CA2034251A1

CA2034251A1 - Rubber mixtures and vulcanized compounds prepared therefrom

Info

Publication number: CA2034251A1
Application number: CA002034251A
Authority: CA
Inventors: Wolfgang Hesse; Erhard Leicht; Richard Sattelmeyer
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1990-01-20
Filing date: 1991-01-16
Publication date: 1991-07-21
Also published as: EP0440036B1; ZA91373B; GR3020220T3; JPH0770373A; KR910014442A; DE59107907D1; ATE139247T1; EP0440036A1; ES2089037T3; DE4001606A1

Abstract

Abstract of the discloure:

Rubber mixtures and vulcanized compounds prepared there-from having improved adhesion to reinforcing supports Preparation of vulcanizable rubber mixtures and vul-canized compounds thereof having improved adhesion to reinforcing supports baaed on steel cord and textile fibers, in which effective amounts of modified novolaks are added to the rubber mixtures as adhesion promotors, which are prepared by simultaneous reaction of polyhydric phenols with aldehydes and unsaturated hydrocarbons with acid catalysis at elevated temperature and do not contain any troublesome proportions of free starting phenols.
Rubber mixtures according to the invention can be proces-sed in an easy and advantageous manner without liberating toxic substances which might be detrimental to persons working in processing and/or to the environment. Rubber mixtures according to the invention which can be prepared, processed and vulcanized by known methods by thorough mixing of the components are highly suitable for the manufacture of industrial rubber articles containing reinforcing supports based on steel cord and textile fibers such as, for example, tubings, fan belts, conveyor belts and tires, the resulting vulcanized compounds having excellent adhesion properties to the reinforcing supports mentioned in combination with a surprisingly good constancy of properties upon exposure to moisture.

Description

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Description 139.170 Rubber mixtures and vulcanized compounds prepared there-from having improved adhesion to reinforcing supports The invention relates to rub~er mixtures which are vulcanizable and have improved adhesion to reinforcing supports based on steel cord and textile fibers. The rubber mixtuxes contain modified novolaks as adhesion promotors, which are obtainable by simultaneous reaction of polyhydric phenols with aldehydes and unsaturated hydrocarbons with acid catalysis at elevated temperature by known methods and do not contain any troublesome proportions of free starting phenols. The novolaks thus modified have the further advantage of preventing the exposure of environment and worktrs to toxic levels when they are used in the preparation and processing of rubber mixtures, which is the case, as is known, when comparable rubber mixtures containing or releasing free resorcinol are prepared and processed, due to the toxic material liberated.

When industrial rubber articles are manufactured, the use of resorcinol for achieving good adhesion of the rubber to reinforcing supports, such as, for example, textile fiber fabrics or steel cord, is part of the prior art, as described, inter alia, in a number of patents, for example US Patent No. 4,148,769, DE-AS 2,002,023, and DE-AS 2,414,789. According to the prior art, resorcinol or precondensation products prepared therefrom can be used as adhesion promotors having specific activity, in order to achieve the desired good adhesion properties. The resorcinol precondensation products used are products such as can be obtained, for example, by the condensation reaction of formaldehyde or compounds releasing formal-dehyde with resorcinol or mixtures of resorcinol and a further phenol. Since the preparation of the - 2 - ~ 3l~c~
precondensation products, as i8 known, takes place in the presence of a substantial stochiometric excess of resor-cinol or resorcinol + phenol, relative to the formal-d~ahyde, the products thus finished contain a substantial proportion of free phenolic starting components which, in plarticular in the case of resorcinol, can be up to 20 ~
by weight, relative to the precondensation product prepared.

However, with respect to rubber manufacture, the hitherto known prior art in general has the ~erious disadvantage that the incorporation of resorcinol or its precondensa-tion products in the rubber mixture leads to extensive smoking and fuming, since the mixing is carried out at relatively high temperatures and in particular the free resorcinol escapes into the atmosphere to a large extent.
HoweYer, due to the toxic properties of resorcinol, this is highly undesirable. This serious disadvantage also occurs when precondensation products of resorcinol are used. Noreover, resorcinol and its mixed condensation products with phenol have only low affinities for the customary unpolar rubber types, due to their pronounced polar structure.

Accordingly, the object of the present invention was to overcome the disadvantages mentioned when resorcinol or its precondensation products are used and to provide a product which can be used during rubber manufacture in rubber mixtures without difficulties and without losing the advantageou~ effect of resorcinol and without pollut-ing the environment or representing a health risk to persons involved in the rubber manufacture.

Surprisingly, it has now been found that the abovemen-tioned disadvantages can be overcome by replacing the resorcinol or its precondensation products with aldehydes in the rubber mixtures with modified novolaks, which are prepared by simultaneous reaction of polyhydric phenols with aldehydes and unsaturated hydrocarbons in the 2 ~ 3 L~ vj 1 presence of acid catalysts at elevated temperature and do not contain any troublesome proportions of free starting phenols.

~y virtue of the invention, the abovementioned dis-advantages can be avoided and the pollution of the environment and the exposure of the workers handling these substances to toxicologically unsafe materials can be substantially reduced. The rubber mixtures prepared according to the invention can be processed more easily and advantageously, compared with the corresponding mixtures having known adhesion promotors based on resor-cinol, and, due to the less polar structure of the modified novolaks used they also lead to increased ad-hesion of the rubber mixtures to the reinforcing sup-ports. The observation that the adhesion is improved according to the invention and, after intensive exposure of the vulcanized rubber mixtures to moisture, shows a smaller decline at higher temperatures than the hitherto known comparable vulcanized systems is also of signifi-cant importance.

Accordingly, the invention relates to rubber mixtures which are vulcanizable and have improved adhesion to reinforcing supports based on steel cord or textile fiber and contain vulcanizable rubber, adhesion promotors, vulcanizing agents, curing agents, fillers and customary additives, and to the vulcanized compounds of these rubber mixtures, which contain modified novolaks as adhesion promotors, which are prepared by simultaneous reaction of polyhydric phenols with aldehydes and un-saturated hydrocarbons in the presence of an acid catalyst at elevated temperature and do not contain any troublesome proportions of free starting phenols.

The modified novolaks used according to the invention can be prepared by known methods, as described, for example, in German Patent No. 2,254,379.

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The phenol components which can be used for preparing the modified novolak resins used according to the invention are polyhydric mono- or multinuclear phenols, preferably those carrying one or more hydroxyl groups on the same aromatic ring, such as, for example, pyrogallol, hydro-q~inone, catechol, resorcinol, preferably resorcinol.
Suitable polyhydric and polynuclear phenols are also condensation products of monohydric phenols and oxo compounds. These can be added as such or prepared in situ during the reaction.

The aldehydes which can be used are alkanals or aryl-alkanals, preferably (C1-C10)-alkanals or (C7-C10)-aryl-alkanals, such as, for example, formaldehyde in aqueous solution or in the form of paraformaldehyde or trioxane, acetaldehyde, also in the form of substances releasing acetaldehyde, higher aldehydes, such as, for example, butyraldehyde, hexanal, octanal, nonanal. Formaldehyde and compounds releasing formaldehyde are particularly preferred.

The unsaturated hydrocarbons which can be used are natural or synthetic compounds having one or more carbon-carbon double bonds, in the latter case also those having conjugated double bonds. The natural unsaturated com-pounds which can be used are unsaturated fatty acids, fatty oils derived therefrom, fatty amides or fatty alcohols.

Suitable ~tarting compound~ are furthermore unsaturated natural products based on terpene, for example turpentine oil, rosin. The synthetic unsaturated hydrocarbon compounds which can be used are alkenes, dienes or even higher unsaturated hydrocarbons, such as, for example, butene, isobutene, isooctene, isononene, isododecene, or di-unsaturated compounds, such as, for example, buta-diene, isoprene, chloroprene, dichlorobutadiene, dicyclo-pentadiene. Acetylenically unsaturated compounds, suchas, for example, acetylene or (Cl-C1O)-alkyl- or di(Dl-C10)-alkylacetylenes are also suitable. Examples of suitable compounds are prefera~ly vinylaromatics, in particular vinyltoluene, and particular preferably styrene.

Examples of suitable rubber types which are vulcanizable with sulfur are preferably natural rubber, polyisoprene, polybutadiene, styrene/butadiene rubber, acrylonitrile rubber, butyl rubber, ethylene/propylene/diene terpolymer rubber or mixtures thereof, such as conventionally used in the tire industry or for the manufacture of industrial rubber goods. The further customary components can be, for example, fillers, such as carbon blacks, silicas, chalk, kaolins and pigments of inorganic or organic nature, such as titanium dioxide, iron oxide and phthala-cyanine dyes. Further customary components are, forexample, vulcanizing agents from the group comprising sulfur and sulfur-donating compounds and vulcanizing aids, such as, for example, accelerators and activators.
~he use of additives, for example from the group compris-20 ing stearic acid, zinc oxide, anti-ageing agents, tacki-ness-increasing resins, as well as metal compounds for the additional improvement of rubber adhesion to steel cord, for example salts of cobalt or nickel of single-chain carboxylic acids, preferably naphthenic acids or isooctanoic acid, is also customary. Furthermore, additions of mineral oils and plasticizers, such as, for example, phthalic esters is also customary for improving processing.

~ he modified novolak resins used according to the inven-tion can be cured by adding conventional curing agents, such as, for example, hexamethylenetetramine or methylene donors in the form of melamine resins of suitable com-position. For this purpose, the melamine resins can be used in pure form as crystalline or liquid products or, preferably, as flowable products after previous adsorp-tion on suitable solid support materials, which can be processed in an advantageous manner.

2~3~2l3 1 The vulcanizable rubber mixtures according to the inven-tion can be prepared in the usual manner, for example in internal mixers or on mixing rolls. For the purpose of improved distribution of the adhesion promotors according to the invention, it may be advantageous to increase the mixing temperature during any desired mixing phase to a value above the melting range of the added adhesion promotor resin. Furthermore, it is important that the curing a~ents are, if possible, only incorporated at the end of the mixing process at moderately high temperatures (in general at 80 to 100C) so as to avoid a premature reaction with the crosslinkable resin components.

As for the rest, the selection of components and deter-mination of their relative amounts in percent in the vulcanizable rubber mixtures can be carried out by known criteria. The mixing of the components at elevated temperatures can also be carried out in a known manner, for example at 100 to 160C, inter alia, for example in conventional heatable internal mixers or on conventional heatable mixing rolls. Likewise, the w lcanization of the vulcanizable rubber mixtures according to the inven-tion can be carried out in a known manner at standard temperatures in conventional heatable apparatuses, if desired under pressure.

The proportion of the adhesion promotors according to the invention in the vulcanizable rubber mixtures is not critical and can be preferably up to 20 % by weight, in particular up to 15 ~ by wei~ht, particularly preferably 0.5 to 10 ~ by weight, relative to the vulcanizable rubber mixture. The additional use of other known adhesion promotors is in general also possible, as long as they do not cause or have any disadvanta~eous or detrimental effects.

The composition of the modified novolaks used according to the invention as adhesion promotors can be varied in a wide range. The molar ratio of the components polyhy-~ ~j 3 ~ ;.3 1 dric phenols: unsaturated hydrocarbons s aldehydes ispreferably 1 : 0.1 : 0.3 to 1 : 1.5 : 0.95, in particular 1 : 0.4 : 0.5 to 1 : 1.1 to 0.8. Particularly preferred components are resorcinol, formaldehyde and styrene. To S further characterize the modified novolak resins usable according to the invention as adhesion promotors, for example, the melting point, the viscosity of their solutions in a suitable organic solvent and the hydroxyl number of the resin can additionally be used, these values being in the usual range for novolaks. Prefera-bly, those resins are used which are present under standard conditions in solid small-particle form. Their melting point should at least be hlgh enough to prevent the products from agglomerating upon storage at room temperature and during transport.

The vulcanizable rubber mixtures according to the inven-tion which can be prepared, processed and vulcanized by known methods are highly suitable for the manufacture of industrial rubber articles, preferably of those contain-ing reinforcing supports based on steel cord or textilefiber. They can be used in particular for the manufac-ture of tubings, fan belts, conveyor belts and tires, since they have not only excellent adhesion properties to the reinforcing supports mentioned but also a surpri-singly good consistency of properties upon exposure tomoisture.

The invention is illustrated in more detail by the examples which follow. pbw denotes parts by weight and ~ denotes percent by weight, unless stated otherwise.

~xample 1 Preparation of a modified novolak 770 pbw of resorcinol, 140 pbw of toluene and 0.75 pbw of concentrated sulfuric acid are heated under reflux in a reaction vessel equipped with stirrer, thermometer and - 8 - 2~3 reflux condenser with water separator. After reaching 140C, 454 pbw of a 37 % strength by weight aqueous formaldehyde solution and 364 pbw of styrene are added dropwise separately to the mixture from two separate metering devices over a period of three hours with stirring at such a rate that the amounts of styrene and aqueous formaldehyde run in per time unit are propor-tional to the respective total amount. During the addition, the temperature of the mixture increases to 150C. After heating under reflux for 3.5 hours, 382 pbw of an aqueous phase containing 0.37 % of formaldehy~e and 0.15 % of resorcinol have been deposited in the water separator. 0.96 pbw of magnesium oxide is then added to the batch, and the solvent is distilled off at 200C
under a pressure which has been reduced to 100 mbar. The distillation residue is then cooled to give 1170 pbw of a red-brown resin having a melting point of 130C. The free resorcinol content of the resin is < 1 % by weight.
The viscosity of a 50 % strength by weight solution of the re~in in methoxypropanol is 5300 mPa s at 20C.
:
~xample 2 Example 1 is repeated, except that 340 pbw of 37 %
strength by weight aqueous formaldehyde solution are used instead of 454 pbw and 582 pbw of styrene are used instead of 3~4 pbw. This gives 1387 pbw of a red-brown resin having a melting point of 80C. The free resor-cinol content of the resin is ~ 1 % by weight. The viscosity of a 50 % strength by weight solution of the resin in methoxypropanol is 552 mPa ~ at 20C.

ExEmple 3 The modified novolaks prepared according to Examples l and 2 are each tested for their effects in vulcanizable rubber mixtures. For this purpose, a rubber base mixture A is first prepared at about 150C by thorough mixing of the starting components. Of this rubber base mixture A, 9 ~ ~ 3 i~
s~itable amounts are removed Ln each case and mixed in a further mix.ing step B at temperatures above 100C with the adhesion promotor to be tested and then, in a third mixing step C, the sulfur, the accelerator and the curing agent are mixed in at temperatures below 100C according to the following compositions:

Rubber based mixture A comprising:
100 pbw of natural rubber 40 pbw of carbon black 15 pbw of active ~ilica 5 pbw of zinc oxide 1 pbw of stearic acid 1 pbw of anti-ageing agent Mixing step B:
162 pbw of rubber base mixture A are initially introduced in each case and at a temperature of 130C
2.5 pbw of adhesion promotor resin from Example ~= Example 3) or from Example 2 (= Example 4) or customary resorcinol ~= Comparative Example 1 not according to the invention) are mixed in.
`:
Mixing step C:
164.5 pbw of mixing step B are initially introduced in each case and at a temperature of 90C
4.0 pbw of sulfur 0.8 pbw of 2-(4-morpholinylthio)benzothiazole and 3.85 pbw of melamine resin powder ~65 % strength on support material are mixed in.

The behavior of the respective mixture upon mixing in the respective adhesion promotor in the mixing step B and during the subsequent mixing in mixing step C and also in the subsequent vulcanization of the respective vulcaniz-able mixture obtained from mixing step C at 145C over a period of 60 minutes in a test specimen is described in Table 1.

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_ 10 --The following characteristic propertie~ are tested by methods customary in practice on the vulcanized compound~
from Examples 3 and 4 and Comparative Example l resulting from the vulcanization as test specimen~:
a) - tear strength and elongation at break according to b) - breaXing force and degree of coverage in the steel cord adhe~ion test.

The results are listed in Table l in summarized form.

Of particular importance are the results of the steel cord a~hesion test, in which, according to a customary method, steel cord of dimensions 4 x 0.25 mm having a copper content of the brass plating of about 67 % is embedded in special molds in the vulcanizable rubber mixture, which is then vulcanized. The vulcanized speci-mens are then stored at 23C and 50 % relative humidity (r.h.) and at 90C and lO0 ~ relative humidity (r.h.).
The force necessary to tear out the steel cord filaments from the w lcanized rubber mixture is then measured and the degree of coverage of the steel cord is evaluated.
This is understood to mean the wire area of the pulled-out cord covered with rubberl which is estimated in steps of lO % and rated by numerical values from l to 10.
Consequently, the numerical ratings 1, 2, 3 ... lO denote lO, 20, 30 ..... lO0 % of covered wire area. The results given in Table l are the average measured values of ten individual tests in each case.

As can be seen from the measurement values in ~able 1, using the vulcanized rubber mixtures according to the invention from Examples 3 and 4, higher tear-out forces and degrees of coverage were obtained than in the com-parative system using resorcinol from Comparative Example 1. Moreover, in Examples 3 and 4 according to the invention virtually no irritations of the workers due to smoke formation or odor during processing and also virtually no pollution of the environment due to toxic 2 ~ t`~ L~

material, compared with Comparative Example 1 in which resorcinol is used, take place. This is clearly evident from the test results listed in Table 1 below.

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T~ible 1 Tests carried out IExample 3 Example 4 Comparative _ Example 1 Vulcanized rubber mixture containing adhesion promotor resin from Example 1 Example 2 Resorcinol _______________________________________ Test result~
_____________________. ~_______________________________________ Preparations of mixtures B and C
and vulcanization:
Smoke formation none none very strong Odor rubber rubber strong odor odor resorcinol odor _____________________ _________________________________________ Vulcanized compound:
Tear strength (MPa) 23.0 20.8 13.4 Elongation at break 349 374 329 - (%) (= % of longi-tudinal elongation, relative to the length of the starting test specimen) Steel cord adhesion:
Force/degree of coverage ~(N/cm)/
numerical rating]
af ter 3 days of storage 346/9 362/9 252/9 at 23C and 50 % r.h 7 days of storage 155/3 170/3 141/1 at 90C and 100 % r.h.

Claims

1. A rubber mixture which is vulcanizable and has improved adhesion to reinforcing supports based on steel cord or textile fibers and contains vulcanizable rubber, adhesion promotors, vul-canizing agents, curing agents, fillers and customary additives, and a vulcanized compound of this rubber mixture, which contains modified novolak as adhesion promotor prepared by simultaneous reaction of polyhydric phenols with aldehydes and unsaturated hydrocarbons in the presence of an acid catalyst at elevated temperature and does not contain any troublesome proportions of free starting phenols.

2. A rubber mixture of claim 1 wherein the adhesion promotor was prepared from resorcinol, an aldehyde of 1 to 10 carbon atoms and an unsaturated hydrocarbon.

3. A rubber mixture of claim 1 wherein the adhesion promotor was prepared from resorcinol, formaldehyde or a compound releasing formaldehyde and a vinylaromatic hydrocarbon.

4. A rubber mixture of claim 1 wherein the molar ratio of polyhydric phenols : unsaturated hydrocarbons : aldehydes during the preparation of the adhesion promotor is 1 : 0.1 : 0.3 to 1 :

1.5 : 0.95.

5. A rubber mixture of claim 1 wherein the molar ratio of polyhydric phenols : unsaturated hydrocarbons : aldehydes during the preparation of the adhesion promotor is 1 : 0.4 : 0.5 to 1 :
1.1 : 0.8.

6. A rubber mixture of claim 1 which contains up to 20% by weight of adhesion promotor, relative to the vulcanizable rubber mixture.

7. A method of preparing an industrial rubber articule con-taining reinforcing supports based on steel cord or textile fibers comprising vulcanizing a rubber mixture of claim 1 to said reinforcing supports.

8. The method of claim 7 wherein the industrial rubber article is selected from the group consisting of tubings, fan belts, conveyor belts and tires.

9. An industrial rubber article containing reinforcing supports based on steel cord or textile fibers by vulcanizing a rubber mixture of claim 1 to said reinforcing supports.