S. M. Moschiar

Miscibility of epoxy monomers with carboxyl-terminated butadiene-acrylonitrile random copolymers

Abstract Cloud-point curves for mixtures comprising epoxy monomers of the DGEBA type (diglycidyl ether of bisphenol A), rubbers of the CTBN type (carboxyl-terminated acrylonitrile-butadiene random copolymers) and, eventually, hardeners such as diamines are reported. It is shown that the miscibility of epoxy monomers with a particular rubber is very sensitive to the molar mass of the epoxy molecule. The application of a simple Flory-Huggins lattice model, regarding both components as monodisperse, leads to the location of the critical point, coexistence curves and the estimation of the interaction parameter per unit volume (Λ). A correlation of the type Λ=Λ0 + ΛTT was found, with a negative ΛT. The decrease in miscibility observed when using a CTBN containing less acrylonitrile is explained from the change in the solubility parameter. Using CTBN as an adduct with the epoxy monomer leads to an increase in miscibility, which is explained by the copolymer effect. Systems based on epoxy-diamine copolymers prepared in the presence of CTBN adducts showed a complex behaviour including two maxima in cloud-point curves.

Polyurethane-ductilized epoxy resins

Amine-cured epoxy resins were modified to improve their impact properties. Urethane prepolymers (PUs), in which terminal isocyanate groups were blocked with nonylphenol (NP) for easy handling, were used as modifiers. The synthesis of the elastomers were carried out at different NCO : OH ratios: 1 : 1, 2 : 1, and 3 : 1 (PU1, PU2, and PU3). Characterization of these materials by GPC and FTIR indicated that PU1 has a negligible amount of NCO-terminated chains and no unreacted toluenediisocyanate (TDI). PU2 and PU3 have free-blocked TDI in the mixture, even after distillation under a vacuum. The molecular weight and polydispersity of the prepolymer increases as PU3 < PU2 < PU1. Copolymerization was carried out by crosslinking with a mixture of cycloaliphatic amines, which react with the epoxy ring and with the NCO groups by deblocking and urea formation. Dynamic mechanical tests were used to measure the glass transition temperatures (Tg) of the copolymers. Two Tg were found if PU1 was the epoxy modifier, indicating that phase separation took place. This was correlated with a structure of PU1 of linear chains with a negligible amount of reactive groups. Flexural and compression properties showed negligible changes for PU2- and PU3-modified epoxy, but the critical strain energy release rate (G1C) was improved if PU2 was the modifier. This behavior was explained by the linkage of elastomeric chains into the epoxy network. The PU1–epoxy copolymer showed a completely different behavior, with the bending modulus (Eb) reduced to almost one-half with respect to that of the epoxy matrix and with largely improved impact properties. This difference was attributed to the separation of an elastomeric phase, which favors the formation of shear bands in the epoxy matrix.

Analysis of the build-up of polyurethane networks from toluenediisocyanate and castor oil considering intra-molecular reactions

Abstract The build-up of polyurethane networks prepared by reacting toluenediisocyanate (TDI 80:20) with an excess of castor oil over the stoichiometric balance has been analysed using a fragment approach. This includes a kinetic scheme giving the evolution of the different species and fragments into which the network may be divided together with a recursive method to obtain statistical parameters. Intra-molecular reactions leading to the smallest cycles are taken into account in the analysis. Sol fraction and concentration of elastically active network chains in the gel fraction are estimated as a function of the stoichiometric imbalance and compared with experimental results previously reported. A reasonable fit is shown for an effective bond length in the smallest cycle equal to 0.23 nm.

Analysis of kinetic parameters of an urethane–acrylate resin for pultrusion process

An urethane–acrylic resin for a pultrusion processing application was studied. The concentration of Perkadox 16 and methyl methacrylate (MMA) was changed in the formulation mixture. A calorimetric study was performed in a DSC equipment. Isothermal runs from 42 to 60°C were performed to obtain a kinetic model for the polymerization reaction. Conversion of vitrification as a function of temperature was determined and the total heat of reaction as a function of MMA content was also measured. A general kinetic model was applied. An autocatalytic model and master-curve approach with an order of reaction of n + m = 2 and an activation energy of 95.9 kJ/mol were found. By the application of the Kissinger model for dynamic runs, an activation energy of 88.8 kJ/mol was obtained.