Bent Blade Runnability of double Coated SBS Paperboard, 2005 TAPPI Coating Conference

The relatively large dispersion particle size and high glass transition temperature (Tg) of polyvinyl acetate binders are suspect as primary causes for its poor runnability on double coated SBS paperboard. Pilot coater trials that simulate mill conditions reveal that even with larger and higher Tg binder particles present, basecoats having lower critical surface energy (CSE) contribute positively to good bent blade runnability with respect to blade scratches. Contact angle determinations were used to estimate the CSE of coated paperboard by the Zisman method that uses a series of fluids of decreasing surface tension. Mixed binder systems of polyvinyl acetate (PVAc), styrene n-butyl acrylate (SA) and styrene butadiene (SB) were designed to constrain the theoretical composite weight average Tg to 40ºC and particle size to between 180 – 190 nm. Basecoated paperboard with increasing amounts of SA, 20% and 40%, were found to have a lower degree of wettability; and few if any topcoat blade scratches for the higher amount of SA. Topcoats showing improved runnability had a glossy wet surface for several feet above the metering blade. In contrast multiple fine and wide blade scratches resulted when topcoats of 100% PVAc were metered over basecoats containing 100% PVAc. These topcoats had a dry look within two feet above the metering blade.