Modification of Epoxy Resins

Essentially, the methods of epoxy resin modification are based on attaining a dispersion of second-phase particulates in the epoxy matrix the second phase may be either rubbery or rigid in nature, or both. Substantial enhancement of fracture toughness has been achieved in the hybrid systems. 

Since cured epoxy resins are inherently brittle materials, modification is necessary to make them usable in many applications. Hence, various methods have been employed to overcome the tyranny of the Griffith equation in this class of materials. 

Improved toughness can be achieved by additions to the epoxy/hardener formulation [145,146], However, if these modifiers are fully compatible with the cured epoxy, they act as plasticizers and cause a considerable reduction in the glass transition temperature (Tg) of the system. This results in toughened systems at an appreciable sacrifice in heat distortion temperature and, therefore, elevated-temperature properties. 

This mechanism is unlikely to operate, however, in the case of highly cross-linked systems, since the chain length between crosslinks is too short to effectively produce the fibrils of oriented chains acting as bridges between the two surfaces of the craze [ 149]. 

The efficiency of modifiers depends on their ability to form diffusion barriers consisting of co-continuous (interpenetrating) phases within the intrinsically hydrophobic matrix. 

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Schematic Representation of Siloxane Modification of Epoxy Resins 69n5)...

After these preliminary studies, McGrath and co-workers have used ethylpiperazine terminated siloxane oligomers with varying molecular weights and backbone compositions throughout their studies 69 114,11S). Modifications of epoxy resins with siloxane oligomers were performed in two steps as depicted in Reaction Scheme XXIII. 

This section focuses on the modification of epoxy resins by blending with acrylonitrile butadiene (nitrile) resins. These are true alloyed blends since the nitrile rubber usually contains no groups that are normally reactive with epoxy groups. The nitrile molecules and the epoxy molecules intermingle as a blend to provide a single-phase alloy. If a large elastomer concentration is used, no phase separation will occur to form precipitates. 

Ho, T., et al., Modification of Epoxy Resins with Polysiloxane TPU for Electronic Encapsulation, Journal of Applied Polymer Science, vol. 60, 1996, p. 1097. 

Howell, B. F., Modification of Epoxy Resins, in Polymer Modification Principles, Techniques, and Applications, J. J. Meister, ed., Marcel Dekker, New York. 

It is these solid carboxylic nitrile elastomers which began to show utility in the modification of epoxy resins. Processing needs for solid elastomer Inclusion, particularly in liquid epoxy resins, have not always been advantageous. Associated problems include gel, viscosity threshold limitations and achieving desired rubber levels in excess of 5-6 phr. Sometimes processing must be carried out in selected solvents, not always a desirable or tolerable step. 

Wide-ranging documentation exists which covers modification of epoxy resins using carboxyl-terminated polybutadiene/acrylo-nitrile liquid polymers in which addition esterification (alky-... 

The chemical literature reveals relatively new and varied interests for nitrile elastomer-modification of epoxy resins in diverse areas of coatings and primers. Desirable properties such as impact resistance, mandrel bend and adhesion improvement are attainable with little or no sacrifice in critical film properties. In some instances, it is documented that proper elastomer modification of select epoxy coatings will enhance corrosion and moisture resistance. This may relate to better film/substrate adhesion durability. The combined literature, journal and patent, has led to continuing study of nitrile elastomer modified epoxy coatings. 

THE MODIFICATION OF EPOXY RESINS BY METAL/ CARBON NANOCOMPOSITES SUSPENSIONS... 

Wan Wang, H.-H., Chen, J.-C. Chemical modification of epoxy resin by reaction with anhydride-terminated polyurethaneimide. Polym. - Plast. Technol. Eng. 33 (1994) 637-651. 

OOKar Karger-Kocsis, J., Gremmels, J. Use of hygrothermal decomposed polyester-urethane waste for the impact modification of epoxy resins. J. Appl. Polym. Sci. 78 (2000) 1139-1151. 

OlFel Fellahi, S., Chikhi, N., Bakar, M. Modification of epoxy resin with kaolin as a toughening Agent. J. Appl. Polym. Sci. 82 (2001) 861-878. 

Modification of epoxy resins by such modifiers is usually achieved in two ways by integrating the rubber into the epoxy pol5rmer or by copolymerizing the epoxy and rubber obgomers simultaneously with the process of epoxy resin curing at 393 or 433 K. 

For modification of epoxy resins with rubbers containing double bonds, the principal difficulty lies in the formation of hydroperoxide groups. These in turn can fragment into radicals, which cause undesirable and uncontrolled side-reactions. 

These findings allow control of the modification of epoxy resins by rubbers. For example, to obtain ERC with improved adhesion strength it is necessary to use a quantity of the rubber at which thermod3mamic incompatibility with the epoxy oligomer is observed. 

As with other polymers and resins, the modification of epoxy resins eliminates a number of disadvantages including brittleness, low thermal stability and high flammability to give the required properties for end applications. Considerable effort has therefore been made to improve the thermal and mechanical properties of vegetable oil-based epoxy resins. These may be grafted or blended with other flexible resins such as polyester, polyacrylate and polyurethane to improve their flexibility and other performance characteristics. 

K. Dusek, et al., "Modification of Epoxy Resins with Liquid Rubbers Having Carboxyl Groups. Part I. Preparation of the Mixtures, Curing and Morphology of the Cured Resins," Chemicky Prumysl, 30/55, No. 11, 591-597 (1980). 

Liquid crystals exhibit a partially ordered state (anisotropic) which falls in-between the completely ordered solid state and completely disordered liquid state. It is sometimes referred to as the fourth state of matter . In recent years, interest in liquid crystalline thermosets (especially liquid crystalline epoxy) has increased tremendously [33-44]. If the liquid crystal epoxy is cured in the mesophase, the liquid crystalline superstructure is fixed permanently in the polymer network, even at higher temperature. Liquid crystal epoxies are prepared using a liquid crystal monomer [33-38] or by chemical modification of epoxy resin [43] which incorporates liquid crystal unit in the epoxy structure. Liquid crystalline epoxy resins with different types of mesogen such as benzaldehyde azine [33], binaphthyl ether [34, 35], phenyl ester [36, 37] and azomethine ethers [38, 39] have been reported. Depending on the chemical nature of the mesogen, the related epoxies display a wide range of thermomechanical properties. The resins can be cured chemically with an acid or amine [40, 41] or by photochemical curing in the presence of a photo-initiator [3]. Broer and co-workers [42] demonstrated the fabrication of uniaxially oriented nematic networks from a diepoxy monomer in the presence of a photo-initiator. 

Figure 5.1 Chemical modification of epoxy resin with carboxyl-terminated poly (ethylene glycol) adipate (CTPEGA). Reprinted with permission from D. Ratna, A.B. Samui and B.C. Chakraborty, Polymer International, 2004, 53,1882. 2004,...

Ho, T. H. Wang, C. S. Modification of epoxy resin with siloxane containing phenol aralkyl epoxy resin for electronic encapsulation application. Eur.Polym J., 2001, 37, 267. 

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