Aliphatic polyisocyanates

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. 

These requirements are usually met with two-pack paints based on hydroxyl-rich polyester or acrylic resins in the pigmented pack and aliphatic polyisocyanates in the activator pack. Cure with this type of finish is relatively fast and complete even at low ambient temperatures. An alternative finish is an acrylic lacquer, similar to the lacquer used for refinishing motor cars. These finishes are applied to the assembled aircraft by operators protected by air-fed hoods and using airless or conventional spray guns. High durability pigments are included. 

Typically used for the coating of polyester and polyamide substrates with plasticised PVC. Chemically one-component bonding agents (e.g. aromatic polyiso-cyanurate) and two-component bonding agents (e.g. aliphatic polyisocyanate) liquids. 

Polyurethane-acrylic coatings with interpenetrating polymer networks (IPNs) were synthesized from a two-component polyurethane (PU) and an unsaturated urethane-modified acrylic copolymer. The two-component PU was prepared from hydroxyethylacrylate-butylmethacrylate copolymer with or without reacting with c-caprolactonc and cured with an aliphatic polyisocyanate. The unsaturated acrylic copolymer was made from the same hydroxy-functional acrylic copolymer modified with isocyanatoethyl methacrylate. IPNs were prepared simultaneously from the two-polymer systems at various ratios. The IPNs were characterized by their mechanical properties and glass transition temperatures. 

Aliphatic polyisocyanates The most important aliphatic polyisocyanates are given in Figure 4.7. 

MDI- or NDI-based PUs usually have better mechanical properties than PUs obtained from aliphatic polyisocyanates. The effect of various diisocyanates on the thermal stability of their PU elastomers is given below for a polycap-rolactone (mol. wt. 2000)/diisocyanate/BDO type elastomer of molar ratio 1 2.6 1 and synthesized using the prepolymer technique. 

HMDI was originally produced by Du Pont as a coproduct in the manufacture of Qiana fiber. Du Pont subsequently sold the product to Bayer. In the 1990s MDA is hydrogenated by Air Products for Bayer (see Amines, AROMATIC-METHYLENEDIANILINE). Commercial HMDI is a mixture of three stereoisomers. Semicommercial aliphatic diisocyanates include / mf-cyclohexane-l,4-diisocyanate (CHDI) and w-tetramethylxylylene diisocyanate (TMXDI). A coproduct in the production of TMXDI is / -isopropenyl-a,a-dimethylbenzyl isocyanate (TMI), which can be copolymerized with other olefins to give aliphatic polyisocyanates. 

A variety of urethane coating systems has been introduced based on hydroxyl-containing acrylic and methacrylic ester copolymers that contain hydroxyethyl acrylates or methacrylates and that are generally cured with aliphatic di- or polyisocyanates. Depending upon the type and relative amounts, acrylate or other saturated esters, for example, adipate-isophthalic esters, are also used in combination with aliphatic polyisocyanates. 

Polyester resins combined with aliphatic polyisocyanate resins have outstanding weather resistance, good substrate adhesion, and high flexibility even under very large temperature changes (aircraft finishes). 

Aliphatic and cycloaliphatic polyisocyanates can be used to produce lightfast, weather-resistant coatings for extremely severe conditions. Aromatic polyisocyanates are of limited suitability for exterior applications and react more quickly than aliphatic polyisocyanates. The different reactivities of aromatic and aliphatic polyisocyanates influence their use because they affect drying and curing. 

The reaction between the isocyanate and the hydroxyl groups results in 100% cross-linking if the reactants are converted in stoichiometric proportions. Optimum paint films are generally formed when NCO OH equals 1. However, it may be advantageous to deviate from this ratio (under- or over-cross-linking). With aliphatic polyisocyanates, a catalyst (accelerator) is generally necessary. Small amounts (0.05-0.5%) of tertiary amines or metal-containing compounds are normally added... 

Aqueous two-pack polyurethane paints are obtained by combining, for example, hydroxyl-containing polyacrylate dispersions with water-emulsifiable, low-reactive aliphatic polyisocyanates. 

Polyurethane coatings have three main advantages high mechanical resistance, outstanding chemical resistance, and (in the case of aliphatic polyisocyanates) excellent lightfastness and weather resistance. The properties of the paint systems which harden at ambient temperature are unsurpassed. They can also be combined and formulated in a large number of ways. Specific properties can be optimally adjusted even for extreme conditions. 

Aliphatic polyisocyanate Aromatic polyisocyanate in ethyl acetate Contains no solvents... 

Chem. Descrip. Aliphatic polyisocyanate based on hexamethylene diisocyanate... 

Chem. Descrip. Acrylic resin Uses Modifier, tlexibilizer tor other acrylics Features Exc. adhesion to plastic, steel, and metallic substrates when formulated with aliphatic polyisocyanates top urethane durability Properties Dens. 8.6 Ib/gal 60% solids G-Cure 869 [Cognis/Coatings Inks]... 

Light cured compositions that contain acrylated polyurethanes have a general reputation of possessing good abrasion resistance, toughness, and flexibility. Furthermore, these properties can be varied by picking either aliphatic polyisocyanates, or aromatic ones, depending upon need. Usually, prepolymers derived from aromatic isocyanates yield better adhesion to the substrate than do the aliphatic ones. 

TPU MWCNT MWCNT-HPU 0.5,1 One-pot polycondensation to graft HPU to CNTs 40-60/- CNTs added to P(EO-THF), mixed with aliphatic polyisocyanate, dibutyltin dilaurate, degassed, cast into molds = 0.5 >1 l.MO-12 1.210-7 [376]... 

Principally, the major aliphatic polyisocyanates comprise hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and dicyclohexylmethane 4,T-diisocyanate (H12-MDI). Aliphatic isocyanates are used if a non-yellowing sealant is required. Table 3 contains a list of the most common isocyanates used in adhesives and sealants. 

The chromophores are closely related to those in peptides. Dissymetric perturbations of the amide chromophores of the main chain arise solely from side chain effects the aliphatic polyisocyanate has enhanced rotational strength as compared to its model compound (5 )-(-)-A,A -diacetyl-2-methylbutylamine. In the aliphatic polymer, the main chain has inherently symmetric chromophores which acquire optical activity from dissymetric perturbation of their environment by the side chain. In the aromatic polymer an additional Cotton effect also arises from interactions among the aromatic side chains. This enhancement may be explained by a conformational preference resulting from favored spatial arrangements of the asymmetric side chain but the study was complicated by the fact that the polymer is insoluble in most organic solvents except in chloroforms and by the specific interactions between this solvent and the urea-like main chain (XIXc). 

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