Static Control Flooring

Static Control Floorings: Trends in Technology
By: Peter Song and Tom Murphy, General Polymers (Published in Plant Services March 2000)

I. Introduction to Electrostatic Discharge

Electrostatic Discharge (ESD) has been an industrial problem for centuries. Today, the problem of ESD is not only a significant cause of failures within the electronics industry, it also impacts productivity, product reliability, manufacturing profitability and human beings safety in many other fields, such as petroleum and chemical processing, munitions plants, textile, clean rooms and hospitals.

Controlling electrostatic discharge begins with understanding how ESD occurs in the first place. Electrostatic charge is most commonly created by the contact and separation, or friction, of two similar or dissimilar materials. A charge generated by friction is also called “triboelectricity”, from the Greek work “tribos”, which means rubbing together. When friction takes place between two materials, an imbalance of electrons on the surface of a material produces an electric field that can influence other objects at a distance. The simple act of standing up can increase the body’s stored energy by as much as 1,500 volts. Electrostatic Discharge is defined as the transfer of electrostatic charge between objects at different electrostatic potentials caused by direct contact or induced by an electrostatic field.

Although the human body has been perhaps the most common ESD source, interestingly, a person does not feel these electrostatic discharges until the body is charged to approximately 3,000 – 4,000 volts. But, in electronics industry, even a charge of 3 millivolts/meter is sufficient to cause ESD damage.

Today, there are many ESD products in use, such as air ionizers, tote boxes, special packaging, wrist straps, footwear, table mats and floors. Most ESD control professionals agree that the first choice of defense against ESD event is to direct accumulated static charges away from humans, manufacture equipment/devices and products. This is usually accomplished through the use of equipment grounding and personnel grounding, combined with utilizing static control floors. The easiest way to achieve localized protection of products in an assembly or manufacturing environment is to ground the work surface.

Personnel grounding is usually accomplished by use of a wrist strap connected to a ground. For people walking or working in a standing position, however, this is impracticable. Static control floors, therefore, are the one most effective approach to address an entire ESD-control system. This article describes the types of static control floors available, why a static control floor can prevent an ESD event from occurring and how a facility’s owner chooses an appropriate floor system to control ESD in different circumstances. Test standards and related measurements associated with static control floorings are also described in this paper.

II. Static Control Floors

Static control flooring can be defined as a flooring system that can drain and/or dissipate static charges by grounding personnel, equipment or other objects contacting the floor surface or that controls the generation and accumulation of static charges. The resistance to the movement of electrons across the material’s surfaces defines static control floorings into the following two categories1-4 :

i) Conductive Floor has a resistance of 2.5 x104 – 106 ohms per 3 ft. It can drain static charge dissipating a 5,000 – volt charge to zero in o.o5 seconds.

ii) Static Dissipative Floor has a resistance of 106 – 109 ohms per 3 ft. It adds no static electricity to the environment and drains off a 5,000 – volt charge to zero in less than 0.2 seconds.

A conductive floor material, because it has low electrical resistance, allows electrons to flow easily across its surface or through its volume. If a charged conductive floor is grounded, or coupled to another conductive object, such as a steel pipe or post that is grounded, the charge accumulated on the floor will be uniformly distributed throughout the floor and rapidly dissipated to ground. Conductive flooring prevents the build-up of a static charge, eliminating the potential of an ESD event.

Static dissipative materials have electrical resistance between insulative and conductive materials. Like a conductive floor material, the charges generated triboelectrically on the static dissipative floor can be transferred to ground but the process of this transferring takes longer than in a conductive floor.

In terms of flooring installation, static control flooring systems can be classified into three groups:

a) Permanently installed floor materials, such as tile, carpet, polymer sheet and resinous flooring;

b) Periodically applied topical coatings on existing floor surfaces, like acrylics or waxes; and

c) A movable island of material placed over existing floors, like a plastic, or rubbery or carpet mat.

The advantages and disadvantages of these flooring systems are compared in Table 1.

 Table 1   Comparison of Static Control Flooring Systems

 

System

Advantages

Disadvantages

Tile (vinyl)

Low initial cost;

Attractive finish;

Easy installation.

Requires frequent conductive or dissipative topical treatments ;

Poor chemical resistance;

Seams collect dirt.

Carpet

Attractive appearance;

Easy maintenance;

Favorite use in offices.

Not suitable to the areas where static charge voltage is higher than 2000 v;

Non-hygenic and not durable.

Topical treatments

 

Lower material cost;

Easy to apply.

Do not have conductive features;

Worn away easily and frequent recoating is required;

High labor cost within floor service life.

Mat

Movable;

Grid patterns make less surface dirt and contamination;

Slip or skid resistance provided.

Coverage area is limited;

Resistance will change with wear;

Black is most common color.

Resinous Flooring

(epoxy, vinyl ester

urethane, etc.)

Seamless;

Both conductive and dissipative ranges;

Variable thickness for traffic condition;

Chemical resistant;

Durable and long lasting;

Low total cost per year of service life;

Easily cleaned,

Withstands wet environments.

Higher initial cost;

More extensive installation procedures;

Topcoats may scratch or scuff when abraded.

With their unique advantages, resinous static control flooring systems are becoming more applicable in industries where electrostatic discharge is a critical concern. The following sections discuss the nature of these floors and their typical applications.

III. Resinous Static Control Floorings

Resinous static control floor systems consist of liquid resin, as a binder, conductive additives, and other fillers or aggregates. Polymer scientists5,6 have found that several kinds of polymers, with a pi-conjugated structure on the polymer backbone, can conduct electricity when doped with charge-transfer agents. There are no inherent conductive resins, without adding foreign conductive materials. Therefore, adding conductive additives into a resin base is still sole approach to formulate static control resinous flooring.

Based on resistance, there are two types of resinous floors: conductive floor with a resistance of 2.5 x104 – 106 ohms and static dissipative floor with a resistance of 106 – 109 ohms. The systems can be also categorized into a thin film coating at 5 – 20 mils, a self-leveling slurry system at 60 –80 mils and a 3/16” – ¼” thick trowel applied resurfacer. These flooring systems can be formulated in a different type of base resin, such as Bisphenol A and/or Bisphenol F epoxy resin, epoxy novolacs, acrylics, vinyl esters and urethanes.

A conductive floor has a much lower electrical resistance than a dissipative floor. It will carry the static charges to a ground quickly and efficiently as to prevent accidental discharge and ignition. If the floor is too conductive, an operator on the floor can become too effectively grounded and will suffer electrical shock. In another case, frequent contact between tools and equipment, or dropping the tools on the floor, will cause spark and ignition. For those circumstances, a sparkproof conductive flooring system is highly recommended.

Dissipative flooring systems have greater resistance to electric current flow than conductive floorings. At a working environment dealing with high test voltages, such as facilities where electronic components are manufactured or assembled, a dissipative floor should be installed so that the static charges can be gradually transferred to ground, protecting personnel from electrical shock while at the same time protecting sensitive electronic equipment.

Facility managers and business owners must establish specifications for individual application in their facilities based on their needs for static control floors. Table 2 gives a brief application guide of the static control floors.

Table 2   System Selector Guide for Resinous Static Control Floors

 

APPLICATIONS

FLOORING SYSTEMS

Munitions plants

Spark-proof conductive

Storage silos

Spark-proof conductive

Manufacturing processing with highly flammable or combustible liquid,  gases or powder, such as petroleum, chemical and pharmaceutical industries

Spark-proof conductive

Other industrial facilities where spark may not be of concern but ESD needs to be prevented

Conductive

Electronics manufacturing, packaging, assembly and test areas

Static dissipative

Electronics maintenance / repair shops

Static dissipative or conductive

Data processing areas/computer facilities

Static dissipative

Clean room

Static dissipative

After determining whether a conductive or dissipative system is required for the application, consideration must be given to the types of traffic and exposure to which the floor will be subjected. In addition, the existing condition of the substrate will dictate the system to be installed. An area containing spalls, and uneven surfaces or expected to have heavy traffic will require a trowel applied static control flooring. New slabs and those in good condition which are expected to experience normal foot and/or fork lift traffic will use a self-leveling systems. For all other applications, with foot traffic only, a coating system can be utilized.

All the floor systems must be connected to a permanent earth ground to achieve a condition known as an electrical equal-potential plane(EP) in which all static charges can be dissipated in any direction. Concrete itself cannot become an EP because air voids, compositional variations, aggregates and other additives present in concrete all work against achieving uniform conductivity. Therefore, a static control floor must be connected to a true earth ground through direct, uninterrupted contact with properly prepared grounding points. Metal floor joints, metal equipment bases and steel posts or columns may be utilized if they have been tested to confirm permanent continuity with an earth ground. Generally, a minimum of one grounding point per every 1000 square feet is sufficient for proper dissipation of static charges.1

IV. ESD Measurements

There are three test standards available for the evaluation of static dissipative or conductive floors and they are ANSI/ESD-S7.14, ASTM F 1503 and NFPA 99 (56A)1 . These test methods describe three types of measurements to be taken, which are summarized7 below:

(1) - Point -to-point resistance --- Two 2.5 inch diameter electrodes, each weighing 5 lbs, are placed 3 ft apart on the floor. The resistance in ohms is read on a properly calibrated Megohmmeter.

(2) - Point-to-groundable point resistance ---- An electrode with a 2.5 inch diameter and a weighing 5 lbs is connected to a Megohmmeter and placed on the surface being tested. The other megger lead is connected directly to a groundable point on the surface being tested.

(3) - Surface resistance --- Two parallel metal electrodes of equal length and cross section are placed on the surface being tested. The distance between the electrodes should be the same as the length of the electrodes. Resistance is read on a Megohmmeter connected to the two electrodes and is expressed in ohms/square.

For quality control and lab procedures, the point-to-point test is most convenient. The measurements of point-to-groundable point test on smaller lab samples usually vary considerably from readings on a practical large floor. Based on these test results a facility manager can check if the flooring conforms to the specification when initially installed and track continual performance of the floor periodically. Providing floors with good maintenance is always the best solution to lasting service life for any type of floor. The standard of NFPA 99 describes appropriate maintenance for a conductive floor is very important to maintaining conductive property through its service life. There are four maintenance guidelines addressed,1 which are suitable for static dissipative floors.

i) The surface of conductive or dissipative floors shall not be insulated by a film of oil or wax. Any waxes, polishes, or dressings used for maintenance of conductive floors shall not adversely affect the conductivity of the floor.

ii) Floors that depend upon applications of water, salt solutions, or other treatment of a nonpermanent nature for their conductivity are not acceptable.

iii) Cleaning instructions for conductive and dissipative floors shall be established, such as a daily basis cleaning, non-abrasive brush or pads being used and requirements for cleaners, then carefully followed to assure that conductivity characteristics of the floor are not adversely affected by such treatment.

iv) The floor’s resistance shall be periodically tested to ensure it still falls the range as initially specified.

Summary

There are certain manufacturing and environmental conditions that require the use of a static control system. Addressing these conditions through the use of a floor treatment is the most effective means of controlling static discharge. When selecting the flooring materials , consideration must be given to the conditions of use including wet or dry exposures, traffic patterns and wear, chemical exposure, and impact tolerance. Further selection criteria requires consideration of the condition of the existing substrate and resurfacing requirements. At this point, a decision can be made as to the use of conductive versus static dissipative materials. After installation the floor must be properly maintained and standard operating procedures must include routine testing to insure personnel safety and minimized materials loss liability.