E. Marchese, V. Kapeliouchko
Ausimont S.p.A., p.le Donegani 5/6, 15047 Spinetta Marengo (AL), Italia
During last years, a great deal of publications has been devoted
to nanostructured materials (1,2). Examples of such materials include either
inorganics like ceramic, metal, semiconductor composites, either polymeric
or polymer/inorganic nanostructures (3). For constructing these nanocomposites
it is necessary to obtain polymer nanoparticles (<50nm) by industrially
based technology. Such nanoparticles could also be extremely useful as
construction material in quickly developing nanotechnology applications.
Polytetrafluoroethylene (PTFE) because of its unique combination of high heat and chemical resistance together with high purity, lubricity, dielectric properties etc. has been an indispensable material for industry (4), and for this reason there is a strong necessity for synthesis of PTFE nanoparticles that can be used for nanocomposites exalting conventional PTFE premium characteristics.
Basically PTFE is manufactured by suspension or emulsion polymerization (4). Suspension polymer is a some-millimeters-dimension reactor bead that after postreatment and milling could arrive to some microns.
Emulsion polymerization latex has an average particle size of about
150?300 nm (4), and by using low reaction conversion it is possible to
obtain 100nm particles (5). In relation to these conventional methods,
Ausimont?s technology of perfluorinated microemulsion polymerization (6,7)
permits to obtain particles from 10 to 100nm. In comparison with other
cases of PTFE microemulsion polymerization (8,9,10) which use low conversions,
high surfactant concentration etc., the Ausimont?s technology gives industrially
PTFE nanoemulsions (10...100nm) at high conversion and latex concentration
(20-40 wt.%) and relatively low surfactant content.
Through Ausimont technology, for the first time applied systematically
to the polymerisation of PTFE, we were able to obtain nanoemulsion of PTFE
and slightly modified PTFE with no need of liquefying the monomer, nor
to use very high ratio of surfactant to polymer, as requested by usual
polymerisation in microemulsion. We found that, varying the amount of PFPE
microemulsion and with the nature and quantity of comonomer, it is possible
to get not only very tiny PTFE particles but also very different morphologies
.
Two extreme cases can be described :
There are two principal classes of PTFE composites materials: one
of them use PTFE as filler and another is filled PTFE. Some examples of
using PTFE as filler are:
A large portfolio of filled PTFE compositions has been developed based
on fillers as glass fibers, carbon/graphite, bronze, molybdenum disulfide,
mica etc.(14,15). These filled compounds show enhanced physical properties
such as compressive strength, creep resistance, wear and abrasion resistance
etc.
We think all these classes of speciality composite materials could benefit by using PTFE nanoparticles that enhance characteristics such as higher contact surface, better rheological properties, homogeneity etc.
Further, PTFE nanoemulsions could be useful to create substantially new materials,-nanocomposites, particularly in the nanotechnology field. For example, it has been described that PTFE nanoemulsions could be advantageously used for ultra-low dielectric constant thin film for ULSI applications (16-18).
One of the principal problems in using PTFE for nanocomposites is the
difficulty for adhesion with other materials (19). The microemulsion polymerization
together with various "core-shell" techniques, studied and available in
Ausimont for different fluorinated and hydrogenated monomers combinations,
permits to obtain materials with modified or grafted shell with good potential
for getting nanostructured materials suitable for nanotechnology applications.