Heat-setting

2 Draw Ratios PLA can be drawn over a wide range of draw ratios. The optimal draw ratio is dependent upon the type of polymer used, the as-spun orientation level, and the desired tensile properties for the product. PLA staple 

Finish should be applied before the tow enters the dryer or heat setter. Finish should be selected based on the planned downstream processing [5a]. Depending on the subsequent processing and finish types, application levels can range from 0.35% up to 0.8% of the weight of fiber [4]. 

Minimal mechanical pressure should be used to crimp PLA to decrease the potential of fiber damage. The primary process variable for increasing crimp level is the tow temperature entering the crimper. The pressure inside the crimper is used as a secondary control parameter to increase the crimp level. 

Heat setting is carried out to provide dimensional stability to the spun fibers at the temperatures of use. During heat setting, 

TABLE 20.4 Typical Tow Temperatures Before and During Crimping 

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Heat-resistant fib( Heat-resistant polymers Heat-set inks Heatshield Heat shields Heat stability Heat stabilizers... 

Dacron, spun Odon 81, spun Dacron, spun and heat set polypropylene 01, filament Odon 39B, spun... 

Control cabinet 2, extmder 3, flat die 4, chill roU 5, septet (seven roUs) 6, hot plate 7, septet (seven roUs) 8, heat-setting oven 9, trio (three roUs) 10,... 

Mechanical Properties. Polyester fibers are formed by melt spinning generally followed by hot drawing and heat setting to the final fiber form. The molecular orientation and crystalline fine stmcture developed depend on key process parameters in all fiber formation steps and are critical to the end use appHcation of the fibers. 

At HOY speeds, the rate of increase in orientation levels off but the rate of crystallization increases dramatically. Air drag and inertial contributions to the threadline stress become large. Under these conditions, crystallization occurs very rapidly over a small filament length and a phenomenon called neck-draw occurs (68,75,76). The molecular stmcture is stable, fiber tensde strength is adequate for many uses, thermal shrinkage is low, and dye rates are higher than traditional slow speed spun, drawn, and heat-set products (77). 

Filament. Eully drawn flat yams and partially oriented (POY) continuous filament yams are available in yam sizes ranging from about 3.3—33.0 tex (30—300 den) with individual filament linear densities of about 0.055 to 0.55 tex per filament (0.5—5 dpf). The fully drawn hard yams are used directly in fabric manufacturing operations, whereas POY yams are primarily used as feedstock for draw texturing. In the draw texturing process, fibers are drawn and bulked by heat-setting twisted yam or by entangling filaments with an air jet. Both textured and hard yams are used in apparel, sleepwear, outerwear, sportswear, draperies and curtains, and automotive upholstery. 

Oriented polypropylene film (OPP) may be classified as heat-set and non-heat-set, blown and tentered, coextmded and coated. Orientation improves the cold-temperature resistance and other physical properties. Heat-set biaxially oriented polypropylene film (BOPP) is the most widely used protective packagiag film ia the United States. It is used to wrap bakery products, as lamination pHes for potato and com chips, and for pastas and numerous other flexible pouch and wrapping appHcations. Nonheat-set OPP is used as a sparkling, transparent shrink-film overwrap for cartons of candy. 

Web Heat-Set Publication and Commercial Inks. Almost all heat-set inks are now printed on web offset presses, and are based on vehicles containing synthetic resins and/or some natural resins. These are dissolved in hydrocarbon solvent fractions which are specially fractionated for use in the ink industry. They vary in boiling range between 180 and 300 °C. Small percentages of alkyd resins (qv) may be contained in these inks. They dry in less than one second by means of solvent evaporation in a heatset oven. These ovens utilize high velocity hot air to raise the web temperature to 120-150 °C. 

The multiplicity of nylon blends, processing systems, and uses requires a large variety of staple types. Tex per filament may be 0.1—2 (1—20 den), the cross section may be round or modified, the luster may be bright or dull, crimp may be present or absent, and the fiber may be heat-set or not, depending on its use. The staple length is about 4 cm for cotton system processing, 5—7 cm for the woolen system, 8—10 cm for the worsted system, and about 18—20 cm for carpet staple. 

Most of the textured apparel and industrial yams are woven or knitted directly into fabric. The carpet BCF yams can be tufted directly off package into loop pile or velvet constmctions. For the textured saxony constmctions, the BCF and the spun staple yams must be ply-twisted and heat-set. The heat-setting temperature for nylon-6 and nylon-6,6 is 180—220°C in hot—dry atmosphere, and 120—140°C in saturated steam. The yams are twist-set in pressurized autoclaves or continuously on the Superba and Suesson machines (121). Before setting the twist, the yam is heated and relaxed for predevelopment of the bulk. 

Fabrics composed of synthetic polymer fibers are frequendy subjected to heat-setting operations. Because of the thermoplastic nature of these fibers, eg, polyester, nylon, polyolefins, and triacetate, it is possible to set such fabrics iato desired configurations. These heat treatments iavolve recrystaUization mechanisms at the molecular level, and thus are permanent unless the fabrics are exposed to thermal conditions more severe than those used ia the heat-setting process. 

To produce easy care wool—polyester blend garments, heat setting of the polyester component will impart set stability to repeated machine washing. A minimum of 20—30% polyester is needed for adequate stability. A fabric shrink-resist treatment such with Synthappret BAP may be necessary to give the required shrink-resistance for easy cate performance. If the polyester content is increased, particularly above 50%, and a suitable fabric constmction is used, heat setting alone will give easy care performance. 

High energy dyes are requited in the Thermosol appHcation and in those instances in which the dyed fabric is subjected to a heat-setting treatment. Each energy type has a characteristic rate of exhaust and, as a consequence, only disperse dyes of the same energy class are used in mixes for shade matching purposes. 

In North America, a special, high conductivity, low permeability, "hot-pressed" carbon brick is utilized almost exclusively for hearth walls. Because of their relatively small size and special, heat setting resin cement, and because the brick is installed tightly against the cooled jacket or stave, differential thermal expansion can be accommodated without refractory cracking and effective cooling can be maintained. Additionally, the wall thickness is generally smaller than 1 m, which promotes the easy formation of a protective skull of frozen materials on its hot face. Thus hearth wall problems and breakouts because of carbon wall refractory failure are virtually nonexistent. 

The cellulose esters with the largest commercial consumption are cellulose acetate, including cellulose triacetate, cellulose acetate butyrate, and cellulose acetate propionate. Cellulose acetate is used in textile fibers, plastics, film, sheeting, and lacquers. The cellulose acetate used for photographic film base is almost exclusively triacetate some triacetate is also used for textile fibers because of its crystalline and heat-setting characteristics. The critical properties of cellulose acetate as related to appHcation are given in Table 10. 

Other Considerations. Some carrier-active products, especially o-phenylphenol and methylnaphthalenes, have an adverse effect on the lightfastness of the finished dyeing. The reason for this is not clear, but the effect is readily estabhshed. This problem is overcome by submitting the dyed material to temperatures higher than those normally required in drying. Under the conditions (150—175°C) that are usually required to heat-set dyed fabrics or to cure resins apphed in finishing operations, the residual carrier is volatilized. 

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