Thermal drawing

Prokunin, Sevruk, and Fridman 37,68,69) have suggested an additional characteristic of rheological and technological properties of thermoplastics used as raw materials in such processes where an important stage is the longitudinal deformation of melts, for example, to produce films, fibers, flat threads, thermal drawing of sheet blanks, etc. 

Many of the so-called thermoplastic, biodegradable natural polymers, known as poly(hydroxyalkanoates), and some synthetic polymers such as poly(lactides) can be made into fibers by cold drawing, melt spinning or thermal drawing [2, 3]. These polymers can also be extended as sheets rather than fibers. These sheets are known as films which are not tme textiles because they are not made of fibers [1]. Nonetheless, these films of poly(hydroxyalkanoates) or poly(lactides) have the same chemical properties as the corresponding fibers, so for convenience, films are often used in biodegradation studies. 

The preform-drawing method is a batch process where a polymeric preform is fabricated first, which is then followed by thermal drawing of the preform into the fiber. A schematic diagram of the process is shown in Figure 5.1. In this method, a cylindrical polymer rod consisting of a core and cladding layers, usually prepared by radical polymerization in bulk under a clean environment, is positioned vertically in the middle of the furnace where its lower portion is heated locally to the drawing temperature. The furnace temperature appropriate for the preform is... 

Figure 7.11 Thermal drawing of a core-only preform into a single-index fiber (pictures glass As2Se3].

Kaufman J. J., Tao G. M., Shabahang S., Deng D. S. S., Fink Y, and Abouraddy A. R, Thermal drawing of high-density macroscopic arrays of well-ordered sub-5-nm-diameter nanowires. Nano Lett, 11, 4768-4773 (2011). 

Thermal drawing is a versatile rapid protot5rping method that can freely form microneedle structures with an ultrahigh aspect ratio without relying on any complex and expensive processes (16). However, thermal drawing is only applicable to thermoplastic materials and most natural biomaterials are incompatible with this method. 

A step mode of heat treatment of fibers under tension or in the free state in a vacuum, inert atmosphere, or in air is usually used. Table 10.3 shows how the physical-mechanical properties of fibers formed from a LC melt of a polyester based on a mixture of tere- and isophthalic acids and l,4-bis(3,5-dimethyl-4-hydroxybenzoyl)benzene [66] change as a function of the heating mode (24-h duration of each stage). The strength of the fiber increases by approximately four times as a result of the heat treatment, and this increase takes place with a simultaneous increase in the elongation at break, which is very important In many cases, thermal drawing of the fiber is less effective than its heat treatment in the free state. To reduce the duration of the heat treatment, it has been proposed that the fiber be preliminarily treated with aqueous solutions of alkali metal halides, of which potassium chloride and iodide are the most effective [67]. 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

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). 

In conventional tenter orientation, the sequence of steps is as described above (MD—TD). In some cases it is advantageous to reverse the draw order (TD—MD) or to use multiple draw steps, eg, MD—TD—MD. These other techniques are used to produce "tensilized" films, where the MD tensile properties are enhanced by further stretching. The films are generally unbalanced in properties and in extreme cases may be fibrillated to give fiber-like elements for special textile appHcations. Tensilized poly(ethylene terephthalate) is a common substrate for audio and video magnetic tape and thermal transfer tape. 

Most wrought alloys are provided in conditions that have been strengthened by various amounts of cold work or heat treatment. Cold worked tempers are the result of cold rolling or drawing by prescribed amounts of plastic deformation from the annealed condition. Alloys that respond to strengthening by heat treatment are referred to as precipitation or age hardenable. Cold worked conditions can also be thermally treated at relatively low temperatures to affect a slight decrease in strength (stress rehef annealed) to benefit other properties, such as corrosion resistance and formabiUty. 

Calculate the thermal withstand times /s,f s under cold and hot conditions and also at different 4, say 200%, 300% and 400% etc. of as shown below. After determining the corresponding safe thermal withstand times, according to the above formula, draw the graph (Figure 2.16), vs... 

But they are also rated for the same fault level for which the system is designed as they are connected directly to the system. This is a safety requirement. Similarly, in a draw-out sw itchgear assembly, the 1,/C and O/G power contacts of a module and its mounts (insulators and supports) being already protected may be. suitable only for the thermal rating of their feeders. 

Duty cycles Continuous duty (CMR) (S ) Periodic duties Factor of inertia (FI) Pleating and cooling characteristic curves Drawing the thermal curves Rating of short motors Equivalent output of short time duties Shock loading and use of a flywheel... 

A secondary fan draws the air and forces it through the secondary heat exchanger, where the reduced air volume temperature is raised to the required desorption temperature. The preheated air is then used to desorb the air in one portion of the wheel. As the air exits the desorption section the organic concentration is approximately 10 times the concentration of the original process stream. This low volume, higher concentration stream then enters the induced draft section of a catalytic or thermal recuperative oxidizer, where the organics are destroyed. 

Isotherm A line in a flow system or on a graph connecting points of equal temperature, or a mathematical or graphical relationship between two variables at constant temperature. Or a display using lines on a drawing to show constant-temperature contour lines, as from thermal imaging with infrared techniques. 

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