Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 95131319 2 t 9 0 ~ S 1 Pcr/CA95loo288
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~: METHOD AND APPARATUS FOR RECYCLING WASTE
MATERIALS INCLUDING THERMOPLASTIC MATERL~LS
FIFl n OF TE~F INVE~TION
This inventions relates to a method and o~ US which
5 may be used for recycling waste materials including thermoplastic waste
m~t~ lc. The process may be used to prepare a granular feed material
which may be used in, e.g., injection molding and extrusion molding
processes.
BA~-KGROUND TO THF 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
15 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
20 recyclable products.
Various different processes have been d.~1~e~ 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
25 fibre and plastic waste material and heating and kneading the mixture to
form a l~ rll~ mass. The kneading may be ~n~1lrte~1 using a sigma
blade double arm mixer which is insulated and jacketed for heating with
hot oil up to ~rl~ Lulr~ of about 500F.
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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;
5 subjecting the mass of granular material to a compacting process to
produce bodies having properties of mP~'hAni~l cohesion and subjecting
the bodies to a shaYing process to give rise to a mass of shavings of small
particle size.
The article "Aufbereitung von thermoplastischen Abfallen
uber die Schmelze" in "Plastverarbeiter" pages 252 to 257, discloses a
method of recycling waste material including thermoplastic material by
the use of a screw extruder. The waste material may include plastic and
other waste material such as straw and other filler material.
While prior art devices disclose mixing plastic and non-
15 plastic material together to form a granular material, in some cases thegranular 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, ~1iffi('111tiP~ may be encountered in ensuring that a proper mixing
20 of the plastic and non-plastic material occurs and in m~intAinin~ the
equipment in proper running condition.
SUMl~ Y OF T~ INV~TION
In accordance with the instant invention, there is provided a
method of recycling waste material including the first fraction of
25 particulate thermoplastic material and a second fraction of particulate
material at least some of which at the melting temperature of the
thermoplastic material comprising the steps of introducing said fractions
into an apparatus having at least one shearing zone, the thermoplastic
material comprising at least 60 wt % of the total weight of the fractions;
30 during the passage of said fractions through said apparatus, subjecting the
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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 dispersed in, and
surrounded by, said first fraction; and, restricting the flow of said fractions
5 through said apparatus at atleast one position downstream from the
position at which said first fraction is plasticized to increase the pressure insaid 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
15 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.
20 The screw extruder may comprise at least two contra-rotating
on~it~ in~11y 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
25 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 immP~ tPly proceeding forward pitch screw member. By using
the process of the instant invention, a plastic mass, which may include
30 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 mP( hsmi~:~l properties that are at least 50%
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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
5 generate sufficient heat to melt the plastic material. Accordingly, the
process may require input of only m~l~h~ni~ energy to recycle the waste
material.
I~SC'RrPTION QF Tl:~E l~RAWT~GS ~
These and other advantages of the instant invention will be
10 more fully and completely understood in ronn~(tion 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.
~ RTPTIONOFT~Pl~ RFnI~MRODrME1~T
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
149C [300] to about 288C [550F], more preferably from about 204C [400]
to about 260C [500F] and, most preferably, from about 204C [400~ to about
232C [450FJ). 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, poly~ yl~lle and cellulosic and acrylic resins. Preferably, the
thermoplastic material does not include material which may deleteriously
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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 if plastic 1 contains no or only very
small metal parts. The metal parts are preferably removed since the metal
may cause ~ llld~Ul~: 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 ~1iml~ncif~n of any particular piece is less than about 5 cm [2"~
and more preferably, is less than about 3.8cm [1 1/2"].
As shown in Figure 2, waste material, at least some of which
is substantially i~nAff~ t~.1 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 th~rml~cf~ing
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 mAnllfAftllre of furniture,
components for fumiture and ~imf~nSionA~ 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.
~he 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,
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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
lZ 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 examp~e,
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 dispersed 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. /O 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
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about 0 to about 15 wt. % and, most preferably, from about 0 to about 10 wt.
%.
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
5 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
10 extruder units 7 and 8. Screw extruder units 7 and 8 are intPrm~hing 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 thereon screw members 7e
and 8e as well as 7f and 8f respectively.
Extruder 6 may also include one or more orifice plates 23
which are shown in more detail in Figure 4. Qrifice 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 by, 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
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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
5 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 inh~rm~shin~ of the screws and the height of the
thread members on the screws), the degree to which the passage of
10 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
15 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,
20 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
25 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
30 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.
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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
5 in~rm~qTlin~ 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
10 dispersed.
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
15 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
20 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
25 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 in~Prm~qhin~ screw members 7c
and 8c which have a reverse pitch. Screw members 7c and 8c therefore
30 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
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compared to saew 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
5 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 1~
The use of the orifice plates and/or the reverse pitch screw
members restricts the flow of the recycled materials through grinder 6 at
atleast one position downstream from the position at which the
thermoplastic material is plasticized so as to increase the pressure in said
shearing zone. For example, the pressure in zone 11 may vary from about
15 35 Kg/cm2 [500] to about 175 Kg/cm2 [2500 psi] and, preferably, the pressure
is about 175 Kg/cm2 [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
20 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
25 homogenous mass of recycled material may exit outlet port 17 and be fed,
optionally, to a pressure inf~n.~ifi~r unit 18. Pressure in~on~ifi~r 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
30 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
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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
5 invention.
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