Saturday, 9 March 2019

Polymeric Materials

Polymeric Materials

Plastic Materials
Plastic materials are the ultimate tribute to man’s creativity and inventiveness. Plastics are true man-made materials. Like any other materials, they have their origin in nature. The structure of plastic materials is based on basic chemical elements such as carbon, oxygen, hydrogen, nitrogen, chlorine, and sulfur. These elements are extracted from the air, water, gas, oil, coal, and even from living plants. It was man’s inspiration to take these elements and combine them through various chemical reactions in an almost unending series of combinations to produce the rich variety of polymers known today as plastics.

It is possible to create different polymers from different combinations of elements and create almost any property desired for an end product. These new polymers have similar properties to existing conventional materials but they offer greater design freedom and cost incentives for manufacturing. There are some plastics with significant property improvements over existing materials, while other polymers can only be described as unique materials with exceptional properties previously unknown to the industrial world.

There are plastics that will melt at 200 °F, while other plastic materials can withstand up to 1,000 °F. The heat shields that protect astronauts travelling in space are plastic materials based on the technology known as “ablative plastics”. There are polymers used for shields that can stop a bullet. There are flexible plastic films that protect grocery products and there are rigid plastics rugged enough to serve as support beams in a building.

Plastics are among the best electrical insulating materials known to mankind. However, we find another type of special plastic material capable of conducting electricity. Plastic composite materials are used for golf club shafts, while other flexible polymers are used as upholstery materials for furniture. There are impact resistant and transparent polymers used as windshields for airplanes, automobiles, and shower doors. There are also transparent packaging materials used to protect consumer items.

The number of permutations possible in combining chemical elements to create plastics with different properties is almost endless. It is this diversity that has made plastics so applicable to such a broad range of end uses and products today. This polymer diversity makes it difficult to grasp the idea of a single family of materials that can provide an infinite range of properties, characteristics, and transformation processes.

Beginning of Plastics
Plastic materials have played an important role in the development of this modern civilization. These polymers have an extensive versatility of properties and process automation while offering several cost advantages. It is surprising to realize that a little more than a century ago there were no such plastic materials any where around the world. The plastics industry dates its beginning back to 1868, when John Wesley Hyatt mixed pyroxylin, made from cotton and nitric acid, with camphor to create an entirely different and new product called “Celluloid”. This material was the first commercial plastic material. The development of celluloid was in response to a competition sponsored by a manufacturer of billiard balls. It came about to overcome a shortage of ivory used to produce billiard balls.With the need for a new material and a production method for this application, celluloid was developed and the plastics industry was born.

First photographic celluloid film

Celluloid quickly moved into other markets, including new applications such as shirt collars, cuffs and shirt fronts, dolls, combs, buttons, and window curtains used on early automobiles. However, the most important celluloid application was the first photographic film used by Eastman to produce the first motion picture film in 1882. This material is still in use by the motion picture industry today, under its chemical name of cellulose nitrate.

First phenolic applications

The plastics industry took its second major step 41 years later. Dr. Leo Hendrik Baekeland introduced the first phenol formaldehyde “ Phenolic” in 1909. This was the first plastic material to achieve world acceptance. What is more important, he also developed techniques for controlling and modifying the phenolformaldehyde reaction. This technology made it possible to produce useful items, such as marbleized clock bases or electric iron handles, under heat and pressure from phenolic. This process of liquefying the material to form various shapes under heat and pressure is the same process that is still in use by the industry to produce thermoset plastic materials.

The third major step in plastic’s development took place in the 1920s with the introduction of cellulose acetate. This polymer was similar in structure to cellulose nitrate but safer in processing and use. Urea-formaldehyde can be processed like phenolic, but into light colored articles that were more attractive than the phenolic’s black and brown colors.

Polyvinyl chloride (PVC) became the second largest selling plastic for such applications as flooring, upholstery, wire, and cable insulation, tubing, hoses, and fittings.

Polyamide or nylon (Du Pont’s trade name) was first developed as a fiber material. Nylon represents one of the most important new developments in the plastic industry. The research development work of W. T. Carothers in the late 1920s made possible the introduction of the nylon technology.

The tempo of plastic’s development picked up considerably in the 1930s and the 1940s. Each decade newer, more exciting, more versatile plastics came into existence. In the 1930 s, the acrylic resins were introduced for signs and transparent articles. The introduction of polystyrene made this polymer the third largest selling plastic for house wares, toys, and for applications in the packaging industry.

Melamine resins were also introduced for use in dishware, paints, and wet strength paper. Melamine later became a critical element (as a binder) in the development of decorative laminate kitchen counter tops, table tops, and panels.

During the World War II years of the 1940s, the demand for plastics accelerated as did research into new plastics that could aid in the defense effort. Polyethylene, today the most important type of plastic, was a war time development that grew out of the need for a superior insulating material for applications such as radar cables. The thermoset polyester resins were also introduced a decade later. Radical changes in the boat building industry were also a war time development introduced for military use.Acrylonitrile-butadiene-styrene (ABS) is best known today as the plastic material used for applications such as appliance housings,  First phenolic applications refrigerator liners, safety helmets, tubing, telephone handsets, and luggage.

The original ABS research work was a crash program during the war for the development of synthetic rubber. By the beginning of the 1950s, plastics were on their way to being accepted by designers and engineers as basic industrial materials. This decade also saw the introduction of polypropylene following the Nobel Award winning work of Karl Ziegler in Germany and Giulio Natta in Italy for “ordering” the molecular arrangement of plastics. Also highlighting this decade was the development of acetal and polycarbonate; two plastics that, along with nylon, came to form the nucleus of a subgroup in the plastic’s family known as the “Engineering Thermoplastics”. Their outstanding impact strength, thermal and dimensional stability enabled engineering plastic resins to compete directly with metal materials.

The 1960s and 1970s also had their share of new plastics’ introductions. The most important contribution was the thermoplastic polyesters used in exterior automotive parts, under the hood applications, and electrical and electronic components. Polyester bottles internally coated with high nitrile barrier resins (outstanding resistance to gas permeation) developed the new drink bottle packaging applications. During this time span, another subgroup of the plastic’s family called “High Performance Plastics” found new markets; this group includes such materials as polyimide, polyamide-imide, aromatic polyester, polyphenylene sulfide, and polyether sulfone. These materials historically met their objectives in the demanding thermal needs of aerospace and aircraft applications; reinforcing the vision of the plastic’s industry that the future is, indeed, plastics.

Polymer Families
Plastic materials are the result of the combination of carbon elements reacting with oxygen, hydrogen, nitrogen, and other organic and inorganic elements. These polymers have the ability to change into a liquid (melt), and are capable of being formed into shapes by the application of heat and pressure.
Thermoplastic material analogy,wax candle

Plastics are a family of materials, not a single kind of material. Plastics have an extensive number of polymers and compounds with each kind of material having its own unique and special type of properties. Most plastics fall into one of the following groups: thermoplastics, thermoplastic elastomers, liquid injection molding elastomers, thermosets, and thermoset rubbers.

Thermoplastic resins consist of a long chain of molecules, either linear or branched, having side chains or unattached groups to other polymer molecules. Usually, the commercial shapes of the thermoplastic materials are pellets, granules or powders. These materials can be repeatedly melted by heat under pressure so they can be formed, then cooled and harden into the final desired shape. Chemical changes do not take place during the transformation process.Shows a simple analogy for molding plastic resins, a wax block that can be liquified by heat, poured into a mold, then cooled to become a solid again.

Thermoplastic elastomer (TPE) resins are rubbery materials with the characteristics of a thermoplastic and the performance properties of a thermoset rubber. TPEs are processed using the same thermoplastic equipment and methods, such as extrusion, injection molding, and blow molding.

Liquid injection molding compounds are a family of unique products. Generally, these materials use two liquid formulations in a 1 : 1 ratio. These compounds produce precision elastomeric molded parts efficiently. They use a liquid metering, mixing, and delivery system, a specially modified injection molding machine, and a high temperature precision mold.

Thermoset materials have a reactive portion between the chain cross link and the long molecule’s network during polymerization. The linear polymer chains bond together to form a three-dimensional network. Therefore, once polymerized or hardened, the material cannot be softened by heating without degrading some linkages of the material. Thermoset materials in commercial form are supplied as resins, powders, and liquid monomer mixtures or as partially polymerized molding compounds. In this uncured condition, they conform to the finished shape with or without pressure and can be polymerized with chemicals or heat.

Thermoset plastics analogy,concrete

Shows one of the analogies for the thermoset materials as the chemical transformation of concrete. When the cement powder blends with water and sand, the mixture becomes a thick paste compound. This mixture is then transferred to a cavity for curing and hardening to become a solid object (concrete). The chemical reaction transforms the product into concrete. The transformation processes of the concrete items are irreversible. Reprocessing concrete forms or returning to cement, sand, and water are not possible. The concrete becomes a new, different and strong material. Thermosetting materials are not reprocessable or recyclable.

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