Epoxy resins: How to select the right grade?

Epoxy resins: A type of thermoset

A thermosetting resin, also known as a thermoset, is a polymer that cures or sets into a hard shape using curing methods such as heat or radiation. The curing process is irreversible. It introduces a polymer network crosslinked by covalent chemical bonds. Upon heating, thermosets remain solid until the temperature reaches the point where they begin to degrade. This mechanism is opposite to thermoplastics. 

A few examples of thermosetting resins are:
 

Among them, epoxies or epoxy resins are one of the most common thermosets. The alternate terms used for epoxy resins are "epoxide" (Europe), α-epoxy, and 1,2-epoxy. 

Epoxy resins are a broad group of reactive compounds. The presence of an oxirane or epoxy ring characterizes these reactive groups. 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. Hence, the presence of this functional group defines a molecule as an epoxide. 

Chemical structure of epoxy resin (Diglycidyl ether of bisphenol-A)

The molecular base can vary widely, resulting in various classes of epoxy resins. They are successful as they offer diversity in molecular structure that can be produced using the same chemical method.

Today, epoxies are widely used in structural and specialty composites applications. Due to their high strength and rigidity (because of the high degree of crosslinking), they are adaptable to nearly any application. Further, epoxy resins can be combined with varied curing agents and modifiers. This is done to achieve the properties required for a specific application.

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Synthesis of epoxy resins

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. Diglycidyl ether of bisphenol A (DGEBA) is the most widely used epoxy resin monomer.
 

The oxirane group of an epoxy monomer reacts with curing agents such as: 
 

They combine with other suitable ring-opening compounds, forming rigid thermosetting products. The cured epoxies are brittle in nature due to the high degree of crosslinking. They contribute to weakening epoxy impact strength and other relevant properties. Hence, it is necessary to modify epoxy monomers to improve their flexibility, toughness, and thermal properties. 

The synthesis of epoxy resin from bisphenol A and epichlorohydrin is shown in the figure below.
 

Synthesis of epoxy resin from bisphenol A and epichlorohydrin

 

 

Primary types of epoxy resins

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

• Phenolic glycidyl ethers
• Aromatic glycidyl amines
• Cycloaliphatics

The following section provides a detailed account of each of these classes.

Phenolic glycidyl ethers

The condensation reaction between epichlorohydrin and phenol forms phenolic glycidyl ethers. The structure of the phenol-containing molecule and the number of phenol rings distinguish different types of epoxy resins.  

Diglycidyl ether of bisphenol-A (DGEBA) is one of the most widely used epoxy resins today. Modifying the ratio of epichlorohydrin to BPA during production can generate high molecular weight (HMW) resin. This HMW increases viscosity, and hence these resins are solid at room temperature. 

Other variations in this class include: 
 

• Hydrogenated bisphenol-A epoxy resins: They offer improved UV stability and weathering resistance compared to conventional BPA-based epoxies.
• Brominated resins: They are flame retardants and are mostly used in electrical applications. They are produced from tetrabromo-bisphenol-A.
• Diglycidyl ether of bisphenol-F (DGEBF): They offer lower viscosity and improved chemical resistance. Thus, making it suitable for high-solids and solvent-free formulations.
• Diglycidyl ether of bisphenol-H (DGEBH): They show promising weather resistance.
• Diglycidyl ether of bisphenol-S (DGEBS): They are used to obtain a thermally stable epoxy resin.

Phenol and cresol novolacs are two other types of aromatic glycidyl ethers. They are produced by combining either phenol or cresol with formaldehyde, producing a polyphenol. The polyphenol is subsequently reacted with epichlorohydrin. This generates epoxy resin with high functionality and high cured Tg.

Aromatic glycidyl amines

Aromatic glycidyl amines are formed by the reaction of epichlorohydrin with an amine. The aromatic amines are suitable for high-temperature applications. 
 

The most important resin in this class is tetraglycidyl methylene dianiline (TGMDA). It offers:   

• excellent mechanical properties,
• high glass transition temperatures, and
• suited for advanced composite aerospace applications.

Triglycidyl p-amino-phenol (TGPAP) is another type of glycidyl amine. It exhibits low viscosity at room temperature. Hence, it is 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 and tetraglycidyl meta-xylene diamine. The primary disadvantage of this class is the cost, which can be higher than that of bisphenol-A resins.

 

Cycloaliphatics

Cycloaliphatic epoxy resins contain an epoxy ring that is internal to the ring structure. They are designed for applications requiring:
 

• high-temperature resistance,
• good electrical insulation performance,
• UV resistance, and
• high glass transition temperatures in the range of 200°C

Cycloaliphatic epoxy resin formulations are used to fabricate many fiber-reinforced structural components. Examples include diglycidyl ester of hexahydrophthalic acid and 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane.
 

 
Now that we have understood the basics, manufacturing, and the main types of epoxy resins, let's learn their properties in the next section.

Properties of epoxy resins

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

The key properties of epoxy resins are listed below:

For thermosetting epoxies, the ideal range of certain material properties are as follows:
 

• Tensile strength ranges from 90 to 120 MPa
• Tensile modulus ranges from 3100 to 3800 MPa
• Glass transition temperature (Tg) ranges from 150 to 220 °C

Epoxy resins have two main drawbacks, which are their brittleness and moisture sensitivity.

 

Role of additives in enhancing material properties

Epoxy resin formulations rely on a range of additives to achieve performance while maintaining cost efficiency, . Each additive class serves a specific role in modifying mechanical properties, processing characteristics, functional performance, or long-term durability of the cured system. 
 

1. Reinforcing fibers: Improve mechanical properties so that epoxies can be used in structural applications. Examples include glass, graphite, and polyaramid.  
    
2. Fillers: Play an important role in epoxy resin formulations. Increasing filler content generally increases viscosity. It also makes processing more difficult. Specific gravity usually increases. However, some fillers like hollow glass or phenolic microballoon, create syntactic foams of significantly reduced density. 
    

3. Non-reinforcing fillers:

   • Powdered metal: Improves electrical and thermal conductivity
   • Alumina: Improves thermal conductivity
   • Silica: Helps in cost reduction and strength enhancement
   • Mica: Improves electrical resistance
   • Talc and calcium carbonate: Promotes cost reduction
   • Carbon and graphite powder: Increases lubricity

    

4. Nanoparticle-reinforced epoxy composites: They have generated considerable industrial interest over the past decades. These materials have a high specific strength-to-weight ratio, low density, and enhanced modulus. These properties permit them to contend with selected metals. 
    
5. Rubber additives: They are used to increase flexibility, fatigue resistance, crack resistance, and toughness in epoxy resins. The liquid rubbers 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. Eventually, the particle size in the cured matrix decreases. Unreactive rubbers are not used in epoxy composite applications.  
    
6. Thermoplastic additives: They are used to increase the fracture toughness of epoxy resins. Only relatively low molecular weight TPs can be dissolved in epoxy resins. Commonly used thermoplastics are phenoxy, polyether block amides, PVB, polysulfone, polyethersulfone, polyimide, polyetherimide, and nylon. As compared to rubbers, thermoplastics are more effective tougheners in highly crosslinked matrices. They do not tend to affect Tg and modulus. However, high loadings of TP lead to an increase in solvent sensitivity and a decrease in resistance to creep & fatigue. 
    
7. Flame retardants: They are added to epoxy resins to incorporate flame-retardant characteristics. The presence of halogens and char-forming aromatics in the epoxy-curative-based resin decreases flammability.
    
8. Colors and dyes: A wide variety of colorants can be used with epoxies. These include inorganic pigments except chrome greens, natural siennas, zinc sulfide white, etc., and organic pigments such as carbon blacks.

Factors to consider while incorporating additives

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

Now that we are through the key properties of epoxies and how various additives influence the material properties, let's see the sustainability aspect of the polymer.

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)

The following table highlights the major differences between epoxy resins and polyesters

Finally, we will explore the key market applications of epoxy resins, highlighting how their properties make them suitable for use accross wide range of industries and end-use requirements.