Fiberglass insulation manufacturing has grown to be one of the largest uses for recycled glass. Because fiberglass does not have glass's clarity, there is an assumption among some that fiberglass raw materials can tolerate higher levels of contaminants than container manufacturing. In fact, metal, organic, and ceramic contaminants can be costly to fiberglass quality and production equipment. During fiberglass production, raw batch materials and glass cullet are continuously added to the furnace. After melting and "fining," the molten glass is spun into fibers by a process called fiberizing. To ensure production of consistent fibers, cullet must meet specifications for major and minor oxide chemical composition, color consistency, and contaminant levels. The chemical composition of cullet as raw batch material for fiberglass should fall within the ranges listed in the table, published in this document. Variations in major oxide concentration can affect both the viscosity and emissivity of the glass melt. Viscosity is a measure of flow resistance, while emissivity is the ability of a material to emit heat through radiation. Both of these properties affect the attenuation ability of the molten glass during fiber formation. In addition, a shift in a major oxide concentration can cause fluctuations in the electrical resistivity of the melt. This requires an adjustment of transformer settings controlling the power input to the furnace, as well as creating changes in heat flow patterns, current paths, and temperature distribution, all of which are undesirable during stable furnace operations. Minor and trace oxide composition of the batch cullet is also very important. Oxides such as CoO, Cr2O, and FeO can influence infrared heat transmission, heat transfer during melting, and rate of fiber cooling. These factors will affect fiber attenuation, leading to variability in the fiber diameters and lengths. This Best Practice document can be downloaded from: www.cwc.org/gl_bp/gbp3-0103.htm
