射出成形における溶融PET(Polyethylene Terephthalate)よりの揮発物質の検討

抄録

Sink or burn marks can appear on injection molded plastics when the air vents in molds become clogged by volatile substances emitted by the molten plastic. The clogging caused by volatiles from PET resin were experimentally reproduced in a glass capillary and the causes of the phenomena were analyzed. When using a nitrogen gas flow, the deposits inside the glass capillary were almost identical with those in the injection molding air vent. The composition of the deposits were as follows; MHET:BHET:B-2:C-2:C-3=87:100:22:66:30 (mold clogs) and 63:100:12:61:21 (glass capillary). The similarity of the compositions is a strong indication that the air vent clog phenomenon was reproduced by the glass capillary experiment using a nitrogen gas flow. A large amount of TA is deposited when air is used instead of nitrogen gas. This is quite different from the deposits accumulated in the injection molding air vent slots. This difference is not unexpected because the short injection time prevents the melted resin from being exposed to air.
The volatility of the main monomers and oligomers that are generated from PET resin in descending order is: TA>BHET>MHET>C-3. On the other hand, the amount of each in PET resin in decreasing order is: C-3>C-2>B-2>BHET>TA>MHET, with the MHET amount being much smaller than BHET. This order is different from that of the mold clogs. The simple monomers (BHET and TA), which are the main volatile substances in PET resin, are conducted in exactly the same manner with PET resin. Though BHET is entirely stable in the nitrogen gas, it changes to MHET in air easily. Therefore, It was concluded that a part of the BHET in the PET resin changed to MHET by the existence of absorbed oxygen within the PET resin and the amount of MHET in volatile substances was comparable with BHET.