Progress in Rubber,Plastics and Recycling Technology

A new experimental procedure for incorporation of model contaminants in polymers was developed as part of a general scheme for testing the efficiency of functional barriers in food packaging. The aim was to progressively pollute polymers in a controlled fashion up to a high level in the range of 1000 ppm and use them as input for the core layer of a tri-layer package, in order to monitor migration phenomena difficult to study in real conditions. To this end, a contamination recipe was initially formulated by a set of selected surrogates. The experimental procedure developed, led to satisfactory results as far as homogeneity and final concentration in the polymers concerned. High temperature data were also used in order to evaluate the efficiency of typical thermoforming processes in reducing possible volatile and non-volatile substances from recycled polymeric materials.

Blends of polystyrene (PS) with polypropylene (PP) are usually developed to overcome the inherent brittleness of PS. However, PS with PP are immiscible and (in the absence of a compatibiliser) incompatible. The present study investigated the effects of styrene-b (ethylene-co-butylene)-b-styrene (SEBS) on the mechanical properties and compatibility of PS-rich PS/PP blends. Using a Brabender PL2000 twin-screw extruder, blends of PS/PP in various compositions ranging from 100-60 wt% PS with and without SEBS were prepared and injection moulded. The overall results clearly showed that the mechanical properties of PS/PP blends are dependent on blend composition (ratio of PS/PP) and SEBS content. The impact strength and elongation at break of the PS/PP blends increase with SEBS content, at the expense of tensile strength and flexural modulus. The improvements in impact strength and elongation at break with the addition of SEBS are due to the improved interfacial adhesion between the dispersed phase (PP) and matrix phase (PS).

The improvement in miscibility of the PS/PP blend with the addition of SEBS is supported by DMA analysis. This showed that the 60/40 PS/PP blends possess two endothermic peaks whereas 60/40/25 PS/PP/SEBS blends have a single endothermic peak at 102 °C, indicating that they have an improved miscibility.

The effectiveness of SEBS in enhancing the blends depends on the blend composition. A significant improvement was observed upon addition of more than 10 phr of SEBS into the 70/30 and 60/40 PS/PP blends, but not much improvement in the case of the 90/10 and 80/20 PS/PP blends. However, a higher SEBS content is more effective at higher PS contents, as illustrated by the 90/10/25 PS/PP/SEBS blends having higher impact strengths than 60/40/25 PS/PP/SEBS. The optimum blend, based on achieving a balance between toughness (impact strength) and stiffness (flexural modulus), is 90/10/25 PS/PP/SEBS, followed by 80/20/25 PS/PP/SEBS.

The morphology and mechanical properties of nylon (copolyamide 6/66)/ acrylonitrile-butadiene rubber (NBR) blends have been studied with special reference to the effect of blend ratio and crosslinking systems. Morphological investigations of the blends using scanning and transmission electron microscopies show that a uniform and finer dispersion of the elastomer phase is achieved by dynamic crosslinking. The effects of various crosslinking systems such as sulphur and dicumyl peroxide on the morphology and mechanical properties of these blends were analysed. Morphological stability of the blends upon annealing has been investigated and the mechanical properties of the blends have been discussed. Attempts have been made to correlate the morphology with the mechanical properties of the dynamically vulcanized blends. The stability of the blend morphology during annealing has been examined.

For both environmental and economic reasons, there is a continuing broad-based interest in the recycling of scrap rubber and therefore in the development of recycling technologies. In the present work three different sizes of mechanically ground Indian crumb rubbers were used in a natural rubber (NR) based tyre tread cap compound. Varying doses of crumb rubber were used as an additional compounding ingredient. The minimum torque and Mooney viscosity values were marginally increased with increasing dose and mesh size of crumb rubber. In a Rubber Process Analyser (RPA) study, the Payne effect before curing was marginally higher for compounds containing crumb rubber, while, after curing a comparable Payne effect was observed with respect to the control compound without having crumb rubber. A marginal deterioration in tensile strength, fatigue to failure and abrasion properties was observed. At all levels of usage, the 100 mesh size ground crumb rubber retained the properties better than other lower mesh size crumb rubbers.

The de-vulcanization of rubber waste poses a challenging economical, environmental and social problem. We propose a new de-vulcanization process to re-use the rubber waste. It is a mechano-chemical process (MCP), where the waste is de-vulcanized by a combination of mechanical shearing, heat (110 °C) and the use of a de-vulcanizing agent (diallyl disulfide). A new look at the de-vulcanization mechanism and the influence of the de-vulcanizing agent on the mechanical properties of the ultimate re-vulcanized rubber is also presented. One of the most interesting observations is that the retention of tensile strength of the re-vulcanized rubber with respect to the original tyre was 34.9% when de-vulcanized in the absence of diallyl disulfide and 72.4% in its presence. The formation of extra crosslinks in those re-vulcanized rubbers containing disulfide was confirmed from crosslink density data and from TGA results. DMA analysis revealed that the storage modulus also increased for re-vulcanized rubber containing the disulfide.