Fibrillated Films

Commercial spun bonded or flash spun products have been made primarily from PE plexifilamentary film-fibril strands and have typically been produced using trichlorofluoromethane as a spin agent. 

Plexifilamentary film-fibril strands of fiber-forming TPX having a tenacity of 1 p den-1 have been produced. Also blends of TPX with PE and PP can be fabricated (12). 

Spin agents used are the same as those noted above for plexifila-mentary, film-fibril materials. 

The recent improvements for flash spinning plexifi-lamentary film-fibril strands of a fiber forming PO consist of ... 

Utilization of a 4.7, hydrocarbon/cosolvent spin liquid with a highly reduced ozone depletion hazard as compared to the halocarbon spin liquids currently used for making such strands commercially. The resulting plexifilamentary film-fibril strands have increased tenacity and improved fibrillation compared to strands flash spun from 100% hydrocarbon spin liquids [32]. 

It is only polymers with weak intermolecular forces, producing films at high stretch ratios, with pronounced anisotropy, that are favorable choices for film fibrillation, film-slitting, or film-cutting techniques. For these reasons the PO polymers, especially HDPE and isotactic PP, have gained growing importance for such applications. 

Among non-conventional technologies, film tape and film fibre processing are the most important. Film-to-tape and film-to-fibre processing are economically justifiable when the film process is simple and inexpensive. Only polymers with weak intermolecular forces (HDPE and PP) producing film at high stretch ratios and pronounced anisotropy are favourable for film fibrillation or film cutting techniques. 

The equipment for the slit-film fiber process is shown in Figure 15 (29). An olefin film is cast, and as in melt spinning, the morphology and composition of the film determine the processing characteristics. Fibers may be produced by cutting or slitting the film, or by chemomechanical fibrillation. 

The film is fibrillated mechanically by mbbing or bmshing. Immiscible polymers, such as polyethylene or polystyrene (PS), may be added to polypropylene to promote fibrillation. Many common fiber-texturing techniques such as stuffer-box, false-twist, or knife-edge treatments improve the textile characteristics of slit-film fibers. 

Several more recent variations of the film-to-fiber approach result in direct conversion of film to fabric. The film may be embossed in a controlled pattern and subsequently drawn uniaxiaHy or biaxiaHy to produce a variety of nonwoven products (47). Addition of chemical blowing agents to the film causes fibrillation upon extmsion. Nonwovens can be formed directly from blown film using a unique radial die and control of the biaxial draw ratio (48)... 

In conventional tenter orientation, the sequence of steps is as described above (MD—TD). In some cases it is advantageous to reverse the draw order (TD—MD) or to use multiple draw steps, eg, MD—TD—MD. These other techniques are used to produce "tensilized" films, where the MD tensile properties are enhanced by further stretching. The films are generally unbalanced in properties and in extreme cases may be fibrillated to give fiber-like elements for special textile appHcations. Tensilized poly(ethylene terephthalate) is a common substrate for audio and video magnetic tape and thermal transfer tape. 

Fibrillated tape, again particularly successful with polypropylene, in which oriented film is stretched so much that fibrillation occurs. 

An important application for polypropylene is film tape. This is made by slitting unoriented film (cast or blown) into tapes 2 or 3.5 mm wide and stretching under heat about seven-fold. With cast film the orientation is more completely monoaxial and there is a tendency for the film to split along its length (fibrillate). Tubular film does not self-split so easily and also has a somewhat softer feel. Such tapes may be woven into sacks and these have... 

Film tapes may also be made drawn up to 10 1 ratios and then fibrillated by pin rollers, embossed rollers or some other technique. Fibrillated tape has been a substantial replacement for sisal. 

As one example, in thin films of Na or K salts of PS-based ionomers cast from a nonpolar solvent, THF, shear deformation is only present when the ion content is near to or above the critical ion content of about 6 mol% and the TEM scan of Fig. 3, for a sample of 8.2 mol% demonstrates this but, for a THF-cast sample of a divalent Ca-salt of an SPS ionomer, having only an ion content of 4.1 mol%, both shear deformation zones and crazes are developed upon tensile straining in contrast to only crazing for the monovalent K-salt. This is evident from the TEM scans of Fig. 5. For the Ca-salt, one sees both an unfibrillated shear deformation zone, and, within this zone, a typical fibrillated craze. The Ca-salt also develops a much more extended rubbery plateau region than Na or K salts in storage modulus versus temperature curves and this is another indication that a stronger and more stable ionic network is present when divalent ions replace monovalent ones. Still another indication that the presence of divalent counterions can enhance mechanical properties comes from... 

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

Fig. 29 Fractured morphology of spherulitic objects in a thin film of PET crystallized at 220 °C [36]. On the fractured surface many small particles with a diameter of 0.2 0.3 xm are seen while on the spherulite surface there is a fibril structure 0.2 05 xm thick... 

Sc(naphthenate)3/ROH/AlR3 (1/2/7) has been found to exhibit an activity similar to the lanthanide series catalyst [114], The cis PA film obtained with it showed an electrical conductivity of 14.4 S cm 1 when the polymer was doped with I2 at a ratio of (CHI0.04)n, and the TEM measurement suggested the formation of ca. 20-30 nm fibrils. 

Fibers are commercially made from uniaxially drawn film. The film is extruded, slit into tapelike strips, drawn, fibrillated, and wound. As in the case of spinning, the drawing produces preferred alignment of the polymer chains along the axis of pull. If the drawing precedes slitting, the fiber gives some cross orientation and is less apt to split. 

Fibrillation can be achieved mechanically by drawing and pulling thin sheets of polymer. This is compounded if twisting is also involved. A rough idea of this process can be demonstrated by cutting several ribbons of film from a trash bag. Take one and pull. It will elongate and eventually form a somewhat thick filament-like material. Do the same to another strip but also twist it. 

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