Page 242 - Biodegradable Polyesters

220 9 Environment-Friendly Methods for Converting Biodegradable Polyesters

PET+PP
Twin-screw extruder

Stretching rolls Cooling bath

Stretching rolls
S 2
Pelletizer Hot bath S 1

Figure 9.3 Schematic of the industrially relevant extrusion and drawing line.

1) Mixing and extrusion: The matrix and reinforcing polymers are dried and
mixed before being compounded and extruded. This forms an isotropic, con-
tinuous blend filament.
2) Drawing and fibrillation: The blend filament is drawn through pairs of rollers
(Figure 9.3), thus creating highly oriented microfibrils with properties biased
predominantly along a linear dimension or symmetry axis [16]. The drawing
ratio is defined as the ratio of the linear speeds (S /S )ofthe twosetsofrollers
1
2
used to draw the filament and gives an indication as to the amount of align-
ment imparted to the blend. Next, the filament is either collected on a spool
or pelletized.
3) Matrix consolidation through thermal treatment: The drawn filaments or pel-
lets are formed into a composite at a processing temperature, T proc ,which
lies between the melting temperatures of the polymers involved. Control of
this temperature is critical to the successful creation of MFCs as it ensures
the formation of an isotropic matrix while still retaining the highly oriented
reinforcing fibrils. If T is too high, the fibrils will melt and the reinforc-
proc
ing effect will be lost. Depending upon the post-processing method, the final
composite structure can exhibit either quasi-isotropic or anisotropic tenden-
cies depending on the production method.
The result of the described process is a microfibril-reinforced composite mate-
rial with mechanical properties superior to those of the plain matrix polymer [17].
It should be noted that variations in the MFC manufacturing process do exist [17].
An important characteristic feature of the MFC concept is the fact that neat
nano- or microfibrils can be isolated from the drawn blend by removing the sec-
ond blend component using a selective solvent. This possibility has been proved
manifold [12–21]. The diameter of the microfibrils depends on the size of the pre-
ceding spherical particles and can be controlled by varying the melt blending and
drawing conditions. In this way, nanofibrillar composites (NFCs) were prepared
using a blend of polypropylene (PP) and poly(ethylene terephthalate) (PET), where
the reinforcing fibrils have a diameter between 50 and 150 nm [19]. The isolated via
selective extraction of PP neat PET nanofibrils have been used for preparation of