Composites: Properties

You have read about the composition of dental composites in earlier class notes This article will speak about the properties. 

Properties of Composites

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.

Properties of microhybrid and microfilled composites [1]

Polymerisation shrinkage (% linear)

1.0-1.7

2-3

Thermal conductivity (10-4 cal/sec/cm2 [oC/cm])

25-30

2-15

Linear coefficient of thermal expansion (x 10-6 /oC)

25-38

55-68

Water sorption (mg/cm2)

0.3-0.6

1.2-2.2

Radiopacity (mm Al) $

2.7-5.7

---------

Compressive strength (MPa)

200-340)

230-290

Diametral tensile strength (MPa)

34-62

26-33

Flexural strength

90-140

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Elastic modulus of compression (GPa)

8-14

3.5

Flexural modulus (GPa)

5-18

---------

Knoop hardness (Kg/mm2)

55-88

22-36

Bond strength to enamel & dentin with bonding agent (MPa)

14-30

14-30

$ If claimed. Enamel is 4.0 mm & dentine is 2.5 mm aluminium.

Polymerisation shrinkage of microhyrid is less than microfilled due to lesser amount of resin matrix. It may reach to the extent that it can break the bond strength between enamel or dentin and the composite restorations, leading to microleakage. To prevent it, two methods are suggested; first is incremental addition of composite material while restoring the tooth and the second is fabrication of restoration in the laboratory using lab composite and then cementing it with a resin luting agent.

Thermal conductivity of composites are much lower than metallic restorative materials, and nearly matches to the enamel and dentine. They provide good thermal insulation to dental pulp.

Thermal expansion of composites are more than the tooth structure. Therefore, composite restorations show more dimensional change with intraoral temperature change. Since microfilled composites contain more amount of resin than that of hybrid, the thermal changes are more in microfilled.

Water sorption by microfilled composites are almost 3-4 times more compared to microhybrid composites. Therefore, the microfilled composites are more prone to discolouration than microhybrid. Water sorption is followed by swelling of the composite; however, this property can not be used to compensate for the polymerisation shrinkage. The degradation of the properties caused by sorption is irreversible.

Radiopacity: Microhybrid composites are almost always radiopaque and microfilled composites are mostly radiolucent except one brand, Heliomolar which contains ytterbium fluoride making it radiopaque. Composites are radiopaque when compared to dentin whereas radiolucent when compared with enamel.

Compressive and Flexural strength of microhybrid composites are better than microfilled composites. Compressive strength increases linearly with the increase in the volume of the filler content. The failure of the composite restoration takes place when it bends or under tension. Therefore, a dentist should take special interest in tensile and flexural strength.

Elastic modulus, in layman terms, stiffness of the composites are controlled by the volume of the fillers. Therefore, the elastic modulus of the hybrid composites is two to four times more in comparison of microfilled composites. It is an important feature in applications where high biting forces are applied or where wear resistance is necessary. However, it has been noted that the bond failure between dentin and microhybrid in class V restorations were more when compared to microfilled. It is postulated that the lower modulus of elasticity of microfilled composites passes less stress to the dentine-composite bonds resulting in less number of failures.

Hardness is proportional to the volume fraction of the fillers rather than the hardness of the filler itself. Hence microhybrid composites are always a choice over microfilled composites where higher abrasive wear resistance is required. Higher filler content resulting in increased hardness is also necessary to prevent the nonrecoverable penetration in a restoration.

Bond strength of both, microhybrid as well as microfilled composites with enamel and dentin are same. The bond strength and the stresses created during polymerisation are of the almost same magnitude; therefore chances of breakage of bonds are always present. If it happens, the resulting microleakage may lead to sensitivity, discomfort, discolouration of the restoration, recurrent caries and pulp pathology. This phenomenon can be prevented by incremental addition of composite during a restorative procedure or using laboratory made composite restoration. A bond strength of 14-20 MPa is considered sufficient for a successful restoration.  



  





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