Processing, thermoplastics polymer flow

The melt-spinning process used to convert mesophase pitch into fiber form is similar to that employed for many thermoplastic polymers. Normally, an extruder melts the pitch and pumps it into the spin pack. Typically, the molten pitch is filtered before being extruded through a multi-holed spinnerette. The pitch is subjected to high extensional and shear stresses as it approaches and flows through the spinnerette capillaries. The associated torques tend to orient the liquid crystalline pitch in a regular transverse pattern. Upon emerging from the... 

The construction of a mold-filling model has been considered in the theory of thermoplastics processing. A rapid increase in viscosity also occurs in the flow of these materials, but the effect is different than in flow during reactive processing. The increase in viscosity of thermoplastic polymer materials is due to physical phenomena (crystallization or vitrification), while the increase in viscosity of reactive liquids occurs due to chemical polymerization reactions and/or curing. This comparison shows that the mathematical formulation of the problem is different in the two cases, although some of the velocity distributions may have similar features. 

All polymers can be divided into two major groups thermoplastics and thermosets) based on their thermal processing behavior. Thermoplastics soften and flow when heated. Upon cooling, thermoplastic polymers harden and assume the shape of the mold. Examples of commercial thermoplastics include polystyrene, polyolefins (e.g. polyethylene and polypropylene), nylon, poly(vinyl) chloride (PVC), and poly (ethylene) terephthalate (PET). Thermoplastics make up 80% of the plastic produced today and these polymers are linear or branched in their structure. 

Let us discuss the results of studies [13, 16-21], obtained through studying isothermal flows of keroplasts. In compliance with the above-mentioned facts these results can be applied to the description of the rheologic behaviour of compositional polymer materials with various disperse inert fillers. At displacement speeds corresponding to the speeds realized under the conditions of processing thermoplastic compositions, the Newton flow area was obtained on the flow curves (FC) of sevilene-based keroplasts but not with other keroplasts (polyethylene and polystyrene-based). 

Thermoplastics and thermosets. All polymers can he divided into two major groups thermoplastics and thermosets) based on their thermal processing behavior. Thermoplastic polymers soften and flow under the action... 

The most common cyclic process for thermoplastics is injection molding. Modern machinery is of the reciprocating screw type. In this process, the screw rotates and loads a metered dosage of polymer toward the tip. A ring valve at the tip of the screw prevents backflow during forward movement of the screw during the injection part of the cycle. The molten polymer flows... 

Rheology of the molten fluoropolymers is of critical importance in processing these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin (in flow) exceeds the critical... 

The most commonly used type of inhibited films is produced by joint processing of thermoplastic polymers and highly dispersed Cl powders. When the melting points, as well as the sublimation and thermal destruction temperatures, of the Cl exceed the viscous flow temperature of the thermoplast, the film acquires the structure of a polymer composite filled with solid inhibitor particles [101]. The particles may partially dissolve in the polymer melt and exert a plasticizing effect on the polymer. Introduction of 1-2% of NBA into PE elevates melt flow by 1.5-1.8 times [10]. Aggregation of solid Cl particles may present essential difficulties for their joint processing with polymer melts and result in structural defects or impair the quality of the film as a whole [4]. 

Rheological Properties Thermoplastics are processed and shaped in the molten state. This can be loosely defined as a state in which a polymer flows under the action of heat and pressure. Molten polymers are non-Newtonian fluids, as opposed to the simpler Newtonian fluids. In the latter, the stress a (force per unit area) is proportional to the shear rate y (velocity per unit length) with a proportionality factor [I (viscosity) which is constant at a given temperature. Newtonian fluids follow the law... 

Figure 1 Typical stages in thermoplastics processing. (1) Solids flow (2) screw filling and solids conveying (3) melting (4) polymer melt shearing/pumping (5) shear flow in channels (6) die swell (7) elongational flow (8) volume change under pressure (9) thermal conduction and (10) shrinkage.

Plasticizers are organic substances of low volatility that are added to plastics compounds to improve then-flexibility, extensibility, and processability. They increase flow and thermoplasticity of plastic materials by decreasing the viscosity of polymer melts, the glass transition temperature (Tg) the melting temperature (Tn), and the elasticity modulus of finished products [53]. 

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