CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Global technical development depends ultimately on the effective utilization of the existing and new materials. The new materials may be the combination of two or more components to cater for particular needs. The composite materials come under this category. Composite materials are the macroscopic combination of two or more distinct materials with enhanced properties. The aim of using the composite material is for high strength to weight ratio and to meet the applications with specific properties. Fibers are a class of hair-like materials that are continuous filaments or discrete elongated pieces. Fiber-reinforced composite materials consist of fibers of high strength and modulus which are bonded in a matrix. The fiber has better interface with the matrix. In composites, both fibers and matrix retain their physical and chemical identities. But they produce a combination of properties that cannot be achieved either by fiber or matrix when they are used alone. In general, fibers are the principal load-carrying members. The matrix in a composite serves the following purpose, Keep the fibers in the desired location and orientation. Act as a load transfer medium. Protect the fibers from the environmental damage like temperature and humidity.

2 2 Historical examples of composites are plentiful in the literature. Plywood (invented by the Egyptians, approx. in 1500 BC) and reinforced concrete (invented by the Romans, approx BC), also natural fiber reinforced clay (used by men before iron was invented) are in essence of composite materials. The composite bows comprising of bark, sinew, bone, wood, horn, metal and glues are believed to have appeared in the hands of Assyrian archers as early as 1800 BC. Assyrians warred with the Egyptians, Babylonians and other civilizations and used the power of the composite bows to make a significant impression on their rivals. The modern era of composites did not begin until scientists developed plastics. Until then, natural resins derived from plants and animals were the only source of glues and binders. In the early 1900s, plastics such as vinyl, polystyrene, phenolic and polyester were developed. These new synthetic materials out performed resins that were derived from nature. However, plastics alone could not provide enough strength for structural applications. Reinforcement was needed to provide the strength and rigidity. In 1935, Owens Corning introduced the first glass fiber, fiberglass. Fiberglass, when combined with a plastic polymer creates an incredibly light weight and strong structure. The first commercial composite boat hull was introduced in This is the beginning of the Fiber Reinforced Polymer (FRP) industry. In the 1970s, the composites industry began to mature. Better plastic resins and improved reinforcing fibers were developed. Kevlar fiber has become the standard in armor due to its high tenacity. Carbon fiber was also developed around this time and it is a promising replacement for metal as the new material of choice. The composite materials find application in aerospace, automobile industry, marine vessels, structures, building, construction industry, chemical plants, corrosion resistant products, consumer durable products, sports goods etc.,

3 3 The composites industry is still evolving with much of the growth and focused around renewable energy. Additionally, composites are on the path towards being more environmentally friendly. Resins will incorporate recycled plastics and bio-based polymers. The synthetic fibers pollute the environment since they are not biodegradable. Development of natural fibers reinforced composites is highly attractive. Composites are materials made from a binder, usually a resin and a reinforcement fiber. Composites in which the resin and/or fiber are made from renewable resources are often called bio-composites. Issues such as recyclability and environmental safety are becoming increasingly important in the introduction of materials and products. Natural fibers have a number of techno-economical and ecological advantages over synthetic fibers like glass fiber. Combination of interesting mechanical and physical properties together with their environmentally friendly character has created interest in a number of industrial sectors, notably the automobile industry. Glass and carbon fibers have been used widely as reinforcement materials, but their non-recyclability becomes a significant disadvantage at the end of their lifetime. They are also found to be hazardous to health. The natural composites can be very cost-effective material especially for building & construction of industrial panels, false ceilings, partition boards etc, packaging, automobile, railway coach interiors and storage devices. It helps to make the best quality industrial yarn, fabric, net and sacks. Wind turbine blades are constantly pushing the limits on size and are requiring advanced materials, designs and manufacturing. In the future, composites will utilize even better fibers and resins, many of which will incorporate nanomaterials. The research activities will continue to develop improved materials and ways to manufacture them into products.

4 4 In spite of these advantages the natural fibers have some limitations and they need to be overcome to make it competitive to the synthetic fibers. The transition towards a bio-based economy and sustainable development as a consequence of global warming offers high prospects for natural fiber reinforced bio-composite materials. Changing to a bio-based economy requires substitution of common raw materials from renewable (plant and animal based) resources. It will help to improve cultivation of fiber plants and also economy of the country. 1.2 NATURAL FIBERS AND THEIR SIGNIFICANCE Natural fibers can be defined as bio-based fibers of vegetable and animal origin. This definition includes all natural cellulosic fibers (cotton, jute, sisal, coir, flax, hemp, abaca, ramie, etc.) and protein based fibers such as wool and silk. Practically in all countries natural fibers are produced and used to manufacture a wide range of traditional and novel products from textiles, ropes, nets, brushes, carpets, mats, mattresses to paper and board materials. The growing environmental concern on global warming have inspired the automobile, structural, construction, packing industries etc., to search for sustainable materials that can replace conventional synthetic polymeric fiber. Natural fibers seem to be a good alternative since they are readily available in fibrous form and can be extracted from plant leaves at very low costs. Natural fibers are subdivided based on their origins, coming from plants, animals or minerals. Generally, plant or vegetable fibers are used to reinforce plastics. Various research works are being carried out with the natural fibers like bamboo, coir, jute, flax, sun hemp, ramie, kenaf, roselle, straw, rice husk, sugar cane, grass, raphia, papyrus and pineapple leaf fibers. A single fiber of all plant based natural fibers consists of several cells. These cells are formed out of crystalline micro fibrils based on cellulose, which are connected to a complete layer, by amorphous lignin and hemicellulose. Many

5 5 of such cellulose-lignin/hemicellulose layers in one primary and three secondary cell walls stick together to a multiple layer composites. These fibers are called lingo-cellulosic fibers. The natural fibers are classified as shown in Figure TYPES OF NATURAL FIBERS Natural fibers are available from plant, animal and mineral sources. Natural fibers can be classified according to their origin. Animal fibers generally contain proteins such as collagen, keratin and fibroin. Examples for the animal fiber are Alpaca, Angora, Byssus, Camel hair, Cashmere, Catgut, Chiengora, Guanaco, Human hair, Llama, Mohair, Pashmina, Qiviut, Rabbit, Silk, Sinew, Spider silk, Wool, Vicuna, Yak etc. Mineral fibers can be particularly strong because they are formed with less number of surface defects, asbestos is a common one. The plant based fibers are known as vegetable fibers. They mainly contain cellulose in their structure. The examples include cotton, jute, flax, ramie, sisal, and hemp etc., The natural fibers can be further categorized into the following classification Fruit / Seed Fibers The fruits and seeds of plants are often attached to hairs or fibers or encased in a husk that may be fibrous. These fibers are cellulosic based and having commercial importance. Cotton, the most important natural textile fiber is one among such type. Coir or coconut fiber belongs to the group of hard structural fibers. It is an important commercial product obtained from the husk of the coconut. Seed fiber is applied in less demanding applications such as stuffing of upholstery. Coir is used to make ropes, mats and brushes. Borassus fruit fibers which are lingo cellulosic in nature are extracted from the Borassus fruits. The fleshy substance of the fruit is reinforced by the Borassus fruit fibers.

6 Leaf Fibers Leaf fibers are obtained from leaves of plants (flowering plants that usually have parallel-veined leaves, such as grass, lilies, orchids and palms), used mainly for cordage. The fiber generally traverses the length of the leaf and is often the densest near the leaf undersurface. Such fibers are usually long and stiff. The leaf elements are harvested by cutting at the base with a sickle-like tool and bundled for processing by hand or by machine decortications. In the latter case, the leaves are crushed, scraped, and washed. The fibers are generally coarser than the bast fibers. Commercially useful leaf fibers include abaca, cantala, henequen, sisal, banana, agave etc Bast Fibers Bast fiber or skin fiber is plant fiber collected from the skin or bast surrounding the stem of certain, mainly dicotyledonous plants. They support the conductive cells of the phloem and provide strength to the stem. In the phloem, bast fibers exist in bundles that are glued together by pectin. The retting process separates the valuable fibers in the phloem. Often bast fibers have higher tensile strength than other kinds and are used in high-quality textiles. Most of the technically important bast fibers are obtained from herbs cultivated in agriculture, as for instance flax, hemp, kenaf and ramie Stalk Fibers Fibers are actually the stalks of the plant. e.g. straws of wheat, rice, barley and other crops including bamboo and grass. Tree wood is also such a fiber. The stalk of the plant contains two types of fiber, the outer bast fiber which can be processed into long strands and the inner woody core or hurds, which are typically processed into material resembling wood chips. They are used as reinforcing members in composites.

7 7 Natural fibers Plant fibers Animal fibers Mineral fibers Silk Wool Animal hairs Asbestos Ceramics Fruit/Seed fibers Leaf fibers Bast fibers Stalk fibers Examples: Cotton fibers, Coir fibers Borassus fruit fibers, Tamarind fruit fibers, Arecanut husk fibers etc. Examples: Flax fibers, hemp fibers, kenaf fibers, Abaca fibers, Jute fibers, Ramie fibers etc. Examples: Abaca fibers, Cantala fibers, Henequen fibers, Sisal fibers, Banana fibers, Agave fibers etc. Examples: Straws of wheat, rice, barley and other crops including bamboo, grass, wood. etc. Figure 1.1 Classification of natural fibers

8 8 Table 1.1 Mechanical properties of synthetic fibers Mueller (2003) Fiber Density (g/cm 3 ) Diameter (µm) Tensile strength (MPa) Young s modulus (GPa) Elongation at break (%) E-glass S-glass Aramide (normal) Carbon Table 1.2 Mechanical properties of natural fibers Malkapuram (2008) Fiber Density (g/cm 3 ) Diameter (µm) Tensile strength (MPa) Young s modulus (GPa) Elongation at break (%) Jute Hemp Kenaf Flax Ramie Sunn Sisal Cotton Kapok Coir Banana PALF

9 9 Table 1.3 Chemical composition of natural fibers Malkapuram (2008) Fiber Cellulose Lignin Hemi Pectin Wax Moisture (wt%) (wt%) Cellulose (wt%) (wt%) Content (wt%) (wt%) Jute Hemp Kenaf Flax Ramie Sunn Sisal Henquen Cotton Kapok Coir Banana PALF BENEFITS OF NATURAL FIBERS High specific strength to weight ratio. It is a renewable resource and less energy is used in the extraction of fibers. Minimum production cost. No wear of tooling and skin irritation during extraction. Good thermal and acoustic insulating properties. They are durable, eco-friendly and bio-degradable.

10 LIMITATIONS OF NATURAL FIBERS Supply and demand cycles are based on product availability and harvest yields. Moisture absorption, which causes swelling of the fiber. Quality variations based on growing sites and seasonal factors. Restricted maximum processing temperature. Low durability and poor fire resistance. 1.6 POLYMER MATRIX MATERIALS The role of the matrix in a fiber-reinforced composite are: (1) to keep the fibers in the desired location, (2) to keep the fibers in the desired orientation (3) to transfer stresses between the fibers, (4) to provide a barrier against an adverse environment, such as chemicals, moisture and to protect the surface of the fibers from mechanical degradation (e.g. by abrasion). The matrix plays a minor role in the tensile load carrying capacity of a composite structure. But the selection of a matrix has a major influence on the compressive, interlaminar shear as well as in-plane shear properties of the composite material. The matrix provides lateral support against the possibility of fiber buckling under compressive loading, thus influencing the compressive strength of the composite material. The interaction between fibers and matrix is also important in designing damage-tolerant structures. The processing and defects in a composite material depend strongly on the processing characteristics of the matrix. Polymer matrix is a long chain molecule containing one or more repeating units of atoms joined together by strong covalent bonds for which classification is shown in Figure 1.2.

11 11 Polymers (Long chain molecules) Plastics (Rigid Materials) Rubbers (Flexible Materials) Thermo Plastic Elastomers Vulcanized Rubbers Thermoplastics (Uncross linked- Heat revesible) Thermoset plastics (Cross linked Rigid) Examples: Polyamides, Acrylics, Polycarbonates, Polyethylene, ABS, Poly Vinyl Chloride, Poly Ether Ether Ketone etc., Examples: Epoxy, Polyester resin, Melamine formaldehyde, Phenol formaldehyde, Vinyl ester, Cynate ester, Furans etc., Figure 1.2 Classification of polymers Thermoset Polymers The molecules of the thermoset polymers are chemically joined together by cross-links, forming a rigid, three-dimensional network structure. Once these cross-links are formed during the polymerization reaction (also called the curing reaction), the thermoset polymer cannot be melted by the

12 12 application of heat. But they may degrade if the temperature is high enough to break the molecular chains. Polyester, Vinyl ester, epoxies, cross linked acrylics, Phenolics, Polyurethanes, Furans, Polyimides etc., are the most commonly used thermoset materials used in making the composites. Thermosets are generally brittle and addition of fiber can improve their toughness. They have good creep resistance. Toughness can also be improved by blending elastomers into the thrmosets. Good wet out between the fiber and the matrix can be attained without the aid of either high temperature or pressure. Thermoset polymers are having better thermal stability and chemical resistance. Thermoset PMC are being made and used for the last forty years and they find applications in a wide range of products ranging from aircraft, satellites, rockets, automobiles, machine elements and consumer goods Epoxy Resin The epoxy matrix consists of three member ring having one oxygen atom and two carbon atoms in its chemical structure. The epoxy resins contribute to the strength, durability and chemical resistance of the composite. Epoxy is a copolymer and is formed from two different chemicals. These are referred to as the resin and the hardener. The resin consists of monomers or short chain polymers with an epoxide group at either end. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-a, though the latter may be replaced by similar chemicals. Each NH group can react with an epoxide group from distinct prepolymer molecules, so that the resulting polymer is heavily cross linked and is thus rigid and strong. The process of polymerization is called curing and can be controlled through temperature, choice of resin and hardener compounds. The structure of epoxy resin is shown in Figure 1.3

13 13 Figure 1.3 Structure of epoxy Hardeners The hardener consists of polyamine monomers, for example triethylenetetramine. When these compounds are mixed together, the amine groups react with the epoxide groups to form a covalent bond. Amine hardeners react with the epoxy resins, contributing to the ultimate properties of the cured epoxy resin system. Amine hardeners provide: gel time, mixed viscosity, demould time of the epoxy resin system. Physical properties such as tensile, compression, flexural properties, etc., of the epoxy resin system are also influenced by epoxy hardeners. The performance of epoxy hardeners in the epoxy resins system depend on the chemical and physical characteristics of the epoxy. The chemical characteristics of the epoxy resins that influence epoxy hardeners are: viscosity and kind of diluents and fillers in epoxy resins. The physical characteristics of the epoxy resins system influencing the behaviour of epoxy hardeners in the epoxy resins system are: temperature of the work area, temperature of the resin system (i.e. the heated resins) and moisture. The structure of epoxy hardener is shown in Figure 1.4. Figure 1.4 Structure of hardener

14 Chemistry The curing process is a chemical reaction in which the epoxide groups in epoxy resin react with hardener to form a highly cross linked, threedimensional network. In order to convert epoxy resins into a hard, infusible and rigid material, it is necessary to cure the resin with hardener. Epoxy resins cure quickly and easily at any temperature from o C depending on the choice of hardener. In the structure of unmodified epoxy prepolymer, n represents the number of polymerized subunits and is in the range of 0 to 25. When epoxy is mixed with the appropriate hardener, the resulting reaction is exothermic and the oxygen on the epoxy monomers is flipped. This occurs throughout the epoxy and a matrix with a high stress tolerance is formed that glues the materials together (Figure s 1.4 and 1.5). Figure 1.5 Chemical reaction between Bisphenol-A and Epichlorohydrin 1.7 BORASSUS FRUITS The palm tree containing Borassus fruits are available all over the world, especially abundantly in India. The palm tree is a native of tropical Africa but cultivated and naturalized throughout India. The palm is a large tree which may grow up to 30 m height and the trunk may have a circumference of 1.7 m at the base. There may be fresh leaves. Leaves are leathery, grey green, fan-shaped, 1-3 m wide, folded along the midrib and

15 15 are divided at the center. Their strong stalks, m long, are edged with hard spines. The palm trees are usually grown well in the dry areas and are drought resistant. The life of the trees will be more than 100 years. In India, it is planted as a windbreak on the plains. It is also used as a natural shelter by birds, bats and wild animals. The coconut-like fruits are three-sided when young, becoming rounded or more or less oval, cm wide and capped at the base with overlapping sepals. When the fruit is very young, this flush is hollow, soft as jelly and translucent like ice and is accompanied by a watery liquid, sweetish and potable. The Borassus fruit fiber is a cellulosic fiber. The cellulose is a long chain polysaccharide made up of glucose monomer units, which are alternately rotated to 180 degrees. Cellulose molecules align to form micro fibrils of diameter of about 3 4 nm. The micro fibrils have both crystalline and non-crystalline regions that merge together. The hemicelluloses, lignin etc. bound the cellulose into fibril aggregates of diameter roughly nm. Hemicellulose binds to the surface of the cellulose micro fibrils, while lignin cross-links the hemicellulose molecules of adjacent micro fibrils. Hemicelluloses are short chain, amorphous polysaccharides with monomer units with acidic groups. They include xyloglucans, xylans, glucomannans and galactoglucomannans. Lignin is an amorphous, complex phenolic compound. Matured Borassus fruit contains cellulosic semi-solid flush which is reinforced by the Borassus fruit fibers. The botanical name of Borassus fruit fiber is Borassus flabellifer of family palmae (Figure 1.6).

16 16 Figure 1.6 Palm tree containing Borassus fruits 1.8 SIGNIFICANCE OF NATURAL FIBER COMPOSITES Natural fiber composites are emerging as promising replacements for synthetic fiber polymer composites. Natural fibers offer both cost savings and a reduction in density when compared to glass fibers. Though the strength of natural fibers is not as great as glass, the specific properties are comparable. These natural fiber composites demonstrate high strength and high toughness and have been developed for a range of rigorous environments. In addition, these composites can easily substitute conventional materials in several areas such as the automotive industry, building industry, consumer goods and sports goods. Many automotive and household components are produced using natural composites, mainly based on polyester and fiber like flax, hemp, pineapple, coir and sisal. The application

17 17 of natural fiber composites in this industry is lead by motives of price and weight reduction. 1.9 NEED FOR NATURAL FIBER COMPOSITES Driven by increasing environmental awareness, automakers in the 1990s made significant advancements in the development of natural fiber composites, with end-use primarily in automotive interiors. A number of vehicle models, first in Europe and then in North America, featured natural fiber-reinforced thermosets and thermoplastics in door panels, package trays, seat backs and trunk liners. Promoted as low-cost and low-weight alternatives to fiber glass, these agricultural products, including flax, jute, hemp and kenaf induced the start of a "green" industry with enormous potential. There remains, however, a general consensus about the main advantages of natural fiber reinforcements, including lower weight, availability, ease of recycling, thermal and acoustic insulation and carbon dioxide neutrality (when burned, the natural fibers reportedly give off no more carbon dioxide (CO 2 ) than they consumed while growing). On an average, the production of natural fiber suitable for composites is some 60 percent lower in energy consumption than the manufacture of glass fibers. It is equally necessary to reduce environmental impacts such as global warming, which are generated by consumption of petroleum, a non renewable resource. The energy and environmental comparisons of the natural fibers with the synthetic fibers are the motivational factors promoting bio-fiber products SCOPE OF THE PRESENT WORK The Borassus fruit fibers are inexpensive, naturally available, renewable, eco-friendly and hence, the investigation of its potential properties to the technical world is essential. An attempt is made in this research work to study the properties of Borassus fruit fiber reinforcements in composites with

18 18 and without alkali treatment and to introduce them as a natural reinforcement to the composites. This work has the following objectives: 1. To safely extract Borassus fruit fibers from the fruits. 2. To find the best alkali treatment percentage required for the fibers. 3. To study the physical, chemical and mechanical properties of raw and alkali treated fibers. 4. To visualize the surface morphology of raw and alkali treated fibers. 5. To study the chemical compounds of the fibers through Fourier Transform Infrared Spectrometry analysis. 6. To make the chopped Borassus fruit fiber reinforced epoxy composite specimens with different fiber lengths such as 1 mm, 3 mm, 5 mm, 7mm and 10 mm for both raw and alkali treated Borassus fruit fibers. 7. To explore the mechanical properties such as (tensile strength, compressive strength, impact strength, flexural strength, machinability), Water absorption, Thermo gravimetric analysis, Fourier Transform Infrared Spectrometry, Wear analysis, Surface morphology using Scanning Electron Microscope of both raw and alkali treated chopped Borassus fruit fiber-epoxy composites with different fiber lengths. 8. To find the tribological properties of Borassus fruit fiber reinforced composites and to visualize the surface morphology of the worn surfaces. 9. To manufacture the application products by reinforcing the alkali treated Borassus fruit fibers in epoxy.

19 To introduce less weight, high strength, durable composites to this technical world by Borassus fruit fiber-epoxy composites through the above applications OUTLINE OF THE THESIS Chapter 1 describes introduction about the Natural fibers and their significance, types of Natural fibers, Benefits & Limitations of Natural fibers, Polymer Matrix Materials, need for Natural fiber Composites, scope for the Present work and Organization of thesis. Chapter 2 describes the review of literature which discusses Mechanical Properties of natural fibers, Mechanical Properties of Natural fiber Composites, Tribological Behaviour of Natural fiber composites and Applications. Chapter 3 discussed about the Extraction of fiber, Alkali Treatment, Physical, Chemical and Mechanical Test of Borassus fruit fiber, preparation of Matrix and Mould, Tensile, Compressive, Impact, Flexural, Water Absorption, Machinability, FTIR, SEM and TGA and Wear Tests. Chapter 4 presents the Analysis of Borassus Fruit fiber, Analysis of Borassus Fruit fiber Epoxy composites (Raw and Alkali treated), Wear analysis of Borassus Fruit fiber Epoxy composites (Raw and Alkali treated) and the results are discussed in detail. Chapter 5 discusses the Application of Borassus Fruit fiber - Epoxy Composites with the fabrication of Two wheeler Bumper, Tumbler gear, Portable Gas Cylinder, Door Model and Solid Rod Model. future work. Chapter 6 Summarizes the thesis and provides suggestions for