Processing of bioresorbable and other polymers for medical applications - Durability and Reliability of Medical Polymers - page 51

∑ single-phase mixtures of partly incompatible polymers, and
∑ multiphase mixture of incompatible polymers.

The morphological development of these polymer mixtures is determined
by two possible mechanisms:

∑ dispersive or intensive mixing and

∑ distributive or extensive mixing.

Dispersive or intensive mixing involves the reduction of the size of a
component having a cohesive character within a continuous liquid phase.
This component can be a solid phase or non-compatible liquid phase.
agglomerates are broken and dispersed in the liquid phase. The cohesive
character of the added component is the result of Van der Waals forces and
surface tension. The most common example of dispersive mixing is the
dispersion of carbon black in a rubber compound. incompatible droplets
tend to assume a spherical shape in order to minimize interphasic surface.
it is necessary to impose a stress high enough to achieve surface tension.
Distributive or extensive mixing involves the distribution of droplets of a
compatible liquid in the primary phase. By imposing a strain, the interfacial
area increases and the local dimension of the secondary phase decreases.
Large strains are insufficient to ensure a homogeneous mixing, because the
type of mixing device, the initial orientation and the position of the two
fluids have a significant role.
The final properties of a polymeric material strongly depend on the mixing
process. Mixing efficiency must also be evaluated, because the amount of
power necessary to obtain the highest mixing quality can be unrealistic or
unachievable. Static mixers are pressure-driven continuous-mixing devices,
through which the melt is pumped, rotated and divided with no use of moving
parts. The twisted tape static mixer is the most common static mixer. in this
device, the polymer melt is shorn and rotated 90° by a dividing wall, thus
increasing the interfaces between fluids. This sequence is repeated until a
homogeneous mixture is obtained.
Internal batch mixers are high-intensity mixers that generate complex
shearing and elongational flows. They work well with dispersions of solid
particles in polymeric matrices, because intensive mixing is able to break
up solid agglomerates. solid dispersion is a function of rotor speed, mixing
time, temperature, and rotor blade geometry. Mixing is also achieved in the
extruder screw channel, as said before. Mixing can be enhanced by adding
pins (Osswald and Hernández-Ortiz, 2006) in the flow channel on the screw
or barrel surface (Fig. 3.3). Pins reorient surfaces between fluids and split the
flow, creating new surfaces. An extruder with adjustable pins on the barrel
surface is called a QSM-extruder (QSM stands for Quer Strom Mischer ,
the German for crossflow mixing). Pin-type extruders are necessary when
operating with high-viscosity mixtures such as rubber compounds.

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