Additives feature | Magnetite
Table 1: Using magnetite is effective way to increase the density of a polymer compound Mineral Weight (%)
Volume (%) Density (g/cm3
CaCO3 Talc
Magnetite
CaCO3 Talc
Magnetite
40 40 40
68 68 80
18.2 18.2 10.3
41 41 41
1.23 1.23 1.34
1.64 1.64 2.66
) Density
Increase (%) 37 37 49
82 82
196
Blocking radiation Magnetite can also be used as a radiation blocking filler in plastics. Unusually, it blocks several different types of radiation including x-rays, gamma rays, microwaves, and radar. Magnetite is able to block so many types of radiation because of its unique combination of several disparate properties. For example, blocking x-rays or gamma rays relies on density; the higher, the better. Magnetite has been used for decades as aggregate
in concrete for the radiology units of hospitals where magnetite aggregate allows walls to be made 40% thinner compared to standard concrete while still providing effective blocking. So, using it to do the same thing in plastic is a natural progression. The unusual microwave and radar blocking efficacy
Magnetite processing at LKAB Minerals
does not rely on density but on a totally different property, namely electrical conductivity. Materials with very high or low electrical conductivity do not absorb microwaves. Highly conductive materials such as metals reflect microwaves. Good insulators, like plastics, allow microwaves to pass straight through unimpeded. For effective blocking by absorption, one needs an intermediate level of electrical conductivity. Magnetite, classified as a semi-metal with a band-gap
of just 0.1 eV has just the right level of electrical conductivity to provide outstanding absorption. Which brings us to the next topic…
Microwave and induction heating A study by the US Department of the Interior showed that magnetite heated quickest of 160 materials tested in a microwave oven. Other researchers wondered why magnetite is so very effective at adsorbing microwaves and converting that energy to heat. They performed a detailed study and discovered the reason. Microwaves, like other forms of electromagnetic
radiation, consist of an electric and a magnetic component. Uniquely, magnetite has a combination of moderate electrical conductivity plus magnetic properties so is able to absorb both the electric and magnetic components of the electromagnetic energy. One of the oldest applications is foot warmers where
magnetite filled elastomer is used as an insole. Just 30 seconds in the microwave heats the insole due to the microwave susceptibility provided by the magnetite. In addition, the exceptionally high specific heat capacity of magnetite means more heat can be stored and then released to ensure warm feet. A newer application is in pot-hole repair. In colder
climates, it is difficult to achieve satisfactory results using conventional repair technology because the warm bitumen cools too quickly so that proper wetting and adhesion to the road is not possible. It has been demon- strated that adding magnetite to the pot-hole repair compound allows it to be microwave heated in-situ. This allows the entire area to get warm which in turn provides better adhesion and extends the durability of the repair. Future developments are expected to include
magnetite in plastic cookware. This would allow induction heatable plastic cookware, speed up heating in microwave ovens and also help heat transfer when cooking in conventional ovens.
Magnetic attractions Magnetite is known for and named after, its magnetic properties. As a ferri-magnetic material, it is not actually magnetic itself, but it is attracted to a magnet. This effect is exploited in many applications. For example, magnetite paint is used in the home or
office to allow magnets to stick to the wall. Surprisingly, although magnetite is black, its pigmenting strength is low, so it is still possible to colour such paints by adding pigment or dye. Magnetite can be added as a tracer to plastics to
make the material detectable. This is useful in the food processing industry when one needs to be able to find and remove any plastic contaminants that may have
64 COMPOUNDING WORLD | August 2015
www.compoundingworld.com
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