Melt viscosity also

The sharp melting poiat and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. Melt decompression, or suck-back, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

Although it is generally true that, as a polymer s chain length increases, its melt viscosity also increases, there are some nuances to this observation. By increasing the chain length, the number of repeat units increases. This creates more entanglements and raises the viscosity. One... 

The high melt viscosity of Larc-TPI at 350°C (10 Pas) prohibits its use as an adhesive for bonding metallic or laminate skins on core honeycombs to make high strength sandwich structures. The work of Chow et al. discussed in Section 4.3.2.2 demonstrates that the melt viscosity of the poly(isoimide) form 21 of Fare TPI is of the order of 10 Pas at 240-250°C, compared to 2 x 10 Pas for polyimide 22 [25]. A semi-crystalline form of Larc-TPI has been obtained by chemical imidisation of the polyamic acid with acetic anhydride and triethylamine [63]. The differential scanning calorimetric curve exhibits an endotherm at 274°C due to melting of the crystalline sites. The value of the initial inherent viscosity (0.22 dl g ) indicates, however, that the lower melt viscosity also results from lower molecular weight polymer. 

Thermoplastic melt viscosity also depends on pressure [132-136]. A4scosity generally increases with increasing pressure and can be correlated generally by an equation of the type... 

Melt viscosity also is a function of pressure, but the change is negligible until one operates in the range of 5000 to 25000 psi, where viscosity increases by factors of 10 to as much as 500 can occur. Because static pressures in this range are encountered with injection molding and some extrusion operations, the effect cannot always be ignored. 

Fabrication and Processing. PVDF is available in a wide range of melt viscosities as powder or pellets to fulfill typical fabrication requirements latices are also commercially available. 

The various lubricants formulated into PVC to improve the processing can also enhance the performance of the stabilizet. In pigmented apphcations, calcium soaps, eg, calcium stearate, ate commonly used as internal lubricants to promote PVC fusion and reduce melt viscosity. This additive is also a powerfiil costabilizer for the alkyl tin mercaptide stabilizers at use levels of 0.2 to 0.7 phr. Calcium stearate can significantly improve the eady color and increase the long-term stabiUty at low levels however, as the concentration increases, significant yellowing begins to occur. 

Styrenic block copolymers (SBCs) are also widely used in HMA and PSA appHcations. Most hot melt appHed pressure sensitive adhesives are based on triblock copolymers consisting of SIS or SBS combinations (S = styrene, I = isoprene B = butadiene). Pressure sensitive adhesives typically employ low styrene, high molecular weight SIS polymers while hot melt adhesives usually use higher styrene, lower molecular weight SBCs. Resins compatible with the mid-block of an SBC improves tack properties those compatible with the end blocks control melt viscosity and temperature performance. 

As the polymer molecular weight increases, so does the melt viscosity, and the power to the stirrer drive is monitored so that an end point can be determined for each batch. When the desired melt viscosity is reached, the molten polymer is discharged through a bottom valve, often under positive pressure of the blanketing gas, and extmded as a ribbon or as thick strands which are water-quenched and chopped continuously by a set of mechanical knives. Large amounts of PET are also made by continuous polymerization processes. PBT is made both by batch and continuous polymerization processes (79—81). 

Melt viscosity is also affected by pressure (43,67,68). The compression of a melt reduces the free volume and therefore raises the viscosity. For example, the viscosity of low density polyethylene increases by a factor of roughly 10 over a static pressure range of 34—170 MPa (5,000—25,000 psi). 

Unlike shear viscosity, extensional viscosity has no meaning unless the type of deformation is specified. The three types of extensional viscosity identified and measured are uniaxial or simple, biaxial, and pure shear. Uniaxial viscosity is the only one used to characterize fluids. It has been employed mainly in the study of polymer melts, but also for other fluids. For a Newtonian fluid, the uniaxial extensional viscosity is three times the shear viscosity ... 

Cone—Plate Viscometer. In a cone—plate viscometer (Fig. 28), alow angle (<3°) cone rotates against a flat plate with the fluid sample between them. The cone—plate instmment is a simple, straightforward device that is easy to use and extremely easy to clean. It is well suited to routine work because measurements are rapid and no tedious calculations are necessary. With careful caUbration and good temperature control it can also be used for research. Heated instmments can be used for melt viscosity measurements. 

Polyamides. Coating powders based on polyamide resins have been used in fusion-coating processes for along time (1). Nylon-11 [25587-80-8] h.2LS been used almost exclusively however, more recently, coating powders also have been sold based on nylon-12 [24937-16-4]. The properties of these two resins are quite similar. Nylon-6 [25038-54-4] and nylon-6,6 [32131 -17-2] are not used because the melt viscosities are too high. 

Blends with styrenic block copolymers improve the flexibiUty of bitumens and asphalts. The block copolymer content of these blends is usually less than 20% even as Httie as 3% can make significant differences to the properties of asphalt (qv). The block copolymers make the products more flexible, especially at low temperatures, and increase their softening point. They generally decrease the penetration and reduce the tendency to flow at high service temperatures and they also increase the stiffness, tensile strength, ductility, and elastic recovery of the final products. Melt viscosities at processing temperatures remain relatively low so the materials are still easy to apply. As the polymer concentration is increased to about 5%, an interconnected polymer network is formed. At this point the nature of the mixture changes from an asphalt modified by a polymer to a polymer extended with an asphalt. 

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