Coatings Clinic: electrical properties 1--liquid paints.

The electrical properties of liquid paints govern their ability to be sprayed effectively with electrostatic equipment. The main property of interest is the ability to conduct electric current, electrical conductance, but what we normally measure is resistance or resistivity. Resistance is the reciprocal of conductance and is the property of opposing the passage of electric current. The magnitude of measured resistance depends on the composition of the material and the dimensions of the detector. Units are ohms or megohms (1 megohm = [10.sup.6] ohms). Resistivity, also called specific resistance or volume resistivity, is the resistance between opposite faces of a centimeter cube of the liquid. Units are ohm-cm or megohm-cm. If the paint has too low a resistance (is too conductive) overcharging will occur and the spray equipment may short out. Overcharging also can cause "wrap back" of paint onto the gun and even onto the person doing the spraying. If the paint has too high a resistance (conductivity is too low), transfer efficiency will be low, perhaps even lower than conventional air spray.

The remedy to these problems may seem simple: measure the resistance of the paint and adjust as necessary. Unfortunately, this is not easy to do. Accurate, precise measurement of the resistance of coatings is difficult and small amounts of impurities (especially water) greatly reduce resistance. Resistance of liquid coatings can be lowered by addition of polar solvents, but adding nonpolar solvents only raises resistance slightly if at all. Finally, the coatings industry uses the terms resistance and resistivity interchangeably even though this is incorrect and causes a great deal of confusion. Many tables for solvents are labeled resistivity, but are in units of megohms (resistance). In most cases, the values are for resistance. If alcohols are listed as being in the region of 0.02 to 0.2 and hydrocarbons as >20, then the table is in terms of resistance. If the values are 100 to 130 times higher, then they are for resistivity.

Resistance or restivity can be measured with a number of probe and meter combinations, but the only standard test of which I am aware is ASTM D5682, "Electrical Resistivity of Liquid Paint and Related Materials." This method originally was based on the Ransburg paint resistance tester commonly used to evaluate paints for sprayability, but could be done with any concentric cylinder probe and resistance meter combination. The repeatability (within-laboratory precision) for this method is 12%, which is barely acceptable, but the reproducibility (between-laboratory precision) is a disappointing 70%. The poor reproducibility probably is due to water pick-up and other contamination of the interlaboratory test samples when they were shipped to the cooperators and handled by them. This is a very real problem, however, and could happen any time that solvents or paints are transported and stored.

Depending on the spray gun, a paint resistance of 0.04 to 1 megohm usually is considered acceptable for good electrostatic spraying. All paint components affect resistance, but the solvent blend normally has the greatest influence. Polar solvents have low resistances (high conductivities) and non-polar solvents have high resistances. Although, tables of solvent resistances may be found in various handbooks and solvent supplier literature, it is better to periodically measure the resistances of solvents being used in the lab and plant. Literature values such as the ranges shown in Table 1 should only be used as guidelines or estimates.

There is another parameter that affects electrostatic spray, the dielectric constant, which is a measure of the ability of a material to store a charge from an applied electric field. Electrostatic spray droplets must be able to hold their charge in order to be attracted to the work piece and to spread properly. The dielectric constant rarely is measured or even considered in the coatings industry. Fortunately, it tends to parallel resistivity (but in an inverse fashion). Hydrocarbons have the lowest dielectric constants, followed by esters. Polar solvents have high values (see Table 1). Water has a very high value of ~ 80.

Table 1--Electrical Properties of Solvents

Solvent Type                      Resistance, Megohms  Dielectric
                                                        Constant

Alcohols ([C.sub.1] - [C.sub.4])       0.02-0.22         18-40
Glycol ethers                          0.01-0.15         10-24
Ketones ([C.sub.2] - [C.sub.5])        0.05-1.5          12-28
Glycol ether esters                     1.6-13            6-9
Esters ([C.sub.2] - [C.sub.4])          19->20            5-6
Aromatic hydrocarbons                    >20             2-2.4
Aliphatic hydrocarbons                   >20             1.9-2

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].

By Clifford K. Schoff

Schoff Associates