Search results for "Extrusion"

Effect of Processing Conditions on the Degradation and on the Recycling of Polycarbonate

2003

Polycarbonate, like other polycondensation polymers, can be easily recycled without significant loss of properties only with careful drying. Indeed, the presence of humidity strongly decreases the molecular weight of the polymer by hydrolytic chain scission. In this paper, the degradation and the recycling of an extrusion grade polycarbonate sample has been investigated considering mainly the effect of the humidity. Some degradation is observed also for the material reprocessed in dry conditions and due to the thermomechanical stress acting on the melt. The chain scission is strongly reduced if the processing is carried out under nitrogen flow. In these conditions less degradation effects …

Rheological Behavior Under Shear and Non-Isothermal Elongational Flow of Biodegradable Polymers for Foam Extrusion

2013

The production of many items, in particular for food packaging applications, is based on foam extrusion and thermoforming. These operations require the use of polymers which can grant some specific rheological properties, both under shear and elongational flow. In this work, the behavior of some biodegradable polymers [Mater-Bi® and poly(lactic acid)] under shear and non-isothermal elongational flow was investigated and compared with a traditional, non-biodegradable polymer, in order to assess their suitability for industrial-scale foam extrusion and thermoforming. The rheological characterization evidenced the differences between the different biodegradable polymers and the reference polys…

Correlation between processability and properties of a high density polyethylene by a rheological approach

2002

The thermal-oxidative and thermal-mechanical stability of high density polyethylene (chromium based catalyst technology) was examined at many different temperatures using a rheological approach. The changes in molecular structure, which take place during processing, have been studied using a Clextral co-rotating twin-screw extruder in comparison with dynamic measurements performed with a rotational rheometer under definite conditions of temperature, strain and frequency and in presence of air. In order to evaluate the degradation response, an investigation of elastic modulus G' as a function of frequency ω on the residual sample after ageing has been carried out. The molecular weight increa…

1978

The thermal degradation mechanism of three aromatic polymers containing double bridged phenylether units was investigated by direct pyrolysis in the ion source of a mass spectrometer. Thermal degradation reactions were followed directly by this method detecting the thermal and electron impact induced fragments. The results obtained have provided evidence that the double bridge between the aromatic rings is the preferred fragmentation site in these polymers. Furthermore in the double sulfur bridged polymer III, extrusion of S2 from the polymer backbone takes place above 400°C. The synthesis of polymers studied is reported in the text. Der Mechanismus des thermischen Abbaus von drei aromatisc…

Processability and Properties of Re-Graded, Photo-Oxidized Post-Consumer Greenhouse Films

2005

The recycling of post‐consumer plastics leads, in general, to secondary materials having properties worse than those of the reclaimed material and certainly worse than those of the same virgin polymer. This is because of the degradation undergone by the objects during their use and because of the thermo‐mechanical degradation undergone during the reprocessing operations. The change of the molecular architecture is responsible for this worsening of properties. The use of stabilizing systems can slow the degradation during the melt processing but cannot give any improvement of the final properties of the material. In order to enhance the properties of the recycled plastics, some rebuilding of…

Recycling and Thermomechanical Degradation of LDPE/Modified Clay Nanocomposites

2013

The special features of polymer nanocomposites are continually gaining interest both in academia and in industry. The latter is especially experiencing an increase in interest towards this class of polymer composites, and this is leading to the first concerns about their reprocessing and reciclability. Here, a systematic study on the behavior of LDPE/Cloisite 15A systems upon reprocessing in a single-screw extruder is described, focusing on rheological, mechanical, and morphological properties changes. The investigated systems show a very complex thermomechanical degradation behavior, with the degradation of the clay modifier and different degradation paths involving the polymer matrix; all…

Liquid crystal polymer-based blends: “Universal grade” polymers?

1992

Polymeric materials can be easily processed with different processing operations thanks to suitable rheological properties. Melt flow index, or grade, which is a measure of the fluidity of the polymers, is the most commonly parameter used to characterize the polymers from the point of view of their melt viscosity. Injection molding requires high grade materials, whereas low grade values are best suited for extrusion. Of course, it would be no doubt of interest if single “universal grade” materials could be used for all processing operations. In this paper it is demonstrated that the blends of low grade polymers with small amounts of liquid crystal polymers can be conveniently used for all p…

Closed-loop recycling. A case study of films for greenhouses

2010

Abstract The degradation undergone by plastic films used for greenhouse coverage is the main cause of the worsening of the processability, of the mechanical properties and of the durability of secondary materials obtained by their recycling. This means that this secondary material cannot be processed to produce films for the same applications (closed-loop recycling). In this work different strategies are proposed to improve the processability, some mechanical properties and the resistance to photo-oxidation of these secondary materials. In particular, homopolymer blends or multilayer films where the secondary material is used in combination with virgin polymer give the best results.

Durability of a strach-based biodegradable polymer

2007

Abstract The photo-oxidation and the photo-stabilization of a commercial biodegradable polymer have been investigated in order to establish the possibility of using this polymer as raw material for films for agriculture. The degradation has been followed by measuring the mechanical properties as a function of photo-oxidation time and in particular by following the elongation at break. The virgin polymer, made from maize starch and a synthetic biodegradable polyester, shows poor resistance to the UV irradiation as observed by the fast decay of the elongation at break, but the presence of small amounts of conventional UV stabilizers strongly improves the durability of this polymer. The UV sta…

Polymer mechanical recycling: downcycling or upcycling?

2004

Recycled plastics are considered as materials with physical, mechanical and aesthetic properties worse than the virgin ones. In fact, this is true because degradation during lifetime and during the recycling operations changes the structure and the morphology of the polymers and then the final properties of the secondary material. Several “strategies” are under investigation, and in some cases also applied, in order to avoid further degradation during recycling and, if possible, to repair the macromolecules broken during lifetime and then to restore or to improve the initial properties. In this paper the more important techniques, already used in industry or under investigation, according …