Opacity of Paper

OPACITY OF PAPER

Introduction

Opacity is conveniently thought of as the ability of paper to hide printing on its reverse side, or on sheets underneath. In other words, the greater the opacity of a paper, the smaller the showthrough of print from the sheet below.

If all else remains constant, the opacity of a sheet will increase with higher basis weight, addition of a mineral filler, lower degree of bleaching, lower density, less beating, less wet pressing, and coating of the sheet. The amount and type of filler added to the furnish has a profound effect on opacity. Opacity is generally measured with opacimeters which use the socalled "contrastratio method." This method is based on measurement not of transmitted light, but of reflected light. Opacity values range from a maximum of 100% for perfectly opaque papers (board) to nearly 0% for transparent ones (glassine).

Significance

The determination of opacity is of vital importance to both the manufacturer and the consumer. Lack of opacity means too much show-through of the print underneath, and this interferes with readability. Complete hiding of print underneath is usually impractical and too expensive, and so a small amount of showthrough must be tolerated. The consumer and manufacturer must cooperate in specifying and supplying papers with practical and reasonable opacity limits.

Definitions

Two slightly different measures of opacity have been defined and are widely used in the paper trade: TAPPI Opacity and the generally more useful Printing Opacity. Both are defined as a ratio of certain reflectance values, namely:

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Ro =light reflectance of a single sheet backed by a black cavity. The subscript zero refers to the fact that the black cavity absorbs all light entering it; thus, the cavity reflects no light and has a reflectance of zero.

R =light reflectance of an opaque stack of sheets ("infinitely thick" stack). This quantity is given a special name, reflectivity.

R0.89 =light reflectance of a single sheet backed by the Tappi standard backing of reflectance 0.89.

TAPPI Opacity and Printing Opacity are defined through these reflectances as follows:

Tests

Measurement of Printing Opacity

1.On the ThwingAlbert Digital Opacimeter (Model 628), turn the switch from POWER OFF to STANDBY, and allow the instrument to warm up and stabilize for 30 min. Then switch from STANDBY to OPERATE.

2.Place the black cavity over the measurement port. Set the digital readout to +0.0 with the ZERO ADJUST control. Check this zero adjustment periodically throughout the day.

3.Place an opaque pad of paper over the measurement port. Set the digital readout to +100.0 with the 100% ADJUST control.

4.Then place a single sheet, backed by the black cavity, over the measurement port. Read the Printing Opacity directly on the digital readout.

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Measurement of TAPPI Opacity

5.If necessary, repeat steps (l) and (2) above.

6.Place a single sheet backed by the white body (TAPPI standard backing of reflectance 0.89) over the measurement port. Adjust the digital readout to +100.0 with the 100% ADJUST control.

7.Rotate the black cavity behind the sample and read the TAPPI Opacity directly on the digital readout.

A Practical Definition of an Opaque Stack

How can you be sure that your stack of sheets is thick enough to qualify as an opaque stack, that is a stack so thick that for all practical purposes no light is transmitted through it? Follow this procedure: Start out with a stack you think might qualify. Place it against the specimen port of the opacimeter, back your stack with the black cavity, and adjust the digital readout to 100. Next, double the thickness of your stack and place this thicker stack, backed by the black cavity, against the specimen port. If the digital readout remains unchanged at 100, you can safely assume that your original stack of sheets was indeed an opaque one.

Opacity Measurement on Paper of Low Brightness

With samples of low brightness and reflectivity, e.g. unbleached kraft, the adjustment of the digital readout to 100, as required in steps (3) and (6) above, may not be possible. In such a case, adjust to the highest digital readout obtainable and note this value. Continue with the procedure and obtain a readout for the single sheet backed by the black cavity. Divide this value by the first readout noted above and multiply by 100. This gives the desired opacity in percent.

Report

Report the means determined from five measurements of Printing Opacity and of TAPPI Opacity, expressed as a percentage. Also report 95% confidence intervals.

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References

ASTM D 589

TAPPI T 425

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