22
G. Martínez-Barrera et al.
Releasing of residual styrene of unsaturated polyester resins generate environmental pollution, in this case those related to the odor. Whereby, gamma radiation is
showed as an alternative process for resolve such problem. In the case of irradiated
resins, at the first stage free radicals are generate after scissions of polymers chains;
then either radical or cationic mechanism can occur. Moreover, the number of chain
scissions as well as the release of the local “strain” energy released can improve the
mechanical properties [17].
As it is known, the absorbed energy after irradiation process depends mainly
of the dose and the molecular weight of the polymers. Nevertheless, sometimes is
possible to control the polymerization in terms of the polymer thickness, because at
the initial reaction the gamma rays act in homogeneous way throughout the polymer.
Moreover, it can happen at low temperature without the use of catalysts or additives.
Thus, total polymerization can be controlled in terms of the dose rate [16, 24, 25].
In non-irradiated resins, the polymerization generates rough surfaces with aligned
structures, showed by the arrows in Fig. 1a. While, for gamma irradiated resins, the
surface is more homogeneous and the aligned structures are more narrow, see the
arrows in Fig. 1b [25].
According to the irradiation dose, at 3 kGy, polyester resin is in gel state. At this
stage, more cross-linking through to their C=C bonds are produced, and the polyester
resin becomes gradually in a viscoelastic gel. But for higher doses, high viscosity
is obtained and the gel becomes insoluble, as it is shown for the lines in Fig. 2a.
The remaining liquid can be separated from the gel, after the microgels are obtained.
They are constituted by spherical particles, as it is shown inside the circles in Fig. 2b,
which are produced between polyester insaturations and some styrene molecules [18,
25].
At the total conversion, both the gel fraction and the styrene monomer are present.
Is to say, below the reaction temperature different materials are obtained, corresponding to the glass-rubber transition. But, above such temperature, a 3-D network is
obtained by interconnection between vinyl monomers and microgels. Thus, polyester
resin changes from gel to a hard solid [26].
Fig. 1 SEM images of polymerized resin: by using catalyst (a), and by gamma rays (b)
G. Martínez-Barrera et al.
Releasing of residual styrene of unsaturated polyester resins generate environmental pollution, in this case those related to the odor. Whereby, gamma radiation is
showed as an alternative process for resolve such problem. In the case of irradiated
resins, at the first stage free radicals are generate after scissions of polymers chains;
then either radical or cationic mechanism can occur. Moreover, the number of chain
scissions as well as the release of the local “strain” energy released can improve the
mechanical properties [17].
As it is known, the absorbed energy after irradiation process depends mainly
of the dose and the molecular weight of the polymers. Nevertheless, sometimes is
possible to control the polymerization in terms of the polymer thickness, because at
the initial reaction the gamma rays act in homogeneous way throughout the polymer.
Moreover, it can happen at low temperature without the use of catalysts or additives.
Thus, total polymerization can be controlled in terms of the dose rate [16, 24, 25].
In non-irradiated resins, the polymerization generates rough surfaces with aligned
structures, showed by the arrows in Fig. 1a. While, for gamma irradiated resins, the
surface is more homogeneous and the aligned structures are more narrow, see the
arrows in Fig. 1b [25].
According to the irradiation dose, at 3 kGy, polyester resin is in gel state. At this
stage, more cross-linking through to their C=C bonds are produced, and the polyester
resin becomes gradually in a viscoelastic gel. But for higher doses, high viscosity
is obtained and the gel becomes insoluble, as it is shown for the lines in Fig. 2a.
The remaining liquid can be separated from the gel, after the microgels are obtained.
They are constituted by spherical particles, as it is shown inside the circles in Fig. 2b,
which are produced between polyester insaturations and some styrene molecules [18,
25].
At the total conversion, both the gel fraction and the styrene monomer are present.
Is to say, below the reaction temperature different materials are obtained, corresponding to the glass-rubber transition. But, above such temperature, a 3-D network is
obtained by interconnection between vinyl monomers and microgels. Thus, polyester
resin changes from gel to a hard solid [26].
Fig. 1 SEM images of polymerized resin: by using catalyst (a), and by gamma rays (b)