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Thermosetting Epoxy Polymer

Epoxy Resins: A to Z Technical Review of Thermosetting Polymer

After their first commercial production in late 1940s, epoxy resins comprise of wide family of materials today. Thanks to their high strength, versatility and excellent adhesion to variety of surfaces, epoxy resins have gained wide acceptance in diverse applications (coatings, electrical, casting resins, composites, etc.). Explore thermosetting epoxy resins in detail along with their key properties and understand what makes them an ideal choice in so many applications. Also, learn about different types of resins available and how to further improve their properties. But, let’s first start by understanding thermosets.

Overview

What is a thermoset?

What is a thermoset?

A thermosetting resin, or thermoset, is a polymer which cures or sets into a hard shape using curing method such as heat or radiation. The curing process is irreversible as it introduces a polymer network crosslinked by covalent chemical bonds.

Upon heating, unlike thermoplastics, thermosets remain solid until temperature reaches the point where thermoset begins to degrade.

Phenolic resins, amino resins, polyester resins, silicone resins, epoxy resins, and polyurethanes (polyesters, vinyl esters, epoxies, bismaleimides, cyanate esters, polyimides and phenolics) are few examples of thermosetting resins.

Among them, epoxies or epoxy resins are one of the most common and widely used thermosets today in structural and specialty composites applications. Due to their high strength and rigidity (because of high degree of crosslinking), epoxy thermoset resins are adaptable to nearly any application.

But what make epoxy resin a versatile resin for these applications. Let’s learn it in detail…

What makes Epoxy Resin versatile?

What makes Epoxy Resin versatile?

The term "epoxy", "epoxy resin", or "epoxide" (Europe), α-epoxy, 1,2-epoxy etc. refers to a broad group of reactive compounds that are characterized by the presence of an oxirane or epoxy ring. This is represented by a three-member ring containing an oxygen atom that is bonded with two carbon atoms already united in some other way.
Epoxy Chemical Structure

Hence, the presence of this functional group defines a molecule as an epoxide – where molecular base can vary widely resulting in various classes of epoxy resins. And they are successful because they offer the diversity in molecular structure that can be produced using the same chemical method.

Further, epoxy resins can be combined with varied curing agents, modifiers to achieve the properties required for a specific application.


Epoxy resins are typically formed by the reaction of compounds containing at least two active hydrogen atoms (polyphenolic compounds, diamines, amino phenols, heterocyclic imides and amides, aliphatic diols, etc.) and epichlorohydrin.

The synthesis of diglycidyl ether of bisphenol A (DGEBA), the most widely used epoxy resin monomer, is:

Synthesis of Epoxy Monomer from Bisphenol A and Epichlorohydrin
Synthesis of Epoxy Monomer from Bisphenol A and Epichlorohydrin

The oxirane group of an epoxy monomer reacts with different curing agents such as aliphatic amines, aromatic amines, phenols, thiols, polyamides, amidoamines, anhydrides, thiols, acids and other suitable ring opening compounds; forming rigid thermosetting products. The cured epoxies are brittle in nature due to the high degree of cross-linking, and they contribute to weakening epoxy impact strength and other relevant properties.

Hence, modification of epoxy monomers is necessary to improve their flexibility and toughness as well as thermal properties.

The three primary classes of epoxies used in composite applications are:

  • Phenolic glycidyl ethers
  • Aromatic glycidyl amines and
  • Cycloaliphatics

Phenolic Glycidyl Ethers


They are formed by the condensation reaction between epichlorohydrin and a phenol group. The structure of phenol-containing molecule, number of phenol rings distinguish different types of epoxy resins. A showed above DGEBA (diglycidyl ether of bisphenol-A) is one of the most widely used epoxy resins today.
Bisphenol A

Modifying the ratio of epichlorohydrin to BPA during production can generate high molecular weight resin. This HMW increases visocity and hence these resins are solid at room temperature. Other variations in this class include hydrogenated bisphenol-A epoxy resins, brominated resins produced from tetrabromo bisphenol-A, diglycidyl ether of bisphenol-F, diglycidyl ether of bisphenol-H, diglycidyl ether of bisphenol-S etc. Brominated resins are flame retardant and mostly used in electrical applications. Further, DGEBH shows promising weather resistance and DGEBS is used to obtain thermally stable epoxy resin.

Phenol and cresol novolacs are another two types of aromatic glycidyl ethers. They are produced by combining either phenol or cresol with formaldehyde produces a polyphenol. This polyphenol is subsequently reacted with epichlorohydrin to generate the epoxy resin with high functionality and high cured Tg.


Aromatic Glycidyl Amines


They are formed by reaction epichlorohydrin with an amine, with aromatic amines suitable dof rhigh temperature application. The most important resin in this class is tetraglycidyl methylene dianiline (TGMDA)

TetraGlycidyl-MethyleneDiAniline (TGMDA)

TGDMA resins offer excellent mechanical properties and high glass transition temperatures and suitable for advanced composited for aerospace applications.

TGPAP – Triglycidyl p-amino-phenol is another type of glycidyl amine. It exhibits low viscositry at RT and hence commonly blended with other epoxies to modify the flow or tack of the formulation without loss of Tg.

Other commercial glycidyl amines include diglycidyl aniline, tetraglycidyl meta-xylene diamine. The primary disadvantage of this class is cost which can be higher as compared to Bis-A resins.


Clycloaliphatics


Cycloaliphatic epoxy resins are designed applications requiring high-temperature resistance, good electrical insulation performance and UV resistance. They contain an epoxy ring that is internal to ring structure.

Cycloaliphatic epoxy resin formulations are used to fabricate many fiber-reinforced structural components. Formulations incorporating these resins can exhibit high glass transition temperatures in the range of 200°C.

An important and widely used cycloaliphatic epoxy resin is the diglycidyl ester of hexahydrophthalic acid and 3,4-Epoxycyclohexylmethyl-3',4'-epoxycyclohexane.
Diglycidyl Ester of Hexahydrophthalic Acid
Diglycidyl Ester of Hexahydrophthalic Acid


Key Properties of Epoxy Resins

Key Properties of Epoxy Resins

We list below the key properties offered by Epoxy Resins.

  • 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:


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

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.

Epoxy Resins vs Polyester Resins

Epoxy Resins vs Polyester Resins

Epoxy Polyester
  • Extremely strong and good flexural strength
  • The hardener and the temperature determine epoxy resin cure time
  • Resistant to wear, cracking, peeling, corrosion and damage from chemical and environmental degradation
  • Has a bonding strength of up to 2,000 psi
  • Epoxy is moisture resistant after curing
  • Brittle and prone to micro-cracking
  • Generally, costs slightly less than epoxy resin
  • Off-gases VOCs and has strong, flammable fumes
  • Bonding strength of polyester resin is generally less than 500 psi
  • Once cured, polyester resin is water permeable, meaning water can pass through it eventually


Overall, Epoxy resins have performance advantages over polyester and vinyl esters in five major areas:

  • Better adhesive properties (the ability to bond to the reinforcement or core)
  • Superior mechanical properties (particularly strength and stiffness)
  • Improved resistance to fatigue and micro cracking
  • Reduced degradation from water ingress (diminution of properties due to water penetration)
  • Increased resistance to osmosis (surface degradation due to water permeability)


Recycling and BioBased Epoxy Systems

Recycling and BioBased Epoxy Systems

As discussed above, Epoxy thermosetting composites are high-performance materials with significant industrial applications. However, recycling of thermosets and their filling matters are challenging. However, significant research and development has been done to enable recycling of thermosets, thus allowing the plastics to be broken down and reformed.

There are some new developments in epoxy thermosets which can be recycled up to some extent, but their commercial importance is not tapped fully yet.

Further, advances in bio-based thermoset resin systems have attracted significant attention given their environmental benefits. Some of the bio-sourced thermosets include:

  • Natural oil-based (soybean, linseed, castor…)
  • Isosorbide-based
  • Furan-based epoxy systems
  • Phenolic and polyphenolic epoxies
  • Epoxidized natural rubber
  • Epoxy lignin derivatives
  • Rosin-based resins


Find Suitable Epoxy Resin

View a wide range of epoxy resins available in the market today, analyze technical data of each product, get technical assistance or request samples.

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