Leather test piece is placed into water to heat up slowly, which begins shrinking suddenly when the water temperature reaches a certain value. The temperature at this critical point is called as Leather Shrinkage Temperature.
After shrinkage, the physical(mechanical) properties of leather will have some drop. Leather is treated with damping and hot-pressing in the process of shoemaking. When temperature exceeds a certain limit, shrinkage and deformation occur on the leather, which causes the strength to lower, and service life to shorten. Especially for shoemaking craftsmanship using vulcanization and moulding, leather withstands high temperature as much as 100-120℃, so the leather’s shrinkage temperature has to be higher than the processing temperature, otherwise leather will have deformation during the treatment. In a word, measuring leather’s shrinkage temperature with a Leather Shrinkage Temperature Tester is very necessary.
Leather’s shrinkage temperature is in close connection with tanning method and tanning extent, for example, the shrinkage temperature of raw hide which is not tanned is 50-68℃, chrome leather is 95-120℃, vegetable leather is 70-85℃. In the case of the same tanning method, the degree of tanning can be judged according to the values of shrinkage temperature. Leather is not tanned ripely and it is not qualified If leather can’t reach the specified shrinkage temperature.
- Cut four pieces of test sample, ensuring the longitudinal edges of two of which are parallel to backbone line, and the longitudinal edges of other two test pieces are normal to backbone line.
- Sample size: Cut sample piece 50mm*3mm in dimension If sample thickness is less than 3mm. Cut sample piece 50mm*2mm in dimension If sample thickness is more than 3mm.
- Puncture 2 small holes along the centre line parallel to the longitudinal edge of the rectangle, ensuring the each of the centers of two holes is 5mm away from the closest end respectively. Two holes of sample are hung on the hook and small nail respectively.
- Soak Specimen: 5.5±0.5ml distilled water at temperature of 20±2℃ is poured into the test tube filled with test pieces, immersing the specimens into the water totally with the help of glass bar or other auxiliary tools.
- Vacuuming: Erect the tube in the desiccator for vacuuming, maintaining the absolute pressure below 4kPa for 1-2min.
- Let stand there and stop pumping, letting the air enters into the desiccator. Keep the test pieces original for 1-6h before test.
- Hang the test piece on the hook and small nail through 2 holes punctured.
- Add enough distilled water with 30-50℃ into the beaker, until water level is at least 30mm above the top of the test piece.
- Actuate electromagnetic stirrer and heat the water simultaneously, maintaining the heating rate at 2℃/min as close as possible.
- Record the water temperature and pointer position every 30s, until the specimen shrinks obviously or the water boils fiercely. Record the temperature of boiling water If test piece doesn’t shrink when water boils.
- Find the temperature when the pointer starts moving to the shrinkage direction for half of one scale(followed by significant movements), which is the leather shrinkage temperature.
Expression of Results
1.If shrinkage of specimen occurs below the boiling point, take the temperature at the time of shrinkage as the specimen’s shrinkage temperature. The mean value of shrinkage temperatures to the nearest 1℃ is recorded as result in report.
2.Report that shrinkage temperature is equal or greater than the water boiling point(mark the the boiling temperature) If specimen doesn’t shrink after water boils. Generally shrinkage temperature index is eligible If the chrome leather is resistant to boiling water.
3.For leather whose shrinkage temperature is up to 100℃, the pressure device can be used or glycerinum can be used as a substitute for water If the exact shrinkage temperature is intended to be determined.
1.If leather shrinkage temperature which is measured doesn’t exceed 10℃ more than initial water temperature in the light of above method, the result shall be discarded and prepare the new test pieces and reduce the initial water temperature to conduct determination again.
2.If the shrinkage doesn’t occur when the water boils in the beaker, and operators still want to measure the shrinkage temperature, using pressure device to raise the boiling point of water or switching to glycerinum instead of water are two alternative solutions. Test results have slight difference, which needs to be marked in report.
3.Because the shrinkage temperature of dry sample is different from the soaked sample, so all leather test pieces must be drenched sufficiently before determination(Vacuumize the desiccator to extract the air in the leather firstly, water can be pressed into the interior of leather by atmospheric pressure while the pressure is restored, so that the test piece can get drenched quickly).
4.Before testing, It is possible that pointer movements occurring on the dial of leather shrinkage temperature tester, even to the reverse direction toward the shrinkage orientation can be observed. The reason is that the dimensions of instrument and test piece have changes(elongation) after being heated, besides, water flow also can lead to bending of test piece. Operators must pay attention to inspect and record to distinguish the difference of pointer movements due to above reasons or actual shrinkage. It is hard to confirm the starting point of leather shrinkage, so shrinkage temperature only can be definite when positive shrinkage phenomenon happens(pointer moves toward the shrinkage direction for at least half of one scale ).
5.The performance of stir shall be evaluated before use. Fit a test piece into the instrument, and hang two calibrated thermometers, whose mercury spheres are close to the specimen, and at the same level with the upper and lower ends of test piece respectively. Heat after adding 350ml±10ml, and record the temperatures of two thermometers every 3min, calculating the temperature difference between the upper end and lower ends of test piece. This stir is qualified If there is no gap exceeding 1℃ among these data.
The factors influencing the leather shrinkage temperature are not only related to the conditions of leather Itself, but also closely associated with determination conditions, which are described as follows.
1.Conditions of leather Itself
A.For the same type of leather in a certain period, The shrinkage temperatures of dry specimen and wet specimen may be different.
B.Different tanning methods result in different shrinkage temperatures.
C.Different degrees of tanning also give rise to different shrinkage temperatures. For example the leather which is not tanned ripely can’t reach the defined shrinkage temperature.
D.When leather contains free acid, damp and hot conditions have evident destruction to lower the shrinkage temperature.
2.Conditions of Determination
A.Heating rate has distinct effect on the testing data. If the heating rate is too fast, the result is higher. If the heating rate is too slow, It may cause the changes in tanning status, leading to deviation of results. So water heating speed has to be controlled stringently when conducting leather shrinkage temperature test. The heating rate is advisable for 2℃ per minute.
B.Size of specimen and clamping status also have influence on the leather shrinkage temperature. The bigger the size of test piece, the higher shrinkage temperature due to longer time of progressive contraction of collagen fibers. If the sample clamping is too tight, collagen fibers is subjected to a certain stress, which is different than natural shrinkage, to affect the test results.
C.Instrument’s sensitivity shall conform to requirements. Testing values can be influenced due to too much friction of axle to drive the pointer, low sensitivity of thermometer, inappropriate stirring and heating devices.
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