Heat Transfer in Polymers

Heat Transfer in Polymers Martin Rides, Crispin Allen, Angela Dawson and Simon Roberts 19 October 2005

Heat Transfer in Polymers - Summary Introduction Thermal Conductivity Heat Transfer Coefficient Industrial Demonstrations: Corus, Zotefoams Standards for Thermal Properties Measurement Outline of Heat Transfer Project 2005-08 Summary AOB

Heat Transfer in Polymers Heat transfer is: key to polymer processing still inadequately understood key to increasing throughput - process times dominated by the cooling phase significant in affecting product properties, e.g. warpage, inadequate melting, thermal degradation

Aim of the project To help companies measure and model heat transfer in polymer processing To enable measurement of more accurate data and encourage the use of improved models for numerical simulation software This should lead to: Right first time design Reduce cycle times / higher productivity (faster processing) Energy saving Fewer failures in service Reduce waste- e.g. reduce warpage and hot spots during processing Resulting in reduced costs and improved quality

Tasks in the DTI Project Heat Transfer Coefficient New facility Thermal Conductivity Uncertainty analysis Extension of method to new materials Simulation To identify the important data To help design equipment Moldflow & NPL s own software Industrial Demonstrations Zotefoams Corus Dissemination Web site, IAGs, PAA Newsletter articles, trade press articles, measurement notes, scientific paper

Related Eureka Project: AIMTECH An associated Eureka project (AIMTECH) is progressing Its aim is to improve productivity of injection moulding Main focus is on the moulds - reduce cycle times by using copper alloy moulds in injection moulding NPL Role Measurement of the thermal conductivity of polymer melts (T,P) Understanding the role of the mould/melt interface: Modelling heat transfer and the effect of uncertainties Six UK companies involved 25k co-funding contribution Close fit with the DTI project

Thermal conductivity measurements

Line source probe apparatus Thermal conductivity probe Sample Measurement cylinder Guiding bar Heater bands Lower Piston Distance holder Measures thermal conductivity at industrially relevant processing pressures and temperatures

Thermal conductivity HDPE at atmospheric pressure and 1000 bar 0.50 0.45 Repeatability (95% confidence level) of thermal conductivity test data for one operator testing HDPE HCE000 from 170 C to 50 C at 1000 bar pressure (green) and at ambient pressure (blue) 0.40 Thermal Conductivity, W/mK 0.35 0.30 0.25 0.20 0.15 1000 bar repeatability 8% ambient pressure repeatability 16% 0.10 40 60 80 100 120 140 160 180 Temperature, C

Thermal conductivity Amorphous polystyrene (PS) under pressure 0.35 0.33 0.31 Thermal conductivity of polystyrene (AAATK002) on cooling from 250 C to 50 C at pressures of 200, 800 and 1200 bar Thermal conductivity, W/mK 0.29 0.27 0.25 0.23 0.21 0.19 0.17 0.15 200 bar 800 bar 1200 bar 0 50 100 150 200 250 300 Temperature, C

Thermal conductivity Semi-crystalline polypropylene (PP) under pressure Thermal Conductivity, W/mK 0.35 0.33 0.31 0.29 0.27 0.25 0.23 0.21 0.19 0.17 0.15 Thermal conductivity of polypropylene (AAATK004) on cooling from 250 C to 50 C at pressures of 200, 800 and 1200 bar 200 bar 800 bar 1200 bar 0 50 100 150 200 250 300 Temperature, C

Thermal conductivity: implications of results Increase in pressure gives increase in thermal conductivity and increase in crystallisation temperature for semi-crystalline polymers Thermal conductivity data strongly dependent on both pressure and temperature: 50% over range 50 C to 250 C 43% over range 200 bar to 1200 bar Implies single-point thermal conductivity value may be in error by up to 50% Important to use pressure and temperature dependent thermal conductivity data for accurate process modelling will improve, for example, warpage and hot-spot prediction More accurate data based on industrial processing conditions - improvements in commercial modelling packages - possible cost benefits

Paper accepted by Polymer Testing The Effect of Pressure on the Thermal Conductivity of Polymer Melts A. Dawson, M. Rides and J. Nottay Department of Engineering and Process Control, National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK http://www.npl.co.uk/materials/polyproc/iag

Heat transfer equipment for measurement of thermal conductivity and heat transfer coefficient

Heat Transfer Coefficient It is the heat flux per unit area (q) across an interface from one material of temperature T 1 to another material of temperature T 2 : Boundary condition for process simulation h = q/(t 1 T 2 ) units: Wm -2 K -1 In injection moulding & compression moulding Polymer to metal Polymer-air-metal (GASM, ) In extrusion & film blowing Polymer to fluid (eg air or water) This project has built apparatus to measure heat transfer coefficient and will investigate the significance of different interfaces to commercial processing

Heat Transfer Coefficient (heat transfer across an interface) Features of apparatus Room temperature to 275 C, pressure to at least 500 bar Polymer samples 2 mm to 25 mm thick Interchangeable top plate to investigate Different surface finishes Effect of mould release agents Option to introduce a gap between polymer & top plate Shrinkage, sink marks Instrumented with temperature measurement devices and heat flux sensors

Heat transfer apparatus Side view cold plate sample hot plate

Heat transfer apparatus fibre optic thermocouple port displacement transducer heater element PTFE seal mould face air gap sample thermocouple ports outer guard ring cooling pipes thermocouples heat flux sensors pressure transducer port

Heat transfer: thermal resistance model When materials are place in series, their thermal resistances R i (m²k/w ) are added R = δt 1 Q h = + i i j x λ j j T + δτ x 1 x 2 x 3 j = 1 j = 2 j = 3 λ 1 λ 1 λ 1 i = 1 i = 2 i = 3 i = 4 h 1 h 2 h 3 h 4 h heat transfer coefficient, W/(m 2.K) λ thermal conductivity, W/(m.K) x thickness, m Q T

Heat transfer 160 0.003 140 T/C Bottom T/C Top Flux Top HTC001 0.0025 120 Temperature, C 100 80 60 0.002 0.0015 0.001 Heat flux, V 40 20 PMMA sample introduced Air gap introduced 0.0005 0 700 900 1100 1300 1500 1700 1900 Time 0

Heat transfer 80 0.0025 Temperature, C 70 60 50 40 30 T/C Bottom T/C Top Flux Bottom 0.002 0.0015 0.001 0.0005 Heat flux sensor, V 20 0 10 Heaters on PMMA, 3 mm Air gap PMMA, 30 mm 0-0.0005 10:48:00 12:00:00 13:12:00 14:24:00 15:36:00 16:48:00 18:00:00 Time

Thermal conductivity measurement Temperature, C 90 80 70 60 50 40 30 20 10 T/C Top T/C Bottom Flux Bottom No specimen Polystyrene Line source method = 0.185 W/(m.K) at 20 MPa & 70 C Heat transfer equipment derived value = 0.170 W/(m.K) Polystyrene specimen 0.0014 0.0012 0.001 0.0008 0.0006 0.0004 0.0002 Heat flux sensor, volts 0 09:36 10:48 12:00 13:12 14:24 15:36 Time 0

Heat transfer coefficient: effect of specimen thickness Specimen thickness, mm 1.03 2.15 3.02 Thermal conductivity, W/(m 2.K) 0.25 0.28 0.27 T/C Top T/C Bottom Flux Bottom Temperature, C 90 80 70 60 50 40 30 20 10 0 PTFE, 1 mm PTFE, 2 mm PTFE, 3 mm HTC044 09:36 10:48 12:00 13:12 14:24 15:36 16:48 Time 0.0025 0.002 0.0015 0.001 0.0005 0-0.0005 Heat flux sensor, volts

Heat transfer coefficient: effect of air gaps Air gap, mm 0.363 0.725 1.088 Equivalent polymer thickness, mm 3.0 5.8 11.1 Thermal resistance, (m²k)/w 0.015 0.030 0.057 T/C Top T/C Bottom Flux Bottom Temperature, C 90 Start of test HTC045 0.0014 80 0.0012 70 0.001 60 50 0.0008 40 0.0006 30 0.72 mm 0.0004 20 0.36 mm 0.0002 10 1.09 mm air gap PMMA sample 0 0 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00 Heat flux sensor, voltage Time

Heat transfer coefficient: effect of air gap Equivalent polymer thickness, mm. 12 10 8 6 4 2 Equivalent polymer thickness, mm Thermal resistance, (m²k)/w 0.06 0.05 0.04 0.03 0.02 0.01 Thermal resistance, m²k/w 0 0 0 0.2 0.4 0.6 0.8 1 1.2 Air gap thickness, mm

Numerical modelling Effect of an air gap Effect of vertical thermocouple on distort the temperature field

Air Gap Mould at 50 C with air gap of 0, 0.5 & 1 mm Polymer at 250 C

Heat Transfer Equipment Summary Measurements of thermal conductivity of polymers yielding reasonable values. However, need to address signal noise and improve data analysis procedure further validate instrument through thermal conductivity measurements Effect of air gap on thermal resistance quantified

INDUSTRIAL TRIALS Corus & Zotefoams

Industrial Demonstrations Aim is to investigate DSC method and demonstrate practical benefits of heat transfer measurements and modelling Corus Thermal conductivity of plastisol coated steel before and after solidification Zotefoams Heat transfer during cooling of polyolefin foam

Corus Use DSC method to measure thermal conductivity of bilayer Plastisol/steel Measure before and after solidification Data useful in predicting optimum line speeds Earlier work had shown that the polymer layer was significant in terms of heat transfer

DSC method for thermal conductivity DSC pan Indium or eutectic Heat Flow (mw) 65 60 55 50 Heat Transfer for sapphire m K x m x = K i m i 2 h h x i Mulitlayer sample (Khanna et al (1988)) where m is the gradient, h is sample thickness 45 145 150 155 160 165 170 175 Temperature ( C)

DSC method for thermal conductivity Thermal TEST ID Specimen ID Specimen gradient Specimen thickness conductivity value - literature Measured thermal conductivity* Error mw/ C mm W/mK W/mK DSC098 PTFE + Indium 2.11 1.026 0.25 0.27 9% DSC089 PTFE + Indium 2.34 1.027 0.25 0.33 34% DSC087 PTFE + Indium 2.48 1.030 0.25 0.38 51% DSC088 PTFE + Indium 2.37 1.030 0.25 0.34 38% average PTFE + Indium 2.32 1.03 0.25 0.33 33% 95% confidence $ 0.09 DSC092 PTFE + Indium 1.77 1.693 0.25 0.32 27% DSC090 PTFE + Indium 1.75 1.767 0.25 0.32 29% DSC099 PTFE + Indium 1.61 1.769 0.25 0.27 10% DSC091 PTFE + Indium 1.77 1.780 0.25 0.33 33% average PTFE + Indium 1.73 1.75 0.25 0.31 25% 95% confidence $ 0.05 DSC093 ref. PMMA + Indium 1.60 1.208 0.18 0.18 3% DSC110 ref. PMMA + Indium 1.64 1.208 0.18 0.19 7% DSC111 ref. PMMA + Indium 1.51 1.208 0.18 0.16-9% average PMMA + Indium 1.58 1.21 0.18 0.18 1% 95% confidence $ 0.01 *Calculated assuming thermal conductivity of 0.18 W/(mK) for PMMA reference sample $ 95% confidence = 2 x standard deviation

DSC method for thermal conductivity 0.4 Thermal conductivity, W/(m.K) 0.3 0.2 0.1 y = -0.0284x + 0.3613 R 2 = 0.1077 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Specimen thickness, mm

DSC method for thermal conductivity Essential to use reference specimen of similar thermal conductivity as specimen to be measured Repeatability 95% confidence levels up to approximately 25% Essential to carry out several repeat tests to improve on statistical significance of data Not appropriate for testing molten/liquid samples (e.g. plastisols) having constant and known sample thickness

Foams Problem: waviness in foams thermal issue Model heat transfer Measure (T, heat flux) over time Measure shrinkage Use to predict curvature

Foams Problem: waviness in foams thermal issue

Heating & cooling of foam block 120 100 Forced air heating curve Non-forced cooling curve Temperature, C 80 60 40 20 0 17:42 18:00 18:18 18:36 18:54 19:12 19:30 19:48 Time, hr min

Numerical modelling of cooling of foam block λ = 0.004 W/m.K ρ = 16 kg/m 3 C p = 1555 J/kg.K 27 C 140 C 27 C 0.04 m

Foams Initial observations and results suggest: uneven cooling of foam sheet resulting in variation in shrinkage with position causing waviness of sheets Industrial trial to be performed 20 21 October 2005 Numerical modelling trials ongoing

Standards for Thermal Properties Measurement of Plastics

Differential scanning calorimetry standards ISO TC61 SC5 WG8 Thermal Properties ISO 11357 Plastics - Differential scanning calorimetry (DSC) ISO 11357-1: 1997 Part 1: General principles (being revised) ISO 11357-2: 1999 Part 2: Determination of glass transition temperature ISO 11357-3: 1999 Part 3: Determination of temperature and enthalpy of melting and crystallization ISO 11357-4: 2005 Part 4: Determination of specific heat capacity ISO 11357-5: 1999 Part 5: Determination of characteristic reaction-curve temperatures and times, enthalpy of reaction and degree of conversion ISO 11357-6: 2002 Part 6: Determination of oxidation induction time ISO 11357-7: 2002 Part 7: Determination of crystallization kinetics

Plastics thermal conductivity standards ISO TC61 SC5 WG8 Thermal Properties ISO 22007 Plastics Determination of thermal conductivity and thermal diffusivity ISO/NWIP 22007-1 Part 1: General principles ISO/CD 22007-2 Part 2: Transient plane source hot-disc method (Gustafsson) ISO/CD 22007-3 Part 3: Temperature wave analysis method ISO/CD 22007-4 Part 4: Laser flash method

Plastics thermal conductivity standards Proposal to develop Line Source Method for Thermal Conductivity as part of ISO 22007 series Method currently standardized as: ASTM D 5930-01, Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique However this does not make provision for: effect of applying pressure to minimize measurement scatter, and effect of pressure on thermal conductivity ISO TC61 SC5 WG8 Thermal Properties.

Plastics thermal conductivity standards Proposed intercomparison of thermal conductivity methods To be carried out, in support of standardisation activity, including at least: Transient plane source hot-disc method (Gustafsson) Temperature wave analysis method Laser flash method Line source probe To be led by NPL

Summary Heat Transfer Heat transfer coefficient apparatus now being used Design assisted by numerical modelling studies Effect of uncertainties investigated (report available) Melt thermal conductivity measurements Nano-filled materials Powders/granules Effect of pressure Effect of uncertainties investigated (report available) ISO Standards being developed New IAG members facility on website http://www.npl.co.uk/materials/polyproc

The next 6 months Complete commissioning trials on heat transfer coefficient equipment Complete industrial demonstrations (Corus & Zotefoams) Dissemination of thermal conductivity and heat transfer coefficient measurement work

Heat Transfer Project 2005-08

Heat transfer project H1 2005-08 H1: Measurement methods for heat transfer properties data for application to polymers Objectives: Development of the method for the measurement of heat transfer properties across surfaces (particular interest has been expressed in the effect of the solid/air interface) Industrial case study to demonstrate the value of reliable heat transfer data Support development of standards for measurement of thermal properties of plastics, including an intercomparison of thermal conductivity methods that are being proposed for standardisation Assessment of uncertainties in heat transfer data and effect on modelling predictions Development of a new user-friendly web-enabled modelling facility, to facilitate industrial adoption of the above

Heat Transfer Coefficient - future activity Investigate effect of: Air gap between polymer & top plate (simulating shrinkage and sink marks) (Cinpres) and compare with numerical modelling predictions Different surface finishes/mould materials (RAPRA) Effect of mould release agents

Potential areas of relevance for future work to increase understanding of heat transfer: Effect of air gaps (EGM) Effect of mould materials (RAPRA) Gas assisted injection moulding (GAIM) Water assisted injection moulding (WAIM) Additives, fillers e.g. effect of nano-particles on thermal conductivity Micro-moulding

Potential areas of relevance for future work to increase understanding of heat transfer: Effect of nano-particles on heat transfer in polymers Effect of dispersion of nano-particles on heat transfer Measurement of heat transfer within micro-fluidic systems Developing techniques for measuring heat transfer properties of foams Curing of fibre/matrix composites and cross-linking of rubbers

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