"Molten polyethylene terephthalate (PET) polymer
is first extruded onto a chill roll drum to form a
film.
* This film then is biaxially oriented by being
stretched first in the machine direction (MD)
and then in the transverse direction (TD).
* The orientation is accomplished by passing the
film over rollers that run at increasingly faster
speeds (MD orientation), then fed into a tenter
frame, where it is pulled at right angles (TD
orientation). This stretching rearranges the PET
molecules into an orderly structure to substantially
improve the film's mechanical properties.
(Films stretched in the machine direction only are
uniaxially oriented; films stretched in both
directions are biaxially oriented.)
* Finally, the film is heat-set to stabilize it. It
will not shrink again until exposed to its original
heat-set temperature."
Larry Woolf
General Atomics
www.ga.com
www.sci-ed-ga.org
-----Original Message-----
From: John Denker
Sent: Friday, February 24, 2006 10:59 AM
Subject: [Phys-l] homogeneity and isotropy
Ditto for a piece of "polaroid" polarizing plastic. Also in my
experience cellophane tape is homogeneous but anisotropic.
Also Mylar ... apparently the polymer molecules have some preferential
orientation. I hypothesize the material underwent uniaxial stretching
during manufacture. In contrast Kapton is
isotropic: http://www.lgarde.com/people/papers/spacestructs.html