it s about composite properties

1. Properties of composites
Craig ch 9

2. Physical properties
• 1) working and setting time
for light-curing composites curing is “on demand”
the setting reaction continues for 24 hours
studies report that 25% of unsaturated carbon double bonds in
methacrylate-based composites remain unreacted.
A thin layer of air-inhibited material on the surface for bonding to the
next layer
It is useful to protect the surface with transparent matrix

3. • Although for some composites the final physical properties may not
be reached until about 24 hours after the reaction is initiated, enough
of the mechanical strength is attained immediately after curing so the
restoration can be finished and polished with abrasives and is
functional.
• Within 60 to 90 seconds exposure to the ambient light the surface
may loose its capability to flow against the tooth
• The setting time for chemically activated composite resins range from
3 to 5 minutes.

4. • 2) polymerization shrinkage and stress
volumetric shrinkage results in the development of contraction stresses as
high as 13 Mpa between the composite and tooth structure, leading to a
very small gap that can allow marginal leakage of saliva and microorganisms.
Recurrent caries and marginal staining
This stress can exceed the tensile strength of enamel and result in stress
cracking and enamel fractures along the interfaces.
Because the development of shrinkage stress depends on the volumetric
shrinkage strain and the stiffness of the composite at the time of shrinkage,
low-shrinkage composites might exhibit high stress if the composite has a
high elastic modulus.
Solution is incremental technique (2mm)

7. Thermal properties
• The linear coefficient of thermal expansion (α) of composites
ranges from 25 to 38 × 10 -6/℃ for composites with fine
particles to 55 to 68 × 10 -6/℃ for composites with microfine
particles.
• The α value for composites are less than the mean of the
constituents added together ; however the values are higher
than those for dentin (8.3 × 10 -6/℃) and enamel (11.4 × 10 -
6/℃).
• Higher values for microfilled composites are related to the
greater amount of polymer present

8. • Thermal changes are also cyclic in nature and can lead to material
fatigue and early bond failure.
• If a gap forms, the difference between the thermal coefficient of
expansion of composites and teeth could permit percolation of oral
fluids.
• The thermal conductivity of composites with fine particles [25 to 30×
10 -4 cal/s/cm2 (℃/cm)] is greater than that of composites with
microfine particles [15 to 20 × 10 -4 cal/s/cm2 (℃/cm)] because of
the higher conductivity of the inorganic fillers compared with the
polymer matrix.

9. • However, for highly transient temperatures, the composites do not
change temperature as fast as tooth structure and this difference
does not present a clinical problem.

10. Water sorption
• The water sorption of composites with hybrid particles (5 to 7
μg/mm2) is lower than that of composites with microfine particles
(26 to 30 μg/mm2) because of the lower volume fraction of polymer
in the composites with fine particles.
• The quality and stability of the salin coupling agent is also important
• Expansion associated uptake of water could relieve some
polymerization stress, but it is slow when compared to polymerization
shrinkage and stress development.

11. • In the measurement of hygroscopic expansion starting 15 mins after
the initial polymerization, most resins require 7 days to reach
equilibrium and about 4 days to show the majority of expansion.
• Because composites with fine particles have lower values of water
sorption than composites with microfine particles, they exhibit less
expansion when exposed to water.

12. Solubility
• The water solubility of composites varies from 0.25 to 2.5 mg/mm3.
• Inadequate light intensity and duration can result in insufficient
polymerization, particularly at greater depths from the surface.
• Inadequate polymerization greater water sorption and solubility 
early color instability.

13. • During the storage of microhybrid composites in water, the leaching of
inorganic ions are associated with the breakdown in interfacial bonding.
• Silicon leaches in the greatest quantity (15 to 17μg/mL) during the first 30
days of storage in water and decreases with time of exposure.
• Microfilled composites leach silicon more slowly and show an100%
increase in amount during the second 30-day period (14.2 μg/mL).
• Boron, barium and strontium, which are present in glass fillers, are leached
to various degrees (6 to 17 μg/mL) from the various resin-filler systems.
• Breakdown and leakage can be a contributing factor to the reduced
resistance to wear and abrasion to composites.

14. Color and color stability
• Change of color and loss of shade match with surrounding tooth
structure are reasons for replacing restorations.
• Stress cracks within the polymer matrix and partial debonding of the
filler to the resin as a result of hydrolysis tend to keep increase
opacity and alter appearance.
• Discoloration can also occur by oxidation and result from water
exchange within the polymer matrix and its interaction with
unreacted polymer sites and unused initiator or accelerator.
• Composites are resistant to color change caused by oxidation but are
susceptible to staining.

15. Mechanical properties
• Factors:
1. The state of matter of the second (dispersed) phase
2. The geometry of the second phase
3. The orientation of the second phase
4. The composition of the dispersed and continuous phases
5. The ratio of the phases
6. Bonding of the phases

16. • Strength and modulus
The flexural and compressive moduli of microfilled and flowable
composites are about 50% lower than values for multipurpose hybrids
and packable composites, which reflects the lower volume percent
filler present in the microfilled and flowable composites.
• The modulus of elasticity in compression is about 62 GPa for
amalgam, 19 GPa for dentin, and 83 GPa for enamel.

17. • Knoop hardness
Knoop hardness for composites (22 to 80 Kg/mm2) is lower than enamel
(343 Kg/mm2) or dental amalgam (110 Kg/mm2).
The knoop hardness of the composites with fine particles is somewhat
greater than values for composites with microfine particles because of the
hardness and volume fraction of the filler particles.
moderate resistance to indentation under functional stresses for more
highly filled composites, but these difference Does not appear to be a major
factor in resisting functional wear.
Can be misleading on composites with large particles (>10 µm)

18. • Bond strength
To etched enamel and primed dentin is between 20 to 30 Mpa
Composite can be bonded to existing composite restorations, ceramics,
and alloys when roughened and primed.
In general the surface to be bonded is sandblasted (microetched) with
50-µm alumina and then treated with a resin-silane primer for
composites, a silane primer for silica-based ceramics, and acidic
phosphate monomer for zirconia, or a special alloy primer.
Bond strength to treated surfaces are typically greater than 20 MPa.

19. Clinical properties

20. • Depth of cure
Depth of light penetration depends on the wave length of the light, its
irradiance, and the scattering that takes place within the restoration.
Influencing factors:
1. Concentration of photo initiator
2. Filler content and particle size
3. Shade and opacity (depth of cure in more opaque shades:1mm)
4. Light intensity
5. Tip distance from restoration surface (optimum: 1 mm)

21. • Microfilled composites with smaller and more numerous particles
scatter more light than microhybrid composites with larger and fewer
glass particles. longer exposure time is needed to obtain adequate
polymerization of microfilled composites.
• Standard exposure time is 20 seconds for depth of 2 to 2.2 mm
(adjacent)
• 40-second exposure improves degree of cure at all depths, and is
required for darker shades
• Step the light across the surface of large restorations.
• Exposure time with large emitting tips should be up to 60 seconds.

22. • Radiopacity
Modern composites include glasses having atoms with high atomic number,
such as barium, strontium, and zirconium.
Some fillers such as quartz, lithium-aluminum glasses, and silica, are not
radiopaque.
Some microhybrid composites achieve some radiopacity by incorporating
finely divided heavy-metal glass particles. Others use ceramic particles
containing heavy metal oxides.
In the nonofilled composite, radopacity is achieved by using nanometric
zirconia (5 to 7 nm) or by incorporating the zirconia in the nanoclusters along
with silics.

23. • Wear rates
Clinical studies have shown that composites are ideal for anterior
restorations
Newer formulations minimize attrition and abrasion, but marginal
degradation is still evident and is attributed to improper preparation design,
inadequate adhesion, polymerization contraction of the composite, and
marginal microcracks.
Acceptable composites for posterior applications require clinical studies that
demonstrate , over a 18-month period, a loss of surface contour less than 50
μm.
The nonofilled composites have been shown to exhibit wear resistance
similar to that of natural human enamel in a 3-year and 5-year study.

24. Biocompatibility
• Nearly all of the major components of composites (Bis-GMA,
TEGDMA, and UDMA among others) are cytotoxic in vitro if tested as
the bulk monomer but the biological liability of a cured composite
depends on the extent of release of these components from the
composite.
• A dentin barrier markedly reduces the ability of the components to
reach pulpal tissue, but these components can traverse dentin
barriers, albeit at reduced concentrations.
• Components of composites are known allergens, and there has been
some documentation of contact allergy (dentists and dental
personnel)

25. • There has been some controversy about the ability of the
components of composites to act as xenostrogens.
• Studies have proved that bisphenol A is estrogenic in in vitro tests
that measure this effect using breast cancer cell growth.