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Hygroscopic and thermal stability of high precision replicated epoxy composite mirrors

Abstract. Replicated lightweight composite mirrors are gaining increasing attention for space applications due to potential weight savings, cost reductions, and faster manufacturing times over traditional glass mirrors. However, dimensional stability remains one of the most critical issues for these resin-based high precision optics. Composite mirrors with a surface figure error (SFE) of 0.03λ were manufactured using a UV-cured replicated layer (RL). The effects of cure state and environmental exposure on the stability of the mirror were evaluated by measuring changes in SFE using interferometric imaging. Higher stability was observed for the replicated epoxy mirror that underwent a secondary cure, after the initial UV cure. SFE for these mirrors increased by only 5-nm (RMS) when exposed to 100% RH for 12 days. The mirrors that were UV cured alone under identical exposure conditions increased in SFE by over 500% of that value. The increased hygroscopic stability is consistent with a reduced amount of unreacted polar epoxide groups that can hydrogen bond with absorbed moisture diffusing into the network. In addition, the mirror with the secondary cure remained stable up to 125°C while the mirror with a lower degree of cure exhibited warpage under identical thermal condition due to additional cure shrinkage.