ddmt 1st quiz

1. Selection of the Materials 2. Evaluation of the Materials

The science of dental materials has two main benefits:

Direct Restorative materials

Use intra-orally to fabricate restorations or prosthetic devices directly on teeth

Indirect Restorative materials

Made extra-orally in which the materials are formed indirectly on casts or other replicas of the teeth.

SHELF LIFE

It is the length of time a dental material can be stored without deterioration.

STORAGE

Must be stored as per manufacturer’s instructions.

Over storage should be avoided.

MIXING TIME

The recommended time for mixing a material until the required consistency / homogenous mixture is achieved. It’s the time from the addition of powder to water until mixing is completed.

WORKING TIME

It’s the time during which a material can be mixed/manipulated ideally with no thickening effect. Time available to use a workable mix.

SETTING TIME

Time required for a material to reach a certain level of rigidity/elasticity.

Starts when the mixing begins until the material hardens (Setting reaction is complete)

SORPTION

The taking up and holding of one substance by another. Sorption is used especially as a general term for absorption and adsorption

ADSORPTION

The process in which liquid or gas molecules adheres firmly to the surface of solid or liquid.

ABSORPTION

The process in which a liquid or gas molecules penetrate into the solid material.

DIFFUSION

The process by which molecules intermingle as a result of their kinetic energy of random motion.

OSMOSIS

The diffusion of solute from the region of low concentration to the region of high concentration through semi permeable membrane is called osmosis.

SOLUBILITY

It is the measurement of the extent to which a material will dissolve in a given fluid. e.g. saliva or water.

EROSION

The process which combines the chemical process of dissolution with a mild mechanical action.

In dentistry erosion is used to describe the destruction of natural hard tissue by acids (either occurring naturally or present in food/drinks)

ADHESION

Force of attraction between molecules of different substances.

COHESION

Force of attraction between molecules of same substance.

SURFACE ENERGY/SURFACE TENSION

The increase in energy per unit area is called as surface energy or surface tension.

Interfacial tension that exists between the two surfaces due to unbalanced intermolecular forces.

For adhesion to exist the surfaces must be attracted to one another at their interface

WETTING/WETABILITY

Interfacial tension between a liquid and a solid resulting in a contact angle of less than 90°.

Adhesion is negligible when the surface molecules of the two materials are separated by a distance greater than 0.7nm.

CONTACT ANGLE OF WETTING

The extent to which an adhesive can wet the surface of adherent can be determined by measuring the contact angle between the adhesive and the adherent.

1) Conductor 2) Insulator 3) Electric conductivity 4) Galvanism

Electrical Properties of Dental Materials

CONDUCTOR

A conductor is a material that allow the flow of electrical current in one or more directions. A metal wire is a common electrical conductor

INSULATOR

The materials that offer high resistance to the flow of electric current. Are called insulators.

ELECTRICAL CONDUCTIVITY

The ability of a material to conduct an electric current is called electric conductivity.

GALVANISM

Galvanic action occurs when two electrochemically dissimilar metals are in contact and a conductive path occurs for electrons and ions to move from one metal to the other

BOILING POINT

The temperature at which a liquid boils and turns to vapor.

MELTING POINT

The temperature at which a given solid will melt.

FREEZING POINT

The temperature at which a liquid turns into a solid when cooled.

DEW POINT

The atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

A higher dew point indicates more moisture in the air.

Dew point greater than 20 °C (68 °F) is considered uncomfortable and greater than 22 °C (72 °F) is considered to be extremely humid.

HEAT OF FUSION

Heat of fusion is the energy required to change a gram of a substance from the solid to the liquid state at melting temperature.

HEAT OF VAPOURIZATION

Heat of vaporization is the energy required to change a gram of a liquid into the gaseous state at the boiling point Is called the "heat of vaporization

THERMAL CONDUCTIVITY

It is the measure of the ability of a material to allow the flow of heat.

THERMAL DIFFUSIVITY

Thermal conductivity of a substance divided by the product of its density and its specific heat capacity.

SPECIFIC HEAT

The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.

COEFFICIENT OF THERMAL EXPANSION

Change in length per unit original length per degree rise in temperature is called coefficient of thermal expansion.

COLOUR

Combined intensities of the wavelengths present in the beam of light determine the property color.

HUE

The property associated with colour of an object (i.e. Red, Green, Blue)

VALUE

The amount of lightness or darkness of a colour is called value (i.e. from bright to dull)

CHROMA

Degree of saturation of a particular hue. Higher the chroma = more intense the colour.

TRANSPARENCY

The property of a material that allows the passage of light in such a manner that the object may be clearly seen through.

TRANSLUCENCY

The property of a material that permits passage of light but disperses the light so the object cannot be seen through.

OPACITY

The property of a material that prevents the passage of light.

FLUORESCENCE

The phenomenon of emission of light by a substance that has absorbed light or other electromagnetic radiations. It is an emission of light (photons) by a substance that has absorbed light of higher energy.

Rheology

is the science of flow and deformation of matter and describes the interrelation between force, deformation and time. - It is the study of the manner in which materials respond to applied stress or strain. - The term comes from Greek „rheos‟ meaning to flow.

VISCOSITY

Resistance of a liquid to flow. Viscosity is dependent upon interatomic bonding. Water molecules has weak interatomic bonding thus flows easily as compared to oil/honey

Creep

is defined as the time dependent plastic strain of a material under static load or constant stress.

Flow

implies a greater deformation produced more rapidly with a smaller applied stress.

VISCOELASTICITY

The behavior that is intermediate between viscous liquid and elastic solid.

NEWTONIAN BEHAVIOR

When shear strain rate is proportional to shear stress, the behavior is called Newtonian behavior.

PSEUDOPLASTIC BEHAVIOR

Material is called pseudoplastic when viscosity decreases with increase in shear rate.

THIXOTROPIC BEHAVIOR

Material is called thixotropic when it exhibit a different viscosity after deformation

DILATANT

Behavior seen in liquids that show higher viscosity with increase in shear rate.

TENSION

Tension results when a body is subjected to two sets of forces directed away from each other in a straight line.

TORSION

Torsion results from the twisting of the body.

BENDING

Bending results by applying bending movement

STRESS

When a force acts on the body, a resistance is developed to the external force applied which is equal in magnitude/intensity and opposite in direction to the applied force and is called as???

STRAIN

Relative deformation of an object due to stress.

PROPORTIONAL LIMIT

It is the maximum stress at which the stress is equivalent/proportional to strain and above this limit the plastic deformation of a material occurs.

YIELD POINT

The point beyond which stress causes a material to undergo permanent deformation. Yield point is always slightly higher than proportional limit.

POISION’S RATIO

If a cylinder is subjected to a tensile or compressive stress, there will be simultaneously an axial or lateral strain.

STRENGTH

is the maximum stress that a material can withstand without sustaining a specific amount of plastic strain. OR Stress at the point of fracture.

SHEAR STRENGTH

Maximum shear stress at the point of fracture.

FLEXURAL STRENGTH

Defined as “force per unit area at the point of fracture when a material is subjected to flexural loading” Also called as “BENDING STRENGTH” or “MODULUS OF RUPTURE”

IMPACT STRENGTH

is the reaction of a stationary object to a collision with a moving body.

is defined as energy required to fracture a material under an impact force.

FATIGUE STRENGTH

Determined by subjecting a material to cyclic stress of maximum known value and determining the number of cycles required to cause failure of the material.

TOUGHNESS

The energy required to fracture a material is called toughness.

BRITTLENESS

It is opposite of toughness.
• When a material fractures at or near its proportional limit.
• Should not be confused with the lack of strength.
• Porcelain, Dental stone & Cements are examples of a brittle material

RESILIENCE

• It is the amount of energy absorbed by a material when it is stressed not to exceed its proportional limit.
• Measured in terms of modulus of resilience (amount of energy stored in the body)
• Modulus of resilience=Proportional limit/Modulus of elasticity

HARDNESS

In mineralogy, relative hardness of a substance is based upon its ability to resist scratching.

RELAXATION

Change in shape due to release of stresses is referred as relaxation.

PERMANENT DEFORMATION

After crossing the elastic limit with continuous stress the resulting change in strain (dimension) is permanent.

DUCTILITY

Ability of a material to deform plastically under a tensile stress before fracture. e.g. metal drawn readily into long thin wires.

MALLEABILITY

The ability of a material to sustain plastic deformation, without fracture under compression. • Gold is the most ductile and malleable pure metal, followed by silver. • Platinum is ranked third in ductility. • Copper ranks third in malleability

Gypsum

is a rock mineral mined in various parts of the world. For dental purposes, it is nearly pure calcium sulfate dihydrate

Calcination

- during heating in the manufacturing process the, gypsum loses 1½ moles of water of crystallization and is converted to the hemihydrate form of CaSO4

Impression plaster

(Type I) - used to get a negative copy

Model plaster

(Type II)

Dental stone

(Type III)

Dental stone of high strength

(Type IV)

Dental stone of high strength and high expansion

(Type V)

Calcination

heating the gypsum in order to remove the water of crystallization

Hand method
Mechanical method
Hand-mechanical combination method

3 methods of mixing gypsum and water

Hand method

single rotary motion for 1 minute

Mechanical method

use of a mixer for gypsum products for 20-30 seconds; advantage - no entrapment of air bubbles

Hand-mechanical combination method

the rotary part is mechanical while you hold the bowl; for 1 minute; has a cover

– Initial setting time (IST)

is also called the working time

• the material can still be mixed and poured into the impression

8-16 minutes from start of mixing.

Final setting time (FST) -

is defined as the time at which the material can be separated from the impression without distortion or fracture; the time at which the chemical reaction is complete → there is a second change, a decrease, in temperature.

Gilmore needle

small needle (1/4 lb) from time of mixing until needle no longer penetrates surface → IST

b. big needle (1 lb) no longer penetrates surface, is known as the FST.

Vicat needle

weighs 300 gm, 1 mm in diameter and 5 cm length

- when needle no longer penetrates bottom of plaster → IST.