can have a three dimensional network structure however the pulp fibers are relatively short in length and have small dimensions in the transverse direction their average length is less than about 5 mm and their average diameter is less than about 200 micrometers preferably less than about 50 micrometers they typically have relatively high surface area greater than about 1 square meter per gram when determined by the bet method as further explained below microcellular foams can be obtained by flash spinning and are usually prepared at relatively high polymer concentrations in the spinning solution i e at least 40 wgt synthetic fiber forming polyolefin polymethylpentene is preferred but the synthetic fiber forming polyolefin may also include polymethylpentene blended with either polyethylene or polypropylene blends of pmp with both pe and pp can also be used also relatively low spinning temperatures and pressures that are above the cloud point pressure can be used microcellular foam fibers may be obtained rather than plexifilaments even at spinning pressures slightly below the cloud point pressure of the solution spin agents used are the same as those noted above for plexifilamentary film fibril materials nucleating agents such as fumed silica and kaolin are usually added to the spin mix to facilitate spin agent flashing and to obtain uniform small size cells microcellular foams can be obtained in a collapsed form or in a fully or partially inflated form for many polymer solvent systems microcellular foams tend to collapse after exiting the spinning orifice as the solvent vapor condenses inside the.