Display Light Guide Swelling Due to Moisture Absorption

Display light guides, commonly constructed of a polymer such as polymethyl methacrylate (PMMA), are known to be susceptible to moisture absorption, swelling, and warping in the field when exposed to elevated ambient humidity levels. This work presents a quantitative theoretical framework for calculating water absorption over time. In addition, a simple theory is laid out, which connects mass absorption of water to linear expansion of the polymer. Finally, a simple quantitative analysis of out-of-plane warping of the light guide is presented. Experimental data from laboratory PMMA coupons and two different sizes of large displays are used to establish the values of key parameters of the theoretical model. General purpose results for any size of PMMA display are presented. The approach can easily be adapted to light guides fabricated using other polymers.