Chapter 10 Priming to improve adhesion GILES DILLINGHAM * Brighton Technologies Group, Cincinnati, OR, USA 1. Introduction The joining of materials through adhesive bonding, the encapsulation of materials with a protective coating of paint, or the modification of material characteristics through incorporation of a distinct phase with complimentary properties are all processes that produce composite materials or structures. The presence of interfaces with properties that determine to a large extent the overall performance of a material or structure is one of the defining characteristics of a composite material. The interface that is created between the two dissimilar phases in a composite structure is both a chemical and physical discontinuity. Even though it may be many molecular layers removed from a free surface, an interface retains the important surface characteristic of an increased chemical potential over the bulk phase due to a non-equilibrium intermolecular force field. This increased interfacial chemical potential is further augmented by the presence of residual stresses resulting from unequal dimensional changes in the constituent phases during processing, and stress concentrations that occur during loading due to the discontinuity in moduli of the constituent phases. These all contribute to the non-equilibrium state of an interface and represent destabilizing influences that contribute to the tendency of composite materials or structures to fail in a manner controlled by the interfacial properties. The volume of substance in a composite material that exists in a non- equilibrium state due to its proximity to an interface has been termed an in- terphase [1]. The interphase is a zone of distinct composition and properties formed by chemical or physical processes such as interdiffusion of mutually soluble components or chemical interaction between reactive species. * Corresponding author. E-mail: [email protected]
Transcript
Chapter 10
Priming to improve adhesion
GILES DILLINGHAM *
Brighton Technologies Group, Cincinnati, OR, USA
1. Introduction
The joining of materials through adhesive bonding, the encapsulation of materialswith a protective coating of paint, or the modification of material characteristicsthrough incorporation of a distinct phase with complimentary properties areall processes that produce composite materials or structures. The presence ofinterfaces with properties that determine to a large extent the overall performanceof a material or structure is one of the defining characteristics of a compositematerial.
The interface that is created between the two dissimilar phases in a compositestructure is both a chemical and physical discontinuity. Even though it maybe many molecular layers removed from a free surface, an interface retainsthe important surface characteristic of an increased chemical potential over thebulk phase due to a non-equilibrium intermolecular force field. This increasedinterfacial chemical potential is further augmented by the presence of residualstresses resulting from unequal dimensional changes in the constituent phasesduring processing, and stress concentrations that occur during loading due tothe discontinuity in moduli of the constituent phases. These all contribute to thenon-equilibrium state of an interface and represent destabilizing influences thatcontribute to the tendency of composite materials or structures to fail in a mannercontrolled by the interfacial properties.
The volume of substance in a composite material that exists in a non-equilibrium state due to its proximity to an interface has been termed an in-terphase [1]. The interphase is a zone of distinct composition and propertiesformed by chemical or physical processes such as interdiffusion of mutuallysoluble components or chemical interaction between reactive species.
Improving the properties of composites frequently requires addressing theissues of interfacial strength and stability. One approach to improving thesecharacteristics is to intentionally introduce an interphase region to improve ini-tial adhesion, provide chemical stabilization against degradation in aggressiveenvironments, and perhaps provide for a broad transitional zone in mechanicalproperties between the phases. An interphase engineered to accomplish thesegoals that is applied as a separate manufacturing step is referred to as a primer.
Primers belong to a class of surface engineering distinct from surface pretreat-ments such as the electrochemical etching or anodization of metal surfaces, andsurface functionalization processes for plastics such as flame treatment, coronadischarge or plasma treatment. Such treatments focus on either improving thestability of one of the phases against degradation through corrosion or hydration[2], increasing the chemical compatibility of the two phases at the interface byproviding specific chemical functionality, or providing a surface morphology op-timized for mechanical interlocking of the constituent phases [3,4]. With surfacepretreatments such as these, the effects generally extend only a few molecularlayers at most from the interface into either bulk phase. While an interphase maybe created in situ through the interaction between the surfaces, the dimensions aregenerally on a molecular level.
In addition to providing a means for engineering an interphase for improvedmaterial performance, primers can be employed to facilitate a manufacturingprocess. In the case of metals prepared for painting or adhesive bonding bycleaning and etching processes, the clean oxide surfaces are highly reactive andsubject to rapid contamination through handling or simply through exposure toatmospheric moisture and airborne contaminants. Application of an appropriateprimer to these surfaces effectively passivates the surface against oxidation andcontamination prior to final adhesive bonding or painting. In these situations, anexcellent primer may be nothing more than a solution of an appropriate polymerthat is soluble in the final coating or adhesive.
Primers may be divided into several broad categories based upon the type ofinterface they are designed to improve. Table I shows one such classification.Further discussion in this chapter is based upon this classification scheme.
2. Primers for structural adhesive bonding of metals
2.1. Coupling agents
Coupling agents are multifunctional compounds designed to provide a means forchemically coupling to both the inorganic surface and to the organic adhesiveor coating. The most widely used coupling agents are based on organosiliconchemistry, although titanates and zirconates have also enjoyed modest technical
Priming to improve adhesion
Table I
Classification scheme for adhesive and coating primers
Metal-polymer, structural adhesive bonding
Metal-polymer, protective or decorative coating
Adhesive bonding and painting of polymers
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Coupling agentsNon-corrosion-inhibiting primersCorrosion-inhibiting primersInorganic barrier primersRubber to metal primers
Galvanic systemsBarrier systemsInhibiting systems
Primers for cyanoacrylate adhesives,chlorinated polyolefin primers
and commercial success. These compounds may be applied directly to the sub-strate surface from dilute solution or may be compounded into an adhesive orcoating to generate a self-priming formulation. The technology is very rich andhas stimulated a tremendous volume of research over the last 3 decades, whichhas resulted in a good understanding of the mechanisms of adhesion promotionthrough organosilanes. On a larger scale, this work has resulted in a much greaterunderstanding of the phenomenon of adhesion in general. It is entirely possiblethat the widespread adoption of fiber-reinforced plastics would not have occurredwithout the development of organosilane coupling agent technology to providemoisture resistance to the glass/polymer interface in these materials. An excellent(though slightly dated) review of coupling agent technology may be found in [5].
Silane coupling agents are generally synthesized through addition of siliconhydrides to unsaturated organic molecules:
X3SiH+CH2=CH-R ----+ X3SiCHrCHrR
where X is a hydrolyzable group such as Cl, -OCH2CH3, or -OCH3 . In practice,organosilanes are frequently synthesized from the chlorosilane precursor which issubsequently hydrolyzed in the appropriate alcohol to form the ethoxy or methoxyanalogue. This avoids the generation of HCl (the byproduct of Si-Cl hydrolysis)in the end user's manufacturing process. Ethoxysilanes are gaining favor overmethoxy versions due to toxicity considerations of the methanol generated duringhydrolysis.
Coupling to a mineral surface requires the presence of active hydroxy Is onthe substrate. The coupling reaction is a multi-step process that proceeds from astate of physisorption through hydrogen bond formation to actual covalent bondformation through condensation of surface hydroxyls with silanols:
The formation of actual covalent bonds between the mineral surface and thesilanol would certainly help explain the resistance of organosilane-modified inter-faces to degradation in the presence of moisture, but the detection of such bondsis extremely difficult. The challenge is detection and structural analysis of what isessentially a monomolecular layer buried between two bulk phases. The presenceof covalent bond formation is widely accepted in the case of glass and silica sub-strates where the chemistry of silanol condensation is well understood. Similarly,condensation of silanols with metal hydroxides to form low concentrations ofM-O-Si species has been demonstrated in several instances using techniques suchas static secondary mass spectrometry (SSIMS) [6-9]. However, the significanceof these occurrences to the observed adhesion enhancement in adhesive jointsprimed with organosilanes is not clear. There are well-developed arguments thatcovalent bond formation is not necessary for obtaining a strong, water-resistantinterface.
Steric considerations limit the ability of more than one silanol per organosilaneto interact with surface hydroxyls. The remaining silanols readily condense withneighboring silanols to form polysiloxane networks whose degree of crosslinkingand three dimensional structure largely determines the film properties. Filmsof coupling agents are generally a fraction of a micron in thickness, so itbecomes necessary to consider the bulk structure of the coupling agent in orderto understand its properties and interactions with the organic overlayer. Due tothe size scale of this aspect of the coupling agent film, the bulk film structureis somewhat more accessible to analysis than the mineral surface-organosilaneinterface and consequently is a little better understood.
As primers for adhesive bonding, organosilanes are typically hydrolyzed insolution to the corresponding silanol prior to application. In most cases, thesesilanols begin to homopolymerize in solution and therefore have a limited shelflife. One result of this condensation is that the structure of the final film has its ori-gins in the solution history prior to film deposition. Hence knowledge of solutionproperties can be critical to primer performance. As the film dries and the water ofcondensation is removed through evaporation, the rate of polymerization greatlyincreases and a polysiloxane network is formed on the surface. The structure ofthis network determines not only the mechanical properties of the polysiloxanefilm, but its ability to interact with the organic overlayer through interpenetrationand interdiffusion. Critical parameters that determine the film structure includetype of solvent and concentration, pH of the solution, solution structure (e.g. theamount of condensation that has occurred prior to film deposition), film thickness,and drying conditions.
Organosilanes are available with a wide variety of organofunctional sub-stituents, and coupling agent formulation generally starts with a screening of po-tential substituents based on potential reactivity or solubility with the organic coat-ing or adhesive. Table 2 shows a list of some representative commercial silanes.
Priming to improve adhesion
Table 2
Representative commercial silane coupling agents
437
Organofunctional group
Vinyl
Chloropropyl
Epoxy
Methacrylate
Primary amine
Diamine
Mercapto
Cationic styryl
Chemical structure
CH2=CHSi(OCH3h
ClCH2CH2CH2Si(OCH3 h
oCH2CHCH20CH2CH2CH2Si(OCH3 h
CH3
CH2 = C-COOCH2CH2CH2 Si(OCH3 h
NH2CH2CH2CH2Si(OCH3 h
NH2CH2CH2NHCH2CH2CH2Si(OCH,h
HSCH2CH2CH2Si(OCH3 h
CH2 =CHC6H4CH2NHCH2CH2NH(CH2 hSi(OCH3 h =HCl
In primer formulations for adhesive bonding of metals, the coupling agentsthat are most frequently used are those based on epoxy and amine functionalities.Aqueous solutions of aminosilanes have been successfully used for obtainingstable adhesive bonds between epoxy and steel [10] and epoxy and titanium[11,12], while epoxy functional silanes are preferable for applications involvingaluminum substrates [13,14]. A simple solution of 1% epoxy functional silane inwater is currently used for field repairs of military aircraft [15] where phosphoricacid anodization would be extremely difficult to carry out, and performance isdeemed quite acceptable.
While both amino- and epoxy-functional silanes are capable of chemically re-acting with thermosetting adhesives, such as epoxies and urethanes, such chemicalreaction may not be a prerequisite for excellent adhesion. In a classic paper, Sungand coworkers [16] demonstrated that aminosilanes provided significant reinforce-ment for polyethylene/sapphire (AhO) adhesive joints as long as the degree ofsilane crosslinking was not excessive and the film thickness was about 1000 A,conditions shown to be necessary for interdiffusion. Because the aminosilane wasbelieved incapable of reacting with the polyethylene at the lamination temper-ature (l49°C), the mutual solubility of aminosilane and polyethylene was usedto explain the adhesion enhancement. In later work, Chaudhury and coworkers[17] showed that a combination of interdiffusion and chemical crosslinking wasresponsible for adhesion enhancement in aminosilane-primed aluminum/PVCadhesive joints.
2.2. Resin-based primers for aerospace adhesives
The most common aerospace adhesives include nitrile epoxy systems, epoxypolyamides, epoxy phenolics, and various unmodified epoxies [18]. Polyimides
438 G. Dillingham
have seen some application in high temperature applications as well. While allof these adhesives generally adhere quite well to properly prepared unprimedsurfaces, the use of primers can provide distinct advantages in mechanical andphysical properties. Of equal importance is the protection from contamination andphysical damage that a primer film provides to a delicate prepared oxide surfaceduring handling and storage prior to bonding. Application standards require thatpretreated aluminum surfaces must be bonded within 2 and 16 h of treatment[19,20], in some cases within 72 h [21].
Commercial primers for structural adhesive bonding of aircraft are generallyformulated to match a given adhesive and marketed as part of a packagedadhesive/primer system. Primers in this class are frequently prepared by dilutingthe adhesive polymers in a suitable solvent and applying to the prepared substrateas a film between about 0.0001 and 0.0025 inch thick. Frequently, an inert dyewill be included in the product to help identify a surface that has been primedwith a uniform film. The addition to these formulations of corrosion inhibitors isalmost universal, and the result has been a significant improvement in long-termdurability. In the past decade, environmental considerations have resulted in atrend towards water-borne primers for these applications, and performance ofthese systems can be excellent as well [22].
As long as the primer and adhesive have similar moduli, the improvements inmechanical properties of adhesive joints prepared from primed adherends are de-rived largely from the improvement in substrate wetting of the primer. A properlyprepared metallic substrate will have a clean, high energy surface. If the sur-face is relatively smooth, then complete wetting by a viscous adhesive is readilyachievable. However, the best pretreatments for aluminum also provide a micro-scopically rough morphology that is believed to aid in mechanical interlockingwith the adhesive [23]. Grit blasting is frequently recommended for steel surfacesas a method for cleaning and removal of scale and weak oxide. It is difficultfor a viscous adhesive to completely wet these microscopic features to obtain avoid-free bond line. A low viscosity primer based on a solvent-diluted adhesiveor an adhesive latex dispersed in water can readily flow into these asperities andplanarize the adherend surface with a thin protective film that the adhesive resincan wet and interdiffuse with during cure.
While primers are believed to improve the performance of adhesively bondedaluminum structures primarily through improved corrosion resistance and im-proved wetting of the microscopically rough adherend surface by the adhesive,modification of the mechanical properties of the adhesive near the substrate canhave a large effect on both the stress distribution and total strain energy. Finite ele-ment analysis has shown [24] that a deformable primer layer (i.e. one that is morecompliant than the adhesive) has the effect of reducing both the stress concentra-tion and the magnitude of the maximum stresses through the thickness of the ad-hesive layer in a loaded single lap shear joint. Perhaps more significantly, although
Priming to improve adhesion 439
the presence of a more compliant primer layer does not significantly alter the totalstrain energy release rate (G T = G1+Gil), it increases the mode mixity (GIl Gil).The crack-opening mode strain energy release rate is enhanced at the expense ofthe shear mode. This is the mode responsible for failure in single lap-shear joints.
High-strength aluminum such as the 2000 series alloys are susceptible topitting corrosion due to the galvanic couples formed between the matrix metaland the strengthening precipitates. These alloys will sometimes be clad with purealuminum (alclad) to protect the surface from corrosion. Interestingly, problemswere noted obtaining reproducibly durable adhesive bonds to these substratesusing the sodium dichromate/sulfuric acid etch (FPL process) that was standardin the 1960s [25,26]. As a result, clad alloys garnered a reputation for being poormaterials for adhesively bonded structures. It was subsequently shown that theirreproducibility was due to differences in oxide morphology between the edgesand center of FPL-treated panels unless the etching solutions were seeded withcopper [27]. For this reason, although properly etched or anodized clad alloys maybe readily primed and adhesively bonded, these materials have fallen from favorfor these applications.
2.3. Corrosion-inhibiting primers
Several studies have established a general relation between the corrosion resis-tance of metals and durability of adhesive bonds [28]. One approach to controllingthis corrosion in adhesive bonds to aluminum is through the inclusion in the primerof a carefully controlled quantity of finely ground corrosion inhibitor, usually achromate salt (based on zinc, strontium, or barium) [29] at a loading that is on theorder of 10% of the solution by weight. This concept represents one of the mostsignificant advances in the history of primer technology. Table 3 shows the type ofimprovement in salt spray resistance afforded by a corrosion-inhibiting primer.
Corrosion-inhibiting primers based on this technology have been in continuousservice since they were first utilized with nitrile epoxies in the late 1960s. Theseinhibitors function by passivating the aluminum. In this process, water permeatingthe adhesive bondline carries a certain amount of inhibitor to the oxide surface.
Table 3
Effect of corrosion inhibiting primer on strength retention after exposure to salt spray [29]
Exposure time (days)
Initial3090
120
Lap shear, psi
Non-corrosion inhibiting (BR 123)
587534901460
o
Corrosion inhibiting (BR 127)
5680589049704480
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Because of the high oxidizing potential of Cr6+, the cathodic reaction becomesreduction of the hexavalent chrome to the corresponding trivalent state, instead ofreduction of water to form hydroxide ions and hydrogen [30-32].
To be effective, there must be a certain minimum concentration of inhibitor atthe interface to be protected. Therefore, there must be sufficient inhibitor in theprimer, and these inhibitors need to be soluble enough in water to enable transportof inhibitor to the oxide surface as water permeates the adhesive joint. However,too high of a solubility will rapidly deplete the primer layer of inhibitor resultingin a loss of protection. One of the fortuitous properties of zinc and strontiumchromates is the limited solubility of these compounds in water (about 1.2 gil at15°C [33]).
The substitution of water-borne versions of these primers is increasing asenvironmental restrictions on the use of organic solvents become stricter. Theseare generally aqueous emulsions of epoxy novolac or phenolic based resinsstabilized by surfactants [34]. Non-ionic surfactants are preferred, as they arenon-hygroscopic in the dried primer films. Hygroscopic ionic surfactants couldresult in excessive water absorption by the primer film in service.
Formulation of water-based primers is significantly more complex than thesolvent-borne versions. Rather than simple solutions of resins, curing agents, andcorrosion inhibitors in suitable solvents, the presence of water makes necessarythe inclusion of many other ingredients such as pH buffers, anti-foaming agents,surfactants, and fungicides [35]. Frequently, a small quantity of an organic solvent,such as an alcohol, is still required, so that water borne primers are generally notcompletely free of volatile organic compounds (VOCs).
Water-borne primers mayor may not contain chromate-based corrosion in-hibitors. The limited solubility of chromate salts in water makes them less thanideal for use in water-based primers, and much work has gone into developingalternatives [36], but the performance of recently developed water based primersusing strontium chromate as the corrosion inhibitor is excellent, however, andappears equivalent to the solvent-borne analogues [37].
Although chromate-based corrosion inhibitors work extremely well, their usehas been increasingly limited since 1982 due to their potential carcinogenicity.The most common reported adverse health effect due to exposure to Cr6+ is lungcancer, although other ailments may result from exposure as well [38]. There isspeculation that chromates will be completely banned, but it is likely that they willmerely continue to come under increasingly stringent legislative control ratherthan be banned entirely, similar to the situation with VOCs [39]. This situationhas spawned a large amount of research to identify and develop replacements forchromate-based corrosion-inhibiting pigments that are potentially less toxic. Zincphosphates are one example. They provide some corrosion protection to aluminumalloys and are non-toxic. One of the best in a recent comparative study [40] ofzinc phosphate corrosion-inhibitor performance on chromate-conversion coated
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Table 4
Comparison of tensile lap shear strengths: solvent-based chromate-inhibited primers versus non-chromate and low VOC primers [42]
Primer Adhesive Tensile lap shear strength (psi)(source) RT (73°F) 180°F 180°F wet 30 day salt fog 60 day salt fog
XEA 9290 FM73 5255 4791(Hysol) AF 163-2K 6217 6079
2024-T3 aluminum was calcium strontium zinc phophosilicate. Other potentialnon-chromate corrosion-inhibiting pigments include cerium and molybdenumsalts or organic inhibitors. A recent review of these compounds can be found in[41]. Another interesting non-chromate inhibitor that is currently marketed in awater-based primer for structural adhesive bonding is ion exchanged silica. Thisinhibitor in completely insoluble in water and only releases active inhibiting Ca2+ions when potentially corrosive electrolyte species are present.
Non-chromate and low-VOC containing primers are capable of excellent per-formance. Table 4 shows the results of a comparison of various environmentallyacceptable primers with 250°F curing adhesive with the 'standard' BR 127solvent-based chromate inhibited primer [42].
The need to identify replacements for chromate-based technologies has alsoresulted in the development of alternatives to resin-based corrosion-inhibitingprimers. These are primarily inorganic barrier coating techniques, such as sol-gelprocesses, which mayor may not include corrosion inhibitors. These are discussedin a later section.
2.4. Structural adhesive bonding ofsteel
Large-scale adhesive bonding of steel is of great interest to the automotive andappliance industries because of the opportunities it provides for design flexibility,weight savings, and manufacturing economy. Because of its economic importance,
442 G. Dillingham
the needs of the automotive industry have especially dictated development ofadhesive and primer technology for steel.
Primer technology seems to play a much less important role in the adhe-sive bonding of steel than in aluminum, however, and the adhesive resin-basedcorrosion-inhibiting primers that are an integral part of the adhesive bonding ofaluminum are not used as frequently in these applications. This may stem fromthe fundamental differences in the aerospace versus consumer products industries.The expense of aircraft dictates that the airframes must have a long service life,at least twice the projected lifetime of about 10 years for an automobile. The lowvolume production of aircraft allows the use of multistep manufacturing proce-dures that are more difficult to automate. Furthermore, the service environment foraircraft is more severe than for automobiles, and reliability issues have potentiallymore devastating consequences.
As discussed above, silane primers have been investigated extensively foradhesive bonding of steel substrates and have been shown to be quite effective, butthe extent to which these are in current commercial use is not known.
2.5. Bonding to electroprimed steel
Much of the steel used in automobile manufacture is electroprimed prior tofabrication and painting. In this process, a thin layer of zinc phosphate crystals(ca. 0.6 urn) is deposited onto a clean steel surface, and then overcoated withan electrodeposited organic primer (10-40 urn). The zinc phosphate providescorrosion inhibition during service while the organic primer improves the qualityof the subsequent paint finish by functioning as a primer-surfacer, filling sandingmarks and other small surface imperfections.
While these primers were not developed specifically for preparing surfacesfor adhesive bonding, the ability to obtain strong and durable adhesive bonds toelectroprimed steel is important if manufacturing operations require componentsto be bonded to previously primed structures. They appear to perform the functionof adhesive primers very well. With properly formulated adhesives, bonds toelectroprimed substrates were actually stronger than to the unprimed substrates byas much as 30% [43]. In this case, 'properly formulated' referred to adhesives thatdid not contain compounds capable of chemically degrading the primer, causingvoids in the primer phase due to gas evolution. Fig. I shows the results of tensiletesting of single lap shear joints constructed of solvent-cleaned, grit-blasted steelbonded with an imidazole-cured epoxy novolac.
2.6. Primerless adhesive bonding ofsteel
Issues with adhesive bonding of steel frequently hinge on surface cleaning anddeoxidation, and preventing oxide growth during storage and handling prior to
Priming to improve adhesion 443
Fig. I. Comparison of unprimed and electroprimed single lap-shear adhesive joint strengths forsteel coupons bonded with imidazole-cured epoxy [43].
bonding. During shipping, storage, and manufacturing, steel is generally coatedwith a film of oil to protect against the otherwise rapid formation of ferric hy-droxide (rust). One approach for obtaining strong, durable bonds to this steelrequires removal of the oils immediately prior to coating the exposed surfacewith a protective primer, paint, or adhesive. This involves in-line cleaning sys-tems with their attendant maintenance and waste disposal issues. An alternativeapproach that has met with considerable success has been the development ofadhesives and coatings capable of absorbing and/or displacing the protective oil.For epoxies, DGEBA resin by itself is not capable of absorbing contaminatingoils to a significant extent [44], but certain curing agents or phase-segregatedtougheners can accomplish this goal. Polyamide curing agents with oleophilicfatty acid moieties can absorb significant amounts of oil and provide strong anddurable adhesive bonds to oily steel surfaces. Furthermore, the amount of oildisplacement by a particular formulation can be related to the amine number ofthe curing agent [45]. Oil displacement or absorption can also be accomplishedthrough inclusion of liquid carboxy-terminated butadiene acrylonitrile (CTBN)rubber in dicyandiamide-cured DGEBA epoxy adhesives as long as a high curingtemperature is used [46].
2.7. Inorganic primers
Inorganic coatings as primers for adhesive bonding and pamtmg have beeninvestigated for almost two decades. The basic mechanism by which they en-
444 G. Dillingham
hance adhesion and protect an adhesive/substrate interface against deteriorationis different from organic primers discussed above. Organic primers depend uponcorrosion-inhibiting pigments to electrochemically protect the substrate from cor-rosion. They are poor barriers to diffusion, and the inhibitors are actually inertuntil water saturates the interface region. In this sense, water ingress activatesthe corrosion-inhibition process in these materials. In contrast, inorganic primersprobably do not electrochemically inhibit corrosion, but rather function as barriersto diffusion of electrolyte to the primer/substrate interface.
Inorganic primers are generally based on either aqueous sol-gel coatingsor plasma polymerized coatings. Both produce highly crosslinked amorphousoxide films that are rough on a microscopic scale. This roughness has beenimplicated in the excellent adhesion enhancement provided by these films [47]and may be the source of what is potentially the most significant economic andenvironmental advantage: excellent performance can be obtained without oxideconversion processes. This eliminates one of the most costly and environmentallydamaging aspects of preparing metals such as aluminum for adhesive bonding.
Sol-gel primers use inorganic or metal-organic precursors (generally aluminum,silicon or titanium alkoxides) whose chemistry is closely related to the silanecoupling agents discussed previously. These precursors are dissolved in alcohol,then hydrolyzed by the addition of water:
Si(OR)4+H20 ---+ HO-Si(ORh +R-OH
Each alkoxy group hydrolyzes with a distinct rate constant, with the first hydroly-sis proceeding much faster than subsequent ones [48].
Film formation proceeds through condensation of hydroxyls to create a three-dimensional oxide network:
Elevated temperature drying accelerates this process.A simple embodiment of this concept [49] involves dissolving I% sec-butyl
aluminum alkoxide in toluene, applying to an acid-etched and anodized aluminumsurface, and drying at 75°C. Atmospheric moisture accomplishes hydrolysisafter solvent evaporation, and the films are believed to be a stable form ofamorphous Boehmite (AIOOH). Crack propagation rates in wedge test specimensprepared with adherends treated in this manner were improved over the organic-primed controls, and furthermore substrates prepared using a chromic acid etch(FPL process) prior to the inorganic primer application provided performanceessentially equivalent to a phosphoric acid anodized (PAA) surface [50].
These inorganic polymers can be copolymerized with organofunctional silanesto modify the physical properties or to include specific reactive functionalities to
A typical comonomer could be a polydimethylsilane to increase the toughnessof the polymerized film, or an amine functional silane for chemical coupling withan epoxy adhesive.
Inorganic or hybrid inorganic-organic films suitable for primers can also besynthesized using plasma polymerization. The plasmas used in these processesare low pressure gases subjected to an electric field. Cascading collisions betweenneutral gas molecules and the electrons and ions accelerated by the field generatea gas rich in active species, including electrons, ions, electrically excited species,and free radicals. Because the gas in a plasma is low pressure, however, gas phasecollisions are infrequent and the degree of ionization of the gas is low. The fractionof gas molecules that are activated is typically only around 10- 10
• Because of theirlow mass, electrons in the plasma have much higher average velocities than therelatively massive ions and therefore the flux of electrons striking a surface in theplasma is much higher than the flux of ions. This causes surfaces in contact witha plasma to charge negatively with respect to the plasma. This negative chargeaccelerates ions from the gas to the surface with sufficient energy (several eV)to rapidly clean and dehydrate metal surfaces. The resulting surface is extremelyclean and in an active state due to the absence of adsorbed contaminants. After thisinitial cleaning, monomer is introduced into the plasma. This rapidly chemisorbsonto the clean metal surface. Subsequent ion flux to the surface creates freeradicals that are the source of film growth.
Molecules that are normally unreactive can be readily polymerized in such aprocess. Examples include organic gases such as ethane and various organosilanes.Monomers such as hexamethyldisiloxane can be readily polymerized to formtightly adherent films having a silica-like structure:
(CH3hSi-0-Si(CH3h +O2 ----+ Si02 +H20 +CO2
The outer surfaces of these plasma polymers are terminated with hydroxylgroups and have high surface energies. They are readily wet by adhesives andform strong and durable adhesive bonds [51].
The basic mechanism of adhesion between an inorganic primer and a substrateis distinct from that of an organic primer. Organic primers adhere to an oxidesubstrate through a combination of mechanical interlocking with the texturedoxide and secondary atomic interactions, probably through surface hydroxyls. Asthe surface energy of a clean oxide is largely polar, the polar-polar (or acid-base)interactions are the predominant intermolecular interactions at the primer-oxideinterface. These linkages are susceptible to hydrolysis upon exposure to water,and corrosion and attendant failure at the primer-oxide interface is the commonfailure mode. With inorganic primers, however, there is an increasing body of
446 G. Dillingham
evidence that these materials are capable of condensing with surface hydroxyls tobond with the oxide through primary chemical linkages. Corrosion and failure atthis primer-oxide interface is very uncommon.
The adhesive-primer interface is distinct for inorganic primers as well. Organicprimers depend upon formation of a diffuse interface with the primer throughinterdiffusion, and overcuring of the primer film can result in poorer performancedue to reduced interpenetration. Inorganic primers, however, present a rough,hydroxylated oxide surface for mechanical interlocking with the adhesive. Acrucial difference between this oxide surface and that on prepared metal surfacessuch as aluminum is that the sol-gel or plasma polymerized oxide is stable anddoes not corrode during exposure to electrolyte. As long as the interactionsresponsible for adhesion between the adhesive and the inorganic primer arestable to the environment, the adhesive joint will maintain its integrity. Improveddurability at the interface between inorganic primers and organic adhesives can beobtained through modifying the structure of the primers for specific, higher-energyinteractions with the adhesive.
2.8. Surface roughness effects in inorganic primerfilms
Surface roughness has been implicated in many studies as being a critical factor inobtaining strong, durable adhesive bonds, and the microscopically rough surfacepresented by properly etched or anodized aluminum or grit-blasted steel is an im-portant factor in the adhesion enhancement afforded by these processes. Inorganicprimer films, whether derived from sol-gel or plasma polymerization routes, havesurface morphologies similar in many ways to that of anodized aluminum, andmay owe a significant fraction of their effects to this morphology.
Quantifying the effect of surface roughness or morphology is difficult, however.Surface preparations that provide different degrees of surface roughness alsousually produce surfaces that have different oxide thicknesses and mechanicalproperties, different compositions, or different contaminant levels. The problemof separation of these variables was circumvented in a recent study [52] by usinga modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2shows the sawtooth profile created by this process.
Adherends textured in this fashion were used as substrates for wedge tests,with the furrows running perpendicular to the direction of crack propagation.Surface analysis indicated a consistent surface composition regardless of theprofile. The performance of wedge test specimens prepared using this techniquewere compared to similar samples prepared by grit-blasting, which produced asurface with a thicker oxide and more hydrocarbon contamination than the micromilled surfaces. Failure in all samples after exposure to 50°C/95% RH wasinterfacial, but the fracture energies showed an exponential dependence on the
Priming to improve adhesion 447
Fig. 2. SEM cross-section of a 60° ultramilled aluminum bonded to epoxy resin. Also indicated isa diagrammatic representation of the ultramill profile showing the base angle [52].
surface morphology. Fig. 3 shows a linear relationship on a semilog plot betweenthe fracture energy after 150 h aging and tan ex (ex is defined in Fig. 2). Thisfigure shows that surface texture alone can provide a 100x increase in the fractureenergy of an adhesive joint aged in a humid environment.
A mechanical analysis of the near surface stresses in a wedge test as a functionof surface geometry shows that tan ex is equal to the ratio of the shear stress to thepeel stresses (Fig. 4).
Fig. 3 suggests that surface textures that enhance the surface peel componentcan promote bond degradation under aggressive aging conditions. The authorssuggested that interfacial voids generated by peel stresses ahead of the crack tipfacilitated moisture ingress and accelerated bond degradation.
2.9. Attachment site density effects in inorganic primerfilms
In addition to the beneficial effects of creating a textured surface, inorganicprimers can provide a very high density of specific chemical attachment sites for
448 G. Dillingham
1000
-NE-....,- 100uu(II,..
"10
21.50.5o 1
tan (0:)
Fig. 3. G lscc determined from crack-length data at 150 h as a function of resolved vertical andhorizontal component of the surface profile angle [52].
d
-c:OJcoa.Eoo
peel componentFig. 4. The forces at the epoxy-aluminum interface resolved into shear and peel components.Shear component/peel component = tan(a) [52].
an organic overlayer such as a paint or an adhesive. At one level, this increasesthe reversible work of adhesion (Wa) between the coating or adhesive and thesubstrate. However, the enhancement in fracture energy of the adhesive jointthat results from an increase in Wa stems from the long-range influence that theinterfacial bonds exert on the adhesive fracture energy G1 of the adhesive far fromthe interface.
Priming to improve adhesion 449
Cognard showed that the fracture energy of an adhesive joint is a function ofcrack velocity. In the range of crack velocities between 10-7 and 10- 1 ms", therelationship can be expressed by:
(I)
The term ex K 2v", derived from reptation theory, describes the velocity-dependent energy necessary to fracture the bulk adhesive. K 2 is the 'consis-tency' which relates the viscosity to the shear rate for a non-newtonian fluid.ex = tira L 2/ h", with r being the chain radius, L the chain length, a the densityof chains crossing over the fracture plane, and h is the distance between the chainand reptation tube.
Go is the 'static toughness', or fracture energy at zero crack velocity. Thisnumber can be obtained through wedge test measurements. The crack length in awedge test is inversely proportional to the strain energy release rate through thefollowing equation:
Ed2h 3 3(a+0.6h)2+h2G 1 = - - (2)
16 [(a +0.6)3 +ah2]2where h is the adherend thickness; E is the adherend Young's modulus; d isthe crack opening displacement by wedge insertion; and a is the crack lengthmeasured from point of wedge contact.
Eq. 2 shows that the elastic strain energy decreases with the inverse fourthpower of the crack length. When a wedge is driven into the specimen, the crackgrows at a velocity defined by Eq. I until the strain energy has decreased to Go.Crack growth ceases at this point until some change in the system reduces Gbelow that defined by the crack length. Changes that could bring about furthercrack growth include substrate corrosion at the interface or plasticization of theadhesive by moisture.
Go is related to the reversible work of adhesion Wa obtained using contact anglemeasurements, but in general is greater than Wa. This is because once an interfaceis formed and the adhesive solidifies, strain energy is required to mechanicallydisrupt the interface. This strain energy arises because of the physical connectionbetween the attachment sites between the adhesive and the substrate and theconnectivity between this interface and the adhesive bulk.
Wa quantifies the specific, discrete interactions that exist between a wettingliquid and a substrate. These interactions may be Van der Waals, acid-base, orcovalent. The reversible work of adhesion is the product of the areal density ofthese interaction sites (or attachment points) and the energy per attachment point:
(3)
where v is the areal density of attachment sites; and U; is the energy per interfacialattachment point.
450 G. Dillingham
Table 5
Wedge test fracture energy (from Eqs. I and 2) vs. adherend surface treatment (from ref. [3])
Surface treatment
NoneSolvent wipeAlkaline cleanAsuclean a
900100020006000
All failures were cohesive within the adhesive. Adhesive: E610 nylon epoxy (Asulab SA).a Asuclean is a proprietary formulation of Asulab SA.
This interaction energy is reversible because removal of the wetting liquid fromthe surface only requires the disruption of these interaction sites. Solidification ofthe liquid into an adhesive changes the requirements for dewetting, however.
When an adhesive solidifies and the joint is loaded through such an interface,stress is transferred from the substrate to the adhesive through these attachmentpoints, and the adhesive resin adjacent to these attachment points is loaded. Thestrain energy in this system is expressed by the following equation [53]:
G=vNUx (4)
where N is the number of stressed network links in the adhesive per interfacialattachment point; U« is the energy per stressed network link. And by combiningEqs. 3 and 4:
(5)
when crack growth just stops in a wedge test, G = Go.Eqs. 1-5 hold whether failure is interfacial or cohesive within the adhesive.
Furthermore, Eq. 5 shows that the reversible work of adhesion directly controls thefracture energy of an adhesive joint, even if failure occurs far from the interface.This is demonstrated in Table 5, which shows the static toughness of a series ofwedge test specimens with a range of adherend surface treatments. All of thesesamples failed cohesively within the resin, yet show a range of static toughnessvalues of over 600%.
3. Primers for rubber to metal bonding
The applications of rubber as an engineering material almost invariably involvebonding to a rigid substrate or reinforcement. In some instances these bonds needto be established to a fully cured thermoset rubber or a molded thermoplasticrubber, and a wide variety of adhesives suitable for this purpose are available. In
Priming to improve adhesion 451
some instances, surface treatment (such as oxidation or chlorination of natural orNBR rubbers) or primers (in the case of olefin-based thermoplastic rubbers) arenecessary.
Many applications of thermoset rubbers require them to be processed in anuncrosslinked state, for example by injection molding, compression molding,or transfer molding. In these processes, bonds between the rubber and a rigidsubstrate must be established during vulcanization. Establishing a suitably strongand durable bond between a thermoset rubber and a substrate during vulcanizationalmost invariably requires an adhesive or primer. This has primarily been attributedto the absence of polarity in the rubber [54].
This is not an issue when vulcanizing certain sulfur-cured natural rubbersagainst brass, however. It was known as early as 1862 that coating metal witha layer of electrodeposited brass created strong bonds to rubber vulcanizedin contact with the surface [55]. The mechanisms of adhesion are still beingactively researched and debated in the literature, but appears related to bothchemical bonding due to formation of CuxS-Sv-NR bridges [56] and mechanicalinterlocking with a porous, dendritic sulfide film formed in situ during thevulcanization process [57]. More recent work emphasizes the potential importanceof both mechanisms [58]. This process is extremely important in the tire industryfor obtaining adhesion of natural rubber compounds to steel tire cords. The brassplating plays a dual role in this instance. The steel in brass plated prior to drawinginto wires, and the lubricious nature of the brass surface improves the drawingprocess considerably. However, adhesion is very sensitive to rubber formulationand the plating process generates large volumes of hazardous waste. These factorslimit the utility of brass interlayers for rubber-metal adhesion, and in manyapplications solvent or water-borne organic primers and adhesives are used.
Systems for bonding rubber to a substrate during vulcanization that do notexploit the reactivity of sulfur and brass are generally two layer primer/adhesivesystems. The bond to the substrate results from strong interactions between apolar and/or reactive polymeric primer and the surface. This primer is then coatedwith a less polar adhesive or cover cement that can interdiffuse and perhapscrosslink with both the primer and the rubber stock during vulcanization. Thegraded structure that results provides a smooth transition between the rigid, polarmetal and the compliant, non polar rubber. One of the earliest embodiments of thisconcept was in a patent to Hugh Lord (founder of the Lord Corporation) in 1930[59]. In this system, the primer and cover cement contained the same ingredientsbut in different proportions (Table 6).
Solutions of the two recipes were blended in varying proportions to providetie coats of continuously varying composition. The patent shows an example ofeight plies or layers of graded composition between the rubber and the metalsubstrate. Because of the high fraction of reactive filler, the material closest tothe metal substrate would be the most rigid and polar. The stiffness and polarity
452
Table 6
An early primer and stock formula for rubber-metal bonding [59]
G. Dillingham
Component
Raw rubberZinc oxideIron oxideSulfurHexamethylene tetramineLime
Primer recipe
3050155
1.25
Stock recipe
1003
5I
would progressively decrease over the thickness of the adhesive layer to providea smooth transition in polarity and modulus. This was claimed to prevent stressconcentration near the interface.
The concept of graded layers transitioning from a rigid, polar primer to acompliant, non-polar adhesive is still employed in most primer/adhesive systemsfor rubber to metal bonding, although the number of layers is generally only two.Since the mid 1900s, the primer coats have been generally based on phenolic orepoxy resins with chlorinated rubber and metal oxides (zinc or titanium), whilethe adhesives (or cover cements) have been based on halogenated rubber with acrosslinking agent [60,61].
The phenolic resins used in primer layers for rubber adhesion are usually basedon the condensation products of aromatic alcohols (such as phenol, cresol, orresorcinol) and hexamethylene tetramine. They can be applied as a separate primerlayer or in some applications blended in the rubber stock. These resins functionas adhesion promoters for a wide range of substrates, including steel, brass-platedsteel, various polymers, and natural cellulosic fibers. The mechanisms of adhesionpromotion have not been well documented. The polymerization mechanism iscomplex and the resulting polymers are highly functional. The excellent adhesionthat results may be related to the ability of the phenolics to condense withhydroxyl groups during polymerization and/or the ability of the hexamethylenetetramine to inhibit corrosion of steel and chelate copper in the brass. The stronginterface between the primer and halogenated cover cement is certainly relatedto interdiffusion of these layers, and perhaps co-polymerization with the reactivehalogenated rubber.
The reactivity of the halogenated rubber in cover cements was shown in aninvestigation that demonstrated that halogenated polymers are labile and capableof reacting with unsaturated rubber under vulcanizing conditions, even withoutcrosslinking agents [62]. Infrared spectra of solution cast films of natural rubberand chlorinated natural rubber show little change after heating to 170°C for 30min. Fig. 5 shows the spectra of the natural rubber before and after heating.Appearance of bands above 1700 and near 1150 em-I with heating is due to
Priming to improve adhesion 453
0.90
0.81
0.72
063c 0.54.Q0.~.0<{
0.27
0.18
0.09
0.001800 1600 1400 1200 1000 800
em"
600 400
Fig. 5. Transmission IR spectra of natural rubber, solvent cast from xylene. (a) Before heating,and (b) after heating at 170De for 30 min [62].
carbonyl products of oxidation, but the effect is relatively slight. Fig. 6 showsthe results of heating chlorinated rubber under identical conditions. This materialshows even less oxidation than the unchlorinated rubber.
But heating of a mixed film produced dramatic results. The two polymers arenot miscible, and solution cast films phase separate. But heating a solution castfilm of a I: I wt% blend of natural rubber and chlorinated natural rubber resultsin a homogeneous film that is oxidized to a much greater extent than either ofthe components. Fig. 7 shows the infrared spectra before and after heating. Thespectrum of the mixed film before heating appears to be a linear combination of thespectra of the chlorinated and unchlorinated rubber. Heating resulted in extensiveoxidation, and, furthermore, the films became single phase. This suggests that it isnot merely the polarity of chlorinated rubber that is responsible for its utility as aprimer component in rubber to metal bonding agents, but its ability to chemicallyreact with natural rubber. The miscibility that results may be a byproduct of thisreaction.
There have been other approaches to obtaining rubber/metal adhesion besidesprimers or additives consisting of phenolics or epoxies plus halogenated elas-tomers. For example, carboxylated polymers (olefins and diolefins copolymerizedwith acrylic acid monomers) have shown excellent adhesion to metals. Very lit-tle carboxyl is necessary, and polymers with carboxyl contents as low as 0.1%show good adhesion when laminated to bare steel. When these materials possess
454 G. Dillingham
0.90
0.84
0.78
0.72
c:.2ts.....0lJ).0
0.54<t:
0.48
0.42
0.36
0.301EOO 1600 1400 1200 1000 800 600 400
em.Fig. 6. Transmission IR spectra of chlorinated natural rubber, solvent cast from xylene. (a) Beforeheating, and (b) after heating at 170De for 30 min [62].
Fig. 7. Transmission IR spectra of a I : 1 wt% blend of natural rubber and chlorinated naturalrubber, solvent cast from xylene. (a) Before heating, and (b) after heating at 170De for 30 min[62].
Priming to improve adhesion 455
sufficient diene content for vulcanization to rubber along with sufficient carboxylcontent, solution-deposited coatings are suitable primers for bonding rubber to gritblasted steel [63,64]. These materials may be blended with phenolic/halogenatedrubber primer systems to improve adhesion of rubbers to metals and variousnatural and synthetic fibers [65]. More recently, it has been shown that plasmapolymerized olefins deposited onto steel under conditions that retain significantresidual unsaturation can provide tremendous adhesion between natural rubberand steel [66]. These systems have some potential for replacing both solvent andwater-borne primer/adhesive systems with a more environmentally benign pro-cess. Other recent work also indicates that mixtures of appropriate organosilanecoupling agents can provide significant adhesion of sulfur cured rubber to a varietyof metal substrates [67].
4. Primers for protective and decorative coatings on metals
Encasing metals such as steel and aluminum in protective or decorative organiccoatings requires addressing many of the same technological problems of struc-tural adhesive bonding. The coatings must demonstrate good initial adhesion andgood durability under adverse environmental conditions. Because the require-ments for the outer coating surface properties are very different from those ofthe film near the substrate/film interface, coating systems frequently include aseparate primer layer.
While the mechanical challenges faced by a coating system are generallyless demanding than those faced by an adhesive, the environmental challengescan be significantly greater. An adhesive bond is protected from the elementsby the adherends except at the perimeter of the joint. For adhesive joints, therate-controlling step in the degradation of bond strength is the rate of moisturediffusion through the bondline edge, and at least in the case of epoxy/steeladhesive joints, debonding can be accurately predicted to occur when the localconcentration of water exceeds a critical level [68,69]. With coating systems,because the relative amount of exposed surface is so much greater than for anadhesive, the rate-limiting step in degradation becomes the rate of reactions (suchas substrate corrosion) occurring at the interface. These rates are determined bythe intrinsic reaction rates and the relative rate of transport of corrosive speciesand inhibitors through the film to the interface.
With the exception of coupling agent technology, primers for structural adhe-sive bonding have received little theoretical treatment in the literature beyond adiscussion of mechanisms of corrosion inhibition by primer additives and limiteddiscussion about statistical techniques for primer formulation. Perhaps because ofthe much more widespread use and greater economic importance of corrosion-protective coatings, the design and function of primers for these systems have
456 G. Dillingham
been thoroughly discussed in the literature and placed on firm theoretical footing.A consideration of the principles developed in paint technology could potentiallyaid in the advancement of primer technology for structural adhesive bonding.
Primers for protective coatings may be divided into three broad classes basedon the mechanism of substrate protection: barrier primers that function by pre-venting the ingress of moisture and electrolytes, primers that protect the substrategalvanically in the presence of electrolytes, and primers that contain electrochem-ical inhibitors to passivate the substrate. Each of these approaches requires adistinct primer film structure due to the different mechanisms of protection.
A primer film consists of an organic binder and dispersed filler particles. In theterminology of paint formulators, the fillers are generally referred to as pigments.The pigment volume concentration (PVC) determines the structure of the primerfilm in the following manner. At low PVC, there is no connectivity betweenthe pigment particles and the substrate, and the film is void-free. The PVC canbe adjusted over a wide range without substantially increasing particle-particleor particle-substrate contact. However, when the PVC reaches a percolationthreshold, particle-particle and particle-substrate contact increases dramatically.At this point the binder is still void-free, and the pigment volume concentrationis labeled the critical pigment volume concentration (CPVC). At PVC higherthan the CPVC, interstitial voids appear because of a lack of sufficient binder.While PVC is a formulating variable, the CPVC is a constant for any givenbinderjpigment combination.
The PVC can be readily expressed mathematically as follows. The dry filmvolume consists of the sum of the individual volume elements of the film com-ponents. The total volume of the film (Vr) may be represented by the followingexpression:
Vr = Pv + Va + Vh + Vc + v,where P; is pigment volume; Vh is binder volume between filler particles; Va isbinder volume adsorbed to filler; Vc is binder volume adsorbed to substrate; andVv is void volume.
The PVC is simply Pvj Vr and is determined by the volume of filler added tothe formulation. The CPVC occurs when the binder particles are close packedand Vv = O. At the CPVC, there is just enough binder to coat all of the fillerparticles and the substrate with at least monomolecular layer of binder, and fill theinterstices between particles.
The ratio PVCjCPVC (or reduced CPVC) is a simple but profound index ofprimer film structure that is perhaps the most important parameter in all of painttechnology [70]. The relationship of the reduced CPVC to film structure can beappreciated by reference to Fig. 8. This figure shows a representation of a dryprimer film with a range of pigment volume fraction that increases from left toright.
Priming to improve adhesion 457
Fig. 8. Structure of dry primer film with inhibiting pigment showing range of filler volumefractions. From ref. [70].
Fig. 9. Barrier primer film structure as a function of PVC. From ref. [70].
The CPVC can be readily identified in Fig. 8 as the point where the fillerparticles are close packed with just sufficient binder to coat the filler and substrateleaving no interstitial voids. At this point, PVC/CPVC = 1. Below the CPVC,there is excess binder and the filler particles are not close packed. Above theCPVC, interstitial voids exist due to insufficient binder.
The optimum PVC that provides the best corrosion protection for the substrateis a strong function of the corrosion protecting mechanism. Figs. 8-10 representprimer films based on three different protection mechanisms, and indicate theappropriate formulating windows for each type of primer.
Fig. 8 shows a primer formulated with a corrosion-inhibiting pigment such as achromate. As discussed previously, some permeability to moisture is necessary forthese pigments to dissolve and be transported to the interface where passivation ofthe substrate can occur. Optimum performance is generally found at PVC/CPVCjust below 1 [71].
458 G. Dillingham
Fig. 10. Galvanic primer film structure as a function of PVc. From ref. [70].
Barrier primers are represented in Fig. 9. These systems require thick, im-permeable films based on binders such as epoxies with platy pigments such asstainless steel flake or aluminum flake. These pigments provide a torturous paththat increases the diffusion length from the surface to the substrate. Because manyof the pigments utilized in barrier primers can be galvanically more noble than thesubstrate, it is generally important to maintain electrical isolation of the particlesfrom each other and from the substrate. The requirements for pigment isolationand low void fraction dictates that while the PVC must be high enough to providesubstantial barrier to diffusion, the PVCjCPVC « 1 to maintain low void volumeand electrical isolation of the pigment.
Galvanic systems are represented in Fig. 10. In primers of this type, the pigmentmust anodically corrode to protect the substrate, which requires the passage ofcurrent between the pigment particles and the substrate. While true contact isnot possible because of the presence of a thin binder sheath on the substrateand pigment particles (Vb and V-J, the contact must be as close as possible. Thiscorresponds to a PVCjCPVC = 1. For zinc dust, a common anodic pigment,the theoretical CPVC is 65.6% [70]. Lower PVC than this results in excessiveelectrical resistance of the films and poor protection. Higher PVCs result ininterstitial void volume, which is necessary for protection. When the voids fillwith electrolyte, the galvanic cells that result allow sacrificial corrosion of the zincand protect the substrate. Therefore, these pigments work best at PVCjCPVC > 1.
5. Primers for adhesive bonding of plastics
Polymeric surfaces are fundamentally different from metal oxide surfaces, andconsequently the technical challenges to obtaining strong and durable adhesive
Priming to improve adhesion
Table 7
Surface energies of polymeric and metal oxide surfaces
Surface energyCorrosion resistanceInitial bond strengthBond durabilityFunction of primers:
HighLowHighLow- Protect treated surface- Provide corrosion resistance
LowHighLowHigh- Protect treated surface- Provide high initial bond strength
bonds to polymers are very different. Table 7 compares the surface energy forseveral polymers and metals; Table 8 contrasts a few of the differences that affectthe strength and durability of adhesive bonds to these surfaces. The high surfaceenergy of a clean metal oxide ensures strong intermolecular interactions with liq-uid adhesives, resulting in complete wetting and good initial bond strength. It alsomakes the surfaces susceptible to rapid contamination after cleaning, and absorp-tion of water to the adhesive/oxide interface is an unavoidable thermodynamicconsequence. Debonding and corrosion frequently ensue. Primer technology foradhesive bonding of metals addresses these issues: metal primers provide protec-tion for a clean oxide surface prior to adhesive bonding and corrosion protectionfor the adhesive/oxide interface during use.
The primary challenge facing adhesive bonding of metals is to obtain sufficientdurability of a bonded structure. Initial bond strength in metal-polymer adhesivejoints is almost invariably excellent. Challenging the application of adhesives inpolymer-polymer joining, however, is the problem of obtaining a joint that is
460 G. Dillingham
sufficiently strong from the outset. This requires sufficient wetting followed by theestablishment of strong adhesion across the adhesive-adherend interface.
While polymeric surfaces with relatively high surface energies (e.g. polyimides,ABS, polycarbonate, polyamides) can be adhered to readily without surface treat-ment, low surface energy polymers such as olefins, silicones, and fluoropolymersrequire surface treatments to increase the surface energy. Various oxidation tech-niques (such as flame, corona, plasma treatment, or chromic acid etching) allowstrong bonds to be obtained to such polymers.
Adhesive bonding of polymers is a case of polymer-polymer bonding. Thestrength of a polymer-polymer interface is developed from one of two mecha-nisms. If molecular mobility is sufficient, a diffuse interface is created throughinterdiffusion and entanglement, and the strength of the structure depends onthe cohesive strength of the interpenetrated interphase region. If interdiffusionsufficient to produce entanglement is not possible, however, adhesion resultsfrom similar mechanisms to those in polymer-metal adhesive bonding: specificintermolecular interactions acting across a well-defined interface.
Interdiffusion is the dominant mechanism in adhesive bonding of polymers,such as polyolefins, which have significant molecular mobility but lack the high-energy functional groups necessary for establishing strong chemical interactionsacross an interface. Adhesive bonding of many engineering thermoplastics as wellas surface-treated polyolefins results from specific molecular interactions betweenthe adhesive and polar functional groups on the polymer surface.
Because of these characteristics of polymer-polymer interfaces, primers em-ployed for adhesive bonding of these materials have very different properties fromthose used for adhesive bonding and painting of metals. Low surface energy andlack of reactive surface functionality in many polymers necessitates some sort ofsurface treatment in order to obtain wetting and adhesion. Two types of primersare generally used in these cases: surface 'activators' for bonding with cyanoacry-late adhesives, and chlorinated polyolefin adhesion promoters for use with otheradhesives and paints such as are commonly used in the United States for obtainingadhesion to toughened polypropylenes (TPOs). In both cases, a solvent carrierwhich permits penetration of the substrate by the active primer components iscritical. In this manner, these primers act to functionalize the polymer surface andprovide specific attachment sites for the adhesive.
Cyanoacrylate adhesives cure by anionic polymerization. This reaction is cat-alyzed by weak bases (such as water), so the adhesives are generally stabilized bythe inclusion of a weak acid in the formulation. While adhesion of cyanoacrylatesto bare metals and many polymers is excellent, bonding to polyolefins requiresa surface modifying primer. Solutions of chlorinated polyolefin oligomers, tran-sition metal complexes, and organic bases such as tertiary amines can greatlyenhance cyanoacrylate adhesion to these surfaces [72]. The solvent is a criticalcomponent of these primers, as solvent swelling of the surface facilitates inter-
Fig. II. Effect of polyolefin primers on bond strength of ethyl cyanoacrylate to plastics.All assemblies tested in accordance with ASTM D 4501 (block shear method). ETFE =ethylene tetrafluoroethylene copolymer; LDPE = low-density polyethylene; PFA = polyper-fluoroalkoxyethylene; PST = polybutylene terephthalate, PMP = polymethylpentene; PPS =polyphenylene sulfide; PP = polypropylene; PS = polystyrene; PTFE = polytetrafluoroethylene;PU = polyurethane. From ref. [73].
penetration of the primer and substrate. The effectiveness of polyolefin primersincreases as the crystallinity of the plastic they are used on decreases, since themore amorphous substrates facilitate interdiffusion. Fig. 11 shows the effect ofprimer on ethyl cyanoacrylate/polymer adhesion for various plastics [73].
Adhesion of paints and adhesives to TPOs is especially problematical due to thealiphatic nature of the substrate material. In Europe, plasma and corona treatmentis employed to render these surfaces wettable and obtain strong adhesion byadhesives and paints in automotive manufacture. In the United States, however,primers based on solvent-borne chlorinated polyolefin oligomers (CPOs) havebecome the treatment of choice for these substrate materials. The VOC emissionsfrom these primers are considerable (as in all solvent-borne adhesives), but the less
PP transcrystalline region
PP spherulites
PPbulk
Fig. 12. Schematic of TPO surface regions.
462 G. Dillingham
capital intensive and more easily controllable nature of a sprayable liquid primersystem is seen as a major advantage. These primers may be water or solventborne and composed of chlorinated polyolefins plus tackifying resins, solvents,and fillers. They are typically applied to a dried film thickness of 2.5-7.5 J..Lm [74].
Similar to the primers developed for cyanoacrylate resins, the solvent carrierplays an important role in facilitating interdiffusion of the primer and the substrate.Fig. 12 shows a schematic view of the top few microns of an injection moldedTPO surface.
Adhesion development depends on diffusion of the CPO component of theprimer through the crystalline boundary layers followed by swelling and entangle-ment with the rubber rich layer [75].
References
I. Adhesion International 1993. In: Sharpe, L.H. (Ed.), Proceedings of the 16th AnnualMeeting of The Adhesion Society, Inc., Williamsburg, VA, Feb. 21-26, 1993. Gordon andBreach, 1996.
2. Pocius, AY., J. Adhes., 39, 101 (1992).3. Venables, J.D., J. Mater. Sci., 19, 2431 (1984).4. Clearfield, H.M., McNamara, D.K. and Davis, G.D. In: Lee, L.H. (Ed.), Adhesive Bonding.
Plenum Press, New York, 1991, p. 203.5. Pleuddemann, E.P., Silane Coupling Agents. Plenum Press, New York, 1982.6. Gettings, M. and Kinloch, AJ., J. Mater. Sci., 12,2511 (1977).7. Davis, S.I. and Watts, J.E, Int. J. Adhes. Adhes., 16,5 (1996).8. Abel, M.-L., Digby, R.P., Fletcher, I.w. and Watts, J.E, Surf. Interface Anal., 29(2), 115-
125 (2000).9. Reichlmaier, S., Hammond, 1.S., Hearn, M.l. and Briggs, D., Surf. Interface Anal., 21, 739
(1994).10. Boerio, r.r. and Williams, 1.W., App!. Surf. Sci., 7, 19 (1981).II. Schrader, M.E. and Cardamone, J.A, J. Adhes., 9, 305 (1978).12. Boerio, F.I. and Dillingham, R.G. In: Mittal, K. (Ed.), Adhesive Joints: Formation, Char-
acteristics, and Testing. Plenum Press, New York, 1984.13. Boerio, F.I. In: Patrick, RL (Ed.), Treatise on Adhesion and Adhesives, Vol. 6. Marcel
Dekker, New York, 1989.14. Theidman, w., Tolan, r.c., Pearce, P.I. and Morris, e.E.M., J. Adhes., 22, 197 (1987).15. Kuhlbander, R.I. and Mazza, J.J., SAMPE Symposium Series, 38 (1993).16. Sung, N.H., Kaul, A, Chin, I. and Sung, e.S.P., Polym. Eng. Sci., 22(10), 637 (1982).17. Chaudhury, M.K., Gentle, T.M. and Pleuddemann, E.P., J. Adhes. Sci. Techno!., 1(1), 29
(1987).18. Millard, E.C. In: Thrall, E.W. and Shannon, R.W. (Eds.), Adhesive Bonding of Aluminum
Alloys. Marcel Dekker, 1985, p. 99.19. Defence Standard 03-2/Issue 3. Cleaning and preparation of metal surfaces, British Min-
istry of Defence, March 1995.20. British Standard Aerospace Series, BS EN 2334. Chromic-sulphuric acid pickle of alu-
minum and aluminum alloys. BSi, 1977.
Priming to improve adhesion 463
21. Boeing Process Specification, BAC 5555 Issue M. Phosphoric acid anodizing of aluminumfor structural bonding. Boeing Airplane Company, 1995.
22. Lindner Meyler, K.L. and Brescia, J.A., J. Adhes. Sci. Technol., 9(1), 81 (1995).23. Rider, AN. and Arnott, DR, J. Adhes., 75, 203 (2001).24. Schmueser, D.W., J. Eng. Mater. Technol., 112, 321 (1990).25. Sharpe, L.H., Apply. Polym. Symp., 3, 353 (1966).26. Bethune, A.W., SAMPE J., Aug., 4-10 (1975).27. Pocius, AV., J. Adhes., 39(2-3),101 (1992).28. Minford, J.D. In: Patrick, R.L. (Ed.), Treatise on Adhesion and Adhesives, Vol. 5. Marcel
Dekker, New York, 1981, p. 45.29. Politi, R.E. In: Skeist, I. (Ed.), Handbook of Adhesives, 3rd edn. Van Nostrand Reinhold,
New York, 1990, p. 713.30. Boises, D.B., Northern, B.J. and McDonald, W.P., Corrosion Inhibitors in Primers for
Aluminum, Paper 31, National Association of Corrosion Engineering, Houston, TX, 1969.31. Davis, G.D. and Shaffer, O.K. In: Pizzi, A. and Mittal, KL. (Eds.), Handhook ofAdhesive
Technology. Marcel Dekker, NY, 1994, p. 113.32. Kendig, M.W., Davenport, AJ. and Isaacs, H.S., Corros. Sci., 34, I (1993).33. Handbook of Chemistry and Physics, Vol. 69, CRC Press, Boca Raton, FL, 1988-1989.34. Bascom, W.O. In: Engineered Materials Handbook, Adhesives and Sealants, Vol. 3, ASM
International, 1990.35. Bishopp, JA, Parker, M.J. and O'Reilly, TA, Int. J. Adhes. Adhes., 21, 473-480 (2001).36. Reinhart, T.J. In: Allen, K.w. (Ed.), Adhesion 2, Novel Concepts for Priming Metallic
37. Kohli, D.K and Sitt, H.W., Proc. Int. SAMPE Symp. Exh., 42(2), 1225 (1997).38. EPA Federal Register, National Emission Standards for Hazardous Air Pollutants for
Source Categories: Aerospace Manufacturing and Rework Facilites 60, 170 p. 45947,1995.
39. Twite, R.L. and Bierwagen, G.P., Prog. Org. Coat., 33, 99-100 (1998).40. MacQueen, R.C., Miron, RR. and Granata, R.D., 1. Coat. Technol., 68, 857 (1996).41. Twite, RL. and Bierwagen, G.P., Prog. Org. Coat., 33, 99-100 (1998).42. Kuhbander, R.I. and Mazza, J.J. In: Proceedings of the 38th Annual SAMPE Symposium,
May 1993, pp. 785-795.43. Foister, RT., Gray, RK and Madsen, P.A, J. Adhes., 24, 17-46 (1987).44. Hong, S.G. and Boerio, F.J., J. Appl. Polym. Sci., 55, 437 (1995).45. Hong, S.G., Cave, N.G. and Boerio, F.J., J. Adhes., 36, 265 (1992).46. Siebert, AR., Tolle, L.L. and Drake, R.S., CTBN-modified epoxies work in poor bonding
conditions. Adhes. Age, 29, 19 (1986).47. Zheng, H., Du, Y, Damron, M., Wright, J. and Tang, M., Metal Finishing, 1998, pp. 35-38.48. Leyden, D.E. and Atwater, J.B., J. Adhes. Sci. Technol., 5(10),815-829 (1991).49. Pike, RA and Lamm, F.P.,J. Adhes., 26, 171-179 (1988).50. Pike, RA, Int. J. Adhes. Adhes., 6, 21 (1986).51. Turner, RH., Segall, I., Boerio, F.J. and Davis, G.D., 1. Adhes., 62(1), 1-21 (1997).52. Rider, AN. and Arnott, DR, J. Adhes., 75, 203-208 (2001).53. Cognard, J., J. Adhes., 57,31-43 (1996).54. Alstadt, D.M., Rubber World, 133, 221 (1955).55. Buchan, S., Rubber to Metal Bonding. Crosby Lockwood and Son, London, 1948.56. Haemers, G., Adhesion, 4th edn. Barking, London, 1980, p. 175.57. Van Ooij, W.J., Ruhber Chem. Technol., 57, 421 (1984).
59. U.s. Patent 1,749,824, 1930.60. Cook, r.w., Edge, S. and Packham, D.E., Int. J. Adhes. Adhes., 17, 333-337 (1997).61. Lindsay, J., Materials World, May, 266-268 (1999).62. Cook, r.w, Edge, S. and Packham, D.E., J. Mat. Sci. Lett., 16,445-447 (1997).63. Frank, C.E., Kraus, G. and Haefner, AJ., Ind. Eng. Chem., 44,1600-1603 (1952).64. Frank, C.E., Kraus, G. and Haefner, AJ., J. Polym. Sci., 10,441 (1953).65. Weber, C.D., Fox, L.A. and Gross, M.E. In: Skeist, 1. (Ed.), Handbook of Adhesives, 3rd
edn. Van Nostrand Rheinhold, 1990, pp. 270-283.66. Tsai, YM., Boerio, EJ. and Kim, D.K., J. Adhes., 61,247 (1997).67. Jayaseelan, S.K. and Van Oiij, W.L, In: Rubber Bonding 2000 Conference Proceedings,
Rapra Technology Ltd., UK, p. I.68. Gledhill, RA, Kinloch, AJ. and Shaw, S.w., J. Adhes., 11, 3 (1980).69. Boerio, FJ. and Williams, J.w., In: Proceedings of the 37th Annual Conference. SPI Rein.
Plastics/Composites Inst., Sec. 2D, 1982.70. Hare, c.n., J. Coat. Technol., 72(910), 21-27 (2000).71. LeBlanc, 0., Surf. Coat. Int., Aug., 288 (1991).72. Yang, J. and Garton, A, J. Appl. Polym. Sci., 48, 359-370 (1993).73. Courtney, PJ. and Verosky,C., Medical Device and Diagnostic Industry, Sept. 1999.74. Ryntz, R.A., Automot. Eng., 101(5), 37-40 (1993).75. Prater, TJ., Kaberline, S.L., Holubka, J.w. and Ryntz, RA, J. Coat. Technol., 68(857),
