Note: Descriptions are shown in the official language in which they were submitted.
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FLOWABLE ELASTOMER GRANULES
FIELD OF THE INVENTION
This invention relates to a process for producing polar, flowable rubber
granules with
the aid of powdering agents, which are rendered hydrophobic at the surface.
BACKGROUND OF THE INVENTION
The provision of elastomers or of polymers in general as granular material is
very de-
sirable, because this form of supply permits further processing in
continuously
operating machines. In most cases, higher throughputs are thus achieved. In
many
processing operations, only continuous mixers (extruders) are available, so
that only
those raw materials, which permit a continuous feed, can be processed.
In order that there can be elastomers available for these processing
operations, it is
necessary to provide granular material.
However, the problem arises in that sticky polymers can indeed be granulated
by
means of the known techniques, but that the granules rapidly agglutinate or
become
blocked. This problem occurs, in particular, with elastomers which, owing to
their
comparatively low or non-existent crystallinity and to low glass transition
tempera-
tures, are both sticky and may have a cold flow.
Typical representatives of such polymers are copolymers of ethylene and vinyl
ace-
tate or of ethylene and acrylates.
In the production of granules of these polymers, it is prior art, after or
during the
granulation process, to add powdering agents, which are intended to prevent
the sub-
sequent agglutination. These powdering agents are mostly inorganic substances
or
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mineral powders such as silicas, talc and various silicates, such as
wollastonite, kao-
linite or mica or even finely-ground thermoplastics such as PE. The latter is
de-
scribed, for example, in EP-A 100,434.
EP 575,900 describes the use of micronized PE waxes for separating
polyisobutylene
granules.
WO 93/23458 describes the use of anti-sticking agents for ethylene-acrylate
copoly
mers containing at least 50% incorporated ethylene. These agents are low-
molecular
compounds having definite melting points.
It is also possible, prior to the granulation of the polymer melts, to add
certain release
agents which, after the granulation process, migrate to the surface and there
form a
film which prevents agglutination.
These agents are typically also used as slip agents in the production of
films. Appar-
ently, these substances only work within a narrow application range. U.S.
Patent
4,510,281 (Reissue 32,325), EP 68 148 claim, for example, the use of ethylene
bisoleylamide only up to EVA copolymers having a VA content of 55%.
What is common to all these documents, however, is the fact that exclusively
hydro-
philic or untreated powdering agents are used.
Polar polymers, in particular, place high demands on the powdering agents and
these
demands are not satisfied by the disclosures in the prior art. The
disadvantages of the
known powdering agents include a high water absorption of the granular
material, the
large quantities of powdering agent which are required for an adequate release
effect
(more than 30 phr powdering agent in EP-A 100,434) and, in the case of organic
powdering agents such as carbon black or polyethylene, either the
impossibility to
manufacture colored articles or handling problems due to dust explosion
hazards.
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Ensuring a flowability in the case of EVA, however, is a problem,
particularly, where
the VA content is high, that is, distinctly higher than 45% and up to 95%.
SUMMARY OF THE INVENTION
Accordingly, the object was to find powdering agents which exhibit a
satisfactory
activity in the case of polar polymers and copolymers.
A further object was to decrease the water absorption of the powdered
granules.
A further object was to improve the dye receptivity of powdered polar
polymers.
Another object was to avoid the disadvantages of the powdering agents known in
prior art. Another object was to provide an alternative process to the
processes
known in prior art for the production of flowable granular materials.
These objects were completely or mainly achieved by a process for producing
flow-
able polar rubber granules, characterized in that a powdering agent which is
rendered
hydrophobic at the surface is used.
DETAILED DESCRIPTION OF THE INVENTION
Polar rubbers are defined as rubbers containing polar repeat units in the
polymer
chain. These induce a dipole moment in the repeat units. Typical polar
monomers are
chloroprene, acrylonitrile, vinyl chloride, vinylidene chloride, vinyl
fluoride, acrylic
acid and their analogues, such as methacrylic acid, methyl methacrylate, vinyl
ace-
tate, acrylamide, methacrylonitrile, as well as vinylpyrrolidone, and
vinylcarbazole.
These monomers and other polar monomers are well known to the person skilled
in
the art. Polar and non-polar monomers are frequently copolymerized to form
polar
rubbers. Suitable non-polar monomers are, for example, a.-olefins such as
ethylene,
propylene, butene, hexene, octene, diolefins such as butadiene, pentadiene,
hexadi-
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ene, octadiene, methyloctadiene or isoprene. Other non-polar comonomers are
known
to the person skilled in the art.
Preferred polar rubbers are polychloroprene, acrylonitrile-butadiene
copolymers,
styrene-acrylonitrile-butadiene copolymers, ethylene-vinyl acetate copolymers
(such
as LEVAPREN~) and ethylene-acrylate copolymers (such as VAMAC~) or mix-
tures of these.
The polar rubbers are generally used in uncrosslinked or partially crosslinked
form.
The person skilled in the art also refers to partially cross-linked rubbers as
being pre-
vulcanized, the degree of cross-linking being low enough not to impede the
good
processing properties.
All particulate materials which decrease or completely neutralize the
stickiness of the
rubber particles are suitable for use as powdering agents. These materials
must not
change in shape at the storage temperature; in particular, the melting point
or the
glass transition should be above the storage temperature. At the same time,
the pow-
dering agents must not interact adversely with the rubber particles during the
further
processing or the final application of the rubber.
Powdering agents can generally be classified as organic and mineral powdering
agents. Within the scope of this invention, mineral powdering agents are
preferred, as
under adverse conditions organic powdering agents can give rise to dust
explosions.
The chiefly preferred organic powdering agents are polyolefin powders, such as
poly-
ethylene or polypropylene, and carbon black.
The chiefly preferred mineral powdering agents are unreinforced and reinforced
fill-
ers known to the person skilled in the art, such as silica, in particular,
precipitated
silicas, talc, clays, mixed silicates, in particular aluminium-containing
silicates, metal
oxides, in particular ZnO, Mg0 and, optionally precipitated, carbonates, in
particular
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calcium carbonates. The above-mentioned organic and mineral powders may, of
course, also be used in mixtures.
The powdering agents have to be rendered hydrophobic at the surface in order
to
achieve the significant improvement compared with prior art.
The person skilled in the art is acquainted with prior art processes for
hydrophobing
fillers. Conventional products which are suitable for the invention. are, for
example,
from the processes for the treatment of, in particular, siliceous fillers with
polysul-
fides of silyl ethers, in particular, bis(triethoxysilylpropyl) tetrasulfide,
described in
U.S. Patents 4,514,213 and 4,704,414. The processes for the treatment of
silicas with
alcohols disclosed in U.S. Patents 2,736,669 and 2,801,185 are also suitable.
Hydrophobic fillers, in particular, hydrophobic silicas, are also commercially
avail-
1 S able.
Suitable powdering agents are particulate and have average particle diameters
in the
range of 2 to 50 Vim, in particular, of 2 to 20 pm, in each case based on the
so-called
DSO value (ASTM C 690-1992, Multisizer 100 p.m capillary).
The required quantity of the hydrophobic powdering agent is clearly below the
quan-
tities of 30 to 100 phr (EP-A 100,434) given for the known powdering agents.
As a
rule, a quantity of below 10 phr, or even less, of the hydrophobed powdering
agent is
sufficient. The precisely required quantity of hydrophobed powdering agent is
deter-
mined by the intended storage temperature, the size of the rubber particles,
the inher-
ent stickiness of the rubber and the intended use of the rubber granules.
Mineral
powdering agents are preferred, as they lessen the hazard of dust explosions.
Moreo-
ver, it is, of course, unnecessary that all the powdering agent present be
localized on
the rubber particles. Frequently, it can be advantageous to compensate for the
cold
flow of the rubber particles by means of additional free powdering agent.
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For many applications, it is advantageous to use a product which is white or
capable
of being dyed. For this reason, mineral powdering agents are often preferred
to car-
bon black or polyolefms, which have a poor dye receptivity.
S The person skilled in the art knows of numerous processes for producing
rubber
granules. At this point, reference may be made to the so-called underwater
granula-
tions. Here, a melt of the rubber to be granulated is pressed through a
perforated plate
and the granules are cut with a rotating knife. This is preferably.carned out
under
water, to ensure the rapid cooling of the granules and to avoid agglomeration.
It is also appropriate to use a bale of rubber granulated by means of a so-
called
Nielander mixer (U.S. 3,623,703). This process is described in EP-A2,100,434,
page
9 ff., to which reference is made herewith. A portion of the powdering agent
is pref
erably added during the granulating step.
The advantages of the described process are mainly the clearly decreased
quantity of
powdering agent required, which minimizes the effect on subsequent processing
steps. Rubber granules having improved flowability are produced. These rubber
granules absorb significantly less water than do conventional granules and are
emi-
nently suitable for continuous processing, for example, in extruders. The
rubber
granules according to the present invention can easily be recognized by the
fact that
they float in water.
The rubber mixtures according to the present invention may, of course, contain
still
further rubber auxiliaries, such as reaction accelerators, antioxidants, heat
stabilizers,
light stabilizers, antiozonants, processing agents, plasticizers, tackifiers,
blowing
agents, dyes, pigments, waxes, extenders, organic acids, reaction retardants,
metal
oxides, as well as activators, such as triethanolamine, polyethylene glycol,
hexane-
triol, which are known and conventional in the rubber industry. The rubber
auxilia-
ries are admixed in conventional quantities and depend on the intended use in
each
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case. Conventional quantities are, for example, quantities of from 0.1 to 50
wt.%,
based on the total weight of the rubber used.
Besides the previously mentioned rubber auxiliaries, the known cross-linking
agents,
such as sulfur, sulfur donors or peroxides, may also be added to the rubber
mixtures
according to the present invention. The rubber mixtures according to the
present in-
vention may, in addition, contain vulcanization accelerators, such as mercapto-
benzothiazoles, mercaptosulfenamides, guanidines, thiurams, dithiocarbamates,
thio-
ureas and/or thiocarbonates. The vulcanization accelerators and the cross-
linking
agents mentioned are conventionally used in quantities of from 0.1 to 10 wt.%,
pref
erably 0.1 to 5 wt.%, based on the total quantity of the rubber used in each
case.
These cross-linkable rubber mixtures are also provided by the present
invention.
The rubber mixtures according to the present invention can be vulcanized at
conven-
tional temperatures of 100°C to 200°C, preferably 130°C
to 180°C (optionally under
a pressure of 10 to 200 bar).
The further mixing of the rubbers with the other rubber auxiliaries, cross-
linking
agents and accelerators mentioned can be carried out in a conventional manner
by
means of suitable mixing units, such as rolls, closed mixers and mixer-
extruders.
The resulting mixtures may optionally be compounded and vulcanized in a conven-
tional manner as described in more detail, for example, in Encyclopedia c~f p
c-lymer
Science and Engineering, Vol. 4, page 66 ff (compounding) and Vol. 17, page
666 ff
(vulcanization). To improve the heat stability and the stability in storage,
preferably
the known phenolic, amine, sulfur-containing or phosphorus-containing
antioxidants
are added. These are described in more detail, for example, in Ullmanns En-
zyklopadie der technischen Chemie, Volume 8, page 19 ff.
The following Examples serve to illustrate the invention.
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F.1~ A MPI .F.C
The polar rubbers investigated were various ethylene-vinyl acetate copolymers
which
are commercially available under the trade name Levapren~ (Bayer AG).
S
In detail, the products used were:-
Levapren~ S00 HV, an ethylene-vinyl acetate copolymer produced by Bayer AG,
with a vinyl acetate content of 50%,
Levapren~ 600 HV, an ethylene-vinyl acetate copolymer produced by Bayer AG,
with a vinyl acetate content of 60%,
Levapren~ 700 HV, an ethylene-vinyl acetate copolymer produced by Bayer AG,
with a vinyl acetate content of 70%.
The following were used as powdering agents:-
Wessalon S, a precipitated silica from Degussa AG, having an average diameter
of 6
~m (D50 value), as a hydrophilic powdering agent,
Talc Luzenac No. 1 from Luzenac, likewise as an example of a hydrophilic
powder-
ing agent, having an average diameter of 15 pm (D50 value),
Sipernat D 10, a precipitated and hydrophobically modified silica from Degussa
AG,
having an average particle diameter of 5 pm (D50 value),
Sipernat D17, a precipitated and hydrophobically modified silica from Degussa
AG,
having an average particle diameter of approximately 10 um (D50 value),
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Coatylene HA 1681, a hydrophobic LDPE powder from Herberts, having an average
particle diameter of 17 Vim, melting point 105°C.
To investigate the flowability, the corresponding products were each removed
from
the production process immediately after the powdering and various powdering
agents were added in a simple powder mixer. In each case, 0.30 wt.% was added.
The
samples taken were used in the form of dry granules of 4 mm in size and on
their
surfaces they still bore 0.1 % talc, which is used as a granulating aid during
the un-
derwater granulation process.
Description of the production conditions for Levapren (T, P, etc.)
Ethylene-vinyl acetate copolymers with the trade name Levapren~ are produced
by
means of radical polymerization by a medium-pressure solution process. Further
de-
tails may be found in EP 341,499, EP 307,755, EP S 10,478 and EP 632,067.
The Levapren types used were produced as described above, freed from monomers
and solvent via degassing steps and the melt was pressed through the holes in
the
plate of an extruder head. The material was granulated underwater into pellets
of
approximately 3 m in size by means of a rapidly-acting knife. The water
contained
approximately 2% talc as a granulating aid and was also used as a transporting
me-
dium for the granules. The water was centrifuged off, the granules were dried
in an
air current and then, in each case, various powdering agents were added.
The flowability was investigated in each case by putting 300 g of granules
into a
plastics cylinder of 10 cm in diameter with a movable bottom plate and loading
the
granules with a 5 kg weight. After storage for a given period of time at given
tem-
peratures, the weight and the bottom plate were removed and a determination
was
made of the quantity of granules which fell out freely.
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Example 1
Various powdering agents were added to Levapren S00 HV as described above, the
samples were stored for 6 days at 40°C and then the flowability was
determined.
The granules containing Wessalon S and talc were firmly blocked and could be
re-
moved from the test cylinder only after vigorous shaking and finally scraping
by
hand. The two other samples fell unobstructed out of the cylinder. and were
accord-
ingly still flowable.
Wessalon S Talc Sipernat D10 Sipernat D17
after one minute:
10% free-flowingSO% free-flowingup to 100% free-flowing,
free-
90% blocked 50% blocked flowing some granules
blocked
Example 2
Various powdering agents were added to Levapren 600 HV as described above, the
samples were stored for 3 days at 40°C and then the flowability was
determined.
Wessalon S Talc Sipernat D10 Sipernat D17
after one minute:
had not fallen had not fallenup to 100% 80% free-flowing
free-
flowing
after three
minutes:
flowable after flowable afterup to 100% up to 100% free-
free-
shaking, but shaking, but flowing flowing
partially blockedpartially
blocked
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Example 3
Various powdering agents were added to Levapren 600 HV as described above, the
samples were stored for 5 days at 40°C and the flowability was
determined.
Wessalon S Talc Sipernat D10 Sipernat D17
immediately:
had not fallen 10% free-flowingup to 100% free-90% free-flowing
flowing
after three
minutes:
very blocked flowable afterfree-flowing free-flowing
shaking, but
partially
blocked
The sample containing Wessalon fell as a coherent block. The sample containing
talc
fell out of the cylinder after vigorous shaking. The samples containing the
Sipernat
powdering agents were freely flowable. The sample containing Sipernat D 10
fell out
of the cylinder somewhat more rapidly.
Example 4
Various powdering agents were added to Levapren 700 HV as described above, the
samples were stored for 4 days at 40°C and then the flowability was
determined.
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Wessalon S Talc Sipernat D10 Sipernat D17
immediately:
had not fallenas Wessalon up to 80% 100% free-flow-
S free-
flowing ing
after three
minutes:
very blocked, as Wessalon free-flowing free-flowing
S
partially flowable
after vigorous
shaking
The samples powdered with Wessalon S and talc were definitely blocked. The sam-
ples containing the Sipernat powdering agents remained flowable. The sample
con-
taming Sipernat D10 fell out of the cylinder somewhat more rapidly.
Example 5 (Comparison - similar to EP-A 068 148)
During the production of Levapren 600 HV, ethylene bisoleylamide was metered
into
the screw-type processor, a solution in vinyl acetate at a concentration of 5%
first
being added in order to achieve easier metering. The quantity of ethylene bi-
soleylamide was so calculated that the end product contained 4000 to 5000 ppm.
The product was granulated under water and packed into 25 kg sacks and stacked
onto pallets. These pallets were stored for 3 months at RT and the extent of
the
blocking was observed.
To this end, the pallets were repacked and the degree of blocking assessed on
the
basis of the sack in the lowest - that is, the most heavily loaded - position.
The sacks
were partially blocked and no longer freely flowable.
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Example 6
25 kg of Levapren 700 HV was powdered with 0.25 phr Wessalon S. A further 25
kg
was additionally powdered with Coatylene 1681 and both samples were put into
polyethylene sacks, packed in cardboard and both stored for 14 days in a hot
cabinet
at 40°C.
The product powdered with Wessalon S was completely baked and could not be fur-
ther processed.
The subsequently powdered product remained flowable and was therefore suitable
for further processing.
Example 7
Water absorption of the powdering agent:
Wessalon S had a water content of 4 to 5%, measured as loss on drying after 2h
at
105°C.
Sipernat D17 had a water content of 2%, measured as loss on drying after 2h at
105°C.
Hydrophobic powdering agents are also advantageous, because these do not
agglom
erate in the presence of moisture. Agglomerated powdering agents can lead to
defects
and voids when used in thin layers.
Although the invention has been described in detail in the foregoing for the
purpose of
illustration, it is to be understood that such detail is solely for that
purpose and that
variations can be made therein by those skilled in the art without departing
from the
spirit and scope of the invention except as it may be limited by the claims.
