Making Informed Choices in Wet Bench Fire Safety

As the costs and consequences of failure grow, so does the list of fire-safe, approved materials.

In the precision manufacturing and process industries, the term “wet bench” generally refers to cleanroom process equipment that contains, dispenses, rinses, or in some manner processes or utilizes corrosive chemicals. The sheet plastics traditionally used in the construction of wet benches, polypropylene or polyvinyl chloride (PVC), provided good construction properties and corrosion resistance along with relative ease of fabrication and welding. These early materials, however, did not offer a high level of fire or flame resistance. The introduction of fire retardant polypropylene in the mid-1990s improved the flame resistance of wet bench materials.

Changing Standards

Until recently, the industry relied on UL94-V0 and V5a and ASTM E-84 standards to evaluate the performance of plastics when exposed to flame. UL94 measures the flammability of plastics used as components in appliances and other devices. E-84, also known as the Steiner Tunnel Test, measures the tendency of building materials to spread flame and produce smoke. None of these standards addressed specific concerns about plastics used in cleanrooms.

Risk underwriters and insurers, facing financial losses from major fires in wafer fabrication plants, demanded a reduction in fire risk from their insureds. Wet bench plastics were identified as culprits in accelerating flame spread and worsening damage to work in process and equipment from excessive smoke. In 1997 Factory Mutual Research Corporation (FMRC) released a new standard to measure the performance of plastics in these areas. This standard, titled Cleanroom Material Flammability Test Protocol 4910, references a fire propagation index (FPI) to measure a plastic’s fire propagation potential and cites a smoke damage index (SDI). Together, these indices are used to evaluate a plastic’s suitability for cleanroom use.

Using the FM4910 standard, plastic sheet manufacturers began to develop and modify sheet formulations to meet it. Submitted products were tested by FMRC. Successful candidates were given official listings as compliant with FM4910. In 2000 Underwriters Laboratories (UL) released |L2360, Test Methods for Determining the Combustibility Characteristics of Plastics Used in Semi-Conductor Tool Construction. This protocol also measures the fire propagation and smoke generation qualities of wet bench plastics. In addition, it provides a class rating of 1, 2, or 3, depending on the material’s FPI and SDI levels. Manufacturers have submitted materials to UL for testing and several sheets and resins have been UL2360 listed.