Fibrids

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

Mention may also be made of an application in which careful control of polymer morphology has led to the production of novel materials. By treatment of solutions of high-density polyethylene, products are obtained with a celluloselike morphology and which are known as, fibrides or synthetic wood pulp. They are used for finishing paper and special boards to impart such features as sealability and improved wet strength. They are also reported to be used for such diverse applications as tile adhesives, thixotropic agents, battery separators and teabags ... 

Last not least, in some cases changing the morphology of a polymer is the only way to achieve certain properties (e.g. polymer fibrids). 

The latest results of a controlled crystallization of macromolecules are the polymer fibrids which are a completely new modification of synthetic polymers as far as the micro- and macro-structure is concerned. They exist of small fibers having a length of up to some millimeters, which are highly oriented, and which have a macro-morphology similar to that of cellulose pulp. 

By special processes, like polymerization under shear conditions or flash-spinning of polymer solutions, it is furthermore possible to obtain materials which do not only have this new shish-kebab micro-morphology, but also a new macro-morphology, namely the polymer fibrids.35... 

Figure 9. Electromicrograph of a high density poly- Figure 10. Light micrograph of polyethylene fibrids.

Starch fibrids are produced by spraying alkaline starch dispersion into an agitated, concentrated solution of ammonium sulfate.41,42 Amylose fibers or films are likewise produced by extmsion of a hot paste into a coagulating bath. Starch fibrids (starch pulp) can be utilized as a bonding additive in paper, but its use is rare. 

Aramid para-aramid, 2.8 Pa strength, 70 GPa modulus chopped staple or fibrid (pulp) 10-150 g/m2 Tensile strength up to 8 N/mm Density 80 kg/mm Improved impact resistance Smooth finish Good wear resistance Can be blended with conductive fiber Superior temperature resistance Aerospace adhesive carriers Automotive improved stone impact resistance Defence radar cross section Recreation skis, snow and surf boards, surf boards, canoes Industrial substrate for friction products wear resistance for high-speed rolls Electrical printed circuit boards... 

MAJOR APPLICATIONS The dopes of PBA can be utilized for the preparation of films, filaments, fibrids, and coatings. Wet-extruded, tough, clear, flexible films can be applied to substrates like glass, ceramics, metals, concrete, and polymeric materials.The high-temperature resistance of the polyaramids make them suitable for asbestos replacement in heat-resistant work wear. The service life is longer for asbestos and the wearing comfort is greater. PBA has been superseded by poly( p-phenylene terephthalamide). ... 

The outstanding thermal and mechanical properties that can be derived from these compositions led to the exploration, as well as commercial realization, of various product forms. Currently these product forms include fibers, fibrids and pulps, films, papers, and particles. 

The excellent thermal properties of these materials led to high volume applications where these materials were used as binders or as short fiber reinforcing agents. This required the development of both fibrids and pulps. This chapter discusses both the processes of formation as well as the principles of applications of these forms. 

Various nonwoven structures have been developed as well. The least important among sheet structures are films. There are two film products (see Section 13.3) based onp-aramids and none on m-aramids. The significant cost differential is the most likely reason for this situation. On the other hand a very large market has been developed for papers based on both p-aramids and m-aramids. In general, these papers are based on short fibers (floe) and a binder (fibrids), but other components have been explored as well. A very small market exists for particles other than fibrids and pulps. 

Papers are available in many forms varying in thickness, degree of densification, and composition (additive type or floe to fibrid ratio). Pressboards, which differ from paper in thickness and rigidity, are likewise available in several thicknesses and degrees of densification. The product of choice will depend on many factors including end-use thermal and mechanical performance requirements, formability or ease of fabrication, and the desired degree of saturability. 

Aramid cores are made from paper (typically 1.5 mil in thickness) comprising w-aramid floe and fibrids, similar to the papers used in electrical applications discussed in the previous section. Adhesive node lines are printed on paper sheets that are then stacked, pressed, and heated to cure the adhesive. The resulting block is expanded. The adhesive-free areas form the hexagonal cells of the honeycomb configuration. The core is dipped several times in an epoxy or phenolic resin solution until the desired density and mechanical property levels are reached. The core is then cut into slices of the desired thickness. Face sheets are glued to each side of the core. The most common face sheet today is a composite of carbon fiber and epoxy resin. 

Fibrids Short, irregular fibrous products, made by mixing a dilute polymer solution with a non-solvent with agitation. They can also be made by flash spinning and breaking up the resulting filaments. Used in felts, in papermaking, for filtration product, etc. Also see fibrets. 

Paper. An equally important use of MPDI fiber are products in the form of paper. Paper can be made from a mixture of MPDI fibrids and fioc (45). The mixture is slurried in water and processed on a conventional paper machine. The primary market for these papers is as electrical insulation in motors, generators, and transformers. The key properties are chemical resistance, good insulating characteristics, and high temperature stability. 

Yarns, films, fibrids, papers and pulp are the usual aramids commercial forms. 

Fibrids are film-like particles that are formed when aramid solutions are precipitated in a non-solvent under high shear [20,21]. The dimensions of as-formed fibrids are aronnd 100 mm X 700 mm x 0.01 mm. Fibrids have a high surface area, around 200-300 m /g, and can function as a thixotrope or a reinforcing agent in composite, sealing, coating, and elastomer applications [22,23], and also in aramid papers. 

Liang R, Han L, Doraiswamy D and Gupta R K (2000) Fundamental characterizations of structured fibrid suspensions, Proc Idf Intern Congr Rheology, Cambridge, UK, Aug 20-25, vol. 4, pp. 136-138. 

Gohlke U and Baum E (1979) Fibride aus aromatischen polyamiden, Acta Polymerica 30 170 175. 

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