FIBKKS (TEXTILE) 



(7) Air pockets. Both elongated, spindle- 

 shaped and unelongated air pockets occur. 

 Usually they are characterized by a wide 

 light refractivity zone and a pronounced 

 stress figure. Fibrillar spUtting in torsion and 

 tensile tests is usually the result. With orig- 

 inally negatively birefringent fibers, such as 

 Orion, a positive form of birefringence may 

 occur through strong fibrillar splitting if 

 there is a skin. 



(8) Fibrillar fiber structure. Most synthetic 

 fibers have a fibrillar structure. In most cases 

 this can be seen only with an electron micro- 

 scope from rephcas or thin cross sections. In 

 Orion and polyvinylalcohol the fibrillar 

 structure is to be clearly seen with a Ught 

 microscope. With Terylene and Dacron it is 

 visible with a Ught microscope, but is clear 

 only under an electron microscope. The fi- 

 brillar structure of nylon can be seen only 

 with an electron microscope. 



(9) Surface structure. This refers in the 

 first place to variations in cross section. In 

 addition, however, small structures such as 

 the fairly common spiral grooves are im- 

 portant. Frosted fibers may have an irregu- 

 lar surface through the protrusion of frosting 

 crystals. These can best be detected by 

 shadowing the fiber slightly and examining it 

 with a normal reflector microscope. 



(10) Lacunose effect. Elongation of syn- 

 thetic fibers until necks are formed may 

 cause more or less spindle-shaped opaque 

 spots in the fibers. These spots consist of a 

 large number of small cracks in close prox- 

 imity. Except in the case of cold drawing, 

 the effect is also found normally in some 

 fibers. 



Nylon (Fig. 4b). Homogeneous, optically 

 "empty" fiber. In polarized light, variations 

 in double refractivity A = 56-62. There is 

 sometimes a skin. The cross section is round. 

 In zinc chloroiodide nylon turns yellow and 

 later orange to brown. In saturated phenol 

 solution it is soluble. In 50% formic acid 

 nylon is insoluble at 80°C, Perlon is soluble. 



Orion (Polyacnjlic) (Fig. 5a). The cross 



section is dumb-bell shaped to irregular. 

 This causes striation in longitudinal speci- 

 mens. In polarized light a positively bire- 

 fringent skin and a negatively double refract- 

 ing pith can be disthiguished. Double 

 refractivity varies from A = -f-3 toA = —10. 



In various types the "pith" contains dark 

 particles which are spindle-shaped to round. 

 These may be double refractive, but also iso- 

 tropic. Air pockets are also found. In satu- 

 rated phenol solution the fiber is hardly visi- 

 ble. Upon heating, visibilit}'' increases. In 

 dimethylformamide the fiber dissoh-es when 

 heated. 



Acrilan {Foly acrylic). The cross section is 

 mostly kidney-shaped. Usually there is a 

 pronounced skin. The pith often contains 

 isotropic particles and air pockets. The fiber 

 is weakly negative birefringent A = approx. 

 4. 



X 51 (Polyacrylic) . The surface is fairly 

 smooth. The continuous filament is optically 

 clear, the staple fiber is less transparent. The 

 cross section is round with a very pro- 

 nounced, fairly thick skin. The birefringence 

 is fairly low: A = approx. 4. 



Dynel (Polyacrylic) (Fig. 5a). Fairly 

 smooth fiber with a ribbon-shaped cross sec- 

 tion. Under a polarizing microscope bire- 

 fringence varies from A = approx. +6 to 

 A = approx. —6. 



Polyvinylalcohol 11 {"Kuralon") (Fig. 

 5a,b). Fiber with the most pronounced skin 

 and pith. The pith contains manj'' spindle- 

 shaped to round bodies which are partly 

 double refractive, partly amorphous. In some 

 cases there are air pockets. 



The cross section is dumb-bell shaped to 

 irregular. The fiber is weakly birefringent 

 and })oth positive and negative birefringence 

 is found. 



Dacron, 



Terylene 

 (Polyester) ^ 



Fiber with a fibrillar structure 

 which can only be seen in longi- 

 tudinal sections. Otherwise ho- 

 mogeneous. Birefringence 115 

 Cross section : Round 



361 



