[34] In general, epoxies are known for their excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties and very good electrical insulating properties. They have been extensively used with concrete and cementitious systems. The resins are low to medium viscosity at room temperature, which makes them easier to process than EPN or ECN resins. Temperature is sometimes increased in a step-wise fashion to control the rate of curing and prevent excessive heat build-up from the exothermic reaction. Epoxies are sold in hardware stores, typically as a pack containing separate resin and hardener, which must be mixed immediately before use. The epoxide functional group is also collectively called epoxy. 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 purchases epoxy raw materials from the major producers and then compounds (blends, modifies, or otherwise customizes) epoxy systems from these raw materials. [13], Epoxidized vegetable oils are formed by epoxidation of unsaturated fatty acids by reaction with peracids. One downside of high purity liquid grades is their tendency to form crystalline solids due to their highly regular structure, which then require melting to enable processing. Epoxy use is a main source of occupational asthma among users of plastics. [49], Water-soluble epoxies such as Durcupan[50][51] are commonly used for embedding electron microscope samples in plastic so they may be sectioned (sliced thin) with a microtome and then imaged. [22] Mechanical properties of epoxy resins are generally not improved by use of diluents. There are various methods of toughening them.[36]. for high voltage electrical insulators. Significant advances in understanding yellowing of epoxies were achieved by Down first in 1984 (natural dark aging) [39] and later in 1986 (high-intensity light aging). As aromaticity is not present in these materials as it is in Bisphenol A and F resins, the UV stability is considerably improved. Overall reactivity potential for different hardeners can roughly be ordered; aliphatic amines > cycloaliphatic amines > aromatic amines, though aliphatic amines with steric hindrance near the amino groups may react as slowly as some of the aromatic amines. [40] Down investigated various room-temperature-cure epoxy resin adhesives suitable for use in glass conservation, testing their tendency to yellow. This means that conventional nucleophilic hardeners such as amines are hardly suitable for crosslinking. [6][7][8], Condensation of epoxides and amines was first reported and patented by Paul Schlack of Germany in 1934. The same applies to transformers, bushings, spacers, and composites cables connecting the windmills to the grid. Aromatic amines were widely used as epoxy resin hardeners, due to the excellent end properties when mixed with a parent resin. Epoxy resins yellow with time, even when not exposed to UV radiation. Epoxy materials tend to harden somewhat more gradually, while polyester materials tend to harden quickly, particularly if a lot of catalyst is used. Reaction of polyepoxides with themselves or with polyfunctional hardeners forms a thermosetting polymer, often with favorable mechanical properties and high thermal and chemical resistance. Depending on the properties required, the ratio may be anything from 1:1 or over 10:1, but in usually they must be mixed exactly. Some (non-crosslinked) epoxy resins with very high molar mass are added to engineering thermoplastics, again to achieve flame retardant properties. 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. Epoxy homopolymerisation is often used when there is a requirement for UV curing, since cationic UV catalysts may be employed (e.g. Many properties of epoxies can be modified (for example silver-filled epoxies with good electrical conductivity are available, although epoxies are typically electrically insulating). The only variable available to the user is to change the rate of this process using a catalyst, often Methyl-Ethyl-Ketone-Peroxide (MEKP), which is very toxic. They are also sold in boat shops as repair resins for marine applications. for UV coatings). Epoxy coatings are often used in industrial and automotive applications since they are more heat resistant than latex-based and alkyd-based paints. Epoxy adhesives can be developed to suit almost any application. Use of a difunctional or polyfunctional amine forms a three-dimensional cross-linked network. The resulting material has ether linkages and displays higher chemical and oxidation resistance than typically obtained by curing with amines or anhydrides. These resins typically display low viscosity at room temperature (10200 mPa.s) and are often referred to as reactive diluents. Hofer, Arnold; Schneider, Hildegard, and Siegenthaler, Nikolaus (1996) "Epoxy resin mixtures containing advancement catalysts". [17][18] The resulting materials may be monofunctional (e.g. This is expressed as the "epoxide equivalent weight", which is the ratio between the molecular weight of the Rorar and the number of epoxide groups. There are two common types of aliphatic epoxy resins: those obtained by epoxidation of double bonds (cycloaliphatic epoxides and epoxidized vegetable oils) and those formed by reaction with epichlorohydrin (glycidyl ethers and esters). When cured, the epoxy resin leads to a thermosetting plastic with high chemical resistance and low water absorption. Common classes of hardeners for epoxy resins include amines, acids, acid anhydrides, phenols, alcohols and thiols. These resins do however contain hydroxyl groups throughout the backbone, which may also undergo other cross-linking reactions, e.g. [28] Curing of epoxy resins is an exothermic reaction and in some cases produces sufficient heat to cause thermal degradation if not controlled.[29]. These high-performance adhesives are used in the construction of aircraft, automobiles, bicycles, boats, golf clubs, skis, snowboards, and other applications where high strength bonds are required. The usual route to higher molecular weight epoxy resins is to start with liquid epoxy resin (LER) and add a calculated amount of bisphenol A and then a catalyst is added and the reaction heated to circa 160C (320F). This parameter is used to calculate the mass of co-reactant (hardener) to use when curing epoxy resins. The cured epoxy is an electrical insulator and a much better conductor of heat than air. [63], This article is about the thermoset plastic materials. [46] By contrast, polyester resins can only bond using the first two of these, which greatly reduces their utility as adhesives and in marine repair. for chips on board ), LEDs, high-tension electrical insulators, paint brush manufacturing, fiber-reinforced plastic materials, and adhesives for structural[3] and other purposes. Novolaks are produced by reacting phenol with methanal (formaldehyde). [43] In addition, for offshore and onshore wind energy installations, epoxy resins are used as protective coatings on steel towers, base struts and concrete foundations. Since many novolacs are solids, this class of hardeners is often employed for powder coatings. Glycidylamine epoxy resins are higher functionality epoxies which are formed when aromatic amines are reacted with epichlorohydrin. [41] They found that the molecular reason for epoxy yellowing was a thermo-oxidative evolution of carbonyl groups in the polymeric carboncarbon backbone via a nucleophilic radical attack. Cycloaliphatic epoxides are therefore usually homopolymerized thermally or UV-initiated in an electrophilic or cationic reaction. Metal cans and containers are often coated with epoxy to prevent rusting, especially for foods like tomatoes that are acidic. In some cases, blends of dianhydrides and monoanhydrides can improve metering and mixing with liquid epoxy resins.[30]. 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). Also aliphatic glycidyl epoxy resins usually have a low viscosity compared to aromatic epoxy resins. Hardeners which show only low or limited reactivity at ambient temperature, but which react with epoxy resins at elevated temperature are referred to as latent hardeners. plant derived glycerol used to make epichlorohydrin). This process is known as "advancement". [47] Although they increase cost of mortars and concrete when used as an additive, they enhance properties. However, the commercial use of fluorinated epoxy resins is limited by their high cost and low Tg. This class also displays lower viscosity at room temperature, but offers significantly higher temperature resistance than the aliphatic epoxy diluents. Variations offering high thermal insulation, or thermal conductivity combined with high electrical resistance for electronics applications, are available.[35]. [4][5], The health risks associated with exposure to epoxy resin compounds include contact dermatitis and allergic reactions, as well as respiratory problems from breathing vapor and sanding dust, especially when not fully cured. [31] The applications include coatings, adhesives[32][33] 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). Typical fiber reinforcements include glass, carbon, Kevlar, and boron. These highly viscous to solid resins typically carry 2 to 6 epoxy groups per molecule. Epoxy resins made of (long-chain) polyols are also added to improve tensile strength and impact strength. printed circuit boards). One-component products generally have shorter shelf-lives than standard 2-component systems, and products may require cooled storage and transport. [25] The study of epoxy curing is usually caried out by using Differential scanning calorimetry. Insufficient heat during cure will result in a network with incomplete polymerisation, and thus reduced mechanical, chemical and heat resistance. Epoxies are also used in producing fiber-reinforced or composite parts. 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. They can be used as adhesives for wood, metal, glass, stone, and some plastics. The global epoxy resin market was valued at approximately 9.1 (USD Billion) in 2021 and is predicted to gain revenue of about 14.7 (USD Billion) by 2028, is set to record a CAGR of nearly 6.8% over the period from 2022 to 2028. [53] It is also used in jewelry, as a doming resin for decorations and labels, and in decoupage type applications for art, countertops, and tables. There are two main areas of marine use. by secondary hydroxyl groups in the epoxy resin. Amines are also blended, adducted and reacted to alter properties and these amine resins are more often used to cure epoxy resins than a pure amine such as TETA. [27] However, good properties are obtained by reacting the linear epoxy resin with suitable curatives to form three-dimensional cross-linked thermoset structures. Epoxy resins diluents are typically formed by glycidylation of aliphatic alcohols or polyols and also aromatic alcohols. Until they are mixed the two elements are relatively inert, although the 'hardeners' tend to be more chemically active and should be protected from the atmosphere and moisture. Important epoxy resins are produced from combining epichlorohydrin and bisphenol A to give bisphenol A diglycidyl ethers. [58][59], Liquid epoxy resins in their uncured state are mostly classed as irritant to the eyes and skin, as well as toxic to aquatic organisms. Ciba's epoxy business was spun off as Vantico in the late 1990s, which was subsequently sold in 2003 and became the Advanced Materials business unit of Huntsman Corporation of the United States. While some epoxy resin/ hardener combinations will cure at ambient temperature, many require heat, with temperatures up to 150C (302F) being common, and up to 200C (392F) for some specialist systems. The reaction of epichlorohydrin and novolaks produces novolaks with glycidyl residues, such as epoxyphenol novolak (EPN) or epoxycresol novolak (ECN). These modifications are made to reduce costs, to improve performance, and to improve processing convenience. Materials like wood, and others that are 'low-tech' are glued with epoxy resin. 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. So it is good practice to mix smaller amounts which can be used quickly to avoid waste and to be safer. In Europe, wind energy components account for the largest segment of epoxy applications, about 27% of the market.[44]. The rate of cure of polyesters can therefore be controlled by the amount and type of catalyst as well as by the temperature. Such epoxies are commonly used in optics, fiber optics, and optoelectronics. Temperature resistance generally increases in the same order, since aromatic amines form much more rigid structures than aliphatic amines. This process is commonly referred to as curing or gelation process. Increasing the ratio of bisphenol A to epichlorohydrin during manufacture produces higher molecular weight linear polyethers with glycidyl end groups, which are semi-solid to hard crystalline materials at room temperature depending on the molecular weight achieved. Epoxy is the family of basic components or cured end products of epoxy resins. Higher molecular weight diglycidyl ethers (n 1) are formed by the reaction of the bisphenol A diglycidyl ether formed with further bisphenol A, this is called prepolymerization: A product comprising a few repeat units (n = 1 to 2) is a viscous, clear liquid; this is called a liquid epoxy resin. Epoxies have been researched and used for construction for a few decades. The chemistry of epoxies and the range of commercially available variations allows cure polymers to be produced with a very broad range of properties. Aliphatic, cycloaliphatic and aromatic amines are all employed as epoxy hardeners. There are several dozen chemicals that can be used to cure epoxy, including amines, imidazoles, anhydrides and photosensitive chemicals. Polyester resins are typically low strength unless used with a reinforcing material like glass fibre, are relatively brittle unless reinforced, and have low adhesion. Epoxies are also used as a structural glue. 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. [45] The chemical reactions in both cases are exothermic. Thiols have a characteristic odour, which can be detected in many two-component household adhesives. This route of synthesis is known as the "taffy" process. The secondary amine can further react with an epoxide to form a tertiary amine and an additional hydroxyl group. The rate of the reaction can be changed by using different hardeners, which may change the nature of the final product, or by controlling the temperature. Normal gelcoat formulated for use with polyester resins and vinylester resins does not adhere to epoxy surfaces, though epoxy adheres very well if applied to polyester resin surfaces. [21] The use of the diluent does effect mechanical properties and microstructure of epoxy resins. As adhesives, epoxies bond in three ways: a) Mechanically, because the bonding surfaces are roughened; b) by proximity, because the cured resins are physically so close to the bonding surfaces that they are hard to separate; c) ionically, because the epoxy resins form ionic bonds at an atomic level with the bonding surfaces. The risk has been shown to be more pronounced in epoxy resins containing low molecular weight epoxy diluents. 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate). Over the past few decades concern about the possible adverse health effects of many aromatic amines has led to increased use of aliphatic or cycloaliphatic amine alternatives. First a hydroxy group reacts in a coupling reaction with epichlorohydrin, followed by dehydrohalogenation. This coupled with high reactivity, plus high temperature resistance and mechanical properties of the resulting cured network makes them important materials for aerospace composite applications. High purity grades can be produced for certain applications, e.g. The largest volume type of circuit boardan "FR-4 board"is a sandwich of layers of glass cloth bonded into a composite by an epoxy resin. While the resulting network does not typically display high temperature or chemical resistance, the high reactivity of the thiol group makes it useful for applications where heated curing is not possible, or very fast cure is required e.g. The formulation may then be reacted in the same way as pure bisphenol A. This process is known as catalytic homopolymerisation. [11] Devoe&Raynolds, which was active in the early days of the epoxy resin industry, was sold to Shell Chemical; the division involved in this work was eventually sold, and via a series of other corporate transactions is now part of Hexion Inc.[12]. [26], In general, uncured epoxy resins have only poor mechanical, chemical and heat resistance properties. Primary amines undergo an addition reaction with the epoxide group to form a hydroxyl group and a secondary amine. In the electronics industry epoxy resins are the primary resin used in overmolding integrated circuits, transistors and hybrid circuits, and making printed circuit boards. 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), and concrete reinforcing rebar. The global market is made up of approximately 50100 manufacturers of basic or commodity epoxy resins and hardeners. These resins typically have lower viscosity and a higher mean epoxy content per gram than bisphenol A resins, which (once cured) gives them increased chemical resistance. Its reactivity to hardeners is comparable to that of bisphenol A. There are hundreds of ways that these formulators can modify epoxiesby adding mineral fillers (talc, silica, alumina, etc. They can be made flexible or rigid, transparent or opaque/colored, fast setting or slow setting. For dianhydrides, and to a lesser extent, monoanhydrides, non-stoichiometric, empirical determinations are often used to optimize dosing levels. [23] Biobased epoxy diluents are also available.[24]. Kinetic studies have shown the reactivity of the primary amine to be approximately double that of the secondary amine. The final product is then a precise thermoset plastic. For the chemical group, see. German Patent 676117. Epoxy systems are used in industrial tooling applications to produce molds, master models, laminates, castings, fixtures, and other industrial production aids. They are therefore added to other epoxy resins as reactive diluents or as adhesion promoters. Epoxies typically are not used in the outer layer of a boat because they deteriorate by exposure to UV light. Also known as mercaptans, thiols contain a sulfur which reacts very readily with the epoxide group, even at ambient or sub-ambient temperatures. All quantities of mix generate their own heat because the reaction is exothermic. 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. [62] Safe disposal also needs considering but usually involves deliberate curing to produce solid rather than liquid waste. Important industrial grades are triglycidyl-p-aminophenol (functionality 3) and N,N,N,N-tetraglycidyl-bis-(4-aminophenyl)-methane (functionality 4). In general, epoxy adhesives cured with heat will be more heat- and chemical-resistant than those cured at room temperature. Research is ongoing to investigate the use of epoxies and other recycled plastics in mortars to enhance properties and recycle waste. Epoxy has a wide range of applications, including metal coatings, composites,[2] use in electronics, electrical components (e.g.
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