High-performance polymers can compete with metallic systems in harsh processing environments and may offer potential benefits in certain applications
The petroleum-refining and natural-gas industries are constantly evolving, spurred by the imperative within energy markets to be continually on the lookout for ways to improve established processes and develop more efficient new ones. Engineers involved in these industry sectors must be cognizant of the many material options that are available that can maximize efficiency and decrease downtime. Material engineers can help the energy industry by carefully selecting the right material for a particular application. Important factors to consider while choosing a material for a project include chemical resistance, mechanical properties, thermal properties, weight, availability and cost.
Metallic systems are found across the refining and gas-processing industries due to their excellent mechanical and thermal properties. Contrary to a common conception that plastic materials are used in less aggressive applications, there are high-performance polymers that can compete in harsh processing environments. Like metals, certain polymers display more robust mechanical, chemical and thermal properties than their commodity-plastic counterparts, due to the chemical structure of the polymer chain. In general, plastics are more lightweight than metals, which can allow easier repair and installation for engineers. Plastic components, such as pipes, do not rust and do not need to be cleaned or electropolished like metallic ones. Certain high-performance plastics can also display broader chemical inertness than metals, especially in acidic, low-pH process media.
Polymeric materials are being considered in applications where they can provide longer-lasting solutions than metallic materials. Hydrocarbon-processing industries value materials that exhibit performance at high temperatures, resistance to hydrocarbons, low permeation rates for various gases and good creep resistance. These industry sectors already utilize high-performance polymers in a variety of applications — such as protective jacketing for umbilical or downhole wire and cable, multilayer piping, plastic-lined steel and reinforced thermoplastic pipe (RTP) and composite tapes.
This article will highlight three high-performance polymers that are highly valued in the energy market today and discuss why their properties can offer benefits. A short summary of the materials to be mentioned and their general properties is listed in Table 1.
PVDF
Polyvinylidene fluoride (PVDF) is a high-performance polymer commonly used in petroleum- and gas-processing. PVDF is a fluoropolymer material, which by definition means it contains repeat units of fluorine. Fluoropolymers generally exhibit excellent chemical resistance, weatherability and performance at high temperatures. Compared to other fluoropolymers, PVDF is highly crystalline, which adds good mechanical properties, abrasion resistance and rigidity. PVDF homopolymers are recommended in systems for the petroleum and gas industries up to 130°C continuously. PVDF systems can be used to replace expensive metallic alloys, like Hastelloy or Inconel, and show exceptional resistance to chlorinated and brominated chemicals, hot acids, alcohols and hydrocarbons. Freestanding pipes, lined-metal pipes, valves, pumps, gaskets and O-rings made with PVDF are utilized across the broader chemical process industries, because the material can be processed easily on standard extruding or injection-molding equipment. PVDF is also compliant with American Petroleum Institute Standard API-17J for flexible pipeline applications.
For applications that require higher flexibility, PVDF copolymers are a common solution. PVDF copolymers contain an additional monomer, hexafluoropropylene (HFP). This comonomer creates a more flexible, impact-resistant material that increases chemical inertness. This process also eliminates the need for plasticizers that can leach out over time, extending the material’s lifespan. Production of heterogeneous PVDF copolymers, in which the rate of HFP reaction is controlled, result in a flexible polymer that can withstand both higher and colder temperatures [1]. The material also conforms to API 17E specifications for subsea umbilical cables.
PVDF resins are well known to be resistant to chemicals prevalent in the oil-and-gas and chemical process industries, including both aliphatic and aromatic hydrocarbons, ethanol, chlorinated, brominated and sulfuric compounds, as well as hot acids. Table 2 demonstrates the material’s retention of tensile strength after a six-month exposure to harsh chemistries commonly found in industrial manufacturing settings. PVDF 740 is a homopolymer grade, PVDF 2850 and 2800 are two copolymer grades. Heptane and toluene were tested at 80°C.
PVDF has a well documented record of permeation resistance to gases common in the petroleum, gas and chemical industries. The material displays good barrier properties to sour environments, particularly those containing H2S and CO2, and outperforms similar fluoropolymers regarding permeation by air, oxygen, nitrogen and helium. Table 3 documents gas permeation of PVDF in environments similar to those typically encountered in the petroleum-refining and natural-gas industries.
PVDF retains much of its tensile properties at elevated temperatures. This is an important property for offshore engineers, as elevated temperatures allow easier transport of fluid in reinforced thermoplastic pipes (RTPs) and flexible pipes. Table 4 documents the mechanical properties of PVDF at high temperatures.
PVDF can be found in a variety of critical applications in the oil-and-gas and refining industries. Offshore flexible flow lines, umbilical cables, choke and kill lines and gas-station piping all incorporate layers of PVDF because of its excellent permeation resistance and good creep resistance. Downhole cables utilize PVDF copolymer jacketing for its flexibility and performance at high temperatures. The material is also used for liners, both steel and fiberglass, to provide higher chemical resistance for transporting fluids like crude oil. In a similar manner, RTPs containing an inner layer of PVDF provide chemical resistance and can be placed on a spool for ease of maintenance and installation.
Polyamides
Long-chain polyamides are a class of materials known for broad chemical resistance, abrasion resistance, ease of processing, performance at high and low temperatures, high dimensional stability and low density. There are many different grades of polyamide, but some, such as PA11, are essential to the oil-and-gas industry due to the aforementioned properties. PA11 is a polymer used in a multitude of different applications, including in automotive, sportswear and electronics, along with oil and gas and chemicals. The material is unique due to its renewable source – castor oil. The 11-aminoundecanoic acid monomer is derived from castor oil, which is in turn extracted from castor beans, a drought-resistant crop that does not contribute to deforestation. This bio-based origin is a key contributor to the material’s success and utilized by many companies looking for more sustainable material solutions.
PA11 resins have very low density (as low as 1.02 g/cm3), which is lower than that of many other high-performance polymers and much lower than metals. Polyamide resins can be up to six times lighter than metals, providing a lightweight solution for the oil-and-gas industry that saves energy and time during maintenance and installation. PA11 has very good resistance to hydrocarbons and alcohols, allowing it to be used throughout the process of collecting, transporting and refining crude petroleum. The material also exhibits good impact resistance and abrasion resistance compared to other engineering polymers. PA11 can also be produced with a wide range of flexibilities due to the inclusion of plasticizers, ranging from a modulus of 1,200 MPa for non-plasticized grades and a modulus of 150 MPa for plasticized grades. The maximum recommended continuous usage temperature for PA11 in the oil-and-gas industry is 90°C in dry conditions and 65°C in wet conditions.
PA11 piping is commonly used for gas gathering systems. This is due to the polymer’s good barrier properties, as well as its resistance to rapid crack propagation. A gas gathering pipe made with PA11 can operate at twice the pressure of a polyethylene (PE) pipe. PA11 can be manufactured into fittings, couplings, valves and risers, providing a complete system, and cathodic protection is not required. PA11 pipe can be joined via butt fusion, on the same equipment used to install polyethylene (PE) pipe.
Table 5 gives the recommended operating pressure for PA11 pipe for gas-gathering systems. Dry gaseous hydrocarbons and wet gas containing aliphatic hydrocarbons (including diesel) have no adverse effect on the expected service life of PA-11 piping system. A design factor (DF) of 0.5 is recommended. For water, the recommended DF is 0.5. In case of high aromatic hydrocarbons (like gasoline), an additional chemical DF of 0.5 should be applied.
The gas permeation resistance of PA11 is highly documented, and it is a material specified by many material engineers designing flexible multilayer pipe. Table 6 documents the permeability of PA11 exposed to various gases common to the oil-and-gas industry.
The oil-and-gas and related process industries utilize PA11 in several applications. It is commonly specified for flexible flowlines and risers due to its resistance to hydrocarbons and good mechanical properties. PA11 has a lifetime prediction model published in API 17 TR 2. The material is also found in anti-wear tables for offshore flexibles, choke and kill lines, downhole cables and liners for these same reasons. The lightweight and flexible nature of PA11 makes it an ideal candidate for RTPs, where these pipes can be delivered in rolls for easier transport and installation. The material is specified for fuel containment piping, natural-gas piping, and offshore umbilical cables for its good barrier properties.
PEKK
Certain materials, such as poly ether ketone ketone (PEKK) are designed for especially demanding applications that require high performance, lightweight materials. PEKK is part of the poly aryl ether ketone (PAEK) family, a class of polymers built from ketone and ether connections. The ratio of ketone to ether in the polymer backbone affects the material’s final properties. PEKK has a ketone-to-ether (K-to-E) ratio of 2, whereas its close relative poly ether ether ketone (PEEK) has a ratio of 0.5. This difference in K-to-E ratio results in PEKK having a higher tensile strength and better barrier properties than PEEK. PEKK resins can also vary in their ratio of terephthaloyl to isophthaloyl units, resulting in different polymer grades that can vary in melting temperature and rate of crystallization.
PEKK resins exhibit excellent properties in many different facets, including high strength and stiffness, high temperature resistance, excellent chemical resistance and barrier properties and good processability. The material has a very wide processing window that allows it to be compatible with a large variety of processing methods, including extrusion, injection molding, compression molding, additive manufacturing (3-D printing) and more. PEKK resins can be used for certain continuous use applications in the oil-and-gas industry up to 250°C.
The barrier properties of PEKK on various gases and fluids has been shown to outperform other high-performance materials. For example, PEKK barriers can outperform PEEK when exposed to CO2 or H2S at elevated temperatures, offering up to 2.5 times as much protection (Figure 1).
FIGURE 1. The graph shows the barrier properties of PEKK and PEEK for carbon dioxide and hydrogen sulfide
The material also displays high tensile creep resistance at elevated temperature, with injection-molded PEKK lasting up to 100 times as long as injection-molded PEEK at the same temperature and stress (Figure 2).
FIGURE 2. The creep properties of PEKK and PEEK polymers at elevated temperatures under constant temperature and stress are shown here
These properties make PEKK suitable for use in the oil-and-gas and related industries, in applications such as O-rings, bearings, seals, valves, compressor parts, electrical connectors, wire coating or other components where high performance is needed at elevated temperatures.
Developments in additive manufacturing (3-D printing) are constantly evolving, and the material’s unique ability to be crystallized after printing allows it to be used in applications unsuitable for similar materials. O-rings for the oil-and-gas industry have been 3D-printed via fused filament fabrication (FFF) that exhibit high crystallinity and low porosity and nearly match the properties of extruded or compression-molded PEKK [2], resulting in another viable option of which engineers can take advantage.
Concluding remarks
Development of high-performance polymers has resulted in materials that can provide lightweight solutions for industrial applications without sacrificing chemical or thermal stability. Solutions exist within many polymer families, including fluoropolymers, polyamides and PAEKs. With a thorough understanding of the engineering materials at their disposal, engineers in the chemicals and energy industries can help maximize efficiency of their processes, creating lightweight and effective solutions to help deliver energy and products.
Edited by Scott Jenkins
Acknowledgement
Figures were provided by Arkema.
References
1. Henry, J., D. Kreh and others. High-Performance Polymers in Wire and Cable, Arkema, 2017.
2. Bergamini, X., P. Bussi and others, Oil & Gas Seals Fabrication via FFF: Closing the Gap with Conventional Manufacturing Methods, Whitepaper, Arkema, 2022.
Author
Jason Knapp is a sales and marketing engineer for Arkema Inc. (155 King of Prussia Road, Radnor, PA 19087; Phone: 610-733-5769; Email: [email protected]). Knapp focuses on fluoropolymers in the oil and gas industry. He graduated from Penn State University with a bachelor of science degree in materials science and technology. Knapp has authored numerous articles on the importance of material selection for chemical processes. Knapp is a member of the American Petroleum Institutte (API) and ASTM International standards committees on thermoplastics and is an American Welding Society (AWS)-certified plastic welder.