Internal bond strength (Scott type)
This method defines a test that measures the energy required to rapidly delaminate a sheet-type specimen. The “Z” directional rupture is initiated by the impact of a pendulum having both a controlled mass and a controlled velocity that exceeds 6000 times the velocity of tensile strength and other dead-weight testers. The geometry of the apparatus causes the tensile stress to be rotational in nature with negligible shear stress on the specimen.
The method is suitable for both single and multi-ply paper and paperboard, including coated sheets and those that are laminated with synthetic polymer films. Because sample preparation entails pressing double-coated tape to both sides of the test specimen under relatively high pressures, this method may not be suitable for testing high strength handsheets or low basis weight (below 40 g/m2), porous, soft or low-density materials, such as tissue. Limitations include materials that permit significant migration of the tape’s adhesive into the sample with potential tape-to-tape bonding, or materials that could be structurally damaged or collapse during the press cycle. To determine the applicability of the test method, it is important to visually inspect both sides of a set of at least 10 delaminated samples. The ability to rupture a sample within the measurement range of the instrument is an insufficient criterion. For interpretations of observations, see Section 9.2. Because energy is absorbed during the elongation and stretching of the sample’s fiber network prior to rupture, this internal bond test responds to the semi-elastic nature of paper and paperboard. The test is a measurement of strain energy per unit sample area, which is proportional to the area under the stress-strain curve. Strength measurements by this method do not correlate with “Z” direction tensile strength tests (ZDT) that measure the maximum (peak) stress in T 569 om-14 Internal bond strength (Scott type) / 2 a slow, constant rate of elongation or dead-weight rupture. Also, the constant rate of elongation specified in TAPPI T 541 “Internal Bond Strength of Paperboard (z-Direction Tensile),” (ZDT) and “X” and “Y” plane tensile tests per TAPPI T 494 “Tensile Breaking Properties of Paper and Paperboard (Using Constant Rate of Elongation Apparatus),” is several orders of magnitude less than the rate attained in this method.