What are the methods for determining tear strength?
Tear strength is the amount of force needed to tear a polymer sample in a straight line. It is measured in n. How to figure out tear strength in general:
1) Trouser tear method
It can be used to test the tear strength of soft and hard films (including embossed films) as well as thin films less than 1 mm thick at the standard experimental speed. During the experiment, a sample with a crack that is half as long as the sample itself is put through a full tensile test on the two “trouser legs” formed by the crack. The tear strength of the test data is calculated by the uniform force needed to completely tear the sample along the length direction of the sample. If you don’t need to show the results as tear strength, you can just show them as tear force.
2) Elmendorf method
It is a way to measure the force needed to cut regular cracks and stretch regular lengths in a thin, soft sheet or film of plastic under regular loading conditions. Hard materials like PVC, nylon, and polyester film can’t be used with this method. During the test, have the sample with regular cutting accept the tear force generated by the regular pendulum and read it directly from the dial. This force value is the tear strength of each sample, expressed in n.
The Elmendorf method can be used on soft PVC and polyolefin films, but the repeatability of experiments on films with a large elongation will be very low because the elongation will change and the tear will be at an angle. Hard materials like rigid polyvinyl chloride, polyamide, and polycool film are not covered by these rules.
How a tear test works
The test is done on a single sample or on several samples of the same size. Most of the time, the length of the sample after the first notch should be 43 mm. To completely tear the sample, a counterweight with the right amount of energy is used. The energy needed to tear the sample is the difference between the up-and-down swing angle and the vertical position of the single pendulum’s center. A digital encoder measures changes in angle, which the microprocessor quickly turns into the average force needed to tear the paper.
Factors affecting paper tear
“Tearing degree” is the amount of force needed to tear a fixed length of material. There are three different kinds of tears: inner, edge, and plane. Internal tear is the amount of force needed to tear a pre-cut sample a certain distance. If we don’t say otherwise, when we talk about “tearing degree,” we usually mean “internal tearing degree.” Edge tear is the force needed to pull a sample apart without a notch for a certain distance (the tear starts from the edge of the specimen).
Factors that influence paper tear
1) The total number of fibers involved in the page breakage process.
2) The nature of the fiber itself (i.e., the length of the fiber and its strength).
3) The frequency and strength of fiber-to-fiber bonding.
The number of fibers that cause a page to break is based on how many grams the paper is and how flexible it is. A stiff sheet of paper will focus power on a small number of fibers in a small area, while a flexible sheet of paper will spread power over a larger area or more fibers.
In loose paper, the tearing force is spread out over a large area around the direction of tearing. This is especially true for fibers that are long and strong. When the fibers of a piece of paper are close together, the same tearing force is used at the breaking point, and the stress is focused on the tearing point to pull the fibers apart. This kind of stress distribution has a big effect on how well paper won’t tear.
Application of the tear tester
Tear strength tester for plastic film
The plastic film can be torn in a certain way. If the film is torn in different places while the goods are being moved, the problem will happen at the point where the force is concentrated. External factors can have an effect on the packaging bag, making it more likely that the contents will get damaged or leak. So, it is important to test how well the plastic film doesn’t tear.
So, we can use the plastic film tear resistance tester, which is also called the film tear strength tester, film tear resistance tester, film tear strength tester, and plastic film tear resistance tester. Check how easily plastic film can be torn.
The plastic film tear strength tester lets the rising pendulum fall, which tears the pattern with the energy of the falling ball. The sensors and computer control system then figure out the specific energy and tear force. The test method for film tear is based on the energy function conversion, which turns the impact energy (work done) into the tear resistance of the film.
The Elmendorf method is used by most machines that test the strength of film to tear. On the basis of the standard operating principle, they use the specified method to test the force value required by the tear pattern. This works well for things like films, flakes, soft PVC, non-woven fabrics, and so on that are hard to tear.
Fabric tear tester
Using the Elmendorf tear method, the Elmendorf tear tester (double pendulum) can measure how much force it takes for different sheet or film samples to stretch to a certain length under a certain load and a certain crack. It can tell how strong a tear is when it is hit with a force of 200–30,000 CN, which is the same as the drop hammer. Now, the tool cuts a small notch in the sample in the shape of a tongue. When the notch is made bigger, the tear strength is measured.
The fabric tear tester is easy to use and flexible in how it is made. It can be used to test a wide range of samples, both light and heavy. The instrument can be used to test the impact tear strength of textiles, woven fabrics, nonwovens, and coated fabrics, as well as the tear strength of silk, cotton cloth, all types of paper, paperboard, and single-layer and multi-layer corrugated paperboard.
How to reduce the large difference between the test values of the tear tester?
When comparing the results, the test results of the same sample when facing the same direction on the instrument should be consistent. The following are some of the possible causes.
Temperature and humidity are different.
First, the sample needs to be tested after the temperature and humidity have been changed. During the test, the same temperature and humidity should be kept. Temperature and humidity will change the sample’s brittleness, softness, hardness, ability to absorb water, and other characteristics, which will change the sample’s tear value.
Samples come in different sizes, especially in terms of width. The standard gives two ways to cut samples: with a constant radius or with a rectangle. The data tested by the two samples is not comparable, so the comparison data should be fixed first, and the method for cutting the samples should be the same. Or, when using the same method of cutting, the sample must be the same size, especially in terms of its width, which is directly related to the tear value. The sample should be cut flat and wrinkle-free. Also, the sample needs to be chosen.
Sample processing
The length of the slit created by the punching tool varies.
The tear tester has a knife that can be used to cut the sample. If the punching knife’s slit length is different, the sample will also be cut differently. There is no way to compare the tear values that were tested this way. So, when you buy the tool, you should make sure that the sample it cuts is standard.
The specimen’s direction (longitudinal and transverse) is different;
When making film or paper, the assembly line is the production line. Many samples are split into groups that go up and down or left and right. Vertical and horizontal materials have different textures, so the degree to which they tear is also different. So, check to see if the test sample is the same in both the vertical and horizontal directions.
The number of layers of the sample is different (or the thickness is different);
The number of test layers can be set during the tear test, and the test can be done with more than one layer. Check to see if the sample has the same number of layers during the test. Different thickness: If the sample thickness is different, it will also change the tear value.
The range of the pendulum used is inappropriate (the value should be 20% to 80% of the range);
The standard says that the range of the test must be between 20% and 80% of the total range. If it goes outside the range, the pendulum capacity should be changed. If the capacity of the pendulum chosen is wrong, the test data are wrong.
The instrument is placed on an unstable test table.
If there is a lot of electromagnetic interference or vibration on the test table, or if there are other things that can change things, the instrument will be affected. If the table is tilted, the pendulum’s gravitational potential energy will be thrown off.
The level and balance of the instrument are not adjusted properly;
Before the instrument test, the level must be adjusted.
The zero point of the instrument is not corrected properly.
The tear meter will be affected by wind resistance as it swings, so it is important to make sure the zero point is correct.
How to use the Elmendorf tear tester
The Elmendorf tear tester is especially good at testing the tear resistance of films, sheets, soft PVC, polyvinylidene chloride (PVDC), waterproof rolls, woven materials, polyolefin, polyester, paper, paperboard, textiles, and non-woven fabrics. It is not good for rigid materials like UPVC and nylon, nor is it good for films that stretch well.
Steps
- Leveling
Loosen the four lock nuts by hand, turn the four leveling screws clockwise or counterclockwise until the horizontal bubbles on the horizontal table are in the middle position, and then tighten the four lock nuts in a clockwise direction.
- Put the pendulum in place.
Using an inner hexagonal wrench, take out the screw, and then take out the gland. Take out the basic pendulum body, put it on the pendulum shaft along the two locating pins, lift the pendulum with your left hand, and let go of the pendulum.
- Empty pendulum test
If you lift the body of the pendulum with your left hand, the pendulum release rod will pop out and hold it.
Press the “sample clamping” key, clamp the sample, press the punching knife again, and then click “test” to let the pendulum body swing back and forth until it stops.
- Change the balance on the left and right
When you lift the body of the pendulum with your left hand, the pendulum release rod will pop out and hold it, and the real-time swing angle will show “180.0”;
- Sample pretreatment
GB 2918 says that the sample must be set up at a temperature of 23.2 °C and a relative humidity of 50.5 % RH for at least 12 hours before the test, unless the specification for the material being tested says something different.
- Sample cutting
The most common types of samples are rectangular samples and samples with a constant radius.
- Five samples should be taken in both the long and short directions of the material, unless the material’s specifications say otherwise.
The arbitration sample is a constant radius sample because it can be used over and over again. After cutting the sample, make a 20 mm by 0.5 mm notch in the middle of its length side. The notch should be smooth and free of nicks.
Note that some testing machines have tools that are always there to help. Use this tool to cut the sample once it is on the fixture of the testing machine. At this point, you should always check if the tool is sharp and the size of the notch.
- When the force needed to tear the film is low, it is okay to take samples with two or more layers to meet the standard. The single-layer samples from each group should be cut next to each other, but each group’s samples should be about the same distance apart.
Note that some of the specimens with more than one layer may tear diagonally in the opposite direction, which will make the results higher. Even if the reading is less than 20, you must use a single-layer specimen and a pendulum with less weight.
- Test samples are clamped, torn, and the range of an appropriate pendulum is checked.
Place the sample in the fixture and make sure it goes all the way to the bottom of the fixture. To clamp the sample, click “sample clamping.” Then, press down on the punching knife and use it to cut a 20mm slit.
Click the “Test” button to let go of the body of the pendulum and do a few full tear tests. Check to see if the value of the tearing force is between 20% and 80% of the full range of the pendulum. If not, change the number of layers in the torn sample or change the weight to one that gets heavier.
When changing the number of torn sample layers, you should also change the number of torn sample layers in the “parameter setting” column.
When replacing the weight-increasing weight, you have to readjust the instrument’s front-to-back level and left-to-right balance, set the “upper point” again, and change the pendulum’s capacity in the “parameter setting” column.
- Enter “test main interface.”
Open the host, click the screen, and go to the instrument’s “test main interface.” You can choose how many layers there are, how much weight each layer can hold, and how far each layer can swing. You must also decide how fast gravity moves and how far each layer can move.
- Start the test.
Click the test button on the test interface, and the instrument will start the test by itself by letting go of the pendulum.
- Print results
The test results are shown after the test is done, and the test report can be printed.
- History query: Click the “Query” button to ask about the history of the instrument or to print the data from the past.
FAQ:
What does the vertical and horizontal tear in paper mean?
The force needed to pull open a piece of paper with a small cut in it beforehand and reach a certain length is measured as the tear strength of the paper (MN).
A number that shows how strong paper and paperboard are. Means the amount of force needed to rip a certain length of fabric. Millinewtons are the unit (MN).
What is the difference between horizontal and vertical tearing degrees of paper?
Means the amount of force needed to tear a piece of paper at a certain distance. Most of what affects the degree of tearing is the average length of the fiber, followed by how well the fibers stick together, how they are arranged, how strong the fibers are, and how they are woven together. Most of the time, a paper’s transverse tear is bigger than its longitudinal tear.
What makes the longitudinal tear of papermaking paper higher than the transverse tear?
The longitudinal tear is larger than the transverse tear because the tension in the longitudinal direction is greater than the tension in the transverse direction. When the paper is wet, you should think about how the raw materials have changed, how heavy the wool cloth is, and how well the forming mesh lets air through. The amount of liquid should be changed because the mesh pulp should be more uniform and have less tension than the paper. One by one, the very hard ones should be thrown out.
How to set the paper tear strength tester to the right level?
Take the pointer away and use your hand to press the release button on the bottom hem. When the pendulum is still, its calibration line should be lined up with the calibration line of the frame. If there is a gap, the pendulum zero adjuster can be used to fix it.
When the pendulum is in its starting position, the line on the rack should line up with the line that says “0” on the right side of the pendulum. The sample clamp adjuster can be used to make sure the two sample clamps are flush with each other. The pointer should be set to the initial position.
Quickly press and hold the release key until the pendulum swings back. At this point, the pointer should show that the scale is at zero.