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Recent Advances in the Rigidization of Gossamer Structures 273
rived from bis-phenol A can be selected and adequately formulated [52]. The
epoxy monomer DGEBA and its acrylated derivative EPDA (Chart 1) polymerize
upon appropriate photo-initiation by a cationic or a free radical mechanism,
respectively.
Chart 1. Examples of monomers derived from bis-phenol A
For an efficient exploitation of the incident UV-visible light, the absorption
spectrum of the photo- initiating package has to be adjusted to the spectral
characteristics of both the curable material and the light source [53]. Inthe
case of a representative structure where the radiation curable material is sandwiched
between protecting thermoplastic films, the light-filtering effect of the
latter shall be overcome by minimizing the screen thickness, or alleviated by
selecting an initiator exhibiting a long wavelength absorption, above the cutoff
line of the enveloping film.
The transmission spectra of Fig. 6 clearly show the cut-off line at 320
nm by a10 µm-thick PET film, whereas polyimide films do not allow the
penetration of light for wavelengths shorter than 400 nm. Additionally, the
dramatic reduction of transmitted light can be calculated at various operating
wavelengthsasafunction of film thickness.
Absorbance
2
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
PET film
10 µm
polyimide films
7, 12, 25, 50 µm
200 400 600 800
Wavelength (nm)
Fig. 6. UV-visible absorption spectra of protective thermoplastic films