Denture base resins/ cosmetic dentistry training

SEMINAR
ON
DENTURE BASE RESINS
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Contents
Introduction
History
Requirements
Classification
Composition
- heat cure
- autopolymerising resin
-other resin materials
Aspects of manipulation
Recent advances(modified acrylics)
Summary and conclusion
References www.indiandentalacademy.com

Introduction
Denture base is that part of a denture
that rests on the foundation tissues and
to which teeth are attached.Although
individual denture bases may be formed
from metals or metal alloys, the majority
of denture bases are fabricated using
polymers.

History
1851 – Vulcanite was developed.
1853-1st
denture base from vulcanite was
constructed by Nelson Charles Goodyear,
1891- use of vulcanite as a denture base material was
considered “universal”.
1869- Celluloid was introduced, developed from plant
framework.
It had natural gingival colour but short shelf
life.therefore, NEW MODE CONTINOUS combination
product, of rubber denture base material, a celluloid
gum work area and porcelain teeth.

1910-1940
Other polymers:vinyl acrylics,Polystyrene,
Epoxy,Nylon, Vinyl styrene, Polycarbonates,
Polysulfonates-unsaturated Polyester,
Polyurethane,Polyvinyl acetate- ethylene,
Hydrophilic polyacrylate,Silicones,Light
activated urethane dimethacrylate,Rubber
reinforced acrylics, and Butadiene- reinforced
acrylics.
1930-1940 the Phenol formaldehyde products
were 1st
to compete with vulcanite.
Self cure were introduced after world war II.

1936- Vernonite or
poly methylmethacrylate(PMMA).
1st
practical replacement for Vulcanite.
1946- 95% of dentures were made with
Vernonite

Plastic other than acrylic resins
Nylon: found unsatisfactory owing to its poor ability to
resist oral conditions-swelled badly.
-if left outside water will shrink.
The epoxy resins:drawbacks
-high rate of water sorption with dimensional
change,fracture,loss of posterior teeth,heavy deposition
of various stains and calculus,colour change and some
fouling odour.
-Phenol formaldehyde(Bakelite):proved too difficult to
process.
•Color instability.
Vinyl resins:low resistance to fracture.
•Failure were common.

Polycarbonates:Contains 10% glass fibers of
150um length and 10 um diameter.
•Good impact strength- nine times more than
PMMA.
•More difficult to mould dentally, since injection
moulding technique is required.
•Greater water sorption,higher flexibility,lower
hardness,lower adhesion to acrylic teeth.
Acrylics: Better aesthetic quality
•Easy to process using inexpensive techniques.
•Shrink at the rate of 1um in eight hours. if left
outside

Requirements
1. Biological:
- non toxic and non-irritant.
2. Chemical:
- inert , in-soluble and non-absorbent.
3. Physical:
- Aesthetically satisfactory.
- The softening temperature should be higher than
the temperature of liquids and food ingested.
- Dimensionally stable.
- Low value of specific gravity or density.

- High value of thermal conductivity: to maintain the
healthy mucosa,and to retain normal reaction to hot
and cold stimuli.
- It should be radio-opaque.
4. Mechanical properties:
- A high value of modulus of elasticity- For greater
rigidity.
- A high value of elastic limit to prevent permanent
deformation.
- A combination of both above would allow the base
to be fabricated in thin sections.

- Sufficient flexural strength to resist fracture.
- An adequate fatigue life and a high fatigue limit.
- Good impact strength.
- Sufficient abrasion resistance to prevent excessive
wear.
5. Miscellaneous
- Relatively inexpensive.
- Easy to manipulate and fabricate.
- Easy to clean.
- Easy to repair.

CLASSIFICATION
I. According to ISO 1567
Type Class Description
1 1 Heat processing (powder and liquid)
1 2 Heat processing (plastic cake)
2 1 Self cure (powder and liquid)
2 1 Self cure (powder and liquid pour type
resins)
3 - Thermoplastic blank or powder
4 - Light-activated materials
5 - Microwave materialswww.indiandentalacademy.com

II. According to material system
1. Metallic – stainless steel denture base
2. Non-metallic – acrylic resins, etc.
III. According to mode of polymerization
1. Addition polymer – e.g. acrylic resins, polyvinyl
chloride, polymethyl methacrylate
2. Condensation polymer – e.g. bakelite, nylon
IV.According to method of activation
1. Thermal – heat cure acrylics
2. Chemical- self cure acrylics
3. Light activated
4. Microwave energywww.indiandentalacademy.com

Acrylic resin materials
Polymerisation reaction:
The polymerisation of PMMA involves the
conversion of low molecular weight
monomer to high molecular weight polymer.
Free radical addition polymerisation which
involves activation, initiation ,propagation
and termination.

Initiation
Free radical
Monomer moleculewww.indiandentalacademy.com

Propagation

Termination

Chain transfer

Composition of heat cure denture base resin
Powder
a. Prepolymerised polymethyl methacrylate in the form
of beads or granules.
b. Initiators – Benzoyl peroxide (0.5-1.5%), to initiate the
polymerization of the monomer
c. Plasticizer – Dibutylpthalate (8-10%), increases the
rate of dissolution of polymer in monomer.
d. Pigments – Cadmium selenide (pink), Mercuric sulfide
(red),Rouge/ Ferric oxide(brown).
e. Synthetic fibers:Acrylic or nylon to give veined
appearance. www.indiandentalacademy.com

f. Inorganic particles-Glass fibers zirconium silicate,
silicone carbide etc. to increase the stiffness and
decrease the coefficient of thermal expansion.
g. Opacifiers-Heavy metal compounds, e.g. salts of
Barium and Strontium.
Liquid
a. Methyl methacrylate monomer- It has a limited shelf
life
b. Inhibitor- Hydroquinone, added to improve the shelf
life of monomer
c. Cross linking agent- Ethylene glycoldimethacrylate, it
improves the mechanical properties, more resistant to
surface crazing.

d. Plasticizer- Dibutylpthalate, to produce a softer and
more resilient polymer.
Gel types – e.g. Vinyl acrylics
•Liquid and powder have been mixed to form a gel and
shaped into a thick sheet.
•Chemical accelerators cannot be used.

Self cure resins:
Dispensed as powder and monomer liquid.
Composition:
Polymer: PMMA in the form of beads.
Copolymer-PEMA
Initiator (< 2%) Benzoyl peroxide
Plasticizer dibutyl phthalate
Colour pigments: salts of Mercury and Iron
Dyed synthetic fibers
Inorganic particles glass fibers, Zirconium Silicate
Radio-Opacifiers-salts of Barium .

Monomer:
Methylmethacrylate monomer.
Inhibitor- hydroquinone
Cross linking agent- glycol dimethacrylate
Plasticizer- dibutylpthalate
Activator- dimethyl para toludine
Setting reaction:
PMMA +initiator+MMA+inhibitor+activator
(powder) (liquid)
free radicals(R°)

R° + M RM°
Liberation of heat.
The rate of polymerisation is influenced by
particle size.
Properties:
Free of residual monomer- 3-5%
Polymerisation of self cure is never completed.
The effects of free residual monomer:released
from the denture thus irritate the mucosa.
it will act as plasticizer and make the resin
weaker and more flexible.
The degree of polymerisation is inferior.www.indiandentalacademy.com

Shorter working time due to gradual increase in
viscosity.
Curing is done at room temperature. So better
fit and more stable.
The color stability is not good.
Easy to manipulate.
Manipulation: Compression moulding technique
-Allowed to polymerize at room temperature in a
pressure chamber.
-Dough forming time is 5 mins.
-Trial closure only 1 or 2 .
-For complete polymerisation kept for 2 ½
hrs or
overnight. www.indiandentalacademy.com

Benefits:
- Eliminates the processing stresses.
- Less thermal contraction.
Disadvantages:
More residual monomer
Low mechanical properties.
Colour instability.
Uses: Occasionally used for denture bases.
-Preparation of special trays.
-Denture repairs, relining and rebasing.
-For removable orthodontic appliances.

property Heat cure Self cure
Method of
activation
heat Chemical
Composition No DMPT Present
Polymerisation
reaction
Heated to 60 to
70°C
On mixing
Degree of
polymerisation
high low
Mechanical Superior inferior

Compressive
strength(Mpa)
75 65
Impact
strength(MPa)
1.5x106
at
17.2MPa
Less
Tensile
strength(MPa)
65 lesser
Residual
monomer
.2-.4% 3-5%
Thermal
shrinkage
higher Lesser
crazing greater Better dimensional
stabilitywww.indiandentalacademy.com

Water solubility 0.02 mg/cm 2
0.05mg/cm 2
Water
sorption(mg/cm 2
)
0.6 0.7
Aesthetic quality Good color
stability
No good color
stability
Processing
shrinkage
0.53% 0.26%

Aspects of manipulation
1. Preparation of the mould
2. Selection of separating medium
3. Powder liquid ratio
4. Polymer monomer interaction
5. Dough forming time
6. Packing
7. Bench curing
8. Polymerization cyclewww.indiandentalacademy.com

1. Preparation of the mould
CAP
COPE
DRAG

2. Selection of separating medium
Used to separate the denture base material from the
mould surface.
Formerly, Tin foil was used, but it was a time
consuming and difficult process.
Causes: a. Dimensional inaccuracy
b. Poor reproduction of details
Tin foil substitutes:
-Cellulose lacquers
-Evaporated milk
-Solutions of alginate compoundswww.indiandentalacademy.com

- Soap
- Sodium silicate
- Starch
The most commonly used separating medium are
water soluble alginates.
Composition:
1. Sodium/potassium alginate- 2-3%
2. Di/tri sodium phosphate- 0.7%
3. Preservatives- 0.3%
4. Glycerin- 4%
5. Alcohol- 7%
6. Balanced with water- 85%

Setting reaction:
Potassium alginate + calcium sulphate calcium
alginate + potassium sulphate
Functions:
1. To prevent the diffusion of water from the mold into
the un polymerized packed dough.
2. To prevent the diffusion of the monomer from the
unpolymerised packed dough into the mould material.
Precautions: The coating should be uniform.
-De waxing must be done thoroughly.
-The resin teeth should not be coated.www.indiandentalacademy.com

: Powder liquid ratio
It controls the workability of the mix and the
dimensional change on setting.
•Too much powder: Under wetting of the polymer
beads
•Too much of monomer-Excessive Polymerisation
shrinkage and loss of quality of fit.
•To reduce the volumetric Polymerisation shrinkage
from 21% to 7% and linear shrinkage to 0.5%
- Use of prepolymerised resin is recommended
- 3:1 polymer and monomer ratio.

Polymer monomer interaction
The powder is slowly added to the liquid to get a
workable mass which passes thro five distinct stages:
(1) Wet, sand like stage
(2) Stringy stage/ a tacky fibrous stage.
(3) A smooth Dough like stage:On molecular
level:Increase in no. of polymers chains, a large
amount of undissolved polymer remains.
clinically, no longer tacky and does not stick to mixing
spatula.The latter phases of this stage is ideal for
compression moulding.

(4) Rubbery or elastic stage:dissipation of monomer.
clinically, rebound and stretches.
(5) Stiff stage:evaporation of free monomer.
clinically, the mixture appears dry and is resistant
to mechanical deformation.
DOUGH FORMING TIME:
The time required for resin mixture to reach a dough
like stage.According to ADA-12 it is less than 40 mins
& clinically it reaches in 10 mins.
WORKING TIME:According to ADA-12,the dough to
remain moldable for atleast 5 mins.

Packing
The dough is packed into the mould
cavity.
•Packed at sandy or stringy stage,
too much monomer will be present
between the powder particles.
•If too early : porosity.
•If done at rubbery or stiff stage, material too viscous
to flow.
•Over packing:Excessive thickness and mal positioning
of prosthetic teeth.
•Under packing: Noticeable porosity.
•The acrylic dough is packed into the flask by use of
pneumatic, hydraulic or mechanical press.

Bench curing
After the final closure of the flask , it is allowed to
stand for 1 hour.
Objective: For more uniform dispersion of monomer
throughout the mass.

Polymerisation cycle or Curing cycle:
The heating process used to control polymerisation is
termed polymerisation cycle.This process should be well
controlled to avoid the effects of uncontrolled
temperature rise, such as boiling of the monomer, or
denture base porosity.The following techniques are
there:
1.Involves processing the denture base resin in a
constant temperature water bath at 74°C(165°F) for 8
hours or longer, with no terminal boiling treatment.(slow
curing)
2.Processing in a 74°C water bath for 8 hour and then
increasing the temperature to 100°C for 1 hour.

3.Processing the resin at 74°C for approximately 2 hour
and increasing the temperature of the water bath to
100°C and processing for 1hr.(fast curing)
BENCH COOLING:
It is cooled slowly as rapid cooling may result in warpage
because of differences in thermal contraction of resin
and investing stone.
Bench cooled for 30mins and subsequently immersed in
cool tap water for 15mins.

Processing defects of dentures
Rough or irregular surface.
Porosity
Dimensional changes
Processing stresses
Crazing
Fractures

Rough or irregular surface
Improper wetting of wax denture by plaster mix.
Entrapment of air bubbles in investing plaster.
Low liquid powder ratio.
Incomplete de waxing.
Insufficient or improper coating of separating
agent.
delayed packing.
Insufficient material in the mold.

porosity
Undesirable effect on the strength and optical,
hygienic properties.The causes are:
Polymerisation shrinkage termed contraction
porosity-appear as irregular voids throughout,
and on the surface of denture.
causes:insufficient material, insufficient
pressure.
Gaseous porosity shows fine uniform bubbles.
Particularly in thicker regions.
Localised porosity-improper mixing of
components or early packing.www.indiandentalacademy.com

Granular porosity-granular effect on the
denture surface, due to low powder liquid
ratio.
Blotchy, opaque surface – when mixing
vessel’s lid is not covered.
Rapid heating
Insuffiecint
mixing
Insuffient
pressure

Processing stresses
Polymerisation shrinkage.
Mechanical stresses on repeated drying and
wetting of the denture, causing alternate
contraction and expansion.
During deflasking internal stresses are
developed.
Difference in the coefficient of thermal
expansion of acrylic resin and investing
material.

Crazing
May appear on the surface and has weakening effect.
Delayed packing.
Constant cycle of drying and wetting done by patient.
it is around the tooth neck Difference in the coefficient
of thermal expansion of porcelain teeth and acrylic
resin.(1:10)
Solvent action-Use of MMA monomer during denture
repair.
Sudden cooling.
During polishing .

fractures
Dentures may break:
On impact(extra orally)
Due to fatigue, form repeated bending
of the denture in service.(if parallel
cracks).(intra orally)
Presence of notches.
Inadequate curing time.
Excessive grinding and polishing

INJECTION MOULDING TECHNIQUE
In this technique, for injection of resin, a hollow
sprue connects the mould cavity created by
wax boil out to an external opening on the flask
and a high pressure injection cylinder is
connected to the opening.
The pressure is maintained during the
polymerisation cycle.

Advantage:
-increased dimensional accuracy.
-Elimination of trial closures.
-It can be used for microwavable and pour type
resins.
Drawbacks:
-inadequate spr uing will lead to under filled moulds.
-Expensive equipment.
-Injector is difficult to clean.

Pour type denture resins/ pour and cure
resins/ fluid resins
These are cold cure resins.
Difference in the size of polymer.
Slurry mixed which is poured into agar-
hydrocolloid mold and allowed to polymerize
under pressure at 0.14MPa for 30 –45 mins..
Centrifugal casting and injection molding
techniques are used.

Fluid resin technique
Employs a pour able resins,
chemically activated resin.
Dispensed as a powder
liquid system.(2.5:1by wt.)

Uses:
Three methods of duplicating dentures with pour
type autopolymerising resin.The method differ
in the type of flask and the investing media.
1. Modified denture flask method (Brewer and
morrow, 1975)
2. The pour –resin flask method(Boos and
carpenter 1974)
3. The cup and flask method(Wagner, 1970
Singer, 1975)
-in the preparation of the saddles for partial
dentures, in which trial packing is difficult.

Advantages:
- Improved adaptation to underlying soft tissues
- Decreased probability of damage to prosthetic
teeth and denture bases during deflasking
- Reduce material cost
- Simple procedure
Disadvantages:
- Noticeable shifting of teeth
- Air entrapment within the denture base material
- Poor bonding between the denture base
material and teeth
- Technique sensitive

Modified acrylics
Objective is to improve the impact strength,
fatigue resistance, or radiopacity.
The impact strength or high impact strength
materials -improved by adding
-Elastomers: able to absorb energy.
-Use of acrylic-elastomer copolymer e.g.
methylmethacrylate-butadiene or
methylmethacrylate- butadiene-styrene
copolymers.
Increased by 10 fold.
Drawback is greater cost.www.indiandentalacademy.com

THE FATIGUE RESISTANCE:or fibre
reinforced acrylics
1. adding carbon fibers:
Positioning of fibers is critical,placed in that part of
denture which is under tensile stress.
bonding between the fibres and acrylics is difficult to
achieve, further weaken the denture therefore the
technique is a complicating factor.
Unaesthetic.
2.other reinforcing fibres:
Kevlar (aramid),nylon, glass fibres
Ultra high modulus polyethylene polymers(UHMPE)
3. Metal inserts(wires,plates,fillers)

Advantages:
-Increase in impact and flexural strength
-Improvement in fatigue resistance.
-Minimizing denture fractures.
Drawbacks:
-Tissue irritation from glass fibres
-Straw colored Kevlar fibres.
-Increase in production time.
-difficulties in handling.
-Metal inserts failure due to stress concentration.

RADIOPACITY:Incorporating atoms of high
atomic numbers than the C,H, and O atoms of
which acrylic resin is made.
Radiopaque additives comments
Metal inserts or powdered
metals
May weaken the base & is
unaesthetic.
Inorganic salts(barium
sulphate), Bismuth(10 to
15%)
At 8% insufficient
radiopacity
At 20% weaken the basewww.indiandentalacademy.com

Co-monomer containing heavy
metals e.g barium acrylate
Polymer has poor
mechanical
properties
Halogen - containing Co-
monomer or additives e.g.
Tribromophenylmethacrylate
Bromine containing Co-
monomers.
Additives may act as
plasticizer
Co-monomers are
expensive.
Most promising ones.

ALTERNATIVE POLYMERS:
Polycarbonates and certain vinyl polymers.
Indications: allergic patient.
when greater impact strength is required.
Processing: Injection molding.
So special equipment is required.
RAPID HEAT POLYMERISED RESINS OR
HYBRID ACRYLICS:polymerized in boiling
water immediately after being packed into a
denture flask.

procedure: after placing the denture in boiling
water , boiled for 20 mins and then bench
cooling .
The initiator is formulated form both chemical
and heat activated initiators to allow rapid
polymerisation.
Polymerisation shrinkage-0.97-0.43% double
the conventional ones.

Hydrophilic acrylics
consist of co-polymer of methylmethacrylate
and hydroxyethyl methacrylate.
This material is easily wetted so enhanced
denture retention.
Drawback:
Poorer mechanical properties than PMMA when
material is saturated with water.

Microwave activated resins
Electromagnetic waves.
A special glass fibre reinforced plastic flask.
Processing technique: compression moulding.
microwave energy in an oven at 500-600W
curing time as short as 3 mins.
Advantages:
-short processing time.,easy and clean .
-Dimensional accuracy.
- comparable physical properties.www.indiandentalacademy.com

Disadvantages:
- high capital cost.
- Flasks easily fracture.
- Porosity if condition are not controlled.

Light activated denture base resins
Single component denture base resin
supplied in pre mixed sheet or rope forms
It is a composite having a matrix of urethane
dimethacrylate, microfine silica and high
molecular weight acrylic resin monomer; no
methyl methacrylate is present
a) Organic filler- acrylic resin beads
b) Activator- visible light
c) Initiator- camphoroquinone

The acrylic is polymerized in a light chamber
with blue light of 400 to 500nm.
The denture base rotates in the chamber.
Setting reaction is free radical addition
polymerisation.
A bonding agent – mixture of acrylic monomers,
including methyl methacrylate;cured by visible
light.
Advantages:
No flask required.
Short processing time.
Improved fit, comparable impact strength &
hardness

Disadvantages:
High capital cost.
Inferior bond to resin denture.
Lower elastic modulus and slightly
lower flexural strength-increase
deformation.
Recently, they have been trying the use
of lasers for polymerisation of resins.

Summary and conclusion
Denture bases are responsible for
artificial tooth fixation,stability and
distribution of masticatory forces over a
large tissue bearing area.so far acrylic
resins are most commonly used and still
the research is going on the resins
which will fill all the requirements of
dentist and patient.

References
Anusavice Kenneth J.: Phillips Science of Dental
Materials. 10th
edition, W.B. Saunders, 1999.
Combe E.C.: Notes on dental materials: 6th
ed.
Churchill Livingstone, 1992.
Craig Robert G. and Powers J.M.: Restorative dental
materials. 11th
ed. Mosby Inc. 2002.
Gladwin Marcia, Bagby Michael: Clinical aspects of
dental materials, Lippincott, 2000.
Vermilyea SG, Powers JM and Koran A. the
rheological properties of fluid denture base resins. J

Austin AT, Basker RM. Residual monomer levels in
denture bases. Br Dent J 1982;153:424
McCabe JF and Walls AWG: Applied dental materials,
8th
ed. Blackwell Science Limited, 1998.
O’Brien WJ: Dental material and their selection, 2nd
ed.
Quintessence, 1997.
Van Noort Richard: Introduction to dental materials,
Mosby 1994.
Devlin H, Watts DC. Acrylic “allergy”. Br Dent J
1984;157:272
Elahi JM, Abdullah MA. Effects of different
polymerisation techniques on dimensional stability of
record bases. J Prosthet Dent 1994;71:150-3

•Consani RLX, Dometti SS and Consani S. effect of new
tension system, used in acrylic resin flasking, on the
dimensional stability of denture bases. J Prosthet Dent
2002;88:285-9
•Morata H, Toki K, Hong G and Hamda T.effect of tissue
conditioners on the dynamic viscoelastic properties of a
heat polymerised denture base. J Prosthet Dent
2002;88:409-14
•Yannikakis S,Zissis A,Dent,Polyzios G & Andreopuolos
A.Evaluation of porosity in microwave-processed acrylic
resin using a photographic method.J Prosthet Dent
2002;87:613-9
•Rueggeberg FA.From vulcanite to vinyl, history ofwww.indiandentalacademy.com

Keenan PL,Radford DR & Clark RKF.dimensional
change in complete dentures fabricated by injection
moulding and microvave processing. J Prosthet Dent
2003;89:37-44
Zappini G,Kammann A & Wachter W.comparison of
fracture tests of denture base materials. J Prosthet
Dent 2003;90:578-85

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