Adhesion Mechanism

1. Mechanism of adhesion

2. Adhesive bonding and cementing
Adhesive bonding in wood, glass and ceramics is also called
gluing, pasting and cementing.
Joining of materials by inter-atomic or intermolecular bond
through chemical reaction is called adhesive bonding.
Adhesive bonding is the process of joining materials with the aid
of a substance, acting as a chemical agent, capable of holding
those materials together by surface attachment forces.
Chemical used for a joint is called an adhesive while substrates
are called adherends.

3. ADHESIVE BONDING

4. “Joining of Materials and structures”, R W Messler, Elsevier Butterworth–Heinemann, 2004

5. Function of adhesive
• To join components.
• Comprehensive bonding with substrate.
• Stress distribution is on large area.
• Stress distribution is uniform.
• Viscoelastic nature of polymer gives
flexibility in the joint.
• Provide sealing affect to stop leakage.

6. Advantages of adhesive bonding
• High load carrying
• Low stress concentration
• Suitable for thin and thick structures.
• Little or no change in chemistry.
• Suitable for dissimilar materials.
• Sealant, insulator of heat and electricity.
• High strength to weight ratio.
• Absorb shock and vibration loads.
• Reduce galvanic and crevice corrosion.

7. Disadvantages of adhesive bonding
• Sensitive to peal and cleavage
• Require careful joint preparation.
• Limited shelf life or working time of
adhesives.
• Sometime curing time is too long.
• Repair is almost impossible.
• Sensitive to heat and organic solvents.
• Direct inspection is not possible.
• Adhesive are prone to bacterial attack.

8. Adhesive bonding
Atoms seek to give or take electrons to stable
themselves. Due to exchange of electrons they
are +/- charged. At the joint interface when both
+&- forces become equal ionic bond is formed.
With covalent sharing of atoms covalent
molecule is established.
Metallic bonds (delocalized)
Secondary bonding (van der waal’s)

9. Natural Adhesives
Animal-based adhesives (e.g., casein, collagen, gelatin)
Plant-based adhesives (e.g., pitch, natural rubbers, asphalt)
Mineral-based adhesives (e.g., sodium silicate, mineral-based sol-gels, calcium
carbonate)
Synthetic Adhesives
Synthetic Organic Adhesives
- Chemically-activated adhesives (e.g., cyanoacrylates, epoxies)
- Heat or radiation-activated adhesives (e.g., one-component epoxies)
- Evaporation or diffusion adhesives (e.g., phenolics)
- Thermoplastic hot-melt adhesives
- Pressure-sensitive (contact) adhesives
Synthetic Inorganic Adhesives
- Portland cements
- High-alumina, calcium aluminate cements
- Mortars (e.g., gypsum)
- Refractory cements
- Dental cements
- Glassy frits
ADHESIVES

10. Adhesion Theories
• Mechanical Adhesion
• Specific Adhesion
–Electronic Theory
–Diffusion Theory
–Adsorption Theory
–Covalent Bonding Theory
–Weak Boundary Layers
»Fourche, 1995

11. 1. The mechanical theory of adhesion
For an adhesive to function properly it must penetrate the microscopic
asperities (e.g., peaks and valleys, open pores, and crevices) on the
surface of adherends, and displace any trapped air at the interface.
Mechanical interlocking and anchoring of adhesive, no chemical bonding.
Open celled polymeric foams, porous ceramics, PMCs, wood and metals having
porous native oxide layer. Etched glasses and abraded metals.
Chemical etching and mechanical abrading important steps in adhesive
bonding.
1. Enhancing mechanical interlocking or anchoring.
2. Creating clean and wettable surface.
3. Increase in bond area due to an increase in the surface area.
4. Formation of a chemical reactive surface.
Theories of Adhesive bonding

12. • Mechanical Theory.
• According to this theory, adhesion occurs by the penetration
of adhesives into pores, cavities, and other surface
irregularities on the surface of the substrate. ... A positive
contribution to the adhesive bond strength results from the
“mechanical interlocking” of the adhesive and the
adherends.

14. 2. Diffusion theory of adhesion
When two materials are at least partially soluble in one another, they can
and do form a solution at their interface.
- Solid state diffusion (both in the solid form, slow)
- Liquid-solid diffusion (liquid adhesive and solid adherend, fast)
Stronger bonds result in chemically similar materials e.g., polymers.
Inter-diffusion and entanglement of long polymeric chains.
Used in wood and polymers. Difficult to apply for metals or ceramics.
- Solvent cementing of thermoplastics.
- Fusion bonding of thermoplastics.
Theories of Adhesive bonding

15. Diffusion Theory
Adhesive
Adheren
d

16. Diffusion Theory
• Occurs as interdiffusion of polymeric
adhesives and adherends at the interface
– Consequence: The disappearance of the interface
• Only possible when both adhesive and
adherend are polymers
– Polymers must be compatible

23. 3. Adsorption theory

24. Theories of Adhesive bonding
3. Adsorption theory of adhesion
Adhesion due to secondary bonding between adhesive and
the adherends.
Wetting as an indication of adhesion.
The degree of wetting is controlled by the balance between the surface
energy or surface tension of the liquid–solid interface versus the liquid
vapor and solid–vapor interfaces it replaces.
It is believed that permanent adhesion results primarily from the
forces of chemical bonding.

26. Adsorption Theory
Ө
Vapor
Liquid
Solid
γLV
γSV
γSL
Development of Intermolecular Forces
“Thermodynamic Adsorption Theory” = “Wetting
Model”