Urea phases

Flexible foams are three-dimensional agglomerations of gas bubbles separated from each other by thin sections of polyurethanes and polyureas. The microstmetures observed in TDI- and MDI-based flexible foams are different. In TDI foams monodentate urea segments form after 40% conversion, foUowed by a bidentate urea phase, which is insoluble in the soft segment. As the foam cures, annealing of the precipitated discontinuous urea phase... 

The top two stationary phases in Figure 12 are the C g and Cg phases, which are the most frequently used phases in reversed-phase LC. Below that, three phases with an embedded polar group are shown the carbamate phases (e.g., SymmetryShield), amide phases (e.g.. Discovery RPAmide C g) and urea phases (such as Prism or Spectrum). It should... 

Figure 3. System characterization for shape and position of absorbance-derived signals for urea unfolding gradients alone. Upper panel depicts me average of 8 runs measured at each detector and the associated high/low statistical error limits computed at a confidence interval of 99.9%. Inset to upper panel depicts measured urea phase delay between the averaged runs. The lower panel depicts the averaged data phase-corrected. The lower panel inset depicts the residual differences in urea concentrations between the two absorbance detectors.

In addition, the excess of ammonia reduces the required synthesis pressure, which is related to the strongly nonideal behavior of the vapor-liquid equilibrium. At a NH3 CO2 ratio around 3, a pressure minimum azeotrope can be observed for the reactive system [13]. Starting from the composition at the pressure minimum, an increase of the NH3 CO2 ratio results in a lower pressure rise than a decrease. This contributes to the fact that ammonia is better soluble in the liquid water/urea phase than carbon dioxide [13]. For this reason, in most of the industrial urea processes, the molar NH3 CO2 ratio in the reactor is adjusted to be around 3 1 or higher. 

Then N-Boc-O-benzylserine is coupled to the free amino group with DCC. This concludes one cycle (N° -deprotection, neutralization, coupling) in solid-phase synthesis. All three steps can be driven to very high total yields (< 99.5%) since excesses of Boc-amino acids and DCC (about fourfold) in CHjClj can be used and since side-reactions which lead to soluble products do not lower the yield of condensation product. One side-reaction in DCC-promoted condensations leads to N-acylated ureas. These products will remain in solution and not reaa with the polymer-bound amine. At the end of the reaction time, the polymer is filtered off and washed. The times consumed for 99% completion of condensation vary from 5 min for small amino acids to several hours for a bulky amino acid, e.g. Boc-Ile, with other bulky amino acids on a resin. A new cycle can begin without any workup problems (R.B. Merrifield, 1969 B.W. Erickson, 1976 M. Bodanszky, 1976). 

In 1987, Toray Industries, Inc., announced the development of a new process for making aromatic nitriles which reportedly halved the production cost, reduced waste treatment requirements, and reduced production time by more than two-thirds, compared with the vapor-phase process used by most producers. The process iavolves the reaction of ben2oic acid (or substituted ben2oic acid) with urea at 220—240°C ia the presence of a metallic catalyst (78). 

Reaction 1 is highly exothermic. The heat of reaction at 25°C and 101.3 kPa (1 atm) is ia the range of 159 kj/mol (38 kcal/mol) of soHd carbamate (9). The excess heat must be removed from the reaction. The rate and the equilibrium of reaction 1 depend gready upon pressure and temperature, because large volume changes take place. This reaction may only occur at a pressure that is below the pressure of ammonium carbamate at which dissociation begias or, conversely, the operating pressure of the reactor must be maintained above the vapor pressure of ammonium carbamate. Reaction 2 is endothermic by ca 31.4 kJ / mol (7.5 kcal/mol) of urea formed. It takes place mainly ia the Hquid phase the rate ia the soHd phase is much slower with minor variations ia volume. 

In this condenser, part of the stripper off-gases are condensed (the heat of condensation is used to generate low pressure steam). The carbamate formed and noncondensed NH and CO2 are put into the reactor bottom and conversion of the carbamate into urea takes place. The reactor is sized to allow enough residence time for the reaction to approach equiUbrium. The heat required for the urea reaction and for heating the solution is suppHed by additional condensation of NH and CO2. The reactor which is lined with 316 L stainless steel, contains sieve trays to provide good contact between the gas and Hquid phases and to prevent back-mixing. The stripper tubes are 25-22-2 stainless steel. Some strippers are still in service after almost 30 years of operation. 

Subsequent chlorination of the amide takes place ia a two-phase reaction mixture (a dispersion of diamide ia hydrochloric acid) through which a chlorine stream is passed. The temperature of this step must be maintained below 10°C to retard the formation of the product resulting from the Hofmann degradation of amides. Reaction of the A/,A/-dichloroamide with diethylamine [109-89-7] ia the presence of base yields /n j -l,4-cyclohexane-bis-l,3-diethylurea (35), which is transformed to the urea hydrochloride and pyroly2ed to yield the diisocyanate (36). 

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

Pigment Blue 15 [147-14-8] 74160 copper phthalocyanine condensation of phthaUc anhydride with urea, in presence of copper ions, with or without added chlorophthahc anhy-dride subsequent conversion to alpha-phase and stabili2ation, if necessary... 

Thiourea will react with neutralised formalin at 20-30°C to form methylol derivatives which are slowly deposited from solution. Heating of methylol thiourea aqueous solutions at about 60°C will cause the formation of resins, the reaction being accelerated by acidic conditions. As the resin average molecular weight increases with further reaction the resin becomes hydrophobic and separates from the aqueous phase on cooling. Further reaction leads to separation at reaction temperatures, in contrast to urea-formaldehyde resins, which can form homogeneous transparent gels in aqueous dispersion. 

There is also growing interest in multi-phase systems in which hard phase materials are dispersed in softer polyether diols. Such hard phase materials include polyureas, rigid polyurethanes and urea melamine formaldehyde condensates. Some of these materials yield high-resilience foams with load deflection characteristics claimed to be more satisfactory for cushioning as well as in some cases improving heat resistance and flame retardancy. 

Note Rhodamine B is a universal reagent that can be used on silica gel, talc, starch [5] and cellulose layers, just as on urea [1] or silver nitrate-impregnated [7] phases. Liquid paraffin-impregnated silica gel and RP layers are less suitable, since the background to the chromatographic zones is also intensely colored. It is often possible to increase the detection sensitivity by placing the plate in an atmosphere of ammonia after it has been sprayed or dipped, alternatively it can be oversprayed with sodium or potassium hydroxide solution. 

BMIM][PFg] and urea-appended (dashed lines) or thiourea-appended (solid lines) TSILs as the extracting phase. 

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