E17.
Compare the physical and mechanical properties of unfilled resins traditional
composites, and microfilled composites. The properties compared should include
surface finish, thermal behavior, compressive strength, tensile strength,
modulus of elasticity, hardness, water absorption, color stability, and
resistance to toothbrush abrasion.
Notice
the difference between weight % and volume %. Weight % depends on density and
may not reflect how much filler there is in a composite (e.g., use uranium, and
the weight fraction can be very high despite a low volume fraction).
Manufacturers often market products based on weight % filler rather than volume
%. Remember, though, that it is the volume fraction filler that was used to
predict properties such as Young's modulus and tensile strength.
| Property | Small Particle Filled | Hybrid | Heterogenous Microfilled |
| Weight % filler | 80-85 | 75-80 | 35-55 |
| Volume % filler | 55-65 | 50-60 | 25-35 |
| Linear shrinkage (%) | 1-2 | 1-2 | 2-3 |
| Thermal expansion (ppm/ºC) | 19-26 | 30-40 | 50-60 |
| Water sorption (weight-%) | 0.5-0.6 | 0.5-0.7 | 1.4-1.7 |
| Modulus of Elasticity (GPa) | 15-20 | 7-12 | 3-6 |
| Tensile Strength (MPa) | 75-90 | 70-90 | 30-50 |
| Compressive Strength (MPa) | 350-400 | 300-350 | 250-350 |
As seen from the table, the volume fraction rarely exceeds 65 vol.%, simply because of the fact that random filler packing is used, not the maximal (74%) close packing.
As seen from this table, the volume fraction filler of the microfilled composite is only about half the volume fraction filler as it was in the small particle filled resin. That difference in volume fraction filler explains why the microfilled composite has inferior properties. However, remember that most microfilled composites belong to the heterogeneous microfilled composites (prepolymerized composite chips mixed with a curable matrix). That explains the lower tensile strength of the microfilled composites (poor filler surface bonding), but also a lower than expected setting contraction (large portion of the resin is prepolymerized. This table also shows that the filler fraction is slightly higher in small particle filled composites than it is in hybrid composites. The reason is simply that the small particle filled composites use broader filler particle size distribution.