DESCRIPTION

The present invention is in relation to a rubber compound for the preparation of an innerliner layer.

The innerliner consists of a rubber inner layer used in tubeless pneumatic tyres, i.e., pneumatic tyres that are devoid of an air chamber, in order to ensure the sealing thereof under the pressure of the air contained within the cavity of the pneumatic tyre.

Moreover, the innerliner must ensure that oxygen remains, as much as possible, confined within the cavity and does not spread into portions of the pneumatic tyre thereby leading to degradation phenomena. As is known to a person skilled in the art, increasing the thickness of the innerliner layer, if on the one hand ensures an improvement in terms of impermeability, on the other hand involves a series of disadvantages in relation to the increase in weight and to a compromise in relation to rolling resistance.

In this respect, one of the needs for the pneumatic tyre industry is that of conferring them an increasingly lower rolling resistance, and a contribution to this need is sought precisely in decreasing the thickness of the innerliner. In fact, a reduced thickness of the innerliner layer necessarily translates into a lower quantity of material used and therefore, a lower weight for the pneumatic tyre, with positive effects on the overall energy consumption of the vehicle and on the rolling resistance. From the foregoing, it would immediately appear that a decrease in the thickness of the innerliner might only be possible if a high degree of impermeability were to be imparted to it.

Carbon black is known to be used as a filler within the compounds of the innerliner. In particular, the carbon black of the N660 series is used. This carbon black ensures the required levels of reinforcement for the innerliner layers. In fact, as is known to a person skilled in the art, the innerliner must meet those mechanical characteristics that ensure the effective implementation thereof. If the integrity of the innerliner were to be compromised then the functionality of the entire pneumatic tyre would be lost.

For a more complete understanding of the scope of the present invention, it must be considered that for reasons of environmental sustainability, within the pneumatic tyre industry there is an increasing need to be able to replace part of the materials of fossil origin, such as the carbon black, with materials from renewable sources.

The need was therefore felt to have a solution that would make it possible to improve the impermeability of the innerliner layer, in order to be able to reduce the thickness thereof and, at the same time, be able to replace the carbon black (a fossil material) with a material from a renewable source . In this respect, the use of lignin has long been known in the pneumatic tyre industry. Lignin is an organic substance that binds the cells and fibers that constitute wood and the lignified elements of plants.

After cellulose, it is the most abundant renewable source of carbon on earth. Although it is not possible to define the precise structure of lignin as a chemical molecule, it is however possible to identify lignin as a polymer comprising the following three phenylpropanoids : p-coumaryl alcohol; coniferyl alcohol ( 4-hydroxy-3-methoxycinnamyl alcohol); sinapyl alcohol ( 4-hydroxy-3 , 5-dimethoxycinnamyl alcohol). There are different types of lignin that are commercially available and that differ therebetween as a function of the different extraction process by which the different raw material used were obtained. The Kraft process and the Sulfonation process are two examples of processes dedicated to the extraction of lignin.

In particular, Kraft Lignin is a byproduct of the Kraft process that is utilized to chemically extract cellulose from wood. This is obtained by means of precipitation, lowering the pH of spent liquor from the Kraft process. Phenol, alcohol and carboxyl hydroxyls are the main identifiable functional groups within Kraft lignin, whilst thiol groups are present to a lesser extent.

In contrast the Sulfonation process leads to lignin being obtained that is characterized by the presence of hydroxyls and high concentrations of sulfonic groups.

As it may immediately seem to a person skilled in the art, the use of lignin represents a significant advantage in terms of sustainability. In fact, lignin is a natural product that constitutes a byproduct of wood in the production of paper. In this respect, it should also be specified that the disposal of lignin is a limiting step within the paper production chain.

Although the partial replacement of carbon black using lignin leads to advantages in terms of impermeability, it does however turn out to be extremely disadvantageous in terms of mechanical properties.

The inventors of the present invention have surprisingly found that lignin, if properly treated, can be used for the partial replacement of carbon black within innerliner compounds, ensuring both that the levels of the mechanical properties are maintained and an unexpectedly large improvement in terms of impermeability.

The object of the present invention is a rubber compound for the preparation of an innerliner layer, said compound comprising a cross linkable unsaturated chain polymer base comprising at least one halobutyl rubber, a reinforcing filler and a vulcanization system; said compound being characterized in that said reinforcing filler comprises from 15 to 45 phr of HTC lignin.

Preferably, said reinforcing filler comprises from 25 to Here and hereinafter, vulcanization system means a complex of ingredients comprising at least sulfur and some accelerating compositions that, during the preparation of the compound, are added in a final mixing step and are aimed at promoting the vulcanization of the polymer base once the compound is subjected to a vulcanization temperature.

Here and hereinafter, HTC lignin refers to lignin that has been subjected to a hydrothermal carbonization process.

Hydrothermal carbonization (Hydro Thermal Carbonization - HTC) is a thermochemical process for converting biomass under relatively low conditions of temperature and pressure in the presence of water. The temperature of the hydrothermal carbonization reaction is generally of between 150 and 250°C, whilst the pressure is generally of between 10 and 40 bar. It has been found that as a result of the hydrothermal carbonization process, the structure of lignin is modified and that the principal changes concern demethylation and dealkylation, as well as the splitting of the b-O-4 links. HTC Lignin has, therefore, a thermally more stable and complex cross linked structure than the original lignin.

Preferably, HTC Lignin has a surface area of between 30 and 55 m ² /g; a distribution of D50 particles of between 1 and 4 pm; a distribution of D90 particles of between 5 and 8 pm; a pH of between 7.5 and 10.

The surface areas were measured by means of nitrogen absorption according to the ASTM D6556 standard. The D50 and D90 values were obtained by means of the LLS technique (Malvern Mastersizer 2000).

Preferably, said lignin is HTC Kraft lignin.

Preferably, said reinforcing filler comprises from 15 to 40 phr of carbon black having a specific surface area of between 30 and 50 m ² /gr.

A further object of the present invention is an innerliner layer obtained using the compound of the present invention.

A further object of the present invention is a pneumatic tyre comprising the innerliner according to the present invention .

A further object of the present invention is the use of HTC Lignin within a compound for the implementation of an innerliner layer, in an amount of between 15 and 45 phr.

The following are non-limiting examples given purely by way of illustration.

Three compounds were produced wherein two thereof are comparison compounds (A and B) and one is a compound according to the invention (C) .

The comparison compound A represents a compound commonly used for the manufacture of the innerliner; the comparison compound B differs from comparison compound A in that part of the carbon black was replaced with untreated Kraft lignin; the compound of the invention C differs from the comparison compound A in that part of the carbon black was replaced with

HTC lignin. Herebelow, the procedure is given for the preparation of the compounds described in the examples. This procedure does not represent a limitation for the present invention.

- 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 ingredients listed in Table I, excluding the vulcanization system (sulfur, stearic acid and vulcanization accelerator), reaching a fill factor of between 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 thus obtained was processed once again in a mixer operated at a speed of between 40-60 revolutions/minute. Subsequently, the compound is discharged once a temperature of between 130-150°C had been reached.

(final mixing step)

The vulcanization (sulfur, stearic acid and vulcanization accelerator) system was added to the mixture obtained from the previous step, reaching a filling factor of 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 between 100-110°C had been reached. Table I shows the compositions in phr of the compounds according to the invention.

TABLE I

Halobutyl rubber is a bromobutyl rubber.

CB refers to the carbon black of the N660 series.

The Kraft lignin used in the examples is marketed under the trade name "Kraft Lignin" by the StoraEnso company.

The HTC Kraft Lignin is marketed under the name "SQ-243" by the SUN COAL company.

HTC Kraft Lignin has a surface area of 55.0 m ² /g; a distribution of D50 particles of 2.7 pm; a distribution of D90 particles of 9.0 pm; a pH of 8.6.

Here and hereinbelow the pH values for the HTC Kraft lignin were measured in accordance with the ISO 787/9 standard .

MBTS is the English acronym for the mercaptobenzothiazole disulfide () and is used as a vulcanization accelerant.

Using the compounds of Table I respective samples were produced that represent the innerliner layers. The samples were subjected to mechanical tests and oxygen impermeability tests . The mechanical properties were measured in accordance with the ISO 37 standard.

The oxygen impermeability test was performed on materials with a thickness of 0.7 mm and using a conventional apparatus as MOCON® OX-TRA® (model 2/61) . The measurements were performed at a temperature of 25°C.

Table II lists the results obtained from the tests described above.

In order to more immediately highlight the advantages deriving from the present invention, the values were indexed to the respective values for the comparison compound A.

In particular, in Table II the higher the values, the better the relevant characteristics.

TABLE II

The values reported in Table II show that the replacement of carbon black with HTC Lignin, not only ensures that the levels of the mechanical properties are maintained but gives the resulting innerliner layer better impermeability to oxygen than the one that might have been expected. In fact, in a completely unexpected manner, it was found that the compound comprising HTC lignin has considerably greater impermeability to oxygen than the compound comprising the lignin that has not been subjected to the hydrothermal carbonization treatment.

In summary, the present invention offers the great advantage of replacing, within the compounds for the innerliner, a fossil-derived material with a material from renewable sources, and of ensuring, at the same time, an improvement in terms of impermeability to oxygen without compromising, in any way, the mechanical properties.