Stretching and Foaming of Polymers

Stretching and foaming are some of the physical processes that can be used to subsequently modify the properties of polymers. 

Stretching denotes a monoaxial or biaxial mechanical stress of a molded article close to the glass transition temperature. This leads to a controlled orientation of the molecular chains in the direction of stretching and thus to a substantial change in some physical properties. Fibers and foils made of synthetic polymers gain their optimal properties only by this mechanical post-treatment. Stability, stiffness, and dimensional stability of fibers, for example, increase nearly proportionally with the stretch ratio, whereas stretchability decreases. In practice, the stretch ratio is between 1 2 and 1 6, depending on the polymer material and the desired properties. 

Natural fibers (wool, silk, jute, sisal, cotton) contain macromolecules that have already been aligned into fiber direction during the enzymatically catalyzed biosynthesis. Hence, stretching is not necessary and often not possible. 

For the production of foamed plastics two methods can be essentially differentiated. 

The inflation of the thermoplastic melt causes an orientation of the polymer chains by multi-axial stretching, whereby several mechanically characteristic values, especially the tenacity of the polymer foam, are positively affected. 

During the prepolymerization phase several observations/tests can be made, for example  

With the mmiomer-free final products (c) one can determine, for example  

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Based on this analysis it is evident that materials which are biaxially oriented will have good puncture resistance. Highly polar polymers would be resistant to puncture failure because of their tendency to increase in strength when stretched. The addition of randomly dispersed fibrous filler will also add resistance to puncture loads. From some examples such as oriented polyethylene glycol terephthalate (Mylar), vulcanized fiber, and oriented nylon, it is evident that these materials meet one or more of the conditions reviewed. Products and plastics that meet with puncture loading conditions in applications can be reinforced against this type of stress by use of a surface layer of plastic with good puncture resistance. Resistance of the surface layer to puncture will protect the product from puncture loads. An example of this type of application is the addition of an oriented PS layer to foam cups to improve their performance. 

Re-entrant foam provides a counter-intuitive demonstration of processing (5). Polyurethane can be isotropically compressed in a mold and heated to about 170 °C. The microstructure of the resulting solid yields a material that bulges in cross section when stretched More information on polymers will be available from John Droske s complementary NSF-funded project (described in the preceding section). 

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

The density of chemicaUy-blown LDPE foam was altered by varying the amount of blowing agent, degree of crosslinking of the polymer, and the foam expansion temperature. A theory was proposed for the equilibrium density, based on the gas pressures in a Kelvin foam structure, and a rubber-elastic analysis of the biaxial stretching of the cell faces. 20 refs. 

Poisson ratio The ratio of transverse contraction strain to longitudinal extension strain in a stretched bar. The maximum possible value is 0.5 (around 0.5 for rubber, 0.33 for aluminum, 0.28 for common steels, and 0.1-0.4 for polymer foams). 

Alpla Technik produced bottles with a density as low as 0.68 g/cm in developmental work. Addition of pigment slightly increases the density. The foam cells cannot be seen with the naked eye, but are visible with a microscope. The cell diameter is between 50 and 100 microns in the bottle neck and base, and about 30 microns in the walls where the polymer is stretched. The surface layer is solid HDPE, with microbubbles in the core. Two-layer versions of the bottle have a smoother bottle surface but are somewhat heavier than monolayer bottles. 

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