Век химии (Английский язык для студентов химического профиля). Учебное пособие

•Millable gums, which can be processed on standardrubber processing equipment;

•Cast elastomers, both solid and microcellular cast systems, both heatand room-temperature curable;

•Fibers of the type known widely as Spandex, usually produced by solution dry spinning of polyurethane polymers;

•Thermoplastic urethanes or TPU's, solid polymers injection moldable on conventional plastic processing equipment, and;

•RIM and RRIM elastomers, low density microcellular elastomers processed on reaction injection molding equipment.

The history of polyurethane elastomers began more then 150 years ago and continues today as new application ideas spur growth and diversification, while ongoing technical development works to make those ideas commercial realities.

The first 90 years of this history could aptly be called the Era of Discovery. It began in 1849, when Wurtz first isolated an isocyanate molecule, and takes us through 1937, when Otto Bayer first discovered the diisocyanate polymerization reaction. One goal of this early research was to duplicate or surpass the properties of nylon.

Through the 40s, 50s and 60s, a few key urethane producers built upon this fundamental understanding of the urethane reaction to develop process technology, products and markets for elastomers. This stage can appropriately be called the Era of Development Highlights included:

•Introduction of Adiprene B isocyanate cured millable gums by Du Pont in 1954;

•Introduction of Adiprene castable systems by Du Pont as a rubber replacement in 1958, and;

•Introduction of Spandex under the name "Fiber K" by Du Pont in 1958.

By the late 60s polyurethane elastomers were entering a period of accelerated growth and development.

This, then, is the Era of Response, when a fast-changing world created new market niches for elastomers. At the beginning of this era, in 1970, U.S. consumption of non-cellular polyurethane was about 56 million pounds in four major categories - millable gums, liquid castables, thermoplastic elastomers and fibers.

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Now automotive applications may represent the major end use by volume. But they by no means reflect the broad application diversity of elastomers, which by now have found their way into uses from helicopter covers to artificial hearts, from cattle tags to running tracks.

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PHELOLIC, NON-STAINING ANTIOXIDANTS

Phenolic antioxidants – a historical background. At the early beginning of manufacturing rubber goods, the producers were faced with rapid deterioration. It resulted in either hardening, brittleness or in sickness of the rubber goods. Around 1910 the first type of antioxidants were discovered, mainly naturally existing materials like creosote, asphalt and coal tar pitch. These were used until roughly 1925 when phenol, cresol, hydroquinone and aniline were used as antioxidants. Between 1925 and 1940 a big number of patents on antioxidants were issued, most of them based on secondary aromatic amine derivatives.

It was not until after 1940 when an increasing worldwide production of synthetic rubber, and consequently the development of tires based on synthetic rubber, called for a more demanding type of antioxidant, not only protecting the rubber goods and tires against oxygen and heat, but also against the severe cracking caused by ozone. The result was the development of a new class of antioxidants, p- phenylen-diamine derivates, which was soon after called antiozonant, due to their ability to protect rubber goods against cracking.

After 1945 the existing large synthetic rubber production had to be diverted to civilian use. The known and common used antioxidants and antiozonants were up to this stage staining types of chemicals. With the new approach to civilian use, there was a demand for nonstaining antioxidants because synthetic rubber for civilian use included natural, white or light colored rubber goods, for example:

•sports rubber goods;

•surgical rubber goods;

•latex products, including foam;

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•white side-walls;

•footwear;

•sheetings;

•flooring tiles;

•sponge rubber;

only to name a few. With the development of non staining antioxidants, it was possible to protect rubber against deterioration, caused by oxygen, heat, light and certain metals like copper and manganese, and still maintain the, natural colour of the rubber.

Antioxidants economical background. Antioxidants are rubber chemicals added to the rubber in the range of 1 to 2 %. Although the price of these antioxidants is ranging from $5 to $10/kg., the cost of protecting the rubber is only a few cents. It is estimated that the lifetime of rubber goods is three to five times longer when protected with antioxidants and one can get some ideas on the indirect value of antioxidants. Adding to this indirect value, costs of failure like idle machinery, break down of cars, blown tires and so on, then it is easy to understand that the costs of antioxidants becomes insignificant compared to the value of the improved rubber goods.

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CHEMICAL PROCESSES

When we follow the sequence of processes in the chemical plant we can see that solid raw materials go to crushers, grinders and screens and then by conveyors to the reaction vessels. For liquid raw materials, the storage tank is the starting point, after which pumps, and perhaps proportioning devices, are required. The same is true of gases which must be propelled by means of fans, blowers and compressors; frequently they are to be cooled in coolers and washed. The devices used for the cleaning of a gas include mainly dust collectors, dry cyclones, filters and scrubbers.

Materials must be mixed, liquids as well as solids. This process in various types of mixers and agitators. At the other stages, solids are to be separated from liquids in filters, centrifuges or simply by settling

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the suspended solid in a settling tank or in continuous mechanical setting units generally known as thickeners. Solutions of solids are evaporated in evaporaters; they crystallize in crystallizers and they are dried in driers. Mixtures of liquids, on the other hand, are usually separated in distillation equipment.

Auxiliary equipment, such as vacuum pumps, heat exchangers and reaction vessels, especially kettles, are required in many of these operations, as well as numerous types of instruments for measurement and control.

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DEVICES FOR TRANSPORTING AND COMPRESSING

GASES AND LIQUIDS

The transportation of a liquid or gas is a common operation. Devices used for this purpose are: fans, pumps and compressors.

FANS. A fan is a machine for applying power to a gaseous fluid to increase its energy content. This energy enables movement or flow of the gas against various degrees of resistance.

The function of a fan is to move air or gas through distribution systems and apparatus required for conditioning gas medium.

The fan consists of a rotating member called the wheel or impeller and a stationary member called the housing. The housing is provided with an intake opening (inlet) and with a discharge opening (outlet). The flow of air or gas is caused by the pressure differential created by the energy transmitted to the gas by the rotating wheel.

If no resistance to flow exists, as in the case of a fan in free space with no inlet and no outlet duct, the fan will provide the gas with velocity energy only, and no compression or rarefaction occurs. When either inlet or outlet duct is added, frictional resistance is imposed and partial compression occurs on the outlet side, whereas partial rarefaction occurs on the inlet side.

The extent of the resistance imposed at the discharge governs the quantity of gas delivered by the fan. Fans are volume blowers. A fan may have 6 or 8 flat steel blades or 60 very small curved blades. The latter belong to the multi-bladed type. Fans are generally belt-driven,

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usually from a special motor. The capacity of any fan is computed on the basis of free air, that is, air at atmospheric pressure and at 150 C.

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GAS COMPRESSORS

These machines are employed for bringing gases to a higher pressure level and for transporting them. The number of types of compressors is large: rotary compressors, piston compressors, diaphragm compressors and others.

Piston compressors have in principle the same construction as piston pumps. But the piston speeds and the number of strokes per unit of time are much higher. These compressors can be constructed for any capacity and delivery pressure.

The most usual type is that with lubricated cylinders. The presence of solvents in the gas does not present so much difficulties in this case as in the rapidly rotating vane compressor. But there are cases when no lubricating oil is admitted into the cylinders, e.g. in compressing oxygen. Some other lubricant must then be used or the cylinders must not be lubricated at all. In the latter case the cylinder liner or the piston ring must be made of graphite. The maximum speed is about 200 ft/min. and the maximum pressure is about 300 lb/min.

Diaphragm compressors is of almost the same construction, as the diaphragm pipe for liquids. The diaphragm can be flat or shaped as a tube or bellows and may be constructed of steel or rubber. The absence of the leaky stuffing box makes these compressors suitable for handling valuable or toxic gases. But it is expensive and vulnerable.

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FILTERING DEVICES

The separation of comparatively small amounts of solids from larger amounts of liquids is done by settling, followed by decantation of the supernatant clear liquid, or by the use of a filtering device. The

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separation of very small amounts of liquids from large amounts of solids is done by draining, or by centrifuging.

Settling out the solid is the cheapest method, and is frequently used. If the operation is extensive, a large capacity in settling tanks is required: these act also as storage tanks. If the liquor is hot, the tank may be insulated by strips of wood. As a rule, this process is too slow; also the mud collected in the bottom of the tank still contains too much water, and is usually sent through a filter, a combination of settling and filtering.

Filtering devices include gravity filters, pressure filters, more commonly called filter-pressers, and suction filters. Several points must be considered when studying a filtration problem: whether the cake or the filtrate is the valuable part, whether or not washing is required, and whether the operation should be continuous or intermittent. These considerations will have weight in the choice of the device.

The suspension to be filtered is called a slurry, if its content in solids is not so high that it does not flow and cannot be pumped; slurries contain from less than 1 per cent to 40 per cent of solids are more properly called sludges. The solid separated in the filter device is called the cake, the clear liquor running off is the filtrate. filtering devices such as filter-presses and suction filters are really frames for the support of the filtering medium, the filter cloth, which may be cotton duck of various thicknesses, muslin, paper, wool flannel, or metal wire woven into cloth, such as iron wire, nickel wire, and monel wire.

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DUST COLLECTORS

The weight of quantity of solids which air or gas can carry varies directly as the velocity; the higher velocity, the greater the carrying capacity. The velocity of a dust-laden gas travelling in a pipe may be lowered by enlarging the pipe; at such enlarged places, the gas will drop part or all of its load. The enlargement need not be very great to produce a considerable reduction, for example, if a 6 inches pipe is

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widened to 24 inches a velocity of 160 feet per second becomes 10 feet per second. The dust collector is such an enlargement. It is made up of two truncated cones; the upper cone flares out downward; the lower cone meets the upper one and flares in downward, so that the box is widest half-way up (посередине). The pipe bringing the gas is so placed that the air is delivered along the wall and acquires a circular motion as it travels spirally around and down the box. The dust drops out and collects at the bottom, while the comparatively clean gas leaves by an internal pipe set halfway down the collector. The size of the dust collector must fit the volume of air or gas handled, so that the required reduction of velocity is reached.

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BAG FILTERS

The bag filter is a dust-arresting device consisting of bags made of cloth, with a number of them within a housing so set up that the dirty air enters the inner part of the bag, depositing its solids there, while the clean air passes through, or so that the dirty air surrounds the bags and deposits its load on the outer side, while clean air travels through to the inner side, to be pulled away by a fan. Alternatively, the bag filter may be fed by a blower, so that the dirty air is under pressure, and the clean air also. In any scheme, the dust is arrested by the cloth, and the coating is removed periodically by rapping or shaking the bag; the housing is mounted on hoppers in which the deposits accumulate. As example of the bag filters, the multi-bag filter will be described. Its individual bags are tall cylinders 6 inches across and 9 feet 6 inches high, made of cotton sateen cloth; both napped and unnapped cloth (ворсистая и неворсистая ткань) are used. Each cylinder bottom fits over a collar in the floor, and is held in place by a quick detachable bottom band, while its top is fastened to the bagshaking shaft. Three individual bags are sewn together as one element is referred to as a single filter bag; it measures 17 inches across. The dust-laden air travels over the hoppers to the inside of the bag, and is pulled through its meshes which retain the dust in the form of a

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coating. Periodically, a motor-operated shaker mechanism gives the bags an upward and downward motion, loosening the coat of dust, which drops to the hoppers below. The bags are mounted in a housing, which is on the suction side of fan, so that any repair necessary may be made in dust-free air.

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COOLERS

In chemical engineering, water cooling is widely used and gas washing, quenching and atomizing are equally important accompaniments of process apparatus.

Except in comparatively few cases where an ample supply of pure cool water is available, it is necessary to cool artificially the water used for industrial purposes, such as for continuous process work in chemical plants. Methods generally available for cooling water include: a natural pond of large area, a cooling tower, or a spray cooling plant arranged over a pond. Where water supply is abundant, or where there is sufficient space, the first method may be adopted; but for ordinary industrial establishments this is generally impossible because of the large space required for the pond. Moreover, this is a very inefficient method of dissipating heat contained in water because a still pond limits the surface exposed to the air for cooling; unless the air temperature is considerably lower than that of the water, cooling is slow. In most places a natural cooling pond system cannot be used in hot weather. A cooling tower occupies less space than a natural pond and is more suitable.

With a cooling tower, the water is introduced into the top of the tower and flows either over a series of baffles, or over packing, commonly in the form of wooden slats and the interfacial area is increased. The air current is produced either by natural convection caused by the lower density of the gas within the tower, or by means of a forced draught produced by a fan of blower. In most cooling towers, the air current is in an upward direction and counter-current flow of the air and water is obtained. Sometimes, however the air

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current is produced by the wind blowing horizontally into the tower. Counter-current flow is not then obtained but fresh air is introduced throughout the length of the tower. Higher gas velocities are obtained in forced draught systems and therefore the transfer coefficients are greater.

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POLYMERS

Polymers are substances whose molecules have high molar masses and are composed of a large number of repeating units. There are both naturally occurring and synthetic polymers. Among naturally occurring polymers are proteins, starches, cellulose, and latex. Synthetic polymers are produced commercially on a very large scale and have a wide range of properties and uses. The materials commonly called plastics are all synthetic polymers.

Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain. In many polymers, only one monomer is used. In others, two or three different monomers may be combined. Polymers are classified by the characteristics of the reactions by which they are formed. If all atoms in the monomers are incorporated into the polymer, the polymer is called an addition polymer. If some of the atoms of the monomers are released into small molecules, such as water, the polymer is called a condensation polymer. Most addition polymers are made from monomers containing a double bond between carbon atoms. Such monomers are called olefins, and most commercial addition polymers are polyolefins. Condensation polymers are made from monomers that have two different groups of atoms which can join together to form, for example, ester or amide links. Polyesters are an important class of commercial polymers, as are polyamides (nylon).

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POLYETHYLENE TEREPHTHALATE

Polyethylene terephthalate (PET), or polyethylene terephthalic ester (PETE), is a condensation polymer produced from the monomers ethylene glycol, HOCH2CH2OH, a dialcohol, and dimethyl terephthalate, СНзO2С–С6H4–СО2СН3, a diester. By the process of transesterification, these monomers form ester linkages

between them, yielding a polyester. PETE fibers are manufactured under the trade names of Dacron and Fortrel. Pleats and creases can be permanently heat set in fabrics containing polyester fibers, so-called permanent press fabrics. PETE can also be formed into transparent sheets and castings. Mylar is a trade name for a PETE film. Transparent 2-liter carbonated beverage bottles are made from PETE. (The opaque base on some bottles is generally made of HDPE.) One form of PETE is the hardest known polymer and is used in eyeglass lenses.

POLYETHYLENE

Polyethylene is perhaps the simplest polymer, composed of chains of repeating –CH2– units. It is produced by the addition polymerization of ethylene, CH2=CH2 (ethene). The properties of polyethylene depend on the manner in which ethylene is polymerized. When

catalyzed by organometallic com pounds at moderate pressure (15 to 30 atm), the product is high density polyethylene, HDPE.

Under these conditions, the polymer chains grow to very great length, and molar masses average many hundred thousands. HDPE is hard, tough, and resilient. Most HDPE is used in the manufacture of containers, such as

milk bottles and laundry detergent jugs. When ethylene is polymerized at high pressure (1000-2000 atm), elevated temperatures (190210 °C), and catalyzed by peroxides, the product is low density polyethylene, LDPE. This form of polyethylene has molar masses of 20,000 to 40,000 grams. LDPE is relatively soft, and most of it is used in the production of plastic films, such as those used in sandwich bags.

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