CA1113367A - Rubber composition having improved adhesion to metal cord - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Effective bonding of a vulcanized composite of rubber to a brass coated steel cord is improved by the addi-tion of small amounts of an organic polymer which contains polysulfide linkages in the backbone of the polymeric chain.
Under conditions where heat and water ordinarily decrease the adhesion of the composite, rubber compounds with small amounts of the polysulfide have considerably increased ad-hesion to brass-plated steel cord.

Description

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Background o~ the Invention The present i~vention concerns an improved me-thod for adhering brass and brass~plated metal to rubber.
It also concerns a new class of rubber compou~ds having im-pro~ed adhesion characteristics to brass plated metal.
The problem o~ securing adequate adhesion o~rubber to metal has been investigated extensively by those skilled in the various aspects of ru~ber manu~acturing.
The best known reference on this subject, Buchan, Rubber ~ LI~a~3~ (Crosby~ Lockwood & Son, ~ondon, 1948~ des-cribes the now widespread practice o~ vulcanization of rub~
ber onto a brass~plated metal substrate. The use of bonding agents such as isocyanates, rubber halogens, a~d thermoplas-tlcs, between the metal and the rubber is found in some ap-plications.
l'he alteration of -the rubber compound i-tself to ; - : :
improve its adheslon to the metal substrate~has~been consi-dered5 and~one such alteration is disclosed in Canadian Pa- `
tent 793,7940 An acidic compound and a free radical curing system is incorporated into rubbers and rubbery copolymers undèr the teachings o~ the Canadian patent.
.
Compounds of various rubbers, natural and syn-thetic, with small amounts o~ certain polymeric polysul~ides, are described in British Patent l,144,634. Advantages taught in the Bri-tish patent are reduced sul~ur requirement, ease o~
compounding, and nonblooming vulcanlzates.
The adhesion of polysulfides to metals (aircraft - ~uel tanks~) is mentioned in U. S Pa-tent 3,099,643, and - their use as a cold setting adhesive for glass, wood and metals is discussed in Jorczak and Fettes, "Pol~sulfide , ~
:' , Liquid Polymers,"Industrial and En~ineering__Chemistr~ Vol.
~3, pp. 324, 327 (February, 1951).
Summary oE -the Invention The objects of this invention are: tl) to provide an improved method of adhering rubber compounds to brass-plated metal substra-tes; and (2) to pro~ide a metal rein~or-ced rubber compound wi-th improved adhesion. Other objects will become apparent as the description proceeds.
The above stated objects are realized through the use of rubber compounds containing small amoun-ts of polysul-fide polymers.
In accordance with their invention, a vulcanizable composite o~ a rubber composition and brass~
plated steel reinforcing material, the polymeric portion of which is comprised of a polysulfide polymer and ano-ther polymer selected ~rom the group consis-ting of natural rubber and synthe-tic diene rubbers in which -the amoun-t of polysulfide i5 from 0.1 to 10~ preferably abou-t 0.5 -to about 7. 4, volume percent of the total polymer content.
For the purposes of -this inven-tion, the polymeric portion is comprised of a polysulfide polymer and a rubber preferab~y selected from the group consisting of natural rubber; blends of natural rubber and SBR rubber; blends of natural rubber and solution polybu-tadiene rubber; SBR
rubber; emulsion polybutàdiene; e-thylene~ propylene~ dicyclo- ~;
pentadiene terpolymer, and polyisoprene. ~-~
For this invention the reinforcing material is desirably selected from the group COi'lSiSting of brass- r plated s-teel wire and fabric woven :Erom brass-plated steel wire.

-Y ~" '--L_'--For purposes of this application, the -te~Q r~
sulfide polymer" is defined to mean any organic elastomeric polymer containing polysulfide linkages in the polymeric chain. This class of polymers includes bu-t i5 not limi-ted to Thiokols, sul~ur modified polychloroprenes, and sulfur containing polymers oF diene monomers alone or copolymerized with one or more other polymerizable unsaturated compounds as described in U.S. Patent 2,234,204. A more detailed description of these polymers follows.
Thiokols are polymers obtained by -the reaction between polysulfides of an alkali metal (e.gO sodium polysulfide) and one or more organic dihalides (e.g.
ethylene dichloride). Other monomers such as trichloropropane may he incorporated into the polysulfides in minor amo~mts. They are available commercially in several varieties o~ solid and liquid polymers.
The particular Thiokols utilized in the develop-ment of -this inv~ntion are represen-tative of polysulfide poly-- -2a-mers commercially available. Their precise chemical compo-sition is unknown to this applicant, and the molecular struc-tures given may be subject to sligh-t variations having little, if any, effect on their properties. They are as ~ollows:
m iokol A - a reaction product of sodium tetra-sulfide and ethylene dichloride having the segmental molecu-lar structure ~CH2CH2S4-)n. A more detailed description can be ~ound in UO S0 Patents 1,890,191; ~,923,392 and Re. 19, ~07. U. S. Pate~t 19890,191 define~ the polymer as a product comprising a compound composed of not less than 70 percent sul~ur in chemical combination with CnH2n groups correspond ing to olefins ha~ing less than four carbon atoms, the per-cent being weight percent.
m iokol FA - a copolymer o~ ethylene dichloride 9 sodium polysul~ide and bis(2-chloroethyl) formal with hydroxyl end groupsO Copolymers of organic dihalides (e.g. ethylene dichloride and bis(2-chloroethyl) formal) are disclosed in U. S. Patents 2,~63,614 ~Example 7 and page 10, column lj lines 3 to 7 and 34) and 2, 363,615~ The fact that they con-tain hydro~yl terminals is discussed in Fettes and Jorczak, "Polysulfide Polymers," ~ , Vol. 42, pp. ~217, 2218, (Novem~er, 1950) and in U~ S. Pa- -tent 2,606,173.
Except for the hydroxyl end groups, these copoly-mers are defined in U. S~ Patent 2,363,6149 as a copolymer which is ~ubstantially a chemical combination of a polymer of the ~nit [RS1 to 6] and a polymer o~ the unit LR'S1 to 6]~ R
and R' being radical~ having structures selected from the groups .

~33~ 7 -- C ~ C --(representing carbon atoms separated by intervening structure) and C - C - ~

(representing adjacent carbon atoms) where R and R' have dif-feren-t specific structuresO In -the case of Thiokol FA, R is -C2H~ and ~' is -C~I2CH20CH20CH2~H2-m iokol ST - a copolymer of sodium polysulfide~
bis(2-chloroeth~1~ formal, and trichloropropane ~which pro-duces branching in the polymer chains) with thiol end groups.
U0 S. Patent 2,363,614 (at page 8 to page 11, right column, line 56) reveals the copolymeri2ation of 1,2,3-trisubstituted propane with disu~stituted dlethyl formal in a solution of sodium tetrasulfide. Such a polymerization is described in detail in ~xamples I and XI of U. S. Patent 2,466,963. The reductive cleavage reac-tion revealed in U S. Patent 2,466, 963 is~ according to Bertozzi, "Chemistry and Technology of Elastomeric Polysulfide Polymers", ~L~Yl:~Y~ eb-L _~
, Vol. 41, pp. 114l 116 (February, 1968), used in the synthesis of Thiokol ST elastomer.
Thiokol ST is comprised o~ the same generic type of polymer units which Thiokol FA is comprised of with the further limitations tha-t the sulfide linkages are disulfide -~
linkages9 R is the unit -CH2-CH-CH2- and R' i~ the unit -cH2cH2ocH2ocH2cH2 -Liquid Thiokols such as m iokol LP-31 - a liquid copolymer of bis(ethylene oxy) methane groups and polysulfide linkages with thiol end groups, having -the general structure~
.

33~ ~

Hs(c2H4-o-cH2-o-c2~4ss)xc2H4-o-c~2 0 C2 L~
LP-31 has a molecular weight of approximately 8000. Other LP-type polymers have molecular weights ranging from approxi~
mately 500 to 4000. Molecular weights ~or a liquid polysul-fide can be as high as 50,000 (as opposed to 100,000 to 200,000 for most solid polysulfides such as Thiokol A and FA) according to U~ S. Patent 2,875,182. The preparation of liquid LP-type polymers is described in Example XVII of U.S~
Patent 2,466,g63 and in U. S. Patent 2,875,182, Example 60 In U~ SO Patent 2t466,963, LP-type polymers are described as polythiopolymercaptans in a liquid form at ordinary temperatures (e.g. 25C.) comprising a series of segmeric units having the general formula -SRS- linked to- ;
gether to form a polymer wherein R is a radical having a structure selected from the group consisting of -C- desig-nating a single carbon atom, - C - C ~ designating two adjacent carbon atoms, and ~- -- C ... C --;~ "' designating two carbon atoms joined to and separated by in-tervening ætructure. In the case of Thiokol LP-31, R is one of the last type of radical9 in particular -CH2CHz-O-CH2-0-CH2CH2~
The words "~hiokol LP", "Thiokol ~A", "Thiokol A"
and "Thiokol ST" are trademarks of Thiokol Chemical Corpora-tion.
Sulfur modified polychloroprenes are described in UO SO Pa-tent 1,950,439. That patent describes sulfur and certain thiuram disulfides as catalysts for controlling -the :

type of polychloroprene produced, maximizing yields, and :
controlling reaction rate. However, i-t was la-ter discovered :
that the sulfur actually becomes part o~ the polymer itself ~see Mochel, ~. E.~ "Structure of Neoprene" ? ournal of r.~ Dg~ Vol. VIII, pp. 583-592 (1952 and Klebanskii et al, J. Pol~m. Sci, Vol. 30, pp. 363-373 (1958)]. 1.
From a reading of the above references, sul~ur modified polychloroprenes can be defined as the class of polymers obtalned by polymerization of 2-chloro-1,3~butadiene in -the presence of sulfur or thiuram disulfides as lis-ted in U.S. Paten-t 1,950,4~9, page 2, column 2, lines 2 through U.S. Patent 2,234,204 describes sulEur containing polymers of "butadiene hydrocarbons" (defined at page 3, column 2~ lines 1 through 8). The "bu-tadiene hydroca~bons"
may be used alone, in admixture wi-th each other, or with l~
.
one or more other polymerizable unsa-turated organic com-. pounds, examples of which.are given at page 3, column 2, lines.].7 through 35. Su~icient examples are included in U.S. Patent 2?23h,204 to illustrate ~his class of polymer.
Typical of the class is -the polymer containing 100 par-ts by weigh-t of 1,3~bu-tadiene, 50 parts by weigh-t of acryloni-trile5 and 0.6 parts by weight of sulfur (Example No. 7 in the patent referred to).
The term "polysulfide polymer" also includes : those polymers of UGS. Pa-tent 3,373,146 having polysul~ide ~
linkages in the polymer chain. This patent discloses low mo~~.ecular weight (usually 500 to 10,000~ mercaptan :~
terminated copoly- :

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mers of sul~ur and at least one diene monomer. The defini-tion of these polymers is ~ound at column 2, lines 17 through 31 of the reference patent.
The preceding descriptions of polysulfide polymers are not intended to be all-inclusive or limiting but are merely illustrative.
Also9 the term "compound" as used herein means the composition o~ matter formed by combining one or more rubbery polymers selected ~rom the group consisting of natural rub-ber, synthetic diene rubber, and polysulfide polymer with conventional compounding ingredients, which ingredients typi-cally include plasticizer, fatty acid, vulcanizing agent, accelerator, age resistors, lubricant, and reinforcing filler.
In the practice of this invention, the rubber is mixed in the conventional manner, for example, on a mill or in a Banbury, with the usual compounding ingredients (e.g. ~ -carbon black, processing oil, zinc oxide, fatty acid, sulfur, accelerator, antioxidant, antiozonant, plasticizer, and wax).
j From 0.1 to 10.0 volume percent of the polymeric content (i.e.
; 20 the natural rubber, polyisoprene, etc.) is a polysulfide polymer, and the concentration of sulfur normally in the mix-ture can be decreased. The resulting vulcanizable compound is pressed onto the metal substrate (e.g. brass-plated steel wire) which operation can be done on a rubber calender.
Various other operations (which depend upon the desired end product) follow. For example, tire carcass stock is cut and combined with other rubber components (e.g.
bead, tread stock~ and sidewalls) on a tire building machine.
These operations are followed by molding and cur-ing under controlled pressure and temperature.
The principal benefit derived from the incorpora tion of polysulfide polymer into the rubber compound is an increase in the adhesion of the brass-plated metal su~strate to the rubber in aged samples. In experiments, the adhesion between the metal and the rubber in the modif'ied samples is 1.4 to 3.9 times that of unmodified stock.
There are other benefits incidental to the substi-tution of polysulfide polymers for other rubber. ~en they are used in natural rubber stocks they reduce the tendency to reversion as shown in Rheometer tests. Experîments with natural rubber have shown that the amount of sulfur can be reduced, and heat build-up in the rubber on flexing is re-duced.
~ Description of the Preferred Embodiments The technique and materials of this invention are useful in any application wherein the bonding of rubber to : .
~ brass-plated metal or brass is important. They are particu-. .
~ larly ad~antageous in the manufacture of tire carcass stock. ~
, ~
The term ~'carcass" refers to the fabric-reinforced parts o~
the tire also called body plies and belt. A typical applica-tion of wire reinforced rubber is the belt ply which is bet-ween -the radial plies and tread in a steel-belted radial tire.
Because tires are subject to many varying stresses under a wide range of temperatures, adhesion of the carcass rubber to its reinforcement is critical. Tires which are retreaded many times such as truck and earthmover tires require a durable carcass to outlas-t several tread applications. -~
Any sul~ur vulcanizable natural or synthetic rubber 33~ 7 may be utilized with polysulfide polymers in manufacturing compounds o~ this invention. The rubbers typically used include natural rubber ~e.g. smoked sheet) and diene rl1bbers such as SBR, solution polybutadiene, emulsion polybutadiene, synthetic polyisoprene, ethylene propylene dicyclopentadiene terpolymer, and blends of the above.
The preferred amount of polysulfide is dependent ; upon the type of polymer to be utilized in the compound, the type of polysul~ide and the condition for which the end pro-duct is designed. However, it has been found that ~olume percents ranging from 0.5 to 7.4 percent polysulfide are pre-ferred.
Processing of the stock is the same as is normally done without the polysulfide polymer with certain exceptions.
Before adding polysulfide to the compound mixture when using Thiokol A, Thiokol FA or Thiokol ST, some rubber should be incorporated into the polysulfide (e.g~ on a mill). It has been found that a preliminary milling of natural rubber with polysulfide in a weight ratio of 18 parts natural rubber to 10 parts polysulfide facilitates dispersion of the polysulfide into the compound and improves tensile strength and tear strength over stocks made without this prior mixing.
Also, as an aid in mixing of the polysulfide with the other ingredients of the compound, a masterbatch of carbon black and polysulfide polymer can be made using approximately 30 volumes of carbon black per 100 volumes of polysulfide.
m e carbon black and polysulfide masterbatch is easier to add to an internal mixer such a~ a Banbury, and it helps to insure an even distribution of carbon black throughout the compound.

3~`7 In the case of Thiokol LP, prior mixing with carbon black or other rubb0r is unnecessary beca~se the liquid Thiokols are easily incorporated into the composition.
The polysulfide or a masterbatch o~ polysulfide poly-mer with rubber or carbon black preferably should be added to the other compounding ingredients last, along with the sulfur.
This practice helps to prevent scorch.
The following examples are presented not to limit but to illustrate the compounds and methods of this invention.
Unless otherwise stated parts are parts by weight per 100 parts by weight o~ total polymer and percentages are volume percent of total polymer content. ~
Various vulcanized compounds have been manufactured ~-experimentally and tested for original and aged adhesion and - 15 heat build-up. The method o~ measurlng adhesion of brass plated metal to the compound was as follows: test specimens ;~ were prepared by curing in a mold a rectangular block of~ polymer compound with dimensions o~ 12 mm. x 12 mm. x 75 mm.
,',! ~ into which had been embedded two brass plated steel cords 5 one at either end of the block. The mold was so designed that the wires were embedded axially and symmetrically, and insertion length of the wire in-to the block was always 19 mm. The wires did not go completcly through the block nor did they touch each o-ther.
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Su~ficient wire was left protruding from the ends of the block to allow placement of a sample in the jaws of a tensile tester such as a Scott tester or an Instron tester.
The two jaws or clamps of the testing apparatus held the two wire ends. The rubber itsel~ was not held~. Force required .

to pull one of the wires out of the block was measured with a ~ixed jaw separation rate (5 cm. per minute was used).
Heat build-up in a compound was measured by the Goodrich Flex Test, QSTM ~623.
The compo~ds used in evaluating the effectiveness of the polysulfide polymers were natural r~bber compounds designed for use in tire carcass stock. Where applicable, ::
reduction in sulfur or to-tal parts of sulfur used is shown in the data tables which follow. ; ;~
EX.AMPLE I
A standard natural rubber compound, which shall be referred to as Standard Compound A, was modified by replac-ing part of the natural rubber in the compound with vary-: ing amounts of Thiokol A and by varying the amount of sul-fur used. Test results are shown in Table I. Adhesion test specimens were cured 85 minutes a-t 13~ C.
Table 1 : : Modi~ications of Standard Compound A
Natural Rubber Vol.%100 95.6 94.4 92.6 92.6 20 : miokol A ~ Vol.% 0 4 4 5.6 7.4 7.4 ~:: Sul~ur (parts) 6.5 5 5 5.5 5.5 4.0 Adhesion (Original) in Newtons 483 562 403 492 501 Adhesion (Aged 10 days in H20 at 90C) in Newtons 203 697 763 783 698 Temperature Rise, C. in Goodrich Flex Test36.4 34.2 32.8 33.1 34.2 : All aged adhesions of the Thiokol stocks were ap-, proximately three times that of the control with~ut Thiokol~
EXAMPLE II
me procedure ~ollowed in this example is similar to the previous example except that a different Standard Compound, Standard Compound B, was used, and lower concen-, , .

. . .

trations of sulfur were tried. Thiokol A was again used as a substitute for a portion of the natural rubber content.
Results appear in Table 2. r' Table 2 Modifica-tions of Standard Compound B
Natural Rub~er Vol.~ 100 95.6 94.4 92.6 92.6 Thiokol A Vol.% 0 4.4 5.6 7.4 7.4 Sulfur (parts) 5.0 L~, 0 4 o 0 3 . 52 . 25 Adhesion (original) in Newtons 622 934 867 907 836 Adhesion (aged 10 days in H 0 at 90C.) in Newton~ 448 687 690 710 647 ~-Tempe2rature Rise, ~C.) in Goodrlch Flex Test 38.9 34.7 32,8 32. 234 . 7 All original and aged a &esions of the Thiokol con-taining stocks exceeded that of the control. As in the pre- ;~
vious example, the Thiokol containing stocks demonstrated a ~, lower heat rise than the control.
EXAMPLE III
An experiment was done at 2~1 volume percent poly-.~
sulfide polymer in Standard Compound A with a portion of ;
~t the carbon black ln the compound replaced by Hi-Sil 233 (a hydrated silicate filler marketed by PPG Industries, Inc.) and at a reduced sulfur leve]. Tests were performed as in the previous two examples with the addition of an adhesion test on a sample aged 10 days in a nitrogen atmosphere at 121C. and 551. Kilopascals gauge pressure. Resul-ts are reported in Table 3.

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~3i33~7 Original adhesion in three o~ the four Thiokol-con--taining s-tocks was improved o~er the controls, and aged ad-hesion of all the Thiokol-containing stocks was superior to the controls. Comparison of the two controls reveals that the use of Hi-Sil 233 filler produced greater hea-t rise in the Goodrich Flex Test~ All of the Thiokol-containing stocks show0d less heat rise than the control containing Hi-Sil.
Adhesion testing has also been done on compounds con-taining blends of natural rubber wi-th solution polybutadiene and blends of natural rubber with SBR rubber. The original and aged adhesion test results for these blends demonstrated improved adhesion for compounds containing polysulfide poly-mer comparable to the data given above for natural rubber ~;
compounds.
m e curing characteristics of several compounds of this invention were measured on an oscillating disk cure meter (ASTM D2084). Tests were run at 149C. with a stock containing 3.7 volume percent Thiokol ST in Standard Com-pound A and at 135C. with stocks containing 7 volume per-cent m iokol A in Standard Compound A and var~ing concen-trations o~ sulfur in the compound. In the tests at 7 vo-lume percent Thiokol A sulfur con-tent in the control was 6.5 parts while that in -the polysulfide containing stocks varied from 4.0 to 5.5 parts~ Although the natural rubber control stocks without polysulfide polymers exhibited a tendency to reversion, the stocks containing polysulfide polymers mixed with natural rubber did not.
Reversion is a decrease in measured -torgue or modulus after maximum torque has been reached. It is measured as the -;

33~'7 time required to fall to 98 percent of maximum torque after maximum tor~ue has been reached.
Polysulfide con-taining s-tocks also exhibited a con- ;
tinuing vulcanization effect resul-ting in higher maximum tor~ue than the controls and longer times to reach maximum torque. This phenomenon was true at all sul~ur levels test-ed~
While certain representative embodiments and details ; have been shown for the purpose of illustrating the inven-tion, it wlll be apparent to those skilled in this art that various changes and modifications may be made -therein with-out departing from the splrit or scope o~ th- invention.

' ,

Claims

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

1. A vulcanizable composite of a rubber compos-ition and brass-plated steel reinforcing material, the polymeric portion of which is comprised of a polysulfide polymer and another polymer selected from the group con-sisting of natural rubber and synthetic diene rubbers in which the amount of polysulfide is from 0.1 to 10 volume percent of the total polymer content.
2. The composite as recited in Claim 1 in which the polymeric portion is comprised of a polysulfide polymer and a rubber selected from the group consisting of natural rubber; blends of natural rubber and SBR rubber; blends of natural rubber and solution polybutadiene rubber; SBR rub-ber, emulsion polybutadiene; ethylene, propylene, dicyclo-pentadiene terpolymer, and polyisoprene.
3. The composite of Claim 2 in which the rein-forcing material is selected from the group consisting of brass-plated steel wire and fabric woven from brass-plated steel wire.
4. The composite as recited in Claim 3, in which the polysulfide polymer constitutes from 0.5 to 7.
volume percent of the total polymer content.
5. The composite according to Claim 4 in which the polysulfide polymer is a polymer composed of not less than seventy weight percent sulfur in chemical combination with CnH2n moieties corresponding to olefins having less than four carbon atoms.
6. The composite of Claim 5 in which the poly-sulfide polymer has the segmental molecular structure -CH2CH2S4-.
7. The composite according to Claim 4 in which the polysulfide polymer is a copolymer comprised of a chemical combination of a polymer of the unit RSX and a polymer of the unit R'SX, wherein R and R' are radicals having structures selected from the group consisting of (representing carbon atoms, separated by inter-vening structure) and (representing adjacent carbon atoms), where R and R' have different specific structures, and x is 1 to 6.
8. The composite according to Claim 7 in which R is C2H4, R' is -CH2CH2OCE2OCH2CH2-, and the end groups are hyaroxyl.
9. The composite according to Claim 7 in which x is 2, R is -CH2-CH-CH2, R' is -CH2CH2-O-CH2-O-CH2CH2-, and the end groups are -SH.
10. The composite as recited in Claim 4 in which the polysulfide polymer has a molecular weight of about 500 to 12,000, thio end groups, and exists at 25°C, as a li-quid which comprises a series of segmeric units having the formula -SRS- wherein R is a radical selected from the group consisting of methyl, ethyl, and designat-ing two carbon atoms joined to and separated by intervening structure.
11. The composite of Claim 10 in which R is -CH2CH2-O-CH2OCH2CH2-.
12. A vulcanizate prepared from the composite of

Claim 1.