Latent accelerator

Quaternary Phosphonium Compound Latent Accelerators for Anhydride-Cured Epoxy Resins... 

For most commercial applications, it is necessary to add some form of accelerator to the formulation to speed the rate of cure. Both acidic and basic accelerators can be used. Consequently, several latent accelerators have appeared on the commercial scene in recent years. Included among these are quaternary ammonium halides such as benzyltrimethylammonium chloride (9 ), stannous octoate ( ), zinc stearate (1 ), extra-coordinate siliconate salts(11), triethanolamine borate (12), triethanolamine titanate (13), and various other metal chelates (14). However, all of these materials have been rejected for one reason or another and... 

Phosphonium Latent Accelerators in Epoxy-Anhydride Resins... 

Table III. Viscosity (Storage) Data for Quaternary Phosphonixun Latent Accelerators in Epoxy-Anhydride Resins...

Latent Accelerator Concentration (% w/w) in Epoxy-Anhydride Resin (Days at 25 C) for Resin ... 

Comparison with Other Latent Accelerators. To illustrate the effectiveness of these quaternary phosphonium salts as accelerators for anhydride cured epoxy resins, a comparison of MTOP-DMP and TBPA with other well-known accelerators is given in Table V. The data were obtained from work done in this laboratory using the same resin systems. To make the comparison valid, the concentration given for each of these materials has been converted from phr to moles/200 g resin . Thus, the comparison is made on a molecular concentration basis instead of weight. Gel times were... 

Comparison of these data (Table V) show, quite clearly, how much more reactive the phosphonium salts are in the resin system than compounds such as substituted imidazoles and organo-zinc salts (e.g., zinc stearate and octoate). The only accelerator that shows substantially more potency than the quaternary phosphonium salts is uranyl nitrate. However, this compound is not a latent accelerator in the sense that storage lifetimes are only of the order of a few days. 

The Initiation Mechanism with Phosphonium Compounds. Although further experimental data are needed to give a fuller understanding of the reaction mechanisms involved in the latent acceleration effect of these quaternary phosphonium compounds in epoxyanhydride resins, there is definite indication, at this stage, that the mechanism does not involve the decomposition of the phosphonium compound to the free phosphine species (16). The initiation mechanism probably involves the formation of hydrogen-bonded phosphonium-epoxy or phosphonium-anhydride complexes which rearrange on the application of heat to form activated species resulting in polymerization of the epoxy-anhydride components ( > ... 

Quaternary Arsonium and Stibonium Compounds. More recently, experimental work in this laboratory has revealed that quaternairy arsonium compounds (18) are extremely effective as latent accelerators for epoxy-anhydride resius. Surprisingly, it has also been found that quaternary stibonium compounds are very much less effective as accelerators than quaternary arsonium, phosphonium or ammonium compounds. In fact, the order of gelation reactivity of these compounds with epoxy-anhydride resins would appear to be as follows ... 

Quaternary phosphonium compounds have been found to be extremely effective latent accelerators for anhydride-cured epoxy resins employed in electrical insulation applications. Used at very low concentrations (0.01 to 0.25% level) in these resins, they have been found to give very fast gel times at elevated temperatures combined with very good storage stability properties at room temperature. Results show that the quaternary phosphonium compounds having non-halide anions give the best compromise of gel reactivity, storage stability and electrical properties. Comparison with other types of latent accelerators reveal that the quaternary phosphonium compounds are among the most effective additives yet discovered for epoxy-anhydride resin systems. 

Uses Latent accelerator for dicyandiamide in epoxy formulations... 

Organometallic conq)ounds have been explored as tiie latent catalysts for various epoxy resin systems [7,8,9]. Metal acetylacetonates (AcAc s), in particular, are foimd to be effective latent accelerators for epoxy and anhydride cure reactions [10,11,12]. Based on the epoxy/ anhydride/ metal AcAc system, underfill materials have been developed for flip-chip applications [5,6,13]. Metal AcAc s are unique as catalysts for epoxy cure reactions in that they not only provide high cure latency, but also offer a wide range of cure temperatures. 

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