Investigation of surface properties explaining the exceptional environmental durability of adhesively bonded grit-blasted aluminum

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Investigation of surface properties explaining the exceptional

environmental durability of adhesively bonded grit-blasted aluminum

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IN V E ST IG A T IO N O F SU R FA C E PR O PE R T IE S E X PL A IN IN G T H E E X C E PT IO N A L E N V IR O N M E N T A L D U R A B IL IT Y O F A D H E SIV E L Y

B O N D E D G R IT -B L A ST E D A L U M IN U M PA R T S

D . G allant

Structural A dhesives and C orrosion G roup, A lum inium Technology C entre, Industrial M aterials Institute, N ational R esearch C ouncil C anada (A TC /IM I/N R C ), 501 bou\. U niversité Est, Saguenay, Q uebec, C A N A D A , G 7H 8C 3

D anick.G allant@ cm c-m c.gc.ca

Introduction M echanicaIly deoxidized alum inum surfaces generaIly provide adhesively bonded joints w ith a low durability w hen subm itted to environm ental degradation conditions. A lthough results in agreem ent w ith this statem ent have often been reported in literature, it rem ains that the relative efficiency of different deoxidizing procedures is not yet clear, especiaIly w hen a single adhesive is considered for aIl these treatm ents.

O verview of results This com m unication w ill present the influence of the deoxidation m ethod applied to alum inum prior to its assem bly into SLS specim ens (see Fig. 1) using a single 2-com ponent epoxy adhesive (Loctite H ysol 9460). This adhesive chem istry is generaIly reported as being highly sensitive to the surface state. A cataplasm a test (Jaguar standard JN S 30.03.35) w as used to accelerate the degradation of adhesively bonded specim ens. A s presented in Fig. 2, phosphoric acid

anodizing, generaIly considered as the reference treatm ent for adhesive bonding, provides assem blies w ith the highest strength and an acceptable durability. Surprisingly, am ong deoxidation procedures em ployed, alum ina grit blasting provides adhesively bonded joints w ith an outstanding durability. Figs. 3 and 4 show that alum ina grit blasting treatm ent does not foIlow the general trends observed w ith other surface treatm ents prior and after degradation of assem blies, respectively. The expected causes explaining these observations w ill be discussed.

Figure C aptions:

Fig. 1: G eom etry of A I-A l SLS specim ens em ployed (dim ensions are in m m ). Fig. 2: D ependence of surface treatrnent on the SLS strength of adhesively bonded specim ens before and after cataplasm a degradation.

Fig. 3: R elationship betw een SLS stress (before cataplasm a degradation) and the polar com ponent of the surface free energy (yp) rneasured, as a function of the surface treatm ent.

Fig. 4: R elationship betw een % SLS loss strength (during cataplasm a degradation) andl, as a function of the surface treatm ent.

Fig.l

- ---lE _50

--

~

--35

_ B efore degradation

c:::::J A fier degradatlon _Fig.2

f11 11 ~ ~ ~ ~ 30 "i25 ~ s: 20 S, c ~ 15 Vi <Il ~ 10 34 35 100 " 32 ~ 30 E '"30 80 _-8'" 0.. 28 25 .<: ~ ~ _~ ë>

.~

so ₂₆ c: '" 20 ..., ~ ii ~ .é. 60 '" '" 24 c, '" '" 15 e-'" ..J 22 .2 1/) 1/) 40 20 10 ...J _• 1/) 18 ~ 20 16 III l' ~ ~ '" ~ if J~ o.c;:;~ . ..s-<8~ ~.~ .~ ~ ~~(;-O) s$rz; Ilf .s Q;J' ~'" ₀ § .. .§Q; "'('(JI §4' _t ~t? 0 8~ ~o ~ 8~ ...~ «s Ç1jit.,$ ~ '<i,," Fig.4 10 20 30 40 rP,Owens (rnJ/rn2)

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Aluminium

Technology

Centre

Investigation of Surface Properties Explaining the

Environmental Durability of Adhesively Bonded

Grit-Blasted Aluminum

D. Gallant, A. Ruest & V. Savard

Structural Adhesives and Corrosion, Aluminum Technology Centre, Industrial Materials

Institute, National Research Council Canada.

501, University Blvd, Saguenay, QC, Canada, G7H 8C3. danick.gallant@cnrc-nrc.gc.ca

1. Introduction

It is well known that surface pretreatment of substrates is a key step to ensure the hydrolytic

stability of adhesive joints. For many years now, highly efficient aluminum surface pretreatments have been used in numerous fields, ranging from basic industrial to advanced aerospace industry applications. However, for many small and medium enterprises (SMEs), anodizing or conversion coatings are not realistic solutions due to their cost and the technical/scientific resources they require.

During recent projects in collaboration with SMEs, simple surface pretreatment approaches were used to meet the requirements of some non-load-bearing and fatigue-free applications involving adhesive bonding. Surprisingly, among the surface pretreatments investigated, alumina

grit-blasting was found to be very efficient in providing durable adhesive joints. Without being an extensive study on the mechanisms that support durability issues, this poster proposes some explanations that might account for these findings.

2. Methodology

The single-lap-shear (SLS, see Figure 1) strength of adhesively bonded aluminum specimens was evaluated for eight (8) different surface pretreatments, from simple acetone degreasing to phosphoric acid anodizing (PAA), according to standard ASTM D3933. A two-component (2K) room-temperature curing epoxy adhesive (amine hardener) was used to evaluate the relative efficiency of surface pretreatments. This adhesive

chemistry is well known for its sensitivity to the quality of surface preparation [1]. The mechanical strength of SLS assemblies was evaluated using an electromechanical test system (Alliance RT/100 model from MTS), before and after cataplasma degradation (Jaguar standard JNS 30.03.35). The surface morphology of

aluminum samples was studied using profilometry (CHR450 from STIL). Chemical modifications brought to aluminum during surface pretreatments were investigated using X-ray photoelectron spectroscopy (Axis-Ultra XPS spectrometer from Kratos Analytical) and diffuse reflectance infrared spectroscopy (Nicolet 6700

spectrometer from ThermoFisher Scientific). The surface free energy parameters were calculated from water, formamide and diiodomethane contact angles, using the Owens, Wendt, Kaelble and Uy theory (DSA100

from Krüss). Finally, polypropylene and adhesive chemistry were investigated using attenuated total reflectance infrared spectroscopy (ATR-IR) (Nicolet 6700 spectrometer from ThermoFisher Scientific).

Figure 1. Typical Al-Al SLS specimen (dimensions are in mm).

3. Results & Discussion

3.1. Influence of Surface Pretreatment on the Durability of Adhesive Joints

Figure 2. Influence of surface pretreatment on the SLS

stress measured before and after cataplasma.

• Before environmental degradation of adhesive joints, cohesive failure of the adhesive was

observed for P2 etch and PAA pretreatments. Following cataplasma, mixed failure mode (i.e. cohesive + interfacial) was observed for PAA

and interfacial debonding was obtained from the P2 etch treatment.

• SLS specimens prepared from grit-blasted (50m white alumina) aluminum substrates were virtually unaffected by cataplasma

degradation. Visually, the failure mode seemed to be interfacial in either degraded or

non-degraded states. However, according to many studies, a greater fraction of the fracture path occurs in the Al oxide [2-5].

• SLS specimens assembled with aluminum coupons treated with Scotch-Brite 7447B were found to be the most affected by

cataplasma degradation.

3.2. Surface Morphology and Surface Free Energy of Scotch-Brite 7447B

Abraded and Grit-Blasted Samples: Comparison of the Least & Most

Durable Pretreatments

Figure 3. Influence of mechanical surface pretreatments on surface roughness.

Scotch Brite 7447B

R

_a

= 0.280.02 m

Grit-Blast 50m alumina

R

_a

= 1.90.1 m

(1.70.1 m in Ref. 6)

3.3. Durability of Grit-Blasted Aluminum Substrates: Literature Overview

According to literature:

1) Changes in surface area and path length for moisture diffusion are unlikely to be the only factors contributing to the bond

durability performance of adhesive joints made of grit-blasted aluminum coupons [4].

2) The influence of roughness on adhesion durability is not insignificant [3-5]. However, mechanical interlocking does not

seem to be responsible for the durability observed, especially in the opening mode [3].

3) Grit-blasting process introduces chemical changes on the surface of aluminum substrates [6].

4) Surface free energy of grit-blasted aluminum depends on the composition of alumina used as blasting media [6]. 5) For grit-blasted Al specimens, a greater fraction of the fracture path occurs in the Al oxide [2-5].

environmental exposure

(b) X-Ray Photoelectron Spectroscopy & Diffuse Reflectance Infrared Spectroscopy

(c) Evidences of H-Bonding Interactions between Grit-Blasted Aluminum Surfaces and

Epoxy Adhesive

According to literature, in the case of epoxy resins, the significant chemical interactions that affect bond

strength are believed to occur between the hydroxyl (OH) groups on the metal oxide surface and OH groups in the adhesive [8,9]. In the present study, in order to support this mechanism, plasma treated polypropylene (PP) material, mainly rich in OH groups (see Figure 6), was used to model the behaviour of OH groups from grit-blasted aluminum. Figure 7 shows that SLS stress measured for Al-PP assemblies is linearly related to the OH concentration measured on the surface of different 2K epoxy adhesives.

Grit-blasted surfaces significantly deviate from the tendency observed with other surface treatments: very high polar

component, but low strength loss.

Figure 4. Influence of the polar component

of surface free energy on the % of SLS strength loss following cataplasma

degradation of specimens.

XPS (top):

• The presence of Al

oxide/hydroxide/oxyhydroxide is more

significant on the surface of grit-blasted Al coupons.

Infrared spectroscopy (left):

• Absorption bands in the 1200-1000 cm-1

interval are attributable to OH-deformation in boehmite, -AlO(OH); [7]

• Absorption bands in the 3700-3000 cm-1

interval are attributable to OH-stretching in boehmite, -AlO(OH). [7]

References [1] R.G. Dillingham et al. (2008) J. Adhesion 84, 1007. [2] D.R. Arnott et al. (1993) Appl. Surf. Sci. 70-71, 109. [3] A.N. Rider et al. (1999) Surf. Interface Anal. 27, 1055. [4]

A.N. Rider et al. (2004) J. Adhesion Sci. Technol. 18, 1123. [5] A.N. Rider (2006) Int. J. Adhes. Adhes. 26, 67. [6] A.F. Harris et al. (1999) Int. J. Adhes. Adhes. 19, 445. [7] A. Corina Geiculescu et al. (2003) Thin Solid Films 426, 160. [8] R.G. Schmidt et al. (1986) Adv. Polym. Sci. 15, 33. [9] A.E. Yaniv et al. (1983) Surf. Interface Anal. 5, 93.

XPS spectra of grit-blasted Al XPS spectra of Scotch-Brite treated Al

Infrared spectra of grit-blasted Al

Figure 5. XPS and IR spectra of aluminum surfaces.

Figure 6. ATR-IR spectra of PP before and after

plasma treatment.

Figure 7. Relationship between Al-PP SLS

stress and OH band intensity for different 2K epoxy adhesives.

Substrate

Adhesive H-bonding

4. Conclusion

H-bonding between -AlO(OH) from the surface of grit-blasted Al and OH groups from the 2K epoxy adhesive is expected to contribute in providing durable joints. It is proposed that water (H-OH) is not efficient in the

displacement of existing bonds through the formation of new H-bonds.

O-H vibration