High active magnesium oxide for production of pneumatic tire innerliner

High active magnesium oxide for production of pneumatic tire innerliner

20 Jan 2021

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 of a pneumatic tire is a key part responsible for keeping the tire pressure stable and constant, therefore, responsible for driving safety.

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.

Brucite+ produces two grades of highly active magnesium oxide, which are used as an acid acceptor and scorch retarder in the production of industrial rubber goods based on halogen-containing rubbers. 

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
Table 1. Parameters of magnesium oxide species

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
Table 2. Formulation of investigated compounds

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
Table 3. The cure characteristics of the investigated compounds, rheometer 160°С, 60 minutes

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
Table 4. Mechanical properties of the investigated compounds (vulcanization 20 minutes at 160°С)

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
Table 5. Mechanical properties of the investigated compounds after ageing 120 hours at 125°С

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.

Más artículos
Detalles
25 Aug 2021
Carbon footprint - key environmental issue for humanity
How to measure a company’s carbon footprint? How to reduce it and to optimize an organization’s operations without a negative impact on the environment?
Carbon footprint - key environmental issue for humanity