Epoxy resins are the most common of all the thermoset polymer systems. They are adhesives, laminating resins, coatings for grinding wheels, fire-retardant coatings, and insulating materials. Epoxy resins are used in both liquid and solid forms. Solids are often preferred because they do not evaporate or degrade as much as liquids do over time and because they are generally less expensive to make. Both types, however, can be used for similar purposes.
Epoxy resins are available in a wide range of viscosities, from super-thin liquids down to solid blocks. The epoxy can be mixed with other materials before curing to make it stronger, tougher, or more flexible when cured. When used as an adhesive, it can be mixed with other substances to form weak bonds or stronger bonds that are resistant to heat, water, and chemicals.
Epoxies are thermosetting polymer resins where the resin molecule contains one or more epoxide groups. The chemistry can be adjusted to perfect the molecular weight or viscosity as required by the end-use. There are two primary types of epoxies: glycidyl epoxy and non-glycidyl. Glycidyl epoxy resins can be further defined as either glycidyl-amine, glycidyl ester, or glycidyl ether. Non-glycidyl epoxy resins are either aliphatic or cyclo-aliphatic resins.
One of the most common glycidyl epoxy resins is created using Bisphenol A (BPA) and is synthesized in a reaction with epichlorohydrin. The other frequently used type of epoxy is known as novolac based epoxy resin.
Epoxy resins are cured with the addition of a curing agent, commonly called a hardener. Perhaps the most common type of curing agent is amine-based. Unlike in polyester or vinyl ester resins, where the resin is catalyzed with a small (1-3%) addition of a catalyst, epoxy resins usually require the addition of the curing agent at a much higher ratio of resin to hardener, often 1:1, 2:1, or as low as 5:1. Epoxy resin can be "toughened" with the addition of thermoplastic polymers. In the field of fiber-reinforced polymers, or plastics, epoxy is used as the resin matrix to efficiently hold the fiber in place. It is compatible with all common reinforcing fibers including fiberglass, carbon fiber, and Kevlar.
Common Products for Fiber Reinforced Epoxy
Epoxies are used in paints & coatings industry for manufacturing insulating and protective coatings for vessels, satellite systems, spacecraft, and aircraft. Owing to their strength, they find applications in the renewable energy sector for coating steel and to protect the structures for hydroelectric power stations.
In the wind energy sector, composites manufactured using epoxy resins and reinforcing fibers have become an essential component of large-sized windmill rotor blades. Epoxies are known to augment the strength-to-weight ratio of turbines' structure, which makes it possible to manufacture longer blades.
Epoxy resins are also used to manufacture floorings. Public buildings such as hospitals, food processing plants require hygiene with utmost importance. Epoxy coatings are widely used to protect floors as strong cleaners can be used for cleaning purposes without damaging the floor. Also, floorings made from epoxy resins have inherent anti slip structure thus preventing accidents. Epoxies are used as sealants for domestic applications and provide strong and durable sealing.
Products commonly manufactured with epoxy resins are:
Skis and snowboards
Prepreg and autoclave
Boats, kayaks, and canoes
Wind turbine blades
Marine and Automotive Racing components
The same epoxy resin likely can't be used for each of these processes. Epoxies are fine-tuned for the desired application and manufacturing process. For example, protrusion and compression molding epoxy resins are heat-activated, while an infusion resin might be an ambient cure and have a lower viscosity.
When compared to other traditional thermoset or thermoplastic resins, epoxy resins have several distinct advantages, including:
Low shrinkage during cure
Excellent moisture resistance
Excellent chemical resistance
Good electrical properties
Increased mechanical and fatigue strength
No VOCs (volatile organic compounds)
Long shelf life
Disadvantages of epoxy resins include long cure times and cost... epoxies are generally more expensive than other resin systems. Epoxy resins do not typically have a good UV resistance. If the resin is exposed to sunlight, then it will break down, and the strength of the bond will be compromised. The resin may also break down and lose its toughness if exposed to moisture or if it stays wet for an extended period of time.
Epoxy resins can be altered and impregnated into the fiber and be in what is called a B-stage. This is how prepregs are created.
With epoxy prepregs, the resin is tacky, but not cured. This allows layers of prepreg materials to be cut, stacked, and placed in a mold. Then, with the addition of heat and pressure, the prepreg can be consolidated and cured. Most Epoxy prepregs and the epoxy B-stage film must be kept at a low temperature to prevent premature curing, which is why companies using prepregs must invest in refrigeration or freezer units to keep the material cool. There are, however, some prepreg options that will remain stable at room temperature for at least 6 months. Fibre Glast offers a room-temperature stable prepreg in both carbon fiber and fiberglass reinforcement options.
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 the flexibility or toughness of the cured adhesive. This results in bond lines that can resist more peel and cleavage stress, as well as impact.
One component heat curing film adhesives are typically based on epoxy resin formulated with curatives and modifiers. They are very high performance adhesives providing high strength, high fatigue resistance, and high temperature resistance. These curing film adhesives require cold storage and have a limited shelf-life after warming to room temperature. They are especially suited for bonding and laminating large areas. Epoxy film adhesives find most of their applications in the aerospace industry for the assembly of components such as aircraft panels and helicopter rotor blades. To obtain optimal performance and durability, aluminum substrates are usually chemically treated.
Two component epoxy adhesives are found in all market segments. The pot life (time adhesive can be processed and bonded after mixing) can vary from a few minutes to several hours. Assemblies must be fixtured until the adhesive has cured sufficiently to have enough strength for handling and additional processing. Final cure and ultimate strength are obtained over hours to weeks depending on the formulation. High ambient temperature accelerates the rate of cure and shortens the pot life. Low ambient temperature slows the rate of cure and extends the time before assemblies can be further processed. In general, adhesives that cure faster have lower final strength than those that cure more slowly. The major advantage of two component epoxy adhesives is that they are suitable for bonding nearly all substrates - metal, plastic, glass and ceramic, wood and wood products, and many types of rubber. In general, they have high resistance to physical and chemical influences. Depending on the type, they can withstand continuous temperatures from 200oF (95°C) up to 390oF (200°C). Cured adhesives are typically hard and rigid and range from brittle to tough depending on formulation
Epoxy resins are important resources that secure modern living. They provide enormous benefits in their industrial and construction uses, thus extending the lifetimes of various types of structures in ever-changing environments.