Composites

• Class of materials that can replace biological tissue in both appearance and function • Largely successful but has certain limitations • Restoration of anterior and posterior teeth • Conservative class 1 or 2s • Pit and fissure sealants • Cementation of fixed prostheses • Bonding of ceramic veneers

• Very large posterior restorations • Bruxism?? • Poor isolation

-Aesthetic, plastic adhesive restorative material consisting of co-polymerized methacrylate-based resin chains embedding inert filler particles and requiring a separative adhesive (bonding agent) to micro/nano-mechanically bond them to either enamel or dentine

• Ability to withstand oral cavity environment • Easily shaped to the anatomy of the cavity • Match the natural tooth colour • Bonds directly to the tooth tissue

Class I and II: Multipurpose, nanocomposites, highly filled composites for posterior restorations Class III and IV: Multipurpose, nanocomposites micro filled composites (hybrids) Class V: Flowable composites

Major constituents: • High molecular weight monomers (matrix phase) • Fillers (discontinuous phase) Minor constituents: • Diluents or viscosity modifiers (dimethacrylate monomers with low molecular weight and viscosity) • Inhibitors, intiators, accelerators, radio-opacifiers and stabilizers • Silane coupling agent (filler is treated with the coupling agent)

-High molecular weight monomers are either aromatic or aliphatic dimethacrylates • Bis-GMA is very viscous • Aromatic • Difunctional methacrylate with two phenyl groups which provide rigidity • Hydroxyl groups present provide H bonding • UDMA is lower viscosity but still quite high viscosity • No phenyl groups • Non-aromatic • TEGDMA and EDMA added to reduce viscosity • Diluents • On polymerisation, the dimethacrylate monomers allow for extensive cross linking to occur • Cross linking renders them stronger, with an increased Tg and lowers fluid uptake

-Compounds added to prevent premature polymerization during storage, handling and placement Examples of common inhibitors include hydroquinone, PMP and BHT

-Dimethylaminobenzoates (DMAB) increase the reactivity of the photo-initiator, speeding up curing time

-HMBP provide colour stability by eliminating the UV action on amine initiators

-Various ferric and titanium oxides

-Aluminium, titanium or zirconium oxides

• Inert filler particles made from barium, silica, quartz or strontium glass derivatives • Coated with an organo silane coupling agent to make the matrix and glassy phase compatible • Increased filler particles increases viscosity • Increased filler particles reduces volumetric shrinkage • Improves the coefficient of thermal expansion • More irregular shapes, larger, harder filler particles increases the wear resistance of the composite (surface hardness) • Improves mechanical properties such as modulus of elasticity, abrasion resistance and strength) • Finer, more spherical particles improve aesthetics • Decreases water sorption • More polishable

• Improves adhesion between filler and polymer matrix • Silane coupling agent has a polymerizable group along with a hydrophilic end which may have the ability to condense with hydroxyl groups present on the surface of the ceramics

• Camphorquinone (a diketone) most common (yellow in colour) • Along with tertiary amine as an activator • Absorbs 480nm blue light • Forms free radicals • Initiation of addition polymerization Camphorquinone is a bright yellow solid thus can be disadvantageous for obtaining different shades. TPO therefore currently being tested as an alternative and some composites are using mixture of the two

• 1-phenyl-1,2 propanedione is a photosensitizer of potential value that helps in reducing colour problems associated with visible light cured dental resins • Acts synergistically to produce a more efficient photoinitiation reaction • Camphorquinone is a bright yellow solid thus can be disadvantageous for obtaining different shades. TPO therefore currently being tested as an alternative and some composites are using mixture of the two

• Free radical addition polymerisation reaction of methacrylates • Free radicals can be generated by heat, light or chemical activation Chemical Reaction: Two paste system, initiators such as benzoyl peroxide and tertiary amine (N, N dimethyltoulidine) producing free radicals or Photoinitiation: One paste system, photoinitiators such as camphorquinone Camphorquinone + amine activator produces free radicals

• Matrix is the phase formed on polymerisation of the methacrylate monomers • Weakest • Least wear resisistant • Can absorb fluids • Responsible for shrinkage

Chemical activation • Gradual increase in viscosity • Limited working time • Mixing may incorporate air bubbles may inhibit polymerisation Light activation (photoinitiation) • No mixing involved • Command set, light attenuation, limited depth of cure • Degree of cure: light intensity, exposure time and thermal energy • Incremental curing

• Triggered by free radicals • Free-radicals generated on irridation of a light-sensitive initiator (initiation) • Free radical attacks the double bond of the methacrylate groups, creating monomer free radicals leading to the generation of a chain (propagation) • Extent of polymerisation is determined from the ratio of uncured monomer or remaining double bonds to the amount initially present ??? • Degree of cure/conversion. Usually 40-78% -Cross linked networks usually formed. Dimethacrylate monomers can form linear polymers, but in the presence of dilutent monomers such as EGDMA and TEGDMA, cross linking occurs. Crosslinked networks influence the resultant physical and mechanical properties

• During polymerisation, there is a massive increase in the viscosity of the system • Results in restricting the mobility of the longer chains, thus formed thereby increasing the free radical concentration • Increases rate of reaction, often known as autoacceleration or the Tromsdorff Norish effect • Rapid vitrification • Limits the diffusion of monomers and leads to entrapped monomers/free radicals leading to less than 100% conversion • Predominant polymerisation occurs during irradiation, however, a very small amount of reaction may occur post setting due to the free volume relaxation

-Polymerisation leads to certain amount of shrinkage Mechanical properties • Amount of filler • Type of filler • Filler-resin coupling Surface Properties • Hardness • Roughness • Abrasion Resistance • Water sorption • Thermal Properties • Colour and Appearance • Adhesion

• Filler particles • But studies also show varying elastic modulus, microhardness and compressive strengths for lots of different types of composites with similiar filler weight content • Suggesting natuve resin matrix governs the properties as well

• Effect on physical and mechanical properties • Determined by infrared spectroscopy • Often indirectly evaluated by determining microhardness with a reasonable correlation • Higher the degree of conversion, the higher the microhardness • But crosslinking also affects the hardness, so there are some exceptions to this rule • Mechanical properties usually improve with higher degree of conversion • Lowering in water uptake and monomer diffusion with increase DC