Note: Descriptions are shown in the official language in which they were submitted.
2~3~
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
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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
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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
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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.
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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-
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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
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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,
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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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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
DIN 53504
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
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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.
~ ~ .3 ~ 2 ., _L
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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.
