Method for making free flowing coated rubber pellets

Free flowing coated pellets are prepared by blending a first polymer with a second polymer wherein the second polymer is a polymer which has a crystalline or semicrystalline melting point which is at least 10.degree. C. above the melting or softening point of the first polymer, the two polymers being insoluble in one another in the melt state; (2) intimately blending the two polymers; and (3) extruding the blend through a die having an outlet die face which is maintained at a substantially lower temperature than the extruder melt temperature. In its preferred embodiment the first polymer is an elastomer and the second polymer comprises at least one crystalline or semicyrstalline plastic polymer. A typical composition comprises an EPM or EPDM rubber and a mixture of polyethylene and a crystalline copolymer of ethylene and propylene.

BACKGROUND OF THE INVENTION 
Many elastomers are tacky or exhibit cold flow in their green or uncured 
state. As a consequence, these materials cannot be transported in bulk as 
free flowing pellets but must be shipped in bales. This practice requires 
that the ultimate elastomer processor must be equipped to cut up or mill 
the bales. The necessary equipment is generally large scale, expensive 
equipment. Additionally, the bales cannot be readily preblended with other 
materials. The necessity for baling results in high handling and shipping 
costs. In order to facilitate handling and processing of elastomers, it 
has been considered desirable to produce elastomer pellets. Generally, 
however, elastomer pellets exhibit "blocking" or cold flow characteristics 
which result in solidification into a solid mass after a short storage 
time, especially at warm temperatures. 
Numerous attempts have been made to formulate elastomeric pellets which 
will remain free flowing until they are to be processed. Dusting the 
elastomeric pellets with inorganic materials, e.g., clay, talc, etc., has 
been found to extend the time over which the pellets are free flowing. 
Improved results have been achieved by dusting a coating with selected 
organic materials such as hydrocarbon waxes (British Pat. No. 901,664) or 
powdered polyethylenes and polypropylenes (British Pat. No. 928,120). 
However, because of the discontinuity of the dust coat, the coated pellets 
eventually flow together to form a solid mass. 
By blending the elastomer with a crystalline type polymer such as 
polyethylene, polypropylene or copolymers of ethylene and propylene, it 
has been possible to produce free flowing elastomer containing pellets. 
However, the elastomer content of the pellet must be less than about 65%. 
The product is, of course, not suitable for use in all elastomer 
applications. 
Another coating approach to the problem has been the coating of elastomer 
pellets with emulsions containing a tack free coating material. Coating is 
accomplished either by dipping pellets into the emulsion or spraying the 
emulsion onto the pellets. In either case the emulsion coating must be 
dried, and where the emulsion contains a solvent the solvent must be 
recovered. Drying and solvent recovery requirements result in increased 
costs. 
Melt-coating methods for producing free-flowing elastomer pellets have also 
been suggested. According to U.S. Pat. No. 3,669,772 to Bishop, coating 
can be accomplished by using a die, similar to wire coating die, into 
which a strand of rubber to be coated is fed simultaneous with melt 
coating material. A continuous melt coated strand of rubber issues from 
the coextrusion die outlet, is cooled in a liquid cooling bath, and is 
subsequently pelletized. This melt-coating method not only adds 
significantly to rubber manufacturing costs, but has limitations from the 
standpoint of efficiently producing large quantities of coated pellets. 
Pellets of rubber have been coated with various coating materials by 
heating the rubber pellet to a temperature which is higher than the 
melting point of the coating material, and then contacting the heated 
pellet with the coating material which is preferably in the form of a fine 
powder. The heated pellet fluxes the coating material on the surface of 
the pellet to form a substantially continuous coating. The hot coated 
pellet is then cooled. 
A study of bicomponent mixtures has shown that upon extrusion of the 
mixtures, stratification will occur. See Soulborn, J. H. and Ballman, R. 
L.; "Stratified Bicomponent Flow of Polymer Melts in a Tube", Applied 
Polymer Science, No. 20, 175-189 (1973). The authors attribute 
stratification to differences in the melt viscosity of the components. 
What is required by the art is a commercially viable method of producing a 
free flowing rubber pellet which contains a major portion of rubber. To be 
commercially viable, the process for producing the free flowing pellets 
must be readily adapted to conventional rubber pelletizing techniques. 
SUMMARY OF THE INVENTION 
It has surprisingly been found that a rubber pellet composition comprising 
an elastomer and plastic insoluble in the elastomer can be caused to coat 
itself with a plastic skin by control of composition, extrusion conditions 
and die temperature. The elastomer-plastic blend is extruded at a 
temperature above the melting point of the plastic, and pelletized as it 
exits from a die, the die having a temperature gradient across the die 
from inlet to outlet, the gradient being such that the die outlet 
temperature is substantially lower than the extrusion melt temperature 
(die inlet temperature). The resultant product is a pellet coated with a 
skin of plastic.

DETAILED DESCRIPTION 
This invention relates to a method for preparing a free flowing elastomer 
pellet. More particularly, it relates to a method for preparing an 
elastomer pellet which is free flowing by virtue of the fact that it is 
encased in a skin comprising a solid plastic material. 
In the practice of this invention an elastomer is blended with a semi 
crystalline or crystaline plastic material which has a melting point of at 
least 10.degree. C. higher than the softening point of the elastomer, 
preferably at least about 15.degree. C. than the softening point of the 
elastomer, preferably at least 30.degree. C., more preferably at least 
40.degree. C. The elastomer/plastic blend is then extruded through a die 
in which the die outlet is maintained at least 10.degree. C. below the 
melting point of the plastic in order to develop a temperature gradient 
across the die from die inlet to die outlet, preferably at least 
20.degree. C., more preferably at least about 30.degree. C. below the 
melting point of the plastic. 
Not wishing to be bound by theory, it is believed that as the melt 
temperature is reduced across the die, the difference in viscosity between 
the elastomer and the plastic is increased thereby causing stratification 
in a manner so as to cause the plastic to be concentrated along the 
surface of the die orifice while the central core becomes elastomer rich. 
Shear plays an important part in the stratification process as does the 
wall effect because of their effect on the velocity profile of the two 
components of the melt, and hence, the composition differences throughout 
the melt exiting the die. The melting point of the polymer is a function 
of shear and pressure, and is higher in the dynamic system of an extruder 
die than the static melting point of the polymer. As used in the 
specification and claims, "melting point" will mean the normal static 
melting point or softening point of the polymer. 
The elastomer/plastic polymer composition is extruded through a 
multi-orificed strand die in the aforedescribed manner and pelletized 
either by use of a strand pelletizer or by using a die face pelletizer. In 
one embodiment a conventional strand die is modified by having the die 
outlet plate cored so that it can be water cooled. In a preferred 
embodiment the die cooling is accomplished by using an underwater 
pelletizer. Typical of these underwater pelletizers is the mini underwater 
pelletizer (MUP) manufactured by Gala Industries, Inc., Eagle Rock, VA. 
Since the stratification process by which a pellet coated with a skin of 
plastic is formed requires a finite time the L/D ratio of the die outlet 
holes is an important criterion in carrying out the process of this 
invention. The L/D ratio can be about 2 to about 20, preferably about 2.5 
to about 12, more preferably about 3 to about 10, e.g., about 3.5 to about 
8. The die outlets through which strands of elastomer/plastic blends are 
extruded can be converging tubular outlets which have a larger diameter 
inlet than outlet. In that event, the L/D ratio is based on an average 
outlet diameter over the length of the channel. 
The length of the outlet channel can be about 1 inch to about 4 inches, 
preferably about 1.5 to about 3.5 inches, more preferably about 2.0 to 
about 3.0 inches, e.g., about 2.5 inches. The diameter of the die outlet 
orifice can be about 0.05 to about 0.200 inches preferably about 0.075 to 
about 0.150 inches, e.g., about 0.125 inches. 
A critical parameter in carrying out the process of this invention is the 
temperature gradient across the die from the inlet to the outlet. While no 
particular temperature gradient is required, at some point within the die 
the melt temperature must be reduced to a temperature which is preferably 
at about the melting point of the plastic in order to insure that there is 
a significant difference between the viscosity of the plastic melt and the 
viscosity of the elastomer melt. It is not essential that the melt 
temperature of the composition be below that of the plastic melt point. In 
a preferred embodiment, however, the melt temperature of the composition 
is reduced to a temperature which is below the melting point of the 
plastic component. In the preferred method of carrying out the process of 
this invention, an underwater pelletizer is used and the temperature 
gradient across the die is created by cooling the face of the die. 
The maximum temperature differential across the die is achieved by 
operating at or about the plugging temperature of the system. The 
"plugging temperature" is that temperature at which some of the die outlet 
orifices begin to be plugged by solidified polymer. Some plugging of a 
multi-orifice die can be tolerated up to the point where flow rate is 
decreased below economical rates. Generally, the outer outlet holes in the 
die will plug first. A multi-orifice die will have twenty or more outlet 
holes, e.g., 50-100. It is possible to operate the die at the plugging 
temperature with as much as about 20-30% of the holes plugged. 
The plugging temperature is determined by gradually cooling the die or die 
face to the point where outlet hole plugging begins to occur. Operation at 
the plugging temperature achieves the maximum stratification and plastic 
skin development in the elastomer pellet. 
Where an underwater pelletizer is used, the cooling water temperature will 
be about 20.degree. C. to about 50.degree. C. The "extruder melt 
temperature" (the die inlet melt temperature) will be about 160.degree. C. 
to about 250.degree. C. and will depend on the elastomer and plastic 
selected. The appropriate extruder melt temperature for various 
plastic/elastomer compositions is known to those skilled in the extrusion 
art. 
The process of this invention is particularly suited to those elastomers 
which are tacky in their solid state or exhibit cold flow. Illustrative, 
non-limiting examples of the elastomers to which this invention may be 
applied are high molecular weight elastomers having a Tg of less than 
0.degree. C., e.g. ethylene-propylene rubber (EPR), terpolymers of 
ethylene, propylene and a non-conjugated diene (EPDM), natural rubber, 
polyisobutylene, butyl rubber, halogenated butyl rubber, 
arylonitrile-butadiene rubber (NBR) and styrene butadiene rubber (SBR). 
The plastics which may be utilized in the practice of this invention have a 
crystalline melting point of at least 70.degree. C. Illustrative of those 
plastic polymers are high density polyethylene (HDFE), low density 
polyethylene (LDPE), polypropylene (PP), linear low density polyethylene 
LLDPE, syndiotactic polybutadiene resin (SBD), polybutene-1 and 
crystalline copolymers of ethylene and other alphaolefins. The plastic and 
elastomer must be insoluble in one another in the melt state. 
The elastomer-plastic polymer composition of this invention can comprise 
about 15 to about 90 weight percent plastic polymer, e.g., about 20 to 
about 80 weight percent. Where the product desired is an elastomeric 
product the plastic polymer comprises about 15 to about 35 weight percent 
of the composition; preferably about 15 to about 30 weight percent, most 
preferably about 20 to about 28 weight percent, e.g., about 25 wt.%. 
The plastic and polymer may be blended in any conventional manner and fed 
to an extruder. For example, an elastomer bale can be shredded and blended 
with plastic polymer powder in a ribbon blender and subsequently fed to an 
extruder. Preferably a mixing extruder, e.g., twin screw extruder is used 
for the extrusion to insure complete mixing of the elastomer and plastic. 
The mixture is extruded out of a conventional multi-orificed die in which 
the die face is maintained at a temperature of at least about 10.degree. 
C. below the melting point of the plastic polymer. Preferably the die face 
is maintained at a temperature at least about 10.degree. C. below the 
melting point of the plastic polymer; more preferably at least about 
20.degree. C.; most preferably at least about 30.degree. C. below the 
melting point of the plastic. Of course, in view of the high melt 
temperature of the polymers the entire die plate cannot be maintained at a 
single temperature, and there will be a temperature gradient across the 
die from its internal inlet surface to its outer face at the outlet of the 
die. 
To demonstrate the effectiveness of the instant invention, an 
elastomer-plastic polymer composition having the formulation shown in 
Table I was extruded through a conventional multi-orificed strand die and 
cooled by passing the polymer strands through a water bath. Subsequently, 
the strands were pelletized. Additionally the same formulation was 
pelletized using an underwater pelletizer. 
The underwater cut pellets had a plastic skin, a lower coefficient of 
friction and were more free flowing than the conventional strand 
pelletized material. Table II compares the coefficient of friction of the 
two products, and Table III shows the pressure/strength ratio for the 
compositions. The pressure/strength ratio is the ratio of the 
consolidation pressure to yield strength under the shear required to 
create pellet flow. A higher ratio is indicative of a more free flowing 
pellet. 
TABLE 1 
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Elastomer/Plastic Composition 
Elastomer.sup.1 : 40% by weight 
Plastic Polymers 
HDPE.sup.2 16% by weight 
Polypropylene.sup.3 44% by weight 
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.sup.1 An ethylene propylene copolymer containing 43% ethylene, having a 
glass transition temperature of 55.degree. C., and having a mooney 
viscosity of 25 (1 + 8 at 127.degree. C.). 
.sup.2 AB 55-100; a 10 melt index HDPE polymer. 
.sup.3 An isotactict polypropylene reactor copolymer of propylene and 
ethylene having a crystalline melt temperature of 160.degree. C.. 
TABLE II 
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Wall Friction Angle (Degrees) 
Process Stainless Steel 
Aged Carbon Steel 
______________________________________ 
A Conventional 22 22 
strand cut 
B Underwater cut 
13 18 
(skin) 
______________________________________ 
TABLE III 
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Yield 
Consolidating Strength Pressure/ 
Process Pressure (psi) 
(psf) Strength ratio 
______________________________________ 
A 386 135 2.49 
B 272 18 15.1 
______________________________________ 
In preparing the underwater die cut pellets of this invention, the MUP was 
operated with an extruder melt temperature of 408.degree. F., an extruder 
pressure of 1600 psi and cooling water temperature of 105.degree. F. The 
extruder output rate was 125 lbs/hr. 
It is evident from the above data that pellets made according to the 
process of this invention are more free flowing than strand pelletized 
material even where the compositions are identical. FIGS. 1A and B are SEM 
micrograph comparisons of the strand formed pellets (FIG. 1A) and the 
underwater cut pellets (FIG. 1B). The rubber phase of the composition was 
extracted with hexane. As can be seen from the micrograph of FIG. 1B, the 
product produced according to the process of this invention has a skin 
which is substantially all plastic polymer while the pelletized strands 
are essentially of uniform composition throughout. 
In order to demonstrate that the skin of the pellets of this invention are 
substantially all plastic polymer, the pellets were treated with hexane to 
extract the rubber phase. A comparison of FIGS. 2A and 2B show that rubber 
was extracted from the surface of the conventional strand pelletized 
material, whereas substantially no rubber was extracted from the skin of 
the pellets prepared according to the process of this invention.