Additives feature | Magnetite
Table 2: Properties of magnetite compared to common fillers such as calcium carbonate and talc
Property Colour Density
Mohs hardness
Attraction to a magnet Electrical conductivity Chemical composition Volumetric heat capacity Microwave heatable Radiation blocking
Typical mineral filler White
2.5-3.0 g/cm3 2-3 No
Insulator
Magnetite Black
5.2 g/cm3 5.5-6 Yes
Conductive
Carbonates and silicates Oxide 2.1 kJ L-1 K-1 No No
3.8 kJ L-1 Yes Yes
K-1 The thermal conductivity of magnetite is 5.1 W m-1
, which is much higher than other mineral fillers such as glass fibre or barium sulphate. Common plastics have low thermal conductivity, in the range of 0.2-0.4 W m-1
K-1 K-1
with magnetite being particularly effective giving composites with values as high as 1 W m-1
K-1 .
found their way into the food from processing machin- ery, for example. Of course, the magnetite would need to be safe for food contact and grades are indeed available with such high purity that they pass US and EU regulations for use in food contact plastics. Adding magnetite helps in production as well. One
example is floor mats for automobiles where the increased density adds vibration damping, while the material’s magnetic properties allow robots to lift and place the mats accurately. Magnetic putty is sold as a novelty item providing a
fun way to experience the properties of magnetite. One well known use of magnetite is in ferro-fluids and magneto-rheological fluids which respond to a magnetic field. These are used in loudspeakers where the magnetic field holds the fluid in place and the high thermal conductivity of the magnetite helps keep the voice coils cool. They are also used to form magnetic seals for pumps, gearboxes, motors and even submarine shafts. Such seals are particularly useful for shafts that are not round where normal mechanical seals do not perform well. Such fluid was first made by NASA where natural magnetite was ground very fine in oil with dispersant added. Nowadays, such fluids are made using synthetic magnetite prepared by a precipitation reaction.
Electrical and thermal conductivity Pure magnetite has marked electrical conductivity and it is classed as a half-metal due to its low band gap. Resistivity is just ~1.0 x 104
mW cm, which opens up new
possibilities for conductive plastic formulations. This has been reported in several polymer types including PP and PA 6 where resistivities as low as 10 kW.m were achieved when the loading was 47 volume %. Applications include EMI blocking and as a permanent, non-migrating, anti-static material. Once magnetite has rendered the plastic conductive, it is then possible to electroplate metals on the surface to make decorative coatings.
66 COMPOUNDING WORLD | August 2015
Thermally conductive plastics are sought after for heat management in electronic devices. Magnetite filled plastics are also used in luxury products such as perfume bottle caps where the magnetite serves multiple roles. It adds heft for a high quality feel and the thermal conductivity provides an “organic” feel that designers appreciate. The high thermal conductivity has been shown to significantly decrease cycle time when moulding parts, thus giving higher productivity. Furthermore, the even distribution of heat prevents hot spots and degradation. Another important thermal property is the heat
capacity, which is the amount of heat required to heat up a material and the amount of heat it can store. Magnetite is used as a “battery” for heat. It can be charged up and then release the heat over an extended period of time. Thus, it has been used for decades in night storage heaters and saunas. In recent years there has been a lot of focus on energy efficient houses. The remarkably high heat capacity of magnetite makes it ideal for buffering temperature fluctuations in build- ings, thereby lowering the heating and cooling costs. It turns out that the heat capacity per unit volume of virtually every solid is the same, so the only way to achieve higher heat storage is magnetite, which has a value some 70% higher than other materials.
Optical properties and safety One glance will tell you that magnetite is black. However, there is more to it than that. Natural magnet- ite fractures during milling to form angular particles. Coarser grades in particular have a metallic sheen and are added to plastics as a form of decoration. One example is in solid surfaces such as kitchen counter tops. The magnetite particles are embedded in an acrylic matrix and confer a black granite-like appearance. One would assume that magnetite filled plastics
would come in any colour you want as long as it’s black. Surprisingly, it turns out that plastics filled with natural magnetite can be coloured over a wide palette of colours by adding dye or pigment. The reason is that magnetite has low tinting strength, i.e. it is a weak pigment. Coarser grades of magnetite are especially suitable for making coloured plastic materials. With regard to material safety, magnetite is present in the cells of organisms and animals including humans.
www.compoundingworld.com
. This can be increased by adding fillers
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84