Bursting Strength

BURSTING STRENGTH

Introduction

The bursting strength of paper or paperboard is a composite strength property that is affected by various other properties of the sheet, principally tensile strength and stretch. Generally, bursting strength depends upon the kind, proportion, and amount of fibers present in the sheet, their method of preparation, their degree of beating and refining, upon sheet formation, and the use of additives.

Bursting strength is measured by means of a Mullen tester. The test specimen, held between annular clamps, is subjected to an increasing pressure by a rubber diaphragm, which is expanded by hydraulic pressure at a controlled rate, until the test specimen ruptures. The pressure reading at the instant of rupture is recorded as the bursting strength. The units of expression are pounds per square inch or "points". The measurement is variously termed the "Mullen", "pop test", or simply the "burst". The burst test for paperboard differs from that of lighter papers in that a heavier Mullen tester is used with a larger diaphragm opening.

Significance

Bursting strength is used widely as a measure of strength in many kinds of papers, primarily as an indication of the suitability of certain fibers and the extent of beating and refining. Its main reason for existence is that it is one of the oldest of the strength tests, it is easy to run, it appears to correlate with some use requirements, and everyone has the equipment. While interpretation is empirical, the bursting strength serves as a quick check on the utility of some papers which do not have an absolute tensile requirement. The bursting strength test also is used for combined containerboards, both corrugated and solid fiberboard as a requirement of the Uniform Freight Classification, Rule 41, for shipping containers. The bursting strength of the containers is generally controlled by the strength of the linerboard so this test is critical as a paper mill control test. Burst test results on a double-face or double-wall corrugated board are highly

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variable and correlate poorly, if at all, with most of the performance values of the finished containers.

Burst Factor and Burst Index

In addition to bursting strength, one frequently reports the burst factor and/or the burst index. The burst factor is calculated according to

The burst index is calculated according to

Note that 1 kPa = 1 kN/m2, where kPa stands for kilopascal and kN for kilonewton. Grammage is the basis weight in units of gf/m2.

The Mullen Burst Testers

Two quite different Mullen testers are available commercially and are both used in the Paper Testing Laboratory:

• The Model C Mullen tester; this is a comparatively light instrument for the testing of paper.

• The Model A Mullen tester; this is a much heavier instrument for the testing of paperboard.

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The light Model C instrument for testing paper has two gages, one ranging from 0 to 30 psi, the other from 0 to 120 psi. Never exceed the range of a gage, because serious damage to the gage may result. Obviously, this instrument is intended for testing papers of bursting strength less than 120 psi. The circular specimen area tested has diameter 30.50 ± 0.05 mm.

The heavy Model A tester for paperboard also has two gages, with ranges 0 - 200 psi and 0 - 800 psi, respectively. The specimen area tested has diameter 31.50 ± 0.03 mm, different from the light instrument. The Model A tester also has a much heavier molded rubber diaphragm, whereas the Model C tester has a fairly thin and pliable rubber diaphragm.

Because of these differences between the two instruments, they do not have a common scale. This means that a paper sample would have a totally different value of bursting strength when tested on the Model A tester. It is therefore a serious mistake to measure the bursting strength of paper on the Model A tester, as the test value obtained is meaningless.

In both instruments, the specimen is firmly clamped between two concentric platens, each having a circular opening in the center. The specimen exposed between the openings is the test area. Pressure is applied at a uniform rate to the underside of the specimen by the rubber diaphragm which is expanded by hydraulic pressure. A maximum reading gauge is included in the hydraulic system. The bursting test value is the maximum gauge pressure required to produce rupture of the specimen. The observed gauge value includes the hydraulic pressure required to distend the rubber diaphragm in addition to that required to rupture the specimen.

Tests

Make ten acceptable tests for each sample, five tests on each side. Make no tests on areas containing water marks, creases, imperfections, or visible damage.

Operating Instructions

Model A Mullen Tester for Paperboard

• Place the handle with the red knob, which operates the clamp, in the central or vertical position.

• Turn on the motor switch, which is located at the bottom of the front side.

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• Place the sample to be tested over the diaphragm plate.

• Lower the clamping plate by turning the the handle with the red knob clockwise to "CLOSE".

• The first test on a new sample should always be done with the gauge of greater range (0 to 800 psi). Make sure the valve to this range is OPEN, and the valve to the other gauge (0 to 200 psi) is CLOSED.

• Make sure the red maximum hand on the gauge you want to use is at zero; otherwise, return it gently to zero by manually turning the knurled metal knob in the center of the gauge counterclockwise.

• To burst the sample, proceed as follows. Find the operating lever with the black knob, located at the far right of the instrument. Move the operating lever to the LEFT and hold it there, all the while keeping your eyes on the gauge. If either the sample bursts or the pressure indicator needle comes too close to the maximum pressure of the gauge, it is extremely important that you quickly move the operating lever to the RIGHT as far as it will go and release it immediately; failure to move the operating lever to the right immediately the sample bursts may result in a ruptured diaphragm. The instrument will automatically return to the starting position.

• Record the bursting pressure indicated by the red maximum hand of the gauge used in the test.

• Return the maximum hand of the gauge gently to zero by manually turning the knurled metal knob in the center of the gauge counterclockwise.

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• Open the clamp by turning the handle with the red knob counterclockwise to the "OPEN" position.

• Remove the old sample. The instrument is now ready for the next test. The motor is kept running continuously; do not turn it off between tests.

Model C Mullen Tester for Paper

• Make sure that the valve to the pressurized air supply line is OPEN. This valve is located to the left and above the nearest Lorentzen & Wettre tensile tester.

• Turn on the motor switch.

• Push the brass ring located at the top of the clamp assembly upward to raise the clamping plate.

• Place the sample to be tested over the diaphragm plate.

• Lower the clamping plate by pushing the brass ring downward.

• The first test on a new sample should always be done with the gauge of greater range (0 to 120 psi). Make sure that the valve to this gauge is OPEN, and the valve to the other gauge (0 to 30 psi) is CLOSED.

• Make sure the red maximum hand on the gauge you want to use is at zero; otherwise, return it gently to zero by manually turning the knurled metal knob in the center of the gauge counterclockwise.

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• Record the bursting pressure indicated by the red maximum hand of the gauge used in the test.

• Push the brass ring upward and remove the old sample. The instrument is ready for the next test. The motor is kept running continuously; do not turn it off between tests.

Report

Report the mean bursting strength in points (lb/in2) as the arithmetical mean of all ten tests on a sample, the burst factor, and the burst index. Also report 95% confidence intervals.

References

ASTM D 774

TAPPI T 403, "Bursting Strength of Paper"

TAPPI T 807, "Bursting Strength of Paperboard and Linerboard"

TAPPI T 810, "Bursting Strength of Corrugated and Solid Fiberboard"

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Mechanical Properties