CA1182615A - Process for increasing the strength of the bond between rubber and metals - Google Patents

Abstract

PROCESS FOR INCREASING THE STRENGTH OF THE BOND
BETWEEN RUBBER AND METALS

Abstract of he Disclosure 1. A process for increasing the strength of the bond between rubber and metal by the addition of a bond-increasing compound, optionally a formaldehyde releasing compound and optionally a resorcinol compound to the rubber, followed by vulcanization, characterised in that a compound corresponding to formula (1) or formula (2):

(1) (2) is added as bond-increasing compound to the rubber in quantities of 0.1 to 20 parts by weight per 100 parts by weight of rubber, the groups in the formulae having the following meaning:

R1 to R11 may denote hydrogen, hydroxyl, C1-C18-alkyl, C5-C12-cycloalkyl, C1-C16-alkylthio, C6-C14-aryl, C6-C14-aryl-C1-C4-alkyl, C1-C18-alkoxy, C5-C12-cycloalkoxy, C6-C14-aryloxy, C6-C14-aryl-C1-C4-alyloxy and halogen , and X may denote C1-C4-alkylene, sulfur, dithio, oxygen or N-C1-C4-alkyl.

Description

~ ~1. 5126~
PROCESS FOR INCREASING THE ~TRENGTH OF THE
BOND BETWEEN RUBBER AND METALS
.. .. _ . ... . .
This invention relates tc a ~rocess for lncreasing the bond strenath between rubber and metals wherein cobalt phenolates are added to the rubber.
Many technical rubber articles such as tyres, conveyor belts or high pressure tubes are provided with reinforcing inserts of high carbon steel, frequently ir. the form of steel cord.
To ensure high performance and long life of the articles, a firm and durable bond between the metal and rubber is necessary.
It is only possible to achieve this withou-t the addition of an adhesive if the filaments of the steel cord are plated with a thin layer of ~-brass, or some other alloy containing zinc and copper as its main constituents, or pure zinc.
The plated cord is directly incorporated by vulcanization in the rubber mixture, which generally contains additives to increase the strength of the bond.
The most usual additives for improving the bonding power, hereinafter referred to as "bonding agentsl', may be divided into two groups according to their chemical structure.
The first group includes all bonding agents which ; 25 only act as multicomponent systems. These all contain highly active silicates.
The other components are resorcinol or resorcinol-formaldehyde condensation products and formaldehyde-releasing compounds such as hexamethylenetetramine, 30 etherified or esterified methylol melamines in various degrees of etherification or esterification, and their condensation products (German Auslegeschriften No.
1,301,475 and No~ 1,301,478).
Although these systems provide good bonding 35 Le A 20 688 .~

s characteristics, they occasionally produce vapours and noxious odours in the process of vulcani~ation and they considerably impair the facility with which the substances can be processed in mixing machines, e.g.
on roller mixers, since resorcinol tends to undergo sublimation, particularly at temperatures near its melting pointu The second group includes organometallic compounds, especially cobalt compounds. Cobalt soaps in particular, such as those also conventionally used as desiccating agents in the lacquer industry, have been used for some time~
Various cobalt salts, e.g. cobalt stearate, cobalt linolate and cobalt naphthenate, are discribed in French Patent No. 1,323,934. Organic boron compounds contain-ing cobalt as d~scrihed US Patent No~ 3,296,242 are also suitable. Copper, nickel, lead and zinc are also suitable metals (see German Offenlegungsschrift ~o.

2,303,674 and ~S Patent No. 4,154,911).
There are marked differences between the effects produced by the bonding agents of the first group and o~
the second group.
When organometallic compounds are added to the rubber mixture, they generally improve the bond between the metal and rubber after vulcanization and have a long lasting effect in delaying corrosion of the metal component.
Their main disadvantage, however, is that the strength of the bond may be severely reduced, particularly under conditions o high temperature vulcanization (180 to 240C);
reversion is also a serious problem.
Bonding agents belonging to the irst group provide a rubber-metal bond which has better resistance to reversion and the bonds are also relatively resis-tant to the ef~ects o heat and mois~ure.
Le A 20 688 .. ~, . . , ~

%~

The bonding mixtures are therefore frequently mixed with bonding agent combinations, using either individual components of a bonding system or the whol.e bonding system (German Offenlegungs-schriften No. 1~720~144 and 2~841~401)o It would be deslrable to provide a bonding system which shows little or no reversion after prolonged ageing by heat and is subject to little or no loss of bond strength.
I~ has now surprisingly been found that the bond between rubber and metals such as raw steel or steel plated with brass or zinc may be substantially improved and good resistance to ageing and reversion may be obtained by adding a phenolic cobalt compound.
Accordingly, an aspect oE the present invention provides a process for increasing the strength of the bond between rubber and metal by the addition of a bond-increasing compound, followed by vulcanization, wherein a compound corresponding to formula (-1) or formula ( 2) ~3 ~ 2 Co . 2 H20 (1 or 2 ~2 ~ C o - --O

X ~ (2) R7 ~8 Rg Rlo is added as the bond-increasing compound to the rubber in quantities of 0.1 to 20 parts hy weight per 100 parts by weight of rubber, the groups in the formulae having the following meaning:
Rl to Rl1 each denotes hydrogen, hydroxyl, Cl-Clg-alkyl, Cs-C12-cycloalkyl, Cl-Cl~-alkylthio, C6-C14-aryl, C6-C14-aryl-Cl-C4-alkyl, C~-Clg-alkoxy, Cs-Cl2-cycloalkoxy, C6 C14-aryloxy, C6-Cl~-aryl-Cl-C4 alkyloxy or halogen, and X
denotes Cl-C4-alkylene, sulfur, dithio, oxygen or N-Cl-C4-alkyl.
The bond-increasing compound is added preferably from 0.3 to 10 parts by weight in particular from 0.5 to 5 parts by weight, per 100 parts by weight of rubber.
These compounds do not manifest the disadvantages of the above mentioned bonding agents containing metal.
Another aspect of the invention provides a rubber com-position which comprises the compound of formula (1) or (2).
Still another aspect of the invention provides an article comprising a reinforcing metal bonded to vulcanized rubber which comprises the compound of formula tl) or (2).
An adclitional increase in the bond strength may be ~ .

obtained by adding a formaldehyde-releasing compound in quantities of 0.01 to 10 phr, preferably 1 to 3 phr, to the rubber in addi-tion to the compounds of formu]ae (1`1 and (2~.
In addition, resorcinol or its derivatives, such as, for example, dimethoxybenzene, diacetoxybenzene, dibenzoyloxy-benzene, dipropoxybenzene, dipropionyloxybenzene or di-(trimethyl-silyloxy)-benzene, or sulphonic esters, phosphoric acid esters, phosphorous acid esters, urethanes and carbonates or resorcinol may be added to the rubber in quantities of from 0.01 to 10 parts by weight, preferably from 0.5 to 5 parts hy weight, per 100 par-ts by weight of rubber.
The resorcinol compound may be added to the rubber separately or in combination with the formaldehyde-releasing com-pound, or resorcinol precondensed with formaldehyde may be added to the rubber in quantities of from 0.01 to 10 parts by weight, preferably from 3 to 5 parts by weight, per 100 parts by weight of rubber, the precondensate used containing 1 to 3 mol of formal-dehyde per mol of resorcinol.
Silicates may also be added in the usual quantities.
The groups Rl to Rll in formulae (1) and (2) have the following meaning and may be identical or different:
hydrogen, hydroxyl Cl-Clg-alkyl, Cs-C12-cycloalkyl, Cl C16-alkylthio, C6-C14-aryl, C6-C14-aryl-Cl-C4-alkyl, Cl-C18-alkoxy, Cs-C12-cycloalkoxy, C6-C14-aryloxy, C6-C14-ary]-Cl-C4-alkyloxy, and halogen (Cl, Br or I, preEerably Cl).
In formula (1), the groups Rl, R2, R4 ancl Rs are prefer-ably hyclrogen and R3 is preferably C6-C12-alkyl or C6-C14-aryl-Cl-(:4-a:Lk,yl .

- 4a -~ 5 --F~
In formula 2 r the groups R6, R7, Rlo and Rll are preferably hydrogen and the groups R8 and Rg preferably Cl-C4~alkyl, benzyl or halogen or R6, R8~ Rg and Rll are preferably Cl-C~-al~yl or halogen and the groups R7 and Rlo are preferably hydrogen, Cl-C4-alkyl, phenyl or halogen.
X denotes Cl-C4--alkylene such as methylene, ethylidene, propylidene or butylidene, or sulphur, dithio, oxygen or N-Cl-C~ alkyl-The compounds of formulae 1 and 2 may be present as isomeric mixtures.
The groups Rl to Rll may have the following specificmeanings:
hydrcgen, methyl, ethyl, propyl, isopropyl, n-butyl, sec.-butyl, isobutyl~ tert.-butyl, n-pentyl, isopentyl, sec.-pentyl, neopentyl, n-hexyl, isohexyl, sec.hexyl, cyclohexyl, n-heptyl, isoheptyl, tert.-heptyl, octyl, 2-ethylhexyl, nonyl, decyl, undecyl, dodecyl, tridecyi, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, methylcyclohexyl, cyclohexylmethyl, naphthyl~ anthracenyl, naphthylmethyl, cycloheptyl, cyclooctyl, phenyl, benzyl cyclononyl, cyclodecyl, cycloundecyl, and cyclododecyl.
The alkoxy and alkylthio groups may be the same as those mentioned under the alkyl groups but with oxygen or sulphur added.
The follo~ing compounds are mentioned as examples:
Compounds 2 to 43 may be present in the ortho-, meta-or para-form or as mixtures of isomers.
Bis-(phenoxy)-cobalt dihydrate Bis-(methyl-phenoxy)-cobalt dihydrate 2 30 Bis-(ethyl-phenoxy) cobalt dihydrate 3 Bis-~propyl-phenoxy)-cobalt-dihydrate 4 Bis-(isopropyl-phenoxy)-cobalt dihydrate 5 Bis-(butyl-phenoxy)-cobalt dihydrate 6 Bis-(pentyl-phenoxy)-cobalt dihydrate 7 ~5 Le A 20 688 Bis-~tert.-butyl-phenoxy)-cobalt dihydrate 8 Bis-(isobutyl-phenoxy)-cobalt dihydrate 9 Bis-(secO-b~ltyl-phenGxy)-cobalt dihydrate 10 Bis-(hexyl-phenoxy)-cobalt dihydrate 11 Bis-(heptyl-phenoxy)-cobalt dihydrate 12 Bis-(octyl-phenoxy)-cobalt dihydrate 13 Bis-(2-ethylheptyl)-phenoxy cobalt dihydrate 14 Bis-(ncnyl-phenoxy)-cobalt dihydrate 15 Bis-(decyl phenoxy)-cobalt dihydrate 16 Bis-(undecyl-phenoxy)-cobalt dihydrate 17 Bis-(dodecyl-phenoxy)-cobalt dihydrate 18 Bis-(tridecyl-phenoxy)-cobalt dihydrate 19 Bis-(tetradecyl-phenoxy)-cobalt dihydrate 20 Bis~(pentadecyl-phenoxy)-cobalt dihydrate 21 Bis-(hexadecyl-phenoxy)-coablt dihydrate22 Bis-(heptadecyl-phenoxy)-cobalt dihydrate 23 Bis-(phenyl-phenoxy)-cobalt dihydrate 24 Bis-(benzyl-phenoxy)-cobalt dihydrate 25 Bis-(naphthyl-phenoxy)-cobalt dihydrate 26 Bis-(cyclohexyl-phenoxy)-cobalt dihydrate 27 Bis-(cycloheptyl-phenoxy)-cobalt dihydrate 28 Bis-(cyclononyl-phenoxy)-cobalt dihydrate 29 Bis-(cyclodecyi-phenoxy)-cobalt dihydrate 30 Bis-(cycloundecyl-phenoxy)-cobalt dihydrate 31 Bis-(cyclododecyl~phenoxy)-cobalt dihydrate 32 Bis-!phenoxylphenoxy)-cobalt dihydrate 33 Bis-(butyloxy-phenoxy)-cobalt dihydrate 34 Bis-(methoxy-phenoxy)-cobalt dihydrate 35 Bis-~decyloxy~phenoxy~-cobalt dihydrate36 Bis-(benzyloxy-phenoxy~-cobalt dihydrate37 Bis-(chloro-phenoxy)-cobalt dihydrate 38 Bis-(cyclohexyloxy-phenoxy)-cobalt dihydrate 39 Bis-(phenyl-thio-phenoxy)-cobalt dihydrate 40 , Le ~ 20 6a8 ~' Bis-(methylthio-phenoxy)-cobalt dihydrate 41 Bis-(butylthio-phenoxy)-cobalt dihydrate 42 Bi.s-(hydroxyphenoxy)-cobalt-dihydrate 43 (5,5'-Dimethyl-dlphenylmethane-2,2'-dihydroxy)-cobalt dihydrate 44 (5,5'-Dichloro-diphenylmethane-2 r 2'-dihydroxy)-cobalt dihydrate 45 (5,5',3,3'~tetramethyl diphenylmethane-2,2'-dihydroxy)-cobalt dihydra-te 46 (5,5',3,3'-Tetramethyl-diphenyl-(l,l isobutane)-2,2'-dihydroxy)-cobalt dihydrat~ 47 (5,5'-Dimethyl-diphenyloxy-2,2'-dihydroxy)-cobalt dihydrate 48 (5,5'-Di-cyclohexyl-diphenylmethane-2,2'-dihydroxy) cobalt dihydrate 49 Bis-l2-naphthoxy)-cobalt dihydrate 50 Bis-(2,4-di-tert.-butyl-phenoxy)-cobalt dihydrate 51 (5,5'-Di-nonyl-diphenyl-sulphide-2,2'-dihydroxy)-cobalt dihydrate 52

3,3',5,5'-Tetramethyl-diphenylsulphide-2,2'-dihydroxy)-cobalt dihydrate 53 3,3',5,5'-Tetramethyl-diphenyl-disulphide-2,2'-dihydroxy)-cobalt dihydrate 54 Pyrocatechol-cobalt dihydrate 55 Bis-(3-nonyl-2-hydroxy-phenyl)-methane-cobalt dihydrate 56 Bis-(4 nonyl-2-hydroxyphenyl)-me-thane-cobalt dihydrate 57 3-Phenylpyrocatechol-cobalt dihydrate . 58 The products corresponding to formulae 1 and 2 are prepared by mixing a cobalt salt of a suitable acid with the required, optionally substituted, phenol in -the absence of a solvent, and slowly heating the mixture. The tempera~

Le A 20 688 , - 8 -ture of the mixture is raised slowly so that initially only water is released from the water of crystallisation of the cobalt salt, and as the temperature is further raised to the region of 220 to 350C, preferably 220 to 270C, the acid, which is generally lower boiling, i.e.
more volatile than the aforesaid substituted or unsubstituted phenol, is split off and displaced by the phenol in the heat and the product of formula 1 or 2 is formed.
The acid may undergo decarboxylation under the reaction conditions so that, instead of being split off as such, it may be released in the ~orm o~ carbon dioxide and possibly also ketones and other h~drocarbons. Suitable cobalt salts for this process include cobalt fo mi~e, cobalt acetate, cobalt propionate, cobalt butyrate, etc. The phenols used in this process of preparation are those on which the products shown in the Table are based. The components are generally used in stoichiometric quantitiesr although a slight excess of substitited phenol, up to 10~ by weisht, may be used to accelerate the reaction, the excess being either distilled off under vacuum after the reaction or left in the product to reduce an otherwise high melting point. A certain excess of substituted phenol is advisable also if partial dealkylation of the substituted phenol is liable to occur under the reaction conditions.
The products are generally obtained in the form of dihydrates but may lose water of crystallisation under prolonged heating in the upper temperature region (~~' 270C).
The structure of the products was determined by ESR and ESCA spectroscopy.
The products may be decomposed quantitatively by water into the corresponding phenols and cobalt hydroxides.
The product may contain a certain quantity of unreacted cobalt salt if the reaction has been incomplete.

he A 2~ 688 The products, even i~ they have varying particle sizes, may be pxotected against hydrolysis by enveloping them in the usual manner with water-repellent substances such as, for example, paraffin oil, wax or water-repellent polymers.
The synthesis of an isomeric mixture of o-, m- and p-nonylphenoxy-Co-dihydrate is described below as an example of the preparation of these products.
1155 g of nonyl phenol are mixed with 622.5 g of commercial cobalt acetate and the mixture is slowly heated with stirring. Water of crystallisation initially sep-arates, and subsequently acetic acid, the reaction temperature being raised to 250C. At this temperature, the reaction mixture is stirred until a deep navy blue, completely homogeneous liquid is obtained. The liquid is cooled to 200C, excess nonyl phenol is drawn off under vacuum and the residue is completely cooled down under vacuum. The product solidifies at 110 to 115C.
By contrast, bisphenol-Co products linked by sulphur bridges are readily prepared in aqueous or aqueous-alcoholic solution from the corresponding bisphenol, a cobalt salt and the appropriate quantity of sodium hydroxide solution.
The preparation of 3,3',5,5'-tetramethyl-diphenylsulphide- ¦
2,2~-dihydroxy-cobalt dihydrate is described below by way of example.
A solution of 274 g of 3,3',5,5'-tetramethyl-2,2'-dihydroxy-diphenylsulphide and 80 g of sodium hydroxide in 600 ml o~ water was added dropwise to 249 g of cobalt acetate.4 H20 in 800 ml of water at room temperature. The reaction mixture was stirred for 2 hours, suction-~iltered, washed and dried under vacuum. 351 g of productmelting above 300C were obtained.
Similar bisphenol sulphides and bisphenol disulphides containing different substituents may be reacted in analogous manner.
Le ~ 20 688 ..... ~......... .

Formaldehyde-releasing compounds are understood to be substances which are capable o~ splitting off ~ormaldehyde when heated, e.g. to temperatures in the region of 40 to 200C, in particular under conditions of vulcanizationJ optionally in the presence of water. When methylol ethers or methylol esters are used, condensation reactions may also occur, accompanied by the liberation of alcohols-or acids.
These products will hereinafter brie~ly be referred to as "~ormaldehyde-releasing compounds".
The following are examples of formaldehyde-releasing compounds: trimeric methylene aminoacetonitrile, l,aza-3,7-dioxabicyclo~3,3,0~ octane, oxazolidines, bis-(1,3-oxazolidino)-methane, octahydro-1,3-benzoxazole t tetrahydro~
1,3-oxazine, dialkylaminomethylalkylether, and diallyl-amino-methylalkylether (see e.g. selgian Patent No. 621,923), e.g. 4~4-dimethyl-1,3-oxazolidine, bis-(~,4-dimethyl-1,3-oxazolidino)-methane, N-n-butyl-5(6)-cyanooctahydro-1,3-benzoxazole, 3-n-hutyl-tetrahydro-1,3-oxazine, diisopropyl-aminomethyl-ethylether, diallyl-aminomethyl-ethyl ether, he~a-(methoxymethyl)-melamine, N-methylolcarboxylic acid amides, e.g. N-methylolacetamide, N-methylolbutyramide, N-methylolacrylamide, N-methylolmethacrylamide, N-methylol-succinimide and N-methylolmaleic acid imide.
The following are ~urther exarnples of formaldehyde-releasing compounds: 1,8-di-(methyleneamino)-p-methane;
azomethines such as ~ dimethyl-benzyl-azomethine (see US Patent No. 2,512,128) and cyclotrimethylenetriamines, e.g. N,N',N"-trimethyl-cyclotrimethylenetriamine and N,N',N"-triethyl-cyclotrimethylenetriamine; diaminomethane 3~ substituted on both nitrogen atoms, e.g. bis-tdi-tcyano-methyl)-amin~ -methane and bis-(diallylamino)-methane;
imidazolines substituted on both nitrOgen atoms, such as N,N'-diphenyl~imidazolidine or N,N'-dibenzyl-imidazolidine;
and heY~ahydropyrimidines substituted on both nitrogen atoms, e.g. N,N'-di-n-hexyl-hexahydropyrimidine (see Belgian Patent No~ 62~,519). J
Le ~ 20 688 r Other formaldehyde-releasing compounds used according to the invention include methylolmelamines such as hexa-methylolmelamine in which all or part of the hydroxyl groups may be etherified or es-terified. Hexamethylol-melamine need not be used in its pure form but may be used in the form of products having a somewhat lower forrnalde-hyde content or containing higher molecular weight conden-sation products. The hexamethylolmelamine may be prepared by known methods, e.g. by the reaction of approximately l mol of melamine with approximately 6 mol of aqueous formaldehyde solution (see "Helvetica chimica acta", 24, page 3lS ~, Swiss Patent No. 197,486 and ~ouben-Weyl, "Methoden der organischen Chemie", Vol.8, page 242).
Instead of hexamethylolmelamine, its esters or ethers may be used, as already mentioned above; these may be regarded as masked methylol compounds. Both in the present case and in the following cases described it is suitable to use ethers or esters, in particular the lower alkyl ethers such as, for example, methyl, ethyl, propyl, 2G butyl and allyl ethers; from 1 to 6 hydroxyl groups may be etherified. Suitable esters include in particular the lower aliphatic carboxylic acid esters such as acetates and propionates. Methylol melamines may, of course, also be used; these should not contain more than 5 and preferably contain 3 to 5 methylol groups per mol, and the methylol groups may be partly or completely ether-ified or esterified. The compounds may be prepared by known methods of reacting melamine with the desired quantity of formaldehyde, optionally followed by etherification or esterification of the methylol compounds obtained (see Houben-Weyl, 'IMethoden der organischen Chemie", Vol.8, page 358~. These processes generally do not yield chemically uniform compounds but mixtures of various methylol compounds, which are also suitable. What has Le A ~O 688 been said o~ hexamethylolmelamine applies analogously to the esters and ethers. The ~ollowiny are examples of such compounds: pentamethylolmelamine acetate and pentamethylolmelamine propionate.
Te-tramethylolhydrazodicarbonamide may also be used.
Its hydroxyl groups may optionally be partly or completely etherified or esterified. Tetramethylolhydrazodicarbon-amide, which is prepared by the reaction of hydrazodicar-bonamide with 4 mol of formaldehyde (see Houben-Weyl, "Methoden der organischen Chemie", Vol. 14/2, page 352) is preferably used in its crystalline form. Instead of using the pure compound (m.p. 149C), resinous condensation products containing ~arying quantities of ~ormaldehyde may be used. Tetramethylolhydrazodicarbonamide acetate l~ and propionate are examples of its esters and ethers.
The ~ollowing compounds may alsG be used as formal-dehyde-releasing compounds: tetramethylol-acetylene-diurea, in which the hydroxyl groups may be completely or partly etherified or esterified. Tetramethylol-acetylene-diurea need not be used in its pure form but may be used in the form of products having a somewhat lower proportion of formaldehyde or containing higher molecular weight condensation products. Tetramethylol-acetylene~diurea may be prepared by known methods, e.g. as described in Houben-Weyl~ "Makromolekulare Chemie", 2, page 353.
The following are examples of its esters and ethers:
tetramethylol-acetylene-diurea-tetramethylether, and tetramethylol-acetylene-diurea-tetraacetate. Other suit-able formaldehyde-releasing compounds include methylol 3~ compounds, in particular N-methylol compounds and their derivatives, in which the hydroxyl groups are optionally completely or partly etheri~ied or esteriEied, e.g. N9N'-dimethylol-urea, dimethylol-urea-dimethylether, N,N'-dimethylol-urone-dimethylether, methylene-bis-(methylol-urea-methylether) and dimethylol-urea-di-n-butyl ether.
N-substil:uted l,3,5-dioxazines which May be substitut-Le A 20 688 - ~3 -ed on the nitrogen atom by straight-chained or branch-chained, saturated or unsaturated, substituted or unsubstituted alkyl, cycloalkyl, aryl or aralkyl groups, and wherein the substituent may also contain further dioxazine rings, are also suitable. The following are examples of such com-pounds: N-allyl-, N-butyl-l N-isobutyi-, N-cyclohexyl-, N-phenyl-, N-(~ hydroxyethyl)- and ~-acetic acid-ethyl-ester-1,3,5-dioxazine, and N,N'-ethylene-bis-(1,3,5-diox-azine).
The dioxazines may be prepared by the known method of reacting the corresponding amines with an excess of formal-dehyde. The mixtures of compounds with condensation products containing varying quantities of ~ormaldehyde such as are obtained from such a method of preparation may be 1~ used insteaà of the pure compounds.
Paraformaldehyde and hexamethylenetetramine may also be used. Exceptionally firm bonding has been obtained with the following compounds: hexamethylolmelamine, hexam~thylolmelamine-pentamethylether, mixtures of hexa-~o methylGlmelamine-tetramethylether and trimethylether, pentamethylolmelamine-trimethylether, tetramethylolhydrazo-dicarbonamide, tetramethylol-acetylene-diurea, N,N'-dimeth-ylol-urea, N-methylol-dicyandiamide, methyleneamino-acetonitrilet N-allyl-dioxazine, N-phenyl-dioxazine, l-aza-3,7-di-oxa-bicyclo~3,3~-octane and hexamethylenetetramine.
Rubber compositions containing the above combination of bonding agents have excellent adherence to iron, cGpper, brass, zinc, bronze, aluminium and other reinforcing metals. Typical rubbers suitable for such mixtures o~
bonding agents are those of the diene type such as natural rubber, polyisoprene, polybutadiene, s-tyrene-butadiene copolymers, acrylonitrile-butadienerubber, chloroprene rubber, EPDM and mixtures of these types of rubber.
The rubber mixtures should contain the usual con-stituents such as reinforcing carbon blacks, inactiveLe A 20 68 2~$

and active fillers such as silicates and zinc oxides, processing auxiliaries, sulphur and vulcanization accel-erators.
Particularly suitable accelerators are the sulphen-amides derived from 2-mercaptobenzothiazole, such as, for example, N-cyclohexyl-thiobenzothiazole, N-morpholino-thiobenzothiazole, and N,N-dicylohexyl-thiobenzothiazole.
Other accelerators may, of course, also be used, either alone or in combination. Examples include thiurams, mercaptobenzothiazole and dithiocarbonates.
Vulcanization retarders may also be added. The cobalt compounds according to the invention themselves act as accelerators and may therefore be used without additional acceleratingcomponents ~see German Offenlegungsschrift No. 2,736,680).
A suitable vulcanization temperature is chosen, e.g. 120 to 220C, preferabiy 140 to 180C.
The advantages of the present invention for the production of very high strength bonds with steel cables or steel cords having an untreated, brass plated, zinc plated or blank surface are illustrated by the Examples which ~ollow.
The following compositions are used for the mixtures:
Mixture A
Natural rubber (RSS l) 60`parts by weight Poly-cis-butadiene 40 " " "
Active silicate 15 " " "
Carbon black N 330 35 " " "
Zinc oxide 6 " " "
Stearic acid l part by weight Phenyl~ -naphthylamine l " " "
Aromatic mineral oil plasticiser 4 parts by weight Colop}lony 2 Sulphur 4 " " "
35 N,N-dicyclohexyl-thioben20thiazole 0.7 part by weight ~e A 20 688 __ I

26~L~

Mlxture B
Natural rubber (RSS 1)60 parts by weight Poly-cis-butadiene 40 " " "
Carbon black N 33055 " " "
Zinc oxide 6 " " "
Stearic acid1 part by weight Phenyl-~-naphthylamine1 " " "
Aromatic mineral oil plasticiser 4 parts by weight Colophony 2 " " "
Sulphur 4 " " ~
N,N-dicyclohexyl-thiobenzothiazole 0.7 part by weight Mixture C
Natural rubber (RSS 1)100 parts by weight Carbon biack N 33055 " " "
Zinc oxide 6 Stearic acid1 part by weight Phenyl-~ naphthylamirle Aromatic mineral oil plasticiser 4 parts by weight Colophony 2 " " "
Sulphur 4 " " "
N,N-dicy~ohexyl thiobenzothiazole 0.7 part by weight Mixture D
Natural rubber (RSS 1)lOG parts by weight Carbon black N 326 43 " " "
Carbon black N 539 20 " " ~
Colophony 3 " " "
Phenyl~ -naphthylamine1.5 Zinc oxide 10 " " "
Sulphur 7 " " "
N,N dicyclohexyl-thiobenzothiazole 0.7 Mixtures A, B and C are prepared on laboratory mixing rollers at a roller temperature of 40C. In the case of mixture D, the basic mixture free from sulphur compounds Le A 20 688 2~
~ 16 - .
F
and accelerators is first mixed in a laboratory internal mixer at 70C and sulphur, accelerator and optionally bonding agent are subsequently added on laboratory mixing rollers at a roller temperature of 60C.
Samples measuring 20 x 15 x 6 mm are prepared to test for the bond strength by the T-test method ~see Bayer-Mitteilungen fur die Gummi-Industrie, No. 29, page 69).
Steel cords having blank, brass-plated or zinc-pla~ed surface are used in a construction measuring 7 x 3 x 0.15mm.
Vulcanization of the test samples is carried out at 150C, corresponding to the tgo value. To test for the resistance of the rubber-steel cord bond to reversion, the samples are vulcanized at 180C for 45 minutes longer than corresponds to the tgO-value. To test for ageing, the samples are kept several days in a Geer furnace at 100C or exposed to superheated steam at 120C for several hours.
The bonding values are determined at a test temperature of 80C, using tensile testing apparatus at a draw-off speed at the clamps of lOG mm/min. The values are given in terms of the maximum force in N/20 mm required to pull the cord out of the sample of rubber~ At least

4 test samples of the same construction are used for one measurement and the average obtained from these individual values is used for assessment.
Examp 1 e The properties of the compounds claimed for bonding brass-plated steel cord are illustrated in E~ample l.
Table 1: Some representative cobalt phenolates for bonding brass plated steel cord (using vulcan-izates of B type mixture; the cobalt content is 0.5 parts by weight/lO0 parts by weight of polymer)~
Le A 20 688 .. .... ;

32~

Vulcaniza 15UC/t90 150C/t90 180C~45' 180C/45' 180C/45' Ageing - 3 d H - 3 d HL 8 h steam _ 417N~ 262N/ 363N/ 201N/
20 mm 20 mm 20 mm 2G mm Cobalt naphthen-ate (com- - ~
parison 495 483 L08 102 '50 ,. " ~, ,.
Compound " "
Compound 47 440 323 ~15 416 166 Compound r . __ _ _ . ._ _~_ _ . _ _ _ Table 1 shows thatJ with the test samples which have been vulcanized according to the tgO-value, firm bonding is generally obtained even without the use of bonding additives.
Substantially better bonding of the rubber to metal is nevertheless obtained if a bonding agent is also added to the mixture.
Under conditions of reversion and ageing, consider-able weakening of the bond rapidly sets in when conven--tional cobalt~bonding agents are used~ In the case of cobalt naphthenate, the residual bonding level is signif-icantly lower after 45 minutes vulcanization at 180C than in samples free from any bonding additive.
By contrast, the residual bonding values obtained after vulcanization are substantially higher when the bonding additives according to the invention are used.
Even when the test samples are stored for 8 hours in superheated steam at 120C, the residual bonding strength, for example in the case of compound 46, is still 166N~20 mm whereas in the case of cobalt naphthenate virtually no bond remains.
Le A 20 688 '1 63L~i The superiority of the group of compounds claimed according to the invention over the cobalt bonding agents used in the present state of the art becomes even clearer when mixtures with high sulphur contents, such as are conventionally used as bonding mixtures for the belt inserts of radial tyres, and with high cobalt contents are compared (Table 2)~ ~
Table 2: Comparison of bonding action of cobalt phenol-ates with cobalt naphthenate in mixtures of type D with high sulphur contents (cobalt content: 0.5 parts by weight/100 par-ts by weight NR).
Vulcaniz- 150C~tgo 150C/tgo 180C/45l 180C/45' . _ . _ 15Ageing - 5 d HL - 5 d HL

.
- 41lN/2Cmm 29lN/20mm 374N/20mm 239N/20mm cobalt naphthen-ate (com-parison) 463 " 444 " 58 " 88 Compound 469 " 42g " 314 " 287 "
Compound 416 " 365 " 274 " 289 "
Compound 18 458 " 398 " 303 " 18~ "
_ . .. _ _ . .. _ _ A similarly advantageous spectrum of activity is found in mixtures containing highly active silicates (Table 3).
Table3: The resistance to separation of some cobalt phenolates in vulcanizates containing highly active silicates (mixture type A) (cobalt content: 0.5 parts by weight/lO0 parts by weight of rubbe.r).

Le A 20 688 Vulcaniz- 150C/t90 180C/45' 180C/45' atior, Ageing - 3 d HL/100 C
-- -- ~ .....
Compound 52 392 N/20n~ 19~ N/2C n~ 207 N/20 mm Compound 53 494 N/20n~; 306 N/20 mm 330 N/20 n~
Compound 54 427 N/20mm 301 N/20 mm 264 N/20 n~
.. _ ~ _ . ...... .. _ Example 2 In contrast to the bond obtained on brass plated steel, the use of bonding additives in-the rubber mixture is absolutely essential for articles containing zinc-plated steel inserts.
; 15 The effectiveness of the bonding action varies sharply with the vulcanization conditions.
Thus, for example, many con~ercially available bonding agents produce a complete loss of adherence under conditions of over vulcanization.
Table 4 shows that the compounds described do not have this disadvantage. They maintain a high level of bonding strength even when vulcanization is carried out for 45 minutes at 180 C. The value obtained is fre-quently higher by a factor of 3 - 4 than that obtainable with cobalt naphthenate.
Table 4: Bonding on zinc-plated steel cord (0~5 parts by weight of cobalt/100 parts by weight of polymer) ' t I.e A 20 688 r ;

26~

Bonding Cobal-t Mixture150 C 180 C/45' Additive content t90 _ - D 55 Co naphthenate (comparison) 0.5 B 343 N/20 47 N/20 mm mm ComPcund 500.5 C 305 Compound 57O.S C 335 Compound 490.5 B 305 158 Compound 560.5 B 305 194 Compound 54005 D 313 Compound 150.5 D 375 160 Compound 250.5 D 311 131 Compound 180.5 D 339 128 Compound 510.3 D 106 . . , _ . _ Example 3 The compounds described provide good bonding values even on raw steel (Table 5).
Table 5: Bonding on raw steel cord (Cobalt contento 0.5 parts by weight/100 parts by weight of polymer).
BondinsType of Bond strength under additive mixture vulcanization 150C/tgo Cobalt naph-thenate (comparison) D 62 N/20 mm Compound 50 C 124 Compound 25 B 106 Compound 54 B 134 Compound 47 B 165 Compound 49 B 126 Compound 15 D 112 Compound 18 D 95 3S Le A 20 688 F
Example 4 The following Example illustrates that vulcanizable rubber mixtures containing both the cobalt naphthenates mentioned above and formaldehyde donors based on methylol-S melamine respond particularly favourably under conditions of ageing in steam and hot air. ~i In the laboratory experiment carried out by the process described, the following results, for example, were obtained for compound 47:
The vulcanized mixture without methylol compound has a bonding value of 430 N/20 mm immediately after vulcanization. This value falls to 369 N/20 mm after 3 days' ageing in hot air. In rubber mixtures which in addition contain resorcinol and methylol compound as bonding agents, the corresponding figures are: 423 N/20 mm before ageing and 406 N/20 mm after ageing.
The differences become even more marked under condi-tions of ageing in superheated steam at 120C. Vulcan-izates free from methylol but containing cobalt have only 25% of their initial bonding value after 15 hours' ageing.
When the vulcanisates contain compound 47 as cobalt component and a methylol-melamine mixture consisting mainly of hexamethyiolmelamine pentamethylether, the bonding value only falls to 75% of its initial value. ~-Le A 20 683 :`:

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for increasing the strength of the bond between rubber and metal by the addition of a bond-increasing compound, followed by vulcanization, wherein a compound corresponding to formula (1) or formula (2):
(1) (2) is added as the bond-increasing compound to the rubber in quantities of 0.1 to 20 parts by weight per 100 parts by weight of rubber, the groups in the formulae having the following meaning:
R1 to R11 each denotes hydrogen, hydroxyl, C1-C18-alkyl, C5-C12-cycloalkyl, C1-C16-alkylthio, C6-C14-aryl, C6-C14-aryl-C1-C4-alkyl, C1-C18-alkoxy, C5-C12-cycloalkoxy, C6-C14-aryloxy, C6-C14-aryl-C1-C4-alkyloxy or halogen, and X denotes C1-C4-alkylene, sulfur, dithio, oxygen or N-C1-C4-alkyl.

2. A process according to claim 1, wherein in addition to the bond-increasing compound, a formaldehyde-releasing compound is added.

3. A process according to claim 1, wherein in addition to the bond-increasing compound, a resorcinol compound is added.

4. A process according to claim 1, 2 or 3, wherein the bond-increasing compound is added in quantities of from 0.3 to 10 parts by weight per 100 parts by weight of rubber.

5. A process according to claim 1, 2 or 3, characterized in that the bond-increasing compound is added in quantities of from 0.5 to 5 parts by weight per 100 parts by weight of rubber.

6. A process according to claim 1, 2 or 3, wherein, in the bond-increasing compound corresponding to formula (1), the groups R1, R2, R4 and R5 denote hydrogen and the group R3 denotes C6-C12-alkyl or C6-C-14-aryl-C1-C4-alkyl.

7. A process according to claim 1, 2 or 3, wherein, in the bond-increasing compound corresponding to formula (2), the groups R6, R7, R10 and R11 denote hydrogen and the groups R8 and R9 denote C1-C4-alkyl, benzyl or halogen.

8. A process according to claim 1, 2 or 3, wherein, in the bond-increasing compound corresponding to formula (2), the groups R6, R8, R9 and R11 denote C1-C4-alkyl or halogen and the groups R7 and R10 denote hydrogen, C1-C4-alkyl, phenyl or halogen.

9. A rubber composition comprising 0.1 to 10 parts by weight based on the rubber of a compound corresponding to formula (1) or (2):

(1) or (2) the groups in the formulae having the following meaning:
R1 to R11 each denotes hydrogen, hydroxyl, C1-C18-alkyl, C5-C12-cycloalkyl, C1-C16-alkylthio, C6-C14-aryl, C6-C14-aryl-C1-C4-alkyl, C1-C18-alkoxy, C5-C12-cycloalkoxy, C6-C14-aryloxy, C6-C14-aryl-C1-C4-alkyloxy or halogen, and X denotes C1-C4-alkylene, sulfur, dithio, oxygen or N-C1-C4-alkyl.

10. A rubber composition according to claim 9, which further comprises a formaldehyde-releasing compound or a resorcinol com-pound or both.

11. A rubber composition according to claim 9 or 10, wherein in the compound corresponding to formula (1), the groups R1, R2, R4 and R5 denote hydrogen and the group R3 denotes C6-C12-alkyl or C6-C14-aryl-C1-C4-alkyl.

12. A rubber composition according to claim 9 or 10, wherein in the compound corresponding to formula (2), the groups R6, R7, R10 and R11 denote hydrogen and the groups R8 and R9 denote C1-C4-alkyl, benzyl or halogen.

13. A rubber composition according to claim 9 or 10, wherein in the compound corresponding to formula (2), the groups R6, R8, R9 and R11 denote C1-C4-alkyl or halogen and the groups R7 and R10 denote hydrogen, C1-C4-alkyl, phenyl or halogen.

14. An article comprising a reinforcing metal bonded to vulcanized rubber, wherein the rubber comprises 0.1 to 20 parts by weight based on the rubber of a compound corresponding to for-mula (1) or (2):

(1) or (2) the groups in the formulae having the following meaning:
R1 to R11 each denotes hydrogen, hydroxyl, C1-C18-alkyl, C5-C12-cycloalkyl, C1-C16-alkylthio, C6-C14-aryl, C6-C14-aryl-C1-C4-alkyl, C1-C18-alkoxy, C5-C12-cycloalkoxy, C6-C14-aryloxy, C6-C14-aryl-C1-C4-alkyloxy or halogen, and X denotes C1-C4-alkylene, sulfur, dithio, oxygen or N-C1-C4-alkyl.

15. An article according to claim 14, wherein the metal is iron, copper, brass, zinc, bronze or aluminum.

16. An article according to claim 14, wherein the metal is a steel cord which is untreated or brass- or zinc plated.