CA1250678A - Natural rubber containing compositions with increased tear strength - Google Patents

Abstract

NATURAL RUBBER CONTAINING COMPOSITION
WITH INCREASED TEAR STRENGTH
Abstract of the Disclosure Inclusion of selected amounts of particular liquid carboxy-terminated unsaturated butadiene-acrylonitrile copolymers (sometimes referred to here-after as CTBN) increases the tear strength of natural rubber containing compositions. Contrary to conventional alternatives, the composition of the invention allows the inclusion of preferred amounts of reinforcing filler and the use of normal cross-linking conditions without deleterious effect on tear strength and instead is characterized by increased tear strength compared to a natural rubber composition without the CTBN additive.

Description

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 ...

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~

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 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~

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.

Claims (13)

1. WHAT IS CLAIMED IS:
   l. A curable but uncured elastomeric composition, said composition comprising (a) 100 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 synthetic rubber, (iii) from about 5 to about 25 parts by weight of liquid carboxy-terminated butadiene-acrylonitrile copolymer having a functionality ranging from about 1 . 5 to about 2.5, an acrylonitrile 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 centipoise, (b) from about 30 to about 120 parts by weight of reinforcing filler.
2. 2. The composition of claim 1, wherein no synthetic rubber is present.
3. 3. The composition of claim 1, wherein the carboxy-terminated butadiene-acrylonitrile copolymer has an acrylonitrile content ranging from about 15% to about 25% by weight and Brookfield viscosity at 27°C. ranging from about 125,000 to about 175,000 centipoise.
4. 4. The composition of claim 3, wherein the reinforcing filler is selected from the group consisting of carbon black and silica.
5. 5. The composition of claim 4, wherein the reinforcing filler is carbon black used in an amount ranging from abut 30 to about 75 parts by weight.
6. 6. The composition of claim 1, wherein the synthetic rubber is styrene-butadiene rubber and is present in an amount ranging from about 45 to about 65 parts by weight.
7. 7. A cured elastomeric composition of improved tear strength, said composition comprising (a) 100 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 synthetic rubber, (iii) from about 5 to about 25 parts by weight of carboxy-terminated butadiene-acrylonitrile copolymer having a functionality ranging from about 1.5 to about 2.5, an acrylonitrile 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 centipoise, (b) from about 30 to about 120 parts by weight of reinforcing filler, and (c) a curing system.
8. 8. The cured composition of claim 7, wherein the reinforcing filler is selected from the group consisting of carbon black and silica.
9. 9. The cured composition of claim 8, wherein the reinforcing filler is carbon black used in an amount ranging from about 30 to about 75 parts by weight.
10. 10. The cured composition of claim 9, wherein the carboxy-terminated butadiene-acrylonitrile copolymer has an acrylonitrile content ranging from about 15% to about 25% by weight and a Brookfield viscosity at 27°C.
    ranging from about 125,000 to about 175,000 centipoise.
11. 11. The cured composition of claim 10, wherein the curing system is an accelerated sulfur system comprising from about 0.5 to about 7 parts by weight sulfur and from about 0.5 to about 2.5 parts by weight accelertor.
12. 12. The cured composition of claim 11, wherein no synthetic rubber is present.
13. 13. The cured composition of claim 11, wherein the synthetic rubber is styrene-butadiene rubber and is present in an amount ranging from about 45 to about 65 parts by weight.