Plastics Suitable for Chemical Environments

The two most common polyolefin materials that are used for applications involving chemical environments are polyethylene (HDPE) and polypropylene (PP).

Polyethylene and polypropylene: 

• Have good chemical resistance at room temperature
• Are easy to fabricate with ordinary woodworking tools
• Bond well via thermoplastic welding
• Are available in a number of different grades, including formulations with enhanced toughness and/or enhanced chemical resistance

It is important to note that polyolefins tend to be soft (and get softer at elevated temperatures), and have relatively high coefficients of thermal expansion and poor creep characteristics. Specifiers should consider these dimensional stability issues when designing polyolefin parts. (Because of their low strength and stiffness, polyolefin chemical tanks are often reinforced with metal structural elements.)

Applications for polyolefins include tanks, food processing equipment, and clean-room and laboratory cabinets.

Rigid vinyls including PVC and CPVC are much stronger and stiffer than polyolefins—and more dimensionally stable (with lower coefficients of thermal expansion) than either polyethylene or polypropylene. Unlike polyolefins and fluoropolymers, rigid vinyls are easy to bond with solvent cements, which simplifies the assembly of piping systems. Rigid vinyl parts can also be joined via hot gas welding.

PVC 

• Has good mechanical properties at an economical cost
• Available in impact-modified grades for improved toughness compared with the standard grades
• Available in a transparent grade that has superior chemical resistance compared with many other transparent plastics. Clear PVC is often used for chemically resistant safety glazing and machine guards.

CPVC 

• Exhibits a higher glass transition temperature and higher heat deflection temperature than standard PVC
• Can be used at somewhat higher operating temperatures than PVC
• Available in impact-modified grades that have improved toughness compared with the standard grades
• Applications include chemical tanks, fume hoods and laboratory workstations

Fluoropolymers are well known for their superior chemical resistance, and are particularly valuable for elevated temperature environments—as well as applications such as pharmaceutical and semiconductor manufacturing that require high purity. Fluoropolymers are often used in tanks, valves and piping applications that involve harsh chemicals. Fluoropolymer sheet materials are frequently used to manufacture wet benches for the semiconductor industry.

Materials including PTFE (polytetrafluoroethylene), Kynar® PVDF, and Halar® ECTFE:

• Stand up to a wide range of chemical compounds
• Are available in both hard and soft grades

High-performance plastics such as PPS (polyphenylene sulfide) and PEEK (polyetheretherketone) are stronger, stiffer and more dimensionally stable than polyolefins, rigid vinyls or fluoropolymers.

PPS and PEEK:

• Stand up to most industrial chemicals
• Are available in friction and wear formulations with additives that make them suitable for use as bearing surfaces
• Maintain much of their room-temperature strength and stiffness at elevated temperatures
• Are often specified for use in high-temperature environments where aggressive chemicals are present

PPS is less expensive than PEEK, but PEEK has superior mechanical properties compared with PPS and it is able to operate at higher temperatures than PPS.