This page contains a Flash digital edition of a book.
Tribology | technical article Tackling friction with graphite


Replacing metals with plastics is a smart idea, offering benefits including lighter weight, improved durability, reduced corrosion, design flexibility and very often a lower overall production cost. However, to provide a real alternative to metals, polymers typically need to be compounded with suitable functional additives that allow them to fulfill the requirements of each application. Graphite is emerging as a potential multi-functional


filler due to a novel combination of properties such as electrical and thermal conductivity, lubricity, low density, and chemical inertness1


. Moreover, graphite is


available in large quantities and at relatively low prices compared to other functional fillers such as carbon nanotubes, carbon fibres, molybdenum disulphide or boron nitride. Graphite can, for example, increase the thermal conductivity of polymers by two orders of magnitude, allowing the production of plastic heat sinks for applications such as LEDs or geothermal heat exchange pipes. Graphite powders have also been used for many years as a solid lubricant in self-lubricating polymer formulations, either alone or in combination with other fillers such as molybdenum disulphide, PTFE, carbon fibres or glass fibres.2 High loading levels (more than 30wt%) of graphite


are typically necessary to achieve the desired proper- ties. The effect of graphite type, particle size and loading level on tribological properties of polystyrene (PS) has been recently investigated3


and it has been


determined that good results could be obtained using Timrex KS44 primary synthetic graphite at a 30% loading level. Other investigations have indicated that the special high aspect ratio C-Therm graphite can impart high thermal conductivity to polymer compounds at much lower loading levels compared to alternatives


A comparison of the lubricating and wear performance of different graphite-loaded PS compounds shows that thermal conductivity and structure play an important part in tribological modification, according to Raffaele Gilardi


such as KS44 graphite.4 This latest work for the first time examined the


tribological performance of C-Therm versus KS44 in PS (a polymer typically used for tribological applications such as water meter valves). Friction coefficient and wear of compression moulded polymer compounds was evaluated by the ball-on-three-plates method (Anton Paar MCR302 Rheometer equipped with a tribology cell T-PTD 200) using different balls (unhardened steel 1.4401 and polyamide PA66).


Graphite-filled PS against steel ball Figure 1 shows the results of tribological tests with a steel ball rotating on graphite-filled PS plates at a fixed speed (500 rpm corresponding to 0.235 m/s) and varying normal force. For unfilled PS, the friction coefficient increases to around 0.5 at very low forces, and a similar behaviour is observed for PS filled with 10wt% KS44 and 10wt% C-Therm (the latter only slightly better perform- ing). This clearly indicates that a 10wt% graphite


Below: Graphite structure and thermal conductivity both contribute to tribological performance in lubricated polymer compounds, according to research by Imerys


www.compoundingworld.com


November 2016 | COMPOUNDING WORLD 39


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  |  Page 95  |  Page 96  |  Page 97  |  Page 98