- Reduction in water vapor permeability is both fundamentally important and a practical necessity in the polymer industry for packaging and protective applications. The barrier of polymers is significantly improved by inclusion of impermeable lamellar fillers, increasing the diffusion path of gas or water vapor molecules. The most common “barrier-modifier” for polymers is clay, which is less effective for reducing water vapor permeability since it absorbs water, swells and also degrades other properties (e.g., mechanical strength). In this study we focus on graphene as the barrier filler in polyurethane, examining the influence of its lateral size and concentration on both water vapor permeability and the mechanical properties. We demonstrated optimal barrier and mechanical enhancements at low graphene loading of 2 and 1 wt%, respectively. We tested few lateral dimension of graphene and found that the larger size (25 μm) exhibited an optimal barrier enhancement (50% reduction). In addition we found correlation between the onset of the barrier and an increase in the polymer viscosity.