Copyright © 2011 by Saunders, an imprint of Elsevier Inc. Physical Properties of Dental Materials...

Copyright © 2011 by Saunders, an imprint of Elsevier Inc.Copyright © 2011 by Saunders, an imprint of Elsevier Inc.

Physical Properties of Dental Materials

Chapter 3

Copyright © 2011 by Saunders, an imprint of Elsevier Inc.

Physical Structure Dental materials are built

from atoms and molecules.

Molecules are formed when two atoms are brought together, but how they bond will depend on their microstructure.

Two types of bonds have been identified: Primary bonds Secondary bonds


Primary BondsPrimary bonds are the strongest because they

involve the exchange of electrons.3 types of primary bonds are known:

Ionic bonds give electrons to another atom. They are brittle and are poor electrical conductors. They are easily broken.

Covalent bonds occur when two atoms share electrons. They are strong and hard with a high melting point. They are not easily broken.

Metallic bonds have a shared lattice of positive electrons, which are attracted to a negatively charged “cloud” of electrons.


Ionic Bond

Covalent Bond Metallic Bond


Secondary Bonds

• Secondary bonds are much weaker than primary bonds.

• No sharing or transfer of electrons occurs with secondary bonds.

• This weakness often leads to deformation or fracture.– Example of secondary bond: Hydrogen Bond

which exist in water.


Physical Properties of Dental Materials

• The physical structure of a material may take on 3 basic forms:– Solid: has shape and volume– Liquid: has volume but not shape– Gas: has neither volume nor shape


Properties of Dental Materials(cont’d)

• Most materials are mixtures of more than one state of matter.– Plaster is a combination of a solid and a liquid.– Fluoride foams are a mixture of gas and liquid


Solids

• Density is a measure of the weight a material has in comparison with its volume.

• Hardness is used to define a material’s resistance to wear and abrasion.

• Elasticity is the ability of a material to recover from deformation.


Solids (cont’d)

• The maximum amount of stress a material can withstand without breaking is known as its ultimate strength.

• Stiffness is the resistance to deformation

• The greatest stress a structure can withstand without permanent deformation is called its proportional limit.


Solids (cont’d)

• Resilience is the resistance of a material to permanent deformation.

• Toughness is the ability of a material to resist fracture.

• Ductility is the amount of dimensional change a material can withstand without breaking.


Solids (cont’d)

• Malleable describes a solid that is easily compressed into a thin sheet. (gold)

• The combination of malleability and ductility gives a metal the ability to resist fracture or abrasion which is known as edge strength.


Liquids

• Molecules in a liquid state are not confined to a specific pattern; they flow.

• Viscosity of a liquid is its resistance to flow.

• Values of viscosity for thin fluids are low, and those for thick fluids are high.

• Thixotropic materials are liquids that flow more easily under mechanical forces.


Application

• Materials are classified by their application: how they are used or fabricated and by their expected longevity.

• Materials may be:– Therapeutic – Preventive – Restorative


Therapeutic Materials

• Therapeutic materials are used in the treatment of disease and include materials such as:

– Medicated bases– Topical applications for periodontal disease


Preventive Materials

• Preventive materials are directed toward the prevention of oral disease and the promotion of oral health.

• Preventive materials include:– Pit and Fissure Sealants– Fluorides


Restorative MaterialsRestorative materials make up the largest

classification and include: Filling Inlay Crown Bridge Implant Partial or complete denture

Restorative materials are further classified: Direct Indirect


Composition

• Components and the reactions of components may classify a material.

• Most materials combine two components chairside to form the resultant material.

• Many materials are classified as both a catalyst and a base.– Catalyst is responsible for the speed of the

reaction and is often the liquid component.– Base is main ingredient of a material


ReactionWhen components are mixed together, 2

reactions can occur: Physical (evaporation or cooling of a liquid) Chemical (creating new primary bonds)

Mixing time is the time it takes to bring components into a homogenous mix.

Working time is the time permitted to manipulate the material in the mouth.

Initial set time begins when the material can no longer be manipulated in the mouth.

Final set time occurs when the material has reached its ultimate state.


How reaction of materials are initiated:

Chemical set materials are set through the timed chemical reaction between catalyst and base (limited working time).

Light-activated materials use a blue light source to initiate the reaction stage (allows for unlimited working time).

Dual set materials begin with initiation of the blue light source and continues with chemical set reaction (also allows clinician more control over working time)


Manipulation

The manipulation of a material’s components is an important consideration for the dental auxiliary.

Manufacturers are now producing materials in a variety of forms to address inconsistencies in mixing and time demands of different materials: Pre-dosed materials to standardize the amount of

catalyst and base in the mix. Automix materials standardize the amount of catalyst

and base and produce a consistent homogenous mix.


Shelf Life

• The deterioration and change in quality of a material over time.

• Factors that affect shelf life:– Date of expiration– Storage conditions (temperature and humidity)– Type of storage container used

*Always refer to manufacturers’ directions and recommendations!


Summary

• A material’s characteristics are defined by its properties.

• As materials become more complex, it is important to mix the materials following the manufacturer guidelines.

• Use of pre-set doses is the most dependable way to produce a consistent, accurate mixture each time.

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