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Non-uniform crystalline state of a PET bottle

Plastic bottles must have strength to resist heat and mechanical shock, and these strengths are said to depend on the degree of crystallinity and stretching orientation. A two-dimensional X-ray diffraction image directly reveals information about preferred orientation and the crystalline state of a sample. If the orientation of the molecules is random, the diffraction image is ring-shaped; however if some preferred orientation is present, the image is in the shape of an arc.

Rigaku's R-AXIS RAPID II uses a large-format imaging plate (IP) as a detector (Fig. 1), allowing a wide-range two-dimensional diffraction pattern to be collected in a single exposure, making it ideal for preferred orientation and crystallinity investigations.

PET Bottle 1
Figure 1: Schematic diagram for measuring a two-dimensional diffraction pattern using an IP

Three sections were cut from a PET (polyethylene terephthalate) plastic bottle (Fig. 2) and placed on the R-AXIS RAPID, oriented so the top of the bottle points upward. In the shoulder portion [1], an arc-shaped diffraction pattern is obtained, indicating that the material is strongly oriented. On the other hand, for a section between the shoulder and side [2], and for the side portion itself [3], a ring-shaped diffraction pattern is obtained, revealing that the orientation is weaker than that of 1, and that the directions of the orientations of 2 and 3 are different. From the comparison of the diffraction patterns of 1 to 3, it is found that the molecular orientation is different depending on the portion of the molded product; so it is probable that each portion has different material strength.

PET bottle 2
Figure 2: Difference of two-dimensional diffraction images depending on the portions of a plastic bottle