3. INTRODUCTION - Denture
base resin
Choice of material for denture base fabrication
since 1940s
AVAILABLE in Powder and liquid system
Composition:
The liquid
The powder
4. Powder : Polymethyl methacrylate
colorless transparent solid
The polymer can be tinted to provide almost any color,
shade, and degree of translucency.
Its color, optical characteristics, and dimensional
properties remain stable under normal intraoral
conditions, and its physical properties have proven to
be adequate for dental applications.
Easy processing
Liquid – Methyl methacrylate
5. Classification of Denture base resin
Type of polymerization
Type I – Heat-polymerizable polymers
Type 2 – Auto-polymerizable polymers
Type 3- Thermoplastic
Type 4- Light activated
Type 5- Microwave cured materials
7. Heat activated denture base
resin
COMPOSITION:
- POWDER AND LIQUID
- GEL
- SHEETS
- CAKES
Commercial names: Stellon (DPI), Trevelon (Dentsply), Lucitone
(Bayer)
8. Heat activated denture base
resin
The Powder: The powder may be transparent or
tooth colored or pink colored
Polymethyl Methacrylate - Major component
The Initiator- Benzoyl Peroxide
Opacifiers- Zinc or titanium oxide
Plasticizer- Dibutyl phthalate ( plasticity- softer, flexible)
Dyes- Compounds of mercuric sulfide, cadmium sulfide, etc
Others-Inorganic fillers- improves physical properties and acrylic
fibers- to simulate small capillaries
9. Heat activated denture base
resin
The liquid: The liquid is supplied in tightly sealed
amber colored bottles to prevent premature polymerization
by light or ultraviolet radiation on storage
Methyl methacrylate
Plasticizer- Dibutyl phthalate
Cross-linking agent: Glycol dimethacrylate
Inhibitor: Hydroquinone Prevents undesirable polymerization
or “setting” of the liquid during storage and retard the curing
process and thereby increase working time
10. Mechanism of crosslinking
agent
Methyl methacrylate possesses one carbon-carbon
double bond per molecule and glycol dimethacrylate
possesses two double bonds per molecule.
As a result, an individual molecule of glycol
dimethacrylate can participate in the polymerization of
two separate polymer chains that unites the two
polymer chains.
If sufficient glycol dimethacrylate is included in the
mixture, several interconnections can be formed and
solvent swelling may occur.
These interconnections yield a netlike structure that
provides increased resistance to deformation.
13. Compression molding
technique
Steps
1. Preparation of the waxed denture pattern.
2. Preparation of the split mold.
3. Application of separating medium.
4. Mixing of powder and liquid.
5. Packing.
6. Curing.
7. Cooling.
8. Divesting, finishing and polishing
14. 1. Preparation of waxed
denture pattern
Wax pattern is constructed
Wax will be replaced by acrylic
15. 2. Preparation of the split
mold
Waxed denture is invested in dental flask with dental
stone or plaster called flasking
Master cast is coated with thin layer of separator to
prevent adherence of the dental stone to the master
cast during the flasking procedure.
Flasking is done using 3 pour technique
16. 3 pour technique
The lower portion of a denture flask is filled with
freshly mixed dental stone, and the master cast is
placed into this mixture.
Dental stone is contoured to facilitate wax elimination,
packing and deflasking procedures.
After initial set, the stone is coated with a separator
Upper portion of the flask is positioned above the lower
portion. Surface tension reducing agent is applied to
exposed wax surfaces
17. Second mix of dental stone is prepared and poured in
the denture flask.
Dental stone is added until it covers all surfaces of
denture and arranged tooth
Incisal and occlusal surfaces are minimally exposed to
facilitate deflasking procedures.
After it has set another coat of separator is placed.
Remainder of the flask is then poured with the stone
and covered with the lid.
19. Dewaxing: After the plaster has set, the denture flask is
immersed in boiling water for 5 min. The flask is then
removed from the water and the appropriate segments
are separated.
The record base and softened wax remain in the lower
portion of the denture flask while the prosthetic teeth
remain firmly embedded in the investing stone of the
remaining segment
The record base and softened wax are carefully
removed from the surface of the mold. Residual wax is
removed from the mold cavity using wax solvent. The
mold cavity is subsequently cleaned with a mild
detergent solution and rinsed with boiling water
20. 3. SELECTION AND APPLICATION
OF A SEPARATING MEDIUM
Application of separating medium onto the surfaces of
the mold cavity.
Function:
1. Prevents water from the mold entering into acrylic resin
2. Prevent monomer penetrating into mold material
3. Helps in easy retrieval of the denture from the mold
21. Failure to place separating
medium can lead to two major
difficulties:
1. If water is permitted to diffuse from the mold surface
into the denture base resin, it can affect the
polymerization rate as well as the optical and physical
properties of the resultant denture base.
2. If dissolved polymer or free monomer is permitted to
soak into the mold surface, portions of the investing
medium can become fused to the denture base.
These difficulties often produce compromises in the
physical and esthetic properties of processed denture
22. Types of separating media
Tinfoil
Cellulose lacquers
Solution of alginate compounds
Calcium oleate
Soft soaps
Sodium silicate
starches
23. Tin foil
It was the material used earlier
Very effective
- However, its manipulation is time consuming and
difficult
- Replaced by other separating media called tin foil
substitutes
24. Sodium Alginate solutions
It is most popular
It is water soluble
They produce thin insoluble calcium alginate films
Composition:
2% sodium alginate in water, glycerine, alcohol, sodium
phosphate and preservatives
25. Precautions
Waxes remaining on the mold surface will interfere with
the action of separating medium
Mold should be warm not hot as it might not form
continuous film if hot
Avoid coating the teeth as it will prevent bonding of
teeth with the denture base.
26. Mode of application
A fine brush is used to spread the separating medium
onto the exposed surfaces of a warm, clean stone mold.
Separator should not be permitted to contact exposed
portions of acrylic resin teeth, since its presence
interferes with chemical bonding between acrylic resin
teeth and denture base resins.
There should not be “pooling” of separator, and the
solution is permitted to dry.
One or two coats may be applied
27. 4. Mixing of powder and
liquid
POLYMER-TO-MONOMER RATIO
3:1 by volume or 2:1 by weight
The measured liquid is poured into a clean dry mixing
jar
Powder is slowly added for better wetting
Mixture is stirred and left for some time in closed
container
28. If excess monomer
- greater polymerization shrinkage
- more time is needed to reach packing consistency
- Porosity can occur
If less monomer
- Not all polymer is wetted thus granular
- Dough will be difficult to manage
29. Physical stages
After mixing the material goes through various physical
phases.
No polymerization reaction takes place during these
stages
A plastic dough is formed
31. Stage I : Wet sand stage
The polymer gradually settles into the monomer forming a
fluid, incoherent mass
32. Stage II – Sticky stage
The monomer attacks the polymer by penetrating into
the polymer.
The mass is sticky and stringy ( cobweb like) when
touched a pulled apart
33. Stage III – Dough or gel stage
The monomer diffuses into the polymer, it becomes
smooth and dough like.
It does not adhere to the walls of the jar
It contains undissolved polymer particles suspended in a
plastic matrix of monomer and dissolved polymer.
The mass is plastic and homogenous and can be packed
into the mold at this stage
34. Stage IV- Rubbery stage
The monomer disappears by further penetration into
the polymer or evaporation
The mass is rubberlike, non plastic
It cannot be molded
35. Stage V- Stiff stage
The mass is totally unworkable and is discarded
36. Working time
The working time is the time from Stage II and the
beginning of Stage IV.
The ideal packing phase is the dough stage
Working time is affected by temperature.
In warm condition the working time is less thus the jar
is chilled to prolong the working time
37. 5.Packing
The mixture is packed into the flask in dough stage
- If it is packed at the sandy or stringy stages, too much
of monomer will be present between the polymer
particles, and the material will be of too low viscocity
and will flow out of the flask too easily. It may also
cause porosity in the final denture base.
- If packed at the rubbery to stiff stage, the material
will be too viscous to flow and metal to metal contact
of the flask halves will not be obtained. It may result in
movement or fracture of the teeth, Loss of detail,
Increase in the vertical height of the denture
38. Trial closure
The acrylic dough is packed into the flask in slight excess
The excess is removed during the trial closure packing with
a damp cellophane film used as a separator for the upper
half of the flask
A hydraulic or mechanical press may be used to apply
pressure
The closing force is applied slowly during the trial packing to
allow excess dough called flash to flow out of the flask.
The flask is opened flash is trimmed away.
The separating film is removed.
The final closure of the flask or metal to metal contact of the
flask halves is then completed in the press.
The flasks are then transferred to a holding clamp which
maintains the pressure throughout the curing process
39. Curing ( polymerization)
After final closure the flasks are kept in room
temperature for 30-60 minutes called bench curing
Purpose of bench curing:
1. Equalizes the pressures throughout the mold
2. Uniform dispersion of monomer throughout the mass of
dough
3. Longer exposure of resin teeth to the monomer
producing better bond of the teeth and base.
40. Curing cycle
Curing cycle or polymerization cycle is the process of
heating used to initiate and control the complete
polymerization of the resin in the mold
The time depends on the thickness of the resin in the mold
THICKER- long curing cycle to avoid porosity
1. Long curing cycle
2. Short curing cycle
41. Long Curing Cycle:
a. 74 c for 8 hours
b. 74 c for 8 hours then boil for 1 hour
Short Curing Cycle
a. 74 c for 2 hours, then boil for 1 hour
42. Cooling
The flask should be cooled slowly called as bench
cooling.
Fast cooling can result is warpage of the denture due to
differential thermal contraction of the resin and gypsum
mold.
Cooling overnight is ideal
However, bench cooling for 30 minutes and then placing
in cold tap water for 15 minutes is satisfactory.
43. Deflasking
The cured acrylic is retrieved from the flask called
deflasking
Flask is opened and mold is retrieved
Deflasking should be done with great care to avoid flexing
and breaking of the denture.
44. Finishing and polishing
The denture is smoothened using progressive grades of
sandpaper
Finely ground pumice in water
45. Injection molding technique
A special thermoplastic resin is used (Thermoplastic
resins are materials that soften to a liquid in high heat
and then harden again when cooled)
Special bath is required for curing
46. Injection molding technique
A sprue hole and a vent hole are formed in the gypsum
mold with the help of sprue formers
The soft resin is contained in the injector and is forced
into the mold space as needed
It is kept under pressure until it is hardened
Continuous feeding of the material under pressure
compensates for shrinkage
47. Advantages
1. Dimensional accuracy
2. No increase in vertical dimension
3. Homogeneous denture base
4. Low free monomer content
5. Good impact strength
48. Disadvantages
1. Higher cost of equipment
2. Mold design problems
3. Less craze resistance (surface defect)
4. Special flask is required
49. 2. Chemically activated
denture base resin
does not require the application of thermal energy
Polymerization can be completed at room temperature
Chemically activated resins often are referred to as
cold-curing, self-curing, or autopolymerizing resins
52. Uses
1. For making temporary crowns and bridge
2. Construction of special trays
3. Denture repair, relining and rebasing
4. For making removable orthodontic appliances
5. For adding a post dam
6. For making temporary and permanent denture bases
7. For making inlay and post core patterns
54. Mechanism of reaction
Activator- Tertiary amine such as dimethyl p-toluidine to
the liquid
Causes decomposition of benzoyl peroxide producing
free radicals and polymerization is initiated
Polymerization progresses in a manner similar to that
described for heat-activated systems
55. The degree of polymerization is less in chemically
activated than in heat-activated systems
There is a greater amount of unreacted monomer which
causes
- First, it acts as a plasticizer, resulting in decreased
transverse strength of the denture resin.
- Second, the residual monomer serves as a potential
tissue irritant, thereby compromising the
biocompatibility of the denture base.
56. COLOR STABILITY:
Color stability of chemically activated resins generally is
inferior due to the presence of tertiary amines within
the chemically activated resins.
Such amines are susceptible to oxidation and
accompanying color changes that affect the appearance
of the resin. Discoloration can be minimized by
stabilizing agents that prevent such oxidation.
WORKING TIME:
The working time for chemically activated resins is shorter
than for heat-activated materials.
57. Advantages
Better initial fit as curing occurs in room temperature
so less thermal contraction
Easy to use for repairing as heat cure causes warpage
(change in shape or fit)
58. Disadvantages
Color stability is inferior to heat cure resin due to
oxidation of the tertiary amine.
Properties are inferior to heat cure as degree of
polymerization in self cure
59. Manipulation of self cure
1. Sprinkle on technique
2. Finger adapting technique
3. Fluid resin technique
60. 1. Sprinkle on technique
- Also known as salt and pepper technique
- Powder and liquid is applied alternatively from droppers
- Powder is sprinkled on the cast and then wet with
monomer
61. The appliance is constructed section by section until
completion
To improve the strength it is placed in pressure pot at
20 psi for around 20 minutes
62. 2. Finger adapted method
• Powder and liquid is proportioned and mixed in a
porcelain jar
• When it reaches dough stage it is removed and adapted
on to the cast and manually molded to the desired
shape
• Curing is completed in a pressure pot
63. 3. Fluid resin technique
A special resin is available for this technique
Pour type of denture resin have high molecular weight
powder particles that are much smaller and when they
are mixed with the monomer, the resulting mix is very
fluid.
Thus they are referred to as fluid resin
They have low powder liquid ratio 2:1 or 2.5:1 makes it
easier to mix and pour
Polymerization is done under pressure at 20 psi at 45 c
for 25 minutes
65. Method of flasking and curing
Agar hydrocolloid or silicone is used for the mold
preparation instead of gypsum
A special flask and resin is used is used
Involves preparing the mold with silicone, creation
of channels for pouring and venting
Fluid resin is poured through one channel in a thin
stream till excess is seen through the vent
Polymerization is done under pressure at 0.14 Mpa
(20psi) at a temperature of 45 c for 25 minutes
66. Advantages
1. Better tissue fit
2. Fewer open bites
3. Less fracture of porcelain teeth during deflasking
4. Reduced material cost
5. Simple laboratory procedure for flasking, deflasking and
finishing of the dentures
67. Disadvantages
1. Air inclusion
2. Shifting of teeth during processing
3. Infraocclusion
4. Occlusal imbalance due to shifting of teeth
5. Incomplete flow of denture base material over neck of
anterior teeth
6. Formation of films of denture material over cervical
portions of plastic teeth that had not been previously
covered with wax
7. Poor bonding to plastic teeth
8. Technique sensitivity
9. Low mechanical properties than conventional heat cure
68. LIGHT-ACTIVATED DENTURE
BASE RESINS
Single-component denture base resins are
supplied in sheet and rope forms and are
packed in lightproof pouches to prevent
inadvertent polymerization.
COMPOSITION:
Urethane dimethacrylate microfine silica,
and high-molecular-weight acrylic resin
monomers.
Fillers- Acrylic resin beads
Activator- Visible light
Initiator-Photosensitizing agent such as
Camphorquinone
69. Light activated denture base
- Supplied in premixed sheets of having a clay like
consistency
- Supplied in various colors like pink, transparent
- obtained in light tight packages to avoid premature
polymerization
70. The most recent generation of light-activated denture
base resins consists of three resins.
3 forms
1. base-forming resin
2. tooth-setting resin
3. contouring resin
71. The base-forming resin is adapted to the dental cast. The
cast and denture base are placed into a high-intensity light
chamber to induce polymerization.
The tooth-setting resin is used to attach the prosthetic
teeth to the polymerized base. A high-intensity light source
is used to polymerize the tooth-setting resin, thereby
maintaining prosthetic teeth in the desired positions.
The contouring resin is used to generate the desired final
surface form. The resultant prosthesis is placed into the
light chamber to complete the denture base fabrication
process. The denture is then removed from the cast,
finished, and polished in a conventional manner.
72.
73. Manipulation
- Denture base material is adapted to the cast while it is
in a plastic state.
- The denture base can be polymerized without teeth and
used as baseplate
- The teeth are added to the base with additional
material and the anatomy is sculpted while the material
is still soft
- It is polymerized in a light chamber with blue light of
400-500 nm from high intensity quartz halogen bulbs
- The denture is rotated continuously in the chamber to
provide uniform exposure to the light source
75. Chemical stages of
polymerization
1. Induction:
Induction is the time during which the molecules of
the initiator becomes activated to form free radical
which reacts with monomer and start to transfer the
energy to other monomer
Impurities can increase the length of this period
Higher the temperature, shorter the length of the
induction period
Initiation energy for activation of each monomer unit is
16000-29000 calories per mol in the liquid phase
76.
77. Heat activation:
- Free radicals liberated by heating benzoyl peroxide will
initiate the polymerization of methyl methacrylate
monomer
Chemical activation:
- Consists of two reactants, when mixed they undergo
chemical reaction and liberate free radicals. E.g. use of
benzoyl peroxide and dimethyl p-toluidine in self cured
dental resins
Light activation:
- Photons of light energy activate the initiator to generate
free radicals, e.g. Camphorquinone and an amine react to
form free radicals, when they are irradiated with visible light.
78. 2. Propagation:
After the growth has started only 5000-8000 calories per
mole are required
Process continues rapidly with evolution of heat
The chain reactions should continue with the evolution of
heat until all the monomer has been changed to polymer.
The resulting free radical-monomer complex then acts as a
new free radical center when it approaches another
monomer to form a dimer, which also becomes a free
radical.
This reactive species, in turn, can add successively to a
large number of ethylene molecules so that the
polymerization process continues through the propagation
of the reactive center.
79.
80. 3. Chain transfer:
The chain termination can result from chain transfer. The
active state is transferred from an activated radical to an
inactive molecule and a new nucleus of growth is created.
An already terminated chain can be reactivated by chain
transfer resulting in continued growth
81. 4. Termination:
The chain reaction can be terminated either by direct
coupling of two chain ends or by exchange of a hydrogen
atom from one growing chain to another
82. Inhibition of Polymerization
Inhibitors:
These react with the activated initiator or any activated
nucleus, or with an activated growing chain to prevent
further growth, e.g. hydroquinone (0.006%) is added to
prevent polymerization of monomer during storage.
Oxygen
Presence of oxygen also inhibit polymerization
83. Co-polymerization
In order to improve the physical properties it is
advantageous to use two or more chemically different
monomers as starting materials
The polymers thus formed may contain units of these
monomers. Such a polymer is called copolymer and its
process of formation is known as copolymerization
84. Importance of
copolymerization
To improve the physical properties of resin
- Small amounts of ethylacrylate with methyl methacrylate to
alter the flexibility
- Block and graft polymers show improved impact strength. They
modify the adhesive properties of resins and their surface
characteristics.
85. Cross linking
The formation of chemical bonds or bridges between
the polymer chains is called cross linking
Application:
1. Increases rigidity and decreases solubility and water
absorption
2. Improve resistance to solvents, crazing and surface
stresses
88. Methyl methacrylate monomer
It is a clear, transparent, volatile liquid at room
temperature.
It has a characteristic sweetish odor
Physical properties:
Melting point = -48 c
Boiling point = 100 c
Density 0.945 gm/ml at 20 c
Heat of polymerization = 12.9 Kcal/mol
Volume shrinkage during polymerization= 21%
89. Polymethyl methacrylate
1. Taste and odor:
They are tasteless and odorless
Poorly made dentures with porosity absorb food and
bacteria, resulting in an unpleasant taste and odor
2. Esthetics:
It can be pigmented easily to duplicate oral tissues
It is compatible with dyed synthetic fibers
3. Density:
Polymer has a density of 1.19 gm/cm3
90. 4. Strength:
Adequate compressive and tensile strength for complete or
partial denture applications
Compressive strength- 75 Mpa
Tensile strength- 48-62 Mpa
Self cure resin have low strength, it is affected by:
1. Composition
2. Technique of processing
3. Degree of polymerization
4. Water sorption
5. Subsequent environment of the denture
91. Impact strength:
It is a measure of energy absorbed by a material when it is
broken by a sudden blow. It can be increased by
modification with rubber.
Chemically activated- 13 (J/m)
Conventional heat cured acrylic resin- 15 (J/m)
Rubber modified- 31 (J/m)
Polyvinyl resins- 30 (J/m)
Fatique strength:
Ability of the denture to withstand large number of small
cyclic loading such as mastication.
93. 6. Modulus of elasticity:
Sufficient stiffness for use in complete and partial dentures
Self cure have slightly lower values
7. Creep:
When load is applied an initial deflection is observed
If the load is sustained additional deformation is observed
over time. The additional deformation is called creep
Self cure resin have higher creep rates
94. 8. Dimensional stability:
Well process acrylic resin denture has good dimensional
stability
9. Water sorption:
Acrylic resin absorb water 0.7mg/cm2 and expand
This partially compensates for the shrinkage
10. Solubility
- Insoluble in water and oral fluids
- Soluble in ketones,esters
- Alcohol causes crazing in some resins
- ISO stipulates solubility should not exceed
Heat cure- 1.6ug/mm2
Self cure- 8 ug/mm2
95. 11. Thermal properties:
Polymethyle methacrylate:
125 c = softens
125 – 200 c= depolymerize
450 c = polymer will depolymerize to monomer
Thermal conductivity:
They are poor conductors of heat and electricity. Patient
wearing acrylic dentures do not feel any temperature of
food thus reducing the pleasure.
Heat distortion temperature for PMMA= 71-91 c
It is of concern during repairing or polishing of dentures
96. 12. Color stability:
Self cure have lower color stability then heat cure. Self
cure slightly gets yellow.
13. Biocompatibility:
Completely polymerized resins are biocompatible
14. Adhesion:
Adhesion of acrylic to metal and porcelain is poor and
mechanical adhesion is required.
Adhesion to plastic denture teeth is good (chemical
adhesion)
Adhesion to metal or ceramic can be improved by treating
with silane coupling agent
97. 15. Radiopacity:
Radiopacity is desirable property to enable easy location of
fragments.
Inclusion of heavy metals like bismuth or uranyl 10-15%
16. Shelf life:
Acrylic in powder and liquid form have best shelf life than
in compare to that obtained in gel form.
98. 17. Residual monomer:
- During polymerization process the amount of residual
monomer decreases rapidly initially and then later more
slowly
The highest residual monomer level is observed with self
cure resin at 1-4% shortly after processing
long cure cycle used then residual monomer is 0.4%.
Short cure cycle- 1-3%
To reduce residual monomer in heat cured resin, it should
be processed for longer time.
Gradual temperature raise, temperature should be raised
to boiling only after most of the polymerization is
completed or else porosity may result.
99. 18.POLYMERIZATION
SHRINKAGE
Methyl methacrylate monomer is polymerized to form
Polymethyl methacrylate
The density of the mass changes from 0.94 to 1.19
g/cm3 .
This change in density results in a volumetric shrinkage
of 21%.
In addition to volumetric shrinkage, one also must
consider the effects of linear shrinkage.
Linear shrinkage exerts significant effects upon
denture base adaptation and cuspal interdigitation.
100. POLYMERIZATION SHRINKAGE
Processing shrinkage has been measured as 0.26% for a
representative chemically activated resin, compared
with 0.53% for a representative heat-activated resin.
However, there are several other factors that affect the
overall dimensional characteristics of processed denture
bases including the
type of investing medium,
method of resin introduction, and
temperature used to activate the polymerization
process
102. Porosity
Porosity results in:
1. Makes the appearance of denture unsightly
2. Proper cleaning is difficult thus hygiene compromised
3. Weakens the denture base. There is stress
concentration in the porosity area causing denture
warps
Types
1. Internal porosity
2. External porosity
103. Internal porosity
Appears as voids or bubbles within the mass of acrylized
resin
Confined to the thicker portion of the denture base
May not occur uniformly
Causes:
Vaporization of monomer when the temperature of the resin
increase above the boiling point of monomer i.e 100.8 C
The center of the thick portion of acrylic cannot conduct
away the heat where temperature rises above the boiling
point causing porosity called localized subsurface porosity.
Dentures with excessive thickness should be cured using long
curing cycle
104. Localized subsurface porosity:
Seen in the center of the flask rather than near the metal
surface. The metal of the flask conducts heat away from the
periphery with sufficient rapidity to prevent a substantial
temperature rise. As a result, the low-molecular weight species
does not boil and porosity does not develop. In contrast, resin
specimens occupying central positions in the mold are
surrounded by larger amounts of gypsum. Because gypsum is a
poor thermal conductor, heat cannot readily dissipated.
Consequently low-molecular-weight species are vaporized and
noticeable porosity is produced.
105. External porosity
Surface porosity occurs due to:
1. Lack of homogeneity:
If the dough is not homogenous at the time of
polymerization, some regions of the resin mass will
contain more monomer than others.
During polymerization, more monomer containing
regions shrink more than adjacent regions, and the
localized shrinkage tends to produce voids. The resin
appears white.
Proper polymer-to-monomer ratios and well-controlled
mixing procedures and dough like stage has more
homogenous mix so it is wise to delay
1. Lack of homogeneity
2. Lack of adequate pressure
3. Air entrapment
106. 2. Lack of adequate pressure:
Lack of pressure during polymerization or inadequate
amount of dough in the mold during final closure causes
bubbles. They assume irregular shapes.
These voids may be so abundant that the resultant resin
appears significantly lighter and more opaque than its
intended color.
Remedy: Use required amount of dough. Check for excess
or flash during trail closure. Flash indicated adequate
material.
107. 3. Air entrapment:
A final type of porosity is most often associated with fluid
resins. Such porosity results from air entrapment during
mixing and pouring procedures. If air inclusions are not
removed, sizable voids may be produced in the resultant
denture bases. Therefore careful mixing, spruing, and
venting are essential.
108. Crazing
Formation of surface cracks on denture base resin
May be macroscopic or microscopic in size
Crazing weakens the resin and reduces the esthetic
qualities
Crack formed can cause fracture
Causes:
1. Mechanical stresses
2. Attack by solvent
3. Incorporation of water
109. Crazing is mechanical separation of the polymer chains
under tensile stress
Cracks are at right angles to the direction of tensile
stress
Crazing is visible around the porcelain teeth in the
denture and is due to contraction of the resin around
the porcelain teeth during cooling after processing
Water incorporation during processing will form stresses
due to evaporation of water after processing causing
crazing
110. Prevention of Crazing
1. Using cross linked acrylics
2. Tin foil separation medium
3. Metal molds
111. Denture warpage
- Deformity or change of shape of the denture which can
effect the fit of the denture.
- May occur during processing
Causes:
1. Release of stress incorporated during processing
2. Rise in temperature during polishing
3. Immersion of denture in hot water
4. Rapid cooling
5. Curing shrinkage
6. Packing during rubbery stage
7. Improper flasking
8. Re-curing of denture after relining
112. Repair of acrylic resin
Repair can be done using
- Heat cure
- Self cure
Heat cure resin-
Use of heat cure resin will tend to warp the denture during
processing
Self cure resin-
Insignificant warpage
113. Infection control
Ethylene oxide gas is suitable for disinfecting the
material
Phenol and gluteraldehyde solutions are avoided as
polymeric material can absorb such liquids
114. Care of Acrylic dentures
Proper care and maintenance of denture is essential
Methods:
1. Dentures should be stored in water when not in use.
Drying can cause dimensional changes
2. Avoid hot water for cleaning can cause distortion due to
release of stress
3. Abrasive dentrifices should not be used it will get
scratched and abraded.
