thermal expansion of polyurethane foam

23
Thermal Expansion of Polyurethane Foam Bradley A Lerch *‡ Research Engineer Roy M. Sullivan Research Engineer Closed cell foams are often used for thermal insulation. In the case of the Space Shuttle, the External Tank uses several thermal protection systems to maintain the temperature of the cryogenic fuels. A few of these systems are polyurethane, closed cell foams. In an attempt to better understand the foam behavior on the tank, we are in the process of developing and improving thermal-mechanical models for the foams. These models will start at the microstructural level and progress to the overall structural behavior of the foams on the tank. One of the key properties for model characterization and verification is thermal expansion. Since the foam is not a material, but a structure, the modeling of the expansion is complex. It is also exacerbated by the anisoptropy of the material. During the spraying and foaming process, the cells become elongated in the rise direction and this imparts different properties in the rise direction than in the transverse directions. Our approach is to treat the foam as a two part structure consisting of the polymeric cell structure and the gas inside the cells. The polymeric skeleton has a thermal expansion of its own which is derived from the basic polymer chemistry. However, a major contributor to the thermal expansion is the volume change associated with the gas inside of the closed cells. As this gas expands it exerts pressure on the cell walls and changes the shape and size of the cells. The amount that this occurs depends on the elastic and viscoplastic properties of the polymer skeleton. The more compliant the polymeric skelton, the more influence the gas pressure has on the expansion. An additional influence on the expansion process is that the polymeric skeleton begins to breakdown at elevated temperatures and releases additional gas species into the cell interiors, adding to the gas pressure. The fact that this is such a complex process makes thermal expansion ideal for testing the models. This report focuses on the thermal expansion tests and the response of the microstructure. A novel optical method is described which is appropriate for measuring thermal expansion at high temperatures without influencing the thermal expansion measurement. Detailed microstructural investigations will also be described which show cell expansion as a function of temperature. Finally, a phenomenological model on thermal expansion will be described. * Mechanics and Lifing Branch, NASA-GRC, 21000 Brookpark Rd, Cleveland, USA 44135 Mechanics and Lifing Branch, NASA Glenn Research Center, MS 49/7, 21000 Brookpark Rd., Cleveland, USA 44135 Corresponding Author; Contact Email: [email protected] Session-PaperNumber

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Page 1: Thermal Expansion of Polyurethane Foam

Thermal Expansion of Polyurethane FoamBradley A Lerch∗ ‡

Research EngineerRoy M. Sullivan†

Research Engineer

Closed cell foams are often used for thermal insulation. In the case of the Space Shuttle,the External Tank uses several thermal protection systems to maintain the temperature of thecryogenic fuels. A few of these systems are polyurethane, closed cell foams. In an attemptto better understand the foam behavior on the tank, we are in the process of developingand improving thermal-mechanical models for the foams. These models will start at themicrostructural level and progress to the overall structural behavior of the foams on thetank.

One of the key properties for model characterization and verification is thermal expansion.Since the foam is not a material, but a structure, the modeling of the expansion is complex.It is also exacerbated by the anisoptropy of the material. During the spraying and foamingprocess, the cells become elongated in the rise direction and this imparts different propertiesin the rise direction than in the transverse directions. Our approach is to treat the foam asa two part structure consisting of the polymeric cell structure and the gas inside the cells.The polymeric skeleton has a thermal expansion of its own which is derived from the basicpolymer chemistry. However, a major contributor to the thermal expansion is the volumechange associated with the gas inside of the closed cells. As this gas expands it exertspressure on the cell walls and changes the shape and size of the cells. The amount that thisoccurs depends on the elastic and viscoplastic properties of the polymer skeleton. The morecompliant the polymeric skelton, the more influence the gas pressure has on the expansion.An additional influence on the expansion process is that the polymeric skeleton begins tobreakdown at elevated temperatures and releases additional gas species into the cell interiors,adding to the gas pressure. The fact that this is such a complex process makes thermalexpansion ideal for testing the models.

This report focuses on the thermal expansion tests and the response of the microstructure.A novel optical method is described which is appropriate for measuring thermal expansionat high temperatures without influencing the thermal expansion measurement. Detailedmicrostructural investigations will also be described which show cell expansion as a functionof temperature. Finally, a phenomenological model on thermal expansion will be described.

∗Mechanics and Lifing Branch, NASA-GRC, 21000 Brookpark Rd, Cleveland, USA 44135†Mechanics and Lifing Branch, NASA Glenn Research Center, MS 49/7, 21000 Brookpark Rd., Cleveland,

USA 44135‡Corresponding Author; Contact Email: [email protected]

Session-PaperNumber

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Ther

mal

Exp

ansi

on o

f Pol

yure

than

e Fo

am

Brad

ley

A. L

erch

and

Roy

M. S

ulliv

anN

ASA

Gle

nn R

esea

rch

Cen

ter

pres

ente

d at

the

43rd

Ann

ual T

echn

ical

Mee

ting

of th

e So

ciet

y of

Eng

inee

ring

Scie

nce

The

Penn

sylv

ania

Sta

te U

nive

rsity

Uni

vers

ity P

ark,

PA

Aug

ust 1

4, 2

006

Page 3: Thermal Expansion of Polyurethane Foam

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Aer

odyn

amic

Hea

ting

Tank

Wal

l

Virg

in S

OFI

Foa

m

Hea

t Aff

ecte

dC

har L

ayer

Abl

ated

Mat

eria

l

70 -

200

o F20

0 -5

00 o F

-423

o F

> 50

0 o F

-423

–70

o F

Tem

pera

ture

s

Expe

cted

Foa

m T

empe

ratu

re P

rofil

e

Page 4: Thermal Expansion of Polyurethane Foam

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Impo

rtanc

e of

CTE

•A

l-sub

stra

te h

as a

CTE

of 2

x 1

0-5 /°

C•

Pol

ymer

foam

s ha

ve a

n ap

prox

imat

e C

TE o

f 1

x 10

-4/°

C•

Ther

mal

stra

in (ε

) equ

als:

ε= Δα

ΔT

= 0.

045

whe

re Δ

T =

580

°Cσ f

oam

~ U

TSfo

am

•Th

erm

al s

trai

ns a

re s

igni

fican

t and

can

ca

use

crac

king

Page 5: Thermal Expansion of Polyurethane Foam

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Goa

l

The

purp

ose

of th

is st

udy

is to

inve

stig

ate

the

mec

hani

sms o

ffo

am th

erm

al e

xpan

sion

to g

uide

mod

el d

evel

opm

ent

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Mea

sure

men

t of T

herm

al E

xpan

sion

LVD

T

Forc

eLa

ser

Det

ecto

r

Hea

t Lam

psFo

am

Con

vent

iona

l CTE

Rig

Non

-Con

tact

ing

CTE

Rig

Cer

amic

Rod

Furn

ace

Foam

Page 7: Thermal Expansion of Polyurethane Foam

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Dis

torti

on o

f Foa

m D

ue to

Tem

pera

ture

Cha

nges

As-

cut

Hea

ted

to

170

C

Hea

ted

to

300

C

Page 8: Thermal Expansion of Polyurethane Foam

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Pro

blem

s A

ssoc

iate

d W

ith T

herm

al M

easu

rem

ents

•Te

mpe

ratu

re m

easu

rem

ents

and

ther

mal

gra

dien

ts-F

oam

is a

ther

mal

insu

lato

r

•A

niso

tropy

of f

oam

mic

rost

ruct

ure

-El

onga

ted

cellu

lar s

truct

ure

-K

nit L

ines

-La

rge

cells

and

voi

ds

Twis

ting,

war

ping

, and

dis

torti

on o

f foa

m sa

mpl

es

Page 9: Thermal Expansion of Polyurethane Foam

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Foam

Mic

rost

ruct

ure

Figu

re 3

.5 –

Cel

l Geo

met

ry, N

CFI

24-1

24

•97%

air;

den

sity

= 0

.03

g/cc

•Pol

ymer

ic c

ell w

alls

•Due

to it

s mic

rost

ruct

ure,

mat

eria

l is a

niso

tropi

c (p

osse

ss

diff

eren

t mat

eria

l pro

perti

es in

diff

eren

t dire

ctio

ns)

Para

llel-t

o-ris

e di

rect

ion

Perp

endi

cula

r-to

-ris

e di

rect

ion

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Kni

tline

s

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Res

ults

Page 12: Thermal Expansion of Polyurethane Foam

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Ther

mal

Exp

ansi

on P

erpe

ndic

ular

to R

ise

Dire

ctio

n

0

0.050.

1

0.150.

2

0.25

050

100

150

200

250

Tem

pera

ture

(C)

Strain

Blo

ck 3

Blo

ck 3

Blo

ck 2

F

KS

C-1

Perm

anen

tvo

lum

e ch

ange

α=

.000

2/°C

α=

0.00

5/°C

Page 13: Thermal Expansion of Polyurethane Foam

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Ther

mal

Exp

ansi

on P

aral

lel t

o R

ise

Dire

ctio

n

0

0.02

0.04

0.06

0.080.

1

020

4060

8010

012

014

016

018

020

0

Tem

pera

ture

(C)

Strain

Sam

ple

1

Sam

ple

2

Sam

ple

3

Sam

ple

4

Perm

anen

tvo

lum

e ch

ange

α=

0/°C

α=

0.00

3/°C

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Ther

mal

Exp

ansi

on in

Bot

h D

irect

ions

0

0.050.1

0.150.2

0.25

050

100

150

200

250

Tem

pera

ture

(C)

Strain

Per

pend

icul

ar to

Ris

e

Par

alle

l to

Ris

e

Page 15: Thermal Expansion of Polyurethane Foam

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Mec

hani

sm o

f Foa

m E

xpan

sion

•H

ot s

tage

exp

erim

ents

con

duct

ed o

n fo

am s

ampl

es•

Indi

vidu

al fo

am c

ells

exa

min

ed in

SE

M d

urin

g he

atin

g an

d co

olin

g•

Exp

ansi

on a

nd c

ontra

ctio

n of

cel

ls o

bser

ved

as a

fu

nctio

n of

tem

pera

ture

•B

oth

who

le c

ells

and

face

s w

ere

exam

ined

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Hot

Sta

ge E

xper

imen

ts

Afte

r hea

ting

Bef

ore

test

Tem

p=15

0 C

Cel

l exp

ansi

ondu

e to

ther

mal

expo

sure

Cel

l exp

ands

m

ostly

in th

ew

idth

dire

ctio

n(p

erpe

ndic

ular

to ri

se)

Tem

p (°

C)

bloc

kle

ngth

wid

th20

00

015

015

-25

6.6

28.8

205-

13-4

.613

.6

cell

stra

in (%

)

Rise

dire

ctio

n

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Cel

l Wid

th T

herm

al E

xpan

sion

-0.6

0

-0.4

0

-0.2

0

0.00

0.20

0.40

0.60

050

100

150

200

250

300

Tem

pera

ture

(C)

Strain (mm/mm)

Cel

l 1 w

idth

Cel

l 2 w

idth

Cel

l 3 w

idth

Cel

l 4 w

idth

Cel

l 5 w

idth

Face

6 w

idth

Cel

l 7 w

idth

Cel

l 8 w

idth

Face

9 w

idth

Face

10

wid

th

Cel

l 11

wid

th

Cel

l 12

wid

th

(Per

pend

icul

ar to

Ris

e)

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Cel

l Len

gth

Ther

mal

Exp

ansi

on

-0.8

0

-0.7

0

-0.6

0

-0.5

0

-0.4

0

-0.3

0

-0.2

0

-0.1

0

0.00

0.10

0.20

050

100

150

200

250

300

Tem

pera

ture

(C)

Strain (mm/mm)

Cel

l 1 le

ngth

Cel

l 2 le

ngth

Cel

l 3 L

engt

h

Cel

l 4 L

engt

h

Cel

l 5 L

engt

h

Face

6 L

engt

h

Cel

l 7 L

engt

h

Cel

l 8 L

engt

h

Face

9 L

engt

h

Face

10

Leng

th

Cel

l 11

Len

gth

Cel

l 12

Leng

th

(Par

alle

l to

Ris

e)

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Bas

is fo

r Mod

el D

evel

opm

ent

•El

onga

ted

cell

stru

ctur

e•

Pore

pre

ssur

e in

crea

ses w

ith te

mpe

ratu

re•

Poly

mer

softe

ns w

ith te

mpe

ratu

re

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Ther

mal

Beh

avio

r of P

olym

er S

kele

ton

Tem

p.le

ngth

stra

inle

ngth

stra

in(°

C)

(μm

)(%

)(μ

m)

(%)

1758

064

010

762

7.7

51-0

.21

1744

-0.2

347

-0.2

8

Liga

men

t 1Li

gam

ent 9

1

9

Porti

ons o

f cel

ls w

ith

no in

fluen

ce

from

por

e pr

essu

re

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Ther

mal

Beh

avio

r of P

olym

er S

kele

ton

-0.5

0

-0.4

0

-0.3

0

-0.2

0

-0.1

0

0.00

0.10

0.20

050

100

150

200

Tem

pera

ture

(C)

Strain (mm/mm)

1 2 3 4 5 6 7 8 9 aver

age

Line

ar (f

it av

g)

α =

-0.0

0029

/°C

Liga

men

t No.

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Con

clus

ions

•H

igh

tem

pera

ture

ther

mal

beh

avio

r of t

he fo

am is

a

com

plex

pro

cess

driv

en b

y:-C

ell a

niso

tropy

-Por

e pr

essu

re-P

olym

er d

egra

datio

n•

Mod

eled

usi

ng s

oils

por

e pr

essu

re a

ppro

ach

and

mic

rom

echa

nics

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Ack

now

ledg

emen

ts

•M

SFC

, KS

C a

nd M

AF

for s

uppl

ying

foam

Cen

ter f

or S

urfa

ce A

naly

sis

of M

ater

ials

–C

WR

U fo

r pe

rform

ing

hot s

tage

exp

erim

ents

•C

hris

Bur

ke (Q

SS

, Inc

) for

dev

elop

ing

lase

r CTE

rig

and

cond

uctin

g th

erm

al e

xper

imen

ts•

Pat

Rog

ers,

Bria

n S

teev

ean

d S

cotty

Spa

rks

for t

heir

cont

inue

d su

ppor

t and

inte

rest

in th

e w

ork