Anhydride hardeners

The mechanism of anhydride hardening is complex but the first stage of reaction is believed to be the opening of the anhydride ring by an alcoholic hydroxyl group (or salt or a trace of water), e.g. Figure 26.6. 

Table 26.5 summarises the characteristics of some of the anhydride hardeners. 

Anhydride hardener Parts used phr Typical cure schedule Physical form Max. HDT of cured resin °C Use... 

The epoxidised polybutadiene resins available to date are more viscous than the diglycidyl ethers except where volatile diluents are employed. They are less reactive with amines but have a similar reactivity with acid anhydride hardeners. Cured resins have heat distortion temperatures substantially higher than the conventional amine-cured diglycidyl ether resins. A casting made from an epoxidised polybutadiene hardened with maleic anhydride and cured for two hours at 50°C plus three hours at 155°C plus 24 hours at 200°C gave a heat... 

Typical recipes of compositions, widely used in the electrical industry and in the production of reinforced plastics, are given in Table 1.2, while the basic properties of epoxy plastics cured by amine and anhydride hardeners can be found in Table 1.1. 

Acid anhydride hardeners, in general, result in brittle systems after curing with a relatively high level of gas evolution on irradiation. 

In addition of polyamines, acid anhydride hardeners are used but the structure of cured resins is less understood. 

Curing with anhydride hardeners requires a long curing time and a high temperature. The cured resins are excellent in mechanical and particularly in electrical properties at temperatures above Tg. Anhydride hardeners are extensively used. Many practical studies on mechanical and electrical properties 381 but few systematic studies on the structures of the cured resins and physical properties have been reported 27,39 41). The reasons why such systematic studies are scarce are as follows Most of the acid anhydride hardeners are cyclic anhydrides of dibasic acids. The main structure of the cured resins is such as shown in Fig. 1 b. Differences among the structure of the cured resins with different hardeners are only based on the differences in the R segment and consequently, cured resins with rather different mechanical properties cannot... 

The anhydride hardened epoxies generally have better dielectric loss properties above the Tg than do the novolac epoxies. Consequently, a higher device Junction temperature can be tolerated because of less leakage at the plastic/chip interface and because the required heat dissipation can be obtained via the filler. This is, unfortunately, obtained at the expense of moisture resistance because the moisture resistance of the anhydride-hardened epoxies is not as good as the novolac epoxies. 

Epoxy- anhydride High molecular weight epoxy resins cross-linked with anhydride hardeners Good Very good Internal white for three piece cans... 

N. Grittner, W. Kaminsky and G. J. ObsL Fluid bed pyrolysis of anhydride-hardened epoxy resins and polyether-polyurethane by the Hamburg process. J. Anal. Appl. 

Anhydride-cured resins generally have better thermal stability. Pyromellitic dianhydride with higher functionality produces tightly cross-linked products of high heat-distortion temperatures. Heat-distortion temperatures as high as 290°C have been quoted. Table 4.28 summarizes the characteristics of some of the anhydride hardeners. 

TABLE 4.28 Properties of Some Anhydride Hardeners used in Low-Molecular-Weight Bisphenol A-Based Epoxy Resins... 

However, it is also known that, DGEBA epoxy resin, amine hardeners, dicarboxylic (phthalic) anhydride hardeners, and reactive epoxy diluents are all common causes of allergic contact dermatitis. And since all are health hazard causing organic pollutants, no remnants should be left in the composite, and must be decreased or avoided completely [73, 74]. 

For example, an anhydride hardener (with tertiary amine accelerator) will produce a polyester -type structure with low polarity, whereas an amine hardener will form a P-hydroxyl amine network with high polarity. Hydrogen bonding of water molecules at these sites enhances the concentration of moisture which can be absorbed. However, with catalytic curing agents, polyether stmctures of relatively low polarity will form. Since it is quite common to use mixed curing agents for composite matrices, the cured... 

Boquillon, N. and Fringant, C. (2000) Polymer networks derived from curing of epoxidised linseed oil influence of different catalysts and anhydride hardeners. Polymer, 41(24), 8603-8613. 

Melamine-formaldehyde resin, etherified acrylated Sucrose benzoate Vinyltriethoxysilane Vinyltrimethoxysilane Vinyltris (2-methoxyethoxy) silane hardener, composites 4,4 -Diaminodiphenyl sulfone hardener, concrete surface Zinc hexafluorosilicate hardener, cosmetics Alkenyl succinic anhydride hardener, decorative coatings o-Tolyl biguanide hardener, elastomers m-Divinylbenzene hardener, emulsions Chromium acetate (ic) hardener, epoxies... 

Hydrofuramide Phthalic anhydride Succinic anhydride Tetrahydrophthalic anhydride hardener, rodenticides Hydrofuramide hardener, rubber... 

Anhydride Hardeners for Epoxy Resins, Publieation 762-5M, Allied Chemical Co., (1962)... 

Anhydride hardeners These materials are organic polycar-boxylic anhydrides. Most require severe curing cycles. They provide thermal stability superior to that of the amines. Anhydride-cured epoxies are often brittle and require a flexibi-lizer, which will result in reduced heat and chemical resistance. 

Use of anhydride hardeners to crosslink epoxidised oils has received more attention. Boquillon and Fringant [19] studied the kinetics of the polycondensation of epoxidised linseed oil with various aromatic and cycloaliphatic anhydrides at 150-170 °C and characterised the ensuing networks by measuring their Tg and flexural modulus (which both increased with increasing proportion of anhydride) as well as dynamic mechanical properties and their crosslink density. These materials, based partly on renewable resources, displayed adequate properties in terms of applications of thermosets associated with a relatively low Tg (35-110 °C). A similar investigation in which epoxidised vegetable oils substituted with <50% of bisphenol A diglycidyl... 

The long-chain hardener segments are built up by a simple condensation reaction between a moles of a dicarboxylic acid and a-1 moles of a glycol, as shown in Fig. 5. Further reaction of the carboxylic groups with diepoxide groups then produces long-chain, flexible epoxide compounds, which require the use of corresponding hardeners. The best results have been obtained with anhydride hardeners. 

Polyamine and organic acid anhydride hardeners serve as co-reactive hardeners, which become incorporated into the epoxy resin, whereas tertiary amines, such as 2,4,6-tris-(dimethylaminomethyl)phenol (tris-DMP) and N,N-dimethylbenzylamine, imidazoles, and boron trihalide amine complexes are catalyst-type curing agents, which may not be chemicaUy bound to the resin molecules during epoxy curing reactions (Guin and Work 1995 Muskopf and McCoUister 1987). 

Table 4. Organic acid anhydride hardeners (Guin and Work 1995 Jolanki et al. 1997b)...

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