Epoxy resin is defined as a molecule containing more than one epoxide group, also called an oxirane or an ethoxyline group. Epoxide resins are thermosetting polymers. They are commonly used as adhesives, coatings, and for potting or encapsulating assemblies or components. Epoxy resins are known for exceptional adhesion to a variety of materials, low shrinkage rates, high resistance to moisture and tolerant to thermal and mechanical shock loads.
How Epoxies Work:
The adhesion for which epoxy is famous is due to the strong polar bonds it forms with the surfaces it comes in contact with. The reactive groups of molecules in the epoxy resin formulations are the terminal epoxide groups and the hydroxyl groups. It is precisely the availability of these reactive hydroxyl groups that contribute directly to epoxy’s legendary adhesion to most substrates.
Epoxy resins must be crosslinked in order to develop the coating's required characteristics. This crosslinking process is achieved by chemically reacting the resin with a suitable curing agent or hardener.
Coatings vs. Adhesives:
The key parameters governing epoxide performance include viscosity, epoxide equivalent weight and molecular weight. Epoxy coatings are fundamentally of the same basic chemistry as epoxy adhesives. The difference arises with the selection of the various additives, viscosity reducers and tougheners that make the resin/hardener system more applicable to the requirements of a coating.
Resin (“Part A”) Chemistry:
Epoxy resins are created by reacting two separate chemical compounds. These compounds dictate which of the two epoxy chemical families is to be produced; glycidyl epoxy resin, or non-glycidyl epoxy resin.
The Glycidyl Family:
In the glycidyl epoxy family there are glycidyl ethers, glycidyl-esters and glycidyl-amines. All of the glycidyl epoxies are prepared via a similar condensation reaction between epichlorohydrin and either a dihydroxy compound, or a dibasic acid, or a diamine.
We will start with the diglycidyl ether as the first member of the glycidyl family as it is the most common. The first member of the reactants is a chemical called bisphenol-A (sometimes called “Bis-A” in the industry), or it can be bisphenol-F (also known as “Novolac”). The second part of the resin reaction is a chemical called epichlorohydrin.
Probably 98% of all epoxies are Bis-A epoxies. Bis-A is the chemical product of combining one acetone unit with two phenol groups. Phenol is a man-made chemical, although it is also found naturally in animal waste and decomposing organic material. It was originally produced from coal tar and called carbolic acid. Structurally it contains a benzene ring with an attached hydroxyl group (a carbon ring with an attached OH). Acetone is an organic ketone (.i.e. it contains a carbonyl C=O group attached to two organic methyl groups) primarily used as a solvent or chemical intermediate.
Also called chloropropylene oxide, epichlorohydrin production is predominantly focused on the production of epoxy resins. This volatile, colorless liquid with its chloroform-like odor, is the key intermediate for all epoxy resin production.
The Bisphenol-A/Epichlorohydrin Reaction—Common Commercial Epoxy Resin:
When Bis-A is combined with epichlorohydrin, in the presence of a catalyst. a new compound is created. This reaction removes un-reacted phenol and acetone and attaches two glycidyl groups (pronounced “gly-see-dull”) to the ends of the Bis-A, creating a “diglycidyl ether of bisphenol A” (called DGEBA), which is standard epoxy resin. The glycidyl group on both ends of the Bis-A are also referred to as an oxirane or an “epoxide group.”
The properties of any given DGEBA resin depends on the value of n, which is the number of repeating units, commonly known as the degree of polymerization. The number of repeating units depends on the stoichiometry (the ratio of Bis-A to epichlorohydrin) of the synthesis reaction. The value n defines the compound’s molecular weight and can range anywhere from 0 up to 25 in most common commercial products.
Bis-F, or Novolac epoxy resins are diglycidyl ethers of phenolic novolac resins (DGEBF). Phenols are reacted in excess with formaldehyde in the presence of an acidic catalyst to produce phenolic novolac resin. Novolac epoxy resins are synthesized by reacting phenolic novolac resin with epichlorohydrin in the presence of sodium hydroxide as a catalyst.
Novolac epoxy resins generally contain multiple epoxide groups. The number of epoxide groups per molecule depends upon the number of phenolic hydroxyl groups in the starting phenolic novolac resin, the extent to which they are reacted and the degree of low molecular species being polymerized during synthesis. The multiple epoxide groups allow these reins to achieve high cross-link density resulting in excellent temperature, chemical and solvent resistance.
Novolac epoxy resins are widely used to formulate the molding compounds for microelectronics packaging because of their superior performance at elevated temperature, excellent mechanical properties, superior electrical properties, and heat and humidity resistance.
Novolac epoxies are more expensive than regular epoxies but exhibit higher heat distortion temperatures.
While a Bis-A epoxy will begin to soften in the 120-160ºF range, Novolac epoxies can improve on this value by about 25ºF. All epoxies will re-harden when the elevated temperatures fall below their transition temperature. But Bis-F/Novolac epoxies are unique in that they will continue to cure when exposed to temperatures of about 150ºF for a few hours. After this curing they can withstand up to about 300ºF assuming a dry environment.
With regard to chemical resistance, a good quality Bis-A epoxy can resist 70% sulfuric acid but a Bis-F Novolac epoxy can comfortably survive 98% sulfuric acid.
Hardener (“Part B”) Chemistry:
The addition of a hardener to the “Part A” resin initiates a chemical reaction in which the epoxide groups in epoxy resin react with the curing agent (hardener) to form a highly cross-linked, three-dimensional network. Epoxy resins cure quickly and easily at practically any temperature from 5-150oC depending on the choice of curing agent.
The curing agent selection plays the major role in determining many of the properties of the final cured epoxy. These properties include pot life, dry time, penetration and wetting ability.
A wide variety of curing agents for epoxy resins is available depending on the process and properties required. The commonly used curing agents for epoxies include amines, polyamides, phenolic resins, anhydrides, isocyanates and polymercaptans. The cure kinetics and the “glass transition” temperature of the cured system are dependent upon the molecular structure of the hardener. The mix ratio of the epoxy-hardener system also affects the properties of the cured material. Different types and amounts of hardener control cross-link density and thus, vary the cured structure.
Amine-Based Curing Agents:
Amines are the most commonly used curing agents. Primary amines are organic materials containing a nitrogen atom linked to two hydrogen atoms. Tertiary amines are generally used as catalysts, commonly known as accelerators for cure reactions. Use of excessive amounts of catalyst achieves faster curing, but usually at the expense of working life, and thermal stability.
In epoxy formulations, the active hydrogen of the amine reacts with the epoxide group of the resin. The structure of the amine-containing organic compound and the number and type of amine groups in the compound are what determine the rate of crosslinking and the coating's properties.
Amines are basically ammonia with one or more hydrogen atoms replaced by organic groups and include:
- Aromatic Amines
- Aliphatic Amines
- Cycloaliphatic Amines
- Aliphatic Amine Adduct
Amine-based curing agents are considered more durable and chemical resistant than amide-based curing agents but most have a tendency to “blush” in moist conditions. Blushing refers to the development of a waxy surface layer on actively curing epoxy as the result of a reaction between the amine curing agent and moisture in the air.
Aromatic Amine Curing Agents:
In the aromatic amines, the amine group is separated by rigid benzene rings rather than flexible chains of molecules as in the aliphatic amines. Coatings produced with them have good physical properties like impact resistance as well as high resistance to heat and chemicals. But being aromatic in nature, they produce dark coatings. They are used to produce chemical & solvent resistant coatings. When they are formulated specifically with novolac/phenolic epoxy resins, they produce coatings that can resist high temperatures. Aromatic amines are generally modified for use as curing agents, which, while reducing their heat resistance, retain their chemical resistance. They have good resistance to water and hence work well in damp conditions & low temperatures.
Aliphatic Amine Curing Agents:
The aliphatic amines like DETA (Diethylene Triamine), TETA (Triethylene Tetramine), TEPA (Tetraethylene Pentamine), and EDA, contain short, linear chemical chains between amine groups. Coatings produced with them tend to have highly crosslinked layers with good resistance to heat and chemicals, including solvents. However, they are rather brittle and possess poor flexibility & impact resistance. Because of their reaction with moisture, they are not suitable for use under damp conditions.
Here is a good time to introduce the concept of “adduct epoxies.” Adduct epoxies are two part epoxies but the curing agent actually contains a bit of the epoxy resin. In effect, the “mixture” has started to cure even before the two parts are mixed. They perform much like other epoxies, but have improved overall physical properties. These include, but are not limited to better color stability and curing at slightly cooler temperatures. Cure time can be much faster than with regular epoxies.
Polyamine Adduct Curing Agents:
Thus, a common modification to aliphatic amines is to form a polyamine adduct by pre-reacting the curing agent with a small amount of the epoxy resin. This gives high molecular weight polyamines that produce coatings with low vapor pressure, with more practical mixing ratios and less formation of amine bloom than the simple aliphatic amines. Adducting has little effect on other properties. Polyamine adducts can be prepared from either aliphatic or aromatic polyamines.
Amide and Polyamide Curing Agents:
An amide is basically ammonia with a hydrogen atom replaced by a carbon/oxygen and organic group.
Amides are surface tolerant and less troubled by moisture. The “amide” curing agent has a molecular structure which typically consists of four hydrogen arms. These hydrogens react with the oxirane (epoxide group) ring unit on the ends of the epoxy resin. The result is a new carbon-hydrogen bond, this time using the hydrogen from the curing agent and freeing the epoxy group's hydrogen to unite with the group's oxygen to form an OH (hydroxyl) pendant. This hydroxyl group contributes to the epoxy's outstanding adhesion to most substrates. The aromatic ring unit, to which the hydroxyls attach, helps provide the desirable thermal and corrosion properties. Because there are at least four hydrogens on the curing agent, they can react with four epoxy resin groups, which causes giant crosslinked structures.
Polyamides are formed by the reaction of aliphatic polyamines and dimer acids of either tall oil fatty acids or from soya or castor oil. As with the amines, adducting polyamides is common practice and produces coatings with good low temperature curing and reduced tendency for amine bloom. Good color and good chemical resistance can be achieved using these adducted polyamides. They generally produce coatings with excellent adhesion, water resistance & flexibility. Unmodified polyamides produce coating layers that are much more open in terms of their chemical structure because of the large distances between amine groups in the chemical chain. Consequently they are more flexible. The downside of this open structure of the curing agent results in coatings with low resistance to chemicals, solvents and acids. However, their resistance to water and corrosion are enhanced because of their surface wetting and adhesion properties.
Amidoamide Curing Agents:
When an aliphatic polyamine is reacted with a monofunctional fatty acid rather than a dimer acid, then an amidoamide is formed. These curing agents are less volatile and have less irritation potential than polyamines, and they have properties that are similar but inferior to polyamides. For instance, polyamides are better in water resistance and provide better adhesion than the amidoamides.
Cycloaliphatic Curing Agents:
These curing agents generally provide better water/moisture resistance, better weatherability, low blush and water spotting, and better chemical resistance.
The cycloaliphatic structure refers to a six-member carbon-atom ring in the backbone of the curing agent instead of the more common carbon-carbon inter-amine linkages.
The cycloaliphatic curing agents have the amine groups connected to the rings. They are adducts in that they both use diglycidyl ether of Bisphenol-A for their pre-reaction. Additionally, they contain benzyl alcohol which is a volatile plasticizer that acts as a molecular lubricant thus promoting cure.
While the cycloaliphatic ring is somewhat more resistant to UV degradation compared to the more common carbon-carbon linkages of amine-based curing agents, the presence of the aromatic ring structure of diglycidyl ether of Bisphenol-A is retained which breaks down fairly readily on UV exposure.
Modified Cycloaliphatic Amine Curing Agents:
Modified cycloaliphatic amines from IPDA are probably the most used curing agents for epoxy resins today. Because of their low viscosities, they can be used in low VOC (volatile organic compound) coatings. They produce coatings with a fast cure rate, short pot life and are also suitable for low temperature cure. They provide very good resistance to chemicals, solvents & water, which makes them suitable for use in portable water tanks for example.
Other Curing Agents:
In addition to the above types of curing agents there are many other curing agents based on polyamines, and there are also non-amine based curing agents. Finally, either aromatic or aliphatic isocyanates may also be used as curing agents for epoxy resins. The isocyanates react through the hydroxyl groups of the epoxy resin and provide very good low temperature curing, good flexibility, good impact & abrasion resistance as well as good adhesion.
The usefulness of epoxy resins in many engineering applications is often limited by their brittle nature and poor thermal conductivity. The term toughness is a measure of a material's resistance to failure—the total amount of energy required to cause failure.
There are several approaches to enhance the toughness of epoxy resins. They include chemical modification of the epoxy backbone to make it a more flexible structure, increasing the molecular weight of the epoxy, lowering the cross-link density of the matrix, incorporation of a dispersed toughener phase in the cured polymer matrix, and incorporation of inorganic fillers into the resin.
Among these approaches, toughening via the dispersed toughener (flexibilizer) phase has been shown to be most effective. The flexibilizers can be reactive or non-reactive rubber.
Various types of thermoplastic polymers as well as reactive rubbers are employed to enhance the toughness of epoxy resin systems. Thermoplastic polymers, such as polyetherimide, polysulphone, polyethersulphone, and polycarbonate have been studied to modify epoxy resins. These studies show significant improvement in the toughness of epoxy resins.
The reactive rubbers used for toughening epoxy resins include, liquid acrylonitrile-butadiene copolymers with various terminal groups, polysiloxanes, polyepichlorohydrin, and polyurethanes.
Although liquid acrylonitrile-butadiene copolymers with carboxyl- (CTBN) and amine- (ATBN) terminated groups have been widely used for epoxy toughening, the relatively high glass transition temperature of the copolymer limits their low-temperature applications. In addition, these copolymers also increase the CTE value (Coefficient of Thermal Expansion) of the molding compound. Also the presence of the unsaturated structure of butadiene in the system is prone to thermal instability and thus unsuitable for long-term use at higher temperatures.
Polysiloxanes (silicones) have excellent thermal stability, moisture resistance, good electrical properties, low stress and lower glass transition values. However polysiloxanes are not compatible with epoxy resins. Addition of compatibilizers such as, methylphenylsiloxane enhances the compatibility but at the same time raises the glass transition temperature of the polysiloxane modifier restricting its low temperature applications.