BULKING THICKNESS AND BULKING FACTOR
Introduction
The measurement of bulking thickness is used for finding the average thickness of paper when placed in a pile, as for use in books. The bulking thickness is only approximately the same as single-sheet thickness for ordinary papers for it usually measures from 2 to 6 percent lower because of the nestling and compacting that occurs on stacking.
Students need not carry out this test; its description is included here for purely informative reasons.
Significance
Bulk or bulking thickness is an important property for papers to be bound into book or pamphlet form. Often a series of books will be designed so that all volumes will be equally thick even though the texts will vary widely in length.
Method of Test
The bulking thickness is the average single sheet thickness in thousandths of an inch (mil) when a pile of several sheets is placed under a steady pressure of 7.10

0.15 psi between two circular and parallel surfaces (about 0.25 in
2
) of a micrometer. If a thickness gage is used, a pack not less than 0.10 inches thick shall be tested in not less than ten different places evenly distributed over the pack. Place the pack between the jaws of the micrometer and lower the pressure foot gently upon the surface of the paper with its edge at least one-half inch from the edge of the paper. The test shall be made on conditioned papers in a conditioned atmosphere.
The apparatus and calibration are described in the references.
Report
Report the mean bulking thickness in mil (1 mil = 1

10
- 3
in) by determining the average thickness of the pack divided by the number of sheets in it. State this value to the nearest 0.05 mil.
Note
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A bulking factor may be obtained by dividing the bulking thickness by the basis weight. This factor is the same as the apparent specific volume, i.e., the reciprocal of the apparent specific gravity.
(See test method for Apparent Density for discussion of units.)
References
ASTM D 527
TAPPI T 426
28