Pdf on thermoplastics

Download Download PDF. Translate PDF. The molecules that compose plastics are long carbon chains that give plastics many of their useful properties. In general, materials that are made up of long, chainlike molecules are called polymers.

Plastics can be made hard as stone, strong as steel, transparent as glass, light as wood, and elastic as rubber. Plastics are also lightweight, waterproof, chemical resistant, and produced in almost any color. More than 50 families of plastics have been produced, and new types are currently under development. Colorful Plastics Assorted, colorful plastic items line a market stall in the Indian city of Mumbai Bombay. Plastics, synthetic resins made of large organic chains, or polymers, are extremely durable and lightweight.

Petroleum is refined to produce single organic molecules, called monomers, that are then combined to form resinous polymers. These polymers are molded or extruded to make plastic articles. For instance, nylons are plastics that are separated by different properties, costs, and the manufacturing processes used to produce them. Also like metals, some plastics can be alloyed, or blended, to combine the advantages possessed by several different plastics. For example, some types of impact-resistant shatterproof plastics and heat-resistant plastics are made by blending different plastics together.

Plastics are moldable, synthetic chemically-fabricated materials derived mostly from fossil fuels, such as oil, coal, or natural gas. The raw forms of other materials, such as glass, metals, and clay, are also moldable. The key difference between these materials and plastics is that plastics consist of long molecules that give plastics many of their unique properties, while glass, metals, and clay consist of short molecules.

Many people sleep on pillows and mattresses filled with a type of plastic—either cellular polyurethane or polyester. At night, people sleep under blankets and bedspreads made of acrylic plastics, and in the morning, they step out of bed onto polyester and nylon carpets. The cars we drive, the computers we use, the utensils we cook with, the recreational equipment we play with, and the houses and buildings we live and work in all include important plastic components.

Telephones, textiles, compact discs, paints, plumbing fixtures, boats, and furniture are other domestic products made of plastics. In the volume of plastics produced in the United States surpassed the volume of domestically produced steel. The aerospace industry uses plastics to make strategic military parts for missiles, rockets, and aircraft. Plastics are also used in specialized fields, such as the health industry, to make medical instruments, dental fillings, optical lenses, and biocompatible joints.

They are lighter than many materials of comparable strength, and unlike metals and wood, plastics do not rust or rot.

Most plastics can be produced in any color. They can also be manufactured as clear as glass, translucent transmitting small amounts of light , or opaque impenetrable to light.

Plastics have a lower density than that of metals, so plastics are lighter. Most plastics vary in density from 0. Plastic can also be reinforced with glass and other fibers to form incredibly strong materials. For example, nylon reinforced with glass can have a tensile strength resistance of a material to being elongated or pulled apart of up to Mega Pascal 24, psi.

Plastics have some disadvantages. When burned, some plastics produce poisonous fumes. Because of their molecular stability, plastics do not easily break down into simpler components. As a result, disposal of plastics creates a solid waste problem see Plastics and the Environment below.

One type of plastic, known as polyethylene, is composed of extremely long molecules that each contain over , carbon atoms. These long, chainlike molecules give plastics unique properties and distinguish plastics from materials, such as metals, that have short, crystalline molecular structures. Although some plastics are made from plant oils, the majority are made from fossil fuels.

Fossil fuels contain hydrocarbons compounds containing hydrogen and carbon , which provide the building blocks for long polymer molecules. These small building blocks, called monomers, link together to form long carbon chains called polymers. The process of forming these long molecules from hydrocarbons is known as polymerization. The molecules typically form viscous, sticky substances known as resins, which are used to make plastic products.

Ethylene, for example, is a gaseous hydrocarbon. When it is subjected to heat, pressure, and certain catalysts substances used to enable faster chemical reactions , the ethylene molecules join together into long, repeating carbon chains.

These joined molecules form a plastic resin known as polyethylene. Joining identical monomers to make carbon chains is called addition polymerization, because the process is similar to stringing many identical beads on a string.

Plastics made by addition polymerization include polyethylene, polypropylene, polyvinyl chloride, and polystyrene. Joining two or more different monomers of varying lengths is known as condensation polymerization, because water or other by-products are eliminated as the polymer forms.

Condensation polymers include nylon polyamide , polyester, and polyurethane. For example, elastomers are plastics composed of long, tightly twisted molecules. These coiled molecules allow the plastic to stretch and recoil like a spring. Rubber bands and flexible silicone caulking are examples of elastomers. These side chains give plastics some distinguishing characteristics. For example, when chlorine atoms substitute for hydrogen atoms along the carbon chain, the result is polyvinyl chloride, one of the most versatile and widely used plastics in the world.

The addition of chlorine makes this plastic harder and more heat resistant. Different plastics have advantages and disadvantages associated with the unique chemistry of each plastic.

For example, longer polymer molecules become more entangled like spaghetti noodles , which gives plastics containing these longer polymers high tensile strength and high impact resistance. However, plastics made from longer molecules are more difficult to mold. This table lists the monomers for several major plastics, as well as the properties and uses of each type of plastic.

These terms refer to the different ways these types of plastics respond to heat. Thermoplastics can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics, on the other hand, harden permanently after being heated once. The reason for the difference in response to heat between thermoplastics and thermosetting plastics lies in the chemical structures of the plastics.

Thermoplastic molecules, which are linear or slightly branched, do not chemically bond with each other when heated. Instead, thermoplastic chains are held together by weak van der Waal forces weak attractions between the molecules that cause the long molecular chains to clump together like piles of entangled spaghetti.

Thermoplastics can be heated and cooled, and consequently softened and hardened, repeatedly, like candle wax. For this reason, thermoplastics can be remolded and reused almost indefinitely. Thermosetting plastics consist of chain molecules that chemically bond, or cross- link, with each other when heated. When thermosetting plastics cross-link, the molecules create a permanent, three-dimensional network that can be considered one giant molecule.

Once cured, thermosetting plastics cannot be remelted, in the same way that cured concrete cannot be reset. The different molecular structures of thermoplastics and thermosetting plastics allow manufacturers to customize the properties of commercial plastics for specific applications. Because thermoplastic materials consist of individual molecules, properties of thermoplastics are largely influenced by molecular weight. For instance, increasing the molecular weight of a thermoplastic material increases its tensile strength, impact strength, and fatigue strength ability of a material to withstand constant stress.

Instead, many properties of thermosetting plastics are determined by adding different types and amounts of fillers and reinforcements, such as glass fibers.

Thermoplastics may be grouped according to the arrangement of their molecules. Highly aligned molecules arrange themselves more compactly, resulting in a stronger plastic.

For example, molecules in nylon are highly aligned, making this thermoplastic extremely strong. The degree of alignment of the molecules also determines how transparent a plastic is. Thermoplastics with highly aligned molecules scatter light, which makes these plastics appear opaque.

Thermoplastics with semialigned molecules scatter some light, which makes most of these plastics appear translucent. Thermoplastics with random amorphous molecular arrangement do not scatter light and are clear. Amorphous thermoplastics are used to make optical lenses, windshields, and other clear products. Air, heat, and molds are used to shape the plastic during its formation.

Plastic is an increasingly popular manufacturing material because it is relatively durable, inexpensive, and versatile.

Many different processes are used to make plastic products, and in each process, the plastic resin must be softened or sufficiently liquefied to be shaped.

A Forming Thermoplastics How Thermoplastics are Formed Thermoplastics are plastics that can be hardened and melted more than once.

In the calendering process, continuous plastic sheets are formed by forcing hot plastic between successive sets of heated rollers. Injection molding uses a screw to push plastic through a heated tube into a mold. Extrusion is a continuous process that heats plastic pellets in a long barrel. A screw pushes the heated plastic through a die opening to form objects such as garden hose and piping.

In thermoforming a hot plastic sheet is draped over a mold and a vacuum draws the plastic down into the mold.

Blow molding forms containers from soft, hollow plastic tubes that have a mold fitted around the outside. The tube is heated, and air injected into the heated tube expands the plastic against the walls of the mold. Although some processes are used to manufacture both thermoplastics and thermosetting plastics, certain processes are specific to forming thermoplastics. For more information, see the Casting and Expansion Processes section of this article. The mold is then opened and the plastic cast removed.

Thermoplastic items made by injection molding include toys, combs, car grills, and various containers. A2 Extrusion Plastic Pellets and Extrusion Early in the manufacturing process, small pellets of nylon a synthetic resin are stirred and melted. Once melted, the blue plastic mixture will be forced into the desired shape in a process called extrusion. Extrusion is a continuous process, as opposed to all other plastic production processes, which start over at the beginning of the process after each new part is removed from the mold.

In the extrusion process, plastic pellets are first heated in a long barrel. In a manner similar to that of a pasta-making or sausage-stuffing machine, a rotating screw then forces the heated plastic through a die device used for forming material opening of the desired shape.

As the continuous plastic form emerges from the die opening, it is cooled and solidified, and the continuous plastic form is then cut to the desired length.

Melted thermoplastic forced through extremely fine die holes can be cooled and woven into fabrics for clothes, curtains, and carpets. A3 Blow Molding Blow molding is used to form bottles and other containers from soft, hollow thermoplastic tubes. First a mold is fitted around the outside of the softened thermoplastic tube, and then the tube is heated.

Next, air is blown into the softened tube similar to inflating a balloon , which forces the outside of the softened tube to conform to the inside walls of the mold. Once the plastic cools, the mold is opened and the newly molded container is removed.

Blow molding is used to make many plastic containers, including soft-drink bottles, jars, detergent bottles, and storage drums. A4 Blow Film Extrusion Blow film extrusion is the process used to make plastic garbage bags and continuous sheets. This process works by extruding a hollow, sealed-end thermoplastic tube through a die opening. As the flattened plastic tube emerges from the die opening, air is blown inside the hollow tube to stretch and thin the tube like a balloon being inflated to the desired size and wall thickness.

The plastic is then air-cooled and pulled away on take-up rollers to a heat-sealing operation. The heat-sealer cuts and seals one end of the thinned, flattened thermoplastic tube, creating various bag lengths for products such as plastic grocery and garbage bags.

For sheeting flat film , the thinned plastic tube is slit along one side and opened to form a continuous sheet. The air inflates the plastic tube like a balloon, until a bag with the desired shape, size, and wall thickness is formed. A5 Calendering The calendering process forms continuous plastic sheets that are used to make flooring, wall siding, tape, and other products. These plastic sheets are made by forcing hot thermoplastic resin between heated rollers called calenders.

A series of secondary calenders further thins the plastic sheets. Paper, cloth, and other plastics may be pressed between layers of calendered plastic to make items such as credit cards, playing cards, and wallpaper. Products made from thermoformed sheets include trays, signs, briefcase shells, refrigerator door liners, and packages. In a vacuum-forming process, hot thermoplastic sheets are draped over a mold. Uses of the concave mirror and the convex mirror in our daily life. Advantages and disadvantages of using robots in our life.

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Related Audiobooks Free with a 30 day trial from Scribd. Elizabeth Howell. Plastics - Classification Elastomers thermosets Thermoplastics 5. Glass transition temperature Amorphous polymers do not have a specific melting point.

At low temp. The temperature at which this transition occurs is called Glass transition temperature Tg. Elastic deformation 2. Viscous deformation 3. Maxwell Model of Viscoelastic deformation 4. Voigt or Kelvin Model of Viscoelastic deformation Draw diagrams on page This is known as orientation.