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The measurement of the thermal conductivity of amorphous polymers above glass transition temperatures.

Dawson, A; Rides, M; Allen, C R G (2007) The measurement of the thermal conductivity of amorphous polymers above glass transition temperatures. NPL Report. MAT 7

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The polymer processing industry is sensitive to low labour cost competition. The industry needs to remain cost effective by improving efficiency when processing polymers into existing and newly developed products. A reduction in cycle times during the manufacture of an injection moulded product would lead to increased output rate of the final product and increased profit margins. Modelling is used to predict cooling times during the injection moulding cycle and to reduce them through improvements in mould design leading to shorter cycle times. For these models to be reliable then the data input into the model has to be accurate. Polymers have low thermal conductivities and because of this, heat transfer data is key to predicting accurate cycle times.

The thermal conductivity behaviour of amorphous polymers at temperatures above the glass transition temperature (Tg) for the polymer is not well described. In the literature, different experimental techniques e.g. the line source probe and the hot plate technique produce conflicting data giving conflicting trends describing thermal conductivity behaviour above Tg nominally for the same polymer. A novel measurement instrument, the heat transfer coefficient (HTC) apparatus, has been designed and built in order to attempt to resolve the measurement issues for thermal conductivity of amorphous polymers above Tg. The instrument has an adjustable upper plate to allow good thermal contact with the specimen across a wide range of temperatures.

Thermal conductivity measurements have been made on poly(methyl methacrylate) (PMMA) to determine the repeatability and precision of the HTC instrument. The repeatability for PMMA tested at set temperature 60°C was determined as 1.5 %. The value compares with a repeatability of 8 % for the line-source probe technique. For PMMA at 60°C the mean thermal conductivity value of 0.189 W/(m.K) compares with an accepted standard value for the specimen of 0.192 W/(m.K) differing from the known specimen value by 1.6 %. Therefore the HTC instrument could be used to establish reliable thermal conductivity data for modelling predictions of cooling time during the injection moulding of amorphous polymers.

Experimental data used to form the current model predicting thermal conductivity behaviour for amorphous polymers above the Tg was reviewed. Thermal conductivities of PC and PS were measured from 53°C to 180°C and show an increase in thermal conductivity with temperature above the Tg for both polymers. This trend is in agreement with data from the line source probe technique. A model predicting an increase in thermal conductivity with increase in temperature above Tg was reviewed.

Item Type: Report/Guide (NPL Report)
NPL Report No.: MAT 7
Keywords: Polymer, amorphous, glass transition temeprature, themal conductivity, steady state, non-steady state, line source probe
Subjects: Advanced Materials
Advanced Materials > Thermal Analysis of Materials
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4116

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