Plasticizers are added to thermoplasts or elastomers
to make them more flexible, improve processability, or allow them to foamed.
Generally, plasticizers are low-molar-mass liquids, and only seldom are they
low-or high-molar-mass solids. Elastomers are mostly plasticized with mineral
oils: typical rubber tires, for example, contain about 40% mineral oil.Phthalic
esters dominate plasticizers for thermoplasts, and here, di (2-ethyl hexyl)
phthalate (“dioctyl phthalate “, DOP) is the most used.Polymeric plasticizers
are only used in a relatively small number of cases. They are mostly polyesters
or polyethers. High-molar-mass polyesters are used for polymer blends, but
low-molar-mass polyesters are used as actual plasticizers. Since the latter are
produced by polycondensation, they have a broad molar mass distrubution, and
thus monomer and oligomer components. High monomer factions mean low polymer
fraction, but quite high oligomer fractions. In such cases, they are called
oligomeric plasticizers.
A distinction is also made between primary and
secondary plasticizers. Primary plasticizers interact directly with the polymer
chains, where sencondary plasticizers are actually only diluents for the
primary plasticizers. For this reason, secondary plasticizers are also called
extenders. This, depending on the polymers, a given plasticizer can act as
either a primary or a secondary plasticizer. For example, heavy oils are
extenders for PVC, but primary plasticizers for elastomers.
Eighty to eighty-five percent of all plasticizers are
used to produce plasticized PVC. The phthalates are preferentially used to
plasticize certain polyurethanes, polyester resin, and phenolic resin.
Phosphaste esters are good plasticizers for melamine resins, unstaturated
polyesters, phenolic resins, polyamides, and cellulose acetate. A total of
about 500 different plasticizers are commercially available on the market.
Plasticizers incerease the chain segment mobility by
different molecular effects. Polar plasticizers produce the gauche effect with
polar polymer chains, that is, they increase the gauche conformation fraction
at the expense of the trans conformations, and so reduce the mean rotational
energy barrier. Acting as more or less good solvents, plasticizers dissolve
helix structure and crystalline regions. In addtion, chain segments become more
separated on account of the dilution effect. On the other hand, solvation does
not increase chain mobility since a solvent sheath acts like a substituent and
consequently increase the rotational energy barrier.
Because of the increased chain segment mobility, the
glass transition temperatures, moduli of elasticity, tensile strengths, and
hardness are decreased, whereas the extension at break is increased. The change
in these parameters can thus be used as a macroscopic meaure of the effectivity
of the plasticization. Of these parameters, only the glass transition
temperature depends solely on the polymer chain mobililty, all other parameters
contain contributions from other effects. Thus, measurements on plasticization
effectively using glass transition temperatures, moduli of elasticity, tensile
strengths, elongations at break, and hardness can not yield identical results.
To increase segment mobility, the plasticizer must be
able to form a thermodynamically stable mixture with the polymer, that is, it
must be compatible with the polymer, but solvents which are too good stifffen
the chain by solvation. Thus, plasticizers must be solvents which are as poor
as possible.
