2,4-TDI is prepared in three steps from toluene, which is doubly nitrated with nitric acid to give dinitrotoluene. This step determines the isomer ratio of the ultimate TDI. Hydrogenation of the dinitrotoluene produces the corresponding isomers of diaminotoluene (TDA). Finally, the TDA is subjected to phosgenation, i.e. treatment with phosgene to form TDI. This final step produces HCl as a byproduct and is a major source of industrial hydrochloric acid.
HENCE THE NAME AS IT IS PREPARED FROM TOULENE AND IS DOUBLY NITRATED ETC.
Distillation of the crude TDI mixture produces an 80:20 mixture of 2,4-TDI and 2,6-TDI, known as TDI (80/20). Differentiation or separation of the TDI (80/20) can be used to produce pure 2,4-TDI and a 65:35 mixture of 2,4-TDI and 2,6-TDI, known as TDI (65/35).
2) The melting point if polyurethane is 240 degrees Celsius.
The melting point if polyurethane is 240 degrees Celsius. The processing temperature must be 227 - 260 degrees Celsius. The molding pressure is 69 - 138 MPa.
3)Why is the melting point of sugar low.
This is a great chemistry question. I hope you are interested in studying chemistry! First sugar melts at 146 degrees Celsius (or about 295 degrees Fahrenheit). I
4) POLYURETHANE USES
Polyurethane products have many uses. Over three quarters of the global consumption of polyurethane products is in the form of foams, with flexible and rigid types being roughly equal in market size. In both cases, the foam is usually behind other materials: flexible foams are behind upholstery fabrics in commercial and domestic furniture; rigid foams are inside the metal and plastic walls of most refrigerators and freezers, or behind paper, metals and other surface materials in the case of thermal insulation panels in the construction sector. Its use in garments is growing: for example, in lining the cups of brassieres. Polyurethane is also used for moldings which include door frames, columns, balusters, window headers, pediments, medallions and rosettes.
Polyurethane formulations cover an extremely wide range of stiffness, hardness, and densities. These materials include:
- Low-density flexible foam used in upholstery, bedding, and automotive and truck seating
- Low-density rigid foam used for thermal insulation and RTM cores
- Soft solid elastomers used for gel pads and print rollers
- Low density elastomers used in footwear
- Hard solid plastics used as electronic instrument bezels and structural parts
- Flexible plastics used as straps and bands
Polyurethane foam is widely used in high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, seals, gaskets, carpet underlay, and hard plastic parts (such as for electronic instruments).5)
One of the most desirable attributes of polyurethanes is their ability to be turned into foam. Making a foam requires the formation of a gas at the same time as the urethane polymerization (gellation) is occurring. The gas can be carbon dioxide, either generated by reacting isocyanate with water. or added as a gas or produced by boiling volatile liquids. In the latter case heat generated by the polymerization causes the liquids to vaporize. The liquids can be HFC-245fa (1,1,1,3,3-pentafluoropropane) and HFC-134a (1,1,1,2-tetrafluoroethane), and hydrocarbons such as n-pentane.
When water is used to produce the gas, care must be taken to use the right combination of catalysts to achieve the proper balance between gellation and blowing. The reaction to generate carbon dioxide involves water molecule reacting with an isocyanate first forming an unstable carbamic acid, which then decomposes into carbon dioxide and an amine. The amine reacts with more isocyanate to give a substituted urea. Water has a very low molecular weight, so even though the weight percent of water may be small, the molar proportion of water may be high and considerable amounts of urea produced. The urea is not very soluble in the reaction mixture and tends to form separate "hard segment" phases consisting mostly of polyurea. The concentration and organization of these polyurea phases can have a significant impact on the properties of the polyurethane foam.
High-density microcellular foams can be formed without the addition of blowing agents by mechanically frothing or nucleating the polyol component prior to use.
Surfactants are used in polyurethane foams to emulsify the liquid components, regulate cell size, and stabilize the cell structure to prevent collapse and surface defects. Rigid foam surfactants are designed to produce very fine cells and a very high closed cell content. Flexible foam surfactants are designed to stabilize the reaction mass while at the same time maximizing open cell content to prevent the foam from shrinking.
An even more rigid foam can be made with the use of specialty trimerization catalysts which create cyclic structures within the foam matrix, giving a harder, more thermally stable structure, designated as polyisocyanurate foams. Such properties are desired in rigid foam products used in the construction sector.
Careful control of viscoelastic properties — by modifying the catalysts and polyols used —can lead to memory foam, which is much softer at skin temperature than at room temperature.
Foams can be either "closed cell", where most of the original bubbles or cells remain intact, or "open cell", where the bubbles have broken but the edges of the bubbles are stiff enough to retain their shape. Open cell foams feel soft and allow air to flow through so they are comfortable when used in seat cushions or mattresses. Closed cell rigid foams are used as thermal insulation, for example in refrigerators.
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