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Plastic
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If suitable, processing can be done at low ambient temperatures - the
curing is then very delicate, without overheating. |
Dielectric heating
Dielectric heating includes radio frequency and
microwave heating.
Plastics are characterized by a low temperature conductivity. Conventional
heating, by incidence of an outer heat source, heats the material surface
at first. The resulting temperature gradient complicates and slows soaking
of the whole material volume, possibly influencing its characteristics
negatively. On the other hand, the dielectric heating
influences the whole material volume simultaneously.
Combined dielectric heating and
conventional heating
Though the dielectric heating can be used
separately, from the technological and economic view it is often
advantageous when combined with the conventional heating.
Dielectric heating is frequently used for
pre-heating, as the plastics, as a rule, are characterized
by a low thermal conductivity. As an example, the conventional heating to
a specified temperature, namely by larger profiles, takes a long time.
Therefore the deep dielectric heating effect,
enabling fast reaching of the specified temperature in the whole material
volume, is very efficient. It enables namely the substantial enhancement
of production capacity. As a rule, the temperature homogeneity contributes
to the higher quality of the final product. Here at, the total energy
consumption is lower.
On the contrary, the dielectric heating is often
used in final technology phase for a very delicate, highly efficient
tempering.
Drying of plastics
Drying is probably the most widespread MW
heating application in the plastics industry. The MW and RF drying
principle totally differs from conventional drying. Distinctly from the
surface conventional drying, during the DE drying the temperature rises in
the whole volume simultaneously, accompanied by fast steam generation.
Here at, the DE energy engages the regions with higher moisture
preferably.
Most of moisture evaporates before it leaves the material. This lowers the
migration of material to the surface. The temperature gradient (inverse to
the conventional drying) accelerates drying process, and the surface does
not harden.
The DE drying technology enables a universal design and processing of
various forms and profiles of loose and bulk materials, without any
substantial modification of drying cavities and lines - the design concept
is mostly similar.
The drying of hygroscopic plastic granulates represents an extremely
important application. As much as 30-times shorter drying process before
extrusion can be reached instead of usual 2 to 12 hours. Another
outstanding feature in comparison with conventional heating is an easy
attainable extremely low moisture level.
Approximately 1 kilowatt of MW power is sufficient for evaporating 1 litre
water per 1 hour.
Polymerization of plastics
Microwave heating can be advantageously used for
temperature homogenization of initial mix of monomers and additives for
polymerization.
It finds its use by the two main groups of polymers:
- thermoplastics, gained by simple
polymerization
- thermosets, gained by polycondensation
reaction.
In comparison with conventional heating, a
substantial reduction of molecular weight of the plastic product can be
reached by the microwave polymerisation of PMMA, PMA, PS (polymethylmetacrylate,
polymethylacrylate and polystyrene) at comparable reaction conditions.
Microwaves can be successfully applied to polymerize plastic material
spread in a thin layer on paper, subsequently used in the manufacture of
filters for car engines.
Polyesters are frequently used for production of reinforced
materials, i.e. in France, Germany, Sweden and in other countries,
microwave heating has been used for reticulation of resin coating on glass
fibres.
Polyurethanes originate by the reaction of polyalcohols with
polyisocyanates. microwave heating starts polymerisation, while the
endemic heat supports the effect of microwaves. The possible applications
include e.g. varnishing of glass bottles and impregnation of corrugated
cardboard.
Epoxy resins result from condensation of epichlorhydrin with bisphenol,
stimulated by drying reagents, e.g. polyamines. In Japan, an application
using polymerisation of epoxy matrices filled with carbon or kevlar by
microwave heating, has been used in the aircraft industry.
Bonding of plastics
For bonding (glueing and welding) of plastics, microwave heating and
particularly RF heating, one of the oldest and most succesfull
applications, are used. Many lines are in operation, e.g. for welding of
parts of casings for blood preservation for blood transfusion, and of
other plastic materials. The attained advances enable automatic control of
the welding cycle also for processing of sofisticated, mass produced
materials.
An experiment - welding of thermoplastics by directed microwave energy -
represents a novelty. The thermoplastics reinforced by long fibres cannot
otherwise be welded by existing methods and must be glued. The Microwave
welding raises the strength of samples up to three times in comparison
with gluing and seems to be a suitable technology option.
Microwave heating is proved in two variants - bonding by direct heating of
two polymer layers and other materials, or by using a suscepting
interlayer.
Forming of plastic boards
By forming of plastic boards, microwave heating or
pre-heating enables a homogene temperature in the whole volume of
material, e.g. PVC, polystyrene and
polypropylene. The commonly used thickness up to 6 mm can be processed in
seconds. Moreover, equally as in other MICROWAVE applications, the heating
intensity in partial regions of boards can be varied by the continuous
change of the radiated microwave power.
Shrinking of plastic semi-products
The main advantage of microwaves used for shrinking of plastic
semi-products (plastic films, hoses etc.) is the exact control of the
temperature. There is no standby power, and there is no heat inertia after
switch-off; therefore no overheating occurs that would otherwise cause
deterioration by partial baking, crozzling etc. This application can
be used for the packaging and protecting layers technology.
Recycling of plastics and waste
An interesting application of plastics recycling has been proved:
microwave heating can be used for production of
a new material with enhanced fireproofing property, usable e.g. for heat
insulation.
Microwaves can be also used for disposal of dangerous waste,
containing high portion of plastics, i.e. from
medical installations. The waste is reliably neutralised to the common
municipal waste level.