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Epoxy
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=== Bisphenol-based === [[file:Diglycidether.svg|thumb|left|Synthesis of bisphenol A diglycidyl ether]] The most common epoxy resins are based on reacting [[epichlorohydrin]] (ECH) with [[bisphenol A]], resulting in a different chemical substance known as [[bisphenol A diglycidyl ether]] (commonly known as BADGE or DGEBA). Bisphenol A-based resins are the most widely commercialised resins but also other [[bisphenol]]s are analogously reacted with epichlorohydrin, for example [[Bisphenol F]]. In this two-stage reaction, epichlorohydrin is first added to bisphenol A (bis(3-chloro-2-hydroxy-propoxy)bisphenol A is formed), then a bisepoxide is formed in a condensation reaction with a stoichiometric amount of sodium hydroxide. The chlorine atom is released as [[sodium chloride]] (NaCl) and the hydrogen atom as water. 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: [[file:Synthesis Bisphenol A diglycidyl ether higher Mw.svg|thumb|Synthesis of bisphenol-A-diglycidyl ether with a high [[molar mass]]]] A product comprising a few repeat units (''n'' = 1 to 2) is a viscous, clear liquid; this is called a liquid epoxy resin. A product comprising more repeating units (''n'' = 2 to 30) is at room temperature a colourless solid, which is correspondingly referred to as solid epoxy resin. Instead of bisphenol A, other bisphenols (especially [[bisphenol F]]) or brominated bisphenols (e. g. [[tetrabromobisphenol A]]) can be used for the said [[epoxidation]] and prepolymerisation. [[Bisphenol F]] may undergo epoxy resin formation in a similar fashion to bisphenol A. 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. Important epoxy resins are produced from combining [[epichlorohydrin]] and [[bisphenol A]] to give [[bisphenol A diglycidyl ether]]s. [[File:Epoxy prepolymer chemical structure.png|thumb|Structure of bisphenol-A diglycidyl ether epoxy resin: ''n'' denotes the number of polymerized subunits and is typically in the range from 0 to 25]] 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. This route of synthesis is known as the "taffy" process. 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 {{convert|160|C|F}}. This process is known as "advancement".<ref>Hofer, Arnold; Schneider, Hildegard, and Siegenthaler, Nikolaus (1996) "Epoxy resin mixtures containing advancement catalysts", {{US Patent|5521261}}.</ref> As the molecular weight of the resin increases, the epoxide content reduces and the material behaves more and more like a [[thermoplastic]]. Very high molecular weight polycondensates (ca. 30,000–70,000 g/mol) form a class known as phenoxy resins and contain virtually no epoxide groups (since the terminal epoxy groups are insignificant compared to the total size of the molecule). These resins do however contain hydroxyl groups throughout the backbone, which may also undergo other cross-linking reactions, e.g. with aminoplasts, phenoplasts and [[isocyanate]]s. Epoxy resins are polymeric or semi-polymeric materials or an [[oligomer]], and as such rarely exist as pure substances, since variable chain length results from the polymerisation reaction used to produce them. High purity grades can be produced for certain applications, e.g. using a distillation purification process. 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. An important criterion for epoxy resins is the [[Epoxy value]] which is connected to the epoxide group content. This is expressed as the "''epoxide equivalent weight''", which is the ratio between the molecular weight of the monomer and the number of epoxide groups. This parameter is used to calculate the mass of co-reactant (hardener) to use when curing epoxy resins. Epoxies are typically cured with [[stoichiometry|stoichiometric]] or near-stoichiometric quantities of hardener to achieve the best physical properties.
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