Guest Submit By Jeremy Pasatta, Advanced Polymer Coatings
All epoxy hardeners within the final sequence of weblog posts (amines, mercaptans and anhydrides) required the epoxy to be combined with the hardener simply earlier than curing and are subsequently thought-about two-part formulations. The frequent factor between all these hardeners is that they start to react instantly with the epoxy resin and have a restricted working time, wherever from 5 minutes to a day. However, latent curing brokers may be combined with epoxy resins, and since the reactivity charge at room temperature may be very low, these formulations may be saved at room temperature for weeks and even as much as a 12 months and not using a vital enhance within the viscosity of the formulation. However, when uncovered to elevated temperatures, the latent curing brokers turn out to be extremely reactive with the epoxy and treatment the formulation in a timeframe of 5 minutes to 60 minutes. The latency could also be as a result of curing agent being insoluble within the epoxy resin and solely dispersed within the formulation, or it might be soluble and intensely unreactive at room temperature. Table 1 summarizes the variations between bodily and chemical latency.
Table 1. Physical latency versus chemical latency
Some of the primary benefits of latent hardeners are as follows:
- High Tg
- Activated by warmth (treatment on demand)
- Curing temperatures may be lowered with accelerators
- One element, no mixing required earlier than utility
- Unaffected by improper mixing of elements A and B
- Usually solely requires catalytic quantities (5 – 10 phr)
One of probably the most generally used latent hardeners in each epoxy adhesives and composites is dicyandiamide, generally known as risky, whose construction is proven in Table 1. Dicy, like different hardeners, is normally micronized to cut back and management the particle. dimension, which helps not solely with dispersion, but in addition with reactivity. In common, the smaller the particle dimension, the extra floor space is on the market for solubilization, and thus the extra reactive the latent curing agent shall be. Typical particle sizes for micronized cubes are 5 or 10 µm. The peak curing temperature of risky with epoxy resin is round 200°C and cures inside half-hour at this temperature. This curing temperature is commonly too excessive for a lot of industrial processes, resembling automotive structural adhesives or prepregs, so a urea accelerator is commonly added to decrease this peak curing temperature. The temperature may be lowered from 200°C to 150°C. A typical loading stage for epoxies is 8 phr risky and three phr urea accelerator. Figure 1 reveals a DSC scan for an epoxy resin containing solely risky and a mix of risky and urea accelerator, clearly exhibiting the discount in peak curing temperature.
Figure 1. DSC plot of epoxy with Dicy and mixture of Dicy with urea
There are a number of urea accelerator buildings that exist, every with a special stability of reactivity and latency. In common, the upper the reactivity (or discount in exothermic peak temperature), the shorter the latency or shelf life. Figure 2 compares eight completely different urea buildings that had been combined with epoxy resin and risky and saved at room temperature and monitored for viscosity enhance. The epoxy and dicy-only formulation reveals no viscosity enhance over 60 weeks at room temperature, and there are some urea buildings that present comparable stability efficiency, whereas some urea buildings enhance in viscosity after solely 8 weeks. In common, the urea buildings that enhance in viscosity after 8 weeks when combined with risky and urea additionally give the bottom onset temperature for curing.
Figure 2. Stability of Dicy/Urea Accelerator Combinations
If there’s a course of that requires speedy curing at temperatures beneath 100 °C, the construction of the amine as proven in Figure 3 can obtain that, along with a particularly lengthy latency time.
Figure 3. Latent stable amine for low curing temperature and lengthy latency
With the amine construction proven in Figure 3, it’s doable to acquire 95% conversion of epoxy in 10 minutes at 110°C, and 90% conversion in about 25 minutes at 80°C. There isn’t any mixture of risky and urea that may treatment epoxies in such a short while at such a low temperature.
Latent hardeners are clearly the selection for one-part heat-activated epoxy techniques. Latency is achieved by way of bodily insolubility or lack of chemical reactivity at room temperature, and the reactivity, solubility and latency should all be rigorously weighed. In the subsequent weblog submit, we are going to take a look at biobased hardeners for epoxy resins, which formulators more and more have to make their merchandise extra sustainable.