factors affects strength of adhesion in dentistry directly dealing with our clinical work in many different times and this is considerd measuers in some cultuers to detects the skill of dentist so we introduce this to you..
Factor Affecting The Strength Of Adhesive Junction
1_Wetting
To create a bond between two materials, the adhesive must make
intimate contact with the substrates surfaces, so that no air voids (which would weaken the bond) are formed.
An adhesive’s ability to contact a substrate depends on the adhesive’s wettability to that particular substratum. Health wetting is the capacity to fully cover the soil, So that the maximum benefit of whichever adhesive mechanism is activated is obtained.
The ability or inability of fluids to humidify a surface is frequently found in
daily life.
PTFE (polytetrafluoroethylene), as used in non-stick saucepans, is an
example of a surface which is extremely difficult to wet with water, When water
is put on a surface with PTFE, It forms globules that do not spread across the
surface in one even layer, That is a poor example of wettability.
The contact between the substratum and the adhesive is controlled by a driving
force that seeks to disperse the adhesive over the substratum, and spreading
resistance that depends on the adhesive ‘s viscosity, surface irregularities and
contaminant presence, The driving force is provided by both adhesive and
substratum surface energies.
Surface Energy :The molecules are exposed to attractive forces in all directions in the bulk of a solid or liquid, so that the molecule is in dynamic equilibrium with the molecules around it.
Nevertheless, this delicate equilibrium is broken on the surface, resulting in a net inward attraction directed at the large number of molecules in the material mass, It is that inward force which gives rise to a material’s surface energy.
The surface energy in liquids is called surface tension.
One of the consequences of surface tension is that liquids appear to take on a
spherical form ideally than any other.
This arises because a sphere has the
minimum surface area (and thus the minimum surface energy) for a givenv olume of liquid, making it possible to minimize the total energy stored in the liquid.
Whereas a liquid’s surface tension is a real surface stress, the work in stretching and not forming the surface is done in the case of a solid.
The measurement of a solid’s surface energy is not realized as readily as it is with liquids.
An approach that has now gained broad acceptance is one pioneered by Zisman who introduced the critical surface energy concept.
The angle of contact :When a solid and a liquid come into contact, the angle between the liquid surface and the solid surface is called the contact angle and depends on the liquid’s surface tension and the solid’s surface energy.
This angle will be 0 ° which is the optimal situation for adhesion to occur, In this case the surface is completely covered with the adhesive and it can achieve the maximum bond strength. The driving force which gives rise to the tendency,or otherwise, of a fluid to spread over a solid surface depends on the liquids surface tension and solid’s surface energy, At the point where the liquid ‘s surface meets the solid ‘s surface, their surface tensions must balance to be in
balance(6).
2-Irregularities On The Adherends
Until adhesive bonding, surface prepration of the adherents plays an important role in improving the strength and longevity of the bonded joints.
Surface preparation is one of the significant parameters that is
directly related to the bonded joint consistency.
A surface treatment of adherends
should ensure the following aspects to obtain a solid and robust bonded joint:
elimination of all pollutants (lubricants, dusts, loose corrosion layers, micro-
organisms) from the surfaces, good wettability of the surface, Surface energy, good surface activation of bonding elements etc.
Different chemical and physical surface
treatments are available but proper surface treatment selection is very important.
Surface irregularities of substrate is consedired as a double edge weapen due to it may be uniform ,wide and shallow so do good wetting thus good adhesion , or may be deep, more narrow and non uniform so do bad wettinf thus bad adhesion(7) .
3-Cleanliness On The Adherend Surface
The adherend’s cleanliness of the surface thus greater the adhesion , The surface of the substrate should be clean from debris or any contamination that prevents adhesion , and dry surfaces is butter than wet surfaces of dentin or enamel ,Adhesives should be fill the irregularities which render surface smooth allowing contact proper or intimate(8).
4-Thickness Of Adhesive Film
shortly The thickness of the adhesive is very important where the adhesive film is more thin because of the presence of intimate contact with the surfaces and less air pupil is favorite. And the thicker adhesive film does weak adhesive junction because it contains more papules of air(9).
5-Stresses
Due To Setting Contraction Of Adhesive: mainly caused be setting
contraction of adhesive ,the contact stresses in a bonded joint are calculated by considering the adhesive as an elastic rectangle confined by plates representing the adhesives.
The interface is typically cohesive, so the contact area is a perfectly adherent region surrounded by cohesive areas where slip at constant shear stress
occurs(10).
6-Thermal Stresses
the materials are expand and contract with changes in
temperatures , the degree of dimentional change depends on the temperature
variation and coefficient of thermal expansion of the material , The greatest
thermal stress occurs when the substrates have different CTEs , The substratum with the higher CTE contracts more in these situations, and compresses the other substratum and vice versa.
This internal load transfer from one substratum to the other is accomplished by inducing shear stress through the adhesive thickness (11).
Bonding to tooth structure
1– Bonding To Enamel
Enamel is the hardest tissue in the human body consists of 95% mineralized inorganic substance,hydroxyapatite , Arranged in dense crystalline structure and low protein and water content.
Concentrating on the mineral portion (hydroxyapatite) of enamel is very important for bonding to enamel.
Buonocore, in French 1955, the adhesion of acrylic resin to acid etched enamel was first revealed. He used phosphoric acid in 85 % Ethching, later Silverstone revealed that the optimum phosphoric acid concentration should vary between 30-40% to achieve a satisfactory adhesion in the enamel.
The 37%phosphoric acid is usually used for 15 to 30 seconds , If the concentration exceeds 50% monocalcium phosphate monohydrate may become precipitated while dicalcium phosphate monohydrate is present at concentrations below 30%Precipitation that is interfering with adhesion.
2-Bonding To Dentin
Dentin bonding has proven more difficult and less reliable and predictable than enamel bonding. This is primarily because of morphological, histological variations And the variations in composition between the two:
95% inorganic hydroxyapatite by volume in enamel and 50% in dentin. Dentin included more water, Hydroxyapatite crystals have a regular pattern in enamel while hydroxyapatite crystals have pattern in dentin Inorganic matrix arranged randomly, The presence of the smear layer makes it more difficult to wet dentin through the adhesive, Dentin contains dentinal tubules that contain vital pulp and odontoblast processes,This makes the dentin a responsive structure, dentin is a dynamic tissue
that shows changes due to the Fluid present in dentinal tubules constantly flows outward to aging and caries or operating procedures which reduces the adhesion of the composite resin to the dentin bond.
So bonding to dentin involve three general steps are conditioning ,priming and bonding(12).
refrences
6-Themes U. 1.9: Principles of adhesion [Internet]. Pocket Dentistry. 2020[ cited 22 May 2020]. Available from: https://pocketdentistry.com/1-9 principles-of-adhesion/
7-Ghumatkar A, Budhe S, Sekhar R, Banea M, Barros S. Influence of Adherends Surface Roughness on the Adhesive Bond Strength. Latin American Journal of Solids and Structures. 2016;13(13):2356-2370.
8-Baum L, Phillips R, Lund M. Textbook of operative dentistry. Philadelphia:W.B. Saunders; 1995.
9-Manuja N, Nagpal R, Pandit I. Dental Adhesion. Journal of Clinical Pediatric Dentistry. 2012;36(3):223-234.
10– Franco A, Royer-Carfagni G. Contact stresses in adhesive joints due to differential thermal expansion with the adherends. International Journal of Solids and Structures. 2016;87:26-38
11-Adams R, Coppendale J, Mallick V, Al-Hamdan H. The effect of temperature on the strength of adhesive joints. International Journal of Adhesion and Adhesives. 1992;12(3):185-190.
12-Enamel, the hardest tissue in the human body consists of 95% mineralizedi norganic substance, hydroxyapatite
