part i: introduction · 2013-12-20 · natural or vegetable-origin adhesives are subject to attack...

Part I: Introduction

1 Introduction and Adhesion Theories

1.1 Definition of Adhesivesand Adhesive Bonding

An adhesive is a material that is applied to thesurfaces of articles to join them permanently by anadhesive bonding process. An adhesive is a substancecapable of forming bonds to each of the two partswhen the final object consists of two sections that arebonded together.1 A feature of adhesives is the rela-tively small quantities that are required compared tothe 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

“Adhesion refers to the state in which twodissimilar bodies are held together by intimateinterfacial contact such that mechanical force orwork can be transferred across the interface. Theinterfacial forces holding the two phases togethermay arise from van der Waals forces, chemicalbonding, or electrostatic attraction. Mechanicalstrength of the system is determined not only bythe interfacial forces, but also by the mechanicalproperties of the interfacial zone and the two bulkphases.”

There are two principal types of adhesive bonding:structural and nonstructural. Structural adhesivebonding is bonding for applications in which theadherends (the objects being bonded) may experi-ence large stresses up to their yield point. Structuraladhesive bonds must be capable of transmitting stresswithout loss of integrity within design limits.3 Bondsmust also be durable throughout the useful servicelife of a part, which may be years. A structural bondhas been defined as having a shear strength greaterthan 7 MPa in addition to significant resistance toaging. Nonstructural adhesives are not required tosupport substantial loads but merely hold lightweightmaterials in place. This type of adhesive is some-times called a “holding adhesive.” Pressure-sensitivetapes and packaging adhesives are examples ofnonstructural adhesives.

The distinction between structural and nonstruc-tural bonds is not always clear. For example, is a hot

melt adhesive used in retaining a fabric’s pliesstructural or nonstructural? One could argue thatsuch an adhesive may be placed in either classifica-tion. However, the superglues (cyanoacrylates) areclassified as structural adhesives even though theyhave poor resistance to moisture and heat.

1.2 Functions of Adhesives

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 struc-tures that are mechanically equivalent to or strongerthan conventional assemblies at lower cost andweight. In mechanical fastening, the strength of thestructure is limited to that of the areas of the membersin contact with the fasteners.4 It is not unusual toobtain adhesive bonds that are stronger than those ofthe strength of adherends.

Smooth surfaces are an inherent advantage ofadhesively joined structures and products. Exposedsurfaces are not defaced and contours are notdisturbed, as happens with mechanical fasteningsystems. This feature is important in function andappearance. Aerospace structures, including heli-copter rotor blades, require smooth exteriors tominimize drag and to keep temperatures as low aspossible. Lighter weight materials can often be usedwith adhesive bonding than with conventionalfastening because the uniform stress distribution inthe joint permits full utilization of the strength andrigidity of the adherends.4 Adhesive bondingprovides much larger areas for stress transferthroughout the part, thus decreasing stress concen-tration in small areas.

Dissimilar materials, including plastics, arereadily joined by many adhesives, provided thatproper surface treatments are used. Adhesives can beused to join metals, plastics, ceramics, cork, rubber,and combinations of materials. Adhesives can also be

Applied Handbook of Adhesives Technology and Surface Preparation

Copyright � 2011 Elsevier Inc. All rights reserved. 3

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formulated to be conductive. The focus of this bookis on adhesives for bonding plastics, thermosets,elastomers, and metals.

Where temperature variations are encountered inthe service of an item containing dissimilar materials,adhesives perform another useful function. Flexibleadhesives are able to accommodate differences in thethermal expansion coefficients of the adherends andtherefore prevent damage that might occur if stifffastening systems were used.

Sealing is another important function of adhesivejoining. The continuous bond seals out liquids orgases that do not attack the adhesive (or sealant).Adhesives/sealants are often used in place of solidor cellular gaskets. Mechanical damping can beimparted to a structure through the use of adhesivesformulated for that purpose. A related characteristic,fatigue resistance, can be improved by the abilityof such adhesives to withstand cyclic strains andshock loads without cracking. In a properlydesigned joint, the adherends generally fail in fatiguebefore the adhesive fails. Thin or fragile parts canalso be adhesive bonded. Adhesive joints do notusually impose heavy loads on the adherends, as inriveting, or localized heating, as in welding. Theadherends are also relatively free from heat-induceddistortion.4

1.3 Classification of Adhesives

Adhesives as materials can be classified ina number of ways such as chemical structure orfunctionality. In this book, adhesives have beenclassified into two main classes: natural andsynthetic. The natural group includes animal glue,casein-and protein-based adhesives, and naturalrubber adhesives. The synthetic group has beenfurther divided into two main groups: industrial andspecial compounds. Industrial compounds includeacrylics, epoxies, silicones, etc. An example of thespecialty group is pressure-sensitive adhesives.

1.4 Advantages andDisadvantages of JoiningUsing Adhesives

The previous discussion highlighted a numberof advantages of adhesive bonding. This sectionwill cover both advantages and disadvantages,

recognizing that some of the points have already beenmentioned.

1.4.1 Advantages

� Uniform distribution of stress and larger stress-bearing area 5,6

� Join thin or thick materials of any shape

� Join similar or dissimilar materials

� Minimize or prevent electrochemical (galvanic)corrosion between dissimilar materials

� Resist fatigue and cyclic loads

� Provide joints with smooth contours

� Seal joints against a variety of environments

� Insulate against heat transfer and electricalconductance (in some cases adhesives aredesigned to provide such conductance)

� The heat required to set the joint is usually toolow to reduce the strength of the metal parts

� Dampen vibration and absorb shock

� Provide an attractive strength/weight ratio

� Quicker and/or cheaper to form than mechanicalfastening

1.4.2 Disadvantages

� The bond does not permit visual examinationof the bond area (unless the adherends are trans-parent) 5,6,7

� Careful surface preparation is required toobtain durable bonds, often with corrosivechemicals

� Long cure times may be needed, particularlywhere high cure temperatures are not used

� Holding fixtures, presses, ovens, and autoclaves,not usually required for other fastening methods,are necessities for adhesive bonding

� Upper service temperatures are limited toapproximately 177�C in most cases, but specialadhesives, usually more expensive, are availablefor limited use up to 371�C

� Rigid process control, including emphasis oncleanliness, is required for most adhesives

� The useful life of the adhesive joint depends onthe environment to which it is exposed

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� Natural or vegetable-origin adhesives aresubject to attack by bacteria, mold, rodents, orvermin

� Exposure to solvents used in cleaning or solventcementing may present health problems

1.5 Requirements of a Good Bond

The basic requirements for a good adhesive bondare [6]:

� Proper choice of adhesive

� Good joint design

� Cleanliness of surfaces

� Wetting of surfaces that are to be bondedtogether

� Proper adhesive bonding process (solidificationand cure)

1.5.1 Proper Choice of Adhesive

There are numerous adhesives available forbonding materials. Selection of the adhesive type andform depends on the nature of adherends, perfor-mance requirements of the end use, and the adhesivebonding process.

1.5.2 Good Joint Design

It is possible to impart strength to a joint bydesign.8 A carefully designed joint can yielda stronger bond by combining the advantages of themechanical design with adhesive bond strength tomeet the end use requirements of the bonded part.

1.5.3 Cleanliness

To obtain a good adhesive bond, it is important tostart with a clean adherend surface. Foreign mate-rials, such as dirt, oil, moisture, and weak oxidelayers, must be removed, else the adhesive will bondto these weak boundary layers rather than to thesubstrate. There are various surface treatments thatmay remove or strengthen the weak boundary layers.These treatments generally involve physical orchemical processes, or a combination of both.9

1.5.4 Wetting

Wetting is the displacement of air (or other gases)present on the surface of adherends by a liquid phase.The result of good wetting is greater contact area

between the adherends and the adhesive over whichthe forces of adhesion may act.10

1.5.5 Adhesive Bonding Process

Successful bonding of parts requires an appro-priate process. The adhesive must not only be appliedto the surfaces of the adherends but the bond shouldalso be subjected to the proper temperature, pressure,and hold time. The liquid or film adhesive, onceapplied, must be capable of being converted intoa solid in any one of three ways. The method bywhich solidification occurs depends on the choice ofadhesive. The ways in which liquid adhesives areconverted to solids are:6

� chemical reaction by any combination of heat,pressure, and curing agents;

� cooling from a molten liquid;

� drying as a result of solvent evaporation.

The requirements to form a good adhesive bond,processes for bonding, analytic techniques, andquality control procedures have been discussed inthis book.

1.6 Introduction to Theoriesof Adhesion

Historically, mechanical interlocking, electro-static, diffusion, and adsorption/surface reactiontheories have been postulated to describe mecha-nisms of adhesion. More recently, other theories havebeen put forward for adhesive bonding mechanism(Table 1.1). It is often difficult to fully ascribeadhesive bonding to an individual mechanism. Acombination of different mechanisms is most prob-ably responsible for bonding within a given adhesivesystem. The extent of the role of each mechanismcould vary for different adhesive bonding systems.An understanding of these theories will be helpful tothose who plan to work with adhesives.

An important facet of adhesion bonds is the locusof the proposed action or the scale to which theadhesive and adherend interact. Table 1.1 showsa scale of action for each mechanism, which isintended to aid in the understanding of these mech-anisms. Of course, adhesiveeadherend interactionsalways take place at the molecular level, which isdiscussed later in this chapter.

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The microscopic parameter of interest inmechanical interlocking is the contact surface of theadhesive and the adherend. The specific surface area(i.e., surface area per unit weight) of the adherend isan example of one such measure. Surface roughnessis the means by which interlocking is thought to workand can be detected by optical or electron micros-copy. In the electrostatic mechanism, the surfacecharge is the macroscopic factor of interest. Thecharge in question is similar to that produced ina glass rod after rubbing it with a wool cloth. Diffu-sion and wettability involve molecular and atomicscale interactions, respectively.

Readers who wish to gain an in-depth under-standing of the interaction forces, adhesionmechanism, and thermodynamics of adhesion arerecommended to consult Fundamentals of Adhesion,edited by Lieng-Huang Lee.11 This reference providesa qualitative and quantitative treatment of adhesion,complete with derivation of force interactionequations.

1.6.1 Mechanical Theory

According to this theory, adhesion occurs by thepenetration of adhesives into pores, cavities, andother surface irregularities on the surface of thesubstrate. The adhesive displaces the trapped air atthe interface. Therefore, it is concluded that anadhesive penetrating into the surface roughness oftwo adherends can bond them. A positive contribu-tion to the adhesive bond strength results from the“mechanical interlocking” of the adhesive and theadherends. Adhesives frequently form stronger bondsto porous abraded surfaces than they do to smoothsurfaces. However, this theory is not universallyapplicable, since good adhesion also takes placebetween smooth surfaces.

Enhanced adhesion after abrading the surface ofan adherend may be due to (1) mechanical inter-locking, (2) formation of a clean surface, (3)formation of a highly reactive surface, and (4) anincrease in contact surface area. It is believed thatchanges in physical and chemical properties of theadherend surface produce an increase in adhesivestrength.13 It can be debated whether mechanicalinterlocking is responsible for strong bonds or anincrease in the adhesive contact surface enhancesother mechanisms. More thorough wetting and moreextensive chemical bonding are expected conse-quences of increased contact surface area.

There is supportive data in the literature that relatejoint strength and bond durability to increasedsurface roughness. There are also contrary observa-tions indicating that increased roughness can lowerjoint strength.14

1.6.2 Electrostatic (Electronic)Theory

This theory proposes that adhesion takes place dueto electrostatic effects between the adhesive and theadherend.15,16,17,18 An electron transfer is supposed totake place between the adhesive and the adherend asa result of unlike electronic band structures. Electro-static forces in the formof an electrical double layer arethus formed at the adhesiveeadherend interface. Theseforces account for the resistance to separation. Thistheory gains support from the fact that electricaldischarges have been noticed when an adhesive ispeeled from a substrate.13

The electrostatic mechanism is a plausible expla-nation for polymeremetal adhesion bonds. Thecontribution of the electronic mechanism in nonme-tallic systems to adhesion has been calculated and

Table 1.1 Theories of Adhesion

Traditional Recent Scale of Action

Mechanical interlocking Mechanical interlocking Microscopic

Electrostatic Electrostatic Macroscopic

Diffusion Diffusion Molecular

Adsorption/surface reaction Wettability Molecular

Chemical bonding Atomic

Weak boundary layer Molecular


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found to be small when compared with that ofchemical bonding. 19,20

1.6.3 Diffusion Theory

This theory suggests that adhesion is developedthrough the interdiffusion of molecules in betweenthe adhesive and the adherend. The diffusion theoryis primarily applicable when both the adhesive andthe adherend are polymers with relatively long-chainmolecules capable of movement. The nature ofmaterials and bonding conditions will influencewhether and to what extent diffusion takes place. Thediffuse interfacial (interphase) layer typically hasa thickness in the range of 10e1,000 A (1e100 nm).Solvent cementing or heat welding of thermoplasticsis considered to be due to diffusion of molecules.13

No stress concentration is present at the interfacebecause no discontinuity exists in the physicalproperties. Cohesive energy density (CED, Eq. (1.1))can be used to interpret diffusion bonding, as definedby Eq. (1.2). Bond strength is maximized whensolubility parameters are matched between theadhesive and the adherend.

CED ¼ Ecoh

V(1.1)

d ¼ffiffiffiffiffiffiffiffiffi

Ecoh

V

r

(1.2)

Ecoh is the amount of energy required to separatethe molecules to an infinite distance, V is the molarvolume, and d is the solubility parameter.

A relevant example is the adhesion of polyethyleneand polypropylene to a butyl rubber. The adhesivebond is weak when two polymers are bonded attemperatures below themelting point of the polyolefin.Bond strength increases sharply when the adhesionprocess takes place above the melting temperature ofpolyethylene (135 �C) and polypropylene (175 �C).Figure 1.1 illustrates the bond strength (peel strength)as a function of bonding temperature. An inference canbe made that at elevated temperatures, the interdiffu-sion of polyolefins and butyl rubber increases, thusleading to higher bond strength.

1.6.4 Wetting Theory

This theory proposes that adhesion results frommolecular contact between two materials and thesurface forces that develop. The first step in bond

formation is to develop interfacial forces between theadhesive and the substrates. The process of estab-lishing continuous contact between the adhesive andthe adherend is called wetting. For an adhesive to weta solid surface, the adhesive should have a lowersurface tension than the critical surface tension of thesolid. This is precisely the reason for surface treat-ment of plastics, which increases their surface energyand polarity.

Van der Waals forces are extremely sensitive to thedistance (r) between molecules, decreasing by theinverse of the seventh power (1/r7) of the distancebetween two molecules and the cubic power of thedistance between two adherends. These forces arenormally too small to account for the adhesive bondstrength in most cases.

Figure 1.2 illustrates complete and incompletewetting of an adhesive spreading over a surface.Good wetting results when the adhesive flows intothe valleys and crevices on the substrate surface. Poorwetting results when the adhesive bridges over thevalley and results in a reduction of the actual contactarea between the adhesive and the adherend, result-ing in a lower overall joint strength.13 Incompletewetting generates interfacial defects, therebyreducing the adhesive bond strength. Completewetting achieves the highest bond strength.

Most organic adhesives readily wet metal adher-ends. On the other hand, many solid organicsubstrates have surface tensions lower than those ofcommon adhesives. The criteria for good wettingrequires the adhesives to have a lower surface tension

50 100 150 200Bonding temperature, ºC

Peel

stre

ngth

, lb/

in

321

5

10

15

Figure 1.1 Peel strength of polypropylene and butylrubber vs. bonding temperature: (1) adhesive failure;(2) adhesive/cohesive failure; (3) cohesive failure.2


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than the substrate, which explains, in part, whyorganic adhesives such as epoxies have excellentadhesion to metals but offer weak adhesion onuntreated polymeric substrates such as polyethylene,polypropylene, and fluoroplastics.13 The surfaceenergy of plastic substrates can be increased byvarious treatment techniques to allow wetting.

1.6.5 Chemical Bonding

This mechanism attributes the formation of anadhesion bond to surface chemical forces. Hydrogen,covalent, and ionic bonds formed between theadhesive and the adherends are stronger than thedispersion attractive forces. Table 1.2 lists examplesof these forces and their magnitudes. In general, thereare four types of interactions that take place duringchemical bonding: covalent bonds, hydrogen bonds,Lifshitzevan der Waals forces, and acidebaseinteractions. The exact nature of the interactions fora given adhesive bond depends on the chemicalcomposition of the interface.

Covalent and ionic bonds (Table 1.2) are examplesof chemical bonding that provide much higheradhesion values than that provided by secondaryforces. Secondary valence bonding is based on theweaker physical forces exemplified by hydrogenbonds. These forces are more prevalent in materialsthat contain polar groups such as carboxylic acidgroups than in nonpolar materials such as poly-olefins. The interactions that hold the adhesive andthe adherends together may also receive contribu-tions from mechanical interlocking, diffusion, orelectrostatic mechanisms.

The definitions of intermolecular interactions arelisted below:

Dipole (polar molecule): A molecule whosecharge distribution can be represented by a centerof positive charge and a center of negative charge,which do not coincide.

Dipole e dipole forces: Intermolecular forcesresulting from the tendency of polar moleculesto align themselves such that the positive end ofone molecule is near the negative end of another.

Hydrogen bonding: A special type of dipoleedipoleinteraction that occurswhen a hydrogen atom that isbonded to a small, highly electronegative atom(most commonly F, O, N, or S) is attracted to thelone electron pairs of another molecule.

London dispersion forces (dispersion forces):Intermolecular forces resulting from the small,instantaneous dipoles (induced dipoles) that occurbecause of the varying positions of the electronsduring their motion about the nuclei.

Polarizability is defined as the ease with whichthe electron cloud of an atom or molecule is dis-torted. In general, polarizability increases withthe size of an atom and the number of electrons

Figure 1.2 Examples of (a) good and (b)poor wetting by an adhesive spreadingacross a surface.13

Table 1.2 Examples of Energies of Lifshitzevan derWaals Interactions and Chemical Bonds

Type Example E (kJ/mol)

Covalent CeC 350

IoneIon Naþ . Cl� 450

Ionedipole Naþ .CF3H

33

Dipoleedipole CF3H .CF3H

2

Londondispersion

CF4 . CF4 2

Hydrogenbonding

H2O . H2O 24


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on an atom. The importance of London dispersionforces increases with the atom size and number ofelectrons.

Covalent chemical bonds can form across theinterface and are likely to occur in cross-linkedadhesives and thermoset coatings. This type of bondis usually the strongest and most durable. However,they require that mutually reactive chemical groupsshould exist. Some surfaces, such as previouslycoated surfaces, wood, composites, and some plas-tics, contain various functional groups that underappropriate conditions can produce chemical bondswith the adhesive material. There are ways to inten-tionally generate these conditions, such as by surfacetreatment of plastics with techniques like corona orflame treatment.

Organosilanes are widely used as primers on glassfibers to promote the adhesion between the resin andthe glass in fiberglass-reinforced plastics. They arealso used as primers or integral blends to promoteadhesion of resins to minerals, metals, and plastics.Essentially, during application, silanol groups areproduced, which then react with the silanol groups onthe glass surface or possibly with other metal oxidegroups to form strong ether linkages. Coatings con-taining reactive functional groups such as hydroxylor carboxyl moieties tend to adhere more tenaciouslyto substrates containing similar groups. Chemicalbonding may also occur when a substrate containsreactive hydroxyl groups, which may react with theisocyanate groups from the incoming coating inthermoset polyurethane coatings.

Most likely, chemical bonding accounts for thestrong adhesion between an epoxy coating anda substrate with a cellulose interface. The epoxygroups of an epoxy resin react with the hydroxylgroups of cellulose at the interface.

1.6.5.1 AcideBase Theory

A special type of interaction, the acidebaseinteraction, is a fairly recent discovery. It is basedon the chemical concept of a Lewis acid and base,which is briefly described. The acid/base definitionwas proposed separately by J. N. Bronsted andG. N. Lewis. Restatement of these definitions byLewis in 1938 led to their popularity and acceptance.The Lewis definitions are “an acid is a substancewhich can accept an electron pair from a base; a baseis a substance which can donate an electron pair

[21].” By this definition, every cation is an acid inaddition to chemical compounds such as BF3 andSiO2. Conversely, anions and compounds like NH3,PH3, and C6H5CH2NH2 are bases. According to theacidebase theory, adhesion results from the polarattraction of Lewis acids and bases (i.e., electron-poor and electron-rich elements) at the interface.This theory is attributed to the work by Fowkeset al.,22,23,24,25 Gutmann,26 and Bolger and Michaels[27].

In BF3, the higher electronegativity of fluorineatoms preferentially displaces the shared electronsaway from the boron atom. Thus, a bipolar moleculeis created that has positive charge on the boron sideand negative charge on the fluorine side. On the otherhand, NH3, by a similar analogy, has a negativenitrogen end that renders it a Lewis base. Naturally,the positive boron end of BH3 and negative nitrogenend of NH3 interact.

A special case of acidebase interaction ishydrogen bonding such as among water moleculesthat exhibit both acidic and basic tendencies. Table1.2 shows that the hydrogen bond strength, whilesubstantially less than ionic and covalent bondenergies, is one of the most significant among thesecondary interactions. The reader can refer to inor-ganic chemistry texts28,29 to learn about Lewis acidsand bases and their chemical reactions. In summary,the interactions between compounds capable ofelectron donation and acceptance form the founda-tion of the acidebase theory of adhesion.

1.6.6 Weak Boundary LayerTheory

This theory was first described by Bikerman. Itstates that bond failure at the interface is caused byeither a cohesive break or a weak boundary layer.30

Weak boundary layers can originate from the adhe-sive, the adherend, the environment, or a combinationof any of these three factors.

Weak boundary layers can occur in the adhesive oradherend if an impurity concentrates near thebonding surface and forms a weak attachment to thesubstrate. When failure takes place, it is the weakboundary layer that fails, although failure appears totake place at the adhesiveeadherend interface.

Polyethylene and metal oxides are examples oftwo materials that may inherently contain weakboundary layers. Polyethylene has a weak, low-molecular weight constituent that is evenly


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distributed throughout the polymer. This weakboundary layer is present at the interface andcontributes to low failing stress when polyethylene isused as an adhesive or an adherend. Some metaloxides are weakly attached to their base metals.Failure of adhesive joints made with these materialsoccurs cohesively within the oxide. Certain oxides,such as aluminum oxide, are very strong and do notsignificantly impair joint strength. Weak boundarylayers, such as those found in polyethylene and metaloxides, can be removed or strengthened by varioussurface treatments. Weak boundary layers formedfrom the bonding environment, generally air, are verycommon. When the adhesive does not wet thesubstrate, as shown in Figure 1.2, a weak boundarylayer (air) is trapped at the interface, causing a reduc-tion in joint strength.13,31

1.7 Definition of Failure Modes

A hypothetical adhesion bond is shown inFigure 1.3. Assume that the bond is tested in thetensile mode in which the two adherends are pulledapart in a direction perpendicular to the bond. Thereare different possibilities for the occurrence offailure. The surfaces involved in bond failure arecalled the locus of failure.

If the bond failure occurs between the adhesivelayer and one of the adherends, it is called adhesivefailure (Figure 1.3a). A failure in which the separa-tion occurs in such a manner that both adherendsurfaces remain covered with the adhesive is calledcohesive failure in the adhesive layer (Figure 1.3b).Sometimes the adhesive bond is so strong that thefailure occurs in one of the adherends away from thebond. This is called a cohesive failure in the adherend(Figure 1.3c). Bond failures often involve more thanone failure mode and are ascribed as a percentage tocohesive or adhesive failure. This percentage iscalculated based on the fraction of the area of the

contact surface that has failed cohesively oradhesively.

It is important to determine the exact mode(s) ofbond failure when a problem occurs. Determinationof the failure mode allows action to be taken tocorrect the true cause and save time and money.

Tables 1.3e1.5 show the result of analyses ofseveral bonds between a substrate and a polyvinylfluoride film using an acrylic adhesive. All surfaceswere analyzed by electron spectroscopy for chemicalanalysis (ESCA). ESCA yields chemical analysis oforganic surfaces in atomic percentage, with theexclusion of hydrogen, which is undetectable by thistechnique. To determine the type of bond failure,ESCA results for the failed surfaces are comparedwiththose of the adhesive and the polyvinyl fluoride film.

In a pure cohesive failure, the two surfacesinvolved should have virtually identical chemicalcompositions, which is nearly the case in Tables 1.3and 1.4. In a 100% adhesive failure, one of thesurfaces should have the same chemical compositionas the adherend and the other the same as the adhe-sive. The examples presented in Tables 1.3 and 1.4illustrate cohesive failure cases for polyvinyl fluoride(adherend) and the adhesive. Table 1.5 gives anexample of an adhesive failure. One can see from thechemical composition that the adhesive and poly-vinyl fluoride surfaces have been separated ina “clean” manner.

1.8 Mechanisms of Bond Failure

Adhesive joints may fail adhesively or cohesively.Adhesive failure is an interfacial bond failurebetween the adhesive and the adherend. Cohesivefailure occurs when a fracture allows a layer ofadhesive to remain on both surfaces. When theadherend fails before the adhesive, it is known asa cohesive failure of the substrate. Various modes offailure are shown in Figure 1.3. Cohesive failure

AdherendAdhesive

(a) (b) (c)

Figure 1.3 Schematics of adhesive bondfailure modes: (a) adhesive failure; (b)cohesive failure in the adhesive layer;(c) cohesive failure in the adherend.


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Table 1.3 Surface Chemical Analysis (ESCA) in a Cohesive Failure of Adhesive Bond

Atomic Concentration (%)

F O N C Si

As-is adhesive (control) nd 26.0 2.1 71.6 nd

As-is film (control) 29.3 6.6 nd 64.4 nd

Polyvinyl fluoride film nd 24.9 2.5 72.6 nd

facing the substrate

Substrate facing the nd 25.0 2.1 72.9 nd

polyvinyl fluoride film

nd, not detectable.

Data were provided by Dr. James J. Schmidt at the DuPont Company, 2003.

Table 1.4 Surface Chemical Analysis (ESCA) in a Cohesive Failure of PolyvinylFluoride


F O N C Si



Polyvinyl fluoride film 31.0 4.0 nd 63.2 1.7


Substrate facing the 30.0 5.4 nd 62.6 2.0


nd, not detectable.


Table 1.5 Surface Chemical Analysis (ESCA) in a Adhesive Failure of PolyvinylFluoride


F O N C Si



Polyvinyl fluoride film 31.6 2.1 nd 66.4 nd


Substrate facing the nd 26.4 3.2 70.5 nd


nd, not detectable.



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within the adhesive or one of the adherends is theideal type of failure because with this type of failurethe maximum strength of the materials in the jointhas been reached. In analyzing an adhesive joint thathas been tested to destruction, the mode of failure isoften expressed as a percentage cohesive or adhesivefailure, as shown in Figure 1.3. The ideal failure isa 100% cohesive failure in the adhesion layer.

The failure mode should not be used as the onlycriterion for a useful joint.3 Some adhesiveeadherend combinations may fail adhesively, butexhibit greater strength than a similar joint bondedwith a weaker adhesive that fails cohesively. Theultimate strength of a joint is a more importantcriterion than the mode of joint failure. An analysisof failure mode, nevertheless, can be an extremelyuseful tool in determining whether the failure wasdue to a weak boundary layer or due to impropersurface preparation.

The exact cause of premature adhesive failure isvery difficult to determine. If the adhesive does notwet the surface of the substrate completely, the bondstrength is certain to be less than maximal. Internalstresses occur in adhesive joints because of a naturaltendency of the adhesive to shrink during setting, andbecause of differences in physical properties ofadhesive and substrate. The coefficient of thermalexpansion of adhesive and adherend should be asclose as possible to minimize the stresses that maydevelop during thermal cycling or after cooling froman elevated temperature cure. Fillers are often used tomodify the thermal expansion characteristics ofadhesives and limit internal stresses. Another way toaccommodate these stresses is to use relativelyelastic adhesives.

The types of stress acting on completed bonds,their orientation to the adhesive, and the rates atwhich it is applied are important factors in deter-mining the durability of the bond. Sustained loadscan cause premature failure in service, even thoughsimilar unloaded joints may exhibit adequatestrength when tested after aging. Some adhesivesbreak down rapidly under dead load, especiallyafter exposure to heat or moisture. Most adhesiveshave poor resistance to peel or cleavage loads. Anumber of adhesives are sensitive to the rate atwhich the joint is stressed. Rigid, brittle adhesivessometimes have excellent tensile or shear strengthbut have very poor impact strength. Operatingenvironmental factors are capable of degrading anadhesive joint in various ways. If more than one

environmental factor (e.g., heat and moisture) isacting on the sample, their combined effect can beexpected to produce a synergistic result of reducingadhesive strength. Whenever possible, candidateadhesive joints should be evaluated under simulatedoperating loads in the actual environment the jointis supposed to encounter.

References

1. Modified from ASTM D 907-82, StandardDefinitions of Terms Relating to Adhesives,published in Vol. 15.06dAdhesives, 1984Annual Book of ASTM Standards.

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

3. Mittal KL. Adhesive Joints: Formation,Characteristics and Testing. Netherlands: BrillAcademic Publishers; 2003.

4. Staff written. Joining techniques. Section 4,Machine Design. Fastening and Joining Refer-ence Issue 1976;48(26):155e62.

5. Sharpe LH. The materials, processes and designmethods for assembly with adhesives. MachineDesign 1966;38(19):179e200.

6. Petrie EM. Plastic and elastomer adhesives(Chapter 7). In: Harper CA, ed. Handbook ofPlastics and Elastomers. New York: McGraw-Hill; 1996.

7. De Lollis NJ. Adhesives for MetalsdTheoryand Technology. New York: Industrial Press;1970.

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

9. Ebnesajjad S, Ebnesajjad CF. Surface Prepara-tion Techniques for Adhesive Bonding. Norwich,NY: William Andrew Publishing/Noyes; 2006.

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