materials testing | Equipment Right:
C-Therm’s TCi Gen III Thermal Conductivity
Analyzer is now in its third generation
offers is the flexibility to test a wide range of sample geometries. It cites the example of a large polymer producer that uses the test samples from its tensile testing regime to also test the in-plane and through- plane thermal conductivity using the C-Therm TCi equipment.
Sink opportunities In a recent blog on the C-Therm website, Managing Director Adam Harris wrote that work is increas- ingly being devoted to replacing heavy and expensive metallic heat sinks in electronics applications with lower cost, lighter heat sinks made from thermoplastic compounds containing thermally conductive additives. “However, such materials have historically possessed very high percolation thresholds, the point at which an additive in such a mixture begins to have an appreciable and scalable effect upon the thermal conductivity of the mixture, before which there is little, if any appreciable improvement in thermal conductivity,” he says. Very high loadings of additives (30 wt% or more)
have therefore been needed to achieve the desired thermal conductivity. This can sacrifice mechanical properties, and sometimes the electrical properties and makes the composite heavier. Research in recent years has, therefore, focused on ways of reducing the required additive loading. Harris highlights work carried out by scientists at the Chinese Academy of Forestry and the Chinese Academy of Sciences to improve the thermal conductiv- ity of a polylactic acid/polycaprolactone (PLA/PCL) blend through the addition of graphene. The additive doping requirement was substantially reduced by trapping the graphene material at the interface of the polymer blend.
Harris says that using a C Therm TCi Thermal Conductivity Analyzer, the Chinese researchers were able to study the effect of graphene content on various
Thermal conductivity of PLA/GE, PCL/ GE and PCL/ PLA/GE composite materials as a function of graphene (GE) loading. The percolation threshold is circled
Source: C-Therm 46 COMPOUNDING WORLD | September 2016
biopolymers. The graphene loading was found to improve the thermal conductivity of PLA and PCL, but only marginally. However, once trapped at the interface of the polymer blend, the thermal conductivity of the material substantially improves and resulted in a four fold improvement in the thermal conductivity of the material with merely a 0.5 vol% doping of graphene. “Remarkably, the percolation threshold of the PCL/ PLA/GE blend was 0.11 vol%, representing the lowest percolation threshold for a thermally-conductive polymer composite (TCPC) yet discovered,” Harris says.
Real-life experience Testing materials in controlled laboratory conditions is extremely useful, but those conditions do not always reflect those that materials and finished products will encounter in the world outside. Humidity, for example, can have a significant effect on properties of many plastics, especially polyamides. Increasingly however, test equipment is available that mimics real-life conditions in the lab. Anton Paar highlights an instrument to characterise
single components of compounds (additives and resins), and also the finished compound itself, under controlled humidity conditions. “With a rheometer from the MCR series, combined with special accessories and a humidity cell, information about the production, processing and even the end product can be gained,” the company says. Tensile tests, dynamic mechanical analysis and curing behaviour can all be measured on the same instrument. The curing behaviour of resins under controlled humidity conditions is ideally measured with a “modi- fied ring geometry,” the company says. The sample is applied between two parts that expose a comparatively big part of the sample to humidity. The humidity is created in a special closable chamber. Hard materials such as cured composites can be characterised with the Solid Rectangular Fixture (SRF), which measures torsional oscillatory deformation of a rectangular specimen. In combination with the humidity chamber,
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