Flame-Resistant Rubber Blends – A New Approach for Optimizing Properties

In many applications, such as in cables or seals, rubber blends must possess both excellent mechanical properties and good flame-resistant properties. This article shows how flame resistance can be easily determined by TGA measurements and how the combination of mechanical and thermogravimetric measurements can be employed to optimize properties.

Introduction

In the development of flame-resistant rubber blends, many properties have to be optimized through the right choice and combination of constituents. In traditional compounding techniques, a high content of inactive fillers is used to improve flame-protection properties. This, however, always has a negative effect on mechanical properties.

In the following sections, we present a physically motivated concept that offers the possibility of specifically improving both flame protection and mechanical properties.

Samples and Properties

In a first step to characterize flame-retardant properties, five test compounds were prepared in which the amount and type of filler were specifically varied. The composition of the samples is summarized in Table 1.

EPDM containing 20 phr oil was used as the polymer matrix in all the samples. The fillers added were aluminum hydroxide, Al(OH)₃, with a density of 2.4 g/m³ and a specific surface area of 6 m²/g and two types of precipitated silica with a density of 2 g/cm³ and specific surface areas of 120 m²/g and 20 m²/g. The weight ratio of active (specific surface 120 m²/g) to inactive silica (20 m²/g) was kept constant at 3:1.

The fillers were chosen so that a strongly flame-retarding filler like aluminum hydroxide with comparatively poor mechanical properties was combined with a mechanically high-strength but weakly flame-resistant filler like silica in the vulcanizate.

Conclusions

A linear correlation analysis enabled a quantitative relationship between mechanical properties, such as hardness, stress for 300% deformation, and the limiting oxygen index (LOI) and type and amount of added fillers to be established. The excellent correlation between the results from thermogravimetric analysis (TGA) and LOI shows that the TGA is a simple and accurate way to determine flammability. Furthermore, TGA offers the possibility of investigating reactions occurring during thermal decomposition and the resulting decomposition products.

_{Flame-Resistant Rubber Blends – A New Approach for Optimizing Properties | Thermal Analysis Application No. UC 443 | Application published in METTLER TOLEDO Thermal Analysis UserCom 44}