Melting point shrinkage

Melting point Shrinkage after 250 °C none none <0.09 none... 

Some cast (unoriented) polypropylene film is produced. Its clarity and heat sealabiUty make it ideal for textile packaging and overwrap. The use of copolymers with ethylene improves low temperature impact, which is the primary problem with unoriented PP film. Orientation improves the clarity and stiffness of polypropylene film, and dramatically increases low temperature impact strength. BOPP film, however, is not readily heat-sealed and so is coextmded or coated with resins with lower melting points than the polypropylene shrinkage temperature. These layers may also provide improved barrier properties. 

Type metal, another tin—antimony—lead alloy, is used primarily in reHef or letterpress printing. Antimony is added to increase hardness, minimize shrinkage, permit sharp definition, and reduce the melting point of the alloy. There has been a substantial decrease in the use of type metals as a result of the emergence of less expensive typesetting techniques. 

Hot blown plastic films usually do not possess sufficient orientation to be heat shrinkable at temperatures below 100°C. For certain shrink packaging applications low temperature shrinkage is essential and can be obtained by biaxial orientation below the melting point. Radiation crosslinking was found to facilitate orientation of these copolymers greatly because of the much greater bubble stability which results from crosslinking. 

Melting point, crystalline, °C Specific gravity Refractive index nf15 Mold shrinkage, average, %... 

Nylon 6/6 is the most widely used, followed by nylon 6, with similar properties except that it absorbs moisture more rapidly and its melting point is 21C (70F) lower. Also, its lower processing temperature and less crystalline structure result in lower mold shrinkage. Nylon 6/6 has the lowest permeability by gasoline and mineral oil of all the nylons. The 6/10 and 6/12 types are used where lower moisture absorption and better dimensional stability are needed. Nylons 11 and 12 have better dimensional stability and electrical properties than the others because they absorb less moisture. These more expensive types also are compounded with plasticizers to increase their flexibility and ductility. With nylon toughening and technology advancements supertough nylons became available. Their notched lzod impact values are over 10 J/m (20 ft-lb/in), and they fail in a ductile manner. 

Slow cooling from the melt leads to the B-form, which has a melting point of 228°C. Annealing above 228 C converts the fi-form to the orform. The -j-form Is metastable and arises from orientation. In this modification the polymer backbone has a planar zig-zag structure and Is extended 1.6 times with respect to the alpha-form helix. Upon annealing It reverts to the alpha-form with concomitant shrinkage. As would be expected, this reversibility has a strong influence upon the properties of polyplvalolactone. 

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