Gelation of polymer colloids in a mechnically agitated column

7
Introduction 
 
 
 The term, colloid, derives from the Greek word "κολλα", which 
means, "glue". Thomas Graham (1805-1869) has given the name, colloids, 
to a substance that could not diffuse through a membrane. Nowadays a 
colloid refers to dispersion of particles in a liquid medium where the size of 
the particle is in the range between 10 and 1000 nm. These kinds of systems 
are spread everywhere. Examples are ink, foodstuffs, etc. Here, we are 
dealing with polymer colloids. 
One of the well-known colloids is the sap from a particular tree, 
which has been used since years to produce natural rubber. This sap is called 
"latex". It is a stable suspension of small rubber particles that are insoluble 
in the suspending medium. Now, the term, latex, is commonly used for any 
stable suspension of polymeric particles. 
When polymers are produced using emulsion polymerization 
processes they are generally in the form of small particles suspended in a 
(often aqueous liquid medium), i.e. polymer latexes. Successive step will be 
either the direct use of the latexes (such as for coating or painting) or 
extraction of the polymers from the suspensions. In the latter case a 
coagulation process in a mechanically agitated vessel with a suitable 
coagulant such as salt or acid is commonly used to produce granules from 
the primary particles, as shown schematically in Figure 1. The coagulation 
“Nessun pazzo è pazzo se ci si pone nelle
sue ragioni.” 
(G.G.Marquez)
8
process is rather complex, involving several physical and chemical 
phenomena, but it may be divided into three stages. 
 
1. Aggregation of primary particles to form clusters; 
 
2. Growth of the clusters, and when they reach a certain size, they 
stick together to form a three dimensional network, generally 
referred as gel phase; 
 
3. Breakage of the gel phase under the turbulent shear to form 
granules. 
 
The difference between gel and a granule is that the former has very open 
structure and the filling liquid dominates its volume, while the latter has a 
structure more compacts containing only very little residual liquid. 
 It is intuitively understandable that the transition from stage (2) 
to stage (3) is rather inhomogeneous. In particular, when the gel is formed, it 
is generally in the form of big blocks. Due to high viscosity of the gel, the 
breakup of the gel and formation of granules start first for the gel blocks in 
the region of the agitators and then from those in the other regions when 
they move progressively to the region of the agitators. The complete 
granulation generally takes at least a few minutes. Under such conditions, 
the first formed granules must be continuously sheared until all the gel 
blocks are broken and converted to granules. Then, the physical properties of 
the granules formed earlier must be different from those formed later. 
Moreover, due to the inhomogeneity in stage (3), the breakup process of the 
gel blocks is generally out of control, and can be different from batch to
9
batch. Thus, it is difficult to obtain good reproducibility of the products from 
such coagulation processes. 
 In order to overcome the above drawbacks and better control the 
coagulation process, it is proposed in the literature (Whitlock, 1962; Higuchi  
 
 
 
et al., 1999) to separate the gelation phase from the granulation phase, i.e., 
changing from one-step to a two-steps coagulation process, as shown 
Figure 1 – A schematic illustration of the industrial coagulation process for 
polymer latexes in a mechanically agitated vessel.
10
schematically in Figures 2a and 2b. In this way, gels of small pieces are 
generated in the gelation step, and then transported to a granulator where 
they are converted to granules either batchwise or continuously. 
It is evident that for such two-steps processes, the granulation 
step must depend on the gelation step and the structure of the formed gels. 
 
 
 
 
 
The present study is devoted to developing and understanding the 
gelation process. In particular, a mechanically agitated gelation column is 
proposed and constructed, where a polymer latex can be fed continuously at 
a desired flow rate and converted promptly into small pieces of gels. The 
main purposes of this study are to explore the feasibility of the designed 
gelation column and to understand how the structure of the formed gels 
depends on the various operation parameters such as particle volume 
fraction in the latex, pH, temperature, fluid dynamics, etc. 
Gelation +Granulation
Latex Granules 
Figure 2a – One-step process for polymer latex coagulation.
Figure 2b – Two-steps process for polymer latex coagulation. 
Gelation Granulation
Latex
Granules
Gelation Granulation
Latex
Granules
11
 The thesis is organized as follows. Chapter 1 gives a brief 
introduction to the gelation phenomena of polymer colloids where the 
concept of colloidal stability, DLVO theory, and kinetics of aggregation 
towards gelation are discussed. Chapter 2 illustrates in details the gelation 
column that we constructed and its operation principle. The methods used to 
characterize the structure of gels are also described in Chapter 2. In Chapter 
3, all the experimental results are presented, and proper explanation for the 
observed phenomena are supplied.
9
 
Introduction to Gelation Phenomena.                   
 
 
1.1 Stability of Polymer Colloids. 
 
 
 Let us consider a piece of material in the bulk state subdivided 
into many particles. There is change in standard free energy, 
0
f
G ∆ , 
relative to the bulk state:  
 
sl sl f
A G ∆ γ ∆ =
0
                       (1.1) 
 
where γ
sl
 is the solid-liquid interfacial surface tension or free energy, 
expressed in Jm
-2
, and ∆ A
sl
 is the increase in interfacial area. 
0
f
G ∆ can be 
either positive or negative. If it is positive, the colloidal state is unstable 
relative to the bulk state, and the colloid is referred as lyophobic (where 
lyo- refers to the surrounding medium). If it is negative it is said to be 
lyophilic, and it is thermodynamically stable. Unstable colloidal system 
Chapter 1 
Che cosŁ la realta? Ci che la
maggior parte delle persone ha
ritenuto dovrebbe essere  (P.Cohelo)
10
may be made metastable when a sufficiently large energy barrier is 
constructed between the two states, as shown in Figure 1.1. This energy 
barrier is the core of our discussion. When this does not exists at all, or is 
small relative to the thermal energy of particles, aggregation occurs. 
 
 
 
1.1.1 Van der Waals Forces 
 
 
 Aggregation occurs because of the universal van der Waals 
attractions among particles. The van der Waals interactions are due to 
dipole-dipole interactions through space.  
 Every neutral molecule attracts each other primarily because 
thermal oscillations of the nuclei relative to their electronic clouds result 
in transient dipoles. These, in turn, induce dipoles in neighboring 
Figure 1.1 - Kinetic stability of a lyophobic colloid. In the case of lyophilic the
situation will be reversed.
11
molecules. Hamaker, taking into account all the microscopic attractions 
between two spheres at close distance in vacuum, has derived the 
expression for the attractive potential energy (Goodwin et al. 1986): 
 
()() ()
            +
− +
+
+
− +
− =
2 2 2
1
1
1 2
1
1
1 1
1
12 x
ln
x x
A
W
H
vdw
   (1.2) 
 
 
 
where A
H
 is the Hamaker constant, x is the dimensionless distance of 
separation between two particles, defined as D/2a, a is the radius of a 
particle and D is distance of separation between two particles. One can 
also use the following simple expression for short distances ( a D << ): 
 
D
a A
W
H
vdw
12
− ≈      (1.3) 
 
The negative values of 
vdw
W indicate a favorable free energy change as 
particles approach each other. When particles are immersed in a 
dispersion medium, their interactions will be different from that in the 
vacuum and is reduced by the self-attraction of the medium. In this case 
A
H
 must be estimated based on the Hamaker constant of both particles 
and dispersion medium, 
11
A and 
22
A , as follows: 
Figure 1.2 - Dipoles between close molecules are the origin of van der Waals
attractions. 
e
-
+
e
-
+
12
 
2
2
1
22
2
1
11 H
A A A       + =          (1.4) 
 
Note that A
H 
is generally considered to be independent of ion 
concentration in the medium.  
 
 
1.1.2 Electrostatic Repulsion Forces 
 
 
 To overcome the van der Waals attractions a charge is given 
to the surface of the particles by means of surfacebound ionogenic 
groups or of absorbed ionic surfactants, either anionic or cationic. Since 
the particles have the same surface charge, they repulse each other, and 
aggregation can be avoided. Note that this is not a simple electrostatic 
repulsion because of the existence of ions in the surrounding medium. 
These form an electrical double layer (Israechvili, 1992; Russel et al., 
1989) composed of the more or less fixed surface ions and a diffusive 
layer of moving ions, as schematically shown in Figure 1.3. 
 Overall in the fluid medium the electro neutrality is 
maintained, but locally the charged surfaces generate an electric field. 
The determination of the potential ψ x () of this electric field involves the 
Boltzmann and Poisson equations. The former gives the distribution of 
charges in a field:  
 
) T k / e z exp( n n
B i i i
ψ − =
0
                            (Boltzmann equation)      (1.5)