Introduction
Titanium dioxide (TiO2) is a white pigment. It gives a coating whiteness and hiding power1. Especially when volume prices of systems are used, the pigment is considered expensive. The high volume price of TiO2 is related to its high density.
Scattering by solid particles
Particles in a matrix, like a binder system in a coating, change the direction of light when the particles and the medium have a different refractive index ‘n‘. This phenomenon, called scattering, results in both white color and hiding power of the coating.
A pre-condition to obtain whiteness is that the particles do not absorb visible light. Titanium dioxide complies with this demand.
A few key properties govern the scattering efficiency of particles
First, scattering is strong when the difference in refractive index of particle and matrix, Δn = np – nm, is big. For binders used in coatings, the refractive index is around 1.6. TiO2 is preferably used as scattering source because the pigment has a high refractive index, depending upon the crystal structure of the pigment.
Secondly, for a specific wavelength of light, λ, there is an optimum particle size. The diameter of the particles ‘d‘ should be close to half of the wavelength of the light.
This implies that rutile TiO2 particles having a diameter of around 280 nm scatter visible light, with λ ≈ 380 – 700 nm, most efficiently.
Optimizing the scattering efficiency of TiO2
After selecting the right TiO2 pigment, with respect to crystal structure and particle size, several measures can maximize the efficiency of expensive TiO2 pigment.
First, during the dispersion process, the primary pigment particles must be separated from each other as much as possible. Also, to prevent flocculation, the particles must be stabilized with dispersant2,3.
By improving the separation process and/or by changing the type of dispersant, a considerable amount of money can often be saved.
Secondly, it is important that the TiO2 particles are distributed uniformly over the complete system. This is called spacing. An approach to prevent crowding, the opposite of spacing, is to use a suitable filler. The filler particles will fill the spaces between the TiO2 particles and act as spacers.
For a filler to provide spacing, the filler particles must be small. It is also preferable when the filler has a low color. An example of a spacer filler is Polygloss® 90 from KaMin. The average particle size of this ultra-fine and bright kaolin filler is around 300 nm.
Another concept that can be used to obtain maximum scattering power is to use TiO2 particles that have a thick layer of post-treatment. Ti-Pure™ Select TS-6300 from Chemours is a white pigment that consists of rutile TiO2 cores that are post-treated with a thick layer of oxides. The diameter of the TiO2 cores is in the range of 300 nm, and the average diameter of a complete particle is 530 nm. The thick oxide shell provides spacing—the TiO2 cores cannot come close to one another because the post-treatment prevents this from happening.
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References
- Titanium Dioxide: An Introduction, Marc Hirsch, 23 September 2016.
- Understanding Dispersants, Marc Hirsch, 19 February 2016.
- The Differences Between Wetting Agents and Dispersants, Jochum Beetsma, 10 April 2015.
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Can we replace TiO2 by high brightness Talc , what is the effect in Paint in what properties. how many % we can replace TiO2 by Talc.
Can we use Talc ink industry. How many % & what the specs of Talc. Any coating is required on Talc & what type of coating???
Hi.
The article is a good and interesting. I do have a question. What ratio will you recommend the filler (eg. Polygloss® 90) and TiO2 should have to maximize the scattering?
Dear Mr/Mrs Bhawanha,
Talc can be used as spacer filler as long as the grade complies with the 2 key demands: low color and small particle size. Depending on the specific application, the talc particles are (not) post-treated. Please realize that talc is a soft filler. This implies that the filler will effect the mechanical properties, like abrasion resistance and scratch resistance.
Dear Kim,
Thanks for your positive feedback.
Most important is that all particles (both TiO2 and filler) are separated from each other in the dispersion process and stabilized against flocculation. If this is the case, maximum scattering efficiency is obtained when TiO2 and spacer filler are mixed on 1-to-1 basis, calculated on VOLUME. The density of the 2 solids is needed to calculate the mixing ratio on volume basis.
Please be aware that the Pigment Volume Concentration (see articles about PVC on the UL website) will increase when filler is added. This might influence other properties, like gloss.
Best is to make a series of paints, with increasing dosage of filler and test the key properties.
Dear Kim,
Thanks for your positive feedback.
Most important is that all particles (both TiO2 and filler) are separated from each other in the dispersion process and stabilized against flocculation. If this is the case, maximum scattering efficiency is obtained when TiO2 and spacer filler are mixed on 1-to-1 basis, calculated on VOLUME. The density of the 2 solids is needed to calculate the mixing ratio on volume basis.
Please be aware that the Pigment Volume Concentration (see articles about PVC on the UL website) will increase when filler is added. This might influence other properties, like gloss.
Best is to make a series of paints, with increasing dosage of filler and test the key properties.
Hi Mr Beetsma,
A nice article. Could you please say what is the refractive index of Polygloss® 90 and how is the cost in compare to paint grade TiO2? Can we dose 5-10% of Polygloss® 90 to an exterior coating? Do you have Polygloss® 90 of 400-500nm particle size?
Warm regards,
Jana
Dear Mr/Mrs Jana,
Thanks for your positive comment..Please be aware that I am not related to KaMin: I am an independent coating consultant.
Polygloss 90 is a pure kaolin, having refractive index of 1.56-1.57. This implies that the filler will be (almost) transparent in most binder systems.
Pure kaolin is an inert filler. This implies that it is suitable to be used in exterior coatings. However, outdoor durability should always be tested when a formulation is changed.
For the following you should contact KaKin company:
– Price.
– Kaolin product with other particle size.
The contact details of KaMin can be found on the UL Prospector website.
Greetings, Jochum Beetsma.
Sir, Its really very nice article. I got many useful information from this.I want to know sometime while facing i am not getting proper dispersion in water based paint. Even after extra grinding with extra dispersing agent. Can you please tell me how to avoid this and what are the probable troubleshooting of this problem
Thanks.
Please be advised that there is a product called Omyaspace specifically for TiO2 spacing. It is a product with a fine calcium carbonate particles tested successfully to enhance the efficiency of TiO2 for opacity by spacing. It is cheaper than other TiO2 spacing agents. Please feel free to let me know if you need further information.
Dear Jack,
There are several options with respect to fillers that can be used to space TiO2 particles. I took Polygloss 90 as an example because this filler can be found on the UL Prospector website.
Greetings, Jochum.
Dear Mr/Mrs Mahtab Alam,
Thanks for your nice comment.
When you are disappointed with respect to dispersion & stabilization of your pigment, please check the following:
– Is the pigment fully wetted by the waterbased material? If not, please add wetting agent. Background: water has a high surface tension.
– Is the equipment suitable for your pigment concentrate? Preferably use a dissolver for pre-dispersion, followed by a bead mill to obtain the desired fineness.
– Do you use a suitable polymeric dispersant? Please take a look at the articles about dispersants that can be found on the UL website.
Greetings, Jochum Beetsma.