(385ak) Catalytic Dehydration of Polyvinyl Alcohol over Solid Acids in Both Molten and Solvent Systems

Multilayer packaging films often use polyethylene (PE) as the major polymer and polyvinyl alcohol-co-ethylene (EVOH) as one of the layers for its oxygen-barrier properties. Physical recycling of these films is challenging since melts of the film show poor reusability due to polymer segregation. One option to address this problem is selective conversion of EVOH via dehydration under mild acid catalytic conditions, followed by hydrogenation of the resulting carbon-carbon double bonds to enhance compatibility with PE.

In this study we investigate the use of zeolites and other solid acid catalysts to carry out the first step, dehydration of PVA. Not surprisingly, in the molten phase dehydration rates are strongly influenced by catalyst pore size and acid site density indicating strong mass transfer effects. In order to quantify the extent of mass transfer limitations, the catalysts were characterized using IPA-TPD as well as FTIR measurement of adsorbed pyridine and di-tert-butylpyridine to quantify total acid sites and external acid sites, assuming that the di-tert-butylpyridine is able to access sites only in the pore mouths. Experimental rates of EVOH dehydration were then used to determine separately the rates on internal and external acid sites. Comparison of these rates with rates measured in solvent systems was used to further elucidate external mass transfer rates in these complex polymer systems. This approach offers valuable insights for optimizing acid catalysts for recycling multilayer films containing PVA, thus advancing sustainability in polymer recycling processes.