2 Basic Chemistry of Polyurethanes
CH 3 COOK
2.9 Reaction of Isocyanates with Cyclic Anhydrides
Isocyanates react with cyclic anhydrides to form cyclic imides [1, 3, 15]:
R N C O + O R N +
CO CO
CO
CO2
CO
Basic Chemistry of Polyurethanes
Table 2.1 shows the relative reaction rates of isocyanates against different HXR.
All the amines are much more reactive than the hydroxyl compounds, the relative order being as follows:
R >> >> Ar R′
R Primary
aliphatic amine
Secondary aliphatic amine
Primary aromatic amine
Secondary aromatic amine
NH2 NH NH2>
Ar R
NH
(2.15)
Primary hydroxyl groups are more reactive than secondary hydroxyl groups and much more reactive than tertiary or phenolic hydroxyl groups:
R > >> R
R′ R′
R R′′
Primary
hydroxyl Secondary
hydroxyl Tertiary
hydroxyl Phenolic hydroxyl CH2OH CHOH COH > Ar OH
(2.16)
Primary hydroxyl groups are around 3× more reactive than secondary hydroxyl groups and 200× more reactive than tertiary hydroxyl groups.
In order to understand the effect of polyol structure on the properties of PU a minimum amount of information about the structure and reactivity of isocyanates is needed.
Oligo-polyols for PU are commercialised in a large number of types and structures.
However, in practice, limited types of isocyanates are used. The most important isocyanates, covering the majority of PU applications are aromatic isocyanates:
toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). Aliphatic isocyanates such as hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) or 4,4′-methylenebis(cyclohexyl isocyanate) (HMDI) are used to a much lesser extent, and only for special applications. TDI is commercialised using a mixture of the 2,4 and 2,6 isomers (TDI 80/20 having 80% 2,4-TDI and 20% 2,6-TDI and TDI 65/35 having 65% 2,4-TDI and 35% 2,6-TDI) or 2,4-TDI as pure isomer. The most important application of TDI is in flexible PU foam manufacture. The structures of commercial TDI are presented in Figure 2.1 [1–3, 6, 12, 13, 23, 27, 28].
Table 2.1 The relative reactivities of isocyanates against different hydrogen
Hydrogen active compound Formula Relative reaction rate (non-catalysed, 25 °C)
Primary aliphatic amine R–NH2 2,500 Secondary aliphatic amine R2 –NH 500–1,250 Primary aromatic amine Ar–NH2 5–7.5
Primary hydroxyl R–CH2–OH 2.5
Water HOH 2.5
Carboxylic acid R–COOH 1
Secondary hydroxyl R2 –CH–OH 0.75
Urea R–NH–CO–NH–R 0.375
Tertiary hydroxyl R3 –C–OH 0.0125
Phenolic hydroxyl Ar–OH 0.0025–0.0125
Urethane R–NH–COOR 0.0025
CH3
N C O O C N N C O
CH3
80%
TDI 80/20
20%
CH3
N C O O C N N C O
CH3
65%
CH3
N C O
N C O
N C O N C O
2,4-TDI
TDI 65/35
35%
Figure 2.1 The chemical structures of commercial TDI
The second most important aromatic isocyanate is MDI, commercialised in various forms and functionalities, the most important being: pure MDI, ‘crude’ MDI and
Basic Chemistry of Polyurethanes Pure MDI, having 2 -NCO groups/mol, is commercialised mainly as 4,4′ isomers, but it is possible to use 2,4 and 2,2 isomers. The main applications of pure MDI (especially the 4,4′ isomer) are: PU elastomers, microcellular elastomers and some flexible foams. The structures of pure MDI isomers are presented in Figure 2.2.
CH2 N C O
O C N
4,4′-MDI 2,4′-MDI
2,2′-MDI
O C N
CH2 N C O
N C O
CH2
N C O
Figure 2.2 The chemical structures of pure MDI
‘Crude’ MDI is a mixture of 4,4′-MDI isomer (around 48–50%) and high-molecular weight (MW) isomers having 3, 4, 5 and higher numbers of aromatic rings, with functionalities in the range of 2–3 -NCO groups/mol (Figure 2.3).
NCO
OCN NCO
Pure 4,4′-MDI
Polymeric MDI
NCO ‘Crude’ MDI CH2
CH2
NCO CH2
Figure 2.3 The chemical structure of ‘crude’ MDI
A high functionality PAPI, obtained after the distillation of one part of pure 4,4′-MDI isomer, has a high functionality, close to 3 -NCO groups/mol (Figure 2.4).
NCO
PAPI
NCO CH2
NCO CH2
Figure 2.4 The chemical structure of PAPI
‘Crude’ MDI and PAPI are especially used in highly crosslinked PU, such as rigid PU foams. PAPI have lower vapour pressures than TDI. Mixtures of TDI with PAPI are also used in many applications, (e.g., in high resilience flexible foams). Aliphatic diisocyanates have a much lower reactivity than aromatic isocyanates. The most important aliphatic diisocyanates are presented in Figure 2.5 [1-3, 6, 23-25].
NCO
HMDI
HDI IPDI
OCN CH2
NCO
NCO
6 NCO
OCN CH2 CH3
CH3
Figure 2.5 The chemical structures of some aliphatic diisocyanates
The characteristics of commercial TDI are presented in Table 2.2 and the characteristics of commercial MDI in Table 2.3.
The reactivity of isocyanates toward active hydrogen compounds is a much more complex problem. As a general rule, the -NCO groups of a diisocyanate have different
Basic Chemistry of Polyurethanes effect is simple: after the reaction of the first molecule of the HXR (e.g., an alcohol), the diisocyanate is first transformed into a urethane isocyanate. The second isocyanate group has a much lower reactivity than the first -NCO group, because the urethane group, due to its electron releasing effect, decreases the reactivity (Equation 2.17).
O C N R N C O + O R N C O
O HOR′ R′ CNH
Diisocyanate Urethane isocyanate
Urethane isocyanate
K1
N +
R C O
O O
CNH HOR′
R′ O R NH
O
CNH O
O
R′ C R′
K2
K1 > K2 (2.17)
Table 2.2 The main characteristics of commercial TDI
Property TDI 80/20 TDI 65/35 2,4-TDI
Form Liquid Liquid Liquid
MW (g/mol) 174.16 174.16 174.16
EW (g/OH group) 87.08 87.08 87.08
bp (°C) at 0.101 MPa 251 251 251
Freezing point (°C) 14.0 8.5 21.4
bp: Boiling point EW: Equivalent weight
Table 2.3 The main characteristics of two commercial MDI
Property Pure MDI Polymeric MDI
Form Solid Liquid
MW (g/mol) 250 >450
Functionality (-NCO groups/mol) 2 2–3
EW (g/OH group) 125 >225
bp (°C), at 665 Pa 194 –
This interesting effect is presented in Table 2.4. The difference between the values of K1 and K2 and the higher reactivity of aromatic isocyanates (TDI and MDI) is shown, as compared to aliphatic isocyanates (HDI and HMDI).
Table 2.4 The different reactivities of -NCO groups in some aromatic and aliphatic diisocyanates against hydroxyl groups
Diisocyanate R K1 K1/K2
2,4-TDI
CH3
OCN
NCO
400 12.121
Pure MDI
CH2
OCN NCO 320 2.909
HDI OCN CH3 6 NCO 1 2.000
HMDI OCN CH2 NCO 0.57 1.425
In PU fabrication, some special techniques are used, such as: prepolymer technique, quasiprepolymer technique and ‘one shot’ technique.