Firstly, when surface of tooth is etched with 37% phosphoric acid, the mineral content of enamel and dentin dissolves in the acid and microporosities are created on the surface of enamel and dentin. So, the surface of tooth becomes roughened. 

Then bonding agent is applied on the roughened tooth surface. Being in liquid form bonding agent easily flows in the microporosities created on the surface of enamel and dentin. When it is cured in gets interlocked in the porosities.

Now when first layer of composite is added over this layer of bonding agent it bonds chemically with the bonding agent. Subsequent layers of composite bond with the previously placed layer of composite chemically.
 
Life of Composite Restoration
Although composites are permanent restorative materials, a composite restoration usually lasts for about 5-8 years depending upon size of cavity and location of the tooth in which filling is placed and But if the restoration is discolored or chipped of it can be repaired easily without the need of removing complete restoration.

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