Fibrous stmctures

All five processes requite plasticization of the nitrocellulose to eliminate its fibrous stmcture and cause it to bum predictably in parallel layers. Mechanical working of the ingredients contributes to plasticization and uniformity of composition. The compositions of representative nitroceUulose-based gun propellants are shown in Table 7. 

Fibrous stmctures represent a grain refinement of columnar stmcture. Stress-reHeving additives, eg, saccharin or coumarin, promote such refinement, as do high deposition rates. These may be considered intermediate in properties between columnar and fine-grained stmctures. 

Historically, strontium metal was produced only in very small quantities. Rapid growth of metal production occurred during the late 1980s, however, owing to use as a eutectic modifier in aluminum—silicon casting alloys. The addition of strontium changes the microstmcture of the alloy so that the siUcon is present as a fibrous stmcture, rather than as hard acicular particles. This results in improved ductility and strength in cast aluminum automotive parts such as wheels, intake manifolds, and cylinder heads. 

In the fibrous acetylation process, part or all of the acetic acid solvent is replaced with an inert dilutent, such as toluene, benzene, or hexane, to maintain the fibrous stmcture of cellulose throughout the reaction. Perchloric acid is often the catalyst of choice because of its high activity and because it does not react with cellulose to form acid esters. Fibrous acetylation also occurs upon treatment with acetic anhydride vapors after impregnation with a suitable catalyst such as zinc chloride (67). 

The sorbent of fibrous stmcture has the best kinetic characteristics in relation to noble metals, for which reaching soi ption balance does not exceed 20 minutes. The rate of soi ption balance establishment depends on the form of nitrogen in functional groups of sorbents used and decreases in a line tertiary nitrogen (linear group) > tertiary nitrogen (heterocycle) > quaternary nitrogen. 

Felt Homogeneous fibrous stmcture made by interlocking fibers via application of heat, moisture, and pressure. 

The Virginia amphibole (identified as actinolite) and the South African amphibole (identified as anthophyllite) were predominately nonasbestiform, whereas the Montana amphibole (identified as actinolite) was predominately asbestiform (Moatamed et al. 1986). Numbers of fibrous amphibole particles in the Virginia samples were reported to be extremely low in comparison to the Montana samples. The infrequent, short fibrous structures were most likely cleavage fragments. The South African vermiculite samples showed a near absence of fibers or rare, short fibrous stmctures. ... 

Chitosan, sodium chondroitin sulfate, and pectin-nanofibrous mats were prepared from the respective polysaccharide/poly(ethylene oxide) blend solutions by electrospray. Unblended polysaccharide solutions showed low processability, i.e., the solutions could not be electrosprayed. The addition of 500 kDa poly(ethylene oxide) to chitosan solutions enhanced the formation of a fibrous stmcture. Sodium chondroitin sulfate/poly(ethylene oxide) and pectin/poly(ethylene oxide) blend solutions were generally too viscous to be sprayed at 25 °C, but at 70 °C the fibrous stmcture was formed [61]. 

The approach of phase separation starts with the dissolution of a polymer into a solution, followed by the gelation of the polymer solution. One phase of the mixture, the solvent, is then extracted in distilled water to leave behind the other phase, polymer, in a highly porous nanofibrous stmcture, which is further freeze-dried to remove excessive water to give a dry fibrous stmcture (Ma and Zhang, 1999). 

Based on the yam volume description, the fibrous stmcture of the yarn, or, more generally, the fibrous stmcture of the unit cell, is described as follows. Consider a point P and the fibres in the vicinity of this point. The fibrous assembly can be characterised by physical and mechanical parameters of the fibres near the point (which are not necessarily the same in all points of the fabric), fibre volume fraction Vf and direction /of them. If the point does not lie inside a yam, then Ff = 0 and/is not defined. For a point inside a yam, fibrous properties are easily calculated, providing that the fibrous stmcture of the yam/ply in the virgin state is known and its dependency of local compression of the yarn/ply, bending and twisting of the yam are given. Searching the cross-sections of the yams, cross-sections S, = S(si) and S,+i =5 (s,+i) (s is a... 

In Chapter 4, VerOTiika Kapsali describes the advancement in the field of biomi-metics in general and its potential use in sports textiles in particular. Biomimetic textiles use textile-related technologies as a platform to transfer properties and functionalities identified in biological systems into fibrous stmctures. This is achieved... 

Figure 12.7 Layer sequences for different fibrous stmctures with pore-size grading in NSN process (a, b) and generated graded scaffold stmcture (c).

This example of vascular grafts devices points out the evolution of fibrous implantable medical devices and highlights the great potential offered by each scale level of fibrous structures for biocompatibility improvements. Fibers as well as whole fibrous stmctures should be considered as implantable devices that have inherent abilities to interact with the biological environment at each of the three predetermined scale levels. Study of characteristics and specificities of fibers, fibrous siuface, and fibrous volume should then provide a more forward-looking approach in the textile substitute s area for design and achievement of smart medical implantable textile devices. 

Discontinuity is the basis of fibrous stmctures that are obtained by fibers interlacing. This implies a number of characteristics that can be exploited to promote biocompatibility of implantable medical devices. 

Through ultrafine fiber and high fiber density, porosity of fibrous stmctures could be decreased and inhibit cellular ingrowth within the substitute fibrous stmcture. The dense fiber network of electrospun membrane presents a barrier for cell entry. It has been used for islets, which are used as an immiinoisolation strategy for disease... 

Fiber interlacing process allows to obtain cohesive fibrous stmctures that demonstrated clear benefits to be used as implantable medical devices with improved biocompatibility features. However, these fibrous cohesive structure features may need to be scaled and organized to better match the applications. These adjustments are mostiy related to textile processes. Besides simple fibrous cohesive structures, textile processes could help to give controlled fibrous patterns with desired scale, orientation, and physical properties. 

While textile processing previously included almost always three to four successive steps to achieve a 3D structure, from fiber to the yam and then from the yam to the surface and volume arrangement, one more recent way to achieve this is 3D printing and 3D deposit of the fibrous component. Even if it is not the only way to obtain desired and optimized stmctures, it points out the willingness to control all the key parameters of fibrous stmctures. Orientation, distribution, and square density of fiber is aimed to be controlled, as well as the fibers properties themselves, their interlacing pattern, at aU scale levels. 

These examples underscore the wide range of material physical characteristics that remain to be explored to improve fibrous implantable medical devices and make them even smarter for each intended application. While some of these smart characteristics could be easily identified, such as radio-opacity, radioresistance, and resistance to sterilization, wettability, the fibrous material area is a vast exploratory field that is not as well known and described as the fibrous stmctures area. 

In practical terms, the electrical resistance measurements on conductive fibers/yams is always problematic due to their soft, flexible, and poor dimensional stabilities. Very few publications have reported research regarding electrical measurements on fibrous stmctures. Usually, the conventional method, in which crocodile clips are attached with a voltmeter, is used for this purpose. Crocodile clips hold the conductive fibers/yarns of specific length and then electrical resistance is measured on particular voltage values. However, the hard grip of crocodile clips damages any conductive coatings or creates internal cracks in the fibrous stmctures, which cause the permanent loss in electrical properties of conductive threads. Consequently, consistent results with crocodile clips cannot be obtained. 

We have developed a novel electrical resistance measruing semp, which can not only be used for fibrous stmctures but also for fabric samples with particular dimensions. The complete semp is shown in Fig. 28.14. 

Also, fibers are controversial. In one currently used handbook,natural, inorganic fibers such as wollastonite or asbestos have been included among fillers whereas other fibers were included in a separate group with only three materials glass, aramid, and graphite. But, mixtures of fibrous and particulate materials are found in many composites today and various natural materials having fibrous stmctures are considered fillers in technical papers. Again our definition includes these examples. 

From a biological viewpoint, almost all of the human tissues and organs are deposited in nanofibrous forms or structures. Examples include bone, dentin, collagen, cartilage, and skin. All of them are characterized by well-organized hierarchical fibrous stmctures realigning in nanometer scale. 

---