Webs of microfibers as taught in the prior art have significantly limited utility utility as thermal insulation. This is true despite the fact that previous investigators of microfibers have almost routinely included thermal insulation in their lists of potential uses for the fibers. See several patents dealing with blown microfibers, including Francis, U.S. Pat. No. 2,464,301; Ladisch, U.S. Pat. No. 2,571,457; Watson, U.S. Pat. No. 2,988,469; and Buntin et al, U.S. Pat. No. 3,849,241 (blown microfibers are very fine, discontinuous fibers prepared by extruding liquified fiber-forming material through orifices in a die into a high-velocity gaseous stream, where the extruded material is first attenuated by the gaseous stream and then solidifies as a mass of the fibers); or see Vinicki, U.S. Pat. No. 3,388,194, which describes microfibers formed by a centrifugal spinning operation. While these previous workers did not report thermal insulation values for a web of microfibers, or give any indication of having measured such values, they appear to have automatically considered that a new fibrous web should be useful as thermal insulation.
The limited value of existing microfiber webs as thermal insulation is true despite another fact, which is not set forth in any known prior literature, namely that microfiber webs provide unique insulating values. For example, a one-centimeter-thick web of polypropylene blown microfibers will give 1.8 clo of thermal resistance, in contrast to the about 0.9 exhibited by a 1-centimeter-thick web of commerical polyester staple fibers..sup.1 (Footnotes and methods of measurement are at the end of the specification).
The reason why existing microfiber webs have only limited value as thermal insulation is that, at least after these microfiber webs have undergone a normal compression history, they are heavier than alternative types of fibrous insulation. This heaviness is an inherent consequence of the very nature of microfibers; their very fine size and comformability causes the microfibers to come together as a dense, fine-pored web. As an illustration, a one-centimeter-thick web of blown microfibers is about five times as heavy as a one-centimeter-thick web of commercial polyester staple fibers. Even if a blown microfiber web only half as thick as a web of polyester staple fibers is used (so as to provide roughly equivalent thermal insulation resistance), the blown microfiber web will still be about two-and-one-half times as heavy as the polyester staple fiber web.
When weight is only of secondary importance (as in glove or boot insulation, for example), thin, dense, blown microfiber webs can be quite useful. But when weight is a more primary consideration, as in such insulated articles as coats, snowmobile suits, sleeping bags, etc., existing microfiber webs will be passed by. Since the latter kinds of uses are major ones, the foreclosing of microfibers from such uses is a severe limitation on their utility in the insulation field.