#in-depth
29 May 2019
Evaluation of the suitability for use of polymeric materials in microwaves

Mori 2A has recently commissioned a study based on scientific literature in order to verify the safety and suitability for microwave use of polymeric materials, with particular attention to polypropylene.
Our products, as indicated by the label, are subject to limitations regarding the maximum temperatures of use for each type of polymer: PP max 80°, PC max 110°, Tritan max 90°.

Some competitors report much higher temperatures, so Mori 2A was interested in understanding if studies conducted and reported in the literature define the heat resistance through microwave radiation of the above polymers and their maximum temperatures of use, compatible with the safety of the object in terms of technological compliance and migration of contaminants.

SUBJECT

Evaluation of the suitability for use of polymeric materials in microwaves: evaluation of scientific literature related to the safety and technological conformity of materials

FOREWORD

The requester of the present study has requested the evaluation of the scientific literature concerning the safety and suitability for microwave use of polymeric materials.

The customer indicates on the label some limitations on the maximum temperatures for each type of polymer: PP max 80°C, PC max 110, Tritan(TM) max 90°C.

Some competitors report much higher temperatures. The customer is therefore interested in understanding whether studies conducted and reported in the literature define the resistance to heat by irradiation with microwave of these polymers and their maximum temperatures of use, compatible with the object safety in terms of technological compliance and migration of contaminants.

In the bibliographic research, particular attention was paid to the studies carried out on Polypropylene.

STANDARDS AND NORMS FOR THE EVALUATION OF SUITABILITY FOR MICROWAVE OVENS

For materials and products intended for use in microwave ovens, the migration test shall be usually carried out in a conventional furnace, after the selection of conditions of appropriate time and temperature. Tests are usually carried out on the basis of the following temperatures and there is also a method for determining the temperature of materials and plastic objects in the plastic-food interface, EN 14233: 2002 (Determination of temperature of plastics materials and articles at the plastics/food interface during microwave and conventional oven heating) in order to select the appropriate temperature for migration testing.

Consequently, the migration tests carried out by the laboratories do not test the migrations directly related to the treatment of polymers with microwaves.

Regarding the microwave resistance of plastic materials, there is a specific standard, EN 15284. This technical standard does not require the use of simulants; the object is simply heated in microwave. The plastic sample is heated under two different conditions. Then the temperatures on the surface of the material are recorded by means of a temperature probe, and one evaluates any physical changes according to the principles set out in the standard. In fact, the standard states a table with possible modifications to be evaluated.

When the materials pass the test, the companies usually insert on the object the symbol of the “microwave safe”.

However, these are individual stress sessions, which do not really correspond to actual use.

Consequently, they also propose internal methods in which materials are stressed several times over, using water inside them.

At the end and during the tests, any physical changes are evaluated according to the same principles as those reported below in standard EN 15284.

The behavior of the material from the point of view of physical changes is then assessed by subjecting it to microwaves, a treatment that involves particular reactions in polymers.

It is clear that in the definition of the conditions of use, both factors, microwave and temperatures, must be considered, and it is therefore advisable to take a brief look at them.

SCIENTIFIC BIBLIOGRAPHY: EFFECTS OF MICROWAVES AND TEMPERATURE ON THE MATERIALS

Microwaves are electromagnetic radiation with wavelengths between those of the shorter radio waves and infrared waves. They have frequencies between 300 MHz and 30 GHz; they are widely used, in both domestic and industrial environments, for heating, thawing and, in part, sanitize food products even if they are already packaged.

The behavior of food contact materials (MOCA) irradiated with these radiations is very diverse and is related to their chemical nature and their molecular organization. If the material contains polar molecules or free ions, these tend to move and to orient themselves according to the orientation of the electric field of the radiation that invests them and that varies with high frequency as shown in the image below:

These movements give rise to shocks that lead to dissipation of kinetic energy which are typical of materials that absorb microwaves.

The table below shows the behavior in response to microwave irradiation of some materials commonly used in food packaging.

Materials intended for use in a microwave oven may or may not be transparent to the consumer, depending on whether they are simply used to heath food or whether they are to be used to assist in cooking. Inert materials include glass, paper and plastics are transparent to microwaves, so they do not heat up directly, and allow the absorption of the maximum possible amount of energy by the food.

The most common packaging materials are polypropylene (PP), which is a good vapor barrier, and the crystalline polyethylene terephthalate (CPET) because, having points of fusion above 210 °C, are suitable for many types of food. CPET has the additional advantage of being suitable for both microwave and traditional ovens.

CPET can be paired with amorphous polyethylene terephthalate (APET), particularly for meat, poultry and fishery products; due to its crystalline nature induced during the process, CPET provides stability at high temperatures

In addition to the behavior of polymers with respect to microwaves, it is essential to evaluate the behavior of polymers with respect to temperature.

As it is well known, temperature has a decisive influence on the physical state of the materials and, as a result, polymeric materials also undergo substantial changes in their properties in terms of temperature function. However, while crystalline materials at the melting temperature pass directly from the solid state to the liquid state, in the case of amorphous or poorly crystalline materials, the passage is more complicated because of the reduced mobility of the constituent molecules.

The amorphous polymeric materials, as the temperature increases, have two transitions in correspondence of two temperatures called “softening” (Tr) and “glass transition (Tg), respectively.

The glass transition Tg is the transition from a situation in which the polymer is relatively rigid (glassy state) to a situation of considerable malleability, consisting of the so-called gummy state. The softening temperature Tr represents the transition from the gummy state to the liquid one.

In the case of a partially crystalline polymer, the glass transition does not change the mechanical properties very much while, on the other hand, they worsen considerably when the polymer, once it has reached the melting temperature, melts.

For all types of polymeric materials there is also another very important parameter, which is represented by the limit temperature of chemical stability (TL), beyond which the polymer undergoes irreversible transformations and / or degradation leading to the loss of its mechanical qualities.

Both the melting temperature and the glass transition temperature are very important parameters for industrial applications of polymeric materials. They define, respectively, the upper limits and lower temperature allowed for numerous applications.

The melting of a polymer takes place within a temperature range, and so, accordingly, there is a range of melting temperatures rather than a single melting temperature.

This is due to the fact that each polymer is composed of molecules that have a certain diversity of molecular weights, and that Tm depends, in turn, on the molecular weight. For most of the polymers, the melting temperature range is normally in the order of a few degrees Celsius.

MICROWAVE, TEMPERATURES AND MIGRATIONS OF CONTAMINANTS

The dynamics of migration from polymeric materials have been scientifically rationalized by using the Fick’s laws of diffusion; the relationships that correlate the diffusion and solubility coefficients to temperature are of an exponential nature and follow the law of Arrhenius; a temperature increase, exponentially rises the spread and therefore the migration of contaminants.

It is therefore essential to define a safety temperature within which the use of the material in microwave is safe.

SCIENTIFIC BIBLIOGRAPHY: COMPARISON OF THE BEHAVIOUR OF DIFFERENT MATERIALS IN MICROWAVE OVENS AND DEFINITION OF CRITICAL TEMPERATURES

The scientific literature reports numerous in-depth studies on the migrations following the use of polymers in microwave ovens. For the analysis of migration, the most studied packaging are the materials made of PVC, PP, PET, PE, PA / nylon and PS. The most interesting chemical compounds include DEHA, ATBC and other plasticizers for the PVC, antioxidants for PP and PE, oligomers and in particular acetaldehyde for PET, caprolactam monomer for the PA and styrene monomer for PS, studies show migrations of BPA from PC materials.

However, the study of migration should not be limited to these chemicals only, as the migration of other chemicals may also occur, depending on the conditions of the study, the nature of chemicals, and the complexity of food. It is not possible to conclude unequivocally that the migration of substances for a particular polymer has a level of higher migration than other polymers, since the amount of substances that migrate in the food depends on the initial concentration of substances in the polymer.

This is the summary of a Review of numerous papers on the subject.

An interesting thesis on a study carried out in Stockholm showed the effects of heating at microwave (and migrations) on polycarbonate, poly(ethylene terephthalate) and food packaging in polypropylene, heated in different food simulants and foods, with respect to the migration of chemical compounds and degradations of the polymer after traditional heating.

Significant degradation of antioxidants has been observed, including Irgafos 168 and Irganox 1010 in PP materials, as well as degradation of the PC polymer and PET, which occurred during microwave cooking after prolonged heating of the 80 °C in food simulants containing ethanol. No degradation was observed during heating at the same temperature.

Prolonged heating to 80°C in the microwave also caused a faster migration of oligomers cyclic from PET and migration of poly(ethylene glycol) from PP to ethanol and isooctane, with respect to migration during conventional heating.

Microwave heating of food in plastic packaging could in some situations, in contact with specific foodstuffs, lead to degradation of the additives incorporated or of the polymer, leading to increased migration of degradation products. Heating methods should therefore not be used during the tests to assess the potential for migration of objects that can be heated by microwaves.

In the Work produced in 2001 by the University of Zaragoza (C. Nerin et all) it is recalled that previous studies (Nerin et al., 2001) have demonstrated that most plastics (PP, PC, etc.) during the heating in a microwave oven for 5 minutes reach temperatures around 90 °C, and that in some cases they could reach temperatures higher than 180 ° C. The work led to the conclusion that plastic containers for heating food in the microwave are not inert and when used to heat food, they can release different compounds, some of which are toxic to humans, when the container itself reaches 100 ° C. Significant differences were found in different plastic materials studied, the polypropylene with 20% talc showed to be a more inert material than PC, SAN, PP copolymer and traditional PP. All materials subjected to temperatures above 100°C have showed releases of volatile substances.

Other work focuses on the microwave heating time of different polymers in different simulants; in the work produced by the Rochester Institute of Technology, the results show that migrations are dependent on the heating time in microwaves and by the polymer. The polystyrene (PS) caused the most relative rapid migration in olive oil while polyethylene terephthalate (PET) has the highest relative migration in the simulant containing 15% ethanol. In addition, acetaldehyde, which can be dangerous for consumers, was found in both 3% aqueous acetic acid and olive oil after 10 minutes of microwave use from materials in PET.

With regard to polypropylene, several works point out that the migration of contaminants is also considerably variable within the same polymer; for example, this is the summary of the work “The significant effect of polypropylene material on the migration of antioxidants from food container to food simulants” in which different types of polypropylene – polypropylene homopolymer (PP), propylene random copolymer (PP-R) and propylene-ethylene copolymer (PP-C) – release substances when subjected to 1h at 80°C in a microwave oven.

About polypropylene, very interesting is the work produced in 2012 at Kasetsart University of Bangkok on different PPs containing different amounts of talc. The work showed that loading of talcum powder particles has improved resistance to high temperatures in microwave applications. In addition, the talc as reinforcement has improved other properties such as compressive strength.

CONCLUSIONS

The works found in literature are very heterogeneous and focus on many themes, such as evaluation of typical contaminants that can be released by different types of polymers, different behaviors with different food simulants of the various materials used, among which mainly PVC, PP, PET, PE, PA/nylon and PS were studied. No studies have been found on Tritan. Publications often focus on techniques and analytical methods used.

It was therefore not possible to find works specifically focused on the definition of maximum temperatures of use for each polymer to be used in the microwave oven, nor a correlation between possible migrations and properties typical of polymers such as Tg, Tm, etc…

However, extrapolating the information published in different studies focusing on subsequent migration after heating of polymers in microwave ovens, it is reasonably possible to deduce that heating times in microwave ovens longer than a few minutes and temperatures higher than 80-100°C depending on the polymer, should be avoided.

With regard to polypropylene, studies often report the migration of antioxidants; some works show that changes in the composition, and additives of the polymer play an essential role for microwave resistance.

Finally, it must be said that, in the light of the bibliographic study, given the variables involved and the research and innovation in material, it is not entirely possible to rule out that some materials can withstand for a few minutes higher temperatures, if the materials have been properly formulated and tested for the specific use.