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Advanced Materials

Classification, properties, applications and processing techniques of composites

von Lokesh K. S. (Autor) Dr. Prasad P. (Autor)

Fachbuch 2018 89 Seiten

Ingenieurwissenschaften - Allgemeines

Leseprobe

CONTENTS

CHAPTER 1
INTRODUCTION TO ADVANCED MATERIALS
1.1 Composite Materials
1.2 Nano Composite Material
1.3 Significance of Nano composites
1.4 Necessity of Advanced Materials
1.5 Category of Composites
1.6 Polymer Matrix Composite
1.7 Metal Matrix Composite
1.8 Ceramic Matrix Composites
1.9 Particle Reinforced Composites
1.10 Fiber-Reinforced Composite
1.11 Benefits And Applications of Composites
1.12 Processing Techniques For Glass, Carbon And Ceramic Fibres

CHAPTER 2
PROCESSING OF PLASTCS AND FIBRES
2.1 Plastic Resins
2.2 Fibres
2.3 Processing of PMC
2.4 Role of PMC’s In Aerospace Industries
2.5 Role of PMC’s In Automotive Industries
2.6 Applications of Polymer Composites

CHAPTER 3
PROCESSING OF METAL MATRIX COMPOSITES
3.1 Introduction
3.2 Reinforcing Materials
3.3 Processing of MMC
3.4 Properties at Interface

CHAPTER 4
PROCESSING OF CERAMIC MATRIX COMPOSITES
4.1 Engineering Ceramic Materials
4.2 Ceramic Matrix Composites
4.3 Oxide-CMC’s
4.4 Non-Oxide CMC’s
4.5 Processing Of CMCs
4.6 Isostatic Pressing
4.7 Cold Isostatic Pressing (CIP)
4.8 Hot Isostatic Pressing

CHAPTER 5
PROPERTIES OF COMPOSITE LAMINATES
5.1 Laminates
5.2 Stacking Sequence
5.3 Classification of Laminates
5.4 Mechanical Characterization
5.5 Composite Laminate
5.6 Joints in Composites
5.7 Mechanically Fastened Joints
5.8 Factors Affecting Mechanical Performance of Composites

Preface

This book is configured to specify the fundamental aspects of new age materials to fulfill the basic requirement to know about brief classification, properties, applications and processing techniques of composites. This work also aims to cover the syllabus prescribed by the University to help undergraduate students of Engineering and technology to study, understand and apply the practical aspects of basics and processing techniques of composite materials.

Concept of composites, applications and processing techniques are clearly detailed in the chapter 1 where chapter 2 covers the concept of polymer resin and preparation of PMC’s and application of PMC’s in different fields.

Chapter 3 highlights the need of MMC’s , Processing techniques of MMC’s , Interface and Interface properties where as the ceramic materials, oxide and non oxide ceramics and processing of ceramics are detailed in the chapter 4 . Chapter 5 deals about laminates and mechanical properties of composites.

Mr. Lokesh K. S.

Dr. Prasad P.

CHAPTER 1

INTRODUCTION TO ADVANCED MATERIALS

1.1 Composite materials

A composite material is a material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. ‘Composition material’ is commonly known as ‘composite’.

The two materials work together to give the composite unique properties. The individual components remain separate and distinct within the finished structure, differentiating composites from mixtures and solid solutions.

Constituents

A composite consist of a matrix and reinforcement. The reinforcement material may be preferred for many reasons: common examples include materials which are stronger, lighter, or less expensive when compared to traditional materials.

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Figure 1.1: Composition of composites

Matrix

The matrix is the monolithic material into which the reinforcement is embedded, and is completely continuous. This means that there is a path through the matrix to any point in the material, unlike two materials sandwiched together.

Matrices are most often weaker and less stiff than the reinforcement (especially if it is polymer). In addition to transferring externally applied loads to the reinforcement, the matrix protects the reinforcement from mechanical, physical, chemical (and biological) degradation, which would lead to a loss in performance

Reinforcement

The reinforcement material is embedded into a matrix. The reinforcement does not always serve a purely structural task (reinforcing the compound), but is also used to change physical properties such as wear resistance, friction coefficient, or thermal conductivity. The reinforcement can be either continuous, or discontinuous.

Reinforcements are used either to increase the efficiency, to reduce the cost. Reinforcements are generally stronger and stiffer than the matrix. The reinforcement provides the “strength” to the composite. Reinforcement is often (mainly) in the form of a fibre (glass, carbon, aramid, etc.). Fibres are good in tension (compare with rope), but poor in compression and shear, therefore need a “matrix” in which the fibre is embedded to “support” the fibre and to transfer externally applied loads to the reinforcement. Fibre geometry as we shall see is important. Fibres may be “short” (i.e. a defined aspect ratio) or “long” (in effect infinitely long)

Non-Synthetic Composites

Natural composites exist in both animals and plants. Wood is a composite – it is made from long cellulose fibres (a polymer) held together by a much weaker substance called lignin. Cellulose is also found in cotton, but without the lignin to bind it together it is much weaker. The two weak substances – lignin and cellulose – together form a much stronger one.

The bone in our body is also a composite. It is made from a hard but brittle material called hydroxyapatite (which is mainly calcium phosphate) and a soft and flexible material called collagen (which is a protein). Collagen is also found in hair and finger nails. On its own it would not be much use in the skeleton but it can combine with hydroxyapatite to give bone the properties that are needed to support the body.

Synthetic Composites

People have been making composites for many thousands of years. One early example is mud bricks. Mud can be dried out into a brick shape to give a building material. It is strong if you try to squash it (it has good compressive strength) but it breaks quite easily if you try to bend it (it has poor tensile strength). Straw seems very strong if you try to stretch it, but you can crumple it up easily. By mixing mud and straw together it is possible to make bricks that are resistant to both squeezing and tearing and make excellent building blocks.

Another ancient composite is concrete. Concrete is a mix of aggregate (small stones or gravel), cement and sand. It has good compressive strength (it resists squashing). In more recent times it has been found that adding metal rods or wires to the concrete can increase its tensile (bending) strength. Concrete containing such rods or wires is called reinforced concrete.

Advanced Composite Materials

The first modern composite material was fibreglass. It is still widely used today for boat hulls, sports equipment, building panels and many car bodies. The matrix is a plastic and the reinforcement is glass that has been made into fine threads and often woven into a sort of cloth. On its own the glass is very strong but brittle and it will break if bent sharply. The plastic matrix holds the glass fibres together and also protects them from damage by sharing out the forces acting on them.

Some advanced composites are now made using carbon fibres instead of glass. These materials are lighter and stronger than fibreglass but more expensive to produce. They are used in aircraft structures and expensive sports equipment such as golf clubs. Carbon nanotubes have also been used successfully to make new composites. These are even lighter and stronger than composites made with ordinary carbon fibres but they are still extremely expensive. They do, however, offer possibilities for making lighter cars and aircraft (which will use less fuel than the heavier vehicles).

1.2 Nano composite Material

Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm), or structures having nano-scale repeat distances between the different phases that make up the material.

The idea behind nanocomposite is to use building blocks with dimensions in nanometre range to design and create new materials with unprecedented flexibility and improvement in their physical properties.

In the broadest sense this definition can include porous media, colloids, gels and copolymers, but is more usually taken to mean the solid combination of a bulk matrix and nano-dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the nanocomposite will differ markedly from that of the component materials.

Size of nanomaterials

Size limits for these effects have been proposed,

- <5 nm for catalytic activity,
- <20 nm for making a hard magnetic material soft,
- <50 nm for refractive index changes,
- <100 nm for achieving superparamagnetism, mechanical strengthening or restricting matrix dislocation movement.

1.3 SIGNIFICance of Nano composites

Nanocomposites are found in nature, for example in the structure of the abalone shell and bone. The use of nanoparticle-rich materials long predates the understanding of the physical and chemical nature of these materials. It is found that the depth of colour and the resistance to acids and bio-corrosion of ‘Maya blue’ paint, attributing it to a nanoparticle mechanism. From the mid-1950s nanoscale organo-clays have been used to control flow of polymer solutions (e.g. as paint viscosifiers) or the constitution of gels (e.g. as a thickening substance in cosmetics, keeping the preparations in homogeneous form). By the 1970s polymer/clay composites were used, although the term "nanocomposites" was not in common use.

In mechanical terms, nanocomposites differ from conventional composite materials due to the exceptionally high surface to volume ratio of the reinforcing phase and/or its exceptionally high aspect ratio. The reinforcing material can be made up of particles (e.g. minerals), sheets (e.g. exfoliated clay stacks) or fibres (e.g. carbon nanotubes or electrospun fibres).

The large amount of reinforcement surface area means that a relatively small amount of nanoscale reinforcement can have an observable effect on the macroscale properties of the composite. For example, adding carbon nanotubes improves the electrical and thermal conductivity. Other kinds of nanoparticulates may result in enhanced optical properties, dielectric properties, heat resistance or mechanical properties such as stiffness, strength and resistance to wear and damage.

In general, the nano reinforcement is dispersed into the matrix during processing. The percentage by weight (called mass fraction) of the nanoparticulates introduced can remain very low (on the order of 0.5% to 5%) due to the low filler percolation threshold, especially for the most commonly used non-spherical, high aspect ratio fillers (e.g. nanometer-thin platelets, such as clays, or nanometer-diameter cylinders, such as carbon nanotubes).

Classification of composites

Composites can be classified as:

1. Ceramic-matrix composites
2. Metal-matrix composites
3. Polymer-matrix composites
4. Magnetic composites

1.4 NeCESSITY OF ADVANCED materials

Composite materials are having many potential applications and become a part of our day-to-day life. Some of the needs of composites are discussed below:

1. Less Weight - Composites are light in weight, compared to most woods and metals. Their lightness is important in automobiles and aircraft, for example, where less weight means better fuel efficiency (more miles to the gallon). People who design airplanes are greatly concerned with weight, since reducing a craft’s weight reduces the amount of fuel it needs and increases the speeds it can reach. Some modern airplanes are built with more composites than metal.
2. Strength - Composites can be designed to be far stronger than aluminum or steel. Metals are equally strong in all directions. But composites can be engineered and designed to be strong in a specific direction.
3. Strength to Weight ratio - Strength-to-weight ratio is a material’s strength in relation to how much it weighs. Some materials are very strong and heavy, such as steel. Other materials can be strong and light, such as bamboo poles. Composite materials can be designed to be both strong and light. This property is why composites are used to build airplanes—which need a very high strength material at the lowest possible weight. A composite can be made to resist bending in one direction, for example. When something is built with metal, and greater strength is needed in one direction, the material usually must be made thicker, which adds weight. Composites can be strong without being heavy. Composites have the highest strength-to-weight ratios in structures today.
4. Corrosion Resistance - Composites resist damage from the weather and from harsh chemicals that can eat away at other materials. Composites are good choices where chemicals are handled or stored. Outdoors, they stand up to severe weather and wide changes in temperature.
5. Impact Strength - Composites can be made to absorb impacts—the sudden force of a bullet, for instance, or the blast from an explosion. Because of this property, composites are used in bulletproof vests and panels, and to shield airplanes, buildings, and military vehicles from explosions.
6. Design Flexibility - Composites can be molded into complicated shapes more easily than most other materials. This gives designers the freedom to create almost any shape or form. Most recreational boats today, for example, are built from fiberglass composites because these materials can easily be molded into complex shapes, which improve boat design while lowering costs. The surface of composites can also be molded to mimic any surface finish or texture, from smooth to pebbly.
7. Part Consolidation - A single piece made of composite materials can replace an entire assembly of metal parts. Reducing the number of parts in a machine or a structure saves time and cuts down on the maintenance needed over the life of the item.
8. Dimensional Stability - Composites retain their shape and size when they are hot or cool, wet or dry. Wood, on the other hand, swells and shrinks as the humidity changes. Composites can be a better choice in situations demanding tight fits that do not vary. They are used in aircraft wings, for example, so that the wing shape and size do not change as the plane gains or losses altitude.
9. Nonconductivity - Composites are nonconductive, meaning they do not conduct electricity. This property makes them suitable for such items as electrical utility poles and the circuit boards in electronics. If electrical conductivity is needed, it is possible to make some composites conductive.
10. Nonmagnetic - Composites contain no metals; therefore, they are not magnetic. They can be used around sensitive electronic equipment. The lack of magnetic interference allows large magnets used in MRI (magnetic resonance imaging) equipment to perform better. Composites are used in both the equipment housing and table. In addition, the construction of the room uses composites rebar to reinforced the concrete walls and floors in the hospital.
11. Radar Transparent - Radar signals pass right through composites, a property that makes composites ideal materials for use anywhere radar equipment is operating, whether on the ground or in the air. Composites play a key role in stealth aircraft which is nearly invisible to radar.
12. Less Thermal Conductive - Composites are good insulators—they do not easily conduct heat or cold. They are used in buildings for doors, panels, and windows where extra protection is needed from severe weather.
13. Durablity - Structures made of composites have a long life and need little maintenance. We do not know how long composites last, because we have not come to the end of the life of many original composites. Many composites have been in service for half a century.

1.5 Category of Composites

Typical engineered composite materials include:

- Reinforced concrete and masonry (brickwork)
- Composite wood such as plywood
- Reinforced plastics, such as fibre-reinforced polymer or fiberglass
- Ceramic matrix composites (composite ceramic and metal matrices)
- Metal matrix composites, and
- Advanced composite materials

Classification Based on Matrix Materials

Based on the matrices used composites can be classified as polymer matrix composites, metal matrix composites, ceramic matrix composites, and carbon and graphite matrix composites.

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Figure 1.2: Classification of Composites Based on Matrix Materials

Classification Based on the Reinforcement Material

Based on the reinforcement material used, composites can be classified as given in the figure below:

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Figure 1.3: Classification of Composites Based on the Reinforcement Materials

Classification Based on the size and orientation of Reinforcement Material

Based on the size and orientation of the reinforcement material, composites can be classified as given in the figure below:

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Figure 1.4: Classification of composites based on the size and orientation of reinforcement

1.6 Polymer matrix composite

A polymer matrix composite (PMC) is a composite material composed of a variety of short or continuous fibers bound together by an organic polymer matrix. PMCs are designed to transfer loads between fibers through the matrix.

Categories

PMCs are divided into two categories: reinforced plastics, and advanced composites. The two categories differ in their level of mechanical properties. Reinforced plastics typically consist of polyester resins reinforced with low-stiffness glass fibers. Advanced Composites consist of fiber and matrix combinations that yield superior strength and stiffness. The PMC is designed so that the mechanical loads that are being applied to the material is being supported by the reinforcements. The function of the matrix is to bond the fibers together and to transfer loads between them.

Composition

Fibers: PMCs contain about 60 percent reinforcing fiber by volume. The fibers that are commonly found and used within PMCs include fiberglass, graphite and aramid.

Fiberglass has a relatively low stiffness at the same time exhibits a competitive tensile strength compared to other fibers. The cost of fiberglass is also dramatically lower than the other fiber which is why fiberglass is one of the most widely used fiber. The reinforcing fibers have their highest mechanical properties along their lengths rather than their widths. Thus, the reinforcing fibers maybe arranged and oriented in different forms and directions to provide different physical properties and advantages based on the application.

Matrix: The properties of the matrix determine the resistance of the PMC to processes that includes impact damage, water absorption, chemical attack, and high-temperature creep. This means that the matrix of the PMC is typically the weak link. The matrix of PMCs consists of resin that is either thermosets or thermoplastics.

Benefits of PMC’s

Some of the advantages with PMCs include their lightweight, high stiffness and their high strength along the direction of their reinforcements. Other advantages are good abrasion resistance and good corrosion resistance.

Disadvantages of PMC’s

- Environmental degradation
- Moisture absorption from environment causes swelling in the polymer as well as a decrease of Tg.
- The moisture absorption increases at moderately high temperatures. These hydrothermal effects can lead to internal stresses in the presence of fibres in polymer composites.
- A thermal mismatch between polymer and fibre may cause cracking or debonding at the interface.

1.7 Metal matrix composite

A metal matrix composite (MMC) is composite material with at least two constituent parts, one being a metal necessarily, the other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite. An MMC is complementary to a cermet (A cermet is a composite material composed of ceramic (cer) and metal (met) materials).

Constituents

MMCs are made by dispersing a reinforcing material into a metal matrix. The reinforcement surface can be coated to prevent a chemical reaction with the matrix. For example, carbon fibers are commonly used in aluminium matrix to synthesize composites showing low density and high strength. However, carbon reacts with aluminium to generate a brittle and water-soluble compound Al4C3 on the surface of the fibre. To prevent this reaction, the carbon fibres are coated with nickel or titanium boride.

Matrix: In structural applications, the matrix is usually a lighter metal such as aluminum, magnesium, or titanium, and provides a compliant support for the reinforcement. In high-temperature applications, cobalt and cobalt–nickel alloy matrices are common.

Reinforcement: The reinforcement can be either continuous, or discontinuous. Discontinuous MMCs can be isotropic, and can be worked with standard metalworking techniques, such as extrusion, forging, or rolling. In addition, they may be machined using conventional techniques, but commonly would need the use of polycrystaline diamond tooling (PCD).

Continuous reinforcement uses monofilament wires or fibers such as carbon fiber or silicon carbide. Because the fibers are embedded into the matrix in a certain direction, the result is an anisotropic structure in which the alignment of the material affects its strength. One of the first MMCs used boron filament as reinforcement. Discontinuous reinforcement uses "whiskers", short fibers, or particles. The most common reinforcing materials in this category are alumina and silicon carbide.

1.8 Ceramic matrix composites

They consist of ceramic fibres embedded in a ceramic matrix. Ceramic matrix composites (CMCs) are a subgroup of composite materials as well as a subgroup of ceramics. The matrix and fibres can consist of any ceramic material, whereby carbon and carbon fibres can also be considered a ceramic material.

Significance of Ceramic Matrix Composites

The motivation to develop CMCs was to overcome the problems associated with the conventional technical ceramics like alumina, silicon carbide, aluminium nitride, silicon nitride or zirconia – they fracture easily under mechanical or thermo-mechanical loads because of cracks initiated by small defects or scratches. The crack resistance is – like in glass – very low. To increase the crack resistance or fracture toughness, particles (so-called monocrystalline whiskers or platelets) were embedded into the matrix. However, the improvement was limited, and the products have found application only in some ceramic cutting tools.

So far only the integration of long multi-strand fibres has drastically increased the crack resistance, elongation and thermal shock resistance, and resulted in several new applications.

Composition

Carbon (C), special silicon carbide (SiC), alumina (Al2O3) and mullite (Al2O3–SiO2) fibres are most commonly used for CMCs.

Recently Ultra-high-temperature ceramics (UHTCs) were investigated as ceramic matrix in a new class of CMC so-called Ultra-high Temperature Ceramic Matrix Composites (UHTCMC) or Ultra-high Temperature Ceramic Composites (UHTCC).

Available Ceramic Matrix Composites

Generally, CMC names include a combination of type of fibre/type of matrix. For example, C/C stands for carbon-fibre-reinforced carbon (carbon/carbon) or C/SiC for carbon-fibre-reinforced silicon carbide. Sometimes the manufacturing process is included, and a C/SiC composite manufactured with the liquid polymer infiltration (LPI) process is abbreviated as LPI-C/SiC.

The important commercially available CMCs are C/C, C/SiC, SiC/SiC and Al2O3/Al2O3. They differ from conventional ceramics in the following properties, presented in more detail below:

- Elongation to rupture up to 1%
- Strongly increased fracture toughness
- Extreme thermal shock resistance
- Improved dynamical load capability
- Anisotropic properties following the orientation of fibers

1.9 Particle Reinforced Composites

Particle reinforcing in composites (or particulate reinforced composites) is a less effective means of strengthening than fibre reinforcement. Particulate reinforced composites achieve gains in stiffness primarily, but also can achieve increases in strength and toughness. In all cases the improvements are less than would be achieved in a fibre reinforced composite.

Forms of Particle Reinforced Composites

There are many different forms of particulate composites. The particulates can be very small particles (< 0.25 microns), chopped fibers (such as glass), platelets, hollow spheres, or new materials such as bucky balls or carbon nano-tubes. In each case, the particulates provide desirable material properties and the matrix acts as binding medium necessary for structural applications.

Benefits of Particle Reinforced Composites

The principal advantage of particle reinforced composites is their low cost and ease of production and forming.

Particulate composites offer several advantages. They provide reinforcement to the matrix material thereby strengthening the material. The combination of reinforcement and matrix can provide for very specific material properties. For example, the inclusion of conductive reinforcements in a plastic can produce plastics that are somewhat conductive. Particulate composites can often use more traditional manufacturing methods such as injection moulding which reduces cost.

Applications of Particle Reinforced Composites

Particulate reinforced composites find applications where high levels of wear resistance are required such as road surfaces. The hardness of cement is increased significantly by adding gravel as reinforcing filler. The most common particulate composite materials are reinforced plastics which are used in a variety of industries.

Automotive: Glass reinforced plastics are used in many automotive applications including body panels, bumpers, dashboards, and intake manifolds. Brakes are made of particulate composite composed of carbon or ceramics particulates.

Consumer Products: Many of the plastic components we use in daily life are reinforced in some way. Appliances, toys, electrical products, computer housings, cell phone casings, office furniture, helmets, etc. are made from particulate reinforced plastics.

Benefits:

- Improved material properties
- Tailored material properties
- Manufacturing flexibility

1.10 Fiber-reinforced composite

A fiber-reinforced composite (FRC) is a composite building material that consists of three components:

(i) the fibers as the discontinuous or dispersed phase,
(ii) the matrix as the continuous phase, and
(iii) the fine interphase region, also known as the interface.

FRC is high-performance fiber composite achieved and made possible by cross-linking cellulosic fiber molecules with resins in the FRC material matrix through a proprietary molecular re-engineering process, yielding a product of exceptional structural properties.

The most common man-made composites are composed of glass or carbon fiber in a plastic resin. Resins can be of the form of thermoset or thermoplastic materials which each have their own unique advantages and disadvantages. The glass or carbon fibers are significantly stronger than the plastic matrix but they also tend to be brittle. A composite construction, therefore, allows one to take advantage of the excellent stiffness and strength properties of glass or carbon by embedding the fibers in a more compliant matrix.

When a composite structure is manufactured, a “dry” fiber and a “wet” resin are used. The dry fiber will be wetted with the resin and placed in resin and dried uniformly.

The failure mechanisms in FRC materials include delamination, intralaminar matrix cracking, longitudinal matrix splitting, fiber/matrix debonding, fiber pull-out, and fiber fracture.

1.11 BENEFITS AND Applications of Composites

Advantages

The biggest advantage of modern composite materials is that they are light as well as strong. By choosing an appropriate combination of matrix and reinforcement material, a new material can be made that exactly meets the requirements of a particular application. Composites also provide design flexibility because many of them can be moulded into complex shapes.

Applications

- Aerospace Industries (Carbon/Epoxy PMCs): Antenna structures, Solar reflectors, Satellite structures, Radar, Rocket engines, etc. of space craft. Jet engines, Turbine blades, Turbine shafts, Compressor blades, Airfoil surfaces, Wing box structures, Fan blades, Flywheels, Engine bay doors, Rotor shafts in helicopters, Helicopter transmission structures, etc. of aircraft.

- Automobile Industry (Epoxy based PMCs): Engines, bodies, Piston, cylinder, connecting rod, crankshafts, bearing materials, etc.
- Springs and bumper systems (Reinforced Thermosets)
- Tooling (Epoxy based PMCs)
- Miscellaneous: Bearing materials, Pressure vessels, Abrasive materials, Electrical machinery, Truss members, Cutting tools, Electrical brushes, etc. Fiberglass reinforced plastic has been used for boat hulls, fishing rods, tennis rackets, golf club shafts, helmets, skis, bows and arrows

More recently, researchers have also begun to actively include sensing, actuation, computation and communication into composites, which are known as Robotic Materials.

1.12 PROCESSING TECHNIQUES for glass, carbon and ceramic fibres

1. Fibre-glass

Glass fibre or Fiberglass refers to a group of products made from individual glass fibers combined into a variety of forms. Glass fibers can be divided into two major groups according to their geometry: continuous fibers used in yarns and textiles, and the discontinuous (short) fibers used as blankets, or boards for insulation and filtration.

Fiberglass can be formed into yarn much like wool or cotton, and woven into fabric which is sometimes used for draperies. Fiberglass textiles are commonly used as a reinforcement material for moulded and laminated plastics.

Fiberglass wool, a thick, fluffy material made from discontinuous fibers, is used for thermal insulation and sound absorption. It is commonly found in ship and submarine bulkheads and hulls; automobile engine compartments and body panel liners; in furnaces and air conditioning units; acoustical wall and ceiling panels; and architectural partitions.

Fiberglass can be tailored for specific applications such as Type E (electrical), used as electrical insulation tape, textiles and reinforcement; Type C (chemical), which has superior acid resistance, and Type T, for thermal insulation.

The Manufacturing Process

i) Raw Materials

The basic raw materials for fiberglass products are silica sand, limestone, and soda ash. Other ingredients may include calcined alumina, borax, feldspar, nepheline syenite, magnesite, and kaolin clay, among others.

Silica sand is used as the glass former, and soda ash and limestone help primarily to lower the melting temperature. Other ingredients are used to improve certain properties, such as borax for chemical resistance.

ii) Melting

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Figure 1.5 Production of Fibre glass

Once the batch is prepared, it is fed into a furnace for melting. The furnace may be heated by electricity, fossil fuel, or a combination of the two. Temperature must be precisely controlled to maintain a smooth, steady flow of glass. The molten glass must be kept at a higher temperature (about 2500°F [1371°C]) than other types of glass in order to be formed into fiber. Once the glass becomes molten, it is transferred to the forming equipment via a channel (forehearth) located at the end of the furnace.

iii) Forming into fibers

Continuous fibres: A long, continuous fiber can be produced through the continuous-filament process. After the glass flows through the holes in the bushing, multiple strands are caught up on a high-speed winder. The winder revolves at about 2 miles (3 km) a minute, much faster than the rate of flow from the bushings. The tension pulls out the filaments while still molten, forming strands a fraction of the diameter of the openings in the bushing. A chemical binder is applied, which helps keep the fiber from breaking during later processing. The filament is then wound onto tubes. It can now be twisted and plied into yarn.

Chopped fibre: Instead of being formed into yarn, the continuous or long-staple strand may be chopped into short lengths. The strand is mounted on a set of bobbins, called a creel, and pulled through a machine which chops it into short pieces. The chopped fiber is formed into mats to which a binder is added

Glass wool: The rotary or spinner process is used to make glass wool. In this process, molten glass from the furnace flows into a cylindrical container having small holes. As the container spins rapidly, horizontal streams of glass flow out of the holes. The molten glass streams are converted into fibers by a downward blast of air, hot gas, or both. The fibers fall onto a conveyor belt, where they interlace with each other in a fleecy mass.

2. Carbon Fibre

The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile (PAN). The remaining 10% are made from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms.

The process for making carbon fibers is part chemical and part mechanical. The precursor is drawn into long strands or fibers and then heated to a very high temperature without allowing it to come in contact with oxygen. Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled. This process is called carbonization and leaves a fiber composed of long, tightly interlocked chains of carbon atoms with only a few non-carbon atoms remaining.

i) Stabilizing

Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern.

The stabilizing chemical reactions are complex and involve several steps, some of which occur simultaneously. They also generate their own heat, which must be controlled to avoid overheating the fibers.

Commercially, the stabilization process uses a variety of equipment and techniques. In some processes, the fibers are drawn through a series of heated chambers. In others, the fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.

ii) Carbonizing

Once the fibers are stabilized, they are heated to a temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with a gas mixture that does not contain oxygen. The lack of oxygen prevents the fibers from burning in the very high temperatures.

The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering.

As the fibers are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form of various gases including water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others.

As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less parallel to the long axis of the fiber.

iii) Treating the surface

After carbonizing, the fibers have a surface that does not bond well with the epoxies and other materials used in composite materials. To give the fibers better bonding properties, their surface is slightly oxidized. The addition of oxygen atoms to the surface provides better chemical bonding properties and also etches and roughens the surface for better mechanical bonding properties.

Oxidation can be achieved by immersing the fibers in various gases such as air, carbon dioxide, or ozone; or in various liquids such as sodium hypochlorite or nitric acid.

The fibers can also be coated electrolytically by making the fibers the positive terminal in a bath filled with various electrically conductive materials. The surface treatment process must be carefully controlled to avoid forming tiny surface defects, such as pits, which could cause fiber failure.

iv) Sizing

After the surface treatment, the fibers are coated to protect them from damage during winding or weaving. This process is called sizing. Coating materials are chosen to be compatible with the adhesive used to form composite materials. Typical coating materials include epoxy, polyester, nylon, urethane, and others.

The coated fibers are wound onto cylinders called bobbins. The bobbins are loaded into a spinning machine and the fibers are twisted into yarns of various sizes.

3. Ceramics

Abbildung in dieser Leseprobe nicht enthalten

Figure 1.6: Classification of non-metallic inorganic fibres

The definition of ceramic is often restricted to inorganic non-metallic polycrystalline solids, as opposed to the noncrystalline glasses. Figure given below shows a general classification of non-metallic inorganic fibres including ceramic fibres.

First ceramics were the pottery objects made up of clay and were developed for refractory insulation. Traditional ceramic has kaolinite as clay mineral that is an alumina whereas modern ceramics include silicon carbide, tungsten carbide and many more. Silicon carbide and aluminium oxide (alumina) fibres are commercially being produced, while several other ceramic fibres are either being made on pilot scale or are under various stages of development.

Ceramic materials are hard, have low densities (compared to metals), high compressive strength and very good thermal resistance and strength at higher temperature.

Production of ceramics fibres

Conventional processes for the fabrication of bulk ceramics, which include powder compaction and sintering, cannot be used for making fine fibres. Also, the conventional spinning and drawing from a melt cannot be used for ceramics as their melting points can exceed 2000°C. The ceramic fibres can be produced by either a direct or indirect process.

Abbildung in dieser Leseprobe nicht enthalten

Figure1.7: Sintering process of producing objects from particles

1. Direct Process

The direct production of fine ceramic fibres requires the spinning of precursors (salt solution, sols or precursor melts) into fibres, which are then heat treated and pyrolysed for a very short time.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1.8: Direct method for obtaining ceramic fibres

[...]

Details

Seiten
89
Jahr
2018
ISBN (eBook)
9783668875906
ISBN (Buch)
9783668875913
Sprache
Englisch
Katalognummer
v452980
Institution / Hochschule
Srinivas School of Engineering – Srinivas Institute of Technology
Note
1
Schlagworte
applications nanocomposites

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Titel: Advanced Materials