N. Tzankova Dintcheva

Recycling of plastics from packaging

Plastic waste coming from the separate collection of packaging film was recycled and characterized. The material is a blend of low density and linear low density polyethylene and after recycling shows mechanical and rheological properties depending on the processing conditions and apparatus. High processing temperature and high residence times strongly enhance the degradation processes and reduce the mechanical properties, in particular the elongation at break. A possible use of this recycled plastic is for the production of low pressure pipes but the properties are lower than those of virgin pipe grade polyethylene. By introducing additives, like antioxidants, inert fillers and impact modifiers, the mechanical properties improve and approach those of virgin pipe grade polyethylene.

Recycling of the “light fraction” from municipal post‐consumer plastics: effect of adding wood fibers

At present the recycling of plastic materials is mostly done using homogeneous polymers. Therefore a separation from a municipal collection of plastic objects is necessary before recycling operations. The easiest way of separation is by flotation in water, i.e. the separation of the different plastics based on the different densities with respect to water. This means that all the plastic materials are separated in a “light fraction” mostly of polypropylene and polyethylene and in a “heavy fraction” mainly of poly(vinyl chloride) (PVC) and poly(ethylene terephthalate). The recycling of the light fraction should, in principle, be easy because of the relative similarity of the chemical structure of the components. The presence of small amounts of polystyrene foam (lighter than water) or of some polymer, such as PVC, or non-polymeric impurities can, however, make the properties of the secondary material quite poor. In this work, the recycling of a light fraction sample has been studied, considering also the effect of the addition of wood fibers, an “environment friendly” filler. Although the similar chemical nature of the two main components, the mechanical properties of the recycled mixture are quite scarce, mainly because of the incompatibility and the possible presence of some heterogeneous particles. The addition of wood fibers (20–40 wt%) leads to a remarkable increase of the elastic modulus while elongation at break and impact strength decrease and the tensile strength remains almost unchanged. Thermomechanical properties are also improved. In order to improve these properties, two functionalized polypropylene samples were used as adhesion promoters. Both polypropylene-grafted maleic anhydride and polypropylene-grafted acrylic acid improve the mechanical properties in particular at very low concentrations. Copyright

Characterization and reprocessing of greenhouse films

Films for greenhouses are an attractive source of post-consumer plastic materials because they are mainly made of polyethylene and can be easily collected in large amounts in small zones. The types of polymers for this application are, however, increasing and the films contain not only additives and stabilisers, but also fertiliser and pesticide residues. Finally, the extent of photooxidative degradation undergone during the use can strongly influence the recycling operations and the final properties of the secondary material. In this work, a complete characterisation of post-consumer films for greenhouses has been carried out and the properties of the recycled material have been correlated with the number of reprocessing steps and compared with those obtained by reprocessing virgin scraps of the same composition. The presence of small amounts of low molecular weight compounds (photooxidized species and pesticide residues) does not compromise the use of the recycled plastic in many applications. The mechanical properties decrease with the number of reprocessing steps and with increasing level of photooxidative degradation but are good enough for many applications.

Reprocessing and restabilization of greenhouse films

From the ecological and economical points of view, reprocessing of polyethylene greenhouse films is a promising solution to reduce discarded materials and to produce useful, or potentially useful, objects for service. Films exposed outdoors, however, show inferior mechanical properties and the melt processing worsens these properties. The addition of stabilizers and antioxidants to recycled plastic products prevents inherent thermal instabilities from occurring within the time frame of proper processing. In this study, we attempt to explore ways for improving product performance by the addition of four different additives when the recycled films undergo intensive shear processing. The process is evaluated by measuring the changes in chemical structure (carbonyl evolution) as well as rheological and mechanical characteristics (tensile properties). The effectiveness of the various additives was estimated and the one with the best antioxidant ability was identified. The analysis of processing conditions allowed us to find that the best results are emphasized by the continuous addition of the stabilizer at each step of reprocessing.

Time–carbonyl groups equivalence in photo-oxidative aging of virgin/recycled polymer blends

Abstract The photo-oxidation behaviour of polymers is strongly dependent on the initial amount of carbonyl groups along the chains. The growing use of recycled post-consumer polymers coming from products used outdoors and then photo-oxidised, both pure and blended with the same virgin polymer, gives rise to an unpredictable behaviour of weathering resistance of products made with these materials. The present work shows that the carbonyl group–exposure time curves can be shifted along the time axis to give a single generalised master plot. It is then possible to predict the formation of the new carbonyl groups by knowing only the initial amount of the same carbonyl groups. The same shift factor cannot be applied to quantitatively predict the changes in mechanical properties of the materials because these characteristics depend on other parameters. Nevertheless, this technique can give useful information on the decay of the mechanical properties.