Flow performance, melt

PPS is well-recognized for its exceptional chemical resistance. There are no known solvents for PPS below 200°C. A comprehensive survey of solvents for PPS has been published (115). Extreme conditions are required to dissolve PPS in both common and exotic solvents. Solution viscosity measurements are made difficult by this high temperature requirement. Inherent viscosity measurements are performed in 1-chloronaphthalene at 206°C at a concentration of 0.4 g of polymer per deciliter of solution. The inherent viscosity of PPS solutions shows a usefiil response to increa sing molecular weight. Table 2 shows a correlation of inherent viscosity measurements with melt flow measurements. 

Free phenol is a major concern in the manufacture of novolac resins. This is true for several reasons. The strongest drivers are probably EPA classification of phenol as a Hazardous Air Pollutant and worker safety concerns. However, free phenol also has significant technical effects on such parameters as melt flow characteristics. In this role, free phenol may undermine the desired effects of a molecular weight design by increasing flow beyond the desired point. Since free phenol is often variable, the effects on flow may also cause variation in product performance from batch to batch. Fig. 18 shows the effects of free phenol on the flow across a series of molecular weights. Free phenol contents between 1 and 10% are commonly seen. In recent years, much work has been aimed at reducing the free phenol. 

MW fraction increases the melt flow, thus improving the processability but at the cost of toughness, stiffness, and stress crack resistance. In addition, the improvement in performance through narrowing the MWD is restricted by the catalyst, the process hardware, and the process control limitations. Dow has developed a reactor grade HDPE of optimized breadth, peak, and shape of MWD... 

The mechanical behavior of plastics is dominated by such viscoelastic phenomena as tensile strength, elongation at breaks, stiffness, and rupture energy, which are often the controlling factors in a design. The viscous attributes of plastic melt flow are also important considerations in the fabrication of plastic products. (Chapter 8, INFLUENCE ON PERFORMANCE, Viscoelasticity). 

The flow patterns resulting from the conditions of a particular fabricating process are very important in influencing product performances. The melting of plastics follows different phases that effect performances. An example is its modulus of elasticity as shown in Fig.7-8. As the temperature increases, the plastic goes through the phases of glassy, transition, rubbery, to melt flow. 

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

This combination gives a very useful blend of mechanical properties. The PBT phase provides melt flow, solvent resistance and the ultimate heat performance of the blend (Tm). The PC phase provides reduced shrink, better dimensional stability, higher heat capability under low load (66 psi HDT) and improved impact strength. Interestingly, the PC also provides improved paint adhesion by being present as a very thin outer layer in molded parts. PBT, by virtue of its solvent... 

The temperature of the screw was measured by several investigators [29-32]. The measurements were performed by mounting thermocouples in an axial hole bored in the center of the screw or by protruding the thermocouples into the melt flow. The sensor signals were then transmitted to a chart recorder using an electrical rotary union. The technology available at the time of these measurements limited the number of sensors in the screw and the quality of the data. 

This brought a bout a keen interest in other methods of intensification in processing. Lately, the directed effect of physical (mechanical) fields on molten polymers has become one such area. These effects, as demonstrated in many works published in the 1970s and in the 1980s, (see for examples [6-9]) result in altered parameters of micro- and macrostress of the system. Molding under conditions of directed physical fields, in particular, in the case of mechanical and acoustic vibration effects upon melts, is performed so that an additional stress superimposed on the polymer s main shear flow and the state of material is characterized by combined stress. 

Under the same test conditions on the basis of melt flow rate, compounds 1 2, 3 and 4 provide significantly better performance than BHT, T and i8 when used in conjunction with phenolic AO-1 (neopentanetetrayl-tetrakis[3,5-di-tert.-butyl]-4-hydroxyhydro-cinnamate). Compounds 2, 2 and 4 also develop less color than BHT and 13. Compounds 6 and 1 develop the most color in the presence of AO-1. ... 

Adequate MW is a fundamental requirement to meet desired properties of plastics. With MW differences of incoming material, the fabricated product performance can be altered. The more the difference, the more dramatic change occurs in the product. Melt flow rate (MFR) tests are used to detect degradation in products. MFR has a reciprocal relationship to melt viscosity. This relationship of MW to MFR is an inverse one as one drops, the other increases or visa-versa. 

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