Influence of the Processes Parameters on the Properties of The ...

Chapter 2.
<strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface
The stability <strong>of</strong> <strong>the</strong> aggregates is due to <strong>the</strong> effects <strong>of</strong> mechanical interlocking that may occur,
especially between particles in <strong>the</strong> form <strong>of</strong> long fibres. Wide size distributions generally lead to close
packing requiring smaller amounts <strong>of</strong> binder and, as a result, <strong>the</strong> formation <strong>of</strong> strong aggregates. The size
distribution <strong>of</strong> particles in an agglomeration process is essentially determined by a population balance that
depends on <strong>the</strong> kinetics <strong>of</strong> <strong>the</strong> various processes taking place simultaneously, some <strong>of</strong> which result in
particle growth and some in particle degradation.
In general, starting with a mixture <strong>of</strong> particles <strong>of</strong> uniform size, <strong>the</strong> following stages may be
identified:
Nucleation in which fresh particles are formed, generally by attrition.
Layering or coating as material is deposited on <strong>the</strong> surfaces <strong>of</strong> <strong>the</strong> nuclei, thus increasing both <strong>the</strong>
size and total mass <strong>of</strong> <strong>the</strong> particles.
Coalescence <strong>of</strong> particles which results in an increase in particle size but not in <strong>the</strong> total mass <strong>of</strong>
particles.
Attrition. results in degradation and <strong>the</strong> formation <strong>of</strong> small particles, thus generating nuclei that reenter
<strong>the</strong> cycle again [Richardson et al., 2002].
5.2 Obtention <strong>of</strong> Composites by <strong>the</strong> Co-grinding Process
Currently, <strong>the</strong>re are two main methods <strong>of</strong> obtaining composite materials. The first and most
common are to extrude various raw materials. The second is to chemically syn<strong>the</strong>size <strong>the</strong> desired composite.
Currently a third method <strong>of</strong> syn<strong>the</strong>sis <strong>of</strong> composite materials is under development: co-grinding.
The products from Extrusion are preheated and introduced upstream <strong>of</strong> an extrusion screw. Within
<strong>the</strong> extruder, <strong>the</strong> temperature gradually increases, leading to a change <strong>of</strong> state products. The use <strong>of</strong> screw
extrusion <strong>of</strong> various morphologies can perform an intimate mixture between different compounds and
disperse within one ano<strong>the</strong>r seamlessly. This technique is widely used for many industrial applications by
using materials in <strong>the</strong> form <strong>of</strong> powder, flakes and granules.
A second method <strong>of</strong> obtaining composite is to syn<strong>the</strong>size chemically. In this case, <strong>the</strong> matrix is
dissolved or suspended in a solvent and <strong>the</strong> load is activated in situ using an oxidizing agent <strong>of</strong>ten. The
composite particles are <strong>the</strong>n filtered and dried. This method is <strong>of</strong>ten used to obtain electrical conducting
polymers [Cassignol et al., 1998; Pouzet et al., 1993]. In However, it is rarely used for composites widely
because it requires facilities very expensive.
Each <strong>of</strong> <strong>the</strong> two techniques for implementing composite materials cited earlier, has limitations
which may be <strong>the</strong>rmal, in <strong>the</strong> case <strong>of</strong> extrusion, or chemical, in <strong>the</strong> case <strong>of</strong> chemical syn<strong>the</strong>sis. In addition,
both techniques have one thing in common is <strong>the</strong> difficulty <strong>of</strong> dispersing <strong>the</strong> filler in <strong>the</strong> matrix. For <strong>the</strong> first,
if <strong>the</strong> operating conditions are not well understood, <strong>the</strong>re has appearance <strong>of</strong> agglomerates, whereas <strong>the</strong>
second technique, agitation alone does not to obtain particle size small enough so as to have properties
homogeneous. Therefore, we decided to explore a new syn<strong>the</strong>tic pathway to obtain composite materials: cogrinding.
This technique consists <strong>of</strong> co-grinding two materials A (polymer) and B (adjuvant/filler) toge<strong>the</strong>r.
In <strong>the</strong> begining, <strong>the</strong>re occurs a phenomenon <strong>of</strong> fragmentation <strong>of</strong> particles <strong>of</strong> different constituents (cf. Figure
2.21) to a size limit. One <strong>of</strong> <strong>the</strong> two components is much more fragmented quickly, here component reaches
its size limit before fragmentation <strong>of</strong> A.
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