Smart Rubbers

Synthesis and Applications

Lorenzo Massimo Polgar, Machiel van Essen, Andrea Pucci, Francesco Picchioni

• Publisher: De Gruyter
• Year: 2019
• eBook ISBN: 9783110639315
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemistry
• Index: Chemistry

Research into "smart rubbers", i.e. elastomeric materials that respond to external stimuli, has increased dramatically recently, predominantly due to the growing need for improved materials for new applications. This book aims to provide an overview over the field of smart rubber research. Examples of the various components involved in smart rubbers are highlighted and discussed. Different types of stimuli and numerous applications are explained.

1 Sustainability in the design of rubber materials

Sustainability becomes an ever more important and unavoidable topic when designing materials or chemical products. This is also the reason for the vast increase in research output on this topic and the number of scientific articles having ‘sustainability’ as a major keyword (Figure 1.1). Sustainable materials can be defined broadly as materials that can be produced and reused indefinitely without affecting the human–ecosystem equilibrium. Three major issues should be taken into consideration for the development of a sustainable material: the production process, carbon footprint and purpose of the material at the end of its product life.

Figure 1.1: The exponential increase in scientific article that focus on sustainability over the last decades as generated from the ‘web of knowledge’.

First, the process to develop and produce the material should be sustainable. Hence, an energy-neutral process is required and all solvents and utility streams must ultimately be recycled without the production of any waste. A good example is the recently developed process to extrude rubbers using supercritical carbon dioxide (scCO2) [1]. Besides being a good and green solvent, scCO2 improves the dispersion of materials such as fillers or other polymers in the rubber matrix [1, 2]. After the process, the harmless solvent is easily fed back into the environment by releasing the pressure. The focus of this book is materials (smart rubbers) so, rather than smart processing, the carbon footprint of a material and its recyclability will be discussed in more detail because they are more related to the material itself. As both components are important, they cannot always be distinguished (Figure 1.2).

Figure 1.2: The environmental axis. The bio-based character and recyclability of materials as individual components of sustainability. Reproduced with permission from Dr. Martijn Beljaars.

Currently, most rubber products consist of elastomers that originate from the lower-left quadrant. These elastomers are usually oil-based and non-recyclable due to the presence of irreversible crosslinks. A significant amount of the rubber products that we use (≈40% of all rubber used [3, 4]) still contain a lot of natural rubber (NR), which is a natural polymeric compound produced from the latex of Hevea brasiliensis (which mainly contains cis-polyisoprene). This amount is ever decreasing as NR is being replaced by synthetic rubbers that have been developed to meet the highly demanding requirements for specific applications. A good example of such a specific application that requires very specific material is sealants. The neoprene group of synthetic rubbers that is generally used for such applications is very stable with respect to NR and is, therefore, used for multiple applications such as wetsuits, laptop sleeves and durable medical devices. The low electrical conductivity of these neoprene rubbers also makes them useful as an insulator in electrical wiring. NR is a poor candidate for such applications because the high number of unsaturated carbon–carbon double bonds makes the elastomers too reactive with other chemicals to be widely used for such applications. Recent developments using biomass as a feedstock for base chemicals have led to the replacement of some synthetic rubbers by bio-based alternatives. Two examples of such bio-based synthetic rubbers are discussed in Section 1.1. Even when using NR or bio-based elastomers in a rubber product, the final products are not completely bio-based due to large amounts of additives such as (reinforcing) fillers or lubricants. Some more ‘green’ and sustainable alternatives for these oil-based fillers are also discussed.

Nevertheless, even if the process and materials used to produce rubbers are completely ‘green’, the materials are spent after some time, resulting in the accumulation of rubber waste. The recyclability of the materials should, therefore, also be considered when designing sustainable rubbers. Unfortunately, the chemical reactions typically used to crosslink elastomers are irreversible and, thus, prohibit reuse of rubber scrap and waste as raw materials. Thermoplastics can be recycled via melt processing, but the three-dimensional (3D) network of crosslinked rubbers prevents melt (re)processing. This problem is particularly evident for rubber tyres of which, due to the inability of recycling them, millions are discarded annually [5]. The bulk of these are dumped in landfills, placing a burden on the environment as well as posing potential health hazards because these become a breeding ground for disease-carrying mosquitoes.

Current trends towards sustainability and the development of products in a cradle-to-cradle fashion (a biomimetic approach to the design of products and systems that models human industry on nature’s processes viewing materials as nutrients circulating in healthy, safe metabolisms) make the recyclability of crosslinked elastomers increasingly relevant [6]. In the last decades, considerable efforts have been devoted to the devulcanisation of various crosslinked rubbers [7, 8, 9, 10, 11, 12]. For some iso-based rubbers, such as NR and butyl rubber (BR), reclaiming processes have been commercially practiced for decades. Also, reclaiming sulfur- vulcanised rubbers using devulcanising agents and high-shear/ temperature processes is now a common technology [13, 14, 15]. It appears to be more difficult to apply this technology to hydrocarbon elastomers with saturated main chains [16, 17, 18], probably because of the higher stability of sulfur crosslinks in an environment with low unsaturation. Nevertheless, some workable reclaiming processes have been developed for these materials [7, 17]. These processes combine thermal and mechanical treatment for selective cleavage of the sulfur crosslinks. Unfortunately, they also cause a considerable amount of scission of the main polymer chain, which is detrimental for the performance of the recyclate [7]. Hence, the amount of devulcanised material that can be reused in new products is limited to ≈25% [17].

A technical solution to this problem is found in the commercially available thermoplastic elastomers (TPE). TPE comprise block copolymers of ‘hard’ and ‘soft’ segments, and thermoplastic vulcanisates (TPV) that consist of blends of ‘hard’ and ‘soft’ polymers [19, 20, 21]. TPE and TPV show advantages typical of rubbery materials and plastic materials, such as easy processing and manufacture.

Unfortunately, these materials have a relatively low stability at high temperatures that limits ...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contents
• 1 Sustainability in the design of rubber materials
• 2 Environment-sensing rubbers
• 3 Optically active elastomers
• 4 Shape-memory elastomers
• 5 Magnetorheological elastomers
• 6 Dielectric elastomers
• 7 Future outlook
• Abbreviations
• Index