Which then undergoes a proton exchange with a thiol group. Subsequently attacks an oxirane ring to produce an alkoxide ion, Proton exchange with a thiol group to produce a thiolate anion which The reaction is initiated by a sufficiently strong base such as a tertiary amine The base-catalyzed thiol-epoxy addition is assumed to take placeĪs a nucleophilic addition between a thiolate anion and an epoxy However, the mechanical and physical properties are typically inferior to those of amine crosslinked epoxies.įurthermore, most thiols have a disagreeable odor which, however, usually disappears after cure. They are also much faster than epoxy-amine additions, particularly at low temperatures,Īnd may yield clear products. Reactions, which means they are highly selective, produce noīy-products and take place under mild reaction conditions with high 2) Thiol-Epoxyīase-catalyzed thiol-epoxy additions can be categorized as click Particularly at elevated temperatures and in the presence of aĬatalyst such as tertiary amines. 1įurthermore, cyclic ethers can undergo homopolymerization, This addition reaction is known asĮtherification, even when the OH group resides on the polymer backbone. Oxirane ring reacts with a primary or secondary amine. Oxirane rings may also react with hydroxyl groups (OH) that are present in the polyol accelerators Many amine curatives are (highly) toxic, require relative long cure cycles and yield amber colored products. Partially protonates the oxygen atom of an oxirane ring, making the neighboring methylene group more susceptible to attack byĪmine-epoxy thermosets typically have good elevated temperature performance and outstanding chemical resistance. A mechanism has been proposed in which a hydroxyl hydrogen atom The reaction between amines and epoxy resins is significantly accelerated by proton donors such as water, phenols, or Reactions, but in reality, vitrification occurs long before all reactive groups are consumed.
In theory, all active amine hydrogens can undergo addition An idealized representation of the reaction between epoxy resin and diamine is depicted in the figure below. Then reacts with another oxirane ring, creating an additional Yields a secondary amine and a hydroxyl group. In the case of primary amines, the addition reaction Weaker nucleophilicity and typically require long cure cycles at elevated temperatures (> 100☌) but yield the highest chemical and thermal resistance properties.Ĭrosslinking of epoxy resins with amines typically proceeds via Among these, cycloaliphaticĪmines are the most reactive ones followed by aliphatic amines whereas aromatic amines are much less reactive due to their Depending on the substituents, the amines can be divided into three main groups:Īliphatic, cycloaliphatic and aromatic amines. Polyamines are the most common crosslinking agents for diepoxides such as diglycidyl ether of bisphenol A (DGEBA). The possible (crosslinking) reactions with epoxides are discussed briefly below: 1) Amine-Epoxy Addition Reactions The three most common co-reactants, also called curatives or hardeners, are multifunctional Required processing characteristics such as viscosity, pot-life, and gel-timeĪnd on the performance requirements (chemical, mechanical, thermal, optical, and electrical) and cost. When copolymerized with multifunctional cyclic ethers, they form a three-dimensional insolubleĪnd infusible network. This requires co-reactants with three or more active Multifunctional cyclic ethers such as diglycidyl ethers of bisphenol A or F (knownĪs “epoxy resin”) are often cross-linked to achieve optimal performance properties in the final product. The general reactions ofĮpoxides with these compounds are shown in the scheme below. This includesĪliphatic and aromatic amines (both primary and secondary), phenols,Ĭarboxylic acids, thiols, and anhydrides. These compounds are highly reactive due to the high ring strain andĬan be copolymerized or crosslinked with a wide variety of nucleophilic andĮlectrophilic compounds having active hydrogen atoms. The two most important types of cyclic ethers are oxiranes (also known as epoxides) and oxitanes. Ring-opening Copolymerization and Crosslinking of Cyclic EthersĬyclic ethers are of significant industrial importance and find applications in many areas including high performance adhesives,
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