According to Cosmetics Historian James Bennett: “In the early 1930s Lady Esther became the top-selling, loose face powder in the United States, a position it maintained for the rest of the decade” (Bennett, 2018) She created that position by promoting her product as the very finest and confirming it with a home test: “Take a pinch of your powder and place it between your front teeth. Bring your teeth down on it and grind firmly. If there is any trace of grit in the powder it will be as instantly detectable as sand in spinach.”(Lady Esther advertisement, 1935)
This was so successful that competitors needed to find new processes and new materials in order to pass the “bite test”. In 1937 chemists started to do research on particle size required to pass the test. One such chemist, M. L. Smith published the results of his experiments on volunteers. As reported by deNavarre, Smith determined that the particles would need to be 12 microns (0.012 mm) or less to pass the test (deNavarre, 1941, p. 354). This was much smaller than the particles produced by the mixing and sifting methods then used by most powder manufacturers. (Bennett, 2018)
“It must be remembered that a “fine powder” is, of course, a relative term. A generation of so ago “200 mesh” was considered very fine. Nowadays a 50-75 micron particle seems rather coarse, and a material needs to be below 10 microns in size to be thought of as an “impalpable” powder. Perhaps in another twenty years even particles of 10 microns might be thought of to be boulder-like, and the public demand may be for something finer still.”
(Hibbott, 1947, p. 271) (Bennett, 2018)
Jet Milling, An Introduction.
Jet mills are particle size reduction equipment which use high pressure compressed air to grind materials to single digit microns. Size reduction is the result of high velocity collisions between the particles of the process material itself. The interior of the chamber is designed to allow re-circulation of the over-size particles, enhancing the incidence and the effect of these collisions. As particles are reduced in size and progressively lose mass, they naturally migrate toward the central discharge port, making precise classification both automatic and precisely controllable. By creating a one step process in milling and classifying, jet mills have continued to increase their presence in new and exciting markets.
There are multiple advantages of using a jet mill to grind dry powders over other particle size reduction technologies. Jet Milling offers:
1. Finest milling and tightest particle size distribution of any dry powder methods;
2. Low contamination due to absence of media, blades, knives or screens; and
3. No temperature rise in the mill due to expanding air creating a cooling effect, especially important for temperature sensitive materials, such as cosmetics.
Jet mills can be powered using many different compressed gases all starting at around 100 PSIG. Commercially compressed air is by far the most commonly used gas but super-heated steam (no moisture is present in the super-heat state) is used in very large installations grinding primarily titanium dioxide pigment. Nitrogen is commonly used when a material must be ground in an inert atmosphere to protect from oxidation and possible fire or even explosion of the product. Argon has also been used for this purpose if the less expensive nitrogen was unsuitable. Light gases such as helium have been experimented with because of the possibility of even higher velocity impacts.
Typically a well designed jet mill will grind friable or crystalline materials down to the 1 to 10 micron average particle size range. Some products, such as some molybdenum compounds, paint pigments and similar products, can be reduced to particles as small as 200 nanometers. Work in the nano size range can also be the deagglomeration of nano size particles. Particles larger than 10 microns are generally hard-to-fracture polymers such as toner compounds or hard waxes, and some organic materials, but if a larger size is wanted, many products can be ground larger than 10 microns by reducing power to the mill or by increasing the rate of feed. Some products are simply polished to remove sharp edges by running at elevated feed rates with low grinding pressure, a process that changes how the material compacts.
Many different methods of milling will claim to achieve less than 10 microns average particle size, but the particle size distribution is what sets the jet mill apart form its competitors. Below are two images, one of a ball milled material, and the other of a jet milled material. One notices that the main curve moves to the left (decreases in size) but also that the distribution narrows. This is particularly important for high value material, where process yield is very important.
Cosmetics Application: The Solution.
“Pulverizers” have, since the 1930s become de rigueur equipment for cosmetic manufacturers to meet their clients’ demand for the finest powders possible. Coty, as early as 1935, started a line of face powders which were milled using air mills, as opposed to mechanical mills, reflected in the brand name “AirSpun” in direct response to the Lady Esther “Bite Test”. (deNavarre, 1941)
Not long thereafter, jet mill companies began designing, engineering and manufacturing fine particle Jet Mills in the 1940s and continued to deliver fine, lustrous materials to the cosmetics markets As Mr. Hibbott had predicted in 1947, cosmetics manufacturers are continuously searching for ever finer particle sizes to meet their clients’ demands and Jet has been there to meet them. Well-designed jet mills can achieve the following particle size results:
Finally, the throughput in both quantity and quality that can expected out of a jet mill allows for excellent production scalability. From a test on a 2 inch jet mill, allowing up to 15 pounds per hour, all the way up to a 36 inch jet mill, producing multiple tons per hour, the jet mill can succeed in research and development laboratories, pilot plants and large scale production facilities. These pieces of particle size reduction equipment can be paired for capacity or redundancy, allowing for increased flexibility in your production plant. The limiting factors, besides virgin material, of course, will be available compressed air, and dust collection systems.
Bibliography
Bennett, J. (2018, 5 11). The Bite Test. Retrieved 5 11, 2018, from Cosmetics and Skin: http://cosmeticsandskin.com/cdc/bite-test.php
deNavarre, M. (1941). The chemistry and manufacture of cosmetics. Boston: D Van Nostrand Company.
Hibbott, H. (1947). Particle size relationships in face powders. The American Perfumer & Essentional Oil Review, 269-271.
Jet Pulverizer Company
The Jet Pulverizer Company is committed to continuous technological advancement of its Micron-Master© Jet Mills and expansion of its Custom Grinding Division to provide customers with value and quality in both equipment and particle size reduction services, specializing in Chemicals, Abrasives, Cosmetics, 3D Printing and Pharmaceutical markets.
