Cluster structure of epoxy-phenolic polymer prepared by curing a mixture of epoxy-4,4?-isopropylidenediphenyl and phenol-formaldehyde oligomers

Abstract

The kinetics of viscosity variation and the concentration and kinetic modes of precondensation of a mixture of epoxy-4,4`-isopropylidenediphenyl and phenol-formaldehyde oligomers in Cellosolve solutions in the presence of phosphoric acid at 333 K were studied. The cluster structure parameters of the chemical network of epoxy-phenolic polymeric coating on a tin plate, obtained by thermal curing of precondensates of an oligomer mixture, were determined using the silver chloride sputtering method.In studying the structure of cross-linked epoxyphenolic polymers we have demonstrated previously [1, 2] that a uniform chemical network is formed on the surface of polymeric coatings at an epoxydiane (EO) to phenol-formaldehyde (PO) oligomer molecular weight ratio close to 4 : 1, the optimal molecular weight of EO (M n ) being about 3500. To improve the compatibility of EO and PO in solutions and to obtain an epoxy-phenolic polymer by thermal curing of the oligomers, precondensation of EO and PO is carried out in the presence of phosphoric acid [3, 4]. On adding H 3 PO 4 to a solution of the oligomers, mono-, di-, and triphosphates of EO are formed [5]. In the course of thermal curing of a mixture of the oligomers, the methylol (butoxy) groups of PO react with the epoxy (hydroxy) groups of EO, and the methylol groups of PO form etheric cross-linking bonds [6,7].In [1, 2, 8], we suggested a cluster mechanism of aggregation of oligomeric molecules in solutions in the course of thermal precondensation of EO/PO mixtures. Following the cluster model [9, 10], one can distinguish three major kinetic stages of precondensation of the oligomers, as well as the corresponding topological types of the chemical network structure of cross-linked epoxy-phenolic polymer. The precondensation mechanism in EO3PO mixed solutions, i.e., the structural evolution of aggregates of the oligomers, involves disaggregation of EO and PO aggregates and formation of mixed EO3PO aggregates and then of an infinite cluster of these macromolecules. Such an approach is useful for optimization of the molecular, concentration, and kinetic parameters of precondensation of the oligomers in solutions [2]. Previously we have shown [8] that the oligomer molar ratio N 1 (EO)/N 2 (PO) and the phosphoric acid concentration N 3 predetermine the shape of the kinetic curves characterizing the viscosity dynamics in EO3PO mixed solutions.A uniform chemical network of the epoxy-phenolic polymer is formed in curing of the oligomer mixture, provided that the density of EO and PO aggregates approaches that of the infinite cluster in the vicinity of the threshold concentration of the particles in the solution [2]. Such a network is not formed in the initial and intermediate stages of precondensation of the oligomers. In this case, the correlation radius x of clusters representing the network junctions initially decreases from~250 to~50 nm and then increases to~500 nm with precondensation time, which is consistent with data on the Rayleigh light scattering in EO and PO solutions [...