Download AKTS-SML Monolayer Version (= Freeware)
Version 4.53 | *.zip | 5.2 MB

(Note: Simply download the above *.zip file and doubleclick on it.)

Download AKTS-SML Viewer Version (Up to 10 layers)
Version 4.53 | *.zip | 11.0 MB

(Note: Simply download the above *.zip file and doubleclick on it.)

Why install SML Software?

• Detect Contaminant Migration Events
• Forecast Migration Accurately
• Improve Packaging Design
• Check Compliance with Legislation

The Software

The version 4 of the program SML is a joint development of the Swiss Federal Office of Public Health (BAG) and the company Advanced Kinetics and Technology Solutions AG (AKTS AG).

SML is a simulation program. Using Finite Element Analysis (FEA) it can be used to predict the amount of a substance (additive, contaminant or residual monomer) that migrates from a polymeric material or article, e.g. packaging to the contacting medium, e.g. food during a given time. This program is firstly designed as a scientific tool for people active in material science: the material producer could check the compliance of a material with specific migration limits (SML), the laboratory specialist could use the calculated concentrations to design the experimental conditions of a migration test. It also includes a non-exhaustive list of starting substances (monomers and additives) used in the fabrication of plastic materials and articles coming into contact with food.

SML-Software focuses on the simulation of release of additives from multilayer materials both in extended temperature ranges and under temperature conditions at which ordinary investigation would be very difficult. These difficulties are prevalent when temperature fluctuates during the observation time. Complex surrounding temperature profiles can be considered such as stepwise, modulated, shock and additionally for real atmospheric temperature profiles (up to 7000 climates). The technique allows the simulation of complex packaging (different geometries and up to 10 layers). By comparison, direct investigation of such diffusion processes would be very complex under these various possible conditions. Calculation of the diffusive process is based on Fick’s law. It considers a Arrhenius Type dependence of the diffusion process from temperature and the last models for estimation of diffusion coefficients like the validated model developed by Pringer with refined Ap constants [2,3]. Diffusion and concentration distribution inside all polymeric material layers can be computed for both: migrants released and components taken up from the contact medium.

[1] COMMISSION DIRECTIVE 2002/72/EC of 6 August 2002 relating to plastic materials and articles intended to come into contact with foodstuffs (OJ L 220, 15.8.2002, p. 18).
[2] T. BEGLEY, L. CASTLE, A. FEIGENBAUM, R. FRANZ, K. HINRICHS, T. LICKLY, P. MERCEA, M. MILANA, A. O’BRIEN, S. REBRE, R. RIJK, O. PIRINGER; Food Additives and Contaminants, January 2005; 22(1): 73–90.
[3] Practical Guide of the European Commission in support of the Plastics Directive 2002/72/EC.

Numerical algorithms

The concentration profile inside the different layers is calculated by Finite Element Approximations. Besides Finite Element Analysis (FEA), there are other methods for solving Partial Differential Equations (PDEs). Monte Carlo Method is one of these. But there are others, the so-called spectral and variational methods, for example. However, FEA is often preferred by practitioners in e.g. solid mechanics or structural engineering, because these methods allow considerable ‘freedom’ in putting computational elements where they want them. This is important when dealing with high irregular geometries or when dealing with complex kinetic reaction process. Spectral methods are sometimes preferred for very regular geometries and smooth functions; they might converge more rapidly than (FEA), but they sometimes do not work well for problems with discontinuities.

Exposure and Compliance 

Exposure

Because consumers within the EU are likely to be exposed to a range of different products or articles containing substances (chemicals) with safety concerns the total consumer exposure to these compounds from all identified pathways forms the basis for exposure assessment. The most important exposure pathways are food, indoor air, and dermal contact with different polymer materials.

Substances with safety concerns are present in final products and articles at a level to accomplish the required technical effect. Due to thermodynamic driving forces interaction between materials and contacting media occurs and these compounds are transferred by release (migration and emission) and/or distribution (partitioning) to food, the ambient air, or directly to the consumer through dermal contact. The amount of compounds transferred is determined by the conditions of use (time and temperature, area to volume ratio) of the final products or articles. Typically exposure factors and/or thermodynamic parameters (partition coefficients, diffusion coefficients) are used to model exposure scenarios. Based on the consumer habits, consumption behaviour and anthropometrics exposure pathways result. The consumer is exposed only to a limited extend to the individual pathways, such that their specific contributions in terms of real exposure levels must be considered.

Interaction

Interaction between polymeric materials and contacting media means transfer of substances (chemicals) from the materials to the media in contact resulting in changes of its quality/properties due to this process. From the point of view of consumer protection the focus has to be on the health risks caused by substances possibly released from the final products and articles. However, migration may lead to a change of the media quality with time and therefore is one of the key topics for quality assurance.

Depending on factors like concentration, mobility and solubility in the polymeric material as well as on temperature and duration of the contact, substances inevitably migrate to a more or less extent, into the contacting medium. Migration causes a change of medium composition with time. Due to this fact the European regulation on food contact materials for example is based on a positive list of substances under exclusion of all others with specific migration limitations on the food side. Migration of a substance from complex materials and articles like multilayer materials or composites are considered as a succession of individual mass transfer processes from which the slowest one defines the overall time scale. In most cases of practical relevance the diffusion process within the contact material is the rate determining step of the migration process. On the other hand the relationship between the solubility of a substance in the material and the contacting medium, i.e. the partitioning process, strongly influences the overall amount of a substance migrating into the medium and consequently the amount of migrant still left in the contacting material.

Compliance

Realistic evaluation of the health risk associated with polymeric materials and articles is necessary. The risk-to-benefit ratio regarding the use of polymeric materials is negligible compared to not using them. Even though this is unarguable, care must be taken to ensure consumer safety. Thus every regulation should consider the following points: i) the transfer of substances from polymeric materials and articles to contacting media may be undesirable and ii) unavoidable mass transfer from state-of-the-art products should not be prohibited, if the substances do not constitute health hazard and are technical unavoidable in an optimized system. The principal optimization criteria for modern products are their technical properties, consumer safety, environmental compatibility, and low cost. Meeting these points benefits the consumer and encourages technological progress.

Beside the acute toxicological properties of substance, their chronic mutagenic and carcinogenic activity is most important. The application to humans of the results of “No Observed Adverse Effect Level” (NOAEL) experiments on animals and the validity of subsequently derived “Acceptable Daily Intake” (ADI) values may be uncertain. “Estimated Daily Intake” EDI levels have additional uncertainties arising from regional and temporal considerations. The relevant exposure varies with season, geography, and socioeconomic or ethnic background. The amount of substances transferred from the polymeric materials and articles contacting media and hence the consumer exposure can vary over a large range. The theory of diffusion can be used to estimate mathematically the quantity of substances transferred. However, many other complex factors affecting migration cannot be anticipated.

Compliance with world wide regulation like FDA's Code of federal Regulation, EU Regulations and Directives, German BfR Recommendations, etc. is compulsory for polymeric materials and articles regarding positive listing and associated limits. The permissible levels of components to which the consumer is exposed are a function of above considerations. As a consequence of these uncertainties, all substances can be divided into categories according to their health risk, which than can be uses as a basis for regulation. One end of the spectrum consists of substances that poses no risk (those substances which are inert or naturally occurring and are toxicological inactive) and the other end are substances that must be excluded from use because they are recognized as toxicological dangerous. Between this two extremes are the remaining substances which in more or less detail require toxicological or migration studies from which useful limits can be derived.

The verification of compliance of food contact materials by the application of recognised diffusion model was introduced recently in the European legislation [1].

References

1. EU legislation Plastics Directive 2002/72/EC

Migration modelling

Introduction

Polymeric materials (Plastics, Coatings, Rubbers, etc.) transfer their components (low molecular weight substances like monomers, additives, etc.) to contacting media to more or less extent, due to thermodynamic driving forces. This phenomenon is called mass transfer and the common wording is migration and/or emission depending on the type of contacting medium, i.e. liquid or gaseous. Due to transfer of substances (chemicals) from the polymeric material to the contact medium the substance amount in the polymeric material decreases and a concentration profile is established. The amount of substance is depleted first at the material/medium interface. The release of the substances from polymeric materials to contacting media obeys in most cases the law of diffusion because the diffusion process is the rate determining step. In case of gaseous contact media the evaporation process may become under certain circumstances the rate determining step. It is obvious that mass transfer can occur as well from the contacting medium to the polymeric material.
The release of substances from the polymeric material was historically assessed by experimental testing under conventional test conditions. Due to advanced understanding of the mass transfer processes and translation into science based computational tools simulation became the method of choice.

Simulation & diffusion models

The complex migration process (mass transfer) is reduced to the rate determining step, i.e. the diffusion process. Correspondingly migration modelling is based on diffusion models. The migration process may be simulated according to real use or according to conventional test conditions. Modelling migration according to conventional test conditions makes a one to one comparison between simulation and experimental results possible
From mathematical point of view the system polymeric material in contact with a medium is

To simplify the mathematical problem it is assumed that the polymeric material and its individual layers as well as the contacting medium are ordered parallel to each other. The diffusion equation is a partial differential equation which can be solved with recognised numerical algorithms. For more details regarding the numerical algorithms employed the reader is referred to the help menu of the SML software.
With the numerical solution of the diffusion equation the migration kinetic, i.e. migration of substances from the polymeric material to the contacting medium with time and the concentration profile of the substance in the system can be calculated.
One has to distinguish between monolayer and multilayer materials. If monolayer materials are in contact with a liquid medium the system is described by two mass transfer constants, the diffusion coefficient of the substance in the polymeric material and the partition coefficient of the substance between polymeric material and contacting medium.
If multilayer materials with n layers are in contact with a liquid medium, all mass transfer constants describing the system must be considered, i.e. n diffusion coefficients of the substance in each layer
As long as the contacting medium is a liquid or a gas the diffusion process in the medium is neglected because the diffusion rates are much lower compared to solids like polymeric materials. If the contacting medium is a solid like some foods or another polymeric material, the diffusion process in the medium must be considered accordingly.

The solution of the diffusion equation requires variables for the simulation of migration kinetics and concentration profiles:

• geometry related variables (thicknesses, contact area, volumes) as well as time and temperature, e.g. according to real use or conventional test conditions
• initial or residual concentration of the migrant in each layer including the contacting medium) e.g. residual amount of monomers or initial concentration of additives.
• mass transfer constants (diffusion- and partition coefficients). These are not available from the literature and must be estimated according to generally recognised and validated estimation procedures based on scientific evidence.

A model is valid if it describes precisely enough the behaviour of the real system. The comparison between time dependent experimental migration data and simulated data is suitable for the validation of diffusion models. This procedure was chosen in the frame of several EU-projects as well as many publications in the scientific literature. It was shown that migration processes in the system polymeric material in contact with a medium can be well described by the solution of the diffusion equation.

Estimation of mass transfer coefficients

Diffusion coefficients

The diffusion coefficient is a time related mass transfer constant which specifies how fast a substance is released from a polymeric material to a contacting medium by diffusion.

Partition coefficients

The partition coefficient is a thermodynamic mass transfer constant which considers the equilibrium concentrations of a substance in the polymeric material and the contacting medium. The partition coefficient defines the maximum amount of substance which can be transferred from the polymeric material to the contacting medium.

For the estimation of diffusion- and partition coefficients there are several scientific approaches:

For the estimation procedures of diffusion coefficients according to Arrhenius and Piringer experimental testing is required to determine material specific parameters like the pre-exponential factor, D0; the activation energy, EA; the polymer specific constant, AP and the polymer specific temperature constant, tau (corresponds to the deviation from the reference activation energy, EA=EA,ref+t·R with R the gas constant). The influence of the migrant size to the diffusion rate may be considered by its molecular weight. The AP-value concept of Piringer was validate in the frame of an EU-project, refined in further EU activities and validated for migration modelling from plastics into food simulants (polymeric material in contact with liquids).

The estimation of diffusion coefficients according to Brandsch is an ab initio technique based on well known thermodynamic parameters like the glass temperature of polymeric materials. Validation of the new procedure is possible against the existing AP-value approach as well as against experimental data.

The estimation of partition coefficients is less advanced. Nevertheless some scientific approaches exist based on a group contribution method developed by Piringer or an empirical approach developed by Brandsch in the frame of the Foodmigrosure EU-project. The empirical approach by Brandsch considers the polarity of the migrant by its octanol/water partition coefficients and sets it in relation to the partition coefficient between polymeric materials and real foods or food simulants. Knowledge of the water solubility may allow for the estimation of worst case partition coefficients between polymeric materials and aqueous media.

AP value concept (Piringer)

The estimation procedure for diffusion coefficients according to Piringer, the so called AP-value concept, requires the experimental determination of the polymer specific constant, AP-value and the polymer specific temperature constant, tau once for each polymer type. For each polymeric material a minimum of time dependent migration experiments must be run at different temperatures. From the experimental data diffusion coefficients are determined. These are translated into AP-values according to the relationship developed by Piringer. From all available AP-values an upper limit value for one polymeric material is calculated as the 95-percentile of that data set. Provided the data set available is big enough, the AP-value for that polymeric material is considered to be validated.
Experimental time dependent migration data, diffusion coefficients and AP-values derived there from are collected and evaluated by the Modelling Task Force at the Joint Research Centre of the EU-Commission.
For most of the polymeric materials used in food contact materials upper limit AP*-values and mean tau-values are listed in the Practical Guide supporting the Plastics Directive 2002/72/EC of the EU-Commission. Upper limit AP*-values result in overestimated migration values in support of consumer safety.

Overestimation and consumer protection

It is in support of consumer safety to develop procedures for estimation of mass transfer constants which systematically overestimate the migration behaviour of real polymeric materials. Due to systematic overestimation the risk of underestimation compared to the real system is minimized.
Due to simulation of one single migration cycle it is easy for food contact materials to implement overestimation. The use of upper limit AP-values in the estimation procedure of diffusion coefficients and lower limit partition coefficients results in estimated migration value which are higher compared to the real ones.

Repeated use

Polymeric materials with repeated use or dynamic flow behaviour of the contacting medium need specific considerations. The experimental conventional test conditions as well as migration simulations account for the repeated use or dynamic flow behaviour by implementation of several migration test cycles. Overestimating the migration in the first migration cycle may cause underestimation in later migration cycles. This can happen only if the overestimation based on diffusion and partitioning is very high and more than 50% of the substance migrates out of the polymeric material in the first migration cycle. In these special cases, which can be easily identified during simulation, only the first migration cycle can be used for comparison with a specific migration limit.

Legal background

Use of simulation (introduction)

Polymeric materials are used in a variety of applications. Through all the following applications direct or indirect consumer exposure with substances (chemicals) from polymeric materials may occur.

• food contact materials and articles
• toys
• textiles
• child use and care articles
• medical devices
• drinking water applications
• household articles
• consumer goods
• construction materials
• automotive applications
• electronics applications
• other materials and articles

Due to interaction between polymeric materials and contacting media, and consumer exposure resulting there from, legal requirements are set at national and international level. For given applications like food contact materials or toys detailed legal requirements in terms of substance specific migration limits are set by the applicable legislation. Historically the experimental approach, i.e. experimental migration testing under conventional test conditions was used for compliance assessment.

Migration modelling is a valuable tool to estimate the type and extend of interaction between polymeric materials and contacting media. Estimating the mass transfer based on the relevant physical and chemical parameters, i.e. mass transfer constants, enable professionals to estimate consumer exposure at all stages of the production chain.

In the last years migration modelling got more and more accepted and was first introduced in legislation for plastic food contact materials and articles allowing for compliance assessment with applicable legislation.

Food contact materials

EU legislation

Migration modelling was introduced in the EU legislation for materials and articles intended to contact food with the 6. Amendment of Directive 90/128/EEC, today consolidated in Directive 2002/72/EC. The consolidated Plastics Directive 2002/72/EC was amended itself four times until now.
Migration modelling was implemented in the EU legislation with the following wording:

Detailed information about the general recognised diffusion models based on scientific evidence can be found in the Practical Guide in support of the Plastics Directive as well as on this web site.
According to Article 8 of Directive 2002/72/EC migration modelling can be used to demonstrate compliance of food contact materials and articles with the applicable legislation. To demonstrate non-compliance, i.e. by official control, the experimental approach is obligatory.

Other applications

Strong efforts are made to get migration modelling accepted for all applications were polymeric materials are used like:

• toys
• textiles
• child use and care articles
• medical devices
• drinking water applications
• household articles
• consumer goods
• construction materials
• automotive applications
• electronics applications
• other materials and articles

Because interaction between plastic materials and contacting media, i.e. migration and emission processes follow the same laws independent from the application one can expect that this objective will be achieved in near future.