Structural adhesive performance in marine environments

PoIj9nier- Ititernurionul 28 (1992) 9-1 7

Structural Adhesive Performance in Marine Environments*

Trevor Horton, Geoffrey M. Spinksx Department of Materials Engineering, University of Wollongong,

PO Box 1144, Wollongong, NSW 2500, Australia

Neils A. Isles Comalco Research Centre, 15 Edgars Road, Thomastown, Victoria 3074, Australia

(Received 22 March 1991; revised version received 31 May 1991; accepted 14 June 1991)

Abstract: Durability of adhcsively bonded aluminium joints was investigated for both stressed and unstressed joints exposed to environments of air, distilled water, 0.5% sodium chloride solution and 5% sodium chloride solution. Testing involved the use of marine grade aluminium, two acrylic adhesives and two epoxy resin adhesives. Surface preparation consisted of a solvent wipe degreasing procedure. The acrylic adhesives used in this study displayed greater bond durability than cpoxy resins. Results also reaffirmed that the combination of stress and environmental exposure is more detrimental to bond durability than environmental exposure alone. The combined effects of stress and dilute salt solution were found to be more detrimental to adhesive bond durability than stress combined with distilled water, or concentrated salt solution. Two possible causes of bond deterioration considered were corrosion of the aluminium adherend and instability of the interface. Either of these explanations could account for the observed results; however, more work is required to verify these theories.

Key iiwrds: aluminium adhesion, durability, hostile environments, epoxy resins, acrylic adhesives.

1 INTRODUCTION

The conventional method for manufacturing aluminium boats is to weld the structure together. For reasons of economics, performance and simplicity, adhesive bonding may be a viable alternative to welding. Problems involved with welding include the production of ‘heat affected zones’ in the aluminium, which can l o c a b reduce the strength of the material, and associated problems with corrosion. Adhesives offer such potential advantages as reduced fabrication costs, more uniform distribution of stress, increased corrosion resistance and weight savings. * Presented at p o { ~ r t i p r ‘9/ /t7~<wZLlt;#nU/ Sjniposiuni, Melbourne, 10-15 February 1991. 1 To whom correspondence should be addressed.

PoljwPr hternutionu~ 0959-8 103/92/$05.00 0 1992 SCI. Printed in Great Britain

However, the durability of adhesive bonds in the marine environment has not been well established.

Adhesive bond durability has been shown to depend upon such factors as adhesive type, surface pretreatment and environmental conditions.’ Surface pretreatments play an important role in bond durabilityS2 In general, to attain maximum durability in bonded aluminium struc- tures, complex surface pretreatments often need to be employed. This limitation can be overcome to a certain extent by the correct selection of adhesive type.

Several previous studies have shown that the combined effects of applied stress with aggressive liquids has a more detrimental effect on adhesive joint Thus the combined effects of stress and water, or salt water solutions, are likely to play a significant role in

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determining the durability of joints in an adhesively bonded aluminium boat.

The adhesive bonding of aluminium has proven reliability in aircraft applications4 but this is yet to be established in boat manufacture. The aim of the investigation is to determine the effects of stress and aqueous environments on the durability of adhesively bonded marine grade aluminium with the use only of simple surface pretreatment. In addition, an attempt is made to identify the mechanism of bond failure in these environments.

2 EXPERIMENTAL DETAILS

2.1 Materials

The aluminium used in this investigation is type 5251- H34. This is marine grade aluminium used in the construction of aluminium boats.

Four commercially available adhesives were used in this investigation. Two were acrylic adhesives (adhesives A and B) and two were epoxy resin adhesives (adhesives C and D).

2.2 Experimental procedure

Individual lap shear specimens were prepared in accordance to dimensions stated in ASTM D1002-72. The 15 mm x 100 mm coupons were machined from 1.6 mm thick aluminium and bonded with a 13 mm overlap. Prior to bonding the surface was degreased by wiping with carbon tetrachloride.

The curing conditions (as recommended by the supplier) for each of the adhesives were as follows:

A. 24 hours at room temperature B. 24 hours at room temperature C. 2 hours at 50 C D. 4 hours at 50 C

Five lap shear specimens of each adhesive were prepared for each different condition to be tested. Initial lap shear strength was determined prior to durability testing by tensile testing in an lnstron model 4302 tensile testing machine at a rate of 1.3 mm/min. Durability ofthe joints was determined by exposing samples to various environments in the stressed and unstressed condition at room temperature.

Creep rupture tests were conducted by stressing the samples to 30% of their initial bond strength while simultaneously exposing them to environments of air, distilled water, 0.5% sodium chloride solution and 5% sodium chloride solution. This was achieved by modify- ing a model M k 111 Unisteel creep machine. Stressed exposure was performed until failure of specimens (to a maximum of 500 hours). Wedge tests were also conducted

in accordance to ASTM D3762-79 for samples bonded with adhesive D, and in environments of distilled water, 0.5% sodium chloride solution and 5% sodium chloride solution.

Strength retention of the joints in the unstressed condition was determined by immersing the samples in environments of distilled water, 0.5% sodium chloride solution and 5% sodium chloride solution. Two exposure conditions were used: continuous exposure and cycled exposure. Cycled exposure consisted of a cycle of 24 hours in solution and 24 hours out of solution. After 500 hours of exposure samples were removed from the water or solution, dried for 24 hours and tested for strength retention in an Instron tensile testing machine. The retained strength is expressed as a percentage of the initial average bond strength. At least five repetitions were conducted to allow averages and confidence limits (at the 95% level) to be calculated.

In addition, bulk adhesive castings were made and tested for weight and property changes following exposure to distilled water and 5% NaCl solution. Tensile tests were conducted to determine the failure stress. Total exposure time was 500 hours. After exposure specimens were removed from the bath, dried of surface water, weighed and tested in the Instron machine. Tensile strength values were compared to those of unexposed samples.

3 RESULTS

3.1 Creep rupture

N o samples failed in the 500-hour time period upon stressed exposure to air. Therefore any failures that occur in the other exposure conditions can be attributed to the additional effects of the particular environment.

Exposure to distilled water (Fig. 1) only proved more detrimental to bond durability than air for adhesive B. No specimens using adhesives A, C or D had failed after 500 hours of exposure. Four adhesive B bonds failed at an average time of 365 hours, with one surviving the full 500 hours. (One adhesive C sample failed after a very short time; however, this result appears to be an anomaly.)

Exposure to dilute salt solution (Fig. 2) in the stressed state proved to be the most detrimental to bond durability. This was especially the case for the two epoxy resin adhesives, C and D. These two adhesives exhibited very poor durability, with average failure times of 13 and 35 hours, respectively. Adhesive B bonds exhibited significantly greater bond durability, with one sample surviving the full 500 hours and the other four failing at an average of 257 hours. Specimens prepared with adhesive A again gave the most durable bonds, with no samples failing within 500 hours.

In general, stressed exposure to the concentrated salt solution was more detrimental to bond durability than

POLYMER INTERNATIONAL VOL. 28, N O . l , 1992

Structural adhesive performance in marine environments

I 500

400

300

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0

1 1

H Sample 1

Sample 2

Sample 3

Sample4

Sample 5

Acrylic A Acrylic B Epoxy C Epoxy D Fig. 1. Time to failure of lap-shear joints upon stressed exposure to distilled water.

environments of air or distilled water, but was less adhesives. For the adhesives that exhibited failures (B, C detrimental than the dilute salt solution. The results are and D), the environments proved to be the most given in Fig. 3. detrimental to bond durability in the following order:

0 3 % NaCI > 5% NaCl >distilled water > dry air Figure 4 summarises the results of the stressed exposure tests. Adhesive A is clearly the most durable of the tested adhesives, with no failures observed within 500 hours in any of the environments tested. Adhesive B exhibited the next best bond durability, followed by the two epoxy resin

An unusual result was that the 0.5% sodium chloride solution was found to be more detrimental to bond durability than the 5% sodium chloride solution. It has

500

400

300

200

100

0 Epoxy D Acrylic A Acrylic B Epoxy C

Fig. 2. Time to failure of lap-shear joints upon stressed exposure to dilute salt solution (0.5%).

POLYMER INTERNATIONAL VOL. 28, NO. 1,1992

Sample 1

0 Sample 2

Sample 3

Sample4

Sample 5

12 Trevor Horton, Geqfrey M. Spinks, Neils A . ls1e.s

Acrylic A Acrylic B Epoxy C Epoxy D Fig. 3. Timc to Failure of lap-shear joints upon stressed exposure to concentrated salt solution ( 5 % ) .

bcen previously suggested 1.3 that deterioration in bond strength is more rapid in a salt water environment than in fresh water. It would also be expected that a more concentrated salt water solution would be more detri- mcntai than more dilute solutions. Unfortunately cxperi- mental results have not been previously published to fully establish either of these correlations.

3.2 Wedge tests

Wedge tests were conducted in an effort to verify the results of the stressed exposure trials given above. Results of these tests (Fig. 5) show that bond durability is again decreased upon exposure to the dilute salt solution. In comparison, the durability of the joint was improved

Acrylic A Acrylic B Epoxy C Epoxy D

Dry Air

Distilled Water

fl 0.5Y0NaCl

5.0% NaCl

Fig. 4. Suniinary of creep rupture data. Average time to failure is shown as a function of the environment to which the joints were exposed.

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Structural adhesive performance in marine environments 13

60

50

h

E E 5 i% 40 g

5 E

v

M

u 30

20

i2 8

d

Y

.I Y C Q) Y

5

t 3;

M c

120

100

80

60

40

20

0

I I

0 10 20 30

Time (hrs) Fig. 5. Crack growth versus time for epoxy D wedgc spccimcns in various environments.

0 Distilled Water

0.5%NaCl

5.0% NaCl


Fig. 6. Strength retention of lap-shear joints after continuous exposure to various environments in the unstressed condition.


14 Trevor Horton, Geqflrey M . Spinks, Neils A . Isles

120

100

80

60

40

20

0

T I I I 1

Distilled Water

0.5% NaCl

5.0% NaCl

Acrylic A Acrylic B Epoxy C Epoxy D Fig. 7. Strength retention of lap-shear joints after cycled exposure to various environments in Lhe unstressed condition.

upon exposure to distilled water or a concentrated salt solution.

The wedge test results are in complete accord with those reported above and verify the finding that the dilute salt solution is the most detrimental of the environments tested.

However, longer exposure times were employed in these studies and this may account for the apparent discrepancy with the results given above. Thus it may be that the 500 hours of cycled exposure employed in this study is an insufficient time in which bond deterioration can occur.

3.4 Bulk adhesive trials 3.3 Strength retention

Unstressed exposure tests were conducted to establish the effect on bond strength of exposure to various aqueous environments in the absence of an applied stress. In general, the epoxy adhesives displayed higher bond strengths than the acrylic-based adhesives. In the absence of an applied stress the aqueous solutions had little effect on the strength of the bond over a 500-hour time period. The results for the continuous and cycled exposure tests are given in Figs 6 and 7, respectively. Considering errors in measurement, there is no statistically significant decrease in bond strength for any of the adhesives in any of the environments for the cycled exposure. A slight decrease in bond strength was observed for adhesive D upon continuous exposure to distilled water and 0 5 % sodium chloride solution.

The results indicate that environmental exposure alone is insufficient to cause creep rupture within 500 hours in the aqueous solutions studied. Contrary to the results presented in the present study, previous investigations have shown that cycled exposure can be more detrimental to the bond durability than continuous e x p ~ s u r e . ~ . ~

To test for the effect of exposure to aqueous environments on the properties of the adhesive material, bulk adhesive trials were conducted. The degree of water absorption in each environment was determined, along with the effects upon bulk adhesive strength.

The changes in weight for each adhesive material upon immersion in both distilled water and a 5% NaCl solution are given in Figs 8 and 9. A continuous increase in mass was observed for adhesives A, C and D in both solutions. Adhesive B showed an initial mass increase, followed by a steady decrease in mass after approximately 100 hours of immersion. Mass increase is due to moisture absorption, while a decrease in mass can be attributed to the leaching of soluble material. Both effects would be expected to influence the bulk strength of the adhesive materials. However, little statistical change in tensile strength was observed for any of the adhesives after 500 hours of immersion in either solution (Fig. 10).

The results indicate that exposure to the aqueous environments studied had little effect on the strength of the adhesive material for an exposure of 500 hours. Therefore the degradation of the adhesive joints under

POLYMER INTERNATIONAL VOL. 28, N O . l , 1992

Structural udhesiue performance in marine environments

v)

t e .I

2.0

1.5

1.0

05

0.0 0 100 200 300 400 500 600

Time (hrs) Fig. 8. Increase in mass of adhesive tensile specimens with time upon exposure to 5% sodium chloride solution.

2.0

1.5

0.5 u f x m

U

0.0

-0.5 0 100 200 300 400 500 600

Time (hrs) Fig. 9. Increase in mass of adhesive tensile specimens with time upon exposure to distilled water.


16 Trcwr Horton, Geoffrey M . Spinks, Nei1.r A . i . r l ~ s

1 120 ' __I

100

80

60

40

20

0

Distilled Water

5% NaCl


Fig. 10. Strength retention. as a perccntagc of initial bond strength, of adhesive tensile specimens aftcr 500 hours o f cxposurc 10 different environments.

creep loads must be a consequence of the breakdown of the interfacial bond between the adhesive and the a I u mi ni um.

4 DISCUSSION

The results reported above reaffirm the general obser- vation that an applied stress in combination with aggressive environments is more detrimental than either effect on its own. In the present case joint failures were the result of the deterioration of the interfacial bond between the aluminium substrate and the adhesive rather than any deterioration in the physical properties of the adhesive material.

An unexpected result, however, was that the dilute salt solution proved more detrimental than either the concentrated salt solution or distilled water. This result was observed in both stressed exposure trials and wedge tests. Apparently the mechanism of failure is sensitive to the concentration of NaCl in solution. Such an effect may either be explained by a corrosive reaction occurring on the surface of the aluminium substrate or by the displacement of adhesive molecules from the aluminium surface by the ingress of the aqueous solution.

4.1 Aluminium corrosion

The role of aluminium corrosion in bond degradation is unclear. Previous studies have suggested that corrosion occurs following adhesive metal interfacial

However, other workers have suggested that corrosion at the interface is a cause of bond deterioration.',"'

I n this investigation corrosion was visible to the naked eye after samples had been exposed for significant periods oftinie in salt solutions. However, it was noted that visible corrosion was not always associated with bond failure. Therefore i t is possible that corrosion of the aluminium is not the cause of bond degradation, although more detailed surface analysis is required to test this assertion.

4.2 Thermodynamic instability of the interface

Another possible cause of bond deterioration is the thermodynamic instability of the interface in the presence o f liquid media. Calculations of the thermodynamic work of adhesion ( WAL)ll.lz demonstrate the instability of the epoxide/aluminium oxide and acrylic adhesive/alum- iniuni oxide interfaces in the presence of water. In both c ;ws the work of adhesion value was negative and consequently the joints were prone to spontaneous debonding. In such instances there is a thermodynaniic driving force for the displacement of the adhesive molecules froni the substrate surface by inolecules of the liquid. The joint would then be unable to support shear stresses and bond failure is likely to be observed.

Unfortunately thermodynamic considerations take no account of kinetics. Thus a negative work of adhesion value does not indicate that a particular joint will show poor durability in a given environment over its antici- pated service life. Nevertheless, thermodynamics can


Struc'turul udlirsivr prrformuncr in murine environmrnts 17

provide useful insights into the possible mechanisms of failure.

Thermodynamic considerations may also explain the effect of NaCl concentration on the bond durability. For a vinylidene chloride-methyl acrylate copolymer/poly- propylene interface, debonding was observed when exposed to a 0.5 %wt solution of sodium n-dodecyl sulphate.13 This corresponded to a negative work of adhesion. However, no debonding was observed upon exposure to a 3 . 5 % ~ solution, for which W,, was positive.

Similar thermodynamic forces could be responsible for the debonding observed in the present investigation. However, further analysis, including measurement of surface energies, is required to verify this theory. Such analyses are currently being conducted.

5 CONCLUSIONS

From the analysis of aluminium bond durability in marine environments the following may be concluded:

( I ) The combination of stress and exposure to severe environments is more detrimental to bond durability than either effect on its own. Under the specific test conditions employed it was observed that the 0.5% sodium chloride solution was more detrimental to bond durability than the 5% sodium chloride solution and distilled water. These results were confirmed by both stressed exposure trials and wedge tests.

(3) The effect of water and salt solution absorption by the adhesive had little effect on the bulk adhesive properties over a 500-hour exposure time. This indicates that joint failure was caused by the

( 2 )

breakdown of the interfacial bond between the adhesive and aluminium.

(4) Two possible causes of bond deterioration considered are corrosion of the aluminium adherend and the thermodynamic instability of the interface. Further investigations are needed to determine the predominant mechanism. This could involve microscopic examination of fracture surface to determine the extent of corrosion and the measurement of surface energies to determine the interface instability. Using a simple surface degreasing pretreatment. the acrylic adhesive grades showed enhanced durability compared with the epoxy adhesive grades tested. However, more appropriate surface pretreatments are likely to significantly improve the durability of the latter. In general, a more thorough understanding of the failure mechanisms of adhesive joints in hostile environments is required to develop the optimum adhesive system.

( 5 )

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Brewis. D. M.. In f h r d i i / i / i , o/ S/ruc,/ irrd .4t//ic,.vIi.c,.\. ed. A. J. Kinloch. Applied Science Publishcrs. London. 1983. p. 21 5. Brewis. D. M.. C l t r r c ~ . .%i. f i d r / i o / . . 2 ( 1986) 761.

Koniia. L. ~4 Olefjord, 1.. Mtr /c r . S(.i. E~ht i / . . 3 (1987) 860. Lord Corporation. Technical data sheet. Minford. J . D.. .I. At/h,.\io/r. 18 (1985) 19. Minliird. J . D.. S.4.ZIPE Q. ( J u l y 1978) 18. Mitiford. J. D.. ,E!c,/t// E/i,qtig Q. (November 1972). Mitiford. J. I).. .4t//ic,.\iw.\ A,qc, ( Ju ly 1974) 24. Vctinblcs. J . D.. J. Mtr /c r . Sci.. 19 ( 1984) 243 I . Glcdhill. R. A. & Kinloch. A. J.. J . A d / i ~ . \ i ~ ~ i , 6 ( I 974) 3 15. Kinloch, A. J.. Dukes. W. A. & Glcdhill. R. A,. PO/J.JIZ. .%/. Techo/ . , 9B (1975) 597. Kinloch. A. J. Ct Bascom. W. D.. In E1i~:1.t./ri~~t~~lit, o/'bfti/cri& Sc~ie~ttc.~, u/rd E/t,yiric,c~i/rg. Vol. I , cd. M. B. Bevrr. Pcrganion Press, 1986. p. 366.

LCC. L. H.. / / / / . .I. At//W.ViflJl t / / l t / .4t//icJ.tii.o.v. 7 ( 19x7) X I .


Structural adhesive performance in marine environments

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