Page 117 - Polymer-based Nanocomposites for Energy and Environmental Applications

Nanoclay and polymer-based nanocomposites: Materials for energy efficiency 93

3.6.1 Processing of polymer nanocomposites foams

It is no different between processing of polymer nanocomposite foams and that of
polymer foams except one of the main issues that is the good dispersion distribution
of the nanoclays throughout the polymer foams. In this case, nanoclay addition and
dispersion in the matrix is usually done before foaming. Moreover, the nanofiller-
reinforced polymer foams may also act as nucleation agents during the foaming pro-
cess to produce finer cell structure and higher cell density [87]. Additionally,
improved gas barrier properties provide another opportunity for the application of
polymer nanocomposite foams. Historically, polymer foams were made for the first
time in 1931 by Dr. Otto Bayer [88]. The polymer foams are processed by the intro-
duction of a thermodynamic instability that causes supersaturation of gas into a poly-
mer matrix, generally done by using either chemical or physical blowing agents [89].
Chemical blowing agents (CBA) are those that decomposing into gases upon being
heated like as air, carbon dioxide (CO2), ammonia, nitrogen, and noble gases such
as argon or other toxic gases [90]. There are two types of chemical blowing agents:
exothermic CBA such as azo compounds (azodicarbonamide and their derivatives)
and 4,4 -oxybis(benzenesulfonyl hydrazide) or endothermic CBA such as sodium
0
bicarbonates and their altered forms [91]. Whereas physical blowing agents (PBA)
undergo an irreversible phase change from a dense to a gas by either liquids (meth-
ylene chloride, ethanol, butane, and pentane) or gases (hydrochlorofluorocarbons)
[85]. Physical blowing agents can be introduced into the polymer matrix using many
ways like (1) physical blending and physical dissolution, (2) physical blending and
chemical decomposition, (3) physical dissolution, and (4) chemical reaction and
encapsulation [92]. The incorporation process of physical blowing agents (PBA) is
suggested in three steps: firstly, the PBA is dissolved and saturated in the polymer
at a high pressure. Secondly, release the pressure at varying temperatures to occur
the phase separation between the dissolved gas and the polymer matrix. In the final
step, the polymer expansion stops and stabilizes the foaming structure [93].The
processing conditions of polymer foams affect both of homogeneous and heteroge-
neous nucleation rates. So, there are four critical fundamental variables in foam
processing: pressure, temperature, saturation time (soaking time), and depressuriza-
tion rate [85]. The basic foam processing called batch process is the first employed
to specify such parameters. This batch process has been developed in the early
1980s, generally made in two stages. In the first step, the polymer is placed inside
high-pressure vessels and nonreacting gas such as CO 2 or N 2 . Therefore, the gas dif-
fuses out into the polymer specimen until the desired concentration [94]. In the second
step, the dissolved gas plays an important role to lessen the glass transition temper-
ature of the specimen when it is placed in heated bath with temperature control;
thereby, the polymer foaming is often observed in specimen when the temperature
is above the glass transition temperature (T g ), starting by a nucleation of bubbles to
end up by bubble growth as seen in Fig. 3.10 [90].
This process is not really practical because it has a disadvantage of high cost
and takes a long time especially in the first stage (from hours to days) depending
on diffusivity, for this reason, and to overcome these drawbacks, continuous extrusion