Properties of Denture base materials /rotary endodontic courses

Methyl Methacrylate Monomer:
 It is a clear, transparent, volatile
liquid at room temperature.
 It has a characteristic sweetish
odour.
Properties of denture resins:
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The physical properties of monomer are:
•Melting point - -48ºC
•Boiling point - 100.8ºC
•Density - 0.945 gm/ml at 20ºC
•Heat of polymerization - 12.9Kcal/mol
•Volume shrinkage during
polymerization - 21%
Properties of denture resins:

Poly (methyl methacrylate):
1. Tasteless and Odorless.
2. Clear transparent resin which can be
pigmented (colored) easily to duplicate the
oral tissues.
Compatible with dyed synthetic fillers.

Poly (methyl methacrylate):
Density: 1.19gm/cm3

Strength:
Compressive and Tensile strength:
 Low in strength.
 Adequate compressive and tensile
strength for complete or partial denture
applications.

Strength:
Compressive and Tensile strength:
 Compressive strength - 75MPa
 Tensile strength - 52MPa
Self cured resins generally have lesser
compressive and tensile strength.

The strength is affected by:
1. Composition of the resin.
2. Technique of processing
3. Degree of polymerization
4. Water sorption
5. Subsequent environment of the
denture.

Impact strength:
 It is a measure of energy absorbed by a materials
when it is broken by a sudden blow.
 Ideally denture base resins should have high
impact strength to prevent breakage when it is
accidentally dropped.
 Addition of plasticizers increase the impact
strength.
 Self cured resins have a lower impact strength.www.indiandentalacademy.com

Acrylic resins are having low hardness.
Easily scratched and abraded.
Heat cured acrylic resin : 18-20 KHN
Self cured acrylic resin : 16-18 KHN
Hardness:

Modulus of Elasticity:
Sufficient stiffness for use in complete and
partial dentures (modulus of elasticity – 2400
MPa).
 Low compared with metal denture bases.
Self cured acrylic resins have slightly lower
values.

Dimensional stability:
Good dimensional stability.
(well processed )
The processing shrinkage is balanced
by the expansion due to water
sorption.

Dimensional stability:
a. Shrinkage:
Acrylic resins shrink during
processing due to two reasons:
1. Thermal shrinkage on cooling
2. Polymerization shrinkage.

Polymerization shrinkage:
The density of the monomer changes
from 0.945gm/cc to 1.19gm/cc.
This results in shrinkage in the
volume of monomer-polymer dough.

Polymerization shrinkage:
The fit of the denture is not affected because
the shrinkage is uniformly distributed over all
surfaces of the denture.
so, the actual linear shrinkage observed is
low.
Volume shrinkage - 8%
Linear shrinkage - 0.53%

b.Water sorption:
 Acrylic resins absorb water (0.6mg/cm2
)
and expand.
 Partially compensates for its processing
shrinkage.
 This process is reversible.
 Repeated wetting and drying should be avoided
as it may resulting warpage of the denture .

Solubility:
Poly (methyl methacrylate) is virtually
insoluble in water and oral fluids.
They are soluble in ketones, esters, and
aromatic and chlorinated hydrocarbons,
Alcohol causes crazing in some resins.

Thermal properties:
a.Stability to heat:
 Poly (methyl methacrylate) is chemically
stable to heat.
 It softens at 125º C.
 It begins to depolymerize between 125ºC and
200ºC
 At 450ºC, 90% of the polymer will
depolymerize to monomer.www.indiandentalacademy.com

Thermal properties:
b. Thermal conductivity:
They are poor conductors of heat and
electricity.

Thermal properties:
c. Coefficient of thermal expansion:
These materials have a high
coefficient of thermal expansion
(81x10-6
/ºC).
Addition of fillers reduces the
coefficient of expansion.

•Color stability:
Heat cure acrylic resins -
good colour stability.
Self cure resins is slightly lower

Biocompatibility:
• Completely polymerized acrylic resins
are biocompatible.
•Pure monomer if inhaled over a long
period is toxic.
• It may also cause allergic reactions in
some individuals.

Precautions to be taken are:
1. Minimize residual monomer content by
using proper processing techniques.
2. Avoid direct handling of acrylic dough
with bare hands.
3. Work in well ventilated areas to avoid
inhalation of the monomer vapour.

Residual monomer:
During the polymerization process the
amount of residual monomer decreases
first rapidly and then later more slowly.

Residual monomer:
The highest residual monomer level is observed
with self cure denture base resins at 1% to 4%
shortly after processing.
When they are processed in less than 1 hour in
boiling water the residual monomer is 1% to 3%.
If they are processed for 7 hours at 70ºC and
then boiled for 3 hours the residual monomer
content may be less than 0.4%.www.indiandentalacademy.com

Adhesion:
The adhesion of acrylic to metal and
porcelain is poor, so mechanical retention is
required.
Adhesion to plastic denture teeth is good.

Shelf life:
It varies considerably.
The acrylic resins dispensed as powder/liquid
have the best shelf life.
The gel type has a lower shelf life and has to
be stored in a refrigerator

In heat cured acrylic before the start of curing the
residual monomer is 26.2%.
In 1 hour at 70ºC it decreased to 6.6% and at
100ºC it was 0.29%
In order to reduce the residual monomer in heat
cured dentures it should be processed for a longer
time in boiling water.

The processing temperature should be
raised to boiling when most of the
polymerization is completed otherwise
porosity may result.

Porosity:
Porosity is a processing error in acrylic resins.
When porosity is present on the surface, it makes
the appearance of denture base unsightly.
Proper cleaning of the denture is not possible, so
the denture hygiene and thus the oral hygiene
suffers. www.indiandentalacademy.com

Porosity:
Even when it appears as internal porosity in
the form of pores or blebs,
it weaken the denture base and areas of
stress concentration,
thus the denture warps as the stresses relax.

Porosity may be:
1. Internal porosity
2. External porosity

Internal porosity:
 Is in the form of voids or bubbles within the
mass of the polymerized acrylic.
 It is usually not present on the surface of a
denture.
It is confined to the thick portions of the
denture base and
 it may not occur equally throughout the
affected area. www.indiandentalacademy.com

Cause:
Internal porosity is due to the vaporization of
monomer when the temperature of the resin
increases above the boiling point of monomer
(100.8ºC) or very low molecular weight
polymers.

In the centre of the thick portion, the heat
cannot be conducted away therefore the
temperature in the thick portions may rise
above the boiling point of monomer.
Inter porosity can be avoided by using long
and low temperature curing cycle.
 The denture is processed for 9 hours at
74ºC, without bringing the water to boil.

External porosity:
It can occur due to two reasons:
1. Lack of homogeneity :
If the dough is not homogenous at the time of
polymerization, the portions containing more
monomer will shrink more than the adjacent
areas.
This localized shrinkage results in voids.
The resin appears white.

External porosity:
It can be avoided or minimized by using
proper powder-liquid ratio and mixing it well.
The mix is more homogenous in the dough
stage,
so packing should be done in the dough stage.

Lack of adequate pressure:
During polymerization or due to lack of
dough in the mould during final closure.
Bubbles appear which are not spherical and
the resin appears white.
A pigmented resin appears lighter in colour
due to lack of pressure during
polymerization.

Lack of adequate pressure:
This is avoided by using the required
amount of dough and distributing it correctly
in the mould cavity.
Check for excess or flash during trial
closure.
Flash indicates adequate material within the
mold cavity.

Problems in addition to porosity are attendant
with rapid initial heating of acrylic dough above
70ºC such as production of internal stress with
subsequent warpage of the denture after
deflasking, and the checking or crazing around
the necks of the artificial teeth.

Crazing:
Crazing is formation of surface cracks on the
denture base resin.
These cracks may be microscopic or
macroscopic in size.
In some cases it has a hazy or foggy appearance
rather than cracks.

Crazing:
Crazing has a weakening effect on the
resin and reduces the esthetic qualities.
Cracks formed on crazing are indicative of
he beginning of a fracture.

Causes:
Crazing is due to
1.Mechanical stresses or
2.Attack by a solvent.

Causes:
In poly (methyl methacrylate) crazing
occurs when tensile stresses are present.
The cracks are at right angles to the
direction of tensile stress.
Crazing is a mechanical separation of the
polymer chains or groups under tensile
stress. www.indiandentalacademy.com

Crazing is visible around the porcelain teeth in the
denture.
The tensile stress is due to the contraction of the
resin around the porcelain tooth during cooling of
the denture after processing.
Due to the action of solvents the cracks are
randomly placed. Eg. Weak solvents i.e. alcohol.

Incorporation of water during
processing will form stresses due to
evaporation of water after processing.
Thus the denture will craze.

Crazing can be avoided by:
1. Using cross-linked acrylics.
2. Tinfoil separating medium
3. Metal Molds.

Properties of Denture base materials /rotary endodontic courses

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Properties of Denture base materials /rotary endodontic courses