RESIDUAL CURING AGENT

The curing process renders the resin essentially inert and nontoxic. At room temperature, full curing may take several days incompletely cured resins may cause skin irritation and sensitization. Respiratory symptoms may result from inhalation of cured epoxy dusts during grinding, presumably due to release of residual curing agent. Skin irritation and sensitization have been associated with epoxy resin exposure. 

The U.S. Food and Drug Administration (FDA) regulates the addition of nitrate and nitrite salts to fish products. Maximum permitted levels vary among products and types of fish, with up to 500 ppm residual sodium nitrate or 200 ppm sodium nitrite permitted as a preservative and color fixative in smoke cured sable fish, shad, or salmon (Code of Federal Regulations, 1981a,b Committee on Nittite and Alternative Curing Agents in Food, 1981). 

TG-MS is an ideal technique for identifying residual volatiles in polymers. The detection of residual volatiles (and of other impurities) can often yield clues as to manufacturing processes. In many cases, such as in the determination of highly volatile materials, of residual solvents or plasticisers, use of TG-MS is requested. Specifically, there are reports on the entrapment of curing volatiles in bismaleimide laminates [145] and elastomers [48], on the detection of a curing agent (dicumylperoxide) in EPDM rubbers and of bromine flame retardants in electronic waste [50], of plasticisers such as bambuterol hydrochloride [142] or TPP and diethylterephthalate in cellulose acetate [143], on solvent extraction and formaldehyde loss in phenolic resins [164], and on the evolution of toxic compounds from PVC and polyurethane foams [146]. 

SAFETY PROFILE Most cured resins have little or no toxicity. If curing is incomplete there may be residues of highly toxic curing agents such as the organic amines m-phenylene diamine, diethylene triamine, tetraethylene pentamine, and hexamethylene tetramine, as well as phthalic anhydride and related compounds. When heated to decomposition they emit highly toxic fumes. See also various epoxy hardeners and POLYMERS, INSOLUBLE. 

Accelerators reduce the cure time considerably by increasing the cure rate. They are not catalysts because they are chemically altered and, in many cases, also react as curing agents. Common sulfur-cure accelerators are amines, dithiocarbamates, sulfenamides, thiazoles, and thiurams. Some accelerators, because of their reactivity, may form toxic compounds, such as 2-(2-hydroxyethylmercapto)benzothiazole from mer-captobenzothiazole (2-MCBT), residues of which may be extractable. Accelerators that are secondary amines may form toxic nitrosamines. 

For some products, elemental sulfur is added to the compound as a curing agent. Sulfur reacts to form C-S-C bonds. This is an inexpensive reaction scheme and it is useful for production of mbber compounds across a very wide hardness range. Residual sulfur compounds lead to odors and acid products that are not well suited to applications where clean materials are desired. 

There are two options for the other component of an epoxy resin system. Use of mono- or di-anhydrides as curing agents, usually catalyzed by a tertiary amine, causes reactions with the residual secondary hydroxyls in the repeating unit of the prepolymer forming esters and free carboxylic acids. The carboxylic acids formed also react with the epoxide end groups forming cross-links and further free secondary hydroxyl groups. Maleic anhydride, phthalic anhydride, or pyromellitic dianhydride are suitable for this process (Eq. 21.27). 

Resin II contains tetraglycidyl methylene dianiline (TGMDA) as the principal epoxide with small amounts of a cresol novolac and DGEBA. DICY as curing agent is present in an amount such that the amine epoxy ratio is 0.25. Diuron is the accelerator. The supplier s recommended standard cure is the same as for SP250. However, in this system approximately 20 percent of the epoxide remains unreacted following the cure cycle (1 ). Selected resin plates were subjected to additional cure at 170°C and 220°C. The samples post cured at 220 C for forty minutes showed no residual epoxide absorbance in the IR spectrum. 

Resin III is based upon DGEBA and an epoxy novolac in a one-to-two weight ratio. The curing agent is 3 percent boron trifluoride monoethylamine complex. The supplier s recommend cure cycle is 0.75 hour at 165 C followed by 4 hours post cure at 177 C. Some samples were cured for 24 hours at 80°C before post cure, while other samples were cured for the standard 0.75 hour at 165 C and subjected to different post cure times (2). There is still residual epoxide absorbance following initiaT 165°C cure. This absorbance decreases during post cure reaching zero after 4 hours. 

An alternative solution involves coproduction of the aminoalcohol and the diamine. In this case, the glycol is used in excess and consequently, removal of residual olefinic nitrile is unnecessary. Using this technique, we have developed a process to coproduce 3-aminopropoxyethoxyethanol (APEE, 2) and di(3-amino-propoxyethoxy) ether (DAPEE, 3). These products have found utility in coatings and epoxy curing agent applications, respectively. 

Case 8 shows the evolution of ammonia that was common with some of the early epoxy formulations that were cured with Dicy. Once the Dicy-curing agent was replaced, results showed no ammonia (Case 9) although some alcohol and ketone were present the source was residual solvents used in the new formulations. 

Figure 2.8 Scanning electron micrograph of unidirectional glass epoxy laminate containing residual dicyandiamide curing agent. After prolonged exposure to hot water, cracks propagated from where the particles had been before they dissolved in the water...

The consequence of soluble inclusions for moisture absorption is shown in Fig. 3.4, where the effect of residual dicyandiamide curing agent (DICY)... 

Conversely, novolac PF resins are produced by the reaction of excess phenol with formaldehyde (P F molar ratio 1 0.8 to 1 1) in the presence of an acid catalyst. Unlike resole resins, novolac PF resins have a more linear structure (Fig. 4) and do not self-cure because they lack the residual reactive methylol groups of resoles. Therefore, an external curing agent such as hexamethylenetetraamine must be added to novolac resins to yield a cross-linked structure. Novalac PF resins cured under acidic conditions are not recommended for wood composites for long-term structural applications. 

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