The result showed that if both the temperature and concentration were increased this would lead to faster destabilisation, so to obtain faster results for study purposes we stressed concentrates at high temperature for many days.
Choosing the best anti-foaming agent means finding a compromise between compatibility and performance. Generally speaking the less stable a defoamer is, the better its de-foamability characteristics, while the contrary is true. This is why compatibility and performances were measured in parallel.
Compatibility
Compatibility of defoamers in concentrates has been measured with three different analysis: visual method, dynamic light scattering and UV-VIS spectroscopy.
The dynamic light scattering technique is a very reliable method but needs optical data and it seemed to be ineffective at very low concentrations. UV-VIS spectroscopy allows for the measurement of what is known as the Tyndall effect, the light-scattering by particles in a colloid observed when the size of dispersed particle is close to the wavelength of visible light. The result is a decreasing of the Tyndall effect intensity over time when concentrates are stressed at high temperature due to defoamer separation.
The most convenient method is the visual evaluation of samples in comparison with reference samples. Here optimised formulations with different defoamers are tested at high temperature and their visual appearance is monitored at the beginning and after 7 days, 15 days, 22 days and 30 days of ageing. Scores vary from 0 the appearance of sample is equal to the blank sample, to 6 where the appearance is equal or worse than the negative reference.
The best defoamer is defined as the one with the lowest score after 30 days of ageing as shown in the table 1.
Performances Moreover, as time passes, defoamer tends to separate and a loss of performances occurs due to a breakdown in the integrity of the fluid. Foam volume and the time of foam halving are measured as the concentrate ages after fixed periods. The defoamer that exhibits a long-lasting effectiveness will be the one that shows the lowest values, as the fig 5 illustrates.
Figure 5: Time of foam halving variation with product ageing
As previously stated, generally the less stable defoamers are the most performant. This is in part because most performant defoamers have to be almost insoluble in the concentrate to properly perform their function. The final choice must therefore be a trade-off between these two parameters: in Figure 6 defoamer number 6 and 8 are the most compatible, while number 7 and number 1 are the most performant.
Table 1: Scores of different defoamers’ visual appearance.
Figure 6: Compatibility and performances scatter plot. The circled point represents the compromise between compatibility and performances.
Continued on page 24 LUBE MAGAZINE NO.169 JUNE 2022 23
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