Evaluating Flooring for Cleanrooms
Choosing a cleanroom floor requires careful consideration of the materials used and the environment it will be used in. Several flooring alternatives are discussed.
By Erick Van Anglen
Cleanroom floors demand the same kind of special attention required for other details of cleanroom design and construction. Among the criteria for cleanroom floors are:
Absence of sites for microorganism growth or propagation;
Minimum particles given off into the airstream;
Resistance to attack by spilled chemicals;
Containment of any spilled chemicals;
Slip resistance;
Electrical conductivity or electrostatic dissipation;
Long-term durability/reliability.
Although few cleanroom floors meet all the requirements, every cleanroom floor must exhibit at least some of these characteristics to qualify under industry and government standards. Over the years, various construction materials and coverings have been employed to meet cleanroom requirements. These include such materials as concrete, terrazzo, vinyl sheeting, tile, thin-film coatings (paint), and most recently, engineered polymer toppings.
Alternatives for Cleanroom Flooring
Concrete is an excellent construction material. Pourable and formable, concrete develops into a hard structural material capable of supporting heavy loads and is durable enough to last for many years. But concrete is porous and subject to wear and to attack from corrosive chemicals. Also, it tends to abrade and give off large amounts of dust, while spilled materials tend to seep into and through it. Even with additives to increase hardness and seal the surface, concrete cannot be exposed in cleanrooms. Still, it forms the base for almost all flooring materials that do provide the required characteristics. These materials must be compatible with the concrete substance and must adhere well for long-term durability.
Terrazzo is one of the world`s oldest construction materials and one of the most permanent of all flooring materials. It consists of marble or stone chips set in mortar, which are then polished to form a hard, tough, continuous surface that stands up to all kinds of abuse. Even so, the surface contains tiny micropockets that provide sites for the growth of microbiologicals. For that reason, it is not satisfactory for pharmaceutical or biotech cleanrooms. It may be used in some electronics applications because it does not generate dust particles and is so durable that repairs are seldom necessary.
Vinyl sheet provides a continuous, impermeable surface that is smooth, easy to clean, and does not release particles into the air. It is used in many pharmaceutical applications; also, desirable electrical properties can be designed into the material for electronics applications. However, if the room is larger than the width of the rolled sheet goods, there will be a seam which can offer sites for the growth of microorganisms. Vinyl sheet is subject to wear, and eventually needs to be replaced–a costly item in any cleanroom. It is also not very chemically resistant and, therefore, cannot be used where chemical containment is required.
Vinyl tile is similar to sheet goods, except that seams between the tiles make cleaning difficult and offer many sites for microorganisms. Vinyl tile is widely used in electronics industry cleanrooms, but only where there is no spilling or splashing of chemicals onto the floor.
Paint is commonly applied to industrial concrete floors to retard dust and protect surfaces against wear and mild chemicals. High performance epoxy and polyurethane floor coatings are relatively easy to mix and apply. (These products should not be confused with flowable engineered copolymer flooring materials.) They adhere well to properly prepared concrete surfaces and effectively extend the useful life of the concrete. However, because painted floors wear and sometimes peel, they also release unacceptable amounts of dust, and provide numerous opportunities for microscopic growth. Even high performance coatings are generally unsatisfactory for cleanroom floors.
Engineered copolymer toppings for concrete meet a range of cleanroom floor criteria, including resistance to corrosive chemicals, abrasion, and impact. Various electrical properties and levels of slip resistance can also be incorporated into the formulations. An engineered copolymer material forms a seamless, pore-free surface able to resist highly corrosive chemicals, including sulfuric acid and caustic soda solutions. These materials bond tightly to concrete, whether in the form of thin-film coatings of 16 to 60 mils or high-build toppings, which can be applied to a thickness of 250 mils (1/4 inch).
Meeting the Needs of Cleanrooms
Preventing the growth of microorganisms is essential in most cleanrooms that deal with food, biologicals, or pharmaceuticals. The best way to combat contamination is by constructing floors without cracks and holes so microorganisms cannot hide and propagate.
Preventing the formation of dust particles from surface coatings is also essential, especially in electronics manufacturing. Not only must a finish protect against wear and impact damage, it must also contribute to the room`s cleanliness. Finished surfaces must be easy to keep clean and impervious to attack by chemicals that could cause minute particles to be released into the atmosphere.
Chemical resistance and containment are major concerns where chemicals and solvents are used and stored. Most states now require some type of secondary containment system to keep spilled chemicals under control to prevent them from contaminating soils both under and around the site. For example, the State of Wisconsin Department of Natural Resources requires that concrete surfaces be covered by an impermeable liner to contain spills and protect surfaces from the effects of spilled corrosive fluids.
To protect the substrate against chemical corrosion, the covering must resist whatever chemicals might be spilled. Therefore, it must be able to withstand any acid, base, or solvent at that location. Because this varies greatly from industry to industry and from plant to plant, any material selected for containment against chemicals should be rigorously tested on-site.
Slip resistance is a requirement in many manufacturing operations, especially where floors are frequently wet. According to the National Safety Council, more than 500,000 injuries occur each year at the workplace due to falls on slippery floors. Many types of anti-slip flooring have been developed for industrial applications, but most of these are unsuitable for cleanrooms. Typically, sand and some other gritty material are embedded in a floor to produce a rough, slip-resistant surface. While this finish provides excellent traction, it can make floors difficult to clean because of surface peaks and ridges, which offer sites for contamination. Therefore, safety floors of this type are not acceptable for cleanrooms.
Electrical properties of cleanroom flooring depend on production requirements and can vary from highly conductive to very resistive, with just enough conductivity to dissipate electrostatic discharges.
“Some estimates indicate that 25 percent of latent failures to electronic devices are ESD-related,” says Bruce Monro, an industrial engineer at AlliedSignal`s 75,000 ft2 General Aviation Avionics plant (Lawrence, KS). “Therefore, controlling static is extremely important.”
According to Monro, “Resistance below 105 Ohms is considered conductive, but we were concerned about too much conductivity. We wanted something that offered a little more resistance in case there was a voltage that could short out through the ground. A highly conductive floor doesn`t provide the same degree of safety as one with a resistance of between 105 and 109 Ohms, which is a true `static dissipative` floor.”
On the other hand, some facilities require extraordinary precautions to eliminate any possibility of a spark-caused explosion.
The Need for Durability
While the cost of materials used in cleanroom construction is always a concern, the durability of those materials is far more of an issue because of the high cost of repairs once the cleanroom is in operation. For example, if the flooring cracks or loses its ability to contain spilled chemicals, it may be necessary to tear up the finish, repair the cracks, and install another floor.
Making this kind of repair while trying to maintain production in the cleanroom by isolating the section under repair with heavy drapes is very expensive. Even then, the cleanroom atmosphere is in jeopardy with the first blow of a jackhammer or the removal of a piece of equipment.
Robert E. Patterson, AIA, Director of Technology at Jacobs-Sirrine Engineers (Portland, OR), says, “In cleanroom construction, it is imperative that the job be installed correctly the first time. Flooring system problems are most often related to poor installation, and it is prohibitively expensive, if not impossible, to correct installation defects without adversely affecting the operation of the cleanroom. Due to this concern, we typically specify that the installed product be warranted by the manufacturer.” n
Erik Van Anglen, technical service director at Rust-Oleum Concrete Protection Systems (Tulsa, OK), has worked for the manufacturer of copolymer-based protective toppings and coatings for 5-1/2 years. Prior to his job with Rust-Oleum, he was a flooring contractor who serviced industrial and commercial property owners for several years.
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The concrete floor of the sub fab area at the Cypress Semiconductor plant expansion (Minneapolis, MN) is covered with a 100 percent solid epoxy coating that protects against any chemical spillage. In addition, it does not shed particles into the airstream.
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All structural steel and ductwork in the sub fab area below the main fabrication level are finished with single-coat, high performance Rust-Oleum 9100 System Epoxy at Cypress Semiconductor`s plant expansion.
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The newly installed UltraPlex ESD floor in the AlliedSignal General Aviation Avionics plant (Lawrence, KS) is bright and highly reflective and provides just the right amount of electrostatic dissipation to meet manufacturing and safety requirements.
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All the steel surfaces in a fabrication area at Cypress Semiconductor`s plant in Minneapolis are finished with Rust-Oleum 9100 System Single-Coat Epoxy.