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Composites: Composition

Composites are tooth coloured restorative materials that are usually recommended for class III, IV and class I cavities with less or no occlusal stress and esthetics are important. Specially designed composites are used in almost 50% of class II restorations, although less durable in comparison to dental amalgam. Composites can be classified as microfilled, nanofilled, flowable, packable, all purpose and laboratory. Composites are used for provisional restorations and core build-ups and in fibre-reinforced posts.

Composition

Composites mainly consist of three components:

  1. Resin matrix
  2. Filler particles 
  3. Silane coupling agents
Other components are 
  1. initiators and accelerators
  2. colourants or pigments.

Resin matrix

An oligomer is a moderate molecular weight organic molecule made from two or more organic molecules. 

Most common resin matrix are, dimethacrylate (Bis-GMA) or urethane dimethacrylate (UDMA), oligomers in the form of viscous liquid. To it, low molecular weight monomers (dimethacrylates) are added to control the consistency of the composite paste. Oligomers and dimethacrylates have double carbon bonds that react to convert them to a polymer.

Fillers

Fine fillers are quartz, lithium aluminium silicate, and barium, strontium, zinc or ytterbium glasses. Microfine fillers are colloidal silica particles. Fine fillers that contain barium, zinc, strontium or ytterbium are radiopaque proportionate to their volume content. Composites that contain colloidal silica as micro fine filler are not radiopaque. Radiopaque composites are indicated in posterior restorations.

Size of the fine filler particles range between 0.2-3.0 microns. The micro fine filler particle size averages 0.04 micron, and it may be present from few percent to 35% by weight. The newly introduced size of nano fillers range between 1-10 nanometres. 

Look at the diagram given below. 

Two dimensional diagram of microhybrid and microfilled composite
Two dimensional diagram of microhybrid and microfilled composite[1]
The first diagram is showing microhybride composite which contains fine and microfine filler particle up to 84% by weight. The microfine filler particles is adjusted in the space between fine filler particles, thus making the total filler concentration of 70% by volume. It improves properties.

The microfilled composites being shown in the second diagram contain microfine fillers with increased surface area; thus allowing 35-50% by volume to produce the acceptable viscosity. There are few composites that contain polymer particles reinforced with microfine particles as large as 10 to 20 micron. This kind of composites contain more microfilled fillers with good working viscosity.

Silane coupling agent

Silanes are bifunctional, silicon-organic compounds that couple inorganic filler particles and resin matrix. They are coated on filler particles and provide enough bond between matrix and filler.   

Initiators and Accelerators

Polymerisation of the composite is mainly done by exposing it by intense visible blue light for 20 to 40 seconds exposer time. The blue light is absorbed by diketone that starts the polymerisation in the presence of an organic amine. Since the blue light is essential to initiate the polymerisation, the diketones and amines can be kept in a single composite paste as it will not cure unless exposed to visible blue light.

The other polymerisation system is self curing-system. The initiator is an organic peroxide and the accelerator is organic amine. These two are kept in separate and are mixed immediately before the placement of the restoration.

Dimethacrylate + Initiator + Accelerator + Filler = Dental composite

Initiator=peroxidase or diketone + Blue light
Accelerator = amine
Filler= treated inorganic reinforced filler 

Pigments

Inorganic pigments are incorporated in tiny amount to provide matching colours. Normally composites are provided in ten or more colours that cover all the shades of the human teeth which varies from yellow to grey. Recently, the composites have been developed matching the colour of the enamel, dentine, cervical, opaque and bleached teeth for special techniques. These composites can be applied in single to multiple layers to get the desired shade.

Properties

The important properties of the composites are as follows:
  1. Polymerisation shrinkage - should be low
  2. Water sorption - should be low
  3. Coefficient of thermal expansion - should be same as tooth
  4. Fracture resistance - should be high
  5. Wear resistance - should be high
  6. Radiopacity- should be high
  7. Bond strength to enamel & dentin - should be high
  8. Colour match to tooth structure - should be excellent
  9. Manipulation - should be easy
  10. Finishing and polishing - should be easy
Few of the above mentioned properties may be important for anterior than posteriors restorations and vice versa. The properties of microfilled and nanofilled composites are same while the microhybrid's differ from both of them.


To read about the properties of composites click below:





Ref:
1. Dental materials properties and manipulation, Robert G. Craig, John M Powers, John C. Wataha