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Description  |
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Published European Patent Application No. 13 330 describes the preparation
of reinforced rubber vulcanisates by vulcanising compositions comprising
natural rubber or synthetic diene rubber with novolak resins in the
presence of from 0.5 to 120% by weight (based on the novolak resin) of
hardeners of reactive melamine resins, which melamine resins are obtained
by reacting 1 mol of melamine with 0.5 to 6 mol of an aldehyde. The
phenolic component of the novolak resin is selected from the group
consisting of phenol, polyvalent polynuclear phenols, alkylphenols,
mixtures of alkylphenols with phenol, and possibly also containing added
amounts of resorcinol, phenylphenol or polyalkyl-phenols. This same
Specification also describes the use of the vulcanisates prepared
according to this process for the production of industrial rubber products
and vehical tires.
It has now been found that, in a process for the preparation of reinforced
vulcanisates by vulcanising compositions comprising natural rubber or
synthetic diene rubber with novolak resins in the presence of from 0.5 to
120% by weight (based on the novolak resin) of hardeners of reactive
melamine resins, which melamine resins are obtained by reacting 1 mol of
melamine with 0.5 to 6 mol of an aldehyde, it is also possible to obtain
reinforced rubber vulcanisates having good properties, more particularly a
high reinforcing effect, if, according to the present invention, instead
of or together with the above-mentioned melamine resins, other
aminoplast-forming compounds containing hydroxyl groups are used, the
total amount of aminoplast-forming compounds also being from 0.5 to 120%
by weight, based on the novolak resin.
Examples of other aminoplast-forming compounds containing hydroxyl groups
include polymethylolureas, mixed condensates of melamines, urea or other
aminoplastforming compounds and formaldehyde, and products obtained by
reacting melamine with more than 6 mol, e.g. 8 mol, of an aldehyde,
preferably formaldehyde, or the ethers, preferably the methyl ethers of
all these products. Examples of other aminoplast-forming compounds which
may be used thus include those wherein more than 1 mol of formaldehyde is
added to some of the NH bonds of melamine, and the non-etherified or
etherified methylol compounds of the available guanamines such as aceto-,
benzo- or formoguanamine. An example of such a product is
tetramethoxymethylbenzoguanamine, which may be obtained in solid form.
The preferred aminoplast-forming compounds are those which are obtained by
reacting 1 mol of urea with 2 to 4 mol of formaldehyde.
Having regard to their compatibility with rubbers, the particularly
preferred compounds are those wherein the methylol groups are at least
partially etherified by being reacted with one or more monohydric alcohols
having 1 to 12, preferably 1 to 8 carbon atoms, such as, for example,
methanol, ethanol, the various propanols and butanols, ethylhexyl alcohol,
n-octanol, nonanol and dodecyl alcohol.
Especially preferred are liquid, etherified, highly methylolated urea
derivatives prepared in a two-step process by reacting 1 mol of urea with
more than 6 mol of formaldehyde at a pH of more than 8 and at a
temperature of 45.degree. to 70.degree. C., and then reacting the reaction
product of this first step with more urea in a second step, so that an
additional 25 mol-% of urea are used to the total molar amount of
formaldehyde minus 4 mol. In order to prepare ethers, the reaction mixture
is preferably further reacted with one or more monohydric aliphatic
alcohols, e.g. those mentioned hereinbefore, according to conventional
methods, under acidic conditions and at a temperature not exceeding
50.degree. l C.
Advantageously, the water content of the reaction product is adjusted to
not more than 5%, preferably less than 1%.
Examples of melamine resins which may be used in conjunction with the other
amine resins include those which can be obtained by reacting 1 of melamine
with 0.5 to 6 mol of an aldehyde, preferably formaldehyde.
Hexamethoxymethylmelamine or corresponding higher condensed polynuclear
products or at least partially etherified trimethylol-, tetramethylol- or
pentamethylol-melamine resins are preferred. The degree of condensation of
all aldehyde resins, more particularly formaldehyde resins, and their
content of free alkylol, more particularly methylol groups, can be
regulated in the usual way by suitable adjustment of the reaction medium.
Under the vulcanisation conditions, the aminoplast-forming compounds used
according to the invention can cure the novolaks contained in the rubber
mixture. If mixtures containing the melamine resins mentioned in European
application specification No. 13 330, or mixed condensates containing
melamine are used, the weight ratio of urea to melamine in the reaction
with aldehyde is advantageously at least 1:1. Moreover, the use of the
amine resins according to the invention has the advantage that the
vulcanisates have better adhesion to metal, e.g. metal reinforcing
elements.
Examples of novolaks which may be used are those wherein the phenolic
component consists essentially of phenol, polyhydric polynuclear phenols,
such as bisphenols, e.g. diphenylolpropane or diphenylolmethane or the
homologues thereof, or mixtures of phenol and alkylphenols, such as
tert.butylphenol, octylphenol or nonylphenol, which are obtained by
condensation with aldehydes, preferably formaldehyde, under acidic
conditions. The phenolic component may also contain added amounts of
resorcinol, phenylphenol or polyalkyl-phenols. These additives may be
incorporated in amounts such that the molar ratio of the total quantity of
additives to the total amount of the other phenols is up to 1:1,
advantageously up to 1:2.
The novolaks may also contain plasticising components, such as, for
example, polyethylene glycol, colophony, tall oil or other plasticisers,
or may be modified with natural resinic acids or olefinically unsaturated
hydrocarbons such as styrene. In theory, it is also possible to use
novolak resins prepared exclusively from alkylphenols having one alkyl
group with 2 to 12 carbon atoms. However, these can only be used in
certain conditions, since they are less hardenable. For practical use,
novolak resins obtained from phenol and mixtures of phenol with cashew
nutshell oil, and from phenol with alkylphenols having 4 to 12 carbon
atoms in the alkyl moiety, more particularly p-tert.butyl-, octyl- or
nonylphenol, are preferred. The proportion of substituted phenols in the
novolak may vary, but the novolak should still be hardenable. Thus, in
general, a proportion of alkylphenol of not more than 70 mol-% of the
total phenolic component will be used.
The proportion of novolak is generally 1 to 30, preferably 3 to 20% by
weight, based on the rubber. In individual cases, larger amounts, e.g.
100% by weight or more, based on the rubber, may also be used. With such a
high proportion of novolak, hard products are obtained which differ
substantially from conventional highly elastic rubber vulcanisates in
their properties.
In principle, the optimum quantities of aminoplast-forming compounds
required to cure the novolak resins have to be determined in preliminary
tests which may be readily carried out. In general, about 1 to 80,
preferably 2 to 50% by weight of aminoplast-forming compounds are
required, to 100 parts by weight of novolak resin.
In the process according to the invention, it is also possible to use solid
powdered products such as methylol aminoplast compounds. In this case, the
effect of these compounds depends, to a critical extent, on the fineness
of the particles, the products used generally having a particle size of
not more than 100 .mu.m, preferably not more than about 50 .mu.m. Products
of this kind can be prepared relatively easily in dissolved form. However,
these types are only suitable for incorporation in rubbers to a limited
extent since they would have to be incorporated with conventional low
boiling solvents such as higher alcohols, water or esters which evaporate
in the course of the mixing process. On the other hand, for example,
certain polyglycols or oily aromatic or aliphatic hydrocarbons or high
boiling polyethers may be co-used.
It is frequently advantageous to reinforce or accelerate the activity of
the aminoplast-forming compounds proposed according to the invention by
the addition of suitable acids. Examples of such acids include mono- or
dibasic organic acids with at least 2 and not more than 20 carbon atoms or
the anhydrides thereof, e.g. phthalic acid, benzoic acid, maleic acid,
fumaric acid, tri- or pyromellitic acid and the anhydrides thereof, or
sulphonic acids such as naphthalenedisulfonic acid of sulfanilic acid.
Instead of, or together with the organic acids, it is also possible to
co-use polyester resins obtained from polycarboxylic acids and polyhydric
alcohols with an acid number of 20 or more, e.g. up to 100, which may also
be modified by monocarboxylic acids and monohydric alcohols. However, the
proportion of monofunctional components should be not more than 25
equivalent-%, based on the acid or alcohol component.
The organic acids or polyester resins used as accelerators may be added in
amounts of from 0.1 to about 5, preferably 1.5 to 3% weight, based on the
rubber. However, it is certainly possible to add larger or small amounts
of acid to obtain a controlled modification of the activity.
Theoretically, curing may also be accelerated by means of strong inorganic
acids such as phosphoric acid or acid esters of phosphoric acid. However,
this method is less appropriate on account of the precautions which must
be taken when handling such acids. Frequently, the co-use of the acids
results in a more intensive reinforcing effect than would be obtained if
they were not co-used. In the vulcanisates prepared according to the
invention, this reinforcing effect is particularly apparent as an increase
in the tension values at 100% expansion and an increase in the hardness of
the vulcanisate.
Instead of, or together with the acids, plasticisers, as conventionally
used in the processing of rubber, may also be added, for example aromatic
polyethers, esters of phthalic acid or the like.
Examples of rubbers which may be used in the invention include, for
example, natural rubber, polyisoprene, styrene rubber, acrylonitrile
rubber, polybutadiene, butyl rubber, ethylene-propylene-diene terpolymer
rubber or mixtures thereof, as conventionally used in the tire industry or
for the production of industrial rubber goods.
As fillers, the standard commercial active or inactive carbon blacks,
silicic acids, kaolins, chalk or other conventional fillers may be co-used
if desired.
For vulcanisation, sulphur is generally used together with the known
accelerators. In many cases, however, the vulcanisation can be done
without sulphur. However, vulcanisation in the presence of sulphur is
preferred. The course of vulcanisation may be adapted to the practical
requirements by a suitable choice of accelerators conventionally used in
rubber technology.
The mixtures produced according to the invention may be prepared in the
conventional way in internal mixers or on mixing rolls. During
incorporation, care should generally be taken to ensure that the mixing
temperature does exceed the melting point of the fusible solid substances,
particularly the novolak resins, at any stage during the preparation of
the mixture, so as to ensure that these substances are fully distributed.
It is also generally important that the aminoplast-forming compounds
proposed as hardeners according to the invention are incorporated in such
a way that premature condensation of these compounds themselves or
premature reaction with the novolak which is to be cured does not occur.
This may be achieved by incorporating them at the end of the mixing
operation at temperatures which are not excessively high. The temperature
depends on the type used, the quantity of acid and the length of time and
is generally 80.degree. C. to 120.degree. C., preferably over 100.degree.
C. In individual cases, it may also be higher, e.g. if no acid is present,
or lower. However, it is also possible to effect curing of the novolak
resins with the aminoplast-forming compounds used according to the
invention in a separate operation by hot-rolling the mixture before the
vulcanisation accelerator is added.
The use of the aminoplast-forming compounds in the process according to the
invention, enables the curing process to proceed favorably during
vulcanisation.
Industrial rubber articles, particularly those with metal inserts, may be
produced from the mixtures prepared according to the invention; these
mixtures may also be used for the production of vehicle tires.
The following Examples serve to illustrate the invention, and in these
Examples the amounts stated are parts by weight.
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Examples 1 to 6
Mixture comparison
1 2 3 4 5 6
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Styrene-butadiene rubber (1)
100 100
100
100
100
100
100
Carbon black N-330 (1)
50 50 50 50 50 50 50
Stearic acid 1.5 1.5
1.5
1.5
1.5
1.5
1.5
Zinc oxide 5.0 5.0
5.0
5.0
5.0
5.0
5.0
Anti-ageing agent
2.0 2.0
2.0
2.0
2.0
2.0
2.0
Sulphur 2.0 2.0
2.0
2.0
2.0
2.0
2.0
Phenol novolak (2)
-- 14.0
14.0
14.0
14.0
14.0
14.0
Mixed melamine-urea ether A (3)
-- 6.0
-- -- -- -- --
Mixed melamine-urea ether B (4)
-- -- 6.0
-- -- -- --
Mixed melamine-urea ether C (5)
-- -- -- 6.0
6.0
-- --
Urea ether (6) -- -- -- -- -- 6.0
6.0
Benzoic acid -- 3.0
3.0
-- 3.0
-- 3.0
Cyclohexylaminothiobenzothiazole
1.0 1.0
1.0
1.0
1.0
1.0
1.0
Tetramethylthiurammonosulfide
0.2 0.2
0.2
0.2
0.2
0.2
0.2
Vulcanisation (.degree.C.)
145 145
145
145
145
145
145
Vulcanisation (min)
30 30 30 30 30 30 30
Tearing strength (MPa)
22.2 21.6
23.4
22.2
20.6
22.8
20.0
Tearing expansion (%)
251 293
312
327
283
315
265
Tension values at 100%
5.3 7.5
7.8
6.0
7.8
6.9
8.4
expansion (MPa)
Tension values at 200%
16.3 15.0
15.0
12.5
14.4
14.1
15.3
expansion (MPa)
Shore hardness A (.degree.)
74 91 92 88 90 88 92
Resilient elasticity (%)
36 33 33 33 33 32 34
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Examples 7 to 10
Mixture 7 8 9 10
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Natural rubber (7) 100 100 100 100
Carbon black N 330 (8)
50 50 50 50
Stearic acid 2 2 2 2
Zinc oxide 5 5 5 5
Anti-ageing agent 2 2 2 2
Sulphur 2,5 2,5 2,5 2,5
Phenol novolak (9) 14 7 14 7
Urea ether (10) 6 3 -- --
Urea ether (11) -- -- 6 3
Cyclohexylaminothiobenzothiazole
0.7 0.7 0.7 0.7
Tetramethylthiurammonosulfide
0.2 0.2 0.2 0.2
Benzoic acid 1.0 0.5 1.0 0.5
Vulcanisation (.degree.C.)
145 145
Vulcanisation (min) 25 25
Tearing strength (MPa)
18.8 25.0 15.3 21.3
Tearing expansion (%)
298 385 242 309
Tension value at 50% expansion (MPa)
3.8 2.8 4.4 3.1
Tension value at 100% expansion (MPa)
6.3 4.7 6.6 5.0
Shore hardness (.degree.)
89 84 92 85
Resilient elasticity (%)
30 31 31 32
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REMARKS ON THE TABLES
1. Styrene-butadiene rubber containing resin and fatty acid with 23.5%
bound styrene; carbon black for high abrasive resistance. The rubber and
carbon black are used as a master batch consisting of 100 parts rubber and
50 parts carbon black.
2. Standard commercial phenol novolak with a melting point (by the
capillary method) of 80.degree. to 88.degree. C.
3. Methylol compound consisting of 1 mol urea, 1.12 mol melamine and 10.72
mol of approx. 39% aqueous formaldehyde solution, prepared by condensation
under alkaline conditions by conventional methods and subsequent
etherification with 32 mol of methanol in the conventional manner at a pH
of <3.
4. Like mixed ether A, but etherified with only half the amount of
methanol.
5. In a two-step process, urea is reacted with eight times its molar
quantity of aqueous formaldehyde solution (approx. 39%) at a pH of >10
according to the usual method. 10 mol % of melamine, based on mols (total
formaldehyde -4), are added to the reaction mixture. The resulting
reaction mixture is substantially concentrated and etherified with 20 mol
methanol, based on the total mols of melamine and urea, under the usual
conditions and then adjusted to a water content of <1%.
6. Reaction product of urea with 3 mol formaldehyde, partially etherified;
viscosity approx. 10 Pa.s at 25.degree. C., based on a solids content of
98.8% at 50.degree. C. for 1 h in vacuo; prepared by the usual method.
7. Sheet quality produced by the SMR method, with a maximum content of
volatile components of 0.8% and a hot scale of not more than 0.6%.
8. Same types as in Examples 1 to 6, but incorporated separately.
9. Standard commercial phenol novolak with a dynamic viscosity according to
DIN 53 177 at 20.degree. C., 40% in monoethylene glycol monoethyl ether,
of 170-250 mPa.s.
10. Dimethylolurea dimethyl ether (cf. Houben-Weyl, Volume XIV/1 (1963),
page 348, Example 4), as a flowing 70% powder prepared by applying the
liquid resin to a silicic acid with a specific surface area of 190 m.sup.2
/g and an average size for the primary particles of 18.times.10.sup.-9 m.
11. See 6., but as a flowing 70% powder according to 10.
DISCUSSION OF THE RESULTS
Examples 1 to 6 show the results obtained with a conventional rubber
mixture based on a styrene-butadiene rubber and using a standard
commercial phenol novolak and the aminoplast compounds according to the
invention as hardeners. The reinforcing effect is demonstrated by
comparison with a vulcanisate to which the phenol novolak-aminoplast
combination claimed according to the invention has not been added.
Examples 1 and 2 shows the reinforcing effect with hardeners consisting of
mixed ethers of melamine and urea containing a smaller amount of urea in
relation to the amount of melamine, the ether according to Example 2 being
less etherified.
Examples 4 and 6 show the increase in hardness and tension values compared
with Examples 3 and 5 and thus demonstrate the good reinforcing effect
obtained by co-using an acid.
A property common to all the phenol novolak-aminoplast combinations claimed
according to the invention is the fact that they significantly reinforce
rubber mixtures. This is clear from the increased tension values,
particularly during slight expansion, and the significant increase,
generally by 14 to 18 points, in the degree of hardness of the
vulcanisates. This also applies to other rubber mixtures, e.g. natural
rubber mixtures (cf. Examples 9 to 12).
The results show that the novolak-aminoplast combinations proposed
according to the invention bring about reinforcing effects which can be
achieved under similar conditions with hexamethylenetetramine as the
hardener, but do not have the disadvantages connected with the use of
hexamethylenetetramine. Thus, for example, no intermediate formation of
ammonia can occur. As a result, any metal inserts are not exposed to
resultant corrosion or reduction in their adhesion to the rubber.
It is not intended that the examples given herein should be construed to
limit the invention thereto, but rather they are submitted to illustrate
some of the specific embodiments of the invention. Resort may be had to
various modifications and variations of the present invention without
departing from the spirit of the discovery or the scope of the appended
claims.
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