Surface properties and adhesion

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

Definitions

1 )Adhesion : is bonding between dissimilar materials through chemical reaction of their atoms and molecules.

2 )Cohesion: is bonding between similar materials.

3 )Adhesive : is the liquid material used to produce adhesion.

4 )Adherent : is the solid substance to which the adhesive is applied.

5 )Wetting : is the ability of an adhesive to wet the surface of the adherent. Its measured by contact angle.

6 )Contact angle: it is the angle formed between adhesive and adherent at their interface. It depends on the surface energy of the adherent and surface tension of the adhesive.

N.B: there is an adhesion between saliva and the palate, also between saliva and the denture. Moreover there is cohesion between saliva on the palate and saliva on the denture.

Types of adhesion:

True adhesion Mechanical

attachmentDef. Bonding between bonding between

dissimilar materials dissimilar materials

through bonging between through mechanical

their atoms and molecules interlocking” no

“ chemical reaction” actual bond is

formed

Examples glass ionomer and amalgam, composite

zinc poly- carboxylate zinc phosphate

with the tooth. Cement with the

tooth.

Mechanism both glass ionomer and a liquid flow into

polycarboxylat contain pores or irregularities

CooH group that react in the surface of a

chemically with calcium solid and set (harden)

of the tooth. Forming a strong

mechanical bond.

Factors affecting the strength of adhesive junction:

1 )Wetting.2 )Irregularities on the adherent.

3 )Cleanliness on the adherent.4 )Thickness of the adhesive.

5 )Stresses due to setting contraction of adhesive.

6 )Thermal stresses. 7) The type of bond formed

Factors affecting the strength of adhesive junction:

1 )Wetting: for adhesive to produce good wetting with the adherent, the contact angle must be zero or less than 90° (forces of adhesion between them are more than the forces of cohesion between adhesive molecules together.

Wetting

Factors affecting wetting:

a) Surface energy of the adherent:

Atoms on the surface of solids have greater energy than atoms inside it because the outer most atoms are not equally attracted in all directions. The surface energy of metal = 0.5 Ed, where E = modulus of elasticity and d is the interatomic distance.

Metal usually have higher surface energy than non metals. The higher the S.E of the adherent, the lower the contact angle and the stronger the strength of adhesive junction.

b) Surface tension of the adhesive:Atoms on the surface of a liquid are strongly attracted to the inside. The force that produces this effect is called surface tension It is 22, 72, 465 dynes/cm for alcohol, water and mercury respectively. Values for S.T are reduced by the increase in temperature and addition of impurities, because both temperature and impurities reduce the cohesive force between liquid atoms o the surface.

c) Viscosity of the adhesive:

Low viscosity of the adhesive is required to allow its easy flow on the surface of the adherent. This increases the strength of adhesion.

2 )Irregularities on the adherent :

This prevent the liquid adhesive form completely wetting the adherent due to the presence of air pockets. This decreases the strength of adhesion.

3 )Cleanliness on the adherent:

Any debris on the surface will prevent direct contact between the adhesive and the adherent. This decreases the strength of adhesion.

4 )Thickness of the adhesive :

The thinner the adhesive film, the lesser the air voids and the stronger the strength of adhesion.

5 )Stresses due to setting contraction of adhesive:

Liquid adhesive undergo contraction during setting. This contraction results in the reaction of stresses at the interface that severely decreases the strength of adhesion.

6 )Thermal stresses:

If the adhesive and adherent have coefficiency of thermal expansion, changes in temperature will produce stresses in the bond. Close matching in coefficiency of thermal expansion is required to minimize stresses and so increases the strength of adhesion.

7 )The type of bond formed:

No doubt that primary bonds between adhesive and adherent produce stronger adhesion than if secondary bonds are formed (soldered joint is stronger than glued joint).

Failure of adhesive junction:

If an adhesive bond is tested in tension, on of three things may happened:

1 )Adherent failure (adhesive – adherent separation)

2 )Cohesion failure of the adhesive.

3 )Cohesion failure of the adherent.

Importance of adhesion in dentistry:

1 )To reduce tooth preparation needed for retention.

2 )To reduce marginal leakage caused by setting contraction of the filling.

3 )To reduce post-operative pain and pulp sensitivity caused by leakage.

4 )To reduce caries by sealing pits and fissures.

5 )Adhesive cements produce better retention of crowns and bridges.

6 )Aesthetic coating to treat enamel defects.

Dental considerations:

The following conditions prevent ideal adhesion in the oral cavity:

1 )The unhomogenous composition between enamel and dentin:

Enamel and dentin are unhomogenous in their composition, being partly organic and

partly inorganic .

An adhesive which would adhere to the organic component would not be able to adhere to the inorganic portion. At the same time materials that would adhere to enamel would not be able to adhere to dentine. Thus adhesion would not be uniform over the entire surface.

2 )Surface irregularities in the prepared cavity:

The surface of prepared cavity is full of pits and fissures. These morphologic roughnesses are further increased by the scratches which are produced by the dental burs used in preparing the cavity. It is difficult to design an adhesive that flow into these minute irregularities and wets the entire surface area of the cavity preparation.

3 )Debris in the prepared cavity:

Microscopically, the tooth surface is covered with debris that is formed when the dentist prepares the cavity (smear layer). Through cleaning of the cavity cannot remove this debris completely. This debris prevent adhesive from complete wetting of an adherent.

4 )Presence of water in prepared cavity:

Of major importance is the presence of water. This is not water from saliva, but a microscopic single molecule layer of water which is always on the tooth surface. This film prevents adhesive from coming into intimate contact with the tooth.

Bonding to enamel and dentin:

1 )Bonding to enamel (Acid-etch technique):

This is done by using 30 to 50% phosphoric acid for 1 to 2 minutes to remove 5µm of the surface enamel.

Acid-etch( phosphoric acid)

Acid etching help bonding to enamel by:

1 )Removal of surface debris.

2 )Producing pores in the surface into which the adhesive penetrates to form tag-like extension, giving mechanical interlocking.

3 )Increasing the surface energy of enamel and so wetting.

4 )Increasing the surface area of the enamel.

Enamel surface etched with 35% and 10% phosphoric acid gels for 15 and 60 seconds. Areas with preferential removal of prism core material and the prism peripheries relatively

intact .

Enamel adhesive is then applied to flow and fill the produced irregularities and bond to enamel by mechanical interlocking, then composite is applied to bond with the adhesive chemically.

2 )Bonding to dentin (dentin adhesives)

The success of acid etching to enamel and obtaining good bonding with composites, led researchers to use phosphoric acid on dentin but this was delayed for fear of harming the pulp and because the composition of dentin is much different form that of enamel, and one would expect it to behave differently when etched with

acid .

The presence of water (10%) and the larger organic components (30% collagen) lowered the surface energy of the dentin and made bonding with the resin impossible. Researchers produce many dentin adhesive. But the most recent are supplied in 3-parts to be used in 3-steps:

a) Etchant or conditioner:Phosphoric acid is applied to dentin for 10-15 seconds only to:

1 -remove the smear layer2 -demineralize the surface (that produce

porous dentin surface)3 )Open the dentinal tubles and the collagen

network.N.B. the same etchant can be used on both enamel and dentin, simplifying the procedure.

Dentinal surface etched with 10% phosphoric acid gel for 15 and 60 seconds. The smear layer was removed and the

dentinal tubules orifices were opened .

b) Primar: such as HEMA is applied to etch dentin. Because most inorganic material (Ca) have been removed by the acid, the primer penetrates into the organic material (collagen fibers) leading to its expansion. This will form the (collagen-resin) hybrid layer.

N.B. HEMA(hydroxyethyl methacrylate)

c) adhesive: low viscosity resin that contains hydrophilic molecules can penetrate inside the porous dentin and the hybrid layer to bond with the primer and the collagen fibers by mechanical interlocking. Composite is then applied to bond with the adhesive chemically.

N.B. hybrid layer: it is a resin reinforce layer formed around intertubular dentine by dentin bonding agent.

Bonding to enamel and dentin:

Etching enamel and dentin cleaning

Applied primer air syringe & enamel

dentin bonding

light curing

An alternative approach is used

self-etch adhesives: containing an acidic-primer, which simultaneously provides the conditioning, and priming of tooth structure.

(One step )all-in-one: )conditioner, primer and bonding agent.) products are designed to simplify the bonding technique, decrease the clinical application time, and minimize the possibility of failures.

self-etch adhesives

)One step )all-in-one