Note: Descriptions are shown in the official language in which they were submitted.
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PRIOR ART
Neubert 3,318,259 discloses the use of resinous butadiene-styrene
copolymer and polymethyl methacrylate as partitioning agents, individually,
but does not suggest that any mixture of them be employed.
THE INVENTION
It is well known that elastomers including natural rubber and
synthetic rubbers such as polyisoprenes, polybutadienes, copolymers of styrene
and acrylonitrile with isoprene or butadiene, etc., made by emulsion, solution
or suspension processes, are tacky. When coagulated from the latex form in
which they occur naturally or in which they are polymerized, the elastomer
particles tend to agglomerate unless they are agitated vigorously. When dry
they are even more tacky and special means must be employed if it is desired
to maintain the elastomer in the form of discrete particles.
It is an object of an aspect of this invention to coat elastomeric
crumb particles with a fused resinous partitioning agent to prevent agglomera-
tion during drying, storage, shipping and handling. The word "-crumb" is
used herein to refer to the individual particles or agglomerates formed in
coagulation processes, or to pellets formed by cutting extruded elastomers,
or to particles formed by grinding larger masses of the elastomers. The
crumb particles produced by coagulation or grinding may generally be con-
sidered as having diameters of about 1 to 15 mm.; and if produced from an
extruded strand may measure as much as 50 mm. in any direction.
In accordance with one aspect of this invention there is provided
a partitioning agent; which is a fused mixture of (a) 50 to 80 parts, by
weight, of a styrene-butadiene copolymer with a styrene content of 80 to 99
per cent, by weight and ~b) 50 to 20 parts of a polymethyl methacrylate.
In accordance with another aspect of this invention there is provided a
process of producing a polymer crumb, which process comprises coating a
tacky polymer crumb with a blend of ~a) 50 to 80 parts, by weight, of a
styrene-butadiene copolymer resin with a styrene content of substantially
80 to 99 percent and (b) 50 to 20 parts, by weight, of a polymethyl
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methacrylate resin, and drying the coated crumb at a temperature not in
excess of substantially 110C.
Various resinous polymers may be employed as fused thermoplastic
partitioning agents for elastomeric crumb materials, such as polystyrene,
polymethyl methacrylate, polyacrylonitrile, polyvinyl chloride, polyethylene,
and various copolymers and terpolymers of styrene, butadiene, acrylonitrile,
methyl methacrylate,
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etc. The satisEactory use of a resinous polymer for the purpose
of this invention depends on its agglomeration temperature. The
agglomeration temperature is that temperature at which particles of
the resin dispersed in water will soften and mass together. For
the process for applying the coating of thermaplastic resin to the
rubber crumb in aqueous slurry as included herein to be practical
and economical it should be conducted at slurry temperatures below
about 99 C. Higher temperatures may be used but they require the
process to be conducted in a pressure vessel under pressure. ~he
coating is best applied at a slurry temperature at or near the
agglomeration temperature of the resinous coating material.
The Neubert patent refers to the use of polymethyl meth- --
acrylate as a partitioning agent. Polymethyl methacrylate, alone,
with an agglomeration temperature over 100 C. can be applied to a
powdered rubber (up to about 1 mm in diameter) at temperatures
under the boiling point of water or slurries of a rubber crumb.
However, when it is attempted to apply the polymethyl methacrylate
resin as a coating to larger-size crumb particles of the elastomer,
under conditions such as included herein, much of the resin coagul-
ates by itself and forms free resin particles rather than forming
a fused coating on the crumb rubber.
Neubert refers to the use of a resinous styrene-butadiene
copolymer with a styrene content of 80-99 per cent by weight. He
refers to it as a preferred partitioning agent. Such copolymer may
be used in the fused blend described herein. Copolymer of a lesser
styrene content, such as 80-85 per cent is preferred for blending
with polymethyl methacrylate. Any such copolymer resin with an
agglomeration temperature such as 55-70 C. may be applied very
readily as a partitioning agent to the surface of crumb particles.
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However, its agglomeration temperature is so low that the resulting
resin-coated crumb particles must be dried at rather low tempera-
tures to avoid their sticking together during drying. A drying
temperature of above about 60 C. would cause the rubber crumb
particles coated with such copolymer to stick together. The use of
such a low drying temperature of about 60 C. is uneconomical
because of the slow rate of evaporation of the water held in the
crumb. On the other hand, polymethyl methacrylate resin, while not
adhering to the surface of the particles as well as the styrene-
butadiene copolymer will permit the crumb coated with it to bedried at a much higher temperature of, for example, 95 C. or
higher, without sticking together to any appreciable extent, due to
its very high agglomeration temperature.
Whereas polymethyl methacrylate is a preferred ingredient
to be used in the partitioning agent of this invention, other resins
of very high agglomeration temperatures could be used in place of
it, such as, for example, polystyrene or polyvinyl chloride, as well
as a copolymer of styrene and acrylonitrile. Any of the other such
polymers may be modified by the inclusion of another monomer such
as acrylonitrile, etc.
The fused resin blend of this invention is applied as a
partitioning agent in a hot slurry of the crumb. The slurry may be
formed by coagulation of the elastomeric latex or by dispersing
pellets or ground crumb in water. This slurry must contain a co-
agulant, either an excess from the coagulation of a rubber latex,
or that adde~ when a slurry of pellets or ground crumb is prepared.
The coagulant may, for example, be aluminum sulfate, magnesium
sulfate, calcium chloride, calcium oxide, barium hydroxide, etc.,
or a mixture of sodium chloride and sulfuric acid used most often
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for SBR coagulation. The concentration of the rubber crumb and the
nature and concentration of the coagulant in the slurry will vary,
and may be anything which is usual in the industry. In the case of
a crumb slurry prepared by coagulating a rubber latex the coagula-
tion and the formation of the fused resin coating may be carried out
either batchwise or continuously.
It has been discovered that blends of polymethyl meth-
acrylate resin and styrene-butadiene copolymer resin have agglomera-
tion temperatures intermediate between those of the individual com-
ponents. Therefore, a blend ratio may be selected which will givea fusion of all or nearly all of the resin blend on the surface of
the crumb particles at a reasonable slurry temperature of, for
example, 90-95 C., and provide a coating with a high enough soften-
ing temperature that the coated crumb may be dried at a high temper-
ature of, for example, 85 C. or higher. Thus, advantages of both
blend components are obtained.
The two components of the partitioning agent are used in
a ratio of about 50 to 80 parts by weight of the styrene-butadiene
copolymer and 50 to 20 parts by weight of the polymethyl methacryl-
ate; and preferably 70 parts of the copolymer and 30 parts of thepolymethyl methacrylate~ Thus the agglomeration temperature of the
blend may vary substantially, and generally will be in the range of
about 85 to 95 C., and preferably substantially 90 C.
In the practice of the invention the slurry of the rubber
crumb particles with coagulant present is heated to a temperature
near or above the agglomeration temperature of the resinous blend
of the partitioning agent. The two resinous components of the blend
in the form of aqueous latices are blended and diluted to any usable
concentration such as, for example, 0.1 per cent to 10 per cent of
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the weight of the latex, more or less, and preferably 2 per cent
by weight total solids, and heated to a temperature near that of
the slurry of the coagulated rubber crumb. Then the hot, dilute
resin latex is added slowly to the hot rubber crumb slurry with
mild agitation. Under these conditions the resins in the latex
blend are coagulated by the coagulant in the rubber crumb slurry
and adhere to the surface of the rubber particles. Because the
slurry temperature is near or above the agglomeration temperature
of the resin blend, the components of the resin soften and fuse, and
surprisingly, instead of the polymethyl methacrylate particles co-
hering to one another the resin blend adheres to the surface of the
rubber particles. The concentration of the blend in the slurry may
vary widely and should be regulated so that the amount of the blend
present is sufficient to coat substantially the entire surface of
the elastomeric particles.
The resin coating thus formed on the surface of the
otherwise tacky rubber crumb particles is an effective partitioning
agent. When the water is drained or filtered from the slurry of
the coated crumb particles and they are dried; the particles resist
sticking together if they are not held at too high a temperature
after becoming essentially dry. The wet particles may enter a
tunnel drier at a temperature considerably above the agglomeration
temperature, such as 110 to 115 or 120 C. or thereabout, provided
that as the water is removed the temperature is lowered, and the
final drying may be accomplished at a temperature not over 85 C.
or 95 C. or thereabout; depending upon the partitioning agent.
Tunnel drying is not necessary, and in any method of drying the
final temperature is critical and should not exceed about 80 to
95 C., or somewhat higher depending upon the composition of the
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partitioning agent, etc.
Usually the ratio of the weight of the coating resin to
the weight of the elastomeric particles would be in the range of
perhaps 1 to 25 per cent or thereabout, and preferably 2 to 10 per
cent, depending on the particle size and the p~rticle surface area.
Several e~amples Eollow. Examples 1 and 2 relate, res-
pectively, to the prior-art use of polymethyl methacrylate and
styrene-butadienc copolymer alone.
EXAMPLE 1
~n SBR 1502 type latex is coagulated with salt and acid.
To 300 ~. of agitatin~ l~te~ containing 60 g. dry weight of rubber,
60 ml. of a solution containing 15 g. of NaCl is added and enough
of a 1 per cent solution of H2S04 to lower the pH to about 5. This
forms a slurry of elastomeric crumb particles of perhaps 1-10 mm.
diameter in water. This slurry is heated to 95 C. An aqueous
latex of polymethyl methacrylate (RHOPLEX B85 manufactured by Rohm
& Haas) with an ags~lomeration temperature of > 100 C. of 3 g. dry
weight (5% of weight of rubber) is diluted to 2 per cent total
solids, heated to 85 C. and added slowly to the agitated slurry of
rubber crumb~ Under these conditions the polymethyl methacrylate
resin is coagulated by the residual salt and acid in the slurry and
a portion fuses on the surface of the crumb particles. When this
ls dried at 95 C. the coated rubber particles do not stick together,
but a considerable portion of the polymethyl methacrylate coagulates
separately and makes the product dusty. This partially fused coat-
ing is not nearly as effective in partitioning the rubber crumb as
if all the resin were fused on the surface of the crumb particles.
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EXAMpLE 2
A coagulated slurry of SBR rubber crumb is prepared as
in Example 1 and heated to 70 C. Eight ml. of a styrene-butadiene
resin latex with 38 per cent total solids, a styrene content of
82O5 per cent, and an agglomeration temperature of 65 C. is
diluted with water to 2 per cen~ solids and added slowly to the
agita~ing slurry. The resin coagulates and almost all of it fuses
together on the surface of the elastomer crumb particles. ~en
this is dried at 65 C. no free resin is evident but the rubber
particles stick togetherO
EXAMPLE 3
A slurry of rubber particles is prepared as in Example 1
and heated to 90 C. A blend of resinous latices of polymethyl
methacrylate with an agglomeration temperature of ~ 100 C. and a
styrene-butadiene copolymer with a styrene content of 82.5 per cent
and an agglomeration temperature of 65 C. is made and diluted to
2 per cent with water. The ratio of polymethyl methacrylate to the
copolymer resin is 30/70 and the agglomeration temperature of the
blend is 90 C. The dilute latex blend containing 3 g. dry weight
of resin is heated to 85~ C. and added slowly to the agitating
rubber crumb slurry. The resins coagulate and fuse on the surface
of the crumb particles. When the particles are dried at a tempera-
ture not over 90 C., there is little or no sticking of the parti-
cles, and little or no free resin present.
EXAMPLE 4
A bale of 1712 SBR-type oil extended rubber is ground to
give a crumb of about 1/8-1/4 inch in particle size. Two pounds
oE this ground crumb is slurried in ten pounds of water containing
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27 grams of MgS04 7H20 coagulant. The slurry is heated to 90 C.
A two ~er cent latex of a 70/30 mixture by weight of styrene-buta-
diene copolymer and polymethyl methacrylate latices containing
four parts of resin per 100 parts of rubber crumb is prepared as
in Example 3 and added to the rubber crumb slurry. The resin coats
the surface of the ground rubber to serve as a partitioning agent.
The water is drained off and the coated crumb is dried at 85 C.
EXAMPLE 5
To a cold NBR latex with 32 per cent bound acrylonitrile
and 68 per cent butadiene; a Mooney viscosity, MlL-4 at 212 F., of
84; and a solids content of 20.5 per cent is added an aqueous dis-
persion of ~ finely ground SRF (ASTM N762) carbon black. The black
amounts to 75 per cent by weight of dry rubber solids. A 1 per cent
solution of sulfuric acid is added to coagulate the black master-
batch in the form of a crumb. If needed to complete the coagulation
of the NBR latex and clear the serum a coagulation aid such as Poly-
amine H, made by Union Carbide, is added. The rubberlblack crumb
.
is washed and fed to an extruder dryer. It is extruded through
a die with holes l/16-inch in diameter and cut with a cutter
into dry pellets of 1/8-inch diameter and about 1/8-inch length.
~orty pounds of these pellets are put into 250 pounds of water,
and eight pounds of sodium chloride and 33 grams of concentrated
sulfuric acid are added. The slurry is heated to 90 C. The 70/30
styrene-butadiene/polymethyl methacrylate resin blend in the form
of a hot 2 per cent latex is prepared as in Example 3 to give 4
parts/lOO parts of the rubber black masterbatch. It is added to the
hot rubber pellet slurry and the resin coats the surface of the
pellets. The pellets are dried at 85 C. without sticklng together
and resist massing during storage.
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EXAMPlE 6
A rubber Iatex is coagulated continuously and the re-
sulting rubber crumb particles are coated continuously with a fused
thermoplastic partitioning agent ln this example. Two thousand
nine hundred seventy pounds of a cold SBR 1713 latex with a solids
of 19.9 per cent, a MlL-4 at 212 F. of 135, and a bound styrene
content of 23.5 per cent is put into an agitated vessel. Five
hundred ninety pounds of a 50 per cent emulsion of a naphthenic,
non-staining processing oil and a suitable antioxidant is added to
the latex. This is 50 parts of oil/100 parts of rubber solids.
The latex/oil blend is heated to 55 C. In another vessel a 0.7
per cent solution of A12(SO4)3-18H20 (alum) is prepared and heated
to 60 F. In a third vessel 2~0 gallons of water are heated to
93 C. with steam. Ninety seven pounds of a styrene-butadiene co-
polymer latex with 36.2 per cent solids, and 39 pounds of a poly-
methyl methacrylate latex of 38.3 per cent solids are added. This
makes a 2.3 per cent solids latex of the 70/30 blend of the two
resin partitioning agents. This dilute latex is maintained at a
temperature of 82 C. or higher. The oil/rubber latex mixture is
run continuously to a 70-gallon coagulation tank at a rate of 0.95
gallons/minute, or a dry rubber rate of 130 pounds/hour. Alum
solution and dilution water are also run continuously into the co-
agulation tank at 1 and 0.9 gallons/minute, respectively. The
contents of this tank are kept at 60-65 C. The alum coagulates
the rubber latex and forms a slurry of the rubber crumb in water
which overflows to a 400-gallon tank. In this second tank the
slurry is heated to 90 C. with steam, and the hot 2.3 per cent ~-
resin latex is added continuously- at a rate of 3.8 pounds/minute
which will give four parts/100 parts of the oil extended rubber.
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The hot slurry overflows to another 400-gallon tank where lt is
maintained at about 90 C., and another four parts of the resin
partitioning agent latex is added continuously at a rate of 3.8
pounds/minute of the dilute latex. The slurry of the coated rubber
particles then overflows to a 100-gallon tank where the coagulation
of the resin latex is completed. Substantially all of the resin
adheres to the surface of the particles. The slurry overflows to a
continuous vacuum filter where the water is drained off the crumb
and washing takes place. The wet rubber particles are then fed to
a continuous apron or tunnel dryer with an air temperature of
85 C. There is some sticking together of the resin-coated crumb
on the apron of the dryer, but the dry material crumbles apart
easily to a; free-flowing rubber crumb.
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