ASTM D726-94(2003)

The air resistance of paper can be used as an indirect indication of such variables as degree of beating, absorbency (penetration of oil, water, etc.), apparent specific gravity, filtering efficiency for liquids or gasses, etc. It is influenced by both the internal structure and the surface finish. Internal structure is controlled largely by type and length of fiber, degree of hydration, orientation, and compaction. Surface finish is governed by the degree of wet pressing, types of felts used in manufacture, and primarily, calendering, as well as by the presence or absence of coating. Air resistance is further greatly influenced by the type and amount of filler, sizing, etc. The air resistance of coated papers is generally so high as to be of doubtful significance. While the measurement of air resistance is a useful control test for machine production, due to the number and complexity of factors outlined above, careful judgment with a consideration for end-product use should govern the adoption of specification limits for air resistance.

In insulating papers, air resistance can be used as a criterion in predicting dielectric strength and absorbency, in uncoated printing papers it can be used as an indication of rotogravure ink absorption, and in filter papers it can foretell the performance in filtering liquids or gasses.

1.1 This test method covers the determination of the resistance of nonporous paper to the passage of air. This test method cannot be used in those cases where the paper cannot be clamped securely against surface and edge leakage, such as, crepe or corrugated papers.

1.2 This test method is applicable to papers that permit the passage of up to 25 mL of air/0.785 in.² in 15 s, and in general, to papers not sufficiently porous to be tested by TAPPI methods T460 and T536.

Note 1—For testing pourous and semiporous paper, refer to TAPPI T460, and T536, respectively. Since the three methods (D 726, T460, and T536) do not give the same results, it is recommended that a specific method be agreed upon in specifications covering paper between the seller and the purchaser, and that the method be chosen to conform to the principle range.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.