Reaction allophanate

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

Amines produce polyurethanes with better mechanical properties than when diols are used for curing. Amines produce polyurethanes with a lower temperature resistance than when diols are used. The use of catalysts has been found to direct the cross-linking reactions away from the biuret to the allophanate reactions. 

R-N=C=0 + R nS-OR" H (urethane) R-jj- -0R" KlHR 120-140°C Tertiary amines. Strong bases. Metal compounds. Allophanate reaction. Reacts slow. 

Saunders and Frisch (2) cite certain catalysts used to Induce an isocyanate-urethane (allophanate) reaction. They are zinc octoate, cobalt napthanate and cobalt octoate and are claimed to yield 95% allophanate. An experiment was designed observing the catalytic effect of these metal complexes under varied concentrations and over time. Ferric acetylacetonate, a catalyst known to Influence an isocyanate-carboxyl reaction, was included 1n the study. The catalysts were added individually and 1n combinations of two into a polyurethane-polyisocyanate system. Concentrations varied from 1.50% to 8.00% by weight. 

In this section, mixed-phase polyurethanes prepared from 1,3-butane diol (1,3-BD), HDI, and MDI are reviewed [28]. When the TPU soft and hard segments are phase-mixed, a second element acting as a fixed phase should be introduced. This second element is typically crosslinks intoduced either by an allophanate reaction or by a multifunctional polyol. Shape memory properties, melt viscosities, dynamic mechanical and thermal properties, and stress relaxations in the glassy and rubbery states are discussed. 

When crosslinks are introduced by an allophanate reaction, a quantitative control of their density is not feasible since the reaction occurs reversibly between free isocyanate groups and main-chain urethane groups, and hence the transition temperature is not closely controlled. Thus, when a reproducible close control of the shape recovery temperature is desired, multifunctional polyols or isocyanates can be used to provide TPUs with well controlled crosslink density. 

Not only are these reactions of importance in the development of the cross-linked polyurethane networks which are involved in the manufacture of most polyurethane products but many are now also being used to produce modified isocycuiates. For example, modified TDI types containing allophanate, urethane and urea groups are now being used in flexible foam manufacture. For flexible integral foams and for reaction injection moulding, modified MDIs and carbodi-imide MDI modifications cU"e employed. 

The water reaction evolves carbon dioxide and is to be avoided with solid elastomers but is important in the manufacture of foams. These reactions cause chain extension and by the formation of urea and urethane linkages they provide sites for cross-linking, since these groups can react with free isocyanate or terminal isocyanate groups to form biuret or allophanate linkages respectively (Figure 27.5). 

Catalysts such as dibutyl tin dilaurate or tertiary amines are added to promote the urethane reaction and/or subsequent moisture cure. Dimorpholine diethyl ether is particularly effective at promoting moisture cure without promoting allophanate side reactions at the application temperature (which leads to instability in the hot melt pot) [29]. 

The allophanate linkage is formed by the reaction of urethane with isocyanate, as shown in the fourth item of Fig. 1 [7], Isocyanates can react with many active hydrogen compounds. The active hydrogen of the urethane linkage is not very reactive, but if reaction temperatures get high enough (usually in excess of 100°C), or in the presence of certain allophanate catalysts, this reaction can actually become favored over the urethane reaction (see pp. 180-188 in [6]). 

Reactions of urethane and urea groups to form allophanates and biurets. 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

Lastly, of course, the main reaction of interest is the formation of urethane groups by reaction of isocyanate groups and hydroxy-groups of the polyester or polyether. Even these reactions do not exhaust the possibilities available to the highly reactive isocyanate group. It will then go on to react with the urethane links to form a structure known as an allophanate (see Reaction 4.13). 

Carbamates (substituted urethanes) are prepared when isocyanates are treated with alcohols. This is an excellent reaction, of wide scope, and gives good yields. Isocyanic acid HNCO gives unsubstituted carbamates. Addition of a second mole of HNCO gives allophanates. 

The desorption of arsenate previously sorbed onto Fe- or Al-oxides or onto an Andisol containing 42% of allophanic materials (Vacca et al. 2002) by phosphate has been demonstrated to be affected by time of reaction, residence time of arsenate onto the surfaces and the pH of the system (Pigna et al. 2006 Pigna et al. 2007, unpublished data). Figure 9 shows the desorption of arsenate at pH 6.0 (phosphate/arsenate molar ratio of 4) when phosphate was added onto the soil (Andisol) sample 1, 5 or 15 days after arsenate (surface coverage of arsenate about 60%). After 60 days of reaction, 55% of arsenate was desorbed by phosphate when the residence time of arsenate onto the surfaces of the Andisol was 1 day, but 35 and 20% of arsenate was desorbed by phosphate with increase in the residence time up to 5 and 15 days. Further, it was also observed that by keeping the... 

One can observe positive deviations in the region of rH < 1 (excess of isocyanate groups) which are due to side reactions (allophanate, urea and biuret groups). In the region of rH > 1 the agreement of wg values is good. In the case of i e, the predicted curves depend not only on the results of the branching theory but also... 

The extent of crosslinking in polyurethanes depends on a combination of the amount of polyfunctional monomers present and the extent of biuret, allophanate, and trimerization reactions [Dusek, 1987]. The latter reactions are controlled by the overall stoichiometry and the specific catalyst present. Stannous and other metal carboxylates as well as tertiary amines are catalysts for the various reactions. Proper choice of the specific catalyst result in differences in the relative amounts of each reaction. Temperature also affects the extents... 

Parfitt, R.L. (1989) Phosphate reactions with natural allophane, ferrihydrite and goethite. 

This addition reaction proceeds readily and quantitatively. Side reactions can give amide, urea, biuret, allophanate, and isocyanurate groupings, so that the structure of the product can deviate from that above such side reactions are sometimes desired (see Sect. 4.2.1.2). 

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