Page 118 - Polymer-based Nanocomposites for Energy and Environmental Applications

94 Polymer-based Nanocomposites for Energy and Environmental Applications

Transfer the polymer specimen after releasing the pressure

Gas
cylinder
Heated
fluid bath
Pressure Polymer
chamber specimen

Fig. 3.10 Microcellular foaming by a batch process.

process was developed to produce polymer foam founded on the same principle of
thermodynamic instability [95]. In this setting, the conventional continuous foaming
processes are of two types: (i) extrusion foaming and (ii) foam injection molding [95].
The first cited called extrusion foaming generally has been done using either by a
single-screw extruder, twin-screw extruder, and a tandem of single-screw extruders
based on the same extrusion process that is established by a rapid depressurization
with a small difference at the level of cell size and cell density [96]. In this process,
both of the polymer grains melt, and the gas injection was occurring into the mixing
zone to ensure a homogeneous mixture of the gas and polymer. Although the nucle-
ation and growth of the microcellular foam structure is achieved by rapid depressur-
ization when the molten polymer escapes from the matrix, the key phase in the
all three continuous extrusion processes is diffusion and mixing the gas into the poly-
mer. In this way, the single-screw extrusion process was equipped with static mixers,
and the twin-screw extrusion process was equipped with kneading elements to
mechanically mix the gas into the polymer [91], while the tandem single-screw extru-
sion process uses a longer residence time to permit for greater diffusion as presented in
Fig. 3.11.
In the other hand, the foam injection molding process is developed to successfully
produce microcellular foams by involving the injection, diffusion, and mixing of the
gas into the polymer [98]. There are generally three manners of foam injection mold-
ing process (Mucell™, IKV, and University of Warwick) with the only variance at the
gas injection mode [99]. The Mucell™ process is based on a direct injection of the gas
inside the injection molder barrel. Cornerwise, the IKV process uses an injection