COMPARISON OF THE COATING STRENGTH OF FOUR BINDER CHEMISTRIES USED IN PAPERBOARD COATINGS, 19PaperCon
This paper discusses a study comparing the coating strength of different latex binder types that are used in paper and paperboard coatings. Typical binders used in paperboard are styrene-butadiene (SB), styrene-acrylate (SA), vinyl acrylic (VA) and polyvinyl acetate (PVAc). Latex binders are used in the coatings to provide adhesion to the basesheet and cohesion between the pigments. Additionally for paperboard, they facilitate acceptable glueability, improve printing press performance and enhance printability. The latex binder can also improve coater runnability, such as the ability to increase the coating solids while providing the appropriate rheology and shear stability on high machine speeds. The binder must also provide the appropriate dry and wet strength as this is important for the printing, finishing and converting of the paperboard as well as for final carton use. In today’s environment, where manufacturing efficiency is measured by the balance of cost and efficiency, a strong coating is required to withstand the stresses of high speed printing equipment and at the same time give sufficient gluability with less efficient adhesives often chosen due to lower cost.
n this study, each binder type was evaluated at 5 different levels in two pigment systems, one containing 20 parts of calcined clay and the other containing 70 parts of calcined clay. Overall results indicate that the SB and SA binders are similar in efficiency when targeting 100% fiber tear in hot melt glue testing. Using a series of predictive models that were generated with the data from this study, we saw that the SB binder has advantages in better dry and wet pick strength and lower Vandercook slope whereas the SA binder has faster wet glue setting. The PVAc binder requires about 9% more binder to achieve similar hot melt glue strength and at the higher binder levels, has lower IGT dry pick and ink density than the SB binder but similar passes to fail, slope and wet strength. In all three cases, the least efficient is the VA binder, but had faster wet glue setting, and at high calcined clay levels, the VA could not achieve 100% fiber tear for hot melt glue within the limits of the predictive models. At the lower calcined clay levels, the VA binder required about 19% more binder to achieve the target hot melt glue strength, had similar passes to fail, slope and brightness, but lower in wet strength, IGT dry pick and ink density compared to the SB binder.
We showed that increasing binder level improved wet and hot melt glue strength, IGT dry pick, passes to fail, and reduces Vandercook slope. Reducing binder level increased coating brightness, regardless of the type of binder. Very high binder levels caused increased water repellency, due to the coating structure being more closed, which resulted in interference in ink laydown.
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