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bioplasticsMAGAZINE_1306

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bioplasticsMAGAZINE_1306

People Films | Flexibles

People Films | Flexibles | Bags New PLA copolymers for packaging films Stress (MPa) 30 25 10 15 10 5 By Vu Thanh Phuong*, Patrizia Cinelli and Andrea Lazzeri University of Pisa, Department of Chemical Engineering, Pisa, Italy Steven Verstichel Organic Waste Systems (OWS) Gent, Belgium *Vu Thanh Phuong is currently on leave from Department of Chemical Engineering, Can Tho University, Can Tho City, Vietnam. http://cet.ctu.edu.vn/cnhoa/en/ 0 0 25 50 75 100 125 150 175 200 225 250 Strain (%) Figure 1: Mechanical properties of films he increasing concern about the environmental impact and sustainability of traditional plastics has led to the development of new materials derived from renewable sources, in particular for use in the production of bags (shoppers). On the market there are many compostable products based on PLA and polyesters, but none of these has mechanical properties comparable to many conventional commodity plastics. Despite the undoubted advantages compared to traditional plastics, PLA is characterized by a glass transition temperature (Tg) of around 60°C, which makes the material too rigid for applications such as packaging film. There are several techniques available to improve the flexibility of PLA, such as copolymerization, mixing with elastomeric polymers, addition of a plasticizer, etc. In particular the copolymerization of PLA and elastomeric polymers through reactive extrusion produces materials with the necessary properties to produce flexible films, but the products that are currently available on the market contain high amounts of elastomeric aliphatic-aromatic copolyesters such as poly (butylene adipate-co-terephthalate) (PBAT) or poly (alkylene succinates) such as poly (butylene succinate) (PBS). Such polyesters are biodegradable, but are not (yet) produced from renewable sources, and they have a very high cost. The products currently on the market, in addition to containing only a small part of raw materials from renewable sources, are opaque due to the limited compatibility of these elastomers with PLA, which is attributed to the substantial difference in the chemical structure of the two components. This causes the formation of a microstructure in two phases, with a continuous phase (usually formed primarily from polyester elastomer) and a dispersed phase (usually made from PLA) in the form of approximately spherical particles with a diameter of several microns. Even if the adhesion between these two phases is generally good, such a microstructure does not allow the passage of visible light and the material is opaque. To overcome these limits the research group has produced a new type of copolymer based on PLA, with a higher content from renewable resources and a lower cost than products actually present on the market. The material is also transparent and has optimum mechanical characteristics for the production of packaging film and for shopping bags. In particular it presents an increased mechanical strength, a good deformability and good elastic recovery, accompanied by being soft to the touch. Specially, the film in a thickness of 15 µm disintegrated almost completely within 4 weeks of composting under industrial conditions 28 bioplastics MAGAZINE [06/13] Vol. 8

From Science & Research figure 2 – Evolution of the visual disintegration (or degradation) of sample UNIPI-05 under industrial composting conditions. At Start After 1 Week After 2 Weeks After 3 Weeks After 4 Weeks (ISO 16929) and proved to be biodegradable under controlled composting conditions (ISO 14855). The new copolymers have been prepared by a process of reactive blending in the molten state, starting from mixtures of: - Polylactic acid (PLA), - Different types of reactive plasticizers - Elastomeric copolyesters such as polybutylene adipate-co-terephthalate-co-(PBAT), or polybutylene-co-adipate-co-succinate (PBAS), etc. After extrusion, the material is then granulated with the common techniques used in the field of compounding and subsequently transformed into a film by means of the known techniques of blown film extrusion. The transparent films were produced by creating new copolymers characterized by a block structure containing polylactic acid (PLA) covalently linked to segments of reactive plasticizers and functionalized elastomeric copolyesters which maintain optimum mechanical characteristics at temperatures below 40°C with a consequent and significant improvement in flexibility within the temperature range mentioned. The molecules of plasticizer are incorporated in a stable manner (internal plasticizer) in the acid polylactic through a covalent bond that is formed during the copolymerization process. This avoids on the one hand that the plasticizer can migrate to the surface of the film, in particular in the presence of water or other polar solvents, and also allows a better compatibilization with the elastomeric polyesters added to ensure good elastic characteristics to the finished product. This copolymer film is not only characterized by excellent mechanical properties, transparency and compostability in industrial processes but it is also economical since it uses only 20 to 30% elastomeric copolyester. Moreover, the reactive plasticizers cost about 4 to 6 USD/kg. This copolymer compostable film will find a strong potential market for packaging. Code PLA (%) Plas1 (%) Plas2 (%) Ecoflex (%) UNIPI-01 82 18 UNIPI-02 84 16 UNIPI-03 64 14 22 UNIPI-04 86 14 UNIPI-05 68 12 20 Table 1. Formulations used to produce the films Figure 3. Photograph of a sample UNIPI-05with a thickness of 15 μm which shows the high degree of transparency of the film. http://materials.diccism.unipi.it www.ows.be bioplastics MAGAZINE [06/13] Vol. 8 29

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