Nylon Fiber

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The two widely used nylon fibers are nylon 66 and nylon 6. The chains, being void of aromatic compounds , tend to fold and lead to fibers that have low modulus and relatively high extension at break. The amide groups, however, allow formation of hydrogen bonds between the NH and CO groups of adjacent chains; this gives the fiber excellent mechanical and thermal stability. The amide groups also attract water, which makes the fiber reasonably hydrophilic (moisture regain 4%) and wettable. Being one of the most stretchable and elastic of the commonly used textile materials, in addition to being acceptably hydrophilic, nylon is a preferred material for tubular bandages and compression hosiery for treatment of venous leg ulcers . The fiber is also used in the construction of monofilament and braided sutures .
Polyamide (PA) or “nylon” fibers were developed in 1938 by Du Pont in the US and in Germany by I.G. Farben. These fibers are based upon amide groups (–C(O)NH–) contained within a chain of carbon atoms making up a backbone of atoms linked by strong covalent bonds ( Table 1 ). However, the molecules are linear and can bend and rotate at each bond so that the molecules are not perfectly aligned parallel to the fiber axis , which results in a low Young's modulus. Around 1947, polyethylene terephthalate (PET) or “polyester” fibers were produced in Great Britain and have become the most widely produced synthetic fibers . Table 1 shows that the polyester molecule contains an aromatic ring , which stiffens the molecule compared to the very flexible polyamide structure. Both polyamide and polyester fibers melt at around 260 °C. Although both of these latter fibers melt at around the same temperature, it is significant that the polyester fiber, which contains the aromatic rings, shows much higher resistance to discoloration at raised temperatures than the polyamide fiber. A fiber which is related to PET fibers is the polyethylenenaphthalate (PEN) fiber. Table 1 shows that this latter fiber is based on a molecule which has two aromatic rings, which further increases the rigidity of the fiber. The three fibers mentioned above are made by drawing from the melt through a spinneret . The drawing rate determines the final properties. An undrawn fiber will have a strain to failure of some hundreds of a percent and tensile loading will produce not only a very high strain to failure but deformation will involve striction of the filament. Striction is when the diameter of the filament is locally reduced under load and in this region the molecular structure becomes aligned parallel to the fiber axis and the loading direction. Further loading induces an increase in the striction until the specimen is reduced in cross section and the molecular structure is largely oriented parallel to the fiber axis. Fibers for textile end-uses are drawn during production so as to produce filaments with enhanced molecular orientation , higher strengths, and longitudinal stiffness but with reduced strains to failure, generally of the order of 30%. Striction is not usually seen with this type of fiber. High-performance versions of PA and PET fibers, which are utilized for industrial applications, are more highly drawn with further reduced strains to failure in the region of 20% for the former and 15% for the latter. PEN fibers are only produced as high-performance industrial fibers and have strains to failure of around 6%. The molecular morphology of these fibers consists of microfibrils , with transverse dimensions of around 5 nm, arranged more or less parallel to the fiber axis. Regions within the microfibrils are composed of well-ordered arrangements of molecules which are folded as in a concertina. The rest of the structure is made up of less well-ordered regions so that the overall molecular structure is semi-crystalline with a crystalline content generally of the order of 40%.
Table 1 . Molecular composition of high-performance organic fibers.
A typical tensile fracture morphology of one of the above thermoplastic fibers is shown in Fig. 1(a) . It can be seen that the fracture surface consists of two regions, an initial inclined region corresponding to initiation at the surface and slow crack propagation normal to the fiber axis. The crack opens because of plastic deformation ahead of the crack. Rapid failure finally occurs when the remaining part of the section can no longer support the applied stress. Under particular cyclic conditions, the same fiber fails in fatigue and one half of the fatigue break is shown in Fig. 1(b) . The crack has initiated at or near the surface but instead of running normal to the fiber axis it has been deviated to run at a slight angle to the fiber axis, penetrating into the fiber section so that the remaining section finally fails by the tensile process. A particularity of this type of failure is that the minimum load has to be lower than a threshold level so that raising the cyclic loading pattern can stop fatigue failure.
Figure 1 . Fracture morphologies. (a) A polyamide 6/6 fiber broken in tension and (b) a polyamide 6/6 fiber broken in fatigue.
Higher melting points or decomposition temperatures and much higher Young's moduli have been achieved by including more aromatic rings in the molecular main chain and by aligning these very rigid molecules parallel to the fiber ( Table 1 ). These types of polymers are made by the technology of liquid crystal spinning, as illustrated in Fig. 2 . Overall, within the melt or solution, the molecular alignment is random; however, orientation is achieved not by drawing but through the shearing of the solution when the solution passes through the spinneret. Molecular alignment in these fibers occurs because of the very stiff molecules that are aligned preferentially parallel to the fiber axis by the shear forces in the spinneret and held together by hydrogen or van der Waals' bonds . The straighter the molecule the higher the modulus achieved. These liquid crystal polymers do not usually lend themselves to melt spinning and are usually spun from a solution although the Vectran fiber, produced since the 1970s by Hoechst-Celenese, is melt spun.
Figure 2 . Spinning from a liquid crystal solution.
Vectran is a liquid crystal polyester-based fiber, which, as can be seen from Table 1 , has a molecular structure that is not perfectly straight. It is produced by the acetylation polymerization of p -hydroxybenzoic acid and 6-hydroxy-2-naphthoic. The good alignment of its molecular structure gives higher rigidity than that found with other polyester fibers but the form of the molecule determines its ultimate rigidity. The fiber is the only melt spun liquid crystal fiber, which is available and finds uses in applications such as sail cloth because of its rigidity and resistance in bending.
The aramid family of fibers are made up of aromatic PAs. Nomex was the first fiber of this family, produced in the 1960s by Du Pont. Nomex is a m -aramid, which accounts for an elastic modulus little different from high-performance PET but with a higher temperature resistance. Nomex is widely used for clothing for potentially hazardous conditions where fire is a danger. It is also widely used in paper form as a honeycomb structure in the aeronautical industry . p -Aramids can possess remarkably high Young's moduli, more than 20 times that of conventional PA fibers and twice that of glass, as shown in Table 2 . This group of fibers, first introduced as the Kevlar fiber from Du Pont, has been available since 1972. These fibers are made from a mesophase or liquid crystal solution of poly-( p -phenylene terephthalamide) (PPTA) dissolved in concentrated sulfuric acid. If an appropriate concentration is made, the solution consists of locally organized PPTA molecules, which align with one another. The structure of aramid fibers is very well ordered; however, they are highly anisotropic , which results in them being weak in all directions except parallel to the fiber axis. As a consequence failure of these fibers is nearly always highly fibrillar.
Table 2 . Comparison of organic fiber properties.
Under tension, Kevlar fibers are 5 times as strong as steel in air for the same weight. Under tension, the fibers show little tendency to deform plastically, but when they are bent, the side experiencing compression undergoes plastic deformation, which allows them to accommodate the imposed strain. This results in the fiber being difficult to cut as well as giving great tenacity and resistance to impact loading . Their highly anisotropic behavior means that the fibers are not used in primary loading structures subjected to compressive forces . Aramid fibers, other than Kevlar, that are commercially available are the Twaron fiber developed by Akzo in Holland, now produced by Teijin Twaron, also in Holland and the Technora fiber from Teijin in Japan. Technora is derived from poly- p -phenylene/3,4'-diphenylether terephthalamide , which, as can be seen from Table 1 , is not a straight molecule and as a consequence the fiber has an intermediate modulus of ∼65 GPa.
In 1998 the Toyobo Company began the commercial production of the Zylon fiber. Its chemical composition is poly( p -phenylene benzobisoxazole) (PBO). The fiber has remarkable properties in tension, as can be seen from Table 2 . It has a Young's modulus, which is 40% higher than that of steel, with the density of an engineering plastic. Its Young's modulus is twice that of Kevlar, since unlike Kevlar, the PBO molecule, shown in Table 1 , is completely straight thus making the most of the covalent bonds in its structure. PBO fibers, however, suffer from poor compressive strength as molecular interchain cohesion is assured by van der Waals' bonds. As can be seen from Fig. 3 , the fiber fibrillates on failure due to its inherent anisotropy.
Figure 3 . Detail of the splitting of a Zylon PBO fiber.
Magellan International in the US has taken over the development of a fiber created by the researchers at Akzo, which at present is called M5. The fiber is said to possess a Young's modulus of 330 GPa, but, above all, superior compressive properties, such as molecular interchain cohesion, are ensured by hydrogen bonds . This leads to a slightly higher specific gravity of 1.7. Compressive strength is said to be 4 times higher than that of the PBO fibers and nearly 3 times higher than Kevlar fibers.
The production of aramid fibers involves complex and expensive chemistry; however, the high-performance properties of the fibers are a result of the molecules being aligned parallel to the fiber's axis. Simpler polymers could, if all their molecules were aligned parallel to one another, give materials with the characteristics of high-performance materials. Such fibers can be made using a sol-gel , in which polyethylene is dissolved in a solvent to make a very dilute solution, which is then spun. Such fibers were first produced in Holland by DSM, under the name Dyneema , and then by Allied (now Honeywell) in the US under the name Spectra. The fibers consist of polyethylene molecules, which are aligned parallel to the fiber axis with molecular interchain cohesion assured by van der Waals' forces . They have properties in tension rivaling those of the aramid fibers with a lower specific gravity (0.97). However their properties in compression are poor. These fibers are limited in temperature to a maximum of 120 °C as they melt around 150 °C and suffer from creep.
Antimicrobial fibres can be mixed with traditional textile materials to produce antimicrobial wound dressings . For example, Silvercel combines the potent broad-spectrum antimicrobial action of a silver-coated nylon fibre with the enhanced exudates-management properties of alginate fibres. Because of the sustained release of silver ions, the dressing acts as an effective barrier and helps reduce infection. As is shown in Fig. 7.2 , the antimicrobial properties are built-in through the use of X-STATIC silver-coated fibres blended into the nonwoven structure. Silvercel dressing has been proven effective in vitro against 150 clinically isolated micro-organisms, including antibiotic-resistant strains.
7.2 . Photomicrograph of Silvercel wound dressing.
Brush cytology is a method used for broad sampling of the mucosal surface . 60 , 61 Cytology brushes, whether reusable or disposable, have a common design. A cytology brush consists of bristles, usually composed of nylon fibers , that branch off a thin metal shaft that runs lengthwise within a protective plastic sheath. The various cytology brushes that are currently available do not seem to vary in terms of performance characteristics. 62 The cytology brush is passed through an accessory channel of an endoscope . The end of the sheath is passed out of the tip of the endoscope, and the bristle portion of the brush is then extended from the sheath. The brush is rubbed back and forth several times along the surface of the lesion, or stricture, and is then pulled back into the sheath. The sheath is then withdrawn from the endoscope, and the brush is pushed out of the sheath, thus exposing the bristles. The bristle portion of the brush may be cut off, placed into fixative, and sent in its entirety to the cytopathology laboratory. Alternatively, the bristles can be rolled against a glass slide in the endoscopy suite. The slides should be sprayed with fixative immediately, or submerged within it, and subsequently delivered to the cytopathologist. If smears are made in the endoscopy suite, little additional benefit is derived from inclusion of the bristles for cytopathologic analysis. 63
Liliana Liverani , ... Aldo R. Boccaccini , in Encyclopedia of Biomedical Engineering , 2019
Porous and biodegradable PLLA scaffolds for vascular tissue engineering applications, with a vessel-like shape, can be produced by a two-step experimental protocol, including a dip coating of a viscous polymer solution around a nylon fiber and a diffusion-induced phase separation (DIPS) by immersion into an antisolvent bath ( Pavia et al., 2013 ). The first step (dip-coating bath, PLLA/dioxane solution) allows the production of the tubular structure of the vascular graft, whereas the second (DIPS) is responsible for the interesting level of porosity and for the surface microporosity as potentially ensuring nutrient and metabolites exchange during blood flow. The as-prepared scaffold can be utilized either as vascular grafts or embedded into a porous scaffold for the regeneration of an injured tissue as a pseudoperipheral circulatory system , with the aim of promoting a rapid vascularization of the whole tissue-engineered construct ( Pavia et al., 2013 ). A cell culture inside the vessel-like scaffolds was carried out, by using endothelial cells (EC), which are the solely components of capillary bed and the first to form during embryonic development . After 21 days, the internal lumen of the scaffold is totally covered by EC, which have organized themselves into a well-differentiated vessel structure. Cells shown to form stable cell–cell interactions and spindle membrane protrusion, characteristic of mesenchymal endothelial phenotype, were not observed. These results suggest that the scaffold produced could be usefully employed in vascular tissue engineering applications ( Pavia et al., 2013 ). In another research study, mouse mesoangioblasts (A6) were seeded onto bidimensional matrices within three-dimensional porous scaffolds of poly ( l -lactic acid) (PLLA) prepared according to the protocol described in ( Carfì-Pavia et al., 2009 ), in the presence or absence of a type I collagen coating. Results show that PLLA films allow direct cell adhesion and represent an optimal support for cell growth ( Carfì-Pavia et al., 2009 ).
Angus C Cameron , ... Sarah Raphael , in Handbook of Pediatric Dentistry (Fourth Edition) , 2013
Orthodontic brackets with a light archwire (0.014″). Orthodontic appliances are particularly useful as the time taken to apply the brackets is half that to set composite resin ( Figure 9.34F ).
Composite resin and nylon fibre (0.6 mm diameter) such as fishing line (20 kg breaking strain).
Splints should generally stay in place for 10–14 days if there are no complicating factors such as alveolar or root fractures. When bone fractures are present, the splint should be retained for 4–6 weeks. If there is a root fracture, then the splint is usually required for 3 months. Avulsed teeth with immature apices that were kept dry prior to replantation may require splinting for up to 4 weeks. The occlusion may need to be relieved when the degree of overbite or luxation is such that the tooth will receive unwanted masticatory force . This can be achieved by minimal removal of enamel , or construction of an upper removable appliance, or placement of composite resin on the molars to open the bite. However, some physiological movement is necessary.
As a general rule, all teeth should be replanted whether wet or dry. Although the prognosis of a dry tooth may be poor, it is usually preferable to have the tooth present during growth than not at all. Always keep options open for future treatment.
Orthodontic splinting is always preferable but obviously requires suitable training and access to equipment. It does not matter which orthodontic bracket system is used. The most important point is that any arch wire placed for splinting is passive and will not move adjacent teeth. There are certain advantages over the use of a composite resin splint, in particular:
Allows the splint to be readily removed and replaced so that the mobility of the teeth can be monitored.
Less time to remove and less chance of damage to the teeth following removal of composite resin (often used to excess).
Rules developed in cooperation with the Institute of Inspection, Cleaning and Restoration Certification (IICRC) and the makers of stain-resist carpet limits the pH of carpet cleaning agents to 10.0 [ 6 ]. Carpets of wool fibers and nylon fibers are dyed under acidic conditions and/or protected by stain blockers and soil protectors applied under acidic conditions. Consequently, formulators of carpet cleaning agents need to be cognizant that carpet color and/or carpet stain and soil resistant properties may be affected by cleaning products with higher pH values. However, research has shown detergent solutions perform better when in the alkaline range [ 7 ]. Therefore, awareness of substances that act as buffering agents to maintain proper pH range is essential for formulating carpet cleaning agents.
John W. Harvey DVM, PhD, DACVP , in Veterinary Hematology , 2012
Numerous steps are required for neutrophil adhesion, chemotaxis, phagocytosis, and ki
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