Sizing Up Fiber Sizings
What glass fiber users need to know about sizings, and how new sizing developments are enhancing the performance of glass-reinforced composites.
By Karen Fisher Mason, Contributing Writer | April 2006
Sizing is essential to glass fiber manufacture and critical to several key fiber characteristics that determine both how fibers will handle during processing and how they perform as part of a composite. "Glass is its own worst enemy," says independent consultant Bob Schweizer (Granville, Ohio), who recently retired after 32 years as a size and reinforcement chemist with Toledo, Ohio-based Owens Corning. Raw fibers are abrasive and easily abraded. "So you need to protect fibers from their neighbor fibers. Without sizing, we'd have nothing but a package of fuzz," he quips.
Source: Fiber Glass Industries
Sizing applicator is operating in a "doubles" process (two bundles or "packages" of fibers). Each package of fibers travels from a bushing above this applicator down to a winder below.
"Without sizing, it would be difficult even to make glass roving," adds Mike Gunther, president of Paradox Technical Consultants (Parker, Colo.), "and it would be virtually impossible for anyone to use it."
Sizing formulations also are the primary means fiber manufacturers have to adapt their glass products for particular applications and distinguish their products from their competitors'. While the performance of glass-reinforced composites certainly depends on compositional parameters, such as fiber volume and, in chopped fiber applications, aspect ratio (length divided by diameter), E-glass fiber itself, for instance, differs little from manufacturer to manufacturer. "You might have different filament or strand diameters, but it's still essentially the same glass, with pretty standard mechanical properties," says Andy Brink, vice president of Hydrosize Technologies Inc. (Raleigh, N.C.). But, with regard to interfacial properties, sizing is the primary variable.
Sizing also is one of the "black box" technologies in composites — glass fiber producers are very reticent to reveal much about the complex formulations they use. A working knowledge of the complex chemistry in sizing formulations is difficult to acquire even without the veil of secrecy. The good news is such knowledge is unnecessary even for many engineers at the fiber manufacturing facility, let alone engineers who design and build composite components. Yet there is much to gain from a general understanding of what sizing is and the ways in which it functions.
MAKE-UP AND FUNCTION
Glass fiber sizing is not a single chemical compound, but a mixture of several complex chemistries, each of which contributes to the sizing's overall performance. The primary components are the film former and the coupling agent. Depending on its formulation, the film former, so called because it forms a film on the glass strands, serves a number of functions, explains Brink, whose company develops and sells film formers. The film former is designed to protect and lubricate the fiber and hold fibers together prior to molding, yet also to promote their separation when in contact with resin, ensuring wetout of all the filaments. Film formers, with some exceptions, are chemically similar to the matrix resin for which the sizing is designed, Brink says.
The coupling agent, almost always an alkoxysilane compound, serves primarily to bond the fiber to the matrix resin. Gunther explains that silanes offer just what is needed to bond two highly dissimilar materials — the glass fiber, which is hydrophilic (bonds easily to water), bonds to a resin that is hydrophobic (insoluble in water and does not bond well to it). Silanes have a silicon end that bonds well to glass, and an opposing organic end that bonds well to resins.
Beyond these two major components, sizings also may include additional lubricating agents, as well as antistatic agents that keep static electricity from building up on the nonconductive fibers as they are formed and converted at high speeds — up to 100 mph, reports Gunther. Including additives for specialized, proprietary functions, a sizing formulation might contain eight to ten or more components. The interaction of these components with each other, with the matrix resin, and within a particular converting/fabricating environment is quite complex, yet reasonably well understood by sizing chemists.
