Physical properties and chemical properties of materials

Shaista Afroz

(MDS, PhD)

Associate professor

Prosthodontics

Dr Z A Dental College

Aligarh Muslim University

Physical properties are based on the laws of

• Mechanics

• Acoustics

• Optics

• Thermodynamics

• Electricity

• Rheology

• Magnetism

• Radiation

• Atomic Structure

THERMAL PROPERTIES

- Thermal conductivity

- Coefficient of thermal expansion and contraction

- Heat of fusion & latent heat of fusion

- Melting and freezing temperature

- Specific heat

THERMAL CONDUCTIVITY

It is the characteristic that determines the rate at

which heat flows through a material.

It is a function of the

• composition which determines the heat capacity

• magnitude of the temperature change

• thickness of the object.

Clinical importance in Dentistry:

1- Metallic filling materials.

2- Metallic denture base materials.

THERMAL DIFFUSIVITY

It is a measure of the rate at which a body with a non uniform

temperature reaches a state of thermal equilibrium.

Clinical importance in Dentistry:

Thermal insulation → Thickness of liner or base

→ 1 / thermal diffusivity

COEFFICIENT OF THERMAL EXPANSION

The change in length per unit length of the material

for a 1°C change in temperature is called the

linear coefficient of thermal expansion(α)

= L final- L original

L original x (°C final- °C original)

Clinical importance in Dentistry:

Close matching of the coefficient of thermal expansion (α) is important

between:

1. The tooth and the restorative materials to prevent marginal leakage.

Opening and closing of gap results in breakage of seal

(marginal percolation):

1. Marginal leakage

2. Discoloration

3. Recurrent caries

4. Hypersensitivity.

2. Porcelain and metal in ceramo-metallic

restorations (crowns and bridges) to provide

metal ceramic bonding.

3. Artificial tooth and denture base to avoid crazing.

HEAT OF FUSION (L)

The amount of heat in calories or joules required to

convert 1 gm of a material from the solid to the

liquid state at the melting temperature.

As long as the mass is molten, the heat of fusion is

retained by the liquid.

LATENT HEAT OF FUSION

It is the amount of heat in calories or joules liberated

when 1 gm of a material is converted from liquid to

solid state.

Importance in dentistry:

During casting, the metal must be heated 100 °C more

than its melting temperature for proper melting

Heat of fusion Latent heat of fusion

SPECIFIC HEAT

It is the quantity of heat needed to raise the temperature of

one gram of the substance 1°C

Metals have low specific heat, while non metals have high

specific heat

Importance in dentistry:

- Because of the low specific heat of dental gold alloys,

prolonged heating is unnecessary, during casting

The science of study of flow and deformation of matter.

Importance in dentistry:

I. Many dental materials are mixed as fluid pastes which

subsequently solidify.

II. The mixed pastes are adapted to the required shape

III. The setting of such materials initially involves a change

in viscosity with time and then the development of an

elastic modulus on solidification

RHEOLOGICAL PROPERTIES

VISCOSITY

Viscosity of a fluid is its resistance to flow.

Viscosity of fluids depends on many factors i.e. the

nature of the substance, temperature, pressure,

etc.

• Newtonian fluid : Ideal fluid demonstrates a

shear stress that is proportional to the strain rate

and thus plot a straight line

Non – Newtonian fluids.

1. Plastic

Some classes of materials behave like a rigid body

until minimum value of shear stress is reached

known as PLASTIC.

E.g. Ketchup – a sharp blow to the bottle is usually

required to produce an initial flow.

2. Psuedoplastic behavior-- Viscosity decreases

with increasing shear until it reaches a nearly

constant value ( Shear thinning ) e.g. polymer,

natural resins , impression materials.

3. Dilatants – Materials whose viscosity increases

with increasing shear rate until constant value

(shear hardening) i.e. becomes more rigid as

rate of deformation increases.

Thixotropic behavior

When some fluids are sheared at a steady rate

viscosity decreases with time. On standing , the

fluid regains its original viscosity, this is called

THIXOTROPY, and is caused by structural

breakdown and reformation.

e.g. fluoride gels, prophylaxis paste, resin

cements, plaster of paris, impression materials

CREEP

Time dependent plastic strain of a material under a

static load or constant stress.

The slow change in the dimensions of an object due

to prolonged (i.e. time dependent) exposure to

stress and high temperature.

FLOW.

Rheology of amorphous material

e.g. waxes has potential to deform under small static

load (own mass)

Metals used in dentistry , for cast restorations or substances for

porcelain veneers have melting points that are much higher than

mouth temperature, and thus are not susceptible to creep

deformation.

An exception is dental amalgam having components with melting

point slightly above room temperature, can slowly creep from

restored tooth

Importance in dentistry:

Galvanism is defined as accelerated corrosion of a metal due

to electrical contact with a more noble metal in a corrosive

electrolyte.

Also called as Wet corrosion.

The resulting current flow can produce nerve stimulation,

unpleasant taste and other physiological reaction.

ELECTROCHEMICAL PROPERTIES

The electrochemical properties of dental materials causes

important phenomenons known as TARNISH and CORROSION.

TARNISH is defined as a process by which a metal surface is

dulled in brightness or discolored through the formation of

chemical film , such as sulfide and an oxide.

CORROSION is defined as the action , process, or effect of

corroding ie, the loss of elemental constituents to the adjacent

environment.

CHEMICAL CORROSION

Direct combination of metal and non metal

components to yield chemical compound.

This is also called as Dry corrosion.

e.g. oxidation of silver-copper alloy particles med

with mercury to prepare dental amalgam

Importance in dentistry:

Stress corrosion

Due to fatigue or cyclic loading in oral cavity.

Concentration cell corrosion

e.g. pitting corrosion or crevice corrosion

The oxygen concentration is lower around amalgam margins

within the crevicular area as compared to outside, so acidity is

greater and plaque accumulation further increases the acidity

which causes corrosion .

Corrosion can be considered somewhat beneficial in case

of amalgam because corrosion products seal the bacterial

leakage from saliva thus improving the longevity of

restoration by decreasing the recurrent caries.

Metallic ions may also serve as bacteriostatic and

bactericidal agents.

However, this type of deterioration can also be harmful

when it occurs in thin sections at solder joints of

appliances .

According to GPT-8, color is defined as

• A phenomenon of light or visual perception that enables one to

differentiate otherwise identical objects.

• The quality of an object or substance with respect to light

reflected or transmitted by it.

• A visual response to light consisting of the three dimensions of

hue, value, and saturation.

• The perception of the Color of an object is the result of a physiological

response to a physical stimulus (light).

• Light is an electromagnetic radiation that can be detected by the human

eye. It can be seen that the visible electromagnetic radiation is in the range

from 400-700 nanometers.

Properties of materials in relation to light transmission and absorption-

Transparency - that allows the passage of light in such a manner that little

distortion takes place.

e.g. glass, pure acrylic resin.

Translucency- which allows the passage of some light and scatters or reflects

the rest . In such manner, the object cannot be clearly seen through them .

e.g. tooth enamel, porcelain, composite and pigmented acrylic resin.

Opacity- that prevents the passage of light. Opaque material absorbs all of

the light. Objects cannot be seen through them.

• When a beam of light encounters or falls on a surface of a medium,

the following may occur :

1- Reflection:

Specular Reflection-

Smooth surface: angle of incidence = angle of reflection

The restoration should have a highly smooth and polished surface to simulate

the tooth structure.

Diffuse Reflection-

Rough surface- the surface appears to have little gloss.

2- Refraction:

It is the change of the direction of a beam of light on entering second

medium.

Refraction results from the difference in refractive indices of the two media.

For perfect matching the refractive index of the restoration should be equal

to the refractive index of the tooth.

Example: Control of refractive index of the filler and matrix phases in

composite resins and porcelain.

3- Scattering

If light rays passing through a medium are obstructed by any different

inclusions it will be redirected in another direction and is attenuated.

↑ OPACITY ↓ TRANSLUCENCY.

Importance in dentistry:

Opacifiers & pigments added to composite resins act as scattering centers

that give rise to opaque shades of the material.

4. Transmission:

Light passing through an optical medium without attenuation

→ completely transmitted.

Total transmission occurs in perfectly transparent materials.

If part of the light is transmitted and part is reflected

( i.e. diffuse transmission), the material appears translucent.

THE THREE DIMENSIONS OF COLOR

Color parameters:

A- Hue: It is the dominant wave length. It represents the color of the material,

i.e yellow, green, red and blue.

B- Chroma: It represents the strength of the color or degree of saturation of the

color (color intensity).

C- Value:

It represents the lightness or darkness of color.

A black standard is assigned a value of 0, whereas a white standard is

assigned 10.

A tooth of low value appears gray and non–vital = DEAD,

Therefore, it is the most important parameter. Because it is intimately

related to the aspect of vitality in human teeth.

CIE L*a*b* COLOR SPACE

Factors affecting color appearance and selection:

According to GPT-8 :

Shade is a term used to describe a particular hue, or variation of a

primary hue such as a greenish shade of yellow.

Tooth color selection is the determination of the color and other attributes

of appearance of an artificial tooth or set of teeth for a given individual.

Shade guide is used for color matching. So, it is important to match colors

under appropriate conditions.

1- Source-

Different sources have different color content.

i.e Incandescent light OR fluorescent light.

Metamerism: It is the change of color matching of two objects under different

light sources.

Metameric pairs: Two objects that are matched in color under one light

source but are not matched under other light sources form metameric pair.

Isomeric pair: They are color matched under all light sources.

Color matching should be done under two or more different light sources.

2- Surroundings:

Colors of wall, lips or clothes of the patient modify the type of

light reaching the object.

3- Object:

• Translucency: High translucency gives a lighter color appearance (higher value)

i.e more vital tooth appearance.

• Surface texture (surface finish): smooth surface appears brighter than rough.

• Presence of scattering centers as inclusions or voids: This increase opacity and

lower the value (more dark).

• Fluorescence: It makes the teeth bright and vital.

• Thickness: The thickness of a restoration can affect its appearance. Increase in

thickness, increase opacity, and lower the value.

• Metamerism

4- Observer:

Eye and color perception

Under low lighting conditions, only the rods are used (scotopic vision).

These receptors allow interpretation of the brightness (but not the color)

of objects to be made. They are most sensitive to blue-green objects.

Color vision is dependent on the cones, which are active under higher

lighting conditions (photopic vision).

The change from photopic to scotopic vision is called Dark Adaptation.

A- Color response: Eye responds differently among individuals.

B- Color Vision: Some individuals may have color blindness and

inability to distinguish certain colors.

C- Color Fatigue: Constant stimulus of one color decreases the

response to that color.

It is measure of the affinity of the liquid for a solid as indicated by

spreading of a drop.

e.g. Wetting of denture base by saliva, wetting of enamel by pit and

fissure sealants .

Wettability of solid by a liquid can be observed by the shape of a

drop of liquid on the solid surface. Shape of the drop can be

identified by contact angle.

.

The degree of wetting depends on the relative surface energies of

the solids and liquids and on their intermolecular attraction.

High energy solids and low energy liquids encourage good wetting

thus liquids generally wet higher energy solids well. e.g. water on

metals and oxides.

On the other hand, liquids bead up on the lower energy solids-

e.g. wax, Teflon, and polymers.

The contact angle is the angle formed by the adhesive with the adherent at their interface.

Hydrophobic surface Hydrophilic surface

The contact angle of water and saliva on denture : it relates to the

retention of the denture.

Contact angle for water on wax is about 110 0 and for water on acrylic is

around 75 0

The spreading of molten solder on the surface of the parts to be

assembled : If the wetting is not adequate, soldering may fail, and if

contact angle of solder is too great, it will not penetrate into the fine

details of metals to be soldered.

ADHESION is defined as the property of remaining in close proximity, as

that resulting from the physical attraction of molecules to a substance or

molecular attraction existing between the surfaces in contact.

BONDING is defined as the procedure of using an adhesive, cementing

material of fusible ingredient to combine, unite, or strengthen .

Attachment of plaque or calculus to the tooth surface can be partially

explained by adhesion attachment.

➢ Leakage adjacent to dental restorative material is severely affected by poor

adhesion and bonding.

➢ Retention of artificial dentures depends on adhesion between the denture and

saliva, and between saliva and soft tissue.

➢ Strong adhesion increases the likelihood that a given restoration or appliance

will be retained on the teeth.

➢ Strong chemical adhesion eliminates the need for excessive removal of tooth

structure to ensure retention, through mechanical undercuts thus chemical

adhesion conserves tooth structures.