Low-viscosity melts

Some plastics caimot be obtained in a low viscosity melt or solution that can be processed into a cellular state. For these cases two methods have been used to achieve the needed dispersion of gas in solid sintering of solid plastic particles and leaching of soluble inclusions from the solid plastic phase. 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

PVC has a unique ability to be compounded with a wide variety of additives, making it possible to produce materials that range from flexible elastomers to rigid compounds, that are virtually unbreakable. Compounds arc also made that have stiff melts for profile extrusion or low viscosity melts fur thin-walled injection molding. 

Another novel technique of making oxide fibres is called the inviscid melt technique [24], In principle, any material in a molten state can be drawn into a fibre shape. Organic polymeric fibres such as nylon, aramid, etc., as well as a variety of glasses can be routinely converted into fibrous form by passing a molten material, having an appropriate viscosity, through an orifice. The inviscid (meaning low viscosity) melt technique uses this principle to make oxide fibres. 

The first important commercial development was a result of the work of Professor Otto Bayer in 1937, who discovered how to make a polymer using diisocyanates employing an additional polymerization technique when working on a polymer fiber to compete with nylon. Initially, the development was considered impracticable. In 1938, Rinke and associates succeeded in producing a low-viscosity melt that could be formed into fibers. This led to the production of many different types of polyurethanes. 

Extrusion coating is the process of applying a thin bonded film of a plastic to a continuous substrate (web) such as paper, board, foil, fabric, as well as a plastic. A high melt temperature is employed with a downwardly directed slit die to produce a low viscosity melt web that adheres to the substrate (Figure 5.15). The plastic must adhere well to the substrate and exhibit good sealing properties. If necessary, the substrate may be given a pretreatment to improve adhesion. 

T-type die with high viscosity melts does not produce a uniform distribution, however it is used with high temperature coating, low viscosity melts that result in an acceptable distribution, and... 

In contrast, magnesite refractories which are comparable with the silica ones as regards the amount of melt, may have lower To.6(1500 —1750 °C) despite a significantly higher pyrometric cone refractoriness. This is due to the granular macro-structure in which the solid particles are separated by layers of low-viscosity melt. 

D. A. Scola and C. D. Simone. Low viscosity melt processable high tem-peratnre polyimides. US Patent 6911519, assigned to University of Con-necticnt (Farmington, CT), June 28,2005. 

In the piston-die technique, the material is confined in a rigid die or cylinder, which it has to fill completely. A pressure is applied to the sample as a load on a piston, and the movement of the piston with pressure and temperature changes is used to calculate the specific volume of the sample. Experimental problems concerning solid samples need not be discussed here, since only data for the liquid/molten (equilibrium) state are taken into consideration for this handbook. A typical practical complication is leakage around the piston when low-viscosity melts or solutions are tested. Seals cause an amount of friction leading to uncertainties in the real pressure applied. There are commercial devices as well as laboratory-built machines which have been used in the literature. 

Polymer melts can be classified based on their viscosity low viscosity melts for polyacylamide polyethylene, polypropylene and polystyrene medium viscosity melts for ABS, cellulose acetate, POM and styrene butadiene and high viscosity melts for polycarbonate, polymethylmethacrylate, polypropylene oxide and polyvinyl chloride. 

The flow path length varies from 40 to 800 mm the wall thickness ranges from 0.5 to 3 mm with the possible flow path increasing as the wall thickness increases (for 0.5 mm thick walls the flow path ranges from 35 to 130 mm). The required specific cavity pressure varies from 18 MPa for low viscosity melts, thick walls to 200 MPa for high viscosity, thin walls. 

Continuous fibers can be formed from viscous (high viscosity) meits [2] [17], and from inviscid (low viscosity) melts [11] [19]. The design of a viable fiberizing process from either meit depends primarily on three important factors (1) the relationship between melt viscosity and temperature, above and below the fiber forming temperature, (2) the liquidus temperature, the highest temperature at which crystals can form, (3) the nature of the crystalline phase at and below the liquidus and the crystal growth rate. 

As-spun mesophase pitch filaments exhibit a high degree of preferred orientation of the discotic molecules along the fiber axis, because the mesogen molecules are oriented under low shear stresses and because of the flow pattern of the melt. The degree of preferred orientation depends on the viscosity of the melt and the final diameter of the fiber. For 100% anisotropic mesophase coal tar pitch, the best results are obtained with a low viscosity melt and small, i.e., 10 ym diameter, fibers [25]. 

When drag flow dominates, extruder output increases linearly with screw speed, and larger screws and deeper channels carry more melt. However, head pressure also rises with screw speed. Although this increases pressure flow, the actual effect depends heavily on melt viscosity. With high-viscosity melts, pressure flow may be minimal and have little effect on extruder output. In contrast, low-viscosity melts produce less head pressure but greater pressure flow. Thus, pressure flow will reduce the expected output. Deeper channels, neutral screws (Fig. 5.26), and shorter metering zones enhance these effects. 

Leakage flow varies with the flight clearance. It is also enhanced by low-viscosity melts and high head pressures. With new screws and barrels, leakage flow is minor and has no apparent effect on extruder output. As the flight clearance increases, leakage flow rises, thereby reducing output. Consequently, the decrease in extruder output over time is used to monitor screw and barrel wear. 

In contrast to extruder output, die output increases with head pressure (Fig. 5.27). Die output is also enhanced by low-viscosity melts and larger die gaps. The match between extruder and die output shifts with operating conditions. The simple die characteristic curve in Fig. 5.27 shows the optimized processing conditions. However, this curve does not consider extrudate quality. Other lines would be required to locate the onset of surface defects, such as melt fracture, and for incomplete melting. 

A large number of techniques have been proposed for the measurement of the extensional viscosity of polymer melts. Only a few of these have been described here details of others may be found in the literature cited. These techniques have achieved a certain degree of maturity, and commercial instruments are available. There is, nonetheless, considerable scope for further improvements, especially in extending existing techniques to low-viscosity melts. 

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