Inverse Vulcanization Research Articles

Composites produced from waste plastic with agricultural and energy sector by‐products

AbstractA three‐stage route to chemically upcycle post‐consumer poly(ethylene terephthalate) (PET) to produce high compressive strength composites is reported. This procedure involves initial glycolysis with diethylene glycol to produce a mixture (GPET) comprising oligomers of 2–7 terephthalate units followed by trans/esterification of GPET with fatty acid chains supplied by brown grease, an agricultural by‐product of animal fat of relatively low nutritional or fuel value. This process yields PGB comprising a mixture of mono‐terephthalate ester derivatives. The olefin units provided by unsaturated fatty acid chains in brown grease were crosslinked by an inverse vulcanization reaction with elemental sulfur to give composites GBSx (x = wt% S, varied from 80%–90%). The compressive strengths of GBS80 (27.5 ± 2.6 MPa) and GBS90 (19.2 ± 0.8 MPa) exceed the compressive strength required of ordinary Portland cement (17 MPa) for its use in residential building foundations. The current route represents a way to repurpose waste plastic, energy sector by‐product sulfur, and agricultural by‐product brown grease to give high strength composites with mechanical properties suggesting their possible use to replace less sustainably sourced legacy structural materials.

Processable crosslinked terpolymers made from elemental sulfur with wide range of thermal and mechanical properties

Inverse vulcanization has emerged as a popular strategy for transforming the waste material, elemental sulfur, into functional polymers with high sulfur content (>50 wt.%, normally). Inverse vulcanized polymers are intrinsically processable and recyclable, and have been demonstrated as promising for applications in many fields. However, the mechanical properties of inverse vulcanized polymers are currently underdeveloped. If this kind of material is to be widely used in some scenarios to replace some traditional plastics, it is necessary to make them with appropriate thermal and mechanical properties that meet basic application requirements. Here, we report a series of terpolymers copolymerized from two distinct organic comonomers and elemental sulfur to obtain polymers with a wide range of glass transition temperatures (−43 °C to 45 °C) that exhibit good mechanical properties, by blending crosslinkers with varying feed monomer ratio and chain length of linear sections, which expands the application opportunities of inverse vulcanization.

Chemical, Thermal, and Mechanical Properties of Sulfur Polymer Composites Comprising Low-Value Fats and Pozzolan Additives

High sulfur-content materials (HSMs) formed via inverse vulcanization of elemental sulfur with animal fats and/or plant oils can exhibit remarkable mechanical strength and chemical resistance, sometimes superior to commercial building products. Adding pozzolan fine materials—fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBFS), or metakaolin (MK)—can further improve HSM mechanical properties and stability. Herein, we detail nine materials comprised of rancidified chicken fat, elemental sulfur, and canola or sunflower oil (to yield CFS or GFS, respectively) and, with or without FA, SF, GGBFS, or MK. The base HSMs, CFS90 or GFS90, contained 90 wt% sulfur, 5 wt% chicken fat, and 5 wt% canola or sunflower oil, respectively. For each HSM/fine combination, the resulting material was prepared using a 95:5 mass input ratio of HSM/fine. No material exhibited water uptake >0.2 wt% after immersion in water for 24 h, significantly lower than the 28 wt% observed with ordinary Portland cement (OPC). Impressively, CFS90, GFS90, and all HSM/fine combinations exhibited compressive strength values 15% to 55% greater than OPC. After immersion in 0.5 M H2SO4, CFS90, GFS90, and its derivatives retained 90% to 171% of the initial strength of OPC, whereas OPC disintegrated under these conditions. CFS90, GFS90, and its derivatives collectively show promise as sustainable materials and materials with superior performance versus concrete.

Evaluation of Animal Fats and Vegetable Oils as Comonomers in Polymer Composite Synthesis: Effects of Plant/Animal Sources and Comonomer Composition on Composite Properties

AbstractRancid animal fats unsuitable for human or animal food production represent low‐value and abundant, yet underexploited organic chemical precursors. The current work describes a strategy to synthesize high sulfur‐content materials (HSMs) that directly utilizes a blend of partially hydrolyzed chicken fat and plant oils as the organic comonomers, following up on analogous reactions using brown grease in place of chicken fat. The reaction of sulfur and chicken fat with either canola or sunflower oil yielded crosslinked polymer composites CFSx or GFSx, respectively (x = wt% sulfur, varied from 85%–90%). The composites exhibited compressive strengths of 24.7–31.7 MPa, and flexural strengths of 4.1–5.7 MPa, exceeding the value of established construction materials like ordinary Portland cement (compressive strength ≥17 MPa required for residential building, flexural strength 2–5 MPa). The composites also exhibited thermal stability up to 215–224 °C. The simple single‐step protocol described herein represents a way to upcycle an affordable and previously unexploited animal fat resource to form structural composites via the atom economical inverse vulcanization mechanism.

Bitumen‐Like Polymers Prepared via Inverse Vulcanization with Shear Stiffening and Self‐Healing Abilities for Multifunctional Applications

AbstractBitumen, which is widely used in various applications, is facing the challenge of being unsustainable. Many strategies are proposed to recycle or replace bitumen. However, most of them can not address the unsustainability of bitumen from the roots. On the other hand, sulfur exists widely on the earth as a kind of industrial waste despite its extensive applications. Herein, a series of bitumen‐like polymers from elemental sulfur is obtained through an inverse vulcanization reaction. The sulfur‐containing polymers exhibit self‐healing, non‐toxic, and adjustable properties depending on the working environment. Based on these features, the applications of sulfur‐containing polymers in self‐healing waterproof sealants, impact‐resistance or shock‐absorption devices, and non‐Newtonian speed bumps are demonstrated. With the method of synthesizing multifunctional bitumen‐like polymers from sulfur wastes, a feasible way is provided to solve simultaneously the challenging problem of bitumen's unsustainability as well as sulfur's utilization, with the advantages including easy massed‐fabrication, non‐toxicity, extremely low cost (≈2 cents per gram) and environment friendliness.

Transesterification‐vulcanization route to durable composites from post‐consumer poly(ethylene terephthalate), terpenoids, and industrial waste sulfur

AbstractDespite improvements in chemical recycling, most post‐consumer plastics are still deposited in landfills where they pose a significant threat to ecological health. Herein we report a two‐stage method for chemically recycling poly(ethylene terephthalate) (PET) using terpenoids and waste sulfur to yield composites. In this method, post‐consumer PET (from beverage bottles) undergoes transesterification with a terpenoid alcohol (citronellol or geraniol) to yield low‐molecular PET oligomers. The terpene‐derived alkenes in these PET oligomer derivatives then served as reaction sites for inverse vulcanization with 90 wt% elemental sulfur to form composite CPS (using citronellol) or GPS (using geraniol). Composition, mechanical, thermal, and morphological properties were characterized by NMR spectroscopy, MALDI, FT‐IR spectroscopy, compressive and flexural strength analysis, TGA, DSC, elemental analysis, and SEM/EDX. The composites CPS (compressive strength = 5.20 MPa, flexural strength = 3.10 MPa) and GPS (compressive strength = 5.8 MPa, flexural strength = 2.77 MPa) showed mechanical strengths comparable to those of commercial bricks (classification C62 for general building). The approach delineated herein thus represents a method to chemically recycle waste plastic with industrial waste sulfur and plant‐derived terpenoids to yield composites having favorable properties comparable to existing building materials.

Current Trends in the Synthesis of Inorganic and Organoelement Phosphorus- and Sulfur-Containing Polymers. A Review

An analysis of literature data on the set of reactions for the production of macromolecules with a high content of phosphorus and sulfur has been carried out, and basic approaches that allow the introduction of these elements into the composition of polymers and polymeric materials have been considered in compliance with the fundamental principles of green chemistry. Methods for synthesis of functional polymers under mild conditions that require minimal energy input from external sources, which can become new growth points for green industrial technologies, are considered. Particular attention focuses on the synthesis of polyphosphazenes and polyphosphoesters for biomedical purposes, as well as on the inverse vulcanization reaction to give polymers used in sorption wastewater treatment, the creation of current sources, and IR optics.

CURRENT TRENDS IN THE SYNTHESIS OF INORGANIC AND ORGANOELEMENT PHOSPHORUS- AND SULFUR-CONTAINING POLYMERS. A REVIEW

The analysis of the literature data on the set of reactions for the production of macromolecules with a high content of phosphorus and sulfur is carried out, and the main approaches allowing to involve these elements in the composition of polymers and polymer materials, considering the fundamental principles of green chemistry, are considered. The methods leading to the formation of functional polymers under mild conditions with minimal energy consumption from external sources necessary for the synthesis, which can become new growth points of green industrial technologies, are considered. Particular attention is paid to the issues of synthesis of polyphosphazenes and polyphosphoesters for biomedical purposes, as well as the inverse vulcanization reaction with the formation of polymers that are used in sorption wastewater treatment, the creation of current sources and IR optics.

Sulfur-rich polymer/Ketjen Black composites as lithium-sulfur battery cathode with high cycling stability

Lithium-sulfur (Li-S) battery is considered as one of the most promising energy storage devices due to its exceptional energy density. However, its development is largely hampered by the shuttle effect of lithium polysulfide and the sluggish electrochemical kinetics of sulfur cathode. In this study, a novel sulfur-rich polymer is synthesized through the inverse vulcanization of sulfur with 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclotetrasiloxane (TVTCSi) and used as cathode material for Li-S batteries, which can effectively immobilize the lithium polysulfides through covalent bonds to impede its shuttling, leading to marked improvement in the cycling performance of the batteries. Furtherly, with the addition of Ketjen Black, the resulting composite cathode exhibited faster electrochemical reaction kinetics and higher cycling stability, which could deliver a reversible discharge capacity of 652 mAh g−1 after 500 cycles at a high current density of 0.5 C,0.5C with quite a low capacity decay rate of only 0.037% per cycle, which indicates broad perspectives of sulfur-rich polymer/Ketjen Black composite as cathode material in lithium-sulfur battery.

The exploration of the inverse vulcanization mechanism of tung oil by controlling the oxygen and moisture presence during reactions

Inverse vulcanization is a cost-effective method for producing high sulfur-content copolymers by combining elemental sulfur with organic monomers, which has rapidly gained popularity due to its simplicity of synthesis and wide range of applications. Although numerous examples of sulfur-rich copolymers have been synthesized at different reaction rates and temperatures using various monomers, the precise reaction mechanism remains unclear. In this paper, we used tung oil containing conjugated triene as a monomer to synthesize sulfur-rich copolymers under six different reaction conditions and investigate the effects of oxygen and moisture. Our study, which employed DSC, XRD, 1H NMR, and XPS characterization methods, revealed that oxygen accelerated the reaction rate and decreased the free sulfur content of the products, while moisture shortened the gel times but increased the free sulfur content. These findings confirm that reverse vulcanization involves two simultaneous mechanisms: the free radical mechanism and the anion mechanism. With regard to the radical mechanism, we discuss the source, ease, and reactivity of radicals and show that the creation of radicals depends on the second monomer rather than sulfur. Tung oil not only acts as a comonomer in the reaction but also plays an initiating role in promoting the dissociation of sulfur chains to generate free radicals for addition of non-conjugated double bonds. The effect of the anion mechanism exceeds that of the radical mechanism once certain factors are stimulated, such as the presence of metal ions and sulfide ions. Understanding the detailed mechanisms involved in inverse vulcanization is essential for selecting optimal monomers, which can enhance not only the synthesis process but also the properties of sulfur-rich materials.

Mechanochemistry Enabled Inverse Vulcanization of Norbornadiene for Optical Polymers and Elastomeric Materials

The emergence of polymeric materials derived from elemental sulfur through inverse vulcanization has recently provoked great interest. Particularly, norbornadiene derived sulfur polymers are predicted to have high transmittance in the long-wave infrared region that can be an excellent candidate for polymeric IR optics. Moreover, owing to the unique cyclic structures of the norbornadiene moiety, it tends to form elastic material upon copolymerization with soft segments which has not been attained for elemental sulfur derived polymers. However, because of the volatility of norbornadiene (NBD) (boiling point is 89 °C), the chemicals could not bear the thermally induced inverse vulcanization process (T > 110 °C). Hence, in this work, for the first time, NBD is subjected to ball milling with elemental sulfur via solid mechanochemical induced inverse vulcanization, which successively affords poly(Sn-r-NBD) with a wide spectrum of sulfur content from 40 to 80 wt %. The synthesis is a green process that is solvent free and releases a traceless side product of H2S without heating. More importantly, the as-fabricated poly(Sn-r-NBD) displays superior transmittance (T > 40%) within the long-wave infrared region, which is obviously better than the most investigated poly(Sn-r-DIB). Furthermore, poly(Sn-r-NBD) derived polymeric material exhibits good elasticity. The present work not only demonstrates a facile and green methodology in fabricating norbornadiene derived sulfur polymers but also promotes the application of the sulfur polymer in high-performance IR optics and elastomeric materials.

Development of Multifunctional Cathode Binder via Inverse Vulcanization for Lithium–Sulfur Battery with Enhanced Capacity Retention

Vinyl-benzaldehyde (VB) monomers containing four functional groups were newly synthesized using the Williamson ether synthesis method. The two aldehyde groups of the synthesized monomers were able to react with amine-terminated macromonomers to form flexible networks. In addition, the two vinyl groups were found to be capable of inverse vulcanization with sulfur by conducting qualitative chemical analysis. The lithium–sulfur battery prepared using the VB-based cathode binder achieved high specific capacity with excellent capacity retention compared to conventional sulfur batteries, demonstrating the potential of the new binder.

Competitive and synergistic effects of metal adsorption in water remediation processes mediated by hybrid copolymers

Copolymers based on vegetable oils and sulfur have recently been developed using the inverse vulcanization technique for water remediation. This study aimed to create a copolymer with heteroatoms, such as nitrogen, to improve adsorption efficiency and examine the competition and synergy between heavy metals in the adsorption process. FTIR, 1H NMR, solid 13C NMR, zeta potential, nitrogen adsorption, Elemental analysis, TEM, and SEM were used to characterize the copolymers synthesized. Once metal adsorption was optimized using the synthesized copolymers, a comparative adsorption study was conducted for the removal of individual heavy metals, Cd2+, Pb2+, Zn2+, Mn2+, and Cu2+ (100 mg/L), using two copolymers, S/CO:80/20 and S/CO/TAA:80/14/6. The copolymers exhibited a higher affinity for soft metals, such as Cd2+, Pb2+, and Cu2+, when they are individually and in a mixture. After 1 h, the S/CO:80/20 copolymer removed 47%, 40%, and 13% of these metals. While with the S/CO/TAA:80/14/6 copolymer removal increased to 10%, 24%, and 53%, respectively respect to S/CO:80/20 copolymer. The introduction of heteroatoms in the copolymers improved their removal efficiency, particularly for heavy metals like Pb2+. Overall, this study demonstrates a promising approach for sustainable water remediation using copolymers.

Thermal and Mechanical Properties of Recyclable Composites Prepared from Bio-Olefins and Industrial Waste

Ordinary Portland Cement (OPC) production consumes tremendous amounts of fresh water and energy and releases vast quantities of CO2 into the atmosphere. Not only would an alternative to OPC whose production requires no water, releases little CO2, and consumes less energy represent a transformative advance in the pursuit of industrial decarbonization, but the greater availability of safe drinking water would lead to significantly improved public health, particularly among vulnerable populations most at risk from contaminated water supply. For any OPC alternative to be adopted on any meaningful scale, however, its structural capabilities must meet or exceed those of OPC. An inverse vulcanization of brown grease, sunflower oil, and elemental sulfur (5:5:90 weight ratio) was successfully modified to afford the high-sulfur-content material SunBG90 in quantities > 1 kg, as was necessary for standardized ASTM and ISO testing. Water absorption (ASTM C140) and thermal conductivity (ISO 8302) values for SunBG90 (<1 wt% and 0.126 W·m−1·K−1, respectively) were 84% and 94% lower than those for OPC, respectively, suggesting that SunBG90 would be more resistant against freeze-thaw and thermal stress damage than OPC. Consequently, not only does SunBG90 represent a more environmentally friendly material than OPC, but its superior thermomechanical properties suggest that it could be a more environmentally robust material on its own merits, particularly for outdoor structural applications involving significant exposure to water and seasonal or day/night temperature swings.

Exploring inverse vulcanisation mechanisms from the perspective of dark sulfur

The build-up of elemental sulfur waste poses problems such that only the advancement of process and product design might act as a solution. Inverse vulcanisation, a process for the generation of high sulfur content polymeric materials may be one such resolution. However, a complete understanding of how these materials form is yet to be fully agreed in this emerging field. Herein is an investigation into the understanding of ‘dark sulfur’ – amorphous, unreacted sulfur, not incorporated into the polymer backbone – in an attempt to understand further the formation mechanisms behind inverse vulcanisation. This research posits theories regarding polymer formation, thermal rearrangement, and the actions of OH to control the degree of product crosslinking, in relation to the quantity of sulfur unreacted into the polymer structure. The detriments and benefits of this dark sulfur in relation to application and general usage are also investigated, showing that a high content of dark sulfur may encourage planktonic bactericidal activity, while also promoting safety considerations from generated species such as hydrogen sulfide and carbon disulfide, concluded as components of this dark sulfur.

On the Mechanism of the Inverse Vulcanization of Elemental Sulfur: Structural Characterization of Poly(sulfur-random-(1,3-diisopropenylbenzene)).

Organosulfur polymers, such as those derived from elemental sulfur, are an important new class of macromolecules that have recently emerged via the inverse vulcanization process. Since the launching of this new field in 2013, the development of new monomers and organopolysulfide materials based on the inverse vulcanization process is now an active area in polymer chemistry. While numerous advances have been made over the last decade concerning this polymerization process, insights into the mechanism of inverse vulcanization and structural characterization of the high-sulfur-content copolymers that are produced remain challenging due to the increasing insolubility of the materials with a higher sulfur content. Furthermore, the high temperatures used in this process can result in side reactions and complex microstructures of the copolymer backbone, complicating detailed characterization. The most widely studied case of inverse vulcanization to date remains the reaction between S8 and 1,3-diisopropenylbenzene (DIB) to form poly(sulfur-random-1,3-diisopropenylbenzene)(poly(S-r-DIB)). Here, to determine the correct microstructure of poly(S-r-DIB), we performed comprehensive structural characterizations of poly(S-r-DIB) using nuclear magnetic resonance spectroscopy (solid state and solution) and analysis of sulfurated DIB units using designer S-S cleavage polymer degradation approaches, along with complementary de novo synthesis of the sulfurated DIB fragments. These studies reveal that the previously proposed repeating units for poly(S-r-DIB) were incorrect and that the polymerization mechanism of this process is significantly more complex than initially proposed. Density functional theory calculations were also conducted to provide mechanistic insights into the formation of the derived nonintuitive microstructure of poly(S-r-DIB).

Long-wave infrared transparent sulfur polymers enabled by symmetric thiol cross-linker

Infrared (IR) transmissive polymeric materials for optical elements require a balance between their optical properties, including refractive index (n) and IR transparency, and thermal properties such as glass transition temperature (Tg). Achieving both a high refractive index (n) and IR transparency in polymer materials is a very difficult challenge. In particular, there are significant complexities and considerations to obtaining organic materials that transmit in the long-wave infrared (LWIR) region, because of high optical losses due to the IR absorption of the organic molecules. Our differentiated strategy to extend the frontiers of LWIR transparency is to reduce the IR absorption of the organic moieties. The proposed approach synthesized a sulfur copolymer via the inverse vulcanization of 1,3,5-benzenetrithiol (BTT), which has a relatively simple IR absorption because of its symmetric structure, and elemental sulfur, which is mostly IR inactive. This strategy resulted in approximately 1 mm thick windows with an ultrahigh refractive index (nav > 1.9) and high mid−wave infrared (MWIR) and LWIR transmission, without any significant decline in thermal properties. Furthermore, we demonstrated that our IR transmissive material was sufficiently competitive with widely used optical inorganic and polymeric materials.

Electrochemical Synthesis of Poly(trisulfides).

With increasing interest in high sulfur content polymers, there is a need to develop new methods for their synthesis that feature improved safety and control of structure. In this report, electrochemically initiated ring-opening polymerization of norbornene-based cyclic trisulfide monomers delivered well-defined, linear poly(trisulfides), which were solution processable. Electrochemistry provided a controlled initiation step that obviates the need for hazardous chemical initiators. The high temperatures required for inverse vulcanization are also avoided resulting in an improved safety profile. Density functional theory calculations revealed a reversible "self-correcting" mechanism that ensures trisulfide linkages between monomer units. This control over sulfur rank is a new benchmark for high sulfur content polymers and creates opportunities to better understand the effects of sulfur rank on polymer properties. Thermogravimetric analysis coupled with mass spectrometry revealed the ability to recycle the polymer to the cyclic trisulfide monomer by thermal depolymerization. The featured poly(trisulfide) is an effective gold sorbent, with potential applications in mining and electronic waste recycling. A water-soluble poly(trisulfide) containing a carboxylic acid group was also produced and found to be effective in the binding and recovery of copper from aqueous media.

Development of a toughened and antibacterial Poly(lactide acid) (PLA) with preserved strength by elemental sulfur-based bio-renewable dynamically crosslinked elastomers

The toughening of PLA is usually realized by blending with elastomers or plasticizers, but often conflicted with lowered mechanical strength and complicated preparation steps. In the current study, inverse vulcanization was used to synthesize a series of dynamically cross-linked polysulfide (SOA) elastomers based on refining by-product elemental sulfur, soybean oil (SO) and allyl glyceryl ether (AGE), which provides dynamically reversible S-S bonds, flexible triglyceride segments and reactive epoxy functions.Melt blended with PLA, the SOAs exhibit great toughening effects and produce droplet-matrix typed blends with good interfacial compatibilization verified by rheology. SOA composition was decisive in dictating toughening efficiency. A high AGE content induces self-polymerization of epoxy groups, resulting in dominance of permanent crosslinking in SOA, giving rise to rigid SOA particles with bad processability and dispersity in PLA, hence poor toughener cavitations under external forces with unsatisfying toughening effect. A high sulfur and low AGE content results in dominance of dynamic crosslinking, providing soft SOA particles in favor of processing and dispersion in PLA matrix with small domains and thin ligament thickness, thus favoring internal cavitation of SOA particles to achieve good toughness. However, the presence of unreacted sulfur with a too high sulfur content can mitigate the toughening effect. In particular, a highly toughened blend with optimized SOA composition (PLA-S50O25A25) was obtained with excellent toughness (elongation at break 235.7% and tensile toughness 76.5 MJ/m3) balanced by only slightly impaired strength (tensile strength 50–60 MPa and modulus ∼ 900 Mpa). Moreover, the PLA-SOA exhibits excellent broad-spectrum anti-bacterial effects. This study reported the unprecedented use of fully sustainable dynamically-crosslinked elastomers as tougheners of PLA, paving new ways for PLA modification with balanced stiffness-toughness and antibacterial activity.

Sulfur-Polymer Nanoparticles: Preparation and Antibacterial Activity.

High sulfur content polymers prepared by inverse vulcanization have many reported potential applications, including as novel antimicrobial materials. High sulfur content polymers usually have limited water-solubility and dispersibility due to their hydrophobic nature, which could limit the development of their applications. Herein, we report the formulation of high sulfur content polymeric nanoparticles by a nanoprecipitation and emulsion-based method. High sulfur content polymeric nanoparticles were found to have an inhibitory effect against important bacterial pathogens, including Gram-positive methicillin-resistant Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa. Salt-stable particles were formulated with the addition of a surfactant, which did not inhibit the antibacterial activity of the polymeric particles. Furthermore, the polymeric nanoparticles were found to inhibit S. aureus biofilm formation and exhibited low cytotoxicity against mammalian liver cells. Interaction of the polymeric particles with cellular thiols could be a potential mechanism of action against bacterial cells, as demonstrated by reaction with cysteine as a model thiol. The findings presented demonstrate methods of preparing aqueous dispersions of high sulfur content polymeric nanoparticles that could have useful biological applications.