Permanent shape

The term plastic is not a definitive one. Metals, for instance, are also permanently deformable and are therefore plastic. How else could roll aluminum be made into foil for kitchen use, or tungsten wire be drawn into a filament for an incandescent, light bulb, or a 100 ton ingot of steel be forged into a rotor for a generator. Likewise the different glasses, which contain compounds of metals and nonmetals, can be permanently shaped at high temperatures. These cousins to polymers and plastics are not considered plastics within the plastic industry or context of this book. 

First-generation polymer composites (e.g., fiberglass) used thermosetting epoxy polymers reinforced with randomly oriented short glass fibers. The filled epoxy resin could be cured into a permanent shape in a mold to give lightweight, moderately strong shapes. 

Hardness measures the resistance of a material to a permanent change of shape. That is, the resistance to shear deformation (not the resistance to a volume change). The precursor to a permanent shape change is a temporary elastic shape change, and a shear modulus determines this. Therefore, the first necessity for high hardness is a high shear modulus. 

Permanent shape, of shape-memory polymer, 22 355-356, 357 Permanent wet drum separators, 15 443 Permanent wet-strength resins, in paper manufacture, 18 115-116 Permanent Yellow FGL, pigment for plastics, 7 366t... 

T hermoset molding materials are those plastic compounds that A when molded form a permanent shape when heat and pressure are applied to them, while confined within a hardened steel mold. 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

Plastics can be placed into two main categories, thermoplastic and thermoset. Thermoset plastics are irreversibly formed into a permanent shape often by applying heat. Thermosets caimot be softened and remoulded on heating and have few applications in food packaging, except for the iimer linings used for can coatings and many adhesives, as used, for example, in multilayer materials. A limited range of food contact materials is made from thermosets, predominantly melamine resins and unsaturated polyesters used in tableware and utensils. 

Explain why neither liquids nor gases have permanent shapes. 

To overcome the problems associated with compressed gases and their uncontrolled expansion, densification must occur slowly and/or dwell times must be designed into the pressing cycle before the compact is released. The same process conditions, potentially enhanced by heating the particulate solids to above ambient temperatures, will also assist in a sufficient conversion of elastic deformation to yield a permanent shape. 

Axens/ExxonMobil Paraxylene-rich Xylenes Toluene In the PXMax process, the feed is a mixture of fresh and recycled toluene. The mixed feed is combined with hydrogen-rich recycle gas, preheated and passed through the reactor where disproportionation reactions are promoted. Benzene and paraxylene-rich xylenes are produced as a result of the combined thermodynamics equilibrium and catalyst permanent shape selectivity. 8 2006... 

The SMP with permanently set shape and dimensions is heated above the transition temperature the polymer is then deformed to the desired shape and dimensions. The new conhguration is allowed to cool under moderate stress to allow consolidation of the temporary shape and dimensions. To revert back to the permanent shape and dimensions, the SMP is heated again above the transition temperature and allowed to cool. 

Shape memory polymers are classified into two chemical and two physical cross-linking categories depending on their differences in fixing mechanism and the nature of permanent shape elasticity (Liu et al., 2007). These categories include ... 

In the second chemical category, permanent shape is set by chemical cross-links formed during processing but a temporary shape is formed when the sample is deformed above the melting temperature of the crystalline components and subsequently cooled below this temperature (Liu et al., 2(X)7). Other than thermal heating, recovery in these SMPs can be triggered by an electric current at low voltage. Liquid... 

Hydrogels are another class of polymers with shape memory properties. They are cross-linked polymers with a hydrophilic portion that has high affinity for water and a hydrophobic part that can be controlled by temperature variations. The cross-linked part is responsible for setting the permanent shape at an elevated temperature, whilst the hydrophobic part assumes a secondary shape at a specific or critical temperature. Heating above these temperatures completes the recovery (Liu et al., 2007). 

The blown-film technique is widely used in the manufacture of polyethylene and other plastic films [14,15]. A typical setup is shown in Figure 2.23. In this case the molten polymer from the extruder head enters the die, where it flows round a mandrel and emerges through a ring-shaped opening in the form of a tube. The tube is expanded into a bubble of the required diameter by the pressure of internal air admitted through the center of the mandrel. The air contained in the bubble carmot escape because it is sealed by the die at one end and by the nip (or pinch) rolls at the other, so it acts like a permanent shaping mandrel once it has been injected. An even pressure of air is maintained to ensure urufoim thickness of the film bubble. 

The pi asllcs maSeriais belsrqing to this group are set i nio permanent shape lufen heS and pressure are applied to them during ft ming. 

Thermoset polymers assume a permanent shape or set once cured. Once set, they cannot be reshaped. They are formed by a large amoimt of cross-linking of linear prepolymers (a small amount of cross-linking will produce elastomers) or by direct formation of networks by the reaction of two monomers. The latter is the more prominent of the two methods. It is a stepwise or condensation method that has been defined as "the reaction of two monomers to produce a third plus a by-product, usually water or alcohol." Because in some cases a by-product is not produced, this definition is no longer exactly correct. The reaction is now referred to as a "stepwise" polymerization. When the reaction results in a by-product, it is called a "condensation reaction." Table 3.1 lists the principal thermoset polymers. 

In this chapter we are concerned with the deformation of ceramics leading to a permanent shape change. This is known as plastic deformation and is both nonrecoverable and irreversible. There are several mechanisms that are responsible for plastic deformation in crystalline materials dislocation motion, vacancy motion, twinning, and phase transformation. In metals at room temperature dislocation motion is the most important of these mechanisms. In Chapter 12 we already noted that dislocations do not move easily in ceramics and this is the reason for their inherent brittleness. Nevertheless, dislocation motion is observed in ceramics under specific loading conditions. In general, plastic deformation of ceramics requires high temperatures and this is important because... 

Shape-memory cycle of a thermo-responsive SMP. The typical shape-memory cycle of a thermo-responsive SMP consists of the following steps 1) start with a SMP in its original shape (permanent shape) 2) heat the SMP above its thermal transition temperature (Ttrans) and deform the SMP by applying an external force, cool well below 3 rans and remove the constraint to obtain the temporary shape with energy stored and 3) heat the pre-deformed SMP above Ttrans at which point the SMP releases the stored energy and recovers the permanent shape (shape recovery). Reprinted and adapted by permission from Cambridge University Press." ... 

Figure 2.5 Electroactive shape-recovery behavior of a 5 wt% MWNT-TPU composite. The sample undergoes the transition from temporary shape (linear, left) to permanent shape (helix, right) within 10 s when a constant voltage of 40 V is applied. Reprinted by permission from Wiley-VCH Verlag GmbH Co. KGaA. ...

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