Key Properties of Epoxy Resins

Epoxies offer:

· High strength

· Low Shrinkage

· Excellent adhesion to various substrates

· Effective electrical insulation

· Chemical and solvent resistance, and

· Low cost and low toxicity

Epoxies are easily cured, and they are also compatible with most substrates. They tend to wet surfaces easily, making them especially suitable for composite applications. Epoxy resin is also used to modify several polymers such as polyurethane or unsaturated polyesters to enhance their physical and chemical attributes.

For thermosetting epoxies:

· The tensile strength ranges from 90 to 120 MPa

· A tensile modulus ranging from 3100 to 3800 MPa

· Glass transition temperatures (Tg) that range from 150 to 220 ?C

Aside from the properties mentioned above, epoxy resins have two main drawbacks which are their brittleness and moisture sensitivity.

Epoxy Composites: Additives for High Performance

Fillers also play an important role in epoxy resin formulations. Reinforcing fibers such as glass, graphite and polyaramid improve mechanical properties to such an extent that epoxies can be used in many structural applications. Other non-reinforcing fillers include:

· Powdered metals to improve electrical and thermal conductivity

· Alumina for thermal conductivity

· Silica for cost reduction, and strength enhancement

· Mica – electrical resistance

· Talc and calcium carbonate – cost reduction

· Carbon and graphite powders to increase lubricity

While compounding with filled systems, some of the important factors to be taken into account include:

· Volume fraction of filler

· Particle characteristics (size, share, surface area…)

· Filler aspect ratio

· Strength and modulus of filler

· Adhesion of filler to the resin

· Viscosity of the base resin

· Toughness of the base rein

Nanoparticle-reinforced epoxy composites have also generated considerable industrial interest over past decades. These materials have high specific strength-to-weight ratio, low density, and enhanced high modulus, which permits them to contend with selected metals. 

The primary aim of reinforcing-blending of epoxies is to achieve the desired properties while maintaining low costs. Increasing filler content generally increases viscosity and makes processing more difficult. Specific gravity usually increases, although some fillers like hollow glass or phenolic microballoons create syntactic foams of significantly reduced density.

Other important modifiers used in Epoxy resin formulations are:

Rubber Additives – They are used to increase flexibility, fatigue resistance, crack resistance, toughness in epoxy resins. The liquid rubbers most often used in epoxy composites are carboxyl-terminated butadiene acrylonitrile copolymer (CTBNs). However, the acrylonitrile content of the rubber is an important consideration when using a rubber modifier. As the nitrile content of a rubber increases, its solubility increases and eventually particle size in the cured matrix decreases. Unreactive rubbers are not used in epoxy composites applications.

Thermoplastic Additives – They are used to increase the fracture toughness of epoxy resins. Only relatively low MW TPs can be dissolved in epoxy resins. Commonly used thermoplastics are phenoxy, polyether block amides, PVB, polysulfone, polyethersulfone, polyimide, polyetherimide, nylon. 

As compared to rubbers, thermoplastics are more effective tougheneres in highly cross-linked matrices and they do not tend to affect Tg and modulus. 

However high loadings of TP lead to increase in solvent sensitivity and decreases resistance to creep & fatigue.

Flame Retardants – They are added to epoxy resins to incorporate FR characteristics. The presence of halogens and char-forming aromatics in the epoxy-curative based resin decreases flammability.

Colors and Dyes – A wide variety of colorants can be used with epoxies such as inorganic pigments except chrome greens, natural siennas, zinc sulfide white etc. and organic pigments such as carbon blacks. – A wide variety of colorants can be used with epoxies such as inorganic pigments except chrome greens, natural siennas, zinc sulfide white etc. and organic pigments such as carbon blacks.