Introduction
A modern automotive pneumatic tire is a unique product consisting of many parts and materials. A number of various rubber compounds are used to produce a modern tubeless tire. Each rubber compound consists of rubbers and a variety of ingredients designed to give the required properties to this element after assembly and vulcanization of the tire.
The innerliner is made of the most gas-proof synthetic rubber to comply with requirements for maintaining tire pressure. It’s a butyl rubber (IIR), generally, its improved modification bromobutyl rubber (BIIR) is used. Since this rubber contains halogen (bromine), hydrogen bromide will be released during the processing of the rubber compound at high temperatures. HBr is an active acid, moreover, it indirectly causes scorching (undesirable vulcanization) of the rubber compound. In order to neutralize hydrogen bromide, 0,15-0,50 parts by weight of highly active magnesium oxide is always added to rubber formulation of the innerliner. Magnesium oxide acts as an acid acceptor and scorch retarder providing the greater processing stability.
Most of the serially produced and marketed grades of high-activity magnesium oxide are products of synthetic origin, obtained by chemical processes from various magnesium containing raw materials. The complexity of production process leads to their high cost.
The product under the brand name MagPro®150 is magnesium oxide with a Specific Surface Area of 150 m2/g, obtained by indirect calcination of milled natural magnesium hydroxide (brucite mineral).
In order to study the applicability of natural based magnesium oxide MagPro®150 for the production of innerliner rubber compounds and the possibility of replacing synthetic MgO grades, a comparative work was carried out at the independent German institute DIK (Deutsches Institut für Kautschuktechnologie).
Materials and formulation
| Species name | MagPro®150 | Competitor 1 | Competitor 2 |
|---|---|---|---|
| Description |
Natural magnesium oxide |
Synthetic magnesium oxide |
Synthetic magnesium oxide |
| MgO content, % | 94.8 | 97.0 | 98.2 |
| CaO content, % | 2.34 | 0.95 | 0.80 |
| SiO2 content, % | 1.57 | 0.20 | 0.35 |
| Fe2O3 content, % | 0.14 | 0.08 | 0.15 |
| Particle size D50, microns | 7.0 | 3.0 | 5.0 |
| Loss on Ignition at 950°C, % | 7.3 | 7.0 | 8.0 |
| Specific Surface Area, m2/g | 152 | 125 | 155 |
The industrial rubber compound formulation of the tire innerliner based on bromobutyl rubber was chosen for the tests; the only changing ingredient was the type of magnesium oxide.
| Ingredient | Tradename | MagPro® phr | Competitor 1 phr | Competitor 2 phr |
|---|---|---|---|---|
| Bromobutyl rubber | EXXON 2222 | 100.0 | 100.0 | 100.0 |
| Carbon black | N660 | 60.0 | 60.0 | 60.0 |
| Naphthenic oil plasticizer | Vivatec 200 | 8.0 | 8.0 | 8.0 |
| Aromatic and aliphatic resins blend | Tudalen 5138 | 7.0 | 7.0 | 7.0 |
| Phenolic tackifying resin | Koresin | 4.0 | 4.0 | 4.0 |
| Stearic acid | 2.0 | 2.0 | 2.0 | |
| Highly active MgO | MagPro®150 | 0.5 | - | - |
| Highly active MgO | Competitor 1 | - | 0.5 | - |
| Highly active MgO | Competitor 2 | - | - | 0.5 |
| Zinc oxide | ZnO RS | 1.0 | 1.0 | 1.0 |
| Sulphur | 0.5 | 0.5 | 0.5 | |
| Mercaptobenzothiazyl disulfide | Vulkacit DM | 1.5 | 1.5 | 1.5 |
| Total: | 184.5 | 184.5 | 184.5 |
Results
The cure characteristics of the investigated compounds, rheometer 160°С, 60 minutes
| Characteristic | MagPro® | Competitor 1 | Competitor 2 |
|---|---|---|---|
| ML (Minimum torque), dNm | 1.21 | 1.18 | 1.16 |
| ML (Maximum torque), dNm | 5.81 | 5.56 | 5.56 |
| MH-ML (delta torque), dNm | 4.60 | 4.38 | 4.40 |
| Ts2, minutes | 1.09 | 0.69 | 0.74 |
| Tc25, minutes | 4.48 | 3.71 | 4.23 |
| Tc90, minutes | 15.57 | 14.16 | 15.41 |
From the data obtained, it can be seen that MagPro®150 in the formulation provides a longer scorch times in comparison with synthetic grades of magnesium oxide.
Mechanical properties of the investigated compounds (vulcanization 20 minutes at 160°С).
| Characteristic | MagPro® | Competitor 1 | Competitor 2 |
|---|---|---|---|
| Tensile strength, MPa | 9.2 ±0.7 | 9.3 ± 0.2 | 8.1 ± 0.4 |
| Elongation at Break, % | 572 ± 46 | 694 ± 15 | 584 ± 33 |
| Modulus 100%, MPa | 1.3 ± 0.1 | 1.3 ± 0.1 | 1.3 ± 0.1 |
| Modulus 300%, MPa | 4.8 ± 0.1 | 4.4 ± 0.1 | 4.5 ± 0.1 |
| Tear strength, kN/m | 14.1 ± 0.4 | 15.1 ± 0.5 | 14.5 ± 0.5 |
| Hardness, units Shore A | 49 ± 1 | 47 ± 1 | 48 ± 1 |
| Tension set after 5 hours at 105°С and 50% strain, % | 43.4 ± 0.6 | 47.1 ± 0.3 | 43.9 ± 0.4 |
The measured differences in cured state are not significant. The obtained values show that the type of magnesium oxide used has practically no effect on the mechanical properties of rubbers.
Mechanical properties of the investigated compounds after ageing 120 hours at 125°С.
| Characteristic | MagPro® | Competitor 1 | Competitor 2 |
|---|---|---|---|
| Tensile strength, MPa | 7.7 ±0.3 | 8.0 ± 0.1 | 6.7 ± 0.2 |
| Elongation at Break, % | 435 ± 17 | 526 ± 14 | 410 ± 22 |
| Modulus 100%, MPa | 1.8 ± 0.1 | 1.8 ± 0.1 | 1.8 ± 0.1 |
| Modulus 300%, MPa | 5.9 ± 0.2 | 5.7 ± 0.1 | 5.6 ± 0.1 |
| Tear strength, kN/m | 9.6 ± 0.2 | 10.0 ± 0.2 | 9.6 ± 0.4 |
| Hardness, units Shore A | 53 ± 1 | 51 ± 1 | 51 ± 1 |
| Tension set after 5 hours at 105°С and 50% strain, % | 30.2 ± 0.2 | 30.4 ± 0.3 | 29.8 ± 0.9 |
It can be seen that the ageing behavior of the investigated compounds after 120 hours at 125°C, is mostly identical and does not depend on the type of magnesium oxide used.
Conclusion
Based on the data obtained, it can be concluded that the natural magnesium oxide MagPro®, produced by calcination of the brucite mineral, provides the same level in rubber performance comparing with widely used synthetic magnesium oxide grades and can be used for the production of pneumatic tire innerliner.
