Method Development for Contactless Resonant Cavity Dielectric Spectroscopic Studies of Cellulosic Paper

This method provides an objective, machine-repeatable alternative to performing traditional fiber analysis of paper, and it exceeds the metrological limits of traditional fiber analysis. Unlike traditional fiber analysis, it is quick, contactless, nondestructive, and requires no specialized training, nor is it dependent on the visual acuity of an individual analyst. Record all manufacturing information provided with the ream of paper, such as basis weight, manufacturer's advertised post-consumer waste content, and manufacturer's advertised brightness.

Wear gloves, and perform handling with tweezers to avoid creasing and contaminating the paper samples. Take an average of 10 thickness measurements along a sheet from the ream, using a caliper. Identify the machine and cross directions of the sheet.

Using a protractor, identify and cut the paper along the desired strip angle between the machine and cross directions. Using a rotary cutter, slice test strips 0.5 centimeters wide by eight centimeters long in the target orientation for the sample. Label samples from one end, and store between glass microscopy slides.

Store under nitrogen atmosphere until testing. Perform accelerated paper fade testing as described in the text protocol. Cut sample strips out of the aged paper samples to fit the resonant cavity.

The typical specimen area is 0.5 centimeters by eight centimeters. The resonant cavity testing fixture consists of an air-filled standard WR-90 rectangular waveguide. Record the temperature and relative humidity.

Take the initial reading of the quality factor, Q-naught, and the resonant frequency, f-naught, of the empty cavity. Now, position the specimen secured in the sample holder above the slot at the center of the cavity. During the measurements, the specimen is inserted into the cavity through the slot in steps of increasing volume, where volume is equal to the height of the inserted sample times the sample width times the sample thickness.

Using the Vernier caliper on the sample mount, insert the sample into the cavity by 50-micron increments. Take readings of the quality factor and resonant frequency at each step until the sample has been lowered 10 millimeters into the cavity. Retract the sample from the interior at the same increments of 50 microns.

Take readings of the quality factor and resonant frequency until the sample has been fully retracted. Store the sample between glass slides, and return them to nitrogen atmosphere. The dielectric loss of the paper samples is obtained from the slope of the perturbation.

The cut orientation of the test sample influences the magnitude of the dielectric response. A comparison of dielectric response by strip angle is shown for virgin, as-received, blue, 24-pound office papers before and after UV fading for 169 hours. Strips cut from non-orthogonal angles along the paper sheet have provided the greatest resolution between paper types, particularly at the 45-degree and 60-degree orientations.

Dielectric loss profiles of cotton-containing bond papers procured by the U.S.Federal Government using 60-degree strips show that different concentrations of cotton fiber can be distinguished. The results of testing 100%cotton bond paper procured by the Federal Government were compared at 46%relative humidity and 49%relative humidity. After 169 hours of UV-accelerated aging, the degradation of the cellulose polymer is discernible as the average dielectric loss values had decreased for both the virgin and recycled varieties.

It is notable that the technique can distinguish between the virgin and recycled materials even after the accelerated aging period. A contour plot-based linear regression fit shows the dielectric loss of white office copier paper based upon the manufacturer's advertised brightness and recycled waste paper content of the analytes. The most important thing to remember when attempting this procedure is to maintain consistent laboratory temperature and humidity conditions.

This method has great promise and direct utility not just to the pulp and paper industry but also to secure credential manufacturing, forensics, customs enforcement, and archaeology and conservation science.