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TAPPI T 569 Internal bond strength






Internal bond strength (Scott type)


Printing, converting and many product applications subject paper and paperboard to impulses, impacts and shock loads into or out of the plane of the sheet. These can cause structural failures such as surface picks, blistering or delaminations within the interior of the sheet. The common denominators of these failures are a) the high velocity of the impact loads b) the short time period during which the material is stressed, frequently one to a few hundred milliseconds, and c) the planar nature of the resultant sheet failure. Test results from this method may correlate with product failures of this type.


The TAPPI T 569 Internal bond strength 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.

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 a slow, constant rate of elongation or dead-weight rupture.


A sandwich consisting of double-coated tape, the paper specimen, and double-coated tape is pressed between a flat metal anvil and an aluminum platen. A pendulum impacts the top inside surface of the platen, causing it to rotate and split the paper specimen in the thin, “Z” direction. The energy absorbed in rupturing the sample is computed by measuring the peak excess swing of the pendulum. The strength equation parameters include a) the potential energy of the latched pendulum, b) the peak excess swing, c) the energy required to accelerate a bare sample angle away from the pendulum, d) frictional losses, and e) internal vibration losses within the pendulum.


Internal bond strength as defined in this method is indicative of the serviceability and processibility of many types of paper and paperboard. These include printing papers, cover, label, release, linerboard, carton, carrier, newsprint and others. Test results often correlate with high-speed surface and internal structural failures encountered in both printing and converting operations.

Some fiber processing and papermaking characteristics can also be correlated with this type of internal bond strength test. These include extent of refining, machine speed, pressing, interply bonding in multiple headbox operations, and furnish composition such as long-to-short fiber mix, recycled content and the effect of dry strength additives. The test responds to both individual fiber strength characteristics and to the degree of interfiber bonding, but does not isolate and distinguish between these factors.

The combination of internal bond strength data with other TAPPI test methods can provide additional strength and processibility information. An example is the combination with porosity data derived from TAPPI T 460 “Air Resistance of Paper (Gurley Method)” or TAPPI T 547 “Air Permeance of Paper and Paperboard (Sheffield Method)” to predict blistering tendencies for coated web offset papers.

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