An Industry Perspective
The use of boric oxide is pervasive in fiberglass formulations because of B2O3's unique influence on properties that are critical to the manufacturing and performance of glass fiber products. Boric oxide has remarkable and relevant glass-forming, fluxing, and viscosity lowering properties. The following review focuses on the use of B2O3 in insulation fiberglass (IFG).
Viscosity, durability, surface tension, and liquidus temperature requirements have been particularly important in establishing the common use of B2O3 in IFG formulations - liquidus representing the lowest temperature at which the glass melt cannot devitrify (crystallize). In most commercial silicate glasses, calcium oxide, magnesium oxide, sodium oxide, and potassium oxide are important tools for adjusting formulations to the desired viscosity, while aluminum oxide is commonly used to enhance glass durability. The following discussion explains why it is difficult simultaneously to optimize the conflicting requirements of viscosity, liquidus, and durability in fiberglass formulations without also using boric oxide.
The fiberglass-making process calls for expensive fiber-forming components made from highly specialized metal alloys which must exhibit resistance to both 'high temperature creep' and corrosion by molten glass. As in many aerospace contexts, these key alloy properties are temperature dependent, the rotary spinners which throw out the molten fibers having a particularly rigorous regime. To save wear and tear (and expense), there has always been a strong drive toward lower operating temperatures for fiberizing equipment through the development of lower viscosity fiberglass formulations.
Boric oxide, sodium oxide, and calcium oxide are all about equally effective at lowering high temperature viscosity when substituted for silica, while aluminum oxide is not effective. (This single component substitution facilitates valid comparison of property trends; in industrial practice, new fiberglass formulations commonly involve complex, multi-component substitutions.)
High temperature viscosity (HTV) is usually taken to represent the optimum temperature point at which commercial fiberization can take place. If the difference between this temperature and the liquidus temperature - at which it is thermodynamically impossible for crystallization to occur - is too small, then crystallization can occur in the cooler regions of the molten glass, and severely disrupt fiber production. There are always temperature gradients in molten glass, particularly as it flows from the furnace through to different parts of the spinner.
In practice, to avoid any possibility of crystallization, the liquidus temperature must be at least 40°C to 120°C below the HTV, depending on the particular fiberization process.
Calcium oxide has a strong tendency to raise liquidus temperature as the HTV decreases, thus limiting its value as a tool for decreasing viscosity. This is not the case with boric or sodium oxide, both of which tend to reduce liquidus temperature at the same time as they reduce HTV.
The durability of insulation fiberglass is very important because the fibers are required to resist loss of strength under end-use conditions and when stored under compression in humid climates. The ability of a glass fiber to resist loss of strength under humid conditions is often referred to as its fatigue resistance parameter, known in the trade as FRP. The higher the FRP index, the more resistant a glass is to moisture attack. Depending on end-use application, for example in attic or loft, an FRP value of 15-16 or higher is typically required for commercial fiberglass products.
Calcium and sodium oxide both decrease the resistance of glass to moisture attack, while boric oxide increases it, as does aluminum oxide.
Over the years, better fiberization efficiency and 'easier' production operations have often been associated with glass formulas which contain higher levels of boric oxide. Figure 4 shows the influence on glass surface tension of four oxide substitutions for silica. Substitutions of aluminum or calcium oxide both increase the surface tension, while boric oxide and sodium oxide both decrease it - which is what the IFG manufacturer wants to achieve.
Combining this information with the data plotted in Figure 3 demonstrates that boric oxide is unique in lowering surface tension while simultaneously improving product resistance to atmospheric moisture. It is believed that lowering the surface tension is at least part of the reason for the empirical performance benefits that support the continuing and relatively high level use of boric acid in fiberglass formulations.
Pragmatically B2O3 appears to improve the recovery of fiberglass insulation after it has - very necessarily - been compressed for transportation reasons. This may be partly a function of bestowing increased durability, but it also seems that boric oxide helps to protect the interface between the glass fiber surfaces and their organic binder.
Boric oxide substitutions for silica are the only ones that have all the required properties, simultaneously decreasing high temperature viscosity, lowering liquidus temperature, increasing moisture resistance, and reducing surface tension. This combination has been the key driving force behind the historical use of B2O3 in fiberglass formulations, and should assure its continued importance for many years to come.