The cutting-edge technologies and sophisticated manufacturing techniques used in the telecommunications, wafer-fabrication and aerospace industries must employ world-class ESD-control programs to maintain their high product yield. These businesses make products that need protection from static-charge levels of <200 V. And only an ESD-control program that starts at the floor eliminates static charges at these levels.
Fortunately, several flooring products including epoxy, carpet, tile, roll flooring and floor coating will help you meet the lower levels of static charge. However, the installation process for each type requires careful planning and preparation of the subfloor for best results.
Typical subfloors are concrete or wood. A wood subfloor should be installed with plywood that meets the underlayment grade specified by the American Plywood Association. The wood subfloor must be dry, smooth, free of vertical movement, horizontal expansion, moisture, oil, dirt, grease and waxes.
The plywood must be installed over cross joints staggered at least 16″ apart. Fasten the plywood to the old subfloor with ring-grooved or screw nails that penetrate 1-1/4″ deep. The nails or screws must be spaced 6″ apart throughout and 3″ apart along the plywood edges. To eliminate irregularities, nail from the center of the area and proceed outward toward the edges.
Leave approximately 1/32″ spacing between the boards. The fasteners should be driven flush with the surface or countersunk no more than 3/64″. Lightly sand any surface roughness, particularly at the joints and around the nails.
Wood flooring types not recommended include tempered and untempered hard board, particleboard, wafer board, strand board and fire-retarded plywood. These products vary in quality and can affect the top flooring.
The best concrete subfloor is a heavyweight type that meets the American Concrete Institute’s 302 Class 4, 5 and 6 specifications. It should have at least a 1″ topping of a heavyweight concrete.
Whichever subfloor you use, it should be level, dry and clean; and be free of depressions, oil, grease, dirt and sealer. The wood subfloor must be smoothed by planing or sanding, and all nails should be countersunk or removed.
For concrete subfloors, patch all cracks and clean and fill control joints with a leveling compound. An adhesive-bond test also should be performed on the concrete to verify the bond is solid and no moisture is present. Concrete that is porous, gritty or dusty must be coated with a primer recommended for your flooring system.
Other permanent floor types such as terrazzo, quarry tile, ceramic tile and marble also may be used as a subfloor. They must be level and free of paint, wax, old adhesive and varnish before a bonding agent and the top flooring system are applied.1
If you use heavy-duty washing agents or strippers to clean the subflooring surface, rinse it adequately to remove all residue, said Lawrence Lepore, vice president of sales for Walter G. Legge. Cleaners and strippers can adversely affect the adhesion of bonding agents.
On-grade subfloor surfaces also should be free from pressure caused by ground waters, continued Mr. Lepore. Water vapor may build up under the substrate and cause it to fail prematurely.
If the moisture vapor transmission rate through the subfloor is greater than 3 lb/24 h/1,000 sq. ft, the adhesion of most flooring materials to the substrate will be adversely affected, said Stephen Koehn, flooring product manager for 3M Electrical Specialties Division. Many companies offer waterproofing for wet floors and provide warranties for up to 20 years. The cost to waterproof a subfloor varies, ranging from several dollars per square foot to as much as $20 per square foot.
Depending on the condition of the subflooring, shot blasting, sand blasting, scarifying, etching and other forms of surface preparation may be necessary to adequately prepare the surface, said Mr. Lepore. New concrete surfaces are commonly neutralized with an acid etching to remove alkalinity caused by calcium carbonate found in mortar and concrete.
Once the subfloor is properly prepared, you must ground the flooring system. Concrete, for example, does not satisfy the requirements for either dissipative or conductive floors as established by ANSI/ESD S7.1 Floor Materials—Resistive Characterization of Materials or NFPA 99 Health Care Facilities. It has air voids, compositional variations and aggregates preventing the necessary uniformity of conductance.
An earth ground must be established and an equipotential coupling device be used to connect the flooring system to ground. Each ground should be tested and verified by an electrician or engineer.
Grounding for floor coatings, tiles and carpeting is accomplished in a similar manner. Here is one recommended procedure:
Drop a #10 or #12 gauge wire inside a wall close to a ground bus, so the wire emerges at the junction of the floor and wall.
Cut a small hole into the drywall or chip into the concrete subfloor.
Intertwine or solder a copper strip approximately 2″ W ´ 6″ L ´ 0.012″ thick to the copper ground wire.
Liberally apply conductive adhesive, completely covering the connected wires at the wall junction.
Lay flat the remaining copper strip in the conductive adhesive on the floor and cover with additional adhesive.
Another grounding method uses the exposed steel columns supporting a building. The copper grounding strips are laid flat on the floor in the conductive adhesive and several inches of the copper are extended up the column and secured with a machine screw (Figure 1).
All concrete pads should be electrically connected across expansion, control and isolation joints by placing a copper grounding strip across each joint. The copper strip should be 2″ W ´ 6″ L ´ 0.012″ thick and laid flat or shaped in a V pattern across the joint. It may be necessary to grind the floor so the copper strip is flush to the concrete surface, ensuring that the top coat is even and smooth.
Carpeting is installed in a manner similar to a floor-coating system. But carpeting uses a grid network of conductive fabric tape laid in glue to electrically contact the back of the carpet. These strips of tape are then joined with a single run of tape near the longest wall and connected to ground.
Once the grounding system is in place, install the carpet, epoxy, tile or roll flooring systems. Each system has its specific procedures and you should follow the manufacturer’s installation advice. For example, with some coatings or epoxy, resistance testing must be performed after each step to ensure the electrical resistance meets your conductive or static-dissipative needs.
The resistance checks are performed with a calibrated megohmeter. The measurements are made with two five-lb electrodes placed 36″ apart and 100 VDC applied to meet the ANSI/ESD S7.1 testing requirement.
The standard requires you to record the date, temperature, humidity, test voltage, duration of the test and the equipment used. It also states that you should take five measurements per floor surface, or a minimum of five tests per 5,000 sq ft. At least three of the five tests should be taken in areas subject to wear, chemical or water spillage.
What About Maintenance?
It is important to maintain your ESD-control floor and to periodically test the surface resistance to ensure that insulative contaminants such as dirt and grime do not build on the surface, said Algis Norkevicius from Charleswater. Testing either point-to-point or resistance-to-ground will indicate if the floor finish needs surface maintenance. However if the surface is clean and has high resistance readings, the floor finish is probably too thin and needs replenishing with a product such as a dissipative floor finish.
For epoxy and vinyl tile, sweep or dry mop daily with a mop head that is free from oils, waxes or chemicals, said Mr. Koehn. Also, scrub as needed to remove scuff marks and other blemishes that affect the resistance values of the floor.
Carpet requires daily vacuuming for moderate to high traffic areas, less for low traffic areas, continued Mr. Koehn. Carpet also should be shampooed every three to six months for moderate to high traffic areas and once per year for low traffic areas. Refer to Table 1 for examples of maintenance procedures and upkeep costs for different flooring systems.
A maintenance program for a low traffic tile and epoxy floor area consists of using a dust mop or push broom nightly to remove accumulated dirt, said Mr. Norkevicius. A damp mop may also be used with a diluted dissipative floor finish.
Floors with moderate to heavy traffic need to be cleaned, have black marks and scuffs removed and be dry-burnished weekly, continued Mr. Norkevicius. Dry burnishing should be done at 1,000 to 2,000 rpm. Also, spray and buff the floor weekly with a diluted floor-finish product. For best results, use a buffing machine at 175 to 1,500 rpm.
The maintenance required for your floor system will vary greatly depending on its design, formulation and type of traffic. Keeping a clean floor, however, is the most important principle and a significant step toward maintaining an effective ESD-control program.
Reference
1. The Norament Flooring System, Installation Guide, Freudenberg Building Systems, February 1996.
Table 1.
Product Type |
Maintenance ($/sq ft/yr) |
Maintenance Procedure |
Estimated Cost/Yr (10,000 sq ft) |
Epoxy (no wax) |
0.10 to 0.15 |
Dry and damp mop as needed with neutral pH cleaner. Scrub with low-speed buffer to remove scuff and tire marks with nonabrasive pad. |
$0.125 sq ft/yr = $1,250/yr |
Vinyl Tile (matte finish, no wax) |
0.10 to 0.15 |
Dry and damp mop as needed with neutral pH cleaner. Scrub with low-speed buffer to remove scuff and tire marks with nonabrasive pad. |
$0.125 sq ft/yr = $1,250/yr |
Vinyl Tile (high luster finish, no wax) |
0.20 to 0.25 |
Dry and damp mop, buff at 175 rpm to 300 rpm with commercial pad. |
$0.225 sq/ft/yr = $2,250/yr |
ESD Carpet |
0.30 to 0.40 |
Vacuum moderate and high traffic areas daily, weekly for low traffic areas. Shampoo high and medium traffic areas two to four times per year and once a year for low traffic areas. |
$0.35 sq ft/yr = $3,500/yr |
Tile (needs wax) |
0.45 to 0.65 |
Apply three to five coats of ESD conductive sealers or wax three to four times/yr. Dry and damp mopping followed by buffing is required. |
$0.55 /sq ft/yr = $5,500 |
Floor Finishes, Coatings, Polishes |
0.45 to 0.65 |
Apply three to five coats of ESD conductive sealers or wax three to four times/yr. Dry and damp mopping, followed by buffing, is required. |
$0.55 /sq ft/yr = $5,500 |
ESD Flooring Products
Floor Finish
Dissipates Static Charge
Statguard® Floor Finish dissipates static charge, prevents triboelectric generation and provides a clear, high-gloss finish that resists wear. The liquid emulsion is applied to hard surfaces or sealed flooring including vinyl, vinyl asbestos, linoleum, rubber, asphalt, sealed or painted wood, terrazzo and concrete. It can be used in clean rooms, electronic manufacturing, assembly and test areas with a relative humidity range from 30% to 65%. Charleswater, (617) 821-8370.
Rubber Flooring Generates
Noraplan Mega AL is a static-dissipative rubber floor covering with a random pattern of color flecks. It resists oil and grease and generates <20 V of static charge. The covering is free of PVCs, asbestos and halogens. It is slip-resistant and needs no waxing. Eight colors are available. Freudenberg Building Systems, (800) 332-NORA.Vinyl Flooring Provides
The VS Smooth, VF Fleck and VE Embossed versions of vinyl flooring provide <15 V of static generation and a resistance-to-ground ranging from 5 ´ 106W to 8 ´ 107 W . The flooring resists stains and chemicals and can be heat or chemically welded together for clean-room environments. It is 0.080″-thick and is available in black, white and gray. The flooring is supplied in sizes from 4’ ´ 6’ to 6’ ´ 60’. Plastic Systems, (508) 485-7390.Epoxy Topcoat Is
Self-Leveling
ConducTop SL 1313 is a seamless, self-leveling epoxy floor coating. It is the top coat of the company’s three-part flooring system. It provides conductive or dissipative qualities when placed over concrete, vinyl tile and similar surfaces. The 40- to 50-mil-thick coating has greater compressive, flexural and tensile strength than concrete. It is available in beige, tan, light gray and dark gray. Sauereisen, (412) 963-0303.
Epoxy Flooring Installed
With Selected Thicknesses
The 8900 ESD Epoxy Flooring System is a multilayer liquid epoxy system offered with electrical properties ranging from conductive to static dissipative. It is available in thicknesses ranging from 8 mils to 125 mils. The epoxy is resistant to chemicals used in the electronics industry and can be applied over new or old concrete floors. The colors include off-white, light gray, oyster gray, tower gray, sky blue and country blue. Typical resistance to ground at 30% RH ranges from 1 ´ 105 W at 10 mils thick to 6 ´ 108 W at 90 mils thick. 3M Electrical Specialties, (800) 665-7862, ext. 74.
Static-Dissipative Tile
Has <300 V Static ChargeStatmate is a 1/8″-thick static-dissipative vinyl tile that contains conductive elements distributed throughout to provide tile-to-tile conductivity. It has a floor-to-ground resistance ranging from 106 W to 108W . Static generation is <300 V at 20% RH when tested with personnel conductive footwear. Applications include electronics manufacturing, assembly and test areas, clean rooms and raised or access flooring. It resists abrasion cracking, chipping and permanent indentations from loads up to 250 psi. VPI, (414) 458-4664.Water-Based Coating
Is Conductive
Elimstat LX is a one-part water-based conductive coating that can be applied over any surface. The acrylic latex formulation resists chemicals and abrasions, and provides a resistance of 104 W . The color selection includes green, gray, brown, black and maroon. Walter G. Legge, (914) 737-5040.
Chemically Resistant Flooring
Generates <15 V Body VoltageChemi-Cote ESD floor coating controls electrostatic discharges and resists chemicals. The uniformly dissipative surface keeps body-voltage generation to <15 V. Voltage decay is 5,000 V in <0.1 s @ 72°F and 12% RH throughout the coating. The coating can be applied over nonconducting floor surfaces, including concrete and existing static-control tiles. Garland Floor, (800) 321-2395.Copyright 1997 Nelson Publishing Inc.
February 1997