Part I: Multi-layer Peel-off Matting
by Dennis K. Baldwin, ALMA, Inc.
The combination of performance and cost effectiveness has resulted in the cleanroom industry worldwide adopting multi-layered adhesive-coated matting in the majority of applications.
Cleanroom matting and flooring has been almost universally accepted for cleanroom applications around the world. However this product group was not developed for cleanrooms, but was instead borrowed from the hospital market.
The need for general cleanliness in the most sensitive areas of the hospitalthe operating room and the intensive care unitcreated the demand for the earliest versions of current cleanroom flooring products. Hospitals needed to control dirt and dust that may contain, or have attached, harmful microorganisms. Early mats made of cloth and paper were introduced in the late 1960s. The multi-layer polymats and washable matting were in use throughout the 1970s.
In the earliest cleanrooms, or “white” rooms, the most critical areas that introduced contamination were shoe bottoms and wheel surfaces that came in contact with the floor prior to entry to the clean area. Some 80 percent of all contaminants by weight enter the cleanroom via this method. To address this problem, designers used a barrier on the floor to remove and hold particles and keep them from the cleanroom.
As our technology continues to progress, we are able to consider the reduction of smaller and smaller particles. Our ability to measure and contain particles went from 30 microns to 10 microns to 1 micron and then to sub-micron levels, now to 0.1 micron.
Our need to concentrate on the smallest of particles, due to the advancement of product and process technologies, makes it that much more critical to utilize one of the most basic control devices: cleanroom matting. The ability to control 0.1- and 0.3-micron particles does not mean that all of the larger particles do not exist, nor that these small particles are not introduced via foot or wheelborne traffic. The greatest concentration of all particle sizes, with the exception of air currents, will still be on or near the floor. When air flow and currents are introduced either through human movement or laminar flow devices, almost any size particle can become airborne.
Preventing the introduction of floor particles at the source will lead to lower overall counts of all particle sizes. This prevention will also reduce the need or increase the effectiveness of more costly forms of contamination control inside the cleanroom. Preventing foot and wheelborne contaminants from entering the cleanroom is vital to increase the quality of the end product and increase yields.
No product used in or around the cleanroom limits the introduction of more particles by weight than cleanroom matting. This includes gloves, wipers, all garments and body barrier control, air handling, etc. In an era of stringent cost control, and examination of all areas of contamination control, the most consistent performer is cleanroom matting.
The use of cleanroom entry matting depends a great deal on the logistics of each operation. Study of the personnel ingress/egress points and traffic flow and direction are critical. Where matting or floor systems are placed, and how they are serviced, can vary dramatically from location to location. For maximum effectiveness, consider these factors: logistics, training, multi-layered versus washable, and cost.
The outer hall area. The sequencing of matting to control the largest gross contamination, and street dirt, will make all downstream control measures more efficient and cost effective. Place matting prior to any gowning or staging area.
Gowning or staging area. The placement of lockers and passthroughs will determine traffic flow and direction. Place matting in the direction of ingress/egress.
Cleanroom entry. Is there an air shower or double doors in use? Where will personnel be standing?
Quantity of personnel. How many people will use this area? How many times will they cross this point? What are the equipment requirements? The answers to these questions will help determine the size of matting and how often it must be changed or cleaned.
Class of adjoining rooms. Is this entryway from a common hallway to Class 1,000 or is it gowning area to Class 10?
Personnel training and protocols are vital in helping you receive the greatest benefit from the product. Using the proper number of steps to maintain maximum footfalls is vital for any type of matting. The correct timing of changing layers or washing must also be in place.
MULTI-LAYERED VS. WASHABLE
There are two main areas to examine when you compare multi-layered adhesive coated matting and washable matting. First is the effectiveness of the product and its ability to function in each application. Second is the cost of the product and the cost of using the product.
The effectiveness of any product must be examined in both controlled laboratory test conditions and the real world of cleanroom operations. It is necessary to state that “all disposable mats are not created equal.” Some “manufacturers” of matting only convert precoated film, while others control the process from start to finish. This difference can create a large discrepancy in the effectiveness between different brands of mats.
Tests and comparisons later in this article reflect expected results with ALMA mats and not necessarily other brands.
Before looking at test results comparing multi-layer adhesive coated mats and washable mats, examining the difference in functionality between washable matting and disposable matting is helpful.
Multi-layer matting consists of sheets of polyethylene material coated with a layer of pressure-sensitive adhesive. The polyethylene film varies from 1.2 to 3.0 mils thick, and the adhesive from 0.2 to 0.3 mils.
When the adhesive comes in contact with the surface of a shoe, shoe cover, or wheel, the pressure-sensitive adhesive removes particles on that surface.
The pressure or weight of the surface embeds the particles into the adhesive. This action is key to combining three different characteristics of the adhesive into play: the tack level, the adhesive thickness, and the adhesive softness. All three work in relation to one another. Because the particles are embedded in the adhesive, the adhesive surface stays fresh, which allows it to continue to perform and function after heavy foot traffic.
If the adhesive does not have any tack it will not attract the particles from the contaminated surface. If the adhesive is not soft enough, the particles will not embed and will stay on the surface. If there is no thickness to the adhesive, the particles will also remain on the surface.
By comparison, the function of a washable mat is similar to that of multi-layer matting in only the first of the three aspects of the adhesive: tack. A washable mat has a surface that attracts particles when feet or wheels come in contact with it. However, there is no thickness and thus no softness, just the surface. When particles come in contact with it, there is no place for them to go; they stay on the surface of the mat. The next contact with a shoe may be with particles from the prior shoe.
If we compare the results head to head, the differences become quite dramatic. In tests performed by Dryden Engineering in June 1997, the particles removed from the multi-layer mat are substantially higher than those removed by the washable mat.
The results show the difference in a controlled situation, but what about at the point of use, over time? When the sheet or layer is removed on a disposable mat it essentially becomes a new mat, a renewed surface. The removal can be done in seconds as many times as necessary in the course of daily operation.
Washable mats, by contrast, are not cleaned or renewed as often. To do so the entry must be closed, equipment assembled, and the cleaning process performed. When the mat is cleaned only once a day or once a shift, particles accumulate, which further reduces the effectiveness of washable matting as the shift or day progresses.
One of the main reasons that washable matting “works,” according to the manufacturer, is the optically smooth surface. Unfortunately, this surface is also soft, and dirt particles are sharp. After a short time the surface is no longer smooth. This further degrades the ability for washable matting to work over time.
The comparison of effectiveness between multi-layer matting and washable matting can be summarized as follows:
- Multi-layered matting initially attracts more particles than washable matting.
- The three functions of the adhesive coatingtack, softness, and thicknesscontinue to allow the multi-layered mat to function throughout the shift.
- Particles that will be removed from washable matting stay on the surface, reducing its ability to perform until it is washed.
- The surface of washable matting changes due to scratching, reducing its ability to attract and retain particles over time.
When comparing the cost of any product, you need to consider all aspects. This means examining that cost of acquisition and the cost of use. The cost of either type of matting will vary with location and requirements.
Initial purchase cost of disposable mats is the cost of a case. This depends on mat and case size and will range from $50 to $300. The cost of washable matting also depends on size. The smallest cleanzone (10'×6'×6″) costs $1,390. Installation of disposal matting takes a few moments to remove the release liner and lay down the mats. Washable matting comes with instructions for the contractor that include checking the sub floor, surface preparation, adhesive and installation procedure, welding, edging strips, and safety concerns. Consultation with your contractor is necessary to estimate installation costs.
The cost-of-use comparison of these two types of mat is where the biggest differential comes into play. From the earlier example, the cost of multi-layer matting may be $300 per case for a case with 480 layers of working surface. This would equate to a product and labor cost of approximately $1,570 over a two-year period. The same entryway with a washable mat system would cost $3,128.70 over the same period, assuming that the mat is washed only once per day. Cleaning once or twice a shift results in two-year costs of $4,865.40 and $8,340.80, respectively.
There is one other substantial, yet hard to quantify, cost. The difference in time between the removal of a layer on the multi-layered mats and the washing of the washable mat is not just a labor cost. It closes the entry, and thus access to the cleanroom. While the washable mat is being cleaned and dried, personnel and capital productivity is lost. In this example, the downtime differential is 18 minutes per day based on one washing, one hour and 12 minutes per day based on twice-per-shift washing. This estimate depends on overall cleanroom labor costs and capital costs of the entire cleanroom.
Cleanroom matting in all forms has been with us since the earliest cleanrooms. They remain one of the most necessary and effective contamination control devices. Barring a revolutionary change in how we operate in and around a cleanroom, they will be part of the cleanroom environment well into the foreseeable future.
The combination of superior performance and cost-effectiveness has resulted in the cleanroom industry worldwide adopting multi-layered adhesive-coated matting for 85 to 90 percent of all applications. It is easier to use, better performing, and the most cost-effective method of preventing foot or wheelborne particles from entering the cleanroom.
Dennis K. Baldwin is vice president and director of sales at ALMA Inc. in Palm Springs, CA.
Washable Polymeric Flooring
By Thomas R. Mulligan, Dychem
Polymeric flooring is best suited for large areas where the user is serious about improving product yields and reducing costs.
Data from 3M Co. and recent independent tests have confirmed that feet and wheels are by far the major source of contamination transferred into the cleanroom or critical environment. Empirical data shows that across the entire range of critical environment industries, donning the appropriate booties, gowns, and gloves solves only about 20 percent of this contamination threat. Therefore best practice dictates using some form of floor contamination control system to minimize the particles being generated by people and carts.
This contamination needs to be addressed prior to the cleanroom for the following reasons:
- To eliminate the threat to products, improve yield, and increase revenue.
- Less contamination entering the cleanroom means a reduced and more effective cleaning schedule inside the cleanroom.
- Cleanrooms operate best when the laminar airflow is disturbed as little as possible.
- HEPA filters work better and require fewer changes.
- To reduce the chance of cross contamination.
- Controlling foot and wheelborne contamination outside the critical area is easier and less expensive.
So which type of system is best: the disposable peel-off mats or permanent, high tack, washable polymeric flooring?
Custom Polymeric Flooring
The ideal contamination control flooring system will remove 100 percent of foot and wheelborne contamination (particles, bacteria, or static) prior to the critical area. This is accomplished by designing the size of the high-tack polymeric floor surface to match the traffic level and cleaning schedule.
Simply clean the polymeric flooring when you clean the other floors with your facility-approved detergents or bactericides.
These large, easy-to-maintain, unavoidable areas remove contamination, require no overt action by personnel and the product’s lifespan reduces costs. They are environmentally responsible compared to small, expensive, disposable adhesive mats. In addition, polymeric flooring can be recycled into less critical uses at the end of its average 4- to 5-year life span.
Polymeric flooring has evolved from 25 years of cleanroom and critical environment experience, extensive research and development and the changing requirements of uses.
Improvements in installation methods with sealed, low profile edges and the use of different color flooring offer different options for each customer's demands.
Peel-off mats are appropriate for applications with low personnel traffic volumes, confined space areas, or where no wet cleaning is allowed.
Polymeric flooring is used wherever a reasonable number of people or carts enter the cleanroom. It is also used where there is a reasonably sized area present to install the flooring. Typically this area is in the gowning room/changing room, airlock, hallway, and entryways wherever wet cleaning is part of the cleanroom operational procedures.
Strengths and Weaknesses
Industry experience and independent studies by both peel-off mat and polymeric flooring manufacturers conclude that a minimum of six steps (three for both feet) and three circumferences of a wheel are needed for full decontamination. Similarly, both companies have empirical studies that show peeling a mat generates static and the release of particles into the atmosphere.
The following is a summary of empirical data comparing the two systems:
Peel-off mat strengths: high brand awareness at (buyer) end-user level; very sticky/tacky; easily understood concept (can be peeled up quickly for a new sheet when a high-profile visitor is on a plant tour); purchased with consumables budget.
Peel-off mat weaknesses (real or perceived): standard sizes are too small to cope with more than a couple of people at one time; difficult to have six footsteps (three for both feet) on these small mats; often remain unpeeled after saturated (“It's not my job!”); performance deteriorates after several footsteps/overstrikes; peeling generates static charge and disperses particles into the atmosphere; operatives avoid having their booties torn off by not stepping on the mats; unsuitable for wheeled traffic (mat wraps around wheel and mat is too small to remove all contamination); costs are high when analyzed on an annual consumption basis; replacement time; disposable costs; inventory cost and warehouse space; low-tech appearance; cannot handle high traffic during shift changes; cannot be recycled.
Polymeric flooring strengths: size; easily addresses the six-footstep requirement; tailored to client needs in terms of traffic volume and cleanliness; contamination remains on surface until cleaning; comfortable to walk on and unavoidable, all operatives are decontaminated; suitable for wheeled traffic, even heavy carts; reduces costs compared to disposable peel-off mats; environmentally friendly, less waste; recyclable; static dissipative.
Polymeric flooring weaknesses (real or perceived): capital purchase; must be cleaned, must be installed; comfort of tack level disconcerting despite maximum contamination removal.
Thomas R. Mulligan is president of North American Operations at Dycem in Warwick, RI.
- Whyte, W; Shields, T. CleanRoom Mats: An investigation of particulate removal, Journal of Environmental Sciences, July/August 1996, pp. 19-27.
- Mainers, L; There’s more to cleaning cleanrooms than meets the eye: CleanRooms, July 1999, pp30.
- Prout, G; A comparative study of two floor cover materials in control of foot and wheel borne contamination: European Journal of Parental Sciences, Vol. 2 1997.
- Barrett, G; Polymeric flooring demonstrates particle retention properties: CleanRooms, Nov. 1996.
- Prout, G; A comparative study of Dycem Clean-Zone and peel off mats: Centre for drug formulation studies: Bath University, England 1995.
- Prout, G; Particulate counts without any floor control system, using Dycem and comparing the efficiency of peel off mats: Case study Henley Medical, Henley Medical Wiltshire, England, March 1999.
MULLIGAN’S FLOORING TYPE ASSUMPTIONS
Peel-off mat assumptions
Size doesn't matter. It does when someone steps over or around a small mat or cannot get three footsteps on the surface.
Step up and down on the mat three times. Impractical.
Gang mats together.
Costly, a waste generator, and gaps in mats are a bacteria trap.
What independent published quantifiable results are offered by peel-off mat companies? None.
How does a peel-off mat work? Multiple layers of polyethylene film are coated with an acrylic adhesive and held in place by a more aggressive adhesive or frames.
Polymeric flooring assumptions
Polymeric flooring must be installed. A one-time installation with self-adhesive panels and edging can be done by the manufacturer or by in-house facilities personnel.
Polymer materials outgas. Independent tests prove no outgassing, with the flooring approved for use even inside cleanrooms.
It requires cleaning. A well designed flooring system is cleaned when you clean your other clean environment floors, using the same detergent and equipment. Damp mop the floor and squeegee dry. Steam cleaning and wet vacs can also be used.
High initial cost. The initial capital expenditure for polymeric flooring with 10 times the coverage of peel-off mats can be recouped from your disposable mat budget in four to ten months (a three- to four-year offering at no cost). Monthly lease programs are available.
How long does it last? It depends upon the size of the area and the volume of traffic. Average replacement time is four to five years.
How does it work? The natural high-tack polymer flooring has a smooth, slightly soft conforming surface which traps, and holds particles. The size of the contamination-gathering surface matches the traffic level and cleaning schedule.
Part III: Raised flooring in cleanrooms
by Firas Alsalih, Tate Access Floors
Guidelines continue to become stricter as they relate to all aspects of the cleanroom industry. As such, product development efforts in the raised floor industry are continuous (for more information on how to specify cleanroom flooring, see “Choosing cleanroom flooring,” CleanRooms, October 1999, page 22).
Raised flooring can be defined as: “A modular, load-bearing flooring system, erected on a system of pedestals (the understructure) at varying and adjustable heights, to create a plenum that can be used for wire/service distribution and air delivery/return.”
Raised flooring was originally developed as a solution to the early problems created by the use of heavy mainframe computers, mostly by governmental agencies. The challenge was to create a flooring system that provided adequate load support, as well as a path to deliver cool air to these heat-generating machines, while providing a plenum to solve the massive wire distribution problems. The answer was raised flooring (then called computer flooring), and an industry was born.
During these same times, the electronics industry also underwent dramatic changes to become the microelectronics industry. The cleanroom industry also evolved, and the meaning of clean changed dramatically. The cleanroom industry needed a floor that not only supported the heavy equipment loads and provided a service distribution plenum, but perhaps most importantly, also allowed the creation of “an enclosed area, designed to control airborne contamination through vertical laminar air-flow, while maintaining a constant temperature, humidity, and air pressure,” or a cleanroom.
This floor, in addition to the above performance features, also had to possess special physical qualities, such as being made of non-ferrous materials (aluminum), have a variety of coverings and/or coatings, and a variety of airflow options.
Types of raised flooring systems
There are various types of raised flooring, but they can best be segmented into two main categories:
Raised floors for general office and equipment/data rooms: The flooring for these applications can be made of various materials, but preferably are made of steel, containing a cementitious core.
Raised floors for cleanrooms: These floors are almost always made of aluminum, due to its lightweight, high strength and non-ferrous qualities. This is the category of flooring we will focus on in the following sections.
A typical raised floor system in a cleanroom consists of various components, which include:
a) The panel: Made from a single unit of die-cast aluminum, typically offered in two sizes, 60cm x 60cm (metric) and 24-in. x 24-in. (imperial).
b) The pedestal: Consisting of two separate units, the base and the head, both also typically made of die-cast aluminum. The head contains panel-locating tabs, for proper panel alignment, and the system is height adjustable to approximately ±1 in.
c) The stringer: Consisting of a box or U channel, connecting each pedestal to its four adjoining pedestals. Stringers provide lateral stability to the overall system.
d) Screws: Screws are used to “corner-lock” or attach the panels to the pedestal (sometimes) for increased vibration control, and to connect the stringer to the pedestal (always).
e) Various components: These include grounding pads, diagonal bracing, dampers, panel lifters, access covers, opening frames and fittings, and urethane adhesive.
There are various options when it comes to panel type, coverings, and coatings. Some of these include:
Solid, Perforated, Grated.
Vinyl (conductive and anti-static), high-pressure laminate (HPL), (conductive and anti-static).
E-coating, Nickel chrome, Electroless nickel, Epoxy coating (standard or conductive).
There are various issues that may be of particular concern. These criteria include: load-bearing capability, ESD performance; chemical resistance; off-gassing performance; air-flow performance and characteristics.
In any cleanroom, all of the above are typically important in varying degrees. Individually prioritizing them will assist in determining the floor most suited for any particular application. Often, a selection of various combinations of components in a cleanroom would be made. A typical selection would have a combination of solid, perforated, and grated panels. The solid and perforated are covered with conductive vinyl, and the grates with a metallic coating, such as nickel chrome.