1.2 - Isocyanates

Several isocyanate types may be found in the market (Table 1.2).

Common name / CAS name		Formula	Structure		Molecular Weight	Mp (°C)	Bp (°C)	Density (g/l)	nToxity
toluene 2,4-diisocyanate (TDI) / 2,4-diisocyanato-1-mehyi-benzene		C9H6O2N2			174,2	21,8	121 (10 mm Hg)	1,061 (20°C)	toxic
toluene 2,6-diisocyanate (TDI) / 2,6-diisocyanato-1-mehyi-benzene		C9H6O2N2			174,2	18,2	120 (10 mm Hg)	1,2271 (20°C)	toxic
65:35 mixture of toluene 2,4 and 2,6-diisocyanate (TDI-65/35)		C9H6O2N2			174,2	5,0	121 (10 mm Hg)	1,222 (20°C)	toxic
80:20 mixture of toluene 2,4 and 2,6-diisocyanate (TDI-80/20)		C9H6O2N2			174,2	13,6	121 (10 mm Hg)	1,221 (20°C)	toxic
4,4’-diphenyl methane diisocyanate (MDI ) / 1,1’-methylenebis(4-isocyanato-benzene)		C15H10O2N2			250,3	39,5	208 (10 mm Hg)	1,183 (50°C)	harmful to health
2,4’-diphenyl methane diisocyanate (MDI)/ 1-isocyanato-2-(4-isocyanatophenyl) methylbenzene		C15H10O2N2			250,3	34,5	154 (1,3 mm Hg)	1,192 (40°C)	harmful to health
2,2’-diphenyl methane diisocyanate (MDI)/ 1,1’-methylenebis(2-isocyanato-benzene)		C15H10O2N2			250,3	46,5	145 (1,3 mm Hg)	1,188 (50°C)	harmful to health
hexamethylene diisocyanate (HDI) /1,6-diisocyanatohexane		C8H12O2N2	OCN-(CH2)6-NCO		168,2	-67	127 (10 mm Hg)	1,047 (20°C)	toxic
isophorone diisocyanate (IPDI) / 5-isocyabato-1-(isocyanatomethy)-1,3,3-trimethylcyclohexane		C12H18O2N2			222,3	-60	158 (10 mm Hg)	1,061 (20°C)	toxic
m-tetramethylxylene diisocyanate (m-TMXDI) / 1,3-bis(1-isocyanato-1-methylethy) benzene		C14H16N2O2			244,3	-	150 (50 mm Hg)	1,05 (20°C)	harmful to health
dicyclohexylmethane 4,4'-diisocyanate (HMDI) / 1,1’-methylebis(4-isocyanato-cyclohexane)		C15H22O2N2			262,3	19-23	179 (10 mm Hg)	-	toxic
triphenylmethane-4,4’,4”-triisocyanate / 1,1’,1”-methylidynetris (4-isocyanatobenzene)		C22H13O3N3			367,4	91	-	-	harmful to health
naphthalene 1,5-diisocyanate (NDI) / 1,5- diisocyanatonaphthalene		C12H6O2N2			210,2	127	183 (10 mm Hg)	1,450 (20°C)	harmful to health
p-phenylene diisocynate (PPDI) / 1,4-diidocyanatobenzene		C8H4O2N2			160,1	96	111 (12 mm Hg)	1,441 (20°C)	toxic

1.2.1 - Isocyanate Reactions

The manufacture of PU's began as a chemists' empirical task, however the pioneers in this field tried to explain the scientific secrets of this research area. Now, looking at the past it is clear that many discoveries have been put forward since Otto Bayer's original work. However there is still a lot of evidence of empirical character in the chemistry of PU's. The observation of the isocyanate group electronic structure indicates that the resonance structures in Figure 1.5 are possible. The electron density is smaller in the carbon atom, average in nitrogen and larger in oxygen. Isocyanate reactions mainly occur through addition to C=N double bond. An active hydrogen atom-containing nucleophilic center attacks the electrophilic carbon atom and active hydrogen is added to the nitrogen atom. NCO-bonded electron acceptor groups increase the reactivity and donor groups reduce it. For that reason, aromatic isocyanates are more reactive than aliphatic ones. Reactivity is reduced by steric hindrance in isocyanate groups, and also in active hydrogen compounds.

The five main reactions making up PU's technology are those of isocyanate with: (1) polyols, to form polyurethanes; (2) amines, to yield polyurea; (3) water, to form polyurea and carbon dioxide which is the main blowing agent in PU foams; (4) urethane and (5) urea groups resulting in the formation of allophanate and biuret cross-linking, respectively (Figure 1.6).

Figure 1.6 - Isocyanate main reactions

1.2.1.1 - Reactions with alcohols

The polymerization reaction (1), between an alcohol and an isocyanate is exothermic and releases about 24 kcal/mol of urethane. Isocyanates reactivity with alcohols is moderate (Table 2.2), being usually catalyzed by bases, mainly tertiary amines or organometals. Reactivity is influenced by structure and primary, secondary and tertiary hydroxyls have decreasing reactivity due to neighboring methyl group steric hindrance. Amine basicity exerts a strong catalyst effect on isocyanate reactions. Hydroxylated compounds with tertiary amino groups (like triethanolamine) exhibit a catalytic effect.

1.2.1.2 - Reactions with amines

Reactions (2) of isocyanates with amines, forming polyurea, are very fast (Table 1.3) and don't require catalysis. Aliphatic amines react more quickly than aromatics of lower basicity, since there isn't any significant steric hindrance. Aromatic amines will be the less reactive as the larger is the electronegativity of the aromatic ring substituents. In addition to electronic effects, the steric hindrances are an important factor. The ortho position substituents of aromatic isocyanates strongly reduce the reactivity. The highly reactive aliphatic amines are used as chain extenders for polyurea, in reaction injection moulding (RIM) (Chapter 4) and in RIM-spray coatings (Chapter 7). On the other hand, the much less reactive aromatic amines, as the methylene-bis-ortho-chloroaniline (MOCA) are used as chain extenders for casting elastomers (Chapter 6).

1.2.1.3 - Reaction with water

The blowing reaction (3), of isocyanates with water, results in urea formation and carbon dioxide gas release. This reaction is very important in PU foam production. The carbon dioxide diffusion to previously nucleated air bubbles blow the foam. The reaction is exothermic and releases 47 kcal/mol of water. The isocyanate reactivity with water and primary hydroxyl group is comparable, however much smaller than with amines (Table 1.3). The catalysis of the blowing reaction is effected with tertiary amines. Initially, carbamic acid is formed and then it decomposes into carbon dioxide and the corresponding amine, which immediately reacts with diisocyanate, yielding urea (Figure 1.7).

Figure 1.7 - Reaction of isocyanate with water

1.2.1.4 - Reactions with ureas and urethanes

The hydrogen of urethane and urea groups can react with NCO, forming allophanate and biuret cross-linking groups. These reactions are reversible and occur at temperatures above 110oC, and when not catalyzed, are slow and very slow, respectively (Table 1.3). They mainly occur during the PU's post cure, where these are kept for a long time at high temperatures (22 hours at 70°C), or days at room temperature, depending on the system.

1.2.1.5 - Reactions with acids

Besides the described five main reactions, isocyanates also react with carboxylic acids, releasing carbon dioxide (Figure 1.8).

Figure 1.8 - Reaction of isocyanates with acids

1.2.1.6 - Self-addition reactions

Isocyanates can also react with themselves, forming dimers, trimers, polymers, carbodiimides and uretoneimines (Figure 1.9). The dimerization of isocyanate to form uretidinediones should be conducted at low temperatures in view of its thermal instability. This explains why the isocyanates dimerization is limited to more reactive aromatic ones. Isocyanates trimerization is of huge commercial importance, and with crude MDI form polyisocyanurates used in rigid foams (Chapter 5). Carbodiimide formation and further reaction with an excess isocyanate to form uretoneimines is also of technical relevance for pure MDI modification, to form a liquid mixture of melting point below 20°C.

1.2.2 - Reactivity

The NCO group reactivity depends on structure. The relative isocyanates reactivities with several active hydrogen-containing compounds are shown in Table 1.3.

1.2.2.1 - Effect of structure on reactivity

The isocyanate structure is important for the NCO group reactivity. Reactivity is increased by substituents that improve the positive load on the NCO group carbon atom. This is why aliphatic isocyanates are less reactive than aromatic ones. These will be the more reactive as the higher is the electro negativity of the aromatic ring substituent. In addition to the electronic effect, steric hindrance is also important. Bulky substituents near the reaction site reduce the reactivity. These steric factors also influence the catalysts specificity, since catalysts need to be close to the reaction site, to exert the desired catalytic effect, which may be impaired by steric hindrance.

1.2.2.2 - Diisocyanates reactivity

Diisocyanate reactions are usually more complicated than monoisocyanate reactions. The initial reactivity of an aromatic diisocyanate is similar to that of a monoisocyanate with a NCO group substituent. So when the first NCO reacts, for instance, with an alcohol, the reactivity of the remainder NCO group is that of a monoisocyanate with an urethane substituent group. As the urethane group is a much weaker activator than a NCO group in the same position, in diisocyanates with both NCO groups in the same aromatic ring the reactivity falls significantly when the reaction reaches 50%. This decrease is still more accentuated if there is another substituent in the ortho position relative to the second NCO, as in TDI.

In TDI the NCO group in the para position reacts much more quickly than NCO in the ortho position. At room temperature, if we consider as 100 the reactivity of the 2,4-TDI para NCO group, the reactivity of the ortho NCO group would be 12. In 2,6-TDI the first NCO group reactivity would be 56 and for the second group it would fall to 17. Though, when temperature approaches 100oC, the steric effects are surpassed, and the reactivity of both positions is similar. Due to this, the TDI type influences some physical properties of foams. In diisocyanates with NCO groups in different aromatic rings (like MDI), or separated by aliphatic chains, the reactivity of NCO groups is the same.

1.2.3 - Commercial Isocyanates