Polyol reaction with diisocyanate

Figure Z21 The general reaction of toluene diisocyanate (TDI) with polyols...

Toluene diisocyanate is used in the manufacture of polyurethanes, which are produced by reaction with polyols and polyesters. 

Polyurethanes (PUs), one of the most commonly used polymers for various blood-contacting biomedical applications, are generally prepared by the polycondensation reactions of diisocyanates with diols or amines [35, 36]. Reactions of diisocyanates with diols result in the formation of urethane linkages while diisocyanates reactions with amines result in urea linkages. Both aliphatic, as well as aromatic diisocyanate monomers, are commonly used for preparing polyurethane biomaterials. Examples include 1,4-butane diisocyanate (BDI), 1,6-hexamethylene diisocyanate (HDI), 4,4-dicyclohexylmethane diisocyanate (HMDI), and 4,4-diphenylmethane diisocyanate (MDl) [37]. Commonly used diols (or termed as polyols) for preparing polyurethanes includes poly ethers, polycaprolactone, and polyesters with molecular weights up to 5000 Da. 

Hie most representative member of this class of polyesters is the low-molar-mass (M 1000-3000) hydroxy-terminated aliphatic poly(2,2/-oxydiethylene adipate) obtained by esterification between adipic acid and diethylene glycol. This oligomer is used as a macromonomer in the synthesis of polyurethane elastomers and flexible foams by reaction with diisocyanates (see Chapter 5). Hydroxy-terminated poly(f -caprolactonc) and copolyesters of various diols or polyols and diacids, such as o-phthalic acid or hydroxy acids, broaden the range of properties and applications of polyester polyols. 

Sheratte55 reported the decomposition of polyurethane foams by an initial reaction with ammonia or an amine such as diethylene triamine (at 200°C) or ethanolamine (at 120°C) and reacting the resulting product containing a mixture of polyols, ureas, and amines with an alkylene oxide such as ethylene or propylene oxide at temperatures in the range of 120-140°C to convert the amines to polyols. The polyols obtained could be converted to new rigid foams by reaction with the appropriate diisocyanates. 

Many diols and polyols like 1, 4-butanediol and hydroxy-terminated polyesters or polyethers or polyesteramides are used for reaction with diisocyanates commercially. 

The reaction rates of systems can be measured. Using heated cells, the reaction between mixtures of polyols and isocyanates can be followed. There are a number of changes to the infrared spectrum that take place. The "-OH" band at 3460 cm 1 will decrease, as will the "-NCO" band at 2270 cm-1, as the diisocyanate reacts with the polyol, leaving only the terminal isocyanate groups. Other urethane bands at 3289, 1730, 1534, and 1230 cm-1 will increase. The ether group (1112 cm-1) will stay the same, because it does not take part in the reaction. It can be used as a reference to normalize the other bands. If a polyol and MDI are studied, it is found that there is only one rate constant, whereas when using 80 20 TDI there are two rate constants. 

If the hydroxyl group (-OH) in a conventional diol is replaced by an amine group (-NH2), polyamines are formed. The polyamines can be used in a similar manner to polyols to form a polyurea on reaction with diisocyanate. These polymers do not have any urethane groups, only urea groups. Because... 

Thermal degradation of foams is not different from that of the solid polymer, except in that the foam structure imparts superior thermal insulation properties, so that the decomposition of the foam will be slower than that of the solid polymer. Almost every plastic can be produced with a foam structure, but only a few are commercially significant. Of these flexible and rigid polyurethane (PU) foams, those which have urethane links in the polymer chain are the most important. The thermal decomposition products of PU will depend on its composition that can be chemically complex due to the wide range of starting materials and combinations, which can be used to produce them and their required properties. Basically, these involve the reaction between isocyanates, such as toluene 2,4- and 2,6-diisocyanate (TDI) or diphenylmethane 4,3-diisocyanate (MDI), and polyols. If the requirement is for greater heat stability and reduced brittleness, then MDI is favored over TDI. 

The uncatalysed reaction of diisocyanates with polyols does not have any significance in the formation of polyurethanes. Reactions are catalysed by acids and organic bases. Among the good catalysts are tertiary amines and organometallic compounds, mainly tin derivatives. 

Some of the most familiar reactions falling into the polycondensation class are those leading to polyamides derived from dicarboxylic acids and diamines, polyesters from glycols and dicarboxylic acids, polyurethanes from polyols and polyisocyanates, and polyureas from diamines and diisocyanates. Similar polymer formations utilizing bifunctional acid chlorides with polyols or polyamines also fall into this class. The condensations of aldehydes or ketones with a variety of active hydrogen compounds such as phenols and diamines are in this group. Some of the less familar polycondensation reactions include the formation of polyethers from bifunctional halogen compounds and the sodium salts of bis-phenols, and the addition of bis-thiols to diolefins under certain conditions. 

When a diisocyanate reacts with a diol, a linear polyurethane is generated, and when it reacts with a polyol, it generates a cross-linked polymer. The isocyanates are commonly prepared by the reaction of phosgene and primary amines as follows ... 

Di-n-butyltin catalysts are being used in the preparation of polyurethane foams. Most polyurethane foams utilize aromatic isocyanates such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) as the isocyanate, and a polyester or polyether polyols as the coreactant. Tertiary amine catalysts are used to accelerate the reaction with water and formation of the carbon dioxide blowing agent. To achieve a controlled rate of reaction with the polyol, an organotin catalyst can be used. Polyurethane foams are not only applied in place, but are also cast in a factory as slabstocks. These foam slabs are then cut for use in car seats, mattresses, or home furnishings. DBTDL is an excellent catalyst in high resiliency slabstock foams. DBTDL shows an excellent reaction profile for this application replacement for DBTDL in such an end-use is difficult and requires a substantial reformulation of the foam. 

The modification of a filler surface with isocyanates is a simple process which involves the reaction of hydroxyl groups on the filler surface with monomeric isocyanate. 2,4-toluene diisocyanate or hexamethylene diisocyanate are commonly used. Since isocyanates are bifunctional they can be further reacted with polyols to form a coating on the surface or they can be used for the reinforcement of polyurethane. A strong covalent bonding can be verified by controlled extraction with the solvent. Bound material will not be removed from the fillefs surface. 

Special methods of incorporation fillers are frequently pre-dispersed in polyol (for better mechanical properties) or plasticizers (to dry while dispersing) hydroxyapatite was modified by reaction with hexamethylene diisocyanate " ... 

Preparation of Cast Elastomers. The cast elastomers were prepared in a two-step procedure. First prepolymers were made from one polyether polyol (poly(oxy-tetramethylene) glycol of 1000 M.W., (POTMG)) and two polyester polyols (adipate polyester of 2000 M.W. (PAG) and polycaprolactone of 1250 M.W. (PCL)) by reaction with the corresponding diisocyanates (MDI, PPDI, CHDI or NDI) at an NCO/OH ratio of 2/1. The temperature was maintained at 80°C and periodic samples were withdrawn to determined the isocyanate content. When the isocyanate content of the mixture reached within 0.3% of the calculated value, the reaction was stopped by cooling. The prepolymer could be kept for a period of six months in the absence of moisture. The isocyanate-terminated prepolymers were then chain-extended with... 

Prepolymers are formed by the reaction of a diisocyanate with an oligo-polyol, at the molar ratio [diisocyanate]/[OH group] of 1/1, in fact only one group of diisocyanate reacts with one hydroxyl group of the polyol. A structure with free terminal -NCO groups called prepolymer is produced (see Equation 2.4) ... 

A reliable method to determine the oligo-polyol reactivity is the study of the kinetics of the oligo-polyol s reaction with phenyl isocyanate, a model for the -NCO groups of toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) ... 

Unfortunately, the high viscosities of PIPA polyols obtained by the reaction of diisocyanates with glycols in polyethers, make the reaction with triethanolamine or diethanolamine preferred at industrial scale. 

The general radical copolymerisation reaction for synthesis of acrylic polyols is shown in reaction 10.1. It is obligatory that one of the comonomers is a hydroxyalkyl acrylate or hydroxyalkyl methacrylate (mainly hydroxyethylacrylate and hydroxyethylmethacrylate) in order to introduce hydroxyl groups (as lateral groups, not as terminal groups) available for the reaction with -NCO groups of diisocyanates (reaction 10.1). 

The very low glass transition temperature (Tg) of polysiloxane chains (Tg = -123 °C) is a very attractive property for using these kinds of polymeric chains to build an oligo-polyol structure with terminal hydroxyl groups [1]. The resulting structure called a polysiloxane polyol gives, after reaction with diisocyanates, polyurethane (PU) elastomers which conserve their high elasticity at very low temperatures [1]. 

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