MadSci Network: Engineering

Re: How would you be able to shrink fabrics made out of polyester?

Date: Sun Nov 28 14:48:53 1999
Posted By: Narayan Variankaval, Grad student, Polymers/Textile and Fiber Engineering, Georgia Tech
Area of science: Engineering
ID: 942688592.Eg

      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

(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). 
FIBERS, ATIRA Communications on Textiles 24, No. 3: 109-117 (Sept. 1990).

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