Maintaining cleanroom walls and floors is critical for overall contamination control and for preserving cleanroom protocol. The cleaning of walls and floors is often overlooked because these surfaces are not immediately adjacent to manufactured product and are assumed to contribute little to product defects. If these surfaces are not maintained, particles and residues can accumulate and can migrate or be transferred to critical areas.
String mops vs. flat mops
Maintenance of cleanroom walls and floors is usually accomplished by mopping to clean the large surface areas involved. Mops have sometimes been described as “large wipers on sticks’ This is not altogether a bad description, since many of the substrate selection criteria and usage techniques developed for wipers can be applied directly to mops.
Fundamentally, there are two types of mops–string mops and flat mops. For optimum contamination control and minimum linting, mop heads for string mops and flat mops should be constructed from laundered knit polyester fabric, a material that minimizes particles, fibers, non-volatile residues and ionic contaminants.
Knit polyester has the further advantage of good liquid absorbency and good abrasion resistance.
A string mop head for cleanroom use consists of a bound assembly of wide, tubular strands of knit polyester fabric. String mops, employed primarily for cleaning floors, are used in conjunction with a wring bucket of cleaning solution and rinse water.
A flat mop head for cleanroom use consists of a flat plastic rectangle to which a conformal, foam pad is affixed and over which a replaceable knit polyester mop cover–also known as a bonnet–is attached.
Flat mops are not used with buckets; cleaning agents are applied to the mop cover with squirt bottles. Multiple mop covers are wetted for later use or pre-packaged, pre-wetted mop covers are employed. (1) Mop covers are changed frequently to ensure that contamination is removed efficiently and effectively. Flat mops are used for cleaning walls and floors.
Occasionally, flat sponge mops are used as is or covered with large polyester knit wiping cloths. This brings with it the risk of contamination from the sponge material, which carries a larger particle, fiber, ion and non-volatile residue burden than polyester knit fabric. The cleaning solutions used with sponge mops will efficiently extract these contaminants and transfer them to walls and ceilings.
How to choose
Floors can be cleaned with either string mops or flat mops. The choice depends upon a number of factors:
Filth level: If the floor is very dirty, then a string mop with cleaning solution may be the quickest, most economical way to pick up most of the contamination. Wallis recommends use of a 16:1 solution of deionized water and a commercial cleaning detergent. (1) This should be followed by string mopping of the floor with pure deionized water to remove residues of the cleaning detergent. Finally, after cleaning the floor with a string mop, the floor should be cleaned again with a flat mop (See the section “How to mop”).
Availability of clean water near the area to be cleaned: If the rinse water in the bucket cannot be changed. frequently and conveniently, then there is a good likelihood that the entire floor will be cleaned with the same bucket of water. This is obviously undesirable for optimum contamination control. Under these circumstances, a flat mop with multiple bonnets is desirable.
Need for disinfection: Aseptic fill areas in pharmaceutical or biotech facilities require disinfection of environmental surfaces during cleaning. This usually calls for application of large volumes of cleaning/disinfecting solution, conveniently applied with a string mop.
String mops are almost never employed on walls. From an ergonomic perspective, string mops can be heavy when wetted with cleaning solutions and are awkward to manipulate when working .on vertical surfaces.
It is difficult to control the amount of cleaning liquid applied to a vertical surface with string mops. Finally, there is a safety issue if aggressive cleaning solutions rain down onto cleaning personnel. For these reasons, flat mops are used almost exclusively for cleaning walls.
How to mop
As with wipers, the most effective way to mop horizontal or vertical surfaces is with linear overlapping strokes, moving from clean or dry areas to dirty or wet areas. (1,2,3) Walls are mopped from the top down because the cleanest area of the wall is assumed to be at the ceiling near the HEPA filters.
Floors are best mopped in small rectangular sections using linear overlapping strokes to minimize recontamination of mopped areas. Doorways or high-traffic areas are likely to be the dirtiest, so they should be mopped last. These areas may need more frequent mopping or special attention to remove visible contamination. Damp mopping is much more effective than dry mopping in removing contamination.
Furthermore, damp mopping reduces the possibility of electrostatic discharge (ESD) events near sensitive devices. Mopping of floors with a string mop presents a unique problem. The typical back-and-forth, “S-shaped” mopping strokes that are most efficient and ergonomically advantageous for cleaning floors unfortunately bring contamination from dirty areas back into clean areas. There is no easy solution to this problem other than to continue with this traditional mopping style with a string mop and wring bucket, then re-mop with a flat mop with linear, overlapping strokes.
American Society of Test and Measurement (ASTM) Standard Practice E2042-99 provides a table of cleaning frequencies for all surfaces in the cleanroom, including walls and floors. (2) As an example, for an ISO Class 5 area, daily damp mopping of floors and walls is recommended.
Water availability and disposal
When using string mops to clean floors, Wallis describes the need to change bucket water frequently, after cleaning every 10 to 15 square feet of floor surface. As many as 50 water changes may be required for one floor, making the task quite onerous. This also raises the issue of the availability of clean water and the disposal of waste cleaning agent and waste rinse water.
Most cleanrooms do not have floor drains to dispose of waste liquids or conveniently-located sinks for the supply of clean rinse water. Consequently, there will be a natural tendency to use the cleaning agent and rinse water far longer than is desirable, and the result will be inadequate cleaning of floors with string mops.
Infrequent changing of the cleaning agent and rinse water will do an effective job of spreading contamination over the entire floor. One solution to this dilemma is to eliminate string mops and buckets entirely and to utilize only flat mops with mop covers. This approach has the advantage that mop covers can be changed quickly, frequently and conveniently, without the need to transport buckets of clean and dirty water back and forth across the cleanroom.
This permits the mopping task to be done faster and results in a cleaner floor. Disposal of a dirty mop cover is much simpler and more convenient than disposal of a bucket of dirty water.
Specific requirements for electronic and regulated industry cleanrooms
Cleanrooms for manufacture of electronic devices–semiconductors, data storage, flat panel displays–are particularly sensitive to particles, so mop heads used for cleaning these environments must incorporate the lowest-particle fabrics such as the laundered, knit polyester fabrics described above.
Cellulosic and foam materials are generally not preferred for these areas. Raised floor systems or perforated floors are often found in these facilities. String mops are unsuitable for such applications because water can drip, through to the sub-floor and set off leak detection devices. Consequently, flat mops with disposable mop covers are preferred for such environments.
Regulated industry cleanrooms, such as those used by the pharmaceutical, biotech and medical device industries, can tolerate a somewhat higher level of particles, but not bacterial entities. Foam or sponge mops are often employed in these areas to apply bactericidal cleaning agents, which are allowed to dry onto floors and walls.
Foam or sponge materials, although they exhibit higher levels of contaminants, have the advantage of holding large amounts of liquid per unit area and, thus, require less frequent re-wetting with the cleaning agent. A key requirement for bacteria-sensitive cleanrooms found in pharmaceutical, biotech and medical device industries is the ability to autoclave consumables such as mops prior to introduction into the cleanroom.
The mop must be constructed from materials that will withstand–perhaps multiple times–the elevated temperature and pressures used in autoclaves.