Polymer Degradation and Stability

Available online 12 October 2024, 111031
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Polymer Degradation and Stability

The Effect of Carbon Black on Degradation of Pipe Grade Black Polyethylene in High Concentration Chlorine Dioxide Solutions

https://doi.org/10.1016/j.polymdegradstab.2024.111031Get rights and content

Highlights

  • •
    Aging in a high concentration ClO2 solution is a streamlined method to assess chemical degradation.
  • •
    Pipe grade polyethylene including carbon black shows inferior resistance to chemical oxidation.
  • •
    Carbon black likely acts as an inhibitor that prevents tie molecule formation.
  • •
    A dual layer black and white pipe may offer increased resistance to chemo-mechanical degradation.

Abstract

In this research, the impact of carbon black on the accelerated degradation of pipe grade polyethylene (PE100) exposed to high levels of chlorine dioxide (150 ppm) is examined. Tensile testing reveals a faster degradation rate in black samples compared to neat samples, indicating a detrimental effect of carbon black aggregates on the polymer's aging process. Rheological analysis shows changes in molecular weight and structure due to chemical degradation and chain scission and can be a reliable method for detecting slight changes in molecular structure. Isothermal crystallization shows a slowdown in crystallization kinetics at first, explained by gel-formation due to crosslinking which hinder the crystallization, and then an increase in the kinetics as apparently the chain scission gets dominant again. Neat samples exhibit a higher density of tie molecules, indirectly revealed by much more fibrils in crack wall observed in FESEM images, suggesting better resistance to chemical degradation while the black sample shows a much less fibrillar crack wake and becomes almost completely devoid of any fibrils at later stages of aging. The fibrils, which essentially offer a load-bearing role against the widening and growth of the crack play a key role in resistance to slow crack growth (SCG). Therefore, the higher SCG resistance is expected for neat grade compared to black samples. The study proposes a dual-layer pipe design with a UV-resistant black outer layer and an oxidation-resistant neat inner layer to prolong the lifespan of PE100 pipes by protecting against UV radiation and chemical reactions. This solution offers increased durability, lower maintenance costs, and environmental sustainability benefits.

Introduction

Over the past decades, plastic pipes have become a popular option for water transportation purposes, either for delivering potable water from treatment plants to urban households or for wastewater transportation and sewage systems. Polyethylene (PE) pipes, especially the pipe grade bimodal PE100 material, has become one of the most popular materials of choice due to ease of handling and manufacturing, excellent mechanical performance, and resistance to chemical elements, resulting in an expected theoretical lifetime of multiple decades [1]. However, it has been reported and widely accepted that addition of oxidating disinfectant agents like free chlorine or chlorine dioxide to water can lead to significant degradation and reduction of lifetime and properties of the PE pipes [[2], [3], [4], [5], [6], [7]]. This degradation occurs due to consumption of antioxidant by the aggressive environment and eventually leads to the failure of the pipe material [[8], [9], [10], [11], [12]].
It has been shown that depending on the condition of the applied stress, the polymer pipes can undergo three different types of failure [13]. At higher stress levels, close to or above the polymer yield strength, the main mechanism for failure is ductile deformation. At lower stress levels, brittle failure due to slow crack growth (SCG) is dominant. The brittle failure itself consists of three stages: crack initiation, slow crack growth and fracture. Lastly, in the presence of aggressive chemicals like chlorine, stress corrosion cracking (SCC) becomes the dominant failure mechanism at even lower stress levels, involving a combined chemo-mechanical degradation process [14]. The figure corresponding to these three distinct failure modes is provided in a supplementary file for this publication.
The degradation steps of the polyolefins exposed to a chlorinated environment include the anti-oxidant depletion, degradation of the surface layer, reduction in molecular weight due to chain scission, micro-crack and crack formation and eventually crack propagation until the final failure stage [9,15]. This poses a serious performance risk for the pipes as detecting an impending pipe failure is not possible, thus it is very important to study the mechanisms involved in this degradation process and come up with new ways to overcome this issue.
Black PE pipes are produced by compounding carbon black (CB) with the base PE, in which the carbon black acts as one of the most effective and robust stabilizers against UV radiation and weathering of PE pipes. Several studies, including authors’ previous works, have investigated various properties of black PE compounds, including morphological and thermal properties [16], rheological and mechanical properties [17], long term creep performance [[18], [19], [20]] and degradation in presence of chlorine dioxide [9]. However, a comprehensive and direct study comparing the chemical degradation of pipe grade PE samples with and without carbon black in an accelerated chlorine dioxide media has not been conducted to the best knowledge of authors. The precise function of carbon black in the degradation process and the potential mechanisms by which carbon black particles might participate have largely eluded researchers in the field of studying chemical oxidation mechanisms in polyolefins, including polyethylene.
The studies by Gholami et al. [18,19] demonstrated that carbon black does indeed decrease the resistance to SCG in PE100 pipes, where it was suggested that the carbon black particles in amorphous areas can act as physical inhibitors for long chains that prevent their entrance from one crystalline region to another to form tie molecules. This leads to a reduction in tie molecule density, a crucial factor in polymer's resistance to slow crack growth, as confirmed by various studies [[21], [22], [23], [24]]. The challenge being posed by adding carbon black as a UV-absorbent additive is striking a balance between UV-stability of the sample and the long-term mechanical performance of the material. While both parameters are equally important, a further understanding of the precise role that carbon black plays in chemical oxidation can help in development of polyethylene materials suitable for use in chlorinated water transportation and distribution.
There have been different studies on changes in the molecular weight distribution in bimodal polyethylene material and how it affects the morphology and properties of the polyethylene [[25], [26], [27]]. Research indicates that when polyethylene is exposed to chlorine oxidizers, it undergoes a complex process involving both crosslinking and chain scission events. As the material ages and degrades under continued exposure, the balance shifts towards chain scission, which becomes the predominant phenomenon over crosslinking. [[28], [29], [30], [31]], since polyethylene has been shown to have a higher tendency for chain scission than crosslinking [32]. On the other hand, less studies have focused on how an unwanted, oxidation-induced change in the molecular weight by chain scission/crosslinking affects the rheological properties and morphology simultaneously.
It is reported that beyond a certain chlorine concentration, the presence (or lack thereof) of stabilizers does not make a difference anymore since the chlorine concentration as the oxidative species completely dominates the oxidation resistance role of the stabilizers [29]. Therefore, in presence of a very high concentration of chlorine species, it is expected that stabilizers are completely overwhelmed by the highly oxidative media and therefore their role as an initial inhibitor can be neglected.
Given that the concentration of chlorine dioxide used in drinking water supply is typically very low (less than 1 mg/L [33]), conducting tests in real-life situations to evaluate pipe grade materials may require months or even years. Therefore, it is crucial to develop accelerated aging tests to study chemical degradation mechanisms within realistic time frames. In this study, black and neat PE100 samples (with exact same properties and molecular characteristics except for the presence of the carbon black particles in black sample) are compared to each other with respect to their aging process and mechanism in a very oxidative environment containing a high percentage of chlorine dioxide. This approach enables us to delve into the impact of carbon black aggregates on the degradation mechanism, paving the way for a deeper exploration of chlorine dioxide-induced effects and the role played by additives like carbon black in the chemical oxidation process.

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Section snippets

Experimental Section

Neat (N) and Black (B) stabilized bimodal pipe grade PE100 (HM-CRP100N and HM-CRP100 Black respectively) produced by Jam Petrochemical Company (Iran) using Ziegler-Natta catalysts under the Hostalen license of Lyondell-Basell and 1-Butene as comonomer were used in this study. The black grade uses the same neat PE100 resin in addition to 2.25 wt% p-type carbon black. Both grades use an identical package of antioxidants and processing agents, and this makes it possible to solely focus on the role

Tensile properties

Figure 1 shows representative specimens following tensile tests, highlighting both non-aged and 144-hour aged neat and black samples. This visual comparison illustrates the maximum strain and the occurrence of fully brittle failure under extreme aging conditions. The complete stress-strain graphs of the neat and black samples are presented in Figure 2(a) and Figure 2(b), respectively, for aging times of up to 144h, after which all samples show almost the same mechanical properties. As it

Conclusion

In this study, black and neat PE100 with same base polymer were exposed to a high concentration chlorine dioxide media to achieve an accelerated aging and assess the effect of carbon black aggregates on the oxidation process and properties of the black compounds. The various characterization tests revealed the following key findings:
Pipe grade PE100 containing carbon black demonstrates a more accelerated degradation compared to neat sample as shown in tensile tests, which indicates a

CRediT authorship contribution statement

Amirhosein Sarafpour: Writing – original draft, Visualization, Methodology, Investigation. Gholamreza Pircheraghi: Writing – review & editing, Supervision, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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