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Adhesion and Cohesion

Adhesion and Cohesion Hindawi Publishing Corporation International Journal of Dentistry Volume 2012, Article ID 951324, 8 pages doi:10.1155/2012/951324 Review Article J. Anthony von Fraunhofer School of Dentistry, University of Maryland, Baltimore, MD 21201, USA Correspondence should be addressed to J. Anthony von Fraunhofer, javonf99@gmail.com Received 18 October 2011; Accepted 14 November 2011 Academic Editor: Cornelis H. Pameijer Copyright © 2012 J. Anthony von Fraunhofer. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The phenomena of adhesion and cohesion are reviewed and discussed with particular reference to dentistry. This review considers the forces involved in cohesion and adhesion together with the mechanisms of adhesion and the underlying molecular processes involved in bonding of dissimilar materials. The forces involved in surface tension, surface wetting, chemical adhesion, dispersive adhesion, diffusive adhesion, and mechanical adhesion are reviewed in detail and examples relevant to adhesive dentistry and bonding are given. Substrate surface chemistry and its influence on adhesion, together with the properties of adhesive materials, are evaluated. The underlying mechanisms involved in adhesion failure are covered. The relevance of the adhesion zone and its impor- tance with regard to adhesive dentistry and bonding to enamel and dentin is discussed. 1. Introduction molecular attraction by which the particles of a body are uni- ted throughout the mass. In other words, adhesion is any at- Every clinician has experienced the failure of a restoration, be traction process between dissimilar molecular species, which it loosening of a crown, loss of an anterior Class V restora- have been brought into direct contact such that the adhesive tion, or leakage of a composite restoration. The procedure is “clings” or binds to the applied surface or substrate. The much the same for any such failure, namely, removal of resi- postsurgical complication of adhesions, involving soft tis- dual adhesive or luting agent and recementation of the resto- sues, will not be discussed here. ration. The clinical notes will describe the problem as, com- In contrast, cohesion is an attraction process that occurs monly, adhesive or cohesive failure based on a simple classi- between similar molecules, primarily as the result of chem- ficationsystemsuchasthatin Figure 1.The causes of such ical bonds that have formed between the individual compo- failures are seldom addressed by most clinicians. nents of the adhesive or luting agent. Thus, cohesion may Adhesion and cohesion are terms that are often confused be defined as the internal strength of an adhesive due to although these subjects are discussed in many standard texts various interactions within that adhesive that binds the mass in dental biomaterials science [1–3]. There are also many together, whereas adhesion is the bonding of one material to excellent texts and monographs on adhesion, cohesion, and another, namely, an adhesive to a substrate, due to a number interfacial reactions [4–6] together with a comprehensive of different possible interactions at the adhesive-substrate treatment in the on-line encyclopedia, Wikipedia. Since ad- surface interface. These differences are shown schematically hesion and cohesion play a very important role in the use of in Figure 2. In dentistry, when a restoration is cemented or luting agents, an in-depth discussion is appropriate in view bonded to a tooth, adhesive forces bind the luting agent to of the communications presented in this issue. the restoration on one side and to the tooth on the other side The Merriam-Webster dictionary has several definitions with cohesive forces operating within the luting agent itself, of the word “adhesion” but the most apposite here is the Figure 3. molecular attraction exerted between the surfaces of bodies The characteristics of chewing and bubble gums clearly in contact. This dictionary likewise has several definitions indicate the difference between cohesion and adhesion. Gum of the word “cohesion” but the most pertinent here is the holds together during mastication because of good cohesion 2 International Journal of Dentistry Applied force Adhesion: attractive forces operate at interface Tooth between dissimilar surfaces Restoration Tooth Tooth Restoration Restoration Luting agent Applied force Adhesive failure Cohesion: internal strength of a material Bulk of adhesive retained on only surface Restoration Tooth Restoration Tooth Cohesive failure Failure occurs within adhesive which is Figure 2: Adhesion and cohesion. retained on both surfaces Figure 1: Adhesive and cohesive joint failures. and, in the case of bubble gum, enables the gum to be blown Cohesive forces operate Tooth with into a bubble. These materials, however, exhibit poor adhe- cemented crown sion in that they do not readily stick to the teeth, oral tissues, or other surfaces, unless mechanical effects intervene. If, for example, gum while being chewed can lodge into under- Tooth Restoration cuts or between teeth, it can get locked in and may be torn away from the bulk of the gum, that is, mechanical inter- locking of the gum within the interproximal area is greater than the cohesive strength of the gum. Likewise, chewed gum does not stick well to smooth surfaces such as glass or Adhesive forces operate polished metal because of its poor adhesion. However, if the masticated and softened gum is pressed onto a rough surface, Figure 3: Adhesive and cohesive forces operating within a cemented the gum will distort and flow into gaps, rugosity and voids restoration. in the surface such that it “sticks,” often very tightly, to that surface, as most of us know when we try to scrape discarded gum off the soles of our shoes. (2) chemical bonds due to crosslinking of the polymer(s) Likewise, zinc phosphate cement has good cohesive within a resin-based material, strength but exhibits poor adhesion to smooth surfaces. In (3) intermolecular interactions between the adhesive particular, it does not bond, chemically to surfaces and its molecules, and bonding or adhesion, that is, its application as a luting agent, is possible only through mechanical interlocking at the inter- (4) mechanical bonds and interactions between the face with the restoration and that with the tooth. Zinc phos- molecules in the adhesive. phate cement, however, does possess good cohesive strength, These molecular interactions, really intermolecular for- even in thin films, so that when used as a luting agent for ces, affect the properties of the uncured (unset) adhesive, restorations subject to high masticatory stresses, it can sup- typically the consistency, flow properties, and viscosity of the port elastic deformation [7]. adhesive. When the adhesive sets or “cures” to a solid mass, In every situation involving an adhesive and a substrate, solidification occurs through bonds formed between the the combination of adhesion and cohesion determines the molecules in the adhesive, through formation of new bonds overall bonding effectiveness. The adhesive bond will fail if and by strengthening of existing bonds. This overall process the adhesive separates from the substrate or there is internal typically consists of crosslinking of short chain molecules to breakdown of the adhesive (i.e., cohesive failure), Figure 1. form longer chains and/or formation of 3-dimensional net- works of molecular chains. The latter is the common mech- 2. Forces in Cohesion anism involved in the setting of zinc oxide-based dental cements. It follows from this that the cohesive strength of an The cohesive strength of a luting agent or adhesive, regardless adhesive is significantly affected by the curing conditions of its chemical composition, is determined by a number of and, when curing/setting occurs under suboptimal condi- molecular forces: tions, the adhesive will lack cohesive strength. Suboptimal conditions during the setting or solidifica- (1) the chemical bonds within the adhesive material, tion process is a common concern in restorative dentistry International Journal of Dentistry 3 and all luting agents, regardless of composition and charac- those that do not, however, wetting depends on the relative teristics, must be protected against the effects of oral fluids surface energies of the adhesive and substrate materials. prior to and during the curing process to avoid detrimental Low surface energy materials such as poly(tetrafluoroethy- effects on the setting reactions. Ingress of saliva and oral flu- lene) or PTFE and silicone materials do not wet and are resis- ids into the adhesive during the setting process will adversely tant to adhesive bonding without special surface preparation, affect the curing reactions of both inorganic and organic hence the use of these polymers to manufacture nonstick adhesive materials, commonly reducing strength, bonding cookware and other nonstick surfaces. efficiency and the degree of cure. Thus, fluid ingress will not Wetting is the ability of a liquid to form an interface with only imperil the integrity and efficacy of the adhesive-sub- a solid surface and the degree of wetting is evaluated as the strate interactions at both the tooth and restoration inter- contact angle θ formed between the liquid and the solid faces but also decrease the cohesive strength of the adhesive. substrate surface. This is determined by both the surface The latter effect is important because the maximum load a tension of the liquid and the nature and condition of the sub- bond can withstand in clinical practice as well as in labora- strate surface. The smaller the contact angle and the lower tory strength tests may be dictated primarily by the cohesive the surface tension of the liquid, the greater the degree of strength of the adhesive, that is, under loading, the bond wetting, that is, the droplet of liquid will spread across the fractures due to cohesive failure of the adhesive rather than substrate surface provided the latter is clean and uncontam- failure of the adhesive-substrate bond. In other words, the inated, as shown in Figure 4. A clean surface allows good cohesive strength of the adhesive, and not the adhesion bet- wetting, that is, the contact angle θ is close to 0 , Figure 4(a). ween adhesive and the substrate, may be the limiting factor Therewillbeagreatercontact angle(θ is greater than 0 but in bond strength tests and in clinical practice. ◦ ◦ ◦ less than 90 , i.e., 0 <θ < 90 ) with a slightly contaminated surface, Figure 4(b), and the contact angle between the 3. Forces in Adhesion liquid and a contaminated surface or one with low surface energy will exceed 90 , Figure 4(c). The latter condition is Adhesion is the propensity of dissimilar particles and/or sur- sometimes referred to as dewetting and the liquid will form facestoadhereorbondtoone anotherand canbedivided droplets on the substrate surface. into three basic types, Table 1. Specific adhesion is achieved through molecular interactions between the adhesive and the The contact angle θ is a function of both dispersive adhe- substrate surface. The intermolecular forces produce specific sion (the interaction between the molecules in the adhesive adhesion although this can really be divided into three dif- and those of the solid, as discussed later) and the cohesion ferent types, namely, chemical adhesion, dispersive adhesion, within the liquid adhesive. If there is strong adhesion to the and diffusive adhesion, to which are added mechanical effects substrate surface and weak cohesion within the liquid, there in effective adhesion. However, a distinction must be made is a high degree of wetting, often termed lyophilic conditions. between weak intermolecular interactions and strong chem- Conversely, a combination of weak adhesion and strong coh- ical bonds. Although chemical bonds can form in a few sub- esion, referred to as lyophobic conditions, results in high strate/adhesive combinations, for example, epoxy resin and contact angles and poor wetting of the substrate surface, that aluminum, they are generally uncommon in dentistry except is, droplets form on the surface rather than a film of fluid. for those that occur between carboxylate-based luting agents A small contact angle indicates more adhesion is present and the calcium within dental hard tissues. When there are because there is a large contact area between the adhesive and chemical bonds within the adhesive joints, they can account the substrate, resulting in a greater overall substrate surface for up to 50% of all interactions although the long-term sta- energy and a high interactive force between the liquid and bility of these bonds is usually dependent on their resistance the substrate. to moisture. These relationships can be put in another way. When the In addition to the intermolecular and chemical adhesion ◦ ◦ surface is wetted, the contact angle is less than 90 (θ< 90 ), forces, micromechanical adhesion also can be involved in the substrate has high surface energy and the adhesion forces the overall adhesion phenomenon. In such cases, the adhe- between substrate and liquid are greater than the cohesive sive can effectively cling to a roughened substrate surface forces within the adhesive (i.e., the surface tension of the and increase overall adhesion, for example, chewing gum liquid, γ) and the liquid can spread over the substrate surface. attached to the soles of our shoes. If the surface has low energy (or is contaminated), θ> 90 and cohesion within the adhesive can exceed the adhesion 4. Mechanisms of Adhesion between liquid and substrate such that there is poor wetting or dewetting, with the liquid forming droplets on the surface. The strength of the adhesion between two materials depends on the interactions between the two materials, and the sur- Surface scientists express things in a different way and face area over which the two materials are in contact. As a refer to interfacial tension using the terms liquid-air inter- result, a number of factors enter into the overall adhesion facial tension, γ (i.e., the liquid’s surface tension), solid- LA system. liquid interfacial tension, γ (i.e., the surface tension bet- SL ween the solid and the liquid, which approximates to the surface adhesion between liquid and solid) and the solid-air 4.1. Contact Angle and Surface Tension. Materials that wet against each other tend to have a larger contact area than interfacial tension, γ (i.e., the surface tension between the SA 4 International Journal of Dentistry Table 1: Basic types of adhesion. Type Characteristics Specific Molecular attraction between surfaces in contact Adhesion arising from mechanical interlocking between the Mechanical adhesive and the substrate surface Optimal bonding between adhesive and substrate surface due to Effective combined effects of specific and mechanical adhesion θ ≤ 45 θ = 0 (a) (b) θ ≥ 105 θ = contact angle between liquid and solid (c) Figure 4: Liquid/surface contact angles for clean, slightly contaminated, and contaminated surfaces. LA Hexane Air Liquid Mineral oil SA γ SL Glycerin Substrate Water γ : Surface tension between solid and air SA 0 1020304050607080 γ : Surface tension between liquid and air LA Surface tension (dynes) γ : Surface tension between solid and liquid SL Figure 6: Surface tensions of common liquids. Figure 5: Interfacial tensions for a drop of liquid on a surface. For a contact angle of θ , these entities are related by solid and air, which approximates to the surface energy of the Young’s equation, solid), Figure 5. Surface tension is commonly expressed as dyne/cm, γ · Cos θ = γ − γ . (1) LA SA SL although it should really be given in the recommended SI units of N/m or J/m . Figure 6 indicates the relative surfaces If there is complete wetting of the substrate surface, that tensions of some common liquids. is, when θ = 0and Cos θ = 1, Young’s equation indicates International Journal of Dentistry 5 PTFE Polyethylene Glass Copper 0.25 nm 0.15 nm 0.4 nm 0 200 400 600 800 1000 Critical surface tension (dyne/cm) 0.2 nm 0.45 nm Figure 7: Critical surface tensions of solids (in dyne/cm). Ionic bond Covalent Metallic Hydrogen Van der bond bond Waal forces bond Figure 8: Bond energies and bond lengths of adhesive forces. that γ = γ − γ or γ ≤ γ . In other words, if the LA SA SL LA SA surface tension of the adhesive (γ ) is less than the surface LA energy of the substrate surface (γ ), the adhesive will spread bonding must be brought very close together and remain in SA this proximity for the bond to be stable. over the substrate. For maximum adhesion, the adhesive must completely cover or spread over the substrate, that is, Although the average lengths of hydrogen bonds are comparable to those of covalent and ionic bonds, they effectively wet it. The contact angle between the adhesive are an order of magnitude weaker. In the case of dental and the substrate is, therefore, a good indicator of adhesive behavior. cements, zinc polycarboxylates provide some chemical bond- ing between the carboxylate molecule of the cement and The value of γ when Cos θ = 1 is the critical surface SA hydroxyapatite mineral in the tooth, whereas bonding with energy (CSE) and equals the value of γ when the liquid SL just spreads over the surface. The critical surface tension of zinc phosphate cements is wholly mechanical in nature. several materials is shown in Figure 7. The very large differ- ence in CSE between say glass and PTFE and polyethylene 4.3. Dispersive Adhesion. In dispersive adhesion or physisor- indicates the difficulty of bonding to the two resins. ption, the surfaces of two materials are held together by van Wetting of the surface occurs when the adhesive surface der Waals forces. The latter are the attractive forces between tension (γ ) is less than the critical surface energy. This is SL two molecules, each of which has a region of small positive often expressed as the adhesion quotient which requires the and negative charge such that the molecules are polar with substrate surface energy (γ ) to exceed the surface tension SA respect to the average charge density of the molecule; it of the adhesive liquid (γ ) by 10 dyne/cm. If the reverse is SL should be noted that there may be multiple poles (regions of true, that is, (γ ≥ γ ), surface wetting is poor, adhesion is SL SA greater positive or negative charge) with larger and/or more reduced and the adhesive tends to pull away from the surface complex molecules. If these positive and negative poles are an during the curing process. inherent property of a molecule, they are known as Keesom The “take home message” here is that the adhesive liquid forces, whereas polarity, that is, a transient effect due to ran- must wet the substrate surface and such factors as surface dom electron motion within the molecules that cause a tem- contamination, surface conditioning, presence of moisture, porary concentration of electrons in one region are known as and the adhesive used all affect the adhesion between sub- London forces. London dispersion forces, which result from strate and adhesive. A small contact angle indicates more ad- statistical quantum mechanics, are particularly useful in ad- hesion is present because there is an interactive force between hesion because they arise without the need for either the the liquid and solid phases. adhesive or the substrate surface to have any permanent pola- rity. Adhesion in surface science commonly refers to disper- 4.2. Chemical Adhesion. If the adhesive and substrate can sive adhesion. form a compound at their interface or union, the ionic Although van der Waals bond lengths are longer than or covalent bonds that are formed result in a strong bond those of other molecular forces, see Figure 8, they are still between the two materials. A weaker bond is formed when short in absolute terms so that these forces only act over very there is hydrogen bonding, that is, a hydrogen atom in one small distances. About 99% of the work required to break molecule is attracted to an electron-donor atom such as nit- van der Waals bonds is performed once the joined surfaces rogen or oxygen in another molecule. Thus, when the surface are separated by more than a nanometer and, as a result, atoms of an adhesive and substrate form ionic, covalent, or the effectiveness of adhesion due to chemical or dispersive hydrogen bonds, chemical adhesion occurs. However, it can bonding is limited. Once a crack is initiated, it propagates be seen that whereas the strengths of these chemical bonds easily along the interface because of the brittle nature of the can be high, Figure 8, their lengths are short and therefore interfacial bonds and, consequently, greater contact surface for bonding to occur, surfaces with the potential for chemical areas often provide little difference in the measured adhesion. Average bond energy (kJ/mol) 6 International Journal of Dentistry This topic will be returned to when the adhesion zone is discussed. 4.4. Diffusive Adhesion. Some materials may merge or inter- mingle at the bonding interface by diffusion, typically when the molecules of both materials are mobile and/or soluble in each other, which typically is the case with polymer chains where one end of a molecule can diffuse into the other Substrate Adhesive material. This form of interaction, known as interdigitation, occurs when a resilient denture liner is processed onto an acrylic resin denture base, or when a fractured denture is repaired with acrylic resin. In such cases, bonding arises from the mutual solubility and interactions between methyl methacrylate (monomer) in the repair (or liner) material and the surface of the poly(methyl methacrylate) or acrylic base with diffusive adhesion (bonding) resulting from sections of polymer chains from the applied material interdigitating with the substrate surface. However, the mobility of the poly- mers strongly influences their ability to interdigitate to achi- eve diffusive bonding. Cross-linked polymers are less capable of diffusion and interdigitation because of their restricted mobility whereas non-cross-linked polymers have greater Adhesion zone mobility and interdigitate more readily. These differences Figure 9: Micromechanical adhesion (schematic). account for the fact that it is easier to bond a resilient liner to a recently processed acrylic base, or even during processing of the denture base, because the acrylic resin has a greater sur- face reactivity, that is, there is greater mobility of its surface viscosity. Water, for example, has a viscosity of 1 centiPoise polymer chains, than when attempting to reline a denture (cP) and that of alcohol is 1.2 cP. Many other fluids, base however, have much higher viscosities, for example, 9.22 cP Diffusive adhesion is also the mechanism involved in sin- for eugenol (oil of cloves), 1490 cP for glycerin and ∼10 cP tering as, for example, when metal or ceramic powders are for honey, and the very large difference in the viscosities of compressed and heated so that atoms diffuse from one par- honey and water explains why water flows far more readily ticle to the next to produce a solid mass. Diffusive bonding than honey. It should be noted that the SI units for viscosity occurs when atoms from one surface penetrate into an adja- are Pa s (Pascal seconds) and are equivalent in magnitude to cent surface while still being bound to their surface of origin. often quoted cP values. This is the mechanism involved in the fusing of porcelain Inevitably, micromechanical adhesion of a luting agent to metal in the fabrication of a PFM crown. Since diffusive to a surface is not simply a matter of wetting (i.e., contact adhesion requires interaction of atomic species between two angles) and the rheological or flow properties of the adhesive. surfaces, the greater the time that the two surfaces can inter- Other factors also enter into micromechanical adhesion, act, the more diffusion occurs and, accordingly, the stronger notably the electrostatic forces (both attractive and repulsive) the adhesion is between the two surfaces. that may be operating between the adhesive and the micro- topography of the substrate as well as a property of the ap- 4.5. Mechanical Adhesion. When uncured, adhesives are fluid plied fluid known as thixotropy. A thixotropic fluid is one that under the action of mechanical forces such as stirring, and they can flow over the substrate, filling the voids, rugo- sity, and pores of the surface and attach or “bond” to that vibration, and even kneading will temporarily transform to a surface by mechanical interlocking. This is often referred to state that has a lower viscosity and which exhibits better flow as micromechanical adhesion and is shown schematically in than the fluid in its static state. Thixotropic behavior is an Figure 9. important characteristic for endodontic (root canal) sealants Micromechanical adhesion is the primary mechanism for which are required to flow into a root canal, often under vib- luting of restorations to teeth with dental cements and prob- ration. Further, thixotropy is often incorporated into indus- ably also contributes significantly to bonding achieved with trial and domestic paints by additives such as silicic acid and resin-based adhesives as, for example, in fissure sealants and is probably present in various dental adhesive and cement direct bonding of restorative resins. The effectiveness of formulations. Thixotropy, when present in an adhesive, pro- micromechanical adhesion is determined in large part by the vides certain advantages to the overall adhesion system. In wetting of the substrate by the luting agent in that poor wet- particular, when a thixotropic adhesive is applied to a sub- ting of the substrate by the luting agent will inhibit good ap- strate surface, it will remain in place, even on vertical sur- position of cement and substrate. Further, the luting agent faces. Further, because adhesive flow is determined in part by must be able to flow into the surface voids, and so forth, and the mechanical forces imposed on the adhesive, there can be for this process to occur, the adhesive must have a low greater control of the adhesive film thickness combined with International Journal of Dentistry 7 improved flow into the microtopography of the substrate rent smear layer of organic debris will form. While this smear surface. layer can provide pulpal protection by reducing dentin per- meability, it hinders bonding. 5. The Adhesion Zone Bonding to dentin involves three stages, namely, condi- tioning, priming, and bonding, although some commercial It follows from the above that the adhesive bonded to a bonding systems combine two or more stages into a single substrate often has a modified molecular structure at the step. The conditioning stage involves modifying or removing bonding interface. This interfacial region is known as the the smear layer by acidic conditioners, the precise approach adhesion zone (Figure 9) and is characterized by the changes being determined by the bonding system used. Priming is the in the adhesive (and sometimes in the substrate) that may key step in dentin bonding because it promotes interactions arise from the bonding interactions. between hydrophobic restorative resins and hydrophilic den- The transition zone, the region between the bonding tin. Primers (dentin bonding agents) are bifunctional mole- interface and the bulk of the adhesive, is the area over cules, one end being a methacrylate group that bonds to resin which the chemical, mechanical, and optical properties of and the other a reactive group that reacts with dentin. Thus, the adhesive differ from those of the bulk adhesive. It varies primers are coupling agents, that is, they are bifunctional in thickness, from a few nanometers up to a few millime- molecules that primarily bond to calcium but may also inter- ters, with the thickness depending on the nature of the sub- act with collagen. The bonding (adhesive) agent is a fluid strate surface, the chemical composition, and physical char- resin that flows over and wets the primed surface to form an acteristics of the adhesive being applied and the curing con- effective bond when cured in situ. ditions. Where there are thick transition zones and/or narrow It should be noted that many manufacturers combine adhesion zones, the behavior of the entire bonding interface many of the conditioning, priming and bonding steps in their may be dependent on the properties of the transition zone systems. If the primer and conditioner are combined as with because the properties, notably strength, of the adhesive self-etching primers, the smear layer is incorporated within may be impaired because of inadequate cohesion within the the primer that directly contacts the dentin and constitutes adhesive. It is considerations such as these that determine, at the adhesive zone. The subsequently applied restorative resin least in part, the selection of the optimum luting agent for bonds to primed dentin when polymerized. An advantage the various combinations of luting agents and restorations with self-etching primers is that the dentin is maintained that were discussed by Pameijer in his review of luting agents in a moist condition throughout the bonding procedure [8]. although enamel etching with such systems is less effective than with phosphoric acid treatment. Alternatively, the pri- 6. Adhesive Dentistry mer and adhesive may be combined so that the applied mate- rial will infiltrate the collagenous network created by condi- Adhesive dentistry, whether it is the cementation (or luting) tioning to form a hybrid (resin-infiltrated reinforced) layer. of a restoration to a prepared tooth or restoration with a Subsequently, applied restorative resin, when polymerized, composite resin, involves the application and curing of an adhesive at the interface between tooth tissue and the restora- bonds everything together. tive material. Consequently, all of the aspects of adhesion and Although high bond strengths (≥20 MPa) to dentin may cohesion discussed above are involved in this process. be achieved, bond failures commonly involve cohesive frac- Restoration with a composite material has three principal ture of the dentin such that these systems are not infallible. steps. The first is the creation of microporosity in ena- They tend to be technique and material sensitive and may mel or dentin by acid etching either through application of require successive treatments for optimal bonding. Further, an etchant or by the in situ action of an etchant/primer/ad- regardless of high bond strengths, which suggest good adap- hesive. The second step is the application of a primer/adhe- tation to the dentin, good bonding and the absence of leakage sive which wets and penetrates the created microstructure are not synonymous and no system provides consistent leak- although because the surface energies of etched enamel and free restorations. etched dentin differ, different primers are required for the two substrates. Finally, a resin is applied to the primed sur- 8. Conclusions face so that when polymerized in situ, it micromechanically (i.e., there is mechanical adhesion) interlocks with the It follows from the above discussion that the performance of substrate microporosity together with a degree of chemical an adhesive in the luting of a restoration to a tooth will be bonding, with some materials exhibiting better chemical ad- dictated by a multiplicity of factors. Ideally, laboratory bond hesion than others. strength test values and the resistance of luted restorations to clinical loads will be maximized when the propagating 7. Dentin Bonding crack that causes bond failure has to travel through the adhesion zone rather than the bulk adhesive. In other Bonding to dentin presents greater problems than to enamel because it has a high organic content, a non-uniform compo- words, optimal retention is achieved when adhesion rather than the cohesive strength of the adhesive determines the sition and it is permeated by tubules. Further, after mechani- cal treatment, a 3–15 μm thick, featureless, and poorly adhe- overall strength of the bond [9]. Nevertheless, the mechanical 8 International Journal of Dentistry properties of the luting agent often can have a marked impact on the resistance of the luted restoration to applied forces when the thickness of the cement film is markedly greater than the width of the adhesion zone, as noted by in vivo determinations of cement film thicknesses beneath restorations [10]. References [1] J. M. Powers and R. L. Sakaguchi, Eds., Craig’s Restorative Den- tal Materials, Mosby-Elsevier, St Louis, Mo, USA, 12th edition, [2] J. F. McCabe and A. W. G. Wells, AppliedDentalMaterials, Blackwell, Oxford, UK, 9th edition, 2008. [3] K.J.Anusavice,Ed., Phillips’ Science of Dental Materials, Saunders-Elsevier, St Louis, Mo, USA, 11th edition, 2003. [4] Y. Hu, “Chapter 9: Friction and Adhesion,” in SEDL/Manuals, Monographs and Data Series / MONO7-EB / MONO10093M, ASTM Standard and Engineering Digital Library. ASTM International, West Conshohocken, Pa, USA, 2008. [5] G. L. Nelson, “Chapter 44: Adhesion,” in MNL17-14TH-EB, ASTM International, West Conshohocken, Pa, USA, 1995. [6] L. A. Girifalco, Statistical Mechanics of Solids,OxfordUniver- sity Press, New York, NY, USA, 2000. [7] B. Habib, J. A. Von Fraunhofer, and C. F. Driscoll, “Compar- ison of two luting agents used for the retention of cast dowel and cores,” Journal of Prosthodontics, vol. 14, no. 3, pp. 164– 169, 2005. [8] C. H. Pameijer, “A review of luting agents,” International Jour- nal of Dentistry, 2012. [9] J. L. Worley, R. C. Hamm, and J. A. von Fraunhofer, “Effects of cement on crown retention,” The Journal of Prosthetic Den- tistry, vol. 48, no. 3, pp. 289–291, 1982. [10] J. W. McLean and J. A. von Fraunhofer, “The estimation of cement film thickness by an in vivo technique,” British Dental Journal, vol. 131, no. 3, pp. 107–111, 1971. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Dentistry Pubmed Central

Adhesion and Cohesion

International Journal of Dentistry , Volume 2012 – Feb 21, 2012

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Pubmed Central
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Copyright © 2012 J. Anthony von Fraunhofer.
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1687-8728
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1687-8736
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10.1155/2012/951324
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Abstract

Hindawi Publishing Corporation International Journal of Dentistry Volume 2012, Article ID 951324, 8 pages doi:10.1155/2012/951324 Review Article J. Anthony von Fraunhofer School of Dentistry, University of Maryland, Baltimore, MD 21201, USA Correspondence should be addressed to J. Anthony von Fraunhofer, javonf99@gmail.com Received 18 October 2011; Accepted 14 November 2011 Academic Editor: Cornelis H. Pameijer Copyright © 2012 J. Anthony von Fraunhofer. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The phenomena of adhesion and cohesion are reviewed and discussed with particular reference to dentistry. This review considers the forces involved in cohesion and adhesion together with the mechanisms of adhesion and the underlying molecular processes involved in bonding of dissimilar materials. The forces involved in surface tension, surface wetting, chemical adhesion, dispersive adhesion, diffusive adhesion, and mechanical adhesion are reviewed in detail and examples relevant to adhesive dentistry and bonding are given. Substrate surface chemistry and its influence on adhesion, together with the properties of adhesive materials, are evaluated. The underlying mechanisms involved in adhesion failure are covered. The relevance of the adhesion zone and its impor- tance with regard to adhesive dentistry and bonding to enamel and dentin is discussed. 1. Introduction molecular attraction by which the particles of a body are uni- ted throughout the mass. In other words, adhesion is any at- Every clinician has experienced the failure of a restoration, be traction process between dissimilar molecular species, which it loosening of a crown, loss of an anterior Class V restora- have been brought into direct contact such that the adhesive tion, or leakage of a composite restoration. The procedure is “clings” or binds to the applied surface or substrate. The much the same for any such failure, namely, removal of resi- postsurgical complication of adhesions, involving soft tis- dual adhesive or luting agent and recementation of the resto- sues, will not be discussed here. ration. The clinical notes will describe the problem as, com- In contrast, cohesion is an attraction process that occurs monly, adhesive or cohesive failure based on a simple classi- between similar molecules, primarily as the result of chem- ficationsystemsuchasthatin Figure 1.The causes of such ical bonds that have formed between the individual compo- failures are seldom addressed by most clinicians. nents of the adhesive or luting agent. Thus, cohesion may Adhesion and cohesion are terms that are often confused be defined as the internal strength of an adhesive due to although these subjects are discussed in many standard texts various interactions within that adhesive that binds the mass in dental biomaterials science [1–3]. There are also many together, whereas adhesion is the bonding of one material to excellent texts and monographs on adhesion, cohesion, and another, namely, an adhesive to a substrate, due to a number interfacial reactions [4–6] together with a comprehensive of different possible interactions at the adhesive-substrate treatment in the on-line encyclopedia, Wikipedia. Since ad- surface interface. These differences are shown schematically hesion and cohesion play a very important role in the use of in Figure 2. In dentistry, when a restoration is cemented or luting agents, an in-depth discussion is appropriate in view bonded to a tooth, adhesive forces bind the luting agent to of the communications presented in this issue. the restoration on one side and to the tooth on the other side The Merriam-Webster dictionary has several definitions with cohesive forces operating within the luting agent itself, of the word “adhesion” but the most apposite here is the Figure 3. molecular attraction exerted between the surfaces of bodies The characteristics of chewing and bubble gums clearly in contact. This dictionary likewise has several definitions indicate the difference between cohesion and adhesion. Gum of the word “cohesion” but the most pertinent here is the holds together during mastication because of good cohesion 2 International Journal of Dentistry Applied force Adhesion: attractive forces operate at interface Tooth between dissimilar surfaces Restoration Tooth Tooth Restoration Restoration Luting agent Applied force Adhesive failure Cohesion: internal strength of a material Bulk of adhesive retained on only surface Restoration Tooth Restoration Tooth Cohesive failure Failure occurs within adhesive which is Figure 2: Adhesion and cohesion. retained on both surfaces Figure 1: Adhesive and cohesive joint failures. and, in the case of bubble gum, enables the gum to be blown Cohesive forces operate Tooth with into a bubble. These materials, however, exhibit poor adhe- cemented crown sion in that they do not readily stick to the teeth, oral tissues, or other surfaces, unless mechanical effects intervene. If, for example, gum while being chewed can lodge into under- Tooth Restoration cuts or between teeth, it can get locked in and may be torn away from the bulk of the gum, that is, mechanical inter- locking of the gum within the interproximal area is greater than the cohesive strength of the gum. Likewise, chewed gum does not stick well to smooth surfaces such as glass or Adhesive forces operate polished metal because of its poor adhesion. However, if the masticated and softened gum is pressed onto a rough surface, Figure 3: Adhesive and cohesive forces operating within a cemented the gum will distort and flow into gaps, rugosity and voids restoration. in the surface such that it “sticks,” often very tightly, to that surface, as most of us know when we try to scrape discarded gum off the soles of our shoes. (2) chemical bonds due to crosslinking of the polymer(s) Likewise, zinc phosphate cement has good cohesive within a resin-based material, strength but exhibits poor adhesion to smooth surfaces. In (3) intermolecular interactions between the adhesive particular, it does not bond, chemically to surfaces and its molecules, and bonding or adhesion, that is, its application as a luting agent, is possible only through mechanical interlocking at the inter- (4) mechanical bonds and interactions between the face with the restoration and that with the tooth. Zinc phos- molecules in the adhesive. phate cement, however, does possess good cohesive strength, These molecular interactions, really intermolecular for- even in thin films, so that when used as a luting agent for ces, affect the properties of the uncured (unset) adhesive, restorations subject to high masticatory stresses, it can sup- typically the consistency, flow properties, and viscosity of the port elastic deformation [7]. adhesive. When the adhesive sets or “cures” to a solid mass, In every situation involving an adhesive and a substrate, solidification occurs through bonds formed between the the combination of adhesion and cohesion determines the molecules in the adhesive, through formation of new bonds overall bonding effectiveness. The adhesive bond will fail if and by strengthening of existing bonds. This overall process the adhesive separates from the substrate or there is internal typically consists of crosslinking of short chain molecules to breakdown of the adhesive (i.e., cohesive failure), Figure 1. form longer chains and/or formation of 3-dimensional net- works of molecular chains. The latter is the common mech- 2. Forces in Cohesion anism involved in the setting of zinc oxide-based dental cements. It follows from this that the cohesive strength of an The cohesive strength of a luting agent or adhesive, regardless adhesive is significantly affected by the curing conditions of its chemical composition, is determined by a number of and, when curing/setting occurs under suboptimal condi- molecular forces: tions, the adhesive will lack cohesive strength. Suboptimal conditions during the setting or solidifica- (1) the chemical bonds within the adhesive material, tion process is a common concern in restorative dentistry International Journal of Dentistry 3 and all luting agents, regardless of composition and charac- those that do not, however, wetting depends on the relative teristics, must be protected against the effects of oral fluids surface energies of the adhesive and substrate materials. prior to and during the curing process to avoid detrimental Low surface energy materials such as poly(tetrafluoroethy- effects on the setting reactions. Ingress of saliva and oral flu- lene) or PTFE and silicone materials do not wet and are resis- ids into the adhesive during the setting process will adversely tant to adhesive bonding without special surface preparation, affect the curing reactions of both inorganic and organic hence the use of these polymers to manufacture nonstick adhesive materials, commonly reducing strength, bonding cookware and other nonstick surfaces. efficiency and the degree of cure. Thus, fluid ingress will not Wetting is the ability of a liquid to form an interface with only imperil the integrity and efficacy of the adhesive-sub- a solid surface and the degree of wetting is evaluated as the strate interactions at both the tooth and restoration inter- contact angle θ formed between the liquid and the solid faces but also decrease the cohesive strength of the adhesive. substrate surface. This is determined by both the surface The latter effect is important because the maximum load a tension of the liquid and the nature and condition of the sub- bond can withstand in clinical practice as well as in labora- strate surface. The smaller the contact angle and the lower tory strength tests may be dictated primarily by the cohesive the surface tension of the liquid, the greater the degree of strength of the adhesive, that is, under loading, the bond wetting, that is, the droplet of liquid will spread across the fractures due to cohesive failure of the adhesive rather than substrate surface provided the latter is clean and uncontam- failure of the adhesive-substrate bond. In other words, the inated, as shown in Figure 4. A clean surface allows good cohesive strength of the adhesive, and not the adhesion bet- wetting, that is, the contact angle θ is close to 0 , Figure 4(a). ween adhesive and the substrate, may be the limiting factor Therewillbeagreatercontact angle(θ is greater than 0 but in bond strength tests and in clinical practice. ◦ ◦ ◦ less than 90 , i.e., 0 <θ < 90 ) with a slightly contaminated surface, Figure 4(b), and the contact angle between the 3. Forces in Adhesion liquid and a contaminated surface or one with low surface energy will exceed 90 , Figure 4(c). The latter condition is Adhesion is the propensity of dissimilar particles and/or sur- sometimes referred to as dewetting and the liquid will form facestoadhereorbondtoone anotherand canbedivided droplets on the substrate surface. into three basic types, Table 1. Specific adhesion is achieved through molecular interactions between the adhesive and the The contact angle θ is a function of both dispersive adhe- substrate surface. The intermolecular forces produce specific sion (the interaction between the molecules in the adhesive adhesion although this can really be divided into three dif- and those of the solid, as discussed later) and the cohesion ferent types, namely, chemical adhesion, dispersive adhesion, within the liquid adhesive. If there is strong adhesion to the and diffusive adhesion, to which are added mechanical effects substrate surface and weak cohesion within the liquid, there in effective adhesion. However, a distinction must be made is a high degree of wetting, often termed lyophilic conditions. between weak intermolecular interactions and strong chem- Conversely, a combination of weak adhesion and strong coh- ical bonds. Although chemical bonds can form in a few sub- esion, referred to as lyophobic conditions, results in high strate/adhesive combinations, for example, epoxy resin and contact angles and poor wetting of the substrate surface, that aluminum, they are generally uncommon in dentistry except is, droplets form on the surface rather than a film of fluid. for those that occur between carboxylate-based luting agents A small contact angle indicates more adhesion is present and the calcium within dental hard tissues. When there are because there is a large contact area between the adhesive and chemical bonds within the adhesive joints, they can account the substrate, resulting in a greater overall substrate surface for up to 50% of all interactions although the long-term sta- energy and a high interactive force between the liquid and bility of these bonds is usually dependent on their resistance the substrate. to moisture. These relationships can be put in another way. When the In addition to the intermolecular and chemical adhesion ◦ ◦ surface is wetted, the contact angle is less than 90 (θ< 90 ), forces, micromechanical adhesion also can be involved in the substrate has high surface energy and the adhesion forces the overall adhesion phenomenon. In such cases, the adhe- between substrate and liquid are greater than the cohesive sive can effectively cling to a roughened substrate surface forces within the adhesive (i.e., the surface tension of the and increase overall adhesion, for example, chewing gum liquid, γ) and the liquid can spread over the substrate surface. attached to the soles of our shoes. If the surface has low energy (or is contaminated), θ> 90 and cohesion within the adhesive can exceed the adhesion 4. Mechanisms of Adhesion between liquid and substrate such that there is poor wetting or dewetting, with the liquid forming droplets on the surface. The strength of the adhesion between two materials depends on the interactions between the two materials, and the sur- Surface scientists express things in a different way and face area over which the two materials are in contact. As a refer to interfacial tension using the terms liquid-air inter- result, a number of factors enter into the overall adhesion facial tension, γ (i.e., the liquid’s surface tension), solid- LA system. liquid interfacial tension, γ (i.e., the surface tension bet- SL ween the solid and the liquid, which approximates to the surface adhesion between liquid and solid) and the solid-air 4.1. Contact Angle and Surface Tension. Materials that wet against each other tend to have a larger contact area than interfacial tension, γ (i.e., the surface tension between the SA 4 International Journal of Dentistry Table 1: Basic types of adhesion. Type Characteristics Specific Molecular attraction between surfaces in contact Adhesion arising from mechanical interlocking between the Mechanical adhesive and the substrate surface Optimal bonding between adhesive and substrate surface due to Effective combined effects of specific and mechanical adhesion θ ≤ 45 θ = 0 (a) (b) θ ≥ 105 θ = contact angle between liquid and solid (c) Figure 4: Liquid/surface contact angles for clean, slightly contaminated, and contaminated surfaces. LA Hexane Air Liquid Mineral oil SA γ SL Glycerin Substrate Water γ : Surface tension between solid and air SA 0 1020304050607080 γ : Surface tension between liquid and air LA Surface tension (dynes) γ : Surface tension between solid and liquid SL Figure 6: Surface tensions of common liquids. Figure 5: Interfacial tensions for a drop of liquid on a surface. For a contact angle of θ , these entities are related by solid and air, which approximates to the surface energy of the Young’s equation, solid), Figure 5. Surface tension is commonly expressed as dyne/cm, γ · Cos θ = γ − γ . (1) LA SA SL although it should really be given in the recommended SI units of N/m or J/m . Figure 6 indicates the relative surfaces If there is complete wetting of the substrate surface, that tensions of some common liquids. is, when θ = 0and Cos θ = 1, Young’s equation indicates International Journal of Dentistry 5 PTFE Polyethylene Glass Copper 0.25 nm 0.15 nm 0.4 nm 0 200 400 600 800 1000 Critical surface tension (dyne/cm) 0.2 nm 0.45 nm Figure 7: Critical surface tensions of solids (in dyne/cm). Ionic bond Covalent Metallic Hydrogen Van der bond bond Waal forces bond Figure 8: Bond energies and bond lengths of adhesive forces. that γ = γ − γ or γ ≤ γ . In other words, if the LA SA SL LA SA surface tension of the adhesive (γ ) is less than the surface LA energy of the substrate surface (γ ), the adhesive will spread bonding must be brought very close together and remain in SA this proximity for the bond to be stable. over the substrate. For maximum adhesion, the adhesive must completely cover or spread over the substrate, that is, Although the average lengths of hydrogen bonds are comparable to those of covalent and ionic bonds, they effectively wet it. The contact angle between the adhesive are an order of magnitude weaker. In the case of dental and the substrate is, therefore, a good indicator of adhesive behavior. cements, zinc polycarboxylates provide some chemical bond- ing between the carboxylate molecule of the cement and The value of γ when Cos θ = 1 is the critical surface SA hydroxyapatite mineral in the tooth, whereas bonding with energy (CSE) and equals the value of γ when the liquid SL just spreads over the surface. The critical surface tension of zinc phosphate cements is wholly mechanical in nature. several materials is shown in Figure 7. The very large differ- ence in CSE between say glass and PTFE and polyethylene 4.3. Dispersive Adhesion. In dispersive adhesion or physisor- indicates the difficulty of bonding to the two resins. ption, the surfaces of two materials are held together by van Wetting of the surface occurs when the adhesive surface der Waals forces. The latter are the attractive forces between tension (γ ) is less than the critical surface energy. This is SL two molecules, each of which has a region of small positive often expressed as the adhesion quotient which requires the and negative charge such that the molecules are polar with substrate surface energy (γ ) to exceed the surface tension SA respect to the average charge density of the molecule; it of the adhesive liquid (γ ) by 10 dyne/cm. If the reverse is SL should be noted that there may be multiple poles (regions of true, that is, (γ ≥ γ ), surface wetting is poor, adhesion is SL SA greater positive or negative charge) with larger and/or more reduced and the adhesive tends to pull away from the surface complex molecules. If these positive and negative poles are an during the curing process. inherent property of a molecule, they are known as Keesom The “take home message” here is that the adhesive liquid forces, whereas polarity, that is, a transient effect due to ran- must wet the substrate surface and such factors as surface dom electron motion within the molecules that cause a tem- contamination, surface conditioning, presence of moisture, porary concentration of electrons in one region are known as and the adhesive used all affect the adhesion between sub- London forces. London dispersion forces, which result from strate and adhesive. A small contact angle indicates more ad- statistical quantum mechanics, are particularly useful in ad- hesion is present because there is an interactive force between hesion because they arise without the need for either the the liquid and solid phases. adhesive or the substrate surface to have any permanent pola- rity. Adhesion in surface science commonly refers to disper- 4.2. Chemical Adhesion. If the adhesive and substrate can sive adhesion. form a compound at their interface or union, the ionic Although van der Waals bond lengths are longer than or covalent bonds that are formed result in a strong bond those of other molecular forces, see Figure 8, they are still between the two materials. A weaker bond is formed when short in absolute terms so that these forces only act over very there is hydrogen bonding, that is, a hydrogen atom in one small distances. About 99% of the work required to break molecule is attracted to an electron-donor atom such as nit- van der Waals bonds is performed once the joined surfaces rogen or oxygen in another molecule. Thus, when the surface are separated by more than a nanometer and, as a result, atoms of an adhesive and substrate form ionic, covalent, or the effectiveness of adhesion due to chemical or dispersive hydrogen bonds, chemical adhesion occurs. However, it can bonding is limited. Once a crack is initiated, it propagates be seen that whereas the strengths of these chemical bonds easily along the interface because of the brittle nature of the can be high, Figure 8, their lengths are short and therefore interfacial bonds and, consequently, greater contact surface for bonding to occur, surfaces with the potential for chemical areas often provide little difference in the measured adhesion. Average bond energy (kJ/mol) 6 International Journal of Dentistry This topic will be returned to when the adhesion zone is discussed. 4.4. Diffusive Adhesion. Some materials may merge or inter- mingle at the bonding interface by diffusion, typically when the molecules of both materials are mobile and/or soluble in each other, which typically is the case with polymer chains where one end of a molecule can diffuse into the other Substrate Adhesive material. This form of interaction, known as interdigitation, occurs when a resilient denture liner is processed onto an acrylic resin denture base, or when a fractured denture is repaired with acrylic resin. In such cases, bonding arises from the mutual solubility and interactions between methyl methacrylate (monomer) in the repair (or liner) material and the surface of the poly(methyl methacrylate) or acrylic base with diffusive adhesion (bonding) resulting from sections of polymer chains from the applied material interdigitating with the substrate surface. However, the mobility of the poly- mers strongly influences their ability to interdigitate to achi- eve diffusive bonding. Cross-linked polymers are less capable of diffusion and interdigitation because of their restricted mobility whereas non-cross-linked polymers have greater Adhesion zone mobility and interdigitate more readily. These differences Figure 9: Micromechanical adhesion (schematic). account for the fact that it is easier to bond a resilient liner to a recently processed acrylic base, or even during processing of the denture base, because the acrylic resin has a greater sur- face reactivity, that is, there is greater mobility of its surface viscosity. Water, for example, has a viscosity of 1 centiPoise polymer chains, than when attempting to reline a denture (cP) and that of alcohol is 1.2 cP. Many other fluids, base however, have much higher viscosities, for example, 9.22 cP Diffusive adhesion is also the mechanism involved in sin- for eugenol (oil of cloves), 1490 cP for glycerin and ∼10 cP tering as, for example, when metal or ceramic powders are for honey, and the very large difference in the viscosities of compressed and heated so that atoms diffuse from one par- honey and water explains why water flows far more readily ticle to the next to produce a solid mass. Diffusive bonding than honey. It should be noted that the SI units for viscosity occurs when atoms from one surface penetrate into an adja- are Pa s (Pascal seconds) and are equivalent in magnitude to cent surface while still being bound to their surface of origin. often quoted cP values. This is the mechanism involved in the fusing of porcelain Inevitably, micromechanical adhesion of a luting agent to metal in the fabrication of a PFM crown. Since diffusive to a surface is not simply a matter of wetting (i.e., contact adhesion requires interaction of atomic species between two angles) and the rheological or flow properties of the adhesive. surfaces, the greater the time that the two surfaces can inter- Other factors also enter into micromechanical adhesion, act, the more diffusion occurs and, accordingly, the stronger notably the electrostatic forces (both attractive and repulsive) the adhesion is between the two surfaces. that may be operating between the adhesive and the micro- topography of the substrate as well as a property of the ap- 4.5. Mechanical Adhesion. When uncured, adhesives are fluid plied fluid known as thixotropy. A thixotropic fluid is one that under the action of mechanical forces such as stirring, and they can flow over the substrate, filling the voids, rugo- sity, and pores of the surface and attach or “bond” to that vibration, and even kneading will temporarily transform to a surface by mechanical interlocking. This is often referred to state that has a lower viscosity and which exhibits better flow as micromechanical adhesion and is shown schematically in than the fluid in its static state. Thixotropic behavior is an Figure 9. important characteristic for endodontic (root canal) sealants Micromechanical adhesion is the primary mechanism for which are required to flow into a root canal, often under vib- luting of restorations to teeth with dental cements and prob- ration. Further, thixotropy is often incorporated into indus- ably also contributes significantly to bonding achieved with trial and domestic paints by additives such as silicic acid and resin-based adhesives as, for example, in fissure sealants and is probably present in various dental adhesive and cement direct bonding of restorative resins. The effectiveness of formulations. Thixotropy, when present in an adhesive, pro- micromechanical adhesion is determined in large part by the vides certain advantages to the overall adhesion system. In wetting of the substrate by the luting agent in that poor wet- particular, when a thixotropic adhesive is applied to a sub- ting of the substrate by the luting agent will inhibit good ap- strate surface, it will remain in place, even on vertical sur- position of cement and substrate. Further, the luting agent faces. Further, because adhesive flow is determined in part by must be able to flow into the surface voids, and so forth, and the mechanical forces imposed on the adhesive, there can be for this process to occur, the adhesive must have a low greater control of the adhesive film thickness combined with International Journal of Dentistry 7 improved flow into the microtopography of the substrate rent smear layer of organic debris will form. While this smear surface. layer can provide pulpal protection by reducing dentin per- meability, it hinders bonding. 5. The Adhesion Zone Bonding to dentin involves three stages, namely, condi- tioning, priming, and bonding, although some commercial It follows from the above that the adhesive bonded to a bonding systems combine two or more stages into a single substrate often has a modified molecular structure at the step. The conditioning stage involves modifying or removing bonding interface. This interfacial region is known as the the smear layer by acidic conditioners, the precise approach adhesion zone (Figure 9) and is characterized by the changes being determined by the bonding system used. Priming is the in the adhesive (and sometimes in the substrate) that may key step in dentin bonding because it promotes interactions arise from the bonding interactions. between hydrophobic restorative resins and hydrophilic den- The transition zone, the region between the bonding tin. Primers (dentin bonding agents) are bifunctional mole- interface and the bulk of the adhesive, is the area over cules, one end being a methacrylate group that bonds to resin which the chemical, mechanical, and optical properties of and the other a reactive group that reacts with dentin. Thus, the adhesive differ from those of the bulk adhesive. It varies primers are coupling agents, that is, they are bifunctional in thickness, from a few nanometers up to a few millime- molecules that primarily bond to calcium but may also inter- ters, with the thickness depending on the nature of the sub- act with collagen. The bonding (adhesive) agent is a fluid strate surface, the chemical composition, and physical char- resin that flows over and wets the primed surface to form an acteristics of the adhesive being applied and the curing con- effective bond when cured in situ. ditions. Where there are thick transition zones and/or narrow It should be noted that many manufacturers combine adhesion zones, the behavior of the entire bonding interface many of the conditioning, priming and bonding steps in their may be dependent on the properties of the transition zone systems. If the primer and conditioner are combined as with because the properties, notably strength, of the adhesive self-etching primers, the smear layer is incorporated within may be impaired because of inadequate cohesion within the the primer that directly contacts the dentin and constitutes adhesive. It is considerations such as these that determine, at the adhesive zone. The subsequently applied restorative resin least in part, the selection of the optimum luting agent for bonds to primed dentin when polymerized. An advantage the various combinations of luting agents and restorations with self-etching primers is that the dentin is maintained that were discussed by Pameijer in his review of luting agents in a moist condition throughout the bonding procedure [8]. although enamel etching with such systems is less effective than with phosphoric acid treatment. Alternatively, the pri- 6. Adhesive Dentistry mer and adhesive may be combined so that the applied mate- rial will infiltrate the collagenous network created by condi- Adhesive dentistry, whether it is the cementation (or luting) tioning to form a hybrid (resin-infiltrated reinforced) layer. of a restoration to a prepared tooth or restoration with a Subsequently, applied restorative resin, when polymerized, composite resin, involves the application and curing of an adhesive at the interface between tooth tissue and the restora- bonds everything together. tive material. Consequently, all of the aspects of adhesion and Although high bond strengths (≥20 MPa) to dentin may cohesion discussed above are involved in this process. be achieved, bond failures commonly involve cohesive frac- Restoration with a composite material has three principal ture of the dentin such that these systems are not infallible. steps. The first is the creation of microporosity in ena- They tend to be technique and material sensitive and may mel or dentin by acid etching either through application of require successive treatments for optimal bonding. Further, an etchant or by the in situ action of an etchant/primer/ad- regardless of high bond strengths, which suggest good adap- hesive. The second step is the application of a primer/adhe- tation to the dentin, good bonding and the absence of leakage sive which wets and penetrates the created microstructure are not synonymous and no system provides consistent leak- although because the surface energies of etched enamel and free restorations. etched dentin differ, different primers are required for the two substrates. Finally, a resin is applied to the primed sur- 8. Conclusions face so that when polymerized in situ, it micromechanically (i.e., there is mechanical adhesion) interlocks with the It follows from the above discussion that the performance of substrate microporosity together with a degree of chemical an adhesive in the luting of a restoration to a tooth will be bonding, with some materials exhibiting better chemical ad- dictated by a multiplicity of factors. Ideally, laboratory bond hesion than others. strength test values and the resistance of luted restorations to clinical loads will be maximized when the propagating 7. Dentin Bonding crack that causes bond failure has to travel through the adhesion zone rather than the bulk adhesive. In other Bonding to dentin presents greater problems than to enamel because it has a high organic content, a non-uniform compo- words, optimal retention is achieved when adhesion rather than the cohesive strength of the adhesive determines the sition and it is permeated by tubules. Further, after mechani- cal treatment, a 3–15 μm thick, featureless, and poorly adhe- overall strength of the bond [9]. Nevertheless, the mechanical 8 International Journal of Dentistry properties of the luting agent often can have a marked impact on the resistance of the luted restoration to applied forces when the thickness of the cement film is markedly greater than the width of the adhesion zone, as noted by in vivo determinations of cement film thicknesses beneath restorations [10]. References [1] J. M. Powers and R. L. Sakaguchi, Eds., Craig’s Restorative Den- tal Materials, Mosby-Elsevier, St Louis, Mo, USA, 12th edition, [2] J. F. McCabe and A. W. G. Wells, AppliedDentalMaterials, Blackwell, Oxford, UK, 9th edition, 2008. [3] K.J.Anusavice,Ed., Phillips’ Science of Dental Materials, Saunders-Elsevier, St Louis, Mo, USA, 11th edition, 2003. [4] Y. Hu, “Chapter 9: Friction and Adhesion,” in SEDL/Manuals, Monographs and Data Series / MONO7-EB / MONO10093M, ASTM Standard and Engineering Digital Library. ASTM International, West Conshohocken, Pa, USA, 2008. [5] G. L. Nelson, “Chapter 44: Adhesion,” in MNL17-14TH-EB, ASTM International, West Conshohocken, Pa, USA, 1995. [6] L. A. Girifalco, Statistical Mechanics of Solids,OxfordUniver- sity Press, New York, NY, USA, 2000. [7] B. Habib, J. A. Von Fraunhofer, and C. F. Driscoll, “Compar- ison of two luting agents used for the retention of cast dowel and cores,” Journal of Prosthodontics, vol. 14, no. 3, pp. 164– 169, 2005. [8] C. H. Pameijer, “A review of luting agents,” International Jour- nal of Dentistry, 2012. [9] J. L. Worley, R. C. Hamm, and J. A. von Fraunhofer, “Effects of cement on crown retention,” The Journal of Prosthetic Den- tistry, vol. 48, no. 3, pp. 289–291, 1982. [10] J. W. McLean and J. A. von Fraunhofer, “The estimation of cement film thickness by an in vivo technique,” British Dental Journal, vol. 131, no. 3, pp. 107–111, 1971.

Journal

International Journal of DentistryPubmed Central

Published: Feb 21, 2012

References