y1 basic principles (lowest priority/ already know)

Question 1

Difference between adhesion and cohesion

Answer 1

Adhesion = the force that binds two dissimilar materials together (when brought in intimate contact) by the attraction of atoms and molecules

Cohesion = the attraction between similar atoms/molecules within one substance

Question 2

Factors affecting adhesion

Answer 2

1. Surface energy (solid) or surface tension (liquid)
2. Viscosity of adhesive (good adhesive is loose enough to fill holes but not too loose that it spreads too far)
3. Wettability of substrate by adhesive (good wettability means adhesive spreads easily)
4. Contact angle of liquid on substrate (perfect wetting occurs when the contact angle is zero degrees).
5. Surface roughness of substrate (roughness increases surfaces area and bond strength but makes it harder for adhesive to penetrate all holes)

Question 3

Types of adhesive bonds

Answer 3

Mechanical
Mechanical interlocking of components or penetration of one phase into the surface of another
Simplest
Needs irregularities on surface

Physical
Dipole-dipole interactions between polar molecules on each surface
Reversible and weak
Heat will break the bonds

Chemical
Covalent/ionic forces at molecular level
Needs presence of reactive groups of both surfaces
Best!

Molecular entanglement
Entanglement between adhesive and substrate
This is a result of good wetting

Combination of all types

Question 4

primary bonds

Answer 4

Covalent bonds
Strongest chemical bond
Atoms share their electrons to achieve an inert gas configuration
Overlapping of atomic electron orbitals
Directional bond

Ionic bonds
Strong electrostatic interactions by atoms ‘swapping’ electrons
Bonding occurs by the formation of ions
Non-directional bond

Metallic bonds
Overlapped orbitals of metallic atoms
Non-directional bond

Question 5

polymerisation

Answer 5

Condensation polymerisation = reaction between 2 molecules to make a polymer and water/small molecule

Addition polymerisation = reaction between 2 molecules to form a polymer with no by-products

Polymer chains are held together by intermolecular forces which are temperature dependent. As temp increases, intermolecular forces decrease.

Glass transition temperature (Tg) = the temp where a rigid material becomes soft and rubbery.

Plasticisers lower Tg by flowing between polymer chains and reducing IFs (molecular lubrication).

Example plasticiser = dibutyl phthalate.
Plasticisers can be bad since they leach out with time. Also Phthalates are carcinogens which leach into patient’s mouth .