Bonding and Applied Surface Phenomena 25-3...Mechanisms of Bonding II. Chemical Bonding Cohesion...
Transcript of Bonding and Applied Surface Phenomena 25-3...Mechanisms of Bonding II. Chemical Bonding Cohesion...
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Bonding and Applied
Surface Phenomena
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Dental applications
• Leakage around restorations.
• Retention of dentures.
• Plaque attachment to tooth structure.
• Joining of metallic components.
• Pits & Fissure sealants.
• Cementation of crowns & bridges
• Bonding of orthodontic brackets.
• Welding of gold foil.
• Enamel and dentin adhesives.
• Fluoride application.
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Mechanisms of Bonding
I. Mechanical attachment
Gross Mechanical Micro-Mechanical
1.Posts & pins.
2.Clasps of RPD.
3.Undercuts.
1.Enamel & dentin
bonding.
2.Pits & fissures
sealants.
II. Chemical Bonding
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Mechanisms of Bonding
II. Chemical Bonding
Cohesion Adhesion
I. Mechanical attachment
• Adhesion : attraction or bonding between dissimilar molecules
of two materials.
• Cohesion: attraction between similar atoms and molecules in two
different surfaces at room temperature when brought in close
proximity
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Mechanisms of Bonding
II. Chemical Bonding
Cohesion Adhesion
Factors affecting: 1. Degree of proximity.
2. Surface contamination and adsorbed
gases.
3. Heating of metallic surfaces
(degassing).
4. Microstructure of the cohering
surfaces, crystalline solids show low
coherence capacity while cold worked
surface have high tendency to cohesion.
Types:
I. Mechanical attachment
Chemical Adhesion
True adhesion
(Chemo-sorption)
Physical Adhesion
1. Formation of surface
oxides. e.g. Au, Ag & Pt.
2. Glass Ionomer.
Capillary phenomena and
its applications. E.g
Denture base
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Adhesion
Adherend (substrate)
Adherend (substrate)
Adhesive
Adhesive
junction
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Definitions Adhesion:
• Adhesive: the material (usually liquid) that used to produce
adhesion.
• Adherend: the material (surface) to which the adhesive is
applied.
• Adhesive junction: the transition layer between the two
surfaces of adhesive and adherend.
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Factors affecting strength of
adhesive junction
1. Type of the bond.
2. Surface roughness.
3. Surface cleanliness.
4. Thickness of adhesive
film.
5. Thermal stresses.
6. contraction stresses of the
adhesive
7. Area of contact.
8. Wettability.
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Factors affecting strength of
adhesive junction
1. type of bond formed:
Chemical (true adhesion)
Primary bonds between adhesive and adherend produce
stronger adhesion than if secondary bonds are formed
(Soldered Joint –metallic bond- is stronger than glued joint -
polarization-).
Mechanical (attachment) By the flow and penetration of a liquid
adhesive into microscopic or sub-microscopic irregularities
in the surface of the adherend or substrate
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2. Surface roughness.
• If roughness is irregular and deep, → air pockets →
prevent adhesive from penetrating→ no intimate contact
between adherend and adhesive→ weak adhesive bond.
• if surface roughness is regular and shallow, → no chance for
air pockets formation → intimate contact between
adherend and adhesive→ good adhesive bond.
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3) Cleanliness on the adherend:
Debris or surface contaminations prevent intimate contact which is necessary to produce adhesion.
** Adhesion to clean and dry surface of enamel and dentin is better than adhesion to wet contaminated one.
4) Stresses due to setting contraction of adhesive:
Contraction of liquid adhesives during setting
→ creation of stresses at the interface
→ decreases the strength of adhesion.
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5) Thermal stresses:
Different thermal coefficients of adhesive and adherend + changes in
temperature → stresses in the bond.
Close matching is required to minimize stresses and increases the strength
of adhesion.
6)Thickness of the adhesive: Thinner adhesive film → stronger adhesive junction,
** less air voids are present.
** Less thermal stresses
** Less stresses due to setting contraction
of adhesive.
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7)Wettability
• Definition: the ability of the adhesive liquid
to spread over the adherend surface.
• Contact angle: measure of spread-ability or
wettability. It’s the angle formed by the
adhesive (liquid) with the adherend (solid) at
their interface.
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Wettability
Factors affecting wettability: 1. Surface energy of the solid adherend
2. Surface tension of the liquid adhesive.
3. Viscosity.
4.Surface irregularities of adheren.
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Wettability
• Factors affecting wettability:
1. Surface energy of the solid adeherant
– In solids, the surface energy is greater than the internal energy, because the outermost atoms are not equally attracted in all directions.
Solids tend to reduce this surface energy by adsorption of atoms or molecules.
- Surface energy wettability - contamination of surface wettability
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Examples:
1. Metals usually have a higher surface
energy and are easy to wet by suitable
adhesive.
2. Waxes are not easily wetted because they
have low surface energy.
3. Teflon used in non stick cooking utensils
has low surface energy.
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Wettability 2- Surface tension of the liquid
adhesive.
1. temperature surface tension
2. impurities surface tension
3. surface tension wettability
4. wettability adhesion
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Wettability 3) Viscosity.
Increasing the viscosity of the adhesive, decreases the wettability.
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Wettability
4)Surface irregularities of adherent.
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• Importance of wettability in dentistry:
- Good wetting by saliva important for denture
retention.
- More natural appearance is achieved if
restorative material are wetted by thin film of
saliva
- To produce smooth surface of casting the wax
pattern is coated by surface acting agent (wetting
agent or debubblizer) before investing this
improve wax wettability thus, producing
smoother surface.
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• Surface active agent (Surfactant)
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Capillary phenomenon
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When adhesive bond is tested under tension there are three
possibilities:-
• Adhesive failure (adhesive-adherend separation).
• Cohesive failure of the adhesive.
• Cohesive failure of the adherend.
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Bonding to tooth structure
Factors contributing to the difficulty in bonding to
tooth structure (obstacles):
• Un-homogenous composition of enamel and dentin
• Tenacious (adherend) microscopic debris in the prepared
cavity. (Smear Layer)
• Surface irregularities of the prepared cavity.
• Inherent wettness of the surface of dentin.
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Mechanical bonding bonding to Enamel
Effect of acid etching on enamel
surface: 1) Removal of surface debris.
2) Producing pores (micropores). for micro-mechanical
interlocking.
3) Increase surface energy of enamel.
4) Increase surface area exposed to the bonding resin.
:
35% phosphoric a.
60 seconds
Enamel surface wash dry bond
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Micro-tags
Micro-pores
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Bonding to Dentin
• Smear Layer:
Thin tenacious adherent layer develops during cutting
or grinding of tooth structure after cavity preparation.
It’s formed of:
a. Dentin chips.
b. Saliva.
c. Bacteria.
d. Collagen.
e. blood.
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Smear layer Conditioned dentin
surface
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Wet dentin surface primed dentin surface bonded dentin surface
Structure
of primer
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Steps of bonding to dentin
Conditioning primer bonding agent
The aim of bonding to dentin is to obtain what’s called
“ HYBRID LAYER ” which is “Resin-reinforced dentin layer”.
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