13. The dilemma of the insulation material manufacturers

Calcium oxide and sodium oxide are not additives that can be struck from a formulation like a colour pigment. Since the earliest glassmaking they have been constitutive components of the melt from which fibres are drawn or spun.

The composition of this melt is not new. Soda-lime-silica glass is the earliest known glass type; it is documented from late Bronze Age Mesopotamia and Egypt and remained the dominant type from the second millennium B.C. into the modern period. The flux was first natron and later plant ash, both sources of sodium; the lime entered through calcareous sand, added shell or limestone and typically amounted to 5 to 10 per cent. Its function is the same as today: it reduces the solubility of the glass in water. [80]

The fibre made from it is younger. Slag and stone wool was first produced in Wales in 1840; commercial operations were running in England by 1885, in Germany soon afterwards, and the first successful commercial plant in the United States began operation in 1897. Glass wool followed in 1932 with the steam-blown process at Owens-Illinois; in 1938 the Owens Corning Fiberglas Corporation emerged from it. [81, 82]

Drawing a filament from viscous glass, by contrast, was already known in Egypt and Venice. What was carried over was not the machine but the formulation: a filament can only be drawn from a melt that has the right viscosity at a manageable temperature. That is precisely what sodium and calcium oxide are for.

Chemically they act as network modifiers. They break up the silicon dioxide network, lower viscosity and melting temperature and make the melt workable in the first place. Sodium oxide is the classical flux, while calcium oxide additionally stabilises the chemical durability of the solidified glass. Without these oxides, pure silicon dioxide would remain a material that melts only far above 1700 °C.

To this is added the raw material side. Cullet, that is soda-lime glass from the packaging cycle, is a substantial input to glass wool production. It brings calcium and sodium with it unavoidably. In stone wool, limestone and dolomite supply the same oxides, and in biosoluble AES wools the alkaline earth basis is even part of the name and a precondition of biosolubility.

The problem therefore does not lie in an admixture but in the material class and in the supply chain. If the basic oxides are named as reaction partners of chromate formation, this concerns not one product but the basis of a large part of today's high-temperature insulation.

And even if the formulation were changed, a second problem would remain. Fewer basic oxides mean a higher melting temperature. The existing plant technology, tanks, linings and energy management, is designed for the present temperatures. A changeover would not be a question of formulation but an investment in plant.

Fibres without appreciable basic oxides do exist nevertheless, and have for decades. High-silica fibres with over 95 per cent silicon dioxide, aluminosilicate fibres in the Al₂O₃-SiO₂ system and polycrystalline alumina fibres, which are not drawn from a melt but spun from a precursor solution and subsequently fired, manage without flux. The difference lies not in the chemistry but in the installed plant: higher melting temperature, different tanks, different linings, higher energy input, lower throughput.

Against this background the reticence of today's market leaders towards in-situ formation is understandable. It may also be self-protection. Understandable does not mean without consequence, however, since the assessment of a hazard does not depend on how costly its removal is.

The question running through this expert analysis thus arises here most sharply: at what point does one know that one can no longer not know?