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Epoxy

[ih-pok-see, ih-pok-]
In chemistry, epoxy or polyepoxide is a thermosetting epoxide polymer that cures (polymerizes and crosslinks) when mixed with a catalyzing agent or hardener. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-A. The first commercial attempts to prepare resins from epichlorohydrin were made in 1927 in the United States. Credit for the first synthesis of bisphenol-A-based epoxy resins is shared by Dr. Pierre Castan of Switzerland and Dr. S.O. Greenlee of the United States in 1936. Dr. Castan's work was licensed by Ciba, Ltd. of Switzerland, which went on to become one of the three major epoxy resin producers worldwide. Ciba's epoxy business was spun off and later sold in the late 1990s and is now the advanced materials business unit of Huntsman Corporation of the United States. Dr. Greenlee's work was for the firm of Devoe-Reynolds of the United States. Devoe-Reynolds, which was active in the early days of the epoxy resin industry, was sold to Shell Chemical (now Hexion, formerly Resolution Polymers and others).

Industry

As of 2006, the epoxy industry amounts to more than US$5 billion in North America and about US$15 billion world-wide. The Chinese market has been growing rapidly, and the market size is more than 30% of the total worldwide market. It is made up of approximately 50–100 manufacturers of basic or commodity epoxy resins and hardeners of which the three largest are Hexion (formerly Resolution Performance Products, formerly Shell Development Company; whose epoxy tradename is "Epon"), The Dow Chemical Company (tradename "D.E.R."), and Huntsman Corporation's Advanced Materials business unit (formerly Vantico, formerly Ciba Specialty Chemical; tradename "Araldite"). In 2007 Huntsman Corporation agreed to merge with Hexion (owned by the Apollo Group). KUKDO Chemical is one of the largest epoxy manufacturers in Asia, and recently their capacity has been increased up to 210,000 MT/Y (Korea 150,000 MT/Y, China 60,000 MT/Y and will be increased totally 300,000 MT/Y by 2009). Nanya Plastic also has the capacity of over 250,000 MT/Y (Taiwan and China), which is mostly for captive use. There are over 50 smaller epoxy manufacturers primarily producing epoxies only regionally, epoxy hardeners only, specialty epoxies, or epoxy modifiers.

These commodity epoxy manufacturers mentioned above typically do not sell epoxy resins in a form usable to smaller end users, so there is another group of companies that purchase epoxy raw materials from the major producers and then compounds (blends, modifies, or otherwise customizes) epoxy systems from these raw materials. These companies are known as "formulators". The majority of the epoxy systems sold are produced by these formulators and they comprise over 60% of the dollar value of the epoxy market. There are hundreds of ways that these formulators can modify epoxies—by adding mineral fillers (talc, silica, alumina, etc.), by adding flexibilizers, viscosity reducers, colorants, thickeners, accelerators, adhesion promoters, etc.. These modifications are made to reduce costs, to improve performance, and to improve processing convenience. As a result a typical formulator sells dozens or even thousands of formulations—each tailored to the requirements of a particular application or market.

The applications for epoxy-based materials are extensive and include coatings, adhesives and composite materials such as those using carbon fiber and fiberglass reinforcements (although polyester, vinyl ester, and other thermosetting resins are also used for glass-reinforced plastic). The chemistry of epoxies and the range of commercially available variations allows cure polymers to be produced with a very broad range of properties. In general, epoxies are known for their excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties and very good electrical insulating properties. Many properties of epoxies can be modified (for example silver-filled epoxies with good electrical conductivity are available, although epoxies are typically electrically insulating). Variations offering high thermal insulation, or thermal conductivity combined with high electrical resistance for electronics applications, are on offer.

Epoxies find significant use in many applications which are described in following sections.

Paints and coatings

"2 part waterborne epoxy coatings" are used as ambient cure epoxy coatings. These non-HAP, two-part epoxy coatings are developed for heavy duty service on metal substrates and use less energy than heat cured powder coatings. These systems use a more attractive 4:1 by volume mixing ratio. The coating dries quickly providing a tough, UV resistant, protective coating with excellent ultimate hardness, and good mar and abrasion resistance. They are designed for rapid dry protective coating applications. Ambient cure 2 Part waterborne epoxy coatings provide excellent physical properties in exterior applications. These products have excellent adhesion to various metal substrates. Their low VOC and water clean up makes them a natural choice for factory cast iron, cast steel, cast aluminum applications and reduces exposure and flammability issues associated with solventborne coatings. They are usually used for industrial and automotive uses as they are high heat resistant (as latex-based and alkyd-based paints usually burn, thus peel, with slight high heat temperatures).

Polyester Epoxies are used as powder coatings for washers, driers and other "white goods". Fusion Bonded Epoxy Powder Coatings (FBE) are extensively used for corrosion protection of steel pipes and fittings used in the oil and gas industry, potable water transmission pipelines (steel), concrete reinforcing rebar, et cetera. Epoxy coatings are also widely used as primers to improve the adhesion of automotive and marine paints especially on metal surfaces where corrosion (rusting) resistance is important. Metal cans and containers are often coated with epoxy to prevent rusting, especially for foods like tomatoes that are acidic. Epoxy resins are also used for high performance and decorative flooring applications especially terrazzo flooring, chip flooring and colored aggregate flooring.

Adhesives

Epoxy adhesives are a major part of the class of adhesives called "structural adhesives" or "engineering adhesives" (which also includes polyurethane, acrylic, cyanoacrylate, and other chemistries.) These high-performance adhesives are used in the construction of aircraft, automobiles, bicycles, boats, golf clubs, skis, snow boards, and other applications where high strength bonds are required. Epoxy adhesives can be developed to suit almost any application. They are exceptional adhesives for wood, metal, glass, stone, and some plastics. They can be made flexible or rigid, transparent or opaque/colored, fast setting or extremely slow setting. Epoxy adhesives are almost unmatched in heat and chemical resistance among common adhesives. In general, epoxy adhesives cured with heat will be more heat- and chemical-resistant than those cured at room temperature.

Some epoxies are cured by exposure to ultraviolet light. Such epoxies are commonly used in optics, fiber optics, optoelectronics and dentistry.

Industrial tooling and composites

Epoxy systems are used in industrial tooling applications to produce molds, master models, laminates, castings, fixtures, and other industrial production aids. This "plastic tooling" replaces metal, wood and other traditional materials, and generally improves the efficiency and either lowers the overall cost or shortens the lead-time for many industrial processes. Epoxies are also used in producing fiber-reinforced or composite parts. They are more expensive than polyester resins and vinyl ester resins, but usually produce stronger and more temperature-resistant composite parts.

Electrical systems and electronics

Epoxy resin formulations are important in the electronics industry, and are employed in motors, generators, transformers, switchgear, bushings, and insulators. Epoxy resins are excellent electrical insulators and protect electrical components from short circuiting, dust and moisture. In the electronics industry epoxy resins are the primary resin used in overmolding integrated circuits, transistors and hybrid circuits, and making printed circuit boards. The largest volume type of circuit board — an "FR-4 board" — is a sandwich of layers of glass cloth bonded into a composite by an epoxy resin. Epoxy resins are used to bond copper foil to circuit board substrates, and are a component of the solder mask on many circuit boards.

Flexible epoxy resins are used for potting transformers and inductors. By using vacuum impregnation on uncured epoxy, winding-to-winding, winding-to-core, and winding-to-insulator air voids are eliminated. The cured epoxy is an electrical insulator and a much better conductor of heat than air. Transformer and inductor hot spots are greatly reduced, giving the component a stable and longer life than unpotted product.

Epoxy resins are applied using the technology of resin casting.

Consumer and marine applications

Epoxies are sold in hardware stores, typically as a pack containing separate resin and hardener, which must be mixed immediately before use. They are also sold in boat shops as repair resins for marine applications. Epoxies typically are not used in the outer layer of a boat because they deteriorate by exposure to UV light. They are often used during boat repair and assembly, and then over-coated with conventional or two-part polyurethane paint or marine-varnishes that provide UV protection.

There are two main areas of marine use. Because of the better mechanical properties relative to the more common polyester resins, epoxies are used for commercial manufacture of components where a high strength/weight ratio is required. The second area is that their strength, gap filling properties and excellent adhesion to many materials including timber have created a boom in amateur building projects including aircraft and boats.

Normal gelcoat formulated for use with polyester resins and polyester resins does not adhere to epoxy surfaces, though epoxy adheres very well if applied to polyester resin surfaces. "Flocoat" that is normally used to coat the interior of polyester fibreglass yachts is also compatible with epoxies.

Polyester thermosets typically use a ratio of at least 10:1 of resin to hardener (or "catalyst"), while epoxy materials typically use a lower ratio of between 5:1 and 1:1. Epoxy materials tend to harden somewhat more gradually, while polyester materials tend to harden quickly.

The classic epoxy reference guide is the Handbook of epoxy resins by Henry Lee and Kris Neville. Originally issued in 1967, it has been reissued repeatedly and still gives an excellent overview of the technology. Some basic tips are given here: www.epoxy.com/install.htm

Aerospace applications

In the aerospace industry, epoxy is used as a structural matrix material which is then reinforced by fiber. Typical fiber reinforcements include glass, carbon, Kevlar, and boron. Epoxies are also used as a structural glue. Materials like wood, and others that are 'low-tech' are glued with epoxy resin. One example, a homebuilt aircraft, would be the RJ.03 IBIS This design is based on a classic wooden lattice structured fuselage and a classic wooden spar, internally stiffened with foam and completely covered with plywood. Except for the plywood covering the wings, everything is glued with epoxy resin.

Art

Epoxy resin, mixed with pigment, is used as a painting medium.

Wind Energy applications

Epoxy resin is used in manufacturing rotor blades of wind turbine. The resin is infused in the core material like Balsa, foam & reinforcing media glass fabric. The process is called VARTM i.e. vacuum assisted resin transfer moulding. Due to excellent properties & good finish, epoxy is most favoured resin for composites.

Chemistry

When epoxies are mixed with the appropriate catalyst, the resulting reaction is exothermic, and the oxygen on the epoxy monomers is "flipped." This occurs throughout the epoxy, and a matrix with a high stress tolerance is formed, and "glues" the materials together.

Cleanup

When using epoxy resin and hardener, vinegar is an effective and safe solvent to clean up tools, brushes, skin, and most surfaces. Acetone can also be used, but it is very volatile and flammable, unlike vinegar. Vinegar is safer for cleaning epoxy resin from human skin than acetone: both liquids will dissolve the resin, but the resin/acetone solution can easily pass through the skin into the bloodstream, unlike vinegar. White vinegar can even clean up epoxy resin that is beginning to cure/harden. DME (Dimethoxyethane) is also a good solvent for epoxy resin and hardener that gives off very little vapor. However, none of these substances is an effective solvent for epoxy that has cured.

Health risks

The primary risk associated with epoxy use is sensitization to the hardener, which, over time, can induce an allergic reaction.

Both epichlorohydrin and bisphenol A are suspected endocrine disruptors.

According to some reports Bisphenol A is linked to the following effects in humans:

  • oestrogenic activity;
  • alteration of male reproductive organs;
  • early puberty induction;
  • shortened duration of breast feeding;
  • pancreatic cancer

See also

References

External links

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