procedure by treating silicates with mineral acids to obtain fine hydrated silica
particles in the course of precipitation [28].
These products (fumed silica) are highly dispersed, amorphous, white in color
and spherically shaped. The average diameters of the primary particles are in the
range of 7–40 nm, according to the AEROSIL
® grade [13]. The company offers
many products made from silica, e.g. hydrophilic fumed silica, hydrophobic fumed
silica or fumed mixed oxides (mixtures of SiO 2 and Al 2 O 3 ) [27].
2.4 Titanium Dioxide
Among the many nanofillers, Titanium dioxide (TiO 2 ) is very often investigated, in
polymeric applications, since it is non-toxic, chemically inert, possess high hardness and has UV filter properties.
Several methods have been developed for generating colloidal titania particles
[29]. One of the popular methods to obtain TiO 2 is the sol–gel method, as with SiO 2
nanoparticles. Using this relatively simple method it is possible to control the
properties of the product, such as size of the particles or phase composition. The
preparation of TiO 2 nanoparticles can be effectively carried out through the hydrolysis and condensation of titanium alkoxides in aqueous media (sol–gel method)
[30, 31]. The reactions are presented in Eqs. (2) and (3) below: where R is ethyl,
i-propyl, n-butyl group.
Ti OR
ð Þ 4 þ 4H 2 O ! 2Ti OH
ð Þ 4 þ 4ROH hydrolysis
ð
Þ
ð 2Þ
Ti OH
ð Þ 4 ! TiO 2 xH 2 O þ 2 À x
ð
ÞH 2 O condensation
ð
Þ
ð 3Þ
Different size and morphology of the TiO 2 strongly depend on the water/
titanium molar ratio and the pH of the solution during the reaction. At high ratios
of H 2 O/Ti, small-sized particles are formed [31]. The second method of producing
TiO 2 nanoparticles, is a hydrothermal process which proceeds in aqueous or
nonaqueous systems. The particles prepared using hydrothermal synthesis, are
expected to have larger surface area, smaller crystalline size, and higher stability
than those obtained by other methods [32].
TiO 2 generally occurs in three crystalline phases: rutile, anatase and brookite
[33]. The possible transformation is accelerated by heat treatment at temperatures
between 450 and 1,200
C and is dependent on several parameters such as particle
size, initial phase, dopant concentration, reaction atmosphere and annealing temperature [34]. Crystalline structure of the TiO 2 , amorphous phase content, morphology and size of the particles significantly affect the activity of the nanofiller and
matrix improvement [29].
TiO 2 has been extensively explored for many years, due to its effect on the
polymer matrix and relatively easy availability. The nano-titanium dioxide used for
modification of the polymer can improve optical [35], UV radiation [36], thermal
66
M. Strankowski
particles in the course of precipitation [28].
These products (fumed silica) are highly dispersed, amorphous, white in color
and spherically shaped. The average diameters of the primary particles are in the
range of 7–40 nm, according to the AEROSIL
® grade [13]. The company offers
many products made from silica, e.g. hydrophilic fumed silica, hydrophobic fumed
silica or fumed mixed oxides (mixtures of SiO 2 and Al 2 O 3 ) [27].
2.4 Titanium Dioxide
Among the many nanofillers, Titanium dioxide (TiO 2 ) is very often investigated, in
polymeric applications, since it is non-toxic, chemically inert, possess high hardness and has UV filter properties.
Several methods have been developed for generating colloidal titania particles
[29]. One of the popular methods to obtain TiO 2 is the sol–gel method, as with SiO 2
nanoparticles. Using this relatively simple method it is possible to control the
properties of the product, such as size of the particles or phase composition. The
preparation of TiO 2 nanoparticles can be effectively carried out through the hydrolysis and condensation of titanium alkoxides in aqueous media (sol–gel method)
[30, 31]. The reactions are presented in Eqs. (2) and (3) below: where R is ethyl,
i-propyl, n-butyl group.
Ti OR
ð Þ 4 þ 4H 2 O ! 2Ti OH
ð Þ 4 þ 4ROH hydrolysis
ð
Þ
ð 2Þ
Ti OH
ð Þ 4 ! TiO 2 xH 2 O þ 2 À x
ð
ÞH 2 O condensation
ð
Þ
ð 3Þ
Different size and morphology of the TiO 2 strongly depend on the water/
titanium molar ratio and the pH of the solution during the reaction. At high ratios
of H 2 O/Ti, small-sized particles are formed [31]. The second method of producing
TiO 2 nanoparticles, is a hydrothermal process which proceeds in aqueous or
nonaqueous systems. The particles prepared using hydrothermal synthesis, are
expected to have larger surface area, smaller crystalline size, and higher stability
than those obtained by other methods [32].
TiO 2 generally occurs in three crystalline phases: rutile, anatase and brookite
[33]. The possible transformation is accelerated by heat treatment at temperatures
between 450 and 1,200
C and is dependent on several parameters such as particle
size, initial phase, dopant concentration, reaction atmosphere and annealing temperature [34]. Crystalline structure of the TiO 2 , amorphous phase content, morphology and size of the particles significantly affect the activity of the nanofiller and
matrix improvement [29].
TiO 2 has been extensively explored for many years, due to its effect on the
polymer matrix and relatively easy availability. The nano-titanium dioxide used for
modification of the polymer can improve optical [35], UV radiation [36], thermal
66
M. Strankowski