Isocyanates equivalent weight

Preparation of the polymers was carried out by a two step method. First, MDI prepolymers were prepared in a 5 liter round bottom flask equipped with a stirrer, thermometer, addition funnel, and nitroggn inlet. The dried polyols were added dropwise tg MDI at 75°C and the reaction temperature was maintained at 85°C. The mole ratios of MDI to polyol listed in Table II gave prepolymers containing 18.67% free NCO groups by weight or an isocyanate equivalent weight of 225. In the case of prepolymers 2 and 4, the DPG was added first and then the 2000 molecular weight polyol was added. 

Many of the commercial reactants used in urethane synthesis are impure materials, for example MDI may be used as pure MDI, a low melting-point (38°C) solid polymeric or crude MDI, which is a dark-coloured low viscosity liquid. Alternatively, MDI and TDI prepolymers may be blended and perhaps have various additives incorporated, all of which may affect the final quantity of free NCO available for reaction. Also, active isocyanate content of a prepolymer decreases with storage, a decrease of about 0-05% per month being normal. For these reasons it is necessary to measure and specify the amount of isocyanate available for reaction at any specific time, and this is done by calculation of the isocyanate equivalent weight of the mixture using the following procedure ... 

Hydroxyl Number. The molecular weight of polyether polyols for urethanes is usually expressed as its hydroxyl number or percent hydroxyl. When KOH (56,100 meg/mol) is the base, the hydroxyl number is defined as 56,100/equivalent weight (eq wt). Writing the equation as eq wt = 56,100/OH No. allows one to calculate the equivalents of polyol used in a urethane formulation, and then the amount of isocyanate required. The molecular weight can be calculated from these equations if the fiinctionahty, is known mol wt = / eq wt. 

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

In this way one mole of water reacts with two moles of isocyanate. For a urethane prepolymer with an equivalent weight of about 300, it only takes 9 grams of water to effectively consume... 

It is important to be aware of the chemical effects of isocyanates. The polynre-thanes you will develop will be combinations of polyols and isocyanates. The ratio of the two compounds will in pait dictate both the physical and chemical properties of the product. As a general rule, the isocyanates are hard segments that impart rigidity to the polymer. The polyol is the so-called soft segment. The various molecular weights (more correctly equivalent weights available in the form of polymeric MDIs) provide certain advantages. Table 2.2 lists a few commercially available polyisocyanates and their physical properties. 

In the reaction shown in Figure 2.6, assume that we are beginning with a PPG with a molecular weight of 1000. There are two hydroxyls (the equivalent weight of the PPG is 500). Two moles of TDI per mole of PPG are required. For 1 kg of PPG, therefore, 542 grams of TDI are required. Excess is usually added to maintain a reasonable reaction rate. Very often, if foam is the desired final product, excess isocyanate is intentionally added to improve the foam characteristics. In that case, the prepolymer is designated as a quasi- or semiprepolymer. 

Hexamethylene diisocyanate was supplied as pure by W. Hopkins of this department (71), having a density of 1.045, and an average equivalent weight of 83.7 0.5 (theoretical equivalent 84.09). The weights of isocyanate used were calculated on the analysis of the particular sample. 

Percent isocyanate = the percent by weight of isocyanate (NCO) groups present. The amine equivalent, AE, is the weight of sample which reacts with 1 gram-equivalent weight of dibutyl amine (reaction 1-13). 

In this way one mole of water reacts with two moles of isocyanate. For a urethane prepolymer with an equivalent weight of about 300, it only takes 9 grams of water to effectively consume the prepolymer. This can lead not only to degraded physical properties, but it also forms gas bubbles in the proplnt which lead to voids and higher than predicted burning rates. This sensitivity to water content also exists with hydroxy-terminated polybutadiene polymers cured with... 

Urethane foams can be classified into two principle types, i.e., flexible and rigid foams. In some cases, flexible foams can be further subdivided into flexible and semi-flexible (or semi-rigid) foams. The differences in physical properties of the two foams are mainly due to the differences in molecular weight per cross-link, the equivalent weight and functionality of the polyols, and the type and ctionality of the isocyanate. 

An example of the jH eparation method at room temperature is as follows (106). A mixture of 161 parts by weight (1.00 equivalent) of 3,3, 4,4 -benzophenone tetracarboxylic dianhydride and 132 parts by weight (1.00 equivalent weight) of polymeric isocyanate was prepared by mechanical blending. 

Preparation of Cast Pads. One hundred parts Adlprene LlOO (an Isocyanate terminated prepolymer containing polyoxytetra-methylene chain segments and having an equivalent weight of about 1025, manufactured by duPont) Is heated to 87-88 C while 12.5 parts MOCA, 4,4 -methylenebls(2-chloroanlllne), Is brought to 124-126. Degassing of the Adlprene LlOO and MOCA at a pressure of 1-2 Torr for about 5 and 1 minutes, respectively, follows. 

Equation 3.8 for EW is very convenient for practical use because it does not depend on the functionality, which is very difficult to determine. The EW of an oligo-polyol is very useful for the required isocyanate quantity calculation. One equivalent weight of an oligo-polyol reacts with one equivalent weight of the diisocyanate (the MW of the isocyanate divided by the number of -NCO groups). The EW of polyols with the same OH are identical, irrespective of the functionality. Thus a diol with an OH of 56.1 mg KOH/g (MW = 2000) and a triol of the same OH (MW = 3000), have the same EW of 1000. 

An example of a polyurethane adhesive used for surface mounting components consists of Mondur M cured with castor oil or with other polyols. The molecular weight and properties of polyurethanes are optimized when one equivalent weight of the isocyanate reacts with one equivalent weight of the polyol. The equivalent weights are based on the percent of hydroxide of the polyol and the percent of isocyanate of... 

As an example, the amounts of Desmodur N-lOO required to react with 300 grams of Desmophen R-221-75 polyol can be calculated (assuming a 1 1 ratio). Desmodur N-lOO has a 22% isocyanate content and Desmophen R-221-75 has a 3.3% hydroxide content. Thus, the equivalent weights are ... 

Two-component adhesives typically consist of low-equivalent-weight isocyanate or prepolymer that is cured with a low-equivalent-weight polyol or polyamine. They may be 100% solids or solvent borne. Since the two components will cure rapidly when mixed, they must be kept separate until just before application. Application is followed quickly by mating of the two substrates to be bonded. 

For example, a polymeric isocyanate containing a mixture of isocyanate groups on analysis may be found to contain 38% NCO and its equivalent weight would prove to be 42/0 38= 110 5. The average functionality of polymeric MDI is in the range 2 1-2 7. 

MDI discussed earlier. This 1 1 equivalent weight ratio is called the isocyanate index. 

The isocyanate equivalent is defined as the weight of sample which will combine with 1 g equivalent weight of phenyl isocyanate. The sample is generally a polyol, amine or other compound that possesses groups capable of reacting with an isocyanate. 

Many suppliers of polyurethane systems provide them in the form of two components and advise on the actual weight of each component to be combined to form the polymer. This is the weight ratio, sometimes and confusingly called the index. Such a practice is to be decried as causing confusion with the chemical term isocyanate index . In many cases there is little difference in value between weight ratio and chemical equivalent weight ratio of index but it is important to understand the difference. Most suppliers supply an NCO-free system for one component and an NCO component (quoted in terms of the OH number) for the other side. 

The equivalent weight of the mixture for reaction with an isocyanate is 105 20/0 46 equiv. = 228 g. That is, 228 g will react with 87 g of TDI on an equivalent basis (equivalent weight of TDI = 87). But in this case we wish to keep the total weight to that originally specified. Therefore, for the above weight 0 46 x 87 or 40 pbw of TDI should be used for a 100-index foam and 1 05 and MO times the amount for the 105 and 110 TDI-index foams. 

EW ratio Ratio between the amine equivalent weight of hydrogen and the isocyanate EXAFS Extended X-ray absorption fine structure, i.e., the fine stmcture present in the X-ray absorption spectra beymid the absorption edge... 

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