Heat Transfer in Polymers - National Physical Laboratoryresource.npl.co.uk/.../iag/october2005/heat_transfer_in_polymers.pdf · Heat Transfer in Polymers. Aim of the project • To

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 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

Heat Transfer in Polymers

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 processingResulting 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

Sample Measurementcylinder

Guiding bar

Distance holderLower Piston

Thermalconductivityprobe

Heater bands

Measures thermal conductivity at industrially relevantprocessing pressures and temperatures

Thermal conductivity

HDPE at atmospheric pressure and 1000 bar

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)

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

40 60 80 100 120 140 160 180Temperature, °C

1000 bar repeatability 8%

Therm

al Co

nduc

tivity

, W/m

K

ambient pressure repeatability16%

Amorphous polystyrene (PS) under pressure

Thermal conductivity of polystyrene (AAATK002) on cooling from 250°C to 50°C at pressures of 200, 800 and 1200 bar

0.15

0.17

0.19

0.21

0.23

0.25

0.27

0.29

0.31

0.33

0.35

0 50 100 150 200 250 300Temperature,°C

200 bar800 bar1200 bar

Ther

mal

con

duct

ivity

, W/m

K

Thermal conductivity

Semi-crystalline polypropylene (PP) under pressure

Thermal conductivity of polypropylene (AAATK004) on cooling from 250°C to 50°C at pressures of 200, 800 and 1200 bar

0.15

0.17

0.19

0.21

0.23

0.25

0.27

0.29

0.31

0.33

0.35

0 50 100 150 200 250 300Temperature,°C

200 bar800 bar1200 bar

Ther

mal

Con

duct

ivity

, W/m

K

Thermal conductivity

• 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 barImplies 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

Thermal conductivity: implications of results

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

Paper accepted by‘Polymer Testing’

Heat transfer equipmentfor measurement of thermal conductivity

andheat transfer coefficient

Heat Transfer Coefficient

• It is the heat flux per unit area (q) across an interface from one material of temperature T1 to another material of temperature T2 :

h = q/(T1 – T2) units: Wm-2K-1

• Boundary condition for process simulation

• 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

hot plate

cold plate

sample

Heat transfer apparatus

displacement transducer

cooling pipes thermocouples heat flux sensors pressure transducer port

mould face

heater element

sample

air gap

outer guard ring

PTFE seal

thermo-couple ports

fibre optic thermocouple port

displacement transducer

cooling pipes thermocouples heat flux sensors pressure transducer port

mould face

heater element

sample

air gap

outer guard ring

PTFE seal

thermo-couple ports

fibre optic thermocouple port

Heat transfer: thermal resistance model

∑∑ +==j j

j

i i

xhQ

TRλ

δ 1

When materials are place in series, their thermal resistances Ri (m²K/W ) are added

i = 2

i = 3

i = 4

i = 1

j = 3

j = 2

j = 1x1

x2

x3

Q

T

T + δΤh1

h2

h3

h4

λ1

λ1

λ1

i = 2i = 2

i = 3i = 3

i = 4i = 4

i = 1i = 1

j = 3

j = 2

j = 1x1

x2

x3

Q

T

T + δΤh1

h2

h3

h4

λ1

λ1

λ1

h – heat transfer coefficient, W/(m2.K)λ – thermal conductivity, W/(m.K)

x – thickness, m

Heat transfer

0

20

40

60

80

100

120

140

160

700 900 1100 1300 1500 1700 1900Time

Tem

pera

ture

, °C

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

Hea

t flu

x, V

T/C Bottom T/C Top Flux Top

PMMA sample introduced Air gap introduced

HTC001

Heat transfer

0

10

20

30

40

50

60

70

80

10:48:00 12:00:00 13:12:00 14:24:00 15:36:00 16:48:00 18:00:00Time

Tem

pera

ture

, °C

-0.0005

0

0.0005

0.001

0.0015

0.002

0.0025

Hea

t flu

x se

nsor

, V

T/C BottomT/C TopFlux Bottom

Heaters on PMMA, 3 mm Air gap PMMA, 30 mm

Thermal conductivity measurement

0

10

20

30

40

50

60

70

80

90

09:36 10:48 12:00 13:12 14:24 15:36Time

Tem

pera

ture

, °C

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

Hea

t flu

x se

nsor

, vol

ts

T/C TopT/C BottomFlux Bottom

No specimen

Polystyrene specimen

PolystyreneLine source method = 0.185 W/(m.K) at 20 MPa & 70 °CHeat transfer equipment derived value = 0.170 W/(m.K)

Heat transfer coefficient: effect of specimen thickness

0

10

20

30

40

50

60

70

80

90

09:36 10:48 12:00 13:12 14:24 15:36 16:48Time

Tem

pera

ture

, °C

-0.0005

0

0.0005

0.001

0.0015

0.002

0.0025

Hea

t flu

x se

nsor

, vol

ts

T/C Top T/C Bottom Flux Bottom

HTC044

PTFE, 1 mm PTFE, 2 mm PTFE, 3 mm

Specimen thickness, mm 1.03 2.15 3.02Thermal conductivity, W/(m2.K) 0.25 0.28 0.27

Heat transfer coefficient: effect of air gaps

0

10

20

30

40

50

60

70

80

90

07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00Time

Tem

pera

ture

, °C

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

Hea

t flu

x se

nsor

, vol

tage

T/C Top T/C Bottom Flux Bottom

HTC045Start of test

PMMA sampleair gap

0.36 mm

0.72 mm

1.09 mm

Air gap, mm 0.363 0.725 1.088Equivalent polymer thickness, mm 3.0 5.8 11.1Thermal resistance, (m²K)/W 0.015 0.030 0.057

Heat transfer coefficient: effect of air gap

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1 1.2Air gap thickness, mm

Equ

ival

ent p

olym

er th

ickn

ess,

mm

.

0

0.01

0.02

0.03

0.04

0.05

0.06

Ther

mal

resi

stan

ce, m

²K/W

Equivalent polymer thickness, mm

Thermal resistance, (m²K)/W

Numerical modelling

• Effect of an air gap

• Effect of vertical thermocouple on distort the temperature field

Air Gap

Polymer at 250 °C

Mould at 50 °C with air gap of 0, 0.5 & 1 mm

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 TRIALSCorus & 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

Heat Transfer for sapphire

45

50

55

60

65

145 150 155 160 165 170 175Temperature (°C)

Hea

t Flo

w (m

W)

(1988)) al et (Khannahh

mmKK

i

x

i

xix

2

⎟⎟⎠

⎞⎜⎜⎝

⎛=

where m is the gradient, h is sample thickness

m

Heat Transfer for sapphire

45

50

55

60

65

145 150 155 160 165 170 175Temperature (°C)

Hea

t Flo

w (m

W)

(1988)) al et (Khannahh

mmKK

i

x

i

xix

2

⎟⎟⎠

⎞⎜⎜⎝

⎛=

where m is the gradient, h is sample thickness

m

Indium or eutectic

DSC pan

Mulitlayer sample

DSC method for thermal conductivity

*Calculated assuming thermal conductivity of 0.18 W/(mK) for PMMA reference sample $ 95% confidence = 2 x standard deviation

TEST ID Specimen IDSpecimen gradient

Specimen thickness

Thermal conductivity

value - literature

Measured thermal

conductivity* ErrormW/°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 $ 0.09DSC092 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 $ 0.05DSC093 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

DSC method for thermal conductivity

y = -0.0284x + 0.3613R2 = 0.1077

0.0

0.1

0.2

0.3

0.4

0.0 0.5 1.0 1.5 2.0 2.5

Specimen thickness, mm

Ther

mal

con

duct

ivity

, W/(m

.K)

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

0

20

40

60

80

100

120

17:42 18:00 18:18 18:36 18:54 19:12 19:30 19:48Time, hr min

Tem

pera

ture

, °C

Forced air heating curve Non-forced cooling curve

λ = 0.004 W/m.Kρ = 16 kg/m3

Cp = 1555 J/kg.K

27°C27°C 140°C

0.04 m

Numerical modelling of cooling of foam block

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 forThermal Properties

Measurement of Plastics

Differential scanning calorimetry standards

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

ISO TC61 SC5 WG8 Thermal Properties

Plastics thermal conductivity standards

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

ISO TC61 SC5 WG8 Thermal Properties

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 websitehttp://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 H12005-08

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

• 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

Potential areas of relevance for future work to increase understanding of heat transfer:

Your:Ideas,comments, suggestions, participation, contributions, …

to steer the project to maximise the benefits to you.

Any other business

Tea & coffee available