Inorganic fillers for silicone rubber

Materials World magazine
1 Dec 2009
coated cables

By comparing inorganic fillers such as quartz, aluminium silicate and vulkasil, researchers at the University of Tehran in Iran, have discovered a way to improve the mechanical properties of silicone rubber (SR).

The findings, which indicate the potential of quartz, in particular, could have implications in electronics, automotive and aerospace as the material is commonly used to coat cables and wires due to its excellent chemical and thermal resistance.

‘Silicone rubbers have many advantageous electrical properties compared with other elastomers as they have strong oxygen and chemical bonds, but their tensile strength and tear resistance are relatively weak’, explains Professor Iraj Rezaeian from the University’s School of Chemical Engineering.

Usually fillers such as natural and synthetic silicas and metallic oxides are added to SR to reinforce these properties, but their efficiency has not been examined in detail.

In his work, a polyblend of hard and soft SR with the ratio of 75/25 (w/w) were prepared, along with samples containing different weight percentages of quartz, aluminium silicate and vulkasil filler materials. The mixing was carried out on a two-roll mill over a period of 20 minutes.

‘We observed that samples containing quartz showed better tensile strength [6.8MPa] and tear resistance [40.58kN m-1] compared with those containing aluminium silicate [5.7MPa and 36.7kN m-1] and vulkasil [6.17MPa and 34.7N m-1], before ageing,’ says Rezaeian.

This is because of the structural compatibility and specific surface area when the material is used in SR compounds. ‘The particle specific surface area for the quartz, aluminium silicate and vulkasil fillers are 2.5, 50 and 175 m2 g-1 respectively’, says Rezaeian.

‘For a silica filler, the higher values correspond to a rougher surface. Our results show that a smoother surface results in better interaction among rubber chains. The links between the rubber and the filler become stronger and the tensile strength of the sample increases.’Scorch times, optimum cure time and the corresponding torque have also been observed. Quartz appears to have the highest resilience, while vulkasil has the least. The researchers attribute this to their pH values, six for vulkasil and 7.5 for quartz.

‘The lower pH value corresponds to a better compatibility of the filler in the compound. As a result, the crosslinking between rubber chains is increased, curing rate is reduced and crosslink density is increased’, says Rezaeian. ‘However, quartz has a more desirable elasticity and lower compression sets.’

Rezaeian and his team will now use the results to optimise quartz fillers.