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The present invention relates to improvements in processes and apparatus
for the manufacture of expanded plastics materials.
It is known that a certain number of plastics materials into which there
has been introduced a swelling or expanding agent, i.e., an agent capable
of being volatilized or decomposed into a gaseous product under the action
of heat, undergo an expansion when they are heated and form a foam.
This is true, for example, of polystyrene. The conventional transforming
technique comprises introducing a certain amount of non-expanded product
into a metal mould and impregnating it with steam under pressure at a
temperature sufficient to supply the heat necessary to initiate the
expansion process.
This process has several disadvantages, namely
(1) it is a cyclic and therefore slow, system,
(2) it requires a large area and apparatus (boiler, press, etc.)
(3) it can only be carried out by specialists,
(4) the dimensions of the parts produce are limited,
(5) the moulds are relatively expensive.
The manufacture of expanded non-polar plastics materials is also known, in
which a non-polar plastics material in the form of particles (balls or
beads) containing a swelling agent, is moistened with a polar liquid and
the moistened particles are subjected to a high-frequency electromagnetic
radiation in a resonating cavity.
This process for the manufacture of plastic materials is advantageously
operated in apparatus which contains at the outlet of an extruder a
spinneret made of non-absorbent material and having a weak coefficient of
friction, which is placed inside a resonating cavity excited by a
high-frequency electromagnetic radiation. The polar liquid is heated and
vaporised under the action of the high-frequency radiation, thus causing
the expansion of the particles.
The process and apparatus described above, and known in the prior art,
enable a greater expansion and especially a more regular expansion to be
obtained than those obtained by the conventional processes (heating in
steam under pressure, for example).
It is often desirable for the particles, balls or beads, used in the
manufacture of expanded plastics materials to be partly expanded, or
pre-expanded. Attempts at pre-expansion have been effected by means of
steam; however, these attempts have the disadvantage of producing
pre-expanded balls or beads containing water and needing drying.
The object of the present invention is to provide a process and an
apparatus for the pre-expansion of plastics material particles, which
enables pre-expanded balls or beads to be obtained which are substantially
or entirely free from water, contrary to the pre-expansion practised by
means of steam, and are dried during the actual operation of
pre-expansion, thus eliminating the need for a drying process following
the pre-expansion operation. In this way, the length of treatment as well
as its cost may be reduced and the pre-expansion treatment can be carried
out continuously. In addition, the process and apparatus according to the
present invention enable on the one hand a considerable reduction in the
storage time of the expanded particles (known as maturing operation)
before the moulding operation, and on the other hand also permit objects
of expanded plastic material with a perfect finish to be obtained.
Accordingly, the present invention provides an apparatus for the
manufacture of pre-expanded particles, which comprises in combination a
heat-insulated resonating cavity, with an ultra high-frequency
electromagnetic source of energy, a ferrite circulater, a wave guide, an
Archimedes screw for the introduction of the plastics material to be
pre-expanded, a pump for regulating the feed of polar liquid and a device
for extracting the pre-expanded particles.
Advantageously, the apparatus of the present invention is provided with a
hot air blower of which the temperature is between 50.degree. and
100.degree. C. The ultra high-frequency generator is preferably one which
functions at between 900 and 10,000 megahertz and has a power between 1
and 200 Kw. The resonating cavity is preferably in the form of a truncated
cone with a circular cross section and is provided with an agitator and a
grid.
In order to obtain a still better distribution of energy in the resonating
cavity, the latter advantageously comprises two wave guides giving a
transverse-electric mode TE.sub.01 and a transverse-magnetic mode
TM.sub.01.
According to an advantageous embodiment of the apparatus of the present
invention, the device for extraction of the pre-expanded particles is
situated at the top end of the cavity.
The device for extracting the pre-expanded particles may consist of a
depression system or may be a mechanical extractor coupled to the axle of
the agitator.
The present invention also provides a process for the pre-expansion of
particles of plastics material containing a swelling agent, in which the
said particles, in the form of balls or spheres, are continuously mixed
with a polar liquid, this mixture is continuously introduced into the
resonating cavity of the apparatus, associated with a source of ultra
high-frequency microwave energy, which vaporizes the polar liquid on the
particles of plastics material, causing their expansion, hot air being
also introduced into the said cavity for pulsating or agitating the vapour
so as to inhibit substantially any condensation on the pre-expanded
particles and causing the instantaneous drying of the said particles,
after which the dried, pre-expanded particles, are extracted from the
cavity.
The polar liquid advantageously consists of water and is introduced in the
proportion of 10 to 40% by weight with respect to the weight of the
particles to be pre-expanded.
The distribution of the ultra high-frequency energy in the resonant cavity
may be improved by crossing two wave polarisations.
The resonating cavity is preferably maintained at a constant temperature,
especially between 70.degree. and 100.degree. C., for the whole period of
the pre-expansion operation.
In addition to the preceding arrangements, the present invention also
comprises other arrangements, which will be evident from the description
which follows.
The present invention relates more particularly to the process and
apparatus for the manufacture of particles of pre-expanded plastics
materials, according to the preceding arrangements, as well as to means
suitable for operating these processes and for the production of these
apparatus, and the combined processes and the chains of manufacture in
which the processes and apparatus according to the present invention are
included.
The present invention will now be further described, by way of example,
with reference to the accompanying drawing, which is a diagrammatic
representation of an apparatus of the present invention.
It must be understood, however, that the example of operation which will be
described hereinafter, like the apparatus described below and shown in the
drawing, are given only by way of illustration of the object of the
invention, but does not in any way constitute a limitation thereof.
The apparatus according to the present invention, in which the
pre-expansion operation and drying of the particles of plastic material
are carried out, comprises:
a microwave resonating cavity consisting of a circular truncated cone 9,
made, for example, of aluminium alloy, excited by a source of ultra
high-frequency electromagnetic energy 1, which is constituted by an
oscillating magnetron at F=2 450 MHz, by means of a circular wave guide 3
in the mode TM.sub.01 ;
a hot air blower 4 75.degree. C.) placed at the base of the cavity;
a feed of plastic material consisting of an Archimedes screw 5 and a hopper
12;
a pump 6 regulating the feed of water;
a mechanical agitator 7, a grid 11 situated in the top part of the cavity,
through which the hot air as well as the evaporated water escape;
and an extractor 8 of the expanded particles. The resonant cavity 9 is
insulated at 10 against heat.
The functioning of the apparatus according to the invention will be
illustrated below, with the aid of a nonrestrictive example, relating to
the operation of the process according to the invention in the said
apparatus.
60 kg/hour of damp beads of polystyrene are introduced by the feed device 5
into the resonant cavity 9 at the same time as 1 kg of water, which is
passed therein by means of the regulating pump 6. The cavity 9 is excited
by the ultra high-frequency energy coming from the source 1 by the
following method: the circular wave guide 3 is excited by the oscillating
magnetron 1 and excites in its turn the cavity 9 in which the vaporization
is effected. The mechanical agitator 7 is started in order to reduce the
agglomerates which could be formed.
The ultra high-frequency microwave energy vaporises the water in suspension
on the beads, which are expanded. The steam and hot air (which pulsates or
agitates the steam and reduces, or even suppresses, the risks of
condensation on the pre-expanded beads) escape through the grid 11, while
the dry pre-expanded beads (at 0-0.5% of humidity) are extracted by the
extractor 8. This extraction may be effected either by means of a
depression system or by means of a mechanical extractor coupled on the
axle of the agitator.
In order to expand 1 kg of polystyrene beads of final density of 20
g/liter, 120 Kc are necessary, i.e., 0.14 Kw/hour at 2 450 MHz. 50 Kg per
hour of dry beads are thus extracted (about 0.2% of water).
As the apparatus works continuously, it is necessary to isolate the
magnetron 1 from the reflected waves which could be formed in the case of
insufficient charge, by means of a ferrite circulator 2. Such an
arrangement prolongs the life of the magnetron.
From the preceding description it follows that whatever may be the method
of operation, realization and application adopted, processes and apparatus
for continuous manufacture of particles (balls or beads) of partially
expanded, or pre-expanded, plastic material are obtained from it, which in
comparison with the processes and apparatus aiming at the same end, but
previously known, have important advantages, besides those which have been
mentioned in the above specification, and especially:
The advantage of permitting a total continuity of the manufacture;
The advantage of permitting the immediate use of the pre-expanded beads,
which brings with it a reduction of the volume of the installations on
account of the suppression of the storage, and consequently, a reduction
in the industrial cost;
The advantage of vaporising the water within the particles, thus causing
the uniform distribution of the material and this at atmospheric pressure,
and the elimination of any risk of agglomeration;
The advantage of permitting a perfectly dry material to be obtained from
the evaporation of the residual water vapour contained in the particles,
by heating through the walls of polystyrene (which is totally transparent
at F=2 450 MHz); and
The advantage of obtaining a final product of which the quality is
appreciably improved compared with that of the products obtained on use of
the processes and the apparatus previously known.
As well as that which has gone before, the invention is not at all limited
to those of its methods of operation, realisation and application which
have just been described more explicitly. On the contrary it includes all
the variants which may occur to the technician in the matter, without
departing from the spirit or the scope of the present invention.
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