Branched epoxy branch concentration

Table II. Branched Epoxy Resins GPC and Melt Viscosity vs. Branch Concentration...

An homologous series of epoxy resins with constant epoxide equivalent weight and increasing branch concentration displayed increased melt viscosity and weight average molecular weight. 

Powder coatings were prepared by extruding stoichiometric quantities of methylene dianiline with epoxy resins varying only in branch concentration. More highly branched resins result in a more tightly cross-linRed thermoset network. 

The first moment of the distribution is Pt0T the total, cumulative molar concentration of polymeric material. As the molecular weight of polymeric species increases, branching and crosslinking reactions yield a thermoset resin. Chromatography analysis of epoxy resin extracts confirms the expected population density distribution described by Equation 4, as is shown in Figure 2. Formulations and cure cycles appear in Table II. 

Epoxy networks may be expected to differ from typical elastomer networks as a consequence of their much higher crosslink density. However, the same microstructural features which influence the properties of elastomers also exist in epoxy networks. These include the number average molecular weight and distribution of network chains, the extent of chain branching, the concentration of trapped entanglements, and the soluble fraction (i.e., molecular species not attached to the network). These parameters are typically difficult to isolate and control in epoxy systems. Recently, however, the development of accurate network formation theories, and the use of unique systems, have resulted in the synthesis of epoxies with specifically controlled microstructures Structure-property studies on these materials are just starting to provide meaningful quantitative information, and some of these will be discussed in this chapter. 

The usual concentration of plasticizers is 20% to 40%, but some systems (plastisols) have 50% to 60% plastification. The diffusion of plasticizers impairs flexibility, making the polymer rigid and brittle. It is important to eliminate the use of toxic additives, which adversely affect the environment through contact with food stock or other polymers. For these reasons a liquid-phase plasticizer may be replaced by a solid-state plasticizer which is mainly based on a short-chain polymer (polyester or epoxy) or a long-branched one. The most ideal plastification may be achieved by an appropriate copolymerization. A typical example is a copolymer made from vinylchloride and vinylacetate, in which a low Tg monomer is inserted into the main chain. 

Mohamed et al. [149] evaluated the use of several types of sulfosuccinate anionic surfactants in the dispersion of MWCNTs in NR latex matrices. Sodium l,5-dioxo-l,5-bis(3-phenylpropoxy)-3-((3-phenylpropoxy)carbonyl) pentane-2-sul-fonate showed the best dispersion capabihty and improved the electrical conductivity of the resulted composites. These results have significant implications in the development of new materials for aerospace applications because the filler s dispersiou directly influences the properties of the final material. Jo et al. [150] obtained pristine MWCNt-Ti02 nanoparticles filled with NR-CllR and epoxidized NR-CUR, concluding that the second blend proved higher thermal conductivity because the epoxy branches in ENR and the functionalized MWCNT form a stronger network. Conductivity in CNTs reinforced with rubber-based blends can be improved when reaching a critical concentration of the filler known as the percolation threshold, when a continuous network structure is formed. Thankappan Nair et al. [151] discussed the percolation mechanism in MWCNT-polypropylene-NR blends. 

Free tertiary amine associates with the secondary hydrojQrl groups and leads to an amino alcohol, M (Rxn. 29). The amino alcohol forms an activated complex, C2, with the epoxy dipole (Rxn. 30). C2 dissociates to form the branched product (Rxn. 31). This mechanism accounts for the observation that branching takes place only after a certain extent of epoxy conversion, i.e., when the fi ee amine concentration increases as it is released fi om the phenol ion pair BPi in Rxn. 24. Free amine is a true catalyst in this branching reaction since it is r enerated. 

High molar mass epoxy prepolymers containing rabber dispersions based on carboxyl-terminated butadiene-acrylonitrile copolymer were prepared from initially miscible solution of low molar mass epoxy prepolymers, bisphenol A and carboxyl-terminated NBR. During chain extension inside a twin screw extruder due to epoxy-phenoxy and epoxy-carboxy reactions, a phase separation process occurs. Epoxy-phenoxy and epoxy-carboxy reactions were catalysed by triphenylphosphine. The effect of reaction parameters (temperature, catalyst, reactant stoichiometry) on the reactive extrasion process were analysed. The structure of the prepolymers showed low branching reactions (2-5%). Low molar mass prepolymers had a Newtonian rheological behaviour. Cloud-point temperatures of different reactive liquid butadiene aciylonitrile random copolymer/epoxy resin blends were measured for different rubber concentrations. Rubber... 

---