20
G. Martínez-Barrera et al.
3 Gamma Radiation
The gamma radiation is electromagnetic energy, which has been used for decades for
modifications of the physicochemical properties of different materials. The ionizing
radiation as gamma rays are, has not neither mass nor charge. This is produced from
isotope sources, Cesium 137 (
137 Cs) or Cobalt 60 (
60 Co) with 0.66 and 1.33 MeV
of energy, respectively. Currently, application of the gamma rays is carried out in an
inert atmosphere (air or N 2 ), at the room temperature, covering different irradiation
dose and using specific dose rates. The gamma irradiator equipments are produced
for limited industries around the world. For example, the 651 PT Gamma beam
irradiator which is produced by the Atomic Energy of Canada Ltd, located at Chalk
River, Ontario, Canada.
The physicochemical changes produced in polymers by gamma radiation happen
through three main processes: (a) scission, or (b) cross-linking of polymer chains, and
(c) grafting. Each process is produced according to the physicochemical properties of
the polymers and the applied dose rate. In the first stage, the ionizing is procedure on
the polymer, through the production of free radicals, that are highly reactive species.
Then, such radicals react with neighboring atoms, for to produce either scission or
cross-linking of polymer chains. For a total polymerization is necessary to analyze
the physicochemical properties of the polymers, including the chemical structure and
its molecular weight [13, 14].
The physical and chemical changes provoke by gamma radiation in polymers
depend of well-defined parameters, which include: (a) irradiation dose, is to say
the absorbed energy per unit mass; and the (b) experimental irradiation conditions,
that include the air or nitrogen atmosphere. Several advantages are observed by the
gamma radiation process; for example, the chemical reactions can initiate at any
temperature and pressure; moreover, this can be applied to different matter states
(solid, liquid or gas). Moreover, in such reactions not use of catalysts is carried out.
In the case of polymers, the scission and cross-linking processes can modify several
physicochemical properties, for example: molecular weight, density, crystallinity
degree, thermal behavior, and mechanical resistance [9, 15].
Modifications of the physicochemical properties of materials can carried out by
different processes, including those related with temperature, chemicals, or with high
energy (ionizing radiation). It is important to mention the advantages of the gamma
radiation over thermal and chemical processes: (I) this produces high rates of radical
formation, than those produced by current initiators; (II) the reaction begins without the use of catalysts or additives; because this does not require any activation
energy; (III) the polymerization can begins at room temperature; (IV) at the initial
reaction there is not a significantly increase of temperature. Because, the overall
activation energy decrease and do not produce thermal explosions, at difference of
those chemical processes; (V) at different stages the temperature and reaction time
can be manipulate; (VI) the termination reaction can be controlled, which allows to
analyze each stage of the polymerization. The termination stage has several characteristics, for example, if the polymer has high viscosity then high activation energy
G. Martínez-Barrera et al.
3 Gamma Radiation
The gamma radiation is electromagnetic energy, which has been used for decades for
modifications of the physicochemical properties of different materials. The ionizing
radiation as gamma rays are, has not neither mass nor charge. This is produced from
isotope sources, Cesium 137 (
137 Cs) or Cobalt 60 (
60 Co) with 0.66 and 1.33 MeV
of energy, respectively. Currently, application of the gamma rays is carried out in an
inert atmosphere (air or N 2 ), at the room temperature, covering different irradiation
dose and using specific dose rates. The gamma irradiator equipments are produced
for limited industries around the world. For example, the 651 PT Gamma beam
irradiator which is produced by the Atomic Energy of Canada Ltd, located at Chalk
River, Ontario, Canada.
The physicochemical changes produced in polymers by gamma radiation happen
through three main processes: (a) scission, or (b) cross-linking of polymer chains, and
(c) grafting. Each process is produced according to the physicochemical properties of
the polymers and the applied dose rate. In the first stage, the ionizing is procedure on
the polymer, through the production of free radicals, that are highly reactive species.
Then, such radicals react with neighboring atoms, for to produce either scission or
cross-linking of polymer chains. For a total polymerization is necessary to analyze
the physicochemical properties of the polymers, including the chemical structure and
its molecular weight [13, 14].
The physical and chemical changes provoke by gamma radiation in polymers
depend of well-defined parameters, which include: (a) irradiation dose, is to say
the absorbed energy per unit mass; and the (b) experimental irradiation conditions,
that include the air or nitrogen atmosphere. Several advantages are observed by the
gamma radiation process; for example, the chemical reactions can initiate at any
temperature and pressure; moreover, this can be applied to different matter states
(solid, liquid or gas). Moreover, in such reactions not use of catalysts is carried out.
In the case of polymers, the scission and cross-linking processes can modify several
physicochemical properties, for example: molecular weight, density, crystallinity
degree, thermal behavior, and mechanical resistance [9, 15].
Modifications of the physicochemical properties of materials can carried out by
different processes, including those related with temperature, chemicals, or with high
energy (ionizing radiation). It is important to mention the advantages of the gamma
radiation over thermal and chemical processes: (I) this produces high rates of radical
formation, than those produced by current initiators; (II) the reaction begins without the use of catalysts or additives; because this does not require any activation
energy; (III) the polymerization can begins at room temperature; (IV) at the initial
reaction there is not a significantly increase of temperature. Because, the overall
activation energy decrease and do not produce thermal explosions, at difference of
those chemical processes; (V) at different stages the temperature and reaction time
can be manipulate; (VI) the termination reaction can be controlled, which allows to
analyze each stage of the polymerization. The termination stage has several characteristics, for example, if the polymer has high viscosity then high activation energy