Adhesion and Surface Phenomenon

Dr Ambreen Azam

MSc Dental Materials, UK

Analyze the surface properties and its effects on adhesion.

Describe the components of an adhesive junction and their role.

What is adhesion & How it takes place. What factors affect adhesion Enlist the Criteria for successful bonding Application of adhesion on tooth

structure

INTENDED LEARNING OUTCOMES

Surface is the outermost layer of an object or a materialWhen two surfaces face each other they form an interface. Related Important concepts: Adhesion Surface energy and surface tension Contact Angle Wettability.

What is Surface?

Different techniques to analyze surface PHOTO EMISSION SPECTROSCOPY Shows presence of contamination on the surface. ELECTRON SPECTROSCOPY Study structure and dynamics of surface atoms AUGER ELECTRON SPECTROSCOPY Provides depth profiles of elements

METHODS TO ANALYZE SURFACE

Surface attachment process between two dissimilar substances is called adhesion.

Formation of bond at interface Cohesion same kind of molecules involved Bonding of similar materials at their interface Like joining two metal pieces together by welding Adhesive & cohesive forces Adhesion and cohesion co-exist

ADHESION

COMPONENTS OF ADHESIVE JUNCTION & TYPES OF ADHESION

Adherend & adhesive together form an adhesive joint. Adhesive is a material used to produce adhesion. e.g.

bonding agent. Adherend is the surface where adhesive is applied Mechanism of surface attachments: Mechanical bonding / Adhesion Chemical bonding/ Adhesion Physical Adhesion Results in Wetting of the surface

Adherend 1Adherend 2

Adhesive Systemor Luting Cement

Interface 2

Interface 1

Tooth Restoration

Adhesive Joint

MECHANICAL ADHESION

May involve penetration of adhesive into microscopic irregularities present in the surface of the substrate.

Micro mechanical adhesion On hardening these adhesive

projections provide anchorage for mechanical attachment.

Substrate 2

Surface Irregularities

Adhesive

Adhesive possess chemical affinity for the adherend surface Covalent and ionic links result in stronger attraction Waander waal,s or secondary bonding results in weaker

adhesion GIC bonds chemically with the tooth structure

CHEMICAL ADHESION

PHYSICAL ADHESION Retention of a denture with the oral mucosa

having a thin film of saliva between the denture and the tissue surface is called physical retention or physical adhesion.

SURFACE ENERGY & SURFACE TENSION Energy at the surface of the solid is greater than that of its

interior. Important consideration for an adherend. Surface energy can be regarded as measure of the excess

energy/ reactivity at the surface of material. When another material attaches to the surface the

surface free energy is reduced. For example; Impurity. Metals have high surface energy and adsorb molecules

more readily onto the surface. For example Tarnish & Corrosion.

Waxes have low surface energies. Interior atom

Surface Atoms .

Due to the intermolecular bonds or cohesive forces between the molecules of a liquid.

It gives them a spherical shape as a sphere has the smallest possible surface area to volume ratio.

Surface tension is the increase in energy per unit area of the surface.

Chemistry of surface layer more important than the bulk properties

SURFACE ENERGY & SURFACE TENSION

Surface active agent (Surfactant) effect the surface tension at the liquid-air interface.

Occupy surface positions displace surface molecules

Reduce cohesive forces over the surface area.

SURFACTANT

Wetting or wettability is the relative affinity of a liquid for the surface of the solid.(Important for the adhesive)

The ease with which the liquid will flow onto the surface the more will be the wettability

Defined as the interfacial tension b/w liquid and the solid substrate having a contact angle of 0 degrees.

Wettability of a liquid is the function of surface tension at interfaces.

WETTING

Wetting can be measured by measuring the angle of contact between the liquid and the solid surface.

It is due to a balance b/w surface and interfacial energies.

Lower the contact angle greater the ability to wet the surface . Complete wetting occurs at 0 degrees. No wetting will occur at 180 degrees.

CONTACT ANGLE

DIFFERENT CONTACT ANGLES

Surface energy will

If surface tension of adhesive is high there is decreased wetting or vice versa.

If viscosity of adhesive is high there is decreased wetting.

Film thickness of adhesives low better flow, better wetting.

Wetting important in adhesive materials

FACTORS EFFECTING WETTING1- ADHESIVE RELATED FACTORS

Surface cleanliness increases surface energy promotes adhesion. Surface irregularities if shallow will promote mechanical interlocking

increasing adhesion but if too deep will prevent adhesive from flowing in to them thus decreasing adhesion.

Surface energy of the adherend should be greater than the surface tension of the adhesive.

Type of bond if it is a primary bond stronger adhesion as compared to secondary bonds

Thermal stresses can generate between adhesive and adherend due to large difference in coefficient of thermal expansion and contraction.

FACTORS EFFECTING WETTING1- ADHEREND RELATED FACTORS

Criteria for successful adhesion Low surface tension of the adhesive. Low viscosity of the adhesive. Low setting contraction of the adhesive. Thin adhesive film. High surface energy of the adherend. Proper surface irregularities of the adherend. Clean surface of the adherend. Closely matching in coefficient of thermal expansion of the

adhesive and the adherend. Primary bond formed between the adhesive and the

adherend.

Failure of the adhesive junction

Adhesive Failure between the adhesive and the adherend

Cohesive Failure within the adhesive or within the adherend

Adhesive failure Cohesive failure

Importance of adhesion in Dentistry Decrease marginal leakage between restoration and cavity

walls. Used in retention of restorations e.g Ceramometallic

restoration The prevention of tooth decay by sealing pits and fissures. Aesthetic coating for treating enamel defects. Complete denture retention through thin film of saliva. Required for bonding agents. Used in Soldering operation. Acrylic teeth in acrylic non metallic denture base

Application Of Adhesion on Tooth Structure Challenges in bonding to tooth structure

Bonding to tooth structure ↔ Exchange process

Replacement of minerals from Tooth by Resin

Primarily micromechanical interlocking

The Challenges of Adhesion in Dentistry

1. Simultaneously bond to enamel & dentin [different substrates.]

2. Work in the presence of

3. Stresses of the restorative materials on the tooth structure

4. Technique sensitive

5. Biocompatible.

1. Moisture.2. Surface irregularities3. Surface contaminants

enam

More homogenous in composition

Organic 2%

Water 12%

• Bonding in enamel • Bonding in dentin

More heterogeneous in composition

Comparison By Volume Enamel Dentine

Organic 25%

Bonding to Enamel by Acid Etching 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 dissolves the smear layer( Debris layer) The acid removes about 5 microns of the surface of enamel and

produce microtags. Adhesive resin flown in to these microtags and forms an adhesive

bond. Surface area & surface energy is increased.

Bonding to Dentine Difficult than enamel more heterogeneous composition Hydrophilic nature due to presence of water within the dentinal

tubules Smear layer also a problem. done in three steps 1. Etching (conditioning)using acid low surface energy. 2 .Priming. Primer has two ends hydrophilic and hydrophobic. Elevates surface energy of dentine ,Makes dentine hydrophobic 3. Bonding. Applying a bonding agent or an adhesive on top of

which the composite resin is applied and cured. 

HYBRID LAYER Etch /dentine conditioning, Priming , Bond or adhesive

application

Two approches to acheive this Total Etch Adhesives (Either 3 or two steps) Self Etch Adhesives (Either 2 or 1 step)

Formtion of a hybrid layer a resin infiltrated/ re-inforced layer at the bulk dentine and resin interface

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Hybrid Layer Formation in Dentine

ETCHANTPRIMER

ADHESIVE RESIN

IntertubularDentine

PeritubularDentine

Dentinal Tubules

HybridZone

Collagen

McCabe Applied Dental Materials 9th Edition. Phillips Science of dental materials 12th edition Craig’s Restorative Materials 13th Edition. Watch : http://www.youtube.com/watch?v=cbTwMoNOwag Watch : http://www.youtube.com/watch?v=Pe12NedfYC0

REFERENCES