2 Introduction to Surface Preparation and AdhesionSina Ebnesajjad

2.1 Definition of SurfacePreparation, Adhesives, andAdhesive Bonding

Surface preparation or treatment is defined as oneor a series of operations including cleaning, removalof loose material, and physical and/or chemicalmodification of a surface to which an adhesive isapplied for the purpose of bonding. In plastics bonding,surface preparation is aimed at increasing the surfacepolarity, improving surface wettability, and creatingsites for adhesive bonding.Metal surfaces are typicallycoveredwith oxide layers, rendering the surface highlypolar and fit for good adhesive bonding. Surfacepreparation is intended to enhance the bondingstrength to metal surfaces. But the main reason forpreparing a metal surface is to improve the durabilityof the bond, especially when exposed to humidity.

An adhesive is a material that is applied to thesurfaces of articles to permanently join the articles bya bonding process. An adhesive is a substance capableof forming bonds to each of the two or more parts ofwhich the final object consists.1 A feature of adhesivesis the relatively small quantities (also thickness) thatare required relative to the weight of the final objects.

Adhesion is difficult to define and an entirelysatisfactory definition has not been found. Thefollowing definition has been proposed by Wu:2

Han

� 2

This

S. E

Adhesion refers to the state in which twodissimilar bodies are held together by intimateinterfacial contact such that mechanical forceor work can be transferred across theinterface. There is unifying theory of adhesionthat relates the physical-chemical propertiesof materials to the actual physical strength ofan adhesive bond.3 The interfacial forcesholding the two phases together may arisefrom van der Waals forces, chemical bonding,or electrostatic attraction. The mechanicalstrength of the system is determined not onlyby the interfacial forces, but also by the

dbook of Adhesives and Surface Preparation, ed. Sina Ebnesajjad. DOI: 10.1

011 Elsevier Inc. All rights reserved.

chapter has been adapted from ‘Introduction to Surface Preparation’ in Surface

bnesajjad & C.F. Ebnesajjad � 2006 Elsevier Inc.

mechanical properties of the interfacial zoneand the two bulk phases.

The primary function of adhesives is to join partstogether. Adhesives accomplish this goal by trans-mitting stresses from one member to another ina manner that distributes the stresses much moreuniformly than can be achieved with mechanicalfasteners. Adhesive bonding often provides structuresthat are mechanically equivalent to or stronger thanconventional assemblies, at lower cost and weight. Inmechanical fastening, the strength of the structure islimited to that of the areas in contact with thefasteners.4 It is not unusual to obtain adhesive bondsthat are stronger than the strength of the adherends(surfaces that are to be bonded together).

2.2 Introduction to SurfaceTreatment

There are a number of reasons for surface treat-ment of materials. The main reasons for applyingsurface treatments prior to bonding are as follows:

(1) To remove or prevent the later formation ofa weak layer on the surface of the substrate

(2) To maximize the degree of molecular interactionbetween the adhesive or primer and the substratesurface

(3) To optimize the adhesion forces that developacross the interfaces to ensure sufficient initialjoint strength and during the service life of thebond

(4) To create specific surface microstructure on thesubstrate

Normally, optimum surface energy and structure isachieved by chemical surface treatment. The chem-ical composition and the morphology of the surfaceare changed, so that the surface energy of thesubstrate is maximized for adhesion. Chemical

016/B978-1-4377-4461-3.10002-1

Treatment of Materials for Adhesion Bonding,

15

16 HANDBOOK OF ADHESIVES AND SURFACE PREPARATION

treatments also increase the chances that hydrogen,dipole, van der Waals ionic and/or covalent bondingcan form at the substrate/adhesive interface. A quickand dirty test for adequate chemical treatment is toplace a bead of water on the surface of the part andnote if the water spreads. If so, the contact angle itforms with the surface is very small. There arespecial fluids called dyne liquids that bracket thenumeric value of the surface energy of a materialsurface and fairly precisely.

Alternative methods to chemical treatment (withsimilar surface changes) include plasma, corona, andflame treatment methods. These act in ways similarto chemical treatments, with less hazardous wastegeneration. Mechanical abrasion is another means ofsurface preparation. This method of surface prepara-tion is superior to other methods because of thesimplicity of the process and significantly less wastegeneration. Mechanical abrasion works because itprovides a clean surface and increases the contact areabetween the substrate and adhesive. In ordinary gluingof objects together, these methods are not neededbecause optimal adhesion is not always necessary.When strong adhesion bond is required, the minimumsurface preparation required is cleaning and removingof dirt and grease from the contact surfaces.5

More recently, novel techniques have beendeveloped to treat the surfaces at the nano scale. Forexample, the interfacial adhesion energy between thelithography resist and the substrate is very importantin nanoimprinting because of problems with theresist sticking or pulling off during separation of themold from the substrate. Substrate surface treatmentswith a self-assembled monolayer or oxygen plasmaprovide good adhesion between a resist coating anda silica substrate.6

2.2.1 Degreasing

Removal of all traces of oily contamination andgrease from the substrate surfaces to be bonded isvital to the formation of strong adhesive bonds.Different methods, which should be carried out eventhough the contact surfaces may appear clean, areavailable for degreasing.

A part may be submerged in trichloroethylene orperchloroethylene vapors even though the vaporsare pure uncontaminated solvent. As the vaporscondense on the part, contaminants are dissolved anddrip off the part with the condensed solvent. Whena vapor degreasing unit is not available, the joint

surfaces can be wiped with a cloth soaked withtrichloroethylene, followed by complete evaporationfrom the joint surfaces. These solvents are toxic inboth liquid and vapor form, requiring the workenvironment to be well ventilated.

Another technique consists of scrubbing the jointsurfaces in a solution of a detergent. Metals can beimmersed in or sprayed with an alkaline degreasingagent, followed by rinsing with clean hot water andthoroughly drying by hot air, steam, or ambient air.

Ultrasonic degreasing is known to produce excel-lent results for small components. Trichloroethylene,acetone, methylene chloride, tetrachloroethylene,and so on are among good solvents for ultrasonicdegreasing. Alcohol, gasoline, and paint thinners arenot recommended. An effective solvent is 1,1,1-tri-chloroethane because of its low toxicity and flam-mability. To verify the cleanliness of a surface that hasbeen degreased, water or dyne liquids are used. Ifa drop of water forms a film of the surface, then it isfree from contamination. If a drop retains its shape,degreasing of the surface must be repeated.

2.2.2 Abrasion

Abraded rough surfaces usually form strongeradhesive joints than do highly polished surfaces,primarily due to larger contact surfaces. A properlyabraded surface should not contain any smooth orpolished areas. Abrasion treatment should be followedby a second degreasing treatment to ensure the removalof loose particles. Grit-blasting removes surfacedeposits such as tarnish, rust, or mill scale from metalsurfaces. If grit-blasting equipment is not available, orthe metal is too thin to withstand blast treatment, cleanthe joint surfaces with a wire-brush, emery cloth, orsandpaper. Painted surfaces should be stripped down tosubstratewith a stripper prior to preparation, otherwisethe adhesive joint will not be strong.

2.2.3 Chemical Treatment

Degreasing alone, or degreasing followed by abra-sion and further degreasing, is sufficient for manyadhesive bonds. To obtain maximum strength, repro-ducibility and resistance to deterioration a chemical orelectrolytic pretreatment is required. Careful attentionshould be paid in the preparation of chemical solutionsto assure correct proportioning of components requiredfor formation of adequate bond strengths. Exposuretime in the solution application is critical. If theapplication is too short, it does not sufficiently activate

2: INTRODUCTION TO SURFACE PREPARATION AND ADHESION 17

the surfaces. Overexposure to the solution builds upa layer of chemical reaction products thatmay interferewith the adhesion bond formation.

Some of the surface preparation treatments usechemicals that have serious health effects, unless usedwith extreme caution. As with all chemicals, consultthe Material Safety Data Sheet (MSDS) before using.Organic solvents must be handled with care becausetheir use may raise a fire and/or toxic hazard. Readthe material safety data sheet before handling. Acidsand sodium hydroxide are aggressive (corrosive)chemicals. Always wear protective clothing anda face shield when using. Read material safety datasheets of all materials before using. Never add waterdirectly to an acid; always add acid to water slowly.

2.3 Requirements of a GoodAdhesive Bond

The objective of any surface treatment method isto enhance the adhesive bond strength and durabilitywhen it is exposed to environmental factors ofservice. There are, however, other considerations forthe formation of an adequate adhesive bond. Thebasic requirements for a good adhesive bond are thefollowing7:

� Proper choice of adhesive

� Good joint design

� Cleanliness of surfaces

� Wetting of adherends (surfaces that are to bebonded together).

� Proper adhesive bonding process (solidificationand cure)

2.3.1 Proper Choice of Adhesive

There are numerous choices available for bondingmaterials. Selection of the adhesive type and formdepends on the nature of adherends, performancerequirements of the end use, and the adhesivebonding process.

2.3.2 Good Joint Design

Imparting strength to a joint by design is possible.8

A carefully designed joint can yield a stronger bondthan one not designed carefully, by combining theadvantages of the mechanical design with adhesivebond strength to meet the end use requirements of thebonded part.

2.3.3 Cleanliness

To obtain a good adhesive bond, starting witha clean adherend surface is essential. Foreign mate-rials such as dirt, oil, moisture, and weak-oxidelayers must be removed, or else the adhesive willbond to these weak-boundary layers rather than to thesubstrate. There are various surface treatments thatmay remove or strengthen the weak-boundary layers.A number of such treatments will be discussed inChapters 5e7. These treatments generally involvephysical or chemical processes or a combination ofboth.7,9

2.3.4 Wetting

Wetting is the displacement of air (or other gases)on the surface of adherend by a liquid phase. Theresult of good wetting is greater contact area betweenthe adherends and the adhesive over which the forcesof adhesion may act.7,10 Surface treatment tech-niques are applied to modify the adherend surface toincrease wettability.

References

1. Modified From ASTM D 907-05, StandardTerminology of Adhesives. Vol 15.06-Adhesives.Annual Book of ASTM Standards; 2005.

2. Wu S. Polymer Interface and Adhesion. 1st ed.New York, NY: Marcel Dekker; 1982.

3. Pocius AV. Adhesion and Adhesives Technology:An Introduction. Munich, Germany: HanserVerlag; 2002.

4. Staff Written, Joining Techniques, Section 4.Machine Design, Fastening and Joining Refer-ence Issue. 1976;48(26):155e162.

5. Brockman W, Geiss LP, Klingen J, Schroder KB,Mikhail B. Adhesive Bonding: Materials, Appli-cations and Technology. Weinheim, Germany:Wiley-VCH; 2009.

6. Yang EJ, Park YB, Lee HJ, et al. Effect ofsurface treatments on interfacial adhesion energybetween UV-curable resist and glass water. Int JAdhes Adhes. 2009;29:662e669.

7. Petrie EM. Plastics and adhesives as adhesives.In: Harper CA, ed. Handbook of Plastics andElastomers. New York, NY: McGraw-Hill; 1975.

8. Handbook of Plastics Joining, Plastics DesignLibrary. Norwich, NY: William AndrewPublishing; 1997.

18 HANDBOOK OF ADHESIVES AND SURFACE PREPARATION

9. Surface Preparation Techniques for AdhesiveBonding. Norwich, NY: William AndrewPublishing/Noyes; 1989.

10. Satas D, Tracton AA, eds. Coatings TechnologyHandbook. 2nd ed. New York, NY: MarcelDekker; 2001.