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Transforming Anaerobic Adhesives into Highly Durable and Abrasion Resistant

Superhydrophobic Organoclay Nanocomposite Films: A New Hybrid Spray Adhesive for

Tough Superhydrophobicity

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2009 Appl. Phys. Express 2 125003

(http://iopscience.iop.org/1882-0786/2/12/125003)

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Transforming Anaerobic Adhesives into Highly Durable

and Abrasion Resistant Superhydrophobic Organoclay Nanocomposite Films:

A New Hybrid Spray Adhesive for Tough Superhydrophobicity

Ilker S. Bayer1�, Andrea Brown2, Adam Steele1, and Eric Loth1;3

1Department of Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A.2Department of Civil and Environmental Engineering, University of California at Los Angeles, CA 90095, U.S.A.3Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904, U.S.A.

Received October 24, 2009; accepted November 16, 2009; published online December 4, 2009

The authors report fabrication of tough nanostructured self-cleaning superhydrophobic polymer-organoclay films from anaerobic acrylic

adhesives displaying strong adhesion to metal surfaces. Both industrial and bio-grade anaerobic adhesives such as bone cements could be

used. Montmorillonite clay filled anaerobic adhesives were modified by blending with a water dispersed fluoro-methacrylic latex in solution

to form abrasion resistant interpenetrating polymer network films upon spray casting. The adhesive films could cure by thermosetting in

oxygen-rich environments. Very high contact angles with low hysteresis were also measured for acidic (pH 2) and basic (pH 11) aqueous

buffer solutions indicating resistance to acidic and basic media. # 2009 The Japan Society of Applied Physics

DOI: 10.1143/APEX.2.125003

Adhesives cured by a redox initiated free radicalmechanism have been used in aerospace andautomotive industry for a long time for locking

and sealing.1) Acrylic anaerobic adhesives belong to thisgroup of adhesives which cure in the absence of oxygen andhence their name.2) Similarly, acrylic medical adhesivessuch as bone cements also cure via anaerobic mechanisms.3)

The two main components of both types of adhesives area liquid methacrylate monomer and a small amount ofhydroperoxide. Industrial anaerobic adhesives, such as 3MScotch-Weld� 3495, may contain aromatic polyester resinssuch as bisphenol-A-fumarate as anti-corrosion agents.

The catalytic decomposition of peroxide initiator mole-cules by metallic cations, arising from substrates, providesfree radicals to induce cross-linking of the adhesive.4,5)

Cumene hydroperoxide (CHP) has been widely used as thepolymerization initiator for many formulations.5) Presence ofample oxygen causes free or complexed metallic cations toremain in their highest oxidation state (i.e., Cu2þ instead ofCuþ) which slows down the redox polymerization reactionsconsiderably.6) Therefore, anaerobic adhesives have notbeen used as metal coatings until now.

In this work, we show that when organically modifiednanostructured montmorillonite is dispersed in anaerobicacrylic adhesives and subsequently blended with waterbornefluoro-methacrylic latex (Zonyl 8740) in alcohol solutions,remarkably durable and wear resistant superhydrophobiccoatings can be fabricated by spray casting. The coatingseasily thermoset on aluminum surfaces under oxygen-richconditions. No post-surface treatment is needed to renderthem superhydrophobic. A large number of commerciallyavailable high-strength anaerobic adhesives can be used forthe purpose including the ones containing liquid polyesterresins.

Dimethyl dialkyl C14–C18 amine functionalized (35–45wt%) montmorillonite clay particles (Nanocor Nanoclay)were initially dispersed in dimethyl sulfoxide (DMSO) at0.25 g/ml concentration. To this, 2.5ml of a custom-madeanaerobic bio-adhesive gel was added. The bio-adhesive for-mulation was very similar to anaerobic bone cement formu-lations mainly comprising a blend of poly(ethylene glycol)

dimethacrylate (PEGDMA) and a polyester functionalacrylic oligomer (Sartomer CN710). PEGDMA is an unsat-urated linear polyether with methacrylate double bonds thatcan be cross linked in situ. Cross-linked PEGDMA isbiocompatible with low cytotoxicity and has been used bothin vitro and in vivo as scaffolds for bone-tissue applica-tions.7,8) The composition of the thixotropic bio-adhesive isgiven as PEGDMA:CN710:CHP:polyamide-wax:propyleneglycol:fumed silica, 75 : 15 : 3 : 3 : 3 : 1 by weight percent.In a typical formulation, the organoclay/bio-adhesivedispersion in DMSO was further diluted with ethanol toa final nanoclay concentration of 0.1 g/ml and adhesiveconcentration of �5% by volume.

At first, this hybrid spray adhesive was coated on polishedaluminum surfaces using an internal mix air brush (Paashe).9)

Spray cast films cured by thermosetting in a conventionaloven at 80 �C within 3 h. Under oxygen deficient conditionsan anaerobic adhesive on a catalytically active metal surfacecures within 3 to 30min depending on the formulation.10) Asshown in Fig. 1(a), however, scanning electron microscopy(SEM) analysis of the surface showed formation of micro-cracks (�10 �m) throughout the film. The fractal-like micro-cracks are believed to originate from the fact that when anadhesive transforms from a viscous liquid state to an elasticsolid or a glassy material under confinement as curingprogresses, pattern formation (interfacial crack propagation)can appear due to weak interfacial adhesion between thesubstrate and the film. As a result, the adhesive becomesunable to resist certain stress levels during cross-linking.11)

In this case, since aluminum is catalytically inactive, thepolymerization of the hybrid adhesive proceeds mostly viathermosetting in the bulk rather than a redox reaction on thesubstrate surface responsible for bonding. It was found that aweak polymer bonding (adhesion) between the aluminumsubstrate and the composite film exists. This observation wasconfirmed by 180� peel tests which indicated that the averagecoating peel strength was �80N/m.

We found that when the diluted organoclay/bioadhesivedispersion was blended with the waterborne fluoro-methacrylic latex12,13) (30% wt 2-(perfluoroalkyl)ethylmethacrylate in water, Dupont Zonyl 8740), directly insolution (1 : 1 by weight), debonding induced crack forma-tion was completely eliminated and the hybrid films became�E-mail address: ibayer1@illinois.edu

Applied Physics Express 2 (2009) 125003

125003-1 # 2009 The Japan Society of Applied Physics

superhydrophobic upon curing within 15min. We argue thatthe elimination of cracks and the unique surface morphologyare due to the formation of an interpenetrating polymernetwork (IPN) as the adhesive monomers cross-link in situin the presence of 2-(perfluoroalkyl)ethyl methacrylatewithout forming covalent bonds between them.14,15) Super-hydrophobic coating surface morphology is shown inFig. 1(b). Figure 1(c) shows the details of the surface texturedue to the effective dispersion of nanostructured montmoril-lonite clay flakes within the IPN film resulting in Cassie–Baxter type self-cleaning superhydrophobicity.9,16) Hierarch-ical surface texture due to nanostructured montmorilloniteclay flakes decorated with ultrafine silica particles is clearlynoticeable. It is believed that the remarkably durablesuperhydrophobicity is due to the precise combination ofhydrophobic material chemistry and hierarchical surfacetopology which has been recently explained throughmechanistic and thermodynamic means as to how naturehas developed such mechanically durable superhydrophobicsurfaces.17)

By spraying two successive films on the self-cleaningsuperhydrophobic film shown in Fig. 1(b), thicker films(�120 �m) were fabricated. These films could be polishedgently with sandpaper and still preserved their self-cleaningsuperhydrophobicity even though several microns of surface

were removed by sanding. Sanding wear marks are clearlyvisible in the SEM image of Fig. 2. Details of the wearmarks are captured as indicated by the arrows. In addition,the leftover debris around the flattened micro-bumps due tosanding is also visible (marked by circles). Gentle surfacepolishing was performed by using a 3M 1000 grit aluminumoxide sand paper mounted on a rotating disk applying�0:06 kg/cm2 downward force.

To quantify the effect of sandpaper polishing on super-hydrophobicity of the composites, up to 40 water contactangle and contact angle hysteresis measurements wereconducted on various locations of a 10� 10 cm2 coatedaluminum foil before and after sanding. In addition,coatings made using an industrial-grade anaerobic adhesive(3M Scotch-Weld� 3495) were also tested for performancecomparisons. On average, static water contact angles aftersanding declined by 5� from 159� to 154� and hysteresisincreased from 4� to 10� still maintaining self-cleaningsuperhydrophobicity. Contact angle hysteresis measure-ments are shown in Figs. 3(a) and 3(b) for both industrialand bio-grade adhesive hybrids. Although increase incontact angle hysteresis indicates a possible wettingtransition from self-cleaning to sticky superhydrophobi-city,9,17) the degree of surface polishing applied here did notcause such a transition as indicated by the slight increase in

(a)

(b)

(c)

Fig. 1. (a) Environmental scanning electron microscope (ESEM)

image of fractal-like cracks forming within the adhesive-organoclay

nanocomposite film upon curing. (b) Hierarchal surface morphology of

the superhydrophobic hybrid adhesive films. (c) Higher magnification

detail of surface topography of superhydrophobic hybrids.

Fig. 2. ESEM image of surface morphology of fluoromethacrylic

latex/bio-adhesive organoclay nanocomposites after sanding with

1000 grit Al2O3 paper. The arrows indicate wear marks caused by

sanding the surface. The leftover debris on the surface after sanding is

also shown marked by the circles.

02468

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Fig. 3. Contact angle hysteresis measurements before, (a), and

after, (b), sanding the composite surfaces.

I. S. Bayer et al.Appl. Phys. Express 2 (2009) 125003

125003-2 # 2009 The Japan Society of Applied Physics

contact angle hysteresis. Droplet roll-off angles on both as-prepared and sanded coatings was less than 4�.

Degree of superhydrophobicity (self-cleaning or sticky)was found to be very sensitive to the percent adhesive inthe final spray solution. As Fig. 4(a) shows self-cleaningnanocomposite coatings with low hysteresis were possiblewhen adhesive concentration was below 5%. This was alsosufficient to render nanocomposites’ superhydrophobicityhighly durable. Between 5 and 7% adhesive concentrationsthe nanocomposites displayed sticky superhydrophobicitywith contact angle hysteresis reaching 60�. In Fig. 4(b), wedemonstrate the remarkable durability of the superhydropho-bic hybrid adhesive films by literally stomping on a coatedaluminum foil with shoes worn by a person weighing about94 kg. Amazingly, the superhydrophobicity of the coatingremains intact after several times of trampling. Super-hydrophobicity was also sensitive to organoclay concentra-tion; however, the starting organoclay concentration of0.25 g/ml in DMSO produced the most stable self-cleaningcoatings. Changes in superhydrophocity by modulating clayconcentration were found to be similar to the coatings studiedby Bayer et al.9) and not reported here for brevity.

Figure 5(a) demonstrates the superhydrophobicity of theadhesive films tested against aqueous pH buffer solutions(Fisher Scientific) ranging from acidic (pH 2) to basic(pH 11). All buffer solution static contact angles exceeded150� with contact angle hysteresis not beyond 5�. Long term

resistance against acidic and basic aqueous liquids wastested by observing pH 2 and 10 liquid blobs placed onthe adhesive coatings for an extended amount of time. Nochange in superhydrophobicity was measured for bothliquids within 3 days of observation. Figure 5(b) showsa still image of a liquid blob of pH 10 placed on asuperhydrophobic adhesive coating at the end of three daysstill at its non-wetting state.

In conclusion, we demonstrated a simple technique tofabricate remarkably durable superhydrophobic hybridanaerobic adhesive films. The films cured to a waterrepellent state inherently on catalytically inactive metalsurfaces such as aluminum. Sufficiently thick films with-stood gentle sanding and preserved their self-cleaningsuperhydrophobicity. Acidic and basic aqueous buffersolution contact angles exceeded 150� with very low contactangle hysteresis indicating resistance to acidic and basicmedia. The coatings can find various applications inbiomedical coatings and in industrial scale flexible anti-corrosion coatings.

Acknowledgments This project was funded by United States

National Science Foundation Center for Compact and Efficient Fluid Power

(www.ccefp.org) and by Grainger Foundation.

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6) B. George et al.: Int. J. Adhes. Adhes. 20 (2000) 245.

7) Z. Zhou et al.: J. Bioact. Compat. Polym. 24 (2009) 405.

8) S. Lin-Gibson et al.: Biomacromolecules 5 (2004) 1280.

9) I. S. Bayer et al.: Appl. Phys. Lett. 95 (2009) 063702.

10) E. Dragoni and P. Mauri: Int. J. Adhes. Adhes. 20 (2000) 315.

11) J. Nase et al.: Phys. Rev. Lett. 101 (2008) 074503.

12) A. Steele et al.: Nano Lett. 9 (2009) 501.

13) I. S. Bayer et al.: Appl. Phys. Lett. 94 (2009) 163902.

14) X. Huang et al.: J. Mater. Sci. 44 (2009) 4522.

15) I. S. Bayer et al.: Appl. Phys. Lett. 93 (2008) 173902.

16) J.-J. Lin et al.: Adv. Mater. 18 (2006) 3248.

17) W. Li and A. Amirfazli: Soft Matter 4 (2008) 462.

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Fig. 4. (a) Effect of bio-adhesive concentration on the degree of

water repellency quantified by static water contact angle and contact

angle hysteresis. (b) Sequence of images form a durability test. The

adhesive composite is being trampled by a person weighing 94 kg.

Original superhydrophobicity of the coating is still maintained. The

movie of the sequence is available at (http://www.youtube.com/

watch?v=HxVnFlKiFRw).

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Fig. 5. (a) Static contact angle and contact angle hysteresis

measurements on the superhydrophobic hybrid adhesive films using

aqueous buffer solutions with a pH range of 2 to 11. (b) Still image of a

pH 10 (highly basic) liquid blob placed on a hybrid adhesive coated

aluminum foil at its non-wetting state after 3 days of observation.

I. S. Bayer et al.Appl. Phys. Express 2 (2009) 125003

125003-3 # 2009 The Japan Society of Applied Physics