Biodegradation of PEEK Piston Rings

Highlights

• PEEK based piston rings of an industrial reciprocating compressor suffered biodegradation and caused catastrophic failure of the compressor.

• The pH test, Xanthoproteic test and light microscopy indicated the presence and the bacterial metabolic activities on the degraded PEEK surface.

• Presence and colonization of rod-shaped bacteria that is identified as Myxococcus Xanthus were observed using a SEM.

• Bacterial cultivation and in-vitro experimental simulation revealed an early stage of bacterial degradation in the form of uniform cracking of PEEK specimen.

• The biodegradation of PEEK is confirmed and is theorized as the enzymatic hydrolysis process due to the metabolism of urease producing ureolytic bacteria.

Abstract

Polyether ether ketone (PEEK) is considered a high performance thermoplastic with excellent mechanical, chemical, and thermal properties. It is generally believed that this material is chemically and biologically inert, and is used for various biomedical and industrial applications, especially in the form of fiber reinforced polymeric composites. Contrary to the general belief, the present work reports the biodegradation of PEEK piston rings that were used in a reciprocating CO₂ compressor. A series of circumstantial and direct evidences were collected by following various tests and characterization methods to confirm the degradation of the piston rings by bacterial attack. The bacterial cells were extracted from the degraded piston rings, cultured in agar medium and then studied using scanning electron microscope. An experimental simulation was carried out by depositing and incubating a bacterial culture on the pristine surface of a PEEK specimen. The simulation experiment revealed an early stage of bacterial degradation in the form of cracking of the PEEK specimen surface. The results of various tests, characterization, and the experimental simulation presented in the paper suggest that PEEK based composites degrade due to enzymatic hydrolysis process by Myxococcus Xanthus, the rod–shaped soil bacteria.

Introduction

Polyether ether ketone (PEEK) is considered a high performance thermoplastic polymer. Pure PEEK melts at about 343°C and exhibits high chemical resistance even at elevated temperatures [1].By blending Polytetrafluoroethylene (PTFE), PEEK can be made self-lubricating [2] By using proper additives and reinforcement, mechanical properties of PEEK and PEEK/PTFE blends can be significantly enhanced. Due to this unique combination of properties, PEEK based polymeric composites are used for making piston rings, rider bands, gaskets, and other industrial sealing components [3]. In particular, PEEK piston rings are preferred to metallic rings in reciprocating compressors to overcome undesirable effects of frictional wear. PEEK is considered biologically inert and is used as a material of choice for many biomedical applications [4]. Due to the excellent combination of properties, there is an increasing demand to use PEEK based composites for various applications in automotive, aerospace, food processing/packaging and electrical/electronic industries [5].

Pure PEEK is a colorless, aromatic, and semi-crystalline thermoplastic that is synthesized via step-growth polymerization by the di-alkylation of bis-phenolate salts [6]. The chemical structure of PEEK is presented in Fig. 1.

Three phenyl rings in PEEK contribute to thermal stability and stiffness, while two ether segments in the chain impart flexibility. The characteristic features of PEEK include high strength, good toughness, high temperature performance, excellent wear resistance, superior chemical resistance, and hydrolytic stability. The physical properties of PEEK are listed in Table 1. PEEK is conventionally processed by extrusion, injection molding and compression molding. PEEK wires can be used in fused deposit modeling [7]. Though PEEK material exhibits excellent chemical and thermal stability, pure PEEK lacks strength, rigidity and resistance to UV light. Additionally, PEEK is an expensive material. Hence, pure PEEK is combined/reinforced with carbon-black/graphite, carbon fiber, glass fiber, and particulates of silica/alumina and other additives to increase the strength and load bearing capacity [2].

Chemical compounds of barium, calcium, and zinc can also be used as anti-fouling additives to increase the service life of industrial PEEK components. The solid lubricant, Molybdenum (MO₂), is also used as filler for improving the wear resistance of PEEK components. Alternatively, pure PEEK is blended with Polytetrafluoroethylene (PTFE) to form PTFE/PEEK blend, as they impart best combination of mechanical and tribological properties, especially for the adhesive wear mode.

Processing of PEEK composites involve, blending of PEEK powder and nano-fillers that are co-dispersed in a suitable solvent, kneading the blend in a twin-screw melt mixer, cold pressed and consolidated by sintering [10]. PEEK based composite materials are extensively used in demanding applications such as aerospace, automotive, electrical, medical and industrial sealing. In addition to the excellent chemical, mechanical, thermal, and self-lubricating properties, pure PEEK is considered bio-inert in nature due to its hydrophobic property; it prevents microbial adhesion and hence inhibits biodegradation.

Fiber-reinforced polymeric composites are known to be susceptible to degradation by microorganisms [11]. In such composites, micro-organisms utilize one or more chemical additives as growth substrates. For example, epoxy/graphite fibers are susceptible to attack by microorganisms. Similar biodegradation process was also observed in polymers such as Polyethylene terephthalate, Polyimide and Epoxy polymers. The hydrophobic nature of Polyethylene terephthalate poses a significant barrier to microbial colonization of the polymer surface, yet undergoes degradation due to hydrolytic enzymes secreted by the microbes [12]. Fungi penetrate into the interior of fluorinated polyimide composites that are reinforced with glass fibers. Epoxy/graphite fibers are susceptible to colonization by fungi, as the fibers embedded in the epoxy resins are susceptible to the growth of microorganisms and formation of microbial biofilms [13].

Owing to their excellent chemical, mechanical and thermal properties, PEEK based composites are considered as suitable replacements for metallic piston rings, O-rings, rider-bands, etc. In modern compressors and pressure vessels, PEEK based composites are widely accepted as the suitable materials of construction for sealing components. However, it should be noted that despite their proven thermal and chemical stability, PEEK based composites may fail during service due to various reasons. It is reported that failure of PEEK based piston rings occurs when used at elevated temperatures, in the presence of moisture. The degradation of PEEK proceeds through an initial ingress of water into the resins, resulting in the de-bonding of fibers from the resins. Similarly, the PEEK components degrade and fragment when the absorbed water molecules encounter foreign particles, impurities, and micro-cracks [11]. The differential pressure or non-uniformity in pressure distribution has been reported as another possible cause for premature failure of piston rings in reciprocating compressor [14].

Though not reported for PEEK materials in particular, polymeric piston rings in general are susceptible to microbial degradation. This is a relatively slow degradation process that takes place in natural environment. During the microbial degradation process, polymeric material undergoes de-polymerization. It is believed that exo-enzymes released by the microorganisms break-down complex polymer chains into short chains or smaller molecular fragments [15]. Natural polymers degrade faster than the synthetic polymers. Polymers made from starch or flax fibers shows greater biodegradability as compared to their synthetic counterparts. It is also reported that fungi can penetrate into composites composed of fluorinated polyimides resins reinforced with glass fibers [13]Synthetic addition polymers such as polyacetal and polyester with hetero-atoms in their backbones may biodegrade. However, the biodegradability of synthetic condensation polymers depends to a greater extent on molecular weight. Biodegradation rate will be higher for polymers having low molecular weight and less crystallinity. Low molecular weight compounds can easily be broken by microorganisms and hence the rate of biodegradation is high. Similarly, amorphous polymers degrade faster than crystalline polymers. The additives to the polymeric base are an easy source of carbon and energy for microbial growth [16] When depleted of simple molecules and nutrients in the vicinity, the microbes may start feeding on the extracellular polymeric substance.

Bacteria and fungi are the common microbes responsible for microbial degradation of polymers. Various types of bacteria participate in the degradation process. Myxococcus Xanthus type rod shaped soil-dwelling bacteria are known to cause degradation of hydrocarbons. M. Xanthus is an abundant, gram-negative, non-pathogenic, and heterotrophic soil dwelling bacterium that is omnivorous in nature. M. Xanthus can kill, lyse and grow on other bacteria for nutrient absorption. M. Xanthus bacteria have a multicellular social lifestyle including social gliding, fruiting body formation and predation. M. Xanthus is known to be producing secondary metabolites such as antibiotics and hydrolytic enzymes to kill and lyse prey cells. The bacteria produce myxospores that are resistant to heat, UV rays and desiccation. A characteristic feature of these bacteria is the production of enzymes called ureases, release of ammonia (NH₃) and precipitation of calcite (CaCO₃) as a metabolic product commonly termed as vaterite. Several factors that favor the growth of bacteria include: nutrients, high alkaline pH, presence of moisture, and suitable temperature [17]_. Carbohydrates, hydrocarbons, urea, polymeric molecules, fillers, and graphite are the common nutrients available for bacteria to survive in an industrial environment. By using Urease, an enzymatic catalyst, the bacteria hydrolyze urea and release CO₂ and NH₃ through cell respiration. M. Xanthus bacteria produce ammonia as a result of enzymatic hydrolysis. However, the higher solubility of ammonia in moisture increases the pH of the medium by forming ammonium hydroxide. The optimum temperature for the catalysis of urea by urease ranges from 20 to 37°C. But when the temperature increases to 55°C enzymatic activity decreases by 47% [18]. This suggests that these bacteria can survive and be active at elevated temperatures in excess of 55°C.

A note-worthy work on the bacterial degradation of polythene was reported by Gauri Singh et al., in which 15 different soil dwelling bacterial species were isolated and used as inoculants for degrading pretreated polythene coupons. The report concluded that Staphylococcus, Pseudomonas and Bacillus species positively degrade both high density and low density polythenes and that the rod-shaped Bacillus group is most effective in degrading polythene [19]. Microbial deterioration and degradation of various polymers and polymeric composites predominantly due to enzymatic actions of different species of fungi and bacteria is reviewed and well documented [20], [21], [22]. However, biodegradation of PEEK based material has not been reported so far. It is generally believed that this material is chemically and biologically inert, and is used for various biomedical and industrial applications especially in the form of fiber reinforced polymeric composites. Contrary to the general belief, the present work reports the biodegradation of PEEK piston rings that were used in a reciprocating CO₂ compressor.

A catastrophic failure of a heavy-duty double–action reciprocating CO₂ compressor in a fertilizer plant has been reported by the authors [23]. Localized damage of the piston rings due to in-situ bacterial attack was suggested as the root-cause for the failure (Fig. 2). Considering that the bacterial degradation of PEEK material is neither reported nor recognized elsewhere, there is no peer-reviewed reference material for the comparison and confirmation of this damage mode. The objective of the present work is to confirm whether the damage of the piston rings indeed occurred by the bacterial activity and to propose a possible degradation sequence for the PEEK based materials.