Fiber of PET

Fine fibers of PET usually appear white, even though the polymer chips that they are made from may appear grey in color. This is a consequence of the high amount of specular reflection of ambient light, emanating from the large surface area of the fibers. 

Figure 13.6 (a) Elongation as a function of wind-up speed for partially oriented yarn, (b-d) Stress-strain curves of fibers of PET blends with 3% copolyester of 1,4-phenyleneterephthalate and p-oxybenzoate (CLOTH) and 3% copolymer of 6-oxy-2-naphthalene and p-oxybenzoate (CO), spun at 3500, 4000 and 4500 m/min (1) PET control (2) 3 % CLOTH (3) 3 % CO the loci of the theoretical extensions of the PET control are shown as dashed curves [17]. From Orientation suppression in fibers spun from melt blends, Brody, H., J. Appl. Polym. Sci., 31, 2753 (1986), copyright (1986 John Wiley Sons, Inc.). Reprinted by permission of John Wiley Sons, Inc. 

Table 5.12. Effect of extrusion temperature and draw-down ratio on mechanical properties of as-spun fibers of PET/60HBA and PET/80HBA. ...

Table 8.8 Effect of Extrusion Temperature on Mechanical Properties of As-Spun Fibers of PET/60 HBA and PET/80 HBA [84]. 

Physically or chemically modifying the surface of PET fiber is another route to diversified products. Hydrophilicity, moisture absorption, moisture transport, soil release, color depth, tactile aesthetics, and comfort all can be affected by surface modification. Examples iaclude coatiag the surface with multiple hydroxyl groups (40), creatiag surface pores and cavities by adding a gas or gas-forming additive to the polymer melt (41), roughening the surface... 

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

Chemical Properties. The hydrolysis of PET is acid- or base-catalyzed and is highly temperature dependent and relatively rapid at polymer melt temperatures. Treatment for several weeks in 70°C water results in no significant fiber strength loss. However, at 100°C, approximately 20% of the PET tenacity is lost in one week and about 60% is lost in three weeks (47). In general, the hydrolysis and chemical resistance of copolyester materials is less than that for PET and depends on both the type and amount of comonomer. 

Other Properties. Polyester fibers have good resistance to uv radiation although prolonged exposure weakens the fibers (47,51). PET is not affected by iasects or microorganisms and can be designed to kill bacteria by the iacorporation of antimicrobial agents (19). The oleophilic surface of PET fibers attracts and holds oils. Other PET fiber properties can be found ia the Hterature (47,49). 

Terephthahc acid (TA) or dimethyl terephthalate [120-61 -6] (DMT) reacts with ethyleae glycol (2G) to form bis(2-hydroxyethyl) terephthalate [959-26-2] (BHET) which is coadeasatioa polymerized to PET with the elimination of 2G. Moltea polymer is extmded through a die (spinneret) forming filaments that are solidified by air cooling. Combinations of stress, strain, and thermal treatments are appHed to the filaments to orient and crystallize the molecular chains. These steps develop the fiber properties required for specific uses. The two general physical forms of PET fibers are continuous filament and cut staple. 

Woddwide, the production capacity for polyester fiber is approximately 11 million tons about 55% of the capacity is staple. Annual production capacity iu the United States is approximately 1.2 million tons of staple and 0.4 million tons of filament. Capacity utilization values of about 85% for staple and about 93% for filament show a good balance of domestic production vs capacity (105). However, polyester has become a woddwide market with over half of the production capacity located iu the Asia/Pacific region (106). The top ranked PET fiber-produciug countries are as follows Taiwan, 16% United States, 15% People s RepubHc of China, 11% Korea, 9% and Japan, 7% (107—109). Woddwide, the top produciug companies of PET fibers are shown iu Table 3 (107-109). 

Poly(ethylene terephthalate). PET is a crystalline material and hence difficult to plasticize. Additionally, since PET is used as a high strength film and textile fiber, plasticization is not usually required although esters showing plasticizing properties with PVC may be used in small amounts as processing aids and external lubricants. Plasticizers have also been used to aid the injection mol ding of PET, but only at low concentrations. 

Some time earlier, Eastman-Kodak has been working on a novel polyester as an entry into the important polyester fiber market and had devised a new ahcychc diol, 1,4-cydohexanedimethanol [105-08-5] effectively made by exhaustive hydrogenation of dimethyl terephthalate. Reaction of the new diol with dimethyl terephthalate gave a crystalline polyester with a higher melting point than PET and it was introduced in the United States in 1954 as a new polyester fiber under the trade name Kodel (5). Much later the same polyester, now called PCT, and a cyclohexanedimethanol—terephthalate/isophthalate copolymer were introduced as mol ding resins and thermoforming materials (6). More recentiy stiU, copolymers of PET with CHDM units have been introduced for blow molded bottie resins (7). 

Polyester fibers are based on poly(ethylene terephthalate) (PET) some modified versions are formed by copolymerization, eg, basic dyeable polyester. The modified forms dye in analogous manner to other fibers of similar charge. 

The engineering applications of PET resins include blow-molded botties, films, mol ding, and extmsion. Resins made for the latter two uses and related purposes are called mol ding resins in this article. The huge volumes of PET resin used for textile filaments and industrial fibers, eg, tire cord, are not included here. The PBT resins are mainly used for mol ding and related appHcations. 

Peculiarities of the Fine Structure of PET Fibers and the Relationship to Their Basic Physical Properties... 

In 1994, the proportion of PET fibers in the world production of synthetic fibers was 62.9% and of chemical fibers was 55.3%, while in the total volume of all kinds of fibers it was 27.4%. Out of PET fibers presently produced, 38% are staple fibers and 52.5% are filament yarns, with a marked tendency toward an increase in the latter. A 55% proportion is anticipated in the year 2000, At present, about 75% of PET fibers are used for textile purposes and 25% for nontextile purposes. 

This dynamic increase in production was accompanied by the qualitative development of PET fibers, which manifested itself in the widening of assortment of the fibers being produced (e.g., staple microfibers and filament yarns of the POY, MOY, FOY, and HOY type) and in the manufacture of second-generation fibers on... 

Characteristic of the microstructure of PET fibers in their final production form is the occurrence of three types of polymer phases crystalline, mesomorphous, and amorphous. The first phase is the result of crystalline aggregation of PET molecules, the second phase—of mesomorphous or, in other words, paracrys-talline aggregation, the third phase—of amorphous aggregation. The mesomorphous and amorphous phases together form a noncrystalline part of the fiber. 

The space lattice does not undergo polymorphous transformation. As with other kinds of fibers, no transformation of the space lattice under the effect of any physical or chemical treatment of PET fibers has yet been found. 

Figure 5 X-ray reflexes of PET fibers differentiated by draw ratio (in %).

The parallelization of crystallites, occurring as a result of fiber drawing, which consists in assuming by crystallite axes-positions more or less mutually parallel, leads to the development of texture within the fiber. In the case of PET fibers, this is a specific texture, different from that of other kinds of chemical fibers. It is called axial-tilted texture. The occurrence of such a texture is proved by the displacement of x-ray reflexes of paratropic lattice planes in relation to the equator of the texture dif-fractogram and by the deviation from the rectilinear arrangement of oblique diffraction planes. With the preservation of the principle of rotational symmetry, the inclination of all the crystallites axes in relation to the fiber axis is a characteristic of such a type of texture. The angle formed by the axes of particular crystallites (the translation direction of space lattice [c]) and the... 

The quantitative assessment of the degree of crystallite orientation by x-ray examination is not free of ambiguity. From a comparative analysis [23] in which results obtained from the consideration of (105) and from three different variations of equatorial reflection were compared, the conclusion was that the first procedure can lead to underrated results, i.e., to the underestimation of the orientation. However, it can be assumed that this does not result from an incorrect procedure, but from ignoring the fact that the adjacent (105) reflex can overlap. The absence of the plate effect of the orientation is characteristic of the orientation of crystallites in PET fibers. The evidence of this absence is the nearly identical azimuthal intensity distributions of the diffracted radiation in the reflexes originating from different families of lattice planes. The lack of the plate effect of orientation in the case of PET fiber stretching has to do with the rod mechanism of the crystallite orientation. 

Overall orientation is understood as the joint arrangement of all the structural elements of the crystalline phase and noncrystalline part of the fiber in relation to the geometrical axis of the fiber. In its essence, the overall orientation of PET fibers, as a result of the crystalline and amorphous orientation, will be characterized by smaller values of the quantitative index of orientation than for the crystalline phase and by greater ones for the amorphous phase. 

The quantitative assessment of the overall orientation of PET fibers is generally made on the basis of fiber optical anisotropy measurements, i.e., measurements of the optical birefringence of the fiber. The determination of the value of optical birefringence makes it possible to determine the value of Hermans function of orientation based on the equation ... 

The overall orientation of PET fibers depends on the production conditions and on further refining treatment. 

The axial elastic modulus of PET fibers E) depends, as with other kinds of fibers, on the value of the elastic modulus of crystalline material ) and amorphous fiber... 

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