WO1995031319A1

WO1995031319A1 - Method and apparatus for recycling waste materials including thermoplastic materials

Info

Publication number: WO1995031319A1
Application number: PCT/CA1995/000288
Authority: WO
Inventors: Kahane Yehuda
Original assignee: R.T. Recycling Technology Inc.
Priority date: 1994-05-17
Filing date: 1995-05-16
Publication date: 1995-11-23
Also published as: IL113770A0; AU2404595A; CA2190351A1

Abstract

A method of recycling waste material including a first fraction of particulate thermoplastic material and a second fraction of particulate material at least some of which is substantially unaffected by thermoplastic processes, comprising the steps of: a) introducing said fractions into an apparatus having at least one shearing zone; b) during the passage of said fractions through said apparatus, subjecting the fractions to shearing forces in said at least one shearing zone to plasticize said first fraction and to intimately mix said first and second fractions to produce a plastic mass wherein said second fraction is disbursed in, and surrounded by said first fraction; and c) restricting the flow of said fractions through said apparatus at at least one position downstream from the position at which said first fraction is plasticized to increase the pressure in said shearing zone.

Description

Title : METHOD AND APPARATUS FOR RECYCLING WASTE MATERIALS INCLUDING THERMOPLASTIC MATERIALS

FIELD OF THE INVENTION

This inventions relates to a method and apparatus which may be used for recycling waste materials including thermoplastic waste materials. The process may be used to prepare a granular feed material which may be used in, e.g., injection molding and extrusion molding processes.

BACKGROUND TO THE INVENTION

In order to reduce the amount of waste which is sent to land fill sites, various types of waste material, including plastic waste material, are separated out for recycling. To some degree, these waste materials themselves are sorted based on the material from which they are made. For example, plastic waste material may be separated from metal, wood and paper waste material which are also to be recycled. In some instances, the plastic waste material itself may be separated based upon the type of constituent plastic utilized in the waste products. In other cases, only certain types of waste material are collected so as to avoid separating out those plastics which may be recycled from those which are not desired as recyclable products.

Various different processes have been developed for recycling waste plastic. For example, Goforth et al (United States Patent No. 5,088,910) discloses a system for making synthetic wood products from waste wood fibre and recycled plastic material by mixing the waste wood fibre and plastic waste material and heating and kneading the mixture to form a homogenous mass. The kneading may be conducted using a sigma blade double arm mixer which is insulated and jacketed for heating with hot oil up to temperatures of about 500°F. Stroppiana (United States Patent No. 5,279,465), discloses a method of recycling waste materials, including thermofusible plastics material comprising the steps of subjecting the mass of waste materials to a grinding process to produce a mass of granular material therefrom; subjecting the mass of granular material to a compacting process to produce bodies having properties of mechanical cohesion and subjecting the bodies to a shaving process to give rise to a mass of shavings of small particle size.

While prior art devices disclose mixing plastic and non- plastic material together to form a granular material, in some cases the granular material only has certain limited uses as a true homogenous mixture may not be obtained. Another problem with some of these processes is that they may be energy intensive and uneconomical. In addition, difficulties may be encountered in ensuring that a proper mixing of the plastic and non-plastic material occurs and in maintaining the equipment in proper running condition.

SUMMARY OF THE INVENTION

In accordance with the instant invention, there is provided a method of recycling waste material including the first fraction of particulate thermoplastic material and a second fraction of particulate material at least some of which is substantially unaffected by thermoplastic processes comprising the steps of introducing said fractions into an apparatus having at least one shearing zone; during the passage of said fractions through said apparatus, subjecting the fractions to shearing forces in said at least one shearing zone to plasticize said first fraction and to intimately mix said first and second fractions to produce a plastic mass wherein said second fraction is disbursed in, and surrounded by, said first fraction; and, restricting the flow of said fractions through said apparatus at atleast one position downstream from the position at which said first fraction is plasticized to increase the pressure in said shearing zone. The flow of the fractions through the apparatus may be restricted by inducing a reverse flow on the fractions.

In an alternate embodiment, the invention comprises a method for recycling thermoplastic material comprising the steps of introducing said material into an apparatus having at least one shearing zone; during the passage of said material through said apparatus, subjecting said material to shearing forces in said at least one shearing zone to plasticize said material; restricting the flow of said fractions through said apparatus by inducing a reverse flow on said material at atleast one position downstream from the position at which said material is plasticized to increase the pressure in said shearing zone.

The apparatus may comprise a plasticizing screw extruder. The screw extruder may comprise at least two contra-rotating longitudinally extending screw extruder units positioned in a casing, said casing having an inlet port and an outlet port, each of said screw extruder units having a plurality of screw members, at least one of said screw members of each screw extruder unit having a reverse pitch, said reverse pitch screw members being positioned to induce a reverse flow on said fractions. The pitch of each successive screw member may be smaller than the immediately proceeding forward pitch screw member. By using the process of the instant invention, a plastic mass, which may include other recyclable materials such as wood, may be prepared in a simple and economic manner. The resulting products have good mechanical properties and preferably have mechanical properties that are at least 50% of those of the parent material, more preferably at least about 70% and most preferably at least about 90%.

In addition, the process is self-propagating in the sense that a heating jacket may not be required. The shearing process itself may generate sufficient heat to melt the plastic material. Accordingly, the process may require input of only mechanical energy to recycle the waste material. DESCRIPTION OF THE DRAWINGS

These and other advantages of the instant invention will be more fully and completely understood in connection with the following description of the following drawings of a preferred embodiment of the invention in which:

Figure 1 is a schematic view of a process for recycling waste material;

Figure 2 is an alternate process schematic of a process for recycling waste material; Figure 3 is a cross-section, along line 3-3 of Figure 2, of an extruder according to the instant invention; and,

Figure 4 is a cross-section along the line 4-4 in Figure 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the instant invention, thermoplastic material may be recycled. The thermoplastic material may be any plastic which melts and becomes moldable at elevated temperatures (e.g. from about 300 to about 550°F, more preferably from about 400 to about 500°F and, most preferably, from about 400 to about 450°F). The thermoplastic material may comprise one or more of the following namely polyvinyl chloride, nylons, fluorocarbons, linear polyethylene (e.g. polyethylene terephthalate) polyurethane prepolymers, polystyrene, polypropylene and cellulosic and acrylic resins. Preferably, the thermoplastic material does not include material which may deleteriously effect the working parts of the machinery. For example, it is preferred that the thermoplastic material does not include polyvinyl chlorides as, on heating, they tend to release chlorine ions which may damage the working surface of the machinery. More preferably, the thermoplastic material comprises one or more of polyethylene, acrylonitrile-butadiene-styrene copolymer (ABS), styrene and polypropylene. Referring to Figure 1, the thermoplastic material which has been collected for recycle (reference numeral 1) is supplied to sorting unit 2 wherein metal elements which may be present in plastic 1 may be removed. Unit 2 may not be required is plastic 1 contains no or only very small metal parts. The metal parts are preferably removed since the metal may cause premature wear of the machinery. Plastic 1 is then crushed in crushing unit 3. As will be discussed in more detail below, plastic 1 is reduced in size so that the particulate material prepared by crushing unit 3 may be fed and worked in extruder 6. Preferably, plastic 1 is crushed so that the longest dimension of any particular piece is less than about 2" and more preferably, is less than about 1 1/2".

As shown in Figure 2, waste material, at least some of which is substantial unaffected by thermoplastic processes, may also be fed to extruder 6. This waste material may comprise wood, wood products such as plywood, paper, rubber, natural and synthetic fibres and thermosetting plastics including polyurethane. More preferably, this waste material comprises wood and wood products. More preferably, the waste material comprises wood such as wood chips, wood pulp and saw dust. The wood may be obtained as waste material from the manufacture of furniture, components for furniture and dimensional lumber. As discussed above with respect to Figure 1, waste material la may be sent to a sorting unit 2a in which metal parts are sorted out. The waste material may then be sent to crushing unit 3a.

The size to which waste material la is ground will vary depending upon the end use of the product of extruder 6. For example, if the material of extruder 6 is for use in extrusion and injection molding, then waste material la is preferably ground sufficiently fine so that it will pass through the injection or extrusion molding machine without causing the machine to clog and without significantly decreasing flow rate of plastic therethrough. Preferably, waste material la is ground so as to pass through a Tyler 10 sieve and, most preferably, so as to pass through a Tyler 12 sieve. After passing through crushing units 3 and 3a, the ground material may be stored in bins until required (not shown). In addition, the ground thermoplastic material may be sent to mixing unit 4 where additives that affect the final end product may be added. For example, colorants may be added to the ground thermoplastic product in mixing unit 4. The ground thermoplastic material is fed through extruder 6.

In the embodiment shown in Figure 2, ground waste material la and ground thermoplastic material 1 are both fed to extruder 6. Ground materials 1 and la may be in individually fed to extruder 6 or they may be combined and fed together through extruder 6. The amount of plastic which is mixed with waste material la is sufficient so that waste material la may be disbursed in plastic material la and completely surrounded thereby. If ground thermoplastic material 1 and ground waste material la are both fed to extruder 6, then the feedstream of ground thermoplastic material and ground waste material may vary from about 60 to about 100 wt. % plastic, more preferably from about 70 to about 85 wt. % plastic and, most preferably the feedstream comprises about 75 wt. % plastic.

The amount of water which is fed to extruder 6 is preferably limited. As will be apparent to those skilled in the art of extruder design, extruder 6 may be modified so as to vent water (both liquid and vapour) which may accumulate in extruder 6. However, it is preferable that the ground thermoplastic material which is fed to extruder 6 is dry or substantially dry. In addition, it is also preferred that ground waste material la is preferably relatively dry. Waste material la may have a moisture content of from about 0 to about 40 wt. %, more preferably from about 0 to about 15 wt. % and, most preferably, from about 0 to about 10 wt.

/o .

As shown in Figures 1 and 2, the waste material is sent via stream 5 to extruder 6 for processing. Extruder 6 contains features which are similar to those disclosed in the extruder disclosed in United States Patent No. 4,599,002 and that patent is incorporated herein by reference.

Referring to Figures 1 and 2, the layout of extruder 6 is generally referred to. Extruder 6 has a longitudinally extending outer casing 9. Positioned within casing 9 are longitudinally extending screw extruder units 7 and 8. Screw extruder units 7 and 8 are intermeshing and rotate in opposite directions. Proceeding from the inlet port (not shown) to exit port 17, extruder 6 comprises a plurality of zones. As shown in Figure 2, extruder 6 may comprise four zones namely zones 10, 11, 12 and 13. In addition, extruder 6 may have a plurality of throttle zones referred to by reference numerals 14, 15 and 16.

Referring to Figure 3, the extruder is shown in more detail. Motor 22 is provided adjacent the inlet end of extruder 6. Motor 22 drives two longitudinally extending axles (30 and 31). A plurality of screw members are fixedly mounted on each axle so that the screw members rotate at the same rate as the axle. Accordingly, one axle has provided thereon screw members 7a, 7b, 7c and 7d. Similarly, the other axle has mounted thereon screw members 8a, 8b, 8c and 8d. Optionally, as shown in Figure 3, each axle may also have mounted thereof screw members 7e and 8e as well as 7e and 8f respectively.

Extruder 6 may also include one or more orifice plates 23 which are shown in more detail in Figure 4. Orifice plate 23 comprises lower fixed portion 32 and upper movable portion 34. Lower fixed portion 32 may be fixably mounted to casing 9 by, for example, a plurality of screws which pass through holes 36. Upper movable plate 34 is also fixedly mounted to casing 9 >y, for example, a plurality of screws which pass through elongated openings 38. When the screws are loosened, portion 34 may be raised or lowered. Once portion 34 is set in the desired position, the screws may be tightened thus locking portion 34 in the desired position.

As shown in Figure 4, portions 32 and 34 define an opening 24 which surrounds the screw extruder units. By raising portion 34, opening 24 is enlarged thus permitting a larger volume of material to pass through orifice plate 23 at any particular time. By lowering portion 34, the amount of material which may pass through opening 24 at any particular time is reduced. Due to the configuration of the screw extruder units (including the degree of intermeshing of the screws and the height of the thread members on the screws), the degree to which the passage of material through orifice plate 23 may be limited is restricted.

The recycled material is fed into an inlet port located in zone 10 of extruder 6. This material may be fed in by passing the recycled material through a hopper which is positioned above an opening provided in the top of zone 10. As will be noted, the pitch of screw members 7a and 8a is relatively large. Accordingly, the contrarotation of screw members 7a and 8a (see the arrows shown in Figure 4) commences the shearing of the recycled material and, at the same time, the grinding of the thermoplastic material into smaller particles. It will be appreciated that the size of the particles, and in particular, the thermoplastic particles, which are. fed through zone 10 will be dependent upon the pitch of screw members 7a and 8a. The larger the pitch, the larger the particles may be. The smaller the pitch, the smaller the particles and the faster the material may be processed through extruder 6.

Zone 11 comprises screw members 7b and 8b. Screw members 7b and 8b have a smaller pitch than screw members 7a and 8a. Generally, when the thermoplastic recycled material enters zone 11, it has been reduced in size and, due to the action of screw members 7a and 8a on the recycled material, the temperature of the thermoplastic material has been raised. In zone 11, this process continues and the material may be raised to a temperature above the softening point of the thermoplastic material. Similarly, zone 12 includes screw members 7d and 8d. Once again, these may be of a smaller pitch than screw members 7b and 8b.

As will be appreciated, the numbers of zones 10, 11 and 12 may be increased or decreased depending upon the size of the particulate material which is fed to zone 10 and the different pitch of the screw members of each successive zone. The shearing action created by the intermeshing of screw member 7b and 8b, as well as 7d and 8d, raise the temperature of the thermoplastic material to a temperature at which the thermoplastic material is moldable or pliable. Ground material la may then be mixed in by the shearing forces into the thermoplastic material to create a homogeneous mixture in which the ground material la is evenly disbursed.

The temperature of the thermoplastic material is raised to a point above the softening point of the thermoplastic material and below the temperature at which the thermoplastic material will degrade. Depending upon the waste material which is present, the temperature may also have to be kept below the temperature of degradation of the waste material. If the shearing results in excessive heat generation, then a cooling jacket (not shown) may be provided surrounding extruder 6. Alternately, in case extruder 6 is shut down, the cooling jacket may double as a heating jacket so as to maintain the thermoplastic material in a softened state while the machinery is shut down or so as to provide additional heat to extruder 6 in case the shearing action does not render the thermoplastic material pliable.

Optionally, as discussed above, gas extractor devices may be provided at the end of each zone 10, 11 and 12 to allow the gases, and any water, which may be present in extruder 6 to be vented. Further, such gas extractor devices may be used to introduce gasses, such as nitrogen, into extruder 6.

Zone 11 is provided with intermeshing screw members 7c and 8c which have a reverse pitch. Screw members 7c and 8c therefore produce a force which pushes the recycled material towards zone 10. Due to the relatively larger pitch of screw member 7b as compared to screw member 7c as well as the difference in length of screw member 7b compared to screw member 7c, the overall force created in zone 11 forces the plastic material through orifice plate 23 and into zone 12. Using this arrangement, the pressure in zone 11 may be increased.

As shown in Figure 3, reverse pitch screw members 7e and 8e may optionally also be provided at the end of work zone 10. Similarly, reverse pitch screw members 7f and 8f may also be provided at the end of zone 12. These optional screw members may be provided if, due to the size of the particles fed to zone 10, additional shearing is required in either of zones 10 or 12.

The use of the orifice plates and /or the reverse pitch screw members restrict the flow of the recycled materials through grinder 6 at atleast one position downstream from the position at which the thermoplastic material is plasticized to increase the pressure in said shearing zone. For example, the pressure in zone 11 may vary from about 500 to about 2500 psi and, preferably, the pressure is about 2500 psi.

A plasticised, homogenous mass of recycled material exits zone 12 of extruder 6 via port 17. This material may be solidified (such as by air cooling or applying heat thereto) and fed to a grinder, hammer mill or the like to create granular material suitable as a feed material for an extrusion machine or an injection molding machine. As will be appreciated, for these uses, the homogenous mass is preferably pelletized to particles having approximately the same shape and size.

In the alternate embodiment shown in Figures 1 and 2, the homogenous mass of recycled material may exit outlet port 17 and be fed, optionally, to a pressure intensifier unit 18. Pressure intensifier unit 18 may be an extrusion molding machine (e.g. a melt extrusion machine). The mass is then fed from intensifier 18 to sieve unit 19 and, finally, to a pelletizer 20. Pressure intensifier 18 may provide additional force to push the homogenous mass through sieve unit 19. Contaminants are extracted in sieve unit 19. Pelletizer 20 produces a stream of pelletized material 21. In a further alternate embodiment, the homogenous mass of recycled material may exit outlet port 17 and be fed to an molding machine such as an extrusion molding machine or a injection molding machine.

It will be obvious to those skilled in the art that other modifications and variations of the process are within the scope of this invention.

Claims

I CLAIM:

1. A method of recycling waste material including a first fraction of particulate thermoplastic material and a second fraction of particulate material at least some of which is substantially unaffected by thermoplastic processes, comprising the steps of:

(a) introducing said fractions into an apparatus having at least one shearing zone;

(b) during the passage of said fractions through said apparatus, subjecting the fractions to shearing forces in said at least one shearing zone to plasticize said first fraction and to intimately mix said first and second fractions to produce a plastic mass wherein said second fraction is disbursed in, and surrounded by, said first fraction; and,

(c) restricting the flow of said fractions through said apparatus at atleast one position downstream from the position at which said first fraction is plasticized to increase the pressure in said shearing zone.

2. The method as claimed in claim 1 wherein the restriction of the flow of said fractions through said apparatus comprises means for inducing a reverse flow on said fractions.

3. The method as claimed in claim 1 further comprising the step of extruding said mass from said apparatus and forming a plurality of relatively small and relatively uniformly sized portions from said mass.

4. The method as claimed in claim 2 wherein said apparatus comprises a plasticizing screw extruder.

5. The method as claimed in claim 4 wherein said extruder comprises at least two contra-rotating longitudinally extending screw extruder units positioned in a casing, said casing having an inlet port and an outlet port, each of said screw extruder units having a plurality of screw members, at least one of said screw members having a reverse pitch, said reverse pitch screw members being matingly positioned to induce a reverse flow on said fractions.

6. The method as claimed in claim 5 wherein the pitch of each successive screw member is smaller than the immediate preceding forward pitch screw member.

7. The method as claimed in claim 1 wherein said first fraction is substantially dry and said second fraction comprises less than about wt. % water.

8. The method as claimed in claim 6 wherein said fractions are raised to a temperature the softening temperature of said thermoplastic material and below the temperature at which said thermoplastic material degrades.

9. A method of recycling thermoplastic material comprising the steps of:

(a) introducing said material into an apparatus having at least one shearing zone; (b) during the passage of said material through said apparatus, subjecting said material to shearing forces in said at least one shearing zone to plasticize said material;

(c) restricting the flow of said fractions through said apparatus by inducing a reverse flow on said material at atleast one position downstream from the position at which said material is plasticized to increase the pressure in said shearing zone.

10. The method as claimed in claim 9 further comprising the step of extruding said plasticized material from said apparatus and forming a plurality of relatively small and relatively uniformly sized portions from said plasticized material.

11. The method as claimed in claim 9 wherein said apparatus comprises a plasticizing screw extruder.

12. The method as claimed in claim 11 wherein said extruder comprises at least two contra-rotating longitudinally extending screw extruder units positioned in a casing, said casing having an inlet port and an outlet port, each of said screw extruder units having a plurality of screw members, at least one of said screw members having a reverse pitch, said reverse pitch screw members being matingly positioned to induce a reverse flow on said fractions.

13. The method as claimed in claim 12 wherein the pitch of each successive screw member is smaller than the immediate preceding forward pitch screw member.

14. The method as claimed in claim 13 wherein said material is substantially dry.

15. The method as claimed in claim 14 wherein said material is raised to a temperature the softening temperature of said thermoplastic material and below the temperature at which said thermoplastic material degrades.

16. An apparatus for recycling thermoplastic waste material comprising: (a) inlet port means, outlet port means and a shearing zone;

(b) shearing means for plasticizing said thermoplastic material;

(c) means for inducing a reverse flow on said material at atleast one position downstream from the position at which said material is plasticized.

17. The apparatus as claimed in claim 16 wherein said shearing means comprises at least two contra-rotating longitudinally extending screw extruder units positioned in a casing, each of said screw extruder units having a plurality of screw members, at least one of said screw members having a reverse pitch, said reverse pitch screw members being matingly positioned to induce a reverse flow on said material.

18. The method as claimed in claim 17 wherein the pitch of each successive screw member is smaller than the immediate preceding forward pitch screw member.