Wednesday, 6 September 2017

Boron Nitride

What is boron nitride? What makes it a good separator?

Boron nitride is a heat resistant refractory compound of boron and nitrogen with the chemical formula BN. It is also chemically stable at elevated temperatures.  It exists in various crystalline forms that are similar to a structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN forms.  It is the form most useful in kiln forming as a smooth release separator, especially for steel.  It is also used as a high temperature lubricant, and has a wide use in cosmetic products.

There is a cubic form that is similar to diamond (called c-BN), but softer.  It has a superior thermal and chemical stability.  There is a harder form called wurtzite, but which is rare. Neither of these is of much use in kiln forming.

Hexagonal BN
Hexagonal BN (h-BN) is the most widely used form of boron nitride. It is a good lubricant at both low and high temperatures (up to 900C, even in an oxidizing atmosphere). Another advantage of h-BN over graphite is that its lubrication properties do not require water or gas trapped between the hexagonal sheet layers. So, h-BN lubricants can be used even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads.  In kiln forming, the high temperature lubricating properties are made use of as separator between metal, ceramic and other supporting materials for the glass.

“Hexagonal BN was first used in cosmetics around 1940 in Japan. However, because of its high price, h-BN was soon abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.”

It has wide application in materials to give them self-lubricating properties.  Boron nitride has the properties of stabilisation of materials, reducing expansion and resistance to electrical conduction, making for wide use in plastics and electronics among a wide variety of other products.

Health and Safety
There are some health issues related to its use.  It is reported to have a weak association with the formation of fibrous material in the lungs and so result in pneumoconiosis when inhaled in quantity in particulate form.  It is best to wear a dust mask when applying and to do it outdoors, as simple ventilation will not prevent dust settlement indoors.