Conventional melting

In the spunbond process, the fiber is spun similarly to conventional melt spinning, but the fibers are attenuated by air drag appHed at a distance from the spinneret. This allows a reasonably high level of filament orientation to be developed. The fibers are directly deposited onto a moving conveyor belt as a web of continuous randomly oriented filaments. As with meltblown webs, the fibers are usually thermal bonded or needled (53). 

Peifluorinated ethylene—piopjiene (FEP) lesin [25067-11-2] is a copolymer of tetiafluoioethylene [116-14-3] (TFE) and hexafluoiopiopylene [116-15-4] (HEP) thus its blanched stmctuie contains units of —CF2—CF2— and units of —CF2—CF(CF2)—. It retains most of the desirable characteristics of polytetrafluoroethylene (PTFE) but with a melt viscosity low enough for conventional melt processing. The introduction of hexafluoropropylene lowers the melting point of PTFE from 325°C to about 260°C. 

Blends of polypropylene (PP) and liquid crystalline polymer (LCP) processed without melting the LCP were compared with conventional melt processed blends. In a first stage, PP was blended with 20 wt% of LCP in a twin-screw extruder with the take-up speed varied to achieve blends with different LCP fiber dimensions. In the second stage, these blends were processed both below and above the Tm of the LCP by extrusion and injection molding. 

Figure 6.26 Comparison of melting dynamics for a conventional melting channel and a transverse barrier melting channel for an LDPE resin at identical rates and screw speeds. The conventional channel is in red while the barrier melting section is in black...

Figure 6.30 Photographs of the conventional melting of a commercial PP resin in an experimental extruder with a glass barrel. In both photographs a melt pool forms near the pushing side of the channel. The screw was rotated at 15 rpm and the barrel temperature was 90 °C. The ruler Is In Inches...

Figure 6.32 Simulation results using the melting model for the conventional melting process that is, melting is occurring in all four melt films...

In this simulation the model assumption is that the melt film is transported into the pores of the bed. Thus the melt film is relatively thin even for this mechanism due to the reducing depth of the channel. The model as developed does not account for any conductive heat transfer into the solid particles from the liquid infiltrate. For most extrusions, the center of the solid bed may be porous while the edges exposed to the melt film are not. The sealed edges prevent the melt from infiltrating the solid bed and the melting process occurs via conventional melting. Conventional melting was observed for the case with the measured bulk density data in Fig. 4.1. 

Differential scanning calorimetry is performed at a heating rate of 2°C/min from 20°C to 100°C. The checkers used a conventional melting point apparatus. 

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