CA1062398A - Elastomer composition for pneumatic tire treads - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

A tire tread having both suitable skid resistance and hysteresis properties comprises an elastomeric tetrapolymer of ethylene an .alpha.-olefin a nonconjugated diene and at least one sterically hindering mono-olefin monomer consisting of a norbornene compound of the formula:

Description

' 1062398 Th~ pre~ent i~rentlon relates to impro~ements in ne~
or recapped pneumatic tires having a tread composed of modified elastomers oi th~ et4ylene-Q_olefin-termonomer typeO
~ la~tomer~ Or the etbylene-propylene-termonomer type~
al~o known under the designation "EP'~" or IIEPD~" are, a~ i~
kno~n~ copolymers Or et4ylene, propylene~ and a noncon~ugated diene. The noncon~ugated diene ~hich may be c~clic or linear~
ie intended to introduce unsaturation~ ~hich make ela~tomer~
Or the etbylene-propylene type capable Or being ~ulcanized ~ith ~ulrur; it ~ay ior instan¢e be dicyclopentadiene, l~S-c~clo-octadiene~ 1~4-he~adiene, ethylidene-norbornene, methylene-nor-bornene~ te~rah~droindene, ~et~yltetrahydroindene~ etc.~ or so~e other diene ~hich is copoly~erizable ~ith ethylene and propylene.
The ~PDMs are ela~tomers obtained ~rom ra~ materials ~uch as ethylene and prop~lene, ~hich are less e~pensi~e than ~utadienc and styrene, since the~ are less hlghly proce#sed pro-du¢ts tb~n the latter.
- ~hese elastomers have certain aa antsges, particularly their resistanco to aging and to o~idation~ related on the one hand to their lo~ degroe Or unsaturation and on the other hand to the position Or the double bQnds ~ith respeot to the chaln.
Thereiore their use ln the produotion Or r~bber articles~ and ~n particular tires~ may be ad~antageous; hlle their general properties aro eatisfaotor~ as co~pared ~ith those Or the cus-to~ary oon~ugated diene elastomers~ the EæD~8 on the other band ~ ha~o a eerious dra~back ~hich up to no~ has liDited thoir field ; o~ use~ namely they have a lo~ skid reslsta~ce~ m~ing lt dir-ncult to use them to ~orm tlre treads.
-~ ~arlous solutlons ha e boo~ proposed ~n order to ro-~0 ~edy thie ma~or tra~back. It hae been atto~pted to cbange the nature and the peroentage Or the ter~onom~r or olse tbe oompos~-tlo~ of the monomors~ but contra~y to ~hat i9 true in the case ``
`~

. : .

.; ~,062398 oi the ordinary con~ugated diene elastomers in ~hich it is po~sible to modi~y the dampening and the ~kid resistance by the nature o~ the component~ o~ the polymer~ ~odiiicatio~ Or the ethylene and prop~lene composition Or the copolymer do not m~ke it possible to obtain products having a su~ilcient skid resistance together ~ith satisractory properties oi utllization and mechanical properties~
Thus with ela~tomers or.the EPDM type, these chanees do not mske it pos~ible to obtain an elastomer ~hich has a sui-iiciently bigh ~kid resistance to constituto a tread ~hich hassuiiicient adheren¢e to the road in all ~eather, ~hate~er the estent to ~hich the tread i8 ~orn. ~ike~ise~ it has been at-t~mpted to increa~e the amount Or carbon Uac~ or oil illler.
However~ ~hen the proportion oi carbon blac~ or oil in the mises i8 inoreased~ ~hile the skid resistance 18, it is truo, impro~ed thereby~ thero i8 at the s~me time obtained a ¢onsiderable re-duction in the ~ysteresis properties and mechanical properties.
It has no~ been found that the abo~e-mentioned dra~-~ao~s can be a~oided in a surprising and economically ad~anta-goous rashion and that the lntended goal oi pro~iding elastomersof ~PD~ t~pe ~or tiro treads ~hich ha~e an excellent skid rosis-tanoe and substantially normal hysteresis propertios as co~-pared ~ith thoso oi a eon~entional EPD~ co~taining a nor~al ill-~lng Or oarbon blae~ and oil i8 reaehed br uslng ~PD~-tJpo elas-to ere modlfled in ac¢ordanee ~ith tho prosent in~ontlon.
Tho tiro or tho tread in aeeordanee ~ith the in~ention i8 charaetorizod by tho iact that the tread is foraod Or an olastomor of tho ot~yleno-o-oleiin-noneon~u4ated diene tetra-~, poly~or t~pe oonta~ning a stericallr hi~dori~g ~ono-oloii~ mo-noDer or a ~ixture oi sterieally hinderl~g mono-olefin monomere.
~y stexicall;y hi~doring mono-olo~in monomer there i8 undorstood a mono~er ~hich, ~hon introduced in small quantit~ ma~os it

-2-.
, , possible to increase the vitreo~i transition temp~ratllr~ of the tetrapolymer.
In other words, the invention consists in using EPDMs, the flexibility of the macromolecular chains of which has been reduced by incorporating sterically hindering mono-olefin monomers.
These elastomers show an improvement in the shock-absorbing properties and in particular an improvement in the skid resistance which do not bring about any substantial reduction in the hysteresis properties.
10The sterically hindering compounds which are ~mployed in accordance with this invention have a high rate of l)olymerizati and consist of norbornene and derivatives of norbornene of the following general formula:
~. IR5 ch Rl~ R2~ R3~ R4~ Rs~ R6~ R7~ R8 represent hydrogen, a - 20 halogen,an aliphatic, cycloaliphatic or aromatic halo~enated or unhalogenated hydrocarbon radical.
The sterically hindering compounds which are preferred are 2-norbornene, 5-methyl-2-norbornene, 5-chloromethyl-2-norbornene, 5-hexyl-2-norbornene, 5,6-dimethyl-2-norbornene, 5-phenyl-2-norbornene, (5,2,1,0 ,6) tricyclo-8-decene (6,2,1,13'6, , o2~7) tetracyclo-9-dodecene. The elastomer obtained contains 3 %
`. to 30 % by weight of these monomers and preferably 5% to 15%.
The skid resistance is, as a matter of fact, a direct function of the amount of these compounds in the polymer.

~;
,~

- 3 -B
;.

Ihe ~-ole~ins u~ed in con~unc~ion ~ith ethylene are those ha~ing the iormula R-CH = C~ in ~hich R repre~ents a hydrogen or an alkyl radicsl ha~ing one to eight carbon atoms.
~8 ~-olefins mention ma~ be made of l-propene, l-butene, l-pentene,

4~ethy1-1-pentene~ he~ene, etcO
As tetrapolymers used there mag be mentioned ror ins-tance those:
oi ethylene-propylene-1,4-hexadiene-2-norbornene Or ethylene-propylene-1,4-he~adiene-5-methyl-2-norbornene oS ethylene-propylene-1,4-he~adiene-5-pheny1-2-norbo~neno Or ethylene-prop~lene-1,4-hexadiene-5-chloro~ethyl-2-norbornene oi eth~lene-prop~lene-eth~lidene norbornene-2-norbornene o$ ethylene-propglene-ethylidene norbornene-5-methyl-2-norbornene Or ethylene-prop~lene-et~ylidene norbornene-5-phenyl-2-norbornene o* eth~lene-propylene-ethylidene norbornene-5-¢hloromethyl-2-: norbornene~ etcO
These polymers are prepared by.methods ~hich are ~nown por seO They can be obtained b~ reacting them on a c~ta1Jtic ~ystem formed oi .a) a co~pound oi a transltlon ~etal belongin~ to Groups IV to VIII oi the Mendelee~ periodic table Or elements~
and b) compounds oi elements Or main Groups I to III Or the ~endelee~ periodic table of elements. The preierred.compounds tho elements oi Groups I to III aro aluminum compounds ot tho for~Nla ~1 RXl~ in ~hich R repreeents a hydrocarbon radlcal or a h~drogen ato~ and Xl and ~ ~hich osy be identical or diirerent, ha~e the s~me mea~ing as R or may represent halogensO
~oro F ticularl~ mentlon ~ay be o~de oi the ~ 1 aluslnum eosquichlorides, the dialkyl alumlnu~ halides~ the monoalkyl al~ nu~ halides and the trial~yl or triaryl aluminu~s.
Suitable compounds of metsls oi Groups IV to ~III

.

.~ , .

Or the periodic sy~tem are titanium tetrachloride, the esters o~ chlorotitanic acids, the derivati~es of ~anadium such as ~anadium tetrachloride, vanadium oxytrichloride, the esters o~
~anadium such as ~anadyl butylate, vanadyl isopropylate, vanadium triacetate, ~anadium acetylacetonate~ etc.
The copolymerization can be carried out in liquified monomers, possibly under pressure, ln the presence of inert diluent# such as aliphatic,.¢y¢loaliphati¢, or aromatic bydro-¢arbons~ possibly _alogenated~ ~ith~n a ~ide temperature rango estending from -20C. to ~60~C. The rea¢tion i8 stopped by the co~ventional pro¢esses and the polymers are re¢overed arter e~a-poration of the diluent by steam distillation.
~ he in~ention i8 illustrated but not limited by the rolio~ing esample~. In these examples the structural ¢haracter-- istics Or the poly~ers ha~e in ~eneral been determined in the folloRing manner: The ¢ompositlon Or propylene is determined in in~rared spectrography~ the composition Or hindering monomers i8 deter~ined b~ nuclear ~ag.netic rosonance; the inherent ~18-co dtios are determinod in ethylene tetrachlorlde ln a con¢en-tration o~ Ool g~ in 100cco~ the skid reslsta~ce indes (SRT
index) at 20Co corre#ponds to the "~et skid indes" meagured ~ith the skid reslstance tester (SRT) marketed bg the Stanley Co~pany~ the higher the indes~ the bettor the s~id resietanceO
.. ~he ~uloanization is eirected at a temperature close to 150-Co ~or about 20 minutesO
EDa~le 1 In thls eDa~ple~ copolymers Or the ethrlene-prop~lene ~ termo~omer (EPD~) type ~hlch are con~entional ~ith respeot to thelr properties are co~pared ~ith an ordlnary con~ugated diene elastomer ao a control. In the case Or the EPDM I/l and I/2 types continuous polymerlzation i# carried out in an 8 liter r~actor ~hile the folloRlng rates Or flo~ in llters/hour ~ere ..

-5-~aintained constant :
For ~PD~ I/l For EPD~ I/2 -~eptane 15.5 Toluene 15.5 Ethylene Z70 Ethylene 300 Propylene 700 Propylene 675 1,4-hexadiene 0.12 Ethylidene o.i norbornene (4 % by weight solution in toluene) CO~OE:NTRA~I!IONS 0~ CJL~
VO (OBu)3: 0.21 m.mole/liter of heptane (for EPDM I/l) 0.047 m.mole/liter of toluene (for EPDM I/2) Cl3Aa2 (C2H5)3: 2.1 m.mole/liter of heptane (for EPDM I/l) 0.47 m.mole/liter of toluene (for EPD~ I/2) The properties are set forth in Table I below.
TAB~E I

EP~ ~ NORDæ~ 10?0* CO~TRO~
_ I/l I/2 I/3 % by weight propylene42 40 41 in polymer ~ r~nt ~ieooslty 2.58 2.85 3.32 Iodine number 5.2 6-3 8.6 (l~i) at 130~C. 60 83 70 ~O~WIA~IONS AND PROPE~IES
olymer 100 100 100 . 100 ~1 40 40 55 38 ~AF Black 7 dulu~ of elongation16 14.4 (100 ~ g/cm2 o8. ~) 32. 6 37 37 34 S~ ~aeY at 20C. 87 82 83 100

-6-, . . . .

' ` 106Z3g8 TABLE I cont'd Test No.
~IP~lM EI~D2M NOR/DEL 1070* CQNTROL

Termonomer1,4-Hexadiene Ethylidene 1,4-Hezadiene norbornene * Nordel 1070 is a trademark of DuPont de Nemours, representing an ethylene-pro~ylene-1,4-hexadiene copolymer.
~ he control con~ist~ of a mixture of 65 ~ by ~eight of SBR 1500 and 35 % by ~elght of polybutadiene.
It can be ~en that vie~ed from the standpoint of their ~ 10 use as tire tread, the~e EPDM polymers have an insufficient - 3kid resistance index of les~ th~n 90.
~amDle 2 In thi~ e~ample, the properties of ~arious conventional polymer6 of ~PDM type are compared ~ith tho~e of conventional csnjugated diene elastomer~. The propertie~ are set forth in ~ Table II.
:. ~
. Natural~or~el Nordel Elastomei SBR 1500 rubber 1040* 1070 .. . . _ . .. _ ~ulc~n1 zing S/Santocure S/Santocure S/TMTMS/MBT S/TMT~S/
. system MBT

; Pormulation: 50 5 5 70 Carbon bla¢k Oil 5 8 40 ~i~'' Polymer 100 100 100 100 . .

~odulu~ of elongatlon ` (100~), ~ cm219.7 26.5 18 13.5 ~ 30 Hy tece~)lo~s32 . 22.5 31.5 33.5 - SRT inde~ at 20oc. 100 70 74 76 L ~
.

It ie noted that the SR~ inde~ of the EPDM~ i8 l~w compared with that of the butadiene-styrene copolymer (SBR 1500) marketed by Shell. It i~ insufficient to ma~e thege EPD~s ~uitable for use in tire tread3.
* Nordel 1040 is an ethylene-propylene-1,4-hexadiene copolymer marketed by DuPont de Nemour~.
Santocure (n-cyclohexyl-mercaptobenzothiazole-sulienamide) $8 marketed by ~onsanto.
TM~æ (tetramethylthiuram~monosulfide) is marketed under the tradename ~oNE~ by the ~augatuck Chemi¢al C~.
~BT (mercaptobenzothiazole) i8 marketed under the trade-name C~PTAX by Vanderbilt Company~ In¢.
Esam~le 3 This esa~ple describes the manufacture of a tetrapolymer - in a¢cordan¢e with the in~ention, namely ethylene-propylene-e-thylidene norbornene_5-phenyl-2-norbornene.
The polymerization is effe¢ted batchwise in a reactor into ~hi¢h toluene 1~ introduced at 200.~ it being then saturated for 25 minutee ~ith agitation by means of a stream of ethylene and propylene ~bi¢h had pre~iouely been mi~ed. ~he rate of flow of ethylene i~ 1.2 liters/minute and that Or propylene 0.6 liters/minute.
.~ . .
~here are then introduoedS
- 1.5 c¢. Or ethylidene norbornene - ~ariabls guantities of 5-pheny1-2-norbornene - 0.25 m.mole of V0 (OBu)3 di~sol~ed in toluene - 2,50 m.mole of Cl3Al2 ~C ~5)3 disso~red in toluene and the rates of flo~ of ethylene and propylcne are modifled by ohanging to 1.6 liter/minute and 0.8 liter/mlnute, respeo-3 ti~ely. The reaotlon i9 stopped after 20 ~inutes by the addit~on ~ o~ acetone~ and the polymers are reco~ered.
- The following formulation i8 used (part~ by ~eight) .

_ ~ .
. - ~ .

- polymer 100 _ carbon blacl~ 65 - oil 25 _ ZnO
- ~tearic acid - ~lfur 2. 2 _ ~hcr~as 0.6 _ MBT 0.2 The properties are ~et forth in Table III.
~ABI E III
.
~est No~ III/l II~/2 II2/3 III/4 _ . .
S_phenyl-2- g- 3 g 5 g- 8 g.
norborne~e introduced into the reac~ion Weight o~ polymer obtained 28 g. 27 gO 2604 g.25.4 g.
% by ~eight of 5- 0 6 9 12 phenyl 2-norbornene in the modified EPDM
~ by weight of propylene 37 35 34 32 ~n the modified EPD~
i~herent ~iscosity 2.50 2.63 2.47 2.53 (dl/g) ~itreous transition 51C. _46C. _44C. _40C.
temperature modul~s of elong~-tlon (100 %), kg/cm2 24.5 25 22 2~.5 ~ystere~is 109~ at 27.7 30.1 29.4 - 32.6 60¢- ( %) SCott fracture: 207 236 248 260 ~up.~ure force ~/cm2) Elongatiqn at 475 512 540 540 rUPtUre (~) S~ inde~ at 20C. 100 112 118 126 . .

... _g_ . . .

- . . . . . _ _ .. . .
.
~ . ' ' ' -~ . . ,; ' ' It can be noted that this EPDM modified in accordance with the invention ~hows a substantial improvement in the 6kla resistance index without substantial los~ of the hysteresis properties ~urthermore, it i~ noted that the increase in the skid resistance index is a direct function of the amount of 5-phenyl norbornene in the polymer.
Example 4 This example relates to the tetrapolymer ethylene-propylene-1,4-hexadiene_5-phenyl-2-norbornene oontaining varying amounts of the hindering monomer.
The polymerization is effected continuously ~ith the following rates of flow in liters/hour:
heptane 15.5 ethylene 175 propylene 450 1,4-hexadiene 0.160 with the following concentration~ of catalyst:
VO(o Bu)3: 0.142 m. mole/liter in heptane C134 2(C2H5)3: 1.14 m. mole/liter in heptane The formulation of the mixture~ i~ identical to that of E~ample 3.
~he properties are ~et forth in ~able IV.

;~ `.

-10_ _ . .
. .

~06Z398 o~
~1 ~ ~N N 1 N
U~ O --:
:' .
~ ~ 2 ~ ~D
P~ N CO g~ o ~1 ~I
. ..
.....

N
N U~ N g~ N ,~
'-'' ~
.'. ~
.1 8 u~ ~
æ æ ~ ~ o ., ~ 3~ b ., .

.

.
. . ... . .
. . ..

There can be noted a substantial ~mpro~rement in the ~kid re~stance index without substantial reduction in the h~s_ tere~i~ properties. The mechanical properties, particularly the Scott fracture, are also improved.
Fig. 1 shows the ch~nge in the SRT inde~ (on the ordinate) as a function of the amount of 5-phenyl-2-n~rbo~nene incorporated (on the ab~cissa).
Fig. 2 sho~s the obange in the hysteresis 1058 at 60C. (on the ordinate) as function of the amount of 5-phenyl-2-norbo~nene incorporatea (on the abscis~a).
Pig. 3 sho~e the change in the S~T index (on the ordinate) as a function of the h~steresis 1088 at 60o0. (on the abscissa). The compromise bet~reen road adherence and hyste-resis 1088 is obtained by mea~s: 1 a) of ethylene-propylene-1,4-hesadiene-5-phenyl-2-norbornene polymers (curve a), or b) of a con~entional 1,4-heYadiene E~!Dlll (~ordel 1660) by modifying the percentages o~ carbo~ black (cur~e b) lrith a formulation comprising 100 parts of pol~rmer and 75 parts of oil.
It ~8 noted that the higher the skid resistanoe index, the poorer the hysteresis properties and that t~e best oompromi~e bet~een road adherence and hyeteresis-propert$es i8 obtained by means o$ tho pol~mers of the in~ention.
E~cam~le 5 Thig e~mple con¢erns the tetrapolymer eth~rlene-pro-pylene-ethylidene norbornene-2-norbornene. ne operates under the ~ame conditions as those used in E~ample 1 for Test V/l~ sdd~ng the 2-norbo~ene lrith a oo~ce~tratio~ of 6 g./l for Te~t V/2.
!l~he ~ollow~g ~reight fomlulat~on i~ employedS polymer 100, carbon bla¢lc 70, oi~ 40~ ZnO 4, steario aoid 1~ sulfur 2~ DS 0.6 IIBT 0.2. ~e properties are set $orth in Table V below:
(TII~DS z tetromethylthiuD-d~sulfide, Dlarketed by Naugotuck . . , .:
., , - . .

~hemical CO.) ~AB~E V
Test No. r/l V/2 % by weight 2-norbornene in the O 11 modified E2DM
% by weight propylene in the 41 38 mod~fied E2D~
Inherent viscosity (dl/g) 3.05 2.98 Mo~ulus of elongatio~ (100 ~) ~g/cm2 . 14.4. 14 Hygtere~is los~ at 60C. (%) 37 40 S¢ott fr~cture: rupture 202 230 for¢e kg/cm2 Elongation at rupture (%) 823 850 SR~ indes at 20C. lQO , 116 E~amPle 6 ~hie exa~ple concerne the tetrapolymer ethylene-pro-pylene-1,4-hexadiene-2-norbornene.
One operates under the same conditions as those used in E~ample 1 for Test VI/l~ adding the 2-norbornene in a con-oentration of 4 gD/l and 5.3g./1 in Tests VI/2 and YI/3, respec-.` 20 ,........... .
: ti~ely. In this example, the influence Or the percentages by .: ~eight of oil and carbon black are shown.
Formulation a) polymer 100, carbon black 100~ oil 75~ ZnO 3, ~tearic acid 0.5~ sulfur 1~ ~BT o.5, T~TDS 1.
~ormulation b) polymer 100, carbon black 70~ oil 40, ZnO 5~ steario acid 1, sul~ur 1~ ~D~S 5~ MBT 0.5-; ~he propertles are set forth in Table VI.
- ~AB~,~
. Te~t No. ~ VI/2 VI/3 `,j ; % by ~elght o~ 2-norbornene 0 6-5 9.5 : in the dified EPD~
.. % by ~eight Or ropyl~ne in the moairiea EP ~ 44 42 39 Inherent ~iscosity (dl/g) 2.65 2.57 2.55 . -13-~ . ,~ . . .. .

~A13I.E VI cond't ~e~t No. I/ rI/2 VI/3 llodulus of ~longation a) b) a) b) a) b) (100 ~), k~cm2 _ 13 16.1 12.7 14.4 12.4 14.5 Hy~tere~is 108~3 at 60C.
( % ) 41.3 32.6 44.1 35.5 45.4 37.3 Sl~ indeY at 20C. 114 100 125 111 1~4 118 Scott fracture: rup- 179 203 179 216 181 233 ture forc~ kg/cm2 ngation at rupture 693 640 730 726 747 737 It is noted that the ~nvention applies to all the modified EPDMs ~hate~er the fo~ulation used for the vulcanizers~
sinoe it i8 the nature of the polymer which i8 modified.
Exam~le 7 In ~his example, there are de~cribed the properties (see !I!able VII) of the tetrapolymer# obtained by the ~anner of procedure set forth in E~;ample 4 with the follo~ng different derlvatlvea of norbornene:
5-chloromethyl-2-norbornene 5,6-dimethyl-2-norbornene 5-he~yl-2-norbornene (5~2~1,02'6) tricyclo-8-decene ,. .

.

.: ~0 :. . .

- , . .

~,Q
n h o~ N N O ~ ~ O

. N ~ ,~

~ ~ N t~ ~ 2 2 "

., ~N C~

HCN O ~ 0 C~i ~ O N N
-~ O ~ ~ ~ ~1 b I ~3 o FJ ,i ~ W `D
~ .

:' . ~
~ ~1 o ~ æ æo~ 8 ~ ~ 3 ~ 7 i ~7 ~` :

4 1 these modified EPDM polymers contatn 2% 1,4-heYadiene a~d 35~ propylene by ~eight. The Yulcanized products studied ha~e the formulation: polymer 100, carbon black 65, oil 25, ~tearic acid 1, sulfur 2.2, T~TMS 0.6, MBT 0.2.
There i9 noted a substantial increase in the skid resistance index (wet skid index) associated with ~ufficient mechanical and hysteresis properties.

.
:

" '' ' .

. , .
, 30 ';

- - . . . ~ . ~ . . _.
.. :
- , ~ ~ .

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A pneumatic tire having a tread, which tread comprises an elastomeric tetrapolymer of ethylene, an .alpha.-olefin, a nonconjugated diene and at least one sterically hindering mono-olefin monomer consisting of a norbornene compound of the formula:

in which R1, R2, R3, R4, R5, R6, R7, R8 represent hydrogen, a halogen, an aliphatic, cycloaliphatic or aromatic halogenated or unhalogenated hydrocarbon radical, the elastomeric tetrapolymer containing from about 3% to about 30% by weight of said sterically hindering mono-olefin monomer.

2. A pneumatic tire according to claim 1, wherein the elastomeric tetrapolymer contains from about 5% to about 15% by weight of said sterically hindering mono-olefin monomer.

3. A pneumatic tire according to claim 1, wherein the .alpha.-olefin is 1-propene.

4. A tread for use on a tire carcass, which tread comprises an elastomeric tetrapolymer of ethylene, an .alpha.-olefin, a nonconjugated diene and at least one sterically hindering mono-olefin monomer consisting of a norbornene compound of the formula:

in which R1, R2, R3, R4, R5, R6, R7, R8 represent hydrogen, a halogen, an aliphatic, cycloaliphatic or aromatic halogenated or unhalogenated hydrocarbon radical, the elastomeric tetrapolymer containing from about 3% to about 30% by weight of said sterically hindering mono-olefin monomer.

5. A tire tread according to claim 4, wherein the elastomeric tetrapolymer contains from about 5% to about 15% by weight of said sterically hindering mono-olefin monomer.

6. A tire tread according to claim 4, wherein the .alpha.-olefin is 1-propene.