SMOOTHNESS OF PAPER

Introduction

Smoothness is an indirect measure of the texture or structural perfection of the paper surface. This means freedom from lumps, wire and felt marks, fuzziness, crush, cockles, interfiber voids, mechanical damage and other gross surface imperfections. In the paper industry, the quality of the surface is commonly referred to as "finish", a term that combines all those characteristics encompassed by the senses of sight and touch. Although "finish" is a useful term, it is an indefinite one from a technical viewpoint as it includes gloss, smoothness, and other less definable properties. Gloss is obviously an optical property while smoothness is a structural property. Increased beating of the stock and the wire shake on paper machines improve smoothness. The type of wire and weave of felts also affect smoothness. While increased wet pressing and calendering improve smoothness, it is far better to form a smooth sheet than to calender it smooth. Filling, surface sizing, and pigment coating generally improve smoothness. The type of pulp and the fiber coarseness also will have an important effect on smoothness.

Significance

Smoothness, or the converse, roughness, is related to the appearance of paper. Rough papers are generally unattractive. Smoothness is very important in printing papers where it correlates well with ink consumption, halftone dot quality, and the overall finish of the printed work.

While the average size of the surface irregularities is the most important factor in smoothness determination, the size, distribution, and arrangement or pattern of surface features are also important. In some cases, small even irregularities give a pleasing appearance such as on matte coated papers.

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Smoothness is important in printing papers and in writing and envelope papers with respect to handwriting, typewriting, and erasure. Papers to be coated require a suitable degree of smoothness in order that the coated product may be of satisfactory quality. Slippage, as a near relative of smoothness, must not be excessive where mechanical processing in automatic equipment is involved. It is evident that this property of paper is directly related to many uses and that paper need be manufactured with a variety of surface textures and smoothness levels.

Method of Test

This method measures the smoothness of a paper surface with a Sheffield Precisionaire Tester. The test specimen is placed upon the smooth glass plate and the gaging head is lowered by means of a short handle so it rests free an the paper sample. No pressure other than the accurately established weight of the gaging head is used. Air under a low pressure of 1.50 psi then escapes, or leaks, across the surface of the paper under the annular lands of the gaging head. The height of the float in the rotameter indicates the air leakage flow across the surface of the paper. The smoother the sample, the less will be the air flow, and the lower will be the float position. The readings are essentially instantaneous, regardless of the degree of smoothness.

Five or more readings on both the wire side and the felt side should be made for each specimen tested.

Report

Report the mean values of the smoothness readings, in Sheffield units, for both the wire side and the felt side of the sample. The felt side is generally the smoother and has therefore the lower value. Also report 95% confidence intervals.

Reference

TAPPI T 538

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