Apparent Density

APPARENT DENSITY

Introduction The apparent density of paper is the ratio of the mass of a paper sample to the volume occupied by that sample, each being determined under prescribed conditions. Apparent specific gravity or apparent density is commonly calculated by dividing the basis weight of the paper by the caliper, but this value varies with the ream size used, a major inconvenience, to say the least. It is much better to express this property in units of grams per cubic centimeter (g/cm 3). It should be remembered that the weighing is done on an area of paper containing fiber, moisture, air space, ash content, and occasionally volatile materials. This accounts for the use of the terms "apparent density" and "apparent specific gravity".

Significance Apparent density is probably the most important property of paper, because it influences almost all mechanical, optical, physical, and electrical properties of paper as well as quality of printing, absorption, flexibility, and the like. Apparent density also has economic significance in those cases where paper or paperboard is purchased on a weight basis and consumed on an area basis. Dense papers are characterized by being relatively hard and nonporous. They are made from highly beaten pulp, or are highly surface-sized papers. They are compact and their fibers are strongly bonded so that the sheet as a whole tends to contract or expand with moisture changes. In soft, bulky, porous papers individual fibers can swell or shrink without much overall dimensional change in the sheet. So we see that the dimensional instability or hygroexpansivity of paper is closely related to the apparent density.

Method of Test The apparent density is determined by measuring the basis weight, W, and the single-sheet caliper, t, of the paper and by taking the ratio W/t. The apparent density should be expressed in units of g/cm3.

Report

29 Report the mean apparent density in g/cm3 as obtained from the average values of basis weight and caliper. Also report a 95 % confidence interval for the apparent density as calculated from the 95 % confidence intervals of basis weight and of caliper.

Reference TAPPI T 411