Principle of adhesion

بسم الله الرحمن الرحيم

Principles of adhesion
Assist proff. Dr. Emad Farhan Alkhalidi
Msc, PhD conservative dentistry

Terminology

Adhesion or bonding: The forces or energies between atoms or molecules at an interface that hold two phases together.
Adherend: The surface or substrate that is adhered.
Adhesive/adherent: A material that can join substances together, resist separation and transmit loads across the bond .
Adhesive failure: The bond that fails at the interface between the two substrates.
Cohesive failure: The bond fails within one of the substrates, but not at the interface.

• Examples of adhesion and cohesion

• Adhesion failure v.s. cohesion failure
• A
• A
• B
• A
• A
• B
A: adherend
B: adhesive
Adhesive failure
Cohesive failure


Factors Affecting Adhesion
• Wetting
Wetting ability of an adhesive depends upon two factors:
Cleanliness of the adherend : Cleaner surface, greater adhesion.
Surface energy of the adherend: More surface energy, greater adhesion

Spreading  good wettability

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Wetting a surface with a material (an adhesive) brings the material into intimate association with that surface. Why?
Use of a fluid that flows into these irregularities provides contact over a greater part of the surface of the solid.

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Wettability
Wettability is a measure of the affinity of a liquid for a solid as indicated by the spreading of a drop.

spreading

 Spreading  good wettability


2. Contact Angle
refers to the angle formed between the surface of a liquid drop and its adherent surface.
The stronger the attraction of the adhesive for the adherent, the smaller will be the contact angle. The zero contact angle is the best to obtain wetting

q = 0°

q

q

3. Surface Energy

The surface tension of the liquid and the surface energy of the adherend, ultimately determine the degree of wetting that occurs.
Generally, the harder the surface is the higher the surface energy will be, which means the adhesive properties of the material will be higher.

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Surface Energy
The energy at the surface of a solid is greater than that of its interior because the outermost atoms are not equally attracted in all directions.
The increase in energy per unit area of surface is referred to as the surface energy or surface tension.

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Question
High- surface energy solids and low-surface energy liquids
 (good or bad) Wetting?
 (high or low) Contact angle?

Good wetting

Low contact angle

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Wetting ability is influenced by:
Cleanliness of the surface
water contamination, oxide film  surface energy of the adherend
Surface energy of adherends
Materials with low surface energy
Teflon (polytetrafluoroethylene), polymers, wax
Materials with high surface energy
Metals
Comparatively low surface energies liquids can spread well on solids of high surface energy.


4. Surface Contamination
The substrate surface should be clean, as contamination prevents the adhesion. Adhesive should be able to fill their regularities making the surface smooth allowing proper or intimate contact .

5. Water

The higher the water content, the poorer is the adhesion.
Water can react with both materials by the high polar group and hydrogen bond which can hamper the adhesion.

Indications for Use of Adhesives

• To treat carious and fractured tooth structure
• To restore erosion or abrasion defects in cervical areas
• To correct un esthetic contours, positions, dimensions, or shades of teeth
• For the repair of fractured porcelain, amalgam and resin restorations

5. For pit and fissure sealants

6. Bond composite restorations
7. Bond amalgam restorations
8. Lutting for crowns
9. Bond orthodontic brackets

Advantages of Bonding Techniques

• Adhesion of composite resin restorations to enamel and dentin
• Minimizes removal of sound tooth structure
• Management of dentin hypersensitivity
• Adhesion reduces microleakage at tooth restoration interface
• As a part of resin cements for bonding cast restorations
• Adhesion expands the range of esthetic possibilities


Advantages of Bonding Techniques
7. Bonding of porcelain restorations, e.g. porcelain inlays, onlays and veneers
8. Reinforces weakened tooth structure
9. Reduction in marginal staining
10. For repair of porcelain or composite
11. Bonding amalgam restorations to tooth
12. Repair of amalgam restorations
13. To bond orthodontic appliances.

Enamel and Dentine bonding Agent

Acid Etch Technique It is essential to enhance the adaptation , retention and seal of the restorative materials to enamel and dentin by the proper application of what is called resin – bonding agent.

This can be done by acid etch technique, in this technique the acid could be used to alter the surface of enamel to render it more respective to adhesion. acid etching removes about 10 (micron) deep by removal of inter rod or interprismatic substance core or periphery.

Microscopically the enamel

after acid etching result with
microscopic under cut.(5-50micron depth). Clinically if the enamel have been properly etched is described as having a ground glass or chalky white in color, this indicate successful etching . the more commercial enamel etching are (30- 40 %) concentration of phosphoric acid solution or gel mostly (37%) and applied for 1 min.





however; several recent studies indicate that the etching time could be reduced to 15 second without significant difference in enamel loss. After that, the acid was followed by the thorough washing with water (10-20) seconds,
if the washing is not enough
the surface may retain some
dissolved monocalicium
phosphate crystals precipitated which may reduce resin bonding strength.




After washing a good drying with air syringe is required. The enamel surface must be kept dry and we have to make good isolation around etching surface . any contamination with saliva or blood may interfere with bonding strength because it may enter inside the microscopic under cut.

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Advantage of enamel etching & bonding
increase retention & reduce the need to under cut with more conservative cavity .
reduce leakage around restoration margin.
Reinforcement of cusp of weak tooth.




acid etched enamel




a


b


c


d

Enamel bonding agent

These are low viscous unfilled resin apply directly over the etched enamel , it can flow in the microscopic under cut and polymerized to form resin enamel mechanical bond. This is know as resin tags. in chemical activated bonding agent, the composite is applied or placed when the bonding agent still fluid. The 2 resins will polymerize together .





e


f


g
Etch enamel with acid

Restore tooth with composite resin

while in light activated bonding agent can be polymerized before placing the light cure composite to ensure complete curing of tags layers .



the enamel bonding agent has less filler or without filler to flow in these undercut thus facilitate the entrance of composite to enamel.

Acid etched enamel

Restore tooth with composite resin

Dentin Bonding Agent

Adhesion of restoration material to enamel has become a routin of modern dentistry .
an early system for bonding composite to enamel were based on acid etching and micromechanical interlocking with bonding agent (unfilled) .

Dentin Bonding

Bonding to dentin has been proven more difficult and less reliable and predictable than to enamel. This is basically because of difference in morphologic, histologic and compositional differences between enamel and dentin

Enamel vs Dentin

In enamel, it is 95 % inorganic hydroxyapatite by volume, in dentin it is 50 percent. Dentin contains more water than does enamel

Enamel vs Dentin

1. Hydroxyapatite crystals
• have a regular pattern in enamel whereas in dentin, hydroxyapatite crystals are randomly arranged in an organic matrix
2. The presence of the smear layer
• makes wetting of the dentin by the adhesive more difficult
3. Dynamic structure of dentin
• shows changes due to aging, caries or operative procedures.


Enamel vs Dentin
4. Fluid present in dentinal tubules constantly flows outwards which reduces the adhesion of the composite resin.
5. Dentin contains dentinal tubules which contain vital processes of the pulp, odontoblasts.
• This makes the dentin a sensitive structure

Conditioning of Dentin

For removal or modification of the smear layer, many acids or/and calcium chelators are used

Conditioning of Dentin - Acids

• Commonly used 37 % phosphoric acid
• Not only removes the smear layer but also exposes the microporous collagen network into which resin monomer penetrates
• Usually, it forms exposed collagen fibrils which are covered with an amorphous layer, a combination of denatured collagen fibers and the collapsed residual collagen layer.

Priming of Dentin

Primers are agents which contain monomers having a hydrophilic end with affinity for exposed collagen fibrils and a hydrophobic end with affinity for adhesive resin
Commonly used primers have HEMA and 4-META monomers, dissolved in organic solvents.

Priming of Dentin

• Primers are used to increase the diffusion of resin into moist and demineralized dentin and thus optimal micromechanical bonding. For optimal penetration of primer into demineralized dentin, it should be applied in multiple coats.
• It is preferred to keep the dentin surface moist, otherwise collagen fibers get collapsed in dry condition resisting the entry for primer and adhesive resin


Moist vs Dry Dentin
• By etching dentin, the smear layer and minerals from it are removed, exposing the collagen fibers
• Areas from where minerals are removed are filled with water. This water acts as a plasticizer for collagen, keeping it in an expanded soft state. Thus, spaces for resin infiltration are also preserved.

Moist vs Dry Dentin

But these collagen fibers collapse when dry and if the organic matrix is denatured. This obstructs the resin from reaching the dentin surface and forming a hybrid layer.

Moist vs Dry Dentin

Wet Bonding
• primers consisting of hydrophilic resin monomers dissolved in water miscible organic solvents like ethanol and acetone are used
1. Acetone-based primers are dependent on a moist dentin surface for hybridization because the acetone displaces water present in the interfibrillar spaces of the collagen network and carry hydrophilic resin along with it for hybridization

Dry Bonding

2. water-based primers are used.
Water-based primers are not dependent on moist dentin because of their ability to self-wet a dried dentin surface and thus separating the collapsed collagen fibers.


Classification of Dental Adhesives
• Based on generations
• First generation bonding agent
• Second generation bonding agent
• Third generation bonding agent
• Fourth generation bonding agent
• Fifth generation bonding agent
• Sixth generation bonding agent
• Seventh generation bonding agent

Classification of Dental Adhesives

• Based on smear layer treatment
• Smear layer modifying agents
• Smear layer removing agents
• Smear layer dissolving agents.

Classification of Dental Adhesives

• Based on number of steps
• Three step
• Two step
• Single step.


Evolution of Dentin Bonding Agents

First Generation

• Developed in 1960s
• Relied on adhesion to smear layer
• No. of steps involved were two; etching of enamel + application of adhesive
• Did not recommend dentin etch
• Low bond strength (2–3 MPa).

First Generation

Second Generation

Developed in 1970s
Did not recommend dentin etching
Low bond strength (4–6 MPa)
Relied on adhesion to smear layer but some of them employed
Use of mild cleansing agent
No. of steps involved were two; etching of enamel + application of adhesive


Second Generation
• Problems with second generation bonding agents:
• 1. Low bond strength
2. Unstable interface between dentin and resin because of the insufficient knowledge about the smear layer.

Third Generation Dentin Bonding Systems

• Employed the concept of conditioning and priming before application of bonding agent
• Involved removal or modification of smear layer
• Three steps application, i.e.
• Etching of enamel + Application of primer + Bonding agent application
• High bond strength and reduced microleakage.

Third Generation Dentin Bonding Systems

Fourth Generation Dentin Bonding Agents

They were made available in the mid 1990s. Fourth generation bonding agents represent significant improvements in the field of adhesive dentistry. These agents are based on total etch technique and moist bonding concept.

Fourth Generation Dentin Bonding Agents

• Mechanism of bonding: The fourth “generation” is characterized by the process of hybridization at the interface of the dentin and the composite resin.
• Hybridization is the phenomenon of replacement of the hydroxyapatite and water at the dentin surface by resin. This resin, in combination with the collagen fibers, forms a hybrid layer.
• In other words, hybridization is the process of resin interlocking in the demineralized dentin surface
• This concept was given by Nakabayashi in 1982.


Fourth Generation Dentin Bonding Agents

Fourth Generation Dentin Bonding Agents

Fourth Generation Dentin Bonding Agents

Components of Fourth Generation Dentin Bonding Agents

• The fourth generation adhesives consist of:
• Conditioner (Etchant):
• Commonly used acids are 37 % phosphoric acid, nitric acid, maleic acid, oxalic acid, pyruvic acid, hydrochloric acid, citric acid or a chelating agent
• Use of conditioner/etchant causes removal or modification of the smear layer, demineralizes peritubular and intertubular dentin and exposes collagen fibrils

Components of Fourth Generation Dentin Bonding Agents

Primer: consist of monomers like HEMA (2-Hydroxyethyl methacrylate) and 4-META (4-Methacryloxyethyl trimellitate anhydride) dissolved in acetone or ethanol
they have both hydrophilic as well as hydrophobic ends which have affinity for the exposed collagen and resin respectively.

primer increases wettability of the dentin surface, bonding between the dentin and resin, and encourages monomer infiltration of demineralized peritubular and intertubular dentin.

Components of Fourth Generation Dentin Bonding Agents

Adhesive: The adhesive resin is a low viscosity, semi filled or unfilled resin which flows easily and matches the composite resin. Adhesive combines with the monomers to form a resin reinforced hybrid layer and resin tags to seal the dentin tubules


Components of Fourth Generation Dentin Bonding Agents

Fifth Generation Dentin Bonding Agents

• Developed in mid 1990s
• Uses two steps, i.e. Total etching + Application of primer and bonding agent
• Primer + Bonding agent are available in single bottle
• High bond strength
• Easy to use
• Reduced postoperative sensitivity

Fifth Generation Dentin Bonding Agents

• The basic differences between the fourth and fifth generation dentin bonding agents is the number of basic components of bottles.
• The fourth generation bonding system is available in two bottles, one consisting of the primer and the other the adhesive, the fifth generation dentin bonding agents are available in one bottle only.
• This makes the fifth generation bonding agents simpler and faster than the fourth generation systems

Sixth Generation Dentin Bonding Agents

• Self-etching primer and adhesive:
• Available in two bottles:
• Primer
• Adhesive
• Primer is applied prior to the adhesive
• Water is the solvent in these systems.
• Self etching adhesive
• Available in two bottles:
•  Primer
• Adhesive
• A drop from each bottle is taken, mixed and applied to the tooth surface, for example, Prompt L-pop.


Sixth Generation Dentin Bonding Agents
Mechanism of bonding: In these agents as soon as the decalcification process starts, infiltration of the empty spaces by the dentin bonding agent is initiated

Advantages of Self Etching Primers

• Comparable adhesion and bond strengths to enamel and dentin
• Reduces postoperative sensitivity because they etch and prime simultaneously
• They etch the dentin less aggressively than total etch products
• The demineralized dentin is infiltrated by resin during the etching process

Advantages of Self Etching Primers

5. Since they do not remove the smear layer, the tubules remain sealed, resulting in less sensitivity
6. They form a relatively thinner hybrid layer than traditional product which results in complete infiltration of the demineralized dentin by the resin monomers. This results in increased bond strength.


Advantages of Self Etching Primers
7. Much faster and simpler technique
8. Less technique sensitive as fewer number of steps are involved for the self etch system

Seventh Generation Bonding Agents

They achieve the same objective as the sixth generation systems except that they simplified the multiple sixth generation materials into a single component, single bottle one-step self-etch adhesive, thus avoiding any mistakes in mixing.

Seventh Generation Bonding Agents

Developed in late 2000s
All in one concept, i.e. components available as single component
Uses self etch primer
Good bond strength No postoperative sensitivity.

Seventh Generation Bonding Agents

Seventh generation bonding agents also have disinfecting and desensitizing properties.

Seventh Generation Bonding Agents

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