An analysis into Surface Tension – The missing link to a good prime coat?
I hope the title caught your attention! Below is a summary of the findings presented for the EICF European Casting Conference held in Porto in April of 2018.
As part of the work, we investigated what affects the use surfactants have on the prime slurry surface tension and resultant prime coat adhesion via plate weights.
What is surface tension?
Surface tension is an interesting phenomenon. Surface tension is the tension caused by the attraction of the particles on the surface layer. Many liquids have very different surface tensions based on the internal molecular forces within the liquid as per figure 1.
Interestingly, it can cause phenomena enabling insects to walk on water and retains water on leaves for increased absorption during heavy rainfall.
Surface tension is important for prime slurries, as you can see within figure 2, the wetting of the material onto the surface changes based off the surface tension of the material. The use of a surfactant acts to reduce this wetting angle and improve the wetting of the material.
Secondly, the use of a wetting agent improves the wetting of the refractory within the slurry. The dispersion and coating of the refractory is required before the slurry can stabilize viscosity and flow. Figure 4, shows a quick lab trial with the different wetting behaviour of two identical slurries with and without wetting agent.
How do you measure surface tension?
There are many ways of measuring surface tension, these depend on whether the material is liquid or solid. Below is a brief synopsis of the most popular methods of measurement:
Capillary Method – The surface tension of liquids can be calculated by measuring the contact angle of liquids in a capillary tube as per Figure 5.
Sessile drop point – This method uses a drop of liquids of known surface tension and a solid substrate. The resultant contact angle made between the materials can measure the surface energy of the solid. This method can be sued to measure the surface of the liquid or the surface energy of the substrate solid.
Wilhelmy Plate – This method analyses the force required to break the surface tension when the plate is slowly removed from the liquid. The plate is of a known perimeter length and the surface tension can be calculated.
Du Nuoy Ring – This method uses a ring instead of a plate as above to measure the force required of a sample. There are subtle differences between this method and the Wilhelmy plate. Namely, there is a non- equilibrium present in the Du Nuoy method vs. the Wilhelmy plate. The du Nuoy method the ring is pulled through the surface, while in the Wilhelmy method, the plate is stationary, and the liquid is static.
Platinum iridium metal is generally used as the ring which has a wetting angle of zero. This makes the surface tension of the sample directly related to the force required to lift the ring form the liquid. This is the preferred method for measuring surface tension of binders. Figure 9 shows the setup Remet UK have within their laboratory.
Base binder Surface Tension
The surface tension of many binders is consistent and are independent of silica surface tension or whether the binder is aluminized. The small particle binders have slightly less surface tension, but they are all equivalent once surfactant is added to the binder. This was an interesting finding which shows that the wetting of these binders is independent of the particle size within the colloid in the presence of a wetting agent.
Effect of surfactant Concentration
The surfactant within the slurry has the function of reducing this surface tension. Analysis was conducted on the concentration levels required to fully wet the slurry out and ensure the wax is fully coated. The results for this testing are outlined below within Figure 11.
Finally, we reviewed if the presence of the refractory, polymer and other additives had on the surface tension of the binder. Samples were mixed together and then the supernatant was tested after a one-week period. There was found to be no degradation of the surface tension over this time. However, it is known that during dipping, the surface tension can change over time and wetting agent may be required to be added periodically to ensure good wetting of wax parts.
So, now that we know what surface tension is, how it is measured, and the effect so slurry and surfactants on the value, how does this property effect the process?
To analyses this, two samples of slurry were taken, one fully wetted, the other un‑wetted. These were mixed simultaneously and then measured for plate weight over time. As per the Figure 12 below, the slurry retained after 2 minutes was 11% higher for the wetted slurry! This points at the wetting of the plate and subsequent retention higher for a wetted slurry.
So, given those results – Should we dump in loads of surfactant into the slurry? The simple answer is no. There is a fine balance between the slurry being wetting and the subsequent bubble formation. As shown below, although surfactant can reduce the surface tension, the knock-on effect is increased bubble formation which is detrimental to a casting.
This article has introduced the concept of surface tension for slurries and binders, it’s effect on prime slurry characteristics and how it changes within a slurry. Remet UK will be further developing this method and gaining results to increase the knowledge gained through this test for our customers.< Back to insights