CHAPTER VI
Applied surface phenomena and adhesion
Science of DENTAL MATERIALS(1)
CHAPTER VI
Adhesion
Prof.Taheya Moussa
0
CHAPTER VI
Applied surface phenomena and adhesion
Applied surface phenomena and adhesion
Adhesion and Cohesion
Cohesion: is the bonding of similar materials at their interface.
Examples of cohesion:
Bonding two pieces of pure gold together under pressure is an example of cohesion. The
bonding in such case results from metallic bond and is called pressure welding.
Adhesion: is the bonding of two unlike materials at their interface
Examples of adhesion:
Denture retention is accomplished by the adhesive action of a thin film of saliva
between the soft tissue and the denture base.
Adhesive: is a material used to produce adhesion. (Liquid) e.g. Bonding agent
Adherend: or substrate is the substance to which the adhesive is applied.(Solid) e.g.Tooth
-For adhesion to take place, the materials being joined must be in close contact
(intimate contact). Thus if two solid materials are to be joined, the adhesive between them
must be applied in the liquid state to produce a thin layer.
Types of adhesion: (according to the bond between adherend and adhesive)
I)
Chemical (true) adhesion:
- Caused by primary bonds.
- Example for chemical adhesion:
Chemical bond of polycarboxylate or glass ionomer cements with tooth structure, as
both contain carboxylic group that reacts with the calcium of the tooth structure to form
primary chemical bond.
II)
Mechanical interlocking (attachment):
If a liquid flows into pores of a solid surface and is allowed to set, it will lead to
mechanical interlocking. The bond, which results between the hardened liquid and the
solid surface, may be strong and the phenomenon is called attachment.
However in dentistry bonding caused by mechanical interlocking (attachment) is often
referred to as adhesion.
III) Physical forces:
Example: upper denture retention.
1
CHAPTER VI
Applied surface phenomena and adhesion
Factors affecting the strength of the adhesive junction:
“Factors affecting adhesion”
1) Wetting or wettability:
It is the ability of the liquid to spread over a solid surface.
Wetting is measured by contact angle.
Contact angle:
It is the angle between the surface of the liquid and the surface of solid.
The smaller the contact angle (the more acute), the better is the wettability.

 = acute
Good wetting
Good adhesion
 = 90
Very poor wetting
No adhesion
 = obtuse
No wetting
No adhesion
N.B; Good wetting means good adhesion, as good wetting indicates strong attraction
between the liquid and solid surface molecules. (As in above figure)
Factors affecting wetting:
a) The surface tension of the adhesive (S.T.):
It is the force of attraction between the surface atoms of the liquid.
Water
Increasing S.T. of the adhesive, decreases the wettability.
S.T. of the adhesive
wetting
Therefore, using adhesive liquid of low surface tension will increase its wettability
to a solid.
b) The surface energy of the adherend (S.E.) (the reactivity of the
solid surface):
It is the surface free energy of the atoms at the solid surface.
Increases S.E. of the solid, increases wettability,
S.E. of the solid
N.B.1: Surface energy and surface tension:
wetting
2
CHAPTER VI
Applied surface phenomena and adhesion
-Atoms and molecules at the surfaces of liquids and solids possess more energy
than do those in the interior. In case of liquid, this energy called surface tension. Also
in case of solids, it is called surface energy , which is greater than the internal
energy, because the atoms at the surface are not equally attracted in all directions.
- Surface tension of adhesive is decreased by heating and presence of impurities.
-High surface energy of solid increased its attraction force to atoms and molecules of
adhesive liquid.
N.B.2: Surface Free-Energy Exchange:
-If the surface tension value of the liquid is greater than the surface energy value of
the solid
the liquid molecules stay bound together leading to Poor wetting,
producing poor bond.
-If the surface energy value of the solid is higher than the surface tension value of
the liquid
the liquid uniformly wet the surface leading to good wetting,
producing good bond.
So for proper adhesion, surface energy of adherend should be greater
than surface tension of liquid.
c) The surface irregularities of the adherend (surface roughness):
-If surface roughness is irregular, deep and very narrow, so air pockets formation is
more common, where air pockets will prevent the adhesive from penetrating into
that area, therefore no intimate contact between adherend and adhesive will be
formed
Weak adhesive bond.
-But if surface roughness is regular and shallow, so there will be no chance for air
pockets formation
intimate contact between adherend and adhesive
Good adhesive bond.
Regular & shallow roughness
Irregular & deep roughness
Adhesive
Adherend
Air pocket
Debris
d) The viscosity of the liquid adhesive:
Increasing the viscosity of the adhesive, decreases the wettability.
Viscosity
wetting
2) Stresses due to setting contraction of the adhesive:
3
CHAPTER VI
Applied surface phenomena and adhesion
Most liquid adhesives undergo contraction as they solidify. This contraction often
results in creation of stresses at the interface.
So, the less the contraction of the adhesive during setting, the less will be the
stresses and the stronger will be the joint.
3) Thermal stresses:
If the adhesive and the adherend have large difference in the coefficient of thermal
expansion and contraction, change in temperature will produce stresses in the bond.
So, for stronger bond, close matching of the thermal expansion is
required to minimize stresses.
4) The thickness of the adhesive film:
Thin adhesive film allows;
- More intimate contact.
- Less thermal stresses.
- Less stresses due to setting contraction of adhesive
- -Less air bubbles within adhesive layer.
So, the thinner the adhesive film, the stronger is the adhesive junction.
5) The type of bond formed:
If bonding between the adhesive and the adherend occurs by primary
bonds, it will be stronger than secondary bonds.
6) Cleanliness:
Any debris or surface contaminations prevent the adhesive from coming into the
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 (uncleaned) surfaces as debris prevent intimate
contact.
4
CHAPTER VI
Applied surface phenomena and adhesion
Summary for factors essential for proper adhesion;
Adherend should have
- High S. E.
- Proper surface irregularities
-Clean surface.
Adhesive should have
- Low S.T.
Low viscosity
- Low setting contraction
-Thin film thickness.
-Close matching in coefficient of thermal expansion of both
-Primary bond formed between both
Types of failure in adhesion
When an adhesive bond is tested under tension there are three possibilities;
1) Failure of adhesive bond
(Adhesive failure )Fracture of bond at adhesive-adherend interface indicating
weak bond.
Adherend
Adhesive
Adherend
2) Failure of the adhesive layer
(Cohesive failure) Fracture within adhesive layer indicating weak bond.
Adhesive
Adhesive
3) Failure of the adherend
(Cohesive failure) Fracture of adherend.
Adherend
Adherend
Strong adhesive bond
i.e. Failure of the adherend indicates the presence of strong adhesive bond.
5
CHAPTER VI
Applied surface phenomena and adhesion
Importance of adhesion in dentistry
123456-
Decrease marginal leakage between restoration and cavity walls.
The prevention of tooth decay by sealing pits and fissures.
Complete denture retention through thin film of saliva.
Soldering operation.
Ceramo-metallic restoration.
Acrylic teeth in acrylic non metallic denture base.
Adhesion in the oral cavity
It is very difficult to produce adhesion in the oral cavity
"Obstacles for adhesion to tooth structures"
1- The inhomogeneous composition of enamel and dentin.
2- Surface irregularities in the prepared cavity.
3- Debris in the prepared cavity.
4- Presence of water in the prepared cavity.
Bonding to tooth structure
Surface treatments should be performed in order to help bonding of
restorative materials to enamel and dentin.
Two mechanisms of adhesion (bonding) may be distinguished: chemical (glass ionomer)
and mechanical (composite filling materials).
The most widely used technique is acid etching of enamel and dentin to produce
mechanical bonding between the tooth and composite filling materials.
-Bonding to enamel; (acid etching technique)
- The most commonly used acid etch is 30-50 % phosphoric or citric acid
- Etch the surface of enamel by applying the acid for 15-30 seconds.
- The acid removes about 5 microns of the surface of enamel and produce micro tags
into which the adhesive will penetrate and bonded to enamel,
-Then apply composite filling material that bonded to the adhesive.
Bonding agent
Tooth structure
6
CHAPTER VI
Applied surface phenomena and adhesion
Micro tag
Acid etching helps bonding to enamel by:
1- Removal of surface debris “produce clean surface”.
2- Producing pores by selective demineralization of inorganic part in the enamel
surface into which resin penetrates to form tag-like extensions, giving
mechanical interlocking.
3- Increasing the surface energy of the enamel, causing better wetting.
4- Exposure of greater surface area of the enamel to the resin.
- Bonding to dentin ;( which is more difficult than enamel bonding):
- Dentin poses greater obstacles to adhesive bonding than does enamel due to;
i) Presence of higher amount of water so it is strongly hydrophilic.
ii) Presence of smear layer which will prevent proper adhesion.
N.B; Smear layer(debris layer) is a 5-10 microns thickness layer formed of collagen,
cutting tooth structure, blood, saliva and bacteria (resulting during cavity preparation.)
- Dentin bonding involves three distinct processes:
1. Etching (conditioning). 2. Priming. 3. Bonding
1. Etchant:
- Dentin etching is done by applying the acid etchant for 10 to 15 seconds.
- Etching will lead to:
a. Partial or complete removal of the smear layer (debris layer).
b. Demineralization of dentin surface ----------micro tags.
On the other hand, etching of dentine by acid will lead to
c. reduction of surface energy of dentin, as the demineralization of dentin will lead to
exposure of more collagen that have low surface energy.
2. Primer:
- It is used to elevate the surface energy of dentin to improve wetting.
3 .Dentin bonding agent:
-Bond the hydrophilic primed dentin surface to hydrophobic resin composite
restoration,
Because composite resins are hydrophobic, dentin bonding should contain both
hydrophilic and hydrophobic materials. The hydrophilic part should be designed to
interact with the moist dentin surface, whereas the hydrophobic part bond to the restorative
resin.
. -The resultant bond is micromechanical rather than true chemical adhesion.
7
CHAPTER VI
Applied surface phenomena and adhesion
- The final successful bond that is aimed to be produced, should have a continuous
layer along the dentin surface called hybrid layer (resin infiltrated dentinal layer),
which is a resin reinforced layer part is tooth and part is resin and resin tags.
Hybrid layer and resin tags
Cross section SEM
composite
•Resin must flow into
•Collagen forming hybrid layer
dentine
Resin tag
Questions - Bank
State true or false and correct the wrong statements:
1. Increased surface energy of the adherend increases wettability
2. Soldered junction is weaker than glued junction
3. The adhesive bond of a restorative material to dentin is stronger than that to enamel
4. The adhesive bond of a restorative material to dentin is weaker than that to enamel
5. For acid etching of enamel, 70% phosphoric acid is routinely used for 40 seconds
6. Etching of dentin will lead to increase in its surface energy and removal of smear layer
7. More natural appearance is achieved if an esthetic restorative is not wetted by saliva
8. A thick adhesive junctions means a stronger one
9. Etching of dentin lowers its surface energy
10. Enamel etching allows chemical bonding between tooth and resin based composite
11. The wetting power of a liquid is the ability to spread on the surface of the solid.
12. Primer is used only to remove the smear layer.
13. The hydrophilic part of dentine bonding agent bonds to tooth structure
14. Bonding of glass ionomer cement with tooth structure is an example for mechanical
attachment
Give reasons
1. Tooth structure poses obstacles to proper adhesion
8
CHAPTER VI
Applied surface phenomena and adhesion
2. Acid etching is done to enamel surface before application of a composite filling
material
3. Adhesive liquid should have low surface tension
4. Surface irregularities is a double-weaponed factors affecting wettgin
5. Waxes are not easily wetted
6. Thick adhesive film should be avoided
7. Using of the primer after etching the dentine surface
Choose the correct answer:
1. Among the factors that present difficulties to produce adhesion to the tooth structure are
a. Smear layer
b. Heterogeneous composition of enamel and dentine
c. Smooth surface of the prepared cavity
d. a & b
e. All of the above.
2. Etching of dentine will lead to
a. Increasing the surface energy of dentine surface
b. Removal of smear layer
c. Demineralization of dentin surface
d. b & c
e. all of the above.
3. Which of the following is a desirable quality for a dental adhesive
a. High viscosity
b. High surface tension
c. Many air bubbles
d. Thin adhesive film
4. More natural appearance is achieved if restorative materials are
a. Wetted by thick film of saliva
b. Wetted by thin film of saliva
c. Not wetted by saliva
5. Which of the following decreases the surface tension of a liquid
a. Addition of detergents(impurities).
b. Increase in temperature
c. Increase in thickness
d. a & b
e. None of the above.
6. For stronger adhesive junction
a. Surface energy of the adherend must be higher than surface tension of adhesive
b. Surface energy of the adherend must be lower than surface tension of adhesive
c. Deep irregularities must be made at the adherend surface
d. High difference in coefficient of thermal expansion between adhesive and adherend.
9
CHAPTER VI
Applied surface phenomena and adhesion
7. Bonding to tooth structure is hindered by
a. Inhomogeneous composition of enamel and denting
b. The absence of water film
c. The presence of smear layer
d. a & c
e.All of the above.
8. Strong adhesive junction is indicated by
a. Failure at the interface
b. Failure within the adhesive
c. Failure within the adherend
d. a & c
e. b & c
9. Spreading of a liquid on a surface indicates that
a. The surface has been poorly wet by liquid
b. The surface energy of the solid is probably quite high and/ or the surface tension of
the liquid is quite low
c. The surface energy of the solid is probably quite low and/ or the surface tension of
the liquid is quite high
d. The surface has been contaminated by a very low surface energy material prior to
the drop’s placement.
10. Metals usually have
a. Poor surface energy
b. High surface energy
c. Low surface tension
11. The acid etching of enamel leads to the following except
a. Removal of surface debris
b. Increased surface energy
c. Increased surface tension of the adhesive
12. Thermal stress may cause failure of the adhesive junction when
a. There is no or slight difference between the joined materials in their coefficient of
thermal expansions
b. There is a large difference in their coefficient of thermal expansion
c. Thermal expansion has no effect at all
13. Stresses which may affect adhesion are
a. Transverse stresses
b. Setting stresses
c. Compressive stresses
d. Thermal stresses
10