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Flame Retardant

I   INTRODUCTION

Flame Retardant, material added or applied to a product to increase the resistance of that product to fire. Flame retardants, also called fire retardants, are less flammable than the materials they protect, burn slowly, and do not propagate fire. Some flame retardants prevent the spread of flame; others burn and thereby create a layer of char that inhibits further combustion.

Flame retardants are generally added to wood, paper, plastics, textiles, and composites to meet governmental regulations for buildings, aircraft, automobiles, and ships. Flame retardants can be incorporated into a material either as a reactive component or as an additive component. Reactive-type flame retardants are preferable because they produce stable and more uniform products. Such flame retardants are incorporated into the polymer structure of some plastics. Additive-type flame retardants, on the other hand, are more versatile and economical. They can be applied as a coating to wood, woven fabrics, and composites, or as dispersed additives in bulk materials such as plastics and fibers.

The chemicals in a flame retardant determine how it works. Most flame retardants contain elements from any of three groups in the periodic table of elements: group IIIa (including boron and aluminum); group Va (including nitrogen, phosphorus, arsenic, and antimony); and group VIIa (including fluorine, chlorine, and bromine). Elements of different groups that are combined in a single flame retardant may work more effectively together than they would separately.

II   GROUP IIIA FLAME RETARDANTS

Flame retardants that contain boron or aluminum increase the amount of char, or burnt material, formed in the early stage of a fire. The char forms a protective layer that prevents oxygen from reaching the inner layers of the material and thus sustaining the fire (see Combustion). Chemicals commonly used for this purpose include borax, boric acid, and hydrated aluminum oxide.

III   GROUP VA FLAME RETARDANTS

Phosphorus can function as a flame retardant in both its solid phase and its liquid phase. Phosphorus-containing compounds such as phosphoric acid work by forming a surface layer of protective char. Nitrogen is used mainly in combination with phosphorus; such combinations have proved effective in cellulose, polyester, and polyurethane products. Arsenic, because of its toxicity, is now rarely used in flame retardants. Antimony by itself is ineffective as a flame retardant and is used only in combination with halogens, especially bromine and chlorine.

IV   GROUP VIIA FLAME RETARDANTS

Bromine works as a flame retardant in its gaseous phase. Bromine-containing compounds are incorporated into flammable materials. When these materials are exposed to flame, the bromine dissociates from the material and forms a heavy gas. This dissociation disperses heat, and the bromine gas forms an insulating layer around the material. The layer prevents flames from spreading by inhibiting access to oxygen and by slowing the transfer of heat. Chlorine works in a similar manner in both its liquid and gaseous phases. The most important fluorine-containing flame retardants are the chlorofluorocarbons, which are used as blowing agents in polyurethane and polystyrene foams. The use of bromine and chlorine in fire retardants is somewhat restricted, however, because a high concentration of these elements can diminish the flexibility, mechanical properties, and durability of materials.




Welding

I   INTRODUCTION

Welding, in engineering, any process in which two or more pieces of metal are joined together by the application of heat, pressure, or a combination of both. Most of the processes may be grouped into two main categories: pressure welding, in which the weld is achieved by pressure; and heat welding, in which the weld is achieved by heat. Heat welding is the most common welding process used today. Brazing and soldering (see Solder) are other means of joining metals.

With the development of new techniques during the first half of the 20th century, welding replaced bolting and riveting in the construction of many types of structures, including bridges, buildings, and ships. It is also a basic process in the automotive and aircraft industries and in the manufacture of machinery. Along with soldering and brazing, it is essential in the production of virtually every manufactured product involving metals.

The welding process best suited to joining two pieces of metal depends on the physical properties of the metals, the specific use to which they are applied, and the production facilities available. Welding processes are generally classified according to the sources of heat and pressure used.

The original pressure process was forge welding. Forge welding was practiced for centuries by blacksmiths and other artisans. The metals are brought to a suitable temperature in a furnace, and the weld is achieved by hammering or other mechanical pressure. Forge welding is used rarely in modern manufacturing.

The welding processes most commonly employed today include gas welding, arc welding, and resistance welding. Other joining processes include thermite welding, laser welding, and electron-beam welding.

II   GAS WELDING

Gas welding is a nonpressure process using heat from a gas flame. The flame is applied directly to the metal edges to be joined and simultaneously to a filler metal in wire or rod form, called the welding rod, which is melted to the joint. Gas welding has the advantage of involving equipment that is portable and does not require an electric power source. The surfaces to be welded and the welding rod are coated with flux, a fusible material that shields the material from air, which would result in a defective weld.

III   ARC WELDING

Arc-welding processes, which have become the most important welding processes, particularly for joining steels, require a continuous supply of either direct or alternating electrical current. This current is used to create an electric arc, which generates enough heat to melt metal and create a weld (see Electric Arc).

Arc welding has several advantages over other welding methods. Arc welding is faster because of its high heat concentration, which also tends to reduce distortion in the weld. Also, in certain methods of arc welding, fluxes are not necessary. The most widely used arc-welding processes are shielded metal arc, gas-tungsten arc, gas-metal arc, and submerged arc.

A   Shielded Metal Arc

In shielded metal-arc welding, a metallic electrode, which conducts electricity, is coated with flux and connected to a source of electric current. The metal to be welded is connected to the other end of the same source of current. By touching the tip of the electrode to the metal and then drawing it away, an electric arc is formed. The intense heat of the arc melts both parts to be welded and the point of the metal electrode, which supplies filler metal for the weld. This process, developed in the early 20th century, is used primarily for welding steels.

B   Gas-Tungsten Arc

In gas-tungsten arc welding, a tungsten electrode is used in place of the metal electrode used in shielded metal-arc welding. A chemically inert gas, such as argon or helium, is used to shield the metal from oxidation. The heat from the arc formed between the electrode and the metal melts the edges of the metal. Metal for the weld may be added by placing a bare wire in the arc or the point of the weld. This process can be used with nearly all metals and produces a high-quality weld. However, the rate of welding is considerably slower than in other processes.

C   Gas-Metal Arc

In gas-metal welding, a bare electrode is shielded from the air by surrounding it with argon or carbon dioxide gas or by coating the electrode with flux. The electrode is fed into the electric arc, and melts off in droplets to enter the liquid metal that forms the weld. Most common metals can be joined by this process.

D   Submerged Arc

Submerged-arc welding is similar to gas-metal arc welding, but in this process no gas is used to shield the weld. Instead, the arc and tip of the wire are submerged beneath a layer of granular, fusible material formulated to produce a proper weld. This process is very efficient but is generally only used with steels.

IV   RESISTANCE AND THERMITE WELDING

In resistance welding, heat is obtained from the resistance of metal to the flow of an electric current. Electrodes are clamped on each side of the parts to be welded, the parts are subjected to great pressure, and a heavy current is applied briefly. The point where the two metals meet creates resistance to the flow of current. This resistance causes heat, which melts the metals and creates the weld. Resistance welding is extensively employed in many fields of sheet metal or wire manufacturing and is particularly adaptable to repetitive welds made by automatic or semiautomatic machines.

In thermite welding, heat is generated by the chemical reaction that results when a mixture of aluminum powder and iron oxide, known as thermite, is ignited. The aluminum unites with the oxygen and generates heat, releasing liquid steel from the iron. The liquid steel serves as filler metal for the weld. Thermite welding is employed chiefly in welding breaks or seams in heavy iron and steel sections. It is also used in the welding of rail for railroad tracks.

V   NEW PROCESSES

The use of electron beams and lasers for welding has grown during the second half of the 20th century. These methods produce high-quality welded products at a rapid rate. Laser welding and electron-beam welding have valuable applications in the automotive and aerospace industries.


Brazing

Brazing, a method of joining two metal surfaces by using nonferrous filler metal heated to above 430° C (800° F), but below the melting point of the metals to be joined. The kinds of filler metal used include brass, bronze, or a silveralloy; the filler metal distributes itself between the surfaces to be bonded by capillary action. Brazing is different from welding; in welding, partial melting of the surfaces may occur, and the filler metal is not distributed by capillarity. Brazing differs from ordinary soldering only in the temperature of the operation; ordinary, or soft, solder melts at temperatures below 430° C, but brazing alloys, sometimes called hard solder, melt above that temperature.

In general, brazing requires careful cleaning of the surfaces to be joined and the use of flux, such as borax, to reduce any oxide film on the surfaces. In mass production, furnaces are often used to heat the parts to be brazed, or the parts are brazed by dipping in baths of molten filler alloys. For single, nonrepetitive operations, the joint is usually heated with a gas, oxyacetylene, or oxyhydrogen torch.

Electric Arc

Electric Arc, type of continuous electric discharge, giving intense light and heat, formed between two electrodes in a gas at low pressure or in open air. It was first discovered and demonstrated by the British chemist Sir Humphry Davy in 1800.

To start an arc, the ends of two pencil-like electrodes, usually made of carbon, are brought into contact and a large current (about 10 amp) is passed through them. This current causes intense heating at the point of contact, and if the electrodes are then separated, a flamelike arc is formed between them. The discharge is carried largely by electrons traveling from the negative to the positive electrode, but also in part by positive ions traveling in the opposite direction. The impact of the ions produces great heat in the electrodes, but the positive electrode is hotter, because the electrons impinging on it have greater total energy. In an arc in air at normal pressure, the positive electrode reaches a temperature of 3500° C (6332° F).

The intense heat of the electric arc is often utilized in special furnaces to melt refractory materials. Temperatures of about 2800° C (5072° F) can easily be obtained with such a furnace. Arcs are also used as a high-intensity light source. Arc lights have the advantage of being concentrated sources of light, because some 85 percent of the light intensity comes from a small area of the tip of the positive carbon electrode. Such lamps were formerly much used for street lighting, but are now chiefly employed in motion picture projectors. Mercury-vapor lamps and sodium-vapor lamps are enclosed arc lamps in which the arc is maintained in an atmosphere of mercury or sodium vapor at reduced pressure.


Date: 2015-01-02; view: 895


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