Insulation fiberglass (IFG) manufacturing requires optimum:
- Surface tension
- Liquidus temperature
Liquidus is 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.
It is difficult to meet all these requirements simultaneously; optimizing the conflicting requirements of viscosity, liquidus, and durability in fiberglass formulations is a challenge unless the manufacturer also uses boric oxide.
Boric Oxide Enables Fiberization in Glassmaking
The fiberglass-making process calls for expensive fiber-forming components made from highly specialized metal alloys that must exhibit resistance to both high-temperature creep and corrosion by molten glass.
In many aerospace contexts, these key alloy properties are temperature dependent, and the rotary spinners that throw out the molten fibers have a particularly rigorous regime. To save wear and tear (and expense), there has 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 equally effective at lowering high-temperature viscosity (HTV) when substituted for silica; 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 is usually taken to represent the optimum temperature at which commercial fiberization can take place. If the difference between this temperature and the liquidus temperature is too small, then crystallization can occur in the cooler regions of the molten glass and severely disrupt fiber production.
However, 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 104°F (40°C) to 248°F (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.
Improving Fiberglass Durability
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 (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; however, boric oxide increases the FRP, as does aluminum oxide.
As an added benefit, boric oxide appears to improve the recovery of fiberglass insulation after it has been compressed during transportation. This may be partly a function increased durability the fibers get from boric oxide, but it also seems that boric oxide helps to protect the interface between the glass fiber surfaces and their organic binder.
Improving Fiberglass Manufacturing Processes
Over the years, better fiberization efficiency and easier production operations have often been associated with glass formulas that contain higher levels of boric oxide. When glassmakers substitute aluminum or calcium oxide for silica, both increase the surface tension, while boric oxide and sodium oxide both decrease it—which is what the IFG manufacturer wants to achieve.
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 use of boric acid in fiberglass formulations.
The Ideal Solution for Manufacturing IFG: Boric Oxide
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 boric oxide in fiberglass formulations, and should assure its continued importance for many years to come.