Unit – 5
Plastic
• Thermoplastics have the simplest molecular structure, with chemically independent macromolecules
• By heating, they are softened or melted, then shaped, formed, welded, and solidified when cooled.
• Multiple cycles of heating and cooling can be repeated without severe damage, allowing reprocessing and recycling.
• Often some additives or fillers are added to the thermoplastic to improve specific properties such as thermal or chemical stability, UV resistance, etc.
• Composites are obtained by using short, long or continuous fibres.
• Alloys of compatible thermoplastics allow applications to benefit from the attractive properties of each polymer while masking their defects.
• Some thermoplastics are crosslinkable and are used industrially in their two forms, thermoplastic and thermoset; for example, the polyethylenes or the vinylacetate-ethylene copolymers (VAE) (the links created between the chains limit their mobility and possibilities of relative displacement). Thermoplastic consumption is roughly 80% or more of the total plastic consumption.
Plastics, also known as thermosetting resins, are rigid polymeric materials that are resistant to higher temperatures than ordinary thermoplastics. They are petrochemical materials that irreversibly cure. The cure may be brought on by heat, generally above 392°F (200°C), chemical reaction or suitable irradiation. It is used as adhesives as well as in semiconductors and integrated circuits. The International Union of Pure and Applied Chemistry (IUPAC) defines a thermosetting plastic as petrochemicals in an indulgent solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing.
Thermosetting plastics are normally made up of lines of polymers which are highly cross-linked. The heavily cross-linked structure produced by chemical bonds in thermoset materials is directly responsible for the high mechanical and physical strength compared with thermoplastics or elastomers. However, it provides poor elasticity or elongation of the material—once hardened, a thermoset resin cannot be reheated and melted to be shaped differently. The cross-linking process eliminates the risk of the product remelting when heat is applied, making thermosets ideal for high-heat applications such as electronics and appliances. Since their shape is permanent, they tend not to be recyclable as a source for newly made plastic.
Common thermosetting plastics or resins include:
Advantages of ceramics
Disadvantages of ceramics
Types of ceramics
They are mainly of two types based on their atomic structure.
They can also be classified into three different material categories.
Structure of ceramic
Structural Ceramics can be classified into three distinct categories
Oxides e.g., Alumina, Zirconia and their derivatives
Non-Oxides e.g., Carbides, borides, nitrides.
Composites
Particulate and fibre reinforced, combination of oxides and non- oxides.
Properties of ceramics
Examples of ceramics
Applications of ceramics
A composite material is a combination of two materials with different physical and chemical properties. When they are combined they create a material which is specialised to do a certain job, for instance to become stronger, lighter or resistant to electricity. They can also improve strength and stiffness. The reason for their use over traditional materials is because they improve the properties of their base materials and are applicable in many situations
Agglomeration, the sticking of particles to one another or to solid surfaces, is a natural phenomenon. For powders and bulk solids, agglomeration can be unwanted, resulting in uncontrolled build up, caking, bridging, or lumping. It can also be a beneficial process, utilizing the controlled enlargement of particles to improve powder properties and obtain high-quality products.
Size enlargement through agglomeration is one of the four unit operations of mechanical process engineering (powder and bulk solids technologies). It involves combining particles to create products with new particle sizes. Products can come in many different forms—granules, tablets, briquettes, extrudates, pellets, bricks, or compacts—and/or they may have brand-related names.
Cermets
A cermet is a composite material composed of ceramic and metallic materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder for an oxide, boride, or carbide. Generally, the metallic elements used are nickel, molybdenum, and cobalt. Depending on the physical structure of the material, cermets can also be metal matrix composites, but cermets are usually less than 20% metal by volume. Cermets are used in the manufacture of resistors, capacitors, and other electronic components which may experience high temperature. Cermets are being used instead of tungsten carbide in saws and other brazed tools due to their superior wear and corrosion properties. Titanium nitride, Titanium carbonitride, titanium carbide and similar can be brazed like tungsten carbide if properly prepared however they require special handling during grinding.
Reinforced concrete is a combination of traditional cement concrete with reinforcements (steel bar). This combination is made to use the compressive strength of concrete and tensile strength of steel at the same time, hence, work together to resist many types of loading. The term reinforced is used because the steel reinforces the concrete and makes it an even stronger construction material.
Reinforced concrete is a combination of traditional cement concrete with reinforcements (steel bar). This combination is made to use the compressive strength of concrete and tensile strength of steel at the same time, hence, work together to resist many types of loading. The term reinforced is used because the steel reinforces the concrete and makes it an even stronger construction material.
Advantages of Reinforced Concrete
1. Strength
Reinforced concrete has very good strength in tension as well as compression. This makes concrete a desired construction material.
2. Economical
Concrete constituents are widely available worldwide and inexpensive. Similarly, the production cost of concrete is very low. There is an overall economy by using reinforced concrete because its maintenance cost is low due to the long-lasting nature of reinforced concrete.
Reinforced concrete durability, resilience, low maintenance requirements and energy efficiency, concrete structures reduce operating costs related to operational energy consumption, maintenance, and rebuilding.
3.Durability
Reinforced Concrete structures are durable if designed and laid properly. The material is not affected by weather such as rainfall and snow, and they can last up to 100 years.
Due to low permeability, concrete can resist chemicals dissolved in water such as sulfates, chloride and carbon dioxide, which may cause corrosion in concrete, without serious deterioration.
That is why reinforced concrete is ideal to underwater and submerged applications like for building structures, pipelines, dams, canals, linings and waterfront structures.
4.Fire Resistance
The nature of concrete does not allow it to catch fire or burn. It can withstand heat for 2–6 hours enabling sufficient time for rescue operations in case of fire. Reinforced concrete buildings are more fire resistant than other commonly used construction materials like steel and wood. It is suitable to fireproof steel and used in high temperature and blast applications.
Fibre-reinforced plastic (FRP) is a composite material made up of polymer that is supported with fibres for added strength. It is commonly used in industries such as aerospace, construction and marine to build structures that require added resistance to force in order to prevent deformation. Fibre-reinforced plastic is useful in terms of corrosion protection because it helps in preventing corrosion due to force application and deformation such as stress corrosion cracking.
Fibre-reinforced plastic may also be known as fibre-reinforced polymer.
Common Fibres include:
Fibre reinforced plastics find wide applications in the automotive, aerospace, construction and marine sectors. Glass fibre reinforced plastics are a very good option for the power industry as they are devoid of any magnetic field and can offer considerable resistance to electric sparks. The uses are diversifying, a phenomenon evident in the entry of carbon fibres in sports goods, gliders, and fishing rods, along with Japan’s application of FRPs to hydraulic gates.
Properties of composite materials:
TYPES OF COMPOSITE MATERIALS:
1.Polymer Matrix Composites (PMCs):
It consists of polymer resin within the type of a matrix, the variety and the greatest amount is being used.
Glass fibre-reinforced polymer composites (GFRP) are the largest amount of produced carbon fibre-reinforced polymer composites (CFRP).
2.High-Performance Composites.
They are Aramid Fibre Reinforced Polymer Composites (AFRPs).
3. Metal Matrix Composites (MMCs):
It is also extra ductile compared to matrix reinforcement.
Reinforcement can improve strength, abrasion resistance, creep resistance, thermal conductivity and dimensional stability of composite composites.
Metal matrix composites are additional resistance to extreme working temperatures, non-flammability, and corrosion of organic fluids.
4.Ceramics Metal Composites (CMCs):
For use in high temperature and severe stress applications, e.g., automobile and aircraft gas turbine engines.
Metal matrix composites, at present though generating a wide interest in research fraternity, are not as widely in use as their plastic counterparts. High strength, fracture toughness and stiffness are offered by metal matrices than those offered by their polymer counterparts. They can withstand elevated temperature in corrosive environment than polymer composites. Most metals and alloys could be used as matrices and they require reinforcement materials which need to be stable over a range of temperature and non-reactive too. However, the guiding aspect for the choice depends essentially on the matrix material. Light metals form the matrix for temperature application and the reinforcements in addition to the aforementioned reasons are characterized by high moduli. Most metals and alloys make good matrices. However, practically, the choices for low temperature applications are not many. Only light metals are responsive, with their low density proving an advantage. Titanium, Aluminium and magnesium are the popular matrix metals currently in vogue, which are particularly useful for aircraft applications. If metallic matrix materials have to offer high strength, they require high modulus reinforcements. The strength-to weight ratios of resulting composites can be higher than most alloys. The melting point, physical and mechanical properties of the composite at various temperatures determine the service temperature of composites. Most metals, ceramics and compounds can be used with matrices of low melting point alloys. The choice of reinforcements becomes more stunted with increase in the melting temperature of matrix materials.
Fiber-reinforced polymers (FRPs) are a type of composite plastics made of a matrix or binding agent reinforced by a fiber material. The FRP process thus entails two steps: making the fibrous material and bonding the material with a polymer plastic.
Fiber Manufacture
Fiber preforms are manufactured through weaving, braiding, stitching, and knitting. Weaving is used in making both two-dimensional and three-dimensional fibers and is suitable for the manufacture of high-value and narrow-width products.
It has its downsides, though. For one, weaving multilayer fibers is time-consuming and fairly more expensive. Also, it is difficult to create fabrics with fibers that are oriented to each other at anything other than 90-degree angles.
Braiding is way better in that aspect as it allows for fibers to be aligned at 45 degrees to each other. Through two-step braiding, manufacturers can create right about any shape of preform.
Knitting typically produces two-dimensional fabric, but the creation of multilayer fabric is possible with machines fitted with more than one needle bed.
Stitching is perhaps the most straightforward technique of creating preforms and is revered for its suitability in both prepreg and dry fabric stitching.
Forming Processes
Bonding the fiber and the polymer plastic is the second and most critical part of FRP manufacture. It can be achieved through several processes, including compression molding, bladder molding, wet layup, mandrel molding, chopper gun, autoclave and vacuum bag, filament winding, and pultrusion.
We are going to focus on pultrusion, which is arguably the most popular method of FRP production at the moment.
Reinforcement Wet-Out
The fiber reinforcements are held in continuous filament mats (CFM) creels and are delivered in rolls that have been split to required widths. The work of the CMF creels is basically to stage the reinforcements before they are fed into guide plates.
The guide plates, on the other hand, position the rovings and unrolled mats before putting them in the liquid resin bath. The resin bath is a concoction of a resin, catalysts, fillers, wetting agents, and pigments.
The pultrusion resin bath's interior is built to optimize the reinforcement wet-out. The wet-out is optimized even further by keeping the reinforcements separated in the resin bath.
As the saturated reinforcements exit the bath, they are shaped into flat sheets before being put in the performer.
Stage 2: Reshaping
In the preformer, the sheets of reinforcements and roving are reshaped to resemble the die cavity as closely as possible. That is primarily the work of the preformer: to shape the reinforcements before they enter the die.
The preformer is a very crucial step in pultrusion as it dictates the robustness of the final product. It is a fairly lengthy process that calls for little to no human intervention. Some FRP manufactures skip this process, including the resin bath phase, and use resin injection instead.
That said, either process aims to have the reinforcements adequately saturated before entering the heated die.
Stage 3: Composite Curing
The cross-section of the wetted reinforcements is typically bigger than the cavity of the die so that it squeezes in and is compacted into the desired shape and size.
The surface of the die's cavity is hardened by chrome-plating or nitriding to prevent abrasion from the reinforcements squeezing through it. Resin curing acts as the limiting factor for the line speed.
Extremely thick parts may run as slowly as four inches per minute, while thinner parts could exceed 100 inches in one minute. Parts that are not cured well may offer inferior mechanical properties.
Stage 4: Cooling
As it exits the cavity, the composite is usually very hot (between 300 F and 400 F), from all the heating in the die, and must be cooled before a puller grips it.
The cooling is accomplished by natural convection, i.e. stretching out the distance between the puller and the die or by forcing cold water or air on the part that has not been gripped by the puller.
Most FRP manufacturers use two types of pulling systems – the reciprocating type and the caterpillar counter-rotating type – to ensure the process is moving continuously.
The pull forces weathered by these systems can be anything between 200 pounds and 100,000 pounds.
Stage 5: Cutting the Parts
The last step of the process involves cutting the parts while the line is in motion. This is accomplished by using a travelling cut off saw, which moves in the same speed and direction as the part being cut.
Upon making the cross cut, the saw moves back to its original position and waits for another cut to be triggered. The part cut length and other parameters such as line speed, gripper forces, and die temperatures are all calibrated and changed from a control panel.
Different part sizes have unique requisite settings that must be entered before the processing begins.
References:
1. Materials Science and Engineering by V.Raghavan, Prentice Hall of India Pvt.Ltd.
2. Elements of Materials Science & Engineering by Van Vlack, Pearson
3. Mechanical Metallurgy by Dieter, Tata MacGraw Hill
4. Composite Material science and Engineering by K. K. Chawla, Springer
5. Material Science and Metallurgy, by U. C. Jindal, Pearson
