Molecular Dynamics Simulation And Experiment Investigation On Migration Of Additives From PET And PP Materials Into Food

Migration of chemical additives from plastic food packaging into food had raised widespread concern in communities.In the last decades,researchers have carried out a lot of toxicological and migration experiments to monitor and evaluate the hazards of additive's migration.Migration experiments are commonly time-consuming,expensive and often hard to carry out at extreme conditions.Consequently,mathematical model as an alternative to predict the migration of additives have been developed.Most models are empirical or semiempirical equations,which are used to estimate the dynamic parameter and predict the migration level of additives from polymer packaging.However,the availability of physicochemical properties for a wide range of substances and polymers is relative rare,which will lead to the rough prediction.More importantly,both experimental method and mathematical model are lack of the intrinsic acknowledge about the micro structure of polymer and migration mechanism of additive.In recent years,with the ever-growing computational power and the development of mechanics theory,molecular dynamics(MD)simulation based on classical mechanics has been applied to investigate the diffusion of small molecules(gases and water)in polymers.The greatest advantage of MD simulation is that it provides material's structural and dynamic details that are difficult to be obtained with experimental techniques,which elucidate the physiochemical phenomena at molecular level.However,the dynamic research about large molecule is relative rare,there is also no systematic attempt to investigate the transport properties of additives from packaging polymer to food using MD simulation to our knowledge.In this paper,MD simulation technique was used to investigate the diffusion of additives in polyethylene terephthalate(PET)and polypropylene(PP)material,and the migration of additives from PET and PP material to simulated food.The overall goal is to obtain the diffusion coefficients of additives in the migration process,understand the influencing factors and mechanical mechanism of migration from a molecular point of view.Amorphous models that contained additive molecules and PET or PP chains were constructed to simulate the diffusion of additives in polymer matrix.Diffusion coefficients of the additives were evaluated from the limiting slope of the mean square displacement as a function of time based on Einstein equation.In addition,different influencing factors for additive's diffusion were examined including the molecular size and structure,the interaction energy between additive molecule and polymer,free volume and the self-diffusion of polymer chain.The results indicate that the diffusion of additive is attributed to the combined effect among these factors: the molecule with large weight/size needs large free volume holes to diffuse as well as to expose lower diffusion coefficient;the strong interaction between molecule and polymer leads to the decrease of diffusion rate;higher temperature makes the polymer chain more flexible and generates greater segmental motions in comparison with low temperature,higher mobility chain can induce larger free volume fluctuation and faster redistribution of free volume,which provide more opportunities for the molecule to diffuse.The movements trajectories of additive molecules at different simulation time suggest that additive molecules with large size and complex structure are not hopping continuously.The larger additive molecules are entrapped in the voids corresponding to local energy minimums for a long time,which are separated by an energy barrier;occasionally when a hole sufficiently large to accommodate the migration molecule is formed adjacent,and the molecule has obtained enough energy,the molecule can undergo spatial motion from one site/configuration to the other,where the energy barrier to be overcome.Few overlapping paths and a relative wide movement range reveal a greater diffusion coefficient for the more active molecule.On the basis of single-phase MD model,a two-phase MD model was established to simulate the migration of additives from PET or PP material to simulated food.The diffusion coefficients and activation energies of additives were calculated based on Einstein and Arrhenius equation,respectively.By calculating the molecule structure parameters,interaction energy between additive molecule and polymer chain,solubility parameter of additive,polymer and food simulant,and the free volume of polymer-food simulant system,the relationship between diffusion coefficient and some factors including the migrant molecule,polymer,the polarity of food simulant and temperature were discussed in detail.The trajectories of additive molecules in two-phase MD model suggest that the migration mechanism of additives is similarly to the diffusion mechanism,the larger additive molecules are entrapped in the local voids for a long time.The two-phase MD model can reveal the microscopic movements of additives between polymer and food simulant.An analytical method for the quantitative determination of 27 kinds of chemical additives in plastic food packaging was developed based on high performance liquid chromatography.Then,PET and PP samples that contained additives were formulated.According to Regulation(EU)No.10/2011,the migration of additives from plastic materials were evaluated by immersing the PET or PP sample in water,acetic acid solution,ethanol solution and isooctane at different temperature,respectively.The partition coefficients of additives were determined from the experimental data considering the final values of concentration as being close to equilibrium.Experimental diffusion coefficients were calculated according to a mathematical model based on Fick's second law.Predicted diffusion coefficients were calculated by the commonly used Piringer model and Limm-Hollifield model.Results show that Piringer model is better than Limm-Hollifield model for estimating the diffusion coefficients of additives.By comparing the diffusion coefficients obtained by MD simulations,experiments and Piringer model,the accuracy of MD simulation technique for estimating the diffusion coefficients of chemical additives were evaluated.Results show that the diffusion coefficients of additives obtained from two-phase MD model were generally within one order of magnitude of the experimental values,and the activation energies from MD simulations were also close to experiments.It indicates that the migration of additives from polymer material to food simulants can be described by the classical mechanics method;the two-phase MD model offers fairly good predictive ability than the single-phase MD model and Piringer model for the additive's migration;MD simulation technique can be used to evaluate the safety of packaging materials.