GLOSS OF PAPER, 75o (TECHNIDYNE)

Introduction

The terms gloss, glare, finish, and smoothness are often used more or less interchangeably. They do, however, connote different meanings, and briefly one can make these qualitative distinctions:

Finish is a general term and refers summarily to the entire set of a paper's surface characteristics.

Smoothness refers to the absence of surface irregularities as judged either visually, by the sense of touch, or a paper's behavior under use conditions, e.g. in a printing operation. Smoothness is a surface structural property. See p. 4-13 for further discussion of smoothness and a method for its quantitative measurement (Sheffield smoothness).

Gloss is an optical property of paper. Everybody can tell a paper of high gloss from a paper of low gloss. Even an untrained person will, after brief inspection, judge a cast coated paper as having much higher gloss than a bond paper. Such a person might describe the cast coated paper's surface as "smooth, polished, shiny, lustrous, and glossy" and the bond paper's surface as "comparatively rough, matte, and dull." In physical terms, gloss is related to the manner in which paper reflects light. In our specific example, both the cast coated and the bond paper reflect a large part of the incident light diffusely. But the cast coated paper, because of its smooth and polished surface, also reflects a small part specularly, i.e., like a mirror. It is this specular component in the reflected light that creates the impression of gloss.

Glare expresses much the same quality as gloss. But whereas gloss connotes the esthetically pleasing side of the phenomenon, glare implies the psychologically unpleasant side. An example of glare is an objectionable, intense, blinding reflection which, for example, makes reading difficult.

It is clear that gloss (optical property) and smoothness (surface structural property) must be related in the sense that higher smoothness makes for greater gloss, but the relationship is by no means simple. Both notions are complex and depend on a great many other variables, so a

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unique quantitative relationship between gloss and smoothness is neither expected nor actually found.

As mentioned above, gloss is due to the fraction of specularly reflected light. This fact is exploited in the visual qualitative ranking of papers with respect to gloss as well as in the Technidyne Glossmeter (Model T480A) used here for quantitative measurement of gloss. In the first instance, the observer views the mirror image of a light source produced by the paper surface. The more perfect the mirror image, the greater the perceived gloss.

In the Technidyne Glossmeter (Model T480A), parallel light is projected onto the paper surface at an angle of 75o with respect to the normal to the paper surface and 15o with respect to the surface itself. The photodetector is positioned so as to capture and measure the specularly reflected light. It will also receive a small fraction of the diffusely reflected light. The instrument indicates the measured light intensity as a percentage of the light reflected from a black glass plate, which serves as a gloss standard.

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Significance

The papermaker has three main production techniques at his disposal to impart high gloss to paper: First, the drying of paper in contact with a highly polished metal surface such as a Yankee dryer; paper so produced is called machineglazed or M.G. paper. Second, the coating of paper; the gloss achieved will depend on the coating technique employed, the base paper used, type of filler, particle size and shape, particle orientation, and the type and amount of adhesive. Third, calendering and supercalendering. Yet another method of producing extremely high gloss is to glue thin glossy plastic film onto a paper surface.

Gloss is often desirable because of its esthetic appeal. High gloss cover papers are an example. Gloss can impart distinction and elegance to the appearance of a publication.

Printing operations depend critically on paper finish in general. Smoothness and the concomitant gloss are particularly important requirements in some situations, e.g. offset printing.

Method of Test

Press the red power switch on the instrument front panel to turn on the power to the instrument. The digital display should illuminate immediately. Place a piece of paper over the sample opening on the top of the instrument to see that the main illuminator lamp has been lighted. Before proceeding with calibration, wait 15 minutes for the instrument to stabilize.

Calibration: Make certain that the diagnostic switch on the back of the instrument is set in the OPERATE (CAL UNLOCK) position. Also make certain that the selector switch on the front panel is in the raised (75o), not the depressed (20o), position.

Of the two black glass standards to be found in the storage case, that with the yellow label on the back should never be used; instead, use the one with the white label. Place the black glass standard on top of the instrument, over the sample opening. The standard should be oriented such that the writing on the label is legible to the operator. Never touch the measurement area of the standard. Handle the standard by the edges and corners. After use, immediately return the standard to the storage case.

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With the black glass standard over the sample opening, press the CAL push button switch on the front panel. The digital display should read the assigned value for the black glass standard (at 75o) within ±0.1. If not, hold the CAL switch down and press the UP (­) or DOWN (¯) switches until the assigned value for the black glass standard is displayed. Then release the CAL switch. This completes calibration and you can begin testing your samples. With time as you test, the instrument may drift out of calibration. You can quickly recalibrate the instrument by simply placing the black glass standard over the sample opening and by pressing the CAL switch. The instrument will return immediately to the assigned value.

Tests: On 5 specimens, measure the gloss on the wire side and on the felt side of the sheet. Furthermore, for each side, measure the gloss in the machine direction and the cross-machine direction, for a total of 5´2´2 measurements.

Report

Calculate the composite average of the machine and crossmachine direction values for the wire side and another average for the felt side. Report the average values to the nearest unit for the wire and for the felt sides. Also report 95% confidence intervals.

References

ASTM D 1223

TAPPI T 480

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