the consumption of the phenolic antioxidant was 2.5 times faster when exposed water containing chlorine dioxide than on exposure to chlorinated water. The subsequent polymer degradation was an immediate surface reaction. It was confirmed by differential scanning calorimetry, infrared spectroscopy and size exclusion chromatography that in the surface layer which came into contact with the oxidising medium, the amorphous component of the polymer was heavily oxidized leaving a highly crystalline powder with many carboxylic acid chain ends in extended and once-folded chains. Scanning electron microscopy showed that propagation of cracks through the pipe wall was assisted by polymer degradation.

Keyword
polyethylene pipes, chlorine dioxide, degradation
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-31465 (URN)10.1016/j.polymdegradstab.2011.02.009 (DOI)000290078200009 ()2-s2.0-79953673200 (Scopus ID)
Note

QC 20110316 Uppdaterad från submitted till published (20110523).

Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2017-12-11Bibliographically approved
3. Antioxidant consumption in squalane and polyethylene exposed to chlorinated aqueous media
Open this publication in new window or tab >>Antioxidant consumption in squalane and polyethylene exposed to chlorinated aqueous media
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2012 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 97, no 11, 2370-2377 p.Article in journal (Refereed) Published
Abstract [en]

Squalane stabilized with 0.2 wt.% of Irganox 1010 and a medium-density polyethylene containing 0.1 wt.% of the same antioxidant were exposed to two different aqueous media (water solutions containing either 10 ppm Cl-2 or 10 ppm ClO2, both buffered to pH = 6.8) at different temperatures between 30 and 70 degrees C. The squalane phase was characterized by differential scanning calorimetry (oxidation induction time, OIT) and infrared spectroscopy, and the aqueous media were analysed after concentrating the analytes using liquid-liquid extraction by liquid chromatography, mass spectrometry and infrared spectroscopy. OIT measurements were carried out on the polyethylene samples after exposure to the chlorinated aqueous media. Exposure of the squalane systems to water containing ClO2 resulted in discolouration by the formation of quinoid structures and a faster depletion of the antioxidant than exposure to water containing Cl-2. The activation energy for the loss of antioxidant activity on exposure to ClO2-water was very low (<10 kJ mol(-1)) in the squalane test (no diffusion control) and 21 +/- 2 kJ mol(-1) at a depth of 1-2 mm from the surface of polyethylene plaques (diffusion control). Calculation from earlier published OIT data from a HDPE exposed to Cl-2-water yielded an activation energy for the loss antioxidant activity of 68 kJ mol(-1). The antioxidant degradation products obtained from the exposure to the ClO2 aqueous medium were found at a higher concentration, were more polar and exhibited a higher proportion of low molar mass species than those obtained after exposure to the Cl-2 aqueous medium. The important chemical difference between ClO2 and Cl-2 is that the former is a one-electron oxidant whereas the latter preferentially reacts by hydrogen substitution. Possible further reactions, in agreement with the observations made, are proposed.

Keyword
Polyethylene, Squalane, Irganox 1010, Chlorinated aqueous media, Degradation
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-107088 (URN)10.1016/j.polymdegradstab.2012.07.038 (DOI)000310655000034 ()2-s2.0-84869220170 (Scopus ID)
Funder
Formas, 245-2007-473
Note

QC 20121210

Available from: 2012-12-10 Created: 2012-12-06 Last updated: 2017-12-07Bibliographically approved
4. Assessing the Long-­term Performance of Polyethylene Stabilised WithPhenolic Antioxidants Exposed to Water Containing Chlorine Dioxide
Open this publication in new window or tab >>Assessing the Long-­term Performance of Polyethylene Stabilised WithPhenolic Antioxidants Exposed to Water Containing Chlorine Dioxide
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2013 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 32, no 2, 359-365 p.Article in journal (Refereed) Published
Abstract [en]

The addition of chlorine dioxide disinfectant to tap water prevents the spread of infection but has a serious drawback in that it degrades materials used in piping, including pipes made of polyethylene. Efficient methods are required to assess the long-term performance of different combinations of antioxidants and polyethylene grades. We have previously presented a screening method which exposes solutions of phenolic antioxidants in squalane (a liquid, low molar mass analogue of polyethylene) to 70 °C water containing either chlorine dioxide or chlorine. This method assesses the stability of the antioxidants towards these aqueous chlorinated media by determining the oxidation induction time through differential scanning calorimetry. The same experimental set-up with two modifications was used in developing a new method. A 0.3 mm thick polyethylene tape replaced the squalane phase and the supply of fresh water containing chlorine dioxide (10 ppm at pH = 6.8) was continuous; this required minimum attention from the operator over the longer exposure time periods used. Tapes of medium-density polyethylene containing 0.1 wt.% of six different phenolic antioxidants were studied. A linear relationship was established between the times to reach antioxidant depletion in the polyethylene tape samples and the times in the squalane samples (with the same antioxidants at the same concentration). A linear relationship was also found between the initial antioxidant consumption rates in polyethylene and squalane. Infrared spectroscopy and scanning electron microscopy of drawn samples revealed the onset of surface oxidation and surface embrittlement in tape samples exposed beyond the time for antioxidant depletion.

Keyword
Polyethylene, Phenolic antioxidants, Chlorine dioxide
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-104734 (URN)10.1016/j.polymertesting.2012.12.003 (DOI)000316513300026 ()2-s2.0-84872131697 (Scopus ID)
Funder
Formas, 245-2007-473
Note

Updated from submitted to published. QC 20130122

Available from: 2012-11-09 Created: 2012-11-09 Last updated: 2017-12-07Bibliographically approved

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