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^J AFML-TR-74-88

DEVELOPMENT OF HIGH TEMPERATURE ADDITION-CURED ADHESIVES

PQ

N. BILOW R. H. BOSCHAN

H. RAECH

ADVANCED MATERIALS DEPARTMENT MATERIALS AND PROCESSES LABORATORY

AEROSPACE GROUP HUGHES AIRCRAFT COMPANY

CULVER CITY, CALIFORNIA 90230

TECHNICAL REPORT AFML-TR-74-88

MAY 1974

D D C gnriffn3

6 I9?4 i I DEC 6 IS

lEisannaaJ B

Distribution limited to U.S.Govemment agencies only; test and evaluation, May 1974. Other requests for this document must be referred to the Air Force Materials Laboratory, Nonmetallic Materials Division, Composite and Fibrous Materials Branch, AFML/MBC, Wri^it-Patterson AFB, Ohio 45433.

AIR FORCE MATERIALS LABORATORY WRIGHT-PATTERSON AIR FORCE BASE, OHIO 45433

^ « - - — ■ ■ ■ ■ - -■ ■--'- — - -

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NOTICE

When Government drawings, specifications, or other data are used for any purpose other than in connection with a definitely related Government procure- ment operation, the United States Government thereby incurs no responsibility nor any obligation whatsoever; and the fact that the government may have for- mulated, furnished, or In any way supplied the said drawings, specifications, or other data, is not to be regarded by implication or otherwise as in any manner licensing the holder or any other person or corporation, or conveying any rights or permission to manufacture, use, or sell any patented invention

that may in any way be related thereto.

This technical report has been reviewed and is approved for publication.

Proj^t Monitor

FOR THE COMMANDER

T. J. REINHART, JR., Chief Composite and Fibrous Materials Branch Nonmetallic Materials Division

^ris^xiÄ^^^^ ^rr.i^rt^. AIR FORCE/56780/8 November 1974 - 200

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AFML-TR-74.88

DEVELOPMENT OF HIGH TEMPERATURE ADDITION-CURED ADHESIVES

N. BILOW R. H. BOSCHAN

H. RAECH

Distribution limited to U.S.Government agencies only; test and evaluation, May 1974. Other requests for this document must be referred to the Air Force Materials Laboratory, Nonmetallic Materials Division, Composite and Fibrous Materials Branch, AFML/MBC, Wright-Patterson AFB, Ohio 45433.

- 1111 ■ in - I Ulli

—— __^

•

FOREWORD

This technical report was prepared by Hughes Aircraft Company

^n".UlUc M..'ri.l. Dlvl.l.., Air Force Material. Laboratory, wth Mr. T. .1. Aponyi (AFML/MBC)«! the Project Engineer.

ThU re. , rt tov-ers the work performed from 1 March 1973 to .8 F.bru.ty 1974' It is .ubmitted in partial fulfilln.ent of the contract.

Ih. work »as performed in the Advanced Materials Department of the

llryt^Luber was provided by Mr. Raymond E. Lawrence and Mrs. Deborah Smythe.

This report was submitted by the authors in March, 1973.

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ABSTRACT

Two systems were evaluated as adhesives which cure via addition reactions. These were (1) nitrile substituted polyphenyl ethers cured with terephthalonitrile N, N-dioxide (TPNO) and (2) acetylene terminated polyimide (HR600) cured thermally. The thermally cured acetylene terminated polyimide proved to be superior to the TPNO cured cyano substituted polyphenylene oxide. Lap-shear strengths of 6A14V titanium bonded with HR600 adhesive exceeded 3000 psi at ambient temperature on powdered aluminum-filled specimens reinforced with heat cleane« 112 glass fabric and 2800 psi at ambient temperature, 2600 psi at 4S0oF and 2100 psi at 500oF on inlilled specimens bonded with 70 percent HR600 prepreg on 112 heat- cleaned glass fabric. Weld-bonding of titanium specimens using aluminum powder filled HR600 adhesive proved to be successful and showed superior fatigue resistance over spot-welded specimens.

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CONTENTS

I

II

1U

IV

INTRODUCTION 1

SUMMARY

CYANO SUBSTITUTED POLYPHENYLENE OXIDE ADHESIVES CURED WITH TEREPHTHALONITR1LE N. N' -DIOXIDE

7 A Background * Q B Effect of Carrier Cloth on Lap Shear Strength J C Thermal Aging Tests '.'.'.'.'.'. iq D Syntheses of Reactive Diluents ...'...'.'. 20 E Discontinuation of Work

THERMALLY CURED ACETYLENE TERMINATED POLYIMIDE ADHESIVES ' il

A Background ' B Lap Shear Strengths of Acetic Anhydride and

Cresol Imidized Polyimide • C Effects of Aluminum Filler Level and Priming Upon

Lap Shear Strength ** D Adhesive Prepreg Studies ^ E Aluminum Powder Filled Adhesive Prepreg •■ F T-Peel Tests . . . 50 G Weldbonding -n H S/N Tests

EXPERIMENTAL

Preparation of 4, 4'-Diacetyl Diphenyl Ether (Experiment No. G1344-90) J!

(Experiment No. H1060-I6) / ' ' * ,* * ' ,\* ' ' * Attempted Preparation of 4, 4'-Bis(l-chloro-2-formylvxnyl)

Diphenyl Ether and 4, 4"-Diethynyl Diphenyl Ether (Experiments No. G1344-92 and -95)

Preparation of 4, 4,Bis( 1-chloro-2-formylvinyI) Diphenyl Ether (Experiment No. G1343-08) bi

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CONTENTS (Continued)

VI.

4, 4'-Diethynyl Diphenyl Ether (Experiment No. G1343-11) .... 62 General Procedure for Preparing Titanium Lap Shear

Specimens Containing Cyano-Substituted Polyphenylene Oxide/TPNO Adhesive 62

Imidization of Amic Acid Precursor to Acetylene- Terminated Polyimide (Experiment No. Gl 344-94) . 63

Preparation of HR 600AC Polyimide Prepregs ^4

REFERENC ES 65

VI

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LIST OP^ ILLUSTRATIONS

Figure

1

n

9

10

1 1

Page

Lap Shear Strengths of Cyano Substituted Polyphenylene Oxide/TPNO Adhesive at 450oF H

Lap Shear Strengths of Cyano Substituted Polyphenylene Oxide/TPNO Adhesive at 500oF 12

Lap Shear Properties, CSPPO/TPNO Polymer, Titanium Adherend, Unfilled Resin, Heat-cleaned 1 12E Glass Cloth Reinforced 14

Lap Shear Properties, CSPPO/TPNO Polymer, Titanium Adherend, 15 Weight % Aluminum Powder Filled, Heat- t leaned 112E Glass Cloth Reinforced 15

Lap Shear Properties. CSPPO/TPNO Polymer, Titanium Adherend, 50 Weight % Aluminum Powder Filled, Heat- Cleaned 112E , Glass Cloth Reinforced 16

Lap Shear Strength of CSPPO/TPNO Bonded Titanium Adherend! as a Function of Aging at 450oF. 0-50 Weight % Aluminum Powder Filled, Heat-Cleaned 112E Glass Cloth Reinforced, Aged and Tested at 450 F 17

Lap Shear Strength of CSPPO/TPNO Bonded Titanium Adherend! as a Function of Aging at 500oF. 0-50 Weight % Aluminum Powder Filled, Heat-Cleaned 112E Glass Cloth Reinforced, Aged and Tested at 500 F 18

Influence of Imidization Method on Lap-shear Strength, 50 Weight % 325 Mesh Aluminum Filled, Heat-cleaned 112E Glass Cloth on Titanium Coupons 25

Lap Shear Strengths of HR 600AC as a Function of Filler Content - Ambient Temperature Tests 27

Effect of Filler Content on Lap Shear Strengths of HR 600AC Bonded Titanium-450oF Tests 28

Influence of Various Parameters on Lap Shear Strength of Polyimide Adhesive (cured 4 hours at 500 F. ) 30

vu

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LIST OF ILLUSTRATIONS (Continued)

Figure

12

l I

14

15

16

17

18

19

20

21

11

24

25

26

Influence of Cure Time on Ambient and Elevated Temperature Properties of 70 Percent HR 600AC Prepreg Bonded Titanium, 500oF Cure

Influence of Cure Time on Ambient and Elevated Temperature Properties of 80 Percent HR 600 AC Prepreg Bonded Titanium, 500oF Cure

Influence of Cure Time on Ambient and Elevated Temperature Properties of 70 Percent HR 600AC Prepreg Bonded Titanium, 5250F Cure

Influence of Cure Time on Ambient and Elevated Temperatur« Properties of 80 Percent HR 600AC Prepreg Bonded Titanium, 5250F Cure

Imlaence of Cure Time on Ambient and Elevated lemperature Properties of 70 Percent HR 600AC Prepreg Bonded Titanium, 600oF Cure

Influence of Cure Time on Ambient and Elevated Temperature Properties of 80 Percent HR 600AC Prepreg Bonded Titanium, 600oF Cure

Influence of Cure Time on Ambient and Elevated Temperature Properties of 70 Percent HR 600AC Prepreg Bonded Titanium, 650oF Cure

Influence of Cure Time on Ambient and Elevated Temperature Properties of 80 Percent HR 600AC Prepreg Bonded Titanium, 600oF Cure

Influence of Cure Temperature on Properties of HR 600AC Prepreg Bonded Titanium - Ambient Temperature Tests

Influence of Cure Temperature on Properties of HR 600AC Prepreg Bonded Titanium - 450oF Tests

Influence of Cure Temperature on Properties of HR 600AC Prepreg Bonded Titanium -500oF Tests

Fatigue (S/N) Tests on Spotwelded Titanium Specimens . . .

Fatigue (S/N) Tests on Spotwelded Titanium Specimens Bonded With HR 600AC Thermally Cured Acetylene Terminated Polyimide

Fatigue 'C'V) Tests on Titanium Specimens Bonded wih f}, 600 ^Y Cured Acetylene Terminated Polyimiuc

Failure Modes Observed on Weldbonded Titanium Lap Shear Fatigue Specimens

Page

31

31

32

32

33

33

34

34

35

35

36

51

51

52

54

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IX

1

LIST OF TABLES

Table Page

I Effecl of Carrier Cloth Upon Lap-Shear Strength of Titanium Samples Bonded with Aluminum-Filled Cyarn.)-Substituted Polyphenylene Oxide TPNO Copol rner (1) 9

II Lap-Shear Strengths of Titanium Specimens Bonded With Mixtures of Acetic Anhydride-Imidized Polyimide and C resol-Imidized Polyimide 23

III Lap Shear Strengths of Titanium Specimens Bonded With HR 600AC Prepreg 37

IV Lap Shear Strengths of Titanium Specimens Bonded With 600AC Preoreg 43

V Ambient Temperature Lap Shear Strengths of Titanium Soecimens Bonded with Aluminum Filled HR 600AC F repreg 48

VI Results of T-Peel Tests on Titanium Specimens Bonded with HR 600AC 49

VII Fatigue Test Spotwelded Titanium Specimens 55

VIII Weld/Bond Fatigue Test Weldbonded Titanium Specimens (Spotwelded with Adhesive Bond) 56

IX Fatigue Test Bonded Titanium Specimens HR 600AC Adhesive 57

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I. INTRODUCTION

The coming generation of supersonic aircraft will require structural

adheaivei capable of withstanding extended use in the 450-600OF temperature

ran« . Such adhesives can be prepared only from polymers with aromatic

■tructure». Most conventional aromatic polymers, however, are cured by

condensation processes which produce volatile by-products. These volatiles

tend to expel adhesive from the bond during cure, leaving voids in the adhe-

sive, and thus impairing the strength of the bond.

A number of molecular features are required of a high temperature

structural adhesive. For high cohesive strength, a high molecular weight

crosslinked polymer is necessary, since forces of attraction between cova-

lently bonded atoms are far greater than those between adjacent molecules.

The chemical makeup of the polymer must be such that it consists of units

having known resistance to bond rupture under extreme thermal, oxidative

and hydrolytic conditions. The backbone of the polymer should contain struc-

tural units which prevent excess rigidity, so that stresses due to change in

temperature are not created.

For optimum adhesive strength, the adhesive must have adequate

fluidity prior to cure to provide for a low contact angle (good surface wetting)

with the substrate, in addition to good flow properties for use with fillers and

carrier cloths. Cure reactions involving strictly addition processes will, by

eliminating volatile cure products, greatly reduce or eliminate formation of

both adhesive-adhe rend interface voids and adhesive-matrix voids during

cure. Furthermore, elimination of void formation during cure by utilization

of addition type cure processes will greatly enhance hydrolytic stability by

retarding entry of water vaoor into the polymer matrix.

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To meet the above requirements for a high temperature adhesive

which cures by addition, two types of adhesive systems have been studied in

this program. These are: (I) Cyano- or ethynyl - substituted polyphenylene

oxide prepolymers cured with terephthalonitrile N . N'-dioxide(TPNO), (2)

Homopolymerizable acetylene-terminated polyimide. Both of these systems

cure to resonance-stabilized thermally stable aromatic polymers. Cross-

linking reactions occur during high temperature cure in both systems. In

addition, both systems feature aromatic ether umts in the prepolymers and

in the subsequently tormed polymer matrix to lend a degree of flexibility and

toughness to the polymeric adhesive.

Early In 1971 under AFML contract, Hughes Aircraft Company began

lh» ievelopnu nt of thermally stable adhesives that could be cured at readily

attainable temperatures by acicüüon reactions which liberated no gaseous by-

products. The material, investigated were nitrile-substituted or nitrile-

■ -nnnated polyphenylene oxides and nitrile-terminated polyimide oligomers

c ■. alning terephthalonitrile N.N'-dioxide as a curing agent. Adhesive com-

positions were found that could be cured by heating to 80OC (1760F) without

producing significant quantities of gaseous by-products. Titanium lap-shear

specimens bonded with an adhesive of this type under a pressure of 15 psi

had tensile strengths up to 2500 psi at 450OF. Thus, the adhesive was demon-

strated to have considerable potential for meeting the stated objectives.

In a continuation of work on addition cured laminating resins, the dis-

covery was made at Hughes that a difunctional acetylene substituted polyimide

oligomer readily cured to an intractable polymeric resin at temperatures

slightly above its melting point. This material, which has been designated

as HR600. showed great success as a laminating resin in the preparation »f

glass fiber and graphite reinforced composites.

The extension of this technology to the application of HR600 as a

thermally curing adhesive for titanium substrates was attempted and found

to be successful, and effort was expended on the program to develop an

adhesive system, based upon thermally curing HR600. for bonding of

titanium.

- - - mmmmm.

The adhesive strengths of this polymer displayed a dependency upon

the method of imidization of the amic acid precursor. Two methods of imidi-

zation have been utilized. These are: (1) refluxing in acetic anhydride and

(2) refluxing in a cresol-benzene mixture. The imidized oligomers from

these two methods of imidization were designated, respectively, HR 600A

and HR 600C. Lap shear studies showed that a mixture of HR 600A and

HR 600C was a superior adhesive to either of these alone.

Further development work on this adhesive system was necessary.

The required studies included a study of fillers and carrier cloths, prepara-

tion of glass cloth prepreg adhesives, optimization of cure cycles, and a

study of the use of this adhesive for titanium weldbonding applications. These

.sludie1- were carried out during the past year.

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II, SUMMARY

The work performed on high temperature adhesives during the past

year is a continuation of the adhesive study previously combined with the

devtl pment of high temperature laminating resins under Air Force Contract

Number F33615-71-C-1228 entitled "Development of High Temperature

Laminating Resins and Adhesives Which Cure Through Addition." The adhe-

■ive study part of this program subsequently was continued under Air Force

Contract Number F336 1 5-73-C-50t.2. The studies reported here are a con-

tinuation of those reported in the previous summary report. A FML-TR-72-57.

Part II. June 1973. Two resin systems were examined as addition cured adhesives for

titanium daring the course of this program. The studies on nitrile sub-

stituted polyphenylene oxide cured with terephthalonitrile N.N'-dioxide were

directed toward experiments on fillers, carrier cloths and thermal aging.

Attempted preparation of a reactive diluent to replace the inert solvent xylene

used in the adhesive formulation was not entirely successful due to difficulties

encountered in purification of products. Studies on TPNO cured polynitrile substituted polyphenylene oxide

adhesives containing xylene in the adhesive formulation were discontinued at

the request of the AFML project monitor. Developmental research on the

Hughes developed acetylene terminated polyimide. designated as HR 6,00. as

a high temperature adhesive was emphasized. Adhesive formulations contain-

ing glass carrier cloth and aluminum powder filler gave ambient temperature

strengths in excess of 3000 psi. Glass carrier cloth impregnated with a

N-methylpyrrolidi-ione solution of the polyimide served as an excellent pre-

preg adhesive. With an optimum cure cycle, adhesive lap shear strength in

excess of 2800 psi at ambient, 2b00 psi at 450OF and 2000 psi at 500OF were

PR&CSDIIC PAGE BUNK.NCT FILMED

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ob^rved in A. .b.ence of filler,. W.ldbond Studie, were made and

preiiminary S/N (fatigue, teata Shuwed superior fatigue ""'*"'" °'"'"-^ bonded specimens over spotWded specitnens. Adhe.ive and «eldbond studt s

wlU, this thermally cured acetylene terminated polyimide are current y being

extended under a continuation of Air Force Contract «3616-73-0-5062.

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characteristics for the adhesive in contact with the titanium substrate. The

vehicle chosen for this purpose was xylene. due to the fact that it is an excel-

lent solvent for the cyano-substituted polyphenylene oxide I and it has a boil-

ing point substantially higher than the initial cure temperature of 80ÜC.

Thus, a low temperature curing, low void adhesive was formulated

from I. II and xylene in the relative weight ratio 1:1:0.7. Initial cure was at

80OC (1760F), followed by postcure at l60OC (320OF) and 260OC (500OF).

This adhesive system has been extensively tested for bonding 6A14V titanium

alloy. Experiments performed on previous programs indicated that a

1:1:0.7 weight ratio of oligomer:TPNO:xylene was close to the optimum.

Smaller amounts of xylene appeared to diminish the ability of the adhesive to

wet the metal. Although the xylene remains dissolved in the adhesive during

cure and did not cause channeling, it was desirable to reduce or eliminate

the xylene so that a denser and stronger adhesive bond would be obtained.

Microscopic voids are left in the adhesive after the xylene diffuses out during

postcure. and the strength of the adhesive is adversely affected.

Thus, in order to improve the utility and reliability of the polycyano-

substituted polyphenylene oxide adhesive system, it appeared necessary to

replace the xylene with a reactive diluent, i.e. . one which serves as a solvent

for the prepolymer and which participates in the cure process by becoming

an integral portion of the final polymeric matrix after postcure. Synthesis of

one possible reactive diluent, 4, 4'-diethynyl diphenyl ether, was attempted.

Preliminary studies on the use of fillers and carrier clothe had been

made on the previous program and continuation of this effort was required.

Thermal aging tests were also necessary and this additional work has since

been carried out and is reported herein.

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B. EFFECT OF CARRIER CLOTH ON LAP SHEAR STRENGTH

In order to determine, the optimum carrier cloth for bonding of

Utanium substrates with cyano substituted polyphenylene oxide-TPNO copoly-

mer. a study was made of the effect of carrier cloth on lap shear strengths.

In all cases, the adhesive contained aluminum powder. As expected, the best

of three earner fabrics tried was lUE heat cleaned glass fabric. The

results of these lap-shear tests are shown in Table 1.

TABLE i EFFECT OF CARRIER CLOTH UPON LAP-SHEAR STRENGTH ' OF TITANIUM SAMPLES BONDED WITH ALUMINUM-

FILLED C VANO-bUlibi 11 u i r-.u r-wi^i J OXIDE TPNO COPOLYMER (li

— -i

r i Adhesive Test Carrie r Lap-Shear

Average Lap-Shear

Spec imen

1

rl hickneu in. x- I0J

i r n 1 s l

T« inperature, oF

Cloth (2)

Strength (k psi)

Strength (k psi)

=4 HR 52C 1 i 20 Ambient a I. 3 1. 4

I

3

4

5

16

12

20

17

1.4

1.8

1. 1

1. 3

HR 52C 6 17 450OF a 1.0 1. 2

7 16 1. 5

8 19 1. 3

0 18 1. 1

in 16 1. 3

HR 60A 1 14 Ambient b 1.8 2. 0

2 9 2.2

3 7 2. 1

4 8 2. 1

5 15 1.8 „

(Contin ued next page )

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(Table I. concluded)

Specimen

Adhesive Thickness in. x 103

(mils)

HR 60A (>

7

8

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10

HR )0B )

1 I

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7

8

9

10

15

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9

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20

21

17

17

14

19

19

15

15

Test Temperature,

op

450OF

Carrier Cloth

(2)

Lap-Shear Strength

(k psil

Average Lap-Shear Strength

(k psi)

A mbient

450OF

1.7

2.0

2. 3

1.9

2. 0

1.6

1. 5

1. 3

1. 7

1.6

1.6

1. 0

1.4

1. 2

1.6

2.0

1. 5

1.4

( 1 Cvano substituted polyphenylene oxide: TPNO: Xylene = 1:1:0.7 (wt. -atio)

21 Carrier Cloth la. 1^4/A1100 Glass Fabric, lb) 112E heat cleaned glass fabric, (c) 112E Volan A cleaned glass labnc.

C. THERMAL AGING TESTS

Thermal aging tests on TPNO cured cyano substituted polyphenylene

oxide adhesive (CSPPO) have been completed. The tests were conducted at

ambient, 450OF and 500OF( in 100 hour increments from 0 to 500 hours. All

specimens were reinforced with glass cloth. The specimens represented by

Figures 1 and 2 were reinforced with 904 scrim and aging was conducted with

both filled and unfüled spe^mens. The filled .peclm«. contained about 38%

by weight of 325 mesh aluminum powder. All of these specimens appeared to

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experience a gradual postcure uver about the first 200 hoars of thermal atfing.

As observed previously, 450OF appears to be the optimum temperature for

COntinoui use of thil adhesive. A loss of about 60% in strength is observed

in aging tests at 500OF compared to those at 4S0OF. Filled and unfilled

specimens aged tor S00 hours and tested at ambient temperature remained

unchanged in strength. The specimens represented by Figures 3 and 4 were reinforced with

1 liE heat cleaned ^lass cloth. Aluminum filler levels (325 mesh aluminum)

studied were 0% (Figur« 3), l^o (Figure 4) and 50% (Figure 5).

In contrast with the samples reinforced with 904 icrim, the strengths

observed during aginR in specimens reinforced with 112E cleaned glass

cloth were somewhat more uniform. In general, it appears that lap-shear

■trengthl at 450OF are iomewhat enhanced after a^ing for 500 hours. At

500OF, there appears to be a flight drop in lap-shear strength. Figures 6

and 7 illustrale the effect of aluminum filler level un lap-shear strength in

reinforced specimens aged al 450ÜF and T00UF, respectively.

It is clear from these figure! that 25 weight percent aluminum

powder approaches the optimum level for achieving highest heat resistance.

This is particularly true on long time aging.

The absolute values of Figures 1 through 7 are not high. They do

however, illustrate the unusual stability of this class of polyimide on tempera-

ture aging at temperatures up ro .00ÜF, and particularly at 450OF, where

the strength actually increases after 500 hours.

13

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D. SYNTHESES OF REACTIVE DILUENTS

The strength and reliability of the TPNO cured cyano substituted

polyphenylene oxide adhesives are severely limited by the fact that a volatile

vehicle, i. e. . xylene. is used in the adhesive formulation. While a low cure

temperature. 80OC (1760F). ll applicable in this system, the inert solvent

may be responsible for the erratic lap shear strengths, possibly due to

porous glue lines created when the solvent is driven off during postcure.

In an attempt to alleviate this difficulty, the synthesis of a reactive

diluent to replace the xylene has been attempted. Its function would be to

provide a solvent lor the cyano substituted polyphenylene oxide oligomer so

that adequate fluidity could be maintained in the monomeric state, allowing

the acihesive to flow readily and ac-quately wet the adherends. In the cure

process, this reactive diluent would then become part of the polymeric adhe-

Blve matrix, and no volatiles would be evolved during cure or postcure.

With these objectives in mind, the synthesis of the difunctional

acetylerm monomer. 4.4' -diethynyl diphenyI ether VI, was undertaken. It

was felt that, in addition to functioning as a reactive diluent for the cyano

substituted polyphenylene oxld« oligomer. VI might also be effective as a

reactive diluent for the acetylene substituted polyimlde discussed in

Section IV of this report. The synthesis of 4.4'-diethyny 1 diphenyl ether VI has been carried out

as follows:

o-°o CH.COCl ^QKOHQ-COCH,

IV HCON(CH3)2 • POCl3

I NaOAc * H20

-Q-O^-CCH ^N^OHCCH-CC-Q-O-Q-CC ™C„o

VI

1-!

■ - ■ - ■ ■ ■ - ■ ■' •mmm^m^

PPVMMPI

The infrared spectrum of the final product, 4, 4'-diethynyl diphenyl

ether, showing the C = CH and C = C stretching vibrations at 330Ü cm" and

2110 cm respectively, is consistent with the expected structure. However,

considerable difficulty was encountered In the purification of the bis chloro-

acrolein derivative V and of the final acetylenic ether VI. The ether was

never obtained in sufficiently high purity to warrant its evaluation, and inas-

much as work on the cyano substituted polyphenylene oxide adhesive system

was discontinued, this effort has not been pursued further.

E. DISCONTINUATION OF WORK

Studies on the TPNO cured cyano substituted polyphenylene oxide

adhesive system have resulted in adhesive compositions that could be cured

at 80 C without producing significant quantities of gaseous by-products and

in tensile shear strengths as high as 2500 psi at 450 F for titanium lap-shear

specimens. However, there have been certain drawbacks to the TPNO cured

cyano substituted polyphenylene oxide system. In addition to the problem of

using xylene, certain difficulties are inherent in the use of the TPNO curing

agent. These are the formation of homopolymer and insolubility of the

monomer. For these reasons the AFML Project Monitor requested that

the study of the TPNO cured substituted polyphenylene oxide adhesive be

discontinued.

20

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IV. THERMALLY CURED ACETYLENE TERMINATED POLYIMIDE ADHESIVES

A. BACKGROUND

The successful use of homopolymerizable acetylene-terminated

polyimide VII, designated as HR 600, as a laminating resin in the prepara-

tion of glass cloth and graphite reinforced laminates led to the speculation

that VII would function as a one-component adhesive for 6A14V titanium. As

with the cvano-substituted polyphcnylene oxido, adhesive tests using VII as

an adhesive have been made at ambiont and elevated temperatures and formu-

lation studies were carried out with fillers and carrier cloths. Results of

these tests have been sufficiently promising to justify a major emphasis being

placed on the acetylenic polyimides and the de-emphasis of the cyano sub-

stituted polyphenyl ether/TPNO system.

0 II c

0 II c

o ll

c

0 II r

o II c

o II c

HC=C rj-VOr W (j Vtr <p iCCw- O

II o

VII

Two methods of imidization have been utilized in the synthesis of the

acetylene terminated polyimide prepolymer. They differed in that a cresol-

benzene mixture was used as a solvent in one case, whereas acetic anhydride

was used in the other. Lap shear tests, using 1:1 mixtures of cresol-

imidized (HR bOOC) and acetic anhydride-imidized (HR 600A) oligomer,

showed superior ambient temperature strength to that of mixtures containing

predominantly acetic anhydride imidized material, where ambient tempera-

ture lap shear strengths in excess of 3000 psi were observed.

21

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mmm ••• ■ ■■ i

B LAP SHEAR STRENGTHS OF ACETIC ANHYDRIDE AND CRESOL IMIDIZED POLYIMIDE

Lap-shear strengths have been measured with titanium adherends

using varying mixtures of the acetic anhydride-imidized and c resol-imidized

acetylene-terminated polylmld« adhesive. Aluminum powde r. 325 mesh, was

used as the filler and heat cleaned 112E glass cloth was used as the reinforce-

ment From these results, it is clear that the method of imidization has a

pronounced effect upon the lap-shear strength. The results of these tests are

ghown In Table 11. The results of these tests are illustrated graphically in Figure 8.

Fiourc 8 »howl the lap shear strengths of titanium specimens bonded

,vith mixture of acetic-anhydride-imidized and cresol-imidized HR 600 rein-

,,,..,; ,vith I >;A Cleaned 112E glass fabric. The dramatic effect of using

acetic anhv .ride Imidized over rresol Imldlwd HR 600 at ambient tempera-

,., • is evir'enr figure 8. Curve A). Also evident from Figure 8 is the fact

■ the ;ar shear strengths of these samples at 450OF show very little

ciep, idence upon method of imidization (Figure 8. Curve B). Note that these

result! contrast with those obtained at ambient temperature.

22

MHHÜ mi"! i ilnltfi *"

"—- > ■■ • >■

TABLE II LAP-SHEAR STRENGTHS OF TITANIUM SPECIMENS TBONDED WITH MIXTURES OF ACETIC ANHYDRIDE-IMJDIZED

POLYIMIDE AND CRESOL-IMIDIZED POLYIMIDE.U;

Average Adhesive Test Ratio Lap-Shear Lap-Shear

Specimen Thickness in. x 103

(mils)

Temperatures 0F

a/b Strength (k psi)

Strength (k psi)

HR 63 Al 6 Ambient ■ 3. 1 3. 1 2 6 3 1 3. 1

1 4 5

6 7 6

2.7 3. 3

HR 63 6 7 450OF CO 1. 1 1. 2 / 6 1 2 1.2

8 6

9 7 1.6 1. 1

10 6

HR 63 Bl 7 Ambient 3/1 3.2 2.6

2 J

6 8 2.9

3. 5 3. I

4 7 3. 1 5 7

HR 63 B6 7 450oF 3/1 0.9 I. 3

7 7 1. 5 1. 3

H I 1 1 9 8 1. 5 10 7

HR 63 Cl 2 3 4 5

9 10

7 8 7

Ambient 1/1 2.2 2. 3 1.9 2.2 2.3

2.2

C6 10 45 0oF 1/1 1.4 1.2 1 5

1.5

7 12 8 9

10

15 10 12

1.6 1.9

• '

tn nnti ntiAil nf»vt naae'

23

- ■ - .-. ■ ■ -*

r i" ■■■■> " "■" ■'■' i1"'" ■ ■

(Table II, concluded)

Specimen

Adhesive Thickness

in. x 10 (mils)

3

HR 63 Dl 2 3 4 5

Dfa 7 B )

10

tjf< 63 E t

2 }

4

5

HR 0 5 Eb 7 8 9

10

Footnotes:

0

©

10 8 7

10 9

8 10 12

3 10

7 S

f

6

t) 7 5 6 6

Test Temperature

Ambient

450oF

Ambient

450oF

Ratio a/b

1/3

1/3

Lap-Shear Strength

(k psi)

2. 0 2. 1 2. 0 2.5 2. 5

2.6 2. 2 1. 8 2. 3 2.2

1. 5 0.

1 9

1. B 1. z

0. 6 1 0 I 0 0 . 9 1 . 8

Average Lap-Shear Strength

(k psi)

2.2

2.2

1.4

1. 1

The adhe u-e used for bondmg these specimens COnUtojd 50% alummum powder. 325 mesh. Reinforcement was provided by d

ul

Se of 1 12E heat cleaned fabnc. After Joining the specimens,

the adhesive was cured 4 hours at 500 F.

(.) - Acetic-anhydride-imidized acetylene terminated polyimide.

(b) - Cresol-imidized acetylene terminated polyimide.

24

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C EFFECTS OF ALUMINUM FILLER LEVEL AND PRIMING * UPON LAP SHEAR STRENGTH

Studies were made to show the effects of «rUti« o, a.umlnum ÜUer

level and of priming upon lap shear strength. LJL. 9 .nd .» show the effect of UonV.non, fit.e. leve. and prlmU,

opon Up shea. stfe„Bths of tltaoiu., samp,es hooded with po.^Ue a he-e

ad .einforced with .UE heat Ceaned g.ass Coth. The .ap sheae tests repee

.„ted ,„ rif»» •> we.e ton at ancient te.pe.a.o.e, whUe ^ose tn Ftguee .0

.cte run at 4.0°F, The specimens were pr.toed hy apphcat.on of a 10%

dlm.,hylfomannde Uc,.« of the acetylene-terminated polyimide to the

spectmen, followed by removal of the solvent in a eacoom ovren at F.

M amMent temperature, the lap shear stren.ths of unprimed iP»«^" „uoded with ,« acettc anhydride-tmidiaed acetylen.-terminated po ytm.de

are nearly the same for 0. 25* and 50% by weiBht of aluminum powder

(Curve A, Fl.u« H. By comparison, tn the prin.ed specimens a deftmte

avop in lap shear strength was noted (or the same bonding condtt.ons wt.h

increased aluminum powder filler level (Curve B, F.gure 9). Samples

containing 7»%«.«. anhydride-imidiaed polyimide displayed a sharp

.„crease in lap shear strength at the 50% aluminum filer level (Curve C

Hgure 9). At 450°F, the prtmed sample showed increased strength wt.h

increasing aluminum filler level (Curves A and B. Figure ,0, The nnprtmed

specmens bonded with 50% acetic anhydride-imidUed polyim.de •*"*•*

contrast, a decreasing strength with increasing aluminum filler-leve w.th a

crossover point between 15% and 20% aluminum (Curve C. Figure 10) It seems evident from these data that, in the elevated temperature lap

shear tests, addit.onal curing of the primer takes place. This indicates that

a longer cure of the polyimide adhesive may be necessary.

26

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D. ADHESIVE PREPREG STUDIES

A dimethylformamide solution of HR 600C oligomer was used to

impregnate glass fabric. Lap shear strength data using this prepreg were

compared with lap shear strength data obtained with powdered adhesive pre-

pared from a 1:1 mixture of HR 600A and HR 600C. using (as reinforcement)

heat cleaned 108 Volan A finished glass fabric (108 VHC), heat cleaned 112

Volan A finished glass fabric (112 VHC), and heat cleaned starch-oil finished

112 glass fabric (112 SHC). The results of these lap shear tests are shown

in Figure 1!. The prepreg was tested at ambient temperature and 450 F and

the powder samples were tested at ambient temperature, 260 F and 450 F.

The measured strengths at 260OF are represented by the dotted areas in the

bar chart in Figure 11. The data from Figure 11 indicates the following priorities for scrim

at both ambient temperature and 26U0F: 112 SHC > 112 VHC >108 VHC.

At a 450OF test temperature, the prepreg adhesive system shows a

clear superiority over specimens prepared from powdered adhesive and

tested at the same temperature. This is shown in Figure 11, where the

specimens prepared by powdered adhesive techniques have all declined in

strength, but the prepreg sample increased slightly in strength.

In our previous work, we found that, in room temperature tests,

adhesive formulations containing appreciable amounts of HR 600A resin had

superior adhesive strengths over pure HR 600C. Thus, in view of-the results

with the HR 600C prepreg, a prepreg containing a 1:1 mixture of HR 600A and

HR 600C on 112 SHC was prepared. Two prepregs wer>> made, one contain-

ing 70 percent oligomer, and one containing 80 percent oligomer. Neither of

these prepregs contained filler. The solvent used in preparing the prepregs

was N-methylpyrrolidinone. This solvent was chosen because it was shown

to be superior to dimethylformamide in the preparation of prepregs with

HR600 when used in laminates. An extensive set of titanium lap shear specimens has been made using

these prepregs, with cure temperatures of 500OF, 525 F, 600 F, and 650 F,

At the upper two temperatures, 600OF and 650OF, cure times of two hours

and four hours were studied. At the 500OF and 525 F temperatures, longer

cure times were used.

29

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ROOM TIMWUTUW AND 26C0f

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^ 2000

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figure 11. Influence of various parameters on lap shear strength of polyimide adhesive. (cured 4 hours at 500oF.)

The results of these studies are shown in Figures 12 through 22. In

Figures 12 and 13 lap shear strengths are shown for samples cured at 500 F

for 16, 20, and 24 hours with prepregs containing 70 and 80 percent oligomer.

The spread in the strengths observed with different cure times indi-

cates that a slightly higher cure temperature, such as 525 F, may make

strength less dependent upon cure time. The 525CF cure temperature was

chosen because this was expected to cause an increase in cure rate without

initiating the degradation of adhesive properties. Indeed, the 525 F cure

temperature showed considerably less dependence of lap shear strength upon

cure time at 450OF and maintained the same upper level of strength as at

500OF. This is illustrated in Figures 14 and 15.

Short cure times may be utilized at higher temperatures without much

loss in lap shear strength. This is illustrated in Figures 16 through 19 for

cure temperatures of 600° and 650OF and cure times of 2 and 4 hours. At

the higher cure temperatures, the data points appear to be closer together,

especially at the 450OF test temperature.

30

—- -^—'-^ -'—

i

*x*

2

xcc 3 IM I

irtX

J-

CURE TIME O 16 HRS O 20 HRS A 24 HRS

'0 IOC 300

TEST TEMPERATURE, 0f

400 500

Figure 12. Influence of cure time on ambient and elevated temperature properties of 70 percent HR 600AC prepreg bonded titanium, 500oF cure

3000

o z

a. o

2000

x

<

1000 70 100 200 300

TEST TEMPERATURE,

Figure 13. Influence of cure time on ambient and elevated temperature properties of 80 percent HR 600AC prepreg bonded titanium, 500oF cure.

31

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CURE TIM£ O 16 HltS A 20H»5 Q 24 HltS

1000 ro 100 MO 300

TEST TEMPEIATURE, 0F

400 500

Figure 14. Influence of cure time on ambient and elevated temperature properties of 70 percent HR 600AC prepreg bonded titanium, 5250F cure.

3000

0 2 ■ Z 2000 W

i

1000 70 100 200 300

TEST TEMPERATURE, '

Figure 15. Influence of cure time on ambient and elevated temperature properties of 80 percent HR 600AC prepreg bonded titanium, 5250F cure.

32

. — -. ..— —MMtthMtfiiur '•■■ ■■-"■■ ■-■ ■ ■ ■ -

3000

■

z

3

icon

CURt TIME A 2 HR5 O 4 HRS

•< 100 ."■ 300

TfST TEMPERATURE, 0F

41 ,f V»

Figure 16. influence of cure time on ambient and elevated temperature properties of 70 percent HR 600AC prepreg bonded titanium, 600oF cure.

>m

-

2000

5

3

1000

CURE TIME A 2 HRS O 4 HRS

70

I

100 ?00 300

TEST TEMPERATURE, "f

I 400 soc

Figure 17. Influence of cure time on ambient and elevated temperature properties of 80 percent HR 600AC prepreg bonded titanium, 600oF cure.

^^^Mi"*-^--- - ■■-»■■- —• - _^ ^ ■-—■-'— ■ - ■■ ■- - - ■ - ■ MMMMHI . . ^.

3000 r

z I 2000

I

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o

CURE TIME A 2HRS O 4 HR5

1Ü0C L—- 70 KM 200 30C

TEST TEMPERATURE, °f

40C HO

Figure 18. Influence of cure time on ambient and elevated temperature properties of 70 percent HR 600AC prepreg bonded titanium, 650oF cure.

3000

3 z

I ^

1000

CURE TIME A 2HRS O 4 HRS

70 100 200 300

TEST TEMPERATURE, 0F

400 500

Figure 19. Influence of cure time on ambient and elevated temperature properties of 80 percent HR 600AC prepreg bonded titanium, 650oF cure.

34

■ - - - - ■■■ -

5000

E 2000

1000

80 X RESIN 70 X RESIN

500 600 7nn CURE TEMPERATURE-

Figure 20. Influence of cure temperature on properties of HR 600AC prepreg bonded titanium - ambient temperature tests.

3000,

2000

a. 3

1000

70? « 80X RESIN

o 70T

AM

500 fino

CURE TEMPERATURE, 0F

700

Figure 21. Influence of cure temperature on properties of HR 600AC prepreg bonded titanium - 450oF tests.

35

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3000

2000

i

1000

801 RESIN

702 RESIN

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CURE TEMPERATURE. 0F

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figure 22. Influence of cure temperature on properties of HR 600AC prepreg bonded titanium-500oF tests.

Figures 20 through 22 show the variation of adhesive strength with

cure temperature. The ambient temperature and 450oF strength is seen to

decrease slightly with increasing temperature (Figures 20 and 21), while

the 500oF lap shear strength, though starting at a lower level, is seen to

remain approximately constant (Figure 22).

At the lower cure temperatures, longer cure times are necessary,

but higher strengths are obtained. This same correlation exists for lap

shear strengths measured at ambient and 450oF.

At 500OF test temperature there seems to be very little dependence

of cure time and cure temperature on lap shear strength, i.e., lap shear

strengths fall within a relatively narrow range when different cure times and temperatures are utilized.

The data for Figures 12 through 22 are presented in Tables III and IV.

Note that for a given cure temperature, cure time and resin content, the

lap shear strengths at 450oF range from about 80 percent to over 100 per-

cent of the ambient temperature values, and the strength? measured at

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500oF range from above 60 percent to over 90 percent of the annbient

temperature strengths.

E. ALUMINUM POWDER FILLED ADHESIVE PREPREG

Studies were conducted to examine the effect of 325 mesh aluminum

powder filler on the strength of specimens bonded with HR 600AC prepreg.

For the initial tests, prepregs of heat cleaned 1 1Z glass cloth were prepared

from a formulation of HR 600AC adhesive containing 25% of 325 mesh alumi-

num powder. The aluminum filled prepreg fabrics contained 81 and 89 per-

< pnt adhesive (including the aluminum powder). The data are shown in

Table V, Testa were run at ambient temperature.

TABLE V, AMBIENT TEMPERATURE LAP SHEAR STRENGTHS OP TITANIUM SPECIMENS BONDED WITH ALUMINUM

FILLED" HR 600 AC PREPFEG

1 ( Ave rage Glue

Lap Shear Lap Shear Line Prepreg Sample Bonding Strength Strength Thickness,

Composition Number Area (psi x 103) (psi x 103) Inch

81% Resin 1 0. 559 1. 31 0. 007 81% Resin 2 0. 554 1, 35 0. 006 81% Resin 3 0. 552 1. 33 1. 36 0. 009 81% Resin 4 0. 540 1.46 0. 007 81% Resin 5 0. 519 1. 37 0. 008

89% Resin 1 0. 575 1. 80 0. 009 89% Resin 2 0. 534 : 1. 77 0. 007 89% Resin 3 0. 540 % 1.65 1. 70 0. 009 89% Resin 4 0. 565 1. 65 0. 008 89% Resin 5 0. 494 1. 62 0. 009

""25% by we ight of the adhesive.

48

mmmm ^m

L

The data indicated that the higher oligomer content prepreg gives

better bonding. Thus, it is anticipated that a metal filled prepreg containing

even lower alumimim content will result in even higher lap shear strength

partly due to the resulting thinner glue lines. Better bonding is expected with the aluminum filled prepreg because

of the known enhancement in strength previously observed with aluminum

filled reinforced specimens.

F. T-PEEL TESTS

T-peel specimens have been prepared by bonding one-inch strips of

16 mil 6A14V titanium alloy with the prenreg fabric. T-peel strengths were

determined according to ASTM T-Peel Method D1876.

The peel distance in each case was five mches. The results of these

tests are shown in Table VI. Testing was done at ambient temperature.

It U recognized that bonded aluminum will have comparatively more

peel strength than bonded titanium due to the lower modulus of the aluminum.

which permits it to bend more as it peels. The titanium T-peel specimens

appear to have a relatively high initial peel strength, but a comparatively low-

average peel strength. Due to the relatively high modulus of titanium, it is

TABLE VI. RESULTS OF T-PEEL TESTS ON TITANIUM SPECIMENS BONDED WITH HR 600AC

Specimen

Sample T-Peel Strength (1 bs)

Initial High Low Average

1 3. 6 3. 6 1. 8 2. 7

2 7. 2 7. 2 2. 0 4. 6

3 4. 8 4. 8 2. 0 3. 0

4 6. 6 6. 6 2. 0 4. 3

5 5. 0 5. 0 2. 0 3. 5

Failures in all cases were cohesive

49

apparent that T-peel tests on titanium Hpecimens must be carried Ottt with

thinner titanium foil in order to g-t useful T-peel data. T-peel tests with

thinner titanium foil will be run in future studies.

G. WELDBONDING

Weld schedules have been established for spotwelding of titanium

specimens and both spotwelded and weldbonded specimens have been prepared.

All spotwelded lap shear specimens had a one-inch overlap. Weld schedules

were established on three phase Sciaky Spot Welders with solid state control-

lers, and both spotwelded and weldbonded specimens were prepared for S/N

tosts. The a 'liesive mixture used in the weldbonded specimens had the follow-

ing ( on-position by weight: HR 600A, 25%; HR 600C, 25%; aluminum powder,

123 mesh, 50%; dimethylform.-rmide, sufficient to make a readily applicable

paste. Good spot.welds were obtained on lap shear specimens containing this

( ■ -mulation.

H. S/N TESTS

S/N (fatigue) tests were run to evaluate the fatigue life of aluminum

filled HR 600AC bonded spotwelded titanium. These tests were run on 1.0

inch overlap, 0.050 inch thick titanium specimens 1.0 inch wide. The speci-

mens were adhesive bonded, spotwelded and weldbonded. The adhesive used

in preparing the weld bond specimens was described in the previous section.

The fatigue tests were run at 30 cycles per second. The S/N tests on the

spotwelded, weldbonded and adhesive bonded specimens are shown in

Figures 23, 24, and 25. The weldbonded specimens (Figure 24) clearly show

increased fatigue resistance over the spotwelded specimens, particularly at

the higher loads (above 2000 psi) where the number of cycles to failure for

a given load is superior by about one order of magnitude, and at the lower

loads (300+psi), where failure occurred at 10 cycles in the spotwelded speci-

mens, but did not c-.cur at all in the weldbonded samples. No carrier cloth

was used in any of the specimens used for the fatigue tests.

50

- ■ - - ■■'— - . ^. _ _ . ^^^-^-^ ^. -^ . , .. ^

t>«

T« 74-10 WfLO/»OND tATIGUf TEST «fSUlf5 R -0, HAM -»Hi „ „ ,. AiUD SPtCIMfN DATE LOAD (•

10' 10

CYCLES TO FAH,Ü«E

Figure 23. Fatigue (S/N) tests on spotwelded titanium specimen:

10" i0

CYCLES TO FAILURE

Figure 24. Fatigue (S/N) tosts on spotwelded titanium specimens bonded with HR 600AC thermally cured acetylene

terminated polyimide.

51

-■ --:-— - ■ ■ ■ — -—- -■■ s.-^-.~~ .<*-*<-*-**-** *~k*a*M**^.. MiliiMMMiiiiWMHiMMilMiirilMilliil

300

J50

200

ISO

o o o

T« 74-10 WRD/IONO FATIGUe TFST «ESULT5 11 • 0, «ATE • 30 Mi IOND SPECIMEN

O CD O

... ir io'

CYCLES TO FAILURE

Figure 25. Fatigue (S/N) tests on titanium specimens bonded with HR 600AC thermally cured

acetylene terminated polyimide.

The strengths of the adhesive bonded titanium specimens are

surprisingly low, especially in view of past Instron lap shear strengths,

where values in excess of 2000 psi were observed on bonded titanium speci-

mens in the absence of carrier cloth. As demonstrated in work with laminating resins, the cured HR 600

resin is a relatively high modulus material, and thus may display poor

strength on the fatigue testing apparatus, particularly in view of the fact

that the rate of application of load is greater in the fatigue tester by at least

two orders of magnitude. Lower modulus adhesives to be developed in the forthcoming program

continuation should be less sensitive to this extremely high rate of load

application and thus are expected to be considerably more tolerant of the

test conditions in the fatigue tester.

52

■ ^ - -

FlgttM 26 shows the four observed types of failure modes and

Tables VII. VIII. and IX contain the applied loads, cycles to failure, and

fa.lure modes for spotwelded. weld-bonded and adhesive bonded (no

enforcement) titanium specimens. At the lower appUed loads, faüure was

a Type I sheet failure, whereas at the higher loads failure appeared to be by

partial or complete nugget separation.

53

— ■ ■ - ^

- <; a

V 'A

-r-r-

o

0)

J 0

a 4) £ w o U

c

H 3

e o

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T) V

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TABLE VII. FATIGUE TEST SPOTWELDED TITANIUM SPECIMENS

Spec. No.

1

2

3

4 r-

-

7

8

9

10

1 1

12

13

14

15

16

17

i '

Z0

21

22

23

2.4

Z5

Load (lb)

2760

0-2128

0-2128

0-2128

0 2128

0-1656

0-1656

0-1656

0-1656

0-1656

0 -1104

0-1104

0-1104

0-1104

0-1104

0-552

0-552

0-552

0-5 52

0-552

0-276

0-27b

Not

2760

N Icycles)

120

150

60

75

1890

1080

1650

1290

1320

15,300

11 .010

14,850

13,860

9,450

80,500

56.000

91,000

95,000

79,000

1 ,028,000

I,303,000

tested

Failure

IV

IV

ti

II

IV

IV

[I

IV

II

IV

III

I

I

I

I

I

I

I

I

[

I

I

II

Remarks

Failed statically - No separation

Wrong settings

Tested statically

55

■■■ ' ■ —■ ...„JJ. - ■'■ ■-■ •- "-- ■■ ----

TABLE VIII. WELD/BOND FATIGUE TEST WELDBONDED TITANIUM SPECIMENS (SPOTWELDED WITH ADHESIVE BOND)

Spec. No. Load (lb)

N (cycles)

Failure Mode

Remarks

1 3075 1 II Tested statically

2 2970 1 II Tested statically

.3 0-2417 270 IV

4 0-2418 220 II

5 0-2418 405 IV

6 0-2418 410 IV

7 0-2418 240 II

8 0-1813 1410 II

9 0-1813 12 50 IV

10 0-1813 1920 I

11 0-1813 1260 II

12 0-1813 520 II Specimen weld/bond misaligned

13 0-1209 5000 I

14 0-1209 10,600 I

15 0-1209 4.530 II

16 0-1209 8,940 I

17 0-1209 10,000 I

18 0-604 143.000 I

19 0-604 87,000 I

20 0-604 115,000 I

21 0-604 217,000 I

22 0-604 158,000 I

23 0-302 1,662,000 No failure

24 0-302 1,685.000 No failure

25 Not tested 1

56

: ^_ -■_ ^.. II ■" ■- J

TABLE IX. FATIGUE TEST BONDED TITANIUM SPECIMENS

HR 600 AC ADHESIVE

Spec. No. Load (lb)

N (cycles)

Bond A rea (in2)

Remarks

1 258 1.051 Tested statically

2 288 1.030 Tested statically

3 — - 1.060 Broke while drilling

4 - — 1.040 Broke while applying static load

5 0.218 60 1.051

6 0-218 18 1,062

7 0-218 30 1.092

« 0-164 930 1. 061

9 0-164 60 1.061 Specimen misaligned

10 0-164 2910 1.061

11 0-164 1890 1. 061

12 0-164 1980 1.061

13 0-109 30 1.071

14 0-109 75 1. 040

15 - - - Missing

16 Not tested 1.071

17 Not tested 1.051

18 - - 1.050 Broke while applying static load

19 0-109 30 1. 133

20 0-109 630 1.030

21 0-109 1800 1.061

22 0-109 713,000 1.040 No failure

23 Not tested 1. 0 51

24 Not tested 1.051

25 0-55 1,839.000 1.052

57

■UMMlMtaHlitlMfcjiMaAli - --■ a^MMfe^^^M *^^^^^^^I^MM

V. EXPERIMENTAL

P i-cparalinn of 4, 4'-Diacetyl DIphenyl Ether Experiment No. G1344-90)

The procedure for the preparation of this compound has been modified

from that reported by Dilthey, Bach, Griiterlng and Hausdörfer4 in that

1, 2-dichloroethatte was used as solvent in place of carbon disulfide.

fo a solution of diphenyl ether (85 g, 0.50 mole) in 1 , Z-dichloroethane

(150 mil ..as added anhydrous aluminum chloride (270 g, 2.02 moles . The

mixture was cooled to about -20 C and acetyl chloride (112 g, 1.43 moles)

was added dropwise with vigorous stirring. Evolution of HC1 was fairly

vigorous, and it became necessary to add the acetyl chloride quite slowly.

This addition required ca. Z-l/2 hours. Aftei the addition was complete,

the mixture was slowly allowed to rise to ambient temperature. This required

an additional 3 hours, since HC1 evolution was still quite vigorous during this

period. Stirring was continued overnight at ambient temperature. The tem-

perature was raised to 60 C on a water bath and kept at 60 C for 2 hours, and

cooled. The contents of the flask were poured into about 2 trays of ice. A

large amount of crystalline material appeared as the mixture became diluted

with wate-. This crystalline precipitate was filtered off and recrystallized

from ethanol to yield HT. 3 g. (70 percent) of material, m. p. 100 - 101 .

Reported m. p. for this material is 100 - 101 , The infrared spectrum is

consistent with the structure.

(Experiment No. H1060-16)

Diphenyl ether, (85g, 0. 50 moles) and 1, 2-dichloroethane, 150 ml.

were placed in a 1 liter 3 necked round bottomed flask fitted with thermometer

and mechanical stirrer. The mixture was cooled with a cooling bath while

59

PRECSDirC PAGE BLANECNOT FILMED

"-^ — ■ - ■ " ■■ ■ - --■

aluminum chloride. 270 g. (2.0 moles) was added. The solution was stirred

and maintained at about -20OC while acetyl chloride, 112 g. (1.43 moles) was

added dropwise. After the addition of acetyl chloride was complete (about

1-1/2 hours), the temperature was slowly (ca. 2 hou-s) allowed to rise to

ambient temperature while stirri"S continuously. If the temperature was

allowed to rise too rapidly, evolution of hydrogen chloride became very

vigorous. After the mixture reached ambient temperature, stirring was

continued overnight. The flask was then heated slowly to about 550C to drive

off most of the remaining HC1 and the product was poured over ice and water.

The mixture was filtered and the yellow precipitate was washed with water.

K.i.'rystalli/.atiün from ethanol yielded 89 g. (70%) of product, m. p. 100 C.

l.ttemptfd ^reparation of 4. 4'- Bis (1-chloro-2-formylvinyl) ]TTph^yTETher and 4^'-Diethynyl Diphenyl Ether (Experiments No. Gl?44-C>2 and -"9F)

Dim.Hhvlformamide (200 ml) was placed in a 500 ml round bottomed

flas, cooled to about 150C. Phosphorus oxychloride. (153.3 g. 1 mole) was

added slowly with stirring. To this solution was slowly added a solution of

4,4'-diacetyl diphenyl ether (101.5 g, 0.4 mole) in anhydrous dimethylforma-

mide (250 ml). The temperature of the mixture was kept at 15 - 20 C with

cooling during the exothermic reaction. After addition was completed, the

r.lxture was gradually warmed to about 50 - 550C. An additional exotherm

appeared to occur during the heating. The temperature of the mixture was

maintained at 50° - 60° for 2-1/2 hours, and then allowed to return to

ambient. The contents of the flask were poured into a mixture of sodium

acetate trihydrate (408 g, 3 moles), water (500 ml) and a tray of ice cubes.

The mixture was extracted, first with ether and then with benzene to remove

the organic portion. The combined extracts were dried with potassium car-

bonate, filtered and distilled on the rotary evaporator to remove solvents.

The bulk of this yellow oily residue resisted attempts at cryt, allization.

A solution of this crude residue (53. 5 g) in adequate dioxane to make

200 ml of solution was added slowly to a refluxing solution of sodium hydrox-

ide «40 g, 1 mole) in water (200 ml). Addition was maintained at such a rate

that refluxing was not interrupted. The addition required ca. 45 minutes and

refluxing was continued for an additional 45 minutes. After cooling, the

60

.. ,....^:. .... ..-..■■ . ■ .J. .

mixture was poured onto an ice-water slush and extracted in several portions

with a total of about 1 liter of ether. The ether extract was dried over anhy-

drous potassium carbonate, filtered and distilled, leaving a dark yellow resi-

due. Attempted molecular distillation of this residue at IO|l and about 75 C

yielded no distillate.

Preparation of 4. 4'Bisd-chloro-Z-formylvinyl) Diphenyl Ether (Experiment No. G1343-08)

Phosphorus oxychloride. 91.0 g. 54.5 cc (0.60 moles) was added to

80 ml. of anhydrous dimethylformamide at ca. 10OC. The mixture was stir-

red for an hour after the addition was complete. To this solution was added

50. 8 g. (0. 20 moles) of 4,4' diacetyl diphenyl ether dissolved in 90 ml. of

dimcthvlformamide. The DMF had to be warmed slightly to dissolve the

diacetyl diphenyl ether. The addition was carried out. keeping the reaction

mixture at 15 - 20° by external cooling during the addition, which required

about an hour. After addition was complete, the cooling bath was removed.

and the exothermic reaction created a temperature rise to ca. 50 C. At this

time the cooling bath was used to lower the temperature to ca. 40 C. After

removal of the cooling bath, the temperature remained at 40 - 42 C for

about an hour, then gradually dropped to ambient temperature. After stand-

ing at ambient temperature overnight without stirring, the mixture was slowly

poured into cold aqueous sodium bicarbonate solution. A finely divided yellow-

ish orange precipitate formed which was difficult to filter. The precipitate

was removed and triturated with benzene. The aqueous soluticns were

extracted with benzene and the combined benzene extracts were dried over

anhydrous magnesium sulfate and filtered to yield ca. 2-1/2 liters of benzene

extract which was concentrated on the rotary evaporator to give a semicrystal

line mass of material. This product, assumed to be a mixture of geometric

isomers of 4. 4'- bis(l-chloro-2-formyIvinyl) diphenyl ether, was carried to

the next step without further purification.

61

MWMa^MMMMMHMiH - BMaHflMtoA^^M|^_HH||Mii|aaA||MMMflaaHHy

4. 4'-Diethynyl Diphenyl Ether (Experiment No. G1343-11)

To a solution of 10 g. ol sodium hydroxide in 80 ml. of water at reflux

was added a solution of 23. 3 g. (0. 065 molei of 4, 4' -bis(l-chloro-2-formyl-

vinyll diphenyl ether in 125 ml. of dioxane. After the addition was complete,

an additional 5 g. of sodium hydroxide was added and refluxing was continued

for an additional 2 hours. Most of the dioxane and water were removed on

the rotary evaporator and the residue was extracted with a total of ca. 300 ml.

of ether. After drying the ether extract over magnesium sulfate, filtering

and distilling the ether, a residue remained which was a semi-solid mass,

yield 9. 5 g. (67% crude yield). The residue failed to crystallize. The infra-

red ipectrum showed the characteristic C ■ C-H stretch band at 3300 cm

and the C " C .stretch band at about 2110 cm" . Further characterization of

thll material was» not carried out.

ieneral Procedure for Preparing Titanium Lap Shear Specimens Containing "^Ino-Suhstituted Polyphenylene" Oxide/TPNO Adhesive-

Titanium coupons were cleaned as before in accordance with Hughes

Process (HP) 9-30, as follows:

The titanium surfaces were first wiped with toluene using clean,

silicone-free tissues. They were then pickled in the following solution at

room temperature for 30 ± 2 seconds:

Nitric Acid, 70 percent 15 ± 1. 5 Volume Percent

Hydrofluoric Acid, 50 percent 3 ± 0. 3 Volume Percent

Water, deionized 82 ± 8. 2 Volume Percent

Following this, the specimens were rinsed in tap water for two minutes.

Next, titanium coupons were immersed in the following room-temperature

solution for 2-2 1/2 minutes:

Trisodium Phosphate

Sodium Fluoride

Hydrofluoric Acid, 50 percent

Tap Water

5. 0 ± 0. 5 Volume Percent

0. 9 ± 0. 09 Volume Percent

1. 6 ± 0. 16 Volume Percent

92. 5 ± 9 Volume Percent

62

_«. ■■■ ■- • •

Following this, tnu coupons were rinsed in tap water, then soaked in a fresh

tap water bath at 150° ± 100F (66° ± 60C) for 15 minutes minimum. They

were then spray rinsed with deionized water and dried at 120° to 150oF

(49° - 660C) in a circulating air oven.

A paste was made of oligomer/TPNO/xylene in the -atio 1: 1:0. 7 by

hlendins with a spatula and carefully "buttered ' on the open scrim which had

been placed on a Teflon film. A second Teflon film was placed on top, then

chilled with dry ice. Adhesive coupons approximately 1-1/8" x 5/8" were

then cut out, the Teflon was stripped off and the coupon placed on one

titanium substrate and covered by a second titanium substrate to achieve

a 1/2 overlap. Pressure sensitive tape was then used to align and hold the

■ pecimena in place.

Cure was conducted in a heated platen press at 15 psi according to the

folio" -i hedule:

18 hours at 80 C followed by

4 hours at 160 C followed by

18 hours at 260OC.

Following this, specimens were conditioned 24 hours at room temper-

ature and tested in shear. Five each were tested at room temperature, and

five each were tested at 450 F after 1/2 hour conditioning at temperature.

Imidlzation of Amic Acid Precursor t o A cetylene-Terminated Polyimide (Experiment No. Gl 344-94)

To 20 g. of acid amide (1996-62A) was added redistilled acetic

anhydri 250 ml) and the mixture was heater at reflux for 48 hours. The

major portion of the acetic anhydride was removed by evaporation on the

rotary evaporator and the residue was poured into anhydrous ethanol (300 ml)

and stirred for an hour, then filtered. The precipitate was resuspended in

fresh ethanol and stirred with a high speed stirrer at about 20,000 RPM for

5 to 10 minutes, then filtered to yield a straw-colored finely powdered

material. The yield was 13 g.

63

^ ■ ■■- —--^ - - • — ■ - - - - - M - --- ■ ■ """

Preparation of HR 600AC Polyimide Prepregs

1. Materials

a 112 Glass Fabric, starch-oil finish, heat cleaned. Source - Clark-Schwebe I Fiber Glass Corp.

b. N.N-dimethyl formamide (DMF)

c. N-methyl pyrrolidinone (NMP)

d. Cresol imidized polyimide (HR 600C)

e. Acetic anhydride imidized polyimide (HR 600A)

2. Process

a. Formulation one part by weight HR 600C polyimide oligomer One part by weight HR 600A po^-ide ohgomer 1.6 part by weight solvent (DMF or NMP)

b- Method Preoare impregnation tank suitable for submerging glass fabric n strip form under the surface of the polyimide .Olutlo" in continuous single-pass impregnation mode. Heat solvent to 35n0F, add polyimide and mix until dis- solved. Place solution in impregnation tank,

rut «lass fabric into strips of suitable proportions, and pass'lhr^ugh solution in impregnation tank, in single pas..

Hang in air circulating oven for 15 minutes at 350 F.

of 50 percent - 70 percent resin by weight.

3. I_:ap-Shear Specimen Preparation

a. Clean titanium specimens per HP 9-30.

Cutprepreg to 1-1/8 inches x 5/8 inch coupon.

Place prepreg between titanium coupons to achieve b.

c. 1 /2-inch lap.

d. Hold in position using pressure sensitive tape.

e. Using spring clips to achieve 15 psi pressure.

f. Cure in air circulating oven.

64

■■ ■ ■■■ ■'

1.

2.

3.

4.

VI. REFERENCES

N. Bilow. R. H. Boschan, A. L. Landis and L. J. Miller, "Development of High Temperature Laminating Resins and Adhesives Curable by Addition Processes," Fifth Quarterly Report, April 1972.

L. J. Miller, H. S. Noji, and C. C. Daschbach, "High Temperature Structural Adhesives Having Improved Processability, " AFML-TR- 71-257, February 1972.

N. Bilow, R, H. Boschan and A. L. Landis, "Development of High Temperature Laminating Resins and Adhesives Which Cure Through Addition. " Part II. Summary Report. APML TR-72-57, Part II June 1973.

Dilthey, Bach, Grütering and Hausdörfer, J. Prakt. 350 (1927)

Chem. 1 17,

65

^^^ - ■ ■ - ^. _ - _

UNCLASSIF1£D — »tcoViTTT^nonoH or THI» PAG» "i"" °'" '"'"•■"

REPORT DOCUMENTATION PAGE I Bf PO»'' NUMOtR

AFHL-TR-74-88

1 OOVT «ccessioN NO.

DEVaOPMENT'of HIGH TEMPERATURE ADDITION^RED ADHES1VES

READ INSTRUCTIONS BEFORE COMPLETING FORM

J RECIPieMT'S CATALOG NUMil»

V TYPB Of «CPOKT ■ PEBIOD COVtRED

1 March 1973 to 28 Feb. 1974

7. AVTHOW»)

N. HlOW, R- H. Boschan. H. Raech

6. peRfOPMING ORG. RtPOPT NUMBS"

■» CONTRACT OR GRAN I NUMBE"f«J

T-57^r6RM,NG ORGANHATION NAME AND «UU-.»

Hughes Aircraft Company Aerospace Group Culver City. CA 90230

7 ^<r-7y^5b6^ -ft^EBtsarg; gutNT.PROJECT, TASK

M CON -ROLLING OFF.CE NAME AND ADDRESS

Nonmetallic Materials Division. Air Force Materials Laboratory

Project Nr. 7340. Task Nr. 734002 "Structural "Adhesives" 12. REPOPT DATE

Mav 1974 11. NUMBER OF PAGES

65 _^______1_i --rr cerilRITV CLASS, fo' *!• '»POf'J

Tsi DECLASSIFICA'IONVOOWNGRADINO SCHEDULE

Tr-öTTfRiBuTiON STATEMENT ro/.M.R.por<) evaluation,

Distribution limited to U. S. S^^r^'Huf be referred to the Air Force (K 19W, Other requests ^r J Ms ocum ^ ^^g™^ 3 FibrouS

"—TZ ,„ m,,,!. JO /( dllltrml from R»porO if DISTR.euTiON STATEMENT fof M SM«M .n..r.«in Bloc» JO,

I»-SUPPLEMENTARY NOTES

T, < EY «OROS fCor.lnu. on «v.f •«* " n.c....ry «- K^nHinn addition

ABSTRACT rSSSSon iS -,...-- = '^sT^wM^h cure via addition reactions.

superior to the TPN0 cured cyano ^bstltuted pojyphenyiene o |t

strengths of 6A14V titanium bo"d^i"^mH^??eSsplciSens reinforced with

ambient temperature on powdered aluminum-fll.ed specimen;» J _ FORM 1473 EDITION OF INOVSS^ OBSOLETE UNCLASSIFIED —,: , ^,.r.d)

„ ....■„■.^„^ —■- --—- «MUM

I

UNCLASS1K1ED «CU-'ITY CLASSIFICATION OF TMH P«Gefl»h- T»'« *nl.fd)

heat cleaned 112 glass fabric and 2800 psl at ambient temperature, 2600 psl at ABOOF and 2100 psi at SOQOF on unfilled specimens bonded with 70 percent HR600 prepreg on 112 heat-cleaned qlass fabric Weld-bonding of titanium specimens using aluminum powder filled HR600 adhesive proved to be successful and showed superior fatigue resistance over spot-welded specimens.

UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGEr»T..n D.t. em.red)

aU.S.Government Printing Office: 1974 - 657-015/181

„.i.—^^M -■