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Issue 01/2018

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Foam Microwave-resistant

Foam Microwave-resistant sustainable packaging By: Nuria López Aznar Researcher of the Extrusion department AIMPLAS Paterna/Valencia, Spain The consumption habits of our society mark the current trend of food packaging. Aspects such as food safety associated to the growing variety of processed food with demanding conservation requirements and packaging sustainability are very important in an ever-increasing market both in volume and complexity. Plastic materials have very diverse characteristics that allow them adapting to the requirements demanded by packages destined to contain food. Among the different types of food plastic packaging we find trays, both foamed and rigid, made of different materials, which include: • Foamed polystyrene (PS) trays. They are used to package all kinds of food, from fresh meat products to fruits and vegetables. Their main advantage is low density, since the amount of material per packaging is very low. This kind of packages have thermal limitations, since PS is not suitable for microwaves given that it does not withstand their temperature. • Polypropylene (PP) and rigid polyethylene terephthalate (PET) trays. These trays have been established in the last years as a highly demanded packaging due to its versatility, since nuts, processed and precooked products can be packaged in it. In the project SOST-FOAM, in which AIMPLAS (Valencia, Spain) and the company BANDESUR (Alcalá la Real, Spain) work in a project funded by the Spanish Ministry of Economy and Competitiveness, in which foamed packages for prepared food (IV-range and V-range food) with microwave heating resistance from two very different plastic materials are being developed: PP, material with a softening temperature that allows to withstand retort conditions (120 ºC, 30min). Its currently foaming by using physical agents is a handicap due to its rheological characteristics. In the project, tasks are being done to optimize this foaming. • PLA based materials. These materials are biobased and biodegradable, but they are more expensive than comparable conventional plastics and have low softening temperatures. In the project, modifications to improve its thermal resistance and optimize its foaming are being made. When decreasing the packaging’s density, its cost decreases. The achievement of the planned targets involves a series of technical risks that constitute the highest level of innovation in the project: • The foaming of materials such as PP or PLA involves the modifications of controlled processing parameters in extrusion. It is necessary to go from polymer grades with specific properties to get a product with low density and specific cell size and morphology and foam uniformity. • The solubility of the physical foaming agents used is different according to the polymer used. In PP or PLA, solubility is lower than in traditional materials such as PS. It is necessary to adjust the injection conditions of the foaming agent, its percentage and other aspects in order to get foams with the required quality. • The packages developed must meet the specifications defined for the applications proposed. Their food suitability will be evaluated, including the migration tests required. These technical challenges are related to the technical difficulty of producing foams of other materials with the same behavior as PS foams. There are different parameters influencing directly the quality of the foam obtained: • Influence of rheological properties. The melt strength indicates the strength of the material in melted state and therefore the capacity of withstanding stresses in this state. PS is one of the materials with higher melt strength PP has a low melt strength, although the use of high molecular weights and narrow molecular weight distributions increase the melt strength value. The branching degree, the type and length of branches influence positively on the melt strength. PLA does not have appropriate melt strength values. The introduction of chain extender additives to obtain branching polymer chains contribute to minimize this effect. • Influence of crystallinity. Amorphous materials (PS) have a higher solubility with the foaming agent, so they foam more easily than semi-crystalline polymers such as PP, PET or PLA. • Influence of the type of foaming gas used. There is a trend to replace flammable gases by less harmless gases such as CO 2 . [1] The performance of polymers is different depending on the gas used and therefore the foaming degree obtained. 46 bioplastics MAGAZINE [01/18] Vol. 13

Foam Fig. 1: Foamed PS trays • Influence of the processing conditions. It is necessary to use the optimum melt temperature for the formation of foam, to reach the maximum expansion degree. • Influence of the nucleating additive. For the correct formation of a homogeneous foam, an inorganic filler (talc, calcium carbonate or silica) is needed, acting as a nucleating agent for the formation of the cell-structure. The selection of the nucleating agent and the percentage will define the type of cells. Fig. 2: PP rigid tray The solution provided by SOST-FOAM is the manufacturing of foamed packages made of PP and PLA-based materials, with the following advantages: • Reduced weight with respect to PP and PLA rigid trays due to their foamed structure, what involves a lower environmental impact due to the carbon footprint reduction. • Higher microwave-heating resistance with respect to foamed PS trays. It has a microwave performance similar to rigid trays. Fig. 3. Foamed PP trays. • Improved mechanical properties with respect to foamed PS trays. • Lower environmental impact (measured in terms of carbon footprint) than PS, PP and PLA rigid trays. • Cost reduction with respect to PP and PLA rigid trays. References: [1] Tomasko, D.L.; Burley, A.; Feng, L.; Yeh, S.K.; Miyazono, K.; Nirmal- Kumar, S.; Kusaka, I.; Koelling, K. Development of CO2 for polymer foam applications. The Journal of Supercritical Fluids. 2009. 47: 493-499. Fig. 4: Foamed PLA tray bioplastics MAGAZINE [01/18] Vol. 13 47

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