Biomedical Engineering Reference
In-Depth Information
functional monomers
[16]
. On the other hand, 2-hydroxyethyl methacrylate (HEMA) is a low
molecular weight monomer, which is characterized by its hydrophilic properties and is an important
constituent of most adhesive systems.
The cross-linking monomers provide strength to the adhesive and have hydrophobic properties
which prevent water sorption of the cured adhesive, while functional monomers are responsible
for the demineralization of tooth substrates and provide chemical bond to calcium in the HAP; nev-
ertheless, their ability to decalcify and adhere to the HAP differs from one functional monomer to
another
[16]
.
7.4.2
Solvents
Bonding to dentin requires the addition of solvent to the adhesive composition. The presence of sol-
vent is responsible for the improvement of wetting of the hydrophilic monomer and decrease in the
viscosity of the adhesive for enhancement of the diffusivity of primer or primer/adhesive mixture
into the acid-etching-created microporosities. Three main solvents used in the adhesives irrespec-
tive of their adhesion strategy are acetone, ethanol, and water.
Adhesives that contain acetone or ethanol are recommended in the wet bonding technique.
Although acetone has a higher vapor pressure than ethanol, both have better evaporation during the
air-drying step
[16]
, but both the solvents lack the rewetting capacity of collagen fibers, especially
when etch-and-rinse adhesives are used. On the other hand, water is an excellent rewetting agent
[9]
but hardly gets evaporated during the air-drying step. A water/ethanol mixture is presented in some
etch-and-rinse adhesive systems to improve the evaporation of water
[16]
. Acetone is only found as
a mixture with water in the self-etching adhesives, as water is an essential component in the self-
etching adhesives.
7.4.3
Fillers
Adhesive can be found with fillers (filled adhesive) or without filler contents (unfilled resin).
Unlike resin composite materials, fillers in the adhesive are presented in small amounts compared
to resin composite and the filler size should be small enough to enable the fillers to penetrate into
dentinal tubules or between the collagen fibril spaces.
7.4.3.1
Role of fillers in dental adhesives
Fillers are added to resin adhesives to improve the strength of the adhesive layer. Increasing the
strength of adhesive layer will improve the bond strength of the adhesive
[17]
to either enamel or
dentin, but such improvement was dependent on filler loading
[18,19]
. Increase in microfiller load-
ing up to 40% had not only negatively affected the shear bond strength but also adversely affected
the polymerization rate (PR) of the adhesive layer
[19]
. On the other hand, adding 10% silica nano-
fillers did not affect the degree of conversion (DC), but it significantly improved the cohesive
strength of the adhesive
[20]
.
Increasing the viscosity of the adhesive to a level that does not affect its wetting to tooth sub-
strate is another issue for adding fillers to the adhesives. During the air-drying step, excessive air
drying could result in excessive thinning of the adhesive. Very thin adhesive layer suffers from
incomplete PR due to oxygen inhibited layer
[16]
. Adding filler prevents the excessive thinning of
