In situ friction and wear behavior of rubber materials incorporating various fillers and/or a plasticizer in high-pressure hydrogen

Highlights

• Model rubbers are tribologically studied in situ under 27.6 MPa H₂.

• Friction increases for all model rubbers in the 27.6 MPa H₂ environment.

• Plasticizer results in reduced wear in high pressure H₂ due to lubricating effects.

• H₂ affects plasticizer-rubber interactions in NBR to allow phase separation.

• Wear performance depends in part on filler dispersion/distribution in the material.

Abstract

Polymers are used routinely for equipment and infrastructure in hydrogen vehicle refueling stations, but significant gaps remain in understanding their hydrogen compatibility. The tribological properties of these materials in a high-pressure hydrogen environment is important in preventing component failure and the need for frequent replacement. We present in situ tribological studies on model rubbers, which include common fillers and plasticizer, using an in situ tribometer developed previously. Results suggest a clear, yet complicated, combined effect of the high-pressure hydrogen and the additives on the tribological performance of the chosen materials, as compared to matching experiments performed in ambient air. We find that the additives improved wear resistance in EPDM but deteriorated that in NBR due to disparate additive-polymer interactions.

Introduction

The conversion of stationary power sources and mobile applications from fossil fuels to cleaner, more renewable hydrogen continues [[1], [2], [3], [4], [5]]. However, the broader application of hydrogen on a global scale is impeded by the potential for materials exposed to high-pressure hydrogen for long periods of time to degrade [6]. The United States Department of Energy's Hydrogen and Fuel Cell Technologies Office has launched the H2@Scale program that is supporting work in hydrogen compatibility of materials to improve the durability and reliability of materials for the hydrogen infrastructure. To improve reliability of hydrogen energy, including infrastructure delivery and distribution systems, fueling stations, and automotive fueling systems, the effects of high-pressure hydrogen on their materials of construction must be better understood. For example, some steels and titanium degrade in high-pressure hydrogen environments by forming hydrides, leading to embrittlement and subsequently structural damage [[6], [7], [8], [9]]. Unlike metals, polymers are thought not to react chemically with hydrogen, leading many to conclude that mechanical failure is the primary damage mechanism when polymers are not challenged by other factors such as temperature [6,[10], [11], [12]]. It is widely accepted in the hydrogen energy community that most damage to polymers in hydrogen applications occurs during sudden decompression of high-pressure hydrogen. This type of damage is commonly referred to as rapid or explosive decompression failure (RDF or XDF) [[13], [14], [15]] and has been investigated in several studies driven by growing interest in high-pressure hydrogen applications. XDF is induced by sudden expansion of the hydrogen absorbed within the polymer due to rapid removal of external pressure, which causes bubbles, internal cracking, surface blistering, or even catastrophic failure by exfoliation. Since commercial polymer products are typically not pure materials, the failure mechanism can be influenced by additives (e.g., filler, plasticizer), conditions (e.g., pressure, gas species), microscale morphology (e.g., additive-polymer interaction) and others. Therefore, further understanding of polymer degradation modes is still in need. The hydrogen compatibility program (H-Mat), led by Pacific Northwest National Laboratory (PNNL) and Sandia National Laboratories (SNL) under the H2@Scale program, seeks to address the challenges of hydrogen degradation by elucidating the mechanisms of hydrogen-materials interactions with the goal of providing science-based strategies to design materials (micro)structures and morphology with improved resistance to hydrogen degradation.

Given the gaps in knowledge and data for polymers suitable for hydrogen service, a growing quantity of work has been devoted to such areas as tribology, failure modes, mechanical property variation, and polymer permeability/solubility. Among these, the interest in hydrogen's effects on tribological friction and wear have surged in recent years, as polymers are typically used in dry, sliding components, where frictional wear plays a central role in determining reliability and durability for intended hydrogen applications [[16], [17], [18]]. Tribological factors that affect characteristics of the friction and wear behavior of polymers are of particular interest. Such factors include but are not limited to surface roughness, mechanisms of adhesion, real contact area, normal load, sliding velocity, temperature, humidity, and physical and chemical interactions between the two surfaces in contact [[19], [20], [21], [22]]. Considering the effects of viscoelasticity, a larger variation in friction and wear behavior is expected for most polymers than for metals. Theiler et al. [16,23] described tribological experiments carried out at room temperature in a low-pressure hydrogen environment on a variety of polymer composites. The authors concluded that the apparent coefficient of friction (CoF) decreased to varying extents in hydrogen, and the fillers of choice had a greater influence on the tribological performance of the composites than the polymer matrix had. Sawae et al. [24] investigated high-pressure (40 MPa) hydrogen effects on the wear behavior of unfilled polytetrafluoroethylene (PTFE, a common sealing compound) and 15% graphite-filled PTFE using 316 L austenitic steel as the sliding counterface in a pin-on-disk type tribometer. The results revealed that the hydrogen gas environment enhanced formation of the polymer transfer film, thereby reducing the specific wear rate despite the fillers, but the 15% graphite-filled PTFE had a lower specific wear rate than the unfilled PTFE when exposed to high-pressure hydrogen. Later, Sawae et al. [25] compared graphite-filled PTFE and bronze-filled PTFE friction and wear performance in a 40 MPa hydrogen environment and concluded that the effects of high-pressure hydrogen were relatively small for graphite-filled PTFE. Unlike the pin-on-disk tribometer, a custom-built linear reciprocating tribometer was developed by Duranty et al. [18], wherein a steel pin is rubbed against the surface of a polymer disk in high-pressure hydrogen (26.2 MPa). A commercial elastomer used for sealing purposes, nitrile butadiene rubber (NBR), filled with magnesium oxide (MgO) was studied using the custom-made tribometer, and results indicated that friction between the pin and the NBR was higher during testing in the high-pressure hydrogen environment than in high-pressure argon or ambient air. They attributed this result to three main effects: (1) pseudo-plasticization, in which hydrogen gas can decrease the mechanical strength of the material, making damage more likely to occur, (2) voids created between the filler and the polymer matrix, and (3) hydriding of the magnesium-based fillers within the polymer, which can trigger a change in surface roughness and morphology. However, there have been only few studies in this field and there exists a need for a comprehensive work where more than one polymer will be looked at and the effects of additives on tribological performance of polymers are explored fundamentally in high-pressure hydrogen.

Particulate fillers are often added to polymers for reinforcement, cost effectiveness, or processability [[26], [27], [28]]. Especially in the rubber industry, either carbon black, silica, or a combination thereof has been commercially used to modify the physical properties of rubber compounds to help meet certain requirements for intended uses [28,29]. There are many functional groups such as hydroxyl, carboxyl, lactone, pyrone, ketone, quinone, and phenol on the carbon black surface, but the amount is limited [30]. Silica has abundant hydroxyl groups on the surface, which contribute to strong filler-filler interactions and adsorption of polar materials by hydrogen bonds [31]. Research has shown that bubbles and voids form at the interface between fillers and rubber polymer during exposure to high-pressure hydrogen gas, deteriorating the filler-polymer interaction at the interface, leading to blister fracture of the bulk [6,[32], [33], [34]]. However, the filler effects on tribological performance of polymers in high-pressure hydrogen environments have not been well established. Furthermore, commercial rubber materials often include a variety of plasticizers, such as dioctylphthalate, dioctyl sebecate, and other oil type plasticizers, to maintain or improve rubbery characteristics of the compounds [35,36]. A few studies revealed that large additions of plasticizer bring about a change in CH₄ and CO₂ diffusivity within a polymer matrix as a result of an increase in the mobility of polymer chains [37,38]. Unfortunately, these studies only provide us limited insights, though useful guidelines for subsequent studies, into polymer compatibility with hydrogen gas. Worthwhile noting, outside the rubber industry, carbon nanotubes (CNTs) were reportedly utilized in carbon fibric-reinforced phenolic composites for improving wear resistance due to the improved interfacial bonding of the composites by the incorporation of CNTs [39]. Also, MoS₂ particles were used as additives and friction modifier to improve the wear and friction behavior of carbon fabric reinforced phenolic composites [40]. Kurdi et al. [41] studied the tribological properties of three different high-performance polymers reinforced with different TiO₂ content and compared at different temperatures, suggesting that TiO2 particles reduce the adhesion between polymer and transfer film layers. Yin et al. [42] found that introducing fluorine rubber micro powder into boron phenolic resin-based composites can reduce wear rate by 80.9% owing to the reduction of fatigue wear and the formation of large-area friction film.

To address these gaps, we sought to understand the friction and wear behavior of representative rubber compounds under 27.6 MPa of hydrogen, using a custom-built in situ tribometer previously described by Duranty et al. [18]. In situ measurements were carried out on model material compounds of ethylene propylene diene (EPDM) and NBR—elastomers often used in dynamic sliding components or seals, mostly for hydrogen transportation, delivery and infrastructure—that were loaded with various fillers and/or plasticizers in the present work. In situ measurements are useful because the finite diffusion rates of hydrogen through the polymers introduce time dependence into the polymer degradation upon changing pressure. The first goal of the work described below is to characterize the friction and wear of target elastomers quantitatively. The CoF and specific wear rate are calculated from the frictional force and penetration depth recorded in situ. The second goal is to understand the role of fillers and plasticizers in influencing the tribological performance of EPDM and NBR.