Thermoplastic polymers, flow

At higher shear rates, three types of deviations are observable when compared to ideal Newtonian flow (see Fig. 2.1).The first kind of deviation relates to the existence of a flow threshold (yield point). In the case of a Bingham fluid, flow occurs only when the yield stress is exceeded. The second type of deviation is shear thickening, observed where the viscosity increases with shear rate. This is the case for a dilating fluid, behaviour which is seldom apparent in polymers. Last, where viscosity decreases with increase in shear rate, fluxing is observed and such fluids are usually referred to as pseudoplastic fluids. This last phenomenon is a general characteristic of thermoplastic polymers. Flow effects may also be time dependent. Where viscosity does not depend only on the shear rate, but also on the duration of the applied stress, fluids are thixotropic. Polymers in a molten state thus behave as pseudoplastic fluids having thixotropic characteristics. 

Stabilization of the Cellular State. The increase in surface area corresponding to the formation of many ceUs in the plastic phase is accompanied by an increase in the free energy of the system hence the foamed state is inherently unstable. Methods of stabilizing this foamed state can be classified as chemical, eg, the polymerization of a fluid resin into a three-dimensional thermoset polymer, or physical, eg, the cooling of an expanded thermoplastic polymer to a temperature below its second-order transition temperature or its crystalline melting point to prevent polymer flow. 

Poly(ethylene oxide) [25322-68-3] (PEO) is a water-soluble, thermoplastic polymer produced by the heterogeneous polymerization of ethylene oxide. The white, free-flowing resins are characterized by the following stmctural formula ... 

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... 

For the most part, plastics are man-made since very few plcistlcs are natural, i.e.- nature-made. Natural plastics include large molecular-wei t proteins and similar molecules. Man-made plastics can be classified as either thermoplastic or thermosetting. Each class derives its physical properties from the effects of application of heat, the former becoming "plastic" (that is- it becomes soft and tends to flow) while the latter becomes less "plastic" and tends to remain in a softened state. This difference in change of state derives from the actual nature of the chemical bonds in the polymer. Thermoplastic polymers generally consist of molecules composed of many monomeric units. A good example is that of polyethylene where the monomeric unit is -(CH2-CH2)-. The molecule is linear... 

In practice, synthetic polymers are sometimes divided into two classes, thermosetting and thermo-plastic. Those polymers which in their original condition will flow and can be moulded by heat and pressme, but which in their finished or cured gtate cannot be re-softened or moulded are known as thermo setting (examples phenol formaldehyde or urea formaldehyde polymer). Thermoplastic polymers can be resoftened and remoulded by heat (examples ethylene polymers and polymers of acrylic esters). 

We can further characterize polymers into thermoplastics and thermosets . Thermoplastics consist of linear or lightly branched chains that can slide past one another under the influence of temperature and pressure. These polymers flow at high temperatures which facilitates their molding into useful products. Thermosets consist of a network of interconnected chains whose positions are fixed relative to their neighbors. Such polymers do not flow when heated. 

A thermoplastic polymer can be repeatedly softened by heating, molded to a new shape, and then cooled to harden it. Thermoplastic polymers consist of chains that have no permanent chemical bonds to their neighbors. When we heat them, their molecules take on the properties of a viscous liquid that flows when we apply pressure. When we cool them, they solidify to take on a shape that remains constant until they are once again subjected to heat and pressure. We can dissolve thermoplastic polymers in solvents without destroying any chemical bonds. 

These highly amorphous elastomers have relatively low Tt values (—73 C) and tend to crystallize when stretched. The cold flow of these thermoplastic polymers is reduced when they are crosslinked (vulcanized) with a small amount (2%) of sulfur. Since these polymers of isoprene have a solubility parameter of 8.0 H, they are resistant to polar solvents but are soluble in many aliphatic and aromatic hydrocarbon solvents. The cross-linked derivatives swell but do not dissolve in these solvents. 

The Theory of Spiral (Helical) Flow of Molten Thermoplastic Polymers... 

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. 

The thermoset component in polyurethanes gives them better compression set than most thermoplastic polymers. They also have better cold flow properties. Polyurethanes are tough and more resilient than a large number of other plastics. 

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