Resins epoxide

The epoxide resins (also widely known as epoxy resins and, occasionally, as ethoxyline resins) are characterised by the possession of more than one 1,2-epoxy group (I) per molecule. This group may lie within the body of the molecule but is usually terminal. 

The three-membered epoxy ring is highly strained and is reactive to many substances, particularly by with proton donors, so that reactions of the following schematic form can occur  

Such reactions allow chain extension and/or cross-linking to occur without the elimination of small molecules such as water, i.e. they react by a rearrangement polymerisation type of reaction. In consequence these materials exhibit a lower curing shrinkage than many other types of thermosetting plastics. 

There is, quite clearly, scope or a very wide range of epoxy resins. The nonepoxy part of the molecule may be aliphatic, cycloaliphatic or highly aromatic hydrocarbon or it may be non-hydrocarbon and possibly polar. It may contain unsaturation. Similar remarks also apply to the chain extension/cross-linking agents, so that cross-linked products of great diversity may be obtained. In practice, however, the commercial scene is dominated by the reaction products of bis-phenol A and epichlorohydrin, which have some 80-90% of the market share. 

In comparison to the polyester resins, epoxides have limited and specialist use, and cost at least twice as much. However, they have superior toughness, will operate at slightly higher temperatures, have excellent adhesion to many substrates and are particularly resistant to alkaline environments. In particular, the chain extension and cross-linking reactions do not normally 

The most important epoxide resins are oligomers produced from the reaction of bisphenol A and epichlorhydrin. Bisphenol A is prepared from phenol and acetone, and need not be pure. Epichlorhydrin is made from propylene via the hydrochlorination reaction with propylene oxide. Diglycidyl ether is produced in the reaction of bisphenol A with excess epichlorhydrin (2-3 times stoichiometry), in an alkaline solution to prevent higher homo-logues being formed (n = 1. in Reaction 12). 

Chain extension and cross-linking mechanisms involve attack on the epoxy groups. Higher homologues n 1) are favoured by more alkaline conditions and reduced excess of epichlorhydrin (Reaction 13)  

Cross-linking can proceed through a number of mechanisms. Tertiary amines and boron fluoride complexes catalyse homopolymerization (Reaction 14)  

Acid and acid anhydride hardeners are less volatile and so safer to use. They give lower exotherms, but will react both with the epoxy and hydroxyl groups in the oligomers (Reaction 16)  

Vinyl ester resins are thermosetting resins that consist of a polymer backbone with acrylate or methacrylate termination. The backbone component of vinyl ester resins can derived from an epoxide resins, polyester resins, urethane resin, and so on, but those base epoxide resins are of particular commercial significance. 

Vinyl ester resins are produced by the addition of ethylenically unsaturated carbo acids (methacrylic or acrylic acid) to an epoxide resin (usually of the bisphenol epichlorohydrin type). The reaction of acid addition to the epoxide ring (esterification exothermic and produces a hydroxyl group without the formation of by-products. Appropriate diluents and polymerization inhibitors are added during or after esterification. 

Epoxide resins that have been used to produce vinyl ester resins include  

Vinyl ester resins contain double bonds that react and crosslink in the presence of free radicals produced by chemical, thermal or radiation sources. 

These materials can be conveniently divided into six classes of resins  

Some general articles on the properties of carbon fiber reinforced epoxies have been published in the references given [14-16]. 

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Bromine is used in the manufacture of many important organic compounds including 1,2-dibromoethane (ethylene dibromide), added to petrol to prevent lead deposition which occurs by decomposition of the anti-knock —lead tetraethyl bromomethane (methyl bromide), a fumigating agent, and several compounds used to reduce flammability of polyester plastics and epoxide resins. Silver(I) bromide is used extensively in the photographic industry... 

The reactions of carboxyUc acids and anhydrides with epoxy resins have been extensively studied in a variety of investigations, particularly References 27—31. The general reaction of epoxide resins and anhydrides is... 

W. G. Potter, Epoxide Resins, Springer-Vedag, New York, 1970. 

Interesting developments were also taking place in the field of thermosetting resins. The melamine-formaldehyde materials appeared commercially in 1940 whilst soon afterwards in the United States the first contact resins were used. With these materials, the forerunners of today s polyester laminating resins, it was found possible to produce laminates without the need for application of external pressure. The first experiments in epoxide resins were also taking place during this period. 

NB Daia for the three important ihermosetting materials (phenolics, aminoplastics and epoxide resins) were not covered in the 1998 review on which the 1997 data was based. The 1987 figures for these materials do include a substantial percentage of use in adhesive, surface coating and laminate applications. 

Encapsulation of semiconductors. The usual material is epoxide resin (see Chapter 26) and the preferred method transfer moulding. It has been estimated that by 1980 annual production of such encapsulated parts exceeded 10 billion units. 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

An important development of polymerisation casting is that of reaction injection moulding. Developed primarily for polyurethanes (and discussed further in Chapter 27), the process has also found some use with polyamides and with epoxide resins. 

Low molecular weight liquid nitrile rubbers with vinyl, carboxyl or mercaptan reactive end groups have been used with acrylic adhesives, epoxide resins and polyesters. Japanese workers have produced interesting butadiene-acrylonitrile alternating copolymers using Ziegler-Natta-type catalysts that are capable of some degree of ciystallisation. 

Mention has already been made of epoxide stabilisers. They are of two classes and are rarely used alone. The first class are the epoxidised oils, which are commonly employed in conjunction with the cadmium-barium systems. The second class are the conventional bis-phenol A epoxide resins (see Chapter 22). Although rarely employed alone, used in conjunction with a trace of zinc octoate (2 parts resin, 0.1 part octoate) compounds may be produced with very good heat stability. 

The chemical resistance of PTFE is exceptional. There are no solvents and it is attacked at room temperature only by molten alkali metals and in some cases by fluorine. Treatment with a solution of sodium metal in liquid ammonia will sufficiently alter the surface of a PTFE sample to enable it to be cemented to other materials using epoxide resin adhesives. 

They have found use as hardeners-eum-flexibilisers for epoxide resins (see Chapter 26) and are of interest in the production of thixotropic paints and adhesives. Related higher molecular weight materials are tough and flexible and find use as hot melt adhesives (Versalons). 

Vulcanisation may also be brought about by zinc and calcium peroxides, p-quinone dioxime, epoxide resins, phenolic resins and di-isocyanates. 

The so-called phenoxy resins were a development of epoxide resin technology which had hitherto been used exclusively in the thermosetting resin field (see Chapter 26). As with the most important epoxide resins they are prepared by reacting bis-phenol A with epichlorohydrin to give the following structure (Figure 21.9) ... 

Their main point of difference is that the phenoxies are of much higher molecular weight ( 25 000). The phenoxies are also said to be slightly branched. Like the epoxide resins they are capable of cross-linking via the pendant hydroxyl groups, in this instance by di-isocyanates and other agents. 

Because of their favourable price, polyesters are preferred to epoxide and furane resins for general purpose laminates and account for at least 95% of the low-pressure laminates produced. The epoxide resins find specialised uses for chemical, electrical and heat-resistant applications and for optimum mechanical properties. The furane resins have a limited use in chemical plant. The use of high-pressure laminates from phenolic, aminoplastic and silicone resins is discussed elsewhere in this book. 

The commercial interest in epoxide (epoxy) resins was first made apparent by the publication of German Patent 676117 by I G Farben in 1939 which described liquid polyepoxides. In 1943 P. Castan filed US Patent 2 324483, covering the curing of the resins with dibasic acids. This important process was subsequently exploited by the Ciba Company. A later patent of Castan covered the hardening of epoxide resins with alkaline catalysts used in the range 0.1-5% This patent, however, became of somewhat restricted value as the important amine hardeners are usually used in quantities higher than 5%. 

About half of epoxide resin production is used for surface coating applications, with the rest divided approximately equally between electronic applications (particularly for printed circuit boards and encapsulation), the building sector and miscellaneous uses. In tonnage terms consumption of epoxide-fibre laminates is only about one-tenth that of polyester laminates, but in terms of value it is much greater. 

The first, and still the most important, commercial epoxide resins are reaction products of bis-phenol A and epichlorhydrin. Other types of epoxide resins were introduced in the late 1950s and early 1960s, prepared by epoxidising unsaturated structures. These materials will be dealt with in Section 26.4. The bis-phenol A is prepared by reaction of the acetone and phenol (Figure 26.1). 

Since both phenol and acetone are available and the bis-phenol A is easy to manufacture, this intermediate is comparatively inexpensive. This is one of the reasons why it has been the preferred dihydric phenol employed in epoxide resins manufacture. Since most epoxide resins are of low molecular weight and because... 

Table 26.1 shows the effect of varying the reactant ratios on the molecular weight of the epoxide resins. ... 

The epoxide resins of the glycidyl ether type are usually characterised by six parameters ... 

As indicated in the preceding section, amine hardeners will cross-link epoxide resins either by a catalytic mechanism or by bridging across epoxy molecules. In general the primary and secondary amines act as reactive hardeners whilst the tertiary amines are catalytic. 

In some instances it is desired to produce a more open network from epoxide resins that have been acid-cured. This may be achieved by the oligoesterdi-carboxylic acids of general structure... 

Although the first and still most important epoxide resins are of the glycidyl ether type, other epoxide resins have been commercially marketed in recent years. These materials are generally prepared by epoxidising unsaturated compounds using hydrogen peroxide or peracetic acid. 

Those which have an essentially linear structure on to which are attached epoxide groups—the acylic aliphatic epoxide resins. 

Cyclic aliphatic epoxide resins" were first introduced in the United States. Some typical examples of commercial materials are shown in Table 26.6. 

Miscellaneous Epoxide Resins 765 Table 26.6 Some commercially available cyclic-aliphatic epoxide resins... 

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