TGIC crystal type and application characteristics
This article talks about the two crystalline forms of TGIC
Let's start with the conclusion: the correlation between the melting range data of TGIC and the curing effect of the coating film is not very high. High data values do not necessarily work well, and low values are not necessarily bad.
We can see the factory report COA of TGIC will be marked with a melt range test data such as: 92-98 ℃. First of all, impure crystals, defects, impurities, a variety of crystalline mixture of solid material will have the concept of melting range. Otherwise, our can be called melting point. Melting range A melting process, the test of the speed of heating, the degree of material crushing will affect the test data. Of course, this article is not about these. This article is mainly about the correlation between the melting range of TGIC and the curing effect of powder coatings.
Where do the two crystals come from? The answer is the presence of chiral carbon
The above figure is the micrograph of β-type TGIC crystals, the crystals appear as square and hexagonal arrangement, the actual production of slow-growing β-type crystal particles are mostly square-shaped
The picture above shows the α-type TGIC needle crystal
Look at two more images
The crystallization of the β-type above starts from b-c-d and melts, corresponding to the temperature of 150-153-154°C
The crystals of alpha type above start to melt from b-c-d, corresponding to the temperature of 98-99-100°C
Think about the temperature of our extrusion process, in look at the temperature corresponding to the above chart, you will certainly come to a doubt, β-type crystals in the extrusion process may not be able to dissolve, can not be dissolved in the case of miscible with polyester seems to be sure to be more difficult. There are two examples can be cited, dicyandiamide as epoxy curing agent because of its melting point of more than 190 ℃, to be used must be micronized. Another example is that the formulation of HAA to improve the extrusion effect will also need to increase the extruder temperature to achieve the dispersion effect because of the melting point of HAA up to 120°C. So here we are faced with a situation where TGIC is firstly not going to be micronized because of toxicity (admittedly micronized TGIC is the best), and secondly it is not feasible to raise the extruder temperature to 150°C.
Two types of crystalline SSS RRR β type in accordance with the theory of spatial site resistance of its reaction speed and the final reaction degree is certainly lower than the three epoxy groups are not in a plane of SSR SRR so α type theoretically has a better powder coating applicable performance. The main reason for mentioning the potential resistance here is the high viscosity and poor mobility of the powder coating curing process.
More than 90% of the commercially available coating grade TGIC is not separated from the two crystalline types, so we can generalize that the better the crystalline purity (chlorine content will also be low) the higher the TGIC melt range data will be better. This good use is expressed as the TG of the powder, the weathering of the coating film, the yellowing when curing, the physical properties of the delayed coating film, boiling, etc.