A detailed study of Adhesion mechanism

1. Sudhansu Senapati
Adhesion Mechanism

2. Contents
❑ Paint film
❑ Adhesion and their mechanism
❑ Factor impacting on Adhesion
❑ Properties affected due to Adhesion failure
❑ How to improve Adhesion?

3. Paint film
Paint film
Substrate
Cohesive forces Adhesive forces Paint film molecule Substrate molecule

4. Cohesion vs Adhesion
Cohesion Adhesion
❑ Cohesion is the inner strength of a material, and it is
determined by the strength of molecular forces in the bulk.
❑ Strong Cohesion: Improved durability and film strength.
❑ Weak Cohesion: Abrasion, cracking, chalking, etc.
❑ Adhesion is the strength of the bonds forming between one
material and another (Coating & substrate).
❑ Strong Adhesion: Improve durability and performance
properties.
❑ Weak Adhesion: Blister, flaking, Peeling, etc.

5. Adhesion
Adhesion is the resistance of the coating to be removed from the substrate
Substrate
Coating film
Adhesion
Interface
Surface-to-coating bond
Strong: Better Adhesion
Weak: Inferior Adhesion

6. Adhesion Mechanisms
Chemisorption
Physisorption
Diffusion
Electrostatic Mechanical
interlocking

7. Adsorption (Physisorption) mechanism
❑ Adhesion occurs due to Van der Waals forces between the substrate and paint film molecules.
❑ Van der Waals forces are weaker than normal covalent and ionic bonds.
❑ Bond energy(kJ/mol): Van der Waals (2-15), Covalent (150-1100), Ionic (400-4000)
❑ They are short-range force, hence only interacts with nearest molecule.
❑ When contact between the substrate and paint film molecules is,
❑ More: Good wetting & Good interaction, hence good Adhesion
❑ Less: Lower wetting & Lower interaction, hence poor Adhesion
Substrate
Paint film
Van der Waals
interaction
Paint film
polymer chain
Good Wetting
Better Van der Waals interaction
Bad wetting
Poor Van der Waals interaction

8. Paint wetting
Substrate: Surface energy Paint: Surface tension
❑ Surface energy is the attractive force present between the
molecules at the surface of a solid substance.
❑ Surface tension is the attractive force of the molecules
present at the surface of a liquid towards each other.
High surface energy
Good Wetting
Substrate
Low surface energy
Bad Wetting
Substrate
High surface tension
Bad Wetting
Low surface tension
Good Wetting
Inorganic pigments Organic Pigments Solvent-based Paint Water-based Paint
Substrate Substrate
Surface energy: 20-40
PVC, Acrylic, Polyethylene,
Polypropylene, Ethylene
vinyl acetate, PTFE
Fluoropolymer, Polyvinyl
Fluoride, Teflon, wood, etc.
Surface energy: 200-1100
Copper, Stainless Steel,
Aluminium, Zinc, Glass, etc.
Surface energy: 40-70
Cement, Nylon, Alkyd enamel,
Polyester, Epoxy paint,
Polyurethane paint, etc.
Surface tension: 20-30
Ethanol, Silicon oil, Acetone,
n-Hexane, Heptane, Octane,
Hexadecane, Acrylic resin,
Benzene, Petroleum
Lubricating oil, etc.
Surface tension: 40-72
Mercury(ST-485), Water,
Glycerol, Ethylene glycol,
Nitrobenzene, Epoxy
resin, Formamide,
diiodomethane, etc.

9. Surface energy of materials

10. Surface energy of materials

11. Surface Energy
Surface
Tension
Paint film wetting
Surface tension < Surface energy
Paint or liquid at its critical surface tension ( When contact angle is 0⁰)
No wetting
Wetting
Paint Wetting related to Surface tension & energy

12. Chemisorption mechanism
Substrate
Paint film
Chemical
Bonding
Paint film
polymer chain
❑ Adhesion occurs due to weak chemical bonds between the substrate and paint film molecules.
❑ Covalent chemical bonds occurs across the interface.
❑ Adhesion prompters used to improve chemical bonds between surface and paint molecules.
❑ Adhesion prompters create molecular bridge between surface and paint film molecules.
❑ One end of Adhesion promoters react with substrate and other end react with paint molecules to create a strong durable bond.

13. How Adhesion Promoter works?
❑ Organofunctional silane used as adhesion promoter to improve adhesion between organic coating with substrate.
❑ Organofunctional silane (R-Si-X3) contains;
❑ Si: Silicone
❑ R (Silicon functional group) mostly alkoxy (ethoxy, methoxy, Di alkoxy, Tri alkoxy, etc.)
❑ X (Organic functional group): Acrylate, vinyl, amino, chloro, epoxy, etc.
❑ In presence of humidity the Organofunctional silane hydrolyses to form siloxane bond between coating and substrate.
Substrate
Paint film
Si
X
(OR) (OH)
(OH)
OH OH
Substrate
Paint film
Si
X
(OH)
(OH)
OH O OH
Si
X
O
Substrate
Paint film
Si
X
O
(OH)
O
(OH)
OH OH OH
Hydrolysis:
R-Si-X3 + 3H2O X-Si-(OH)3 + 3R-X

14. Selection of functional group
Silane functional group selection
Coating polymer Silane functionality
Acrylic Acrylic Epoxy
Alkyd, Alkyd urethane Vinylic Methacrylic
Epoxy Amino Epoxy Chloroalkyl Mercapto
Polyster Amino methacrylate Styryl Vinyl
Urethane, Polyurethane Amino Alkanolamine Epoxy Isocyanate
Compatibility to different substrate
Silane
functionality
Aluminium Copper Glass Steel
Amine    
Alkyl    
Epoxy    
Methacrylic   - 
Vinylic   - 
Type of Adhesion
promoter
Paint
Substrate
Glass Metal Mineral Plastic Wood
Organofunctional
silane
Solvent, water-based paint, Alkyd,
Epoxy, Polyester, Polyurethane, etc
    -
Organometallic
Compounds (Zr / Al)
Solvent, water-based paint, Alkyd,
Epoxy, high solid Polyester, Rubber, etc
-  - - -
Phosphates Acrylate, Alkyd & Polyester -  - - -
Silicones & PDMS All organic coatings   
Sucrose derivatives All organic coatings -  -  

15. Electrostatic mechanism
❑ Adhesion occurs due to electronegativity difference between the substrate and paint film molecules.
❑ Adhesive force is created due to attraction of electron across the interface.
❑ Positive and negative charges that attract each other
❑ The electrostatic force is created at interface and resists separation of substrate and paint film.
Substrate
Paint film
δ- δ-
δ-
δ- δ- δ-
δ-
δ-
δ-
δ-
δ-
δ+ δ+ δ+ δ+ δ+

16. Diffusion Mechanism
Substrate
Paint film
Inter
diffusion
❑ Adhesion occurs due to inter-diffusion of substrate and paint film molecules.
❑ It is applicable when both coating and substrate have polymeric chain.
❑ Polymer which are capable to movement, compatible and miscible with each other.
❑ Parameters effecting diffusion process are; Contact time, temperature, molecular weight of polymers.

17. Mechanical interlocking mechanism
❑ Adhesion occurs due to mechanical interlocking between substrate and paint film.
❑ Adhesion improves when paint film,
❑ Penetrates the cavities on the surface
❑ Displace trapped air at the interface
❑ Lock on mechanically to the substrate
❑ Surface roughness aids used to increase teeth to the substrate, hence good mechanical interlocking with paint film and
substrate.
❑ Also, roughness aids increase paint wetting by increasing surface energy of the surface.
Substrate
Paint film
Mechanical
interlocking

18. Factors influencing Adhesion
❑ Poor substrate cleaning: Soils, oils, dust, other contaminants present on the substrate prevents to form bonding
between paint film and substrate.
❑ Surface profile: smooth surface do not hold coating well.
❑ Improper wetting: Bad wetting affect paint and surface bond and proper adhesion.
❑ Paint ingredient: Insufficient crosslinking or uncured coating/ over-curing of coatings leading to poor adhesion.
❑ External environmental condition: Such as Humidity, water, UV exposure, etc.

19. Properties affected due to Adhesion failure
❑ Blistering:
❑ Occurs when a coated object is contact with water.
❑ Blistering is caused by water-soluble materials within or under the
coating, rapid drying of coatings or chemical exposure.
❑ Peeling:
❑ Reduction in bond strength of the paint film due to contamination or
incompatibility of coats.
❑ Flaking:
❑ Causes the paint to become separated from the substrate.
❑ Undercutting:
❑ Involves the corrosion buildup under coating.

20. How to improve Adhesion?
Factors improving
adhesion
Additives Function
Wetting of the surface
Wetting agents Reduces surface tension and improves wetting of paint film on substrate
Surface
preparation
Increases surface energy of the substrate
Substrate-Coating bond
Adhesion
promoters
Improves adhesion through chemical bond between coating and substrate.
Paint flexibility
Solvents Increases drying time of coating to minimize the formation of blisters
Plasticizers
Increases binder flexibility and enhances the coating’s resistance to mechanical
force during coating process.
Pigments &
Extenders
Imparts physical properties, such as hardness and water resistance

21. Thank You!