Rubber-modified epoxy resins

Studies of the particle—epoxy interface and particle composition have been helpful in understanding the rubber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in rubber-modified epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and tf-butyl acrylate—acrylic acid copolymers (309), have been used for toughening epoxy resins. 

Mechanical properties of rubber-modified epoxy resins depend on the extent of mbber-phase separation and on the morphological features of the mbber phase. Dissolved mbber causes plastic deformation and necking at low strains, but does not result in impact toughening. The presence of mbber particles is a necessary but not sufficient condition for achieving impact resistance. Optimum properties are obtained with materials comprising both dissolved and phase-separated mbber (305). 

Daly, J., Pethrick, R. A. Rubber-modified epoxy resins 2. Dielctric and ultrasonic relaxation studies, Polymer, 22, 37 (1981)... 

High temperature epoxy resins are brittle materials, and one method of improving their fracture properties is to incorporate reactive liquid rubbers in the formulations In situ phase separation occurs during cure the cured rubber-modified epoxy resins consist of finely dispersed rubber-rich domains ( 0.1-S pm) bonded to the epoxy matrix. TTT diagrams can be used to compare different rubber-modified systems. 

In this Section, an experimental approach for constructing isothermal TTT cure diagrams has been described, TTT diagrams of representative epoxy systems including high Tg and rubber-modified epoxy resins have been discussed, and perturbations to the TTT cure diagram due to thermal degradation and rubber modification have been illustrated. 

For high temperature and rubber-modified epoxy resins, thermal degradation events and the cloud point curve are included on the diagrams, respectively. Two degradation events have been assigned devitrification, or a glass-to-rubber event and revitrification, which is associated with char formation. The cloud points and depressions of Tg for different rubber-modified epoxies can be compared and related to volume fractions of the second phase and to the mechanical properties of the cured materials. 

Hedreul et al. (1998) examined a model of the cure kinetics of a thermally and microwave-cured rubber-modified epoxy-resin formulation. The phenomenological cure kinetic model used was... 

TEM was used to study the toughening mechanism in rubber modified plastics. A crack tip of a damaged sample of an acrylic rubber modified epoxy resin was observed [Sue et al., 1993]. 

One can rationalize a need for small rubber inclusions in some of the newer approaches to waterborne and high solids epoxy coating systems. Water-thinned epoxy coating compositions are described (48) where the two-component system consists of a nitrile rubber modified epoxy resin in the epoxide component and a styrene/ butadiene/methylmethacrylate latex modifier for an emulsion-based polyamide hardener component. Showing improved adhesion, impact and water resistance, the paint has good wetting characteristics and can be formulated to a high solids content at low viscosity. 

Sayre, J. A. Assink, R. A. Lagasse, R. R. Ibid. 87-9A Sohn, J. E. "Morphology of Solid Uncured Rubber-Modified Epoxy Resins", 181st National Meeting, ACS, ORPL, (March, 1981). 

The amount of elastomer contained in a rubber-modified epoxy resin is usually dictated by the final application and material properties desired. Knowing that one equivalent of epoxide reacts with one equivalent of carboxylic acid and the final concentration of elastomer that is desired, one can add another term to Equation (2) to account for the equivalents of epoxide consumed during the esterification reaction. The equivalent weight of the CTBN elastomers vary from lot to lot but is typically —1800 g eq . Using this number as an example, the expanded equation now reads,... 

Performing both advancement and esterification reactions results in a solid rubber-modified epoxy resin. The advancement and esterification reactions may be performed in several ways concomitantly, esterification followed by advancement, or advancement followed by esterification. 

The objective of this section is to characterize the thermal stability of uncured solid rubber-modified epoxy resins. The effects of extended thermal history on melt viscosity and epoxide equivalent weight are discussed. The influence of the type and concentration of CTBN elastomer in the solid rubber-modified epoxy resin on melt viscosity and EEW is also discussed. Mechanistic considerations are proposed to explain the side reactions which influence the thermal stability of solid rubber-modified epoxy resins. 

Results and Discussion To contrast the thermal stability of rubber-modified epoxy resins, a control experiment was performed to establish the thermal stability of a non-rubber-modified epoxy resin. An advancement reaction was performed (EEW 900 g eq ), the product maintained isothermally at 175C, and the reaction progress followed by thermally quenching aliquots to room temperature at various time intervals. Although previously published reports indicate no significant increase in the EEW of an... 

All of the above solid rubber-modified epoxy resins visually displayed clearly biphasic morphological properties (i.e., discreet rubber domains in a continuous epoxy matrix). If there is a reaction between a rubber moiety and an epoxide, it would best be studied in a homogeneous reaction mixture. Lower molecular weight epoxy resins are more compatible with CTBN elastomers and will form homogeneous solutions at elevated temperatures. Reaction of an epoxide with a reactive moiety contained in the elastomer, R, will most likely obey the following rate law ... 

Although there is some scatter in the plots of EEW versus isothermal aging time, these data are convincing that, within experimental error, there is no significant difference in the rates of loss of epoxides between members of this homologous series of liquid rubber-modified epoxy resins. Further, that the rate of loss of epoxides is so low, despite the homogeneous reaction mixture and the high concentrations of epoxide... 

Conclusion A direct result of these studies was the successful scale-up of the synthesis of solid rubber-modified epoxy resins. It was shown that the presence of elastomers destabilizes epoxy resins such that the reaction between the terminal epoxide and pendant secondary hydroxyl groups is facilitated. Since the final product contains all of these moieties (i.e., elastomer, secondary alcohols, and epoxides) it is impossible to eliminate the undesirable side reaction. However, it is possible and advantageous to design the synthesis such that the magnitude of the hydroxyl-epoxide reaction is minimized. 

The successful scale-up of advancement and modification of rubber-modified epoxy resins is discussed. Mechanisms are proposed for both advancement and esterification reactions as catalyzed by triphenylphosphine which are consistent with experimental results. A plausible mechanism for the destruction of the catalyst is also presented. The morphology of these materials is determined to be core-shell structures, dependent upon composition and reaction and processing conditions. Model studies have been performed to determine the effects of thermal history on the kinetics of reaction. These efforts have resulted in the successful scale-up and use of rubber-modified epoxy resins as functional coatings in the electronics industry. 

Lee Lee, D.-B., Kim, J.-H. Failure analysis on rubber-modified epoxy resin under various loading speed conditions. Key Eng. Mater. 297-300 (2005) 1907-1912. 

Although little research has been reported on the effects of fillers on crack propagation rates, DiBenedetto (1973) and Peretz and DiBenedetto (1972) have investigated the behavior of rapidly growing cracks in epoxy resins filled with glass beads and in a rubber-modified epoxy resin. In both the unfilled and the bead-filled resins, the terminal crack velocity... 

Adhesive systems based on free-radical curing epoxymethacrylates are cited (16). And Dudgeon (17) has covered cationic, heat-curable rubber-modified epoxy resin systems which very likely have latent-cure, structural adhesive capability. 

W.A. Romanchick and J.F. Geibel, "Synthesis of Solid Rubber-Modified Epoxy Resins", ACS, ORPL Preprints, April, 1982, Las Vegas, Nevada. 

R.A. Pethrick, etal., "Rubber-Modified Epoxy Resins ... 

S. J. Shaw, Rubber modified epoxy resins, in Rubber Toughened Engineering Plastics, ed. by... 

Thermoset epoxy resins were toughened by small elastomeric inclusions of a carboxy terminated butadiene-acrylonitrile (CTBN) random copolymer by Visconti and Marchessault [198], who showed the variation in size as a function of CTBN content by TEM and light scattering. A major study of rubber modified epoxy resins has been reported by Manzione et al. [202,203], who showed a range of morphologies which result in a range of mechanical properties, even for a single polymer. An amine cured rubber modified epoxy... 

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