Editing Epoxy (section)

== Curing ==
[[File:VernetzteEpoxidharze.svg|thumb|Structure of a cured epoxy glue. The triamine hardener is shown in red, the resin in black. The resin's epoxide groups have reacted with the hardener and are not present anymore. The material is highly [[crosslink]]ed and contains many OH groups, which confer adhesive properties]] There are several dozen chemicals that can be used to cure epoxy, including [[amine]]s, imidazoles, anhydrides and photosensitive chemicals.<ref>[https://www.threebond.co.jp/en/technical/technicalnews/pdf/tech32.pdf "Curing Agents for Epoxy Resin"]. ''Three Bond Technical News''. Vol. 32, pp. 1–10. December 20, 1990</ref> The study of epoxy curing is usually carried out by using [[differential scanning calorimetry]].<ref>{{cite book |doi=10.1007/3-540-15546-5_5 |chapter=The application of differential scanning calorimetry (DSC) to the study of epoxy resin curing reactions |title=Epoxy Resins and Composites I |series=Advances in Polymer Science |date=1985 |last1=Barton |first1=John M. |volume=72 |pages=111–154 |isbn=978-3-540-15546-1 }}</ref>

In general, uncured epoxy resins have only poor mechanical, chemical and heat resistance properties.<ref>{{cite book |doi=10.1016/B978-0-08-096701-1.00178-6 |quote=In resin manufacturers’ recommended formulations resin : hardener ratios are usually in the band 3:1 to 10:1 by weight. It is the combination of resin and curing agent which produces the cured thermoset epoxy resin. |chapter=Epoxy Resins |title=Comprehensive Polymer Science and Supplements |date=1989 |last1=Hodd |first1=Kenn |pages=667–699 |isbn=978-0-08-096701-1 }}</ref> However, good properties are obtained by reacting the linear epoxy resin with suitable curatives to form three-dimensional cross-linked thermoset structures. This process is commonly referred to as curing or gelation process.<ref name="onepetro.org">{{cite book|doi= 10.2118/176457-MS|chapter= Is Epoxy-Based Polymer Suitable for Water Shut-Off Application?|title= SPE/IATMI Asia Pacific Oil & Gas Conference and Exhibition|year= 2015|last1= Hakiki|first1= Farizal|last2= Salam|first2= Damian Dion|last3= Akbari|first3= Achmad|last4= Nuraeni|first4= Nuraeni|last5= Aditya|first5= Wisnu|last6= Siregar|first6= Septoratno}}</ref> Curing of epoxy resins is an [[exothermic reaction]] and in some cases produces sufficient heat to cause thermal degradation if not controlled.<ref>{{Cite web |title=Epoxy Basics |url=https://entropyresins.com/how-to/resin-and-hardener-basic-instructions/ |access-date=2022-04-27 |website=Entropy Resins |language=en-US}}</ref> Curing does induce residual stress in epoxy systems which have been studied.<ref>{{cite journal |last1=Li |first1=Qiong |last2=Weinell |first2=Claus Erik |last3=Kiil |first3=Søren |title=Parallel measurements and engineering simulations of conversion, shear modulus, and internal stress during ambient curing of a two-component epoxy coating |journal=Journal of Coatings Technology and Research |date=September 2022 |volume=19 |issue=5 |pages=1331–1343 |doi=10.1007/s11998-022-00652-8 |url=https://backend.orbit.dtu.dk/ws/files/357093159/Accepted_manuscript-Parallel_measurements_and_engineering_simulations_of_conversion_shear_modulus_and_internal_stress_during_ambient_curing_of_a_two-component_epoxy_coating.pdf }}</ref> The induced stresses may be alleviated with flexibilisers.

Curing may be achieved by reacting an epoxy with itself (homopolymerisation) or by forming a [[copolymer]] with polyfunctional curatives or ''hardeners''. This curing is what produces the qualities of the substance such as resistance, durability, versatility, and adhesion. In principle, any molecule containing a reactive hydrogen may react with the epoxide groups of the epoxy resin. Common classes of hardeners for epoxy resins include amines, acids, acid anhydrides, phenols, alcohols and thiols. Relative reactivity (lowest first) is approximately in the order: phenol < anhydride < aromatic amine < cycloaliphatic amine < aliphatic amine < thiol.

While some epoxy resin/ hardener combinations will cure at ambient temperature, many require heat, with temperatures up to {{cvt|150|C}} being common, and up to {{cvt|200|C}} for some specialist systems. Insufficient heat during cure will result in a network with incomplete polymerisation, and thus reduced mechanical, chemical and heat resistance. Cure temperature should typically attain the [[glass transition]] temperature (T<sub>g</sub>) of the fully cured network in order to achieve maximum properties. 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.

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''. When using latent hardeners, the epoxy resin and hardener may be mixed and stored for some time prior to use, which is advantageous for many industrial processes. Very latent hardeners enable one-component (1K) products to be produced, whereby the resin and hardener are supplied pre-mixed to the end user and only require heat to initiate curing. One-component products generally have shorter shelf-lives than standard 2-component systems, and products may require cooled storage and transport.

The epoxy curing reaction may be accelerated by addition of small quantities of [[accelerant|accelerators]]. Tertiary amines, carboxylic acids and alcohols (especially phenols) are effective accelerators. Bisphenol A is a highly effective and widely used accelerator, but is now increasingly replaced due to health concerns with this substance. The most widely used accelerator is [[2,4,6-Tris(dimethylaminomethyl)phenol]].<ref>{{cite journal |last1=Fedtke |first1=Manfred |title=Acceleration mechanisms in curing reactions involving model systems |journal=Makromolekulare Chemie. Macromolecular Symposia |date=January 1987 |volume=7 |issue=1 |pages=153–168 |doi=10.1002/masy.19870070114 }}</ref><ref>{{cite journal |last1=Niazi |first1=Mina |last2=Beheshty |first2=Mohammad Hosain |title=A new latent accelerator and study of its effect on physical, mechanical and shelf-life of carbon fiber epoxy prepreg |journal=Iranian Polymer Journal |date=April 2019 |volume=28 |issue=4 |pages=337–346 |doi=10.1007/s13726-019-00704-8 }}</ref>

=== Homopolymerisation ===

Epoxy resin may be reacted with itself in the presence of an anionic catalyst (a Lewis base such as tertiary amines or imidazoles) or a cationic catalyst (a Lewis acid such as a boron trifluoride complex) to form a cured network. This process is known as catalytic homopolymerisation. The resulting network contains only ether bridges, and exhibits high thermal and chemical resistance, but is brittle and often requires elevated temperature for the curing process, so finds only niche applications industrially.
Epoxy homopolymerisation is often used when there is a requirement for UV curing, since cationic UV catalysts may be employed (e.g. for [[UV coating]]s).

=== Amines ===

Polyfunctional primary amines form an important class of epoxy hardeners. Primary amines undergo an [[addition reaction]] with the epoxide group to form a hydroxyl group and a secondary amine. The secondary amine can further react with an epoxide to form a tertiary amine and an additional hydroxyl group. Kinetic studies have shown the reactivity of the primary amine to be approximately double that of the secondary amine. Use of a difunctional or polyfunctional amine forms a three-dimensional cross-linked network.
Aliphatic, cycloaliphatic and aromatic amines are all employed as epoxy hardeners. Amine type hardeners will alter both the processing properties (viscosity, reactivity) and the final properties (mechanical, temperature and heat resistance) of the cured copolymer network. Thus amine structure is normally selected according to the application. 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. Slower reactivity allows longer working times for processors. Temperature resistance generally increases in the same order, since aromatic amines form much more rigid structures than aliphatic amines. Aromatic amines were widely used as epoxy resin hardeners, due to the excellent end properties when mixed with a parent resin. 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. 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. Increasingly, water-based [[polyamine]]s are also used to help reduce the toxicity profile among other reasons.{{citation needed|date=March 2021}}
[[File:N1,N1'-(ethane-1,2-diyl)bis(ethane-1,2-diamine) 200.svg|thumb|150px|Structure of [[Triethylenetetramine|TETA]], a typical hardener. The amine (NH2) groups react with the epoxide groups of the resin during polymerisation.]]

=== Anhydrides ===

Epoxy resins may be thermally cured with anhydrides to create polymers with significant property retention at elevated temperatures for extended periods of time. Reaction and subsequent crosslinking occur only after opening of the anhydride ring, e.g. by secondary hydroxyl groups in the epoxy resin. Homopolymerization may also occur between epoxide and hydroxyl groups. The high latency of anhydride hardeners makes them suitable for processing systems which require addition of mineral fillers prior to curing, e.g. for high voltage electrical insulators. Cure speed may be improved by matching anhydrides with suitable accelerators. For dianhydrides, and to a lesser extent, monoanhydrides, non-stoichiometric, empirical determinations are often used to optimize dosing levels. In some cases, blends of dianhydrides and monoanhydrides can improve metering and mixing with liquid epoxy resins.<ref>{{Cite web|last=Mishra|first=Vinay|date=2020|title=Benefits and Applications of BTDA and Other Dianhydrides in Polyimide and Epoxy Resins|website=[[YouTube]]|url=https://www.youtube.com/watch?v=YKoSez98UlM| archive-url=https://ghostarchive.org/varchive/youtube/20211107/YKoSez98UlM| archive-date=2021-11-07 | url-status=live}}{{cbignore}}</ref>

=== Phenols ===

Polyphenols, such as bisphenol A or novolacs can react with epoxy resins at elevated temperatures ({{cvt|130|-|180|C|disp=comma}}), normally in the presence of a catalyst. The resulting material has ether linkages and displays higher chemical and oxidation resistance than typically obtained by curing with amines or anhydrides. Since many novolacs are solids, this class of hardeners is often employed for [[powder coating]]s.

=== Thiols ===

Also known as mercaptans, thiols contain a sulfur which reacts very readily with the epoxide group, even at ambient or sub-ambient temperatures. 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. for domestic DIY adhesives and chemical [[rock bolt]] [[Anchor bolt|anchors]]. Thiols have a characteristic odour, which can be detected in many two-component household adhesives.

=== Isocyanates ===

The reaction of epoxide groups and isocyanate groups can result in two predominant types of ring structures: [[isocyanurate]] rings (through [[trimerization]] of isocyanate groups) and [[oxazolidinone]] rings (through the reaction of an isocyanate group with an epoxide group). The reaction is carried with the presence of a catalyst at temperatures ranging from 150°C and 180°C. studies have shown that there is a correlation between epoxy equivalent weight (EEW) and the [[glass transition temperature]] (T<small>g</small>) of the final polymer, indicating that higher EEW corresponds to higher T<small>g</small>.<ref>{{Cite web|date=2013|title=Non-sintering isocyanate modified epoxy resin for fusion bonded epoxy applications|website=[[Google Patents]]|url=https://patents.google.com/patent/EP2205653B1/en}}{{cbignore}}</ref>