Expanded Beads of High Melt Strength Polypropylene Moldable at Low Steam Pressure by Foam Extrusion

We explore the foam extrusion of expanded polypropylene with a long chain branched random co-polypropylene to make its production process simpler and cheaper. The results show that the presence of long chain branches infer high melt strength and, hence, a wide foamability window. We explored the entire window of foaming conditions (namely, temperature and pressure) by means of an ad-hoc extrusion pilot line design. It is shown that the density of the beads can be varied from 20 to 100 kg/m3 using CO2 and isobutane as a blowing agent. The foamed beads were molded by steam-chest molding using moderate steam pressures of 0.3 to 0.35 MPa independently of the closed cell content. A characterization of the mechanical properties was performed on the molded parts. The steam molding pressure for sintering expanded polypropylene beads with a long chain branched random co-polypropylene is lower than the one usually needed for standard polypropylene beads by extrusion. The energy saving for the sintering makes the entire manufacturing processes cost efficient and can trigger new applications.

Development of high melt strength polypropylene and its application in thermoplastic elastomeric composition

High melt strength polypropylene (HMS-PP) with a long-chain branched structure is a modified form of polypropylene (PP) which has basic properties of regular PP but with superior melt drawability. This paper reports on the development of gel-free HMS-PP from a linear isotactic PP through the introduction of long-chain branching on its backbone via a reactive extrusion process, using dicetyl-peroxydicarbonate (PODIC) alone or in combination with a coagent. The melt strength and the mechanical properties such as impact and flexural strength of PP showed improvements with the modification with PODIC. 5000 ppm by weight of PODIC was found to provide the best balance of properties. The efficacies of zinc diethyldithiocarbamate (ZDC) and tetramethyl thiuram disulphide (TMTD) as coagents in combination with PODIC to augment properties of HMS-PP further were explored. TMTD offered slightly enhanced performance benefits as compared to ZDC at an optimized concentration of 100 ppm by weight. The application potential of HMS-PP in thermoplastic elastomeric blends of HMS-PP with ethylene-propylene-diene monomer (EPDM) rubber at a fixed ratio of 35/65 by weight was also investigated. Structure-property correlations were established between the extent of long-chain branching in the modified PP and the properties of the resultant thermoplastic elastomeric composition.

Preparation and properties of high melt strength PBS and its environmentally friendly foaming materials

In order to improve the melt strength of Poly(butylene succinate) (PBS) resin, the silane graft-crosslinked PBS copolyester materials were prepared by melt blending method with vinyltriethyl silane as graft material and benzyl peroxide (BPO) as initiator. At the same time, the environmentally friendly compound foaming agent (citric acid and sodium bicarbonate) was used as foaming agent. The results showed that the tensile properties and melt strength of PBS resin were greatly improved after silane grafting and cross-linking, and the graft and cross-linking reaction between PBS resin and silane occurred, forming a three-dimensional space network structure, and the viscosity and elasticity of polymer melt was changed, which increased the entropy elasticity of the material and strengthened the polymer melt strength. The additional amount of compound foaming agent and the cross-linking degree of material had important influence on the diameter and distribution of PBS foaming material.

Investigation of the Thermal and Hydrolytic Degradation of Polylactide during Autoclave Foaming

Polylactide (PLA) is one of the most important bioplastics worldwide and thus represents a good potential substitute for bead foams made of the fossil-based Polystyrene (PS). However, foaming of PLA comes with a few challenges. One disadvantage of commercially available PLA is its low melt strength and elongation properties, which play an important role in foaming. As a polyester, PLA is also very sensitive to thermal and hydrolytic degradation. Possibilities to overcome these disadvantages can be found in literature, but improving the properties for foaming of PLA as well as the degradation behavior during foaming have not been investigated yet. In this study, reactive extrusion on a twin-screw extruder is used to modify PLA in order to increase the melt strength and to protect it against thermal degradation and hydrolysis. PLA foams are produced in an already known process from the literature and the influence of the modifiers on the properties is estimated. The results show that it is possible to enhance the foaming properties of PLA and to protect it against hydrolysis at the same time.