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This test method may be used for the acceptance testing of commercial shipments of yarns and cords. Caution is advised because yarn and cord may contract in length over a period of time due to room temperature retraction. Thermal shrinkage values are reduced proportionately by the amount of room temperature retraction.
Note 18212;Experience, especially with nylon, shows that yarn retraction, which may be observed directly as shortening of length (or indirectly as denier increase), will occur in unrestrained yarn or cord that is not at equilibrium (equilibrium in this case being defined as essentially zero thermal shrinkage yarn or fully relaxed yarn). Normally, retractive forces are present in most wound packages of yarn and cord; thus, unrestrained yarn near the surface is likely, with time, to undergo some retraction. After retraction, such yarns exhibit lower thermal shrinkage values than yarn or cord deeper within the package. The opposite condition of yarn on the surface exists with yarn or cord wound against or near a rigid package core, such as a metal or hardwood wind-up spool. Such core yarn or cord cannot move against this restraint, and thus, will exhibit thermal shrinkage values even several weeks later near to those which were measured immediately from the surface of the freshly wound package. Elevated humidity will accelerate retraction of unrestrained yarn, but moisture content in itself will have little influence on thermal shrinkage. Exposure of untensioned skeins of yarn or cord to 95 to 100 % relative humidity at room temperature for two days and reconditioning under standard laboratory conditions will cause most of the room temperature retraction that is possible within a sample to occur.
In case of differences of practical significance in reported test results from two or more laboratories conduct comparative tests to determine if there is a statistical bias between them. Competent statistical assistance is recommended for the investigation of bias. As a minimum, the parties should take a group of test specimens that are as homogeneous as possible and that are from a lot of material of the type in question. The test specimens then should be assigned randomly in equal numbers to each laboratory for testing. The average results from the two laboratories should be compared using Student''s t-test for unpaired data and a probability level chosen by the parties before the testing is begun. If a bias is found, either its cause must be found and corrected, or future test results must be adjusted in consideration of the known bias.
Thermal shrinkage of nylon, polyester, and other fibers is related to the polymer of origin and its manipulation in processing. Thermal shrinkage measurement can be used to control product uniformity.
The level of thermal shrinkage is critical in the user''s subsequent operations. For example it can affect the drumset (original length of cord) required in tire building to produce a finished, final tire of a particular size.
Thermal shrinkage is critical to final shape and size of fiber reinforced articles. For example, thermal shrinkage affects final size of V-belts and their ability to maintain tension while running.
This test method is similar to the procedures of Methods D885 for the determination of thermal shrinkage of yarns and cords. Shrinkage is measured while the specimen is within an oven and tensioned as specified in Methods D885;D885 however, there are enough vagaries among different units of apparatus for measurement of thermal shrinkage that numerical equivalence between.......
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