Bio‐based β‐myrcene‐modified solution‐polymerized styrene–butadiene rubber for improving carbon black dispersion and wet skid resistance

Sustainable Elastomers for Actuators: "Green" Synthetic Approaches and Material Properties

Elastomeric materials have great application potential in actuator design and soft robot development. The most common elastomers used for these purposes are polyurethanes, silicones, and acrylic elastomers due to their outstanding physical, mechanical, and electrical properties. Currently, these types of polymers are produced by traditional synthetic methods, which may be harmful to the environment and hazardous to human health. The development of new synthetic routes using green chemistry principles is an important step to reduce the ecological footprint and create more sustainable biocompatible materials. Another promising trend is the synthesis of other types of elastomers from renewable bioresources, such as terpenes, lignin, chitin, various bio-oils, etc. The aim of this review is to address existing approaches to the synthesis of elastomers using "green" chemistry methods, compare the properties of sustainable elastomers with the properties of materials produced by traditional methods, and analyze the feasibility of said sustainable elastomers for the development of actuators. Finally, the advantages and challenges of existing "green" methods of elastomer synthesis will be summarized, along with an estimation of future development prospects.

Synthesis and Vulcanization of Polymyrcene and Polyfarnesene Bio-Based Rubbers: Influence of the Chemical Structure over the Vulcanization Process and Mechanical Properties

The overuse of fossil-based resources to produce thermoplastic materials and rubbers is dramatically affecting the environment, reflected in its clearest way as global warming. As a way of reducing this, multiple efforts are being undertaken including the use of more sustainable alternatives, for instance, those of natural origin as the main feedstock alternative, therefore having a lower carbon footprint. Contributing to this goal, the synthesis of bio-based rubbers based on β-myrcene and trans-β-farnesene was addressed in this work. Polymyrcene (PM) and polyfarnesene (PF) were synthesized via coordination polymerization using a neodymium-based catalytic system, and their properties were compared to the conventional polybutadiene (PB) and polyisoprene (PI) also obtained via coordination polymerization. Moreover, different average molecular weights were also tested to elucidate the influence over the materials' properties. The crosslinking of the rubbers was carried out via conventional and efficient vulcanization routes, comparing the final properties of the crosslinking network of bio-based PM and PF with the conventional fossil-based PB and PI. Though the mechanical properties of the crosslinked rubbers improved as a function of molecular weight, the chemical structure of PM and PF (with 2 and 3 unsaturated double bonds, respectively) produced a crosslinking network with lower mechanical properties than those obtained by PB and PI (with 1 unsaturated double bond). The current work contributes to the understanding of improvements (in terms of crosslinking parameters) that are required to produce competitive rubber with good sustainability/performance balance.

Preparation and application of carbon black-filled rubber composite modified with a multi-functional silane coupling agent

Carbon black (CB)-filled rubber composites are widely used for tire tread and other rubber products due to their good abrasion resistance and reinforcement. Numerous studies have been focused on the filler-rubber interaction with the aim of obtaining optimum performances. In order to investigate the effect of modification on properties of diene rubber composites, a modified CB-filled isoprene rubber (IR) composite was prepared with a multi-functional silane coupling agent, 2-aminoethyl-2-(3-triethoxysilylpropyl)aminoethyl disulfide (ATD). This modification significantly enhanced the CB-IR interaction and improved CB dispersion. For the modified CB/IR vulcanizates, the tensile strength, stress at 300% elongation, temperature rise, and dynamic loss coefficient were significantly improved at low ATD dosage (1.5 phr). Such modification provides an effective route to prepare CB-reinforced diene rubber composites with low hysteresis and improved mechanical properties.

INTRODUCING BIOBASED NONPOLAR BOTTLEBRUSH β-MYRCENE SEGMENTS TO IMPROVE SILICA DISPERSION FOR SUSTAINABLE SSBR/SILICA NANOCOMPOSITES

To overcome the problem of fossil fuel depletion and associated environmental issues arising from the use of tire tread elastomers, a convenient, environmentally friendly, and highly efficient strategy was developed to prepare high-performance green solution polymerized styrene–butadiene rubber (SSBR)/silica nanocomposites by improving silica dispersion in the nonpolar polymer matrix via the introduction of a biobased nonpolar bottlebrush segment with two double bonds. Various elastomers containing biobased nonpolar bottlebrush β-myrcene segments were synthesized using an industrially robust anionic polymerization method. Results of rubber process analysis, small-angle X-ray scattering, scanning electron microscopy, and transmission electron microscopy revealed that rubber with myrcene could significantly improve silica dispersibility and inhibit the strong filler–filler interactions, which are due to the formation of hydrogen bonding between the double bonds in the myrcene block and silanol groups on the silica surface and possibly to the spreading or infiltrating of myrcene bottlebrush segments onto silica. Furthermore, for the modified rubber, rolling resistance decreased by 41.7%, tear strength increased by 20.78%, and tensile strength increased by 77.8% with the elongation at break remained practically unchanged as compared with the unmodified silica/SSBR composite. On the basis of aforementioned assessment, we believe that silica-reinforced β-myrcene–based styrene–butadiene integrated rubber is a versatile and promising candidate for future tire tread elastomers.

Sustainable Myrcene-Based Elastomers via a Convenient Anionic Polymerization

Soluble heterocomplexes consisting of sodium hydride in combination with trialkylaluminum derivatives have been used as anionic initiating systems at 100 °C in toluene for convenient homo-, co- and ter-polymerization of myrcene with styrene and isoprene. In this way it has been possible to obtain elastomeric materials in a wide range of compositions with interesting thermal profiles and different polymeric architectures by simply modulating the alimentation feed and the (monomers)/(initiator systems) ratio. Especially, a complete study of the myrcene-styrene copolymers (PMS) was carried out, highlighting their tapered microstructures with high molecular weights (up to 159.8 KDa) and a single glass transition temperature. For PMS copolymer reactivity ratios, rmyr = 0.12 ± 0.003 and rsty = 3.18 ± 0.65 and rmyr = 0.10 ± 0.004 and rsty = 3.32 ± 0.68 were determined according to the Kelen-Tudos (KT) and extended Kelen-Tudos (exKT) methods, respectively. Finally, this study showed an easy accessible approach for the production of various elastomers by anionic copolymerization of renewable terpenes, such as myrcene, with commodities.

Temperature Variation Enables the Design of Biobased Block Copolymers via One-Step Anionic Copolymerization

A one-pot approach for the preparation of diblock copolymers consisting of polystyrene and polymyrcene blocks is described via a temperature-induced block copolymer (BCP) formation strategy. A monomer mixture of styrene and myrcene is employed. The unreactive nature of myrcene in a polar solvent (tetrahydrofuran) at -78 °C enables the sole formation of active polystyrene macroinitiators, while an increase of the temperature (-38 °C to room temperature) leads to poly(styrene-block-myrcene) formation due to polymerization of myrcene. Well-defined BCPs featuring molar masses in the range of 44-117.2 kg mol^-1 with dispersities, Ð, of 1.09-1.21, and polymyrcene volume fractions of 30-64% are accessible. Matrix assisted laser desorption ionization-time of flight mass spectrometry measurements reveal the temperature-controlled polymyrcene block formation, while both transmission electron microscopy and small-angle X-ray scattering measurements prove the presence of clearly microphase-separated, long range-ordered domains in the block copolymers. The temperature-controlled one-pot anionic block copolymerization approach may be general for other terpene-diene monomers.

Maleic Anhydride Modified Dicyclopentadiene Resin for Improving Wet Skid Resistance of Silica Filled SSBR/BR Composites

As commercial rubber in tires, silica-filled solution-polymerized styrene-butadiene rubber/butadiene rubber (SSBR/BR) compounds exhibited preferable wet skid resistance (WSR) properties, which could be further enhanced by the incorporation of some oligomeric resins. However, the untreated dicyclopentadiene (DCPD) resin shows a slight improvement in wet friction even if the good compatibility between DCPD and SBR owing to their common cyclic structures. For this problem to be addressed, we aimed to enhance its resin-silica interaction by reaction with maleic anhydride (MAH). In detail, the effect of MAH content on WSR, curing characteristics, physical-mechanical properties of the silica-filled SSBR/BR composites was investigated. When the MAH content is 4 wt% in the modified DCPD resin, the maximum enhancement of about 15% in tan δ values at 0 °C, as well as that of 17% in British pendulum skidding tester (BPST) index is obtained, indicating a desirable improvement in WSR. In addition of these two commonly used methods, water contact angles of the vulcanizates increase gradually with increasing MAH content, further confirming the remarkable performance of modified DCPD resin in WSR.

Terpene Based Elastomers: Synthesis, Properties, and Applications

The limited source of fossil-fuel and the predominance of petroleum-based chemistry in the manufacturing of commodity polymers has generated tremendous interest in replacing the fossil source-based polymers with renewable counterparts. The field of sustainable elastomers has grown in the past three decades, from a few examples to a plethora of reports in modern polymer science and technology. Applications of elastomers are huge and vital for everyday living. The present review aims to portray a birds-eye view of various sustainable elastomers obtained from the wide family of acyclic terpenes (renewable feedstocks from different plant oils) via various polymerization techniques and their properties, as well as plausible developments in the future applications of sustainable polymers. Not only the homopolymers, but also their copolymers with both green and commercial fossil based comonomers, are reviewed.

Tales of the unexpected. The non-random statistical copolymerisation of myrcene and styrene in the presence of a polar modifier

Terpene monomers such as myrcene are considered sustainable replacements for oil-based dienes in the production of sSBR. We present unexpected data for the anionic copolymerisation of myrcene and styrene in the presence of TMEDA, a polar modifier.