Di isocyanate

The polycondensation of di-isocyanates with polyhydric alcohols gives a wide range of polyurethanes which are used as artificial rubbers and light-weight foams, and have other important properties. Isocyanates are also used as modifiers in alkyd resins. ... 

The third approaeh to synthetic polymers is of somewhat less commereial importance. There is in fact no universally accepted deseription for the route but the terms rearrangement polymerisation and polyaddition are commonly used. In many respects this process is intermediate between addition and condensation polymerisations. As with the former teehnique there is no moleeule split out but the kinetics are akin to the latter. A typical example is the preparation of polyurethanes by interaction of diols (di-alcohols, glycols) with di-isocyanates Figure 2.7). 

Polytetrafluoroethylene contains only C—C and C—F bonds. These are both very stable and the polymer is exceptionally inert. A number of other fluorine-containing polymers cU e available which may contain in addition C—H and C—Cl bonds. These are somewhat more reactive and those containing C—H bonds may be cross-linked by peroxides and certain diamines and di-isocyanates. 

Terephthalazide (VIII) Acid azide N2 85-112 207-311 Generates a di-isocyanate on decomposition which may cross-link some polymer species. Subsidiary reactions may increase blowing power. 

Vulcanisation may also be brought about by zinc and calcium peroxides, p-quinone dioxime, epoxide resins, phenolic resins and di-isocyanates. 

Their main point of difference is that the phenoxies are of much higher molecular weight ( 25 000). The phenoxies are also said to be slightly branched. Like the epoxide resins they are capable of cross-linking via the pendant hydroxyl groups, in this instance by di-isocyanates and other agents. 

The chain length of the polymer is then increased by reacting the wax via the hydroxyl, amino or acid end groups with a di-isocyanate such as hexamethyle-nedi-isocyanate (see Chapter 27 for the appropriate reaction). 

This tri-isocyanate is reported to impart good light stability and weather resistanee in polyurethane eoatings and is probably the most widely used aliphatic isocyanate. A number of other aliphatic polyisocyanates have been introduced recently in attempts to produce polyurethanes with improved light stability. Amine derivatives of diphenylmethane are made by reacting aniline of toluidines with formaldehyde. These can lead to a mixture of di-isoeyanates, the diphenylmethane di-isocyanates (MDIs) of commerce. Triphenylmethane-pp p"-tny tri-isocyanate is produced from leucorosaniline. 

As previously mentioned the initial research on polyurethanes was directed towards the preparation of fibre-forming polymers. Many poly hydroxy compounds and many di-isocyanates were used and the melting points of some of the more linear aliphatic polyurethanes produced cU"e given in Table 27.1. 

The starting point in the prepration of these rubbers is a polyester prepared by reacting a glycol such as ethylene or propylene glycol with adipic acid. This is then reacted with an excess of a bulky di-isocyanate such as 1,5-naphthylene di-isoyanate (Figure 27.3). 

The molar excess of di-isocyanate is about 30% so that the number of polyesters joined together is only about 2-3 and the resulting unit has isocyanate end groups. A typical structure, with P for polyester groups, U for urethane and I for isocyanates would be... 

When polyols, di-isocyanates and glycols are reacted together as described in the previous section they do in fact tend to produce block copolymers as can be seen from the following reaction mechanism ... 

The isocyanate group may be terminal on a polyester chain or may be part of the unchanged di-isocyanate. The density of the product, which depends on the amount of gas evolved, can be reduced by increasing the isocyanate content of the reaction mixture and by correspondingly increasing the amount of water to react with the excess isocyanate (that is excess over that required for chain extension and cross-linking). 

Formulations for one-shot polyether systems are similar to those used for flexible foams and contain polyether, isocyanate, catalyst, surfactant and water. Trichloroethyl phosphate is also often used as a flame retardant. As with polyesters, diphenylmethane di-isocyanate is usually preferred to TDI because of its lower volatility. Tertiary amines and organo-tin catalysts are used as with the flexible foams but not necessarily in combination. Silicone oil surfactants are again found to be good foam stabilisers. Volatile liquids such as trichlorofluoro-methane have been widely used as supplementary blowing agents and give products of low density and of very low thermal conductivity. 

In addition to one-shot processes, quasi-prepolymer systems are used commercially with rigid polyether foams. The quasi-prepolymer is commonly produced using excess TDI rather than diphenylmethane di-isocyanate. Sinee the former isocyanate is light in colour and the latter dark, quasi-prepolymer foams... 

A wide range of polyurethane-type products has become available in recent years for coating applications. These include simple solutions of linear polyurethanes, two-pot alkyd-isocyanate and polyether-isocyanate systems and a variety of prepolymer and adduct systems. The coatings can vary considerably in hardness and flexibility and find use mainly because of their toughness, abrasion resistance and flexibility. Uses include metal finishes in chemical plant, wood finishes for boats and sports equipment, finishes for rubber goods and rain-erosion-resistant coatings for aircraft. One type of coating is potentially competitive with PVC leathercloth. Both alkyd-di-isocyanate and adduct-diisocyanate compositions may be coated on to fabrics from solutions of controlled viscosity and solids content. Such coated fabrics are soft, flexible and, unlike PVC leathercloth, free from plasticisers. 

Some materials, such as arsine, phosphine, toluene di-isocyanate and stibine, may be present in concentrations in excess of their hygiene standards yet undetectable by smell. 

Toluene-2,6-di isocyanate p-Toluene sulphonic acid o-Toluidine... 

There are numerous applications in solvent recovery processes where evaporation equipment are employed. Figure 14 provides an example of a process scheme for toluene-di-isocyanate recovery. This is an example of continuous vacuum evaporation of distillation residues. 

Acetylene works Acrylates works Aldehyde works Aluminum works Amines works Ammonia works Anhydride works Arsenic works Asbestos works Benzene works Beryllium works Bisulfate works Bromine works Cadmium works Carbon disulfide works Carbonyl works Caustic soda works Cement works Ceramic works Chemical fertilizer works Chlorine works Chromium works Copper works Di-isocyanate works Electricity works Fiber works Fluorine works Gas liquor works Gas and coke works Hydrochloric acid works Hydrofluoric acid works Hydrogen cyanide works Incineration works Iron works and steel works... 

Rearrangement polymerisation Here the mechanism resembles condensation polymerisation hut no stnall mbleculfr is split out. In the first example l 4-butane di6l reacts with hexaniethyiehe di-isocyanate to give 6,4-poly-... 

The Poly urea Polymer of 3,3-Dinitro-1,5-Pentane Di isocyanate and 3,3-Dinitro-1J5-Pentanediamine. 

The Polyurethane Polymer of 3,3,5,7,7-Penta-nitro-5-Aza-1,9-Nonane Di isocyanate and 5,5,5-Trinitro-1,2-Pentanediol. 

The Postnitrated Polyurethane Polymer of Ethylene Di isocyanate and Ethylene Dinitramine. 

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