Dental Materials: Surface Energy and Polishing

Study Notes

Surface Energy and Adhesion

Adhesion: the force of attraction between the molecules/atoms of two different surfaces as they are brought into contact
Factors governing adhesive success:
- Wettability of the substrate by the adhesive
- Viscosity of the adhesive
- Morphology or surface roughness of the substrate
Classification of adhesive bonding mechanisms:
- Mechanical Adhesion: mechanical interlocking of the two phases
- Physical Adhesion: secondary forces arise through Dipole-dipole interaction
- Diffusion: one phase penetrates by diffusion into the surface of a second phase and forms a hybrid layer
- Chemical Adhesion: chemical bonding between two phases

Surface Energy

Surface energy: energy on the surface of a solid greater than on the interior
Surface energy is responsible for adhesion
Atoms on the surface are unbalanced and can bond to approaching materials

Wettability

Wettability: the ability of a liquid to spread on a surface
Contact angle: the angle between the drop of liquid and the solid surface
Wettability characteristics:
- Complete wetting: contact angle = 0°
- Partial wetting: contact angle > 0°
- Non-wetting: contact angle = 180°

Cutting, Grinding, and Polishing

Cutting: use of a bladed instrument to remove material
Grinding: use of a bonded or coated abrasive instrument to remove material
Polishing: use of progressively finer particles of polishing media to produce a high-luster finish

Finishing and Polishing

Finishing and polishing is a step-by-step process
Removing scratches and bringing a high luster to a surface
Methodically progressing from open cut instruments to fine grade polishing pastes on a soft surface

Abrasives

Characteristics of ideal abrasives:
- Irregular in shape
- Harder than the surface it is intended to abrade
- High impact strength
- High attrition resistance
Factors influencing rate of abrasion:
- Particle size and shape
- Speed and pressure
- Lubricants

Biological Hazards of Finishing

Solid particles are generated and released into the breathing space of laboratories and clinics
Airborne particles may contain tooth structure, dental materials, and microorganisms
Control measures:
- Infection control procedures
- Ventilation systems
- Personal Protection Equipment (PPE)

Why Materials Fail

Materials can fail due to:
- Mechanical failure
- Physical failure
- Inclusion of dust, particles, moisture, or inclusions
- Bond failure
Desirable properties of dental materials:
- Mechanical properties (strength, toughness, elasticity, hardness)
- Physical properties (thermal, electrical, optical)
- Biological properties (biocompatibility, toxicity)### Corrosive and Abrasive Wear
Corrosive wear occurs when a smooth, soft surface is worn away by a rough, hard surface
Abrasive wear can be either two-body or three-body; two-body may convert to three-body if portions of the sliding surfaces break away and act as an abrasive
Hardness is not an absolute indicator of wear and wear resistance; hard surfaces do not necessarily exhibit greater wear resistance than soft surfaces
Two requirements for abrasive wear:
- There must be a definite difference in hardness between the two surfaces
- The harder surface has to be the rougher one

Wear Testing

In vitro testing lacks predictive value due to acceleration of the test process to produce data in a reasonable period of time
Some in vitro tests for wear:
- Weight loss and volume loss
- Two-body and three-body wear
- Indentation hardness
- Surface profilometry (roughness)

References

Please refer to the original text for a list of references cited

Description

Learn about surface energy, adhesion, cohesion, and wetting in dental materials. Understand how surface energy affects polishing and differentiate between cutting, grinding, finishing, and polishing a dental prosthesis.