From plant phenols to novel bio-based polymers

Abstract

Replacing petroleum-based products with inexpensive, biorenewable, natural materials is important for sustainable development and will have a significant impact on the polymer industry and the environment. Biorenewable aliphatic and cycloaliphatic compounds are less competitive with counterparts with aromatic structures for advanced polymeric materials in terms of rigidity, hydrophobicity, as well as chemical and thermal stability. Biorenewable plant phenols represent a diverse class of chemicals with a great industrial significance due to their unique structures and high abundance. The depolymerization process of lignin into small bio-based phenols is a relatively new approach and has received a considerable attention recently. This process produces key intermediates, phenolic compounds that can be used to develop and design wide range of high performance biorenewable multifunctional polymers and composites. In this review article, the production of biorenewable phenols from natural sources such as lignin by energy-, catalytic-, enzymatic-assisted depolymerization processes will be summarized. The different chemical modifications and polymerization pathways to obtain bio-based polymers (e.g. vinyl ester resins, cyanate ester, epoxy, benzoxazine resins, etc.) will be discussed. In addition, this review article will conclude with an overview of current and potential future applications of the bio-sourced phenolic-based materials in a wide range of automotive, electrical, and medical applications. Overall, the present review article will provide a quantitative experimental basis for the depolymerization process of lignin to produce biorenewable multifunctional phenolic-based polymers to increase our level of understanding of the behavior of this important class of polymeric materials and other similar bio-based polymers.

Introduction

The continuous fluctuating oil prices led by economic and politic factors and increasing environmental awareness have driven numerous efforts and significant researches to find alternative renewable feedstock for chemicals and polymeric materials, most of which are currently derived from fossil feedstock [1], [2], [3], [4]. Therefore, several renewable resources have already been studied and are industrially available, such as plant oils, starch, cellulose, chitosan [5], and isosorbide [6,7]. Several polymer systems have been developed from these renewable resources.

Except for sustainability, several characters have been endowed to the resulting polymers. For example, the inherent biodegradability and low price of plant oils have become the platform chemicals for polymer materials. Although plant oils are not naturally present in polymeric structures, they are precursors of reactive monomers that can be used to produce various polymeric materials, including polyurethanes, polyesters, polyethers, and polyolefins. Plant oilsare suitable for producing hydrophobic polymers and supplementing other natural hydrophilic biological resources, such as carbohydrates and proteins [8]. Plant oils are also suitable for the synthesis of reactive monomers with structures similar to petroleum-based counterparts [9]. Cellulose is the most abundant polymer on earth. Through appropriate chemical and physical treatments, one-dimensional or two-dimensional fiber materials in the nanometer range can be produced from any natural cellulose source [10]. Nanocellulose-based materials have a low carbon footprint, are sustainable, recyclable and nontoxic. Chitosan has good biodegradability, solubility, biocompatibility and low toxicity to mammalian cells, so it is widely used in the fields of medicine and pharmacy [11], food processing [12,13], biotechnology [14], cosmetics, textile industry, wastewater treatment, and agriculture [15].

However, most of these renewable resources mentioned above are aliphatic and cycloaliphatic, which are less competitive with aromatic compound counterparts suitable for creating advanced polymeric materials with outstanding properties, in terms of rigidity, hydrophobicity, as well as thermal and chemical resistance [16]. Until now, many key aromatic compounds were ultimately derived from petroleum-based products. However, With increasing environmental concerns and rapid depletion of petroleum feedstock, much effort from academia and industry has been dedicated to exploit renewable resources, especially naturally occurring phenolic compounds and other bio-based phenolics, for the development of high value-added chemicals and bio-based polymers [17].

Phenolic compounds are a diverse class of chemicals that contain one or more hydroxyl groups on the benzene skeleton and have already showed great industrial significance in field of materials, medical and food ingredients. Usually, they are used as the building blocks for production of versatile elastomers, fibers, coatings, nutraceuticals, and pharmaceuticals. Plant phenolics, such as eugenol, ferulic acid (FA), vanillin, cardanol, etc., are now considered as the most important aromatic renewable resource which have great potential to be excellent alternative feedstocks for the development of new chemicals and polymers. Interestingly, the simplest source of plant phenolics is through direct extraction and isolation from natural resources or transformation of the waste from agro-based industries (e.g. wood, nut shell, fiber, fruit and vegetable peel, and bagasse), which recently was found to be rich in phenolic derivatives but mainly utilized either as a low cost fuel or natural fertilizers in earlier time [18]. Nowadays, these bio-based phenolic derivatives can be used as promising alternatives for many petroleum-based aromatic compounds. On the other hand, lignin as a bio-based substitute of aromatic compounds is drawing an enormous interest today. By applying energy, catalytic, and enzymatic modification methods, lignin can be depolymerized into small compounds, such as phenolics, aldehydes and aliphatic compounds. Those low molecular weight reactive compounds could be further converted into phenolics by using enzymatic, microorganism and chemical synthesis.

Plant phenolics represent an important and long­standing raw materials for different industrial products (see Fig. 1). They have many functionalities which can offer a wide variety of structural modifications to design and produce new bio-based chemicals and renewable materials. Generally, the functionalities of these plant phenolics vary from phenolic hydroxyl groups, aryl groups, and side chains including saturated or unsaturated bonds, carbonyl groups, carboxylic groups, etc., which could be further chemically modified into high-valued chemicals using different kinds of coupling methods. These chemicals may be then used as monomers, oligomers or intermediates which could be further explored for creating a wide range of different polymers. From these bio-based phenolics and phenolic-derivatives, polymers with specific properties have been developed. These polymers included vinyl-based resins, benzoxazines [19], polyimide [20], epoxy [21], polyesters [22], polyurethane [23], and phenolic resins [24]. In addition, different nano-fillers and fibers have been also introduced into these polymer matrices for creating advanced polymer composites [25], [26], [27]. The obtained materials could be used in many applications (Fig. 1), such as antibacterial, adhesives, flame retardants, self-healing, recyclable, shape memory, etc.