Fiber reinforced composites of a novel pendent polyimide: Optimum processing and mechanical properties

Rochester Institute of Technology

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8-1-2002

Fiber reinforced composites of a novel pendent

polyimide: Optimum processing and mechanical

properties

Akshay Kamdar

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Recommended Citation

Fiber

Reinforced

Composites

of a

Novel

Pendent

Polyimide:

Optimum

Processing

and

Mechanical Properties

A Thesis

Presented

to

The Graduate

Faculty

of

Rochester Institute

of

Technology

In

partial

fulfillment

Of

the

requirements

for

the

Degree

of

Master

of

Science in Materials

Science

and

Engineering

Akshay

R

Kamdar

Fiber Reinforced Composites of a Novel Pendent

Polyimide: Optimum Processing and Mechanical Properties

Akshay R Kamdar

Approved:

Dr. Marvin

L.

IIIingsworth

(Advisor)

Dept. of Chemistry

Accepted:

Dr.

K.S.V. Santhanam

Interim Department Chair

Copyright Release Form

Fiber Reinforced Composites of a Novel Pendent

Polyimide: Optimum Processing and Mechanical Properties

I, Akshay R. Kamdar, hereby grant permission to Wallace Memorial Library of

the Rochester Institute of Technology, to reproduce my thesis in whole or in part. Any

reproduction will not be for commercial use or profit.

Date:

~-

02 -

02

ABSTRACT

A

2-component

polyimide [3,4'-ODA +

4,4'-ODPA],

a 3-component

"parent"

polyimide

[3,4'-ODA

+_4,4'-ODPA+ 10 mol% Mellitic aciddianhydride (MAD

A)]

and a

4-component

"pendent"

polyimide [3,4'-ODA+ 4,4'-ODPA + 10 mol % MADA + 10

mol %

Zr]

were synthesized and characterized _using fourier transform infrared

spectroscopy

(FTIR),

differential scanning calorimetry

(DSC)

and thermogravimetric

analysis

(TGA)

techniques. Sincetemperatureandtime arecriticalfactors

influencing

the thermal and mechanical properties ofthe

final

polyimide, it

is

important tohave detailed

information

onthebest time-temperature conditions for imidization and cure in orderto

optimize the _processing and properties of this polyimide. The present investigation

focused

on

developing

these conditions for the polymer. The processing condition of

heating

the poly(amic _acid) at 310 C for 15 minutes was chosen as an optimum

processing condition for the Zr-pendent polyimide. The sample showed complete

imidizationwithout_{any decomposition}atthegiven_processingtemperature.

The polyimide resins were also investigated as a matrix for high performance

composites. Prepregs were fabricated using the conventional

hand

lay-up

technique.

Carbonand/or glass fiberreinforced composite laminateswere

fabricated

in an autoclave

to obtain a 60:40 fiber to resin

by

weight ratio.

Tensile,

flexural,

Izod

impact

and

dynamic mechanical analysis tests were carried out on the composite laminates.

Mechanical properties ofthe pendent polymer were compared with those ofthe parent

(withoutpendent _groups)polyimide. Failuremechanismswere studiedthrough theuse of

optical and_scanning electron microscopy. The Zr-pendentpolyimide composites showed

lower moduli for the mechanical tests performed as compared to the

2-component

and

parent polyimide composites. Comparisons of mechanical datawith

laminates fabricated

ACKNOWLEDGEMENTS

"If

an experiment _{works, something has}gone

wrong"

Finagle 's First Law. Dr.

Illingsworth

_certainly

helped

me prove him wrong through his valuable suggestions,

continuous encouragement and extreme_{patience,especially}whentheresearch struggled.

This thesis would not

have

been a success without the constant support and valuable

insights

of _my committee _members, Dr.

Langner,

Dr. Angela at Xerox

Corporation

and Dr. Carle from the Mechanical

Engineering

Technology

Department.

Special

thanks toDr. Kimfor

helping

me

during

theinitialphasesof_mythesis.

I would like to thank Owens

Corning

and Mr. John Alexion at Composites One forthe_supplyofglass and carbon fibers. I appreciatethe effortsofthestaff attheCenter

for

Composite Materials at the

University

of Delaware for

fabricating

the composite samples and that ofDie Max ofRochester for cutting them. I also thankTom Locke at the Center for Integrated

Manufacturing

Studies

(CIMS)

for his

help

in making notches

in the composite samples. Sincere thanks to Dr. Michael Jackson andPeter Terrana for their

help

withtheSEMatthemicroelectronicsdepartment.

I am grateful to the Material Sciences and

Engineering

department and the

Chemistry

department for awarding mewith a

Teaching

Assistantship. To Tom Allston

and the stock room _staff, your constant support made all thehurdles so _easy to

jump.

I

would liketo _say a special "thank you"

to Brenda Mastrangelo for always _cheering me up.

To my lab co-workers, Wei

Cheng

and Sangeetafortheir

help,

andto _my

friends,

Prakash, Vivek, Eugene, Abhishek,

Ashish, Mayank, Suruchi,

Hrishikesh and

Rajiv,

this thesiswouldnothave beensomuchfunwithout your

help

and_wittycomments.

Writing

this thesis would not have been so

interesting

without

listening

to the tunesofDire Straits andPink Floyd.

And last butneverthe

least,

to _myparents and

family

for

loving

me so much and

TABLE OF CONTENTS

Page

ABSTRACT

iii

ACKNOWLEDGEMENTS

iv

TABLE OF

CONTENTS

v

LIST OF TABLES vii

LISTOF FIGURES ix

CHAPTER

I. INTRODUCTION& BACKGROUND

1.1. Introduction 1

1.2.Background 8c

Theory

3

1.2.1 Polyimides Overview 3

1.2.2Properties & ApplicationsofPolyimides 7

1.2.3 Fiber Reinforced Polymer Composites 8

1.2.4 FabricationofComposites 10

1.2.5Failure Mechanisms in Composites 12

1.2.6 OverviewofResearch PolyimidesunderInvestigation 14

1.3. Hypothesis 16

1.4. Major Objectivesofthis

Study

16

II. EXPERIMENTAL

2.1 Chemicals andMaterials 18

2.2 Synthesis of

2-,

3- and4-Component Poly(amicacid)s 19 2.3

Establishing

Optimum

Processing

Conditions

24

2.4CharacterizationTechniques 25

2.5 FabricationofComposite Laminates ₂₆

IH.

RESULTS

AND DISCUSSION

3.1

Synthesis

of

2-,

3- and4-Component Poly(amicacid)s 35 3.2

Establishing

Optimum

Processing

Conditions 37

3.2.1 Visible Color Changes 37

3.2.2 Fourier Transform Infrared

Spectroscopy

(FTIR)

37

3.2.3 Thermogravimetric Analysis

(TGA)

44

3.2.4 Differential

Scanning Calorimetry

(DSC)

52 3.2.5 Summary: DeterminationofOptimum

Processing

Condition

and

Molding

Cycle 54

3.3 Composite Fabricationand

Processing

55

3.4 Mechanical PropertiesofLaminates 59

3.4.1 Tensile Test (ASTM

D3039/D3039M-95a)

59

3.4.2 Izod Impact Test (ASTM

D256-00)

73

3.4.3 Flexural Test (Three-point

bending)

(ASTM

D790-00)

84 3.4.4 Dynamic Mechanical Analysis

(DMA)

(ASTM D4065-95)....94 3.4.5

Summary

oftheComparisonofMechanical Propertiesof

Laminates Fabricated using Different

Molding

Cycles 101

3.4.6

Summary

(Global

Discussion)

103

IV. CONCLUSIONS 105

V. FUTURE WORK 107

LIST OF TABLES

Table

Page

1 1

Chemical

structures ofadditionpolyimides 6

1.2 Propertiesof selectedconventional andcompositematerials 8

2. 1 Fabric

Specifications

18

2.2

Heating

stepsforthepoly(amic_acid) powder 24

2.3 Autoclaveparameters 29

2.4 Tensiletestparameters(ASTM D3039/D3039

M-95a)

31

2.5 Flexuraltestparameters(ASTM

D790-00)

32

3.1 Infraredabsorptionbandsof

imides

37

3.2 % Degreeofimidization forthe2-componentpolyimideresin 43

3.3 % Degreeof

imidization

fortheparent polyimide resin 43

3.4 % Degreeof

imidization

forthependentpolyimide resin 44

3.5 Comparisonof actual andtheoreticalweight_{using different processing}

conditions 50

3.6 Weightloss

(%)

ofthe threeresinsheldat370 C foronehour 51

3.7a Glasstransitionsobserved

in

the2-componentpolyimides 54

3.7b Glasstransitionsobservedintheparentpolyimides 54

3.8 Resincontent of compositelaminates using Method 1 57

3.9 Resincontent ofcomposite laminates using Method II 58

3.10 Tensiletestfor 2-componentcarbonfiberreinforced composite 60

3.11 Tensiletest for 3-componentcarbon

fiber

_{reinforced composite 61}

3.12 Tensiletestfor 4-componentcarbon

fiber

reinforced composite 61

3.14

Prepregand

autoclavefabricationparameters 66

3.15a

Comparison

oftensilestrength andmoduli of2- and3-component

composite

laminates

_{from different molding}cycles 66

3.1 5b

Comparison

oftensilestrength andmoduli of4-componentcomposite

laminates from

_{different molding}cycles 67

3.16 Impactstrength and resistance forthe2-componentcarbonfiber

composites 74

3.17 Impactstrengths and resistanceforthe3-componentcarbonfiber

composite 75

3.18 Impactstrengths and resistanceforthe4-componentcarbonfiber

composite 75

3.19 Comparisonofimpactstrength and moduliof

2-,

3- & 4-component compositelaminates from

different

_moldingcycles 77

3.20 Flexuralstrengths and modulusofthe2-componentcarbonfiber

composites 85

3.21 Flexuralstrengths and modulus ofthe3-componentcarbon

composites 86

3.22 Flexuralstrengths andmodulusofthe4-componentcarbon

composites 86

3.23 Comparisonofflexural strengthandmoduliof

2-,

3- & 4-component compositelaminates from different moldingcycles 88

3.24 Storagemodulus

(E')

comparison at300 C _{for different molding}

cycles 1 00

3.25 Comparisonof mechanicalpropertiesoflaminates

fabricated

_using

LIST

OF

FIGURES

Figure

Page

1.1 Kaptoncover_{onsolar panel}inthe

spaceship

"Discovery"

1

1.2 Atomicoxygen resistance exhibited

by

polyimides with zirconium

components 2

1.3 Chemicalstructureofanimide group 4

1.4 Formationofpoly(amicacid) 4

1.5 Conversion frompoly(amicacid)topolyimide 5

1.6 Chemical structures of

different

polyimides 15

1.7 Usetemperatures forresin matrixcomposites 9

1.8 Autoclave 11

1.9 Typical vacuum

bag

assembly 12

2.1 Synthesisofpoly(amic _acid)of 3,4'- ODA/ 4,4'-ODPA 19

2.2 ConversionofMellitic AcidtoMellitic Acid Dianhydride 20

2.3 Synthesisofpoly(amic _acid)of3,4'-ODA / 4,4'-ODPA/ 10mol%

MADA 21

2.4 Synthesisof

Zr(adsp)(dsp)

Co-polyimide 23

3.1 TGAgraph ofMADA 35

3.2 Melliticacidand

its

products 36

3.3 FTERspectrafor 2-componentpolyimide 39

3.4 FTJJR.spectraof parentpolyimides ₄₀

3.5 FTIRspectra ofpendentpolyimides ₄₁

3.6 Isolatedreference peaksobserved at 1012 cm"1

forthe

different

3.7

TGA

curves of poly(amic acid)softhe threedifferentresins 45

3.8a

TGAcurves of

2-component

polymer 46

3.8b

TGAcurvesof

2-component

polymer 46

3.9a TGAcurves oftheparent polymer 47

3.9b TGAcurvesoftheparent polymer 47

3.10a TGAcurves ofthependent polymer 48

3. 1 Ob TGAcurves ofthependent polymer 48

3.11 TGAgraph ofthependent poly(amic_acid) 49

3.12 Emulationof_moldingcyclesfortheneat polymer resins. TGAof

2-,

3-,

and4-componentpolyimidesafter

heating

at370 C for 1 hour

under

N2

gas 51

3.13

(a-f)

DSCcurves forthe2-componentpolyimide resin 53

3.14

Molding

cycleforcompositelaminate fabrication 56

3.15 Comparisonoftensilestrength oflaminas 63

3.16 Comparisonoftensilemodulus oflaminas 63

3.17 Stress-Straincurvesfor 2-componentcarbonreinforcedtensile

laminas 64

3.18 Stress-Straincurvesfor 3-componentcarbon reinforcedtensile

laminas 64

3.19 Stress-Straincurves for 4-componentcarbonreinforcedtensile

laminas 65

3.20 Stress-Straincurves for 4-componentglass reinforcedtensile

laminas 65

3.21 Tensiletestopticalimagesof2-componentcarbon

fabric

laminas 68

3.22 Tensiletestopticalimagesof

3-component

carbon

fabric

3.23

Tensile

testoptical

images

of4-componentcarbonand glass fabric

laminas

70

3.24 SEM

images

offailedtensile specimens 71

3.25 SEM

images

of

failed

tensilespecimens 72

3.26 Comparisonof average

impact

resistance

(energy)

of carbonfiber

laminates 76

3.27

Comparison

of averageimpactstrengthsof carbonfiber laminates 77

3.28 Izod

impact

optical imagesof2-componentcarbonfabric laminates 79

3.29 Izodimpactoptical

images

of3-componentcarbonfabric laminates 80

3.30 Izodimpactoptical

images

of4-componentcarbonfabric laminates 81

3.3 1 Izod

impact

opticalimagesof4-componentcarbonfabric laminates 82

3.32 SEM imagesoffailed impactspecimen 83

3.33 SEMimageof an aluminum foilon _aplysurface 84

3.34 Comparisonof averageflexuralmodulusofcarbonfiber

laminates 87

3.35 Comparisonof averageflexural strengthsofcarbon fiber

laminates 87

3.36 Flexuraltestopticalimagesof2-componentcarbon fabric

laminates 89

3.37 Flexuraltestopticalimagesof3-componentcarbonfabric

laminates 90

3.38 Flexural testoptical imagesof4-componentcarbon

fabric

laminates 91

3.39 Flexuraltestopticalimagesof4-componentcarbon

fabric

laminates 92

3.40 SEM imagesoffailed flexural specimen 93

3.42 DMAcurves forparentpolyimidecarbonfiber laminates 96

3.43

DMAcurves

for

pendent polyimidecarbonfiber laminates 97

3.44 DMAcurves forpendentpolyimideglassfiber laminates 97

3.45

Summary

oftheDMAanalysis forthedifferentcompositelaminates

(a)

Storage Modulus Vs Temperature 99

(b)

Loss Modulus Vs Temperature 99

(c)

Tan D Vs Temperature 100

3.46

Summary

of_commonlyobserved

failure

mechanisms forthecarbon

CHAPTER I

INTRODUCTION &

BACKGROUND

1.1 Introduction

Fiber reinforced polymer composites are considered to be an important class of engineering materials _offering unique

flexibility

in design capabilities. The advantages arehighspecific strengths and_moduli, strengthtoweight ratio andfatigue strength.

They

are used in many diverse applications in the aerospace, electronics and automotive

industries.

Currently,

a _majority of aerospace applications _rely upon composites

having

epoxy, bismaleimide or polyimide polymer matrices reinforcedwith one ofanumber of high performance carbon or aramid fibers. The choice of polymer matrix depends primarily on

its

long-term use temperature and the service environment inwhich it will be used. In low earth orbit

(LEO),

atomic oxygen

(AO)

is themost dominant species at altitudes between 200 and 700 km. As spacecraft orbit the earth at _{these altitudes,}

they

collide with atomic oxygen and due to their high orbital _{velocity, these} collisions are energetic enoughtobreakchemicalbonds and oxidize materials. Thisdegradationresults not_{only in}massloss butalso areductioninmechanicalandthermalproperties.

A DuPont polyimide,

Kapton,

has been chosen for a _variety of applications aboardspacecraftin low earth orbit

(LEO),

such asthe InternationalSpace Station (ISS). Its characteristicfeaturesareits low

density,

high

flexibility

and goodthermalstability.

However,

the oxidation rate ofKapton

in LEO

atomic oxygen environment

is

great enoughthatstructural

failure

ofthepart occurs

in

muchlesstimethan the_operating

life

of the spacecraft. The novel polymer under

investigation is

a zirconium pendent

polyimide

based

on

3,4'-oxydianiline

(3,4'-ODA)

and 4,4'-oxydiphthalic anhydride

(4,4'-ODPA).

Films

have

demonstrated

excellent resistance to atomic oxygen

(AO)

by

forming

Zr02,

a white powder that acts a protective layer [2]. A schematic diagram

representing

theprocess of

Zr02

formation has beenshownin Figure 1.2.

atomic

m,

*polymeffirr itoninn-iii-rj\7t\

;-jZJ Zroxl*,

/M

|

..,.. [image:16.529.112.433.73.426.2]

reformsrjAiii.i layer ot pent

Figure 1.2. Atomicoxygenresistanceexhibited

by

polyimideswith zirconiumcomponents

The onset ofimidization forthis polymeroccurs at

C,

the glass transition

temperature

is

-296

C

and the thermo-oxidative decomposition temperature of this

polymer

in

airis-551

C

[3]. The high glasstransitionand

decomposition

temperatures,

coupledwith

its

improved AOresistance,makethisnovel polymer a_strong candidatefor

use as astructural material aboardtheISS.

However,

theproperties ofthisnovel polymer

as acomposite material forstructural applicationshavenot yet

been

investigated.

When polyimides are

thermally

imidized and cured for use either as composite

matrix resins or as photoresists and dielectrics in electronic

integrated

_circuits,

temperature and time become critical factors that

influence

the thermal and mechanical

onthe

best time-temperature

_conditions

for imidization

_{and cure inorderto optimize the}

processing

andproperties ofthispolyimide. It

is

critical to

develop

an _{understanding}of the role of thermal parameters such as

imidization temperature,

glass transition

temperature and cure temperature at the_partiallyor

fully

imidized

and cured stages and

their effect on the

final

mechanical properties and adhesion [4]. Since the optimized

processing conditions

for

thisnovel polymer

is

not

known,

thepresent investigationwill

focus

on

developing

theseconditionsforthepolymer.

1.2

Background &

Theory

1.2.1 Polyimides Overview

Polyimides are a sophisticated

family

of materials

having

applications in

highly

technicalend use fields fromaerospacetomicroelectronics. Their

diversity

is

suchthatit leads them

into

applications as _adhesives,

films, foams,

plastic moldings and resin

matricesfor composites. Theirmajoradvantage, ahighresistanceto

heat,

placesthem in

a niche other polymers cannotenter

[5,

6].

They

are considered_specialtyplasticsbecause

oftheir _{outstanding high} performance _engineering properties. As _such,

they

are priced

well above the _commodity polymers such as polyethylene and polystyrene.

Many

monomers used to prepare polyimides are also _specialty chemicals.

Therefore,

theprice

ofcommercial polyimidescoversa widerange,

$

8.80

-$

1 1 1.00per

kg

[7].

Synthesis ofPolyimides

Polyimides,

prepared from a _variety ofdianhydride and

diamine

monomers, are

characterized

by

_repeating imide structural units in the polymer

backbone

as seen

in

Figure 1.3. Thisstructure contributesto theexceptionalthermal and oxidative _stabilityof

o

II

-C

N R"\/\/\y\^

Figure 1.3 Chemicalstructure_ofan _{imide group}

1.2.1.1 Linear

Condensation

Polyimides

Thereare threemaintypesof polyimides:

(1)

Linearcondensationpolyimides,

(2)

Thermoplastic polyimides, and

(3)

Addition polyimides [12]. Linear condensation

polyimides are prepared from a

two-step

method. In the

first

step, an aromatic

dianhydride

combineswith an aromatic

diamine

inthepresenceof a polarsolventtoform

a poly(amic acid).Anexample

is

presentedin Figure 1.4.

H,N

Pyromelliticaciddianhydride

r\

/

4,4'-Oxydianiline NH,

NMP/DMAc

O II c

HOOC

\

/ru~\

r*

COOH

Poly(amic acid)

Figure 1.4. Formation ofpoly(amic_acid)

In the second _step, the

imide

_group

is formed

by

a _{ring closing} mechanism

involving

cyclodehydration of the poly(amic acid). This

is

achieved

by

a thermal

imidization

process

by

extended

heating

at elevated temperatures of250 - 400 C _or

by

treatment with chemical

dehydrating

agents. The latter method

is

_generally not used

commercially because

oftheproblems associated with

handling

thereagents. Imidization

involves

the release of

by-products

such as water and the loss of residual solvents or

volatiles. The thermal

imidization

_{step is usually} preceded

by

a _processing operation,

whereby a poly(amic _acid) solution

is

used to cast a

film,

_coating or spin a fiber. The

polymer

in its different forms

_{is driedandthensubjectedto specific}

heating

cycles.

HOOC

\J~0-\

/-H

COOH

Heat

-H,0

/

Poly(amicacid)

O

II

c

/N^\

/r^w

/

KaptonPoly(imide)

Figure 1.5. Conversion frompolyfamic_acid) topolyimide

Polyimides synthesized forthisresearch work followthe condensation technique

described above.

Commercially

available

Kapton

(DuPont)

and LARC-IA

(Langley

Research Center- Improved

Adhesive)

made

by

NASA are examples of condensation

polyimides.

Polyimides that are _generally soluble

in

organic solvents are often prepared

by

a

one-step or single-stage method. This method is especially useful

in

the polymerization of unreactivedianhydrides anddiamines and canyield a material with a

higher degree

of

1.2. 1.2

Thermoplastic

polyimides

Thermoplastic

polyimides are obtained

by

introducing

large cyclic side groups

into

the polymer _chain,

by

tailoring

_{random sequences}

into

_the

chain, or

by

the

interposition

of

flexible,

_aliphatic

linking

groups betweenthe aromatic and heterocyclic

moieties.

One

general

drawback

ofthermoplastic polyimides

is

that for use at elevated

temperatures,

their glass _{transition temperatures} mustbe high (> 200

C)

andhencetheir

processing

temperatures must be even

higher.

Ultem,

produced

by

General

Electric,

is

an example of a

commercially

_{availablethermoplasticpolyimide.}

1.2.1.3 Addition

Polyimides

Addition polyimides _usually consist ofshort chain preimidized oligomers with

reactive end groups thatcan react

by

addition polymerization. The end groups usedhave been _{norbornene, maleimide, acetylenic,} allynadic and benzocyclobutene. PMR-15

(Polymerization of Monomer

Reactants),

developed

by

NASA

Langley

is

the most widely used polyimide made

by

this technique. Other resins include the

bismaleimides,

V-CAP

[8],

PMR-1

1,

LARC-13and LARC-160systems.

Table 1.1. Chemicalstructures _ofadditionpolyimides*

Trade Name Chemical Structure

PMR-15 BTDE/NE/DDM [diesterofbenzophenonetetracarboxylicacid +

monoesterof nadic acid +

4,4'-diaminodiphenylmethane]

LARC-13 BTDA/NA/3,3'-DDM [benzophenonetetracarboxylic

dinahydride

+

nadicanhydride+

3,3'-diaminodiphenylmefhane]

PMR-11 HFDE/NE/PPD [hexafluoropropenedianhydride+ monoester of nadic

acid +p-phenylenediamine

LARC-160 BTDE/NE/AP22[diesterofbenzophenonetetracarboxylic acid +

Monoesterofnadic acid +Jeffamine

22]

*

Formoreinformation,refertoWinters, W.; Cavano, PJJO"'

Nat.SAMPETech._Conf., 10₍₁₉₇₈₎661;T.L. St._Clair;

Progar, D.J.,24,hNat. SAMPESymp.,24(1979)1081andT.L. St._{Clair; Jewell, R.A.,}23rd

Nat.SAMPESymp.

Characterization

_ofpolyimides

Polyimides

can

be

characterized _using several techniques

including

analytical,

thermal,

rheological and mechanical. Analytical techniques such as gel permeation

chromatography

(GPC)

and

dilute

solution _viscometry

have

been used to determine the molecular weight of poly(amic acid)s. Verification of the polymer structure and

monitoring

of the polyimide

formation

during

cure can

be

accomplished _{using Fourier}

Transform

Infrared

(FTfR)

Spectroscopy.

Thermal techniques

involving

thermogravimetric analysis

(TGA),

thermal mechanical analysis

(TMA)

and differential

scanning

calorimetry

(DSC)

allow measurement of thermal decompositions and glass transition temperatures. Rheological properties provide _{information regarding} the flow

characteristics ofthe resin and

help

to _{define processing} windows for

fabricating

fiber

reinforced composites. Mechanical and electrical properties are _usually obtained

by

casting and _curing of poly(amic _acid)

films

and _performing the desired tests.

Tensile,

impact

and tear strength are some of the common mechanical tests while dielectric

constant,

dielectric

strength and volume _resistivity are the common electrical tests performed on polyimidefilms.

1.2.2

Properties

and

Applications

ofPolyimides

The thermal and oxidative stabilities of polyimides are _primarily

determined

by

the structural features ofthe polymer. Polyimides derived from aromatic diamines and

aromatic dianhydrides exhibit _outstanding thermal _stability atelevated temperature. The

Tg

determines the method of _processing and the maximum temperature at which a

polymer can

be

used in any given application. The high glass transition temperatures offered

by

these polymers areattributedto their stiff molecular

backbones,

which contain

aromatic rings

[6,

7]. Polyimides exhibit good solvent resistance to acidic or neutral

aqueous environments.

However,

almost all polyimides undergo

hydrolytic degradation

in

the presence of _strong alkaline aqueous solutions. _{Their outstanding} mechanical

properties make them excellent candidates in highperformance applications. Like most

bearings,

grinding

wheels and communicators in electric motors. Polyimide films are

used as

high

temperature

insulation

materials and passivation layers

in

integrated circuits

[7].

They

_{are also used as resin matrices in advanced composites and as photo resists and}

inter level dielectrics in

integrated

circuits

in

themicroelectronic

industry.

1.2.3 Fiber Reinforced Polymer Composites

A

judicious

selection of

fiber,

matrix and

interface

conditions can lead to a

composite with a combination of strength and modulus comparabletoorbetterthanthose

of_{many conventional metallic materials.}

Composites

_{are superior to metals in specific}

strength and specific stiffness. Table 1.2 shows a comparison between selected

conventional and composites materials.

Table 1.2 Properties ofselected conventional and composite materials

[9]

Material

Density

(P),

g/cm3

Tensile

Modulus

(E),

GPa

Tensile

Strength

(a),

MPa

E/p

106N.m /

kg

dip 103N.m/

kg

A16061-T6 2.70 68.90 310.00 25.70 115.00

Ti-6A1-4V 4.43 110.00 1171.00 25.30 26.40

Nylon6/6 1.14 2.00 70.00 1.75 61.40

Unidirectionalhigh

strength carbon

fiber/epoxy

1.55 137.80 1550.00 88.90 1000.00

Unidirectional

E-glass

fiber/epoxy

1.85 39.30 965.00 21.20 522.00

Randomglass

fiber/epoxy

1.55 8.50 110.00 5.48 71.00

1.2.3.1 ResinMatrices

The role ofthe resin matrix is primarily to bind the

fibers

together and

hence it

determines the thermal _stability ofthe composite. The resinkeeps the

fiber in

adesired

location andorientation andacts as aload transfermediumbetween

fibers.

Itprotectsthe

fiber

from environmental damage

before,

during

and after composite processing. The

chief polymers used in high performance composites are

thermosetting

resins. These

The field

of

thermally

_stable

heteroaromatic

_polymers

is

_dominated

by

the

polyimides.

Most recently

their_{use as resin matrices}

has

_{attracted the greatest attention.}

The

needs of the

defense

industry

and the

high

temperature requirements of the

electronics

industry

have

promoted the use of polyimide _coatings,

films

and laminates.

The most

commonly

used polyimide resin matrices for composites are condensation

polyimides,polymerization_{of monomer reactants}

(PMR)

_and

bismaleimides

(BMI)

[9].

It is the

outstanding

thermal _stability and relative ease offabrication that have

established the _{reputation of polyimides as viable}

engineering

materials and_particularly

as resin matrices foradvanced composites. Figure 1.7 showstheusetemperature forresin

matrix composites.

600

SHORT TIME (SECS)

LONG TIME (HRS)

EPOXY BISUALEIUOE POcnuoE

Figure 1. 7. Usetemperaturesforresin matrix composites

[10]

Until now, epoxy based composites were the most

important

matrix materials.

However,

their long-term use

is

limited to temperatures _up to 130 C.

Bismaleimide

(BMI)

resins offer a"half way

house"

intemperatureperformances

between

epoxies and

polyimides. The reason for the intense interest in BMI systems is their _ability to be

fabricated using epoxy-like conditions and without the evolution of

void-producing

volatiles. BMI systems are capable of_performing at temperatures _up to 230 C

[10].

Service temperatures for polyimides depend on the application requirements. For

polyimides

have

shownto exhibit good short-term and

long-term

performanceoverlarge

temperatureranges.

1.2.3.2 Fiber

Reinforcements

Fibers aretheprincipal load

bearing

membersinacomposite.

They

are_usuallyof

high

strength _{and modulus and are}

incorporated into

_the matrix either in continuous

lengths

or as chopped

fibers.

Examples of

fiber

reinforcements are_{glass, carbon, aramid,}

boron,

alumina, silicon carbide and polyethylene fibers. Even though these fibers have high _{tensile moduli,}

they

are _{usually brittle (have low elongation),} with carbon fibers

more

brittle

than aramid or glass

fibers.

1.2.4

Fabrication of

Composites

Thetechniquesmost_commonlyusedforthefabricationof composites are[1 1]:

1)

Manual

Lay-Up

(Wet

Lay-Up

and

Prepreg Method)

2)

Automated Tape Lamination

3)

Vacuum

Bag Molding

4)

Autoclave

Curing

5)

Filament

Winding

6)

Pultrusion

7)

Matched-Die

Molding

8)

ResinTransfer

Molding (RTM)

and

9) Spray-Up

Methods

Prepregs fortheresearch work were made_{using the}manualwet

lay-up

techniqueand

composites were then fabricated

by

autoclave curing. These techniques have been

described below:

1.2.4.1

Prepreg

Fabrication

A prepreg

is

a fiberreinforcement that has been

impregnated

with _{resin, slightly}

prepregstoproduce

laminates depends

uponthe typeofpolyimideusedfor

impregnation,

theequipment

(autoclave

or_press),andtheshape oftheitemand

its

thickness [12].

There

are two _primary varieties of fiber _{reinforcement,} unidirectional

fibers

(tapes)

and woven

fabrics.

These fiber

_{assemblies are}

impregnated

_{either via solution}

coatingprocedures or

by

hot

melt methods. Recentwork hasalso shownthat_prepregcan

be produced

by

powder

impregnation

and

by

the _co-mingling offibers. In the hot-melt

method, resin

is

rolled

into

a thin

film

of

desired

thickness to provide the correct

fiber/resin

ratio. The reinforcement and resin

film

are thenjoined and passed between

heated

rollersto saturatethebundlesof

fibers

_producing aprepreg. In solvent_{coating, the}

fibers

(usually

a woven

fabric)

are passedthrough_{baths containing}a solution oftheresin

in a solvent. Thepreimpregnated

fabric

is thenpassed through aheated ovento remove

most ofthe solvent. _{The prepreg is} rolled at the end ofthe line with a release paper

interleaf [10].

1.2.4.2 Autoclave

Curing

The technique most often used in the lamination of carbon or graphite fiber

reinforced composites is the autoclave_{processing (See Figure 1.8). It is} alarge pressure

vessel equipped with a temperature and pressure control system. The elevatedpressures

and

temperatures,

required for processing ofthe

laminate,

are _commonly achieved

by

electrically

heating

apressurized inertgas such as nitrogento reduce_oxidizingreactions

thatoccurintheresinatelevatedtemperatures. Pressureappliedintheautoclave provides

force to consolidate the plies and to compress vaporbubbles to obtain a product with

minimum voidcontent.

The

first

_step

involved

in

this process

is

the vacuum bagging. The starting

material

is

_normally a_prepreg, which

is

then _{laid up} as plies on a moldsurface. This

is

followed

by

the placement of a porous release

film

and ableeder material. The porous

release

film

permits the excess resinto flow

through,

while thebleedermaterial absorbs

the excess resin. This stack

is

then covered with abarrier

film,

abreather material and a

heat resistant vacuum bag. The barrier material prevents the resin from clogging the

breather and vacuum lines whereas the breathermaterial acts as adistributor for airand

escapingvolatiles and gases. The entire _{assembly is} thenplacedinside the autoclave and

a vacuum

is

applied withinthe

bagging

film to achieve a uniform pressure and to draw

out volatiles created

during

cure. Figure 1.9 shows a typical vacuum

bag

_assembly

[9,

11].

VACUUMLINE

CJUr/.VALVE SAGGING TilM

BHEATHKR

[image:26.529.50.464.22.453.2]

SEALANT

Figure 1.9. Typicalvacuum

bag

_assembly

[11]

However,

the above hand lay-up/autoclave fabrication type of_{processing is} not

only costly but also environmentally unfriendly. NASA is currently

developing

an

automated robotictowplacement

technology

thatcould overcomethesedeficiencies [14].

1.2.5

Failure Mechanisms in Composites

Failurebehaviorofcompositesis complexduetotheiranisotropic nature. It

is

not

onlythe magnitudeofthe stressesthatare

important,

butalso theorientation ofthemain

stresses relativeto the axes ofthematerial's anisotropy. Compositepolymers can

have

a

number of fiber orientations i.e. continuous, discontinuous or _randomly oriented. The

undergo. _{For unidirectional, continuous}

fiber

composites, the

direction

ofthe _externally

applied stress

largely

determines

_the

failure

_{mode of the}

composite.

However,

in

laminates,

comprised _{of a number of plies with}

varying orientations, the

failure

mode

is

theresult of a complex

interaction

of

factors.

Themechanical responseto

loading

is

often

referredtoas

being

either

fiber

or matrix

dominated.

Thetypicalcompositefailuremodes

are

described

below [15]:

1)

Debonding

Debonding

is due

to

interfacial failure

_along the

fiber-matrix boundary.

This

is

characterized

by

the

fracture

surface

showing

_numerous

protruding

fibers

with

little

orno

resin

adhering

_{to them.}

2)

Inter

laminar

Failure

Inter laminar

failure

can occur when the

interfacial

strength between matrix and

fibers is

greater than the matrix cohesive strength. Such failure is manifested

in

the

composite_{exhibiting excessively brittle behavior.}

3)

Fiber

Buckling

Fiber

buckling

can occur

in

compressionwhenthematrix

has inadequate

strength.

Micro

buckling

of

fibers is

a common form of

failure

in the case ofcontinuous

fiber

reinforced composites subjected to compression. This failure has also

been found

to

occur as a result of_curing at high temperatures when there is a significant

difference

betweenthedegreeofcontraction ofthematrix andthe

fibers.

4)

Fiber PullOut

Fiberpulloutarisesduetovariationsinthe

interfacial bond

strength and

localized

load

transferfromthefiberto thematrix.The energy

dissipated

during

fiber

pull outfrom

the matrix

is

largely

dependant inthedegree ofinterfacial

friction

_present, whichinturn

5)

Fiber

Breakage

Fiber

breakage has been

regarded as asignificant _{energy absorbing}mechanismin

the

failure

ofcompositematerials.

6)

Cracking

of

Composites

Cracking

in

composites canbe

initiated

by debonding

atthefiberresininterface.

This can result

in

a transverse _ply crack that

increases in

length,

and upon _reaching a

fiber

continues _along

it

andthenproceedsback

into

thematrix. This phenomenon,where

a

propagating

crack

in

thematrix canbe deflectedanumber oftimeswhenit impingeson

the

fiber

reinforcementis called crackdeflection. Thiscrackdeflectionphenomenon is an

important

mechanism in

increasing

the toughness of the polymer _matrix, since the

fracture

pathlength ismuch_{less readily}achieved.

7)

Micro cracking

Micro cracking ofcontinuous fiber composites is induced

by

thermal expansion

mismatchesbetween the fiber andthematrix _causingconsiderable stresses andthese can

result incomplete _yielding ofthematrix.

Furthermore,

macroscopic thermalmismatches

can also occurbetween cross plies.

Cross-ply

laminates generatehigher residual stresses

than unidirectional laminates because ofthe _{anisotropy in} the thermal expansion ofthe

plies.

1.2.6

Overview

of

Research Polyimides

under

Investigation

Three different types ofpolyimides have been synthesized forthisresearchwork

by

thecondensationtechniquedescribedabove:

1)

2-componentpolyimidebasedon3,4'-ODAand4,4'-ODPA

2)

3-component"parent"polyimidebasedon

3,4'-ODA,

4,4'-ODPAand 10mol

%

Mellitic Acid Dianhydride

(MADA)

to form a copolymer of

(3,4'-ODA

+

4,4'-ODPA)

&

(3,4'-ODA + MADA). MADA acts as a site for pendent _group

attachment_and,

3)

4-component

"pendent"

MADA and 10 mol

%

zirconium pendent _group to form a copolymer of

(3,4'-ODA

+

4,4'-ODPA)

&

_(3,4'-ODA + _MADA + Zr). The Zr-pendent _group

is

attached atthe site oftheMADA.

Figure 1.6shows comparisonswith respecttochemicalstructures madebetween

thependent polyimide and_commerciallyavailableKapton& andLARC-IA.

/

Vo^ \

DuPont Kapton

NASA-LARC-IA

^tX^-oJ^o^

CVr^o.,,

H.

W

,H C:N N:C

Zirconium-pendentpolyimide

Asseen

from

Figure

1.6,

Kapton

has

one ether

linkage

whileNASA'sLARC-IA polymer

has

two ether

linkages

_alongthepolymer chain.This

imparts

a certain degreeof mobility to the polymer

backbone.

The novel research polymer also has two ether

linkages in

90

%

of

its

monomer groups(whichare

identical

to

LARC-IA),

but duetothe

heavy

zirconium pendent_group attached

in

the

remaining

_monomers,

its

_mobilitymay be somewhat

diminished.

The

2-component

polyimide synthesized inthepresentresearch is verysimilarto the

LARC-IA developed

by

_{NASA. LARC-IA}

is

_{prepared from 3,4'-ODA and} 4,4'-ODPA

in

NMP as a 30 %w/w solids solution andthemolecularweight is controlled

by

end_capping groups such as phthalic anhydride

(PA)

whilethe2-componentpolyimide

is

preparedfrom

3,4'-ODA

and4,4'-ODPA in NMP as a 15

%

w/wsolids solution without anyend_cappinggroups.

1.3 Hypothesis

Since the pendent _group structure is massive relative to the mass of the repeat units ofthepolymer_{chain,mobility}ofthechain shouldbe

hindered,

which shouldmake the resin stiffer and more brittle. It is hypothesized that the moduli for a _variety of mechanical properties will be lower for composites with pendent polymer as the resin matrix as compared to those with parent (without pendent _groups) polymer as the resin matrix.

1.4

Major Objectives of

this

Study

There aretwo objectives defined forthis present study. In the

first

objective, the

aim

is

to

identify

the _processing conditions that produce the maximum degree of imidization in the shortest time possible. This would _{potentially be}

important

to

processing industries usingpolyimides, as there wouldbe optimumproduct performance andshortercycletimes to manufacturetheproduct

leading

tohigherproduction rates. To achieve

this,

optimum _processing conditions forthe 2-componentpolyimide

(3,4'-ODA

+

4,4'-ODPA),

3-component "parent" polyimide (3,4'-ODA +

4,4'-ODPA

+ 10 mol

%

4,4'-ODPA + 10 mol

%

MADA + 10 mol

%

zirconium _complex) will be established

by

varyingthe timeandtemperatureparameters of aprogrammable oven.

The second objective

is

to fabricate woven carbon or glass fabric reinforced

composites of the

2-,

3- _{and 4-component polyimide and evaluate} their mechanical

properties

for

structural applications. Comparisons ofmechanical data with laminates

Chapter

II

EXPERIMENTAL

2.1

Chemicals

and

Materials

3,4'-Oxydianiline

(3,4'-ODA)

(TCI,

Tokyo)

and 4,4'-Oxydiphthalic anhydride

(4,4'-ODPA)

(ChrisKev

Company, Inc.)

_{were purified}

by

sublimingthree times at84 C

and 254

C,

respectively. These chemicals were then stored in a _{dessicator containing}

P2O5

to prevent moisture absorption. The

l-methyl-2-pyrollidinone

(NMP)

(Aldrich

Chemical

Co.)

was dried

by

distillation

over

CaH2

just prior to its use.

Benzenehexacarboxylic

acid (mellitic _acid) was obtained from TCL Tokyo. The

zirconium(IV) n-butoxide

[Zr-(0-n-Bu)4]

and

dicyclohexylcarbodiimide

(DCC)

were

obtained from Aldrich and used as received. ER grade potassium bromide

(KBr)

for

infrared

spectroscopic analysis was acquired

from

Aldrich Chemical Co.

Carbon fabric

(Hexcel Style

282)

was received from

Composites

One. Glass

fabric

(Corning

Style

WR24-5X4C)

was obtained from Owens

Corning,

Inc. Other [image:32.529.46.488.398.675.2]

relevant properties are presentedin Table 2.1.

Table 2.1. Fabric Specifications

E- _Glass

Fabric,

_Owens

Corning

Style WR24-5X4C

Weave Style: Plain

Warp

Count: 5 ends/inch

FillCount: 4 ends/inch

Areal Weight: 24 oz./sq.yd.

Avg. Filament Diameter: 17.2 microns

Carbon

Fabric,

Hexcel Style 282 (3K filamentsper

bundle)

Weave Style: Plain

Warp

Count: 12.5 yarns/inch

Fill Count: 12.5 yarns/inch

Areal Weight: 5.8 oz/sq.yd.

Fabric Thickness 8.7mils

2.2 Synthesis

of

2-,

3- _and

4-Component Polyfamic

acid)s

2.2.1

Synthesis

_of

2-component

poly(amic_acid)basedon3,4'- ODAand4,4'-ODPA

A 15 weight % solid solution ofpoly(amic _{acid) using} a 1:1 mole ratio of

3,4'-ODA:

4,4'-ODPA

was achieved.

Triply

sublimed

4,4'-ODPA,

27.3433 g (88.1473 _mmol)

was added to 106 mL ofNMP

in

a 1000 mLroundbottom flask. The reactionflaskwas

capped with a rubber septum and stirred for half an hour and the temperature was

maintained at0

C

_using an

ice-water

bath. A steady flowof nitrogen was maintained in

thereaction

flask.

A solution of_{17.6506 g (88.1472 mmol)}of

triply

sublimed 3,4'-ODA

dissolved

in 65 mL ofNMP was then added dropwise into the flask _using a 10 mL

syringe.

NMP,

75 mL,was used to

rinse

the flask and syringe. Upongradual addition of

the

3,4'-ODA,

the4,4'-ODPA dissolved in thesolution and _ultimately yieldeda_viscous,

homogeneous solution. The poly(amic _acid) was then kept in a cold room and the

solution was _continuouslystirred overnight. The structures forthisreaction are shownin

Figure 2.1.

\ H.N '

P * _lx

NMP,0C

r\

\

/"A_

3,4'-ODA

HOOCv^vs^<Kv^v^CO<

O-OttU

^rd

Poly(amic acid)of3,4-ODA / 4,4-ODPA

Figure 2. 1. Synthesis of

Poly

(amicacid)of 3,4'- ODA/ 4,4 '-ODPA

2.2.2Synthesis_ofMelliticAcidDianhydride

(MADA)

Mellitic acid dianhydride

(MADA)

is

a light brown solid prepared from the

cyclodehydrationofmellitic acid. Themellitic acidwas heated

in

anitrogen atmosphere

at 190-195

acid (~ _{1 1 g)} was placed ina20mLtest tubecovered with a rubberstopper. The testtube

was then

laid

horizontally

in

a

drying

pistol sealed

by

another rubber stopper. The

thermometer,

nitrogen

inlet

and outlettubes passthroughboththe innerandouterrubber stoppers. The sample was rotated

periodically

to achieve uniform

heating

ofthe mellitic

acid

by

turning

the outer rubber _stopper, which inturnrotates theinner stopperand thus

ultimately

rotating

the

inner

tube. The change in chemical structure which occurs is

shown

in

Figure 2.2.

COOH

HOOC

HOOC

COOH

COOH

COOH

0 COOH 0

II

II

.c. ,c

(-2H20)

o

1^ ^^^"

A (-190"C)

\^

^^^^

c c

II

II

0 COOH 0

Mellitic Acid Mellitic Acid Dianhydride

Figure 2.2. Conversion ofMelliticacidtoMellitic Acid Dianhydride

One mole mellitic acid loses 2 moles of waterto _{form MADA. The purity}ofthe

MADA has been confirmed

by

the TGA analysis that shows a _one-step weight loss of

5.88

% due

to the loss of 1 mole of water per mole MADA

forming

mellitic acid

trianhydride(MATA).

2.2.3 Synthesis of3-componentpoly(amic_acid) basedon

3,4'-ODA,

4,4'-ODPA & 10

mol

%

MADA

Synthesis of the 3-component poly(amic _acid)

is

similar to the 2-component

poly(amic_acid) exceptthatinthis case, theMADAand4,4'-ODPAwere addedtogether

to thereactionflask. Amole ratio of1.0:0.9:0.1 of3,4'-ODA: 4,4'-ODPA: MADAwitha

15 weight

%

solid solution was achieved. A total volume of 750 mL solution of

poly(amic _acid) was prepared for _{processing evaluation,} preparation of prepregs and

O o C00H

0 0 NH2

&"

Jo

4,4'-ODPA 3'4'-DA COOH

NMP,0C MADA

COOH

O O -9x HOOC

^"^COOH

COOH 3,4'-ODA/4,4'-ODPA

3,4'-ODA/MADA

Figure

2.3.

Synthesis

ofPolyfamic_{acid) of 3,4 '-ODA / 4,4 '-ODPA / 10}mol

%

MADA

2.2.4

Synthesis

_of

4-component

poly(amic _acid) based on

3,4'-ODA, 4,4'-ODPA,

10

mol

%

MADA & 10mol% Zrcomplex

[Zr(adsp)(dsp)J

Some

ofthe 3-componentpoly(amic _acid) solution (160 mL)prepared above was

then separated in to two portions. One portion was stored for characterizing and

composite

fabrication

purposes while the other part was used to synthesize the

4-component poly(amic acid). The zirconium complex pendent poly(amic _acid) was

synthesized

by

the addition ofthemixed ligand complex

Zr(adsp)(dsp)

to thepoly(amic

acid) backbone

in

the presence ofN,N'-dicyclohexylcarbodiimide (DCC). The reaction

of

Zr(adsp)(dsp)

with carboxylic acid groups on the MADA moieties ofthe poly(amic

acid) was promoted

by

dehydration due to the DCC. Yellow

Zr(adsp)(dsp)

_powder,

3.546 g (4.892 mmol), along with 9.70 mL of NMP was added to the reactor flask

containing the poly(amic _acid) solution (160 mL). A stoichiometric amount of DCC

(0.9955 g, 4.824 mmol) was

first

dissolved in NMP (2.70 mL) solution and then added

dropwiseto the polymer solution viaa5 mL syringe. Additional NMP wasused to

rinse

the syringe and the container three times and added to the reaction flask. As DCC was

added, gel formationwas observed onthesurface. After vigorously shakingthe

flask,

the

geldisappeared. Thereactionwasmonitored atregular

intervals

by

_spottingthesolution

on a silica gel TLC plate and _eluting withmethylene chloride/ ethyl acetate

(93:7).

The

Zr(adsp)(dsp)

andnon-pendentpoly(amic_acid) were usedas references ontheTLCplate

andviewed underaUV lamp. A spotdueto the

free

Zrcomplex was still observed after

added was not attached to the polymer backbone. An additional 10 mole % ofDCC /

NMP solution was added and a

TLC

testperformed again showed no spotdueto the

free

Zr complex, which meant that the _appendingreaction had been completed. The viscous

polymer solution was then stored in a cold room with continuous stirring. The reaction

0.9x

O O COOH

jk..

O

J?

__

I

0JO TX,0

+ lx

H2N<>OHQ

+ _{0.lx 0^(1^0}

O O

NH2

5T5

4,4'-ODPA 3,4'-ODA

NMP,0C

COOH

MADA

' ' _COOH

/ HOOCN^5t.Ov^s_COOH ,

O I O

O O

-9xX

HOOCV^COOH

3,4'-ODA/ODPA

DCC

Zr(adsp)(dsp)

HOOC^T

COOH

COOH

3,4'-ODA/MADA

COOH

O n

0.9xV

HOnc^V^COOH

Variable

Processing

Conditions

HOOC'Y'COOH

c=o

NH

H^ _H

C:N N:Cn

C=N

N=Q

H

M

H

lx

i.-fcr^Cu

Q*pp-P

H H

2.2.5 Precipitation

ofPolyfamic_acid)Solution

Portions

ofthepoly(amic _{acid) solutions made above of2 g}

each, were separated

from

the

bulk

solution for precipitation. The poly(amic _acid) solution was added to a

1000 mL

beaker filled

with

450

mL of anhydrous ether

(ethyl

_{ether) using}adropper. The

solution was

continuously

_{stirred andthepoly(amic}

acid) precipitatedinthe ether. Fresh

ether was added to

facilitate

the removal oftheNMP

from

the polymer. Forlargerscale

precipitations, a

blender

was used tobreak theprecipitated

lumps in

to afiner powderin

fresh

ether solution. The powdered precipitate was then

filtered

and dried in a vacuum

oven

for

2

days.

Amortar and pestle was usedtogrindthe

dried

powder toa

finer

grade.

Precipitation

was carried out on all three polyimides _{for establishing} the _processing

conditions.

2.3

Establishing

Optimum

Processing

Conditions

2.3.1

Heating

Steps

Toestablishtheoptimum_{processingconditions, the time}andtemperature settings

of a programmable oven (Lindberg/Blue Model

2416)

were varied. One ofthe most

popular

heating

cycles inthe literature for

imidization

consists of

heating

the poly(amic

acid) at 100 C for an

hour,

followed

by

200 C

for

an hourand

finally

at300 C for an

hour [10]. Ten samples of each polyimide precursor (i.e. poly(amic _acid),

PAA)

were

subjectedto the

heating

steps reflectedin Table 2.2.

Table 2.2.

Heating

Steps forthepolyfamic_acid)powder

Processing

Condition

Temperature

(C)

Time

(Hours)

Temperature

CC)

Time

(Hours)

Temperature

(Q

Time

(Hours)

1 100 200 --

--2 100 200 300 0.5

3 100 200 300 1

4 100 200 300 1.5

5 100 200 350 0.5

6 100 200 350 1

7 100 ~ 350

0.5

8 - -- 240 0.25 --

--9 - -- -- 310 _0.25

10 -- 310

2.4 Characterization

techniques

Characterization

has

been

performedto

determine

whichset of_processing

conditions

is

superior. If

incomplete

imidization

or

decomposition is

_seen, then those

processing

conditions

have

beenrejected. We

have

characterizedthe

2-component,

parent

and

Zr-pendent

polymers

by

_carrying out Fourier Transform Infrared

Spectroscopy

(FTfR),

Thermogravimetric Analysis

(TGA)

and Differential

Scanning

Calorimetry

(DSC)

on each.

2.4.1 Fourier Transform Infrared

Spectroscopy

(FTIR)

This technique has been usedto reveal theextent ordegree ofimidization

taking

place

in

the sample.

Spectra

have been analyzed

by

aband ratio method. Peak

intensity

ratio

for

the

imide

ring

stretch _occurring at 1779 cm"1

and areference aromatic peak at

1012 cm"1

(etheroxygen-to-aromatic

ring

_{stretching vibration) has been}calculated [16].

Degree of

imidization

has been calculated

by

_normalizing a sampleband ratio with one

showingthemaximum bandratio. Foranalysis, each peak was enlargedto minimizethe

measurement errors [17]. A Bio-Rad Excalibur Series FTS 3000

instrument

was used to

analyzethe samples. ThepolymerandKBrpowder(IR

Grade)

weregroundedtoobtaina

gooddispersionandthespectra were recorded_usingadiffusereflectancecell.

2.4.2 ThermogravimetricAnalysis

(TGA)

TGA indicates

(1)

_{if any} weightlossoccurs dueto incomplete

imidization,

or

(2)

for

the Zr-pendent polymer, the extent ofdecomposition that takes place in the sample

during

processing, if any. The decomposition of the Zr-pendent polyimide can be

calculatedby:

Expected Wt ofundecomposedpolyimide

=

gZr02x

(\

moleZrO,\\ (\ moleZrpendentgroup")x(\ moleZrpendentmer^l x

ftTl

187.3g^ +9

(

474.4g

Yl

ll23.2gZrO^

I

lmoleZr02

)

Kl

moleZrpendent_grp

J

LUmoleZrJ

ll

molenon-JJ pendent mer pendentmer

....Eqn. 2.1

Ifthe difference between the expected weight and the actual weight

is

0,

then

0,

then either the sample has not

been

_completely

imidized

or some

Z1O2

was lost

from

the TGApan due to the airflow and ifthe

difference between

the expected weight

and the actual weight

is

<

0,

it

means that the sample has decomposed at the chosen

processingcondition.

A TA

Instruments

TGA

2050

was usedtoperformtheanalysis. Samplesweighed

between 10 and20 mgand were

heated

at10 C /min inair atmosphere_upto 800 C.

Molding

cycles used

for fabrication

ofthe composite laminates in the autoclave have also been emulated in theTGA furnace to _study

decomposition

taking

place in the

polymer,

if

any. The

2-component,

parent and pendent poly(amic acid)s wereheated up

to 370 Candheldatthis temperature

for

anhour innitrogen atmosphere.

2.4.3 Differential

Scanning Calorimetry

(DSC)

This characterization technique

is

used to

indicate

the effect of _processing on

glasstransition temperaturesofthedifferentpolymers.Itwouldalso

identify

thepresence

of unimidized polymer if any. The analysis was carried out in a nitrogen atmosphere (flow rate of80 mL/min) in a TA Instruments DSC 2010 machine. The samples were

hermetically

sealed and weighedbetween 5 and 10 mg. The

following heating

cycle was

used:

Step

1:

Ramp

@

10 C/minto450 C

Step

2: Isothermal for 10min

Step

3:

Ramp

@

10 C/minto25 C

Step

4: Isothermalfor 10 min

Step

5:

Ramp

@

10 C/minto450 C

Step

6: Isothermal for 10min

Step

7:

Ramp

@

20 C/minto25 C

2.5

Fabrication

of

Composite Laminates

A vacuum oven(National Appliance

Co.,

Model

5831)

was used for making the

prepregs and the laminates were fabricated in an autoclave

(Thermal Equipment

Corporation). The carbon and glass reinforced composites were

fabricated

atthe

Center

2.5.1

_{Fabrication ofPrepregs}

Prepregs

are

defined

as a

reinforcing

material

impregnated

with resin priorto the

moldingprocess and cured

by

theapplication of

heat.

Theprepregs were fabricated using

the conventional

hand

lay-up

technique. The amount ofresin needed for each _ply was

calculated

by

following

theprocedure given

below:

Tomaintain a

fiber

toresin ratio of60:40

by

_weight,

Wresin

needed=

Wfiber

x

40_

Eqn. 2.2

60

Fora 15 weight% solids_{solution, the}weight ofthesolution needed:

0.15=

Wresin

Eqn. 2.3

"resin '

"solvent

Where,

Wresin

=_Weight

oftheresin

(g),

Wsolvent

=_Weight

ofthesolvent

(g),

and

Wresin+WsoiVent

=_Weight

ofthesolution

(g

)

Theprocedure

for

_makingtheprepregsisas follows:

1)

In orderto make a

4-ply

laminate,

asingle sheet of 11"

x 7"

ofthe wovenfabric

wascut.

2)

The sheet wasweighedon a

laboratory

electronicbalance and placedon a plastic

tray

coveredwith a releasefilm.

3)

Theamountofresinandhence theamount of solutionneeded was thencalculated

to obtain the desired 60:40 fiber to resin

by

weight ratio _using the equations

shownabove.

4)

Theweighed amountofsolutionwas pouredina

beaker,

plus some extra solution

was weighedtocompensateforsolutionthatwouldstickonthewalls ofthe tray.

5)

The brushwasdipped inthesolution foraround 5minutestosaturate

it.

6)

Theresin was applied_usingthebrushandtheexcess resin was squeezed out_using

arollerthatalso facilitated_{wetting the}fabric.

7)

Resinwasthenapplied onthereverse side andtheexcess resin was squeezed out.

9)

The prepregwas placedonthefilm and

heated

at 65 C for 30minutes. Thiswas

done to retain some solvent

facilitating

the _mobilityofthe polymer chains in the

presence ofthe_plasticizingaction ofthe solvent.

10)

The prepreg was weighed and

if

the ratio of

fiber

to resin was not _reached, then

more resin was applied

followed

by heating

inthevacuum oven.

1

1)

Theabove process was repeated until the

desired

ratiowasreached.

12)

Thesheet wasthencut

into

fourplieseach

having

adimensionof

7"

x2.75".

13)

Theprepregs were wrapped

in

an aluminum foiland storedinthecoldroom.

For single-ply

laminas,

sheets of 7"

x

4"

of the woven fabric were cut and

prepregs were made_usingthe above procedure. All the_prepregsheets (single plyand

4-ply) had aresincontentof~52

%

by

weightto accountforthesolventlosses andthe loss

of water

during

imidization

taking

place in the autoclave. This was done based on the

following

calculation/ argument:

Fora2-componentpolyimide,

molecular weight oftherepeat unit=_510.44

g/mol

Sincetwomolecules of waterarelostupon

imidization,

loss

in

weightdueto water=_{36 x 100} =~ 7%

510.44

Itwas also observedthat

during

the vacuum oventreatment oftheprepregs, there was a

lossof~4

weight% duetosolvent evaporation. In ordertoachieve40%

by

weightofthe

resin,

resintobeappliedtothefibers= _40%+7%+4% = _{51 %}

2.5.2Fabrication _ofLaminates

Fora_single-ply

lamina,

the prepregwas laidout ontheautoclave

table,

while for

making a

four-ply

laminate,

4 plies were stacked on

top

of each other. The entire

assemblywasvacuumbaggedand placedinsidetheautoclave.

Composite

laminationwas

performed _using a Thermal EquipmentCorporation autoclave under controlledpressure,

Table 2.3.

Autoclave Parameters

Recipe Parameters Units

Heating

Cycle

Cooling

Cycle

Target Temperature F 702 230

Soak Time min 60 1

Heating

Rate F/min 20 5

Target Pressure psi 250 250

Target Vacuum

in.

Hg

30 30

2.5.3 Resin Content ofLaminates

Two methods were employed to calculate theresin content ofthe laminates after

autoclaveconsolidation.

Method I: Theresin content ofthelaminates was found accordingto ASTMD

2584-94,

Standard Test Method for Ignition Loss of Cured Reinforced Resins. A 1 cm x 1 cm

sample was cut from each laminate and the resin content was obtained

by following

the

procedure described below:

An empty ceramic crucible was heated in a furnace at 560 C for about 15

minutes. Itwas thencooledtoroomtemperature inadessicatorand weighed. The sample

wasplaced

in

thecrucibleandweighed precisely. Itwasburned in afurnace at560C for

about 1 hour. Theresin burnedoff_completelyandthe cruciblewas allowedtocoolback

toroomtemperature

in

thedessicator. Theweight ofthe fiberswas recordedandtheresin

contentwascalculated _usingtheformulagivenbelow:

%

Resin Content=_W

iaminate

- W

fibers Eqn. 2.4

W laminate

Where,

W

laminate=Weightofthecompositesample

(g),

Wfibers=Weightofthefibers

(g),

and

Wlaminate- W

Method

II: This is a _very rough technique employed to

determine

the resin content. In

this method, the weight of a comparable size sheet of

fiber

was recorded before the

application of_any resin. The

laminates

were then weighed on an electronicbalance after

fabrication,

and resin content was

determined

_usingequation2.4.

2.5.4

Machining

_of

Composite Laminates

Various specialized techniques such as water-jet or _{laser cutting for machining}

composite

laminates

have

been

developed

[13]. Prior to mechanical

testing,

the

composite

laminates

were machined _using a laser beam and specimen edges were

smoothed_usingsand paper(Grade

320)

toASTMrecommendeddimensions.

2. 6 Mechanical Properties

of

Composite Laminates

Variousmechanical testssuch as

Tensile,

Flexural (three-point

bend),

Izod Impact

andDynamic Mechanical Analysis

(DMA)

were conducted onthecomposite specimens.

Thetensile andDMA tests wereperformedon single _{ply laminas}while the flexural and

Izod

impact

testswere performed on

four-ply

laminates.

2.6.1 Tensile

Testing

Tensiletest

is

a measurement ofthe_abilityofthematerialtowithstand

forces

that

tend to pull

it

apart and to determine to what extent the material stretches before

breaking. The tension test has been used to measure the _{tensile modulus,} strength and

ultimate strain to failure for composites. Tests were performed on single _{ply laminas} on

anMTS 45G Universal

Testing

Machine

(U.T.M.)

_accordingtoASTM

D3039/D3039M-95a,

Tensile Properties ofPolymer Composite Materials. The machine was computer

controlled_usingaTest Works

4

software.

2.6.1.1Specimen Preparation

Bonding

ofendtabsto tensilespecimen

Conventional dog-bone specimens fortensile

testing

have

a

tendency

to split

in

the region where the width changes. For fiber composites, uniform width

(rectangular)

used. Theend-tabs were

bonded

to the testspecimensto minimize stressconcentrationat

load

introduction

regions at the specimen ends. A Hysol

9303

grade adhesive (Dexter

Adhesives, Inc.)

was used to

bond

the end-tabs to the tensile specimen. The procedure

described

belowwas

followed [13]:

1)

Using

a sand paper

(grade

220or 1

80),

theregions ofthepanelwheretheendtabs

are

bonded

were sanded

in

+45

motions.

2) Using

a wire

brush,

the

loose

particles were_carefullyremoved.

3)

Thesurface was cleaned with acetone until alltheloose

fibers

were removed.

4)

The surface ofthe endtabs were sanded and cleaned _according