thermally conductive | Technical feature
in-plane thermal conductivity higher than 5 W/mK is considered suffi cient to decrease the temperature of LED lamps to acceptable values [6].
Carbon nanotubes and conductive carbon blacks have been shown to be not very suitable for reaching
high (>2 W/mK) thermal conductivity levels in thermo- plastics. Among the suggested reasons for the low
Figure 1: Through-plane thermal conductivity of carbon black (E250) and graphite (KS44) loaded HDPE at different concentrations Source: Imerys Graphite & Carbon
thermal conductivity of the resulting polymer com- pounds are the high thermal contact resistance of the particle/polymer and particle/particle interface and the large number of contacts per unit volume of nanoaddi- tives [2]. Other drawbacks of carbon nanotubes in thermally conductivity modifi cation applications are the low maximum concentrations achievable (due both to the increase of viscosity and the high cost of CNTs). When CNTs are loaded at a concentration above 15-20% the viscosity of the compound can be so high that degradation of the polymer will take place during processing. In the case of carbon black, this limit is much higher (above 40% w/w depending on the structure of the carbon black) but, nevertheless, a low thermal conductivity is achieved with such material. Graphite, on the other hand, not only has fewer
contacts, due to the larger particle size, but can be also loaded at much higher concentration (above 50% w/w) to give the best overall thermal conductivity achievable in polymeric compounds (see Figure 1).
Figure 2: In-plane and through-plane thermal conductivity of graphite loaded HDPE compression molded plaques Source: Imerys Graphite & Carbon
Compound properties are highly dependent on the graphite grade selected
Graphite is an anisotropic material and, due to the different thermal conductivity in the different graphite crystalline directions and to its intrinsic fl aky morphol- ogy, compounds made with graphite are extremely anisotropic (see Figure 2). As a consequence, fi nal compound processing greatly affects the degree of orientation and determines the overall material performance. Compression moulding reduces the degree of orientation and decreases the fi nal material anisotropy while injection moulding increases the in-plane component (shown in Figure 3).
It is also important to note that various available
graphite grades may result in different properties; char- acteristics such as purity, particle size distribution, aspect ratio, degree of crystallinity and crystalline texture all play an important role on fi nal compound thermal conductivity. Figure 4 shows the thermal conductivities (in-plane and through plane) of four HDPE compounds produced with different grades of Imerys graphite and injection moulded into 2mm thick plaques. The material made with Timrex SFG graphite has a higher in-plane thermal conductivity than the material made with the KS grade. This is due to the higher crystallinity and particle aspect ratio of the SFG grade compared to the KS grade, as well as the higher degree of orientation of the graphite crystals within the
58 COMPOUNDING WORLD | February 2016
www.compoundingworld.com
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 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94