Editing Epoxy (section)

== Chemistry ==
[[file:Synthesis epoxide Epichlorohydrin.svg|thumb|Coupling reaction of a hydroxy group with epichlorohydrin, followed by dehydrohalogenation]]

Most of the commercially used epoxy monomers are produced by the reaction of a compound with [[Hydroxy group|acidic hydroxy groups]] and [[epichlorohydrin]]. First a hydroxy group reacts in a coupling reaction with epichlorohydrin, followed by [[dehydrohalogenation]]. Epoxy resins produced from such epoxy monomers are called [[Glycidol|glycidyl]]-based epoxy resins. The hydroxy group may be derived from [[aliphatic diol]]s, [[polyol]]s (polyether polyols), [[phenolic compounds]] or [[dicarboxylic acid]]s. Phenols can be compounds such as [[bisphenol A]] and [[novolak]]. Polyols can be compounds such as [[1,4-Butanediol|1,4-butanediol]]. Di- and polyols lead to [[glycidyl ether]]s. Dicarboxylic acids such as hexahydrophthalic acid are used for diglycide ester resins. Instead of a hydroxy group, also the nitrogen atom of an [[amine]] or [[amide]] can be  reacted with epichlorohydrin.

[[file:Synthesis epoxide peracid.svg|thumb|Synthesis of an epoxide by use of a peracid]]

The other production route for epoxy resins is the conversion of [[Aliphatic compound|aliphatic]] or [[cycloaliphatic alkene]]s with [[peracid]]s:<ref name="Ullmann" /><ref name="KunstChem">Kaiser, Wolfgang (2011) ''Kunststoffchemie für Ingenieure.'' Vol. 3. Hanser, Munich. pp. 437 ff. {{ISBN|978-3-446-43047-1}}.</ref> In contrast to glycidyl-based epoxy resins, this production of such epoxy monomers does not require an acidic hydrogen atom but an aliphatic double bond.

The epoxide group is also sometimes referred to as an ''oxirane'' group.

=== 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.

=== Novolaks ===
[[file:Epoxyphenol-Novolak.svg|thumb|General structure of epoxyphenol novolak with ''n'' usually in the range from 0 to 4. The compound is present in the form of various [[Structural isomer|constitutional isomers]].]]

[[Novolak]]s are produced by reacting [[phenol]] with [[methanal]] ([[formaldehyde]]). The reaction of [[epichlorohydrin]] and [[novolak]]s produces novolaks with [[Glycidol|glycidyl residues]], such as epoxyphenol novolak (EPN) or epoxycresol novolak (ECN). These highly viscous to solid resins typically carry 2 to 6 epoxy groups per molecule. By curing, highly cross-linked polymers with high temperature and chemical resistance but low mechanical flexibility are formed due to the high functionality, and hence high crosslink density of these resins.<ref name="Ullmann">{{cite book |doi=10.1002/14356007.a09_547.pub2 |chapter=Epoxy Resins |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2005 |last1=Pham |first1=Ha Q. |last2=Marks |first2=Maurice J. |isbn=978-3-527-30385-4 }}</ref>

=== Aliphatic ===
[[file:Diepoxyester.svg|thumb|left|Structural formula of [[3,4-Epoxycyclohexylmethyl-3’,4’-epoxycyclohexane carboxylate]]]]

There are two common types of aliphatic epoxy resins: those obtained by epoxidation of double bonds (cycloaliphatic epoxides and [[Epoxidized soybean oil|epoxidized vegetable oils]]) and those formed by reaction with epichlorohydrin (glycidyl ethers and esters).

Cycloaliphatic epoxides contain one or more aliphatic rings in the molecule on which the oxirane ring is contained (e.g. [[3,4-Epoxycyclohexylmethyl-3’,4’-epoxycyclohexane carboxylate|3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate]]). They are produced by the reaction of a cyclic alkene with a [[peracid]] (see above).<ref>Hammerton, L. (1996) ''Recent Developments in Epoxy Resins''. Ed.: Rebecca Dolbey. RAPRA Review Reports. p. 8. {{ISBN|978-1-85957-083-8}}.</ref>  Cycloaliphatic epoxides are characterised by their aliphatic structure, high oxirane content and the absence of chlorine, which results in low viscosity and (once cured) good weather resistance, low dielectric constants and high [[Glass transition|T<sub>g</sub>]]. However, aliphatic epoxy resins polymerize very slowly at room temperature, so higher temperatures and suitable accelerators are usually required. Because aliphatic epoxies have a lower electron density than aromatics, cycloaliphatic epoxies react less readily with nucleophiles than bisphenol A-based epoxy resins (which have aromatic ether groups). This means that conventional nucleophilic hardeners such as amines are hardly suitable for crosslinking. Cycloaliphatic epoxides are therefore usually homopolymerized thermally or UV-initiated in an electrophilic or cationic reaction. Due to the low dielectric constants and the absence of chlorine, cycloaliphatic epoxides are often used to encapsulate electronic systems, such as microchips or LEDs. They are also used for radiation-cured paints and varnishes. Due to their high price, however, their use has so far been limited to such applications.<ref name="Ullmann" />

Epoxidized vegetable oils are formed by epoxidation of [[unsaturated fatty acids]] by reaction with peracids. In this case, the peracids can also be formed in situ by reacting carboxylic acids with hydrogen peroxide. Compared with LERs (liquid epoxy resins) they have very low viscosities. If, however, they are used in larger proportions as [[reactive diluent]]s, this often leads to reduced chemical and thermal resistance and to poorer mechanical properties of the cured epoxides. Large scale epoxidized vegetable oils such as epoxidized soy and lens oils are used to a large extent as secondary plasticizers and cost stabilizers for [[Polyvinyl chloride|PVC]].<ref name="Ullmann" />

Aliphatic glycidyl epoxy resins of low molar mass (mono-, bi- or polyfunctional) are formed by the reaction of epichlorohydrin with aliphatic alcohols or polyols (glycidyl ethers are formed) or with aliphatic carboxylic acids (glycidyl esters are formed). The reaction is carried out in the presence of a base such as sodium hydroxide, analogous to the formation of bisphenol A-diglycidyl ether. Also aliphatic glycidyl epoxy resins usually have a low viscosity compared to aromatic epoxy resins. They are therefore added to other epoxy resins as reactive diluents or as [[adhesion promoter]]s. Epoxy resins made of (long-chain) polyols are also added to improve tensile strength and impact strength.

A related class is cycloaliphatic epoxy resin, which contains one or more cycloaliphatic rings in the molecule (e.g. 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate). This class also displays lower viscosity at room temperature, but offers significantly higher temperature resistance than the aliphatic epoxy diluents. However, reactivity is rather low compared to other classes of epoxy resin, and high temperature curing using suitable accelerators is normally required. As aromaticity is not present in these materials as it is in Bisphenol A and F resins, the UV stability is considerably improved.

=== Halogenated ===

Halogenated epoxy resins are admixed for special properties, in particular brominated and fluorinated epoxy resins are used.<ref name="Ullmann" />

Brominated bisphenol A is used when flame retardant properties are required, such as in some electrical applications (e.g. [[printed circuit board]]s). The tetrabrominated bisphenol A (TBBPA, 2,2-bis(3,5-dibromophenyl)propane) or its diglycidyl ether, 2,2-bis[3,5-dibromo-4-(2,3-epoxypropoxy)phenyl]propane, can be added to the epoxy [[formulation]]. The formulation may then be reacted in the same way as pure bisphenol A. Some (non-crosslinked) epoxy resins with very high molar mass are added to engineering thermoplastics, again to achieve flame retardant properties.

Fluorinated epoxy resins have been investigated for some [[High-performance plastics|high performance applications]], such as the fluorinated diglycidether 5-heptafluoropropyl-1,3-bis[2-(2,3-epoxypropoxy)hexafluoro-2-propyl]benzene. As it has a low surface tension, it is added as a wetting agent (surfactant) for contact with glass fibres. Its reactivity to hardeners is comparable to that of bisphenol A. When cured, the epoxy resin leads to a thermosetting [[plastic]] with high chemical resistance and low water absorption. However, the commercial use of fluorinated epoxy resins is limited by their high cost and low T<sub>g</sub>.

=== Diluents ===

Epoxy resins diluents are typically formed by glycidylation of aliphatic alcohols or [[polyol]]s and also aromatic alcohols. The resulting materials may be monofunctional (e.g. dodecanol glycidyl ether), difunctional ([[1,4-Butanediol diglycidyl ether]]), or higher functionality (e.g. [[trimethylolpropane triglycidyl ether]]). These resins typically display low viscosity at room temperature (10–200 mPa.s) and are often referred to as reactive diluents.<ref>{{cite book |doi=10.1007/978-94-011-5862-6_24 |chapter=Diluents and viscosity modifiers for epoxy resins |title=Plastics Additives |series=Polymer Science and Technology Series |date=1998 |last1=Monte |first1=Salvatore J. |volume=1 |pages=211–216 |isbn=978-94-010-6477-4 }}</ref> They are rarely used alone, but are rather employed to modify (reduce) the viscosity of other epoxy resins.<ref>{{cite journal |last1=Jagtap |first1=Ameya Rajendra |last2=More |first2=Aarti |title=Developments in reactive diluents: a review |journal=Polymer Bulletin |date=August 2022 |volume=79 |issue=8 |pages=5667–5708 |doi=10.1007/s00289-021-03808-5 }}</ref> This has led to the term ''modified epoxy resin'' to denote those containing viscosity-lowering reactive diluents.<ref>{{cite journal |last1=Sinha |first1=Animesh |last2=Islam Khan |first2=Nazrul |last3=Das |first3=Subhankar |last4=Zhang |first4=Jiawei |last5=Halder |first5=Sudipta |title=Effect of reactive and non-reactive diluents on thermal and mechanical properties of epoxy resin |journal=High Performance Polymers |date=December 2018 |volume=30 |issue=10 |pages=1159–1168 |doi=10.1177/0954008317743307 }}</ref> The use of the diluent does effect mechanical properties and microstructure of epoxy resins.<ref name="Khalina effect of reactive diluent">{{cite journal |last1=Khalina |first1=Morteza |last2=Beheshty |first2=Mohammad Hosain |last3=Salimi |first3=Ali |title=The effect of reactive diluent on mechanical properties and microstructure of epoxy resins |journal=Polymer Bulletin |date=August 2019 |volume=76 |issue=8 |pages=3905–3927 |doi=10.1007/s00289-018-2577-6 }}</ref> Mechanical properties of epoxy resins are generally not improved by use of diluents.<ref name="Khalina effect of reactive diluent"/> Biobased epoxy diluents are also available.<ref>{{cite journal |last1=Chen |first1=Jie |last2=Nie |first2=Xiaoan |last3=Liu |first3=Zengshe |last4=Mi |first4=Zhen |last5=Zhou |first5=Yonghong |title=Synthesis and Application of Polyepoxide Cardanol Glycidyl Ether as Biobased Polyepoxide Reactive Diluent for Epoxy Resin |journal=ACS Sustainable Chemistry & Engineering |date=June 2015 |volume=3 |issue=6 |pages=1164–1171 |doi=10.1021/acssuschemeng.5b00095 }}</ref>

=== Glycidylamine ===

Glycidylamine epoxy resins are higher functionality epoxies which are formed when [[aromatic amines]] are reacted with [[epichlorohydrin]]. Important industrial grades are triglycidyl-''p''-aminophenol (functionality 3) and ''N'',''N'',''N''′,''N''′-tetraglycidyl-bis-(4-aminophenyl)-methane (functionality 4). The resins are low to medium viscosity at room temperature, which makes them easier to process than EPN or ECN resins. 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.