PVA Fiber

Commercial production of PVA fiber was thus started in Japan, at as early a period as that for nylon. However, compared with various other synthetic fibers which appeared after that period, the properties of which have continuously been improved, PVA fiber is not very well suited for clothing and interior uses because of its characteristic properties. The fiber, however, is widely used in the world because of unique features such as high affinity for water due to the —OH groups present in PVA, excellent mechanical properties because of high crystallinity, and high resistance to chemicals including alkah and natural conditions. 

The People s RepubHc of China introduced Kuraray technology and started production of PVA fiber by a wet spinning process in 1965. Its annual capacity reached 165,000 tons in 1986 (9). The Democratic People s RepubHc of Korea produce PVA and reportedly have an annual production capacity of 50,000 tons (9). 

Pure PVA dissolves in water but does not fluidize by melting. Commercial production of PVA fiber is therefore carried out by wet spinning or dry spinning, utilizing aqueous PVA solution. In either case, purified PVA is dissolved in hot water and the solution is extmded through fine holes of a spinneret the extmded streams are coagulated to form continuous filaments, which are then heat-treated to have adequate mechanical properties. 

Since PVA fiber as spun is soluble in water, it is necessary to improve the water resistance of the as-spun fiber (10). Heat treatment followed by acetalization is a classic method to provide high water resistance. 

Fig. 2. Pliotogiapli of the cross sections of PVA fiber manufactured by wet spinning with a coagulating bath of sodium sulfate (a) and sodium hydroxide...

Fig. 3. Schematic comparison of the stmctures of PVA fibers formed by salt coagulation and alkaU coagulation, (a) Low orientation, low crystallinity (b)...

Boric acid/alkaU spinning has been commercialized in Kuraray Co. and Unitika Co. in Japan, and is reportedly under research and development also in the People s RepubHc of China as a process for producing high strength PVA fiber to be used for replacing asbestos (9). 

Fine adjusting and optimization of each step of this process is stiU underway, and a PVA fiber having a single fiber strength as high as 2 N/tex (21 gf/dtex), which is close to that of aramid fiber, has been reported (18). 

Moisture Absorbency. PVA fiber is more hygroscopic than any other synthetic fiber. The hygroscopicity varies depending on how the fiber is processed after spinning, ie, in heat-drawing, he at-treatment, acetalization, and the like. 

Dimensional Stability. The wet heat resistance of PVA fiber is indicated by the wet softening temperature (WTS) at which the fiber shrinks to a specified ratio. At one time, the WTS was not more than 95°C for nonacetalized PVA fiber, but improvement of WTS has been achieved by improvement in heat-drawing and -treating techniques other methods proposed include suppression of polymerization temperature of vinyl acetate (36) and employment of alkafi spinning (37). 

On the other hand, water-soluble PVA fibers are available on the market. They are stable in cool water but shrink in warm water and dissolve at 40 to 90°C. The dissolution temperature is controlled by the degree of polymerization and hydrolysis of PVA, he at-treatment conditions after spinning, etc. 

PVA fiber is better in dimensional stabifity under dry heat than other synthetic fibers. 

Chemical Resistance. Table 2 shows the chemical resistance of PVA fiber (40). The fiber exhibits markedly high resistance to organic solvents, oils, salts, and alkaU. In particular, the fiber has unique resistance to alkaU, and is hence widely used in the form of a paper principally comprising it and as reinforcing material for cement as a replacement of asbestos. 

Rubber Industry. This is the oldest industrial area in which PVA fiber has been successfully appHed. The fiber is used for reinforcement for... 

PVA fiber is best fit for reinforcement of oil brake hoses for cars that require high reUabiUty, because of excellent mechanical properties and good chemical resistance to pressure—transmission Hquid contained in the hose. 

PVA fiber ropes are widely used in fishing and on ships, because of excellent weather resistance, coiling property, ease of handling, twist stabihty, etc. For this purpose spun yams obtained directly from tow by the Pedok spinning system are used. 

Sewing Thread. Spun yams and filament yams of PVA fiber with their characteristics of low elongation and high strength are used as industrial sewing threads for leather materials such as shoes and bags and for similar items. In Japan, the PVA fiber threads are also used for sewing tatami mattress. 

Nonwoven Fabric. Crimped PVA staple is being used for the manufacture of dry-laid nonwoven. Also, as an example utilising the uniqueness of the fiber, a soft sheet is prepared by shrinking and pardy dissolving in hot water a nonwoven from water-soluble PVA fiber and then insoliibili ing the fabric by acetalization or similar processes. This sheet is used as car wipers, wipers for high grade furniture, and for similar purposes. 

Reinforcement. PVA fiber in the form of short cut chips having a length of several millimeters to several tens of millimeters is widely used as raw material for paper and for reinforcing plastics, cement, and the like, and has been acquiring more and more significance. 

Fiber-Reinforced Plastics. PVA fiber, with its high strength and toughness, has become widely used as reinforcement fiber for FRP. Strength as high as 1.8 N/tex (18 gf/dtex) has been reported (32,41). Penetration in this field will therefore become more active. 

Fiber-Reinforced Cementitious Material. Use of asbestos (qv) has been legally restricted in Europe and the United States as being ha2ardous to health. In asbestos cement, which had consumed 70—80% of total asbestos, PVA fiber has been used in large amount as a replacement for asbestos. PVA fiber has a strength of at least 0.88 N/tex (9 gf/dtex) and can therefore provide the necessary reinforcement for cement the fiber has excellent adhesiveness to cement (qv) and alkaU resistance, and is not a health ha2ard. 

Based on the technology developed for using PVA fiber as a replacement for asbestos in cement products, Kuraray has been developing thick fibers for reinforcing concrete (42). Super-thick fibers with a thickness of 39 tex (350 den) (200 p.m in diameter) to 444 tex (4000 den) (660 p.m in diameter) are now available the 39 tex material is used for reinforcing various mortar-based cement products and the 444 tex material for reinforcing concrete in civil engineering works such as tuimels, roads, harbors, and bays. 

PVA fibers have found wide spread industrial use in cement as replacement for asbestos in cement products, reinforcement of mbber material such as conveyor belts and hydraulic mbber hoses used in cars, ropes, fishing nets, etc. Only a small amount of fibers is used in the production of textiles. 

Several patents (327—329) have been issued that cl aim processes for production of ultrahigh tensile strength PVA fibers, which have a tensile strength comparable to that of Kevlar. 

Putty powder, stannic oxide in, 24 805 PVA fibers, 25 619. See also Poly(vinyl alcohol) (PVA)... 

It is customary to stop the hydrolysis of PVAc before all the acetyl groups are removed. Thus the commercial product, with a degree of hydrolysis of about 88%, is readily soluble in water but is resistant to less polar solvents, such as benzoie and gasoline. PVA fibers (Kuralon) are strong and insoluble in water because of a surface treatment with formaldehyde which reacts with the surface hydroxyl groups to produce polyvinyl formal on the polymer surface. 

We have to pay attention to the iodine concentration at which the specimens are soaked when we discuss the structure and properties of complexes in iodine soaked PVA films. This is due to the fact that iodine only penetrates into the amorphous phase at low iodine concentration, while it penetrates into the crystalline phase as well as the amorphous phase at higher concentrations. Hess and his coworkers [48] found that a new X-ray diffraction peak appears when PVA fibers absorb more than 12% iodine, which is cawed by the penetration of iodine into the crystal phase. As expected, iodine sorption in the different phases of PVA must cause different modes of complex. 

The main applications for PVA are in textile sizing, adhesives, polymerization stabilizers, paper coating, poly(vinyl butyial), and PVA fibers. In terms of percentage, and omitting the production of PVA not isolated prior to conversion into poly(vinyl butyral), the principal applications are textile sizes, at 30% adhesives, including use as a protective colloid, at 25% fibers, at 15% paper sizes, at 15%, poly(vinyl butyral), at 10% and others, at 5%, which include water-soluble films, nonwoven fabric binders, thickeners, slow-release binders for fertilizer, photoprinting plates, sponges for cosmetic, and health care applications. 

These fibers show higher crystallinity than the neat PVA fibers which are electrospun for comparison. The orientation of PVA or nanotubes is not mentioned in this work. In a very recent article, another group also investigated electrospun PVA/MWNTs fibers. The nanotubes and PVA were dispersed into a water/ethanol mixture in the presence of lignosulfonic acid sodium (45). 

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