Diisocyanate-based polyurethanes

Flash Photolysis of Aromatic Diisocyanate-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. 

Future work will be concerned with quantitative measurement of hydrogen abstraction rates of labile hydrogens in the carbamate moieties of several aromatic diisocyanate based polyurethanes. It is expected that experimental conditions will alter significantly the hydrogen abstraction rate. Emphasis will also be placed on measurement of transient intermediates in polyurethane films. Finally, extensive laser flash photolysis experiments will be conducted on polyurethanes based on both 2,A-toluenediisocyanate and 2,6-toluenediisocyanate. Preliminary data suggest that the placement of the methyl substituent can alter the nature of the transient intermediates formed. 

Fluorescence of Naphthyl Carbamate Models. Two compounds (shown below) based on carbamate substituted naphthalene will be considered as models for the naphthalene diisocyanate based polyurethanes under investigation in this paper. The propyl N-(1-naphthyl) carbamate (PNC) and dipropyl N,N -naphthalene-1,5-dlylbiscarbamate (1,5-DN3) model compounds were made by reacting the requisite mono- or diisocyanate with 1-propanol. 

Thus, the hardening of the toluene diisocyanate-based polyurethane system in the presence of microquantities of the surface-active agent KEP-2 occurs at all stages of the process in conditions different from the ordinary hardening mixture of TMP-TDI-oligoglycol. These differences become apparent in kinetic and structural studies of the hardening process. The final product is a polymer with very weak adhesion compared with the initial specimen (hardening temperature 353 K). 

K. Kojio, S. Nakashima and M. Furukawa, Microphase-separated structure and mechanical properties of norbornane diisocyanate-based polyurethanes . Polymer, 2007, 48, 997-1004. 

Fig. 3.4. Temperature dependence of the loss tangent (tan S) of different diisocyanate-based polyurethanes. PU type Capa 225/diisocyanate/1A BDO molar ratio 1 2 6 1 (from Barikanf 1986).

Figure 3.4 compares the loss tangents (tan 5) of different diisocyanate-based polyurethanes. For each PU three relaxation peaks are observed present in the low temperature range and designated as the a, jS and y peaks. The overall effects of these diisocyanate structures are significant, with the major phase changes occurring at about —15 to —33°C (Table 3.11). 

Other polymers that have been subjected to thermal stability studies include cycloaliphatic diisocyanate based polyurethanes (DSC, TGA, ET-IR) [40-42], polyallyl azide [DSC, TGA, FT-IR, nuclear magnetic spectroscopy (NMR)] [43], ethylene vinyl acetate nanocomposites (TGA, XRD) [44], methyl methacrylate - N-phenylmaleimide... 

Laser flash photolysis of model compounds of diphenylmethane-4,4 -diisocyanate based polyurethane low molecular weight model compounds, such as mono- and bis-carbamates, exhibits the formation of diarylmethyl radicals which readily react with oxygen. The formed polymer peroxy (PO2) radicals abstract hydrogen and yield polymer hydroperoxides (POOH) [1000, 1001, 1003, 1006, 1007]. 

N. Yamamoto, A. Nakayama, M. Oshima, N. Kawasaki, and S.i. Aiba, Enzymatic hydrolysis of lysine diisocyanate based polyurethanes and segmented polyurethane ureas by various proteases. Reactive and Functional Polymers, 67(11) 1338-1345, November 2007. 

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