Denture base materials DENTURE BASE: that part of a denture which rests on the oral mucosa and to which artificial teeth are attached.
Ideal properties of Denture base materials
1.It should be compatible with absence of odor and taste. 2.It should be unaffected by oral fluids (insoluble, non absorbent, and inert) 3.It should have adequate mechanical properties; including high modulus of elasticity, high proportional limits, high transverse strength, high impact strength, high fatigue strength, sufficient resilience, hard and with good abrasion resistance.4.It should have other physical properties including; thermal expansion matches that of artificial teeth, high thermal conductivity, low density, and softening temperature higher than the temperature of foods and liquids in the mouth. 5.It should be esthetically satisfactory. 6.It should be easy to process with the minimum of expense and equipments.
7.It should be easy to repair. 8.It should be radio opaque; if a denture or fragment of a broken denture is accidentally inhaled or ingested, it should be capable of detection by x-ray. 9.It should have dimensional stability on processing the denture and in service. 10.It should be easy to clean and resistant to microbial growth. 11.It should be good shelf life.
DENTURE BASE MATERIALS 1. metallic: Gold, nickel -chromium (Ni/Cr), cobalt - chromium (Co/Cr), Titanium alloys. 2. non metallic: acrylic resins, flexible denture base materials.
POLYMERS Before the introduction of acrylic polymers to dentistry in 1937, the principal polymer used was vulcanized rubber for denture bases. This natural rubber was difficult to pigment and tend to become unhygienic due to the uptake of saliva. Polymers used in dentistry include; vinyl acrylics, polystyrene, epoxies, polycarbonates, polyvinyl acetate polyethylene, cis- and trans-polyisoprene, polysulfides, silicones, polyethers, polyacrylic and others.
CHEMICAL COMPOSITION The term polymer denotes a molecule that is made up of many (poly) parts (mers) . The molecules from which the polymer is constructed are called monomers (one part). Copolymers means polymers contain two or more different types of monomers. Eg. Monomer : methylmethacrylate Polymer : polymethylmethacrylate Copolymer : methylmethacrylate- ethylmethacrylate copolymer
POLYMERS' USES : Denture bases Artificial teeth Tooth restoratives Cements Orthodontic space maintainers and elastics Facings on fixed metal crowns and bridges Obturators for cleft palate Impressions Dies Root canal filling materials Maxillofacial prosthesis Custom impression trays Soft denture lining materials. Polymers are either thermoplastic or thermosetting. Thermoplastic refers to polymers that may be softened by heating and solidify on cooling , the process being repeatable. Eg, polymethylmethacrylate, polyestyrene .
Thermosetting refers to polymers that solidify during fabrication but cannot be softened by reheating .eg, cross-linked polymethylmethacrylate, silicones. Cross-linked molecules are a network structure that may result in the polymer's becoming one giant molecule .
ACRYLIC RESINS: Acrylic resins is the most widely used and accepted plastic material in dentistry. Acrylic denture base materials are classified into five groups (types): Table: Classification of acrylic denture base materials according to ISO 1567 :
Type1DescriptionHeat- processing polymers, (powder and liquid or plastic cake )2Autopolymerized polymers ( powder and liquid)3Thermoplastic ( blank or powder)4Light-activated materials5Microwave-cured material
Properties of denture base acrylic resins: 1.Adequate appearance with various pigments, allowing tissue color matching. 2.light weight. 3.low thermal conductivity. 4.low abrasive resistance. 5.they have residual monomer that may develop allergy in some patients. 6.suffecient thickness of the material provides adequate rigidity and strength. 7.insignificant dimensional changes. 8.water absorption followed by drying may cause crazing. Therefore, the dentures should be kept moist at all times.
OTHER POLYMERS: A variety of polymers are used in fully polymerized form without any polymerization carried out by the dentist or laboratory technician. Examples: cis- polyisoprene in the very flexible form is used in surgical gloves, orthodontic elastics and rubber dams for restorative procedures. trans- polyisoprene is rigid and compounded with zinc oxide, waxes and zinc silicate to be used for gutta-percha (endodontic points as part of the filling material for root canals).
Oily organic esters like dibutyl phthalate plasticise (soften) the polymer, and if enough is added, the polymer becomes rubbery. These plasticized acrylics are used as temporary soft liners for dentures. They gradually become hard as the saliva leaches out the oily plasticizer.
Chemical Composition of type 1 and type 2 Acrylic Resins Most materials are supplied as a powder and liquid.
Powder
polymerThe major component of the acrylic powder is beads of PMMA which are produced by polymerizing methylmethacrylate in specific methods. The PMMA is a clear, glass-like material.
initiator
pigments
: inorganic pigments, usually pink, and small red fibers are incorporated to simulate the natural oral mucosa with its minute blood vessel. Salts of cadmium or iron or organic dyes are used.
Inorganic particles
to improve acrylic properties
Opacifiers
to increase the radio opacity of the acrylic
liquid
monomer
Methylmethacrylate: it is a clear, colorless, volatile liquid with a distinct odor that is toxic if inhaled for a prolong period.
Cross-linking agent
Ethylene glycoldimethacrylate: to improve the physical properties
inhibitor♦ Hydroquinone (trace ): to prevent the premature polymerization of the material.
activator
N N' –DIMETHYL-P-TOLUIDINE (approximately 1% ) ): is added only to self curing material to activate the polymerization at room temperature i.e. without heating.