Approaching to the mysterious VantaBlack! !
Hello, this is Teppei in Koyo Orient Japan.
We are the company developing and selling light-absorbing sheets for the internal Stray light reduction. So, we always need to know our competitors, especially we can't escape from the surface treatment “Vantablack”.
This is a surface treatment technology developed by Surrey NanoSystems in the UK. At any rate, its special feature is such a very black color! It has become a kind of symbolic black, so comparing our kit with the Vantablack black color is also the comparison with the stray light reduction champion. In addition, at the exhibition held at the Frankfurt, BMW showed the world's blackest car that was coated by Vantablack. It became a very hot topic.
However, the actual performance is relatively unknown. Looking at review comments, it seems there are many misunderstandings. It may quite limited who could have ever seen the actual Vantablack.
So, in this blog, I would like to introduce Vantablack with mainly through the catalog. And explain the products and light-absorbing materials with also misunderstandings about them.
The catalog used in this blog; you can easily get it at Surrey NanoSystems website. Please click "Downloads" at the top of it.
How much is the total stray light rate of Vantablack?
The tag line of Vantablack is "absorbing up to 99.965% of visible light!" In other words, “It boasts the ability of 99.965% light absorption rate". By the way, "what is the meaning of light absorption rate?"
There are three types of reaction after the light hit any object: reflection, absorbed into the object, and going through it. Among these reactions, the light intensity of reflection and going through themselves can be measured, so these two are the popular indicators. But the light absorption rate was not so familiar with me.
By the way, let me explain the word of “total reflectance rate of the light”. For example, the angle of the reflected light depends on the object surface irregularities or the object properties. If all the reflected lights are collected to one point and measured the intensity of the light at such point, it means the sum of all reflected light measured, then those measurement are called “total hemispherical reflectance” or “total reflectance”. Please see the drawing of a outline below. You can say the total reflectance in the visible light is the index of blackness.
With relating to the above topics, I found the reflectance value of Vantablack in the catalog. “Feature: Total hemisphere reflectance 0.036% (in case of 700 nm)”.
In other words, it is considered if it doesn't have any transmission light, the light absorption rate is calculated “1 minus reflectance 0.036% = 99.965%".
However, I feel the figure of 99.965% is a little suspicious. Because if you see the total reflectance graph at each wavelength in the catalog, you might not feel the result is 0.036% level. Because there are also excerpt value of the total hemispherical reflectance for each wavelength, then let me check them!
It’s not 0.036% at all …. Looking at the graph, you might feel the total reflectance average is a degree of a little over than 0.2% in the visible light wavelengths.
I think this is the proper total reflectance value matched to your feeling when you see the Vantablack. By the way. as a matter of-course, it is the honorable value and it's GREAT. For example, even the black matte with anodic oxide coating used for Stray light reduction of optical equipment, its total reflectance is around 5%.
Vantablack has some limitation of its processing and usage environment
I will explain the detail later, but there are some types in Vantablack. Here I will talk about the blackest Vantablack.
The name of Vantablack is abbreviation of “Vertically Aligned Nano Tube Arrays + Black”, and the name of “vertically aligned carbon nanotubes” itself shows the mechanism of its blackness.
The carbon nanotube forest is grown on the surface of the object by chemical evaporation, and the temperature when the chemical evaporation is done, it is about 430 °C. In other words, the base material needs to have the endurance of heat resistance than 430 ℃. Therefore, the maximum size that can be constructed would be limited by the size of the chemical evaporation furnace also.
In addition, the carbon nanotubes themselves are a little difficult to handle. For example, it has been pointed out that they are toxic almost same as to asbestos if we suction carbon nanotubes. And, because it has not good resistance against touching or rubbing, so the products using Vantablack are inevitably limited to be used in an isolated area.
The reason why ordinary users hesitate to use it may due to the high hurdles like those. I often see the comment like "I want to paint Vantablack on xxxx", but actually it's not so easy.
Vantablack has some types
Talking about Vantablack, I feel the most common misunderstanding is a confusion of some types. Besides the blackest Vantablack described above, the other types of Vantablack are same called “Vantablack" too, but all black processing are completely different and big discrepancy exists of light absorption performance. Then, let me introduce them!
○ Vantablack S-VIS
It seems to be a process that randomly generates carbon nanotubes on the surface of the object by spray coating and a vacuum treatment process under the temperature of 100 ~ 280 ℃. It is said that it can also be applied to 3D shapes, so, there is high possibility that many typical samples seen on the web were made by this process.
If you see the graphs in the catalog, it seems the average of the total reflectance in visible light is around 0.3%. It's a little bigger than Blackest type of 0.2%.
Because this type is also used carbon nanotubes, therefore its durability for touching might be quite low. So, it seems those also should be used in an isolated environment.
This black type does not use carbon nanotubes. The BMW “World's Blackest Car” at the Frankfurt show, it seems that it was painted with this material too.
This type is often misunderstood and confused that it is same with Blackest carbon nanotubes version. For example, if you see the internet, you can easily see the users' some misunderstanding opinions, like “BMW X6 is painted with Blackest Vantablack, so it absorbs 99.965% of the light”.
Judging from the graph in the catalog, the total average of reflectance in visible light seems to be around 1.1%. I feel even this figure, it is quite excellent as low-reflection coatings, but it coated by low-reflection ink, there is a tendency that trade-off relationship may exist between the touching durability and the low-reflection. So, if it is coated by special ink (such as containing microbeads) to make higher durability, it may also inhibit the low reflection performance of Vantablack, because the ink will cover the micro irregularities on the surface too.
And then, I think the above Stray-light reduction process with touching resistance coating may be around 3% total reflectivity from my experience as I have been treating so far.
For above reasons, VBx2, which has excellent low reflection performance, cannot be expected to have good touching resistance. The catalog also says that it is not suitable to use at the area where people can touch.
Thank you for reading above long sentence. But could you have understood a little furthermore about the mysteries about the Vantablack which is the world's blackest black processing?
Even though, carbon nanotube products are a little difficult to use in general environments, because of the production conditions or health care problems, but they still have full of possibilities.
Now as of September, I wrote this blog, it came a news that "MIT succeeded in developing a carbon nanotube material that is 10 times black than Vantablack!" This news might become a big update on the applications using carbon nanotubes in low reflection use.
I'm looking forward to seeing the splendid future of carbon nanotubes, but please take cared about our Fineshut series using a little old-fashioned technology too. One of our products which is “KIWAMI” is thickness 0.37mm urethane sponge. The total reflectance in visible light range is 0.7%! It has double-side tape together, so, you can easily use it by just cutting and sticking.
If you put the "KIWAMI" on the exterior of the car, you may have a possibility to update the record of the BMW “World's Blackest Car”.