Biomedical Engineering Reference
In-Depth Information
Fig. 10.7
Difference of phosphate and phosphonate end groups in 10-MDP and EAEPA
In situ Curing by Irradiation with Light
So far were presented the building stones but the crowning glory is polymerization, a
process that can be performed chemically, the rule for bone cements and in dentistry
where metal of ceramic parts prohibit the transmission of light (see Chap. 11). In
most other cases, light curing is the usual practice. As said above, polymerization
starts when the acryl monomer meets a molecule bearing a
radical
. The reaction is
called
free radical
or
addition polymerization
.
A radical atom has an unpaired electron and is symbolized as [-C
]. The poly-
merization
propagates
because the unpaired electron is transferred to the central
carbon atom of the acryl ester which in turn links by the same mechanism to another
monomer. When two radicals meet, the propagation stops by a process called
termi-
nation by transfer
or
disproportionation
. The polymerization sequence for acrylic
monomers is represented in Fig.
10.8
. Termination happens randomly resulting in
polymer chains with a distribution of chain lengths. Addition polymerization leads
in general to more homogeneous chain lengths, hence with a narrower distribution
than other mechanisms.
But what is the 'initiation rite' for the first radical?
Formation of a radical is not a spontaneous process, so we need a source of
energy and an acceptor, named an
initiator
. For the time being, the main initiator
candidate is
camphorquinone
; the energy source is visible light.
Initiation by this molecule is effective but at low rate. Therefore, booster
molecules such as amines are added to accelerate the reaction. Examples are:
methylethylenediamine
(aliphatic) or
N-phenylglycine
(aromatic). The reactions are
schematized in Fig.
10.8
.
2
Irradiation is performed by halogen light filtered around
a wavelength of 470 nm, a standard for that purpose although Light Emitting Diodes
(LED), emitting a narrower wavelength band near 470 nm, will gradually substitute
the halogen sources.
Camphorquinone is yellowish and is by some manufacturers partially substituted
by
1-phenyl-1,2propanedione
. This initiator is only slightly yellow, which eases
2
For an in-depth description of the mechanisms, see Jakubiak and colleagues [329].
