Fiber diameter, reduction

Nozzle-Collector Distance—The distance between the nozzle and collector has a direct influence on the flight time of the liquid jet. When a solution is used as the spinning fluid, a minimum nozzle-collector distance is required so the liquid jet can have sufficient time for most of the solvent to be evaporated before reaching the collector. In addition, when the nozzle-collector distance increases, the liquid jet will have a longer distance to travel and this favors the fiber diameter reduction (This applied to both solution and melt). However, when the nozzle-collector distance is too large, the liquid jet may not be able to reach the collector due to the insufficient centrifugal force. 

Other materials besides boron and SiC with potential for CVD fiber production are being investigated. They include B4C, TiC, and TiB2 deposited on a heated core filament, generally by the hydrogen reduction of the chlorides.Typical properties of the resulting experimental fibers are shown in Table 19.3. Fiber diameter varies from 20 to 200 pm. 

Coating thickness should be chosen to optimize the benefit in toughness and minimize the loss in strength and some other properties. As a rule of thumb, the thickness of the coating should be kept minimum compared to the fiber diameter in order to eliminate any reductions of composite stiffness and strength in both... 

Evans et al. [25] have confirmed these conclusions of Wickett. In addition, the observation that Japanese hair is easy to perm and that fine Caucasian hair, less than 75 pm in diameter, is more difficult to perm was also confirmed. However, these scientists were unable to identify any common characteristics such as fiber diameter or cystine content that would account for this behavior. The fact that fine hair is more difficult to perm than thick hair may be due to the larger ratio of cuticle to cortex in fine hair and the fact that cortex plays a stronger role in waving than cuticle. This explanation is consistent with the experiments by Wortmann and Kure [2], demonstrating that the cuticle does inhibit the reduction reaction. In addition to pH, other important variables that influence the rate of reduction of keratin fibers by mercaptans are temperature, hair swelling, prior history of the hair, and structure of the mercaptan. 

Hair fibers that have been reduced and reoxidized approach the original fiber diameter. Indeed, Eckstrom [121] has suggested that the milder the conditions of reduction, the closer the fiber will return to its original dry-state diameter on neutralization. 

Fiber diameter determined in the wet or swollen state is sensitive to detect changes produced by damaging treatments like permanent waves or bleaches. The swelling action of permanent-waved hair [30] and in bleached fibers [32] is greater than in unaltered hair and has been used to estimate the relative extent of alteration produced by reduction and reoxidation [31]. For additional discussion on hair swelling, see Chapters 1 and 8. 

During fiber-spinning and film-blowing, one may expect the extensional viscosity increases with the increase of dv/dr. As the local increase of dv/dx leads to the reduction of the fiber diameter, the increase of viscosity could prevent further thinning and thus makes the fiber more homogeneous in thickness and stronger. Such a performance of polymer fluids has been described as spinnability (Petrie 2006). 

Typically, fibers having diameters up to 100 (jm are readily possible fibers with diameters above 2()0 pm are technically speaking no longer fibers but rods. This includes the niobate fibers [46] and the superconducting fibers [14] [50] [33] [63] discussed below. In principle, the float zone method is a containerless process. There is a nominal diameter reduction in the float zone from that of the preform rod to that of the final fiber. Zone (or overall process) stability for the growth of oxide and fluoride fibers is usually achieved with diameter reductions in the range of 1/2 to 1/3 [14]. 

The largest diameter reduction occurs within less than 1 cm of the spinneret surface. Spinning of a mesophase pitch with 40-80% mesophase requires a melt viscosity of 10-40 MPa.s (or log 2.6 poise). This viscosity corresponds to a fiber forming temperature of 340-380°C, and requires a winder speed ranging from 100 to 500 m/min [15] [24]. Extrusion or melt spinning of 100% mesophase pitch melts can be carried out at higher viscosities and with higher winder speeds [6] [25-26]. 

Steam gave an initial loss of up to about 15% and then remained unchanged. The authors believe that steam results in fiber bum-off producing microporosity and a reduction in fiber diameter, whilst CO2 produces microporosity without a reduction in fiber diameter. However, with CO2, internal pitting takes place with a significant reducing effect on fiber strength. 

The third direction which was the concept of diameter reduction has led to the group of nano fibers, although such fibers can also be built up in a bottom-up way [1]. This group is the combination of the two former directions because the aim in this case is both increased strength and functionalization. The decrease of fiber diameter generally leads to an increased strength because of the so-called size-effect [2]. This phenomenon revived the development of nanocomposites and nano-reinforcements where the typical size of reinforcement is in the nanometer or submicron scale. 

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