COMPOSITE CHEMISTRY Dental composite is composed of a resin matrix and filler materials. Coupling agents are used to improve adherence of resin to filler surfaces. Activation systems including heat, chemical and photochemical initiate polymerization. Plasticizers are solvents that contain catalysts for mixture into resin. Monomer, a single molecule, is joined together to form a polymer, a long chain of monomers. Physical characteristics improve by combining more than one type of monomer and are referred to as a copolymer. Cross linking monomers join long chain polymers together along the chain and improve strength.
RESIN MATERIALS BIS-GMA resin is the base for composite. In the late 1950's, Bowen mixed bisphenol A and glycidylmethacrylate thinned with TEGDMA (triethylene glycol dimethacrylate) to form the first BIS-GMA resin. Diluents are added to increase flow and handling characteristics or provide cross linking for improved strength. Common examples are: RESIN
BIS-GMA glycidylmethacrylate
bisphenol
DILUENTS
MMA
methylmethacrylate
BIS-DMA bisphenol dimethacrylate
UDMA
urethane dimethacrylate
CROSS LINK DILUENTS
TEGDMA dimethacrylate
triethylene glycol
EGDMA dimethacrylate
ethylene glycol
COUPLING AGENTS Dental composite is composed of a resin matrix and filler materials. The resin filler interface is important for most physical properties. There are three causes of stress on this interface including: resin shrinkage pulls on fillers, filler modulus of elasticity is higher than resin, and filler thermo coefficient of
expansion allows resin to expand more with heat. Coupling agents are used to improve adherence of resin to filler surfaces. Modification of filler physical structure on the surface or aggregating filler particles create mechanical locking to improve interface strength. Coupling agents chemically coat filler surfaces and increase strength. Silanes have been used to coat fillers for over fifty years in industrial plastics and later in dental fillers. Today, they are still state of the art. Silanes have disadvantages. They age quickly in a bottle and become ineffective. Silanes are sensitive to water so the silane filler bond breaks down with moisture. Water absorbed into composites results in hydrolysis of the silane bond and eventual filler loss. Common silane agents are: VINYL TRIETHOXYSILANE METHACRYLOXYPROPYLTRIMETHOXYSIL ANE
HEAT CATALYST
Polymerization of resin requires initiation by a free radical. Initiation starts propagation or continued joining of molecules at double bonds until termination is reached. Heat applied to initiators breaks down chemical structure to produce free radicals, however, monomers may polymerize when heat is applied even without initiators. Resins require stabilizers to avoid spontaneous polymerization. Stabilizers are also used to control the reaction of activators and resin mixtures. Hydroquinone is most commonly used as a stabilizer. Common heat based initiators are peroxides such as BENZOYLPEROXIDE T-BUTYLPEROXIDE T-CUMYTHYDROXYPEROXIDE
CHEMICAL CATALYST Polymerization of resin requires initiation by a free radical. Initiation starts propagation or continued joining of molecules at double bonds until termination is reached. Chemical
activation of peroxides produces free radicals. Chemical accelerators are often not color stable and have been improved for this reason. The term self cure or dual cure (when combined with photo chemical initiation) describes chemical cure materials. Chemical composites mix a base paste and a catalyst paste for self cure. Bonding agents mix two liquids. Mixing two pastes incorporates air into the composite. Oxygen inhibits curing resulting in a weaker restoration. Chemical accelerators include: Dimethyl p-toludine N,N-bis(hydroxy-loweralkyl)-3,5-xylidine
PHOTOCHEMICAL CATALYST Polymerization of resin requires initiation by a free radical. Initiation starts propagation or continued joining of molecules at double bonds until termination is reached. Early photochemical systems used were benzoin methyl ether which is sensitive to UV wavelengths at 365 nm. UV systems had
limited use as depth of cure was limited. Visible light activation of diketones is the preferred photochemical systems. Diketones activate by visible, blue light to produce slow reactions. Amines are added to accelerate curing time. Presently, different composites use different photochemical systems. These systems are activated by different wavelengths of light. In addition, different curing lights produce various ranges of wavelengths that might not match composite activation wavelengths. This can result in no cure or partial cure. Composite materials must be matched to curing lights. Common photochemical initiators are: CAMPHOROQUINONE ACENAPHTHENE QUINONE BENZYL
PLASTICIZERS Dental composite is composed of a resin matrix and filler materials. Coupling agents are used to improve adherence of resin to filler surfaces. Plasticizers are
solvents that contain catalysts for mixture into resin. They need to be non reactive to the catalyst and resin.
COMPOSITE FILLERS Industrial composites use glass, kevlar, graphite, mica, wood, hollow glass spheres, or the like to modify resin. Dental composites require materials that match tooth color and translucency so an optical index of 1.5 is required. Materials such as strontium glass, barium glass, quartz, borosilicate glass, ceramic, silica, prepolymerized resin, or the like are used. Fillers are placed in dental composites to reduce shrinkage upon curing. Physical properties of composite are improved by fillers, however, composite characteristics change based on filler material, surface, size, load, shape, surface modifiers, optical index, filler load and size distribution.
Fillers are classified by material, shape and size. Fillers are irregular or spherical in shape depending on the mode of manufacture. Spherical particles are easier to incorporate into a resin mix and to fill more space leaving less resin. One size spherical particle occupies a certain space. Adding smaller particles fills the space between the larger particles to take up more space. There is less resin remaining and therefore, less shrinkage on curing the more size particles used in proper distribution.
One micron is a critical filler size. Fillers greater than one micron are visible to the human eye. As resin matrix around filler particles wears, the filler becomes prominent and visible so the composite surface looks rough. Fillers less than one micron do not produce a rough appearing
surface with aging. Fillers greater than one micron are referred to as macrofills and fillers less than one micron are referred to as microfills. A new classification of filler is the nano particles. The nano particles fill between all other particles to further reduce shrinkage. A mixture of different particle sizes is referred to as a hybrid. Distribution of filler particles can be uniform or distributed over a bell curve so a microfill composite might contain many particles greater than one micron but the predominance of particles are one micron or less.
COMPOSITE FRACTURE Dental composite is composed of a resin matrix and filler materials. The resin filler interface is important for most physical properties. There are three causes of stress on this interface including: resin shrinkage pulls on fillers, filler modulus of elasticity is higher than resin, and filler thermo coefficient of
expansion allows resin to expand more with heat. When fracture occurs, a crack propagates and strikes a filler particle. Resin pulls away from filler particle surfaces during failure. This type of failure is more difficult with larger particles as surface area is greater. A macrofill composite is stronger than a microfill composite. Coupling agents are used to improve adherence of resin to filler surfaces. Modification of filler physical structure on the surface or aggregating filler particles create mechanical locking to improve interface strength. Coupling agents chemically coat filler surfaces and increase strength. Silanes have been used to coat fillers for over fifty years in industrial plastics and later in dental fillers. Today, they are still state of the art.
COMPOSITE WEAR There are several mechanisms of composite wear including adhesive wear, abrasive wear, fatigue, and chemical
wear. Adhesive wear is created by extremely small contacts and therefore extremely high forces, of two opposing surfaces. When small forces release, material is removed. All surfaces have microscopic roughness which is where extremely small contacts occur between opposing surfaces. Abrasive wear is when a rough material gouges out material on an opposing surface. A harder surface gouges a softer surface. Materials are not uniform so hard materials in a soft matrix, such as filler in resin, gouge resin and opposing surfaces. Fatigue causes wear. Constant repeated force causes substructure deterioration and eventual loss of surface material. Chemical wear occurs when environmental materials such s saliva, acids or like affect a surface
COMPOSITE CLASSIFICATION
Composite is classified by initiation techniques, filler size, and viscosity. Heat cured composites are polymerized by application of heat. Laboratory heat process fillings are processed under nitrogen and pressure to produce a more thorough cure. Self cured composite means chemical initiation converting monomer to polymer takes place. Core build up materials are commonly self cure. Dual cure means chemical initiation is used and combined with photochemical initiation so either and both techniques polymerize composite. Dual cure composite is commonly used as a cementing medium under crowns. Light cured composite means photochemical initiation causes polymerization. Composite fillers are classified by material, shape and size. Fillers greater than one micron are referred to as macrofills and fillers less than one micron are referred to as microfills. A new classification of filler is the nano particles. The nano particles fill between all other particles to further reduce shrinkage. A mixture of different particle sizes is referred to as a hybrid.
Fillers are irregular or spherical in shape depending on the mode of manufacture. Spherical particles are easier to incorporate into a resin mix and to fill more space leaving less resin. One size spherical particle occupies a certain space. Adding smaller particles fills the space between the larger particles to take up more space. There is less resin remaining and therefore, less shrinkage on curing the more size particles used in proper distribution. Viscosity determines flow characteristics during placement. A flowable composite flows like liquid or a loose gel. A packable composite is firm and hard to displace.