Nowadays, there has been an increase in the production of plastic automotive parts in many countries around the globe. Last three decades, car parts have been reused and repaired due to the high cost of production. However, the expansion of companies in different countries has given cost reductions and an alarming increase in PP production, representing a big problem owing to its long life. PP is the polymer most used in industries, but, it is only recycled 0 to 1% as a consequence of its rigidity and the different composites used on it. The unknown of PP composition is the main limitation in recycling automotive plastics. Companies possess the formulation as an industrial secret; nevertheless, it is well-known that car plastics are based on PP. This unspecified information about the composition has avoided PP exploitation and recycling in other fields to implement circular economy [1]. In 2015, the European Commission Regulation on End-of-Life Vehicles (ELVs) established targets to rise the percentage of recyclability and reusability of cars, generating policies to recycle and/or reuse for car manufacturers. [2]. It is estimated by 2030, the number of plastics circulating will increase from 236 to 417 million tons per year. Mandatory regulation for the 21st century focuses on 100% recycling. The ELVs proposed that companies producing cars in mass, instead of paying fines, paying to the groups dismantling and reprocessing PP parts. In 2018, only 29.1 million tons of plastic waste were collected in Europe [3]. In this sense, automotive manufacturers have committed to reducing their waste, e. g., Volvo stated by 2025, 25% of plastic used in its vehicles would come from recycled materials [4].
For years, unused plastics have been left to lay in massive landfills found all over the world, and each day they are getting fuller. Plastic waste from landfill degrades faster than waste from sea or protected places due to the environment, being a factor in the degradation process [5]. Recycling is a viable solution for reducing the investment and production of raw materials and decreasing the growth and expansion of discarded materials in landfills [6]. Nowadays, finding a way to reuse all that plastic is essential and can be very profitable. Though, as the plastic waste gets recycled, its properties could get downgraded, becoming unuseful. Accurately knowledge of the time when PP-based plastic properties are completely degraded is still an open problem. The plastic parts of vehicles need different processes for recycling, such as grinding, and re-extrusion-injection, which can affect its original physicochemical properties due to the thermal processes [7]. The plastic parts of vehicles need different processes for recycling, such as grinding and re-extrusion injection, which can affect their original physicochemical properties because of thermal procedures. Composites containing recycled polymers exhibit better environmental performance in non-automotive applications than composites containing virgin ones [8]. The quality of recycled products depends on the operation conditions of the process and knowledge of the physicochemical properties of the original automotive part, in most cases, this data are not available because each company makes its formulation. It means before any recycling intervention is necessary to execute a lot of scientific work around the original part to know the thermal, structural, and mechanical properties, among others. Primary, secondary, tertiary, and quaternary recycling are the mainly four ways to recover plastics. Primary recycling is the re-extrusion of material. It is used to produce similar materials to original plastic. The secondary one is mechanical recycling, whereby mechanical means, plastics are recovered for the plastic industry. This technique can only be used on single polymer plastic forasmuch it is more complex to recycle plastic with different composites. Tertiary is chemical recycling; it is used for the production of new petrochemical plastics. The final one is energy production consisting of burning plastics to generate energy by using the calorific values of plastic [9].
Car bumpers are manufactured with elastomers such as ethylene-propylene rubber (EPR) and ethylene propylene diene monomer (EPDM). They are frequently blended with PP to increase their inherent low toughness at low temperatures [10]. Many problems arising from polymers recycling are related to the purity of automotive components, it can be affected not only by the technology of sorting, but also by the adhered elements such as paints and pastes frequently used to fix the parts and formulas [11]. One of the main problems with recycling car parts is their components (PP, EPR, EPDM, or talc) cannot separate during the recycling process. Due to the long lifetime of PP-based products and increasing concern about ecological problems in different countries around the globe, the intervention of national and international organizations has emerged to search for solutions to reduce the environmental impact of end-of-life products [12]. It is crucial to have quick responses for local and international organizations regarding the recycling of products like automotive parts. Damaged PP-based parts of bumpers suffer changes occurred during their useful life, causing oxidation of components, loss of compatibility at the interphase, and progressive deterioration of damping properties during storage time. It indicates a need for optimization of storage processes [13, 14]. Studying the modifications occurring during the environmental aging of thick sheets of PP was carried out using different techniques such as thermal degradation, dynamic-mechanical analysis, scanning electron microscopy (SEM), mechanical tests, and positron annihilation lifetime spectroscopy. Degradation of the material is produced by morphology variation yielding the separation of amorphous oxidized copolymers initially dispersed in the bulk material. The degradation process takes place with an increase in bulk density [15]. Different processes during recycling should be beneficial to regain original properties (re-crystallization) via reestablishing phase compatibilization and spreading oxidized components in the whole volume [13]. Improvement of their properties is directly related to each step in the recycling proceeding.
In the case of a semi-crystalline polymer, its physicochemical properties are governed by the morphology, influenced by its crystallization and intrinsic parameters such as molecular characteristics, molecular mass distribution, and stereo-regularity of the crystallizing polymer, as well as extrinsic parameters such as the processing conditions involving rate of cooling, orientation in the melt, melting temperature, and storage [17]. The crystallization of isotactic PP (iPP) was studied using differential scanning calorimetry (DSC) [18]. The degree of crystallinity and morphology of the matrix can affect the mechanical properties. Fiber-reinforced iPP obtained through thermo-mechanical history and interphase modification (fiber sizing or both matrix coupling) exhibited changes in the interfacial shear strength [19]. However, complementary techniques implementation like X-ray diffraction, Fourier-transform infrared spectroscopy, and inductively coupled plasma, among others, have not been reported in order to understand this phenomenon. The crystallinity of the PP increases when recycling is made by grinding and re-extrusion, inducing an impact on the physical and mechanical properties of PP/ethylene octene copolymer (EOC) blends for different recycling processes. Notwithstanding, it has not been reported an explanation for crystallinity increase.
This work aimed to study the physicochemical properties of recycling bumpers parts formed mainly by thermoplastic olefin (TPO) composites of PP. Re-extrusion-injection changes in the structure were determined by X-Ray diffraction. This technique also was used to identify crystalline compounds in the bumpers. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to study the thermal events during heating and decomposition, respectively. Mechanical properties, dynamic mechanical analysis, and morphology (by SEM) were correlated by ductility and fiber rupture. Fourier-transform infrared spectroscopy (FTIR) was used to identify the molecular composition of the polymeric system. The effect of the re-extrusion-injection process on the mechanical properties was analyzed to propose a methodology and metrology to characterize PP-based materials, knowing the accurate chemical composition and the relation with its properties to find applications in different fields for developing a circular economy framework.