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R. Tesser et al.
In general, most plasticizers are liquids characterized by a very high boiling temperature and an average molecular weight included in the range between 300 and
600, with linear or cyclic carbon chains [50, 59, 127,202, 232]. Hence, the key feature of a plasticizer molecule concerns the small molecular sizes, which allow it
to enter and occupy the spaces between polymeric chains, reducing the secondary
forces among them. As a consequence, a plasticizer modifies the three-dimensional
molecular organization of the polymers, having an effect on the secondary forces
between the chains and lowering the required energy to motion.
There are many classifications related to the plasticizers; they concern: (1) the distinction between internal or external, depending on the presence or not of chemicals
bonds with polymer chains; (2) the definition of primary or secondary.
An internal plasticizer has normally a bulky structure and it provides to reduce
the polymer glass transition temperature and make it more flexible at room temperature (lowering the elastic modulus) [232]. It is a part of the polymer molecules,
copolymerized into the polymer structure, grafted with the original polymer, thereby
making the chains of the polymer more arduous to fit and compact with each other
closely [141]. An external plasticizer, on the other hand, is an additive, which is not
part of the polymer structure. These low-volatile molecules are added to polymers
to produce swelling without chemical reaction. Even if these plasticizers can be lost
by migration, evaporation or extraction, they are most widespread.
Another important classification splits the plasticizers into primary and secondary
[223, 224]. If a plasticizer was soluble in the polymer at very high polymer concentration, it is said ‘primary’. Thus, primary plasticizers must meet the following requests:
(a) be totally soluble in the polymer; (b) not exude from the material; (c) be used
alone. The secondary plasticizer, on the other side, has lower gelation capacity and
compatibility, and it is inclined to exude. These plasticizers are generally used in
addition to primary plasticizers to reduce costs or enhance some properties.
Over the years, several theories have been developed and proposed to shed light on
the plasticizing mechanism [194]. The main ones, as resumed in Fig. 8.10, correspond
to (a) the lubricity theory; (b) the gel theory; (c) the free volume theory. According
to the lubricity theory, the action of the plasticizer is regarded just like a lubricant: it
diffuses into the polymer, inserting between the chains and reduces the intermolecular
frictions. As a result, by flexing the plastic material the macromolecules can slip over
each other. The plasticizers lubricate the movement of the molecules and reduce
the rigid matrix. In this way, the plasticized polymers can be depicted as parallel
alternating layers of polymer and plasticizer. The gel theory was developed straight
after the first one, and it can be viewed like an extension of the lubricity one. It
suggests that the plasticizer act to destroy and replace the intermolecular interactions,
lowering the polymer gel structure and increasing the flexibility. According to this
theory, the plastic is represented as a complex three-dimensional network, where
the plasticizers are bonded to polymer chains by weak secondary forces. From the
definitions of these two theories, similarities and differences between them are clear.
The lubricity theory treats the rigidity of the material as internal frictions between
polymers and the plasticizer lubricates the layers, while the gel theory ascribes it to
the attachments between polymers, and the plasticizer acts to reduce them.
R. Tesser et al.
In general, most plasticizers are liquids characterized by a very high boiling temperature and an average molecular weight included in the range between 300 and
600, with linear or cyclic carbon chains [50, 59, 127,202, 232]. Hence, the key feature of a plasticizer molecule concerns the small molecular sizes, which allow it
to enter and occupy the spaces between polymeric chains, reducing the secondary
forces among them. As a consequence, a plasticizer modifies the three-dimensional
molecular organization of the polymers, having an effect on the secondary forces
between the chains and lowering the required energy to motion.
There are many classifications related to the plasticizers; they concern: (1) the distinction between internal or external, depending on the presence or not of chemicals
bonds with polymer chains; (2) the definition of primary or secondary.
An internal plasticizer has normally a bulky structure and it provides to reduce
the polymer glass transition temperature and make it more flexible at room temperature (lowering the elastic modulus) [232]. It is a part of the polymer molecules,
copolymerized into the polymer structure, grafted with the original polymer, thereby
making the chains of the polymer more arduous to fit and compact with each other
closely [141]. An external plasticizer, on the other hand, is an additive, which is not
part of the polymer structure. These low-volatile molecules are added to polymers
to produce swelling without chemical reaction. Even if these plasticizers can be lost
by migration, evaporation or extraction, they are most widespread.
Another important classification splits the plasticizers into primary and secondary
[223, 224]. If a plasticizer was soluble in the polymer at very high polymer concentration, it is said ‘primary’. Thus, primary plasticizers must meet the following requests:
(a) be totally soluble in the polymer; (b) not exude from the material; (c) be used
alone. The secondary plasticizer, on the other side, has lower gelation capacity and
compatibility, and it is inclined to exude. These plasticizers are generally used in
addition to primary plasticizers to reduce costs or enhance some properties.
Over the years, several theories have been developed and proposed to shed light on
the plasticizing mechanism [194]. The main ones, as resumed in Fig. 8.10, correspond
to (a) the lubricity theory; (b) the gel theory; (c) the free volume theory. According
to the lubricity theory, the action of the plasticizer is regarded just like a lubricant: it
diffuses into the polymer, inserting between the chains and reduces the intermolecular
frictions. As a result, by flexing the plastic material the macromolecules can slip over
each other. The plasticizers lubricate the movement of the molecules and reduce
the rigid matrix. In this way, the plasticized polymers can be depicted as parallel
alternating layers of polymer and plasticizer. The gel theory was developed straight
after the first one, and it can be viewed like an extension of the lubricity one. It
suggests that the plasticizer act to destroy and replace the intermolecular interactions,
lowering the polymer gel structure and increasing the flexibility. According to this
theory, the plastic is represented as a complex three-dimensional network, where
the plasticizers are bonded to polymer chains by weak secondary forces. From the
definitions of these two theories, similarities and differences between them are clear.
The lubricity theory treats the rigidity of the material as internal frictions between
polymers and the plasticizer lubricates the layers, while the gel theory ascribes it to
the attachments between polymers, and the plasticizer acts to reduce them.