What is a thermoset?
What is a thermoset?
A thermosetting resin or thermoset is a polymer that cures or sets into a hard shape using curing methods such as heat or radiation. The curing process is an irreversible process. It introduces a polymer network crosslinked by covalent chemical bonds.
Upon heating, thermosets remain solid until the temperature reaches the point where it begins to degrade. This mechanism is opposite to thermoplastics. A few examples of thermosetting resins are:
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Phenolic resins
- Amino resins
- Polyester resins
- Silicone resins
- Epoxy resins, and
- Polyurethane resins
Among them, epoxies or epoxy resins are one of the most common thermosets. Today they 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.
What makes epoxy resin versatile?
What makes epoxy resin versatile?
The term "epoxide" (Europe), α-epoxy, and 1,2-epoxy are alternate terms used for epoxy resins. They are a broad group of reactive compounds. These reactive groups 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.
Hence, the presence of this functional group defines a molecule as an epoxide. 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.
Further, epoxy resins can be combined with varied curing agents and modifiers. This is done to achieve the properties required for a specific application.
Watch Tutorial: How to Select the Best Curing Agent for Epoxy Systems
How are epoxy resins produced?
How are epoxy resins produced?
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
The oxirane group of an epoxy monomer reacts with curing agents such as:
- Aliphatic and aromatic amines
phenols,
- Polyamides,
- Amidoamines,
- Anhydrides,
- Thiols, and acids.
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 cross-linking. They contribute to weakening epoxy impact strength and other relevant properties. Hence, it is necessary to modify epoxy monomers to improve their flexibility and toughness as well as thermal properties.
What are the primary types of epoxies used in composites?
What are the primary types of epoxies used in composites?
The three primary classes of epoxies used in composite applications are:
Phenolic Glycidyl Ethers
They are formed by the condensation reaction between:
-
epichlorohydrin and
- phenol group
The structure of the phenol-containing molecule and the number of phenol rings distinguish different types of epoxy resins. DGEBA (diglycidyl ether of bisphenol-A) is one of the most widely used epoxy resins today.
Modifying the ratio of epichlorohydrin to BPA during production can generate high molecular weight 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,
- 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 retardants and are mostly used in electrical applications. DGEBH shows promising weather resistance. Further, 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 producing a polyphenol. This polyphenol is subsequently reacted with epichlorohydrin to generate the epoxy resin with:
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high functionality and
- high cured Tg
Aromatic Glycidyl Amines
They 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) that offers:
- excellent mechanical properties,
- high glass transition temperatures, and
- suited for advanced composited aerospace applications.
TGPAP – Triglycidyl p-amino-phenol is another type of glycidyl amine. It exhibits a 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
tetraglycidyl meta-xylene diamine.
The primary disadvantage of this class is the cost which can be higher as compared to Bis-A resins.
Cycloaliphatics
Cycloaliphatic epoxy resins contain an epoxy ring that is internal to the ring structure. They are designed for applications requiring:
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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.
Example: diglycidyl ester of hexahydrophthalic acid and 3,4-Epoxycyclohexylmethyl-3',4'-epoxycyclohexane
Diglycidyl Ester of Hexahydrophthalic Acid
What are the key properties of epoxy resins?
What are the 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. They enhance their physical and chemical attributes.
For thermosetting epoxies:
Epoxy resins have two main drawbacks which are their brittleness and moisture sensitivity.
What are the additives used in epoxy composites?
What are the additives used in epoxy composites?
Incorporating additives: Major types and material properties
Fillers also play an important role in epoxy resin formulations. The primary aim of reinforcing-blending of epoxies is to achieve the desired properties while maintaining low costs. Increasing filler content generally increases viscosity. It also makes processing more difficult. Specific gravity usually increases. Although some fillers like hollow glass or phenolic microballoons create syntactic foams of significantly reduced density.
- Reinforcing fibers improve mechanical properties so that epoxies can be used in structural applications. For example, glass, graphite, and polyaramid.
- 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
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Nanoparticle-reinforced epoxy composites have also generated considerable industrial interest over the past decades. These materials have:
- high specific strength-to-weight ratio,
- low density, and
- enhanced high modulus
These properties permits them to contend with selected metals.
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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. Eventually the particle size in the cured matrix decreases. Unreactive rubbers are not used in epoxy composites applications.
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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 tougheners in highly cross-linked matrices and they do not tend to affect Tg and modulus. However high loadings of TP lead to an increase in solvent sensitivity and decreases resistance to creep & fatigue.
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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.
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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.
Factors to consider while adding additives
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 resin
How are epoxies different from polyesters and vinyl esters?
How are epoxies different from polyesters and vinyl esters?
|
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)
Can epoxy resins be recycled?
Can epoxy resins be recycled?
As discussed above, Epoxy thermosetting composites are high-performance materials with significant industrial applications. However, recycling thermosets and their filling matters are challenging. Significant research and development are going to enable the recycling of thermosets. This thus allows 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.
What are the advancements in bio-based thermosets?
What are the advancements in bio-based thermosets?
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
What are the commercially available epoxy resins?
What are the commercially available epoxy resins?
View a wide range of epoxy resins available in the market today, analyze technical data of each product, get technical assistance or request samples.