It is known that most polypropylene (PP) materials show low resistance to impact especially at low temperatures, which limits its applications in engineering fields. The technical requirements for automobiles, appliances, and other commercial products applications of PP materials envisage good mechanical properties, easy processing, good aesthetics, and low weight and cost. In order to obtain these properties, usually PP is compounding with elastomer, resulting in polypropylene composites with improved properties.
Generally, the elongation, impact strength, and brittle/tough transition temperature represent the most important properties improved when PP is compounded with elastomer. For example, investigated the compatibilized PP/poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) blend with styrene-ethylene/butylene-styrene (SEBS), styrene-butadiene-styrene (SBS), and styrene-isoprene-styrene (SIS) and reported a remarkable increase in impact strength due to the addition of SIS, about 5 times as that of the uncompatibilized blends; the yield strength of the blends with SBS and SIS is enhanced from 33.6 to 39.6 and 37.6 MPa, respectively. A significant improvement on elongation at break from 35.9% to 91.2% and notched IZOD impact from 3.5 kJ/m2 to 7.1 kJ/m2 was obtained when PP was compounded with 10% SEBS.
However, the widespread use of PP in various domains results in a significant amount of plastic waste. Different recycling methods have been used for PP wastes including land filling (causing a negative impact on the environment), mechanical including dissolution/reprecipitation method, chemical method, and incineration. By mechanical recycling, the PP wastes are separated from other resin types, washed to remove dirt and contaminants, and grinded and crushed to reduce the plastics particle size, followed by extrusion by heat and reprocessing into new plastic goods. Chemical recycling leads to the conversion of plastic wastes to original monomers or other valuable chemicals, while, by incineration, energy is obtained. Among all recycling methods, mechanical and chemical recycling are the most used for PP wastes management.
The reuse and recovery rate of recycled PP materials are of great importance for the plastic industry, which depends on oil and has consequences on the environment and the economy. Therefore, the amount of energy and raw materials (such as propylene) required for the manufacturing of new products is reduced, large quantities of raw materials are returned to the economic circuit, and the amount of PP waste deposited at the landfill or incinerators is also diminished.
A large number of papers describe the modification of recycled polypropylene with different additives, fillers, or processing reinforcements for obtaining of wood plastic composites (WPC) with high added value, concrete, and a novel coupling agent for fibrous cellulose/polypropylene composite.
It is known that the mechanical properties of virgin materials are different from those of the corresponding recycled materials. For example, it showed that the tensile strength of the highly recycled polypropylene decreased by about 15% when it is compared to neat polypropylene. In another paper, by investigating the mechanical properties of recycled PP/nylon compounded with SEBS elastomer, it was found that the impact strength was improved with the increase of SEBS content, while the tensile strength of PP/nylon blend was not enhanced by the addition of SEBS. The same results were obtained by Jose and coworkers that modified the PP wastes with rubber in a Brabender Plastograph.
The effect of recycling on the molecular weight, rheological properties and the mechanical properties of the PP-based composites, and the deformation mechanisms were also investigated. The obtained results showed that thermomechanical recycling process led to decrease of the molecular weight, failure stress, and impact energy. Instead, the yield stress and the Young modulus increased until the fifth cycle of reprocessing due to increase of the crystallization rate taking place during the recycling process. Previous researches of our team indicated the incorporation of 10% elastomer as the optimum amount that leads to the improvement of the recycled polypropylene properties.