Note: Descriptions are shown in the official language in which they were submitted.
ios~
It i~ known that rubber powder3 can be produced by
spray-drying a latex or by grinding ~olid rubber. In
either ca~e, the fluidity o~ the rubber powder, once it
has been produced, h~s to be ]kept i~tact by the Presence
of separating agents which prevent coalescence and cohesion.
The quantity in which the separating agent is added is, of
cour~e, dependent upon the taokines~ oi the rubber ~nd
upon the eP~ectiveness o~ the additive. Thus, the ~pray
drying of a latex in the presence o~ at least 10 % by weight
of diatomaceous earth i~ described in US Patent Speciiicatio~s
Nos. 2,315,924 and 3,194,781. In addition to heterogeneous
additive~, powder-~orm rubber~ of the type in que~tion also
contain considerable quantitieR o~ emulsi~iers arising ~rom
their production which are potentially detrimental to the
propertie~ of the materials produced ~rom the rubber. The
relatively high degree of swelling whioh vulcanisate~ ~uch
as the~e undergo in water i5 mentioned as an ex~mple.
According to British Patent Speci~ication No. 1,079,976,
a rubber powder i9 obtained by using a poly~acch~ride,
especially starch~ and an inorganic additive, especially
talcum. In addition, US Patent Speciiication~ No9.
1,215,918; 1,204,405 and 1,148,348 and Briti~h Patent
Speci~ication No. 1,113,348 de~cribe prooe~es ~or the
production oi powder-~orm rubber which, through anti-
sticking agents added to the latex in certain precipitation
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and working-up processes, are sAid to produce the pOwaer-
form rubberl precipit~ted i~ powder or granular iorm, in
that condition.
Another possible method ~ producing powder-~orm
rubber is to ~tart with the solution polymer. In thi~
method, which i~ described in DT-OS No. 2,214,121, more
or le~ large quantities o~ eillers and emul~i~iere are
added during the process which is terminated by a
precipitation ~tep.
In addition, solid rubber h~s already been ground
below its glas~ temperature ior the production o~ rubber
powder (Journal o~ Applied Science, 10 (1966), p~ge~ 955
959)-
In the above-mentioned processes) which generally
start with the late~ of an emulsion or solution polymsr9
mare or less het~x~e~eous a ~ tives are w~rked into the r~bber,
ior e~mple by spray drying. Unfortunately~ the relatively
large quantities oi additives required for maintaining a
high degree oi ~luidity aiXect the character of the rubber
and its processing a~d vulcani~ation properties.
All conventional methods ior the production oi rubber
powder require relatively large quantities of heterogeneous
additives so that inter alia the choice of the s~ating agents
i9 oi critical ~igni~icance eo far a~ the i'urther -
proceesing oi the powders i~ concerned. In particular,
inorganic lubricant powders alter certaln raw material
propertie~ such as by producing high a~h level~, but above
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all - especially in case~ where the powder-form rubber
i8 ~urther processed in solution (~or esumple rOr
producing adhesives~ - becau~le they are insoluble or
swellable in solvents and precipitate ~rom the ~olutions.
Or~c separating agents ~ ~so have the disadvantage
that they adversely a~fect the vulcanisnte values o~
elastom0rs in regard to their resistance to ~welling in
water and in industrial oils and also in machine and
engine oils, or they are dissolved by the corresponding
medium and contaminate it, so that ~econdary damage can
be caused. Another disadvantage of the separating agent
described in the literature is, ior e~ample, the ~a¢t
that they are not heat-re~istant and either ~often or
give off aggre~ive substances or are destroyed by
atmospheric oxygen. For example, the addition o~ polyvlnyl
chloride as a separating agent for powd ~ form nitrile n~r has
long bee~ known and is used on a commercial scale ior the
production of powder-i'orm rubber~. Uniortunately, the
addition o~ PVC limit~ the use$ul~ess o~ an elastomer.
The properties of ethylene-vinyl acetate copolymer~ or
even heat-stabilised acrylonitrile-butadiene rubber
vulcanisates would be greatly aifected by the ~ddition
of PVC ~nd their range of industrial application~ ~would be ~everely reetricted.
It ha~ now been iound that polyacrylonitr~le (PAN)
is a suitable separating agent in ~any respects. Polyacrylonitrile
has the adv~ntage from the commerci.al point o~ view that
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as an organic substance it is obtained during polymerisation in
such a fine particle size of from 5 to 150~ that it may be
directly used without any need for further size reduction. A
polyacrylonitrile powder having a grain size distribution of
from 10 to 80~ is particularly suitable.
The present invention relates to a freeflowing rubber
mixture which is stable against coalescence and cohesion com-
prising a vulcanisable rubber in powder form having a grain
size of from 1 to 3000~ and from 3 to 12~ by weight, based on
the rubber, of polyacrylonitrile having a particle size from
5 to 150~.
The polyacrylonitrile unexpectedly has an excellent
effect as separating agent. In addition to this, it is heat-
resistant and behaves favourably in contact with liquid media.
It does not dissolve or swell in apolar or substantially apolar
liquids such as, for example, industrial oils, so that it may
be used in elastomers of the type employed for the production
of adhesives. Completely clear or clouded solutions are
obtained, according to the polarity of the solvent used. The
separating agent is swollen and shows no tendency towards
sedimentation in these solutions. ;
This applies in particular to so-called special
elastomers, such as nitrile rubber, ethylene-vinyl acetate
elastomers and other elastomers, such as polychloroprene for
example, which are only soluble in polar solvents.
Unexpectedly, the separating agent is also highly
effective in apolar elastomers such as polybutadiene and
styrenebutadiene rubber with high cold flow, so that powderform
rubbers with extremely good free-flow properties are obtained.
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B According to the present invention any_ ~ d-or
synthetic rubber may be stabilized against coalescence and
cohesion. Suitable synthetic rubbers are homo- or copolymers
of conjugated dienes having from 4 to 8 carbon atoms and of
the corresponding monomers wherein one or more of the hydrogen
atoms have been replaced by halogen atoms. Examples for such
monomers are butadiene, isoprene, dimethyl butadiene, 2-chloro-
butadiene (chloroprene) and 2,3-dichloro butadiene. Likewlse
suitable are copolymers of these monomers with vinyl aromatic
compounds like styrene andd~methyl styrene, acrylonitrlle
and methacrylonitrile. Other suitable synthetic rubbers are
ethylene propylene terpolymers and copolymers of ethylene
with vinyl esters of C2-C20-alkane carboxylic acids. Polychloro-
prene and copolymers of ethylene with vinyl acetate, butadiene
with styrene and butadiene with acrylonitrlle are preferred.
In the rubber mixtures according to the present invention the
rubber particles have a grain size of from 1 to 3000 /u
preferably from 5 to 1600 /u.
Unexpectedly, the vulcanisation rate is also unaffected by
PAN. It may even be increased and not decreased as is the case
with some inorganic separating agents.
Another advantage of polyacrylonitrile as a separating
agent is above all the fact that the addition of PAN produces
hardly any changes in the properties, especially the Mooney
viscosities, of the raw materials.
The separating agent may be worked into the rubber, for
example during a grinding process, by precipitation processes
or during spray drying. It is generally added in a quantity
of from 3 to 12 ~ by weight and preferably in a quantity of
from 3 to 7 ~ by weight, based on the weight of rubber.
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When the polyacrylonitrile was added in conjunction
with other separating agents, for example zinc or calcium
salts of fatty acids, it was not possible to observe any
adverse effects upon the maint:enance of fluidity. ~n cases
such as these, the quantity i~l which the separating agent
according to the invention is added may frequently be
reduced.
Further additives which may be present in an amount of
from 3 to 12 % by weight, based on rubber, in the rubber
mixtures according to the present invention are reparating
agents, fillers and other additives known in the art, for
example metal oxides like siliciumdioxide, titanium dioxide
and magnesium oxide; carbon black; talcum, diatomaceous
earth, kaolin, silicic acid and salts of stearic acid like
zinc stearate and calcium stearate.
EXAMPLE 1
An ethylene-vinyl acetate copolymer with ~ vinyl
acetate content of 45 ~ by weight and a Mooney viscosity
oi 22 ME wae ground in the preeence oi 4 ~ by weight oi'
polyacrylo~itrile (P~N, p~rticle 9ize 10 - 80 ~U) in an
industrial 45 kilowatt ba~ile-plate impact mill. The
rubber powder obt~ined had a pack etrength (determined
in ~ccordance with A5TM-D 1937-62 T) oi 30 lb~. ~he
material showed excellent ~luldlty whlch it ret~ined
~or ~ever~l ~onthe.
Grain ei~e d~etributlon o~ the 0 - 0.5 mm = 21
rubber powder: 0.5 - 1 mm = 41
1 - 1.5 mm ~ 38
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The physicAl propertiee of a peroxidlcally croe~linked
vulcani~ate prod~ced irom the rubber powder were sub~tantially
unchanged in comparison with a vulc~ni~at0 oi tha ~tarting
material. ~here was no evidence oi any impairment to the
pero~idic cro~linking process.
A slightly olouded rubber solution was obtained in
toluene, but no sediment wa~ produced, even art0r prolonged
standing.
E~AMPLE 2
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A butadiene-acrylonitrile rubber with an ACN-content
of 28 % by weight and a Mooney viscosity of 59 ME was ground
in the pre~ence o~ 5 % by weight o~ PAN in the ~ame way
a~ described i~ E~ample 1.
A pack strength (determined in aocordance with
ASTM-D 1937-62 T) o~ more th~n 50 lbs was mea~ured ior a
grain ~i~e di~tribution oi the completed powder-~orm
rubber of: 25 % les~ than 0.5 mm
50 ~ ~rom 0,5 - 1 mm
25 ~ ~rom 1 - 1.6 mm
A ~ree-flowing rubber powder with extremely good
iluidity was obtained. The vulcani~ate value~ of the
rubber mi~tures produced irom thi~ powder were satisiactory.
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Styrene-butadiene rubber (SRR 1500) with a Mooney
vi~co~ity oi 51 ME was ground in the presence oi 7 ~ by
weight o~ PAN in the ~ame way as de~cribed in E3ample 1.
A pack strength (ASTM D 1937-62 ~) oi more than 3D lbs was
obtained.
The vulcanisate propertie~ o~ the test specimens
produced ~rom this rubber powder were satid~actory.
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The gr~n size distributio:n of the rubber powder was
as iollows: 24.5 96 up to 0.5 mm
36.5 oh ~rom 0.5 - 1 m~
49.0 ~ ~rom 1 - 1.5 mm
A iree-flowing rubber powder with extremely good
fluidity wa~ obtained.
E~MPLE 4
A polymer of 2-chlorobutadiene with a Mooney vi~co~lty
(according to DIN 53 523) ML 4 (100C) o~ 81 wae ground
in the prese~ce oi 5 ~ by weight oi polyacrylonltrile in
the same way a~ de~cribed in Example 1. A iree-~lowing
rubber powder with a pack strength oi more than 3D lb~
was obtained.
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