Amorphous extrusion temperature

Similarly to the behavior of isotropic poly(ether ester)s the amplitude and position of the relaxation peaks in the loss curve of the extnidates were influenced by the composition of the amorphous phase. This is determined by the concentration and the degree of polymerization of the ester segments. For the extnidates the observed effect was pronounced only in the case of material C. Here, the glass transition temperature, as determined from the maximum of the so-called a-relaxation peak, increased linearly with decreeing extrusion temperature from - 4 C to 17 For the materials A and B the glass transition temperatures were found to be — 59 and - 50 °C, respectively, independently of the extrusion conditions. 

The content of the mesophase significantly decreases in fibres taken at other parameters. At certain conditions the mesophase completely disappears and inside fibres only two-phase structure, crystalline and amorphous, is formed. The high crystalline content was obtained in fibres extruded at low extrusion temperature and taken at high take-up velocities (Spruiell White, 1975 Bond Spruiell, 2001). 

The extrusion of the AlgsNisYio amorphous powder into an amorphous bulk at extrusion ratios of four and seven was tried by changing the extrusion temperature. The extruded bulk was obtained at temperatures above 543 K and the relative density was measured to be 0.969 at 543 K, 0.980 at 573 K, 0.987 at 603 K and 0.996 at 673 K. The... 

The value of the approach using dichroic ratios coupled with a knowledge of the assignments of peaks is further illustrated by measurements on plain polyethylene films [2]. The production process leads to a rather complex orientation pattern. The molten polymer is extruded as a thin, hollow cylinder, in what may be termed the machine direction. It is simultaneously expanded in a plane perpendicular to the machine direction by the application of internal pressure. Additional variables are the extrusion temperature and the rate at which the blown film has been cooled. The resulting orientation behaviour is best studied by x-ray diffraction pole figure measurements [3,4,5] but the infrared approach provides a relatively simple means for obtaining a useful amount of information, particularly for the behaviour of chains in amorphous regions. 

Extrusion Resins. Extmsion of VDC—VC copolymers is the main fabrication technique for filaments, films, rods, and tubing or pipe, and involves the same concerns for thermal degradation, streamlined flow, and noncatalytic materials of constmction as described for injection-molding resins (84,122). The plastic leaves the extmsion die in a completely amorphous condition and is maintained in this state by quenching in a water bath to about 10°C, thereby inhibiting recrystallization. In this state, the plastic is soft, weak, and pHable. If it is allowed to remain at room temperature, it hardens gradually and recrystallizes partially at a slow rate with a random crystal arrangement. Heat treatment can be used to recrystallize at controlled rates. 

Extrusion blow moulding of bottles has been successfully accomplished in reeent years by attention to the points mentioned above. It is to be noted here that UP VC has a much lower average specific heat between the proeessing temperature and room temperature than polyethylene and, being essentially amorphous, no latent heat of fusion. This leads to much less heat needing to be removed on cooling of mouldings and very short cycle times are possible. 

Ultem PEI resins are amber and amorphous, with heat-distortion temperatures similar to polyethersulfone resins. Ultem resins exhibit high modulus and are stiff yet ductile. Light transmission is low. In spite of the high use temperature, they are processible by injection molding, structural foam molding, or extrusion techniques at moderate pressures between 340 and 425°C. They are inherently flame retardant and generate little smoke dimensional stabilities are excellent. Large flat parts such as circuit boards or hard disks for computers can be injection-molded to maintain critical dimensions. 

Characteristic temperatures are always given in terms of temperature differences. For example, the characteristic temperature of the melt of an amorphous polymer in an extrusion operation is AT 7), — 7, or the difference... 

Either crystalline and amorphous types (Chapter 1) can be used with flow molding because the plastic is melted prior to forming. The forming temperature is usually lower than for injection molding or extrusion. Plastics need not be trimmed, as the composite is compression-molded to completely fill the mold cavity (Chapter 14). Very important, the flow molding process permits more complex parts to be formed than solid-state forming. The process cycle time is usually about 1 min. 

The physical state of starch during extrusion can be considered to change from a partially crystalline polymer to a polymer melt which is homogenized by shear. Extrusion may also decrease the molecular size of starch components, which is observed from decreased melt viscosity (Lai and Kokini 1991), and obviously a decreased molecular size results in a decreased glass transition temperature of the extmdate. The dramatic decrease of pressure that occurs as a viscous, plasticized melt exits the die may allow an extremely rapid loss of water, expansion of the melt and cooling to an amorphous solid state. 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

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