Mixed Polyolefins Research Articles

Advances in Nonreactive Polymer Compatibilizers for Commodity Polyolefin Blends.

Recycling mixed polyolefin plastics is a significant challenge due to the limitations in sorting and degraded mechanical properties of blends. Nonreactive compatibilization by adding a small amount of polymeric additive is a widespread approach to restoring the performance and value of recycled plastics. Over the past several decades, synthetic advances have enabled access to low-cost copolymers and precision architectures for deepening the understanding of compatibilization mechanisms in semicrystalline polyolefins. This review covers the design parameters of a polymeric compatibilizer, the testing of blends, the synthetic methods of producing economically viable additives, and surveys the literature of blends of compatibilized HDPE, LLDPE, LDPE, and iPP. From this, readers should gain a comprehension of the polymer mechanics, synthesis, and macromolecular engineering of processable polyolefin blends, along with the field's future directions.

Single-step and stepwise pyrolysis to recover high-value chemicals from mixed polyolefins and polylactide

Biodegradable plastics are increasingly utilized due to environmental concerns but are typically disposed of together with conventional plastics. Valuable chemicals could be recovered from waste plastics by chemical recycling, but the mixing of different types of waste plastic affects the efficiency of the process. In this study, high-value chemicals were produced from a mixture of polyolefins and biodegradable polylactide by single and stepwise pyrolysis processes. The pyrolysis of polyolefins yielded fuel-like hydrocarbons, and the pyrolysis of polylactide yielded lactide and lactic acid. In the stepwise process, oxygenated compounds were only produced in the low-temperature step, and fuel-like hydrocarbons produced in the high-temperature step were not contaminated by oxygenated compounds. The influence of different catalysts on yields was investigated in the single-step and stepwise processes. The yield and selectivity of each product were significantly influenced by the type of catalyst and the composition of the plastic feedstock. Using a mixed feedstock of polyolefins and polylactide, low-temperature pyrolysis with a zeolite LTA catalyst favored the formation of lactide and lactic acid, while diesel-range hydrocarbons were obtained in the second step at a higher temperature. Aromatic and gasoline-range hydrocarbons were preferably produced from the single-step pyrolysis of polyolefins with spent FCC catalyst.

IPP/HDPE blends compatibilized by a polyester: An unconventional concept to valuable products.

Polyolefins are the most widely used plastics accounting for a large fraction of the polymer waste stream. Although reusing polyolefins seems to be a logical choice, their recycling level remains disappointingly low. This is mainly due to the lack of large-scale availability of efficient and inexpensive compatibilizers for mixed polyolefin waste, typically consisting of high-density polyethylene (HDPE) and isotactic polypropylene (iPP) that, despite their similar chemical hydrocarbon structure, are immiscible. Here, we describe an unconventional approach of using polypentadecalactone, a straightforward and simple-to-produce aliphatic polyester, as a compatibilizer for iPP/HDPE blends, especially the brittle iPP-rich ones. The unexpectedly effective compatibilizer transforms brittle iPP/HDPE blends into unexpectedly tough materials that even outperform the reference HDPE and iPP materials. This simple approach creates opportunities for upcycling polymer waste into valuable products.

Advancing the Characterization of Recycled Polyolefin Blends with a Combined Experimental and Numerical Approach to Thermomechanical Behavior.

The blending of polyolefins (POs), such as polyethylene (PE) and polypropylene (PP), is a growing area of research, particularly for recycling mixed polyolefin (MPO) waste through flotation sorting techniques. However, understanding the thermomechanical behavior of these recycled blends is challenging due to limitations in the existing characterization methods. This paper introduces a combined experimental and numerical method to accurately assess the complex mechanical behavior of high-density PE, PP, and their blends. We conducted detailed thermomechanical analyses using a high-speed stereo digital image correlation (DIC) system paired with an infrared camera to capture temperature variations alongside mechanical stress and strain. This approach allowed us to correct for distortions caused by necking and to derive accurate stress-strain relationships. We also applied a cutting-edge unified semi-crystalline polymer (USCP) model to simplify the analysis, focusing on the effects of strain rate and temperature, including self-heating and thermal softening phenomena. Our results, which closely match experimental observations of stress-strain behavior and temperature changes, offer new insights into the thermomechanical properties of PO blends, which are essential for advancing their practical applications in various fields.

The Influence of a Commercial Few-Layer Graphene on the Photodegradation Resistance of a Waste Polyolefins Stream and Prime Polyolefin Blends

This work investigated the photostabilizing role of a commercially available few-layer graphene (FLG) in mixed polyolefins waste stream (MPWS), ensuring extended lifespan for outdoor applications. The investigation was conducted by analyzing carbonyl content increase, surface appearance, and the retention of mechanical properties of UV-exposed MPWS/FLG composites. Despite the likely predegraded condition of MPWS, approximately 60%, 70%, 80%, and 90% of the original ductility was retained in composites containing 1, 4, 7, and 10 wt.% FLG, respectively. Conversely, just 20% of the original ductility was retained in unfilled MPWS. Additionally, less crack density and lower carbonyl concentrations of the composites also highlighted the photoprotection effect of FLG. For prime polyolefin blends, only 0.5 wt.% or 1 wt.% FLG was sufficient to preserve the original surface finishing and protect the mechanical properties from photodegradation. Hence, it was observed that MPWS requires more FLG than prime polyolefin blends to get to comparable property retention. This could be attributed to the poor dispersion of FLG in MPWS and inevitable uncertainties such as the presence of impurities, pre-degradation, and polydispersity associated with MPWS. This study outlines a potential approach to revalorize MPWS that possess a minimal intrinsic value and would otherwise be destined for landfill disposal.

Catalytic conversion of mixed polyolefins under mild atmospheric pressure

The chemical recycling of polyolefin presents a considerable challenge, especially as upcycling methods struggle with the reality that plastic wastes typically consist of mixtures of polyethylene (PE), polystyrene (PS), and polypropylene (PP). We report a catalytic aerobic oxidative approach for polyolefins upcycling with the corresponding carboxylic acids as the product. This method encompasses three key innovations. First, it operates under atmospheric pressure and mild conditions, using O2 or air as the oxidant. Second, it is compatible with high-density polyethylene, low-density polyethylene, PS, PP, and their blends. Third, it uses an economical and recoverable metal catalyst. It has been demonstrated that this approach can efficiently degrade mixed wastes of plastic bags, bottles, masks, and foam boxes.

Upcycled waxes from mixed polyolefins for hot-melt adhesive (HMA) applications

Virgin paraffin waxes are blended with poly(ethylene-co-vinyl acetate) (EVA) to make hot-melt adhesives (HMAs).

Polymer Multi-Block and Multi-Block+ Strategies for the Upcycling of Mixed Polyolefins and Other Plastics.

Due to a continued rise in the production and use of plastic products, their end-of-life pollution has become a pressing global issue. One of the biggest challenges in plastics recycling is the separation of different polymers. Multi-block copolymers (MBCPs) represent an efficient strategy for the upcycling of mixed plastics via induced compatibilization, but this approach is limited by difficulties associated with synthesis and structural modification. In this contribution, several synthetic strategies are explored to prepare MBCPs with tunable microstructures, which were then used as compatibilizer additives to upcycle mixtures of polyolefins with other plastics. A multi-block+ strategy based on a reactive telechelic block copolymer platform was introduced, which enabled block extension during the in situ melt blending of mixed plastics, leading to better compatibilizing properties as well as better 3D printing capability. This strategy was also applicable to more complex ternary plastic blends. The polymer multi-block strategy enabled by versatile MBCPs synthesis and the multi-block+ strategy enabled by in situ block extension show exciting opportunities for the upcycling of mixed plastics.

Polymer Multi‐Block and Multi‐Block+ Strategies for the Upcycling of Mixed Polyolefins and Other Plastics

AbstractDue to a continued rise in the production and use of plastic products, their end‐of‐life pollution has become a pressing global issue. One of the biggest challenges in plastics recycling is the separation of different polymers. Multi‐block copolymers (MBCPs) represent an efficient strategy for the upcycling of mixed plastics via induced compatibilization, but this approach is limited by difficulties associated with synthesis and structural modification. In this contribution, several synthetic strategies are explored to prepare MBCPs with tunable microstructures, which were then used as compatibilizer additives to upcycle mixtures of polyolefins with other plastics. A multi‐block+ strategy based on a reactive telechelic block copolymer platform was introduced, which enabled block extension during the in situ melt blending of mixed plastics, leading to better compatibilizing properties as well as better 3D printing capability. This strategy was also applicable to more complex ternary plastic blends. The polymer multi‐block strategy enabled by versatile MBCPs synthesis and the multi‐block+ strategy enabled by in situ block extension show exciting opportunities for the upcycling of mixed plastics.

Virgin-like High-Density Polyethylene from Recycled Mixed Polyolefins

Virgin-like High-Density Polyethylene from Recycled Mixed Polyolefins

Mixed plastics waste valorization to high-added value products via thermally induced phase separation and spin-casting

Plastic waste is an underutilized resource that has the potential to be transformed into value-added materials. However, its chemical diversity leads to cost-intensive sorting techniques, limiting recycling and upcycling opportunities. Herein, we report an open-loop recycling method to produce graded feedstock from mixed polyolefins waste, which makes up 60% of total plastic waste. The method uses heat flow scanning to quantify the composition of plastic waste and resolves its compatibility through controlled dissolution. The resulting feedstock is then used to synthesize blended pellets, porous sorbents, and superhydrophobic coatings via thermally induced phase separation and spin-casting. The hybrid approach broadens the opportunities for reusing plastic waste, which is a step towards creating a more circular economy and better waste management practices.

The interactions between mixed waste from discarded surgical masks and face shields during the degradation in supercritical water

The widespread use of surgical masks made of polyolefin and face shields made of polyester during pandemics contributes significantly to plastic pollution. An eco-friendly approach to process plastic waste is using supercritical water, but the reaction of mixed polyolefin and polyester in this solvent is not well understood, which hinders practical applications. This study aimed to investigate the reaction of waste surgical masks (SM) and face shields (FS) mixed in supercritical water. Results showed that the optimal treatment conditions were 400 °C and 60 min, achieving a liquid oil yield of 823.03 mg·g−1 with 25 wt% FS. The interaction between polypropylene (PP), polyethylene terephthalate (PET), and iron (Fe) in SM and FS mainly determined the production of liquid oil products such as olefins and benzoic acid. The methyl-branched structure of PP enhanced PET hydrolysis, resulting in higher production of terephthalic acid (TPA). The degradation of PP was facilitated by the acidic environment created by TPA and benzoic acid in the reaction. Moreover, the hydrolysis of PET produced carboxylic acid, which coordinated with Fe3+ to form Fe-H that catalyzed the polymerization of small olefins, contributing to higher selectivity for C9 olefins. Therefore, this study provides valuable insights into the degradation mechanism of mixed PPE waste in supercritical water and guidance for industrial treatment.

Contaminant removal from plastic waste pyrolysis oil via depth filtration and the impact on chemical recycling: A simple solution with significant impact

Recycling of mixed plastic waste via pyrolysis and subsequent steam cracking towards light olefins is a promising solution for the ever-growing plastic waste crisis. However, the pyrolytic recycling pathway is still not established on industrial scale due to the large variety of pyrolysis oil contaminants that hamper the application in (petro-)chemical processes. In short, plastic waste pyrolysis oils are unfeasible for steam crackers without upgrading. In this work, depth filtration for the removal of particulate contamination from a post-consumer mixed polyolefin (MPO) pyrolysis oil was performed using three different filter media with different porosity. Comprehensive analyses using SEM-EDX of the retained particles as well as ICP-OES and GC × GC-FID of the filtered oils allowed to understand the efficacy of filtration. Most of the particulate contamination was removed by filtration leading to a reduction from 69 mg/L in the unfiltered sample to < 2 mg/L in the filtered samples. Particle characterization confirmed that the main contamination is composed of iron, calcium and silicon-based contaminants in combination with carbon-based species. It was confirmed by ICP-OES that after filtration, important contaminants such as nickel, vanadium and lead were reduced below contamination thresholds for steam crackers indicating that filtration is an efficient, and potentially cost-effective upgrading technique for pyrolysis oils. When steam cracking the filtered oils, results show that radiant coil coke formation was reduced by 40–60% compared to the unfiltered oil without changes in product selectivity, confirming the strong impact of particulate contamination on coke formation during steam cracking.

Ni-catalyzed carbon–carbon bonds cleavage of mixed polyolefin plastics waste

The inert carbon–carbon (C–C) bonds cleavage is a main bottleneck in the chemical upcycling of recalcitrant polyolefin plastics waste. Here we develop an efficient strategy to catalyze the complete cleavage of C–C bonds in mixed polyolefin plastics over non-noble metal catalysts under mild conditions. The nickel-based catalyst involving Ni2Al3 phase enables the direct transformation of mixed polyolefin plastics into natural gas, and the gas carbon yield reaches up to 89.6%. Reaction pathway investigation reveals that natural gas comes from the stepwise catalytic cleavage of C–C bonds in polypropylene, and the catalyst prefers catalytic cleavage of terminal C–C bond in the side-chain with the low energy barrier. Additionally, our developed approach is evaluated by the technical economic analysis for an economically competitive production process.

Purification and characterisation of post-consumer plastic pyrolysis oil fractionated by vacuum distillation

Valorisation of post-consumer plastic pyrolysis oil in steam crackers and their application as precursors to chemicals is currently hindered by the complexity of these oils in terms of their boiling range, the risk of fouling by unsaturated compounds, and the presence of contaminations (heteroatoms, metals, and halogens). To gain insights into which of these challenges is actually a potential showstopper, we have studied the vacuum distillation of pyrolysis oil derived from two sorted post-consumer waste plastics, i.e. polypropylene (PP) rigids and mixed polyolefins (MPO) rigids, in a batch fractionating column. The crude pyrolysis oil samples were distilled in three fractions i.e., the light fraction (final boiling point <175 °C), middle fraction (175–360 °C), and heavy fraction (initial boiling point >360 °C). Subsequently, comprehensive characterisation, such as physicochemical properties, the level of contaminations, and chemical composition, was performed. The suitability of the light and middle fractions for steam cracking and precursors to chemical feedstock was assessed. It was found that the density and viscosity values of light and middle fractions from the two pyrolysis oil samples were comparable to fossil naphtha and diesel, whereas total acid number (TAN) values were notably higher compared to petroleum-based oil fractions (>0.5 mgKOH/g). Analyses via GC-MS and ATR-FTIR revealed predominant concentrations of olefins. ICP-OES analysis showed that distillation removes more than 97.5% of metals in all fractions. The chlorine content (>3 ppm), nitrogen (≥0.1%), and oxygen (≥0.1%) exceed thresholds. Hence, this study shows that vacuum distillation effectively improved most, but not all, of the properties of the distilled fractions and can thus be an essential downstream process step for the application of post-consumer plastic pyrolysis oils as feedstock in steam crackers and precursors to chemicals.

Assessment of Melt Compounding with Zeolites as an Effective Deodorization Strategy for Mixed Plastic Wastes and Comparison with Degassing.

The emission of off-odors from mechanically recycled plastics severely limits their re-introduction into the market for the production of new objects, for the same use or even for less demanding applications, thus hindering the implementation of an effective circular economy for plastics. The addition of adsorbing agents during the extrusion of polymers represents one of the most promising strategy to reduce the odorous emissions of plastics, due to its characteristics of cost-effectiveness, flexibility and low energy consumption. The novelty of this work lies in the assessment of zeolites as VOC adsorbents during the extrusion of recycled plastics. They appear more suitable than other types of adsorbents, due to their ability to capture and "hold" the adsorbed substances at the high temperatures of the extrusion process. Moreover, the effectiveness of this deodorization strategy was compared with the traditional degassing technique. Two types of mixed polyolefin wastes, coming from completely different collection and recycling processes, were tested: Fil-S (Film-Small), deriving from post-consumer flexible films of small size, and PW (pulper waste), which is the residual plastic waste obtained from the paper recycling process. The melt compounding of the recycled materials with two micrometric zeolites (zeolite 13X and Z310) resulted as more effective in the off-odors removal with respect to degassing. In particular, the highest reduction (-45%) of the Average Odor Intensity (AOI) was measured for both PW/Z310 and Fil-S/13X systems at 4 wt% of the zeolites' amount, compared with the corresponding untreated recyclates. Finally, by combining degassing and melt compounding with zeolites, the best result was obtained for the composite Fil-S/13X, whose Average Odor Intensity resulted as quite close (+22%) to the one of the virgin LDPE.

Effect of Few-Layer Graphene on the Properties of Mixed Polyolefin Waste Stream

This work demonstrates how the addition of few-layer graphene (FLG) influences the processability and mechanical properties of the mixed polyolefin waste stream (R-(PE/PP)). Three different types of compounds were investigated: (1) R-(PE/PP) with FLG; (2) blends of R-(PE/PP) with prime polyethylene (PE) or polypropylene (PP) or PP copolymer; and (3) R-(PE/PP) with both the prime polymer and FLG. The processability was assessed by measuring the torque during melt extrusion, the melt flow index (MFI), and viscosity of the compounds. Investigations of the processability and mechanical properties of the composites indicate that the presence of FLG can reinforce the composites without hindering the processability, an unusual but desired feature of rigid fillers. A maximum increase in tensile strength by 9%, flexural strength by 23%, but a reduction in impact strength were observed for the compounds containing R-(PE/PP), 4 wt.% FLG, and 9 wt.% prime PP. The addition of FLG concentrations higher than 4 wt.% in R-(PE/PP), however, resulted in higher tensile and flexural properties while preserving the impact strength. Remarkably, the addition of 10 wt.% FLG increased the impact strength of the composite by 9%. This increase in impact strength is attributed to the dominant resistance of the rigid FLG particles to crack propagation.

Multi-Scale Modeling of Plastic Waste Gasification: Opportunities and Challenges.

Among the different thermo-chemical recycling routes for plastic waste valorization, gasification is one of the most promising, converting plastic waste into syngas (H2+CO) and energy in the presence of an oxygen-rich gas. Plastic waste gasification is associated with many different complexities due to the multi-scale nature of the process, the feedstock complexity (mixed polyolefins with different contaminations), intricate reaction mechanisms, plastic properties (melting behavior and molecular weight distribution), and complex transport phenomena in a multi-phase flow system. Hence, creating a reliable model calls for an extensive understanding of the phenomena at all scales, and more advanced modeling approaches than those applied today are required. Indeed, modeling of plastic waste gasification (PWG) is still in its infancy today. Our review paper shows that the thermophysical properties are rarely properly defined. Challenges in this regard together with possible methodologies to decently define these properties have been elaborated. The complexities regarding the kinetic modeling of gasification are numerous, compared to, e.g., plastic waste pyrolysis, or coal and biomass gasification, which are elaborated in this work along with the possible solutions to overcome them. Moreover, transport limitations and phase transformations, which affect the apparent kinetics of the process, are not usually considered, while it is demonstrated in this review that they are crucial in the robust prediction of the outcome. Hence, possible approaches in implementing available models to consider these limitations are suggested. Finally, the reactor-scale phenomena of PWG, which are more intricate than the similar processes—due to the presence of molten plastic—are usually simplified to the gas-solid systems, which can result in unreliable modeling frameworks. In this regard, an opportunity lies in the increased computational power that helps improve the model’s precision and allows us to include those complexities within the multi-scale PWG modeling. Using the more accurate modeling methodologies in combination with multi-scale modeling approaches will, in a decade, allow us to perform a rigorous optimization of the PWG process, improve existing and develop new gasifiers, and avoid fouling issues caused by tar.

Transition metal chalcogenide bifunctional catalysts for chemical recycling by plastic hydrocracking: a single-source precursor approach.

Sulfided nickel, an established hydrocracking and hydrotreating catalyst for hydrocarbon refining, was synthesized on porous aluminosilicate supports for the hydrocracking of mixed polyolefin waste. Zeolite beta, zeolite 13X, MCM41 and an amorphous silica-alumina catalyst support were impregnated with the single-source precursor (SSP) nickel (II) ethylxanthate for catalyst support screening. Application of this synthesis method to beta-supported nickel (Ni@Beta), as an alternative to wet impregnation using aqueous nickel (II) nitrate, provided catalytic materials with higher conversion to fluid products at the same mild batch reaction conditions of 330°C with appropriate agitation and 20 bar H2 pressure. Mass balance quantification demonstrated that SSP-derived 5wt%Ni@Beta yielded a greater than 95 wt% conversion of a mixed polyolefin feed to fluid products, compared with 39.8 wt% conversion in the case of 5wt%Ni@Beta prepared by wet impregnation. Liquid and gas products were quantitatively analysed by gas chromatography–flame ionization detection (GC-FID) and gas chromatography–mass spectrometry (GC-MS), revealing a strong selectivity to saturated C4 (37.3 wt%), C5 (21.6 wt%) and C6 (12.8 wt%) hydrocarbons in the case of the SSP-derived catalyst.

Highly selective conversion of mixed polyolefins to valuable base chemicals using phosphorus-modified and steam-treated mesoporous HZSM-5 zeolite with minimal carbon footprint

Catalytic fast pyrolysis of polyolefinic waste streams was investigated to recover valuable base chemicals at high selectivity. HZSM-5 zeolite with different properties, affected by Si/Al, mesoporosity, phosphorus stabilization, and steaming, were tested and thoroughly characterized. Different feeds, catalyst/feed ratios and reaction temperatures were evaluated in a micropyrolysis reactor coupled to two-dimensional gas chromatography. While unmodified HZSM-5 rapidly deactivated, phosphorus-modified and steamtreated HZSM-5 showed almost no deactivation due to its lower coking propensity during 130 runs with stable conversion towards C5+ aliphatics and high C2-C4 olefins selectivity (~75%) using post-consumer mixed polyolefins. The performance of this direct olefins production route with unprecedented high olefin selectivity was further evaluated in a plantwide context. It was found that it requires ~37% lower energy input than the plastics pyrolysis followed by pyrolytic oil steam cracking, while it results to at least a one order of magnitude lower environmental burden as compared to waste incineration.