DESCRIPTION

The present invention is related to a new class of silica based nanomaterials that may find application as vulcanization activators in rubber compounds and, therefore, as a substitute for ZnO.

Recently, one of the topics for research in the field of compounds for rubber products, such as for example pneumatic tyres, has been concentrated on a possible replacement for zinc oxide (ZnO) as the vulcanization activator .

Due to the possible environmental implications thereof, it is required that the use of ZnO be limited, if not completely eliminated.

The need was therefore felt for an alternative to the use of ZnO in rubber compounds, which could ensure an effective vulcanization process at least equal to that activated by ZnO and that at the same time would not involve issues of an environmental character.

The Applicant has prepared new nanomaterials that can completely replace the presence within compounds of ZnO as the vulcanization activator according to the requirements cited above.

The object of the present invention is the use, as a vulcanization activator in compounds for the preparation of rubber products, of a silica based nanomaterial ; said use being characterized in that said silica based nanomaterial consists of particles of a Zn salt chosen from between Zn ₃ (P0 )2 and Z^SiC^ supported on the surface of silica nanoparticles .

Preferably, the nanomaterial has dimensions that are equal to or less than 100 nm by Transmission Electron Microscopy measurement.

Preferably, Zn ₃ (P0 )2 is present in a quantity of between 40 and 60% by weight in relation to the total weight of the nanomaterial.

Preferably, Zn ₂ Sio is present in a quantity of between 20 and 40% by weight in relation to the total weight of the nanomaterial .

A further object of the present invention is a synthesis method for the preparation of a nanomaterial consisting of particles of a Zn salt chosen from between Zn ₃ (P0 )2 and Zn2Si04 supported on the surface of silica nanoparticles; said method being characterized in that it comprises : a first step, wherein an aqueous solution A is produced with a pH of between 7.5 and 9.5 and comprising Zn (NO3) ₂ · 6H ₂ O at a concentration of between 10 and 50 mM, 1 , 1 , 1-Tri (hydroxymethyl ) ethane at a concentration of between 16 and 160 mM, and silica nanoparticles at a concentration of between 33.3 and 133.3 mM; and

- a second step, wherein to said solution A is added either (i) an aqueous solution comprising (NH ₄ ) ₂ HP0 ₄ at a concentration of between 5 and 50 mM once the same solution A is heated to a temperature of between 30 and 50 °C or (ii) an alcohol solution comprising tetraethyl orthosilicate at a concentration of between 30 and 300 mM once the same solution A is heated to a temperature of between 0 and 10 °C.

Preferably, the method comprises a preliminary step for the preparation of the S1O ₂ nanoparticles, wherein an alcohol solution comprising tetraethyl orthosilicate at a concentration of between 30 and 300 mM and with a pH of between 8 and 11 is maintained under agitation at a temperature between 20 and 40 °C.

Preferably, once the (Ν]¾) ₂ ΗΡθ aqueous solution or the tetraethyl orthosilicate alcohol solution has been added, the resulting solution is maintained under agitation for between 18 and 30 hours.

Preferably, the (Ν]¾) ₂ ΗΡθ aqueous solution or tetraethyl orthosilicate alcohol solution are added dropwise to the solution A.

Preferably, the solvent for the tetraethyl orthosilicate alcohol solution is ethanol.

A further object of the present invention is a silica based nanomaterial consisting of particles of a Zn salt supported on the surface of silica nanoparticles; said nanomaterial being characterized in that said Zn salt is Zn ₃ (P0 ₄ ) 2.

Preferably, the silica based nanomaterial has dimensions that are equal to or less than 100 nm by Transmission Electron Microscopy measurement.

Preferably, Zn ₃ (P0 )2 is present in a quantity of between 40 and 60% by weight in relation to the total weight of the nanomaterial.

The following are examples of non-limiting embodiments given purely by way of illustration. - Synthesis of the S1O ₂ nanoparticles -

A three necked round bottom flask was filled with 50 mL of ethanol and 3 mL of an aqueous solution of ammonia at 28%. The resulting solution was maintained under agitation and heated to a temperature of 30 °C. At this point 1.7 mL of tetraethyl orthosilicate were added and the solution maintained firstly for 5 hours again at 30 °C and then for 16 hours at 25 °C. The S1O ₂ nanoparticles were precipitated by means of centri fugation at 8000 rpm for 10 minutes and then again dispersed twice in a solution of ethanol and water. Finally, the nanoparticles thus obtained were dispersed in 10 mL of ethanol.

- Synthesis of nanomaterials consisting of Zn ₃ (P0 )2 particles supported on silica nanoparticles -

100 mL were prepared of an aqueous solution comprising Zn (NO3) ₂ ^' 6H ₂ O at a concentration equal to 33.4 mM, 1,1,1- tri (hydroxymethylethane ) at a concentration equal to 1%, and the silica nanoparticles dispersed in 10 mL of ethanol as described above. By means of the addition of ammonia the pH of this solution was raised to the value of 8.5. This solution was heated to 50 °C and maintained under vigorous agitation. Subsequently, to this solution were added dropwise 100 mL of an aqueous solution of 20 mM of (Ν]¾) ₂ ΗΡ0 . The period of time for the addition lasted 30 minutes. The resulting solution was maintained under agitation at 50 °C for 24 hours, the nanomaterial was then collected by means of centrifugation, washed three times with water and ethanol, and finally redispersed in 15 mL of ethanol .

Synthesis of nanomaterials consisting of Z^SiC particles supported on silica nanoparticles -

190 mL of an aqueous solution were prepared comprising Zn (NO3) ₂ ^' 6H ₂ O at a concentration equal to 17. 6 mM, 1,1,1- tri (hydroxymethylethane) at a concentration equal to 0.5%, and the silica nanoparticles dispersed in 10 mL of ethanol as described above. By means of the addition of ammonia the pH of this solution was raised to the value of 8.5. This solution was cooled to 0 °C and maintained under vigorous agitation. Subsequently, to this solution was added dropwise a mixture composed of 10 mL of ethanol and 0.4 mL of tetraethyl orthosilicate . The period of time for the addition lasted 20 minutes. The resulting solution was maintained at 0 °C for 24 hours under agitation. The nanomaterial was collected by means of centrifugation, washed three times with water and ethanol, and finally redispersed in 15 mL of ethanol.

- Material characterization -

The Si02 nanoparticles produced were analyzed using the "Transmission Electron Microscopy (TEM)" technique. Using this technique an average diameter for the nanoparticles of 74± 6 nm was measured.

The SEM images obtained show that the dimensions of the nanomaterials do not vary in relation to those of the respective S 1O ₂ nanoparticles insofar as the dimensional contribution of the Zn ₃ (P0 ) 2 or Zn2Si0 particles on the S 1O2 surface is virtually nil.

The nanomaterials thus obtained were tested as vulcanizing activators as a substitute for ZnO, demonstrating the ability thereof to ensure, in the same way as the latter, the efficient vulcanization of rubber compounds .

EXAMPLES OF VULCANIZATION TESTS

Two comparison compounds were prepared (Compound A and Compound B) , together with two compounds according to the invention, wherein the nanomaterials , object of the present invention, were used as vulcanizing activators. In particular, in the first comparison compound (Compound A) a vulcanization activator is not used, in the second comparison compound (Compound B) zinc oxide was used as a vulcanization activator, whilst in the two compounds according to the invention (Compound C and Compound D) the nanomaterial consisting of Zn ₂ Si0 particles supported on silica and the nanomaterial consisting of Zn ₃ (P0 )2 particles supported on silica were respectively used as the vulcanization activator.

The example compounds were obtained according to the procedure below:

- preparation of the compounds -

(1 ^st mixing step)

Before the start of the mixing, a mixer with tangential rotors and an internal volume of between 230 and 270 liters was loaded with the cross-linkable polymer base and the carbon black, thereby reaching a filling factor of 66-72% .

The mixer was operated at a speed of 40-60 revolutions/minute, and the mixture thus formed was discharged once a temperature of 140-160 °C had been reached.

(2 ^nd mixing step)

The mixture obtained from the previous step was reworked in a mixer that was operated at a speed of 40-60 revolutions/minute and, thereafter, discharged once a temperature of 130-150 °C had been reached.

(final mixing step) To the mixture obtained from the previous step were added the sulfur, the vulcanization accelerants, the stearic acid and, where provided for, the ZnO in combination with the stearic acid or the nanomaterial according to the invention, reaching a fill factor of between 63-67%.

The mixer was operated at a speed of 20-40 revolutions/minute, and the mixture thus formed was discharged once a temperature of 100-110 °C had been reached.

Table I shows the compositions in phr of the compounds described above.

The polymer based used is natural rubber.

The carbon black used is classified as N330.

The nanomaterial 1 consists of Z^SiC particles supported on silica.

The nanomaterial 2 consists of Zn ₃ (P0 )2 particles supported on silica.

N-tert-butyl-2-benzothiazylsulfenamide (TBBS) was used as a vulcanization accelerant.

N- ( 1 , 3-Dimethylbutyl ) -N ' -phenyl-p-phenylenediamine ( 6 PPD) was used as an antioxidant. In order to evaluate the rheometric, mechanical and dynamic mechanical properties thereof, respective samples were prepared from the compounds of Table I and subjected to a series of tests.

In particular, the rheometric properties were measured according to the ISO 6502 standard; the mechanical properties were measured according to the ISO 37 standard.

The values obtained from the tests listed in Table II.

TABLE II

The values of Table II demonstrate the vulcanization activator activity of the nanomaterials, object of the present invention. Indeed, the results in relation to the properties indicated above demonstrate how the presence of the nanomaterials of the present invention (Compounds C and D) , comparable to that of zinc oxide (Compound B) and different than an absence of a vulcanization activator (Compound A) , facilitates the correct vulcanization of the compound .

In conclusion, the nanomaterials of the present invention make it possible to eliminate the use of zinc oxide in rubber compounds without, for this reason, compromising in any way the vulcanization of the same compound and, consequently, the mechanical characteristics thereof .