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.
Kashmiri Lal Mittal was employed by the IBM Corporation from 1972 through 1993 Currently, he is teaching and consulting worldwide in the broad areas of adhesion as well as surface cleaning. He has received numerous awards and honors including the title of doctor honoris causa from Maria Curie-Sklodowska University, Lublin, Poland. He is the editor of more than 135 books dealing with adhesion measurement, adhesion of polymeric coatings, polymer surfaces, adhesive joints, adhesion promoters, thin films, polyimides, surface modification surface cleaning, and surfactants. Dr. Mittal is also the Founding Editor of the journal Reviews of Adhesion and Adhesives.
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: firstname.lastname@example.org
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
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 .
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  in Table 1.1, Dinwoodie  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. Lignocellulose source Cellulose (%) Hemicellulose (%) Lignin (%) Other constituents (%)
Hardwood 43-47 25-35 16-24 2-8 Softwood 40-44 25-29 25-31 1-5 Coir 32-43 10-20 43-49 4.5 Cotton 95 2 0.9 0.4 Hemp 70.2 22.4 5 5.7 Henequen 77.6 4.8 13.1 3.6 Jute 71.5 13.6 13.1 1.8 Kenaf 36.0 21.5 17.8 2.2 Ramie 76.2 16.7 0.7 6.4 Sisal 73.1 14.2 11.0 1.7
Table 1.2 Cellulosic component, polymeric state, derivatives and function. Component Polymeric state Derivatives Function
Cellulose Crystalline highly oriented large molecule Glucose "Fiber" Hemicelluloses small molecules Semi-crystalline mannose, xilose Galactose "Matrix" Lignin Amorphous large 3-D molecule Phenyl propane "Matrix" Extractives Some polymeric; Other nonpolymeric polyphenols Terpenes
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...