NASA Technical Memorandum 89045

EVALUATION OF T i - 6 A l - 4 V SURFACE TREATMENTS

FOR USE WITH A POLYPHENYLQUINOXALINE ADHESIVE

;NBSB-TE-89045) E V A L U A T I C N OF Ti-681-49 N87-11010 S U R F A C E I R E A T E E h T S FCB USE H I l H B E G L Y P B E N Y L Q U I E O X A L I N E A d H E S I V E ( N A S A ) 20 p

CSCL l l B Unc las G3/27 43967

DONALD J, PROGAR

OCTOBER 1986

National Aeronautics and Space Administration

Langley Research Center Hampton, Virginia 23665

https://ntrs.nasa.gov/search.jsp?R=19870001577 2020-03-20T14:05:03+00:00Z

INTRODUCTION

Quinoxal ines, and i n particular a monoether polyphenyl quinoxal ine

(MEPPO) , have shown promise as h i g h temperature polymers w i t h good oxidative s tab i l i ty which, a t reasonably h i g h temperatures, exhibit thermoplasticity.

MEPPQ bonded t i t a n i u m alloy (Ti-6A1-4V) has been shown to retain hicih l a p

shear strengths a t 232OC ( 4 5 O O F ) c23.1 MPa (3350 psi)] af ter 8000 hrs of

thermal exposure a t 232OC (45OOF).

1

1

The use of h i g h temperature, h i g h performance polymers as adhesives for

aerospace applications is h igh ly dependent on the durability of the surfaces

joined by the adhesive. Recent efforts t o determine a surface treatment

w i t h long term, h i g h temperature performance for t i t a n i u m a1 1 oy joints has

met w i t h limited success. 9 s4 T i t a n i u m , and i n particular the t i t a n i u m

alloy, Ti-6A1-4V, i s a primary candidate material for spacecraft and h i g h

performance a i rc raf t because of i t s h i g h strenath t o weiqht ra t io and i t s

h i g h temperature capabilities.

In this paper, three surface treatments for Ti-6A1-4V were evaluated

w i t h the MEPPQ adhesive: phosphate-fluoride ( P F ) etch, chromic acid

anodiza t ion ( C A A ) , and sodium hydroxide anodization (SHA). The SHA

treatment has been shown t o have excellent d u r a b i l i t y i n low temperature

adhesive joints as assessed by wedqe specimens exposed t o h i g h h u m i d i t y and

50°C ( 1 2 2 O F ) temperature.'

better results t h a n the CAA treatment.

In this application, the SHA treatment gave

The primary criterion used t o evaluate the MEPPQ/Ti system w i t h three

different surface treatments was their d u r a b i l i t y to 1 ong term thermal

exposure i n air a t 232OC (45OOF) based on l a p shear strengths and the type

of f a i 1 ure.

2

EXPERIMENTAL

Synthesis . The r eac t ion scheme for the formation of MEPPQ is shown i n

f i g u r e 1. The chemicals used t o prepare the polymer were obtained from

commerci a1 sources. The polymer was prepared by addi ng 3 ,3 ' -d i amino- benzidine (0.49 mole) a s a fine powder t o a stirred s o l u t i o n of 4,4'-oxydi-

benzi l (0.50 mole) i n a 1:l mixture of - m-cresol and xylenes. The s o l i d s conten t , based upon the weight of the monomers, was 20% ( w / v ) . The i n i t i a l

r eac t ion exotherm was c o n t r o l l e d by maintaining the temperature a t < 4OoC

(104°F) by external cool ing.

so lu t ion was formed. Polymer was i s o l a t e d by adding a small volume of

so lu t ion t o methanol i n a blender. The yellow f i b r o u s mater ia l was boi led

twice i n methanol.

a s measured on a 0.5% so lu t ion i n - m-cresol a t 25OC (77OF). The g l a s s t r a n s i t i o n temperature a s measured by d i f f e r e n t i a l scanning ca lor imet ry a t

20°C/min (36'F/min) was 2 9 O O C (554OF).

brown b o t t l e a t ambient condi t ions .

After s t i r r i n g f o r 18 hr, a viscous amber

The dried polymer had an inherent v i s c o s i t y of 0.81 d l / g

The polymer so lu t ion was s to red i n a

The 20 w t % s o l i d s so lu t ion was d i l u t e d t o 10 w t % by add i t ion of 1:l

- m-cresol/xylene t o lower the v i s c o s i t y f o r use a s a primer f o r the 112-style

g l a s s c l o t h and a s a primer f o r the Ti-6A1-4V adherends.

Charac te r iza t ion . Lap shear strength (LSS) was obtained according t o

ASTM 0-1002 using a Model TT Ins t ron universal testing machine. Elevated

temperature tests were conducted n a cl am-shell , auartz-1 amp oven w i t h temperatures cont ro l led t o w i t h i n k3"C (k5OF) f o r a l l tests. Specimens were

h e l d 10 mins a t temperature p r i o r t o testing.

4

3

The average bondline thicknesses f o r the MEPPQ adhesive w i t h the CAA

and PF surface t r e a t e d Ti-6A1-4V adherends were 0.26 mn (0.0104 i n . ) and

0.28 mm (0.0112 i n . ) r e spec t ive ly . The average bondl ine thickness f o r the

SHA specimens was 0.18 mn (0.007 i n . ) .

Glass t r a n s i t i o n temperature (Tg) of the adhesive from the f r a c t u r e d

l a p shear specimensvwas determined by thermomechanical ana lys i s (TMA) on a

D u P o n t 943 Analyzer. TMA was run on the f r a c t u r e d specimens i n s ta t ic a i r

a t a hea t ing r a t e of 5"C/min (9OF/min) using a hemispherical probe w i t h a 15

g load.

mechanical and d i e l e c t r i c re1 axation measurements, was reported i n reference

1.

A Tg of 29OOC (554OF1, as determined f o r this polymer by dynamic

Adhesive Tape Prepara t ion . MEPPQ adhesive t ape was prepared by brush

c o a t i n g a 10 w t % s o l i d s so lu t ion i n 1:l - m-cresol/xylene onto a 112 E-glass c l o t h w i t h an A-1100 f i n i s h (v-aminopropyls i lane) . The g l a s s cloth had been

t i g h t l y mounted on a metal frame and d r i ed i n a fo rced -a i r oven f o r 30 m i n

a t 100°C (212°F) p r i o r to coa t ing . The 0.10 mn (0.004 i n . ) t h i c k g l a s s

c l o t h served a s a carrier f o r the adhesive as well a s f o r bondline cont ro l

and an escape channel f o r so lvent . The coated c l o t h was then a i r d r i ed

overn ight (approximately 16 hrs) and heated f o r 1 hr a t each of three

temperatures: 85°C (185"F), 14OOC (284OF), and 185OC (365°F). Subsequent-

l y , each app l i ca t ion of a 20 w t % s o l i d s s o l u t i o n was brush coated onto both

sides of the cloth and the same heat inq schedule used u n t i l the th i ckness

was approximately 0.30 mn (0.012 i n . ) .

a t 185°C (365OF) and 9 hrs a t 22OOC (428OF) t o reduce the v o l a t i l e c o n t e n t

t o 2%.

The t ape was t h e n heated f o r 18 hrs

The percent v o l a t i l e s was determined by hea t inq a 2.54 cm x 2.54 cm

4

( 1 i n . x 1 in . ) p iece of tape i n a fo rced-a i r oven a t 399OC (750OF) fo r 30

min. The weight o f the tape was determined before and a f t e r the heat

t reatment.

Ti-6A1-4V Surface Treatments. The adhesive tape

ti t a n i um a1 1 oy adherends ( T i -6A1-4V, per M i 1 -T-9046E,

a nominal th ickness o f 1.27 mn (0.050 in . ) .

was used t o bond

Type I11 Comp. C) w i th

J

Three surface treatments were inves t iga ted : chromic a c i d anodizat ion

(CAA) , devel oped by Boei ng A i r c r a f t Company; and sodi um hydroxi de anodizat ion (SHA) . were prepared f o r LaRC according t o the Roeinq process s p e c i f i c a t i o n BAC

5890 us ing 10 vdc a t 2 amp.

by Roeing according t o the BAC 5514 process s p e c i f i c a t i o n . The surfaces

t r e a t e d by Boeing were immediately primed w i t h MEPPQ s o l u t i o n suppl ied by

LaRC. The SHA surface treatment was performed i n our l a b o r a t o r i e s us ing the

cond i t i ons which produced the bes t r e s u l t s i n reference 5, i.e., 10 vdc f o r

30 min i n 5M NaOH a t 20 * l 0 C (68 22°F) [ac tua l temperatures used i n our

l abo ra to ry were 25 + 2 O C ( 7 7 *4OF)].

g r i t A1203 p r i o r t o the surface treatment. I n general, s tab le oxides of

t i t a n i u m are formed by these processes which increase the d u r a b i l i t y o f the

adhesive j o i n t t o adverse environments, i.e., humid i ty and heat.

phosphate-f l u o r i de (PF) etch,

The CAA surface t rea ted adherends

The PF treatment o f Ti-6A1-4V was a lso prepared

Adherends were g r i t b las ted w i t h 120

Bonding. The Ti-6A1-4V was suppl ied i n a " four - f ingered" con f igu ra t i on

w i t h each ' ' f inger" being 2.54 cm (1 in . ) wide and approximately 12.7 cm ( 5

in . ) long. Immediately a f t e r the surface treatments, the area t o be bonded

was primed w i t h a 10 w t % s o l i d s s o l u t i o n of MEPPC), a i r - d r i e d f o r 30 min

under a fume hood, placed i n a fo rced-a i r oven, and h e l d f o r 30 min a t each

o f the th ree temperatures: 7OoC (158OF), 140OC (284OF), and 180OC (356OF).

5

The primed adherends were placed i n a s ea l ed polyethylene bag and s t o r e d i n

a d e s i c c a t o r u n t i l needed.

The l a p shear specimens were prepared by i n s e r t i n g the MEPPQ adhesive

t ape between the primed adherends using a 1.27 cm (0.5 in . ) over lap (ASTM

0-1002). A vacuum bag technique was used t o bond the samples. Bonding

temperature was monitored using a type K thermocouple spotwelded t o the

t i tan ium adherend a t the edge of the bond area .

process was used:

The fol lowing bonding

(1) Apply a f u l l vacuum w i t h i n the bag,

( 2 ) Apply 1.37 MPa (200 psi) pressure ( w i t h a hydraul ic press),

( 3 ) Heat a t approximately 20"C/min (37OF/min) t o 371OC (7OO0F),

( 4 ) Hold 371OC (7OOOF) f o r 20 m i n ,

( 5 ) Cool under vacuum and pressure t o 1 5 O O C (302OF) and remove from

bonding press.

RESULTS AND DISCUSSION

Bonding. The p rocessab i l i t y o f MEPPQ i s a t t r i b u t e d t o the

thermoplas t ic na ture of this type of polymer. The adhesive tape was

processed t o conta in very l i t t l e v o l a t i l e material, 2% i n this case , i n

o r d e r t o avoid the formation of v o i d s during bonding.

schedule previously mentioned, the thermoplastic MEPPQ flowed s u f f i c i e n t l y

t o provide a void free adhesive bond w i t h a small amount of squeeze out .

Microscopic examination of f a i l e d specimens v e r i f i e d the vo id f r e e na ture of

the bonded j o i n t s .

Using the bonding

6

Thermal Exposure Tests. The d u r a b i l i t y o f the MEPPQ bonded j o i n t s w i t h

three d i f f e ren t surface treatments was determined by exposing l a p shear

specimens i n a fo rced-a i r oven a t 232OC (450°F) f o r up t o 12 months f o r the

PF and CAA prepared specimens and f o r up t o 18 months f o r the SHA speci-

mens.

( c o n t r o l ) and a f t e r thermal exposure. SHA l a p shear s t rengths were a lso

determined a t RT before and a f t e r exposure.

Lap shear s t rengths were determined p r i m a r i l y a t 232OC (45OOF) before

Resul ts are given i n Table 1 and inc luded i n F igure 2 f o r MEPPQ/PF

The RT LSS values were lower than expected f o r t h i s t r e a t e d Specimens.

adhesive, 19.6 MPa (2851 p s i ) , compared t o those i n re ference 1, 32.7 MPa

(4740 p s i ) . However, the adhesive type o f f a i l u r e i nd i ca tes the adherend surface as the poss ib le reason f o r the lower resu l t s . Low r e s u l t s were

obtained a f t e r thermal aging w i t h t o t a l l oss o f s t rength a t some per iod

between 6 months and 12 months ( i n d i c a t e d by the dashed p o r t i o n o f the curve

i n F igure 2). Again, on ly adhesive type f a i l u r e s occurred.

Resul ts f o r the CAA t rea ted specimens are given i n Table I i and

inc luded i n Fiqure 2. Higher LSS values were obtained f o r t h i s system as

compared t o the PF system except f o r a s l i g h t d i f f e rence f o r the 232°C

(45OOF) cont ro l values 123.1 MPa (3355 p s i ) versus 24.8 MPa (3608 p s i ) ] .

The CAA re ta ins good st rengths up t o 6 months (4380 h r ) b u t s t rength

decreases s i g n i f i c a n t l y a f t e r a 12 month (8760 h r ) exposure 15.8 MPa (838

p s i ) ] .

b u t changed t o adhesive f a i l u r e s fo r the 12 month tes ts .

specimens are shown i n F igure 3. The surfaces o f these specimens were

surface analyzed by Boei ng Aerospace Company us ing x-ray photoelect ron

spectroscopy (XPS). The XPS data on the unexposed and 3 month exposure

F a i l u r e modes were 100% cohesive f o r the con t ro l and 3 month t e s t s

Examples o f f a i l e d

7

specimens indicate a disbond i n the adhesive (cohesive fai lure) whereas the

data for the 6 month exposure specimen implies t h a t the disbonding occurred

a t the primer/oxide i n t e r f a ~ e . ~ The low results a f te r 12 months thermal

exposure a t 232OC ( 4 5 O O F ) make the CAA surface treatment unacceptable for

MEPPQ for long term durability a t elevated temperatures. S imi la r results

were obtained i n a Boeing study where LSS values decreased significantly

between 5000 and 10,000 hrs exposure a t 232°C (450"F).4

Because of the encouraging results obtained w i t h the SHA treatment by

Kennedy, e t . a l . , reference 5, this treatment was investigated for the MEPPQ

adhesive af ter the poor results f o r the PF and CAA treatments were

determined.

i n Figure 2 for the MEPPQ/SHA specimens. The LSS tes t s a t RT and 232OC

(45OOF) were conducted af ter 6.7, 12, and 18 months. A siqnificant

improvement i n LSS retention was obtained af te r the thermal aging w i t h th i s

treatment. The LSS values a t 232°C (45O0F) for the SHA tes t s af ter 12

months are significantly higher t h a n the CAA and PF LSS values: 17.0 MPa

(2465 psi) , 5.8 MPa (838 ps i ) , and 0 MPa, respectively. The failure mode

for the SHA specimens tested a t 232°C (45OOF) af te r 12 months were pr imar i ly

cohesive (Fiqure 4 ) whereas the CAA specimens failed adhesively and the PF

specimens fel l a p a r t (adhesively) when removed from the oven.

specimens were tested a t 232°C (45OOF) af ter 18 months therma

LSS values were lower than those after 12 months exposure, 12 1 MPa (1750

psi) compared to 17.0 MPa (2465 psi) . Failure for the 18 month specimens

were primarily adhesive (Figure 4 ) .

Thermal aqing t e s t results are given i n Table I11 and included

When the SHA

exposure, the

RT LSS tes t s were also conducted for MEPPQ/SHA specimens before and

af te r thermal exposure. A s i g n i f i c a n t decrease i n LSS was obtained between

I 8

6.7 and 12 months:

w i t h this decrease was a change i n fa i lure mode from primarily cohesive to a

100% adhesive failure. The reason for this change of failure mode i s

unknown a t this time b u t a possible casue i s discussed further below.

24.5 MPa (3560 p s i ) to 9.1 MPa (1318 p s i ) . Associated

The Tg's of the MEPPQ adhesive from the fracture specimens are included

i n Table 111. The measured Tg's range from 275°C (527°F) to 332°C (630°F).

As noted before, reference 1 c i tes the Tq as 290°C (554°F) which was

determined by dynamic mechanical and dielectric re1 axation measurements.

A possible cause of the abrupt chanqe i n fa i lure mode noted above i s an

interaction of the solvent system w i t h the treated surfaces. A 1:l mixture

of - m-cresol and xylene (mixed) solvent system was used for t h i s polymer. - Meta-cresol 1b .p . 202°C (396"F)I is a d i f f icu l t solvent to remove from the polymer system because i t is tenaciously held by the polymer.

137°C - 140°C (279°F - 284"F)] is more easily removed t h a n - m-cresol. Perhaps the solvent system interacts i n some manner w i t h time and tempera-

ture ( h i g h ) t o degrade the interface region or surface of the adherend t o

b r i n g abou t an interfacial type failure. An analysis was made using mass

Xylene [b.p.

spectrometry to determine i f any of the solvent system, especially the

- m-cresol, was s t i l l evident i n the adhesive of a fractured lap shear

specimen of an as-fabricated as well as a specimen thermally aged for 18

months a t 232°C (450°F). No evidence of the existence of either solvent was

found. Perhaps surface analysis studies of the treated surface before c

testing and after failure could provide an explanation for the system's

failure.

9

SUMMARY

Three surface treatments for Ti-6A1-4V adherends were evaluated u s i n g a

thermoplastic polymer monoether polyphenylquinoxaline, MEPPQ, which had been

shown i n previous studies t o have good potential as a h i g h temperature

adhesive for aerospace applications. I n i t i a l results based on lona term

thermal exposure a t 2 3 2 O C ( 4 5 O O F ) using the phosphate-fluoride ( P F ) and

chromic acid anodized ( C A A ) treatments w i t h MEPPQ adhesive were not

encouraging.

i n fa i lure mode (cohesive) a t 232°C (450OF) was found for the SHA treated

specimens compared t o the PF and CAA treatments.

long term thermal d u r a b i l i t y was obtained w i t h the SHA treatment of Ti-6A1-

4V, a change i n failure mode from cohesive t o adhesive was noted for the RT

tes t s a f te r 12 months exposure a t 23'2°C (45OOF).

t ive of a degradation of the adhesive-adherend interface.

surface treatment w i t h better lona term d u r a b i l i t y i s s t i l l required f o r

aerospace appl i cati ons .

A significant improvement i n strength retention and a change

Al though an improvement i n

Such a chanqe i s indica-

An improved

REFERENCES

1. Hergenrother, P. M.: Hiqh Temperature Organic Adhesives. SAMPE

Quarterly, - 3 l (1971) . 2 . H i l l , S. G.: Eva lua t ion of High Temperature Structural Adhesives for

Extended Service. NASA CR-170129, Nov. 1981.

10

3. H i l l , S. G.; Peters, P. D.; and Hendricks, C. L.: Evaluat ion o f High

Temperature S t ruc tu ra l Adhesives f o r Extended Service. NASA CR-165944.

J u l y 1982.

4. H i l l , S. G.; Peters, P. D.; and Hendricks, C. L.: Evaluat ion o f High

Temperature S t ruc tu ra l Adhesives f o r Extended Service. NASA CR-169882,

May 1981.

5. Kennedy, A. C.; Kohler, R.; and Poole, P.: A Sodium Hydroxide Surface

Pretreatment f o r the Adhesive Bonding o f T i tan ium Al loys.

Adhesion and Adhesives, - 3 No. 2, 133( 1983).

I n t . J.

6. Moj i , Y. and Marceau, J. A.: Method o f Anodizing T i tan ium t o Promote

Adhesion. U. S. Patent No. 3,959,091, May 1976.

11

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ORIGINAL PAGE ?3 KX3R QUALITY

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15. Supplementary Notes

Use of trade names or manufacturers does not consti tute an off ic ia l endors e i ther expressed or implied, by the National Aeronautics and Space Adminstration.

21. No. of Pages 22. Price 19 A02

16. Abstract

Three surface treatments for Ti-6A1-4V adherends were evaluated u s i n g a t h p las t ic polymer monoether polyphenylquinoxaline, MEPPQ, which had been sha previous studies t o have good potential as a h i g h temperature adhesive f o r aerospace applications. In i t i a l results based on long term thermal exposu 232°C ( 4 5 O O F ) using the phosphate-fluoride (PF) and chromic acid anodized treatments w i t h MEPPQ adhesive were not encouraging. A significant improv in strength retention and a change i n f a i lu re mode (cohesive) a t 232°C (45 was found for the SHA treated specimens compared t o the PF and CAA treatme Although an improvement in long term thermal durabili ty was obtained w i t h SHA treatment of Ti-6A1-4V, an improved surface treatment with bet ter long durabili ty i s s t i l l required fo r aerospace applications.

17. Key Words (Suggested by Authors(s))

Polyqui noxal i ne The rmopl as t i c Surface pretreatment Phosphate-fluori de Anodized Lap shear strenqth

Adhes i ve 18. Distribution Statement

Unclassified - Unlimited

Subject Category 27

b

i

For sale by the National Technical Information Service, Springfield, Virginia 22161 N A S A Langley Form 63 (June 1985)