Emerald Group Publishing Limited
Copyright © 2000, MCB UP Limited
Characterising the cure profile of powder coatings
Keywords Tioxide, Powder coatings, Curing, Measurement, Rheology
One of the important attributes of powder coatings is surface appearance. This is largely determined by the rheology of the molten powder and its cure rate. Titanium dioxide, while an essential opacifier and tint reducer in powder coatings, can have unwanted effects on cure; the incorporation of TiO2 into powders leading inevitably to an acceleration of the gelation rate. With the ever-widening use of Tioxide's specialist industrial grade TR81 in the powder coating field, there was a wish to characterise its behaviour in this important area.
There has been a long-standing desire to study the cure of powder coatings in more depth than the classical tests such as gel time and solvent rubs would allow. Buying a sophisticated rheometer such as the AR1000 (Plate 1) has given Tioxide's Technical Service Department some new capabilities. Among them is the ability to use measurement techniques other than classical flow rheometry, such as oscillation and creep, to access previously unattainable information. In particular, oscillation measurements can be used to study the rheology of curing powder films, allowing several measurements to be combined in one and eliminating some of the variability of previous measurements.
Plate 1 AR1000 rheometer
The oscillation technique involves sandwiching a sample between two platens, typically 4cm in diameter. The top platen is then oscillated to and fro in the horizontal plane by an extremely small fixed amplitude (typically of the order of 10-4 radians). A small amplitude is required so that the formation of structure in the powder coating is not disturbed by the measurement. The torque or stress required to keep the oscillation amplitude constant is monitored.
In the case of measuring the cure of powder coatings the temperature of the platens is also increased linearly and monitored at the same time. As the coating cures more and more stress is required to maintain the required amplitude. Measuring this increase in the applied stress enables the cure profile to be monitored. The scientific parameter used to monitor the cure profile is the storage modulus, G, which is directly proportional to the applied stress. The storage modulus is a measure of a sample's ability to store energy. As this is a primary property of solids the more solid the coating becomes the higher the value of the storage modulus.
With the assistance of Application Rheologists from TA Instruments, an experimental approach was devised. The extended temperature module was used to preheat the disposable plate assembly to 100°C, slightly above the melting point of the powder. A weighed quantity of pigmented polyester/Primid powder was then carefully placed in the centre of the bottom plate. The top plate was then lowered on to the by now molten powder and although some overfilling occurred, by experimentation with sample weights, this could be minimised. An oscillation measurement could then be used to study the evolution of the storage modulus from the molten state through the gel point to final cure. Plate 2 shows results from such measurements on three different powder coatings. The results clearly distinguish that coating RTC30 cures more rapidly than samples RTC4 and TR81.
Plate 2 Comparison of cure profiles for three different powder coatings
Following the development of this basic method, it will be refined in future by using the AR1000's programmable capability to automate the procedure from lowering the top plate onwards, reducing the variability of the method. Also the versatility of the extended temperature module allows for pre-programming of temperature profiles so that the sample may be subjected to any required heating rates. Plate 3 shows the results obtained from such a test when a coating was subjected to five different heating rates throughout the test. Initially the value of the storage modulus falls to zero as the powder melts and forms a liquid. Once the temperature exceeds 125°C a rapid increase in G can be seen as the sample cures.
Plate 3 Powder coating subjected to pre-programmed temperature profile
The method under development could in future improve the reliability of some of the mechanical tests commonly performed on powders as well as providing information on gelation time, melt-flow rheology and extent of cure. All of this information can be obtained from one convenient measurement. The method has already demonstrated that TR81 does not have an excessive influence on extent of cure, enabling Tioxide to continue to promote this grade with confidence for use in powder coatings.