Role of paper face stock properties in determining removal efficiencies of pressure-sensitive adhesive films from recycling operations

Results are presented from a study on the influence of paper face stock properties on the screening removal of two commercial thermoplastic or hot-melt pressure-sensitive adhesives (PSAs). Significantly lower removal efficiencies were found when the adhesives were attached to commercial face stocks as opposed to untreated paper. These differences were only apparent for temperatures within the plateau region of the thermoplastic formulations and were maximized near their shear adhesion failure temperatures (SAFTs). It is proposed that the collapse of PSA films is inhibited by the presence of residual fiber from the laminate face stock, resulting in higher film stress and increased fragmentation. Detachment of the face stock is likely controlled by its strength and adhesion to PSA surfaces during repulping. Removal efficiencies of laminates involving commercial PSAs and fiberizable face stock treated with a poly(amide epichlorohydrin) wet-strength resin were found to correlate with the paper's loss in tensile strength after water saturation (i.e., tensile loss). Removal efficiencies decreased with increasing face stock size (alkenyl ketene dimer) concentration, which should increase PSA-face stock adhesion in aqueous environments. The sigmoidal function previously presented for modeling removal efficiency-temperature data of thermoplastic formulations was found to provide an accurate fit of data for laminates involving various wet-strength and sized face stocks. However, the T-50 or inflection points of these curves were shifted to values lower than the SAFTs for the attached PSAs. T-50 values were found to be proportional to both face stock tensile loss and surface energy.