Guest Publish By Jeremy Pasatta, Advanced Polymer Coatings
Latent hardeners for epoxy resins make it doable to create steady single element formulations utilized in adhesives and composites which have wonderful excessive temperature and mechanical properties. To activate the latent curing brokers, warmth is required to provoke the response. However, industrial processes are always optimised, decreasing the warmth required to remedy and shortening the curing time to maintain the general cycle time low. Therefore, latent epoxy hardeners, comparable to dicyandiamide, additionally use an accelerator to cut back curing time and temperature. The mostly used accelerator for dicyandiamide is a urea-based construction. Common constructions of urea accelerators are proven in Figure 1. The urea portion of the construction is highlighted in crimson.
Figure 1. Common urea accelerator constructions
The alternative of the urea group can have an effect on the next properties of the epoxy formulation:
- Speed of therapeutic
- Curing temperature
- Latency or shelf lifetime of the only element formulation
- Final properties of the cured epoxy, particularly Tg
Table 1 lists a number of the commonest commercially obtainable urea constructions with their melting factors. The incontrovertible fact that the melting factors of the urea accelerators are greater than typical curing temperatures signifies that the accelerators are solely dispersed within the formulation and usually are not soluble or don’t soften within the formulation.
Table 1. Common commercially obtainable urea constructions
It could also be famous that in Table 1 there are two completely different entries for 4,4′-methylenebis(phenyldimethylurea) with barely completely different melting factors. Depending on the beginning uncooked supplies and synthesis situations, the ultimate product might range barely and provides a special reactivity and stability profile. When formulating a one-part epoxy formulation utilizing dicyandiamide and urea accelerators, a typical formulation would comprise:
- 100 elements epoxy resin
- 5-8 elements of dicyandiamide
- 1-5 elements urea accelerator
Dicyandiamide with out accelerators requires warmth to about 180˚C to remedy, often over a interval of 30 to 40 minutes. By including urea accelerators, the curing temperature could be lowered to 120 to 150˚C and might remedy in lower than half-hour, saving the formulator money and time in curing the ultimate elements, which is especially essential for automotive OEMs utilizing one-part epoxy adhesive. In normal, the extra the urea accelerator lowers the curing time and temperature, the much less steady the one-component formulation. The shelf lifetime of extraordinarily lively urea accelerators can vary from 4 to eight weeks, with much less reactive urea accelerators giving one-component epoxy formulations which have a shelf lifetime of greater than a yr. The shelf life of those formulations is usually measured because the time it takes for the viscosity to double. Table 2 ranks the soundness of a one-component formulation containing urea accelerators, with the primary being essentially the most steady.
Table 2. Latency of urea accelerators, most steady to least steady
As talked about earlier, the least latent urea accelerators have a tendency to provide the best lower in remedy time and temperature. Table 3 ranks the urea accelerators by way of their reactivity, measured because the time to succeed in 95 % remedy at 120˚C, as measured by DSC.
Table 3. Reactivity of urea accelerators, most reactive to least reactive
Comparison of Tables 2 and three offers essentially the most reactive urea the least shelf steady formulation and the least reactive urea offers essentially the most shelf steady formulation. An attention-grabbing be aware on the comparability of Tables 2 and three is that the 4,4′ methylenebis(phenyldimethylurea) with a melting level of 220 to 230˚C is the second most steady of all urea constructions, and likewise has the fourth highest reactivity. This is a singular construction that gives an excellent steadiness between reactivity and stability, making it a well-liked alternative in steady single-component formulations.
Proper alternative of accelerators in latent dicyandiamide cured epoxy formulations results in important reductions in remedy time and temperature whereas nonetheless sustaining a formulation steady sufficient for use inside an affordable timeframe. Latent epoxy formulations usually are not the one sort of formulations that generally use accelerators. Accelerators for amine and anhydride cured epoxies shall be mentioned within the following weblog posts.