Chain extenders equivalent weights

Epoxide adhesives comprise epoxy resin, many of which are prepared from phenols and epichlorohydrin, for example, the diglycidyl ether of bis-phenol A or bis-phenol F usually, these resins are a mixtnre of molecular weights blended to fit the applications. The most-common cnratives for epoxy resins are polyanfines (used in stoichiometric amounts), usually a chain-extended primary aliphatic amine, for example, diethylene triamine or triethylene tetraamine or chain-extended equivalents, which react rapidly with the epoxy resin at room temperature. Aromatic amines react slowly at room temperature but rapidly at higher temperatures. Most epoxide adhesives also contain catalysts, typically, tertiary amines. Dicyanimide is the most-common curative for one-component high-temperature-cured epoxide adhesives. Mercaptans or anhydrides are used as curatives for epoxide adhesives for specialist applications, for example, for high-speed room-temperature cures or for electronic applications. A smaller number of epoxide adhesive are cured by cationic polymerization catalysed by Lewis acids photogenerated at the point of application. Lewis acid photoinitiators include diaryliodonium and triarly sulphonium salts. See Radiation-cured adhesives. 

Chain Extension of -U-Polystyrene Hiols. A two-stage chain extension of the. -kJ -polystyrene diols was accomplished by carboxylation of the diols with succinic anhydride followed by chain extension with a diepoxide. The succinic anhydride reaction was carried out 120-130°C under nitrogen. The reaction was monitored bj changes in the carbonyl bands at 1715 and 1740 cm in the infrared spectra of the reaction mixtures. The resulting dicarboxylic acid polymers were chain-extended in bulk at 130°C for 9 hours with Sow s HER diepoxide, equivalent weight =171, using bis( 3,5-diisopropylsalicylato)Cr (III) as the catalyst. 

The prepolymer % NCO content and the equivalent weight of the chain extender materials were determined to calculate the amount of each material to achieve 95% stoichiometry. After careful degassing and mixing, the material was poured into the stainless steel mould and cured for 16 hours continuously at 110 °C. Table 8.3 summarises the experimental details relating to cast elastomer preparation. 

For theoretical purposes a chemical reaction is only considered to occur between Class A and Class B and when materials of Class C are present they are included in the calculation as a chemically inert series of components. In practice these inert components usually contain absorbed moisture or small traces of other active components which may react to a limited extent with the diisocyanate. Hence some additional diisocyanate may have to be included in a practical formulation to counteract this diluent effect on ideal stoichiometry. Determining the equivalent weights for chain extenders (crosslinker and diisocyanates) is performed as follows ... 

POLYOL BLEND USED AS AN EXAMPLE TO CALCULATE THE EQUIVALENT WEIGHT OF A MIXTURE OF POLYOLS AND CHAIN EXTENDER... 

To demonstrate the formulation of a polyol mixture, consider a system of a 100 parts by weight (pbw) batch of polyol. The hydroxyl numbers are chosen to illustrate the concept of formulating, using polyols with differing weights and equivalent weights, plus a chain extender. Table 2.3 gives the composition. 

The chain extender is 1,4-butane diol of equivalent weight 45. Hence summing the hydroxyl groups in the 100 pbw batch ... 

The minicomputer system has been described previously I 5]. The subsequent dat reduction and analysis provides molecular weight averages (M, M2, M2+1) or the equivalent extended chain... 

In a real chain segment-segment correlations extend beyond nearest neighbour distances. The standard model to treat the local statistics of a chain, which includes the local stiffness, would be the rotational isomeric state (RIS) [211] formalism. For a mode description as required for an evaluation of the chain motion it is more appropriate to consider the so-called all-rotational state (ARS) model [212], which describes the chain statistics in terms of orthogonal Rouse modes. It can be shown that both approaches are formally equivalent and only differ in the choice of the orthonormal basis for the representation of statistical weights. In the ARS approach the characteristic ratio of the RIS-model becomes mode dependent. 

The craze density of a hot-drawn craze is not known, but its maximum value can be estimated from an equivalent monodisperse molecular weight, M, on the simple assumption that randomly conformed polymer chains of n backbone bonds (i.e. n = M114 for PE, where M is the segment molecular weight) become fully extended into nearly perfect fibrils. Hence... 

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