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Issue 06/2017

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  • Bioplastics
  • Biobased
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Materials Biobased

Materials Biobased adhesives: Requirements and perspective By: Horst Beck and Andreas Taden Henkel - Adhesive Research / Bio-Renewables Platform Düsseldorf, Germany Biobased adhesives literally constitute an ancient material class. Already 200.000 years ago Neanderthals identified birch pitch as valuable adhesive material, which can be obtained from the bark via pyrolysis under the absence of oxygen – a non-obvious and quite sophisticated technological process [1]. Being liquid at higher temperatures it solidifies under ambient conditions and was eventually used throughout the ages (stone and metal ages) as hotmelt glue to fasten arrowheads or metal tools onto wooden shafts (Figure 1). In the more recent Sumerian and Egyptian history animal-based proteins – especially animal skin, blood (Albumin), fish glues from air bladders and casein from milk – and starch-based binders appeared as the first truly industrial adhesives produced on larger scale. However, during the 20 th century biobased adhesives lost their predominant importance, which is closely related to the scientific progress in synthetic polymer chemistry and the development of phenolic resins, epoxides, acrylics, polyurethanes, silicones, etc. Numerous fossil-based high performance adhesives, typically designed and optimized for one specific application with an individual set of requirements, eventually replaced most biobased systems. Apart from pure performance considerations the steadier and hence more calculable raw material quality and cost of supply of synthetic fossil-based compounds did foster this development over the last decades. On the contrary, recent progress in biotechnology enables the green production of bio-renewable platform chemicals and specific design of functional proteins & peptides, which is expected to significantly impact adhesive development and create numerous new possibilities and applications. In this context this contribution aims to discuss the requirements and perspective of biobased adhesives in our modern world. Hurdles and drivers Recently biobased adhesives regained a lot of attention. As most obvious driver increased sustainability might appear, i.e. the content of renewable carbon. However, this one-dimensional perspective has too many shortcomings and cannot substitute a comprehensive life cycle analysis survey. Renewable carbon content is not interchangeable with reduced carbon dioxide footprint, and neither are biobased components necessarily biodegradable materials or less dangerous in terms of safety & health. With respect to the above mentioned advantages of synthetic adhesives, the regained focus on biobased adhesives can only be put into perspective and justified considering a larger context of requirements in our modern world. Furthermore the current regulations and trends in chemistry, like costintensive registration of new chemicals (REACH, TSCA, etc.) or the anticipated long-term price stability of crude oil, are opposing to ongoing biobased research efforts. Additional issues are insecure availability and potential food to fuel dilemma of biorenewables. Initially the drop-in approach, which simply involves the one-to-one replacement of petro-based molecules with otherwise chemically identical biobased substances, was seen as fast track methodology towards a more sustainable value chain. Unfortunately significant market shares were never achieved, mainly due to higher cost levels [2]. Two further aspects have to be considered in this context: 1.) In order to make a meaningful market claim, the composition of the complete formulation should be close to 100 % biobased content. 2.) The customer awareness for biobased adhesives is relatively low, especially when the adhesive remains a rather invisible part of the finished product, like in cars, handheld devices, etc. An additional hurdle for biobased adhesives is the relatively small market size and the different technological requirements compared to plastics, which leads many bio-related companies to focus on high molecular weight polymers as thermoplastic materials for construction, transportation or packaging. In contrast to that adhesive formulations are typically based on low molecular weight (reactive) precursors which are liquid or enable a low melt viscosity as starting point for the consecutive setting process (curing reaction). As a consequence the availability of biobased raw materials which are suited or even especially designed for adhesives so far remains limited. Performance as key contribution In some niche areas bio-polymer adhesives escaped their replacement by petro-based materials because of a very good fit to the requirements, selected examples include starches as adhesive in the manufacturing of corrugated paper boards, cellulose- and starch ethers for wallpaper glues or rheology modifiers in cement-based formulations and casein as bottle-labelling adhesive with fast setting behavior even on wet and cold bottles. It seems that within this complex framework of requirements and constraints (see “hurdles and drivers”) the terms for the development 30 bioplastics MAGAZINE [06/17] Vol. 12

Materials Figure 1: Arrowhead mounted onto a wooden shaft supported by birch pitch as “Neanderthal hotmelt adhesive”. This biobased glue was used throughout the ages for ca. 200.000 years. The picture shows is a replica made by Henkel Adhesive Research of new biobased adhesives are surprisingly clear – simple drop-in alternatives cannot prevail, and the key to success can only be significantly increased performance benefits originating from novel low to medium molecular weight species. Unfortunately, as stressed out above and closing the loop to the introduction, most biobased adhesives became substituted due to a lack of performance compared to synthetic polymer systems. However, the amazing ability of Mother Nature to undergo strong and sometimes highly specific bonding under ambient conditions was not recognized and valued adequately, mainly due to the absence of modern analytical capabilities. Furthermore, industrial or so-called white biotechnology was basically unknown for the synthesis of biobased platform chemicals. Today more attention is paid to safety, health and environment (SHE), e.g. reduction or elimination of undesired volatile organic compounds (VOC) like organic solvents or residual monomers, and adhesive performance becomes evaluated in a more comprehensive manner, including sustainability and life circle analysis supplementing the well-established purely application dependent technical specifications. In order to achieve new performance levels in the various dimensions interdisciplinary thinking is required, which lead to the employment of biotechnological synthesis methods for novel raw materials, the development of hybrid systems and biomimetic binders. As will be explained the unique synthesis and interaction capabilities found in nature enable new-to-the-word systems with unprecedented property combinations. In the following selected examples will be briefly introduced. Novel platform chemicals via biotechnology Biotechnology is known to humankind for thousands of years, but only in the late 20 th and early 21 st centuries it developed in a thriving discipline with previously unmatched synthetic possibilities supported by genomics and recombinant gene design. White biotechnology works by engineering living cells into micro-factories that — by using sugars, starches or even lignocellulosic-based biomass as a feedstock rather than traditional petrochemicals — produce valuable products via fermentation that can function as stand-alone products (e.g. enzymes, fuels) or serve as platform chemicals for further downstream processing. In 2004 the DOE (US Department of Energy) [3] published an overview about so-called platform chemicals based on the vision of an expert panel. This vision identified already a detailed view how those platform chemicals could be transferred via a complete value chain to end (consumer) products. Recently the European Commission published a report which provides an assessment of the technology development status and market size for the most important platform chemicals, which consists mainly – but not exclusively – out of hydroxyl- or carboxylicfunctionalized molecules [4]. Typical examples are bioderived 1,4-butanediol (BDO), succinic acid, adipic acid or 2,5-Furandicarboxylic acid (FDCA). The progress in this new area of biotechnological derived raw materials is very dynamic and especially the development and upscaling of further downstream derivatives is an ongoing process. Consequently certain biobased platform chemicals with high potential for adhesive applications and/or polymer chemistry in general are not yet available on a commercial scale. Furthermore the value proposition for each of these components can be quite different, ranging from predominately cost-driven considerations (e.g. for BDO or succinic adid, which are at least cost-competitive compared to their petrol-based analogues) to unique chemical and/ or physical characteristics. Following the scope of this contribution, trans-β-farnesene belongs to the latter category and is particularly interesting for adhesive applications [5]. It´s a branched chain alkene which shall be exemplary discussed as modem biotechnological platform chemical with no identical fossil based substitute and hence new-to-the-world performance characteristics. Farnesene can be used as fragrance, cosmetic emollient or fuel, and with respect to polymers and adhesives it´s particularly valuable due to its similar reactivity compared to (gaseous) butadiene, which constitutes the main raw material for synthetic rubber. However, due to its higher molecular weight Farnesene is a liquid monomer, which substantially simplifies the rubber polymerization process and the related reactor design. Farnesene can be polymerized via free racial, cationic or anionic pathways — the latter process enables highly defined bottle-brush Poly(trans-β-farnesene) polyols. This particular backbone structure provides low tendency for entanglements and hence drastically altered viscoelastic properties, i.e. greatly reduced viscosity compared to polybutadiene systems of similar molecular weight [7]. Polyfarnesene bioplastics MAGAZINE [06/17] Vol. 12 31

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