DENTURE BASE RESINS

Dr. lakshya kumarAsstt. Professor

Deptt of ProsthodonticsLecture-9 am to 10 am

20/9/2014

PART 2, 3

Conventional heat cure acrylic resins

Conventional cold cure acrylic resins

High impact resistant acrylic resin

Injection moulding resin

Rapid heat polymerizing resin

Light activated resin

COMPOSITION OF MONOMER

Methyl methacrylate

Main chemical which will polymerize

Hydroquinone (o.Oo3-o.1%)

Inhibitor while storage

Dibutyl phthalate

Plastisizer

Glycol dimethacrylate (1-2%)

Cross linking agent

Styrene vinyl acetate or ethyl methacrylate

As copolymers

PROPERTIES OF MONOMER

Typical smell of its own Clear, transparent liquid Boiling point 100.3*c Good organic solvent Can get evaporated Inflammable Stored in dark colored bottle Light weight Volumetric shrinkage of 21% It is a known allergen

LIQUID Methyl methacrylate monomer Cross linking agent Ethylene glycol

dimethacrylate(5-15%).they are added to avoid crack or craze produced by stresses during drying.

Inhibitor Hydroquinone (trace) to avoid premature polymerization and enhance shelf life.

When MMA polymerizes it shrinks 21% by volume.

Using a 3:1 powder liquid ratio it could be minimized to 6%.

A correctly heat processed denture base could have as little as 0.3% to 2% residual monomer.

Composition of polymer

Polymethyl methacrylate

main ingredient

Benzoil peroxide

0.5-1.5%, initiator

Dibutyl phthalate

8-10%, plasticizer

Zinc or tetanium oxide

opacifier

Composition of polymer

Mercuric sulfide or iron oxide or titanium oxide

- pigments and dyes

Glass fibres or beads or zirconium sulfate

- to increase stiffness

Nylon or acrylic fibres

- in veined type

- capillaries of gum

Different types of polymer powder

Normal pink

-which resembles the normal pink

color of gum

Clear polymer

- no coloring agents are added

- indicated in palatal area

Translucent

Veined or deep veined

Properties of polymer powder

Stable at room temperature

- has long shelf life

Softening temperature

- 125 * c

- 125*-200*c

depolymerization occurs

- 450 *c

converted back to monomer

Absorbs water and soluble in chloroform

Properties of polymer powder

Tensile strength

- 600 kg/sq cm

Appearance

- shiny

- transparent

Produced in two shapes

- spherical

- granular

Properties of polymer powder

Control of particle size

- no.52 sieve mesh

- 300 micrometer

If particle size is smaller

- softening will be quicker

- granular shape softens faster

Manufacturing of polymer powder

Spherical particles

By suspension polymerization

- monomer and water are mixed with an emulsifier i.e, powder talc.

- then the mixture is heated and stirred

- at the end talc is washed off to

get polymerized polymer particles

Granular particles

- solid block

- then it is grinded or milled

CURING CYLES EMPLOYED

A Recommended curing cycles overnight water-bath cures: 1) 7hr at 70 C 2) 14hr at 70 C 3) 7hr at 70 C +1hr at 100C 4) 14hr at 70C + 1hr at 100CDry heat cure: 5)temp in excess of 100C using dry heat system

B Short curing cycles 6) 7hr at 60C 7) 7hr at 60C +1 hr at 90C

(II)PHYSICAL PROPERTIES Should possess adequate strength and resiliency and

resistance to biting and chewing forces impact forces and excessive wear in oral cavity.

Should be dimensionally stable under all conditions of service including thermal changes and variations in loading.

I) Specific gravity: It should have low value of specific gravity in order that dentures should be as light as possible.

II) Thermal conductivity: It is defined as the number of calories per second flowing through an area of 1cm2 in which the temperature drop along the length of the specimen is 1°C/cm.

It should have high value of thermal conductivity

Radiopacity: It is the inhibition of passage of radiant energy. It should be ideally radiopaque

Glass transition temperature: It is the temperature at which molecular motions become

such that whole chains are able to move. It is close to softening temperature. At this temperature sudden change in elastic modulus occurs.

Amorphous polymer below Tg behave as rigid solids while above Tg they behave as viscous liquids, flexible solids or rubbers.

Increased chain branching Decreased Tg.Increased number of cross links Increased Tg

Effect of molecular weight on propertiesIn many polymers the chains are held together by secondary,

or Vander Waals forces and molecular entanglement. Materials of high molecular weight have a greater degree of molecular entanglement, and have greater rigidity and strength and higher values of Tg and melting temperature than low molecular weight polymers.

Effect of plasticizers Plasticizers penetrate between the randomly oriented

chains of polymer as a result of which molecules are further apart and forces between them are less. They soften the material and make it more flexible by lowering its Tg. They lubricate the movements of polymer chains and are sometimes added to help molding characteristics. This

principle is used in producing acrylic soft lining materials. Effect of fillers Modulus of elasticity and strength are generally increased. A degree of anisotropy exist, that is the strength depends

on the orientation of fibres in the polymers.

Viscoelasticity : Polymers show viscoelastic behavior. Elastic behavior is caused by uncoiling of polymer molecules. Plastic behavior is caused by breaking of intermolecular Vander Waals forces

(III) MANIPULATION Should not produce toxic fumes or dust Easy to mix, insert, shape and cure and short setting time Oxygen inhibition, saliva and blood contamination should have

little or no effect. Final product should be easy to polish and easy to repair in case of

breakage.

(IV)AESTHETIC PROPERTIES Should be translucent to match oral tissues Capable of pigmentation No change in color after fabrication.

(v)ECONOMIC CONSIDERATION Cost should be low Processing should not require complex and expensive instruments.

(VI) CHEMICAL STABILITY Conditions in mouth are demanding and only the most chemically

stable and inert materials can withstand such conditions without deterioration.

“No resin has yet met all of these ideal criteria”. Methacrylate polymers fulfill the aforementioned requirement reasonably well.

2)AUTOPOLYMERIZING/COLD CURE POLYMETHYL METHACRYLATE (POUR RESIN)

Composition same as the heat cure version with following differences

1)The powder contains beads of polymer that have a lower molecular wt. and benzoyl proxide (initiator)

2) The liquid contains a chemical activator ,tertiary amine such as dimethyl-para-toluidine.

Upon mixing tertiary amine causes decomposition of benzoyl peroxide.

Dentures processed have more residual monomer (1-4%),but lower dimensional change.

Decreased transverse strength (residual monomer act as plastisizer).

Compromised biocompatibility (residual monomer)

Color stability inferior (teriaty amine susceptible to oxidation), stabilizing agents should be added

Fluid resin and compression molding technique can be employed for the fabrication of denture.

Also used as repair material

3)HIGH IMPACT RESISTANT ACRYLIC Similar to heat cured material but less likely to be broken if

dropped. Produced by substituting the PMMA in the powder with a

copolymer. Copolymer of butadiene with styrene or methyl methacrylate are

incorporated into the beads. Phase inversion resulting in dispersion throughout the beads of

tiny islands of rubber containing small inclusions of rubber/PMMA graft polymer.

Electron micrograph of high impact dentureBase showing size and shape of polystyrene-butadieneRubber inversion phase.

4) Injection molded polymers

These are made of Nylon or Polycarbonate.

The material is supplied as a gel in the form of a putty .

It has to be heated and injected into a mold

Equipment is expensive.

Craze resistance is low .

The SR-Ivocap system uses specialized flasks and clamping presses to keep the molds under a constant pressure of 3000 lbs

5) RAPID HEAT POLYMERIZED POLYMER

Same as conventional material except that they contain altered initiation system.

These initiator allow them to be processed in boiling water for 20 min.

A problem with these is that areas of the base thicker than approx.6mm have a high level of porosity.

Short duration of heating also leaves a higher level of residual monomer,3-7 times greater than conventional heat cured denture base.

6) MICROWAVE POLYMERIZED POLYMERS Resins are the same as used with conventional material and are

processed in a microwave. Denture base cures well in Special polycarbonate flask (instead of

metal). The properties and the accuracy of these materials have been

shown to be as good or better than those of the conventional heat cured material.

Processing time is much shorter (4-5 min).

Microwave resin and non metallic microwave flask

7)Light activated denture base resins This material is a composite having a matrix of urethane

dimethacrylate, microfine silica and high molecular wt acrylic resin monomers

Acrylic resin beads are included as organic fillers. Visible light is the activator, whereas camphorquinone serves as

the initiator for polymerization. Can be used as repair material and as custom tray material. Single component denture base is supplied as sheet and rope

form in light proof pouches.

Light curing unit for polymerizing Dimethacrylate

8) FIBER –REINFORCED POLYMER

Glass, carbon/graphite, aramid and ultrahigh molecular wt polyethylene have been used as fiber reinforcing agents.

Metal wires like graphite has minimal esthetic qualities.

Fibers are stronger than matrix polymer thus their inclusion strengthens the composite structure.

The reinforcing agent can be in the form of unidirectional, straight fiber or multidirectional weaves.

Polymer – monomer interaction When mixed in proper proportions, the resultant

mass passes through five distinct stages 1.Sandy 2. stringy 3. Dough like 4. rubbery 5. Stiff

1.Sandy

During sandy stage, little or no interaction occurs on a molecular level. Polymer beads remain unaltered.

This stage is ideal for compression molding. Hence material is inserted into mold cavity during

dough like stage.

2. Stringy stage Later, mixture enters stringy stage.

Monomer attacks the surfaces of individual polymer beads.

Stage charcterized by stringiness,…3. dough like stage

The mass enters a dough like stage. On molecular level increased number

of polymer chains are formed. Clinically the mass becomes as a pliable dough. It is no longer tacky ( sticky)

4.Rubbery or elastic stage Following dough like stage, the mixture

enters rubbery or elastic stage. Monomer is dissipated by evaporation and by further penetration into remaining polymer beads. In clinical use the mass rebounds when compressed or stretched

5. Stiff Stage Upon standing for an extended period,

the mixture becomes stiff. This may be due to the evaporation of

free monomer. From clinical point, the mixture appears very dry and resistant to mechanical deformation

COMPRESSION MOLD TECHNIQUE

INJECTION MOULD TECHNIQUE

FLUID RESIN TECHNIQUE

MICROWAVE PROCESSING

LIGHT ACTIVATED DENTURE BASE RESINS

Compression mold technique

Primary impressions

Secondary impressions

Jaw relations

Try in stage

acrylization

flasking

dewaxing

packing- under pressure

curing

surround or invest

“ a metal case or tube used in investing procedure”

- metal

- brass

- 3 or 4 parts

Flask

FLASKING

TWO POUR TECHNIQUE

THREE POUR TECHNIQUE

Checking the seating of flask members

Invested flask & cast is washed in clear slurry water

Surface tension reducing agent is applied to the exposed wax

Separating medium is applied

Second mix is mixed

Hand spatulation

Mechanical spatulation-Under reduced atmospheric

pressure-Minimum air inclusion-Reduces finishing time

Stone is coated in occlusal & inter- Proximal areas with stiff brush

Stone is poured in flask Vibrator can be used In absence of vibrator Flask is filled till approximately ¼ of the

flask Stone is removed in incisal & occlusal

surfaces Stone is allowed to set

After the stone is set separating medium is coated

Occlusal & incisal surfaces shouldn’t be coated with the separating medium

Clear slurry is poured till the stone is mixed

Slurry is poured off, flask is filled with the stone

Lid is closed

Flask is clamped

The denture is taken out from the flask. It is then trimed Finally wet polishing is done.

Finishing and polishing

Porosity Crazing Warpage Fracture

Denture defects

Presence of voids within structure of resin Porosity can be of two types

◦ Internal◦ External

Internal porosity is due to voids within the structure usually at thicker portion

Cause – due to vaporization of the monomer (100.8C)

Solution- long low temperature curing cycle is recommended.

porosity

External porosity is due porosities which occur near the surface of denture.

Cause ◦ Lack of homogeneity of dough.◦ Lack of adequate pressure.

Prevention – use proper monomer – powder ratio, packing in dough stage.

Crazing is formation of surface cracks on denture base resin.

Causes –◦ Incorportion of stress◦ Attack by solvent (alcohol)◦ Incorporation of water during processing.

Prevention◦ Avoidance of solvent◦ Proper use of separating media◦ Metal moulds◦ Use of cross linked acrylic

Crazing

CRAZING

Denture warpage is change in shape or fit of denture.

Cause is incorporation of stress in deture◦ Packing in late dough or rubbery stage.◦ Stress induced during curing◦ Improper deflasking◦ Rise in temp while polishing◦ Immersion of processed denture in hot water.

Denture warpage

Denture warpage

Improper deflasking Denture base excessively thin Accidental dropping at time of polishing

Denture fracture

Care to taken at time of dewaxing procedure

Rearticulation to be done after processing the denture to check of occllusal discrepancy.

Change in tooth position

The cured polymer should be stiff enough to hold the teeth in occlusion during mastication and to minimize uneven loading of the mucosa, underlying the denture should not creep under masticatory loads should not deteriorate in the aqueous oral environment and crazes should not form The cured polymer should be biologically inert and slow to foul

For an allergic patient, low free-monomer content may be thought more important than stiffness.

For a patient requiring a soft lining, stiffness is very important if the reduced cross sectional area of the denture, may lead to stability or loading problems.

HEAT CURE Curing the dough before the monomer has

diffused to the core of the beads may result in reduced flexural strength and a tendency for cracks to propagate.

The curing cycle is designed to raise the temperature to a point at which

(1) sufficient benzoylperoxy radicals are produced to overcome the scavenging effect of oxygen,

(2) polymer chains form by free-radical addition polymerization.

Too rapid a rise in temperature produces large numbers of

radicals or radical avalanche, as a result, many growing polymer chains. These chains collide either with other radicals or with polymer chains, producing an increase in branching and cross-linking of the interstitial polymer. This in turn reduces toughness.

Slow cures result in much tougher denture bases, producing fewer cross-links and branches, and having a higher overall molecular weight between cross-links because

Free-monomer content is often lower also, because the steadier rise in internal viscosity

of the curing polymer allows the monomer easier access to the growing free radicals. The cross-linker is more completely polymerized in heat-cured systems; this results in significantly lower creep values due to removal of the plasticizing effect of unreacted pendant crosslinker groups.

Heat-cured systems have one great advantage over autopolymerized and injection molding methods: an increased rate of monomer diffusion at the higher temperature.

Conversely, an increased temperature of cure can also result in the annealing of stresses that build up in the structure due to polymerization shrinkage. If this stress is not released, it can act as the foci for crazes or distortions

Autopolymerizing acrylic

The pour technique for dentures, originally developed during the 1960s, reversible hydrocolloid. The fitting surface of the mold

consists of the plaster model itself; the acrylic teeth occupy their positions in the agar mold in the same way they do in a conventional plaster mold. In general, the creep of these products is greater than that of heat-cured acrylics.

Permanent Soft Lining Materials

Acrylic soft liners

acrylics that have a natural glass-transition temperature at least 25°C less than that of the mouth. The plasticizer used to soften the acrylic can either be unbound to the acrylic and hence free to diffuse out during use, resulting in a loss of resilience,

Silicone soft liners

The silicones used as soft liners can be divided into two types:

room-temperature vulcanizing (RTV) and heat curing. The RTV silicones' greatest drawback is

their lack of adhesion, which is especially a problem around the edges of the attachment between acrylic and silicone

Temporary Soft Liners and Functional Impression Materials

it is their viscoelastic properties that are important,

specifically their ability to flow under masticatory and linguistic forces, spreading the load on the mucosa evenly.

SUMMARY

MCQ DENTURE BASE RESIN

1) In heat cure denture base resins the monomer is

  a) methacrylate  b) ethylmethacrylate c) metyl ethyl methacrylate d) polymethylmethacrylate

2)If curing occurs at temperature more than 100 degree celsius porosity results in which are of denture

a)hard thick central area b) thin palatal area c)thin area of flanges d) porosity is uniformly distributed

3)What is the shrinkage if 3:1 powder:liquid ratio is used

a) 6% b) 8% c) 10% d) 21% 

4) Hydroquinone is added to methyl methacrylate monomer 

a) to prevent polymerization during storage b)to initiate release of free radicles c) to enable polymerisation reaction at room

temperature d)all of the above

5) cross linking of denture base is contributed by 

a)gylcol dimethacrylate b) benzoyl peroxide c) N para toluidine  d) methyl methacrylate

6) use of dimethyl P toluidine is indicated for

a) thermal polymerization of acrylic b) chemical polymerization c) retarding the polymerizing reaction d) to inhibit the action of benzoyl peroxide

7)Porosity present in acrylic denture is usually the result of 

a) prolonged curing cycle b) lack of sufficient pressure applied to flask c) insufficient acrylic resin monomer d) prolonged bench cooling after curing

8)The most important disadvantage of acrylic denture base is

a) porosity b)shrinkage c)tooth breakage d)water absorption

9)In self cure acrylic resin the activator is

a) toluidine b) quaternary ammonium compound c) benzoyl peroxide d) tertiary amine

10) Which of the following may result if an excessive amount of monomer is incorporated into an acrylic resin mixture

a) excessive expansion b) low impact strength c) excessive shrinkage d) excessive brittleness

11) Porosity of denture base is due to a) packing at dough stage b) curing at 160 degree Celsius c) inadequate pressure d) all of the above

12) Subsurface porosity is due to a) thermal changes b) thickness of resin c) lack of temperature d) packing past the dough stage  

13) To prevent porosity in self cure acrylic resin curing should be carried in

a) cold water b) hot water c) under tap water d) under vaccum pressure

14) Acrylic resins are used for a) anterior restorations b) temporary brigdes c) denture bases d) all

15) Packing of heat cure acrylic resin should be done in which stage

  a) Sandy stage b)Stringy stage c)Dough stage d)Rubbery stage