CH663178A5 - METHOD FOR RECYCLING THERMOPLASTIC WASTE, DEVICE FOR IMPLEMENTING THE METHOD AND EXTRUDER WITH SUCH A DEVICE. - Google Patents

Description

The invention relates to a method for recycling or recovery from thermoplastic film waste, such as is produced continuously in the form of edge strips in the production of polyolefin films by cutting or trimming. The invention further relates to a device for carrying out the method and an extruder with such a device.

“Return” of thermoplastic film waste here means the use of material called “recycling”; “Recovery” of thermoplastic film waste means the recovery of the thermoplastic material contained in the waste in the form of finished products or semi-finished products, e.g. Films or other thermoplastic extrudates, and generally with the addition of fresh and / or regenerated thermoplastic granules, optionally with the usual additives, pigments and the like. 25 Film waste is particularly problematic for the recovery of thermoplastic material, among other things because the heating of film waste for the production of granular Regenerate leads to an increased oxidative degradation or to a considerable reduction in the quality of the thermoplastic material due to the relatively large specific surfaces.

The usual recovery of thermoplastic material from film waste is based on the fact that the film waste - if necessary after comminution - is heated to sufficiently high temperatures to form granules or granular extrudate in order to bring it into an at least plastic state; The thermally granulated Régénérât, which is available from film waste, not only has the cost disadvantage of heating, but also an increased quality reduction, e.g. is difficult or unreliable to measure through ongoing measurements of the melt index and 40 increases the risk of non-uniformity in viscosity or melt flow when extruding thermoplastic material with such a reagent.

It seems obvious to circumvent the separate thermal granulation of the thermoplastic film waste by using the sufficiently shredded ("film shredded") waste from thermoplastic film material directly as feed for a production extruder; however, more thermoplastic is usually required for production operations than is available in the form of sufficiently homogeneous film waste material. Therefore, the film waste in this way must be used in a mixture with the usual feed material which has the bulk densities of 0.4 to 0.6 kg / dm3 or more that are usual for granules or Régénérât; Film chips with their typically about three to ten times lower bulk density of typically less than 0.1 kg / dm3 accordingly tend to “float” when mixed with granulate, which does not provide a uniform feed mixture for the extruder.

Known methods or devices for solving this problem, such as those e.g. in GB-A-2 008 421 and the state of the art cited therein are based either on a special treatment of the film chips before introduction into the extruder or on a forced mixing device, which requires additional energy and corresponding devices.

The object of the invention is to produce a sufficiently stabilized feed mixture for an extruder supplied with thermoplastic granules from the granules and the film chips alone by correspondingly guiding the material streams 65.

This object is achieved according to the invention by a method with the features specified in claim 1. Preferred

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Embodiments of the method of the invention have the features specified in claims 2 to 4.

The inventive device for performing the new method has the features mentioned in claim 5. Preferred devices have the features specified in claims 6 to 8.

Finally, the invention relates to a thermoplastic processing system which has an extruder combined with the device according to the invention.

The term "particulate shredded film waste of relatively lower bulk density" is understood to mean one whose bulk density, e.g. expressed in units of kg / dm3, at most a third (33%), preferably less than a quarter (25%) and typically about a fifth (20%) or less of the bulk density of the thermoplastic granules, which is correspondingly a « has relatively higher »bulk density and can consist of customary fresh and / or regeneratively obtained granulated material.

Typically, the bulk density of the shredded film waste is about 0.1 kg / dm 3 or less, while the bulk density of the granules in equal units is about 0.5 or more.

Furthermore, the largest particle dimensions of the film waste particles on average (e.g. determined by sieving) are preferably not more than three times as large and preferably not more than twice as large as the largest average particle dimensions of the granules (e.g. also determined by sieving).

According to the invention, a mechanically stabilized feed current is formed; In connection with the invention, this denotes a state of distribution which shows practically no segregation effects during the continuous feeding, since the particles of the first granulate partial stream exert a "hail-like" precipitation effect on the flakes and these at the end of the falling air / flake stream stabilize in a bed of premix, the latter of which is drawn into the extruder together with the granules of the second partial stream.

The film flakes produced, for example, in a knife mill and guided by a conveying fan through a sieve into a cyclone separator form the falling stream into which the first partial granulate stream is introduced; the air stream can then be guided through appropriate slots at the lower end of the chute and preferably through a bed of thermoplastic granulate in order to retain any residues of film flakes in the granular bed acting as a filter.

Since the method according to the invention is extremely simple to carry out in operational and apparatus terms, it can also be carried out in-line operation at relatively low rates of film waste formation, e.g. in that the feed stream formed according to the invention is fed back to the extruder which also produces the film providing the waste.

The device according to the invention can accordingly be designed as an additional device to a conventional extruder and is characterized by a chute, e.g. an approximately vertical downpipe with a rectangular, circular or polygonal cross-section; the top of the shaft is provided with a source of foil flakes, e.g. a mill connected; the lower end of the chute leads to the feed opening of the extruder and through the machine hopper which surrounds the lower end of the chute; a first upper granulate feed and a second lower granulate feed open into the chute; the chute forms an upper mixing zone between the two granulate feeds for generating a falling stream from a premix of granulate and flakes; on the second granulate feed there is a lower mixing zone for producing a particle bed in which the premix is continuously combined with granulate from the lower granulate feed and can then be introduced into the extruder as a feed stream.

If, according to a preferred embodiment, the device according to the invention is used as part of a thermoplastic processing system for carrying out the method according to the invention and is connected directly to an extruder as a feed system, the feed stream can form a material blanket which rests on the screw of the extruder and which, before being drawn in the extrusion cylinder and, if necessary, during the drawing in of the screw or the screw flight is additionally mixed before the material drawn in is compressed and plasticized in the extruder.

Preferably, the device according to the invention also has an upper granulate storage container, generally designed as a funnel, from which a first granulate line leads to the chute and defines the beginning of the first mixing zone there; this first granulate line generally has a slide or a similarly acting device with which the amount of granules reaching the first mixing zone per unit time can be controlled or regulated. A second granulate line leads from the upper granulate storage container to the machine hopper in such a way that its mouth determines the height of the granulate bed in the machine hopper.

which surrounds the chute. As a result, the refilling of granules can be limited to the upper granulate container and, if required, monitored there automatically, since the granulate bed in the machine hopper is automatically kept at the level defined by the location of its mouth through the second granulate line.

The invention is further explained on the basis of the drawings. Show it:

1 shows the schematic partial view of a thermoplastic processing system suitable for carrying out the method according to the invention with the device according to the invention,

2A shows the semi-schematic broken sectional view of a device according to the invention, and

Fig. 2B shows the device of Fig. 2A in section along A-A.

1 shows the partially broken, greatly simplified section through a thermoplastic processing system 1 for carrying out the method according to the invention. The system 1 comprises the device 10 according to the invention, which mainly consists of the generally hollow prismatic, e.g. is formed with a rectangular cross section or hollow cylindrical, formed chute 11 and the machine hopper 14.

The upper end 110 of the chute 11 is connected to the exit end of a source 12 of thermoplastic film flakes 129, which consists of a device 120, e.g. a knife mill, to form the flakes 129 from the film strips 131, a conveying fan 121 and a cyclone 122, which supplies the upper end 110 of the chute 11 with a falling flow of film flakes 129. While the main part of the air flow generated by the conveying blower 121 flows upward through the line 123 in the cyclone, a part of the air flow can enter the chute 11 with the flakes 129 downward and influence its falling speed.

An upper or first granulate feed 151, e.g. as the mouth of the first granulate line 161, which runs from the upper granulate storage container, e.g. the granulate hopper 16, is branched off and feeds a partial granulate flow into the chute 11, for example under the action of gravity. The amount of granules fed by this partial granulate flow into the chute 11 per unit of time is e.g. with a slide 163 or the like manually or preferably automatically controllable.

The upper granulate feed 151 defines the upper end of a first mixing zone M \ in which the granulate particles 169 form a premix with the flakes 129 in the chute 11; the mixing zone M1 is a comparatively dynamic mixing zone, in which the parts to be mixed move relatively quickly (order of magnitude 10 "to 10 ° m / sec).

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As a result of the relatively higher bulk density of the granulate particles 169 (which bulk density is in turn also a consequence of the shape of the particles, as in the case of the film flakes 129), the granule particles 169 exert a kind of “hailstorm effect” in the sense that the granule particles 169 if the mixing zone M1 falls through, tear the film flakes with it and form a bed with the premix thus formed at the lower end of the mixing zone M1; In this premix bed, the flakes are fixed between the granulate particles in the sense that the granulate particles 169, with their larger mass per particle in relation to the flakes 129, form a mobile granulate matrix, in the spaces between which the flakes or flake groups are “trapped” and become involved move the matrix but can no longer separate from the matrix; the premix now enters the second mixing zone M2, which is relatively static and has a typical movement speed in the order of 10-2 to 10 ~ 4 m / sec.

In the mixing zone M2, the premix bed formed from flakes 129 and the first partial stream of granulate particles 169 formed at the lower end of the mixing zone M1 is combined with the second partial stream of granulate particles 169; this second partial flow is fed from the granulate bed 149, which lies in the machine hopper 14 and surrounds in the lower part of the chute 11 at least up to the height at which the lower granulate feed 152 is arranged.

In general, the chute 11 has a plurality of air outlet openings 114, which are formed by appropriate covers, lamellae, slot widths, grids or the like so that air from the mixing zone M1 through the granulate bed 149 to the outside but practically no granulate particles 169 from the bed 149 in can reach the mixing zone M1; as is readily understandable, the bedding 149 thereby acts as a filter for the air flowing down through the chute 11 and retains any entrained foil flakes 129 in the bedding 149.

At least one lower granulate feed 152, e.g. in the form of at least one pellet passage opening 118 formed in the wall of the chute 11, through which pellet particles enter the mixing zone M2 from the bed 149 and there, with the premix formed in M1, form a mechanically stabilized, practically uniform supply stream of shredded film waste and thermoplastic pellets for the Combine extruder 100. This mechanical stabilization of the mixture is probably based in part on the fact that the film flakes 129 are at least so far fixed at the end of the mixing zone M1 in the continuously forming premix bed of the granulate particles 169 or of the movable matrix formed by them that demixing phenomena are practical no longer occur.

As a result, the film flakes 129 contained in the premix have already lost their free mobility when they enter the mixing zone M2, as a result of the “hail effect” of the granulate particles 169 in each case suddenly and to the extent that they reach the premix bed. It goes without saying that the quantity of granulate particles 169 fed into the mixing zone M1 per unit of time is sufficiently large, i.e. an excess of film flakes 129 in the resulting premix bed should be avoided.

A defined boundary between the lower end of the upper mixing zone M1 and the upper end of the lower mixing zone M2 cannot normally be seen; the transition is rather dependent on the respective working conditions or feed rates and it is of no practical importance whether the ongoing bedding from premixing is still regarded as the lower end of the upper mixing zone M1 or as the upper end of the lower mixing zone M2.

The mixing process which is essential for the mixing zone M2 consists in the continuous combination of the premixing with the second partial granulate stream, which supplies the remaining granulate fraction of the feed stream; The bed of premixing preferably forms an upper covering of the mixing zone M2 or an overlying layer, the thickness of which can be controlled with the aid of the respective feed rates (first granulate stream, flake stream, second granulate stream).

In many normal working conditions (usual rate of film waste during continuous production of typically less than 10% by weight, e.g. about 5% by weight), it is sufficient to control the granulate feed into the lower mixing zone M2 only by controlling the granulate feed in the upper mixing zone M1, ie that the rate of feeding granulate at the upper granule feed 151 is variable or continuously adaptable to the process control, but the rate of granule feeding through the lower granule feed 152 is kept practically constant.

For a corresponding control in a system according to FIG. 1, the actuation of the slide 163 is therefore usually sufficient for a more or less large portion of the granulate portion flowing in at 151 in the total mixture, and a control of the lower granule feed 152 is not necessary.

As control parameters for the slide 163, e.g. serve the optically measured density of the stream of film flakes 129 entering the chute 11. Alternatively or complementarily, the height of the bedding that forms in the second, relatively static mixing zone M2 can also be monitored, since an excessive increase in this height could lead to the first mixing zone M1 being displaced by the second mixing zone M2, i.e. filled and would make the formation of a mechanically stabilized mixture difficult or impossible.

30 If there is a possibility under given operating conditions that the dynamic mixing zone M1 is filled by an upward rising bed of the mixing zone M2, a corresponding sensor in the mixing zone M1 can reduce the feed rates accordingly, e.g. via the slide 163.

35 With the preferred bulk density ratio of granules: film flakes of 4: 1 to 6: 1, the bulk volume ratio of granules: film waste is always greater than 1 at a normal feed rate of film waste of up to 10% by weight, which is why regulation of the film flake flow is generally not is required. If 40 is necessary, the hailstorm effect of the granulate particles in the mixing zone M1 can be supplemented by an intensification of the air flow introducing the film flakes into the chute 11, and the capacity for increasing proportions of film flakes can thereby be increased if necessary.

45 In addition to the first granulate line 161, which supplies the mixing zone M1 with granulate, the upper granulate container 16 shown in a funnel shape in FIG. 1 has a second granulate line 162, which supplies the machine hopper 14 or the granulate bed 149 with filter-like and supplementary action therein with granulate verso . The position of the opening of the granulate line 162 in the machine hopper 14 is expediently chosen such that the desired height of the bed 149 (in particular sufficiently high via the air passage openings 114) is set automatically when the granulate container 16 is filled. The monitoring of the granulate supply 55 can then be restricted to the granulate container 16.

The comminution device 120 can be a commercially available knife mill or a similarly operating beating or cutting system for cutting up the film waste and is expediently adapted to the respective form of the waste (strips, tapes, webs) 60; the appropriate shape of the particles of shredded film waste results from (a) the normal density of the usual thermoplastic materials (0.8-1), (b) the desired, generally preferably less than 0.1, bulk density of the flakes and (c) the desired size ratio of granulate to flake material. The largest dimensions (e.g. determined as the number of sieves) of the flakes should generally not be more than three times larger than the largest dimensions of the granulate particles; typically the largest surface dimensions are on average twice larger

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as the largest granule dimensions; expediently the largest area dimensions are on average almost as large as the largest granule dimensions; a much greater size reduction of the flakes, i.e. lower average values of the largest flake dimensions are possible, but generally have no advantages.

The device 10 or the system 100 shown in FIG. 1 enables trouble-free and economical recycling of thermoplastic film waste; Construction and operation are clearly simple and economical; Typically, with an additional io energy expenditure of at most about 10 kW / hour, the waste generated by a film production system with edge trimming can be recovered practically without loss, and without problems of dust accumulation and thermal degradation.

The thermoplastic feed stream generated as shown in FIG. 1 can preferably form a material blanket lying directly on the extruder screw 102 and additionally mixed in it by the action of the extruder screw 102 or the screw flight, as shown in FIG. 1 by the area 19 designated as screw mixing zone is indicated by arrows; the circumferential screw flight 104 can cause a more or less strong circulation or additional homogenization of the feed stream by pulling a part of it into the extruder, compressing it and converting it to the plastic state 25 as indicated by the cross-hatched plastication zone 109 and not shown (not shown) ) Extruder end pressed out; another part of the feed stream is circulated by the screw flight 104 in the screw mixer zone 19 in the direction of the arrows.

2A shows a schematic broken sectional view of the two mixing zones M1, M2 of a device 20 according to the invention, the beginning of the mixing zone M1 (mouth of the upper granulate feed) being omitted and the falling granulate particles 269 or schematically represented by circles and squares in the middle part Flakes 229 are not shown; the density of the particles and flakes in the area of the screw mixing zone 29 35 also does not correspond to reality, since the screw 204 of the screw 202 increasingly compresses the particles of the feed stream.

The chute 21 consists of an upper shaft part 210 which is inserted into the lower shaft part 211. The lower part 211 has two air passage openings 213, 214; 215, 216, each of a lamella 223, 224; 225, 226 are covered, so that the air flowing down in the chute 21 can exit through the (only partially shown) granulate bed 249 in the direction of the arrows, but the granulate particles in turn cannot get through the air passage openings into the chute 21.

The premix formed in the mixing zone M1 collects as a slowly downward flowing bed of material in the mixing zone M2 and is mixed there with additional granules from the bed 249, which are each passed through a granulate passage 221, 222; 223, 224 can enter the mixing zone M2 in each wall of the lower chute from the machine hopper 24. The opening 222 in the front wall is not shown.

The mechanically stabilized mixture of flakes 229, the hail-like falling granulate particles 269 of the first granulate stream and the granulate particles slipping out of the bed 249 is, as explained above, moved in the direction of the arrows by the action of the revolving screw flight 205 in the screw mixing zone 29 and thereby additionally mixed.

FIG. 2B shows the device 20 in section A-A of FIG. 2A, the flakes and the granulate particles being indicated schematically only in the area of the screw mixing zone 29 in order to clarify the material circulation which also takes place in the axial direction before being drawn into the extruder 200.

The invention is not limited to the recycling of film waste from film production. Rather, according to the invention, thermoplastic film waste can also be processed, which is used in the processing of plastic films, e.g. in textile or packaging technology or in other processes. Preferred examples of typical thermoplastic film wastes are those made from homo- or copolymeric polyolefins, such as polyethylenes of the various types, and copolymers from ethylene and acrylic acid and other thermoplastic polymer compositions with similar extrusion properties.

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Claims (9)

663178 1. A process for recycling thermoplastic film waste into an extruder continuously supplied with thermoplastic granules by forming a feed stream of flaky particles from shredded film waste of lower bulk density and the thermoplastic granulate with higher bulk density, characterized by (a) formation of a falling stream from the flaky particles and air; (b) introducing a first partial stream of thermoplastic granulate into the falling stream of flaky particles and air; (c) depositing the flake particles by means of the first granulate substream to continuously form a bed from a premix of the flake particles and the first granule substream at the end of the falling stream; and (d) continuously supplying a second partial stream of thermoplastic granulate to the premix, and to form a mechanically stabilized, practically uniform feed stream which is fed into the extruder. 2. The method according to claim 1, characterized in that a second bed is formed from the second partial stream of thermoplastic granulate fed in step (d) and the air from the falling stream formed in step (a) is passed through this second bed to the air stream practically completely free of flakes of foil. 2nd PATENT CLAIMS 3. The method according to any one of claims 1 and 2, characterized in that the feed stream is fed into the feed opening (101) of an extruder (100) in order to form a material blanket lying on the extruder screw (102) which, before being drawn into the Extrusion cylinder (103) is mixed by the worm gear (104). 4. The method according to any one of claims 1 to 3, characterized in that the bulk density of the granules is at least three times greater than the bulk density of the comminuted flake waste and that the largest particle dimensions of the latter are on average at most twice larger than that of the former. 5. Device for performing the method according to one of claims 1 to 4, characterized by a chute (11), the upper end (110) for connection to a source (12) for the flakes and the lower end (111) for connection to an infeed opening (101) of an extruder (100) is determined, and by a machine hopper (14) which surrounds the chute (11) near its lower end (111), an upper granule feed (151) and one in the chute (11) lower granulate feed (152) opens in order to form an upper mixing zone (M1) between the two granulate feeds (151, 152) for generating a falling stream from a granulate / flake premix and a lower mixing zone (M2) on the lower granulate feed (152) To produce a particle bed in which the premix continuously combines with granules from the lower granulate feed (152) and as feed into the extruder (10) to be led. 6. The device according to claim 5, characterized by an upper granulate hopper (16), from which a first granulate line (161) to the upper granulate feed (151) and a second granulate line (162) to the machine hopper (14) is branched. 7. The device according to claim 5 or 6, characterized in that the machine hopper (14) surrounds the lower portion of the chute (11), the at least one granulate passage opening (118), which forms the lower granulate feed (152), and at least one Has air outlet opening (114), the latter opening into the part of the machine hopper (14) which is intended to receive an outer granulate bed (149) surrounding the lower section of the chute (11), which feeds the lower mixing zone (M2) with granulate and serves as a filter for thermoplastic flakes. 8. Device according to one of claims 5 to 7, characterized by a device (120) for comminuting film waste to form film flakes, which device (120), preferably via a blower (121) and a cyclone (122), with the upper End (110) of the chute (11) is connected. 9. Thermoplastic processing system, characterized by a 5 extruder (100) which is connected to a device (10) according to one of claims 5 to 8.