Q. What are the different ways to measure electrocoat cure? We use solvent rubs but, depending on the solvent, we obtain better or worse results of cure. How do we interpret this and what other tests can we do? A.R.
A. Proper cure of the electrocoat is of vital importance to coating performance with regard to corrosion protection, durability and resistance to chemical
There are three methods used to determine the degree of cure of an electrocoated part. These three methods provide several levels of testing time, equipment, accuracy and cost.
The first method is the solvent rub test. It was developed by OEMs many years ago as a quick indicator to check cure. It provides instant feedback on the chemical resistance of the electrocoat and, by correlation, feedback on the cure of the electrocoat. The accuracy of this test is totally dependent of the correlation study with the solvent or solvents used and the operators performing it, and because of its shortcomings, sometimes leads to some confusion. The electrocoat supplier must specify the solvent, the rubbing media, the operator pressure to apply during the test, the number of rubs to perform during the duration of the testing, the pattern of the rubs, and ultimately the degree of electrocoat transfer allowed from the coated part to the rubbing media. The more film transfer, the lower the cure of the electrocoat. Although solvent rubs have been around for a long time, they should never be used as the only means to verify cure.
The second method to determine degree of cure is the cumulative cure method. In this method, it is necessary that we run a “Datapaq” through the electrocoat oven with large and small parts to determine the temperature profile of the parts from the time they enter the oven until the time they exit the oven.
These temperature profiles are then compared to the required cure schedule provided by the electrocoat supplier. From this comparison we can then calculate empirically the degree of cure obtained. The premise is simple: calculate the percentage of cure attained at various incremental temperature thresholds and then sum up those percentages to obtain a total “degree of cure.” It is considered proper cure if the total cumulative cure is between 80 and 120 percent, with 80 percent being slightly under-cured and 120 percent being slightly over-cured. The cumulative cure method is very accurate but takes significant time if done by hand. There is some commercially available software capable of performing the calculations and displaying percentage cure.
The third method to determine degree of cure is Thermo Mechanical Analysis (TMA) testing. In this method, a coated part is sent to an accredited laboratory for testing. Typically, the electrocoat supplier has the capability to perform this test. A thermo mechanical analyzer will determine the dimensional variation of a mechanical property at various temperatures and with this calculate the degree of polymer crosslinking and degree of electrocoat cure. The TMA is a very accurate test but requires very expensive instrumentation.
Proper operation of an electrocoat system must include testing intervals to perform all three methods explained above. The frequency of each test will vary depending on the available resources and the wanted quality and accuracy. Establishing long-term correlations between the three tests can lead to long-term savings in testing frequencies.
Something very important to keep in mind is that the acceptable degree of cure for any given part is the end-use of the part involved and the coating properties needed for the specific application. A part that obtains an 83 percent degree of cure may have all the corrosion protection, chemical resistance, hardness and flexibility it needs for the application, but it may slightly yellow while being coated with a white topcoat, thus becoming an unacceptable level of cure for the application. The same parts with the same 83 percent cure may be acceptable if the part will require flexibility during crimping or will not require a topcoat. Conversely, topcoat adhesion may be marginal on a part that obtains a degree of cure of 133 percent while another part with the same cure that doesn’t get topcoated has excellent durability by a customer’s standards.
Let experience and field testing guide you in determining what is the “proper” degree of cure your electrocoat and parts needs.