The first thing to know is that epoxy is a colloquial name for the epoxide functional group, or any of the basic components or cured end products of epoxy resins
Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups.
Epoxy resins may be reacted (cross-linked) either with themselves through catalytic homopolymerisation, or with a wide range of co-reactants including polyfunctional amines, acids (and acid anhydrides), phenols, alcohols and thiols (usually called mercaptans). These co-reactants are often referred to as hardeners or curatives, and the cross-linking reaction is commonly referred to as curing.
Reaction of polyepoxides with themselves or with poly functional hardeners forms a thermosetting polymer, often with favorable mechanical properties and high thermal and chemical resistance. Epoxy has a wide range of applications, including metal coatings, use in electronics/electrical components/LEDs, high tension electrical insulators, paint brush manufacturing, fiber-reinforced plastic materials, and adhesives for structural and other purposes.
What is Epoxy Resin Made of
Epoxy is merely cured epoxy resin. Or in other words, epoxy resin is epoxy adhesive in its liquid form. For any epoxy, there are two primary parts to the equation: the resin and the curing agent. When the resin and the curing agent react together, the hardening process ensues.
The resin itself is made of bisphenol (and there is more than one type) and epichlorohydrin. The most common type of bisphenol is a combination of acetone and phenol. Maybe you’re asking yourself now: where on earth does phenol come from? Well, when it was first discovered it came from coal tar, but nowadays, chemists extract it from petroleum (like so many other useful compounds). Now as far as epichlorohydrin is concerned, well, it’s derived from something called allyl chloride, a chlorinated subchemical of propylene.
Epoxy is one of the best adhesives available for industrial uses. According to adhesives.org, epoxy resins, when cured, provide “rigid but tough bond lines and have excellent adhesion to metals. Chemical and environmental resistance is excellent. Most formulations have a paste consistency and can be applied by trowel or extruded as beads. They easily fill gaps and provide excellent sealing properties particularly against harsh chemicals. They are often used as alternatives to welding and rivets.”
What is Epoxy Resin Made of
Because of their ability to adhere to a wide variety of materials, their high strength, their resistance to chemicals and environments, and their ability to resist creep under sustained load, epoxies are the most widely used structural adhesive. They are available in one component, heat curing and two component, room temperature curing systems. Unmodified epoxies cure to hard, brittle solids. Most adhesive formulations include modifiers to increase flexibility or toughness of the cured adhesive. This results in bond lines that are able to resist more peel and cleavage stress as well as impact.
One component systems typically cure at temperatures from 250 to 350oF (120 to 175F). Cold storage is required to provide sufficient shelf life. They provide rigid but tough bond lines and have excellent adhesion to metals. Chemical and environmental resistance is excellent. Most formulations have a paste consistency and can be applied by trowel or extruded as beads. They easily fill gaps and provide excellent sealing properties particularly against harsh chemicals. They are often used as alternatives to welding and rivets. Some formulations can tolerate processing oil on the substrate and still obtain satisfactory bond strength.
Epoxy Resins vs Polyester Resins
- 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)
Use of Epoxy Resins in Structural Applications
Epoxy resins are of particular interest in structural composite applications because they provide:
- Unique balance of chemical and mechanical properties
- As well as extreme processing versatility
Some of their most interesting applications are found in the aerospace and recreation industries where resins and fibers are combined to produce complex composites structures. Epoxy resins satisfy a variety of non-metallic composite designs in commercial and military aerospace applications including flooring panels, ducting, vertical and horizontal stabilizers, wings and more.
Epoxy composites are also used to produce lightweight parts for automobiles, rails, bicycle frames, golf clubs, snowboards, racing cars and musical instruments. These applications use complex epoxy formulations which will include multiple epoxy resins with modifiers for toughness or flexibility, flame suppression, fillers for strength, pigments for colors, and curing additives that promote curing reactions.
High temperature applications can be improved by the use of higher functionality resins, which increases crosslink densities and improves thermal and chemical resistance. Epoxy resin (VII) based on tris (hydroxyl phenyl) methane is one of the important epoxy resins used in high performance applications. At elevated temperatures, this resin shows excellent:
- Physical and electrical properties
- Moisture resistance
- Formulation stability
- Reactivity and retention of properties