Polymer Melt Viscosity

PET suitable for bottle manufacture is produced by a modified process. Here the high-viscosity polymer melt is subjected to a rapid quenching in water to produce clear amoiphous pellets. These are further polymerised in the solid phase at temperatures just below the T . This is useful to reduce aldehyde content, since aldehyde-forming degradation reactions occur less in the lower temperature solid phase polymerisations. Aldehydes can impart a taste to beverages and it is important to keep the aldehyde content to below 2.5 p.p.m. 

But, such an extensive virtual network has not yet developed in the forming polymer and chain growth dominates shear-induced chain cleavage, eventually producing a much higher viscosity polymer melt. 

An extruder is a suitable apparatus for handhng high-viscosity polymer melts continuously. Mixing of a polymer melt and supercritical CO2 takes place in an extruder because of the shear action and the convection mechanism. During this process, a single-phase solution forms, and the time needed is minimized because the shear actions in an extruder decreases the striation thickness and the diffusion distances. 

The transverse shear stress through the melt will be equal through the thickness and will be proportional to the applied rotation rate for a constant viscosity. Polymer melts are not ideal fluids (i.e., Newtonian and constant temperature), so in actual conditions, the transverse shear rate will not be constant through the thickness of the extrudate between the cylinder walls, but this is a good approximation. 

It has been found that when one phase is predominant the disperse phase in a polymer blend system evolves from a pellet usually to a lacehke sheet and then filaments and ultimately to droplets [76, 147]. The evolution from pellet to droplet occurs rapidly as soon as both phases melt. Lee and White [146,147] find in studies of various polymer blends that if there is a large viscosity difference between the two phases and the low viscosity polymers melt first, the latter will wet the screw and barrel and lubricate the motions of the pellets of the other phase. This delays the position of melting late in the extruder, thus reducing mixing time and producing coarse blends. 

Thermorex, Gear pump for prepolymers and high-viscosity polymer melts, Maag Pump Systems Textron... 

The Prandtl number is simply the ratio of kinematic viscosity (t /p) to thermal diffu-sivity (a). Physically, the Prandtl number represents the ratio of the hydrodynamic boundary layer to the thermal boundary layer in the heat transfer between fluids and a stationary wall. In simple fluid flow, it represents the ratio of the rate of impulse transport to the rate of heat transport, ft is determined by the material properties for high viscosity polymer melts, the number is of the order of 10 to 10 . 

High viscosity polymer melts behave as non-Newtonian fluids, with the viscosity reducing with increase shear rate. 

When mixing two viscous liquids their viscosities should ideally be the same [1]. When two liquids of widely different viscosities flow through a channel the situation may be as shown in Figure 12.1. Mixing a small quantity of a low viscosity additive into a high viscosity polymer melt will be less difficult than mixing a small quantity of high viscosity additive into a low viscosity polymer. 

Even very stiff torsion bars must twist slightly to record the torque. With high viscosity polymer melts, this small twist can lead to significant errors in the strain or transient strain rate imposed on a sample (Gottlieb and Macosko, 1982). For example, consider stress relaxation after a step strain of 100% on a polymer melt with Go = 10 Pa (see Figure 3.3.6). Using a 0.1 rad, 25 mm diameter cone, to achieve a strain of 100% requires an initial torque of 0.4 N-m. However, if the transducer stiffness is 10 N-m/rad then it will twist 0.04 rad, and the true strain in the sample will only be 60%. There are also transient errors, For very viscous samples, the parameter C in eq. 8.2.1 dominates the damping of the transducer, and it can be used to estimate the time constant of the error ... 

Generally, the major component of a blend is the continuous phase, and the minor phase is dispersed in it. At intermediate compositions, the two polymer phases maybe co-continuous. There is a tendency for a low-viscosity polymer melt phase to encapsulate higher viscosity phases. 

The key to the melt blowing process is the spin head. The basic principle is the extrusion of low viscosity polymer melt through a single row of very fine holes placed close together in the order of 1000-4000 holes per m. lliese... 

Reduction in molecular weight of the thermoplastic scrap to low-molecular weight and consequently low-viscosity polymer melts. 

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