Waste Rubber Recycling: A Review on the Evolution and Properties of Thermoplastic Elastomers

Waste rubber - Black pollution reframed as a global issue: Ecological challenges and sustainability initiatives

In contrast to "white pollution" originating from waste plastics, waste rubber is often referred to as "black pollution." The quantity and variety of waste rubber are increasing at an alarming rate, with a considerable fraction entering the global ecosystem via various pathways. This study presents the first critical review of waste rubber research with a focus on the risks associated with toxicant discharge and existing problems in waste rubber disposal, management, and recycling practices. We aim to obtain a comprehensive understanding of current research, particularly regarding the ecological impacts of these wastes, highlight major gaps, and propose the most significant research directions. A total of 192 studies published in journals were critically analysed. The importance of conducting long-term and large-scale experiments and developing efficient waste rubber recycling systems is also emphasised. This study highlights the need to address the challenges posed by waste rubber pollution and offers insights and references for undertaking ecological risk assessments and understanding the mechanisms underlying toxicant behaviour. Suggestions and countermeasures are proposed with ecosystem sustainability as the ultimate goal. Further long-term, comprehensive, and systematic research in this area is required.

Structural, radiation shielding, thermal and dynamic mechanical analysis for waste rubber/EPDM rubber composite loaded with Fe2O3 for green environment

Increasing waste rubber recycling produces a specious range of products for many valuable applications. Waste Rubber/EPDM composite with different concentrations was prepared. Infrared spectroscopy (FTIR) is used to identify the chemical composition. A water absorption test, Dynamic mechanical analysis (DMA), and Thermal Gravimetric Analysis (TGA) were performed. The (75/25) WR/EPDM rubber composite exhibited the best behavior with the highest mechanical performance. Fe2O3 was added to (75/25) WR/EPDM rubber composite. Water absorption, FTIR, TGA, and DMA were investigated. The composite performance was improved with increasing Fe2O3 content. The linear attenuation coefficients (μ) were also measured as a function of the concentrations of Fe2O3 for γ-ray energy 662 keV by using 137Cs point source; the radiation shielding can be denoted by numbers of parameters like mass attenuation coefficient (μm), half value layer (HVL), Tenth value layer TVL and radiation protection efficiency (RPE%), radiation protection efficiency increased as Fe2O3 increased.

Supersoft Norbornene-Based Thermoplastic Elastomers with High Strength and Upper Service Temperature

With over 6 million tons produced annually, thermoplastic elastomers (TPEs) have become ubiquitous in modern society, due to their unique combination of elasticity, toughness, and reprocessability. Nevertheless, industrial TPEs display a tradeoff between softness and strength, along with low upper service temperatures, typically ≤100 °C. This limits their utility, such as in bio-interfacial applications where supersoft deformation is required in tandem with strength, in addition to applications that require thermal stability (e.g., encapsulation of electronics, seals/joints for aeronautics, protective clothing for firefighting, and biomedical devices that can be subjected to steam sterilization). Thus, combining softness, strength, and high thermal resistance into a single versatile TPE has remained an unmet opportunity. Through de novo design and synthesis of novel norbornene-based ABA triblock copolymers, this gap is filled. Ring-opening metathesis polymerization is employed to prepare TPEs with an unprecedented combination of properties, including skin-like moduli (<100 kPa), strength competitive with commercial TPEs (>5 MPa), and upper service temperatures akin to high-performance plastics (≈260 °C). Furthermore, the materials are elastic, tough, reprocessable, and shelf stable (≥2 months) without incorporation of plasticizer. Structure-property relationships identified herein inform development of next-generation TPEs that are both biologically soft yet thermomechanically durable.

The Mechanisms of Plastic Food-Packaging Monomers' Migration into Food Matrix and the Implications on Human Health

The development of packaging technology has become a crucial part of the food industry in today's modern societies, which are characterized by technological advancements, industrialization, densely populated cities, and scientific advancements that have increased food production over the past 50 years despite the lack of agricultural land. Various types of food-packaging materials are utilized, with plastic being the most versatile. However, there are certain concerns with regards to the usage of plastic packaging because of unreacted monomers' potential migration from the polymer packaging to the food. The magnitude of monomer migration depends on numerous aspects, including the monomer chemistry, type of plastic packaging, physical-chemical parameters such as the temperature and pH, and food chemistry. The major concern for the presence of packaging monomers in food is that some monomers are endocrine-disrupting compounds (EDCs) with a capability to interfere with the functioning of vital hormonal systems in the human body. For this reason, different countries have resolved to enforce guidelines and regulations for packaging monomers in food. Additionally, many countries have introduced migration testing procedures and safe limits for packaging monomer migration into food. However, to date, several research studies have reported levels of monomer migration above the set migration limits due to leaching from the food-packaging materials into the food. This raises concerns regarding possible health effects on consumers. This paper provides a critical review on plastic food-contact materials' monomer migration, including that from biodegradable plastic packaging, the monomer migration mechanisms, the monomer migration chemistry, the key factors that affect the migration process, and the associated potential EDC human health risks linked to monomers' presence in food. The aim is to contribute to the existing knowledge and understanding of plastic food-packaging monomer migration.

Microplastics and Tire Wear Particles in Urban Stormwater: Abundance, Characteristics, and Potential Mitigation Strategies

Stormwater has been identified as a pathway for microplastics (MPs), including tire wear particles (TWPs), into aquatic habitats. Our knowledge of the abundance of MPs in urban stormwater and potential strategies to control MPs in stormwater is still limited. In this study, stormwater samples were collected from microlitter capture devices (inlet and outlet) during rain events. Sediment samples were collected from the material captured in the device and from the inlet and outlet of a constructed stormwater wetland. MP (>25 μm) concentration in stormwater varied across different locations ranging from 3.8 to 59 MPs/L in raw and 1.8 to 32 MPs/L in treated stormwater, demonstrating a decrease after passage through the device (35-88% removal). TWPs comprised ∼95% of all particles, followed by polypropylene (PP) and poly(ethylene terephthalate) (PET). The concentration of TWPs ranged from 2.5 to 58 TWPs/L and 1450 to 4740 TWPs/kg in stormwater and sediment, respectively. A higher abundance of MPs was found in the sediment at the inlet of the constructed wetland compared to the outlet, indicating a potential role of wetlands in removing MPs from stormwater. These findings suggest that both constructed wetlands and microlitter capture devices can mitigate the transport of MPs from stormwater to the receiving waterways.

Thermoplastic elastomers based on recycled high-density polyethylene/ground tire rubber/ethylene vinyl acetate: Effect of ground tire rubber regeneration on morphological and mechanical properties

This work investigates the properties of different types of regenerated recycled rubbers (RR1 and RR2) to produce thermoplastic elastomers (TPE) based on recycled high-density polyethylene (RHD) as the matrix. The higher regeneration degree of RR2 (24%) compared to RR1 (15%) was able to better restore the plasticity and processability of the ground tire rubber (GTR). So better entanglement between RR2 free chains and the thermoplastic macromolecules was obtained inducing stronger interfacial interaction leading to higher elongation at break (159%) and impact strength (342 J/m) of the blends filled with 80 wt.% RR2. To further improve the adhesion and achieve rubber-like properties, ethylene vinyl acetate (EVA) was used as a compatibilizer. The microstructure analysis showed that uniform dispersion of the particles and ground tire rubber encapsulation by EVA increased the resistance to crack propagation and failure of the compatibilized blends. The swelling, mechanical and physical properties of the ternary blends (RHD/GTR/EVA) showed that EVA improved the interfacial interactions between GTR and RHD which was confirmed by enhanced elongation at break (203%) and impact strength (379 J/m) by the addition of 10 wt.% EVA.

Study and Characterization of Regenerated Hard Foam Prepared by Polyol Hydrolysis of Waste Polyurethane

In this paper, four different kinds of diols were used for the alcoholysis of waste thermoplastic polyurethane elastomers. The recycled polyether polyols were used to prepare regenerated thermosetting polyurethane rigid foam through one-step foaming. We used four different kinds of alcoholysis agents, according to different proportions of the complex, and we combined them with an alkali metal catalyst (KOH) to trigger the catalytic cleavage of the carbamate bonds in the waste polyurethane elastomers. The effects of the different types and different chain lengths of the alcoholysis agents on the degradation of the waste polyurethane elastomers and the preparation of regenerated polyurethane rigid foam were studied. Based on the viscosity, GPC, FT-IR, foaming time and compression strength, water absorption, TG, apparent density, and thermal conductivity of the recycled polyurethane foam, eight groups of optimal components were selected and discussed. The results showed that the viscosity of the recovered biodegradable materials was between 485 and 1200 mPa·s. The hard foam of the regenerated polyurethane was prepared using biodegradable materials instead of commercially available polyether polyols, and its compressive strength was between 0.131 and 0.176 MPa. The water absorption rate ranged from 0.7265 to 1.9923%. The apparent density of the foam was between 0.0303 and 0.0403 kg/m3. The thermal conductivity ranged from 0.0151 to 0.0202 W/(m·K). A large number of experimental results showed that the degradation of the waste polyurethane elastomers by the alcoholysis agents was successful. The thermoplastic polyurethane elastomers can not only be reconstructed, but they can also be degraded by alcoholysis to produce regenerated polyurethane rigid foam.

Direct Recycling of WC-Co Grinding Chip

Grinding is a finishing process for high precision, high surface quality parts, and hard materials, including tool fabrication and sharpening. The recycling of grinding scraps, which often contain rare and costly materials such as tungsten carbide (WC-Co), has been established for decades. However, there is a growing need for more energy-efficient and environmentally friendly recycling processes. Currently, grinding sludges, which are a mixture of abrasives, lubricants, and hard metal chips, are only treated through chemical recycling. Direct recycling ("reuse" of chips as raw material) is the most effective but not yet viable process due to the presence of contaminants. This paper presents an oil-free dry grinding process that produces high-quality chips (i.e., oil-free and with few contaminants, smaller than 60 mesh particle size) that can be directly recycled, as opposed to the oil-based wet grinding that generates sludges, which require indirect recycling. The proposed alternative recycling method is validated experimentally using WC-Co chips from a leading hard metals' processing specialized company. The contaminant level (oxygen 0.8 wt.%, others < 0.4 wt.%), granulometry (chip D50 = 10.4 µm with grain size < 3 µm) and morphology of the recycled chips' powder is comparable to commercial powders proving the research and industrial potential of direct recycling. The comparison of sintered products using recycled and commercial powder provided equivalent characteristics for hardness (HRA of 90.7, HV30 of 1430), porosity grade (A02-04) and grain size (<3 µm).

Determining the Mechanical Properties of Solid Plates Obtained from the Recycling of Cable Waste

In this article, the possibility of obtaining a solid plate from waste cable sheaths, by mechanical recycling, i.e., grinding, plasticising and pressing, is discussed-waste cable sheaths being pure PVC with a slight admixture of silicone. Press moulding was carried out under the following conditions: temperature 135 °C, heating duration 1 h and applied pressure 10 MPa. The yield point of the obtained solid plate obtained was 15.0 + -0.6 MPa, flexural strength 0.94 MPa, yield point 0.47 MPa and Charpy's impact strength 5.1 kJ/m^2. The resulting solid plate does not differ significantly from the input material, in terms of mechanical strength, so, from the point of view of strength, that is, from a technical point of view, such promising processing of waste cables can be carried out successfully in industrial practice.

Molten-State Dielectrophoretic Alignment of EVA/BaTiO3 Thermoplastic Composites: Enhancement of Piezo-Smart Sensor for Medical Application

Dielectrophoresis has recently been used for developing high performance elastomer-based structured piezoelectric composites. However, no study has yet focused on the development of aligned thermoplastic-based piezocomposites. In this work, highly anisotropic thermoplastic composites, with high piezoelectric sensitivity, are created. Molten-state dielectrophoresis is introduced as an effective manufacturing pathway for the obtaining of an aligned filler structure within a thermoplastic matrix. For this study, Poly(Ethylene-co Vinyl Acetate) (EVA), revealed as a biocompatible polymeric matrix, was combined with barium titanate (BaTiO₃) filler, well-known as a lead-free piezoelectric material. The phase inversion method was used to obtain an optimal dispersion of the BaTiO₃ within the EVA thermoplastic matrix. The effect of the processing parameters, such as the poling electric field and the filler content, were analyzed via dielectric spectroscopy, piezoelectric characterization, and scanning electron microscopy (SEM). The thermal behavior of the matrix was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC). Thermoplastic-based structured composites have numerous appealing advantages, such as recyclability, enhanced piezoelectric activity, encapsulation properties, low manufacturing time, and being light weight, which make the developed composites of great novelty, paving the way for new applications in the medical field, such as integrated sensors adaptable to 3D printing technology.

Artificial turf and crumb rubber infill: An international policy review concerning the current state of regulations

BACKGROUND

Although artificial turf fields are utilized widely around the world, sufficient research has not yet been conducted to assess the potential human and environmental health risks posed by the chemicals contained in the fields' fibers, backing, and often-used crumb rubber infill. Consequently, there is wide variation in governmental policies.

OBJECTIVE

Review the notable policies concerning artificial turf and crumb rubber infill in the European Union, United Kingdom, United States of America, Canada, China, Qatar, and the Global Stockholm Convention of the United Nations.

METHODS

Information was collected that included published papers, technical and policy reports, and grey literature. These were then analyzed by a collaborative group familiar with the environmental policies in their respective countries to extract the pertinent legislative or regulatory information. The group members were primarily identified through their involvement in publications pertinent to artificial turf and crumb rubber infill health research and included environmental health professors, active researchers, and governmental agency officials. Most information on direct policies was taken directly from reports provided to the public by various governmental agencies responsible for their countries' regulations, often available within the respective agency's online archives.

RESULTS

There are significant differences in the regulatory approaches adopted by the investigated countries with regards to artificial turf and its crumb rubber infill. Some regions, such as the European Union, have taken substantial steps to limit the fields' chemical components to which the public and environment are exposed. Other regions and countries have done far less to address the issue. Most policies relate directly to (i) the fields themselves, (ii) the microplastic components of crumb rubber infill, or (iii) the concentrations of harmful polycyclic aromatic hydrocarbons (PAHs), perfluoroalkyl and polyfluoroalkyl substances (PFAS), and heavy metals.

CONCLUSION

While nearly every country acknowledges the potential health risks posed by heavy metals, microplastics, PAHs, and PFAS chemicals, very few have actually implemented artificial turf and crumb rubber infill regulations and/or established adequate surveillance measures to protect those regularly exposed to the fields.

4R of rubber waste management: current and outlook

Excessive accumulation of rubber waste necessitates the need to revisit the effectiveness of the existing rubber waste management system. This review provides an overview of the legislative frameworks, techniques, challenges, and trends of rubber waste management in various countries. The 4R (reduce, reuse, recycle and recover) framework applied in waste management system in some countries appears to be viable for the processing of rubber waste. Certain countries especially some of the European Union (EU) members have implemented extended producer responsibility (EPR) system to manage the collection of rubber waste, particularly used tires. The processing of rubber waste in each level of the 4R hierarchy was then discussed, with detailed elaboration on the most practiced 'R', recycling which encompasses the direct recycling of products, as well as material recycling via physical and/or chemical means. The challenges faced in the implementation of rubber waste management system in different countries were highlighted and recommendations for a more sustainable rubber consumption were provided at the end of this review.

Experimental and Finite Element Simulation of Polyolefin Elastomer Foams Using Real 3D Structures: Effect of Foaming Agent Content

In this study, polyolefin elastomer (POE) foams were prepared without any curing agent using a single-step foaming technique. The effect of azodicarbonamide (ADC) content as a chemical foaming agent on the foams’ morphology and mechanical properties was studied using scanning electron microscopy (SEM), mechanical properties (tension and compression) and hardness. The results showed that increasing the ADC content from 2 to 3, 4 and 5 phr (parts per hundred rubber) decreased the foam density from 0.75 to 0.71, 0.65 and 0.61 g/cm3, respectively. The morphological analysis revealed that increasing the ADC content from 2 to 4 phr produced smaller cell sizes from 153 to 109 µm (29% lower), but a higher cell density from 103 to 591 cells/mm3 (470% higher). However, using 5 phr of ADC led to a larger cell size (148 µm) and lower cell density (483 cells/mm3) due to cell coalescence. The tensile modulus, strength at break, elongation and hardness properties continuously decreased by 28%, 21%, 16% and 14%, respectively, with increasing ADC content (2 to 5 phr). On the other hand, the compressive properties, including elastic modulus and compressive strength, increased by 20% and 64%, respectively, with increasing ADC content (2 to 5 phr). The tensile and compression tests revealed that the former is more dependent on foam density (foaming ratio), while the latter is mainly controlled by the cellular structure (cell size, cell density and internal gas pressure). In addition, 2D SEM images were used to simulate the foams’ real 3D structure, which was used in finite element methods (FEM) to simulate the stress–strain behavior of the samples at two levels: micro-scale and macro-scale. Finally, the FEM results were compared to the experimental data. Based on the information obtained, a good agreement between the macro-scale stress–strain behavior generated by the FEM simulations and experimental data was obtained. While the FEM results showed that the sample with 3 phr of ADC had the lowest micro-scale stress, the sample with 5 phr had the highest micro-scale stress due to smaller and larger cell sizes, respectively.

The effect of the secondary reactions on volatile composition during the pyrolysis treatment of scrap tires

The phenomenon of the secondary reactions of volatiles prevails during the pyrolysis process of scrap tires, but less is known about the influence of volatiles' residence time and temperature on the pyrolytic oil compositions. Experiments on the secondary reactions caused by residence time and temperature of volatiles were carried out on a lab-scale fixed bed reactor. The regularity of the secondary reactions was presented in detail according to the distribution of liquid and gaseous pyrolytic products. Considering the inadequacy of lab-to-industry research, experiments were further carried out on a pilot-scale auger reactor. The results of the pilot-scale system were corresponding well to the regularity obtained in lab-scale experiment, demonstrating the universality of the regularity in this work. At in situ pyrolysis condition without any secondary reactions, limonene content reached up to 46.24% while a high yield of BTEX (50.55%) was obtained at 700℃/60 s. A remarkable increase of methane and ethane was observed at 700℃/60 s, reaching 0.058 and 0.040 g·(g scrap tire)^-1, respectively. This paper provided a novel strategy to selectively produce target products in a simple and economical method. The results were of great significance for guiding the optimisation of pyrolysis parameters in industrial equipment to obtain desired valuable products.

Fire Resistance Evaluation of New Wooden Composites Containing Waste Rubber from Automobiles

Particleboards containing waste rubber (tires and mixtures of isolators and carpets) filler were evaluated from the point of view of its flammability. The assessment of the utilization of these composites in the construction industry was analyzed through the determination of their spontaneous ignition temperatures, mass burning rate and calorific value. Based on the results of spontaneous ignition temperatures, similar values between particleboards and particleboards containing 10%, 15% and 20% of waste tires were obtained. The average time was from 298 s to 309 s and the average temperature was from 428.1 °C to 431.7 °C. For the mass burning rate, there were similar results between particleboards and particleboards containing 10% of waste tires and waste rubber. The time to initiation was 34 s and the time to reaching a maximal burning rate was from 66 s to 68 s. The calorimetry results showed similar properties for the calorimetric value and ash content in particleboards and particleboards containing 10% of waste tires and waste rubber. The calorific value was from 18.4 MJ·kg^-1 to 19.7 MJ·kg^-1 and the ash content from 0.5% to 2.9%.

Mechanical Properties, Crystallization Behaviors and Phase Morphologies of PLA/GTR Blends by Reactive Compatibilization

Different ratios of Polylactic acid/Ground tire rubber (PLA/GTR) were prepared by melt blending and adding dicumyl peroxide (DCP) as a reactive compatibilizer. The compatibilizer could initiate a reaction between PLA and GTR to increase the compatibility and interfacial adhesion of the two phases, as indicated by Fourier transform infrared (FTIR) spectrometry and scanning electron microscopy (SEM). Adding the compatibilizer significantly improved the impact strength of the PLA/GTR blends without compromising the tensile strength. The elongation at the break and notched Izod impact strength of the blend increased by 61.8% and 150%, respectively, but there was only a 4.1% decline in tensile strength compared with the neat PLA. The plastic deformation on the impact fractured surface showed that the improvement of toughness could be attributed to the compatibilization initiated by DCP. Therefore, the improvement of the interfacial adhesion and compatibility of the two phases induced a brittle-ductile transition that occurred in the failure of blends. Moreover, the crystallinity of blends reached 40.5% without a further annealing treatment, which was nearly 24 times of the neat PLA, and the crystallization rate was enhanced simultaneously. These exciting findings suggest that compatibilization can provide a promising avenue for fabricating GTR-toughened PLA blends with balanced stiffness-toughness.

Sustainable Reuse of Waste Tire Textile Fibers (WTTF) as Reinforcements

Waste tire textile fibers (WTTF), as a by-product (10–15% by weight of tires) of end-of-life tires (ELT) mechanical recycling (grinding), are classified as hazardous wastes and traditionally burnt (thermal recycling) or buried (landfilling), leading to several environmental and ecological issues. Thus, WTTF still represent an important challenge in today’s material recycling streams. It is vital to provide practical and economical solutions to convert WTTF into a source of inexpensive and valuable raw materials. In recent years, tire textile fibers have attracted significant attention to be used as a promising substitute to the commonly used natural/synthetic reinforcement fibers in geotechnical engineering applications, construction/civil structures, insulation materials, and polymer composites. However, the results available in the literature are limited, and practical aspects such as fiber contamination (~65% rubber particles) remain unsolved, limiting WTTF as an inexpensive reinforcement. This study provides a comprehensive review on WTTF treatments to separate rubber and impurities and discusses potential applications in expansive soils, cement and concrete, asphalt mixtures, rubber aerogels and polymer composites.

Thin Film Composite Polyamide Reverse Osmosis Membrane Technology towards a Circular Economy

It is estimated that Reverse Osmosis (RO) desalination will produce, by 2025, more than 2,000,000 end-of-life membranes annually worldwide. This review examines the implementation of circular economy principles in RO technology through a comprehensive analysis of the RO membrane life cycle (manufacturing, usage, and end-of-life management). Future RO design should incorporate a biobased composition (biopolymers, recycled materials, and green solvents), improve the durability of the membranes (fouling and chlorine resistance), and facilitate the recyclability of the modules. Moreover, proper membrane maintenance at the usage phase, attained through the implementation of feed pre-treatment, early fouling detection, and membrane cleaning methods can help extend the service time of RO elements. Currently, end-of-life membranes are dumped in landfills, which is contrary to the waste hierarchy. This review analyses up to now developed alternative valorisation routes of end-of-life RO membranes, including reuse, direct and indirect recycling, and energy recovery, placing a special focus on emerging indirect recycling strategies. Lastly, Life Cycle Assessment is presented as a holistic methodology to evaluate the environmental and economic burdens of membrane recycling strategies. According to the European Commission's objectives set through the Green Deal, future perspectives indicate that end-of-life membrane valorisation strategies will keep gaining increasing interest in the upcoming years.

Environmental impacts and leachate analysis of waste rubber incorporated in construction and road materials: A review

In recent years, the recycling of waste tyre rubber in construction and road materials has emerged as a potential innovative solution to the growing waste rubber tyre dilemma. However, to determine the feasibility of any recycling method, it is crucial to assess the potential environmental implications of the proposed method. The environmental conditions waste tyre rubber products are exposed to are often not accurately simulated in leachate studies, leading to incomplete findings. The Toxicity Characteristics Leaching Procedure (TCLP) (1997) and Australian Bottle Leaching Procedure (ABLP) (1992), which have been used in most leachate studies in the past, have been criticised for inadequate replication of site conditions when applied to assess the leachability of modified materials. The objective of this study is to (1) review standard leachate testing methods and subsequently investigate the adequacy of these methods, (2) review all available major research focusing on the leaching characteristics and environmental and health implications of products recycled with waste tyre rubber, (3) prepare recommendations for the improvement of future leachate studies and testing based on the assessment of existing research. The existing leachate analysis studies that assess the environmental implications of different applications of waste tyre rubber have demonstrated that considerable knowledge gaps exist in the current body of knowledge. It was found leachate studies involving the recently published ABLP (2019) and Leaching Environmental Assessment Framework (LEAF) (2017) appeared to better replicate local environmental conditions and yield results of higher integrity and precision due to improved testing procedures. This study recommends that the ABLP and LEAF testing methods be applied to assess the leachability of heavy metals and organic materials (on which minimal research has been conducted) of all currently available products incorporated with waste tyre rubber, as well as in future leachate studies of waste tyre rubber.

Recycled HDPE/Natural Fiber Composites Modified with Waste Tire Rubber: A Comparison between Injection and Compression Molding

With the objective of turning wastes into added-value materials, sustainable and fully recycled wood-plastic composites were reinforced by waste tire rubber particles to show balanced properties and potentially low-cost materials. Recycled high density polyethylene (rHDPE) was compounded (melt extrusion) with flax fiber (FF) and waste regenerated tire rubber (RR) to investigate the effect of mixing ratio, coupling agent (maleated polyethylene, MAPE) and molding process (injection and compression molding) on the properties of hybrid composites. In particular, a complete set of characterization was performed including thermal stability, phase morphology and mechanical properties in terms of tension, flexion and impact, as well as hardness and density. Adding 40 wt.% of flax fibers (FF) increased the tensile (17%) and flexural (15%) modulus of rHDPE, while the impact strength decreased by 58%. Substitution of FF by waste rubber particles improved by 75% the impact strength due to the elasticity and energy absorption of the rubber phase. The effects of impact modification were more pronounced for rHDPE/(FF/RR) compatibilized with MAPE (10 wt.%) due to highly improved interfacial adhesion and compatibility. The results also suggest that, for a fixed hybrid composition (FF/RR, 25/55 wt.%), the injection molded composites have a more homogenous morphology with a uniform distribution of well embedded reinforcements in the matrix. This better morphology produced higher tensile strain at break (12%) and impact strength (9%) compared to compression molded samples.

Molecular Dynamics Simulation of Oxidative Aging Effect on Diffusion Behaviors of Oxygen and Cyclohexane in NBR

The influences of thermal-oxidative aging on the diffusion behaviors of oxygen and cyclohexane in nitrile-butadiene rubber (NBR) at the micro-scale were investigated by molecular dynamics (MD) simulation. The two types of aged rubber models were established on the basis of rubber oxidative chains modified by the introduction of hydroxyl groups and carbonyl groups in rubber chains. The diffusion behaviors of oxygen and cyclohexane in NBR under different conditions were characterized by the fractional free volume (FFV), mean square displacement (MSD), diffusion coefficients, and diffusion trajectory. It turns out that the elevated temperature contributed to the increase in the free volume and diffusion range of oxygen and cyclohexane, while the compressive stress showed the reverse influence. Additionally, the introduction of oxidative polar functional groups (hydroxyl groups and carbonyl groups) in rubber chains lowered the flexibility of the rubber chains and promoted the formation of strong polar interaction, which further inhibits the diffusion of oxygen and cyclohexane.

Recycled (Bio)Plastics and (Bio)Plastic Composites: A Trade Opportunity in a Green Future

Today’s world is at the point where almost everyone realizes the usefulness of going green. Due to so-called global warming, there is an urgent need to find solutions to help the Earth and move towards a green future. Many worldwide events are focusing on the global technologies in plastics, bioplastic production, the recycling industry, and waste management where the goal is to turn plastic waste into a trade opportunity among the industrialists and manufacturers. The present work aims to review the recycling process via analyzing the recycling of thermoplastic, thermoset polymers, biopolymers, and their complex composite systems, such as fiber-reinforced polymers and nanocomposites. Moreover, it will be highlighted how the frame of the waste management, increasing the materials specificity, cleanliness, and a low level of collected material contamination will increase the potential recycling of plastics and bioplastics-based materials. At the same time, to have a real and approachable trade opportunity in recycling, it needs to implement an integrated single market for secondary raw materials.

The Influence of Fly Ash on the Mechanical Properties of Water Immersed All Waste Composites

The paper presents new value-added composite materials prepared by recycling tire rubber, polyethene terephthalate (PET), high-density polyethene (HDPE), wood sawdust, and fly ash. The composites were manufactured through the compression molding technique for three temperatures (150 °C, 160 °C, and 190 °C) previously optimized. The addition of fly ash as reinforcement in polymer blends is a viable route to improve the composite” properties. The paper aims to assess the effect of fly ash on the mechanical properties and water stability of the new all waste composites considering their applications as outdoor products. The static tensile (stress-strain behavior) and compression properties of the composites were tested. The fly ash composites were characterized in terms of wetting behavior and surface energies (contact angle measurements); chemical structure of the new interface developed between composite” components (FTIR analysis), crystalline structure (XRD analysis), surface morphology and topography (SEM, AFM). The addition of fly ash promoted the development of the hybrid interfaces in the new composites, as FTIR analysis has shown, which, in turn, greatly improved the mechanical and water resistance. The novel all waste composites exhibited lower surface energies, larger contact angles, and smoother morphologies when compared to those with no fly ash. Overall, the study results have revealed that fly ash has improved the mechanical strength and water stability of the composites through the formation of strong hybrid interfaces. The study results show optimal water stability and tensile strength for 0.5% fly ash composites cured at 190 °C and optimal compressive strength with good water stability for 1% fly ash composite cured at 150 °C.

Phase morphology, mechanical, and thermal properties of fiber-reinforced thermoplastic elastomer: Effects of blend composition and compatibilization

In this work, recycled high density polyethylene (rHDPE) was compounded with regenerated tire rubber (RR) (35-80 wt.%) and reinforced with recycled tire textile fiber (RTF) (20 wt.%) as a first step. The materials were compounded by melt extrusion, injection molded, and characterized in terms of morphological, mechanical, physical, and thermal properties. Although, replacement of the rubber phase with RTF compensated for tensile/flexural moduli losses of rHDPE/RR/RTF blends because of the more rigid nature of fibers increasing the composites stiffness, the impact strength substantially decreased. So, a new approach is proposed for impact modification by adding a blend of maleic anhydride grafted polyethylene (MAPE)/RR (70/30) into a fiber-reinforced rubberized composite. As in this case, a more homogeneous distribution of the fillers was observed due to better compatibility between MAPE, rHDPE, and RR. The tensile properties were improved as the elongation at break increased up to 173% because of better interfacial adhesion. Impact modification of the resulting thermoplastic elastomer (TPE) composites based on rHDPE/(RR/MAPE)/RTF was successfully performed (improved toughness by 60%) via encapsulation of the rubber phase by MAPE forming a thick/soft interphase decreasing interfacial stress concentration slowing down fracture. Finally, the thermal stability of rubberized fiber-reinforced TPE also revealed the positive effect of MAPE addition on molecular entanglements and strong bonding yielding lower weight loss, while the microstructure and crystallinity degree did not significantly change up to 60 wt.% RR/MAPE (70/30).

A ReaxFF molecular dynamic study on pyrolysis behavior and sulfur transfer during pyrolysis of vulcanized natural rubber

In this work, ReaxFF molecular simulations were performed to study the pyrolysis behavior of chemical cross-linked natural rubber (NR) under non-isothermal and isothermal conditions. Three different sulfur vulcanized NR models were established and simulated to study the effect of inner sulfur structure on NR decomposition behavior and sulfur evolution in comparison with carbon cross-linked structure. To understand the NR decomposition with temperatures, the non-isothermal simulations were performed between 300 and 3800 K at a 50 K ps^-1 heating rate. The results reveal that the decomposition process can be classified into four stages: 1) Structure adjustment; 2) Decomposition of the main carbon chains; 3) Secondary decomposition of heavy tar; and 4) Deep decomposition of light tar. Based on the results of non-isothermal pyrolysis, four different temperatures were selected for the isothermal simulations. Compared with carbon cross-linked NR, sulfur cross-linked structures facilitate the generation of C₂H₄ and C₄H₆ in the gas phase at low temperatures. At higher temperatures, more heavy tar is generated. Regarding the sulfur evolution, the sulfur-containing products mainly include H₂S, thiophene, sulfide, and thiol. The distribution of sulfur-containing products with temperatures follows the similar pattern with the product distribution of main compounds. At higher temperatures, most sulfur exists in the form of thiophene compounds. In particular, the structure with single CS cross-links facilitates the generation of H₂S at low temperatures. The results of this work provide insight into the sulfur transformation and pyrolysis behavior of vulcanized NR.

Ground Tire Rubber Modified by Elastomers via Low-Temperature Extrusion Process: Physico-Mechanical Properties and Volatile Organic Emission Assessment

In this paper, low-temperature extrusion of ground tire rubber was performed as a pro-ecological waste tires recycling method. During this process, ground tire rubber was modified with constant content of dicumyl peroxide and a variable amount of elastomer (in the range: 2.5–15 phr). During the studies, three types of elastomers were used: styrene-butadiene rubber, styrene-ethylene/butylene-styrene grafted with maleic anhydride and ethylene-octene copolymer. Energy consumption measurements, curing characteristics, physico-mechanical properties and volatile organic compounds emitted from modified reclaimed GTR were determined. The VOCs emission profile was investigated using a passive sampling technique, miniature emission chambers system and static headspace analysis and subsequently quantitative or qualitative analysis by gas chromatography. The VOCs analysis showed that in the studied conditions the most emitted volatile compounds are dicumyl peroxide decomposition by-products, such as: α-methylstyrene, acetophenone, α-cumyl alcohol, methyl cumyl ether, while the detection level of benzothiazole (devulcanization “marker”) was very low. Moreover, it was found that the mechanical properties of the obtained materials significantly improved with a higher content of styrene-butadiene rubber and styrene-ethylene/butylene-styrene grafted with maleic anhydride while the opposite trend was observed for ethylene-octene copolymer content.

Recycling NR/SBR waste using probe sonication as a new devulcanizing method; study on influencing parameters

In this work, a vulcanized blend of natural rubber (NR) and styrene butadiene rubber (SBR) (i.e. at weight ratio of 50 : 50) as a model for tire rubber was devulcanized using probe sonication. The effect of processing parameters such as sonication media, power, temperature and time on sol/gel contents and devulcanization percent of rubbery samples was investigated. Moreover, the influence of pre-immersion of vulcanized NR/SBR samples in different liquids (i.e. water, oil and toluene) was assessed for different sonication times (i.e. 10 to 60 min) and powers (i.e. 30 to 60 W). It was found that pre-immersion of rubber particles in oil significantly increased the devulcanization percent. The optimum conditions for devulcanization of the NR/SBR blend via probe sonication were found to be 60 Watts, 20 min, oil and 24 h for sonication power, sonication time, pre-immersion/sonication media and pre-immersion time, respectively. The highest obtained devulcanization percent in this step was about 40%. The effects of two devulcanizing chemical agents (i.e. diphenyl disulfide and VitaX) on devulcanization performances of the samples were also studied. Results showed that higher devulcanization percent (i.e. about 52%) was obtained by using VitaX. It was also observed that VitaX significantly improved re-vulcanization speed (i.e. cure rate index) and decreased scorch time. It was also found that lower content of VitaX (i.e. 0.6 phr) caused better curing properties but lower mechanical properties compared to the higher content (i.e. 1.2 phr).

In this work, a vulcanized blend of natural rubber (NR) and styrene butadiene rubber (SBR) (i.e. at weight ratio of 50 : 50) as a model for tire rubber was devulcanized using probe sonicationP.

Recycling of Plastics from Cable Waste from Automotive Industry in Poland as an Approach to the Circular Economy

This paper presents the contemporary problems of polymer waste recycling, mainly recycling cables from end-of-life vehicles. The authors developed a new material based on mixed polymer waste (ASR) modified with a ductile polymer, mainly recycled low-density polyethylene (rLDPE), to produce moisture-resistant boards with beneficial mechanical properties. The ASR-based compositions without and with homogenization process-including physical, chemical, and shear-assisted compatibilization-were successfully applied and verified by evaluating final recycled product properties. The results showed that recycled polyethylene (rLDPE) was effective as a modifier increasing tensile modulus and flexural strength compare to an ASR mixture. It was found that the adding 5 wt % of polyethylene-grafted maleic anhydride (PE-g-MAH) as a compatibilizer to the ASR mixture significantly increases the homogenization of the components in the ASR matrix. The optimal solution for management cable waste is the manufacture of ASR composites with homogenization using an internal mixer the adding 20 wt % of rLDPE and 5 wt % of PE-g-MAH to the mixed plastics cable waste. The results obtained demonstrate that the hot-pressing with the pre-blending with rLDPE and compatibilizer of the ASR based waste provides a high gain in mechanical and usage properties, enabling the circular economy of plastics from automotive cables.

Waste Reutilization in Polymeric Membrane Fabrication: A New Direction in Membranes for Separation

In parallel to the rapid growth in economic and social activities, there has been an undesirable increase in environmental degradation due to the massively produced and disposed waste. The need to manage waste in a more innovative manner has become an urgent matter. In response to the call for circular economy, some solid wastes can offer plenty of opportunities to be reutilized as raw materials for the fabrication of functional, high-value products. In the context of solid waste-derived polymeric membrane development, this strategy can pave a way to reduce the consumption of conventional feedstock for the production of synthetic polymers and simultaneously to dampen the negative environmental impacts resulting from the improper management of these solid wastes. The review aims to offer a platform for overviewing the potentials of reutilizing solid waste in liquid separation membrane fabrication by covering the important aspects, including waste pretreatment and raw material extraction, membrane fabrication and characterizations, as well as the separation performance evaluation of the resultant membranes. Three major types of waste-derived polymeric raw materials, namely keratin, cellulose, and plastics, are discussed based on the waste origins, limitations in the waste processing, and their conversion into polymeric membranes. With the promising material properties and viability of processing facilities, recycling and reutilization of waste resources for membrane fabrication are deemed to be a promising strategy that can bring about huge benefits in multiple ways, especially to make a step closer to sustainable and green membrane production.

Valorization of Recycled Tire Rubber for 3D Printing of ABS- and TPO-Based Composites

Vulcanized and devulcanized ground tire rubber microparticles have been used as a minor phase in acrylonitrile butadiene styrene copolymer (ABS) and thermoplastic polyolefins (TPO) for the development of materials with desired functionalities by 3D printing. These polymers have been selected because they (i) present part of the plastic waste generated by the automotive industry and (ii) have totally different properties (ABS for its stiffness and robustness and TPO for its softness and ductility). The study aims to improve the circular economy of the automotive industry by proposing a promising route for recycling the generated tire rubber waste. In this respect, emergent technology for plastic processing such as 3D printing is used, as part of the additive manufacturing technologies for the prolongated end of life of recycled plastics originated from automotive waste such as ABS and TPO. The obtained results revealed that (i) the composites are suitable for successful filament production with desired composition and diameter required for successful 3D printing by fused deposition modeling, and that (ii) the optimization of the composition of the blends allows the production of materials with interesting mechanical performances. Indeed, some of the investigated ABS-recycled rubber tire blends exhibit high impact properties as TPO-based composites do, which in addition exhibits elongation at break higher than 500% and good compression properties, accompanied with good shape recovery ratio after compression.

Preparation of High-Performance Composite Hydrogel Reinforced by Hydrophilic Modified Waste Rubber Powder

In order to reduce the environmental pollution caused by waste rubber and to realize the recycling of resources, we proposed a facile method for the hydrophilic modification of waste rubber powder (HRP) and used it to reinforce a composite hydrogel. In the presence of toluene, dibenzoyl peroxide (BPO) diffused into the waste rubber powder. After the solvent was removed, BPO was adsorbed in the rubber powder, which was used to initiate the grafting polymerization of the acrylamide monomer on the rubber-water interface. As a result, the polyacrylamide (PAM) molecular chains were grafted onto the surface of the rubber powder to realize hydrophilic modification. The success of the grafting modification was confirmed by FTIR, contact angle testing, and thermogravimetric analysis. The hydrophilic modified waste rubber powder was used to reinforce the PAM hydrogel. Mechanical tests showed that the tensile strength and elongation at the break of the composite hydrogel reached 0.46 MPa and 1809%, respectively, which was much higher than those of pure PAM hydrogel. Such a phenomenon indicates that the waste rubber particles had a strengthening effect.

A Multifaceted Approach for Cryogenic Waste Tire Recycling

One of the important aspects for degradation of the life quality is the ever increasing volume and range of industrial wastes. Polymer wastes, such as automotive tire rubber, are a source of long-term environmental pollution. This paper presents an approach to simplifying the rubber waste recycling process using cryogenic temperatures. The temperature of cryogenic treatment is ranged from 77 K to 280 K. Liquid nitrogen was used as a cryoagent for laboratory tests. Experimental and numerical studies have been carried out to determine the optimal conditions for the recycling process. Numerical studies were performed using the COMSOL Multiphysics cross-platform software. The optimal force of mechanical shock for the destruction of a tire which turned into a glassy state after cryoexposure was determined experimentally. The chemical and physical properties of the final product (crumb rubber) have been studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. The analysis shows that the morphology and elemental composition of the samples remain practically unchanged, demonstrating environmental friendliness of the proposed process.

Mechanical Behavior of Plaster Composites Based on Rubber Particles from End-of-Life Tires Reinforced with Carbon Fibers

The principal objective of this research project is the disposal of end-of-life tire rubber waste and its incorporation in gypsum composites. As a continuation of previous projects, which established a reduction in the mechanical properties of the resulting products, the behavior of these composites is analyzed with the incorporation of carbon fibers. The density, Shore C hardness, flexural strength, compressive strength, dynamic modulus of elasticity, strength–strain curves, toughness and resistance values and microstructure of the material are studied and compared. The results obtained show a significant increase in the mechanical tensile strength of all of the samples containing fibers. The moduli of elasticity results show a decrease in rigidity and increase in toughness and resistance of the material produced by incorporating the fibers. An optimum dosage of a water/gypsum ratio of 0.6 and incorporation of 1.5% carbon fibers is proposed. This lightweight material, which offers a high mechanical performance, features characteristics which are suitable for large prefabricated building elements in the form of panels or boards.

Effect of Ground Tire Rubber (GTR) Particle Size and Content on the Morphological and Mechanical Properties of Recycled High-Density Polyethylene (rHDPE)/GTR Blends

This work investigates the effect of ground rubber tire (GRT) particle size and their concentration on the morphological, mechanical, physical, and thermal properties of thermoplastic elastomer (TPE) blends based on recycled high-density polyethylene (rHDPE). In our methodology, samples are prepared via melt blending (twin-screw extrusion followed by compression molding) to prepare different series of blends using GTR with three different particle sizes (0–250 μm, 250–500 μm, and 500–850 μm) for different GTR concentrations (0, 20, 35, 50, and 65 wt.%). The thermal properties are characterized by differential scanning calorimeter (DSC), and the morphology of the blends is studied by scanning electron microscopy (SEM). The mechanical and physical properties of the blends are investigated by quasi-static tensile and flexural tests, combined with impact strength and dynamic mechanical analysis (DMA). The SEM observations indicate some incompatibility and inhomogeneity in the blends, due to low interfacial adhesion between rHDPE and GTR (especially for GTR > 50 wt.%). Increasing the GTR content up to 65 wt.% leads to poor interphase (high interfacial tension) and agglomeration, resulting in the formation of voids around GTR particles and increasing defects/cracks in the matrix. However, introducing fine GTR particles (0–250 μm) with higher specific surface area leads to a more homogenous structure and uniform particle dispersion, due to improved physical/interfacial interactions. The results also show that for a fixed composition, smaller GTR particles (0–250 μm) gives lower melt flow index (MFI), but higher tensile strength/modulus/elongation at break and toughness compared to larger GTR particles (250–500 μm and 500–850 μm).

Morphological and Mechanical Properties of Thermoplastic Elastomers Based on Recycled High Density Polyethylene and Recycled Natural Rubber

In this work, thermoplastic elastomers (TPE) are produced by melt mixing of recycled high-density polyethylene (RHD) with two types of off-the-road (OTR) ground tire rubber (GTR). Non-regenerated (NR) and regenerated (RR) rubbers are used to investigate the effect of GTR concentration and regeneration on the blends properties containing up to 90 wt.% GTR. The blend morphology is studied by scanning electron microscopy (SEM) to show incompatibility and low interfacial adhesion between RHD and GTR (NR and RR) especially above 40 wt.% RR. This behavior is attributed to the GTR regeneration process and agglomeration of RR particles with lower surface area and affinity toward RHD compared with NR. In all the compounds, the mechanical properties in tension of RR blends were lower than for NR blends, which is attributed to the degradation of the GTR backbone chains lowering the molecular weight (MW) during the regeneration process. Also, NR has a more important effect on impact strength improvement due to its higher crosslinked structure making the particles more deformable/elastic to absorb the mechanical energy before crack initiation. The experimental results also show that 80 wt.% GTR is the optimum concentration for the production of low cost and eco-friendly TPE based on recycled materials.

Blending Ferulic Acid Derivatives and Polylactic Acid into Biobased and Transparent Elastomeric Materials with Shape Memory Properties

Thanks to its remarkable properties such as sustainability, compostability, biocompatibility, and transparency, poly-l-lactic acid (PLA) would be a suitable replacement for oil-based polymers should it not suffer from low flexibility and poor toughness, restricting its use to rigid plastic by excluding elastomeric applications. Indeed, there are few fully biobased and biodegradable transparent elastomers-PLA-based or not-currently available. In the last decades, many strategies have been investigated to soften PLA and enhance its toughness and elongation at break by using plasticizers, oligomers, or polymers. This work shows how a ferulic acid-derived biobased additive (BDF) blends with a common rigid and brittle commercial grade of polylactic acid to provide a transparent non-covalently cross-linked elastomeric material with shape memory behavior exhibiting an elongation at break of 434% (vs 6% for pristine PLA). Through a structure-activity relationship analysis conducted with BDF analogues and a modeling study, we propose a mechanism based on π-π stacking to account for the elastomeric properties. Blending ferulic acid derivatives with polylactic acid generates a new family of fully sustainable transparent elastomeric materials with functional properties such as shape memory.

Dually Crosslinked Polymer Networks Incorporating Dynamic Covalent Bonds

Covalent adaptable networks (CANs) are polymeric networks containing covalent crosslinks that are dynamic under specific conditions. In addition to possessing the malleability of thermoplastics and the dimensional stability of thermosets, CANs exhibit a unique combination of physical properties, including adaptability, self-healing, shape-memory, stimuli-responsiveness, and enhanced recyclability. The physical properties and the service conditions (such as temperature, pH, and humidity) of CANs are defined by the nature of their constituent dynamic covalent bonds (DCBs). In response to the increasing demand for more sophisticated and adaptable materials, the scientific community has identified dual dynamic networks (DDNs) as a promising new class of polymeric materials. By combining two (or more) distinct crosslinkers in one system, a material with tailored thermal, rheological, and mechanical properties can be designed. One remarkable ability of DDNs is their capacity to combine dimensional stability, bond dynamicity, and multi-responsiveness. This review aims to give an overview of the advances in the emerging field of DDNs with a special emphasis on their design, structure-property relationships, and applications. This review illustrates how DDNs offer many prospects that single (dynamic) networks cannot provide and highlights the challenges associated with their synthesis and characterization.

Biodegradable all polyester-based multiblock copolymer elastomers with controlled properties

A cascade polymerization method is developed here for the synthesis of environmentally-friendly biodegradable all polyester-based thermoplastic elastomers with tunable properties.