Note: Descriptions are shown in the official language in which they were submitted.
2178561
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There have been prior attempts at treating cured
(vulcanized) rubber scrap material to break it down in some
manner to effectively regenerate it, and thereby provide a
rubber material which can be recompounded, and subsequently
molded and cured. The result of this process is a material
usually known as reclaim rubber.
Known processes generally comprise the following:
a) swelling the rubber with oils and then
treating with chemicals such as caustic
soda, and steam distilling the mixture;
b) attacking the rubber as above, and then
treating in a autoclave;
c) applying specific wavelengths of micro-wave
or radio-frequency radiation for lengthy
periods of time; and
d) exposing the rubber to work energy in the
presence of large amounts of various
chemicals.
These processes are not as energy-efficient as is
desirable and costs of producing reclaim rubber by these
processes are high. Moreover, with the known processes the
reclaim rubber has an objectionable smell associated with
it, and often produce undesirable by-products such as
hazardous waste materials.
Various procedures and forms of apparatus are
known for applying mechanical forces to rubber and like
materials for the purposes of mixing, compounding and
blending rubbers, elastomers and plastics. Such mixing,
compounding and blending equipment usually is capable of
inducing high intensity shearing forces in a plastic mass.
Examples of such equipment include BANBURY mixers
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available from Farrell Corporation, Ansonia, Connecticut,
FARRELL continuous mixers, also available from Farrell
Corporation, and twin screw extruders from a number of
manufacturers.
Attempts have been made to regenerate rubber by
applying mechanical forces using such equipment but all
prior attempts of which applicants are aware required
excessively long processing times and expenditure of
excessive amounts of energy and resulted in production of
material that is not satisfactorily regenerated.
Applicants have now found a process whereby
regenerated or reclaim rubber can be produced relatively
inexpensively, that mitigates the objectionable smell
characteristic of known reclaim rubbers, and avoids the
production of undesirable by-products.
Applicants have found that when a mass consisting
substantially wholly of finely divided vulcanized rubber is
subjected to high intensity impact forces, the rubber
becomes rapidly substantially regenerated to the point that
it can be effectively re-vulcanized. This process can be
conducted in short times, is highly efficient, and produces
reclaim rubber at considerably less cost than the known
processes. Moreover, the product is free from or has the
above-mentioned objectionable smell associated with it to a
lesser extent, and produces no by-products presenting
environmental concerns.
The invention also encompasses applying
mechanical forces, for example impact forces, to the finely
divided vulcanized rubber mass at high energy dissipation
rates such that the mechanical working dissipates energy in
the mass at a rate of not less than about 1000 watts per
100 g of the mass.
Applicants have found that the process of the
2178561
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invention may preferably be carried out using a specialty
type of mixer/compounder not commonly used in the rubber
industry, but familiar to those experienced in the art of
mixing and compounding plastic materials. That equipment
has novel application in the present invention. Applicants
have found that exposing cured scrap rubber or the like to
the extremely intense action of a GELIMAT (trademark)
blender (available from Draiswerke Inc., Mahwah, New
Jersey), or essentially similar equipment available from
other sources, results in the re-generation of the rubber
compound. Such equipment may be operated in either a batch
or continuous fashion.
Whereas the equipment cited above as being
familiar to those practiced in the art of mixing,
compounding and blending of rubber is generally considered
to impart shearing forces to the mass, the equipment used
in the present invention is unique in that shearing is not
involved as the primary functional force. Rather, it is
considered that the energy necessary to re-generate the
rubber is imparted to the rubber by an impact mechanism,
although a minor amount of incidental shearing may be a by-
product of the process as the rubber heats up and becomes
more mobile.
The manner or mechanism by which the high
intensity impact action serves to effect regeneration is
not presently fully understood but, without wishing to be
bound by any theory, is considered to result from breakage
of sulfur-sulfur or other crosslinking bonds and possibly
breakage of carbon-carbon bonds.
In case of uncertainty it is in any event
possible to determine by simple trial and experiment
whether a given type of equipment provides sufficiently
intensive impact forces to effect regeneration, by placing
a small quantity of finely divided vulcanized rubber in the
equipment and subjecting it to the mechanical action of the
2178561
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equipment at progressively increasing levels of power
consumption until an indication of regeneration is
obtained. It has been found that a useful indication of
the onset of regeneration is given by the mass becoming
tacky, and demonstrates 'green' or un-cured rubber
characteristics such as the ability to be blended on a mill
into an integral sheet, and the ability to be re-vulcanized
to a high percentage of the original physical properties,
and this therefore provides a readily determined indicator
of regeneration.
In the present invention, the mass is subjected
to mechanical working applying sufficiently intensive
impact forces for a period such that the mass is
substantially regenerated. The degree of regeneration is
best determined by comparing the strength properties, such
as tensile strengths, achieved on curing of the regenerated
mass, with those achieved on curing, under similar
conditions, of a chemically similar virgin (never
vulcanized) rubber. For example, in the case of passenger
car tire crumb, consisting of vulcanized styrene-butadiene
rubber, or a blend of styrene-butadiene and natural
rubbers, the degree of regeneration may be determined by
comparing its tensile strength after curing under standard
conditions with those obtained after curing of a similar
virgin styrene-butadiene rubber or styrene-butadiene -
natural rubber blend under the same conditions.
Preferably, the degree of regeneration as indicated by such
cured tensile strengths, as a percentage based on the cured
tensile strengths achieved with the similarly cured virgin
rubbers, is at least about 350, more preferably 400, and
still more preferably 450.
Moreover, the degree of regeneration can be
demonstrated by the reduction in the viscosity of the
material, as compared with a dry powder blend of tire crumb
and sulfur and other curative chemicals, with respect to
the cure curve obtained when tested on equipment such as a
CA 02178561 2008-04-03
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Monsanto Oscillating Disc Rheometer.
In the process of the present invention, the tire
crumb or other finely divided vulcanized rubber is
preferably present as substantially the sole component of
the mass undergoing regeneration. Preferably at least
about 90%, more preferably at least about 95%, and still
more preferably up to 1000, by weight of the mass consists
of vulcanized rubber.
If desired the vulcanized rubber particles may be
mixed with small quantities of various additives before
subjecting them to the present process. Examples of such
additives include hydrocarbon-based oils and conventional
vulcanization inhibitor compounds such as phthalic
anhydride, potassium phthalimide, phthalimide and
tributylphosphite, which may be added in amounts of
preferably no more than about 50, based on the total weight
of the mixture.
Preferably, the equipment used is such that it
dissipates energy within the mass at a relatively high
rate, so that regeneration is achieved in a short period,
preferably of the order of a few seconds, for example about
2 to about 20 seconds. Preferably the rate of power
dissipation is not less than about 1,000 watts per 100g of
the mass undergoing the high intensity impact action,
otherwise the time taken to achieve regeneration may tend
to be unacceptably long and hence the throughputs
unacceptably low and costly. Preferably, the rate is not
greater than about 145,000 watts per 100g otherwise
degradation of the rubber may tend to occur within such a
short time span that the reaction may tend to be difficult
to control. In the course of mechanically working the
vulcanized rubber mass, it is important to avoid excessive
working such that the rubber is degraded. Such degradation
is indicated by the mass acquiring a crispy texture and a
burnt smell and exhibiting little or no tensile strength on
*Trade-mark
2178561
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curing or being so friable or brittle in texture that it is
difficult or impossible to mix it adequately with curing
agents for the purpose of conducting tests of
vulcanizability. Accordingly in the present invention the
mass is subjected to highly intensive impact forces
sufficient to regenerate the mass without substantially
degrading it, to the point that it can be revulcanized as
stated above. More preferably the rate of energy
dissipation in the mass is about 4,000 to about 35,000
watts per 100g of the mass, still more preferably about
9,000 to about 17,000 watts per 100g.
In the case of many electrically-driven forms of
equipment, the equipment as supplied by the manufacturer
may be provided with an integrating ammeter providing an
indication from which the rate of power dissipation can be
determined or calculated. The power dissipation rate of
other forms of equipment can be readily determined by
conventional procedures well known to those of skill in the
art.
Preferably, the equipment employed comprises a
rotary mixer having a stationary chamber that holds the
mass undergoing regeneration and at least one mixer member
rotating in the chamber. In the case in which the mixing
element or elements extend generally radially within the
chamber, the mechanical action can be described in terms of
the mechanical impact velocity which is defined as the
velocity of the highest velocity portion of the rotary
mixer member with respect to the chamber. Preferably the
mixer provides a mechanical impact velocity of about 20 to
70 meters per second, more preferably about 25 to 50 meters
per second.
The preferred GELIMAT (trademark) apparatus
comprises, for example, a cylindrical chamber, and a shaft
extending co-axially within the chamber. The shaft is
provided with a plurality of radially extending mixing
2178561
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elements, which as viewed laterally of the axis, increase
in width towards their tips adjacent the inner wall of the
chamber, the tips presenting paddle-like surfaces that
extend in planes generally parallel to the axis or slightly
angled thereto, and that face generally laterally of the
axis, so that as the mixing elements rotate with the shaft
the generally flat paddle-like surfaces impact on the
material within the chamber. Particles within the chamber
are thereby subjected to impact against the chamber wall,
the mixing elements and themselves. For further
particulars, reference may be made for example to
literature describing the GELIMAT (trademark) apparatus,
for example Draiswerke "New Developments in Superfast
Compounding", Drais News, Vol. 1 No. 4, 8 pgs. Other
apparatus that subjects finely divided material to high
intensity impact forces may of course be employed.
By virtue of the highly intensive mechanical
impact to which the mass is subjected and of rubber's
properties of kinematic restitution it will usually exhibit
a fairly rapid rise in temperature during the course of the
process, for example as a result of hysteresis or internal
friction losses arising from the rapid deformation and
elastic restoration of shape of the particles following the
impacts discussed above. Preferably, the working is such
that the mass exhibits a rate of temperature rise of about
5 C/sec to about 60 C/sec, more preferably about 10 C/sec
to about 50 C/sec.
Typically, when the mass is subjected to
mechanical working sufficient to achieve substantial
regeneration, its temperature will rise to at least about
190 C. Working that is insufficient to raise the mass to
at least this temperature is usually insufficient to
achieve substantial regeneration. Preferably the
temperature of the mass does not rise above about 320 C
more preferably not above about 300 C during the process,
since exposure of the mass to such temperatures for more
CA 02178561 2008-04-03
8 -
than a brief period may tend to result in degradation of
the rubber.
As noted above, as regeneration commences, the
mass tends to become tacky. The power consumed by the
equipment, at least when operated at approximately constant
speed, tends to increase as the regeneration proceeds and
the mass becomes progressively more tacky. Further, as
noted above, the temperature of the mass tends to increase
during the course of the process. Applicants have found
that, other conditions being equal, it is possible to
monitor the degree of regeneration of the mass by observing
the power consumption or rate of energy dissipation, or the
maximum residence time within the mixing chamber, or both,
and use the observed or measured.values to control the
process. For example, such values may be used to determine
the point at which the mass is exited from the equipment in
the case of batch operation, or to control the rate of
throughput of the material in the case of continuous
operation.
The process of the invention may be used to treat
a wide variety of vulcanized rubber materials. Preferably,
because of its wide availability and relatively low cost,
the vulcanized rubber starting material is tire crumb
obtained in the conventional manner by grinding road
vehicle tires, for example passenger car tires or heavy
service tires, for example truck, bus, aircraft, etc.
tires. Preferably, the tire crumb is substantially free
from tire cord and metal. Other sources of scrap or
discarded vulcanized rubber may of course be employed.
In order to effectively regenerate the rubber
within an acceptably short period, desirably the starting
material rubber is in the form of finely divided particles.
Preferably the particle size of the rubber is not greater
than about 4 mesh (all mesh sizes herein refer to Tyler:*
standard sieve). Preferably, the particle size is not less
*Trade-mark
2178561
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than about 200 mesh, since smaller particle sizes do not
appear to result in significantly increased rates or
degrees of regeneration, and tend to greatly increase the
energy costs. More preferably the particle size is about 6
to about 100 mesh.
The invention will now be described in more
detail by way of example only with reference to the
following Examples and Comparative Example.
Examples 1 to 24 and Comparative Example
Tire crumb was regenerated using a GELIMAT G1
mixer/compounder obtained from Draiswerke, Inc., Mahwah,
New Jersey. This machine has a horizontally extending
cylindrical chamber with a central shaft provided with
staggered generally radially extending mixing elements.
The shaft is rotated, at controllable rpm, by a 600v motor.
Material to be treated, in the present Example finely
divided vulcanized rubber usually in the form of 100g
batches, is introduced into the chamber through a top
hopper with a locking slide. The chamber is equipped with
an ammeter shaft drive measuring energy input or power
dissipation. The outputs of these are fed to a
microprocessor which controls a discharge door provided on
the chamber. The device is settable so that when a given
time or power dissipation peak or level is attained, the
door is opened so that the mass is discharged rapidly from
the chamber under the action of centrifugal force.
A number of runs were conducted at various rpms,
and with discharge set to take place at various times or
amperage peaks as indicated by the above mentioned
equipment. In each case the actual temperature of the mass
exiting the chamber was measured using a digital
temperature probe and this temperature is referred to as
the "eject temperature" in Table 1. Actual measurements of
the temperature of the material in the chamber can be
2178561
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difficult and inaccurate due to the nature of the material,
which is often black. By measuring the temperature of the
ejected material at a range of power consumption levels an
effective calibration can be obtained.
Thereafter, uniform ejection temperatures can be
achieved by monitoring the unit power consumption.
The eject temperatures obtained over a large
number of runs are given in Table 1; together with
calculated maximum rotor element velocity (tip speed) and
power dissipation rates.
2178561
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2178561
- 12 -
A number of runs were made at various rpms and
eject temperatures. The vulcanized rubber batch charged to
the machine for the reported experiments was, unless
otherwise indicated, 8 mesh passenger car tire crumb (free
from steel and tire cord), either alone or in admixture
with an additive. The percentages indicate the content of
additive based on the total weight of the mixture. In each
case the mixture was exited into a metal bucket and stirred
to cool it quickly to reduce risk of degradation or
combustion. Usually the interval between a batch entering
and the reclaim product exiting the machine was about 8 to
about 16 sec.
The degree of regeneration was determined by
mixing the reclaim product in the following formulation:
47.6g reclaim product
0.71g sulfur
1.19g zinc oxide
0.48g stearic acid
0.36g Delac accelerator.
This mixture was blended in a Haake Bukler
Rheocord 600 mixer for 7 min. at 80 rpm at 80 C.
The blended samples were placed in a heated mould
(2mm thick copper frame between two copper sheets) that had
been sprayed with silicone spray (LPS heavy duty silicone
lubricant). The mould was placed into a press and gently
pressed for 30 sec to allow some heating of the rubber.
The pressure was increased to 1400 psi, held for 30 sec,
released to allow any air pockets to dissipate, pressed at
960 psi for 30 sec, again released to allow air to escape,
and then pressed for 6 minutes at the process temperature
of 180 C (350 F) and at a pressure of 960 psi. The plates
were removed and allowed to cool.
After allowing the plates and the process to
CA 02178561 2008-04-03
- 13 -
cool, ASTM standard dumbbell samples were cut using a
standard die. Cutting was done on a polyethylene cutting
board and using the press to provide an even pressure on
the die.
Tensile strengths and elongations at break of the
samples were determined on an Instrori Tensile Testing
machine, calibrated using a 10 lbs weight and set to
stretch the sample at 20 inch/min. The samples were loaded
at an initial jaw spacing of 43.2 mm.
The results indicated in Table 2 are mean values
obtained in each case from a series of five tests.
It was found that when the eject temperature was
less than about 190 C, the mass did not become tacky and
remained friable and insufficiently cohesive to be blended
to form a coherent mixture and therefore cured tensile
strength testing was not possible. When the eject
temperature was above about 250 C, there was risk that the
crumb became degraded and excessively tacky and would stick
inside the primary processing chamber. In such case, the
small portion ejected smoked profusely and quickly turned
crispy. When cool, the sample smelled burnt and had no
tackiness. The crispy degraded samples were not suitable
for blending and could not be tested for cured tensile
strengths.
*Trade-mark
2178561
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CA 02178561 2008-04-03
- 17 -
Notes
~
(a) Santogard PVI (prevulcanization inhibitor), lo (all
percentages based on the total weight of the mixture),
dissolved in 3o hot aromatic oil was added to the tire
crumb before treatment.
(b) 110 g of the tire crumb was added to the GELIMAT
machine instead of 100 g.
(c) 90 g of the tire crumb was added to the GELIMAT
machine instead of 100 g.
(d) The tire crumb was 30 mesh instead of 8 mesh.
(e) lo Santogard PVI was added to the tire crumb before
treatment.
(f) lo phathalic anhydride was added.
(g) 1s potassium phthalimide added.
(h) 1% phthalimide added.
(j) 2% phthalimide and 2% oil added.
(k) 2% oil added.
(1) Results obtained with a portion of the regenerated
rubber product which was stored for one week before
curing.
(m) Results obtained with a portion of the same product as
Example 17, cured the same day.
(u) 1% Santogard PVI added.
*Trade-mark
2178561
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(o) 201 Santogard PVI added.
(p) lo tributylphosphite and 30 oil added.
(q) lo Santogard PVI and 30 oil added.
(n) 1% potassium phthalimide added.
(s) 10 mesh truck tire crumb (having a content of
vulcanized natural rubber) was substituted for the
passenger car tire crumb.
(t) 47.6 g of virgin SBR rubber was substituted in the
formulation given above, and was blended, cured and
tested as described above.
