Isocyanates, 1-functionalize

Nitrocellulose based lacquers often contain short or medium oil alkyds to improve flexibiUty and adhesion. The most commonly used are short oil non drying alkyds. Amino resins or urethane resins with residual isocyanate functional groups may be added to cross-link the coating film for improved solvent and chemical resistance. The principal appHcations are furniture coatings, top lacquer for printed paper, and automotive refinishing primers. 

Polymers. The molecular weights of polymers used in high energy electron radiation-curable coating systems are ca 1,000—25,000 and the polymers usually contain acryUc, methacrylic, or fumaric vinyl unsaturation along or attached to the polymer backbone (4,48). Aromatic or aUphatic diisocyanates react with glycols or alcohol-terrninated polyether or polyester to form either isocyanate or hydroxyl functional polyurethane intermediates. The isocyanate functional polyurethane intermediates react with hydroxyl functional polyurethane and with acryUc or methacrylic acids to form reactive p olyurethanes. 

Blocked isocyanate, for our purposes, will refer to the reaction product of a diisocyanate or isocyanate-terminated prepolymer in which the isocyanate functionality has been reacted with a blocking agent . Once blocked , the diisocyanate can be added to polyols or certain chain extenders, and these materials will not react at room temperature. The concept is shown in the sixth item of Fig. 1. An adhesive formulated with a blocked isocyanate is basically a two-component adhesive that does not react until heated to the activation temperature. When an adhesive is made with a blocked isocyanate together with hydroxyl-containing curatives, the adhesive has a good long shelf life at room temperature. However, once heated... 

Solvent-home adhesives are of two different types reactive and non-reactive. The reactive solvent-home adhesives are usually high molecular weight oligomers with isocyanate functionality. When applied, these adhesives can react further, increasing physical properties. The non-reactive solvent-home adhesives will not react further after application. 

Processing of rigid foams from two part formulations involves combining measured quantities of the polyisocyanate with a polyhydroxyl such that there are no or limited reactive isocyanate functional groups. Moisture is not required to complete the cure. Once the reactants are combined the mixture is poured into a form where expansion and polymerization take place simultaneously. Cure times are usually very fast, on the order of minutes. 

A more complete discussion of the kinetics of isocyanate crosslinking in the presence of humidity along with experimental verification of the rate equations used above has been given by van der Ven, et al. (18). Equations 38 and 39 can be integrated numerically for different values of isocyanate to hydroxy ratio and H. Plots of the consumption of hydroxy and isocyanate functionality are shown in Figure 2 for H-0 and H-0.66 assuming equal Initial Isocyanate and hydroxy levels. High humidity increases the rate of consumption of isocyanate and reduces the consumption of hydroxy. 

HPGPC also was used for quality control of incoming raw materials. Figure 8 shows the chromatograms of two different batches of blocked isocyanate crosslinkers. One was acceptable and the other was too reactive. As can be seen from the HPGPC traces, the level of the component eluted at retention volume 40 is much higher for CX-46 than for CX-48. This component was associated with free isocyanate functionality which in excess would make CX-46 too reactive. With this information, either the necessary adjustment for the presence of excessive free isocyanate functionality could be made or this particular batch from the supplier could be rejected. 

Grelier etal. (1997) synthesized a variety of UV stabilizers based upon hydroxy-phenylbenzotriazoles incorporating an isocyanate functionality. These were reacted with wood using microwave energy. The modified woods were exposed to UV irradiation and the photoinduced discolouration determined. It was found that the best protection was obtained when the UV stabilizer was used in conjunction with polyethylene glycol, or hindered amine light stabilizers. 

The nitration of aromatic hydrocarbons is one of the most widely studied and well-documented reactions in organic chemistry. Aromatic nitro compounds are of huge industrial importance in the synthesis of pharmaceutical drugs, agrochemicals, polymers, solvents and perfumes, and for the synthesis of other industrially important chemicals containing amine and isocyanate functionality. However, early research into aromatic nitration was fuelled exclusively by their use as explosives and intermediates in the synthesis of dyestuffs. The former is the subject of this chapter. 

In the urethane process a small amount of water is added to convert some isocyanate functionalities into CO2 gas and amines. The degree of foaming can be controlled by the amount of water added. 

Note Top reaction sequence represents hydrolysis of isocyanate functions to corresponding amines. The remainder of the reaction scheme represents some of the potential side reactions of hexamethylene diisocyanate and the hydrolysis products of hexamethylene diisocyanate. Extended repetition ofthese reactions results in the formation of high molecular weight polyurea compounds. 

Nozawa et al. (5) prepared reactive methacrylate monomers containing isocyanate functions, (IV), which were used in dental compositions. 

A number of issues related to this reaction should be discussed. First, a polymer is rarely isolated in this form. In the early 1950s a technology was developed that has since come to be known as the one-shot process. While the technique certainly produces a capped polyol, it immediately reacts further to achieve its ultimate form (Figure 2.6, bottom). You will notice that the capped polyol still has isocyanate functionalities as end groups. Regardless of the process, these end groups must continue to react (by the addition of water and/or a catalyst) to complete the process. While this reaction produces one of the most commonly constructed polyurethanes, it is rarely isolated as an end product. 

The last chemical characteristic is the attachment of ligands. Unlike the other properties (reservoir capacity and biocompatibility) that could be incorporated into a one-shot process, this aspect is most conveniently practiced at the prepolymer level. The philosophy is to use some of the isocyanate functionality of the prepolymer to attach active side chains. This is illustrated in two examples that we will discuss again. 

A methyl isocyanate-functionalized resin 8 has been used to sequester excesses of amines or hydrazines from solution phase when these reactants were used in urea-forming reactions or pyrazole-forming reactions.19,21 Finally, the a-bromoketone resin 9 was shown to efficiently sequester thioureas from solution in Hantzsch aminothiazole-forming reactions.27... 

Table 2.2. Extents of reaction in acetone between alcohol and isocyanate functions on different poly(n-butyl acrylate) molecules, extrapolated to 0% reactive functions...

An alternative explanation,1 1 is that halide ion opens the epoxide ring, and that the resultant /J-chloroalkoxide ion attacks the carbon atom of the isocyanate function, as shown in Eq. ( 72). 

Specialty Isocyanates. Specialty isocyanates are organic isocyanates having the isocyanate function attached to a carbonyl group or to elements other than carbon. /t-Toluenesulfonyl isocyanate is used as a drying agent for organic solvents. Arenesulfonyl diisocyanates, such as m-phenylenedisulfonyl diisocyanate, are used as monomers for base-soluble polymers. Arenesulfonyl monoisocyanates are used as intermediates for pharmaceuticals and herbicides. 

Both chemically and toxicologically, the most significant property of isocyanates is the high chemical reactivity of the isocyanate functional group. Industrially, the most significant such reaction is with alcohols to yield urethane (carbamate) compounds, as shown by reaction 15.8.2. Multiple... 

Polyurethanes with isocyanate functionality (for polyurethane adhesives) Chlorinated polypropylene in solvent (for polyolefins)... 

Dozens of isocyanate functional compounds have been synthesized, but only a few find much use in urethane structural adhesives. The choices are largely dictated by a combination of performance, price, and safety considerations. Most of the materials used in adhesives are derived from the aromatic isocyanates, toluene diisocyanate (TDI) and 4,4 -diphenyl-methane diisocyanate (MDI). 

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