MDI based polyurethanes

Table II. Composition and intrinsic viscosity of TDI- or MDI-based polyurethanes...

The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4 -diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. 

SCHEME II - Direct and Indirect Photolysis of MDI Based Polyurethanes... 

In order to demonstrate the use of laser flash photolysis in elucidation of the MDI based polyurethane photolysis mechanism, three polyurethanes, two aryl biscarbamate models, an aryl monocarbamate model, and an aromatic amine were selected. Two of the polyurethanes are based on MDI while the third is based on TDI (mixture of 2,4 and 2,6 isomers in 80/20 ratio). The MDI based polyurethanes all have the same basic carbamate repeat unit. The MDI elastomer (MDI-PUE) is soluble in tetrahydrofuran (THF). The simple polyurethane (MDI-PU) based on MDI and 1,4-butanediol is used in the tert-butoxy abstraction reactions since it does not contain a polyether backbone. (See page 47 for structures of polymers and models.)... 

The results in this paper support an N-C bond cleavage mechanism (Schemes I and II) for the photolysis of both TDI and MDI based polyurethanes. The substituted anilinyl radicals observed no doubt are formed by diffusion from a solvent cage after the primary N-C bond cleavage. Although not specifically shown in this paper, the reported photo-Fries products (6) are probably formed by attack of the carboxyl radical on the phenyl ring before radical diffusion occurs. The solvent separated anilinyl radicals rapidly abstract hydrogens from the solvent to give the reported aromatic amine product (6). 

For MDI based polyurethanes we have provided evidence for formation of a diphenylmethyl radical by direct excitation (248 nm) of the carbamate moiety as well as hydrogen abstraction by a tert-butoxy radical which is produced by excitation (351 nm) of tert-butyl peroxide. The diphenylmethyl radical readily reacts with oxygen. A proposed mechanism which accounts for the production (direct or indirect) and subsequent reaction with oxygen of the diphenylmethyl radical is shown in Scheme IV. The hydrogen peroxide product depicted in Scheme IV has been previously identified by FT-IR (7) we have simply provided a plausible mechanism for its formation. 

In our experience, polyurethanes have proven safe for topical applications. Extractable formulations including surfactants, catalysts, chain extenders, and other components used in manufacturing devices are more problematic and cause the most concern. Furthermore, all toluene diisocyante (TDI)-based polyurethanes are subject to hydrolysis that produces small amounts of toluene diamine, which has been confirmed as a carcinogen. Interesting enough, there is a strong distinction between this confirmed chronic toxicity and the suspected toxicities of MDI-based polyurethanes. 

TDI-based polyurethanes produce the best properties when further chain-extended with amine-based curatives. The overall properties can be increased if only the 100% 2,4 isomer is used. These materials are generally not suited for use with food. MDI-based polyurethanes have good overall properties and as they are predominately cured with a diol, they can obtain FDA approval more readily. 

TG-MS is ideally suited to reveal differences in pressure behaviour during thermal decomposition of materials. This has been illustrated by Mol [144] in TG-MS analysis of toluene diisocyanate (TDI) and methylene bis-4-phenol isocyanate (MDI)-based polyurethanes, where the observed greater increase in pressure for the TDI polyurethane than for the MDI derivative indicates a higher loss of low molecular weight fragments. This is not possible to deduct from the TG curves alone. Such indications are of great... 

The variation of Tg of the soft matrix in segmented polyurethanes as a function of composition or segmental chemical structure has been monitored and used as an indicator of the degree of microphase separation. Factors influencing the phase-separation process in these MDI-based polyurethanes have been summarized by Aitken and Jeffs (69) as follows (a) crystallization of either component, (b) the steric hin-... 

Phase I The properties of the phase I elastomers are shown in Tables I and II. The mechanical properties of all the elastomers, both polyether based (Table III) and polyester based (Table IV) were, in general, comparable with conventionally chain extended MDI-based polyurethane elastomers (3). 

HER is an effective chain extender for MDI-based polyurethane elastomers, with both polyether and polyester-based prepolymers. Good overall properties and green strengths resulted. 

Dalene, M., Jakobsson, K., Rannug, A., Sharping, G., and Hagmar, L., MDA in plasma as a biomarker of exposure to pyrolysed MDI-based polyurethane correlations with estimated cumulative dose and genotype for N-acetylation, Int. Arch. Occup. Environ. Health, 68, 165-169, 1996. 

Polyurethanes show many of the same features as the copolyesters and copolyamides with respect to both hard and soft segment crystallinity. One substantial difference between polyurethanes and the previous two matmals is that not all commercial polyurethane TPEs show hard segment crystallinity MDI-based polyurethanes show crystallinity while TDI-based urethanes do not. 

Figure 3 High-resolution carbon-13 solid state NMR spectra of the MDI-based polyurethane elastomer via cross polarization.

Polyurethanes show many of the same features as the copolyesters and copolyamides with respect to both hard and soft segment crystallinity. One substantial difference between polyurethanes and the previous two materials is that not all commercial polyurethane TPEs show hard segment crystallinity MDI-based polyurethanes show crystallinity while TDl-based urethanes do not. Two crystalline structures have been found for MDl-BD hard segments in the unoriented state with another distinct form found in the oriented state [86,87]. Other studies have examined the effect of chain extender length on the crystal structure [88]. The differences between polyurethanes with chain-folded and chain-extended hard segment crystalhtes has been extensively studied by Eisenbach et al. [89]. 

Polyurethanes prepared via transesterification of soybean and linseed oils with n-butanol in the presence of lipozyme (a lipase), also possess these improved properties, particularly in the case of MDI-based polyurethanes. Mechanical properties such as tensile strength, percentage elongation, elastic modulus, wear resistance, tear resistance. Shore A hardness, and thermostability of vegetable oil-based polyurethane IPNs showed significant improvement at a critical vinyl or acrylic polymer level. 

Table 4.11 Compositions of the family of MDI based polyurethane elastomers (M), with increasing the hard segment content from 20% to 80% [185]...

Comparing the DBDI and MDI based polyurethanes of different recipes in Table 4.15, it is clear that, in recipes otherwise identical (same MD and CE, and equal hard segment molar concentration) the DBDI based PUs display a substantially higher input energy density Eic and lower fractional energy recovery E than the corresponding MDI based PUs [61]. This applied for all MD/CE combinations studied PTHF-DEG (compare PU3 with PU7) PEA-DEG (compare PU6 with... 

---