The tear resistance of paper or cardboard refers to the average force required to rip a pre-cut sample. When the initial cut is made along the length, the resulting measurement is called the longitudinal tear. Conversely, if the cut is across the width, it is referred to as the transverse tear. This test is crucial in evaluating the physical properties of paper and provides insight into its fiber characteristics. By measuring tear resistance, we can indirectly assess the quality and strength of the fibers used in paper production.
The Avery Mendorf Tear Test Method is one of the widely accepted procedures for determining this property. It measures two key parameters: the tear value and the tear index. The tear value is the average force needed to tear a paper sample of a specific size, while the tear index is calculated by dividing the tear value by the basis weight of the paper. These values are typically expressed in millinewtons (mN) and millinewtons per square meter per gram (mN·m²/g), respectively.
In China, the standard GB/T 455-2001 outlines the procedure for determining the tearing resistance of paper and paperboard. The test involves applying a tearing force to a stack of samples—usually four layers—with a defined pre-cut. A pendulum device is used to measure the energy consumed during the tearing process. The results are derived from the pendulum’s movement and the number of layers tested.
The Elliman Dove Tear Tester, as shown in Figure Al, is a commonly used instrument. It consists of a base and a freely swinging pendulum with low friction bearings. The specimen is clamped between two jaws, one fixed and the other attached to the pendulum. The sample must be at least 25 mm wide and 15 mm deep. Proper alignment and preparation of the sample are essential for accurate results.
Sample preparation follows GB/T 450 standards, ensuring no visible defects such as wrinkles or creases. The sample size should be (63 ± 0.5) mm × (50 ± 2) mm, and tests should be conducted separately in both the longitudinal and transverse directions. At least five valid tests should be performed in each direction.
During the experiment, the pendulum is adjusted to ensure readings fall within 20% to 80% of the full scale. The sample is positioned correctly, and the pendulum is released to initiate the tear. The result is read from the scale or digital display, and the process is repeated until five satisfactory results are obtained. Any samples that show excessive skew or peeling are excluded from the final calculation.
For accuracy, the number of layers tested is usually set to four. If results are not satisfactory, the number of layers may be adjusted accordingly, but this should be noted in the report. The tear degree is calculated using the formula: F = (S × P) / n, where S is the average reading, P is the conversion factor (typically 16), and n is the number of layers. The tear index is then calculated as X = F / G, where G is the basis weight of the paper.
Modern electronic tear testers like the SLD-1000Z from Jinan Sanquan Zhongshi Experimental Instrument Co., Ltd. offer automated testing, data storage, and real-time results. These devices reduce human error and improve efficiency, making them ideal for laboratories and quality control departments.
For more information or assistance, contact Chen Meiru at 0531-6781 3036 or 15666775359. Visit http://news.chinawj.com.cn for additional resources.
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