Polyolefin Materials Research Articles

Polysiloxane-Coated PI Nonwoven Separators with Higher Thermal and Electrochemical Stability for Lithium Ion Battery Application

Polyolefin separators are widely used for most of commercial application owing to their excellent mechanical properties, electrical insulation, and internal porous structures. The raw polyolefin materials have economic viability and possess facile mechanical extension suitable for processing as a film [1]. However, the inherent thermal property of polyolefin inevitably results in the severe thermal deformation of the separators upon the elevation of LiB temperature that has been pointed out as a major cause of fire in LiBs with the possibility of short circuits caused by lithium dendrites [2]. There have been trials to secure cycle characteristics by preventing LiB separators from shrinkage. But, even the coated polyolefin separator couldn’t have thermal stability at high temperatures above 180 oC, since the polyolefin was the base film material.Many studies have been performed to achieve high temperature stability with engineering plastics, including polyimide (PI) [1]. As PI has superior thermal stability, it has been widely applied as nonwoven separators. PI nonwoven fabric separators exhibit stability usually above 200 oC. As PI nonwoven separators have also high ionic conductivity and good electrolyte wettability, it is therefore expected to show excellent rate capability and cyclability [3]. Despite those advantages, however, PI nonwoven separators still have several issues to resolve before application of them to LiBs. One of the problems is the existence of large pores [4]. Pores over a certain size is thought to render particles in cathode active materials pass through or Li dendrites penetrate upon overcharging. Then, it can result in leakage current and internal short circuit eventually. Various studies such as particle-coating or ceramic-coating have been studied in order to figure out this problem [26].Other than pore-size distribution or existence of large pores, it must be the electrochemical stability of PI nonwoven separator and the electrochemical cell performance with the separator at high voltage that needs to be studied and examined for the potential application to LiBs [3]. In accordance with the strong demand on high power LiB development, study on the electrochemical resistance of materials and components is highly required [5]. As PI has polar chemical structure in contrast to non-polar chemical structure of polyolefin, it would be worthy to investigate the electrochemical nature and stability under high current or high voltage. In this study, a PI nonwoven separator is modified by coating of polysiloxane to improve its electrochemical stability and properties as well as its porous structure. As the coated PI nonwoven separator should be stable up to 200 oC due to the robust PI frame and the modification via polysilicon-coating is rather simple, the feasibility of this chemical approach is demonstrated and studied including its electrochemical behaviors. Ionic conductivity, electrolyte wettability, and gas permeability of the modified separator is also examined as a set of tests for LiB application. It was electrochemically stable during LSV even at 5 V vs Li+/Li. The polysiloxane-coating maintains or improves the excellent thermal and electrical stability of the PI nonwoven separators. The full cell test demonstrated that the polysiloxane-coating enabled a cyclability of 98.6% after 100 cycles, while the PI nonwoven could not be charged due to an internal short circuit.[1] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, X. Zhang, A review of recent developments in membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci, 7, 2014, 3857.[2] X. Zhang, E. Sahraei, K. Wang, Li-ion Battery Separators, Mechanical Integrity and Failure Mechanisms Leading to Soft and Hard Internal Shorts, Sci. Rep., 6, 2016, 32578.[3] Z. Lu, F. Sui, Y. Miao, G. Liu, C. Li, W. Dong, J. Cui, T. Liu, J. Wu, C. Yang, Polyimide separators for rechargeable batteries, J. Energy Chem., 58, 2021, 170.[4] G. Dong, B. Liu, G. Sun, G. Tian, S. Qi, D. Wu, TiO2 nanoshell@polyimide nanofiber membrane prepared via a surface-alkaline-etching and in-situ complexation-hydrolysis strategy for advanced and safe LIB separator, J. Membr. Sci., 577, 2019, 249.[5] Y. Xiang, J. Li, J. Lei, D. Liu, Z. Xie, D. Qu, K. Li, T. Deng, H. Tang, Advanced Separators for Lithium-Ion and Lithium-Sulfur Batteries: A Review of Recent Progress, ChemSusChem, 9, 2016, 1. Figure 1

Barbier Polymerization-Induced Emission towards Fully Substituted Polyethylene Analogues with Non-Traditional Intrinsic Luminescence.

The properties of polyethylene are highly dependent on the variety and quantity of substitutions. Generally, polyethylene can only be fully substituted with fluorine atoms, mainly e. g., polytetrafluoroethylene and nafion, because atomic radius of fluorine atom is small enough. The preparation of fully substituted polyethylene analogues (FSPEA) and their non-traditional intrinsic luminescence (NTIL) are attractive, especially for substitutions with relatively larger atomic radii than a fluorine atom. Here, Barbier polymerization-induced emission (PIE) is demonstrated as a universal method for the molecular design of NTIL type FSPEAs with intriguing aggregation-induced emission (AIE) behaviors. Through Barbier polymerization of diphenyldichloromethane and different peroxyesters in the presence of Mg in one pot, a series of FSPEAs, including polytriphenylethanol (PTPE), polydiphenylfurylethanol (PDPFE), polydiphenylthiophenylethanol (PDPTE) and polydiphenylnaphthylethanol (PDPNE) have been successfully prepared. Further potential applications for explosive detection, artificial light-harvesting system and white phosphor-converted light-emitting diode are investigated. Therefore, this work opens up a new approach for the molecular design of FSPEA with non-conjugated luminescence, which may cause inspirations to different research fields like polyolefin and luminescent materials.

Selective deuteration along a polyethylene chain: Differentiating conformation segment by segment.

To improve the circularity and performance of polyolefin materials, recent innovations have enabled the synthesis of polyolefins with new structural features such as cleavable breakpoints, functional chain ends, and unique comonomers. As new polyolefin structures become synthetically accessible, fundamental understanding of the effects of structural features on polymer (re)processing and mechanical performance is increasingly important. While bulk material properties are readily measured through conventional thermal or mechanical techniques, selective measurement of local material properties near structural defects is a major characterization challenge. Here, we synthesized a series of polyethylenes with selectively deuterated segments using a polyhomologation approach and employed vibrational spectroscopy to evaluate crystallization and melting of chain segments near features of interest (e.g., end groups, chain centers, and mid-chain structural defects). Chain-end functionality and defects were observed to strongly influence crystallinity of adjacent deuterated chain segments. Additionally, chain-end crystallinity was observed to have different molar mass dependence than mid-chain crystallinity. The synthesis and spectroscopy techniques demonstrated here can be applied to range of previously inaccessible deuterated polyethylene structures to provide direct insight into local crystallization behavior.

Plastically deformable polyethylene with excellent transparency and stress whitening resistance properties fabricated by constructing massive chain entanglement

AbstractStress whitening is one of the most prominent macroscopic defects for polyethylene (PE), which greatly affects the appearance and quality of its commodity. In order to eliminate the stress whitening defects of PE, ultrahigh molecular weight polyethylene (UHMWPE) was blended with PE to fabricated PE/UHMWPE, and the enhanced intermolecular entanglement and stretchability was achieved, which greatly suppressed and impeded the occurrence of heterogeneous plastic deformation and fibrillar bundle structure during applying tensile stress, resulting in the sharply deceased of the nanoscale cavities defects for the sample. And thus, the elimination of stress whitening defects and high‐light transmittance in the visible region with excellent optical transparency performance were successfully achieved for PE/UHMWPE. Moreover, introduction of UHMWPE chains significantly improved the orientation degree and formed a dense oriented crystalline network with fine lamellae for the sample, which highly enhanced the mechanical strength of PE/UHMWPE after applying stress. This work provided a new strategy for developing stress whitening eliminating polyolefin materials with high performance, which was of great significance for sustainable development and environmental friendliness.

High-value recycling and biodegradation of polyolefin materials

The pollution of plastic materials has seriously affected global environmental problems. Polyolefin materials are widely used as raw materials for plastics. This is due to their practical physical properties and low cost. However, there are major challenges in the disposal of waste polyolefin materials. Recycling and degradation have emerged as the two main approaches for the treatment of plastic waste today. Through a comprehensive literature analysis and review of methods, this paper provides an in-depth study of recycling and biodegradation of polyolefin materials. The study is based on a detailed search of several papers through Google Scholar in order to provide valuable insights into the different methods that are used for the recycling and biodegradation of polyolefins. The review summarizes the most effective technologies for recycling and biodegradation, while highlighting recent advances and future directions in the field. In particular, the research has focused on two main approaches: closed-loop recycling and chemical recovery. The latter technology is aimed at non-polluting biodegradation, which has become an increasingly important topic of interest for the scientific community. Given the urgency of the environmental challenges posed by polyolefins, the development of efficient and sustainable recycling and degradation methods is essential to create a circular economy and ensure a sustainable future.

Upcycling of Polypropylene Wastes via Catalytically C−H Modification with Polar Olefins

AbstractWhile polypropylene (PP) is one of the most widely used polyolefin materials, its post‐functionalization has been a continuously researched topic in the polymer field since it could significantly improve physical and chemical properties by introducing polar groups, beneficial for development of the next generation of polyolefin materials. In this work, we describe the development of a visible‐light promoted, environmentally friendly iron‐catalyzed strategy and establishing of the reaction scope for C−H alkylated modification of polypropylene. Under our conditions, various polypropylenes could be functionalized with diverse polar alkenes with good levels of functionalization (LOF). The properties of the resulting polymers were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and tensile testing. Polypropylene wastes could also be upcycled. While the incorporation of the polyglycol groups enhanced hydrophilicity, the installation of the ester groups increased the miscibility with other polymers by acting as a compatibilizer for polystyrene and polyethylene.

The enhanced physical properties of XLPE insulation for high voltage cable based on LDPE/LLDPE or LDPE/HDPE blends

Exploring methods of improving the performance of high-voltage cable XLPE insulation is a hot issue in high-voltage and insulation technology. This paper presented a new method to improve the physical properties of XLPE by doping low-content polyolefin materials, such as LLDPE or HDPE. The new XLPE insulation has significantly improved high-temperature mechanical properties and slightly improved room-temperature mechanical properties. The optimal thermal elongation properties were achieved with a 1 phr content of HDPE and LLDPE, while DC conductivity, DC breakdown strength, and dielectric properties were stable or slightly improved. The properties mentioned above showed a decreasing trend at higher content. This study demonstrated that adding HDPE or LLDPE to LDPE increases the physical entanglement of XLPE molecular chains, which has a similar effect to crosslinks formed by chemical crosslinking in enhancing the mechanical properties of XLPE insulation. This suggests that there is a potential to reduce the concentration of crosslinking agents during XLPE cable manufacturing while maintaining electrical properties and improving mechanical properties at elevated temperatures. It provides a new idea for improving the properties of XLPE insulation for high-voltage DC cables.

The effect of chemistry and thermal fluctuations on charge injection barriers at aluminum/polyolefin interfaces

Charge injection at metal/polymer interfaces is a critical process in many technological devices, including high voltage capacitors and cables in which polyolefin materials, such as polyethylene (PE) and polypropylene (PP), are often used as insulation materials. We use simulations based on density-functional theory to study charge injection at aluminum/PE and aluminum/PP interfaces. Specifically, we investigate the influence of incorporating a variety of polar chemical impurities at the PE and PP chain ends on electron and hole injection barriers. Crucially, we account for the effect of thermal disorder by considering ensembles of thousands of interface structures obtained from ab initio molecular dynamics trajectories at 373 K. We show that the mean injection barrier can change by up to 1.1 eV for Al/PE and 0.6 eV for Al/PP, as compared to the pristine case, depending on which chemical impurity is introduced. We also show that the spread of injection barriers from thermal fluctuations also depends strongly on the chemistry of the impurity. The observed trends can be understood with a simple model based on thermal fluctuations of the dipole moment density associated with the chemical impurity at the interface. We further verify this model by considering larger interface models with lower impurity densities. Our results demonstrate that small chemical modifications, which may arise from oxidation, for example, have a significant influence on charge injection barriers in metal/polyolefin interfaces.

Use graphene coated silica core shell particles to provide rapid, precise, and equivalent chemical composition distribution analysis for polyolefin materials by high temperature thermal gradient interaction chromatography

High temperature thermal gradient interaction chromatography (HT-TGIC) has been widely used to measure chemical composition distribution due to its applicability to separate crystalline and non-crystalline amorphous polyolefin materials. The compatibility of HT-TGIC with various detectors (infrared (IR), light scattering (LS), and viscometer) has also allowed a comprehensive analysis of molecular architecture of polyolefin and recycled plastics. The introduction of an easy-to-fabricate graphene coated onto non-porous silica particles as HT-TGIC column in 2020 showed a superior chromatographic performance over the traditional graphite column. A reduction in peak broadness (∼47 %) under identical experimental conditions was demonstrated in that research. This paper similarly uses a graphene column but with the focus on optimization of experimental parameters (concentration, and thermal cooling and heating rates etc.). Equivalent chemical composition distribution (CCD) data to that obtained by the incumbent graphite column over a wide range of polyolefins products was achieved, in addition to a shortened analysis time from 120 min down to 88 min per sample. The materials studied included semicrystalline linear low-density polyethylene (LLDPE), elastomers, terpolymers, model blends to mimic recycled plastics. The results also suggest that the elimination of substrate pores enable a better HT-TGIC separation. Coupling the ease and reproducibility of the graphene column fabrication process enables long term chromatographic robustness. This not only results in equivalent CCD data compared to the traditional graphite column but also a 27 % reduction in analysis time. These results demonstrate a substantial advancement of technology in the high throughput industrial laboratory setting where fast testing turnaround time is critical. In addition, simple fabrication with commercially available silica particles and graphene nanopowder provides a cost-effective approach to make HT-TGIC columns reproducibly.

Sintering behavior of ceramic ethylene‐propylene‐diene rubber/pyrophyllite/zinc borate composites

AbstractCeramic polymer materials have great future applications in the field of fireproof materials. However, ceramic polyolefins have shortcomings such as few available inorganic fillers, high sintering temperature, and low flexural strength. Therefore, achieving rapid sintering of ceramic ethylene‐propylene‐diene rubber (EPDM) is an urgent problem that needs to be solved. Herein, we prepared ceramic EPDM materials using pyrophyllite as aggregate and zinc borate as flux. The materials were prepared using a mixer and sintered in a muffle furnace at 850 and 1000°C. The flexural strength of the sinter residue was tested by a material testing machine. The microstructure changes during the ceramification process were characterized using scanning electron microscopy (SEM), thermogravimetric analysis (TG), derivative thermogravimetric analysis (DTG), differential thermal analysis (DTA), and x‐ray diffraction (XRD). The results indicated that the EPDM filled with pyrophyllite and zinc borate had a flexural strength of 2.6 MPa after sintering at 850°C. After sintering at 1000°C, its flexural strength further increased up to 12.3 MPa. The SEM results indicated that as the temperature increased, the zinc borate melted and filled in the space among pyrophyllite particles, and the zinc borate and the pyrophyllite were sintered into a dense whole. The TG, DTG and DTA testing results indicated that the melting and decomposition behaviors of the pyrophyllite‐zinc borate mixture changed compared with pure pyrophyllite and zinc borate. The XRD results indicated that during the sintering process, the crystalline structure of the zinc borate and the pyrophyllite gradually transformed into a quartz‐based crystalline structure. It is promising that the ceramic polyolefin materials are prepared by the pyrophyllite/zinc borate combination.

Modeling the Migration Behavior of Extractables from Mono- and Multilayer Polyolefin Films to Mathematically Predict the Concentration of Leachable Impurities in Pharmaceutical Drug Products. Part 2: Conservative Diffusion and Partition Coefficient Determinations.

In the development of a pharmaceutical drug product packaging, an important step is to demonstrate acceptable levels of leachable impurities migrating from the packaging material into the drug product during its shelf life and therapeutic use. Such migration processes can be quantified either by analytical methods (which is often challenging and labor intensive) or (in many cases) through theoretical modeling, which is a reliable, quick, and cost-effective method to forecast the level of leachable impurities in the packaged drug when the diffusion and partition coefficients are known. In the previous part, it was shown how these parameters can be determined experimentally, and subsequent theoretical fitting of the results for a series of low- and high-molecular-weight organic compounds (known leachables) in a series of polyolefin materials was performed. One of the interpretations of these results is that a theoretical calculation can be made only for organic compounds and materials whose diffusion/partition/solubility coefficients were determined experimentally and theoretical fitting was achieved. However, in practice, there will be situations in which other leachable compounds may have to be investigated. In such cases, strictly speaking, it would be necessary to perform the whole experimental and fitting procedure for the new compound before a proper theoretical modeling is possible. But this would make the theoretical calculation of a leaching process from a pharmaceutical packaging material a cumbersome and cost intensive procedure. To address this problem, the pools of diffusion and partition coefficients were used to develop an approach that allows the estimation, without any additional experimentation, of so-called "conservative" diffusion and partition coefficients for a much wider range of potential leachables in the polyolefin pharmaceutical packaging materials and aqueous solutions investigated previously.

Influence of Vinyl Acetate Content and Melt Flow Index of Ethylene-Vinyl Acetate Copolymer on Physico-Mechanical and Physico-Chemical Properties of Highly Filled Biocomposites.

Highly filled biocomposites may be used as biodegradable masterbatches that manufacturers add to traditional polymers for making plastic goods more biodegradable. Biocomposites based on various trademarks of ethylene-vinyl acetate copolymer (EVA) and natural vegetable fillers (wood flour and microcrystalline cellulose) were studied. The EVA trademarks differed both in terms of the melt flow index and in the content of vinyl acetate groups. The composites were created as superconcentrates (or masterbatches) for the production of biodegradable materials based on vegetable fillers with polyolefin matrices. The filler content in biocomposites was 50, 60, 70 wt.%. The influence of the content of vinyl acetate in the copolymer and its melt flow index on the physico-mechanical and rheological properties of highly filled biocomposites was evaluated. As a result, one EVA trademark with high molecular weight that has a high content of VA was chosen because of its optimal parameters for creating highly filled composites with natural fillers.

Monitoring the Chemical Composition of Polyolefins along the Molar Mass Axis with High‐Temperature Size Exclusion Chromatography Coupled with an Infrared Detector (HT SEC‐IR5)

AbstractSeries of ethylene/1‐alkene and ethylene‐propylene‐diene copolymers are separated with size exclusion chromatography (SEC) at 150°C using 1,2,4‐trichlorobenzene as the mobile phase. Methyl (CH3–) and methylene (–CH2–) groups in the separated polymer samples are monitored at specific wavelengths via a filter‐based infrared detector (IR5). It is found that the ratio of CH3/CH2may increase, be constant, decrease or change irregularly along the molar mass axis, depending on the sample under investigation. Because the CH3/CH2profiles may be very different even for a given series of copolymers synthesized with the same catalyst, it is supposed that one or more experimental parameters, which are not strictly controlled throughout synthesis, are responsible for these substantial differences in the chemical composition along the molar mass axis. The correlation between the ratio of CH3– to –CH2– groups and the average chemical composition of the investigated series of polymer samples as well as the reproducibility of the measurements and the limit of detection of the SEC‐IR5 measurements are evaluated. Coupling of SEC with the advanced IR5‐detector enables the convenient and reliable characterization of the chemical composition of many polyolefin materials along their molar mass axis. The monitoring of the chemical composition may help to improve quality of synthesized copolymers.

Heterogenization Strategies for Nickel Catalyzed Synthesis of Polyolefins and Composites

ConspectusPolyolefin is one of the most common synthetic polymers. However, most polyolefins are nonpolar materials, which leads to poor compatibility with polar materials, thus limiting their application in some fields. Polar functionalized polyolefins can lead to significantly improved dyeability, adhesiveness and compatibility with polar fillers, and can realize customized properties of polyolefin materials. Transition-metal-catalyzed coordination copolymerization of olefin and polar functionalized comonomers represents a direct and potentially economic route to synthesize polar-functionalized polyolefin materials, which has attracted great attention in recent decades. Literally hundreds of nickel and palladium catalysts have been synthesized and investigated in olefin-polar monomer copolymerization to achieve high copolymerization efficiency by tuning the molecular structures of the catalysts. In particular, earth-abundant and low-cost nickel-based catalysts hold great potential for industrial applications. However, most research efforts focus on homogeneous copolymerization catalysts, while the industrially preferred heterogeneous systems have remained largely unexplored. With the objective of bridging this gap, our group has recently developed a series of heterogenization strategies for the synthesis of high-performance heterogeneous nickel catalysts taking advantage of hydrogen bond anchoring, ionic anchoring, coanchoring, cocatalyst, and ionic cluster formation. These strategies involve known or easily accessible catalysts, can be easily adapted to various catalytic systems, and can lead to simultaneous enhancement in all copolymerization parameters such as thermal stability, activity, comonomer incorporation ratio, and copolymer molecular weight versus the homogeneous counterparts. In addition, the performance of the catalysts can be tuned by changing the type of the support, thereby facilitating the discovery of high-performance heterogeneous nickel catalysts. Most importantly, great product morphology control can be achieved. This is desirable for industrial polymerization processes because it avoids reactor fouling and results in significant improvements in the safety and operational efficiency of the polymerization processes. Finally, these heterogenization strategies give access to high-performance polyolefin materials (polar-functionalized polyolefins, polar-functionalized polyolefin in-reactor blends and polyolefin-based functional composites) with custom-made properties. Compared with melt blending, polyolefin composites obtained by heterogeneous catalytic in situ polymerization have better material properties. By using functional fillers, customized polyolefin composites with high performance can be prepared in situ. These polyolefin-based materials have potential applications in many fields, such as electrical and thermal conductivity, photodegradation, flame retardancy, barrier, etc. It is expected that the continuous research on the copolymerization mechanism/catalysts, heterogenizatioin strategies and material properties of polar functionalized polyolefins and their composites will eventually achieve commercialization of nickel-catalyzed production of polyolefins and composites.

Heterogenization of nickel catalysts with ionic liquid‐modified supports for ethylene polymerization and copolymerization

AbstractThe preparation of heterogeneous catalysts by loading homogeneous metal complexes on the supports is an effective strategy preferred by the polyolefin industry. In this contribution, we synthesize a series of heterogeneous nickel catalysts using the supporting strategy of ionic liquid‐modified supports. These supported catalysts have polymerization activities up to 1.68 × 107 g mol−1 h−1, polymer molecular weights up to 224.1 × 104 g mol−1, and significantly, higher melting points compared to homogeneous systems. The polyolefin materials prepared by this supporting strategy possess a wide range of tunable mechanical properties, from thermoplastics to elastomers. These heterogeneous nickel catalysts can catalyze the copolymerization of ethylene and methyl 10‐undecenoate.

Comparison of Ethylene Copolymerization with Norbornene and 5‐Norbornene‐2‐methanol by Using Ph2C(Cp)(Flu)ZrCl2/MAO

AbstractBoth cyclic olefin copolymer (COC) and functionalized polyolefins are high‐performance polyolefin materials with different outstanding properties. The introduction of functional groups into COC has the potential to yield materials that combine the advantages of both. In this work, ethylene copolymers with norbornene (NBE) or 5‐norbornene‐2‐methanol (NBMO) are synthesized by using a metallocene catalyst. The activity and the molecular weight of copolymer of the former is much higher than that of the latter. A joint deactivation‐chain transfer mechanism is proposed to interpret these observations. Chain transfer is further discussed. Through regression analysis, it is considered that the chain transfer mode of the two comonomers is the same. Three methods for the calculation of NBMO incorporation are proposed and compared, and a more suitable method is obtained, on the basis of the resonance assignment. The comparison of the copolymerization of ethylene with NBE and NBMO may indicate that NBMO has a higher coordination probability but a slower insertion rate compared to NBE.

A co-anchoring strategy for the synthesis of polar bimodal polyethylene

Since polar groups can poison the metal centers in catalysts, the incorporation of polar comonomers usually comes at the expense of catalytic activity and polymer molecular weight. In this contribution, we demonstrate polar bimodal polyethylene as a potential solution to this trade-off. The more-polar/more-branched low-molecular-weight fraction provides polarity and processability, while the less-polar/less-branched high-molecular-weight fraction provides mechanical and melt properties. To achieve high miscibility between these two fractions, three synthetic routes are investigated: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy (CAS) to heterogenize different homogeneous catalysts on one solid support. The CAS route is the only viable strategy for the synthesis of polar bimodal polyethylene with good molecular level entanglement and minimal phase separation. This produces polyolefin materials with excellent mechanical properties, surface/dyeing properties, gas barrier properties, as well as extrudability and 3D-printability.

Progress in the catalyst for ethylene/α‐olefin copolymerization at high temperature

AbstractEthylene/α‐olefin copolymers are one of the most widely‐used polyolefin materials. With the continuous improvement of polyolefin catalysts, high‐performance polyolefin materials were synthesized by adjusting the chain microstructure, changing the comonomer type and comonomer insertion amount, among which the ethylene/α‐olefin random copolymer elastomer (POE) and olefin block copolymer elastomer (OBC) are the most famous and well accepted by the market. The excellent properties of POE and OBC first depend on their polymer chain microstructure. The chain microstructure of polyolefins is fundamentally determined by the catalysts, polymerization conditions, comonomer feed policies, and reaction engineering. High‐performance ethylene/α‐olefin copolymer elastomers are currently prepared by high‐temperature solution polymerization process, which needs to be carried out at a temperature above the melting point of the polymer and is beneficial to speed up the polymerization reaction rate and control the polyolefin chain microstructure. However, the high‐temperature solution polymerization process launched more stringent requirements for the olefin coordination polymerization catalyst. Systematic reports on catalysts for high‐temperature solution copolymerization of ethylene and α‐olefins are lacking. In this review, we screened some catalysts suitable for the controllable copolymerization and high‐temperature solution copolymerization of ethylene/α‐olefin based on the catalyst's heat resistance, copolymerization activity, comonomer insertion ability, molecular weight, and distribution of the copolymer, including traditional Z–N catalysts, metallocene catalysts, and post‐metallocene catalysts. And the future development of catalysts for high‐temperature solution copolymerization of olefins, catalysts for precise control of polyolefin chain microstructures, and catalysts for olefin copolymerization with polar monomers at high temperature are envisaged.

Improved flame retardancy and ceramifiable properties of PE composites by the combination of aluminum hypophosphite and zinc borate

AbstractCeramifiable polyolefin materials protect circuits by forming compact ceramic layers under fire conditions, and can have an excellent application prospect in the refractory cable field. In this paper, Aluminum hypophosphite (AHP) and zinc borate (ZB) were added to further improve flame retardancy and ceramifiable properties of polyethylene (PE)/silicon powder (SP)/wollastonite (WS)/glass frits (GF) composites. The LOI values of composites with AHP/ZB can reach 23.5%, significantly higher than that of PE/SP/WS/GF composites (19.6%). The thermal stability behavior and char yield behaviors of composites could also be characterized by the TG test. The incorporation of AHP/ZB enhances the flexural strength of residue formed at 1000°C from 0.1 to 20.5 MPa. In addition, a new crystal is formed at high temperatures and is identified as the calcium aluminum phosphate phase [CAP, Ca9Al (PO4)7] by XRD analysis. The flow of molten ZB accelerates the reaction of AHP and WS, and this eutectic reaction promotes the formation and stability of ceramics. Furthermore, the SEM analysis reveals the fluxing effect of ZB at low temperatures and AHP at high temperatures. The incorporation of AHP/ZB with a ratio of 1:1 could effectively avoid the vitrification of ceramics and improve their dimensional stability.

Toughening behavior and synergistic flame retardant effect of a ternary block copolymer with amorphous morphology and thermal softening property in polypropylene

A novel intumescent flame retardant with ternary block copolymer style not only more effectively enhanced flame retardant efficiency but also dramatically increased mechanical properties of polypropylene (PP) composites compared with traditional compounds. The ternary block copolymers were synthesized by the dehydration reaction of polydiphenylsiloxane, piperazine pyrophosphate oligomer and melamine phosphate. Among them, the ternary block copolymer T4-0.8PSi with particular unit ratio presented best comprehensive effect in PP composites. The LOI value of PP composite T4-0.8PSi/PP with 20 wt% T4-0.8PSi reached 30.2 %, and UL94 level of 1.6 mm spline passed V-0 rating. Compared with binary block copolymer B1 and the mixtures B1/0.8PSi and Mix/0.8PSi with similar contents as T4-0.8PSi, T4-0.8PSi obviously reduced the peak value of heat release rate, total heat release, effective heat of combustion and total smoke release. Particularly, it dramatically increased the residue yield and promoted the formation of denser and compact char layer. The ternary block copolymer T4-0.8PSi with appropriate group ratio generated synergistic charring effect in PP. At the same time, the mechanical properties of T4-0.8PSi/PP were dramatically enhanced and the impact strength was increased by 14.5 % compared with B1. Compared with the two mixtures just mentioned, the impact strength of T4-0.8PSi/PP increased by 48.8 % and 103.3 %, and the elongation at break increased by 601 % and 2056 %, respectively. The silicon-containing unit in ternary block copolymer nearly eliminated crystallinity to form amorphous morphology, which made flame retardants deform to droplet-like particles during processing, thus resulting in the more contact area and stronger interaction between the ternary block copolymer and polymer matrix. The research provided a new way for developing polyolefin materials with both high flame retardancy and excellent mechanical properties.