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1174372 
Journal Article 
Reversible post-exposure yellowing of weathered polymers 
Pickett, JE 
2004 
1 
Polymer Degradation and Stability
ISSN: 0141-3910 
ELSEVIER SCI LTD 
OXFORD 
85 
1 
681-687 
WOS:000222549100017 
The commonly observed effect of weathered polymer samples continuing to change color in the dark was studied in some detail for polycarbonate, styrene-acrylonitrile copolymer (SAN), and various combinations with SAN-polybutadiene rubber graft (SRG) copolymer. All of the formulations became more yellow upon storage in the dark after accelerated weathering, and all behaved very similarly. There was a fairly rapid shift of 1-2b units (as defined in CIELAB color space) over the first two weeks followed by a slower, linear shift that continued for many months. This amount of color shift is enough to be visible to the eye in side-by-side comparisons. Most of the additional color generated in the dark could be bleached by exposing the samples to sunlight through a south-facing window. The samples usually reached the original color after 2-4 days of window exposure. Putting the samples back in the dark caused the yellowing to start up again, although the rapid yellowing often was not as much as the first time. Samples aged under natural conditions outdoors also underwent a small color shift upon storage for 5 months and substantial color shifts after two years. The results are consistent with the photo-thermal equilibrium between Z-E isomers as suggested in the literature, although some aspects remain unexplained. (C) 2004 Elsevier Ltd. All rights reserved.