Effects of Gamma Radiation on the Physicochemical Properties …
21
is produced, thus the overall activation energy decreases (measured in kJ/mol) [14,
16–18].
Moreover, (VII) ionizing radiation produce a complete and fast polymerization;
in some of them the catalyst does not have enough energy for a complete polymerization; (VIII) complete polymerization allows better solvent resistance and high shape
stability of the polymer dimensions. It is very important to check out the gel and the
vitreous states through the process, because in many cases, they happen at the same
time; (IX) there is no emission of volatile compounds to the environment [19, 20].
4 Effects of Gamma Radiation in Polyester Resin
At the beginning, the polyester resin is usually in the liquid state. Then, once the
reaction temperature is reached, a change of state is produced, from liquid to gel.
Then, the polymers chains are bonding between them, which is denominated as
a cross-linking process. Chemical reactions are obtained by using some initiators
and promoters, whose weight concentration depending on the resin content. Methyl
ethyl ketone peroxide (MEKP), amine or benzoyl peroxide (BZP) have been used as
initiators, and cobalt naphthenate or dimethyl-para-toluidine (DMPT) as promoters
[21, 22].
There are several studies concerning to partial polymerization of polyester resin, is
to say, the catalyst is insufficient for such activity. Fortunately, gamma radiation can
finish the polymerization process. According to the mentioned advantages, ionizing
radiation does not require any activation energy for its initiation; and the final reaction
can be controlled. Nevertheless, total polymerization depends of the gamma radiation
dose and the type of polymer [17].
During polymerization process, there are reactions between the double-bonds and
the free radicals formed on the chains; such process release strain energy. The chain
stress is a fundamental parameter, because chemical bond scissions can increase and
the recovery radicals decrease, according to the chain stress values. Moreover, there
are connection among the parameters: chain length, strain and bond rupture. For
example, short chain lengths show high strain energy and fast breaking.
In the first stage of the irradiation process on the polymers, scission of the polymer
chains is obtained. Then, radicals are formed in the molecular end groups. Nevertheless, different processes can happen. For example, if there are restrictions of the
surrounding chains over the chains with the radicals, then these may rebound with the
original chain, is to say, the recovery of the chain is obtained. Such phenomenon is
denominated the “cage effects”. It is well known, that at low temperature the recovery
happen. Then, there is a proportional effect between the total number of the scissions
and the number of radicals. But, at high temperature, the “cage effects” happen, is
to say, the initial “stress” of the surrounding chains generate “strain” energy on the
chains. It is known, that ionizing radiation can help to chains to escape from the
restrictions of the surrounding chains. Another phenomenon is called “cage breaks”
through which scissions of polymer chains happen first for short chains [23].
21
is produced, thus the overall activation energy decreases (measured in kJ/mol) [14,
16–18].
Moreover, (VII) ionizing radiation produce a complete and fast polymerization;
in some of them the catalyst does not have enough energy for a complete polymerization; (VIII) complete polymerization allows better solvent resistance and high shape
stability of the polymer dimensions. It is very important to check out the gel and the
vitreous states through the process, because in many cases, they happen at the same
time; (IX) there is no emission of volatile compounds to the environment [19, 20].
4 Effects of Gamma Radiation in Polyester Resin
At the beginning, the polyester resin is usually in the liquid state. Then, once the
reaction temperature is reached, a change of state is produced, from liquid to gel.
Then, the polymers chains are bonding between them, which is denominated as
a cross-linking process. Chemical reactions are obtained by using some initiators
and promoters, whose weight concentration depending on the resin content. Methyl
ethyl ketone peroxide (MEKP), amine or benzoyl peroxide (BZP) have been used as
initiators, and cobalt naphthenate or dimethyl-para-toluidine (DMPT) as promoters
[21, 22].
There are several studies concerning to partial polymerization of polyester resin, is
to say, the catalyst is insufficient for such activity. Fortunately, gamma radiation can
finish the polymerization process. According to the mentioned advantages, ionizing
radiation does not require any activation energy for its initiation; and the final reaction
can be controlled. Nevertheless, total polymerization depends of the gamma radiation
dose and the type of polymer [17].
During polymerization process, there are reactions between the double-bonds and
the free radicals formed on the chains; such process release strain energy. The chain
stress is a fundamental parameter, because chemical bond scissions can increase and
the recovery radicals decrease, according to the chain stress values. Moreover, there
are connection among the parameters: chain length, strain and bond rupture. For
example, short chain lengths show high strain energy and fast breaking.
In the first stage of the irradiation process on the polymers, scission of the polymer
chains is obtained. Then, radicals are formed in the molecular end groups. Nevertheless, different processes can happen. For example, if there are restrictions of the
surrounding chains over the chains with the radicals, then these may rebound with the
original chain, is to say, the recovery of the chain is obtained. Such phenomenon is
denominated the “cage effects”. It is well known, that at low temperature the recovery
happen. Then, there is a proportional effect between the total number of the scissions
and the number of radicals. But, at high temperature, the “cage effects” happen, is
to say, the initial “stress” of the surrounding chains generate “strain” energy on the
chains. It is known, that ionizing radiation can help to chains to escape from the
restrictions of the surrounding chains. Another phenomenon is called “cage breaks”
through which scissions of polymer chains happen first for short chains [23].