Melt-flow start temperature

TABLE 10.1 Melt-Flow start Temperatures of Polyimides and Their Structures... 

Diamine Melt-flow start temperature, Dianhydride op... 

Sharkskin occurs at a lower shear rate than melt fracture, but the term melt fracture is often applied to all regular flow defects including sharkskin. Sharkskin occurs at a critical linear extrusion speed that can be raised by increasing melt temperature. Melt fracture starts at a critical shear stress and can be reduced significantly by reducing the die inlet angle, but, like sharkskin, it is also reduced by raising melt temperature. Linear low-density polyethylene is particularly prone to these defects, but they are minimized by the addition of special additives or blends with other polymers. 

A very important assumption is that channel flow surfaces and melt temperatures are the same. For well designed heads, flow surface temperatures are somewhat uniform. But often there are sections at heads and manifolds, located at mechanical joints or hard to heat areas, that have exposed bare metal surfaces which lead to heat loss. Once surface temperatures start to drop, the melt slows down. If the surface temperature drops below the melting point, the melt freezes. Either situation is a source of color change problems. 

On heat exposure, a thermoplastic or an elastomer undergoes softening, melting, flow of polymer melt, and vaporization (or decomposition), whereas a thermoset vaporizes or decomposes. As the polymer vaporizes/decomposes, a combustible vapor-air mixture is generated that ignites and the polymer starts burning. The vaporization/decomposition characteristic of a polymer is governed by its thermal stability characterized by its vaporization/decomposition temperature, Ty Ti). 

Upon heating, CPE-1 macromolecules lose their rigid conformation. Finally, the smectic phase transforms into the nematic LC phase at 180-250 °C. At such low temperatures, this state is not in equilibrium. As a result, starting from 250°C, two thermodynamically stable crystalline phases are readily formed. In the aimealed samples, the melting temperature of these phases is 340-350°C, and net crystallinity is 35-40%. In the temperature range above 250°C up to melting temperatures, two crystalline phases coexist with a nematic LC (60%) phase. The crystalline lattice serves as a physical network that reinforces the material and does not allow copolyester to flow at temperatures below the melting point. In the case of the as-spun CPE-1 fibers, the flow starts at 250°C, i.e., at temperatures where devitrification is completed, and the smectic phase is completely transformed into the nematic, while the development of the crystalline phases is not yet completed. 

The second heating step should start after thermal equilibrium of the sample/DSC has been achieved. Equilibrium is identified when the ordinate signal (heat flow or temperature) is stable. The second heat is used to compare sample specimens that have been conditioned in the same manner. This heating is used because all samples being compared now have the same controlled heat history. In this step, the sample is heated from below the glass transition temperature through the melting temperature. 

Flow A U-tube in which the two limbs are joined by a capillary section is used. The gel is allowed to set with different heights in the two links and the melting point is the temperature at whieh flow starts from one limb to the other. 

The simulation calculations show additionally, that the maximum temperature does not occur in the joining plane but in the absorbing material (Fig. 8). The melt layer thickness in the transparent part is thus generally thinner than in the absorbing part, and the squeeze flow starts in the absorbing part, and only later spreads to the tfansparent material. The laser absorption in the ttansparent part causes a gradual shift in the location of the maximum temperature towards the ttansparent adherend as the welding time increases. 

The mixture was heated under reflux and a solution of 0.2 g of ethyl iodide in 5 ml of dry tetrahydrofuran was allowed to flow into the reaction medium. When the reaction started, a solution of 6.2 g of 7heated under reflux until the complete disappearance of the magnesium turnings. The reaction medium was then cooled in an ice bath, after which there was added thereto a solution in 45 ml of tetrahydrofuran of 7 g of 6-oxo-benzo[b] -benzofurano[2,3-e] oxepin. The reaction mixture was allowed to stand for 20 hours at a temperature of 20°C, and was then poured into a saturated aqueous solution of ammonium chloride maintained at a temperature of 5°C. The mixture was extracted with ether and the organic portion was washed and dried over anhydrous sodium sulfate. After evaporation of the solvent, 9.4 g of crude product were obtained, which after recrystallization from isopropanol, provided 6.7 g of pure 6-(3-dimethylam nopropyl)-8-hydroxybenzo[b] benzofurano-[2,3-e] oxepin, melting point 160°C (yield, 71 %). 

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