Study On The Relation Between Structure And Plasticizer Migration Of Starch-based Food Packaging Materials

Alongwith increasing attention focused on food safety and protecting environment, theeco-friendly food packaging material with favorable safety and qualified performance hasbecome the research hotspot in the food packaging industry. Consequently, biodegradablenatural polymer packaging based on renewable resources has attracted more and moreinteresting. Thereinto, as an abundant natural resource with low price, starch has been appliedto produce the starch-based food packaging material, which is considered as one of the mostpromising novel packaging. However, it is hardly possible to avoid plasticizer, which is anessential processing agent involved in the design and manufacture of this novel packagingmaterial, migrating from the matrix into the food system. The plasticizer migration wouldchange the structure and performance of the packaging material, which could lead to thegradual decay of the protection to the food inside. Therefore, in this academic dissertation, itwas proposed as the breakthrough that the relations between plasticizer migration andstructural changes of the starch-based film, including the microstructure and the interfacecontacted with food system, as well as during thermal processing. Moreover, the interaction,crystalline structure and ordered aggregation structure on multiple scales, which changedwhen the material contacted with different food simulants and involved in thermal processingwere discussed, as well as the effect from these structural changes. Finally, themicro-structural factors of starch-based film that restricted the plasticizer migration wereobtained.Hydrophobic starch ester with high substitute degree (DS=2.49) and triacetin were chosenas the base material and plasticizer. Modern analytical techniques were applied to detect theeffect from plasticizer distribution and plasticizer-starch ester interaction on the crystallinestructure, microstructure and thermodynamics properties. Results showed that new interaction(mainly as Van der Waals force) formed to enhance C-O in plasticizer and starch estermolecules and activate the whole molecules. Meanwhile, the distance betweenmacromolecular chains was enlarged, and the chain mobility was increased to approach toeach other and arranged in more order, which facilitated crystallite formation dispersed withinthe ordered microregion, restricting the chain mobility to some extent. Simultaneously, the plasticizer molecules previously distributed within crystallite were squeezed to formamorphous “plasticizer-rich domain”. The amorphous region was enlarged to provide largerfree volume for macromolecular chains, thus promoting relaxations on different scale. Inaddition, the dimension of chain aggregation involved in glass-rubber transition increased.When starch-based film contacted with the food stimulants, based on the different affinityamong simulants, plasticizer and film matrix, the diverse swelling behavior occurred,resulting in different migration process. Distilled water and4%acetic acid aqueous with lowaffinity exerted mild swelling effect on the films, and the migration obeyed the Fick’s secondlaw. The non-Fickian model of diffusion then began with the enhanced swelling, while thewhole plasticizer migration obeyed first-order kinetics. Comparatively, the swelling effectfrom65%ethanol aqueous and n-hexane was more obvious, which caused the comparablefilm relaxation and plasticizer diffusion, non-Fickian diffusion thus occurred.Alongwith the plasticizer migration and the permeation of solvent in food simulants, theplasticizer-starch ester interaction and the chemical surroundings for plasticizer were changed,reducing the stability of C-O and stabilizing the plasticizer molecules. At the same time, theinter-and intra-macromolecular interaction was enhanced to make the amorphous chainsshrink, which compressed the whole ordered microregion with smaller and polydispersed size,the crystallite inside was compressed as well. The mobility of macromolecular chains wasdirectly weakened because of the plasticizer migration. The formed compact aggregationstructure and the crystallite exerted more intense restriction on the relaxations within the film.It was meant that when immersed in food simulants, more compact aggregation structureformed to exert more intense effort on plasticizer molecules. Consequently, it was moredifficult for solvent diffusion and further plasticizer migration. Accordingly, the physicalmodel of influencing mechanism was established to describe the structural changes affectingplasticizer migration in different food simulants.During the thermal processing, more distilled water entered into the film matrix toaggravate the swelling, result in aggregation structural changes and thus accelerate theplasticizer migration. Due to the compact structure within the ordered microregion, morewater molecules were promoted to permeate the amorphous region, so as to delaying themacromolecular shrink after plasticizer migration. On the other hand, the chains mobility was enhanced by thermal treatment, which caused the macromolecules at the edge of the orderedmicroregion to become amorphous. The whole ordered microregion was decreased withslightly destructive crystallite inside, the blocking effect on plasticizer migration and waterpermeation was thus weakened. More water entered the film matrix to dissolve plasticizer,and thermal treatment increased the plasticizer mobility as well, consequently, the plasticizermigration was facilitated. Accordingly, the mechanism model describing the structuralchanges affecting plasticizer migration during thermal processing was proposed.The relevance connected plasticizer migration with microstructure and interface structurethe starch-based film, as well as the changes during thermal processing, was proposed in thisdissertation. Moreover, the multilevel structural changes of the material and its effect onplasticizer migration were understood, which could provide basic data and theory forrestraining plasticizer migration from biodegradable starch-based food packaging material andguiding the reasonable design and safe application of this novel packaging material.