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Sustainability 110: pultruded fiberglass windows anyone?

Pultruded fiberglass windows
Pultruded fiberglass windowsPhoto by Rick Zimmerman

Throughout a long career in architecture and design, I have encountered my share of aluminum windows and wood windows, even windows of PVC. But perhaps a sustainable option today is the pultruded fiberglass window.

But, just what is pultruded fiberglass, anyway? It begins with FRP, or fiber reinforced polymer (or plastic). Typically, the fine, short reinforcing fibers are of glass; hence the name fiberglass. These are mixed into a composite slurry with a thermosetting — meaning it sets into its final rigid form under heat — polyester resin. The short glass fibers, arrayed in all directions in the setting resin, provide structural strength in all directions, while the resin acts as a load-distributing binder and stiffener. Roughly 1/5th of final product is comprised of limestone or clay fillers, and a variety of additives, such as lubricants, catalyst, fire retarder or UV inhibitor.

The strength-to-weight ratio of fiberglass coupled with its ability to withstand extreme temperatures and corrosion has recommended its use in such applications as boat hulls, sport vehicles and equipment, rocket nose cones, plane fuselages, and propeller blades, as well as for containers for corrosive materials. You have undoubtedly at some point encountered a shower stall, tub or spa fabricated of fiberglass. Fiberglass has also invaded the construction industry, being used as structural beam members, highway guardrails, sea walls, street light poles, and even manhole covers.

Unlike extrusion, which is a process that pushed a formable material through a die, pultrusion instead pulls the material through a die. Pultruded materials are limited to those of constant cross-sectional profile (matching the die through which they are pulled).

Pultrusions originated in the 1940s for fairly heavy or bulky profiles. As successive improvements enabled the increase of glass fibers to over 60% of the composite, greater product strength could be achieved. Along with that increased strength came the decreased bulk and wall thickness, resulting in ever finer pultrusion profiles. Wall thicknesses of less than 1/10th inch can now be readily achieved, as can very tight tolerances. Pultruded fiberglass profiles thus closely approach the mechanical properties of metal, at potentially lower cost.

Pultruded fiberglass window components are typically manufactured by fusing bundled arrays of fibers (called rovings) for longitudinal strength, then wrapping then in a multi-directional mat for transverse strength. The great appeal of pultruded fiberglass lies in its eventual tensile strength, which can be twice that of steel, three times that of aluminum, and up to ten times that of PVC. Though less stiff than either aluminum or steel, fiberglass is also roughly three times as stiff as wood. Fiberglass maintains these structural characteristics throughout virtually the entire temperature spectrum of typical service. Fiberglass does not out-gas like PVC, and is far better at flexing under severe loads and then returning to is original state.

Factory-painting of pultruded fiberglass products is typically via two coats of polyurethane enamel, providing a hard, durable finish that will resist fading, loss of gloss, chipping, scaling, blistering or discoloration. Finally, fiberglass production consumes less embedded energy than PVC, steel or aluminum.