Aromatic polyurethanes

The photodegradation mechanism of aromatic polyurethanes is generally much more complex than aliphatic polyurethanes, and the products obtained are difficult to separate and identify [12, 204, 567, 680-682, 689, S 97, 999, 1003-1007, 1262, 1317, 1567. 1637, 1652,1836-1838, 1901, 1902, 2105]. 

Polyurethanes with aromatic diisocyanates are more unstable with respect to light than those with aliphatic diisocyanates [1837,1901,1902,2098]. The stability of the different polyurethanes follows the order diphenylmethane-4,4 -diisocyanate (4.77) toluene-2,4-diisocyanate (4.78) hexamethylene-diisocyanate (4.79)  

The absorption spectra of polyurethane change during photoirradiation (Fig. 4.11). An absorption peak at 246 nm (due to intramolecular charge- 

It has been reported that the photo-Fries reaction occurs during irradiation with wavelengths 340nm, whereas formation of quinone structures takes place with wavelengths 340nm [680-682]. 

The random chain scission of aromatic polyurethanes occurs from the excited singlet state (n,n ) [1652] however, the triplet state Ti[n,n ) may also be involved [1643]. The excited singlet state (Sj) has an energy value of 98.6 kcal mol (fluorescence lifetime = 3.2 ns), whereas the triplet 

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Up to this point we have discussed only carbamates la-4a with a single carbamate group on the phenyl ring as model systems for aromatic polyurethane photodecomposition. In polyurethane coatings based on the aromatic diisocyanate TDI two carbamate groups are attached to the phenyl ring. Furthermore commercially available TDI is actually a mixture of 2,4-toluene diisocyanate (2,4-TDI) and 2,6-toluene diisocyanate (2,6-TDI) which when formulated give 2,4- and 2,6-biscarbamates. Model systems for these species would then be biscarbamates of 2,4-TDI and 2,6-TDI (as shown below) and not carbamates such as la-4a. 

P.O.34 is rarely used in polyolefins. In such media, it only withstands exposure to 200°C, and its opaque colorations show insufficient lightfastness. P.O.34 tends to bloom, especially in extrusion products made of low molecular weight LDPE types. The pigment is, however, recommended for a variety of other media. These range from aromatic polyurethane foams to cast resins of unsaturated polyester, in which the pigment slightly delays the hardening process. 

Isocyanates that are produced fi om aliphatic amines are utilized in a limited range of polyurethane products, mainly in weatherable coatings and specialty applications where the yellowing and photodegradation of the aromatic polyurethanes are undesirable [5]. The aliphatic isocyanates are not used more widely in the industry due to the remarkably slow reaction kinetics of aliphatic isocyanates compared to their aromatic counterparts [6]. Due to the slow reactivity of aliphatic isocyanates, it is not practical to use them in the preparation of flexible or rigid foams, which are the main commercial applications for polyurethane chemistry. 

Lvova et al. [100] Caffeine, catechines, sugar, amino acid L-arginine Natural coffee, black tea and different sorts of green teas Glucose oxidase/with glutaraldehyde and BSA, then covered with an aromatic polyurethane membrane Carbon paste screen-printed electrodes/ potentiometric (different active components on each of 30 sensors to construct an array) Iron hexacyanoferrate Prussian Blue, Fein[Fen(CN)6]3... 

The effect of ultraviolet (UV) light on aliphatic clear polyurethanes or aromatic polyurethanes that have had UV stabilizers added can be evaluated using Weather-Ometer . The tests are performed to ASTM D750 or to ISO 4665/3. 

On the other hand, Berlin et al.(124,125) observed a singlet spectrum with a linewidth of about 8-12 gauss after irradiation of aromatic polyurethane. They attributed this spectrum to delocalized electrons in the conjugated double bonds which were produced by the secondary reactions of primary radicals produced by scissions of N—C and C—O bonds. 

Formation of (a) Aromatic Polyurethane, Pellethane, (b) Aromatic Polyurethane Biomer, and (c) Aliphatic Polyurethane Tecoflex... 

Chem. Descrip. Waterborne aromatic polyurethane polymer Uses Urethane for sports flooring when the final coat is crosslinked Features Hard outstanding abrasion resist. 

Wang, F.C., Feve, M., Lam, T.M., and Pascault, J.P. (1994) FTIR analysis of hydrogen bonding in amorphous linear aromatic polyurethanes. I. Influence of temperature. Journal of Polymer Science Part B Polymer Physics, 32, 1305-1313. 

Aromatic polyurethanes exhibit a strong tendency to yellow in sunlight. This may be accounted for by two mechanisms based on formation of different photoproducts. The first one was found to be a photo-Fries reaction caused by a short wave length light [572]. The second one, caused by longer wave length light, is formation of quinonoid structures [573] ... 

This TPE is a segmented polyurethane and is reported to be not significantly different from biomer in chemistry and molecular weight. It is a polytetra-methyleneoxide-based aromatic polyurethane urea with mixed aliphatic and cycloaliphatic diamine chain extenders. A copolymer of... 

Caution with some aromatic polyurethanes methylene dianiline (MDA) can be generated with steam sterilization. 

Rosu, D., Rosu, L., Varganici, C.-D. The thermal stability of some semi-interpenetrated polymer networks based on epoxy resin and aromatic polyurethane. J. Anal. Appl. Pyrol. 100, 103-110 (2013)... 

The hydrolysis reaction is preferably carried out under alkaline conditions. Acid hydrolysis can lead to undesired side reactions and also incomplete reaction. Hydrolysis by water under pressure is also incomplete, especially in the case of aromatic polyurethanes. Alkaline hydrolysis in glass containers can give large amounts of silicate which interfere with subsequent analysis, and for this reason the use of steel or even nickel-coated steel containers is recommended. The hydrolysis of polyester urethanes yields the diamine from the diisocyanate, and the acid salt and glycols from the polyester. Hydrolysis of polyether urethanes yields the diamine and the polyether. If diamines are used as curatives then two diamines will be present in the hydrolysis products. 

Aromatic polyurethanes are made from isocyanates that contain unsaturated carbon rings, for example, toluene diisocyanate. Aromatic polyurethanes cure faster due to inherently higher chemical reactivity of the polyisocyanates [8], have more chemical and solvent resistance, and are less expensive than aliphatics but are more susceptible to UV radiation [1,9,10]. They are mostly used, therefore, as primers or intermediate coats in conjunction with nonaromatic topcoats that provide UV protection. The UV susceptibility of aromatic polyurethane primers means that the time that elapses between applying coats is very important. The manufacturer s recommendations for maximum recoat time should be carefully followed. 

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