GB2075353A

GB2075353A - Producing mixtures of thermoplastic synthetic resins and mineral or organic fillers

Info

Publication number: GB2075353A
Application number: GB8114095A
Authority: GB
Original assignee: SAPCO SYSTEMANALYSE PROJEKT
Current assignee: SAPCO SYSTEMANALYSE PROJEKT
Priority date: 1980-05-09
Filing date: 1981-05-08
Publication date: 1981-11-18
Also published as: BE888734A; DE3017752C2; ES502035A0; KR850000531B1; KR830006378A; ZA813026B; RO86050A; SE8200079L; IT8121589A0; ES8202504A1; JPS57500552A; GR74899B; NL8120135A; WO1981003144A1; KR850000530B1; FR2481991A1; GB2075353B; KR850001468A; HU182749B; IT1138770B

Abstract

In a method for producing mixtures of thermoplastic synthetic materials and mineral or organic charge materials, the materials are vacuum mixed and compressed. According to the invention, the mixtures of synthetic materials and charge materials are already homogeneous in a powder state. They are also resistant without granulation to disaggregation, they are stable and can be shaped as products. Moreover, the rigidity must be obviously high but without fragility. The values of the resistance must approach those of the pure synthetic material. Finally, commercial advantages must be attained. To these purposes, the charge materials must be electrically or electrostatically charged before and/or during the mixing with the synthetic materials. As an alternative, this can also be achieved with the synthetic material and even, for instance, through a continuous stirring within an electric field. The charge remains stable during a long period allowing a previous treatment and a storage of the charge material separated from the mixing operation. A combined loading and mixing reactor, together with an installation (57) for producing a strong grinding of the content, is also part of the invention.

Description

SPECIFICATION Process for producing mixtures of thermoplastic synthetic resins and mineral or organic fillers and apparatus for performing the process The invention relates to a process for producing mixtures of thermoplastic synthetic resins (hereinafter referred to as "plastics") and mineral or organic fillers, wherein the starting materials to be mixed together are mixed under vacuum and compression.

The purpose of mixing mineral or organic fillers with plastics is, on the one hand, to make the raw material cheaper and, on the other hand, to obtain desired properties in the product obtained later. One particular difficulty is in providing the processor of the mixture of plastics and filler with a homogeneous starting material which can be transported and which is stable against unmixing, if the mixture is not to be further processed immediately. Moreover, as a raw material for further processing, the mixture must satisfy certain strength requirements. To achieve these aims, various methods have been tried in the past, which more or less come down to a thermal treatment of the plastics substrate, causing melting or softening of the thermoplastic.

In the known processes, the plastics and filler components are put together and mixed by rolling, kneading, plasticising or extruding. It is known that, in order to form suitably great adhesive forces between the plastics and filler, the pores of both components must be as free from water and air as possible, since, as the distance between the molecular chains the plastics and the filler decreases, the van der Waals forces which bond the two components together increase. If cohesion-inhibiting coverings of gas or moisture are to be removed, the compounding should be carried out under vacuum e.g. in single or double screw extruders in which the substances are continuously conveyed, mixed or compressed. For this, it is necessary to use suitable fun news with degassing apparatus for the addition of the components.

This realisation is the basis for a process for producing a mixture of plastics and filler which is known from German OLS 2334 189. According to this, the fillers are first subjected to intensive pre-drying, which is carried out under vacuum, and then they are mixed with the plastics, again under vacuum, to ensure the exclusion of moisture. During the mixing operation, the process temperature must be controlled so that the plastics particles added in powder form begin to gel on their surface, so that the particles of filler are sintered on to them, with the result that the agglomerates thus formed cannot become unmixed at a later stage.

The disadvantage of this process is that the cost and complexity of the process for drying the filler and for the sintering operation (gelling operation) bears no relation to the degree of adhesion obtained between the plastics matrix and the particles of filler and moreoverthe homogeneity of the mixture thus produced does not satisfy the normal requirements.

Furthermore, any colouring pigments must be added before the mixing process.

In another known process (German OLS 23 32 583), the plastics and filler are added together, with constant kneading, in order to obtain the desired mixture. The heat generated by the constant friction during the kneading operation, which may be further increased by an additional supply of heat from outside, causes the plastics particles to melt and these are mixed with the particles of filler. This operation is continued until substantially all the filler is used up by the mixing operation, so that no free or uncombined filler is left and the particles of polymeric material have been used up by melting, at least to the desired extent. Depending on the components used, the operation has to be repeated several times.

These and other known processes which can be grouped underthe general heading of "thermal methods" on the basis of the physical action, have the disadvantage that the mechanical apparatus required involve high capital costs and operating costs and frequently give only low throughputs. The mixtures produced are not homogeneous, i.e. the proportion of filler in the individual granules is variable. This is generally accompanied by inferior properties forthe mixture and the product obtained subsequently, owing to inadequate adhesive forces between the plastics matrix and the particles of filler.

The mixtures of plastics and filler have a tendency to be brittle or have an excessively high modulus of elasticity, whilst as the proportion of filler increases their strength is reduced. Admittedly, these effects are overcome or at least limited by the addition of so-called adhesive agents, but this involves complicated procedures and consequently high costs, with the result that this option is reserved for the production of special products.

One aim of the invention is therefore to provide a process for producing mixtures of plastics and fillers which are homogeneous even in the powdered state and which are resistant to unmixing and can be processed to form commercial products, without complex granulation processes. Moreover, the materials produced by the process of the invention should have a significantly higher rigidity, but without having the brittleness which usually accompanies it, and at the same time should have a degree of strength similar to that of pure plastics materials and hitherto not obtained with mixtures produced according to the known methods described hereinbefore.Finally, the process according to the invention should involve lower costs than the processes according to the prior art, so that the reduction in costs obtained by the saving in raw materials is not wiped out by excessively high processing costs.

A further object is to provide a suitable apparatus for performing the process of the invention.

According to one aspect of the present invention we provide a process for producing mixtures of at least one thermoplastic plastics and one or more mineral or organic fillers by mixing under vacuum and simultaneous compression, the said process comprising the electrical or electrostatic charging of the fillers before and/or during the mixing with the plastics.

In a preferred embodiment of the present invention the plastics are also electrically or electrostatically charged before and/or during the mixing with the fillers.

The charging operation within the process of the invention may be effected by the intensive friction of particles of filler and/or plastics against one another and on the internal parts and surfaces ofthe reactor in which charging is effected; in this mannerfric- tional heating and corresponding electrical or electrostatic charging are produced.

The charging operation within the process of the invention alternatively may be effected in an electrical field prior to the mixing of the plastics and fillers.

In such a case, it is preferred that the material undergoing the charging operation be constantly circulated during that operation to ensure a uniform distribution of charge.

According to a further aspect of the present invention we provide an apparatus for performing the process of the present invention having a coolable vacuum container in which a mixing apparatus is provided, funnels and valves through which the starting materials may be supplied into the vacuum container and the resulting mixture may be removed from the container and which serve to maintain the vacuum, the said apparatus comprising a combined charging and mixing reactor provided with means for generating intensive friction between at least part of the contents of the charging and mixing reactor, the said reactor being connected to a coolable intermediate volume itself connected to a cooling reactor and a line which is mounted so as to pass back from the cooling reactor into the charging and mixing reactor.

According to a further aspect of the present invention we provide an apparatus for performing the process of the present invention having a vacuum container in which there is mounted a mixing; apparatus, fittings through which the starting materials may be supplied into the vacuum container and the resultant mixture may be removed from the container and which serve to maintain the vacuum, the said apparatus having two or more charging reactors connected to the vacuum container.

According to a yet further aspect of the present invention we provide a material comprising particulate thermoplastic plastics the particles of which are encased by particulate mineral or organic fillers bound thereto by electrostatic and van derWaals forces.

In the procedure according to the invention, the filler is subjected to electrical or electrostatic charg ing. Depending on the type of filler, this charging remains stable for months. Thus, it is possible to carry out pre-treatment of the filler separately from the mixing operation, in terms of both time and place, and to store the mixture in the meantime, if necessary.

During the mixing of plastics and filler, the friction occurring causes charge separation, with the plastics material becoming charged with a polarity opposite to that of the filler. Certainly, with certain combinations of filler and plastics, depending on their position relative to each other in the electrostatic series, it may be necessary forthe plastics to be subjected to preliminary charging so as to receive a charge opposite to that of the filler.

Solely as a result of the opposite charges of the two components, a firmly adhering layer of particles of filler is formed around the entire grain of plastics during the mixing operation; in contrast to the processes described in the prior art, there is no sintering or gelling fo the particles of filler on the possibly softened surface of the plastics particles. This can be demonstrated by the fact that, when the mixture of plastics and filler is dispersed in water, the covering offiller becomes detached from the grain of plastics, since the electrostatic bonding forces are removed in water. Subsequent examination of the re-exposed grain of plastics powder under the electron microscope reveals its virtually undamaged surface.

The thickness of the covering of filler depends on the desired proportion offiller. The total quantity of filler added is bonded in the covering of filler and does not form any agglomerates or clumps which are separate from the plastics or easily removed therefrom. Consequently, the covering of filler produced by the method according to the invention is not susceptible to normal mechanical stress, e.g.

from pressure, impact, shearing forces or friction.

Furthermore, the even formation of the covering of filler ensures that the powder flows freely, which is desirable for further processing.

Thus, the mixture of plastics and filler is resistant to unmixing even when it is still in powder form. As a rule, the mixture is either granulated, if the purchaser so wishes, or is taken directly for processing to form an end product A particular advantage is that, in contrastto the process described in German OLS 2334189, the end products can be coloured at a laterstage, and this is particularly favourable in the case of fibres produced from the material according tithe invention.

Irt addition, the adhesive forces between the plas tics and filler obtained by the electrical charging of the components result in-greatly improved material properties for the filled thermoplastics, which cannot be obtained with mixtures of plastics and filler produced according to the prior art. This is demonstrated by the following comparison of the mechanical characteristics of a standard commercial poly- ropylene mixture containing 40 wt. - percent of talc with a polypropylene mixture also containing 40 wt.

percent of talc but produced according to the pro cess of the invention

T ~ ~ Standard commercial material 0 | Material according to L to the invention.

4, 4, E II 7 6 4, -rl U 4, -U 'n u 4, 4,' a) a) H H a) - a) H 4, (0 4, a) -I 4, a) a) a) Lw H 4, a o H 4, a) 0 a) (0 (0 U 4, H a) a) - a) (0 4, 4, H 4, U a) E (0 H a) 4, a) 4, a) x m c a) o H H x c a) a) a) (0 E - cu z = Z N z o 0 -= 0 - - - , 0 N 1 m ~ d7 a' - - m n ', to - - to ,- -- to E as Q In particular, there is a significantly lower modulus of elasticity and a substantially improved ultimate tensile strength and impact strength.

In performing the process according to the invention, various methods may be used, as can be seen from the Examples described hereinafter. Corres ponding embodiments of apparatus for performing the process are also shown by way of example in the accompanying drawings, where: Fig. 1 is a schematic view of the arrangement in an apparatus according to the invention of units for the charging of the components by friction and for the subsequent mixing operation; Fig. 2 is a schematic view of the arrangement in a further embodiment of the apparatus of the invention of units for the charging of the components in an electrical field and for the subsequent mixing operation; Fig. 3 is a schematic diagram of a combined charging and mixing grid of the apparatus of Figure 2;; Fig. 4 is a schematic view of the arrangement in a further embodiment of the apparatus of the invention of units forthe charging of the components in an electrical field for carrying out the process continuously; Fig. 5 is a section through a particle of the mixture produced by a process according to the invention having a plastic core and a shell of filler; Fig. 6 is an enlarged view of a portion of the phase interface of the particle of Fig. 5; and Fig. 7 is an enlarged view of a portion from the phase interface of the particle of Fig. 5.

Example I (A) 20 kg of a filler, e.g. chalk, talc, kaolin or mica with a moisture content of less than 3 wt. percent are mixed under vacuum in a high-speed mixer under a pressure of 10 millibars, thus producing intensive friction of the particles of filler against one another and against the internal parts and surfaces of the reactor. Consequently, the particles become electrically charged, chalk receiving a negative charge whilst talc, kaolin and mica receive a positive charge corresponding to several kV, e.g. 1-10 kV. The charges are particularly stable if mixing is carried out at peaktemperatures of above 200 C lasting for short periods (e.g. 0.5 seconds), these temperatures being caused either by frictional heat or by additional heating.

The filler pretreated in this way should first be cooled again. Then the electrostatically charged filler is put into an evacuated high-speed mixer together with the desired quantity of plastics, e.g. 20-30 kg of polyethylene, polypropylene or other powdered thermoplastics.

The mass is mixed thoroughly under a pressure of less than about 1000 Pascals, whilst the temperature should not exceed a specific maximum temperature which is to be regarded as an indicator forthe friction which has occurred and hence for the degree of charge separation of the filler and plastics.This maximum temperature is below the softening temperature of the plastics and is dependent both on the combination of plastics and filler used and also on the proportion of these components, for example as shown in the following Table:

Proportion of filler Type of plastics \ 10% 25% 40% > 40% Type of filler Polyethylene 85"C 950C 130"C 1300C Talc 80"C 100 C 140"C 1450C Chalk Polypropylene 80"C 1 05C 135"C t40 C Mica 80"C 95C 140"C 450C Talc Polyvinylchioride 80"C 80"C 80"C 80"C Chalk The mixture of plastics and fillerthus prepared is either stored in powder form or, if so requested or if necessary, granulated in an extruderortaken directly for processing to form an end product The granulation and processing should be carried out under vacuum, so that the intensive wetting ofthe filler by the thermoplastics is not hindered by layers of air adsorbed in cavities or niche-like irregularities in the rough surface of the shell or bubbles enclosed therein, and therefore the adhesive forces can operate advantageously.

The attractive forces produced by the electrical charging result in substantially better properties for the filled thermoplastics, and more particularly in good impact strength with a low modulus of elasticity.

(B) The process of this Example can be carried out using an apparatus as shown in Figure 1.

This apparatus contains a combined charging and mixing reactor 10 with two filling funnels 11, 12. The reactor 10 has two outlets 13, 14 at its base, of which the outlet 14 is connected to a coolable intermediate volume 15, from which a line 16 leads to a cooling reactor 17. From the cooling reactor 17, a line 18 passes back to the combined charging and mixing reactor 10.

The combined charging and mixing reactor 10 consists of a reactor body 19 and a lid 20, which is electrically insulated relative to the reactor body 29, preferably by a layer 21 of polytetrafluoroethylene.

In order to produce the vacuum required, the reactor 10 is connected, via its lid 20, to a vacuum pump 22.

A pressure gauge 23 is provided on the lid 20.

The lid 20 of the reactor 10 is connected to a high voltage generator 24 which can supply voltages from 0 to 10 kV and is infinitely variable. The reactor body 19 is earthed via an earth line 25 to which the mixing mechanism 26 located in the reactor body 19 is also connected. The earthing of the reactor body and mixing mechanism can be interrupted by means of a switch 27. Furthermore, the reactor body 19 is provided with a temperature sensor 28 and a quantometer 29. Cooling coils 30 are arranged in an annu lar configuration around the outside of the reactor body 19.

The feeding funnels 11,12 are also connected to the vacuum pump 22 and are each provided with a pressure gauge 31,32. Both funnels are connected to the reactor 10 via vacuumtight closure means 33,34.

From the outlet 14 of the combined charging and mixing reactor10, a line 35 passestothecoolable intermediate volume 15 which is connected to a vacuum pump36 and has access to a pressure gauge 37. On theoutside,the intermediate volume is surrounded by an nular coolant lines 38. The coolable intermediate volume 15 and cooling reactor 17 are connected to each other via the line 16. The cooling reactor 17 is also connected to the vacuum pump 36 and has access to a pressure gauge 39. The cooling reactor consists of a cooling reactor body 40 with a lid 41, the cooling reactor body 40 being provided on the inside with a non-conducting layer 42, preferably of polytetrafluoroethylene. Coolant lines 43 are arranged in an annular configuration around the outside of the cooling reactor body 40.On the inside, there is an upright shaft 44 on which three stirring arms 45 are mounted, wipers 46 being provided between the individual stirring arms.

(C) The preparation of a mixture of plastics and filler by the process described above in this Example is carried out in the apparatus shown in Figure 1 as follows: A batch filler, e.g. 30 kg of chalk or similar material, is fed into the combined charging and mixing reactor 10 through the evacuated funnel 11 and the product slide valve 33. In the reactor 10, the filler is thoroughly mixed by a high-speed mixer 26, so that the filler heats up and becomes electrically charged.

Charging is monitored constantly by means of the temperature sensor 28 and the quantometer29. To ensure that no layers of the filler to be charged are deposited on the lid 20 of the reactor 1.0,.the lid is in turn electrically charged by the high voltage generator24, with the opposite polarity to that of the charging. of the filler, upwards of a temperature of 70 to80"C forthefiller. The reactor body 19 and mixer 2.6, onthe other hand, are earthed, and if necessary this earthing can be interrupted during the charging operation by means of the switch 2:7 so that constant potentials can be built up.

When the filler has become sufficiently charged, it is conveyed through the outlet 14 and through the line 35 into the coolablaintermediate volume 15.

Here, the filler is cooled by means of water flowing throughthe cooling coils 38, before this filler is conveyed through the line 16 into the cooling reactor 17.

During this cooling operation, the reactor 10 is also cooled, by cooling water flowing through the cooling coils 30, so that a new batch of filler can be introduced into the reactor 10 through the funnel 11.

Here, the charging of the filler takes place in the manner described.

The cooling reactor 17 has four times the volume of the reactor 10, so that four batches of filler can be cooled and stored therein. This is convenient because the charging of the filler takes less time than the cooling operation and the mixing of the filler and plastics which follows.

Once the cooling reactor 17 is filled, the first batch of filler, lying at the bottom, has cooled to such an extent that some of it can be fed through the line 18 and the funnel 11 back into the reactor 10. At the same time, a corresponding quantity of plastics is fed into the reactor 10 through the funnel 12. Here, intensive mixing of the plastics and filler occurs, in the same way as during the electrical charging of the filler. The temperature is constantly monitored by means of the temperature sensor 28. Once a suffi cient charge separation has occurred during the mix ing operation, the mixing is stopped. The finished powdered compound can then be discharged through the outlet 13 and either packaged or fed into a granulating extruder or some other processing machine.

Example II (A) As a variation on the procedure described in Example I, the charging of the filler or, if necessary, of the plastics as well, may also be effected by exposing the particles to a correspondingly strong electrical field. For this, a correspondingly high voltage is applied between the stirring mechanism and outer wall of a reactor designed for this purpose. If is particularly advantageous to construct the stirring mechanism in the form of a grid with spikes placed thereon, since, during so-called point discharging, even relatively small voltages are sufficient to pro duce a strong electrical field locally. The apparatus is therefore comparable with a capacitor, with the ini tially electrically neutral solid components acting as a dielectric.

When a negative voltage is applied to the spikes in the stirrer grid, the spikes emit free electrons which are picked up by the moleculesoffillerand/orplas- tics. An electron excess is produced, causing the par ticles of components to become negatively charged.

Thus, the particles themselves become charge car riers and there is a virtual displacement of the voltage-carrying grid towards the outer wall of the reactor until all the particles have become electri cally charged to the desired extent. Discharging of the particles at the counter electrode, namely the outer wall, is prevented by the fact that the inner wall of the reactor container is lined with an electrically insulating layer.

If the particles are to be positively charged, the polarity of the stirrer grid and outer wall must be reversed accordingly. A strong electrical field with grid spikes as the (positive) anode releases electrons from the molecules of the components, thus causing an electron deficiency, i.e. a positive charge.

In both cases, the amount of charge introduced is monitored by means of the voltage level and the duration of the operation. A uniform distribution of charge is obtained by constantly slowly circulating the material by rotation of the stirrer grid.

After charging, the components pass into a mixing reactor and are mixed in the manner described in Example I and then taken for further use.

Compared with the electrical charging in a highspeed mixer as described in Example I, the method of charging in an electrical field as described above in this Example has the advantage that cooling, particularly of the filler, after the charging operation is no longer necessary. As a result, the extensive stock of apparatus for cooling the material and recycling it into the mixing reactor can be dispensed with.

Moreover, the charging process is easierto control thanks to the possibility of selecting the polarity and the amount of charging, and consequently, thanks to the attractive forces thus produced during the mixing of the plastics and filler, particularly good coating of the grain of plastics can be obtained.

(B) The charging of the components in an electrical field can be effected in an apparatus as shown diagrammatically in Figures 2 and 3.

Two reactors 50, 51 of similar construction, for the electrical charging, are placed on a vacuum container 49 (only one of these two reactors is described hereinafter). The reactor 50 is evacuated by means of a vacuum pump 52, the vacuum being capable of being monitored by means ofthe pressure gauge 53.

The reactor has a reactor body 54 and a reactor lid 55 which is electrically insulated relative to the body 54.

The reactor body 54 is lined on the inside with a layer 56 of electrically insulating material.

Inserted in vacuumtight and electrically insulated manner in the reactor lid 55 is a stirrer cylinder 57 which consists of an electrically insulating material and a conductive core 58. The stirrer cylinder 57 extends only about halfway into the reactor body 54.

As can be seen from Figure 3, the conductive core 58 of the stirrer cylinder is connected to a stirrer 59 in the form of a two-armed grid 59a, wherein the opposing grid surfaces are each tilted at 300 to the vertical. Mounted on the points of intersection of the individual lattice bars forming the grid 59a, there are mounted, at right angles to the grid plane, short spikes 59b of conductive material; these spikes 59b are mounted only on the upper side of the grid 59a tilted through 30 and they point in the direction of rotation of the grid.

A voltage of between 0 and 100 kV is applied to the conductive core 58 and the outer wall 60 of the reactor body 54, this wall being made of conductive material. This voltage is generated by an infinitely regulatable high voltage generator 61. The voltage is transmitted to the rotating conductive core 58 by means of a brush electrode 62.

The reactor 50 is also provided with a vacuum type inlet sluice 63 and an outlet 64, the latter being connected to the vacuum tight product slide valve 33 of the vacuum container49. This vacuum container 49 is furthermore constructed in every respect like the mixing reactor in Example I. The cool able intermediate volume and the cooling reactor, which are not required in the process described above, are not provided in this case.

(C) In the apparatus described hereinbefore, the process according to this Example takes place as follows: A batch of the filler required is fed into the reactor 50 through the vacuum tight material sluice 63. In this reactor, the filler is constantly stirred and moved around by the stirrer 59. Between the stirrer 59 and the outer wall 60 made of conductive material, a voltage of about 80 kV is applied, which is generated by the high voltage generator 61. The polarity of the two conductive components, namely the stirrer and the outer wall, depends on the desired polarity of charging of the filler. The filler is kept in the reactor under vacuum and with constant rotation of the stirrer until all the particles have attained the desired degree of charging. The filler is then passed into the vacuum container 49 through the outlet 64 and the vacuum tight product slide valve 33.Atthe same time, an equivalent quantity of plastics is fed into the vacuum container 49 via the product slide valve 34, and the plastics material may also be electrically charged.

In the vacuum container 49, high-speed! mixing of the two components results in charge separation in the same way as described in Example I, so that as a result of the electrostatic forces of attraction, the desired compound is formed.

Example III (A) Since Examples I and II described hereinbefore relate to apparatus which only permits a discontinuous production, i.e. batch production of the desired mixture of plastics and filler, the apparatus shown in Figure 4 is designed for a continuous process of production. Here, the charging of the components has' to be effected in an electrical field using the method described in Example II, since this does away with the need for cooling and hence intermediate storage of the components.

(B) Consequently, the apparatus for discontinuously producing a mixture of plastics and filler according to Figure 4 consists of a mixing column 65 on which are placed the two charging reactors 50, 51 corresponding to the reactor described in Example II, so that the components charged therein pass into the mixing column 65 through the sluices 66,67. For producing the required vacuum, the mixing column 65 is connected to a vacuum pump 68, whilstthe pressure in the mixing column is monitored by means of a pressure gauge 69.

The mixing column 65 is constructed as a cylinder 70 which merges into a cone 71 tapering downwards in a funnel shape. The inner wall of th cylindrical portion 70 of the mixing column 65 is provided with grooves 72 ascending in a spiral configuration.

Inside the mixing column, a total of three rotors 74 are arranged on a verticaily positioned drive shaft 73, one of these rotors, having a small diameter, being located at the apex of the cone 71, whilst the other two, having correspondingly larger diameters, are mounted at the lower end of the cylindrical portion 70 of the mixing column 65. The direction of rotation of the rotors 74 is selected so thatthe mater ial is conveyed upwards in the ascending spiral.

Finally, atthe base of the cone 71, there is also an outlet 75 which communicates with an apparatus 76 for further processing the mixture of plastics and fil ler, preferably an extruder.

(C) The process of this Example takes place in the apparatus shown in Figure 4 as follows: The components, charged with opposite polarities in the reactors 50 and 51 as described in Example II, are metered through the sluices 66, 67 into the evacuated mixing column 65. In the mixing column, they come into contact with the rotors 74 mounted at the end of the cylindrical portion 70, where they are mixed and then conveyed upwards in the spiral provided on the inner wall of the cylinder 70. During'the resultant upwardly directed migration, the two components, namely the plastics material and the filler, are intensively mixed.

The upwardly directed conveying of the material in the spiral and hence the duration ofthe mixing operation are controlled by metering the supply of components into the mixing column 65 and by-the speed ofthe rotors 74, so that a corresponding amountoffinished compound falls into the cone 71.

Here, it is conveyed to the outlet 75 with further constant mixing by the rotor 74 mounted in the apex of the cone,.and is continuously supplied to the apparatus provided for further processing, which is under vacuum. The total retention time in the mixing column 65should be at least five minutes.

Figure 5 shows an enlarged photograph of a single particle 80 of a mixture of plastics and filler, produced according to one of the Examples described hereinbefore. In the interests of clarity, the particle 80 has been cut open and opened outwards so that it is possible to see the structure of the particle with its plastics core 81 and shell of filler 82. The sharp dividing line 83 between the plastics core 81 and the shell of filler 82 is clearly visible. Thus, the particles of filler have not penetrated into the plastics core and the plastics core has not melted, nor has it combined with the filler.

This is even more clearly visible in Figures 6 and 7 which show part of the dividing line 83 between the plastics core 81 and the shell of filler 82 in two scanning electron micrographs with different degrees of enlargement. There are no plastics constituents 85 between the individual particles of filler 84, as would be the case in a sintering or gelling operation between plastics material and filler. Thus, the adhesion between the plastics and filler is due solely to the forces of attraction produced by the opposing electrical charges.

Claims

1. Aprocessforproducing mixtures of at least one thermoplastic plastics and one or more mineral or organic fillers by mixing under vacuum andsimul- taneous compression, the said process comprising the electrical or electrostatic charging of the filler before and/or during the mixing with the plastics.

2. A process according to claim 1, characterised in that the plastics are also electrically or electrostat ically charged before and/or during the mixing with the fillers.

3. A process accordingto eitherofclaims 1 and 2, characterised in that the charging of the said plas tics and fillers starting materials is effected with intensive friction of the particles against one another and on the internal parts and surfaces of the reactor, thereby producing frictional heat and corresponding electrical or electrostatic charging.

4. A process according to any one of the preced ing claims, characterised in that the fillers are sub jected to the charging operation with a moisture con tent of less than 3% by weight and under a pressure of approximately 1000 Pascals.

5. A process according to any one of the preced ing claims, characterised in that the charging is car ried out at peak temperatures of more than 200"C and of short duration, these temperatures being pro'duced by friction and/or heating.

6. A process according to claim 5, characterised in that the starting materials are cooled after being charged and before being mixed.

7. A process according to any one of claims 1 to 6, characterised in that the electrical charges applied correspond to between about 1 and 10 kilovolts.

8. A process according to either of claims 1 and 2, characterised in that the starting materials are charged in an electrical field before being mixed.

9. A process according to claim 8, characterised in that the starting materials are constantly circu lated during charging in order to obtain a uniform distributed of charge.

10. A process according to either of claims 8 and 9, characterised in that the amount of charge intro duced is selected by variation of the voltage level and/orthe duration of the operation.

11. A process according to any one of claims 1 to 3 and 8, characterised in that the mixing of the charged materials is effected under a pressure of about 10 millibars.

12. A process according to any one of claims 1 to 3 and 8, characterised in that the maximum tempera ture occurring during the mixing of the starting mat erials is set so as to be lower than the softening temperature of the plastics material used.

13. A process according to claim 12, character ised in that the maximum temperature is set at vari ous levels, depending on the nature and quantity of the plastics material and/or filler used (table on page 12).

14. A process according to any one of the preced ing claims, characterised by further processing of the mixture produced, e.g. granulation or spinning, under conditions which prevent or at least'reduce the discharging of the charge, preferably under vac uum.

15. A process according to any one of the preced ing claims, characterised in that the mixture or the product, for example the granulate or spun filament, is coloured.

16. Apparatus for performing a process as claimed in claim 1 having a coolable vacuum con tainer in which a mixing apparatus is provided, fun nels and valves through which the starting materials may be supplied into the vacuum container and the resultant mixture may be removed from the con tainer and which serve to maintain the vacuum, the said apparatus comprising a combined charging and mixing reactor provided with means for generating intensive friction between at least part of the con tents of the charging and mixing reactor, the said reactor being connected to a coolable intermediate volume itself connected to a cooling reactor, and a line which is mounted so as to pass back from the cooling reactor into the charging and mixing reactor.

17. Apparatus according to claim 16, characterised in that the reactor body (19) and lid (20) of the charging and mixing reactor (10) are electrically insulated from each other by means of a layer (21) of non-conducting material.

18. Apparatus according to claim 17, characterised in that the lid (20) is connected to a high voltage generator (24), the output voltages of which are designed to be infinitely variable.

19. Apparatus according to claim 18, characterised in that reactor body (19) and friction generator (26) are insulated and are earthed through an earth line (25) and in that the earthing can be interrupted by means of a switch (27).

20. Apparatus according to claim 16, characterised in that a temperature sensor (28) and a quantometer (29) are mounted in the charging and mixing reactor (10).

21. Apparatus according to claim 16, characterised in that the cool able intermediate volume (15) is connected to a vacuum pump (36) and can be evacuated therewith.

22. Apparatus according to claim 16, characterised in that the cooling reactor (17) has a volume which is a multiple, preferably four times the size of, the volume of the charging and mixing reactor (10).

23. Apparatus according to either of claims 21 and 22, characterised in that the cooling reactor (17) is evacuated and for this reason is connected to the vacuum pump (36).

24. Apparatus according to claim 16, characterised in that the reactor body (40) of the cooling reactor (17) is provided on the inside with a layer (42) of non-conducting material.

25. Apparatus according to claim 16, characterised in that an upright shaft (44) rotating under the effect of a drive is mounted in the cooling reactor (17) and in that stirrer arms (45) are secured to the shaft, with wipers (46) located between them.

26. Apparatus for performing a process as claimed in claim 8 having a vacuum container in which there is mounted a mixing apparatus, fittings through which the starting materials may be supplied into the vacuum container and the resultant mixture may be removed from the container and which serve to maintain the vacuum, the said apparatus having two or more charging reactors connected to the vacuum container.

27. Apparatus according to claim 26, characterised in that each charging reactor (50, 51) is capable of being evacuated and for this purpose is connected to a vacuum pump (52).

28. Apparatus according to claim 26, characterised in that the reactor bodies (54) and lids (55) of the charging reactors (50, 51) are electrically insulated from each other.

29. Apparatus according to any one of claims 26 to 28, characterised in that the reactor body (54) consists of conducting material, but is lined on the inside with a layer (56) of electrically insulating material.

30. Apparatus according to claim 26, characterised in that a stirrer cylinder (57) is mounted in the reactor body (54) from the lid end (55), said stirrer cylinder consisting of an electrically insulating mat erial with a conductive core (58) and penetrating approximately halfway along the reactor body (54).

31. Apparatus according to claim 30, character ised in that the conductive core (58) of the stirrer cylinder (57) is connected to a stirring mechanism (59) constructed as a multi-armed grid (59a), which is also conductive.

32. Apparatus according to claim 31, characterised in that the grid surfaces forming a plane are tilted through about 30 degrees from the vertical.

33. Apparatus according to claim 32, characterised in that short spikes (59b) of conducting material are mounted at right angles to the grid planes, on the upper side of the tilted grid (59a) and pointing in the direction of rotation of the stirring mechanism (59).

34. Apparatus according to either of claims 29 and 30, characterised in that a voltage of up to 100 kilovolts which is generated by an infinitely variable high voltage generator (61) is applied to the outer wall (60) of the reactor body (54) and to the conductive core (58).

35. Apparatus according to claim 26, characterised in that the charging reactors (50, 51) are connected to the vacuum container (49) in vacuum tight manner via sluices.

36. Apparatus according to claim 26, characterised in that the outlet (75) of the vacuum container (65) is directly connected to an apparatus (76) used for further processing.

37. Apparatus according to claim 36, characterised in that the vacuum container constructed as a mixing column (65) consists of a cylinder (70) which merges into a lower portion (71) tapering in a funnel shape, the cylindrical portion (70) being provided on the inside with helically ascending grooves (72).

38. Apparatus according to claim 37, characterised in that, mounted on a drive shaft (73) standing perpendicularly in the centre of the mixing column (65), there are a plurality of rotors (74) one rotor (74) of which is located in the tip of the funnel (71), whilst the others are provided in the lower region of the cylindrical portion (70).

39. A material comprising particulate thermoplastic plastics the particles of which are encased by particulate mineral or organic fillers bound thereto by electrostatic and van der Waals forces.

40. Material as claimed in claim 39 in the form of a mixture of thermoplastic plastics and mineral or organic fillers, produced by a process as claimed in claim 1 or 2, characterised in that the constituents (80) of the mixture consist of a core (81) of thermoplastics material and a casing of filler (82) formed around it

41. Material according to claim 40, characterised in that the casing of filler (82) is held firmly in place on the plastics core (81) solely by the forces of attraction produced by the opposing electrical charges of the plastics and filler and van der Waals forces.

42. Material according to claim 41, characterised in that the surface of the plastics core (81) under the particles of filler deposited thereon is undamaged.

43. Material according to claim 41, characterised in that the casing of filler (82) is mechanically stable.