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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a degassing process and device for a worm extruder or the like.
A known worm extruder or the like has a worm housing with a chamber containing the worm or worms and at least one degassing passage opening into the worm chamber. A return delivery is arranged in the degassing passage for returning to the worm
chamber material which has penetrated into the degassing passage, the return delivery device comprising a conveyor element arranged within the degassing passage and delivering to the worm chamber.
In the processing of synthetic plastic materials with the aid of a worm extruder or the like (especially a worm kneader or a worm injection-molding machine, each with one or more worms), the problem frequently arises that the material fed to the
worm extruder or the like is mixed with water deriving for example from a washing process. In addition or alternatively, the material can contain solvents and diluents such as hydrocarbons, chloro-hydrocarbons, ketones and volatile reaction products
such as alcohols and/or esters. These must be removed in order to obtain optimum quality products from the worm extruder or the like. Consequently, they are gassified and the material degassed before supply to the worm extruder or in the worm extruder. In this context the expression "degassing" means both the elimination of residual moisture (by evaporation or vaporisation) and the removal of dissolved gases and volatile constituents. Degassing in separate appliances preceding the worm extruder has
the disadvantage of greater construction expense and a possible renewed contamination, especially by moistening of the material, on the way from the appliance to the worm extruder.
2. Description of Art
A degassing device is known for example from the periodical "Plastverarbeiter" (`Plastics processor`), vol. 28, No. 5, 1977, pages 233 to 240. In this case the degassing of the material is obtained within the worm extruder with the aid of a
radial degassing passage in the region of the middle of the length of the worm FIGS. 1 and 3). The degassing zone in the region of the degassing passage is preceded by a compression zone. At the transition from the compression zone into the degassing
zone, which is at atmospheric pressure, an expansion of the material takes place leading to a gas emission by the material. It is sought to exclude penetration of the material into the degassing passage in that the core diameter is greatly reduced in
this region, so that the material does not completely fill out the cross-section of the housing. However this measure has not yet proved adequate to prevent a flow of material into the degassing passage. In order to counter this problem it is known
from the initially cited periodical passage to form a degassing pocket into the otherwise cylindrical worm interior chamber, in the region of the degassing passage. Further special developments of the degassing opening in the worm cylinder are known
from the periodical "Kunststoffe" (`Synthetic Plastics`) in volume 64, 1974, pages 175 to 177 FIG. 3). Moreover a so-called By-Pass Extruder is known (for example Wittfoht "Kunststofftechnisches Worterbuch" (`Technical Dictionary of Plastics`), Part 3,
Carl Hanser Verlag, Munich, Vienna 1978, page 367) in which a bypass passage of regulable throughflow opening into the degassing zone is provided for pressure regulation. However these known measures are frequently not fully satisfactory, since the
precise setting of the material level close below the degassing passage opening is achievable only with great difficulty if at all. For safety's sake, consequently, the material level beneath the degassing opening will be set lower than absolutely
necessary, which reduces the throughput of material through the extruder. In the case of materials of high viscosity, for example pastes, hitherto this so-called cylinder degassing could be applied only to an extremely limited extent, since the material
in paste form always penetrates into the degassing passages. This leads firstly to the blockage of the degassing passages and also to a part of the material, namely the material which has penetrated into the passage, no longer being subject to the
intended continuous processing and assuming different material properties. If now this branchedoff material is returned, sooner or later, into the worm chamber, a deterioration of the homogeneity of the material results.
In a known degassing device of the initially stated kind the return delivery device consists of a double worm, that is two interengaging worms, arranged in a degassing housing, which rest along their circumference, except the region of mutual
overlap, in more or less sealing manner on the internal circumferential surface of the housing interior, which in cross-section forms approximately a figure-of-eight. Now it has appeared that in the case of relatively tough compositions in the range
between 0.1 and 10 kPA.s. a completely satisfactory degassing cannot be achieved. Thus what is called flash evaporation can occur, that is a sudden evaporation, in which mass particles of the material to be processed in the extruder are entrained by
the gas current, leading to blockage of the return delivery device.
SUMMARY OF THE INVENTION
An object of the invention is to improve the degassing process and device of the initially stated kind to such effect that it reliably prevents blockage of the degassing passage, especially in the case of material of past type.
This is accomplished in that the free surface (evaporation area) of the material to be processed in the extruder amounts within the degassing passage to more than about 40% of the cross-section of the degassing passage, and in that the minimum
gas passage cross section of the return delivery device amounts to at least about 0.1 times, better at least about 0.15 times, the degassing passage cross-section.
It has appeared that the return conveyor device formed in accordance with the invention permits the removal of even large gas volumes, without the degassing passage being blocked. The reason for this may consist in that by reason of the large
evaporation area a smooth vapour generation occurs avoiding the entraining of particles of composition. By evaporation area here there is understood the surface of the material to be processed in the extruder, within the degassing passage, which surface
is in direct communication with the space outside the worm housing, that is either the surroundings or a vacuum suction installation. Between the evaporation area and the surroundings or the vacuum suction installation there may also be a passage with
reduced passage cross-section, which however must be of such large dimensions that no appreciable excess pressure develops above the evaporation area, so that the vapour generation is undisturbed.
By the term "degassing cross-section" in this context there is understood the pure passage cross-section (that is without conveyor element) which corresponds to the entry area for the extruder material for entry into the degassing passage. The
gas passage cross-section on the other hand is the cross-section presented to the gas current with the conveyor element inserted. In the case of the known return delivery device the evaporation area and the gas passage cross-section are substantially
smaller. This is due for one part to the fact that the two worm cores greatly reduce the available evaporation area and nextly to the fact that by reason of the interengagement of the two worms the space formed between two successive worm turn flanks is
not available as gas passage cross-section, but only a fraction thereof corresponding to the clear interval between the interengaging worms.
According to a first preferred embodiment according to the invention it is provided that the conveyor device comprises a single, preferably rotating, inclined scraper sweeping over at least a part of the degassing passage cross-section,
preferably in the form of a single conveyor worm in the degassing passage, preferably with worm axis substantially parallel to the passage axis, delivering towards the worm chamber. In the case of paste material with relatively high viscosity the
possibility exists of making the conveyor worm diameter smaller than the diameter of the degassing passage, since by reason of the high viscosity and stiffness even material more or less remote from the conveyor worm is transported by it. In an
especially preferred form of embodiment of the invention however it is provided that the axis of the conveyor worm coincides with the axis of the hollow cylindrical degassing passage and that the conveyor worm diameter substantially corresponds to the
passage diameter. This form of embodiment, which is suitable for high viscosities (in the range from 1 to 10 kPa.s.) of the material in paste form ensures that the inner wall of the passage always remains free from deposits of material.
In a further development of the invention it is proposed that the spiral or spirals of the conveyor worm each have at the worm end nearest the worm chamber an end edge running in radial direction towards the core. Consequently the rotating worm
end sweeps, on the material swelling up in the passage, along a flat circular area filling out the passage cross-section, so that the rising material is immediately pressed back into the worm chamber again. Since however at any moment the worm is in
contact with the material only with the end face of the core and with the edge of the worm spiral, a relatively large free evaporation area of the material formed by the remainder of the passage cross-section results. The edge sweeping along on the
material effects a certain beating of the material, which again facilitates the liberation of the gases. Since the formed vapours can flow without hindrance along the turns of the conveyor worm, the result is that the minimum gas passage cross-section
of this return delivery device is equal to the evaporation area.
A worm diameter amounting to about 5 to 20 times, preferably about 8 to 15 times, optimally about 10 times the core diameter (of the conveyor worm in each case) has proved especially favourable.
A maximum passage cross-section and thus a maximum evaporation area of the material is obtained for a given extruder if, as proposed in accordance with the invention, the passage diameter corresponds approximately to the dimension of the worm
chamber perpendicular to the longitudinal axis of the worm chamber and to the passage axis.
A speed of rotation of the conveyor worm amounting to about 20 to 120, preferably about 40 to 80, optimally about 60 revolutions per minute has proved advantageous.
According to a further form of embodiment of the invention it is provided that the return delivery device comprises a conveyor roll in the degassing passage with roll axis substantially perpendicular to the axis of the passage, delivering towards
the worm chamber. This form of embodiment is of universal suitability for pastes in a wide viscosity range (0.1 to 10 kPa.s.) however it has proved especially advantageous for mixtures or pastes with viscosities in the range from 0.7 to 3 kPa.s.
The material returned by the roll is fed back immediately to the remainder of the material flow in the worm chamber, in the correct direction of movement, if in accordance with the invention the direction of rotation of the conveyor roll is
opposite to the direction of rotation of the worm or worms in the worm chamber.
In order reliably to preclude material deposits on the conveyor roll according to the invention a scraper extending along the conveyor roll is provided for material from the worm chamber which has settled on the roll circumference. It is further
proposed that the scraper is arranged above one worm of the worm chamber in such a way that material falling from the scraper is carried away by the worm. This ensures that even the material falling from the scraper is immediately processed further in
the intended manner.
In an especially preferred form of embodiment of the invention the conveyor roll is arranged in the degassing passage in such a manner that a passageway is formed between each of the longitudinal side walls of the degassing passage, which are
substantially parallel to the roll axis, and the roll circumference, and that the scraper is arranged in that passageway which immediately follows the worm chamber in the direction of rotation of the roll. The minimum gas passage cross-section of the
return device corresponds to the total passage cross-section of the two passageways. In most cases it is sufficient if only one of the passageways is free and the other is closed by the scraper, since even the cross-section of one single passageway in
general is completely sufficient to prevent a build-up of pressure above the evaporation area preventing the evaporation process.
With the proposed high paste viscosities the conveyor roll conveys even material lying relatively far from it, so that the diameter of the roll can be made considerably smaller than the corresponding dimension of the degassing passage, which is
preferably of rectangular cross-section. It is preferably provided that the conveyor roll diameter amounts to about 0.4 to 0.9 times, better 0.5 to 0.7 times, optimally about 0.65 times the clear distance between the longitudinal side walls of the
degassing passage, which are parallel to the conveyor shaft axis.
In total a relatively large evaporation area results which is formed by the surfaces of the extruder material which has swelled up into the degassing passage, on both sides of the conveyor roll which dips more or less far into the extruder
material, and also by the material layer surface which adheres to the conveyor roll and finally is removed from the roll circumference by the scraper. Thus the effective evaporation area can be even larger than the degassing cross-section. At least one
open passageway offers an adequate gas passage cross-section for the liberated gases.
If, as proposed in accordance with the invention the roll axis extends substantially parallel with the longitudinal axis of the worm chamber, the roll axis length can be made relatively large, since its size is now independent of the
cross-sectional dimensions of the worm chamber. Correspondingly the evaporation area and the minimum gas passage cross-section are also enlarged.
The degassing device in accordance with the invention can also be used with particular advantage in combination with a vacuum suction device, since here again the return delivery device reliably ensures that the material remains in the worm
chamber. To this end it is suggested that an extractor attachment covering the outer end of the degassing passage be used on the worm housing, being connected to a vacuum suction device. In order that the drive motor for the return conveyor device may
be operated outside the vacuum zone it is proposed that the suction attachment comprises a vacuum duct for a drive shaft coupled with a drive motor, preferably a compressed-air motor, and driving the return conveyor device. By way of example an electric
motor or equally a compressed-air motor can be considered as drive motor.
An especially simple assembly results in the case of a conveyor worm, since then the drive shaft can be rigidly connected with the conveyor worm shaft, preferably made in one piece therewith.
For the case where a conveyor roll is used it is proposed that the drive shaft be connected with the roll shaft through a gearing, preferably a spur wheel gearing.
In order to ensure a thorough degassing or drying, as the case may be, and in order to counteract the formation of deposits in the return conveyor device, it is suggested that the return conveyor device be heatable, preferably by the conducting
of heating medium through a housing wall interior space of a double-walled housing of the return conveyor device.
The invention further relates to an extruder having a degassing device of the kind as described above, which is formed as a double-shaft extruder, preferably with worms rotating in the same direction. In order to obtain the largest possible
evaporation area of the material, that is to say the largest possible passage cross-section, it is proposed that the axis of the passage should extend perpendicularly of the plane defined by the two worm axes, preferably approximately equidistantly from
the two worm axes. The concept "evaporation area" in this context is naturally not limited to the emission of vapour by the material, but to the emission of the gases of all origins.
The invention further relates to an extruder having a degassing device of the kind as described above, which is formed as single-shaft extruder with axially, preferably pulsatingly, movable worm.
In order to gain the most extensive possible degassing in such extruders, and also for example in planet roll extruders, worm kneaders or worm injection-molding machines, it is proposed to use two degassing devices of the kind according to the
invention arranged in tandem in the direction of the longitudinal axis of the worm chamber.
It is of essential importance for a reliable progress of the working process with the aid of the worm extruder or the like that the material be fed to the extruder is of high regularity. For this purpose in accordance with the invention a
quantity-regulating feed device, preferably an eccentric worm pump, is provided at the material feed opening of the extruder.
The invention further relates to a process for the production of peroxid-containing pastes or plastic compositions, especially using the degassing device according to one of the preceding descriptions. Such pastes or plastic compositions contain
organic peroxides in a plastic or liquid matrix, for example a desensitizing agent, or mixtures thereof. Such mixtures of organic peroxides with carrier substances were hitherto produced in batches in kneaders or planetary agitator mechanisms. To
ensure that the paste possesses the necessary high homogeneity, processing times of 5 to 20 hours (in the kneader) are necessary. In the case of very high paste viscosities of 1 to 5 kPa.s. a homogenization in the kneader is often no longer possible to
an sufficient extent. Additional difficulties arise in the case of very viscous pastes, since then cavities occur in the kneader, with the paste forming bridges above the kneading arms and therefore no longer being homogenized. In fact it is possible
to carry out an after-treatment in a two-roll or three-roll mechanism in order to achieve the desired homogeneity, which however is of considerable detriment to the economy of the process.
Mixtures which are obtained by an in situ process from chemical synthesis are used of perference as starting materials for the production or mixing of the peroxide-containing pastes, because such products contain the peroxide in finely
crystalline form in uniform distribution in a matrix. These products of synthesis are mixed with water deriving from the process of washing the composition (generally 4 to 10% water). Moreover such paste pre-products can contain solvents and diluents
such as hydrocarbons, chloro-hydrocarbons and/or ketones and volatile reaction products such as alcohols and/or esters which must be removed in order to obtain optimum quality for the subsequent utilisation of these pastes.
The particle size of the peroxide distributed in the liquid or plastic matrix and the uniformity of the distribution are also of special practical importance. Thus, for example, for the cross-linking of silicone rubber with a peroxide--such as
benzoyl peroxide or dichloro benzoyl peroxide--it is necessary that the crystal size of the peroxide should not exceed 20 .mu.m., because otherwise bubbles can form in the silicone rubber in the cross-linking reaction. If the paste contains agglomerates
of the peroxide, after it is worked into the silicone rubber, inhomogeneous distributions can result having the consequence of inadequate strength values after vulcanisation.
In the known manner of working with production of the pastes in the kneader only about 70% of the batches are obtained in the requisite quality; the remainder must be rehomogenized with a roller frame, in a complicated manner. The production
process by means of kneader or rolling mill is also very labourintensive, since the filling and emptying must take place manually. The usual conveyor worms on the kneader devices are unsuitable since in the emptying of a kneader with a discharge worm
only 80% of the content can be withdrawn at maximum. The remainder remains adhering to walls and kneader arms and to the lid of the kneader.
In accordance with the invention a process is prepared which is capable of distributing extremely fine particles, especially of size below 10 .mu.m., in a liquid or plastic matrix extremely homogeneously without agglomeration, with removal of
volatile constituents, including even moisture from the mixture. The process according to the invention also presents the possibility of automation and of processing with exclusion of atmosphere, which largely eliminates the safety risks in the
processing of organic peroxides.
For this purpose the process according to the invention is characterized in that a plasticisable or plastic mixture of solid and/or liquid organic peroxides and any solid additives with a liquid or plastic matrix is freed from evaporatable
constituents and (at the same time) homogenized in a worm extruder or continuous kneader.
In contrast to the known process, the process according to the invention can be carried out continuously if the mixture to be treated is conducted continuously through an evaporation or drying zone.
The degree of degassing or moisture extraction can further be increased in that the evaporation and/or drying process is carried out in vacuo.
Problem from the penetration of the material into a degassing passage or even entraining of the material with the emerging gases in the case of vacuum degassing are reliably prevented according to the invention by a return device in the degassing
passage of the worm extruder or continuous kneader which return material penetrating into the degassing passage. It is also of particular advantage that the use of the return device renders it possible to select an enlarged degassing passage
crosssection and thus an enlarged evaporation area of the material.
BRIEF DESCRIPTION OF THE DRAWING
For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings of preferred embodiments in which.
FIG. 1 is a roughly diagrammatic lateral elevation of a worm extruder according to the invention,
FIG. 2 is a cross-section on an enlarged scale through a worm extruder similar to FIG. 1 in the region of the degassing device in a first embodiment of the invention,
FIG. 3 is a cross-section similar to FIG. 2 of a second embodiment of the degassing device (section along line III--III in FIG. 4),
FIG. 4 is a section of the arrangement in FIG. 3 along the line IV--IV.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The worm extruder 10 as represented roughly diagrammatically in FIG. 1 consists of a worm housing 14 containing one or more worms 12. For the sake of simplicity in FIG. 1 there is represented only one worm of a two-shaft extruder, as may be seen
in greater detail in FIGS. 2 and 3. At the right end of the housing 14 in FIG. 1 a drive shaft 16 of the worm 12 is conducted out of the housing 14 for connection to a motor (not shown). The material intended for processing in the extruder 10 is fed to
the eccentric 10 by a symbolically represented eccentric worm pump 20 (with feed hopper 22) which opens at the right extruder end in FIG. 1 into the worm chamber 24 accomodating the worm or worms. Water liberated by the compression of the material in
the compacting in the eccentric worm pump 20 and at the right worm end can already be drained away here, which is symbolically indicated by the water outlet passages 26 and 28 in FIG. 1.
The worm 12 consists of a plurality of sections connected for joint rotation, which sections can differ by different pitches or equally by different delivery directions. In the purely diagrammatic example as illustrated a relatively long section
12' at the right end of the worm is followed by a relatively short section 12" with opposite delivery direction which, by reason of its shorter length, merely leads to a build-up of the extruder material in this region. An axial section, designated by A
in FIG. 1 of the extruder 10, can therefore be called build-up or compression zone, which constitutes a vacuum-tight seal of the following zones against atmosphere. On the core 29 of the worm 12 besides the spirals 30 as illustrated still further
spirals can be provided and/or radially protruding pegs for conveying the mixture of the supplied material. The short section 12" is adjoined by a section 12"' which corresponds as regards its delivery direction to the first section 12'. This has the
consequence that the pressure in the material (paste) falls off; accordingly zone A is followed by a decompression zone designated by B. By reason of the pressure drop in the material, water vapour and dissolved gases are liberated. In the region of the
decompression zone B the housing 14 is provided with a radial degassing passage 34 which is part of a degassing device 36, to be explained further by reference to FIG. 2. The gases or vapours liberated in the decompression zone B can escape through this
degassing passage 34. To promote this process the degassing device 36 is connected through a vacuum connector 38 to a vacuum suction device (not shown). The worm housing 14 can also be made heatable, at least in the region of the decompression zone B,
in order to reinforce the evaporation.
The decompression zone B is adjoined by a second build-up zone C, which is followed by a second decompression zone D, which is achieved by appropriate formation of the individual worm sections in this region, as already described above. In the
region of this zone D a second degassing device 36' is provided in correspondence with the first degassing device 36. Again a build-up zone E adjoins for the vacuum-tight closure of the adjoining region against atmosphere. The discharge opening 40 of
the extruder 10 at the left end of the extruder in FIG. 1 is adjoined by a compacting and length-cutting device (not shown).
FIG. 2 shows a first form of embodiment of the degassing device according to the invention, similarly to the symbolic representation in FIG. 1. The degassing device 36 consists of a conveyor worm 42 with core 42a and spiral 42b, inserted into
the cylindrical degassing passage 34. The conveyor worm axis 44 coincides with the cylinder axis of the degassing passage 34. The axis 44 is perpendicular to a plane 53 defined by the two axes 46 and 48 of the two worms 50 and 52 of the double-shaft
extruder 10. The axis 44 is equidistant from the two axes 46 and 48. The diameter c of the passage 34 is slightly smaller than that dimension of the worm chamber 24 of the housing 14 accomodating the two worms 50 and 52 which lies within the plane 53
and is perpendicular to the axes 46 and 48. This dimension, designated by d in FIG. 2, is the largest dimension of the cross-sectional area, represented in FIG. 2, of the worm chamber 24.
The housing 14 is in three parts and consists of an inner housing part 14a of elongated oval cross-section accomodating the two worms 50 and 52 and of two further parts, a lower part 14b and an upper part 14c, commonly enclosing the part 14a.
The mentioned passage 34 passes through both the upper part 14c and the part 14a above the two worms 50 and 52.
The core diameter e amounts to about 1/10th of the worm diameter f which substantially corresponds to the passage diameter c. The core 42a is formed at its lower end with a radial end face 54. The worm spiral 42b here terminates in an edge 56
running in radially towards the core 42a and lying in the plane of the end face 54. On rotation of the conveyor worm 42 in the counter-clockwise direction seen from about (movement arrow F) the edge 56 moves along a circular surface perpendicular to the
axis 44, while the region of the worm spiral 42b preceding the edge 56, inclined in the direction of rotation and slightly upward, ensures a levelling of the paste which has swelled upwards above this circular area (that is into the passage 42). Per se
a rotating or reciprocatingly movable spatula having the edge 56 would suffice for this levelling; however the conveyor worm has the advantage that material which has momentarily swelled up above the edge 56 is conveyed back again into the region below
the conveyor worm.
The speed of rotation of the worm 44 here amounts to about 60 revolutions per minute. The drive of the conveyor worm 42 takes place by means of a compressed-air motor (not illustrated in FIG. 2) which is coupled to the upper end in FIG. 2 of the
worm 42. This end lies outside an extractor attachment 60 which is flanged on to the outside of the worm housing 14 (on the upper side of the housing part 14c), with a cylindrical interior space 62 of the attachment 60 in alignment with the passage 34.
The extractor attachment 60 closes off the passage 34 from atmosphere. Furthermore it is provided with the flange 38, already mentioned in connection with FIG. 1, for connection to a vacuum suction device (not shown). The flange 38 starts from the
external circumference of the cylindrical extractor attachment 60. Opposite to the flange 38 there lies a flange 66 closed off by an observation window 64 of transparent material. The core 42a of the conveyor worm 42 is conducted to the exterior, for
connection to the mentioned compressed-air motor outside the extractor attachment, by way of a rotational-movement-permitting vacuum duct 68 of an upper lid part 70 of the extractor attachment 60. The lid part 78 is detachably connected with the
remainder of the housing through a clip connection 72.
In FIGS. 3 and 4 there is represented a second form of embodiment of the degassing device which is generally designated by 136. Elements in FIGS. 3 and 4 w | | |
