Q: The customer wants to know if they can save expenses by reducing the thicknesses for their tanks. They have several tanks containing different concentration of HNO3 such as 0.015 %, 0.5 %, 1.0%, 1.6% 35% and 65%. The total area to be coated amounts to 600 m2.
Please let us know if they can reduce the thickness according to the concentration, especially for low concentration.
In addition, your data sheet says that the force cure can be completed at lower temperatures than you recommended. Can we cure the coated film at lower temperature by extending the cure hours?
A: There are two critical components of successful long term coating performance; the ability of the coating to resist the permeation of the solution, and the adhesion of the coating to the substrate.
The EE-121 will provide excellent resistance to HNO3 at lower the concentrations and quite possibly the 35% level. The 65% will be difficult and we would recommend a test. In order to get the best resistance, the 121K must have heat applied to reach its fullest cured state. At 72F the product will achieve 85% of its theoretical full cure (polymerization of epoxide groups with amine hydrogens). At 150F it will reach 92%. At 250F it can reach 97-98% of its theoretical cure.
Many issues can inhibit proper cure most notably humidity and rate of cure. High humidity can prompt reaction of the product amines and epoxide groups with hydroxyl groups and hydrogen present in water vapor. This will reduce potential for full cure. This is one of the reasons we recommend dehumidification be used in critical applications.
Rate of cure is also very important in allowing epoxide and hydrogen reactive sites to ‘meet’ in order to react. If the product cures too quickly reacted molecules can inhibit unreactive molecules from reaching each other. This reduces the theoretical cure. Therefore, it is best to allow the product to react as slowly as practical. Generally, you would allow the product to react at room temperature (72-75F) until hard. 8-12 hours. Then apply heat. Allow the heat to increase slowly to 150F and allow to remain for 4-6 hours. Then increase heat slowly again to 250F and allow to remain 4-6 hours.
In order not to stress the coating cool down in the same fashion.
This method will give the best possible results for making samples and for cure the coating in the tank coating.
If you cannot get the tank coating as high as 250F you can still get excellent results by applying the heat slowly to the temperature you can achieve.
The second major aspect of performance is adhesion. To maximize you want to get as clean a surface as possible. A ‘white metal’ abrasive blast will give the best possible results. It is also very important to use an angular abrasive to cut and profile the substrate to get a 3 mil or better angular profile (1 mil = .001 inch = 25 microns). Cleaning is best achieved with finer abrasives (60 – 80 mesh) and the profile is best achieved with medium abrasives (24 – 36 mesh). Often we will blast with a fine grit to clean and the follow with a medium grit to profile. It is also possible to mix the two and blast at one time. We would mix at 25% fine with 75% medium.
It is recommended that a test be done of the EE-121 with a Post Cure sample plate (coupon) to determine its resistance to 65% HNO3. The following are his instructions for Post Curing the coupon:
1) Coat a blasted coupon with EE-121.
2) Put the wet coupon in a cold oven.
3) Heat oven to 121C for 6 hours.
4) Cool coupon in oven until coupon is at room temperature.
5) Test coupon in the application environment.