Cobalt Bearing Polymeric Compositions
The invention relates to polymeric cobalt bearing compounds, in particular for use
as metal-rubber adhesion promoters (RAPs) in such products as tires, belts and
hoses.
Many formulations have been tested for this type of application. According to
current practice, cobalt bearing substances are typically used. Cobalt indeed
appears to have an essential function in achieving the desired adhesion. Known
active substances are a.o. cobalt stearates, naphthenates, resinates, decanoates,
boro-decanoates, and many other forms of acylates. Examples can be found in
GB 1338930, EP 0065476, US 4340515, and GB 972804.
While these substances appear to enhance the metal-rubber adhesion, all of them
also come with significant drawbacks. Most of these RAPs show e.g. a relatively
high bioavailability of cobalt, as indicated by aqueous leaching tests. This is a
concern in view of the well known toxicity of cobalt. Other RAPs, such as those
based on natural products, may offer a rather variable quality only.
As RAPs are added to the rubber, they may, under certain circumstances, and as
an undesired side effect, interfere with the characteristics of the rubber. One
therefore generally tends to minimize the additions of RAPs, while nevertheless
ensuring that a sufficient cobalt concentration is obtained in the rubber. This leads
to the use of RAPs with a relatively high cobalt concentration, a feature tending to
further exacerbate their toxicity.
The goal of the present invention is therefore to provide a RAP formulation
combining a high cobalt concentration with a low solubility of cobalt in aqueous
media.
To this end, and according to the invention, a family of high cobalt (Co)
concentration polymers is presented, which offer low cobalt solubility, yet provide
excellent properties as adhesion promoters.

The polymer according to the invention comprises Co-carboxylate sequences, with
a Co content of at least 3% by weight, and with a mean molecular weight of more
than 2000.
A polymer with a Co concentration of more than 10%, or even of more than 12%,
is preferred.
It is understood that the repeating Co-carboxylate sequences correspond to
polymer molecules with at least 2, and, preferably, 3 or more Co-dicarboxylate
groups, which form an integral part of the backbone of the polymer molecule.
In another embodiment, the polymer further comprises one ore more borate
groups.
The polymer preferably contains residual unsaturation.
The present invention further concerns an unvulcanized elastomeric composition
comprising the abovementioned polymer and rubber. It also concerns a vulcanized
composition comprising the abovementioned polymer.
Another embodiment of the present invention concerns a rubber-metal composite
obtainable by vulcanization of the abovementioned unvulcanized elastomeric
composition in the presence of one ore more metallic parts. The so obtained
object may typically be a tire, a belt, or a hose.
A further embodiment concerns the use of the abovementioned polymer as
rubber-metal adhesion promoter, in particular for the preparation of a composite
comprising an unvulcanized elastomeric composition and one ore more metallic
parts.
A further embodiment concerns the use of the abovementioned unvulcanized
composite for the manufacture of a vulcanized rubber-metal composite, in
particular for the manufacture of a tire, a belt, or a hose.

There is also disclosed a process for the synthesis of the abovementioned
polymer, comprising Co-carboxylate sequences, with a Co content of at least 3%
by weight, and with a mean molecular weight of more than 2000. The process
comprises the steps of:
- selecting a first amount of x mole of one or more mono-carboxylic acid or of a
corresponding precursor, with C4 to C36;
- selecting a second amount of y mole of an n-basic poly-carboxylic acid or of a
corresponding precursor with C4 to C36;
- selecting a third amount of z mole of a Co2+ source;
whereby 1.0 < (x + ny) / 2z < 1.2 and 0 < x / y < 1; and,
- mixing and heating the carboxylic acids and the Co2+ source at 100 to 250 °C,
thereby eliminating volatile reaction byproducts.
Synthesis temperatures above 250 °C are to be avoided as this would result in the
decomposition of carboxylic acids. It is therefore advisable to select polymers
having a melting point of less than 250 °C, as this will allow for proper stirring of
the reaction products using common industrial equipment, while minimizing
thermal decomposition. Moreover, polymers with a mean molecular weight of less
than 10000 are preferred to guarantee a sufficiently low viscosity during synthesis.
The newly developed products comprise an adequately high Co content. They
nevertheless show a low Co solubility in aqueous media, while demonstrating an
excellent activity as RAP. It thus appears that embedding the Co in polymer chains
significantly reduces the water-solubility of the Co, without impairing its activity as
a RAP.
The synthesis methods are based on the reaction between mono- and poly-
carboxylic acids, and a basic cobalt reactant such as cobalt hydroxide. Also,
borate groups can be substituted for part of the carboxylate groups. The resulting
polymers typically show a wide range of molecular weights, as demonstrated by
Gel Permeation Chromatography (GPC).

For the GPC determinations, a PL-GPC-50 from Polymer Laboratories® is used.
Columns are filled with polystyrene gel as stationary phase, and the response is
calibrated using standard supplied polystyrene solutions. Samples are dissolved in
tetrahydrofurane, which is also used as an eluent. Detection is performed by the
standard Rl-detector.
The mean molecular weight is determined by the standard calculation methods
such as commonly used in the formulation of polyesters and oil-modified
polyesters. Such a method is described in "Alkyd Resin Technology", T.C. Patton,
Interscience Publ. 1962, pp. 82, 83, 106, and 107.
The synthesis method allows for many modifications and substitutions. This can
be put to use to adjust the physical and chemical properties of the resulting
compounds. Indeed, when using only mono- and di-carboxylic acids, low melting,
linear structures are obtained. The use of tri- or tetra-carboxylic acids results in
three-dimensional structures, generally also showing a higher melting point. The
melting point is a relevant parameter, defining the preferred technique for
homogeneously mixing the desired amount of RAP with the rubber.
Unsaturated polymeric molecules can be synthesized by choosing correspondingly
unsaturated carboxylic acids as starting products. The so-obtained unsaturated
polymers tend to co-vulcanize, thereby becoming an integral part of the rubber.
This reduces the risk of a detrimental influence of the added RAP on the final
characteristics of the rubber.
The invented compounds can further be used individually or as mixtures, at the
discretion of the user. The substances can further be modified by mixing them with
reactive or non reactive diluents.
Figures 1 to 3 show examples of specific structures believed to be obtained
according to the examples below. The atom designated by M, is cobalt.

Figure 1 shows the structural formula of essentially linear polymers. R1, R2 and
R3 are alkyl groups with 5 to 36 carbon atoms, linear or ramified, unsaturated or
saturated. R2 is the central part of a di-carboxylic acid. Example 1 illustrates the
synthesis of such a compound.
Figure 2 shows the structural formula of tri-dimensional, ramified polymers. R1,
R2, R3, R4 and R5 are alkyl groups with from 5 to 36 carbon atoms, linear or
ramified, unsaturated or saturated. R2', R3', R2", and R3" are the side parts of R2
after diene addition. Example 2 illustrates the synthesis of such a compound.
Figure 3 shows the structural formula of polymers containing boron. R6, R7 and
R8, are substituents according to the structures of Figures 3a, 3b and 3c, with Ri is
R1, R2, or R3. Example 3 illustrates the synthesis of such a compound.
Example 1
The apparatus used for the synthesis comprises a round-bottomed glass reaction
vessel of 2 I capacity, equipped with a stirrer, heating means, a water cooled
condenser, and with provisions for a nitrogen flow.
Are added:
- 350 g neo-decanoic acid; and,
- 590 g dimeric fatty acids.
The mixture is stirred under nitrogen and the temperature is raised to 120 °C.
190 g of cobalt hydroxide is added in small portions over a period of 6 h, during
which the reaction temperature is slowly raised to 160 to 170 °C, so as to maintain
the viscosity within the range for the stirrer to operate normally. The reaction
mixture is kept at 180 °C for 2 h, to finalize the reaction.
The obtained product is a homogeneous, dark bluish molten mass. It is then
poured out to cool to a friable solid, which is then ground or pelletized.

The melting point, as determined by the Ball & Ring method, is 120 °C. The
product contains 11.2% Co (by weight). Its mean molecular weight is 3700.
Example 2
Are added in an apparatus similar to that of Example 1:
- 576 g of tall-oil fatty acids; and,
- 98 g of maleïc anhydride.
The mixture is stirred while the temperature is raised to 90°C. Is then added:
- 40 g of water.
The reaction mix is heated to about 100 °C for 30 minutes. Then, is added in small
portions:
- 190 g of cobalt hydroxide.
After the addition of the first 50 g of cobalt hydroxide, the temperature is slowly
raised to 120 °C, thereby boiling off the excess and reaction water. Then addition
is continued over a period of 6 h, increasing the temperature as needed to allow
stirring. With the final addition, the temperature reaches about 250 °C. This
temperature is maintained for an additional 2 h.
The obtained product is a dark bluish molten mass, which is poured out to cool as
a dark bluish friable solid product. It can be ground to a powder or pelletized.
The melting point is 220 °C, and the Co concentration 14.6%. The mean molecular
weight is 2800.
Example 3
Are added in an apparatus similar to that of Example 1:
- 365 g of neo-decanoic acid;
- 490 g of dimeric fattv acids: and.

- 15.25 g of glacial acetic acid.
The components are mixed and the temperature raised to 80 °C. Then, is added in
small portions:
- 180 g of cobalt hydroxide.
During the addition, the temperature is raised as necessary for the normal working
of the stirrer. Finally, a temperature of 165°C is reached. At this temperature the
reaction mix is kept for 1h.
Then, is added slowly:
- 39.40 g of tributyl orthoborate ester.
The temperature is slowly increased to 230 °C, where it is kept for 1 h.
The obtained product is a dark bluish molten mass, which is poured out to cool as
a dark bluish friable solid product. It can be ground to a powder or pelletized.
The melting point is 150 °C, and the Co concentration 11.4%. The mean molecular
weight is 4900.
Example 4
Experiments are performed to determine the aqueous solubility of the Co in the
invented polymers. These tests are relevant as indicators for the bioavailability of
Co.
Pure water, as well as a physiologic aqueous solution containing 0.9 g/l NaCI, are
used as a solvent. The leaching tests are conducted according to the OECD-105
standard flask method, as described in the OECD Guideline for the Testing of
Chemicals, adopted on 27 July 1995, and in the Official Journal of the European
Communities L 383 A, 54-62 (1992).

The solubility of 3 typical polymers according to the invention is compared to that
of 3 commercial products. The results are summarized in Table 1.

The polymers according to the invention release only very limited amounts of Co.
Results of less than 200 mg/l are considered as acceptable.
Example 5
In this example, the effectiveness of the invented polymers as RAPs is assessed
and compared to that of commercial products.
To this end, the maximum rubber to brass-coated steel adhesion force is
determined for rubber compounds containing different RAPs. All RAPs are added
at a level equivalent to 0.2 phr (per hundred rubber) Co, which is a typical
concentration.


The rubber compounds, including the RAP, are prepared according to industry
standard procedures in a 1.5 I nominal capacity internal mixer. The dump
temperature is 157 to 162 oC. The sulfur and DCBS are mixed according to
industry standard procedures at a temperature of 60 °C on a two-roll open mixer.
Test pieces with 10 cords each are made according to Russian Standard GOST
14863-69, and subsequently aged by keeping them in water with 5% NaCI at 23
°C for 7 days. The vulcanization temperature is 160 °C, for a time of t95 + 8
minutes. This corresponds to about 14 to 18 minutes, depending on the
compound. The cord used is Bekaert® 3*7*0.22 brass coated steel.
The maximum forces needed to pull the wire out of the rubber, both before and
after aging of the test pieces, are reported in Table 3.


The results show a significant enhancement of adhesion when using the RAPs
according to the invention, both before and after aging, comparable in magnitude
to an industry standard product. This level of performance is considered as more
than adequate.
Example 6
This example is performed with materials and in conditions similar to those of
Example 5, yet with the following differences:
- Bekaert® 7*4*0.22 wire is used;
- vulcanization temperature and time are 162 °C and 15.5 minutes;
- RAP concentration is 0.15 phr Co;
- the test method is according to ASTM 2229-04;
- the number of wires per test amounts to 7;
- aging is performed at 100 °C in air for 72 h.
The rubber composition is also slightly different from Example 5, according to
Table 4.


The maximum forces needed to pull the wire out of the rubber, both before and
after aging of the test pieces, are reported in Table 5.

The results show a significant enhancement of adhesion when using the RAPs
according to the invention, both before and after aging, comparable in magnitude
to industry standard products. This level of performance is considered as more
than adequate.

Claims
1. Polymer comprising Co-carboxylate sequences, with a Co content of at
least 3% by weight, and with a mean molecular weight of more than 2000.
2. Polymer according to claim 1, further comprising one ore more borate
groups.
3. Polymer according to claims 1 or 2, having residual unsaturation.
4. Unvulcanized elastomeric composition, comprising the polymer according to
any one of claims 1 to 3, and rubber.
5. Rubber-metal composites obtainable by vulcanization of the composition
according to claim 4, in the presence of one or more metallic parts.
6. Rubber-metal composite according to claim 5, whereby said composite is a
tire, a belt, or a hose.
7. Use of the polymer according to any one of claims 1 to 3, as a rubber-metal
adhesion promoter.
8. Use of the polymer according to any one of claims 1 to 3, for the
preparation of a composite comprising an unvulcanized elastomeric composition
and one ore more metallic parts.
9. Use of the composite prepared according to claim 8 for the manufacture of
a vulcanized rubber-metal composite, in particular for the manufacture of a tire, a
belt, or a hose.
10. Process for the preparation of the polymer according to claim 1, comprising
the steps of:

- selecting a first amount of x mole of one or more mono-carboxylic acid or of a
corresponding precursor, with C4 to C36;
- selecting a second amount of y mole of an n-basic poly-carboxylic acid or of a
corresponding precursor with C4 to C36;
- selecting a third amount of z mole of a Co2+ source, whereby
1.0<(x + ny)/2z< 1,2 and 0