On the Origins of Out-of-Plane Auxetic Response in Paper

Previously, several commercially available papers were examined and an increase in thickness upon stretching in certain types was found. A mechanistic explanation for the origin of this auxetic response was also devised, utilizing earlier reports. In this article, the understanding of this mechanism is applied to examine auxeticity in lab-produced paper handsheets. Only bleached kraft wood fiber pulp is employed, with no fillers, additives, or coatings incorporated. In this way, the contribution of a small number of structural factors to any auxetic response in the handsheets can be directly examined. Key structural parameters of a nonwoven cellulose fiber network in paper-staple fiber length, bulk density, and fiber contact spacing-which are predicted to affect the magnitude and sign of Poisson's ratio previously, are altered by using softwood and hardwood pulps with and without refining to produce a range of handsheets. It is found that longer fibers favor auxeticity while refining hampers it.