Gaskets are exposed to a multitude of different stresses. In addition to the pressure acting on a gasket and the occasionally aggressive media to which it is exposed, temperature is the principal limiting factor when choosing a material. Frequently used materials, such as graphite, approach their limits at around 500°C. Since many applications and processes require temperatures above 500°C, mica-based materials are suitable alternatives due to their exceptional heat resistance. Depending on the types and properties of micas, the upper limit for their use is between 600°C and 1000°C. This enor-mous temperature advantage, combined with the excellent chemical resistance of the material, makes high-quality mica the hidden champion for high-temperature applications.
The quirks of oxidation
As a catalyst of the tendency of a material to undergo oxidation – a reaction of the solid with atmospheric oxygen – the operating temperature is decisive, alongside the concentration of oxygen. For example, the graphite materials in widespread use are characterised by very good tightness and low leakage. However, depending on the quality of the material, oxidation sets in from as low as 350°C. If graphite is exposed to oxidising media for extended periods at high temperatures, it has a pronounced tendency to lose mass. In specific practical applications, this can cause a loss of surface pressure resulting in the failure of the gasket. As the thermogravimetric analysis (TGA) curve in Figure 1 shows, for graphite-based materials, oxidative mass loss becomes disproportionately high at temperatures above 600°C, thus ruling out the use of graphite as a reliable gasket material on technical grounds – loss of sealing integrity after a short time is almost inevitable. This is where high-quality mica-based materials have significant advantages.
Types of mica
Mica is a natural material that belongs to the group of phyllosilicates. It is classified into various types, only a few of which are suitable for making gaskets. These include special phlogopite micas. In these materials, what is known as calcination – the elimination of water of crystallisation with a resulting change in material properties – starts only at relatively high temperatures. In contrast to the muscovite micas, which are used for acoustic and thermal insulation and for which calcination starts at around 600°C, phlogopite micas remain stable up to temperatures of around 1000°C.
Up to 1000°C – phlogopite mica is unbeatable
Phlogopite mica is extremely resistant to heat-induced volume and mass loss, with the critical calcination starting only above 1000°C. Numerous gasket materials use enhanced phlogopite-mica papers, such as UniTherm. The smallest possible amounts of binders, such as silicone resins or impregnating agents, promote the elasticity of the seal and reduce leakage by blocking diffusion and permeation pathways.
Figure 1 shows the thermogravimetric analysis of a high-quality phlogopite-mica product (UniTherm) as a function of temperature compared to a mica equivalent, such as chemically and thermally expanded vermiculite.
The mass loss of the phlogopite material up to 1000°C is less than 5% and merely half that of a comparator material in the critical temperature range. The reason for this is often the high-temperature evaporation of the binder components necessary for producing sheet materials for the gasket industry. The usual methods for determining leakage parameters at room or ambient temperature suggest initially good tightness for materials with larger amount of binders. A very different picture emerges for high-temperature applications, however. At temperatures above 600°C, phlogopite mica and the expanded vermiculite mentioned above are technically equivalent. But when cost and economic aspects are considered, phlogopite mica has the clear advantage.
Figure 1: Thermogravimetric analysis
At 800°C – a real grilling
The leakage measurements were based on spiral wound gaskets (SGs) with various fillers: phlogopite mica (UniTherm SG), expanded vermiculite (expanded vermiculite SG) and pure graphite (graphite SG). The following diagrams show the leakage results as functions of surface pressure for a series of experiments conducted in accordance with EN 13555. Experimental set-up: test pressure: 16 bar; test medium: helium; room temperature (Figure 2); test temperature: 800°C (Figure 3).
Looking only at the curves for the high-temperature materials, the graphs are practically identical at a thermal load of 800°C. From a surface pressure of around 45 MPa, the gasket with phlogopite mica has improved leakage values compared to the other specimens. Nevertheless, it is also clear that maintaining compliance with any specific leakage-rate requirements, such as L0.01 in accordance with VDI 2290, becomes a dim and distant prospect at such temperatures.
Contrary to expectations, the leakage rate for the graphite spiral wound gaskets in the high-temperature test was significantly lower than for the other specimens. However, there were major indications of failure even after a very short time (24 h/60 MPa), up to the complete collapse of the gasket, as a result of which no useful leakage measurements could be determined for graphite.
Deceptive tightness measurement at room temperature
With the aim of reducing the risk of failure for gaskets in high-temperature applications, the proportion of the volatile binding agent was deliberately kept low for UniTherm. These products consequently had worse leakage values in the comparison of room-temperature tightness with products with high proportions of binders. From a technical perspective, however, the leakage values measured at room or ambient temperature need to be viewed critically and questioned. This means that conclusions cannot be drawn as to the high temperatures found in practical application.
Figure 2: Comparison of leakage at room temperature (test pressure: 16 bar; medium: He)
Figure 3: Comparison of leakage at a test temperature of 800°C (test pressure: 16 bar; medium: He)
When choosing a gasket for temperatures up to a maximum of 500°C, products made from graphite have considerable advantages due to their low leakage values and the excellent chemical and physical resistance of the material. But if the application requires prolonged resistance to temperatures greater than 500°C, mica-based materials offer considerably greater security in terms of thermal stability. In particular, phlogopite mica products are characterised by outstanding reliability on extended exposure to up to 1000°C. The quantitative proportion of binder should also be considered. The lower the proportion of binder, the better the performance, because the setting of the gasket and the associated loss of surface pressure on exposure to extreme temperatures is reduced. High-quality but often misunderstood phlogopite micas are therefore the hidden champions for high-temperature applications.