Amorphous Nylon - Chapter

The chapters devoted to structure-permeability properties of the more commercially advanced amorphous nylons (Chapter 5 (50)) and experimental polycarbonates (Chapter 7 (51)) illustrate many of the general principles needed for optimizing the barrier properties of essentially any polymer family. These chapters consider the importance of intersegmental packing and intrasegmental mobility on... 

Unlike nylon, which is highly crystalline, PET fibers are amorphous after spinning. They are like the molecules shown at the top of Figure 22-6 in Chapter 22. In order to make a usable textile yarn or staple fiber our of PET, it must be drawn under conditions that result in orientation and crystallinity. This is accomplished by drawing at temperatures of about 175°F with stretch 300-400%. As with nylon, the conditions of draw (especially... 

Once the rear zone is set, one should lower the front zones to whatever level will still give good molding conditions. With crystalline types such as nylon, PP, PE, and so on, the operator must watch the screw return. If the screw is moving backward in a jerky manner, there is insufficient heat in the rear zone the unmelted resin is jamming or plugging the screw compression zone. The heat energy required to melt crystalline plastics is different from that needed for amorphous plastics (see Chapter 1). 

Solid-state forming basically uses a male metal plug mold that matches a female metal cavity mold and can be used only with crystalline-type resins. Below their glass transition temperatures Tg) amorphous-type resins are generally too stiff to be rapidly formed into stable products. Crystalline types can be permanently deformed at temperatures between their Tg and melting point (Chapter 1). Molecular orientation, the mechanism that allows this to occur, relates to the draw ratio. Draw ratios can vary from 5 1 for PET and nylon to 10 1 for low molecular weight PP. 

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