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Transcript of Dimensional Stability
There are a number of different causes of dimensional change, some of which are connected to one another. Most mechanisms only operate with fibre types that absorb moisture, but relaxation shrinkage can affect any fibre type. The following types of dimensional change are generally recognised: 1 Hygral expansion
2 Relaxation shrinkage
3 Swelling shrinkage
4 Felting shrinkage Hygral Expension Hygral expansion refers to the property of certain fabrics that absorb moisture, where the fabric expands as the moisture content increases, owing to the swelling of the constituent fibres. This is particularly a property of wool fabrics. Hygral Expension Figure shows that the increase in dimensions of two wool fabrics with increasing atmospheric moisture content; in one case the expansion increases with regain almost up to the maximum value for wool, whereas in the other fabric the expansion reaches a maximum at around 20% regain. Hygral Expension Hygral expansion is believed to be caused by the straightening of crimped yarn as it absorbs moisture. This is due to the fact that wool fibres swell to 16% in diameter and 1% in length when wet.
Hygral expansion of a fabric in a finished garment can cause problems when the garment is exposed to an atmosphere of higher relative humidity than that in which it was made. The expansion can cause pucker at seam and wrinkling where it is constrained by other panels or fixed interlinings. Relaxation Shrinkage Relaxation shrinkage in wool fabrics is caused by stretching the wet fabric beyond its relaxed dimensions during drying.
This effect is related to the hygral expansion value of a fabric in that a fabric with a high value of hygral expansion will increase its dimensions more when it is wetted out so that it subsequently needs to contract to a greater extent when it is dried. Merely holding such a fabric at its wet dimensions will thus give rise to a fabric that is liable to relaxation shrinkage. Swelling Shrinkage This type of shrinkage results from the widthways swelling and contraction of the individual fibres which accompanies their uptake and loss of water.Because of the fibre swelling, the yarns made from them increase in diameter which means that, for instance, a warp thread has to take a longer path around the swollen weft threads. Swelling Shrinkage Swelling Shrinkage The overall effect of the swelling mechanism on a fabric's dimensions is dependent on the tightness of the weave. This mechanism is the one that is active when viscose and cotton fabrics shrink. Felting Shrinkage Felting shrinkage is a mechanism of shrinkage that is confined to wool fabrics and it is a direct consequence of the presence of scales on the wool surface as shown in figure Felting Shrinkage Felting is related to the directional frictional effect (DFE) which is found in wool fibres.Shrinkage is caused by the combined effects of DFE and fibre movement promoted by the elasticity of wool. The behaviour is promoted if the fibres are in warm alkaline or acid liquor.
When alternating compression and relaxation are applied to the wet material, the compression force packs the fibres more tightly together but on relaxation of the force the DFE prevents many of the fibres from reverting to their original positions.Wool can be made shrink resistant by treatment to reduce the effect of the scales on friction. Weft Knitted Wool Fabrics Knitted fabrics are similar to woven fabrics in that they are subject to relaxation shrinkage and also to felting shrinkage if they are made of wool.it has been found difficult experimentally to determine when a fabric has reached a totally relaxed state.
A number of relaxed states have been suggested; 1 Dry relaxed state.
2 Wet relaxed state.
3 Finished relaxed state,
4 Fully relaxed state. Weft Knitted Wool Fabrics Marking Out Samples The general procedures for preparing and marking out of samples is laid down in the British Standard.
For critical work the recommended sample size is 500mm X 500mm and for routine work a minimum sample size of 300mm X 300mm is considered sufficient. Marking Out Samples The samples are marked with three sets of marks in eachdirection, a minimum of 350mm apart and at least 50mm from all edges as shown in figure. Marking Out Samples For critical work it is recommended that the samples are preconditioned at a temperature not greater than 5O C with a relative humidity of between 10% and 25%.All samples are then conditioned in the standard atmosphere. WIRA Steaming Cylinder Four warp and four weft samples are tested, each measuring 300mm X 50mm. They are first preconditioned and then conditioned for 24 h in the standard testing atmosphere in order that the samples always approach condition from the dry side. Markers (threads, staples, ink dots) are then put on the fabric so as to give two marks 250mm apart on each sample. WIRA Steaming Cylinder The four specimens are then placed on the wire support frame of the apparatus shown in figure and steam is allowed to flow through the cylinder for at least one minute to warm it thoroughly. WIRA Steaming Cylinder The specimens are then allowed to cool, preconditioned and then conditioned for another 24 h to bring them into the same state they were in when they were marked. They are then remeasured on a flat smooth surface and the percentage dimensional change calculated. The mean dimensional change and direction is reported: Relaxation Shrinkage The international standard for measuring relaxation shrinkage is the determination of dimensional changes of fabric induced by cold water immersion. In the test the strains in the fabric are released by soaking the fabric without agitation in water that contains a wetting out agent. The specimen is conditioned, measured, soaked in water, dried, reconditioned and measured again. Washable Wool When testing washable wool products for shrinkage it is usual to carry out tests that separate any felting shrinkage from relaxation shrinkage. It is important that the contribution of each type of shrinkage to the overall shrinkage is determined because both the cause and remedy for each type are quite different.
Relaxation and consolidation shrinkage
Cubex Washing Programmes Most tests for dimensional change due to washing use the procedures given in BS 4923 or ISO 6330.
The standards specify the level of agitation during heating, washing and rinsing, the washing temperature, the liquor level during washing and rinsing, the washing time, whether there is a cool down after washing and the number of rinses and spin time. There are two sets of programmes in the standard, one for front loading machines designated type A and one for top loading agitator machines designated type B. Therefore a type 5A wash specifies a number 5 wash (4O C, normal agitation) for a front loading machine. The use of a standard detergent without additives is also specified. The Wascator is an industrial washing machine which is commonly used for these testsand it can be programmed for all the main functions such as temperature, liquor level during washing and rinsing, washing time, time and number of rinses as required by the standard. Washing Programmes Dimensional Stability To Dry Cleaning The British Standard method requires the use of a commercial dry cleaning machine. In the test the sample is prepared and marked out according to BS 4931. The total load used is 50kg for each cubic metre of the machine cage made up of specimen plus makeweights. The solvent to be used is tetrachloroethylene containing 1 g/1 of surfactant in a water emulsion, 6.5 litres of solvent being used for each kilogram of load. The machine is run for 15min at 3O C, the sample is rinsed in solvent and then dried by tumbling in warm air. The sample is then given an appropriate finishing treatment, which in most cases will be steam pressing, and it is then reconditioned and measured again. Dimensional Stability To Dry Cleaning Dimensional Stability To Dry Heat This test is intended to predict the behaviour of fabrics when heated in a hot press.
The ISO method recommends that the samples are preconditioned at a low relative humidity before conditioning in the standard atmosphere. The samples are then marked out as shown in figure and the dimensions AB, CD, EF and GH determined. They are then placed in a press heated to 15O C under a pressure of 0.3 kPa for 20s. The samples are conditioned and measured again so that the dimensional change in each direction can be calculated. Dimensional Stability To Dry Heat Figure shows the relative scale of severity for treating wool. Any setting that has taken place under one of the conditions listed can be relaxed by subjecting the material to any of the conditions above it on the list. This is shown diagrammatically in figure where the swelling of the yarns from the dry state (a) to the wet state (b) causes an increase in the length of the path the yarn must take if the fibre centres remain the same.