Hi Dick,

I have about 15 years of test data that shows that the Coefficient of
Thermal Expansion (CTE) for uncoated polyester film base is about 0.0010%
per 1ºF regardless of the film thickness.  The humidity coefficient of
expansion for uncoated base is 0.0008% per %RH for 7 mil polyester and
0.0009% per %RH for 4 mil polyester.

A film product that consists of polyester base coated with relatively thin
layers may have a higher humidity coefficient because these layers absorb
moisture more readily than the polyester.  Modern silver halide films have
humidity coefficients in the range of 0.0009 - 0.0011% per %RH.  The
humidity coefficient of diazo film and thermal imaging film are typically
in the range of 0.0008 - 0.0009% per %RH.

I don't believe it is possible to have a film whose structure is 90%+
polyester with a CTE that is significantly less than the polyester itself.

Bob Seyfert
DuPont Electronics

>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++++
>
>
>
>The problem with silver halide film is it's dimensional stability to humidity
>changes.  Silver film specs quote dimensional stability to be .012% per
deg RH
> change over 24 inches, whereas the thermal coefficient is .0009% per deg
F for
>7 mil film.
>The solution is to switch to a dry process system using dry plot film
which has a thermal coefficient of .00085% per deg F for 4 mil film and a
humidity
> coefficient of .0009% per deg RH.
>
>
>Dick Desrosiers
>[log in to unmask]
>

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