Epoxy resin curing agents Novolac resins

Methylimidazole (EMI) n. An epoxy-resin curing agent with a heterocyclic structure. EMI is used with epoxies formed from epichlorohydrin and bisphenol A or -F, and for novolac epoxy resins. It provides ease of compounding, long pot life, low viscosity, and non-staining characteristics. 

For maximum heat resistance, pyromellitic dianhydride is often used. The composition and properties of metal-to-metal adhesives based on a combination of epoxy novolac and a bisphenol A epoxy resin have been described in Sec. 12.5.1. Depending on the cure temperature and the choice of amine curing agent, strength as high as 3000 psi at room temperature and over 1000 psi in the range of-55 to + 150°C is possible. 

These resins (Resole or Novolac) are used as curing agents or hardeners for epoxy molding compounds for electronics applications such as computer components. 0-cresol-formaldehyde resins have heen also used to modify phenol-formaldehyde resins, and in laminates. 

Resin I contains roughly equal amounts of diglycidyl ether of bisphenol A (DGEBA) and an epoxy cresol novolac. Sufficient dicyandiamide (DICY) as a curing agent is present such that the amine/epoxy ratio is 0.85. Monuron is present as an accelerator. The supplier s recommend standard cure is two hours at 127°C. Previous work (1 ) has shown that this cycle produces a fully cured system, as indicated by the disappearance of the epoxide absorbance band in the infrared spectrum. 

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. 

Carboxylic acid and anhydride curing agents are used to a lesser extent in aerospace applications as compared to the amines which have mechanical properties and cure conditions that can be tailored to a wider variety of specific applications. Anhydrides tend to be somewhat brittle but offer useful service as high as 250 °C with novolac-type epoxy resins. In addition, the aliphatic dicarboxylic acid anhydrides give tough and sometimes flexible properties which are useful in encapsulation applications. 

Expanded materials with excellent high-temperature properties are obtained when cresol novolacs are used as hardeners. A typical formulation is based on mixtures of bisphenol A resins and epoxy novolac resins which are cured by cresol novolacs and accelerated by suitable nitrogen-containing agents. 

Anon, Dow Epoxy Novolac Resins, Technical Bulletin No.296-00279-1098, Dow Chemical, 1998. Anon, Epoxy Resin Curing Agents, Comparative Performance Properties, Henkel Technical Bulletin, 1998. 

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