Polymer melt flow characteristics

Suspension Polymers. Methacrylate suspension polymers are characterized by thek composition and particle-size distribution. Screen analysis is the most common method for determining particle size. Melt-flow characteristics under various conditions of heat and pressure are important for polymers intended for extmsion or injection molding appHcations. Suspension polymers prepared as ion-exchange resins are characterized by thek ion-exchange capacity, density (apparent and wet), solvent sweUing, moisture holding capacity, porosity, and salt-spHtting characteristics (105). 

This example illustrates the simplified approach to film blowing. Unfortunately in practice the situation is more complex in that the film thickness is influenced by draw-down, relaxation of induced stresses/strains and melt flow phenomena such as die swell. In fact the situation is similar to that described for blow moulding (see below) and the type of analysis outlined in that section could be used to allow for the effects of die swell. However, since the most practical problems in film blowing require iterative type solutions involving melt flow characteristics, volume flow rates, swell ratios, etc the study of these is delayed until Chapter 5 where a more rigorous approach to polymer flow has been adopted. 

Polymer melt flow properties determine to a large extent the characteristics of the forming process. The detailed discussion on rheology is provided in Chapter 7 Rheology of Polymer Alloys and Blends. Rheological behavior will only be summarized here in order to provide sufficient information to understand and discuss polymer forming. 

The dimensions of the cylinder, the piston, and the die must be chosen and carefully defined appropriate to a particular application. The temperature and pressure that is applied to the melt and the ouqiut rate must also be accurately controlled and/or measured. ASTM D3835 specifies measurement of a polymer s flow characteristics via a capillary rheometer. 

In blown film extrusion, the number of lines in the film frequently corresponds to the number of ports in the die. As a result, these lines are often referred to as port lines. These port lines are obviously related to the die design but they are also very dependent on the melt flow characteristics of the polymer. High molecular weight polymers have very long relaxation times and are more likely to exhibit port lines or other types of die lines. 

The polymer melt flows through a die to the next process step flat, circular, or slot dies are preferred. The flow through the die must be uniform across the exit plane. However, this is complicated by the nonlinear dependence of melt viscosity on both temperature and shear rate in the die (23,24). The suitability of a material is determined by measuring the flow properties with a capillary rheometer in the temperature and shear-rate range expected. Melt elasticity can cause flow instabilities, which affect haze and thickness (27,28) or the operation of downstream equipment. Exit melt velocity, flow characteristics, and quenching rate may impart significant orientation to the polymer. In some instances, melt orientation is reduced in others, it is maintained by quenching. 

Compositions with terephthalate levels of over 60% would not be as desirable, however, since flow was not measurable above this level (Table 1). The polyesters which were examined all can be prepared with melt flow characteristics similar to those of commercial polysulfone resins as indicated by Figure 2. While the viscosity/temperature relationships for the two types of polymers are different this should not be a serious problem in the utilization of the aromatic polyesters. 

TMA-based polymers i.e., polyesterimides, polyamideimides etc. possess better thermal stability than most of the large volume commodity pol3nners. Although their heat resistance is somewhat lower than polyimide or polybenzimidazole, their superior solubility behavior and melt-flow characteristics make them easily processable. Combination of these two desirable properties is responsible for their commercial interest. 

Certain fluorocarbon processing aids are known to partially alleviate melt defects in extrudable thermoplastic hydrocarbon polymers and allow for faster, more efficient extrusion. Blatz first described the use of fluorocarbon polymer process aids with melt extrudable hydrocarbon polymers wherein the fluorinated polymers are homopolymers and copolymers of fluorinated olefins having an atomic fluorine to carbon ratio of at least 1 2, wherein the fluorocarbon polymers have melt flow characteristics similar to that of the hydrocarbon polymers (4). 

A characteristic flow pattern at the capillary entrance develops when a polymer melt flows at high shear rates from a cylindrical reservoir through a capillary or die, as shown in Fig. 2.12. The qualitative difference between the capillary entry flows of linear and branched polyethylenes has been convincingly presented by Tordella [50] and discussed by others [67-70]. For linear polymers, the converging flow at the die entry fills the available space, whereas for branched polymers there is a large dead space filled by recirculating vortices. 

Polymer molecular weights influence material properties and molecular weight distributions determine melt flow characteristics. 

Melt flow characteristics are important for all polymers and the intended process route. High melt flow indices are appropriate for injection moulding whilst lower values are necessary for extrusion. 

In the study, the mathematical model of the polymer melt flows in the extrusion process of plastic profile with metal insert was developed and the complex melt rheological behavior was simulated based on the finite element method. The melt flow characteristic in the flow channel was analyzed. The variation of the melt pressure, velocity, viscosity and stress versus different metal insert moving rate was investigated. Some suggestions on its practical manufacturing control were concluded based on the simulation results. 

Table II shows the melt flow characteristics of the blends and the transesterified blends of PEC and PC-PDMS. As can be seen from the table, the blends that contained TE, showed an increase in flow compared to the blends, which did not contain catalyst. This slight increase in melt flow was due to the redistribution of molecular weights between the resins and some hydrolysis of the polymer chains, which took place during transesterification.

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