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Polyurethane chemistry is based on the reaction of isocyanates with active hydrogen containing compounds. Isocyanates are compounds having one or more of the highly reactive isocyanate group. This group will readily react with hydrogen atoms more electronegative than carbon.

Electron density is expected to be the greatest on the oxygen atom and least on the carbon item. This results in the oxygen atom having the largest net negative charge, carbon a net positive charge, and the nitrogen, an intermediate net negative charge.

The normal reactions essentially involve addition to the carbon-nitrogen double bond. A nucleophilic center from an active hydrogen-containing compound attacks the electrophilic carbon. The active hydrogen atom then adds to the nitrogen atom. Electron-withdrawing groups attached to the isocyanate molecule increase the reactivity of the NCO group toward the nucleophilic groups. Electron-donating groups reduce reactivity. Thus, in most reactions, aromatic isocyanates are more reactive than aliphatic isocyanates. Steric hindrance effects on either the isocyanate or the active hydrogen compound will effect the reaction.

Formation of flexible polyurethane foam is a complex process involving many ingredients and at least two competing reactions.

The Polymerization Reaction

The polyurethane polymer forming reaction occurs between an isocyanate and an alcohol as follows

Isocyanate + Aocohol = Urethane

This is addition process for which the heat of reaction has been reported to be approximately 24 kcal/mole of urethane. Depending on the choice of starting materials, the R and R' groups may also contain isocyanate or isocyanate-reactive groups respectively. When extended to polyfunctional reactants, this reaction provides a direct route to cross linked polymers.

The hydrogen on the nitrogen atom of the urethane group is capable of reacting with additional isocyanate to form an allophanate group.

Urethane + Isocyanate = Allophanate

Note that the formation of allophanante is a high temperature, reversible reaction. If actually formed in normal flexible foams, the allophanate linkage would serve to cross-link the polymer further. The catalysts generally used in the foam formulation do not promote this reaction, and temperatures greater than 110 Centigrade are necessary for significant allophanate formation.

The Gas Producing Reaction

To make foam, the polyurethane polymer must be expanded or blown by the introduction of bubbles and a gas. A convenient source of gas is the carbon dioxide produced from the reaction of an isocyanate group with water.

Isocyanate + Water → Carbamic Acid

Carbamic Acid → Amine + Carbon dioxide + Heat

The intermediate product of this reaction is a thermally unstable carbamic acid, which spontaneously decomposes to an amine and carbon dioxide. Diffusion of the carbon dioxide into bubbles previously nucleated in the reacting medium causes expansion of the medium to make foam. Further reaction of the amine with additional isocyanate gives a distributed urea.

Isocyanate + Amine → Distributed Urea

The approximate total heat release per mole of water is 47 kcal.

Again, if the isocyanate and the amine molecules are polyfunctional, a cross-linked polymer will result. Another conceptual method of cross-linking the polymer is by reaction of a hydrogen from the distributed urea with a free isocyanate group to form a biuret linkage.

Distributed Urea + Isocyanate → Biuret

Since the reaction is also reversible, there is debate about whether allophanates and biurets actually exist in the final polyurethane foam.

Blowing can also be achieved by the physical addition of a low-boiling nonreactive liquid to a foam formulation. Historically, the most commonly used blowing agents are chlorofluorocarbons, urethane grade methylene chloride and trichloroethane. Vaporization of these liquids by heat from the exothermic reactions produces gas molecules which diffuse into nucleated bubbles and contribute to foam expansion.

For more information visit my site Polyurethanes Information

Tourism attractions in France Paris, what to see, thiongs to do in Paris Sightseeing and day tour

IRTOURING, offer excellent tours to iran, cultural tours to iran, iran tour and Onlive informations about Exotic Locations such as Eifel Tower

How to visit Eifel Tower

The original lifts to the first and second floors were provided by two companies. Both companies had to overcome many technical obstacles as neither company (or indeed any company) had experience with installing lifts climbing to such heights with large loads. The slanting tracks with changing angles further complicated the problems. The East and West lifts were supplied by the French company Roux Combaluzier Lepape, using hydraulically powered chains and rollers. Contemporary engravings of the lift cars show that the passengers were seated at this time but it is not clear whether this was conceptual. It would be unnecessary to seat passengers for a journey of a couple of minutes. The North and South lifts were provided by the American Otis company using car designs similar to the original installation but using an improved hydraulic and cable scheme. The French lifts had a very poor performance and were replaced with the current installations in 1897 (West Pillar) and 1899 (East Pillar) by Fives-Lille using an improved hydraulic and rope scheme. Both of the original installations operated broadly on the principle of the Fives-Lille lifts.

The Fives-Lille lifts from ground level to the first and second levels are operated by cables and pulleys driven by massive water-powered pistons. The hydraulic scheme was somewhat unusual for the time in that it included three large counterweights of 200 tonnes each sitting on top of hydraulic rams which doubled up as accumulators for the water. As the lifts ascend the inclined arc of the pillars, the angle of ascent changes. The two lift cabs are kept more or less level and indeed are level at the landings. The cab floors do take on a slight angle at times between landings.

The principle behind the lifts is similar to the operation of a block and tackle but in reverse. Two large hydraulic rams (over 1 metre diameter) with a 16 metre travel are mounted horizontally in the base of the pillar which pushes a carriage (the French word for it translates as chariot and this term will be used henceforth to distinguish it from the lift carriage) with 16 large triple sheaves mounted on it. There are 14 similar sheaves mounted statically. Six wire ropes are rove back and forth between the sheaves such that each rope passes between the 2 sets of sheaves 7 times. The ropes then leave the final sheaves on the chariot and passes up through a series of guiding sheaves to above the second floor and then via a pair of triple sheaves back down to the lift carriage again passing guiding sheaves.

This arrangement means that the lift carriage, complete with its cars and passengers, travels 8 times the distance that the rams move the chariot, the 128 metres from the ground to the second floor. The force exerted by the rams also has to be 8 times the total weight of the lift carriage, cars and passengers, plus extra to account for various losses such as friction. The hydraulic fluid was water, normally stored in three accumulators, complete with counterbalance weights. To make the lift ascend, water was pumped using an electrically driven pump from the accumulators to the two rams. Since the counterbalance weights provided much of the pressure required, the pump only had to provide the extra effort. For the descent, it was only necessary to allow the water to flow back to the accumulators using a control valve. The lifts were operated by an operator perched precariously underneath the lift cars. His position (with a dummy operator) can still be seen on the lifts today.

The Fives-Lille lifts were completely upgraded in 1986 to meet modern safety requirements and to make the lifts easier to operate. A new computer controlled system was installed which completely automated the operation. One of the three counterbalances was taken out of use, and the cars were replaced with a more modern and lighter structure. Most importantly, the main driving force was removed from the original water pump such that the water hydraulic system provided only a counterbalancing function. The main driving force was transferred to a 320 kW electrically driven oil hydraulic pump which drives a pair of hydraulic motors on the chariot itself thus providing the motive power. The new lift cars complete with their carriage and a full 92 passenger load weigh 22 tonnes.

Due to elasticity in the ropes and the time taken to get the cars level with the landings, each lift in normal service takes an average of 8 minutes and 50 seconds to do the round trip spending an average of 1 minute and 15 seconds at each floor. The average journey time between floors is just 1 minute.

The original Otis lifts in the North and South pillars in their turn proved inferior to the new (in 1899) French lifts and were scrapped from the south pillar in 1900 and from the north pillar in 1913 after failed attempts to re-power them with an electric motor. The north and south pillars were to remain without lifts until 1965 when increasing visitor numbers persuaded the operators to install a relatively standard and modern cable hoisted system in the north pillar using a cable-hauled counterbalance weight, but hoisted by a block and tackle system to reduce its travel to one third of the lift travel. The counterbalance is clearly visible within the structure of the North pillar. This latter lift was upgraded in 1995 with new cars and computer controls.

The South pillar acquired a completely new fairly standard electrically driven lift in 1983 to serve the Jules Verne restaurant. This was also supplied by Otis. A further 4 tonne service lift was added to the south pillar in 1989 by Otis to relieve the main lifts when moving relatively small loads or even just maintenance personnel.

The east and west hydraulic (water) lift works are on display and, at least in theory, are open to the public in a small museum located in base of the East and West tower, which is somewhat hidden from public view. Because the massive mechanism requires frequent lubrication and attention, public access is often restricted. However, when open, the wait times are much less than the other, more popular, attractions. The rope mechanism of the North tower is visible to visitors as they exit from the lift.

Second to the third level

The original lift from the second to the third floor were also of a water powered hydraulic design supplied by Léon Edoux. Instead of using a separate counterbalance, the two lift cars counterbalanced each other. A pair of 81 metre long hydraulic rams were mounted on the second level reaching nearly half way up to the third level. A lift car was mounted on top of the rams. Ropes ran from the top of this car up to a sheave on the third level and back down to a second car. The result of this arrangement was that each car only travelled half the distance between the second and third levels and passengers were required to change lifts halfway walking between the cars along a narrow gangway with a very impressive and relatively unobstructed downward view. The 10 tonne cars held 65 passengers each or up to 4 tonnes.

One interesting feature of the original installation was that the hoisting rope ran through guides to retain it on windy days to prevent it flapping and becoming damaged. The guides were mechanically moved out of the way of the ascending car by the movement of the car itself. In spite of some antifreeze being added to the water that operated this system, it nevertheless had to close to the public from November to March each year.

The original lifts complete with their hydraulic mechanism were completely scrapped in 1982 after 97 years of service. They were replaced with two pairs of relatively standard rope hoisted cars which were able to operate all the year round. The cars operate in pairs with one providing the counterbalance for the other. Neither car can move unless both sets of doors are closed and both operators have given a start command. The commands from the cars to the hoisting mechanism are by radio obviating the necessity of a control cable. The replacement installation also has the advantage that the ascent can be made without changing cars and has reduced the ascent time from 8 minutes (including change) to 1 minute and 40 seconds. This installation also has guides for the hoisting ropes but they are electrically operated. The guide once it has moved out of the way as the car ascends automatically reverses when the car has passed to prevent the mechanism becoming snagged on the car on the downward journey in the event it has failed to completely clear the car. Unfortunately these lifts do not have the capacity to move as many people as the 3 public lower lifts and long queues to ascend to the third level are common. Most of the intermediate level structure present on the tower today was installed when the lifts were replaced and allows maintenance workers to take the lift half way.

The replacement of these lifts allowed the restructuring of the criss-cross beams in upper part of the tower and further allowed the installation of two emergency staircases. These replaced the dangerous winding stairs that were installed when the tower was constructed.

Restaurants

The tower has two restaurants: Altitude 95, on the first floor 311 ft (95 m) above sea level; and the Jules Verne, an expensive gastronomical restaurant on the second floor, with a private lift. This restaurant has one star in the Michelin Red Guide. In January 2007, the multi-Michelin star chef Alain Ducasse was brought in to run Jules Verne.

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oil leak on my 1993-4 Lreg bmw 3 series?

my 1994 BMW 3series has recently got an oil leak it looks like its coming from bottom of the steering column or the intermediate shaft is this an expensive job or simple iv only noticed this because sometimes when i turn corners i can here the steering makin a weird noise like a sort of pump wiv no oil in sorry bout the detail but im no machanic or anythin just askin the question the best detail i can, thanks

Forget the "intermediate shaft" for there is none in your car.
To confirm that the power steering is leaking just open the atf reservoir to see the oil level. Check also the steering hoses from the reservoir to the pump all the way to the gear box, cooler and back to the reservoir.
Replacing the hose(s) is not rocket science. But refilling the atf and bleeding can be tricky.
To bleed, top off with atf then turn steering wheel a few times (without running the engine), top off again then run the engine and turn steering wheel maybe 12 times.

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