Deterioration of UV filters

Again I mention tests we ran some 15 years ago on extruded acrylic
florescent tube filters.  We were asking a similar question (do they
deteriorate as they get yellower) and found some interesting
results.  In plotting transmission curves of new and natural aged
samples, it seems the mid-point of the curves stayed about the same
(about 400nm?), but the slope of the curve increased with age.
Meaning the aged sleeves were both cutting out more blue light *and*
less UV.  This seemed odd until we noticed numerous striae in the
plastic running parallel with the extrusion: microcracks that were
obvious if the plastic was flexed and were more prevalent at the
heat-damaged tube ends.  Conclusion?  The plastic was darkening, but
was also transmitting more UV through small fissures created as the
polymer deteriorated.  Moral? Don't use extruded filters.  I doubt
this problem would occur anytime soon in thicker cast acrylic sheet,
but how thin polyester films might age is a fair question.

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                  Conservation DistList Instance 10:74
                Distributed: Thursday, February 20, 1997
                       Message Id: cdl-10-74-001
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