Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02245330 1998-08-19
Process for producing batches of rubber-based composition
The present invention relates to processes for producing batches
of rubber-based composition and articles made thereof, and more
5 particularly to processes for producing batches of rubber-based
composition and shaped made thereof.
Since the creation of the rubber industry over more than a century
ago, rubber materials have been essentially produced through processes
based on dimes vulcanization. In the early stages, dimes-based
10 materials were produced through plant-latex obtained from particular
tropical trees such as Para rubber-plant (Hevea). Synthetic rubbers have
first been introduced during the second world war to obviate a shortage
in natural rubber supply. Dimes-based rubbers, either natural or
synthetic, are made of macromolecular compounds exhibiting significant
15 flowing characteristic under tensile stress. Actually, such basic
materials do not present great interest for industrial applications. To
prevent macromolecular slipping for the purpose of providing a material
capable of sustaining deformation while recovering its initial state after
stressing, blocking of the macromolecular chains to one another is
20 required. Such molecular chains blocking is obtained through a process
known as vulcanization or curing, which typically consists of cross-
linking macromolecular chains at double bond sites which are present
along the chains, using sulfur as the linking element or an organic
peroxide. Typically, curing occurs through heating diene-based rubber
25 materials at a temperature between about 130°C to 150°C in
the
presence of about 7 - 10% of sulfur. Mechanical properties of sulfur-
cured rubbers vary with cross-linking density which is proportional to
the initial sulfur concentration. In practice, compositions of various
virgin (uncured) rubber types, such as natural rubber, butyl rubber,
30 polybutadiene and neoprene, with specific additives such as curing
starting agents and curing accelerators, have been developed to obtain,
after curing, rubber-based material exhibiting various mechanical
properties such as tensile strength, maximum tensile elongation, tear
strength, embrittlement temperature and resilience.
35 During the past years, due to the generally high cost of uncured
rubbers, reclaimed cured rubber in the form of particles or dust has been
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used as raw material to form a variety of manufactured articles.
Essentially, the technique usually consists of mixing rubber particles
with 1% - 5% sulfur and curing the obtained mix in a suitable mold
heated at a temperature of about 180°C while applying a pressure
5 between 500 pound/sq.in. to 2000 pound/sq.in. with a conventional
hydraulic press. Processes using such technique are generally known to
be more cost effective than processes using uncured rubber as raw
material, which is significantly more expensive than reclaimed rubber.
However, the mechanical properties exhibited by such reclaimed
10 materials are generally inferior, typically characterized by a maximum
tensile strength of about 300 pound/sq.in., a maximum tensile elongation
of about 50%, and maximum tear strength of about 100 pound/in., which
properties are significantly inferior than those exhibited by uncured
rubber-based compositions. Such inferior mechanical behavior is mainly
15 due to porosity characteristics and cross-linking level. Materials made of
rubber particles or dust are characterized by a porosity which is
generally responsible for the appearance of microfissures under
mechanical strength. Furthermore, effective contact areas between
adjacent particles of a reclaimed rubber material are reduced as
20 compared to those observed in virgin raw material. Under such
conditions, cross-linking between macromolecules of adjacent particles
are reduced accordingly, since most free double links of adjacent
particles are not in sufficiently close proximity to be bound, and
accordingly the non-reacted curing agent rapidly becomes in excess.
25 Microfissures appearing within the rubber material initiate flaws therein
which rapidly grow toward material rupture, due to the weakness of
internal forces binding rubber particles together, observed at low cross-
linking level.
A known technique to reduce the inherent porosity of a material
30 made of reclaimed rubber particles consists in adding a resin, preferably
a thermoplastic resin, to bind the rubber particles and therefore reduce
porosity. A certain amount of uncured rubber material can also be added
to increase the number of free double bond available for curing.
Additives such as compatibility agents may also be added. The resultant
35 composition is used to produce rubber materials which have been found
to exhibit improved mechanical properties in the range of 400
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pound/sq.in. for tensile strength, 250% for maximum tensile elongation
and 180 pound/in for maximum tear strength. However, such improved
reclaimed rubber materials still exhibit lower mechanical properties as
compared to materials essentially made of virgin rubber, since the cross-
5 linking level remains substantially unchanged when a thermoplastic
binding resin is used, and because the electrostatic forces acting between
rubber particles and thermoplastic resin are also weak. Such an
improved technique is disclosed in U.S. Patent No. 5,510,419 issued
April 13, 1996, to Burgoyne et al., and it teaches to produce a polymer-
10 modified rubber composition comprising reclaimed cured styrene-
butadiene rubber particles, uncured rubber, a styrene-based
thermoplastic resin, a homogenizing agent, to form a blend wherein the
thermoplastic resin is substantially homogeneously mixed with the cured
and uncured rubbers. Additives including plasticizers, lubricants, mold
15 release agent or viscosity modifiers such as traps-polyoctanamer rubber
may also be added. Use of a batch mixer such as the well known
Moriyama or Bandbury high intensity mixers is proposed to produce a
moldable composition showing a temperature between 120°C and
150°C, which is then transferred to a mold which is preheated at a
20 temperature above a vulcanizing temperature of about 120°C. Another
similar process is disclosed in U.S. Patent No. 5,425,904 issued June 20,
1995, to Smits, which uses rubber latex to treat cured waste rubber
particles with a curing agent to produce an activated moldable
composition. Another similar approach is taught in U.S. Patent No.
25 4,257,925 issued March 24, 1981, to Freeguard, which consists of
swelling reclaimed tire rubber with a monomer and then causing
polymerization thereof.
In order to further improve mechanical properties of reclaimed
rubber materials, the addition of various reactive cross-linking resin
30 binders to the curing agent has been proposed. In U.S. Patent No.
3,489,710 issued January 13, 1970, to Bonotto et al., ethylene-based
flexible resins reactive with a curing agent such as sulfur are mixed with
reclaimed rubber particles in the presence of the curing agent using a
high intensity batch mixer such as a Bandbury mixer. Similarly, in U.S.
35 Patent No. 4,481,335 issued November 6, 1984 to Stark, a liquid sulfur-
curable polymeric binder, namely a homopolymer or copolymer of 1,4-
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butadiene and substituted butadiene, is blended with cured rubber scrap
and sulfur to produce a treated rubber material which can be used in
large proportion as a filler or extender for uncured rubbers. Although
known prior art processes employing reclaimed rubber have heretofore
5 proved to be capable of producing, at lesser cost, shaped rubber-based
products of various quality in terms of mechanical properties, in order to
be practiced on an industrial basis, these processes generally require the
use of large and expensive high intensity mixing equipment, generally
limited to long production cycles to provide a composition homogeneity
10 which is required to obtain uniformity of physical properties, which
requirement may affect the effective productivity of the process.
Furthermore, known processes for mixing curable rubber-based
compositions may not provide proper reduction of humidity within a
batch to be formed, which may cause dangerous high pressure steam
15 discharge upon removal from the mold. Therefore, there is still a need
for a process for producing batches of rubber-based composition and
articles made therefrom which overcome the foregoing drawbacks of the
prior art.
It is therefore an object of the present invention to provide
20 economic and safe processes for the production of moldable rubber
based compositions and shaped products made therefrom, which affords
short production cycle for a large range of shaped product quality
requirements in terms of mechanical characteristics.
According to the above object, from a broad aspect, there is
25 provided a process for producing a batch of rubber-based composition
capable of being formed into an article, comprising the steps of:
feeding a batch of mixable material comprising from about 40%
to about 90% by weight of particles of cured rubber material with from
about 60% to about IO% ;;;;;;by weight of a resinous material into a
30 closed mixing chamber provided on a high intensity mixer including a
central shaft having mixing blades;
rotating the shaft to provide a blade tip speed of from about 17
meters to about 35 meters under conditions effective to intensively mix
and thermokinetically heat the batch of mixable material;
35 continuously monitoring one of temperature and temperature
related parameter of the batch;
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discharging the batch from the mixing chamber when said one of
temperature and temperature related parameter reaches a reference value
ensuring a substantially uniform mix of said rubber material in a matrix
of said resinous material to produce said rubber-based composition.
5 Preferred embodiments of the present invention will now be
explained in detail with reference to the accompanying drawing in
which:
Fig. 1 is a perspective view of a high intensity batch mixer used
to practice the
10 process according to the present invention;
Fig. 2 is a partially cross-sectional perspective view of a front end
of the batch mixer shown in Fig.l; and
Fig. 3 is a graph showing the variation of a mixer motor load
current measured as a function of time, and related thresholds used to
15 control a process according to the present invention.
Referring now to the drawings, a high intensity mixer 10 which
could be used to carry out the process according to the present invention
is depicted, which mixer 10 comprises a base frame 12 including a pair
of longitudinal beams 14 secured to upper ends of front and rear pairs of
20 floor mounted pillars 16 rigidly secured by transverse members 18 and
bracing plates 20. On the rear portion of the beams 14 is secured a motor
base 22 having a mounting plate 24 on which is secured lower parts of
legs 26 supporting an electric industrial motor 28, preferably of a self
cooling type. As shown in Fig. 2, motor 28 is provided with an output
25 shaft 30 connected to a driving end of a flexible coupling 13 of known
construction, which has a driven end connected to a driving shaft 15
extending through a cantilever bearing unit 17 permanently lubricated
with a pumped oil circuit (not shown). The flexible coupling is covered
fox protection with a casing 19 (Fig. 1) provided with an aperture 21
30 giving required clearance for the main driving shaft 15. The cantilever
bearing unit 17 is rigidly mounted through rear and front supports 29
(only rear support being shown in Fig. 2) on an horizontal plate 19 as
part of a sub-frame assembly 21 secured to the front portions of the
beams 14, and having a vertical mounting plate 23 further secured to the
35 horizontal plate 19 through a pair of opposed bracing plates 27.
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As shown in Fig. 2, apparatus 10 further comprises a cylindrical
mixing chamber 32 having outer double peripheral wall 34 and provided
with cooling fluid ports 35 (Fig. 1) to be connected to a cooling circuit
line (not shown} as will be later explained in more detail. Wall 34 is
5 further provided with and optional temperature probe 37. Mixing
chamber 32 further has a circular double end wall 36 defining a mixing
cavity 38. Mixing chamber 32 is secured to the body of a feeding unit
42 having an outer double peripheral wall 39 provided with cooling fluid
ports 41 as better shown in Fig. l, and an input tubular portion 44 in
10 communication with an output annular portion 46 defining a feeding
cavity 48 therebetween, as better shown in Fig. 2. The inner edge of
peripheral double wall 34 of the mixing chamber 32 defines an annular
channel adapted to receive an annular rib protruding from the outer
surface of a first collar 52, which is then rigidly welded or otherwise
15 secured to the double wall 34. The inner surface of first collar 52 is in
turn joined to the outer mating surface of a second collar 54. Collars 52
and 54 are rigidly bolted together. Output annular portion 46 of the
feeding unit 42 tightly engages with an inner opening formed in collar
54, to form an annular inner end wall opposed to circular end wall 36.
20 Input tubular portion 44 defines with the inner rear surface of collar 54
an internal space 33 in fluid communication through a fluid line (not
shown) with internal space 31 within double walls 34 and 36, to form a
cooling cavity as part of the cooling fluid circuit filled with a cooling
fluid such as cold water, for limiting the temperature of the feeding
25 cavity and mixing chamber to a level providing desired process
continuity.
Centrally extending through the output annular portion 46 is a
two-part driven shaft 56 having a forward bladed portion 58 contained in
the mixing chamber 32 and being opposed to a rearward threaded
30 portion 60 contained in the feeding cavity 48 and acting as a conveyor
screw. The cantilever bearing unit 17 provides axial and radial stability
for shaft 56 while allowing rotation thereof about axis 66. Rearward
shaft portion 60 is rigidly secured to the driven end of the driving shaft
15 using a key screw (not shown}, to impart rotation to the rearward
35 shaft portion 60. The outer end of forward shaft portion 58 is formed
with a threaded hole 62 for receiving an aligned bolt 64 securing the
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forward shaft portion 58 to the driven end of the driving shaft 15, to
impart rotation to the forward shaft portion 58. An optional aeration
port 65 is provided at the outer surface of double end wall 36.
Alternatively, instead of using a cantilever arrangement, port 65 may be
5 replaced by a bearing for receiving a rotating rod (not shown) to be
secured to the outer end of the forward shaft portion 58. The outer
diameter of threads 66 provided on the driven shaft 56 is chosen to
closely fit within output annular portion 46, mixing cavity 38 thereby
becoming substantially enclosed. Input tubular portion 44 of feeding unit
10 42 is coupled with a corresponding lower flange 68 of a feeding column
40 provided with a feeding hopper 70 at an upper portion thereof. In the
example shown, a feeding door mechanism 72 provided with a hand-
held actuating arm 74 allows an operator to dump raw materials
previously discharged in the hopper into feeding cavity 48. Ra.dially
15 extending from the surface of the forward shaft portion 58 are a plurality
of mixing blades 75 each having a tip portion maintained in substantially
close proximity to the inner surface of the double wall 34. Located in a
lower portion of the peripheral double wall 34 is a discharge opening 76
delimited by a peripheral sealing edge surface adapted to tightly mate
20 with the corresponding inner surface of a displaceable door 78. Door 78
is rigidly secured to an outer end of a pivoting arm 77 having an inner
end rigidly attached to a clamping sleeve 79 receiving an output shaft 80
of a rotary actuator 81 mounted on the sub-frame assembly 21 and being
driven through a pneumatic cylinder 84 coupled to a linear-to-rotary
25 converting mechanism of a known construction (not shown). Using set
screws 80a, the output shaft 80 is caused to rigidly engage the clamping
sleeve, to selectively displace the door away from a discharge opening
76 to provide discharge of mixed material out of the mixing chamber
upon rotation of the shaft in a first direction, or to displace the door
30 toward discharge opening 76 and maintain door 78 against it in an
opposite direction to tightly obstruct the opening 76 during the feeding
and mixing steps. To cover the discharge area there is provided a chute
82, which comprises front and rear wall 85, 86 as better shown in Fig. 2
and side walls 88, 90 as better shown in Fig. 1, which walls define a
35 discharge outlet for dumping a mixed batch to be transferred to a
forming station.
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Apparatus 10 further comprises a control device such as
parogrammable logic controller (PLC) 92 which receives from line 98 a
current load indicating signal generated by an electrical current
measuring device such as ammeter 96 as part of the motor drive unit 94,
5 or located at a remote location, to derive the estimated temperature of
the mix, as will be explained later in more detail. Alternatively, the PLC
may receive via line 102 and optional temperature probe 3 7, a signal
representing the temperature directly measured within mixing cavity 38.
Due to the generally short time cycle of the process, the temperature
10 within the batch of material being mixed could be significantly different
from the temperature of walls 34 or 36 of mixing chamber 32.
Therefore, a remote temperature sensor should preferably be used, such
as optical infrared sensor as disclosed in U.S. Patent No. 4,332,479
issued June l, 1882, to Crocker et al. PLC 92 also sends to motor drive
15 unit 94 through line 100, a command signal to control the rotation of
motor 28 according to a preset RPM value. The motor drive unit 94
incorporates a feedback device to substantially maintain the preset RPM,
whatever the load current variation may be. PLC 92 sends a command
signal to pneumatic cylinder 84 via line 97 according to a selected
20 predetermined program stored in the PLC, as will be discussed later in
more detail. It should be understood that discharge can also be manually
operated a through manual control mode available on the PLC. Lines
106 and 108 are used to send control signals to a lubricating oil pump
for bearing unit 17 and a valve (not shown) which controls the cooling
25 circuit. Optionally, the hand-held arm 74 of the feeding door mechanism
72 can be replaced by a rotary actuator which can be coupled to the
feeding door mechanism and activated through the PLC via line 104.
The principles underlying the process according to the present
invention will now be explained with reference to the operation of the
30 apparatus described above with respect to Figs. l and 2, and also in
conjunction with Fig. 3. Mixing and heating of the raw material present
in mixing cavity 38 of mixing chamber 32 are simultaneously achieved
as a result of the high speed of blades 75 obtained by rotation of shaft
56. Under centrifugal forces resulting from the rotation of the blades,
35 particles of material are violently projected against the inner surface of
the peripheral wall 34 and the shaft or against one another, such friction
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converting kinetic energy into thermal energy resulting in material
temperature increase at a fast rate toward a target maximum temperature,
typically within 30 to 60 seconds for a target temperature above 180°C,
without any need for further extraneous heat source. The rotation speed
5 in RPM of shaft 56 is preset to a value ensuring that sufficient mixing
and friction energy is provided to the mix, in a short processing time
while still providing the stability required for efficient process control.
The composition homogeneity which is required to obtain uniform
physical properties is also obtained. Such results can be obtained when
10 the blade tip speed is above about 17 meter/sec, and preferably between
about 25 and 30 meter/sec. Experience has shown that with blade tip
speeds above 35 meter/sec, a fast temperature rising rate observed may
render the process difficult to control. Since the blade tip speed is
directly related to the RPM value as a tangential speed at the
15 corresponding radius, the RPM as a standard parameter, is preferably
chosen as a reference control parameter for the process. To substantially
maintain a preset RPM, electrical power or energy required will depend
on the inherent properties of the formulation such as volume of the
batch, density and viscosity of the material, as well as characteristics of
20 the mixer such as mixing cavity radial section and volume, design of
shaft 56 and configurations of blades 75, torque requirement without
load, etc. While the latter characteristics can be considered as
substantially constant, the parameters characterizing properties of the
material to be mixed such as density and viscosity vary during the
25 mixing process, altering load current or energy required to maintain a
preset RPM.
Referring now to Figs. 2 and 3, the process control according to a
preferred embodiment of the present invention will be explained through
a series of examples, wherein a load current measurement to monitor
30 the temperature of the batch material under mixing is used.
Example 1
Referring to Fig. 3, curve 101 which is shown, represents the
variation of load current (in amp) shown on vertical reference axis 102,
and as a function of time (in sec.) on horizontal reference axis 104,
35 measured during mixing of a 1 kg batch of material, from the initial
material feeding step to a final state where the 3 liter mixing chamber 32
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of the high intensity mixer 10 is empty after discharge of the mixed
batch. In this example, 63% by weight of vulcanized (cured) rubber
crumb of either natural or synthetic type were previously pre-mixed with
32% of a thermoplastic polymer resin, selected from polyethylenes
5 (from low to high density) or ethyl-vinyl acetates, optionally with 5
by weight of an additive such as acrylic acid. The rubber crumbs were
selected to pass through a 8 mesh sieve in the instant example.
However, depending on the raw rubber crumb grain size and desired
composition quality, any sieve size between 8 to 60 mesh, and
10 preferably between about 20 to 30 mesh may be used. Depending on the
desired mechanical properties for the formed material, experience has
shown that from about 40% to about 90% by weight of cured rubber
crumbs may be added to from about 60 to about 10 % by weight of
thermoplastic resin. Before feeding the raw material into feeding cavity
15 48 upon activation of feeding door mechanism 72, the load current
drawn by motor 28 running idle at a preset speed of about 3200 RPM
was measured to be about 15 amp. For the composition of this example,
the rotation speed of shaft 56 may be typically set from about 2800 to
about 3600 RPM, which limit values correspond to a blade tip speed
20 from about 22 to 29 meterls, respectively. Once the material is
introduced, friction of the material carried along the path of feed thread
66 upon rotation of shaft 56, creates a counter-torque producing an
initial increase of the load current which rises to a preset feeding value
of about 19 amp in the example shown, which roughly corresponds to an
25 effective introduction of the material into mixing cavity 38.
Conveniently, the timing of the process was chosen to begin as the
measured load current reaches, associated with an initial temperature for
the input material which generally corresponds to ambient storage
temperature. Thereafter, the intensity of the load current increases
30 according to a first substantially linear gradient until the whole batch of
material has been introduced within the mixing cavity 38, which
corresponds to a substantially flat intermediate curve part showing an
average load current value of about 35 amp in the example shown. That
temporary load current stability is associated with a constant mixing
35 behavior exhibited by the batch material as the temperature is rising
toward a first transition temperature where the material enters into a
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viscous state progressively adding to counter-torque, requiring further
increase of the load current above to maintain the RPM at the preset
level of 3200. The increase in viscosity is associated with a melting of
the thermoplastic resin which encapsulates the rubber crumbs which
5 remain in a solid state at that temperature range. In the example shown,
the transition temperature was obtained in about 9 sec. Then, the load
current rises according to a second substantially linear gradient, passing
through a minimum value the purpose of which will be explained later,
to substantially stabilize at a maximum level of about 56 amp in about
10 15 sec for the example shown, which maximum corresponds to a second
transition temperature where the mixed material gradually enters into a
fluidized state requiring less energy to abe mixed. Thereafter, the load
current begins to drop accordingly, while the temperature of the
fluidized matter still increases, until the desired batch temperature is
15 considered to have been reached, corresponding to a predetermined
discharge load current, which parameter can be derived in various ways.
A first way consists in causing PLC 92 to open discharge door 78 for a
preset time following the moment where the load current reaches
through a numerical analysis method programmed in PLC 92. It should
20 be understood that can be set at zero. The preset value for can be
chosen experimentally by a direct measurement of the temperature of the
discharge material using successive test values for. In the example
shown in Fig. 3, was given a value of 2 sec. An alternate way would
consist in programming PLC 92 to open the discharge door 78 for a
25 preset time following the moment where the load current reaches, both
current values being experimentally set while ensuring that the derived
has a value equal to or under. In the example shown in Fig. 3, was given
a value of 8 sec, to correspond to a target discharge temperature for the
mixed matter, of about 170°C. For the composition of this example, a
30 target temperature within a range from about 160°C to about
190°C
ensures the production of a substantially uniform mix of cured rubber
material in a matrix of resinous material to produce a rubber-based
composition which could be later formed into an article. A third way
would consist in causing PLC 92 to open discharge door 78 for a preset
35 time following the start of the mixing, which corresponds to. In the
example shown in Fig. 3, was given a value of 19 sec. It should be
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understood that any combination of the foregoing conditions for
discharge control can be used alternatively, whichever is met first. It
should also be understood that any other practical way using any other
appropriate temperature-related parameter can be used to control batch
5 discharge after mixing. Optionally, the material discharge may be
caused to occurs in case where the load current as measured reaches a
predetermined critical value, indicating a possible malfunction of the
apparatus. As mentioned before, the mixed matter discharge can also be
under the control of the operator using manual functions available on
10 PLC 92, based on manual timing carried out after the introduction of the
material into feeding cavity 48. If the process control is based on direct
temperature measurement through temperature probe 37 as shown in
Fig. 1 rather than being indirectly based on load current measurement,
PLC 92 can be programmed to monitor a temperature signal coming
15 from probe 37, and to command activation of discharge door 78
whenever a target discharge temperature is measured. The discharged
material which was in a viscous state was then ready to be transferred to
a shaping station to be shaped into an article. For a rubber-thermoplastic
composition used in the example described with reference to Fig. 3, a
20 conventional cold-molding press on which is mounted a two-part mold
was used to form the article, the latter being allowed to cool into the
mold under pressure until mold removal temperature was attained.
Example 2
In this example, from about 90% and 99% by weight of cured
25 rubber crumbs of either natur al or synthetic type were previously pre
mixed with from about 10% to 1% of a curing agent such as sulfur,
which is characterized by a minimum curing temperature of about
110°C, and then introduced into 3 liter mixing chamber 32 of high
intensity mixer 10. The rubber crumbs were selected to pass through a 8
30 mesh sieve in the instant example. For the composition of this example,
the rotation speed of shaft 56 may be set from about 2200 to about 3600
RPM, which limit values correspond to a blade tip speed from about 17
to 29 meters, respectively. PLC 92 was programmed to monitor the
temperature as previously explained, to command activation of discharge
35 door 78 whenever a target discharge temperature was considered to have
been reached, corresponding to a temperature for the mixed matter
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selected within a range from about 115°C to about 140°C,
ensuring the
production of a substantially uniform mix of cured rubber with the
curing agent without causing significant further curing to produce a
rubber-based composition which could be later heat-shaped into an
5 article.
Typically, the mixing cycle is completed within about 30 sec. The
discharged composition in the form of a mass of hot particles was then
ready to be transferred to a shaping station to be shaped into an article,
the mold being pre-heated to a molding temperature substantially above
10 a minimum curing temperature. The batch of heat-moldable
composition is maintained within the pre-heated mold for a sufficient
period of time to allow further curing of the composition to produce the
shaped article. For the rubber of the present example, a conventional
hot-molding press on which is mounted a two-part mold pre-heated to a
15 temperature from about 140°C or about 200°C was be used to
shape the
article, the latter being allowed to further cure under heat into the mold
under pressure and then allowed to chill until mold removal temperature
was attained. Pre-heating of the composirion in the mixing stage
followed by molding at a temperature well above minimum curing
20 temperature has been found more energetically effective than known
processes using one-step heating while shaping. Typically, curing is
completed within about 60 min., which is more than twice faster than
the curing time required by known one-step heating shaping processes.
The humidity which remained in the molded batch was sufficiently
25 reduced during mixing to ensure safe removal from the mold.
Example 3
In this example, from about 40% to 90% by weight of cured
rubber crumbs of either natural or synthetic type were previously pre-
mixed with from about 0.1% to about 5% of a bonding agent, such as
30 acrylic acid, Primacor 3460 (Dow Chemicals) a silane or a titanate, and
are first introduced into 3 liter mixing chamber 32 of high intensity
mixer 10. T'he rubber crumbs were selected to pass through a 20 mesh
sieve. In a first step, the rotation speed of shaft 56 was maintained for
about 30 sec to a value of about 3000 RPM corresponding to a blade tip
35 speed of about 24 meters, to cause a substantially uniform distribution
of the bonding agent through the rubber particles to form a coating.
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During that step, the temperature of the mix did not significantly raise.
In a second step, a thermoplastic polymer resin selected from
polyethylene (from low to high density) or ethyl-vinyl-acetates, from
about 60 to about 10 % by weight, was introduced into the mix, while
5 shaft 56 was rotated at a speed set from about 2800 to about 3600 RPM,
which correspond to a blade tip speed from about 22 to 29 meters,
respectively. PLC 92 was programmed to monitor the temperature as
previously explained, to command activation of discharge door 78
whenever a target discharge temperature was considered to have been
10 reached, corresponding to a temperature for the mixed matter from about
160°C to about 190°C to yield a ready to shape matter exhibiting
improved mechanical properties due to the bound rubber-thermoplastic
matter as compared to material wholly made of cured rubber crumbs.
The discharged material in a viscous state was then ready to be
15 transferred to a shaping station to be formed into an article, using a cold
molding press as explained before in conjunction with example 1.
Example 4
In this example, from about 20% to 99% by weight of cured
rubber crumbs of either natural or synthetic type were previously pre
20 mixed with from about 10% to 1% of a curing agent such as sulfur, with
from about 80% to 1% of a low molecular weight polymeric binder
which reacts with the curing agent, such as traps-polyoctenamer
(supplied by Huls AG) with from about 40% to 1% of natural or
synthetic uncured rubber such as a styrene-butadiene. Other polymeric
25 binder such as styrene-1,4-butadiene, a copolymer of 1,4-butadiene and
acrylonitrile or 1,2-polybutadiene may also be used. Known activators
such as zinc oxide or stearic acid, and a curing accelerator such as
tetramethyl thiuran disulfide (TMTD) or benzothiazyldisulfide (MBTS)
were added to control curing of the uncured rubber. This pre-mix was
30 then introduced into 3 liter mixing chamber 32 of the high intensity
mixer 10. The rubber crumbs were selected to pass through a 20 mesh
sieve. For the composition of the present example, the rotation speed of
shaft 56 may be set from about 2200 to about 3600 RPM, which
correspond to a blade tip speed from about 17 to 29 meters,
35 respectively. PLC 92 can be programmed to monitor the temperature as
previously explained, to command activation of discharge door 78
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whenever a target discharge temperature was considered to have been
reached, corresponding to a temperature for the mixed material from
about 115°C to about 140°C, at which temperature fw-ther curing
is
substantially prevented, thereby producing a rubber-based composition
5 which can be later heat-shaped into an article. The discharged
composition in the form of a mass of hot particles was then ready to be
transferred to a shaping station to be shaped into an article, in the same
manner as explained in conjunction with Example 2.
10 It is to be understood that any variant of the preferred
embodiments described above should be considered within the ambit of
the present invention, provided it falls within the scope of the appended
claims.
15
