Flooring - Choosing cleanroom flooring
10/01/1999
The planning and construction of a cleanroom is a rather complicated process. However, questions related to the flooring sometimes are not discussed in depth because the interested parties on the users' side do not have the necessary information or may even be convinced they are sufficiently informed. On the other hand, flooring manufacturers, distributors and installers often have only very little knowledge of the end-users' working conditions and of possible limitations set by the processes or products. The author of this paper, being on the floor producers' side, would welcome more direct discussions with the final users and would invite all those involved to establish such contact.
Staying on the surface. The flooring is one of the biggest surfaces in the room: as big as the ceiling and with as many functions. Additionally, people will walk on and equipment will be moved over, the floor.
People responsible for the interior design of buildings ("clean" or not) tend to perceive the floor as a two-dimensional object. But there is a third dimension to be considered: the vertical section, the "thickness." A floor does have a structure and thereby has many other functions too.
The details. A floor covering material is homogeneous if its structure is the same throughout its full thickness. It is not homogeneous in the chemical sense (i.e. composed of parts all of the same kind). This is one reason why there is no floor covering "made of 100 percent vinyl." Even if it has a homogeneous structure, it will be a mixture of several ingredients (PVC, plasticisers, fillers, pigments, etc.).
A floor covering that has layers of different composition/nature is heterogeneous.
The myth of thickness. It is fundamental to understand that a floor does not have to support a load but only to distribute and transmit it towards the layers of construction below: e.g. to the leveling (if present), to the concrete slab or metallic structure, etc.
The inner strength of a flooring material is measured by laboratory tests (tensile test), the results of which are expressed in force/unit area.
As a consequence, the tensile strength of a material, should this be 2.0 mm or 3.0 mm thick, is the same. On the other hand, a 2.0 mm floor covering made of material "A" can be stronger than a 3.0 mm one made of material "B"-it depends only on the material's composition and structure, not on its thickness.
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Another known feature, the so-called indentation of a floor, is a function of its (constant) intrinsic features and of its thickness. The indentation of a 3.0-mm-thick floor will be approximately 50 percent deeper than the indentation, under the same conditions, of a 2.0-mm-thick floor made of the same material.
So, thicker is not better, at least not as a rule. Where thickness might play a role is in the resistance to abrasion, i.e. it takes more time and energy to consume the whole thickness of a 3.0-mm-thick floor-again, of the same material.
Weight or pressure? A question frequently heard from users is: "Can we use a 3-ton forklift truck on this floor?" It is important to realize that it is pressure that counts, not weight. If the fork-lift truck in question has traditional tires, the total weight will be distributed over a large area and the resulting pressure on the floor is rather low (about 10 kg/cm2). It is not a particular stress for an industrial type floor covering. But an AGV (Automatic Guided Vehicle, a transporting/operating robot) with a total weight of 1-ton, or less, can be a serious problem for the floor (and subfloor) because of the small, hard wheels: the weight is distributed over a much smaller area than in the previous case. Also, the vehicle runs and turns always on the same track. The tendency is toward "moving minienvironments" with higher weights.
Sometimes we are confronted with specifications which have nothing to do with the floor covering: something like "500 kilonewtons per square meter" refers obviously to the structure of the building or the maximum permissible load in a certain area.
Installation: how and when? Hopefully, all other tradesmen, including plumbers, fitters, and painters have left the site when the floor installers finally begin their job. There is only one professional who should appear after the floor layers: the electrician, to connect the ESD-control floor to the equipotential system of the facility.
Problems start when flooring is considered "last" in the sense of "resolving" it within the remaining time and separate from the rest of the budget. Haste and penny-pinching solutions often lead to dissatisfaction. In a cleanroom the floor is either suitable or not, there is no question of an "aesthetic discount." Changing a floor in an already completed cleanroom is a nightmare. The floor here is part of a system and it cannot be removed without jeopardizing the whole. For example, joints and sealings between the floor and other surfaces might suffer from such an action. Changing the floor also means dirt and dust choking filters, depositing on all surfaces and in the air ducts. There is only one chance to select and make the floor, and it has to be done correctly the first time.
From the ground up. The 2-dimensional perception of the floor brings a second inconvenience: the quality of an installed floor is mostly judged by its superficial appearance. The facts are:
- About 80 percent of claims related to floors are caused by improper subfloor preparation.
- The imperfections of the subfloor have a tendency to "migrate" upward in the floor structure.
A too dry, porous concrete slab will lead to the "burning" of the levelling compound layer, which will result in the adhesive curing too quickly, thus causing insufficient adhesion. A floor covering with poor adhesion will be hypersensitive to concentrated loads, shear stress, heat and humidity. All these effects ultimately will be visible on the surface.
Omission of sanding and vacuuming the self-levelling layer will not only cause a "grime-through" of small imperfections but again, adhesion problems might arise.
The right type of adhesive, the right mixture of the resin components and the way they are applied are other critical points. Small mistakes, like spreading the adhesive with an improper blade, leaving the already spread adhesive "open" too long and so on, may cause serious loss of adhesion and of electrical conductivity, in the case of ESD-control floors. (The resistance readings can exceed the required or standard values by 1-10 megohm.)
Installing a floor is a question of craftsmanship. It is wise to entrust the job to installers who are trained and qualified by the floor material producers themselves.
Standards, tests and data sheets
Flooring features are explained by the manufacturer in a data sheet. The values specified are obtained through standardized tests. Materials of different classes cannot be compared, and materials of the same (or similar) classes have similar data. They will have indentations and abrasions within 0.05 mm (which is negligible in practice), all of them will show the highest fire resistance ratings, they will resist-more or less-the same chemical agents. Are these test realistic? Do the data sheets by themselves give us all the answers? Hardly.
Laboratory tests vs. real conditions. Let's take as a first example the indentation test. A standardized weight is placed on the flooring sample, left there for a certain period of time (1 to 24 hours), then removed. The material is left unloaded for relaxation for another period of time. Finally the residual indentation is measured. In real conditions by contrast, the load is either transitory (moving vehicles), or stable at the same place for much longer periods (months, years). After such a long time, the maximum indentation (under load) would be sensible data-but there is no such standardized test.
Abrasion tests do not reflect reality any better. Under the same conditions, a so-called "cushion vinyl" floor material (excellent for domestic use) will show less abrasion than a much "tougher" industrial vinyl floor-only because the higher plasticiser content of the former makes it deflect under the test wheel. However, this excellent abrasion test result alone does not make a cushion vinyl floor suitable for industrial purposes.
Another example is the chemical resistance test. A specific quantity of the reagent is placed on the surface of the flooring sample and covered with an hourglass for a period of time. Next, the eventual trace is "cleaned" (usually with a damp cloth) and the sample is visually examined. If the stain (damage, etc.) is still there, the sample is classed as "non-resistant" (with no further attempts to remove the stain). Who would actually cover spilled chemicals for 24 hours and then try to remove the stains only with a damp cloth the next day?
End-users can resolve this problem rather simply: ask for a flooring sample from the manufacturer/dealer, put it under realistic stress, such as spilling on it the chemical agent used in the facility, in the concentration and quantity that could happen during regular operations. Clean or repair the flooring sample as recommended by its manufacturer, and judge the results.
In "clean" environments electrostatic protection is a common requirement. An ESD-control system in most applications also includes a conductive/dissipative flooring. Conductivity is achieved by two main principles:
- The floor covering contains a migratory additive, which uses the humidity of the room air to form a conductive film over the surface of the floor itself. As a result, conductivity is mostly horizontal across the surface.
- The flooring material contains conductive particles and the layer immediately under it (adhesive, primer, etc.) is also conductive. In this case the conductivity is "vertical," through the material to ground.
The electrical resistance of the first type of flooring material (with the additives surfactants), should be measured "surface to surface," while (that with the conductive particles), should be measured using a "surface to ground" or "surface to groundable point" configuration.
The standard test methods are in this case realistic and reliable, but the user has to know which measuring principle is appropriate.
Outgassing and particulating. Today flooring materials for cleanrooms are tested in two ways. Some years ago, CVCM (Collected Volatile Condensable Materials) type tests, such as ASTM E-595, were first used. This method uses a closed vacuum chamber and a relatively high temperature. These tests had been originally developed for measuring the mass loss of satellites and contamination collected on their optics (mirrors) as happens in open space. Cleanrooms on earth, by contrast, have ventilation and ambient temperature. Besides, this test is only quantitative, not qualitative. A substance that is indifferent for optics might greatly disturb a magnetic or an electronic device - or vice versa .
Test methods under consideration (both by IEST and ISO) will probably use a different principle: dynamic headspace, thermal desorption, gas chromatography, or mass spectrometry.
One more thought about outgassing: auxiliary materials (e.g. adhesives, hardeners etc.) should be checked before acceptance of a system: during installation they might contaminate the clean environment before it is completed and ready for start-up.
For airborne particle counting and room classification, standards are available. It is not yet clear how a floor covering should be tested to see if-and what kind of-particles it releases to the environment. A simple experiment can provide a first practical approach-although it is far from being scientific. Imitating the tensile strength test, pull on narrow strips cut from some flooring materials. It takes a different force to tear them apart (if possible at all). It is already an indication of how easily a material will release particles (higher or lower adhesion/cohesion between the particles).
Think for yourself
The following conclusions can be drawn:
- The technology race impedes standards writers to follow in real time.
- Applications are so different that it is impossible to find a separate standard for each
- Some standards/test methods have only a distant relation to reality
- Standards cannot answer all the questions
- Data sheets based only on standard test methods and results do not differentiate much between materials
As a consequence, users should make their own choice depending on the combination of features important to them.
Close cooperation with manufacturers can bring better results.
The materials most frequently used in clean room constructions are: metals (stainless steel and aluminum), synthetic sheet and tile floor coverings (vinyl and rubber), coatings (or poured floors, epoxy or PU resins) and high pressure laminates. Ceramics are a rare exception, and the disposable cleanroom mats are generally not considered as floor coverings in the classic sense, so they will not be discussed in detail.
Manufacturers' opinions and recommendations will be different from ours and also different between each other.
Flooring vs facility costs. Generally speaking, the relative cost of the floor covering decreases when the level of technology used for the construction, and used later in the facility, increases.
In an average family house the floor covering (material plus installation) will make up 2.0 percent of the total cost. In an office building, this will be about 1.7 percent .
In high-tech construction, the cost of the floor covering can be as low as 1.5 percent of total cost (the price of eventual access panels is not included here).
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László Kende is product manager of conductive/dissipative floor tiles for Forbo-Giubiasco SA.
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Looking at the cross section of the torn sample we can notice if the surface is uneven (brittle material) or if it has significant traces of certain components (such as carbon, for certain ESD-control floors. The above described feature becomes a real issue in the case of access floor panels used in laminar flow areas: not all flooring materials resist perforation without "chipping"; the cross section of the floor is largely exposed; the strong airflow enhances particulation and brings the contaminants along.
Cleanroom flooring:
the standards
- Today there is no internationally accepted special standard for cleanroom flooring.
- ISO and IEST started to work on one
- Other standards apply indirectly.
- ISO 14644-series
- US 209E
- SEMI F21-95
- ASTM F-1227-89
- ASTM E-595-90
- IEST-RP-CC018.2
- IEST-RP-CC022.1
- ANSI/ESD 7.1
- ESD STM 97.1-97.2
- ESD DS20.20
- ESD ADV-2.0
ESD management starts with the floor
by Ir. Rientz W. Bol
Bolidt Synthetic
Products & Systems
The floor is the surface on which business activities take place. The floor has direct contact with everything on top of it. Therefore, allowing static electricity to flow to earth via the floor is logical and practical.
Organizations where ESD management is a major issue choose to have conductive flooring in all production areas and laboratories. The reason for this is operational flexibility. When the floor is the right one everywhere, it is easy to expand cleanroom areas, laboratory areas or sensitive production areas. If an organization wants to enlarge an ESD protected area it is simple to move furniture and equipment. However, if the floor is not right, relocation becomes difficult. First a conductive floor has to be installed before the new ESD protected area can be used. If the floor is already conductive, cleanroom, laboratory and production area expansion can be done quickly and efficiently.
The most demanding industries are producers of electronic devices, producers of sensitive equipment, users of sensitive equipment and environments with danger to explosion and/or fire.
With this background in mind, suppliers of all kinds of floorings are continuously developing systems to avoid the malfunctioning of these devices caused by static electricity. One solution is in situ installed, liquid flooring systems based on conductive synthetic compounds. State-of-the-art systems which have recently been developed show that these systems can even reach resistance properties of less than 107 ohm or even less than 106 ohm while also securing the lower limit.
Ir. Rientz W. Bol is director of Bolidt Synthetic Products & Systems in the Netherlands.
by László Kende,
Forbo-Giubiasco SA, Switzerland