Note: Descriptions are shown in the official language in which they were submitted.
NATURAL RUBBER CONTAINING COMPC)SITIONS
WITH INCREASED TEAR STRENGT~
Technical Field
This invention relates to increasing the tear
strength of cured ~lastomeric compo~iti~ns containing
natural rubber.
It is especially useful in providing cured
elastomeric compositions for use in making articles
containing natural rubber that are subject to dynamic
defoxmation, especially tear stress. These compositionæ
are useful in manufacturing tank track pads ~the rubber
blocks glued to metal tank tracks), belt skims (the rubber
sheets laminated with steel cord to provide the st~el belt
for a tire), and treads for tires for trucks and o-road
vehicles.
Background Of The Invention
Conventionally, articles requiring high tear
strength and tear resistance are fabricated o~ natural
rubber (rather than ~nthetic rubber) with tear strength
bein~ increased further by increasing the ~mount of
reinforcing filler or increasing the cross linking by
utilizing more sulfur.
Summary Of The Invention
It has now been discovered that inclusion of
selected amounts of particular liguid carboxy-texminated
unsaturated butadiene-acrylonitrile copolymers ~sometimes
referred to hereinafter as CTBN~ increases the tear
strength of natural rubber containing compo~itions.
Contrary to con~entional alternativ~s, the composition of
3~
the invention allows the inclusion of preferred amounts of
r~inforcing filler and the use of normal cross linking
conditions without deleteriou~ effect on tear strength and
instead is characterized by increased tear strength
compared to a natural rubber composition without the CTBN
additive. Contrary to conventional alternatives, the
composition of the invention allows the flexibility of
including synthetic rubber in place of some of the natural
rubber while maintaining the high tear strength normally
associated with natural rubber compositioIls.
It has further been discovered that the inclusior
of the selected amounts of the CTBN increases the hardness
of cured compositions and reduces the viscosity (thus
aiding processing) ~f uncured composi-tions.
Compositions herein are elastomeric comp~sitions
which are curable and comprise:
(a) lO0 parts by weight polymer consisting of
~i) from about 95 to about 25 parts by
weight of natural rubber,
¢ii) from 0 to about 65 parts by weight
of syntheti~ rubb~r,
(iii) from about 5 to about 25 parts by
weight of liquid carboxy-terminated
butadiene-acrylonitrile copo~ymer having a functionality
ranging from about 1.5 to about 2.5, an ac~ylonitrile
content ranging from about 10% to about 40% by weight and
a Brookfield viscosity at 27~C. ranging from about 50,000
to about 200,000 centipois~,
~b) from about 30 to about 120 parts by weight
of reinforci~g filler.
The aforementioned ingredients are used with a
conventional curing system to provide a cured compo~ition
within the scope of the invention which exhibits the
aforementioned very desirable attributes.
In certain embodiments of the invention, the
aforementioned ingredients (a) and (b) can be kept apart
~ 7 ~
rom at least part of the curing system until just prior
to curing. Therefore, a composition comprising (a) and
(b) is one embodiment while the cured composition is
another. The former can be sold as a separate item of
commerce to be later combined with the cllring system.
The term "phr" as u~ed hereinafter refers to
parts by weight per hundred parts by weight of polymer
consistin~ of natural rubber, any synthetic rubber present
and the CTBN.
Detailed Description
Th~ greatest tear strengths and re~istances
obtained ~hen the only elastomer presenk lb~sides CTBN) is
natural rubber rather than a blend of natural rubber with
synthetic rubber. The term "natural rubber" is used herein
to mean elastomeric substances obtained from trees and
plants such as the well-known hevea and guayule rubbers.
However, very acceptable tear strengths can be
obtained even wh~ some of the ~atural rubber is replaced
by synthetic rubber and thu~ blends of natural ~nd
~0 synthetic rubber.are also included within the scope o
useful embodiments. The synthetic elastomer can ~e any of
those normally blended with natural rubber, e.g.
polybutadiene, polystyrene-butadiene, neoprene and EPDM
elastomers. CompositiQns including styrene-butadiene
rubber at a le~el of about 45 to about 65 phr weight o:E
polymer are very useul herein.
q~rning to the liquid CTBN ingredient, it
normally i~ a liquid at ambient temperatures (say 5-35C)
and preferably utilized in an amou~t ranging from about 5
to about lS phr. Preferably the CTBN has an acrylonitrile
content ranging from about 15% to about 25% by weight and
a Brookfield viscosity at 27C. ranging from about 125,000
to about 175,000 centipose. Carboxy-terminated butadiene-
acrylonitriles (CTBNs3 for use herein are available
commercially available such as those sold under the
~ 5~ ~7 ~
trademarks by the B. F. Goodrich Company of Akron, Ohio,
U.S.A. Hycar 13~0x8 and Hycar 1300x9. Hycar 1300x8 is
described by it~ manufacturer as hav}ng a functionality o
1.8, an acrylonitrile content of 18% by weight and a ~
Brookfield viscosity o 27C. of 150,000. Hycar 1300x9 is
recited by its manufacturer as having a functionality of
2.3, an acrylonitrile content of 18% by weight a Brookfield
viscosity at 27C. of 160,000.
As indicated above, the CTBN ingredient is used
in amounts generally ranging from about 5 to ahout 25
phr. If less than about 5 parts by weight is utilized,
the tear strength increasing ability is reduced. If more
than about ~5 parts by weight is utili~.ed, there is
~u~ther modulus and tensile ~oss. Since the CTBN
ingredient is normally a liguid, it readily blends with
the other ingredients.
The reinforcing filler typically is selected from
the group consisting of carbon ~lacks, clays, minerals
such as talc, and silica and very preferably is carbon
black utilized in an amount ranging from about 30 to about
75 phr. When silica is used rather than carbon black, it
preferably i~ utilized in an amount ran~ing from about 30
to about 75 phr. Carbon black and silica can be used in
combina~ion; in such case, the amount of carbon black
utilized normally ranges from about 30 to about 75 phr,
and the amount of silica utilized normally ranges from
about 5 to about 30 phr. Generally, the carbon black used
is one or more of the commercially available blacks known
as under their ASTM designations as N-llO, N-220, N-339,
N-330, N~550 and N-660 gr~de carbon black ~ASTM
D-1765-B22).
Turning now to the curing system, it preferably
is an accelerated sulfur curing system comprising from
ahout 0.5 to about 7 phr sulfur and from about 0.5 to
abou~ 2.5 phr accelerator. The amount of sulfur utilized
is related to the article being produced. For tank track
~5~ 7~3
pads, a low sulfur system (about 0.5 to about l.S phr~ is
preferred since this produces lower running temperatures.
For bel skims, a high sulfur system ~about 2 to about 7
phr) is preferred since this produces an article with ~ood
adhesion to steel. Very pref rably it comprises from
about 0.5 to 1.5 phr, accelerator. A preferred accelerator
is sold under the name Amax*by Vanderbilt Chemicals and is
N~oxydiethylene-2-benzothia~ole-sulfenamide. Other
accelerators include, for example tetramethylthiuram
disulfide, mercaptobenzothiazole, and cadmium, zinc or
tellurium diethylthiocar~amate~ Such curing systems are
well-known in the art; see for example, the Vanderbilt
Rubber Handbook, especi~lly pages 337 to 406 and 464-485,
edited by Robert 0. Babbit, RT VandPrbilt Co. Inc.
Norwalk, CT, USA ~1978).
The curing system may also include zinc stearate
as a promoter to reduce the time of curing. The zinc
stearate i~ readily added in the form of ~inc oxide (e.g.
from about 5 to about 15 phr) plus stearic ac.id (e.g. from
about 1 to about 5 phr) ingredients. While the rest of
the curin~ system is ordinarily kept apart from the
elastomer until just prior to curing, the stearic acid and
zinc oxide can be included in a master~atch o elastomer,
CTBN, and reinforcing filler.
Cured products of this invention which include
natural ruhber as the only elastomer present can have, for
example, a ring tear ~test described hereinafter) at room
temperature ranging from 900 to 1000 lbs./in. and at
212~E. ranging from about 450 to about 750 lbs./in.
Inventive cured products including synthetic rubber in
place of part of the natural rubber can have a ring tear
at room temperature about that normally associated with
100% natural rubber elastomer product and æubstantially
better ring tear than is normally associated with 100%
n~tural rubber elast~mer product at 212F.
~' *The asterisk indicates a -trademark.
The advantages herein are readily obtainecl by
substituting the CTBN additive for an equal weight o
natural rubber in an existing formulati~n, e.g. such as
that for tank track pads or belt skims.
The formulations herein are very suitable for
production of tank track pads, belt skims ~nd tire tread~,
e.g. truck and off-road tire treads.
Processing is readily carried Ollt, for example,
by adding all the elastomer into a mixer ~nd mixing, then
introducin~ the ~inc stearate precursors and part of the
reinforcing filler while continuing mixing, then adding
the CTBN additive along with the rest of the reinforcing
filler and mixing, or example, for 4 to 6 minutes to
obtain a temperature of 300-350F., then dropping on a
mill and a~ding curing system ingredients and processing
on the mill for 4 to 6 minutes.
The invention is illustrated in the followin~
specific examples which include the presently known best
embodiment of the invention.
Example I
Four composi~ions are formulated. The
composition of Run 1 is not within the scope of ~he
invention and contained no CTBN. The ~mposition of Run 2
is within the scope of the invention and is simiilar to
that of Run 1 except that 10 parts by weight CTBN (~ycar
1300x9~ i~ used in p~ace of 10 parts of the natural
rub~er. The compositioIl of Run 3 is within the scope o
the invention and has t~e same formulation as that. of Run
1 except that 20 parts by weigh-t CTBN ~Hyc~r 1300x9) i~
used in place o 20 parts of the natural rubber. The
composition of Run 4 is not within the scope of the
invention and has the same formulation as that of Run 1
except hat 10 parts by weight butadiene-acrylonitri le
copolymer ~sometimes referred to hereinafter as BN~,
namely Hycar 1312 obtained from Goodrich, is used in place
...
7 -
12 ~ ~7 8
of 10 parts of the natural rubber (the Hycar 1312 has the
same molecular weight as the Hycar 1300x9 utilized in Runs
2 and 3 but has ~o carboxyl groups~.
Processing for each run is carried out as
follows: The elastomer, in this case the natural rubber,
is added into a Banbury mixer and mixing initiated. After
1 minute, the zinc stearate precursors and one half the
carbon black are introduced. After mixing for one more
minute, the rest of the carbon black and any CTBN or BN is
adde~. Mixing is then carried out for 5 minutes to obtain
a temperature of 320F. The resultant mix is dropped on a
mill where the sulfur and accelerator (Amax) are added and
processing on the mill is carried out for 5 minutes. Test
specimens are cut from the resulting sheet and these
tested as descri~ed below.
The f~rmulations are made up using the ollowing
parts by wei~ht of each ingredient. In the listing ~elow,
NR stands for natural rubber, CTBN for ~ycar 1300x9, BN
for Hycar 1312, CB for carbon black, and SA or stearic
acid.
Run 1 Run 2 Run 3 Run 4
100NR/No CTBN~ 90NR/10 CTBN)~80NR/20 CTBN~90NR/l~ BN~
NR 100 90 80 9
CTBN - 10 20
25 BN - - - 10
CB 40 40 40 40
SA 3 3 3 3
ZnO 10 10 10 10
Amax 1.2 1.2 1.2 1.2
S 2.0 2.0 2.0 2.0
Testing is carried out on samples made up from
each of the formulations for tear strength in Ring Tear
tests. The Ring Tear tests consisted of cutting ring
shaped samples with outside diameter of ~.25 inches, width
of about 0O25 inch and thickness of 0.100 inch, notching
- 8
P~
the rings with a razor blade and testing them by pullin~
on an Instron tester utili~ing a ~aw speed o 20 i~ches
per minute. Testing was carried out at room temperature
at at 212F. (a constant temperature box was utilized to
obtain the testing temperature). Each result is based on
t~e average for four samples. Results are in lbs./inch
with greater ~mounts indicating higher tear strengths.
Test results are as follows:
Run 1 Run 2 Rlm 3 Run 4
lOONR~No CTBNL90NR/10 CTBN)(80NR/20 CTBN~90NR/10 BN)
Ring Tear
(room
temp. 711 965 927 800
Ring Tear
(212F.) 312 500 703 364
The above results show that inclusion of the CTBN
substantially im~roves tear strength of the cured rubber
compound at room temperature and at 212F. and does so
significantly better than BN.
The uncured stock in Runs 2 and 3 has a lower
visc06ity than the control (Run 1) and processes facilily.
Similar results are obtained when silica is used
in place of part or all of th~ carbon olack or when other
accelerators are used in place of Amax.
The formulations of Run~ 2 and 3 can be modified
to contain a high sulfur curing systems~ These are useful
in makiny belt skims and tire treads.
Example II
Two compositions are formulated. The csmposition
of Run 1 is the control. In Run 1 the elastomer is 35
parts by weight NR and 65 parts by weight styrene
butadiene rub~er (hereinafter SBR) and no CTBN is
included. The composition of Run 2 is within the ~cope of
the invention. In Run 2 the elastomer is 25 parts by
weight NR and 65 parts by weight SBR, an~ 10 parts by
weight CTBN was included.
Processing is carried out as in Example I.
The formulatiorls are made up using the following
parts ~y weight o~ each ingredient. In the listing below
the same abbreviations are utilized as in Example I and
SBR stands for styrene butadiene rubber. Th~ CTBN
ingredient utilized is Hycar 1300x9.
Run 1 Run 2
(No CTBN) (lO Parts CTBN~
NR 35 25
SBR 65 65
CTBN 0 10
15 CB 53 53
SA 1.7 1.7
ZnO 5 5
Amax 1.25 1.25
S 1.80 1.8~
Testing.is carried out on samples made up from
each of the formulations, for the following: Tear Strength
(Ring Tear test as described in Example I~; Shore "A'
~ardness (ASTM Testing Procedures D-1415 and D-2240~;
Running Temperature (ASTM Testing Procedure D-623);
Compression Set SASTM Testing Procedure D~395); Curability
(Monsanto Rheometer -300 F.; ASTM Testing Procedure
D-2084); 300% Modulus, Tensile and % Elongation (ASTM
Testing Procedure D-412). The uncured stock is tested for
Mooney Viscosity (ML4~ at 212F. ~ASTM Testing
30 Pr~cedure D-16463.
l- ~o -
Test results are as follows:
Run 1 Rur~ 2
(No CTBN) ( 10 Parts (::TBN)
-
Ring Tear (lbs./in.)
73E.648 695
212~F. 337457
Shore "A" Hardness
73 F . 74 80
212~F. 68 70
Firestone Flexometer-250 lb. Load
% Deflection 11.3 7.3
Running Temperature F. 329 -~71
ML4 at 212F. 87 73
Monsanto Rheometer-300F.
15 TS(2) - Minutes 9.6 7.7
TC(90) - Minute~. 21.8 21.9
Compression Set-22 hrs. at 1~8~F.
% 14.7 33
Ring Stress~Strain-30 min. at 300F.
20 300 Modulus (lbs./sq. in.) 2597 1889
Tensile (lbs./sg. in.) 3642 3022
% Elongation 382 447
The above results show that the inclusion of the
CTBN improves tear ~trength at room temperature and at
212~F., improves hardness at room temperature, and
improYes viscosity (thereby aiding processing) in the
uncured stocX. While the stock with CTBN cures a little
faster (Monsanto Rheometer results), the ~mall difference
~ 3~
is not such as to cause problems. The above results show
an increase in Compression Set and a decrease in 300%
Modulus and Tensile properties but not to s~ch degree as
to eliminate utility.
~hile the foregoing example describe certain
preferred embodiments of the intention, modifications will
be readily apparent to those skilled in the art. Thus,
the scope of the invention is intended to be defined only
by the following claims.
