Part 2: Material Selection for Chemical Process Equipment – Plastics
Part 2 of the seven-part series on material selection for chemical process equipment focuses on plastics. Plastics include a range of organic polymer structures that can include chlorine or fluorine and other functional groups. Plastics are commonly used in applications for tanks, pumps, piping and valving, ventilation systems, and miscellaneous equipment and components. Various plastics provide good to excellent chemical resistance for caustics, acids and some solvents that can be superior to even higher cost metals. In some applications with very aggressive process chemistries, fluoropolymer plastic heat exchangers are used instead of metal heat exchangers (although plastics require significantly larger heat transfer surfaces compared to metals). Factors including temperature, concentrations and combinations of chemistries, and mechanical stresses/loads including fluid pressures, are important considerations for selecting specific plastics for chemical process equipment applications. Different proprietary and non-proprietary plastic formulations for a specific plastic type can provide significant variations in product purity, structure, temperature application range, and chemical resistance. Plastics are very light weight compared to metals, provide thermal and electrical insulation, and can be simple to form and machine. Plastic material prices can vary by more than an order of magnitude from commodity plastics up to more exotic fluoropolymer and imidized plastics. Individual plastic prices* can vary significantly with market demand and raw material supply. Some plastics used in surface finishing process applications include the following types:
Polyvinyl Chloride (PVC) and Chlorinated Polyvinyl Chloride (CPVC)
PVC and CPVC are relatively low cost and provide good chemical resistance for a range of acids, bases, and salt solutions. PVC is attacked by polar solvents. CPVC (modified PVC with chlorine content increased from ~57% to ~67%-74%) is higher in cost but provides added chemical resistance over PVC in many applications. CPVC is also suitable for higher temperatures up to 180°F to 210°F for some products and applications, compared to 140°F for PVC under favorable pressure and chemical exposure. CPVC also provides superior fire-resistance over PVC. PVC and CPVC are common materials for ventilation hoods and ducting up to application specific temperature and chemistry limits. Metals, such as stainless steel (SS), or fiberglass reinforced plastic (FRP) composites with appropriate materials and reinforcement are needed for some higher temperature applications. The density of PVC is ~50% more than PP and HDPE, and PVC has some strength advantages over other common plastics. HDPE pipe walls must be 2.5 times thicker than PVC to achieve the same pressure rating for room temperature water applications, and the tensile and flexural strength of PVC are significantly higher than PP or HDPE.
Polypropylene (PP)
PP is used as a cost-effective material for tanks, including seamless tanks, and other process system equipment and components in a wide range of surface finishing applications. PP is also preferred over PVC and CPVC for piping and ventilation in many parts of the world. PP provides superior chemical resistance and high temperature resistance in many applications beyond PVC and is superior in chemical resistance over PVDF in some applications with strong bases and with some solvent and mixed chemical applications. PP has superior bending stiffness and tensile strength as compared to HDPE. PP has a Vicat heat distortion (softening) temperature in the range of ~289-305°F, compared to ~ 176°F for rigid PVC. Different PP polymer formulations (e.g., homopolymer vs. copolymer) provide different properties, such as low temperature application range, impact strength, hardness, malleability, melting and softening points, and translucence. PP brittleness increases substantially more than HDPE or PVC or PVDF at temperatures approaching and below freezing.
Polyethylene (PE)
PE is available in a range of densities/molecular weights, providing different physical properties in addition to broad chemical resistance. PE can provide similar temperature and chemical resistance for many applications compared to PP (both are polyolefins). PE has superior abrasion resistance to PP and PVC, with a sand-slurry method seven times higher than PP and over 50% more than steel. Some common PE types include the following:
Low-Density Polyethylene (LDPE) and Medium-Density Polyethylene (MDPE): LDPE is very flexible but relatively low in strength – it is commonly used for plastic sheeting. MDPE is stronger than LDPE and provides some additional chemical resistance.
Cross-Linked Polyethylene (PEX or XLPE): The cross-link polymer bonds in this medium-to-high-density PE improve high temperature properties and enhance chemical resistance for some applications.
High-Density Polyethylene (HDPE): HDPE provides significantly more strength and stiffness and higher temperature resistance than LDPE and MDPE and is easy to fabricate and weld. HDPE is available in sheet and pipe form and is molded into complex shapes.
Ultrahigh Molecular Weight Polyethylene (UHMW): Due to a less-efficient molecular packing structure, the density of UHMW is slightly lower than HDPE. UHMW is less rigid and has ~78% tensile strength and ~55% flexural modulus of HDPE but provides superior impact resistance over HDPE. UHMW has a low coefficient of friction and is self-lubricating, making it excellent for parts exposed to friction and wear.
Polyvinylidene (PVDF)
PVDF is a relatively higher cost fluoropolymer thermoplastic that provides superior chemical resistance in many applications (strong bases are one exception) and has a significantly higher service temperature, ranging up to 250°F to 300°F in favorable conditions, compared to CPVC, PP, and HDPE. PVDF is almost twice as dense as PP. PVDF is available in piping, sheet, tubing, film, and plate form. Kynar® (one common group of PVDF plastics) can be homopolymer or copolymer and may also contain application specific additives (e.g., Red Kynar® has pigment added for ultraviolet (UV) protection). High purity 100% homopolymer PVDF is non-leaching and does not support growth of biological impurities, making it suitable for high and ultrapure applications.
Other Fluoropolymer Plastics
Polytetrafluoroethylene (PTFE) is a fluoropolymer plastic with excellent chemical resistance and high temperature resistance extending well beyond PVDF and other commodity plastics. A common brand name is Teflon. PTFE is not melt-processable. PTFE is primarily used as a high-performance coating and is featured more in Part 4 of this series. Perfluoroalkoxy (PFA) is a melt-processable fluorinated copolymer that provides chemical resistance and temperature performance similar to PTFE but at higher cost. Fluorinated ethylene propylene (FEP) copolymer has a slightly different structure than PTFE and is melt-processable but has reduced chemical resistance and temperature performance. FEP is lower cost than PTFE. Ethylene Tetrafluoroethylene Copolymer (ETFE) provides chemical inertness approaching PTFE and with superior melt-processability and mechanical properties beyond FEP and PFA. In addition to coating applications, various fluoropolymers are used in tubing for heat exchangers and other fluid applications with a combination of aggressive chemistries and elevated temperatures, including seals and gaskets and valve components. PTFE, FEP, and PFA are non-combustible, and PVDF and ETFE are self-extinguishing.
Imidized Plastics
These high cost, highest performance plastics include polyamide-imides and polyimides. Imidized plastics provide good to excellent chemical resistance and are superior to other plastics in combined high-temperature application range (up to 500°F) with high load bearing and wear performance. Imidized plastics are available as sheet, rod, or fabricated parts and are also used as a coating material. Example applications include bearings and bushings, pump and valve parts, and seals and specialty components in aerospace, electronics, and semiconductor industries.
Acrylics and Polycarbonates
Acrylic and polycarbonate plastics are used where clear, impact-resistant materials are needed for viewing ports, equipment enclosures/barriers, and other specialized applications. One common acrylic is Plexiglas. One common polycarbonate is Lexan. In addition to having different chemical resistance properties for select applications (e.g., polycarbonates provide superior resistance to chlorine bleach and hydrogen peroxide solutions), these clear plastics differ substantially in physical properties. Polycarbonates are far superior in impact resistance, and acrylics provide superior resistance to scratching.
Additives and Fillers in Plastics
Plastic compositions vary with manufacturer-specific formulations, including ranging from pure homopolymers to varied copolymer structures, and may include various additives and fillers. For example, while homopolymer polypropylene contains only propylene monomers, copolymer polypropylene has ethylene incorporated into the polypropylene polymer chains either randomly (typically up to 6% ethylene) or in regular block patterns (typically 5% to 15 % ethylene). Some plastics, such as PVDF and PP, are provided in commercial high-purity form, and other plastics contain additives and/or fillers. For example, PVC stabilizers can include various metals or rare earths or metal-free compounds (e.g., newer technologies have moved away from lead and tin stabilizers).
A range of additives are used in various plastic products to aid in the polymer material production processes. These include plasticizers to improve flexibility and slip agents for easier release from molds. Additives are also used in plastics to enhance or modify plastic product properties, including long term stability, mechanical properties, chemical resistance, service temperature range, UV light resistance, resistance to biofouling, degree of opacity, product color, antistatic performance, electrical or thermal conductivity, and fire retardance.
Application Considerations for Plastics
With over 100 different grades of application-specific plastics used for chemical process equipment and systems, it is important to define technical design criteria and project cost and non-cost criteria to focus on candidate plastic materials that can be systematically evaluated vs. metals, other materials, and combinations of materials. Application specifics, including ranges and variation in chemical and temperature exposure and mechanical stresses over the design life, can significantly impact the service life of different plastics and impact life-cycle costs for chemical process equipment and process systems. Environmental sustainability is another consideration for evaluating different plastic options for chemical process equipment and components and for evaluating plastics vs. other materials.
Next week, watch for Part 3: Material Selection for Chemical Process Equipment – Other Materials. Part 3 provides information on fiberglass reinforced plastics (FRP), rubbers and specialty elastomers, and advanced composites.
* For example, the polypropylene price increased ~65% between December and March 2017 and decreased ~ 26% between March and May 2017.