Recent Developments in Synthesis, Properties, Applications and Recycling of Bio-Based Elastomers

In 2021, global plastics production was 390.7 Mt; in 2022, it was 400.3 Mt, showing an increase of 2.4%, and this rising tendency will increase yearly. Of this data, less than 2% correspond to bio-based plastics. Currently, polymers, including elastomers, are non-recyclable and come from non-renewable sources. Additionally, most elastomers are thermosets, making them complex to recycle and reuse. It takes hundreds to thousands of years to decompose or biodegrade, contributing to plastic waste accumulation, nano and microplastic formation, and environmental pollution. Due to this, the synthesis of elastomers from natural and renewable resources has attracted the attention of researchers and industries. In this review paper, new methods and strategies are proposed for the preparation of bio-based elastomers. The main goals are the advances and improvements in the synthesis, properties, and applications of bio-based elastomers from natural and industrial rubbers, polyurethanes, polyesters, and polyethers, and an approach to their circular economy and sustainability. Olefin metathesis is proposed as a novel and sustainable method for the synthesis of bio-based elastomers, which allows for the depolymerization or degradation of rubbers with the use of essential oils, terpenes, fatty acids, and fatty alcohols from natural resources such as chain transfer agents (CTA) or donors of the terminal groups in the main chain, which allow for control of the molecular weights and functional groups, obtaining new compounds, oligomers, and bio-based elastomers with an added value for the application of new polymers and materials. This tendency contributes to the development of bio-based elastomers that can reduce carbon emissions, avoid cross-contamination from fossil fuels, and obtain a greener material with biodegradable and/or compostable behavior.

Understanding the Effects of Cross-Linking Density on the Self-Healing Performance of Epoxidized Natural Rubber and Natural Rubber

The demand for self-healing elastomers is increasing due to the potential opportunities such materials offer in reducing down-time and cost through extended product lifetimes and reduction of waste. However, further understanding of self-healing mechanisms and processes is required in order to develop a wider range of commercially applicable materials with self-healing properties. Epoxidized natural rubber (ENR) is a derivative of polyisoprene. ENR25 and ENR50 are commercially available materials with 25 and 50 mol % epoxidation, respectively. Recently, reports of the use of ENR in self-healing materials have begun to emerge. However, to date, there has been limited analysis of the self-healing mechanism at the molecular level. The aim of this work is to gain understanding of the relevant self-healing mechanisms through systematic characterization and analysis of the effect of cross-linking on the self-healing performance of ENR and natural rubber (NR). In our study, cross-linking of ENR and NR with dicumyl peroxide and sulfur to provide realistic models of commercial rubber formulations is described, and a cross-linking density of 5 × 10^-5 mol cm^-3 in sulfur-cured ENR is demonstrated to achieve a healing efficiency of 143% for the tensile strength. This work provides the foundation for further modification of ENR, with the goal of understanding and controlling ENR's self-healing ability for future applications.

Preparation and Characterization of Diene Rubbers/Silica Composites via Reactions of Hydroxyl Groups and Blocked Polyisocyanates

To improve the curing reaction rate and efficiency of sulfur-cured diene-based rubbers, the introduction of some chemical compounds as activators and accelerants is inevitably required, causing potential harm to humans and ecological systems. Moreover, silica is usually employed as a green filling material for rubber reinforcement, and a silane coupling agent is always required to improve its dispersion. Herein, we reported an effective method to cure hydroxyl-functionalized rubbers/silica composites with blocked polyisocyanates, avoiding the use of any other additives. The enhanced dispersion of silica by interaction with hydroxyl groups on molecular chains endowed the composites with high-mechanical performance. The mechanical properties and crosslinking kinetics of the resultant silica composites can be regulated by adjusting the content of hydroxyl groups in the rubber, as well as the amount of the blocked polyisocyanates. The dynamic heat build-up was related to the distance between crosslinking points. A SBROH/B-TDI/silica composite prepared with blocked toluene diisocyanatem (TDI) exhibited comparable tanδ (0.21 at 0 °C and 0.11 at 60 °C) to that of silica composites cured by sulfur with the help of a silane coupling agent (SBR/S/Si69/silica, 0.18 and 0.10), suggesting great applicable potential for new tire rubber compounds.

Off-line analysis in the manganese catalysed epoxidation of ethylene-propylene-diene rubber (EPDM) with hydrogen peroxide

The epoxidation of ethylene-propylene-diene rubber (EPDM) with 5-ethylidene-2-norbornene (ENB) as the diene to epoxidized EPDM (eEPDM) creates additional routes to cross-linking and reactive blending, as well as increasing the polarity and thereby the adhesion to polar materials, e.g., mineral fillers such as silica. The low solubility of apolar, high molecular weight polymers in the polar solvents constrains the catalytic method for epoxidation that can be applied. Here we have applied an in situ prepared catalyst comprising a manganese(ii) salt, sodium picolinate and a ketone to the epoxidation of EPDM rubber with hydrogen peroxide (H₂O₂) as the oxidant in a solvent mixture, that balances the need for polymer and catalyst/oxidant miscibility and solubility. Specifically, a mixture of cyclohexane and cyclohexanone is used, where cyclohexanone functions as a co-solvent as well as the ketone reagent. Reaction progress was monitored off-line through a combination of Raman and ATR-FTIR spectroscopies, which revealed that the reaction profile and the dependence on the composition of the catalyst are similar to those observed with low molar mass alkene substrates, under similar reaction conditions. The combination of spectroscopies offers a reliable method for off-line reaction monitoring of both the extent of the conversion of unsaturation (Raman) and the extent of epoxidation (FTIR) as well as determining side reactions, such as epoxide ring opening and further, aerobic oxidation. The epoxidation of EPDM described, in contrast to currently available methods, uses a non-scarce manganese catalyst and H₂O₂, and avoids side reactions, such as those that can occur with peracids.

Epoxidation of ethylene-propylene-diene rubber (EPDM), based on 5-ethylidene-2-norbornene, to epoxidized EPDM (eEPDM) opens routes to cross-linking and reactive blending, with increased polarity aiding adhesion to polar materials such as silica.

Effectiveness of Epoxy Coating Modified with Yttrium Oxide Loaded with Imidazole on the Corrosion Protection of Steel

The search for highly effective corrosion protection solutions to avoid degradation of the metallic parts is enabling the development of polymeric organic coatings. Of particular relevance, polymeric nanocomposite coatings, modified with corrosion inhibitors, have been developed to provide enhanced surface protection. In this work, yttrium oxide nanoparticles loaded with corrosion inhibitor (Imidazole), used as additives in the formulation of epoxy for coated on the steel substrate. The loading of Y₂O₃ with imidazole was confirmed by field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and Brunauer–Emmett–Teller analysis. UV-Vis analysis demonstrated the pH-sensitive behavior of the imidazole that helps in self-release when necessary. Electrochemical impedance spectroscopy (EIS) of the coated samples revealed that the coating modified with Y₂O₃/IMD provides better corrosion protection compared to coatings containing only Y₂O₃. XPS analysis validated the presence of an imidazole protective film on the steel substrate that enhanced the corrosion resistance of the coated samples

Design of next-generation cross-linking structure for elastomers toward green process and a real recycling loop

Currently adopted cross-linking methods in rubber industry are suffering from variable persistent issues, including the utilization of toxic curing packages, release of volatile organic compounds (VOCs) and difficulties in the recycling of end-of-life materials. It is of great importance to explore a green cross-linking strategy in the area. Herein, we report a new "green" strategy based on hydrolyzable ester cross-links for cross-linking diene-typed elastomers. As a proof of concept, a commercial carboxylated nitrile rubber (XNBR) is efficiently cross-linked by a bio-based agent, epoxidized soybean oil (ESO), without any toxic additives. ESO exhibits an excellent plasticization effect and excellent scorch safety for XNBR. The cross-linking density and mechanical properties of the ESO-cured XNBR can be manipulated in a wide range by changing simply varying the content of ESO. In addition, zinc oxide (ZnO) performs as a catalyst to accelerate the epoxide opening reaction and improve the cross-linking efficiency, serving as reinforcement points to enhance the overall mechanical properties of the ESO-cured XNBR. Furthermore, the end-of-life elastomer materials demonstrate a closed-loop recovery by selectively cleaving the ester bonds, resulting in very high recovery of the mechanical performance of the recycled composites. This strategy provides an unprecedented green avenue to cross-link diene elastomers and a cost-effective approach to further recycle the obtained cross-linked elastomers at high efficiency.

Stress relaxation and thermally adaptable properties in vitrimer-like elastomers from HXNBR rubber with covalent bonds

Widening the scope of skeletons in the chemistry of vitrimer(-like) high molecular weight rubbers, the present study highlights the preparation of vitrimer-like elastomers based on a technically relevant rubber that is characterised by high thermal and oxidation stability. In particular, we prepared covalently crosslinked hydrogenated carboxylated nitrile butadiene rubber (HXNBR) networks that can rearrange their topology due to the exchangeable nature of the crosslinks. By crosslinking with a di-functional epoxide, β-hydroxyl ester linkages are incorporated into the rubber, enabling thermo-activated transesterifications in the presence of the catalyst triazabicyclodecene. At moderate temperatures, the covalent linkages ensure good mechanical properties as well as chemical and thermal stability of the rubber, which is essential for most applications. In addition, bond exchange reactions allow for fast and distinctive stress relaxation at elevated temperatures. Due to the enhanced network mobility above the vitrification transition temperature, the materials exhibit thermally adaptable properties. A comparative study throughout all experiments with catalyst-free samples serving as a reference is made. Shape change experiments reveal a certain malleability of the HXNBR elastomers and improved adhesion properties are shown by means of lap shear tests. In the presence of catalyst, the failure mechanism changes from adhesive to cohesive failure proving the weldability of the material. Furthermore, the samples exhibit thermally triggered repair capabilities as demonstrated by stress-rupture tests. In general, it is shown that already low quantities of exchangeable crosslinks of associative nature impart a promising thermal adaptability into high molecular weight HXNBR rubber.

Photothermal-Induced Self-Healable and Reconfigurable Shape Memory Bio-Based Elastomer with Recyclable Ability

Photothermal-induced self-healable and shape memory materials have drawn much attention due to the rapidly growing technical applications and environmental requirements. As epoxy natural rubber (ENR) is a kind of bio-based elastomer with good mechanical properties, weather resistance, and air impermeability, it is of great significance to incorporate ENR with recyclable, photothermal-induced self-healable and shape memory properties. In this study, we report a simple method to cross-link ENR with dodecanedioic acids (DAs) through esterification reaction, and during the cross-linking process, a little aniline trimer (ACAT, a kind of oligoaniline) was added at the same time. Then, the ENR-DA-ACAT vitrimers that were covalently cross-linked with recyclable, self-healable, and multiple responsive properties were obtained, which also possessed various functions. As a result of the transesterification reactions at elevated temperatures, the ENR-based vitrimers possess the ability to be reprocessed and self-healed, and the mechanical properties could be maintained even after three consecutive breaking/mold pressing cycles. Besides, the vitrimer is also responsive to near-infrared (NIR) light and pH with the introduction of ACAT, and we also find that ACAT can be used as a catalyst to accelerate the transesterification reaction. Moreover, it is demonstrated that the ENR-DA-ACAT vitrimer could also be used to construct the reconfigurable shape memory polymer; the shape fixing ratio and shape recovery ratio are both above 95% in the reconfiguration process, and the multistage shape memory performance can also be achieved by NIR irradiation, which will potentially lead to a wide application for ENR in the field of actuators.

Density functional theory study of mechanism of epoxy-carboxylic acid curing reaction

A comprehensive picture on the mechanism of the epoxy-carboxylic acid curing reactions is presented using the density functional theory B3LYP/6-31G(d,p) and simplified physical molecular models to examine all possible reaction pathways. Carboxylic acid can act as its own promoter by using the OH group of an additional acid molecule to stabilize the transition states, and thus lower the rate-limiting barriers by 45 kJ/mol. For comparison, in the uncatalyzed reaction, an epoxy ring is opened by a phenol with an apparent barrier of about 107 kJ/mol. In catalyzed reaction, catalysts facilitate the epoxy ring opening prior to curing that lowers the apparent barriers by 35 kJ/mol. However, this can be competed in highly basic catalysts such as amine-based catalysts, where catalysts can enhance the nucleophilicity of the acid by forming hydrogen-bonded complex with it. Our theoretical results predict the activation energy in the range of 71 to 94 kJ/mol, which agrees well with the reported experimental range for catalyzed reactions. © 2017 Wiley Periodicals, Inc.

Dual-Triggered and Thermally Reconfigurable Shape Memory Graphene-Vitrimer Composites

Conventional thermoset shape memory polymers can maintain a stable permanent shape, but the intrinsically chemical cross-linking leads to shape that cannot be altered. In this paper, we prepared shape memory graphene-vitrimer composites whose shape can be randomly changed via dynamic covalent transesterification reaction. Consecutive shape memory cycles indicate stable shape memory with undetected strain shift and constant shape fixity and recovery values (Rf > 99%, Rr > 98%). Quantitative characterization of shape reconfiguration by dynamic mechanical thermal analysis (DMA) shows prime reconfigurable behavior with shape retention ratio of 100%. Thus, the arbitrary 2D or 3D newly permanent shape can be easily obtained from a simple plain sample by facile thermal treatment at 200 °C above transesterification temperature (Tv). Besides, it is found that graphene-vitrimers show a ductile fracture in tensile test with a large breaking strain and classical yield phenomenon because of the well-dispersed graphene sheets in the vitrimer that endow effective stress transfer. As the graphene loading increases from 0% to 1%, the yield strength and breaking stain increase from 12.0 MPa and 6% to 22.9 MPa and 44%, respectively. In addition, graphene also serves as energy convertor to convert near-infrared (NIR) irradiation into thermal energy to induce a helix shape sample that is recovered totally within 80 s sequent NIR irradiation. These dual-triggered and reconfigurable shape memory graphene-vitrimers are expected to significantly simplify processing of complex shape and broaden the applications of shape memory polymers.

Chemically crosslinked yet reprocessable epoxidized natural rubber via thermo-activated disulfide rearrangements

Disulfide crosslinks introduced into an ENR matrix enable the thermo-activated reprocessing of the chemically crosslinked rubber, studied in terms of stress relaxation and adhesion experiments.

Catalytic Control of the Vitrimer Glass Transition

Vitrimers, strong organic glass formers, are covalent networks that are able to change their topology through thermoactivated bond exchange reactions. At high temperatures, vitrimers can flow and behave like viscoelastic liquids. At low temperatures, exchange reactions are very long and vitrimers behave like classical thermosets. The transition from the liquid to the solid is reversible and is, in fact, a glass transition. By changing the content and nature of the catalyst, we can tune the transesterification reaction rate and show that the vitrimer glass transition temperature and the broadness of the transition can be controlled at will in epoxy-based vitrimers. This opens new possibilities in practical applications of thermosets such as healing or convenient processability in a wide temperature range.

Metal-catalyzed transesterification for healing and assembling of thermosets

Catalytic control of bond exchange reactions enables healing of cross-linked polymer materials under a wide range of conditions. The healing capability at high temperatures is demonstrated for epoxy-acid and epoxy-anhydride thermoset networks in the presence of transesterification catalysts. At lower temperatures, the exchange reactions are very sluggish, and the materials have properties of classical epoxy thermosets. Studies of model molecules confirmed that the healing kinetics is controlled by the transesterification reaction rate. The possibility of varying the catalyst concentration brings control and flexibility of welding and assembling of epoxy thermosets that do not exist for thermoplastics.