Improving the adhesion properties of polypropylene using a liquid-phase sulfonation treatment

Abstract

This work describes the effects of a stable, short-reaction time, liquid-phase sulfonation technique aimed at improving the adhesion properties of polypropylene. The relationships among SO3 concentration (0.15, 0.2, and 0.3 N), treatment time, surface chemistry, and adhesion debond strength have been investigated for polypropylene (PP) sheets sulfonated with SO3 dissolved in 1,1,2-trichloro 1,2,2-trifluoroethane and neutralized using ammonium hydroxide (NH4OH) or polyethyleneimine (PEI). It was confirmed that PEI neutralized specimens, compared to untreated PP, exhibited a larger increase in debond strength (~269% increase) than similarly treated specimens neutralized with NH4OH (~210% increase). ATR-FTIR spectroscopy indicates the formation of sulfonic acid, ethylenic, ketone and alcoholic hydroxyl groups. These results are supported by X-ray photoelectron spectroscopy (XPS) that show the O : C ratios increasing from 0.03 to 0.25 for both the NH4OH and PEI, and the S : C atomic ratios increasing from 0.0 to 0.05 and 0.06 for the NH4OH and PEI, respectively. Furthermore, XPS examination of PEI neutralized specimens revealed a nitrogenated surface (~6%), providing evidence that PEI had grafted onto the sulfonated surface. The observed increases in adhesion are attributed to formation of polar functionalities and increased wettability (as measured by water contact angle measurements). The neutralizing agent also affects degree of adhesion improvement: the PEI causes larger increases in adhesion compared to the NH4OH. The physical effects of sulfonation were examined using environmental scanning electron microscopy (ESEM), which showed crack formation after 2 min of treatment. Sulfonation times beyond 5 min degraded the polymer surface (severe microcracking and sloughing of the surface layer) decreasing the adhesion debond strength.