|MadSci Network: Engineering|
Angie, The simplest way to shrink polyester fabrics would be to heat them to about 80-90C. I will tell you why and how shrinkage occurs in semicrystalline polymeric materials. To understand shrinkage one must know what causes shrinkage in polymeric materials. Most polymers are semicrystalline i.e. they contain long chains of molecules arranged partly in a random fashion and partly in an ordered fashion. The ordered units are called crystalline units while the randomly organized units are called amorphous which means shapeless. It is this mixture of crystalline and amorphous units that is one reason why shrinkage occurs in polymeric materials. There are some chemical origins of shrinkage in certain polymers like cellulose (wood, cotton etc) that are not relevant to polyester shrinkage and so I will not go into them. Suppose you have a mixture of crystalline and amorphous units. Now as you heat this material, the natural tendency for the amorphous chains is to coil up. It does this because the chains always tend to be in the state of lowest energy. As you heat the polymer you are supplying energy to it. Below the melting point of polyester, which is about 230-240C, only the motion of amorphous chains in the material can dissipate the energy. This motion results in shrinkage. However, chains in a polymer can move only above a certain temperature called the glass transition temperature. This is the temperature above which chains in a polymer can move and exhibit rubberlike behavior and below which the polymer behaves like a glassy material i.e. frozen in. The glass transition temperature for polyester is about 68-81C. So at room temperature polyester is glassy. Chains or segments of chains cannot move. However when you heat this material above Tg (glass transition) then segments of chains can move. Then depending on the temperature to which the material is heated the amount of shrinkage will be different. When the temperature is close to melting then chains start to melt and the shrinkage now occurs due to both melting of the crystalline portion and movement of the amorphous portion. Certain materials can be shrunk by dipping them in water for e.g. cotton. The mechanism of shrinkage is different in this polymer. This is because cotton is primarily cellulose. Cellulose contains a lot of hydroxyl groups i.e. OH like in water. This produces a high degree of hydrogen bonding in cotton. When cotton is immersed in water, the water permeates through the amorphous regions of cotton and breaks the hydrogen bonds. This causes the polymer to shrink. There are no such hydrogen bonds in polyester and so unless water is able to permeate through polyester and disrupt the ordered regions or perturb the amorphous regions in polyester immersing it in water may not cause significant shrinkage in polyester. So the best method to shrink polyester fabrics is (1) in hot air above 80 C (2) wash them in boiling water Significant shrinkage can be achieved only when the material is heated to well above the Tg. Note: Cotton that is not preshrunk will shrink when washed, especially in hot water, for exactly the same reason. If you have any more doubts or if any part of the above answer is not clear to you you can email me with your questions at firstname.lastname@example.org. Reference: (1) Perepelkin K. E.; Belonogova M. N.; Smirnova N. A., Determination of Shrinkage by Textiles Made of Chemical and Flax Fibres by Different Methods. , Fibre Chemistry 29, No. 3: 200+, 6 pages (May-June 1997). (2) Patel N. C. Panchal B. P., THERMAL SHRINKAGE IN POLYESTER STAPLE FIBERS, ATIRA Communications on Textiles 24, No. 3: 109-117 (Sept. 1990).
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