Thermal Properties of Bio-based Polymers

Industrial production of thermoplastics is mostly based on the use of fossil sources. Due to serious environmental concerns, in the past years a number of environmental friendly polymers, produced from short-term renewable resources and often addressed as bio-based polymers or biopolymers, have been developed. Their expected large-scale commercialization will preserve mineral resources for future generations, which also contributes to an increased worldwide interest in this class of materials. Polymers synthesized from short-term renewable resources exhibit a variety of properties, which make them amenable to production of different types of final products, such as films, sheets, molded articles, and fibers. Their commercial development is linked to the final application, which in turn is strongly affected by the property profile. In order to tailor specific properties, the relationship between chemical architecture, processing behavior, physical structure, and the resulting property profile needs to be known. The key to proper design of industrial processing is a thorough knowledge of polymer melting, crystallization, and vitrification. For this reason, in this volume focus is given to these main thermal properties of bio-based polymers. Bio-based polymers formally include also long known/traditional polymers that are typically made from fossil resources, such as bio-polyethylene, bio-polypropylene, or bio-poly(ethylene terephthalate). The latter bio-based "classical" polymers have the same properties as the petrochemical-based analogues, and are therefore not treated in this volume. Three main types of bio-based polymers have been identified: (1) natural polymers directly derived from biomass, such as starches, chitin, chitosan, cellulose and its derivatives, or natural rubber; (2) bio-engineered polymers, synthesized by microorganisms and plants like poly(hydroxyalkanoates); and (3) polymers synthesized from monomers obtained from short-term renewable resources, like poly (L-lactic acid) or poly(butylene succinate). Their thermal properties are summarized in the introductory chapter, where an overview of the commercially available biobased polymers is presented, together with the main features of each class of material. The next two chapters are devoted to quiescent and flow-induced crystallization of poly(L-lactic acid) (PLLA), which is the bio-based polymer that has received the largest attention in recent years. PLLA is not only bio-based, but it is also biodegradable, compostable, and biocompatible. Despite its ability to degrade after disposal, PLLA is extremely robust when used for applications like food packaging, parts in electronic industry, automotive, or in the biomedical sector, with global suppliers now able to produce several kilotons per year. Crystallization of poly[(R)-3-hydroxybutyrate] (PHB) is discussed in the next chapter. PHB is the first produced and most studied poly(hydroxyalkanoate). It represents a class of polymers that is synthesized by a variety of bacteria through fermentation, which leads to a number of special features. These include a perfect isotactic configuration with all chiral carbon atoms in (R)-position, or the absence of catalyst residues and other impurities typically present in the majority of synthetic polymers, often promoting crystallization. These peculiarities make PHB a model compound for the study of polymer crystallization, and therefore its crystallization behavior has received considerable attention over the years. The following chapter focuses on thermal properties of polyamide 11, a semicrystalline high-performance thermoplastic engineering polymer produced from castor oil, with many specific applications in all fields of engineering, including bio-engineering. Its main feature is the balanced property profile in terms of thermal stability, mechanical behavior, and resistance to media, which all led to increasing production volumes in recent years. Last but not least, bio-sourced polymers based on 2,5-furandicarboxylic acid (FDCA) are discussed. These materials have the potential to replace oil-based polyesters, such as PET, in a wide range of applications, including bottles and carpets. Major attention is given to thermal properties of poly(ethylene 2,5- furanoate) (PEF), which is by far the most studied FDCA polyester due to its structural analogy with PET, even foreseen to compete with PET in the near future regarding price and performance, with the added value of sustainability. The volume ends with a chapter on crystallization of bio-based polyester blends. Polymer blending has extensively been used to develop novel bio-based polymer formulations with an attractive combination of properties that combine those of the pure components, with some of them that have already found specific industrial applications. As Editors, we truly thank all the chapter authors for sharing their scientific points of view on the thermal properties of the different bio-based polymers. We would like to express our sincere gratitude also to the many colleagues who actively participated in the review process and invested time and efforts to revise, comment, and improve each contribution. We hope that we have reached the goal to provide an overview of the main thermal transitions (crystallization, melting, and glass transition) of bio-based polymers, as these are essential for their current and future development. We also hope that this volume helps to promote further development of bio-based polymers, with the aim to preserve a sustainable environment for the future generations.