Bulk conductivity of paper

This paper describes some recently completed work on the electrical conductivity of paper. A reliable method of measuring bulk conductivity of paper, where the contact resistance is reduced to negligible values, has been developed. A study of the effect of some papermaking variables, such as the type of pulp, the degree of refining and the fiber orientation, on the bulk conductivity of paper is reported. Finally, an investigation has been made into the current transient phenomena exhibited by paper upon the application of an electric field. These transient currents were interpreted as the transport of ionic species within a water associated fibrous network making up the paper. 

Bulk electrical conductivity has always been controversial for paper since the contact resistance at the paper-electrode interface can be much higher than the resistance of the paper itself. This neglect of contact resistance has produced many erroneous reports of bulk conductivity of paper. 

Two important requirements must be met by a technique designed to provide accurate measurements of the bulk conductivity of paper. First, the contact resistance between the electrode and the paper should be either known or negligible and, secondly, the paper should not be significantly modified by the technique used. The in situ pressure bulk conductivity cell satisfies these requirements, as will be shown in the following sections. 

A study, similar to that described above, was made to compare the results of using optically polished stainless steel electrodes under pressure with Galium-Indium eutectic liquid electrodes in determining bulk conductivity of papers. The Ga-In alloy is believed to make intimate contact with the surface of the paper, thus significantly reducing the contact resistance. 

From these results, it may be concluded that the degree of refining has a relatively small effect on the bulk conductivity of paper. A small change in the electrical properties is also observed for different pulp types in paper. These variations are not significant when compared with the variations resulting from a change in relative water content. For cellulose the conductivity increases with water content by a factor of 1014 from 0% to 20% water content 8). 

This article has addressed a number of issues relating to the electrical properties of paper or fibrous structures. It was shown that reliable measurement methods are now available for estimating both the bulk and surface conductivities of paper. In the case of the bulk conductivity, a new in situ pressure conductivity cell was described which significantlyreduces contact resistance. The surface conductivity can be determined by the application of a modified four-point probe method first used on paper by Cronch<15). It was shown that the degree of refining has a small effect on the bulk conductivity of paper. 

Reproducible and accurate measurements of electrical conductivity of paper are of great importance in a number of technologies. Two commonly measured electrical parameters of paper are its surface and bulk conductivity. A method for measuring surface conductivity has been reported by Greismer 13, and his recommendations were incorporated in D.C. surface conductivity measurements reported by Howlett and Landheer 14. ... 

The orientation of cellulosic fibers has some effect on the conductivity of the paper. The conductivity in the XY plane of the sheet (surface conductivity parallel to most of the fibers) may be quite different from the conductivity along the Z direction (bulk conductivity perpendicular to the fibers). Comparison of surface and bulk conductivity for a given paper sheet can thus yield information which reflects the anisostropy in the structural morphology due to fiber orientation. Bulk conductivity measurements are also important since many paper sheets used in reprographic processes are composed of a conductive base sheet coated with a dielectric material 16. One important specification for these types of papers is the value of the bulk conductivity of the base paper. 

In dentistry, silicones are primarily used as dental-impression materials where chemical- and bioinertness are critical, and, thus, thoroughly evaluated.546 The development of a method for the detection of antibodies to silicones has been reviewed,547 as the search for novel silicone biomaterials continues. Thus, aromatic polyamide-silicone resins have been reviewed as a new class of biomaterials.548 In a short review, the comparison of silicones with their major competitor in biomaterials, polyurethanes, has been conducted.549 But silicones are also used in the modification of polyurethanes and other polymers via co-polymerization, formation of IPNs, blending, or functionalization by grafting, affecting both bulk and surface characteristics of the materials, as discussed in the recent reviews.550-552 A number of papers deal specifically with surface modification of silicones for medical applications, as described in a recent reference.555 The role of silicones in biodegradable polyurethane co-polymers,554 and in other hydrolytically degradable co-polymers,555 was recently studied. 

The increasing importance of multilevel interconnection systems and surface passivation in integrated circuit fabrication has stimulated interest in polyimide films for application in silicon device processing both as multilevel insulators and overcoat layers. The ability of polyimide films to planarize stepped device geometries, as well as their thermal and chemical inertness have been previously reported, as have various physical and electrical parameters related to circuit stability and reliability in use (1, 3). This paper focuses on three aspects of the electrical conductivity of polyimide (PI) films prepared from Hitachi and DuPont resins, indicating implications of each conductivity component for device reliability. The three forms of polyimide conductivity considered here are bulk electronic ionic, associated with intentional sodium contamination and surface or interface conductance. 

In the following section, a new bulk conductivity cell is described that significantly reduces the contact resistance to a level where the measurements of paper bulk conductivity can be made with an accuracy that is limited primarily by the anisotropic structure of the paper itself. A small uncertainty in the measured conductivity arises from compaction ( 10%) of the paper sample in the apparatus caused by the application of 13-8 MPa pressure to the stainless steel electrode system in the cell. This pressure is used to eliminate contact resistance. Despite this uncertainty, measurement errors in the new cell are significantly less than the spread in the conductivity values ( 200)t) determined at different points in a single paper sheet. The variability arises from inhomogeneities in the cellulose fiber network within the sheet. 

Shown in Figure 8 is a plot of conductivity as a function of the pressure applied to the paper samples. In the low-pressure range, up to approximately 5000 psi (3 1.5 MPa), the paper conductivity appears to increase linearly. Above 5000 psi (3 1.5 MPa), there is a distinct increase in the bulk conductivity, followed by another linear region which has a much steeper slope than the dependence found in the low-pressure region. A precise measurement of the paper thickness as a function of pressure 16) showed that the paper sample starts with a thickness of 70 fia prior to the application of pressure and then decreases in thickness to 50 jun at 15,000 psi (103 MPa) in an essentially linear manner. The change in conductivity shown in Fig. 8 is most probably associated with the compression of the cellulose fiber network in the paper. 

For the purpose of making relative measurements of the bulk condutivity in paper, a pressure of 2000 psi (13.8 MPa) was chosen. At that pressure, most paper samples typically compres 5 /tm, from 70 (im to 65 /W16)- As can be concluded from the discussion below, the results of measurements at 2000 psi (13-8 MPa) are in good agreement with the Ga-In liquid alloy method. Samples were contacted on both sides with Ga-In over an area of 0.8 cm2 and were placed in the pressure conductivity cell. A nominal pressure of 3-2 psi (22 kPa) was applied to the electrodes. 

Figure 9 compares the two methods for making bulk conductivity measurements on conductive base paper the pressurized stainless steel electrode method and the Ga-In eutectic liquid metal method. The conductivity is plotted as a function of voltage at 50% RH. For the stainless steel electrodes, at 2000 psi (13.8 MPa), the measured current is a superlinear function of voltage and a strong function of time 16), up to a voltage of approximately 10V. With the Ga-In electrodes this effect is noticeable only for applied potentials up to approximately 3V. Between 10 and 50V, the bulk conductivity measured by both methods is essentially independent of voltage, and the current traces are reproducible and show no time dependence. 

Figure 8. Bulk conductivity as a function of applied pressure for James River conductive paper, using 10V applied to the electrodes at 50% RH. (Reproduced, with permission, from Ref. 16. Copyright 1981, TAPPl.)...

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