Progress in Adhesion and Adhesives

By K.L. Mittal

With the ever-increasing amount of research being published it is a Herculean task to be fully conversant with the latest research developments in any field, and the arena of adhesion and adhesives is no exception. Thus, topical review articles provide an alternate and very efficient way to stay abreast of the state-of-the-art in may subjects representing the field of adhesion science and adheisves.

Based on the success and the warm reception accorded to the premier volume in this series “Progress in Adhesion and Adhesives” (containing the review articles published in Volume 2 (2014) of the journal Reviews of Adhesion and Adhesives (RAA)), volume 2 comprises 14 review articles published in Volume 4 (2016) of RAA.

The subjects of these 14 reviews fall into the following general areas:

1. Surface modification of polymers for a variety of purposes.

2. Adhesion aspects in reinforced composites

3. Thin films/coatings and their adhesion measurement

4. Bioadhesion and bio-implants

5. Adhesives and adhesive joints

6. General adhesion aspects

The topics covered include: surface modification of natural fibers for reinforced polymer composites; adhesion of submicrometer thin metals films; surface treatments to modulate bioadhesion; hot-melt adhesives from renewable resources; particulate-polymer composites; functionally graded adhesively bonded joints; fabrication of nano-biodevices; effects of particulates on contact angles , thermal stresses in adhesively bonded joints and ways to mitigate these; laser-assisted electroless metallization of polymer materials; adhesion measurement of coatings on biodevices/implants; cyanoacrylate adhesives; and adhesion of green flame retardant coatings onto polyolefins.

• Language: English
• Publisher: Wiley
• Release date: Jun 15, 2017
• ISBN: 9781119407478

Book preview

Preface

In 2015 we had brought out the premier volume in this series Progress in Adhesion and Adhesives (although we had not called it Volume 1 as we had no idea what the future plans would be) based on 13 articles published in 2014 in the journal Reviews of Adhesion and Adhesives (RAA). RAA was initiated in 2013 with the sole purpose of publishing review articles on topics of contemporary interest.

With the ever-increasing amount of research being published it is a Herculean task to be fully conversant with the latest research developments in any field, and the arena of adhesion and adhesives is no exception. Thus topical review articles provide an alternate and a very efficient way to stay abreast of the state-of-the-art of a given subject. Moreover, anybody embarking on a new research area or an individual who just wishes to be knowledgeable about a topic are well advised to start with a good review article on topic of his/her interest.

The success of and the warm reception accorded to the premier volume provided us the impetus to bring out this sequel, designated as Volume 2. The current volume is based on 14 critical, concise, illuminating and thought-provoking review articles (published in 2016 in RAA) written by a coterie of internationally renowned subject matter experts, covering many and varied topics within the broad purview of Adhesion Science and Adhesive Technology.

The rationale for bringing out Volume 2 is the same as was applicable to its predecessor, i.e., the RAA has limited circulation so this set of books should provide broad exposure and wide dissemination of valuable information published in RAA. The chapters in this Volume are arranged in the same order as published originally in RAA. The subjects of these 14 reviews fall into the following general areas.

Surface modification of polymers for a variety of purposes.

Adhesion aspects in reinforced composites

Thin films/coatings and their adhesion measurement

Bioadhesion and bio-implants

Adhesives and adhesive joints

General adhesion aspects

The topics covered include: surface modification of natural fibers for reinforced polymer composites; adhesion of submicrometer thin metals films; surface treatments to modulate bioadhesion; hot-melt adhesives from renewable resources; relevance of adhesion in particulate-polymer composites; analysis of damages in functionally graded adhesively bonded joints; surface modification strategies for fabrication of nano-biodevices; effects of particulates on contact angles and adhesion of a droplet; thermal stresses in adhesively bonded joints and ways to mitigate these; laser-assisted electroless metallization of polymer materials; adhesion measurement of coatings on biodevices /implants; cyanoacrylate adhesives; and adhesion of green flame retardant coatings onto polyolefins.

This book consolidating plentiful information on a number of topics of current interest should be valuable and useful to materials science, nanotechnology, polymers, bonding, biomedical, composites researchers in academia, government research labs and R&D personnel in a host of industries. Yours truly sincerely is sanguine that Volume 2 will receive the same warm welcome as its forerunner by the materials science community in general and the adhesionists in particular.

Now is the pleasant task of thanking those who were instrumental in shaping this book. First I am thankful to the authors of review articles for their enthusiastic support for bringing out Volume 2 as they felt that this was a very useful medium for bringing the information to a wider audience. Also, I should thank Martin Scrivener (publisher) for conceiving the idea of these books and for his steadfast interest in and support for this book project.

Kash Mittal

P.O. Box 1280

Hopewell Jct., NY 12533

E-mail: [e-mail]

April 2017

Chapter 1

Surface Modification of Natural Fibers for Reinforced Polymer Composites

M. Masudul Hassan1* and Manfred H. Wagner2

1Department of Chemistry, M C College, National University, Sylhet-3100, Bangladesh

2Berlin Institute of Technology (TU Berlin), Institute of Materials Science and Technology, Polymer Engineering/Polymer Physics, D-10623 Berlin, Germany

*Corresponding author: [e-mail]

Abstract

Recent advances in engineering, natural fibers development and composites science offer significant opportunities for new, improved materials which can be biodegradable and recyclable and can also be obtained from sustainable resources at the same time. The combination of bio-fibers like betel nut, banana, coir, jute, rice straw, tea dust and various grasses with polymer matrices from both non-renewable (petroleum based) and renewable resources to produce composite materials that are competitive with synthetic composites such as glass fiber reinforced polypropylene or epoxide has been getting increased attention over the last decades. This article provides a general overview of natural fibers and bio-composites as well as the research on and application of these materials. A special emphasis is placed on surface modification of natural fibers to attain desired composite properties. The roles of compatibilizers and radiation on the natural fiber-polymer composites are also included. A discussion about chemical nature, processing, testing and properties of natural fiber reinforced polymer composites completes this article.

Keywords: Natural fiber, surface modification, compatibilizer, radiation, hybrid composite, mechanical properties

1.1 Introduction

The demand for natural fiber reinforced polymer composites is growing rapidly due to their high mechanical properties, significant processing advantages, low cost and low density. Natural fibers are renewable resources in many countries of the world; they are cheaper, pose no health hazards and finally provide a solution to environmental pollution by finding new uses over expensive materials and non-renewable resources. Furthermore, natural fiber reinforced polymer composites form a new class of materials which seem to have great potential in the future as a substitute for scarce wood and wood based materials in societal applications [1].

Lignocellulosic natural fibers like jute, sisal, coir, and pineapple have been used as reinforcements in polymer matrices. Natural fibers of vegetable origin include bast, leaves, and wood fibers. They may differ considerably in their physical appearance but they have, however, many similarities that identify them as one family. The characteristics of the fibers depend on the individual constituents and the fibrillar structure. The fiber is composed of numerous elongated fusiform fiber cells. The fiber cells are linked together by means of middle lamellae, which consist of hemicellulose, lignin and pectin. Growing environmental awareness has spurred the researchers worldwide to develop and utilize materials that are compatible with the environment. In this process natural fibers have become suitable alternatives to traditional synthetic or man-made fibers and have the potential to be used in cheaper, more sustainable and more environmentally-friendly composite materials [2–3].

1.1.1 Natural Fibers (NFs): Sources and Classification

Natural organic fibers can be derived from either animal or plant sources. The majority of useful natural textile fibers are plant derived, with the exception of wool and silk. All plant fibers are composed of cellulose, whereas fibers of animal origin consist of proteins. Natural fibers, in general, can be classified based on their origin, and the plant-based fibers can be further categorized based on part of the plant they are recovered from. An overview of natural fibers and some photographs of NFs are presented in Figures 1.1 and 1.2, respectively [4–5].

Figure 1.1 Overview of natural fibers.

Figure 1.2 Photographs of some natural fibers.

Plant fibers are a renewable resource and have the ability to be recycled. The plant fibers leave little residue if they are burned for disposal, returning less carbon dioxide (CO2) to the atmosphere than is removed during the plant’s growth.

Chemically the lignocellulosic fibers comprise cellulose, hemicellulose, lignin, pectin and small amounts of waxes and fat. Several important sources of lignocellulosic materials are listed [6] in Table 1.1, Dinwoodie [7] summarizes the polymeric state, molecular derivatives and function of cellulose, hemicellulose, lignin and extractives (see Table 1.2).

Table 1.1 Chemical compositions of various lignocellulosic materials.

Table 1.2 Cellulosic component, polymeric state, derivatives and function.

1.1.2 Composition of NFs

Natural plant fibers are composed of cellulose fibers, made of helically wound cellulose micro-fibrils, bound together by an amorphous lignin matrix. Lignin keeps the water in the fibers acts as a protection against biological attack and as a stiffener to give stem its resistance against gravity forces and wind. Hemicellulose found in the natural fibers is believed to be a compatibilizer between cellulose and lignin. The cell wall in a fiber is not a homogeneous membrane [8–9]. Each fiber has a complex, layered structure consisting of a thin primary wall which is the first layer deposited during cell growth encircling a secondary wall. The secondary wall is made up of three layers and the thick middle layer determines the mechanical properties of the fiber. The middle layer consists of a series of helically wound cellular micro-fibrils formed from long chain cellulose molecules. The angle between the fiber axis and the micro-fibrils is called the microfibrillar angle. The characteristic value of microfibrillar angle varies from one fiber to another. These micro-fibrils typically have a diameter of 10–30 nm and are made up of 30–100 cellulose molecules in an extended chain conformation and provide mechanical strength to the fiber. Study on jute cellulose, hemicellulose and lignin [10–11] suggests that these consist of units as shown in Figures 1.3–1.5.

Figure 1.3 Structure of cellulose.

Figure 1.4 Structure of hemicellulose.

Figure 1.5 Structure of lignin.

1.1.3 New Trends in the Chemistry of Cellulose

The chemistry of cellulose now under development will make possible the use of cellulose, the most important and widespread polymer, for manufacturing a great variety of materials with new structures and endowed with valuable properties quite different from those of ordinary cellulose products. The transformation of natural cellulose containing one type of reactive groups (primary and secondary alcohol groups) into high molecular weight compounds which, depending on processing conditions, will contain almost any of the known reactive functional groups.

Cellulose reacts as a trihydric alcohol with one primary and two secondary alcohol groups per glucose unit. The relative reactivity of the hydroxyl groups of both low molecular mass carbohydrates and cellulose has been studied [12]. In the former, the 2- and 6-hydroxyl groups are usually the most reactive. With cellulose, certain data indicate a preferential reactivity of the 2-hydroxyl and others of the 6-hydroxyl group. The manifold reactions of cellulose may be conveniently divided into two main kinds: those involving the hydroxyl groups and those involving or causing a degradation of the chain molecules. The former includes the following reactions: (1) Esterification: nitration, acetylation and xanthation. (2) Etherification: alkylation and benzylation. (3) Replacement of –OH by –NH2 and halogen. (4) Replacement of –H in –OH by Na. (5) Oxidation of –CH2OH to –COOH. (6) Oxidation of secondary –OH groups to aldehyde and carboxyl and (7) Formation of addition compounds with acids, bases, and salts. The various possible types of oxidized groups formed in the cellulose molecule are shown in Figure 1.6.

Figure 1.6 Possible types of oxidized groups in cellulose.

1.1.4 Action of Reducing and Oxidizing Agents

Reducing agents have no effect on cellulose while oxidizing agents readily convert it to oxycellulose. For chemical treatment of fibrous materials, various oxidizing agents are widely used: chlorinated lime, sodium hypochlorite, hydrogen peroxide, sodium chlorite, sodium and potassium chromates, and such acids that are capable of oxidizing, such as, for instance, nitric acid. These reagents may cause intense oxidation of cellulose functional groups and breakage of chains as a result of glucosidic linkage rupture. The oxidizing agents first act on the functional groups located on the cellulose fiber surface and then progressively penetrate into the depth of the fiber. There are oxidizing agents which mainly affect the primary alcohol group at the 6th carbon atom, while other oxidizing agents principally react with the secondary alcohol groups at the 2nd and 3rd carbon atoms, breaking the pyran ring. Figure 1.7 represents the oxidation process [13].

Figure 1.7 Effect of oxidizing agents on cellulose.

1.1.5 Drawbacks of Natural Fibers

Most natural fibers are hygroscopic in nature, i.e., they take in or give out moisture to their surrounding atmosphere. When NFs neither absorb nor give out moisture to the air around them they are said to be in equilibrium with that particular atmosphere. The amount of moisture held by NFs can be expressed in two ways: by moisture content, or moisture regain. The equilibrium moisture held by NFs when exposed to atmospheres of different relative humidities shows appreciable hysteresis according to whether absorption from low humidities or desorption from high humidities is concerned [14–16]. In general, the physico-mechanical behavior of NFs depends on the shape and size of cellulose molecule, fibrillar arrangement, various bonds, and interaction of non-cellulosic components of the fiber. The individual fiber filaments of an NF are composed of a number of ultimate cells cemented together by an isotropic, non-cellulosic intercellular substance (hemicellulose, lignin and pectin) which forms a layer of middle lamella in between the fiber cell walls. The walls of the fiber cells are thick and lignified and except for the original cracks, these are relatively smooth in size. The ultimate fiber cells are elongated in the direction of the stem axis with pointed or tapering ends and appear more or less polygonal with well-defined angles in a cross section.

The residual oil is the major contaminant in the NF products and creates greater problems in addition to the natural and inherent defects such as falling off of fiber from fiber products. Another drawback of an NF, which is responsible for its limited use, is that of discoloration due to the development of yellow to brown color after sufficient exposure to light. Moreover, there is a major drawback associated with the application of NFs for reinforcement of resin matrices. Due to presence of hydroxyl and other polar groups in various constituents of an NF, the moisture uptake is high (approx. 12.5% at 65% relative humidity & 20 °C) by dry fiber. All this leads to (i) poor wettability with resin, and (ii) weak interfacial bonding between NF and the relatively more hydrophobic matrices. Environmental performance of such NF composites is generally poor due to delamination under humid conditions. Thus, it is essential to pretreat the surface of the NF, so that its moisture absorption is reduced and the wettability by the resin is improved. Hence cellulosic fibers have some inherent drawbacks which can be briefly enumerated as follows:

poor solubility in common solvents, which makes improvements in fibers and yarns through spinning processes almost impossible;

poor crease resistance, which makes garments made from cellulosic fibers crumple easily during wear;

lack of thermoplasticity, which is a requirement for heat setting and shaping of garments; and

poor dimensional stability which results in distortion of the garment during laundering and ironing. These drawbacks, and the fact that cellulose has encountered stiff competition from synthetic fibers, have directed attention toward improving the properties of cellulose.

Therefore, the limited use of natural fiber composites is also connected with some other major disadvantages still associated with these materials. The fibers generally show low ability to adhere to common non-polar matrix materials for efficient stress transfer. Furthermore, the fibers inherent hydrophilic nature makes them susceptible to water uptake in moist conditions. Natural fiber composites tend to swell considerably with water uptake and as a consequence mechanical properties, such as stiffness and strength, are negatively influenced. However, the natural fiber is not inert. The fiber-matrix adhesion may be improved and the fiber swelling reduced by means of chemical, enzymatic or mechanical modifications.

1.2 Modifications of Natural Fibers

To achieve some improvements, the physical and chemical structures of cellulose must be altered.

1.2.1 Physical Modifications of Natural Fibers

The physical structure of cellulose can be altered either by swelling or by regeneration. Cellulose can be swollen in a suitable swelling agent and then partially deswollen by removal of the swelling agent. There is practically no change in the chemical structure of the cellulose, whether fiber, crumb, or film, but there are considerable changes in the physical form resulting in an enhancement of strength, luster, and reactivity.

1.2.1.1 Plasma Treatment

Plasma has been extensively used as a physical method for the modification of polymers [17–20]. The plasma treatment of natural fibers affects the surface only within a few tens of nm and thus does not affect the bulk properties of fibers [21]. It was observed that the plasma treatment can induce dramatic changes in the surface morphology of natural plant fibers [22]. More specifically, some tiny grains, cracks and longitudinal grooves appeared on the surfaces of the plasma-treated flax fibers, indicating that plasma treatment causes degradation and increases the surface roughness of the flax fibers. Jute fibers were treated with oxygen plasma in different plasma reactors with different plasma powers. It was reported that all treatments increased the tensile strength and flexural strength of the resulting jute fiber-unsaturated polyester composites [23].

1.2.1.2 Physical Activation Processes on Cellulose

1.2.1.2.1 Nonionizing (Low Energy) Radiation

Low-energy, radiation-induced grafting involves the use of ultraviolet or visible light supplied by a suitable source. The energy is used to cause excitation of the sensitizer, causing generation of radical species which may then attack the substrate. It is shown that ultraviolet radiation can be used to initiate grafting. Since this type of radiation is not of sufficiently high energy to break C-C or C-H bonds, a photosensitizer must be added to the system [24]. Sodium 2,7-anthraquinonedisulfonate and 2-methylanthraquinone are used as sesitizers to graft acrylamide, styrene and other monomers onto cellulose (Cellophane) and cellulose acetate films. Approximately 0.5% of the sensitizer (based on monomer) is used [25].

1.2.1.2.2 Ionizing (High Energy) Radiation

With all types of high-energy radiations such as gamma rays, X-rays, alpha particles, and protons, primary event consists of the formation of ions resulting from the scission of C-C or C-H bonds belonging to the cellulose, the monomer, or the solvent. The ions are rapidly converted into free radicals, and in nearly every known case of radiation polymerization or radiation grafting, a radical mechanism, rather than an ionic mechanism, accounts for the initiation and growth steps [26–27]. When polymeric materials are subjected to irradiation by ionizing radiation such as γ-rays from Cobalt-60 (⁶oCo) or high-energy electron beams generated from electron accelerators, active sites, usually free radicals, are formed in the polymeric materials. When these active sites are brought into contact with reactive monomers, either simultaneously during irradiation (direct or simultaneous method) or after irradiation (post irradiation method), the active sites initiate polymerization of the reactive monomers to form chemically different polymer chains (graft chains) bonded to the polymeric materials (polymer substrates). In the presence of monomer, the possible product from the irradiation of cellulose, which will lead to the formation of graft copolymers, can be represented as Figure 1.8.

Figure 1.8 Possible free radicals formation by irradiation of cellulose.

It can be proposed that the localization of the absorbed energy in the cellulose initiates photochemcial reactions, thereby leading to free radical formation. The chain scission by the photon of light is the primary reaction resulting in free radical formation. In the case of cellulose, graft reaction takes place at the main backbone. The formation of free radicals by chain scission is shown in Figure 1.9 and additional modes of radical formation are shown in Figure 1.10.

Figure 1.9 Free radicals formation by chain scission of cellulose.

Figure 1.10 Additional modes of free radicals generation.

1.2.2 Chemical Modifications of Natural Fibers

1.2.2.1 Fundamental or Basic Aspects

Intimate molecular contact at the fiber-matrix interface is necessary to obtain strong interfacial intereaction. Without an intimate molecular contact, the interfacial adhesion will be very weak, and accordingly the applied stress that can be transmitted from one phase to the other through the interface will be very low. Natural fibers are amenable to modifications as they bear hydroxyl groups from cellulose and lignin. The hydroxyl groups may be involved in hydrogen bonding within the cellulose molecules thereby reducing the activity towards the matrix. Chemical modifications may activate these groups or can introduce new moieties that can effectively interact with the matrix. In order to improve the fiber-matrix adhesion a pre-treatment of the fiber surface or the incorporation of a surface modifier during processing is required.

Several processes have been developed to modify polymers and fiber surfaces including chemical treatments, radiation treatment, plasma treatments, surface grafting, etc. which are shown in Table 1.3 [28–59]. These cause physical and chemical changes in the surface layer without affecting the bulk properties [60]. The chemical structure of cellulose can be altered in several ways:

Table 1.3 Various surface treatment methods for natural fibers.

By substitution of the cellulose hydroxyl, the cellulose molecules are altered through introducing side groups, usually by an etherification or an esterification reaction.

By reacting cellulose with bi- or polyfunctional compounds, which results in the production of cross-links or resinification products in the cellulose, thereby stabilizing its structure.

By combining synthetic polymers with cellulose to produce materials with improved properties. This process is known as grafting, usually done by modifying the cellulose molecules through creation of synthetic polymers that confer certain desirable properties on the cellulose without destroying its intrinsic properties.

Much research has been done on grafting polymeric molecules onto cellulose to produce materials with new properties intermediate between those of cellulose and those of synthetics.

1.2.2.2 Grafting Reactions on Cellulose in an NF

Grafting of vinyl monomers with different functional groups (–OH, -Cl, -C≡N, etc.) onto cellulose is a typical free radical polymerization reaction [9, 61] which involves three distinct aspects, namely, initiation, propagation, and termination. Initiation consists of two steps. The first step is to produce free radicals on the cellulose backbone from the initiator. This is generally achieved by abstraction of a hydrogen atom from the cellulose molecule.

The second step entails the addition of a monomer molecule to the cellulose free radical, resulting in the formation of a covalent bond between the monomer and the cellulose and in the creation of a free radical on the newly formed branch. Thus, a chain is followed by many subsequent additions of monomer molecules to the initiated chain, thereby propagating the chain. Termination occurs by combination, where the radicals of two growing polymer chains are coupled Figure 1.11.

Figure 1.11 Possible grafting reaction on cellulose in an NF.

Or by disproportionation where a hydrogen atom is abstracted by one chain from the other Figure 1.12. Termination may also occur by reaction with impurities, initiator, or activated monomer, or by a chain transfer process.

Figure 1.12 Free radical attachment during grafting reaction on cellulose in NF.

1.2.2.3 Mercerization of Fibers

Alkali treatment of natural fibers, also called mercerization [62] is the usual method to produce high quality fibers. Alkali treatment increases surface roughness, resulting in better mechanical interlocking and the amount of cellulose exposed on the fiber surface. In the alkali treatment, the following reaction takes place: Addition of aqueous sodium hydroxide (NaOH) to natural fiber promotes the ionization of the hydroxyl group to the alkoxide.

Graphic

Several studies conducted on alkali treatment reported that mercerization led to increase of the amount of amorphous cellulose at the expense of crystalline cellulose in the network structure [63].

1.2.2.4 Acetylation

Acetylation of natural fibers is a well-known esterification method of introducing plasticization to cellulosic fibers [64]. Acetylation is based on the reaction of cell wall hydroxyl groups of lignocellulosic materials with acetic or propionic anhydride at elevated temperature (usually without a catalyst)

Graphic

It has been shown that esterification improves the dispersion of lignocellulosic materials in a polymer matrix, as well as the dimensional stability and interface of the final composites [63–65].

1.2.2.5 Acrylation Treatment

Acrylation treatment, maleated polypropylene/maleic anhydride treatment, and titanate treatment of cellulosic fibers have also been reported [66]. Through such treatments, the surface energy of the fibers is increased, thereby providing better wettability and high interfacial adhesion [67].

1.2.2.6 Isocyanate Treatment

Isocyanate has a functional group -N=C=O which is very susceptible to reaction with the hydroxyl groups of cellulose and lignin in the fibers, and forms strong covalent bonds, thereby creating better compatibility with the polymer matrix in the composites. The performance of isocyanate as a coupling agent has been studied. Isocyanates provide better interaction with thermoplastics resulting in superior properties. Isocyanate can act as a promoter or as an inhibitor of interaction [68].

Graphic

1.2.2.7 Maleic Anhydride Treatment

Maleic anhydride (MA) is used as a grafting monomer to functionalize biopolymers because of the high reactivity of the anhydride group. MA-functionalized polymer is of considerable importance for application as a copolymer precursor in polymer blends and also as an adhesion promoter in biocomposite applications [69]. During reactive extrusion processing, maleic anhydride reacts with the biopolymer matrix to form maleated biopolymer, which then reacts (Figure 1.13) with the biofiber [30].

Figure 1.13 Possible mechanism of natural fiber with maleic anhydride for compatibilization.

1.3 Composites

A typical composite material is a system composed of two or more materials (mixed and bonded) on a macroscopic scale. Composites consist of one or more discontinuous phases embedded in a continuous phase. The discontinuous phase is usually harder and stronger than the continuous phase and is called the ‘reinforcement’ or ‘reinforcing material’, whereas the continuous phase is termed as the ‘matrix’ [70–71]. Properties of composites are strongly dependent on the properties of their constituent materials, their distribution and the interaction among them. The composite properties may be the volume fraction sum of the properties of the constituents or the constituents may interact in a synergistic way resulting in improved or better properties.

1.3.1 Hybrid Composites

The word hybrid is of Greek-Latin origin. Hybrid composites are the systems where one type of reinforcing or filler material is incorporated or added in a mixture of different matrices (blends) [72], or two or more reinforcing or filling materials are present in a single matrix [73–74] or, also, both approaches are combined. The integration of a variety of natural fibers in a single matrix results in the development of hybrid bio-composites. Reinforcements have been incorporated either by: (i) intermingling of two types of short fibers thoroughly before incorporating them into the polymer in a mixer, or are added alternately into the polymer with or without modification [75–77]. (ii) sandwiching of fibers or their mats or fabrics [77–78] or (iii) using non-woven or woven fabrics of both types of reinforcements, as in the case of glass fiber and liquid crystal (LC) fiber composite systems [49, 77–78]. Hybrid bio-composites are usually designed and processed by combination of a synthetic fiber and natural fiber (bio-fiber) in a matrix or with combination of two natural fiber/bio-fiber in a matrix [79]. The behavior of hybrid composites is a weighted sum of the individual components. The hybrid composite properties are exclusively governed by the length of individual fibers, orientation, fiber to matrix bonding, content, extent of intermingling of fibers, and arrangement of both fibers. Rule of mixtures can be used to determine the properties of the hybrid system consisting of two components. Moreover, successful use of hybrid composites is determined by the mechanical, chemical, and physical stability of the fiber/matrix system. Several researchers have developed hybrid composites by combining natural fibers with polyurethane resin, phenolic, polyester, epoxy, poly (vinyl ester), etc., as polymeric matrices [80]. Table 1.4 shows the reported and exclusive work on cellulosic/synthetic and cellulosic/cellulosic fibers reinforced hybrid composites [45, 81–101].

Table 1.4 Reported work on hybrid composites.

1.3.2 Compatibilization

Polymer matrices (e.g. Polypropylene (PP), Polyethylene (PE), Linear low density polypropylene (LLDPP), High density polypropylene (HDPP), have been used in many applications but, in general, their use is limited by their lack of functional groups. Thus, functionalization reactions have been used to increase their interfacial interactions [102–103]. The grafting process is one of the methods most frequently used to modify polymer matrices. Graft polymerization by hydrogen abstraction from tertiary carbon offers an effective approach to introduce some desirable properties into the polymer, thus expanding its applications without adversely affecting the backbone architecture [104]. Using benzoyl peroxide (BPO) as initiator and glycidyl methacrylate (GMA) as monomer, the copolymerization process occurs via a free-radical mechanism, through the scission of the double bond in the GMA [105].

GMA monomer possesses a dual functionality, containing both epoxy and acrylic groups, providing design and performance versatility required for coating and resin applications. Besides, the acrylic and vinyl functionalities (free-radical reactivity) allow copolymerization with a variety of other vinyl monomers in aqueous and nonaqueous systems. The resulting polymers feature a combination of epoxy functionality with an acrylic backbone. On the other hand, the epoxy group (functional reactivity) enables reactions with amines, carboxylic acids, anhydrides and hydroxyl-containing polymers. It allows structural modification of the polymer backbone that can result in different properties and higher performance. Furthermore, another compatibilizer i.e. the maleic anhydride grafted PP (MAH-g-PP) is a well-known and widely applied graft copolymer used as a modifier to enhance the compatibility between fibers and PP [106–108]. With sufficient fiber and proper compounding conditions, the mechanical properties of composites are increased. Therefore, GMA-g-PP and MAH-g-PP have been used in a wide range of applications as co-monomers in polymers with tailored properties or compatibilization agents for polymer blending as well as for polymer composite preparation. Fabrication of tea dust-PP (TDPP) bio-composites as well as investigation of the effect of different compatibilizers like GMA-g-PP and MAH-g-PP on the prepared TDPP bio-composites properties have been reported [46].

1.3.3 Effect of Radiation on Fiber Composites

Several reports have documented the use of surface modification of natural fibers as well as their use as reinforcement in thermoplastics such as polyethylene (PE) and polypropylene (PP) and thermosets like unsaturated polyester and epoxy resins [31, 41, 78]. Among the physical treatments, ionizing radiation (γ-radiation) can induce surface cross-linking between the natural fiber and matrix [46, 109]. The use of γ-radiation in composite materials offers several advantages, such as continuous operation, minimum time requirement, less atmospheric pollution, curing at ambient temperatures, increased design flexibility through process control, etc. [109]. Polypropylene (PP) is a semi-crystalline thermoplastic polymer and is widely used because it possesses several vital and useful properties such as transparency, high mechanical strength, high heat distortion temperature, low moisture pickup and good dielectric properties [31, 103–104].

1.3.3.1 Effect of γ-Radiation on Hybrid Composites

The use of two or more fibers (hybrid fiber) in the same matrix provides another dimension to the potential versatility of fiber reinforced composite materials. Ionizing radiation (γ-radiation) can induce surface cross-linking between the natural fiber and matrix. Rice straw (Rs) and seaweed (Sw) were subjected to γ-radiation with different intensities (25, 50, 75, 100, 150, 200 krad) before extrusion and then different hybrid composites were prepared using the optimum formulation system (20% Rs, 10% Sw in 70% PP). The effects of γ-radiation on the mechanical properties such as tensile strength (TS), bending strength (BS), impact strength (IS) and elongation at break (Eb) of the irradiated hybrid fiber composites were investigated and it was reported [26–27] that better mechanical properties were obtained for the composite in which both fillers were pre-irradiated at 100 krad and attained maximum strength with TS = 35 MPa, BS = 75 N/mm², IS = 2.7 kJ/mm² and Eb = 68%. It has been mentioned that the effect of high-energy radiation on organic polymers is to produce ionization as well as free radical formation. Due to the radiation, the polymer may undergo cleavage or scission into smaller fragments and, subsequently, rupture of chemical bonds yields fragments of the large polymer molecules. The free radicals thus produced may react to alter the structure of the polymer as well as the physical properties of the polymer material. It may also undergo cross-linking (i.e., the molecules maybe linked together into large molecules). The increase of TS, BS and IS properties with increasing γ-radiation dose may be due to the intercross-linking between the neighboring cellulose molecules that occurs under γ-exposure. This is corroborated by the previous work [45].

1.3.4 Initiative in Product Development of NF Composites

During the last few years, a series of investigations have been carried out to replace the conventional synthetic fiber composites with natural fiber composites [31, 35, 41, 45, 56, 78, 110–111]. For instance, hemp, sisal, jute, cotton, flax and broom are the most commonly used fibers to reinforce polymers like polyolefins [111], polystyrene and epoxy resins [36, 49, 112]. In addition, fibers like sisal, jute, coir, oil palm, bamboo, bagasse, wheat and flax straw, waste silk and banana [13, 35–36, 42–43, 47–48, 49, 112–113] have proved to be good and effective reinforcements in both thermoset and thermoplastic matrices. Nevertheless, certain aspects of natural fiber reinforced composites behaviour are still poorly understood such as their visco-elastic, visco-plastic or time-dependent behaviour due to creep and fatigue loadings [114], interfacial adhesion [112, 115] and tribological properties. Only a little information concerning the tribological performance of natural fiber reinforced composite materials [114–118] is available in the literature. In this context, long plant fibers, like hemp, flax [58, 112], bagasse [119] and bamboo [115–116] have considerable potential in the manufacture of composite materials for triboapplications. Accordingly, extensive studies on preparation and properties of polymer matrix composites (PMCs) replacing the synthetic fiber with natural fibers like jute, sisal, pineapple, bamboo, kenaf and bagasse were carried out [26–27, 32–33, 44–45, 50, 54, 120–123]. Likewise, Lantana-Camara fibers, tea dust (TD), rice straw (Rs) and some other fibers like jute, betel nut etc. may also have considerable potential as reinforcements for polymer composites and these have also been reported in the current decade [26–27, 44–45, 124].

1.4 Properties Evaluation

1.4.1 Lantana-Camara Fiber

1.4.1.1 Morphology of Treated Lantana-Camara Fibers

It is well established that cellulose chains of natural fiber are strongly bound by chemical constituents, lignin and hemicellulose, resulting in the formation of a multi-cellular fiber. It is reported [124] that the surface of untreated Lantana-Camara fiber appears rough due to the presence of lignin, wax, oil, and surface impurities (Figure 1.14a) which have been partially removed with acetone (Figure 1.14b) and further removed with alkali and benzoyl chloride treatments (Figure 1.14c–1.14d). These clean surfaces are expected to provide direct bonding between the cellulose fiber and the matrix such as epoxy resin. The mechanical properties of untreated and treated Lantana-Camara fibers are represented in Tables 1.5 and 1.6. It is seen that the alkali and benzoyl chloride treatments result in separation of the microfibrillar structure (fibrillation) and reduction in thickness of fiber because of the removal of cemented materials (i.e. lignin and hemicellulose) [102–103]. Moreover, these two treatments increase the effective surface area by fibrillation which promotes mechanical interlocking between the fiber and the matrix whereas the acetone treatment does not affect the fiber surface very much.

Figure 1.14 SEM micrographs of Lantana-Camara fibers (a) Untreated, (b) Acetone treated, (c) Alkali treated and (d) Benzoyl chloride treated.

Table 1.5 Mechanical properties of untreated Lantana-Camara fiber-epoxy composite.

Table 1.6 Mechanical properties of treated Lantana-Camara fiber-epoxy composite.

1.4.1.2 Surface Chemical Composition of Modified Fiber

The effect of chemical modifications on the fiber surface was observed using FTIR spectroscopy. FTIR measurements were performed using an IR-Prestige-21 spectrometer. A total of 100 scans were taken from 400–4000 cm–1 with a resolution of 2 cm–1 for each sample. The comparison of the representative FTIR spectra of untreated Lantana-Camara before and after chemical treatment (acetone, alkali and benzoyl chloride treatment) are shown in Figure 1.15. In comparison to the unmodified Lantana-Camara fiber, the alkali treated, acetone treated and benzoylated Lantana-Camara fibers showed a reduction in O-H stretching intensity and shifting of the peak from 3308.5 cm–1 to 3384.2, 3334.7, and 3364.2 cm–1 indicating participation of some free hydroxyl groups in these chemical reactions. The site of reaction was probably at the lignin –OH and C2-OH of the glucopyranose unit in the cellulose component. A strong and sharp band at 1725.3 cm–1 is observed due to C=O stretching of carbonyl groups (>C=O) in hemicellulose component for the untreated fiber, which disappeared in alkali treated fiber. Alkali treatment of Lantana-Camara destroys the C=O unit of the uronic acid residue in hemicellulose, perhaps arising from the intermolecular addition of the alcoholate (-CH2-O-Na+) from cellulose and lignin components to the C=O group. The C=O stretching band at 1763.2 cm–1 in benzoylated fiber shifts to 1752.7 cm–1 in acetone treated fiber. The benzoylation of Lantana-Camara fiber introduces a new absorption peak at about 1763.2 cm–1 owing to the presence of phenyl nucleus [34]. This band in benzoylated fiber is more intense, indicating a combined effect of -O-CO-Ph and –O-CO-CH3 groups arising from benzoylation. The acetone extracted Lantana-Camara fiber spectrum is similar to that of the untreated hemp, although a light green extract was observed during Soxhlet extraction. The band of medium intensity at 831.9 cm–1 due to β-glucosidic linkage in the unmodified Lantana-Camara fiber underwent shift to a lower wavenumber [59, 124].

Figure 1.15 FTIR spectra of Lantana-Camara fiber before and after chemical modifications.

1.4.2 Tea Dust-Polypropylene (TDPP) Composite

1.4.2.1 Tensile Properties of TDPP Composite

The effect of tea dust (TD) content (10, 20, 30, 40 and 50 wt%) (Table 1.7) on the mechanical properties of the resulting composites has been studied and it is reported [46] that as the TD content increases, the stress is more evenly distributed and the tensile strength of the composites increases up to 40% TD content, and after that the composite exhibits a decreased value with increasing fiber content. At lower levels of TD fiber content, the composites show low tensile strength due to low fiber population and low load transfer capacity to one another. As a result, stress gets accumulated at certain points of composites and highly localized strains occur in the matrix. At intermediate level of TD loading (40%), the population of the fibers is just right for maximum orientation and the fibers actively participate in stress transfer. High levels of TD content showed that the increased population of fibers leads to agglomeration and stress transfer gets blocked and the resulting composite property is again found to decrease. It is concluded that 40% TD with 60% PP composite produces highest level of mechanical performances with tensile strength (TS) = 24 MPa, elongation at break (Eb) = 2% and tensile modulus (TM) = 2785 MPa.

Table 1.7 Relative amount (wt%) of tea dust (TD) as filler and polymer matrix in composites to optimize the filler (TD) content in composites.

1.4.2.2 Compatibilizer Effect on TD-PP Composites

1.4.2.2.1 Effect of GMA-g-PP on TD-PP Composites

The effects of different percentages (0.5, 1, 2, 3, 4) of GMA-g-PP compatibilizer were studied using the optimised composite system (40% TD content with 60% PP matrix) (Table 1.8). It is reported that there is increase in TS with increase in compatibilizer dose up to 2% which is expected as found in earlier research [46]. However, a decrease in modulus is noticed due to the fact that migration of too much compatibilizer around the fibres causes self-entanglement among compatibilizer rather than with the polymer matrix, resulting in slippage. The effect of GMA-g-PP as compatibilizer in TD-PP composites may be attributed to the presence of epoxy functions for better bonding with the TD and with the PP matrix. A significant improvement in mechanical properties of the GMA-g-PP treated TD-PP composites was reported due to a better cross-linking among hydroxyl group of TD and epoxy group of GMA-g-PP with polymeric matrix [125–126].

Table 1.8 Relative amount (wt %) of GMA-g-PP compatibilizer and polymer matrix in composites to optimize the amount of GMA-g-PP content in composites.

TD: Tea dust

1.4.2.2.2 Effect of MAH-g-PP on TDPP Composites

It is reported [125–126] from study of the effect of MAH-g-PP on TS and TM for the TD-PP composites containing 40% TD that the tensile strength increases gradually with an increase in the amount of MAH-g-PP compatibilizer from 0.5–3% (Table 1.9). However, in this case maximum tensile strength is produced at 3% MAH-g-PP and the TS value up to 4% MAH-g-PP remains nearly the same. The tensile modulus of the MAH-g-PP treated composites shows a similar trend as the effect of GMA-g-PP on TD-PP composite. The result supports that MAH-g-PP produces good adhesion between TD and polymer matrix which indicates that there is some kind of interfacial contact between TD and PP in the presence of MAH-g-PP. The role of MAH-g-PP as compatibilizer in natural fiber composites has also been reported [126] which corroborates the present discussion.

Table 1.9 Relative amount (wt%) of MAH-g-PP compatibilizer and polymer matrix in composites to optimize the amount of MAH-g-PP content in composites.

TD: Tea dust

1.4.2.3 SEM Analysis of TD-PP Composites

Examination of the fracture surfaces of the composites by SEM gives information on how a compatibilizer affects the interphase of the composites. Scanning electron micrographs for different composite samples TD, TD40, TDM3 and TDG2 which were prepared using different filler/matrices ratios (shown in Tables 1.7 and 1.8) are represented in Figure 1.16a, 1.16b, 16c and 16d. The effect of compatibilizer on the microstructures of TDM3 and TDG2 composites is clearly seen in SEM micrographs. In Figure 1.16b of the composite TD40, there are some void spaces around the TD fibers and some spaces where TD fibers have been pulled out. These may be due to the poor adhesion between TD fibers and polymers matrix in the absence of a compatibilizer. However, relatively less void spaces and a lower proportion of pulled out TD fibers are found in Figures 16(c) and (d), which represent the microstructures of the composites prepared with compatibilizers. These indicate that there is some kind of interfacial contact between TD fibers and PP in the presence of MAH-g-PP as well as GMA-g-PP. This means that the compatibilizer produces good adhesion between TD fiber and polymer matrix [46].

Figure 1.16 SEM micrographs of various composites: (a) TD, (b) TD40, (c) TDM3 and (d) TDG2 (For description of composites, see Tables 1.7–1.9).

1.4.2.4 Outdoor Degradation of TD-PP Composites

To study outdoor degradation of different composites, composite samples were placed on roof and ground. The tensile strengths of the samples were measured periodically and the results of the loss of tensile strength are presented in Table 1.10. The loss of tensile strength is found to increase with increase of storage time on roof. Without compatibilizer composite (TD40) was found to lose its strength faster than compatibilizer treated composites (TDG2 and TDM3). TDG2 composite shows the lowest degradation. In the case of degradation during ground storage, maximum loss of tensile strength for TD40 composite is 42%, whereas that for TDM3 and TDG2 these are about 20% and 14%, respectively. Composite samples were placed indoors at 25 °C and 30% relative humidity to study the indoor degradation. In this case, TD40 showed about 20% tensile strength loss after 120 days and tensile strength loss of the compatibilizer treated composites TDM3 and TDG2 is about 10% and 8%, respectively at 120 days.

Table 1.10 Loss of tensile strength (%) of composites on outdoor exposure.

Note: For description of composites, see Tables 1.7–1.9.

1.4.3 Water Absorption Test

The water absorption capacity of a composite determines the water swelling behaviour of the composite. The results of water absorption are shown in Figure 1.17 as water absorption versus soak time. The water absorption results demonstrated that the γ-irradiated hybrid fiber composite (GHC) absorbs lowest amount of water. A lower absorption of water by the composite indicates that more OH groups of cellulose are blocked from their interaction with the PP matrix [33]. It is also believed that –OH groups of cellulose molecules in the composites are mutually bonded or cross-linked due the effect of γ-radiation [26, 33, 54].

Figure 1.17 Water absorption of various composites: Seaweed composite (SwC), Ricestraw composite (RsC), Hybrid composite (HC) and gamma-irradiated hybrid composite (GHC).

1.4.4 Jute Fiber Reinforced Vinylester Composites

It is reported that [127] treatment of jute fibers by 5% NaOH at room temperature for varying times show an overall improvement in properties both as fibers as well as reinforced composites in Table 1.11. The fibers after treatment were finer, having less hemicellulose content, increased crystallinity, reduced amount of defects resulting in superior bonding with the vinylester resin. As fibers, the improvements in properties were predominant around 6–8 h treatment whereas for composites, it was maximum when reinforced with 4 h-treated fibers at 35% fiber loading. The flexural strength properties of the composites at 35 vol% fiber loading after 4 h alkali treatment was 238.9 MPa in contrast to 199.1 MPa for composites with untreated fibers. This is an improvement of 20%. The improvements, however, were 3 and 2.5% for composites prepared with 2 and 8 h treated fibers, respectively. The trend was similar for lower loadings with treated fibers. The improvement occurred after 23 vol% fiber loading. Similar observations for alkali-treated coir reinforced polyester composites at 19 wt% loading were reported by Rout et al. [128]. The improved properties of fibers with alkali treatment for longer duration (8 h) as a result of dissolution of hemicellulose, development of crystallinity and fibrillation thus created superior bonding with vinylester resin matrix but the increased brittleness of the fibers due to development of crystallinity