Dual-Site Adsorption in Hygro-Expansion of Paper

The effect of moisture on paper is of great relevance for numerous applications ranging from printing to packaging. The uptake of water in the polymer network of the paper fibers via the liquid or vapor phase of water results in hydro- or hygro-expansion of the paper, respectively. Here, we present a detailed atomic force microscopy study of the out-of-plane hygro-expansion of paper by conducting a series of experiments for varying relative humidity. The high spatial resolution of the atomic force microscope enables measuring the out-of-plane hygro-expansion of paper fibers with subnanometer resolution. We find a linear relation between the relative humidity and the out-of-plane expansion of the paper fibers. A sorption isotherm measurement reveals a good fit with the Guggenheim-Anderson-de Boer model. The combination of the two measurements suggests that the vast increase in weight seen in the isotherm is due to water filling the pores between the fibers, where it does not result in additional out-of-plane expansion. In addition, by locking the atomic force microscopy tip at a predefined location on a paper fiber, we monitor the expansion and shrinkage of paper with a temporal resolution in the millisecond range. We find that the kinetics of the out-of-plane hygro-expansion is described using a double exponential function comprised of two exponents with different time scales, which we attribute to two separate adsorption sites.