Durability of Bonds Formed Between Epoxy Adhesive and Aluminium Alloy Treated with Phosphonate Inhibitors

SURFACE AND INTERFACE ANALYSIS, VOL. 24, 583-590 (1996)

Durability of Bonds Formed Between Epoxy Adhesive and Aluminium Alloy Treated with Phosphonate Inhibitors

A. N. Rider and D. R. Arnott Aeronautical and Maritime Research Laboratory, Victoria 3207, Australia

A traditional organosilane coupling agent, y-glycidoxypropyl trimethoxysilane (y-GPS), was compared with a series of phosphonate hydration inhibitors in the role as metal pretreatments for adhesive bonding. The bond durability of epoxy adhesive-bonded double cantilever beam specimens in hydrothermal environments was assessed where the aluminium adherend had been pretreated with either class of compound. The performance of the compounds is related to the structure of the adsorbed film on the metal substrate and its subsequent interaction with the epoxy adhesive in the bonded system. These assessments were conducted using XPS and Fourier transform infrared spectroscopy (FTIR) analysis of the deposited films and XPS analysis of failed bond specimens to determine the zone of fracture. The bond failure mechanisms are discussed in terms of a simple bond degradation model.

INTRODUCTION

The use of adhesively bonded structures in industrial applications relies critically on their ability to maintain strength in a variety of environmental conditions. The high strength of epoxy-based adhesive systems exhibited in dry environments often deteriorates substantially in hot and humid conditions.' A common method used to improve the hydrolytic stability of epoxy adhesives bonded to metal substrates is to apply coupling agents to the metallic substrate.2 In particular silicon-based coupling agents have been used extensively in this role. Whilst organosilane coupling agents represent the most regularly used solution to improved hydrolytic stability of adhesive bonds, a variety of alternatives is a~ailable.~

Venables and co-workers examined a class of phosphorus-based hydration inhibitors which improved the hydrolytic stability of epoxy to metal bonds.4 These compounds differ from traditional coupling agents in that their main effect is to inhibit the degradation of the metal's surface oxide layer, the zone where bond failures often occur, particularly in wet environments. Whilst organosilane coupling agents have been in use for several decades and have been investigated extensively, the phosphonate inhibitors have only recently been applied in the adhesive bonding field and are compara- tively unknown. This work investigates a series of phosphonate hydration inhibitors and compares their performance with a traditional silane coupling agent with the aim of determining any significant differences in the properties of the two classes of compounds.

EXPERIMENTAL

Hydration studies

Coupons of A1-2024 T3 clad aluminium alloy were abraded with P-1200 silicon carbide paper and debris

CCC 0142-2421/96/090583-08 0 1996 by John Wdey & Sons, Ltd.

was removed with high-pressure nitrogen. This prep- aration produced a surface with low carbon contami- nation and a very thin oxide film. One series of coupons was treated by immersion in a 1% aqueous solution of y-glycidoxypropyl trimethoxysilane (y-GPS) for 10 min followed by rinsing in distilled water and drying with nitrogen. The 1% y-GPS aqueous solution was previously stirred for 1 h. A second series of coupons was treated by immersion in a 300 ppm aqueous solution of nitrilotris methylene phosphonic acid (NTMP), adjusted to a pH of 3.5 using dilute nitric acid additions, for 15 min, followed by thorough rinsing in distilled water and drying with nitrogen. The molecular structure of these molecules is shown in Fig. 1. A series of control coupons was left untreated. The specimens were then exposed to a condensing humidity environment at 50 "C and the rate of oxide film thickening monitored with time.

The rate of aluminium oxide film thickening was monitored by ratioing the intensity of the A13+ to A1 peaks from the A1 2p photoelectron line2 using a Kratos XSAM800 photoelectron spectrometer with Mg Ka radiation in FAT mode with a pass energy of 20 eV.

Phosphonate adsorption studies

Coupons of A1-2024 T3 clad aluminium alloy were vapour degreased and treated in a 60 "C solution of alk- aline cleaner prior to immersion in a 65 "C Forest Pro- ducts Laboratory (FPL) chromic acid etch ~olut ion.~ Samples were then rinsed in tapwater and immersed for 15 min in 1, 5, 10, 50, 100, 200, 300 and 500 ppm solutions of the following phosphonate inhibitors: NTMP, (n-buty1)nitrilobis methylene phosphonic acid (nBu- NB M P) and (3-hydroxypropy1)ni trilo bis met h ylene phosphonic acid (3HP-NBMP). The structures of these acids and y-GPS are shown in Fig. 1. The solutions were adjusted to a pH of 3.5, using dilute nitric acid additions, and samples were thoroughly rinsed after removal from solution.

Receiued 14 November 1995 Accepted 5 May 1996

584 A. N. RIDER A N D D. R. ARNOTT

l 0.

I -0

(3-Hydmxypropyl) Niuilobis Mcihylene Pliosphinic Acid

(3HP) NBMP

-o-p=o

I 0'

I ' 0

(n Bulyl) Niwilobis Methylene Phosponic Acid

(n Bu) NBMP

I 0 I I CN2

CHz I /N\

r TH2

\ -0-p-0 O = P - O -

l 0-

I -0

Si CH1O' I 'OCH,

OCH, Nitrilotna

Methylene Phasphonic Acid tiarnma-glycidaxy propyl mmethoxy stlane

y-r. PS NTMP

Figure 1. Molecular structures of the three phosphonate and the organosilane coupling agents used in the study.

X-ray photoelectron spectroscopy (XPS) was used to monitor the concentration of adsorbed phosphonate from the different solutions by ratioing the atomic concentration of the P 2p to A1 2p photoelectron peaks. The spectrometer was operated in FRR mode with a ratio of 53. Quantification of the spectra was performed using the sensitivity factors provided by the manufac-

Figure 2. The XPS measurements conducted on surface-abraded 2024 aluminium alloy with thin overlayers of yGPS and NTMP to assess the rate of growth of aluminium oxide (a) and the rate of desorption of y-GPS and NTMP (b) as a function of exposure to

LUICI. 50 "C-condensing humidity.

Examination of silane solution and films by FTIR

A model 1725X Perkin-Elmer FTIR spectrometer was used to analyse the reaction of a 3% aqueous solution of y-GPS. This was the minimum detectable concentration of silane required for analysis using the experimental set-up available. Small volumes of solution were analysed at different times using a recess-mounted ZnSe crystal trough assembly fitted to a Graseby Specac 11050 series optical unit. This unit enabled six reflec- tions at a 45" incident angle. Detection was performed with an external MCT detector. The spectra acquired were averaged over 200 scans and the silane spectrum was obtained by subtracting the water spectrum from the 3% y-GPS spectrum.

A polished A1-2024 T3 clad specimen was immersed for 10 min in a 1% y-GPS solution which had been stirred for 60 min. The deposited film was water rinsed, dried and analysed using a Graseby Specac 19756 series specular reflectance accessory. The sample was analysed at an angle of 55" using the MCT detector and averag- ing the spectra over 200 scans. A polished, cleaned alu-

minium specimen with a mirror finish was used to acquire the background spectrum.

Adhesive bond durability studies and failure analysis

A1-2024 T3 clad aluminium sheets of 3.2 mm thickness were used to prepare Boeing wedge test specimens (ASTM D3762-79) for adhesive bond durability studies. The 150 x 150 mm plates were prepared as specified for the phosphonate adsorption studies. One series of plates was immersed for 15 min in 300 ppm solutions of one of the following three phosphonate compounds : NTMP, nBu-NBMP or 3HP-NBMP. A second series of plates was immersed for 10 min in a 1% y-GPS solution which had been stirred for 60 min. All plates were thoroughly rinsed in distilled water and dried at 80°C for 1 h. Bonding with FM 73 modified epoxy film adhesive was conducted in an autoclave at 275 kPa and 120 "C for 60 min. Bond durability tests were carried out at 50 "C in a condensing humidity with crack rates monitored over a

DURABILITY OF BONDS FORMED BETWEEN EPOXY ADHESIVE AND ALUMINIUM ALLOY 585

P/AI Ratio for Aluminium Immersed in Phosphonate Solution

0.3 1 I I

0.25 -I 0.2

0 .- Y e 4 0.15 n \

0.1

0.05 0.1 1 10 100 1000

Concentration (ppm)

Figure 3. The P/AI ratio for aluminium immersed in three phosphonate solutions as a function of concentration.

period of weeks. A separate series of plates received a manual abrasion using a scotchbriteTM scouring pad, followed by treatment with the NTMP or silane compound as described above. Control specimens were pre-

pared in which plates only received the FPL etch or scotchbriteTM abrasion treatment.

X-ray photoelectron spectroscopy analysis of failure surfaces, exposed by separating wedge specimens, was

t 1 1 1 I I 1 I

1500 1400 1300 1200 1100 1000 goo aoo 700 cm-’

I I I I 1

3000 2500 2000 1500 1000

cm- ’ Figure 4. (a) The FTlR spectra of 3% y-GPS solution as a function of time. (b) Specular reflectance Al-2024 alloy immersed in 1 YO y-GPS solution for 10 min.

FTlR spectrum of film deposited on

586 A. N. RIDER AND D. R. ARNOTT

100 1000 lo4 lo5 20 1 10

Time (min)

Time (hours) Figure 5. Crack length as a function of exposure time to a 50°C condensing humidity environment for: (a) FPL-etched Al-2024 clad aluminium pretreated in 300 ppm phosphonate solutions or 1% y-GPS aqueous solution; (b) manually abraded Al-2024 clad aluminium pretreated in 300 pprn NTMP or 1 % y-GPS aqueous solution.

performed using the FRR mode, with conditions detailed earlier. The FRR mode enabled focusing of the extracted photoelectron signal from a circular area of -2 mm diameter. This allowed spectra to be acquired at points close to the crack tip of the wedge specimen.

RESULTS

Hydration studies

The rate of aluminium oxide thickening in a 50 "C condensing humidity environment with a thin y-GPS or NTMP overlayer is shown in Fig. 2(a). This is compared with the rate of aluminium oxide thickening in a 50 "C dry oxygen environment. The aluminium oxide film thickness was estimated using the following equation'

(nlA13+] + [Al']) CAI0]

d = l l n

where [A13'] and [All are the relative concentrations of aluminium oxide and aluminium metal, respectively, measured by fitting the components of the A1 2p peak, I is the mean free path of 2.5 nm6 for 1200 eV electrons and n is the atomic density normalization for aluminium oxide.

The difference in the rate of oxide growth in dry oxygen relative to a humid environment indicates that the presence of moisture accelerates oxide growth substantially. The results also indicated that, relative to the untreated surface, the silane- and phosphonate-treated surfaces inhibit oxide growth in the humid environment. The NTMP inhibited growth more effectively. However, both molecules were slowly removed from the oxide surface with time, as indicated in Fig. 2(b), where the level of silicon and phosphorous decreased with exposure time.

Phosphonate adsorption studies

Figure 3 shows the adsorption of the three phosphonate molecules from the different solution concentrations. Above -10 ppm the P/A1 ratio reaches a plateau. A similar adsorption plot was observed by Venables4 and is believed to be an indication of the monolayer cover-


Figure 6. The surface compositions of both fracture faces of separated adherends following bond durability tests in 50°C condensing humidity. The surface treatments are: (a) FPL etch and FPL etch followed by phosphonate or y-GPS; (b) manual abrasion and manual abrasion followed by NTMP or y-GPS.

age for the phosphonate molecule on the aluminium surface. Some scatter was observed in the results; however, all three molecules showed similar ratios of P/Al for the range of concentrations tested.

Examination of silane solution and films by FTIR

Figure 4(a) shows the FTIR spectra taken over 90 min for a 3% y-GPS aqueous solution. The peaks of interest are those at 1197 cm-' and 1061 cm-' due to the SiOCH, bond and 1017 cm-' and 914 cm-l due to the SiOH bond.7 The peak at 1096 cm-' is due to the SiCH,R backbone of the organo-silane molecule. The results indicate that, with time, the methoxy groups are hydrolysed and silanol groups are formed. Hydrolysis has begun to take place within the first 15 min but some methoxy groups are still evident in the 90 min spectrum. There is no evidence of peaks due to the condensation

reaction of the silanol groups, resulting in production of SiOSi siloxane bonds.

Figure 4(b) shows the specular reflectance spectrum acquired for the film deposited on A1-2024 clad aluminium from a 1% y-GPS aqueous solution. The spectrum shows an intense peak at 1122 cm-' with minor peaks in the same region at 1059 and 1143 cm-l. These peaks are in positions which have been attributed to Si-0-Si vibrations*-' and provide an indication of cross-linking occurring in the surface film. The peak at 1059 cm-' was observed by Ishidag and was assigned to the silane hydrolysate dimer. They also tentatively assigned the peak at 964 cm-I to the Al-0-Si interfacial stretching mode.

Adhesive bond durability studies and failure analysis

Figure 5(a) shows the durability data for the FPL- etched surface and the FPL-etched surfaces treated with


the different phosphonate and silane solutions. The results indicate that the silane treatment produces the best durability, whereas the phosphonate treatments show similar durability to the untreated FPL etch surface, within the statistical variation of results. Figure 5(b) shows the durability data for the scotchbriteTM- abraded surface and the scotchbrite-abraded surface treated with NTMP and y-GPS. In contrast to the FPL pretreated surfaces, there is a more obvious difference between the NTMP and the untreated surface. The phosphonate reduces the durability, whereas the y-GPS- treated sample shows marked durability improvement relative to the untreated surface.

Analysis was performed on the complementary failure surfaces resulting from the durability specimens. For all samples failure was adhesive, i.e. at the epoxy adhesive and metal adherend interface. This produced two surfaces : the one which retained the epoxy adhesive, which will be referred to as the side with ‘adhesive’ appearance; and the one which exposed the metal substrate, which will be referred to as the side with ‘metallic’ appearance.

The failure analyses of the FPL-pretreated surfaces are shown in Fig. 6(a). The FPL etch specimen indicated the presence of aluminium and oxygen on the ‘metallic’ side, with the ‘adhesive’ side exhibiting a high carbon level. The FPL etch specimens which received a phosphonate treatment exhibited very similar failure surfaces. The sides with ‘metallic’ appearance all showed the presence of aluminium, oxygen, carbon and phosphorous, presumably from the phosphonate molecule. The NTMP and nBu-NBMP-treated surfaces showed high levels of carbon on the ‘adhesive’ side and small amounts of phosphorus were evident. By contrast, the 3HP-NBMP-treated specimen did not show evidence of phosphorus on the ‘adhesive’ side but showed high levels of carbon. The FPL etch surface treated with y-GPS indicated levels of aluminium, oxygen, carbon and silicon on the ‘metallic’ side. The ‘adhesive’ side showed a high concentration of carbon and small levels of aluminium and silicon. The scotchbrite-treated specimens displayed similar failure surfaces [Fig. 6(b)] to the FPL etch specimens. The only significant difference was the absence of phosphorus on the ‘adhesive’ side of the NTMP-treated specimen.

DISCUSSION

Hydration studies

Clearly the results shown in Fig. 2(a) indicate that both the phosphonate and silane compounds reduce the rate at which the aluminium oxide grows in a humid environment relative to an untreated surface. The growth is presumably a measure of the rate at which the thin native surface oxide hydrates. These results indicate that the untreated oxide reacts immediately with water vapour at 50 “C during the induction period prior to the formation of hydrated aluminium oxides, as discussed by Alwitt.’l The superior performance of the phosphonate inhibitor may indicate that the bond formed between the phosphonate and aluminium oxide is hydrolytically more stable than the corresponding bond

formed between the silane and oxide. Inelastic tunnel- ling spectroscopy (IETS) of phosphonic acid” sug- gested that the molecule bonded ionically with the hydroxyl groups on the aluminium oxide surface, IETS of y-GPS adsorbed on aluminium13 failed to find direct evidence for the formation of a covalent Si-0-A1 bond. Also, Henricksen12 could not find evidence for ionic or covalent bonding for triethoxysilane adsorbed on aluminium.

The difference in results may represent a difference in density of bonds on the surface for the two compounds. Figure 2(b) illustrates that the onset of hydration for the treated surfaces corresponds to the removal of the silane or phosphonate from the surface. The fact that both compounds are removed at similar rates but that the phosphonate-treated surface hydrates at a slower rate may indicate there are a greater number of phosphonate linkages per unit area.

Structure of the adsorbed phosphonate and silane layers

The adsorption studies of the phosphonate molecules shown in Fig. 3 indicate that above 10 ppm the phosphonate compounds reach saturation coverage. The nature of the phosphonate layer is probably that of individual molecules covering most of the aluminium surface. At the solution pH of 3.5 the phosphonate molecule will be doubly ionized.14 However, at this pH Davis14 indicates that the molecules will have the great- est potential for intermolecular hydrogen bonding and consequent multilayer formation. Hence, whilst the solution concentration of 300 ppm used for the durability studies in this work will ensure saturation coverage, there will also be potential for multilayer formation unless the surfaces are thoroughly rinsed after solution treatment.

The silane film shows a marked change from the soh- tion form. The solution studies shown in Fig. 4(a) indicate that, for the solution stirring times used in these experiments, the y-GPS does not form any siloxane linkages, although substantial hydrolysis of the methoxy groups occurs. This means that whilst the aluminium is immersed in the silane solution the silanol groups are present to interact with the hydroxyl groups on the aluminium oxide surface. This may account for the peak observed at 964 cm-’, which could be due to an Al-0-Si bond or the hydrogen bonding of silanol groups to the aluminium hydroxyl groups. However, the data shown in Fig. 4(b) also show that the film has a significant degree of cross-linking, as shown by several peaks due to siloxane bonds. In this respect, the silane film shows a significant difference from the phosphonate film. It is likely that several layers of silane are adsorbed on the aluminium surface,’ but unlike the phosphonate films the siloxane cross-linking reaction binds the film together. The Si-0-Si linkages produce a cohesively stronger film which is resistant to water attack.

Adhesive bond durability studies

The bond durability data in Figs 5(a) and 6(a) show that the phosphonate-treated adherends never improve the bond durability of the untreated FPL surfaces and reduce the durability of a manually abraded surface.


However, the y-GPS specimens all show a significant improvement relative to the untreated adherends.

The difference in performance is not linked to the hydration inhibiting properties of the two compounds. If this was the case, then the phosphonate-treated adherends would have performed at least as well as the silane-treated specimens. The poor performance of the phosphonate inhibitor in the bond durability studies must be related to differences in the structure of the films deposited from the silane and phosphonate solutions. Similarly, the coupling of the silane and phosphonate molecules to the epoxy adhesive may differ.

A simple bond degradation model' was previously proposed to explain the durability performance of epoxy adhesives bonded to aluminium adherends. Essentially, this model described the bond degradation in terms of water diffusing in front of the crack tip of an adhesively bonded stressed double cantilever beam specimen, (Fig. 7). The rate of water diffusion and its concentration in front of the crack tip depended on the density of adhesive to aluminium bonds in this region. This influences the number and size of micro-cavities formed between the adhesive and adherend in the stressed crack-tip region and ultimately determines the rate and mechanism of failure. In conjunction with the failure analysis results and the data describing the structure of the silane and phosphonate films, the bond degradation model will be used to interpret the durability data.

Figure 6(a) shows that, for all the specimens treated with phosphonate and silane, the failure was essentially in the layer between the adhesive and aluminium adherend. Similarly, the FPL etch treatment also led to failure at the adhesive/aluminium interface.

The failure analysis data [Fig. 6(a)] suggest that for the 3HP-NBMP surface, failure was between the phosphonate and adhesive layer. In this instant, the weak- ness at the phosphonate/adhesive boundary will lead to the formation of micro-cavities under stress, which in turn will permit water migration ahead of the crack tip. The water in this area will debond other linkages between the adhesive and phosphonate, and the crack will continue to propagate along this interface. Although the NTMP and nBu-NBMP showed failure at the adhesive/metal interface, the presence of phosphorous on both failure surfaces may indicate that multi- layers of phosphonate have been adsorbed. Here, some

failure has also resulted from water penetrating between the phosphonate layers, causing disbondment in this zone.

The untreated FPL etch failure surfaces [Fig. 6(a)] indicate that, under stress, the micro-cavities form between the adhesive and aluminium. Water migrating in front of the crack front will cause debonding of the adhesive from the metal oxide.

The failure analysis data for the silane-t reated surface [Fig. 6(a)] indicate that the failure zone was in the silane layer. The marked durability improvement for the silane-treated specimen must be due to the density of bonds formed between the adhesive and metal. The increased density of these bonds must be primarily due to superior coupling between the silane and adhesive compared with the phosphonate molecules. This results in the formation of smaller micro-cavities in front of the crack tip and reduced concentration of water. This causes a decrease in the rate at which water de-bonds the coupling agent linkages between adhesive or metal. In contrast to the phosphonate molecules, water is retarded from separating the silane molecules by the siloxane linkages which bind the film layer between the adhesive and metal.

The failure analysis data shown in Fig. 6(b) indicate that for the manually abraded surface failure has occurred between the oxide and adhesive. The manually abraded surface treated with silane performs as well as the silane-treated FPL etch surface. This indicates that the size and distribution of the micro-cavities ahead of the crack tip are similar for both surfaces and are created by the linkages between metal and adhesive produced by the coupling agent layer. However, the reduced durability of the manually abraded surface, relative to the FPL etch surface, indicates that the micro-cavities formed in front of the crack tip between the adhesive and metal must be larger dur: to a reduced number of bond linkages. The increased water concentration ahead of the tip will hasten the debonding of adhesive. The poorer performance of the NTMP-treated surface is related to the size of the micro-cavities in front of the crack tip. The higher water concentration increases the rate at which the phosphonate debonds from the adhesive. This result indicates that the major obstacle to improved bond durability for the phosphonate molecules is the poor bonding between the phosphonate layer and the adhesive.

Figure 7. Diagrammatic representation of the micro-cavities formed in the crack-tip region of a double cantilever adhesive bond specimen, indicating interfacial water migration.


CONCLUSIONS

The comparison of a silane coupling agent, y-GPS, with a series of hydration inhibiting agents, phosphonates, in the role of metal surface pretreatments for adhesive bonding revealed significant differences. The silane coupling agent forms a multilayer film which is strongly cross-linked through siloxane linkages. This property and the ability to couple effectively with an epoxy adhesive significantly improves bond durability in a humid environment. In contrast, whilst the phosphonates inhibited aluminium oxide hydration more effectively than the silane, poor coupling with the epoxy adhesive was the main barrier to improved bond durability. In some cases the phosphonate molecules may have been

adsorbed in multilayer films and provided a zone sus- ceptible to water degradation. Whilst this problem may be overcome with more thorough rinsing of the metal surface after treatment, it highlights a sensitive process parameter which is not relevant to silane films, making phosphonates a less attractive option for in-field appli- cation. Phosphonate performance could possibly be improved by modifying the ligand interacting with the adhesive. A group of similar structure to the epoxy linkage on the y-GPS molecule may be suitable.

Acknowledgement

The authors wish to thank Mr Greg Bain from Albright and Wilson for providing the phosphonate samples used in this work.

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Documents

Durability of Bonds Formed Between Epoxy Adhesive and Aluminium Alloy Treated with Phosphonate Inhibitors