Shrinkage force

Neither entropic, nor chemical shrinkage are exported to be very sensitive to the small amount of comonomer present. More probably, the greater plasticization by the larger comonomer molecules enhances the elongation in the thermoplastic phase, thus reducing the apparent shrinkage. 

From this point of view, improved carbon fiber properties are to be expected from the fibers with highest apparent shrinkage force, where the loss of attained molecular order by relaxation is the lowest. This may be an additional reason for the good performance of the AN/VBr precursor. In fact, the carbon fiber properties of this precursor are the best of those compared in Table 13. However, there is no clear trend relating properties to shrinkage force. As discussed in the preceding sections, the whole process of carbon fiber making is too complex to expect a simple relationship. 

For certain application it may be of interest to use precursor fibers with low overall shrinkage force. Thus, if a non-woven fabric would be subjected to continuous stabilization, one would certainly wish to maintain the over-all shrinkage force low to minimize strain at the bonding points. AN/MMA might be an interesting candidate for such applications. 

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Similar results have been obtained for polypropylene where again the high modulus products show a greatly reduced shrinkage, and comparatively high shrinkage forces. 

Fig. 52. Variation of shrinkage force with time and temperature f<...

PAN based fibers develop shrinkage forces during two different stages of stabilization entropic shrinkage in the early stages, and shrinkage due to chemical reaction in the later stages. 

Fig. 7. Entropic shrinkage force developed by a fiber from AN/MMA (4.2%), on heating in air. Heating rate 30 °C/min...

Fig. 8. Shrinkage forces developed by acrylic fibers during stabilization at constant temperature (schematic)...

Table 13. Overall shrinkage force during stabilization standard screening conditions...

A third benefit from the small molecular volume of VBr is probably a reduced relaxation in the thermoplastic phase of stabilization (i.e. increased overall shrinkage force, cf. Sect. 4.4.3). 

Daley, C. Menard, K. Measurement of shrinkage forces of synthetic fibers held at constant length during solvent exposure. North American Thermal Analysis Society Notes 1994, 26, 56. 

Shrinkage forces Relatively small shrinkage forces High shrinkage forces, may ... 

Despite the extreme importance of the thermal properties on the processing and performance of textile polymers, particularly in filament processing and finishing, there are surprisingly few standard methods of test dealing with the subject. This is perhaps due to the complexity of the subject in terms of the effects of temperature on oriented chain molecules and influence of moisture on the polymer. However, a standard method that is available is ASTM D 5591-95, which is concerned with the thermal shrinkage force of yarn and cord. The instrument specified is the Testrite thermal shrinkage force tester... 

Thermal shrinkage force or thermally induced contraction is critical to many commercial operations, such as tire production, transmission belts, and certain types of conveyor belting and is dependent on the annealing and drawing processes during filament manufacture,... 

ASTM 5591 95 Thermal shrinkage force of yarn and cord using the testrite thermal shrinkage force tester. 

Variation of the shrinkage force at temperatures above Tg shows the presence of the permanent residual network. 

Figure 4 (a) Fracture process zone (area of stress concentration) surrounding the area or volume of the bondline immediately ahead of the crack tip when the joint is subjected to cleavage, shear, or shrinkage forces (b) small process zone and high stress concentration with rigid adherend and adhesive (c) large process zone and low stress concentration with flexible adhesive and adherend. 

Figure 2.28 shows that shrinkage increases with temperature increasing. At any temperature, shrinkage force is an intrinsic property of material, independent of heating rate. This is clearly observed on crosslinking, which reduces shrinkage. 

K. Arakawa, Shrinkage forces dne to polymerization of light-cured dental composite resin in cavities, Polym. Test. 29 (2010) 1052-1056. 

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