Functional Materials from Lignin

Methods and Advances

Xian Jun Loh, Dan Kai;Zibiao Li

• Publisher: WSPC (EUROPE)
• Year: 2018
• eBook ISBN: 9781786345226
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Lignin is one of the most abundant plant-derived feedstock on earth and qualifies as a renewable material. However, lignin is widely recognized as waste byproduct of the cellulosic ethanol and pulp and paper industry. How to properly modify lignin and develop it into functional polymers is a huge challenge, but an attractive research topic in both industry and academia.

This book brings together leading engineering approaches to address the challenges of lignin valorization. It presents the chemistry and properties of different types of lignin, and explores the cutting-edge approaches of lignin modifications. Unlike any existing texts, this book not only summarizes the traditional ways of using lignin, but also presents various potential applications of lignin materials together with advanced processing techniques.

The basis of lignin (its chemistry, types and properties) is described, as are different approaches to modify it. The features of lignin and its copolymers are explored and aligned with their potential applications. In addition to the carbon materials from lignin, the advanced fabrication approaches to engineer lignin-based micro/nano-structural materials are summarized.

Contents:

• Lignin and its Properties (Dan Kai, Li Ping Chow and Xian Jun Loh)
• Types of Lignin and Extraction Processing (Meiling Zhong and Yanjiao Nie)
• Lignin-based Functional Nanocomposites (Yonggang Xiang and Zibiao Li)
• Chemical Modification of Lignin (Zibiao Li, Zhi Xuan Toh, Yonggang Xiang, Enyi Ye and Xian Jun Loh)
• Carbon Precursor from Lignin: Methods and Applications (Zibiao Li, Ros Azlin Binte Rosli, Yonggang Xiang, Enyi Ye and Xian Jun Loh)
• Lignin-based Nanomaterials (Li Zheng, Bo Li and Yi He)
• Advanced Applications of Lignin-based Materials (Yanqin Cheng, Huijie Zhang, Zejun Xu and Lin-ping Wu)

Readership: Advanced undergraduates and graduate students in material science and engineering; chemists and materials scientists.
Key Features:

• Compared to other works, the book focuses on the various approaches to modify lignin or synthesize lignin copolymers, and the functionality that lignin or its copolymers added into the resulting products, such as mechanical reinforcement, UV protection, antioxidant and antimicrobial properties. Advanced lignin materials will also be included

Chapter 1

Lignin and Its Properties

Dan Kai^∗,‡, Li Ping Chow^†,§ and Xian Jun Loh^∗,†,¶

^∗Institute of Materials Research and Engineering,
A*STAR (Agency for Science, Technology and Research),
2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Singapore

†Department of Materials Science and Engineering,
National University of Singapore, 9 Engineering Drive 1,
Singapore 117576, Singapore

^‡[email protected]
^§[email protected]
^¶[email protected]

Abstract

Lignin is a class of complex organic polymers found in the structural tissues of plants. There are variations in the properties of lignin depending on its origin, such as polydispersity molar masses and hyperbranched structures. Lignin occurs via oxidative coupling of monolignols, which are synthesized via the phenylpropanoid pathway. To know the chemistry and structure of lignin is the first step in its valorization. In this chapter, we briefly describe the background, varieties, and complexity of the lignin and its structure. Moreover, the properties of lignin, including thermal properties, solubility, degradability, and antioxidant activities, have been discussed.

1.1Introduction

Lignin is a group of polyphenolic, organic polymers present in plants. It is a macromolecular compound with a average molecular weight of ∼20,000 [1]. Lignin is derived from the word lignum, which means “wood” in Latin. It is found in the cell wall of plants, where it is one of the main constituents together with cellulose microfibrils and hemicellulose formed by oxidative coupling of free lignin monomers secreted directly into the plant cell wall [2]. Lignocellulose is a structure for all three constituents (cellulose, lignin and hemicellulose) together contributing to the strength of the plant. Cellulose is embedded in the matrix of lignin and hemicellulose. Lignin is largely present in the middle lamella. However, unlike cellulose, the formation of lignin is not independent of the type of plant cell but has codependence on the specific function, monomeric composition, subcellular localization of the cell, as well as response to environmental changes [3].

Lignin in plants mainly provides rigidity by strengthening the structure of cell walls, especially in larger roots and stems. It provides additional tensile strength of 25–74 MPa and a Young’s modulus of 2.5–3.7 GPa [3]. Lignin regulates water and other liquid transportation through the plant. It also protects against biological stresses by inhibiting enzymatic degradation of other components [4]. Lignin provides protection for the plant against microorganisms and contains antioxidants that are discussed further in the book. In specialized plant cell types, lignin also waterproofs the cell walls [5]. As cell walls specialize, some will lignify as part of its normal functions. However, some cell walls, which would otherwise not lignify in response to abiotic and biotic stresses in the cells [5], can be triggered to lignify.

The ability of plants to synthesize lignin comes from the evolution of plants. Plants are classified into various phyla and not all of them have the ability to synthesize lignin. They have three non-vascular phyla (known as Bryophytes) and seven vascular phyla, two of which are seedless and five are seeded plants. Vascular phyla are plants that are able to synthesize lignin while non-vascular phyla are unable to do so [6]. The seedless vascular plants include plants such as mosses and quillworts (Phylum Lycophyta) as well as horsetails and ferns (Phylum Pterophyta) [7]. Four of the seeded vascular phyla are called gymnosperms, defined as vascular plants with naked seeds such as conifers, cycads, ginkgos and gnetophytes [7]. The remaining seeded vascular phyla is known as angiosperms that are vascular flowering plants with protected seeds that include monocots and eudicots.

Lignin is the second most abundant biological material, with cellulose being the first. During the papermaking pulping and bleaching process, cellulose is extracted to make paper while the lignin is separated and is a waste material during this process. The abundance of untapped “waste” lignin and the low cost of lignin make it attractive to be explored for further applications given its good properties.

1.2Varieties of Lignin

Lignin has a random three-dimensional network structure, where varieties of lignin are synthesized in plants depending on the plant species and linkages formed between phenylpropane units. Lignin content also varies from species to species where variation of 15–40% was obtained. Furthermore, within the same species, lignin content has some slight variation of a few percentage points as well [8]. It has been noted that lignin content within a plant also varies where lignin is very low in young shoots and high in wood [9]. It is seen in a study of lignin content in Eucalyptus globulus plants that as the plant matures, its lignin composition changes in the order of p-hydroxyphenyl (H), guaiacyl (G), and then syringyl (S) deposited at a later stage of growth [10]. H, G and S are phenolic compounds found in lignin and its structure, which in turn affects the interunit linkages formed between lignin. In another study, it has been concluded that the higher biomass as the tree grows has a negative correlation to the regulation and production of lignin present in larger trees due to a competition for the allocation of carbon to lignin or cellulose and hemicellulose [9].

Lignin can be classified based on its resources. For the seeded vascular phyla, gymnosperms (softwood) lignin is made up of more than 90% guaiacyl units while in angiosperm (hardwood) lignin, it is a copolymer of guaiacyl and syringyl units of varying ratios. Lastly, for lignin present in grass, the lignin content varies generally from 24% to 33% in softwood and 19% to 28% in hardwood [4]. For vascular, seedless plants (grass), they are more like angiosperms with varying ratios of guaiacyl and syringyl units [12]. Lignin can be separated from the wood in the form of milled wood lignin (MWL), dioxane lignin or cellulolytic enzyme lignin (CEL) [11]. Further processing in paper-making industries yield as by-products of chemical pulping such as kraft lignin, Soda lignin and lignosulfonates [6]. Gymnosperm and angiosperm are differentiated based on the seeds, type of leaves and method of reproduction of the plant as mentioned above. They do not necessarily have a higher lignin content over the other.

1.2.1Gymnosperm

Gymnosperm (softwood) comes from the Greek word gymnospermmos, which means “naked seeds”. Gymnosperms encompass any vascular plants that reproduce with an exposed seed, as they do not develop in an ovary. Most of these seeds are borne in a strobilus (a woody cone) that contains the reproductive organs of some nonflowering plants. Gymnosperm seeds can develop either at the end of short stalks or on the surface of the scales of the strobilus. Reproduction for gymnosperm has their pollen blown away from the parent plant by wind [12]. There are 14 plant families and 88 plant genus that belong to the gymnosperm species. Gymnosperms contain four main phyla, conifers, cycads, ginkgo and gnetophytes. The largest group of living gymnosperm plants is coniferous, which usually remains evergreen while the smallest is ginkgo.

Conifers have about 588 living species that can grow in all climates except Antarctica, making them the most economical, ecological and important group in gymnosperm. They have been around since 290 million years ago as seen from fossil records and have diversified since then. Conifers have a high lignin content compared to most hardwoods that can be up to 35% lignin to keep the branches or stems erect. Conifer lignin consists of mainly guaiacyl units and lacks syringyl units. The polymer resulting from the polymerization of lignin units tend to be more condensed with higher levels of carbon–carbon linkages between monomers [13]. Cycads have about 220 species that thrive in tropical and sub-tropical regions. They tend to resemble ferns but differ in terms of their reproduction where cycads are unisexual plants. They have unique seeds like ginkgo where their sperms are motile as a result of evolution. Cycads are shown to not form compression wood and thus have lower lignin content than other gymnosperm groups [14]. Ginkgo has only one living species that is a tree. Lignin content of ginkgo is seen to be high at about 30.7% [15]. Gnetophytes have 68 highly distinctive species composed of shrubs in deserts, vines in tropical rainforests and a single species that grows only two leaves in its entire lifespan [13].

1.2.2Angiosperm

Angiosperms (hardwood) are flowering plants that can be found almost all over Earth except in extreme conditions such as mountaintops, regions around the poles and in deep oceans. There is an estimated 352,000 species of angiosperms. They survive by feeding off other plants by floating and rooted aquatics and vary in size, shape and longevity on land. Trees under this category of “hardwoods” carry their seeds in a fleshy fruit, fleshy seeds, or in nuts. Angiosperms have five characteristics: first, ovules enclosed in a carpel, second, double fertilization resulting in the formation of an endosperm, third, stamens with two pairs of pollen sacs, fourth, gametophyte structure and development and lastly, phloem tissue made of sieve tubes and companion cells. Angiosperms can be easily spotted due to their broad leaves and flowers, which attract insects and birds to the tree. They tend to change color and die every autumn. Reproduction for angiosperm utilizes animals to carry the pollen to other trees for fer...

Table of contents

• Cover Page
• Title
• Copyright
• Preface
• About the Editors
• Acknowledgements
• Contents
• Chapter 1 Lignin and Its Properties
• Chapter 2 Types of Lignin and Extraction Processing
• Chapter 3 Lignin-based Functional Nanocomposites
• Chapter 4 Chemical Modification of Lignin
• Chapter 5 Carbon Precursor from Lignin: Methods and Applications
• Chapter 6 Lignin-based Nanomaterials
• Chapter 7 Advanced Applications of Lignin-based Materials
• Index