Denture Base Polymers

Denture Base Polymers

Dr. Affan Ahmad

• Denture An artificial substitute for missing natural teeth and

adjacent tissues.• Denture Base The part of a denture that rests on the foundation

tissues and to which teeth are attached.• Denture Base Material Any substance of which a denture base may be

made.• Polymer A chemical compound consisting of large organic

molecules built by repetition of smaller monomeric units

• Resin A broad term used to describe natural or synthetic

substances that form plastic materials after polymerization.

Materials Used

• Carved Ivory• Carved wood• Vulcanite• Highly crossed linked Acrylics

Requirements of denturebase polymers

• Should be capable of matching the appearance of the natural oral soft tissues.

• Should have a value of glass transition temperature (Tg) which is high enough to prevent softening and distortion during use.

• Should have good dimensional stability in order that the shape of the denture does not change over a period of time.

• Should have a low value of specific gravity in order that dentures should be as ‘light’ as possible.

• Should have a high value of thermal conductivity would enable the denture wearer to maintain a healthy oral mucosa and to retain a normal reaction to hot and cold stimuli.

• Should be radiopaque.• A high value of modulus of elasticity is advantageous and

is also desirable.• Should have sufficient flexural strength to resist fracture.• Should have an adequate fatigue life and a high value of

fatigue limit.• The ability of a denture base to resist fracture is a function

of the impact strength of the material.• Should have sufficient abrasion resistance to prevent

excessive wear of material by abrasive denture cleansers or foodstuffs.

• Should be chemically inert. • Should be insoluble in oral fluids.• Should not absorb water or saliva since this

may alter the mechanical properties of the material and cause the denture to become unhygienic.

• Should not be harmful to the technician involved in its handling.

• The ‘set’ denture base material should be nontoxic and non-irritant to the patient.

• Water sorption > Base should not be able to sustain the growth of bacteria or fungi.

• Should be relatively inexpensive and have a long shelf life so that material can be purchased in bulk and stored without deteriorating.

• The material should be easy to manipulate and fabricate.

• Cheap processing equipment. • Should be easy to repair.

The material of choice?• Acrylic resin (polymethyl methacrylate) PMMA• Why? • Desirable qualities –reasons why it is used nowadays- :

– Good esthetics– Cheap– Easy to process & use

• Disadvantages:– Not strong enough especially impact strength (denture fall accidentally

and it is broken easily) – Susceptible to distortion with time– Low thermal conductivity leads to the pt frequently burning the mucosa

as they dont actually feeling the heat immediately.– Radiolucent > so they include opacifiers to the composition to make it

radio-opaque.

Classification of denture base polymers according to ISO 1567.

Type Class Description

1 1 Heat-processing polymers, powder and liquid

1 2 Heat-processed (plastic cake)

2 1 Autopolymerised polymers, powder and liquid

2 1 Autopolymerised polymers (powder and liquid

pour type resins)

3 Thermoplastic blank or powder

4 Light-activated materials

5 Microwave-cured material

Composition of Heat cured Material

• Powder:– Beads or granules of PMMA so they are already polymerized not monomers.– Initiator: benzoyl peroxide – Pigments – Opacifiers: titanium/zinc oxide to make it radio-opac– Plasticiser: dibutyl phthalate to make the material softer, added flexibility.– Synthetic fibers: nylon/acrylic to look like blood vessels –small blood vessels-

to give the gingiva a natural appearance.

• Liquid:– Methyl methacrylate monomer– Cross-linking agent: ethylene glycol dimethacrylate. This component improves

mechanical properties add strength.– Inhibitor: Hydroquinone

Composition of Self cured Material

• Powder Polymer: Polymethylmethacrylate beads Initiator: A peroxide such as benzoyl peroxide

(approximately 0.5%) Pigments Salts of cadmium or iron or organic dyes• Liquid Monomer: Methylmethacrylate Cross-linking agent: Ethylene glycol dimethacrylate

(approximately 10%) Inhibitor: Hydroquinone (trace) Activator: N N -dimethyl-p-toluidine (approximately 1%)′

Processing

Mixing and curing (heat curing materials)Powder/liquid ratio of 2.5 : 1 by weightIf the powder/liquid ratio is too high •the mix becomes ‘dry’ .•unmanageable •will not flow when placed under pressure in the gypsum mould.

•Very high powder/liquid ratio•Evaporation of monomer (if container is not covered by lid.>Granular porosity > blotchy, opaque surface.

‘sandy’ ‘stringy’‘doughy’‘rubbery’’hard’

PackingTwo part gypsum mould

Contraction porosity.• Insufficient dough• Insufficient pressureCuringWater bath or air owenCuring Cycle• Heat the flask containing dough for seven hours at

70oC• Three hours at 100oC. • The final three hours at 100oC

Modified acrylic materials

• To improve the impact strength acrylic elastomer copolymer e.g. methylmethacrylate-butadiene methylmethacrylate-butadiene-styrene• To improve fatigue resistance Fiber reinforcement > Carbon fiber inserts, aramid, polyethylene and glass fibres. Polypara-phenylene terephthlalamide can be woven to

produce a commercial material (Kevlar).• To improve radioopacity Barium sulphate