Polymer blending is useful for improving physical properties. Blends of transparent polymers are generally hazy. However, transparency is required in many products such as packages (especially PET bottles). The miscible blends, PET (polyethylene terephthalate)/PBT (polybuthylene terephthalate), maintain transparency in almost all cases regardless of the blending ratio, whereas the immiscible blends, PET/MxNYLON (MXD6 nylon, i.e. polymethaxylirene adipamide) and PET/PMAI (polymethacril imide, i.e. N‐methyl dimethyl glutarimide) become hazy as the blending ratio increases. The reason for this haze is the number and size of the dispersed particles. Differences in the refractive indices of various polymers also have a large influence on haze. Stretching makes even the transparent blends hazy in the case of PET/MxNYLON. One reason for this phenomenon is that stretching increases the size of the dispersed particles in the sheet plane. A second reason is that the difference in the anisotropic refractive indices of the matrix and the dispersed phase is increased by stretching. These effects are very consistent with light scattering theory.
Huge numbers of PET (polylethylene terephthalate]) bottles are produced in the world. Especially in Japan, the number of hot-fillable PET bottles used is extremely large and is still increasing. This type of bottle is generally manufactured by the heat-set method using hot molds after stretch-blow molding. Herein, we examined how the PET sheet stretching condition affects the PET heat-shrinkage behavior at 85"C, which is the hot-filling temperature. Sheets stretched at a higher temperature and higher speed had higher thermal stability for a wider range of draw ratios. This is because those sheets have a higher crystallinity and relaxed amorphous regions. The higher stretch speed gives the sheet a higher crystallinity with self heat generation during rapid deformation. A higher stretch temperature makes the molecular segments relaxed.We were interested in how the thermal stability changes by varying the stretch condition (speed and temperature) and by what structure the thermal stability is achieved. There are a few reports on the thermal stability of fiber (3-5) and film ( 6 4 , none of which deal with the thermal shrinkage at 8 5 9 5°C of biaxialy stretched film. We have examined the thermal stability by stretching PET sheets at various conditions.
EXPERIMENTAL
MeasurementsCrystallinity X , (%) The crystallinity is estimated by a density gradient method (Ikeda Scientific Co.). Carbon tetrachloride and n-heptane were used in the density gradient column. The expression used was X=100 d, (d-d,)/d(d,-d,) (1) where X is the crystallinity (%); d is the measured sample density (lo3 kg/m3); d, is the crystal density (lo3 kg/m3) = 1.455 (8); and d, is the amorphous density (lo3 kg/m3) = 1.335 (9).
Refractive Index nRefractometer (Atago Co.) following Samuels (10).The refractive index was measured using the Abbe
The consumption of hot‐fillable poly(ethylene terephthalate) bottles is extremely large and is still increasing in Japan. This type of bottle is generally manufactured by the heat‐set method using hot molds after stretch‐blow molding. In this study, the method is simulated using a setting application in which sheets can be stretched constraining their sizes on a hot aluminum block. The crystallinities of the sheets are found to depend on the thermal history, i.e., the duration and temperature of the heat‐set cycle. Heat‐setting mitigates thermal‐shrinkage of the sheets which is due to the increase in crystallinity or in the tense segments in the amorphous region. The structure of the heat‐set sheet varies depending on the original stretched sheet. For a sample of low draw ratio, the crystallinity does not increase because of the heat‐set. For a sample of medium draw ratio, the crystallinity increases greatly and the tense segments in the amorphous region also increase because of heat‐set. For a sample of high draw ratio, the crystallinity increases a little but the numbers of the crystallite and tense segments in the amorphous region do not change with the heat‐set.
The thermal shrinkage of heat‐set polyethylene terephthalate (PET) sheets at 85°C after stretching at low speed and low temperature was similar to that for sheets stretched at high speed and high temperature if the crystallinities of these sheets were the same. However, differences between other physical properties, such as yield strength, were observed, which may be due to differences in the amorphous phase. The sheets stretched at the higher temperature and higher speeds may have different structures in the molecular segments in the amorphous phase, more relaxed than in the sheets prepared at the lower speed and temperature.
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