The Influence Of High-pressure Processing On The Diffusion Of Functional Additives In PE Film

High-pressure processing(HPP),a newly developed technique,utilizes high pressure above 100 MPa to process pre-packaged food for prolonging the shelf life of food.Great attention has been focused on the mass transfer properties of packaging under HPP because the packaging material is exposed to HPP conditions which may alter its structure,migration of additives and consequently may cause food safety issues.However,the effect of HPP on mass transfer of polymers is still matter of debate.The research on the mechanism of HPP influencing the diffusion of additives in packaging is still limited and insufficient,which restricts the application of this technique to a certain extent.Combining experimental method and molecular dynamics simulation,this paper studied the effect of molecular weight(MW)of polyethylene(PE)and the nature of additives on the diffusion of additives in packaging materials under HPP.The influence of HPP on the microstructure of PE was analysed and the mechanism of HPP affecting diffusion of additives was discussed.Finally,the effect of high pressure and molecular weight of PE on the diffusion of additives was qualified by a new model.The main research contents and results are as followings.(1)The effect of MW of PE on the diffusion of 2,6-Di-tert-butyl-4-methylphenol(BHT)in PE films under HPPThe diffusion of BHT in PE film with four MW was studied by migration test after conducting HPP with various combinations of pressure and pressure holding time.Results showed that high pressure and molecular weight of PE highly affect the diffusion of BHT in PE film.The diffusion coefficient of BHT decreases with increased pressure and MW of PE.The influence of pressure holding time on the diffusion of PE was not significant.We obtained the MW of PE and the pressure-volume-temperature(PVT)relation of PE,which represents the nature of polymer,from the experiments.The PVT result showed that the resistance of pressure varies with different MW of PE.The volume of PE with a lower MW decreased largely,which represented less free volume,thus the diffusion of BHT decreased significantly.The mechanism of pressure and MW of PE influencing the diffusion of BHT was discussed from two aspects,the free volume and the mobility of PE chains.The volume and free volume of PE was extruded by high pressure.The entanglement of PE increases with its MW,resulting in a large viscosity.Thus the increasing friction between the two chains makes it difficult for the movement of PE chains and restricted the diffusion of BHT.(2)The diffusion mechanism of BHT in PE under HPP by molecular dynamics simulationThe diffusion coefficient and trajectory of BHT,the free volume of PE,the mobility of PE chains and molecular weight dependence were analysed by MD simulation.Results showed that the simulated diffusion coefficient was decreased with pressure and MW of PE on the rise,which is in assistance with the results of the experiments.The diffusion coefficient of BHT in PE with a low MW was largely affected by pressure.The free volume fraction and mobility of PE chains decreased with the increasing pressure and MW of PE.And the molecular weight dependence of the diffusion coefficient of PE chains was well depicted by reptation theory.The frequency and length of jumping of BHT were decreased under HPP.The space was limited for movement of BHT with a high MW of PE.And BHT was trapped in the matrix of polyethylene,wiggling to diffusion.(3)Research on the diffusion of different additives in PE film under HPPThe diffusion of four additives(BHT,UV-P,UV-327,and UV-531)in PE was studied by migration tests after conducting HPP with several pressure levels.Results indicated that the diffusion coefficient of additives decreased with the increasing MW of additives at atmosphere,and decreased under HPP.The diffusion coefficient of an additive with a smaller MW decreased significantly under a low-level pressure.When the pressure went higher,the diffusion coefficient of the four additives decreased by around 50%.However,MW is not comprehensive to explain the diffusion difference between molecules with the same MW.Based on the results of the MD simulation,the topological parameters of additives,such as MW,van der Waals volume,the radius of gyration and the dimensionless shape parameter of additives under HPP have an important influence on their diffusion.Interaction between PE and additives was less affected by pressure.(4)The qualified model of the effect of high pressure and MW of PE on the diffusion of additives in PE filmsAccording to the Piringer diffusion model,a new Piringer-HP model was established to qualify the effect of high pressure and MW of PE on the diffusion of additives.The two significant factors,high pressure,and MW of PE were introduced into Ap,which indicates the character of the polymer.With the new Piringer-HP model,the parameters of this model for this research were calculated.This model has high accuracy with good prediction effect on the diffusion of BHT in various MW of PE film.