Assessing the Influence of Electrostatic Charge Retained on Materials

John Chubb

Measuring both the surface voltage and the quantity of charge transferred can produce meaningful measurements from triboelectric studies.

Illustration by TAISHA PAYTON

Static electricity arises when surfaces in contact are separated. If the charge that arises from differences between the surfaces cannot run to earth quickly enough, then it is trapped—it is static. The "quickly enough" relates to the time for the charge to spread out over the surface of a material or to leak away to earth. If the time for charge movement is very short, the material is described as a conductor; if the time is very long, then the material is described as an insulator. This article focuses on materials on which charge does not move easily over the surface. These materials include many plastics and composite structure materials, such as cleanroom garment fabrics.

Electrostatic charge retained on the surface of a material creates a potential at the surface. This potential is an important parameter to useful applications of static electricity. It is also a critical element in many of the risks and problems that static electricity can cause. It is the surface potential that creates the electric field at nearby items; creates forces of attraction of dust, dirt, and thin films; induces charge; and causes electrostatic discharges. When the material is a conductor, such as a metal, it is well understood that reliable earth bonding is necessary to avoid problems. For nonconducting materials, or those that have poorly conducting surface features, the solution is more complicated. Two important features are the surface potentials that may arise and the length of time that significant surface potentials are present.

The quantity of electrostatic charge transferred to materials by actions such as rubbing and sliding is limited by the intensity of the mechanical operation (speed and pressure) and by the character of the materials involved. The important factor is the maximum surface potential that will arise for the maximum quantity of charge likely to occur in practice. Surface potentials will be limited to a low value if the time for charge dissipation is short compared with the time at which the rubbing surfaces separate. The value will also be low if the charge on the material experiences a high capacitance because this high capacitance suppresses the surface potential.

Both of these factors can be used to determine or to limit the surface potentials likely to occur in practice. Therefore, they also determine whether materials will be suitable for particular applications or will give rise to risks or problems. These aspects are relevant to a wide range of electrostatic topics, such as attraction of atmospheric dust and dirt, cling of thin films, risks of ignition of flammable gases, and indirect damage to semiconductor devices through induced charges.

Assessment of Materials

The logical way to assess the ability of a material to dissipate static charge and to show the capacitance experienced by charge on the surface is to measure the initial peak surface voltage created and note the rate at which the voltage falls as the charge moves away.

A suitable approach would be to rub the surface of a material, quickly remove the rubbing surface, and then observe without contact how quickly the surface voltage created by rubbing decreases to a low value. A variety of materials have been tested using this scuff-charging approach.^1,2 Decay times less than 1/4 second are needed to limit surface voltages to low values in rubbing actions.

A basic problem with traditional tribocharging studies has been that the voltages observed vary from test to test, depending on the pressure and speed of the rubbing action. Measuring the quantity of charge transferred to create individual values of initial peak voltage provides a way to make much more consistent and meaningful measurements. The quantity of charge to achieve a certain surface voltage has the dimension of capacitance.

With simple materials, this capacitance is shown to be independent of the quantity of charge transferred. This independence provides an opportunity to bring some consistency and operator independence to tribocharging studies. A measurement method has been developed for these simple materials.

A surface charge can be expected to experience a capacitance. This capacitance will be partly from the spatial distribution of the charge itself, partly from the influence of nearby earthy surfaces, and partly from the influence of the dielectric constant of the surface material. This article does not address the details of the area and distribution of charge. Rather, the term capacitance loading is used in this article to define the ratio of the capacitance experienced by the charge on the test material compared with the capacitance that a similar distribution of charge would experience on a thin layer of a good dielectric. This definition is key to applying the technique presented in this article. A method to measure the neutralization time of certain materials under certain conditions is provided.

As an experimental technique, the scuff-charging approach has provided some useful insights into the tribocharging behavior of materials (see Figure 1). However, another approach is more suited to practical testing. Such an approach uses a high-voltage corona discharge to put a local patch of charge on the surface. An electrostatic fieldmeter is used for noncontact measurement of both the initial peak voltage and the rate at which it falls away.³

Figure 1. Simple tribocharging studies. (Charge transfer from initially neutral Teflon rod is measured using a Faraday pail.)

Figure 2 illustrates the practical arrangement in which a small cluster of corona discharge points is mounted on the underside of a light movable plate with an electrostatic fieldmeter above. After a corona pulse of a few kilovolts for 20 milliseconds, the plate is moved away within 20 milliseconds, and the fieldmeter observes the initial peak voltage generated by the charge deposited and follows how quickly the charge decays. Measurement can also be made of the quantity of charge transferred to the sample. Figure 3 shows an example of a number of charge-decay curves observed for a paper card. Good reproducibility can be achieved with even fairly low-level signals and fast charge decays by use of a novel stutter-timing approach.

Figure 2. Arrangement for corona charge-decay measurements.

Figure 3. Example of four corona charge-decay studies on 160-g paper. Curve shows variation of surface voltage with time (LH axis). The short lines show how local decay time constants vary during charge decay (RH axis).

The effective capacitance experienced by the charge on the surface is calculated from the quantity of charge transferred to the test surface and the initial peak surface-voltage potential created.

The ability of materials to dissipate triboelectric charge, generated on their own surface by contact or sliding actions, is well matched by the decay of charge deposited from a high-voltage corona discharge.

It has been shown that there is no match between measurements of resistivity and corona charge-decay times.⁴ In addition, the federal test standard method of measuring charge decay (FTS 101C Method 4046) is not suitable for assessing the self-dissipation capability of materials.^5,6 Although a number of standard test methods are available for assessing materials, few have been shown to match practical experiences of charge dissipation or to be suitable for use with a wide variety of materials.

No single approach can answer all questions, and it is important to check methods of assessment against the requirements of applications. For example, for certain materials, spark discharges can occur. For some materials, one side of the material can be shielded from transient electrostatic events (such as sparks) that are occurring on the other side.

Findings

Using this experimental technique, several major points have emerged from measurements. Many simple plastics (e.g., polyethlene sheet and polyester fabric) show low capacitance-loading values, near unity. High voltages are easily generated on such surfaces by rubbing actions.

Capacitance-loading values are fairly independent of quantity of charge. In cases in which capacitance loading is low and a fast charge decay is necessary to avoid problems from static, the charge-decay time needs to be less than 1/4 second.

Fabrics for cleanroom garments are made primarily from polyester. Conductive threads are usually included in the fabric (in stripe or grid patterns) to control static charge on garment surfaces. These threads work principally by providing a high capacitance loading that can suppress surface voltages to low levels even when charge-decay times are long.⁷

Recent experiments have shown a relationship between the voltages that can arise on the surface of inhabited cleanroom garments and the results of corona-charging measurements on sample areas of the garments. In this work and for the particular range of garments tested, it is clear that the parameter of main significance is not charge-decay time but rather capacitance loading. Resistivity is clearly not relevant.

Simple papers (such as newsprint) show charge-decay times that decrease and capacitance loading values that increase with increasing humidity. With more highly finished papers (such as for photocopying), charge-decay times are fairly independent of humidity, but capacitance loading values increase more strongly than for simple papers.

Conclusion

Static electricity has many useful applications, but it can also create many risks and problems.^8–11 The surface potential created by static charge and retained on materials is the parameter of primary practical importance. The potential depends on the quantity of charge transferred to materials and on the character of the materials involved. The maximum surface potential that will arise then depends on both the time for dissipation of charge compared with the time at which the rubbing surfaces separate, and the capacitance experienced by the charge on the material.

Either or both of these aspects can be used to determine or to limit the surface potentials likely to occur. Therefore, they also determine whether materials will be suitable for particular applications or will give rise to risks and problems. The term capacitance loading was introduced to define the ratio of the capacitance experienced by the charge on the test material compared with the capacitance that a similar distribution of charge would experience on a thin layer of a good dielectric.

Measurements of charge-decay time and capacitance loading need to be made using appropriate test methods. Measurements based on the use of corona charging have been shown to provide the basis for instrumentation suitable for a wide variety of materials. Such instrumentation provides results that match well to results observed with tribocharging.

References