Phenomena of monomers and oligomers transfer from heat-setting oriented PET sheet to hot mold

摘要

While the PET bottles are blow-molded and heat-set in the hot mold, the" Mirror-like" aluminum mold surface becomes rough by transferring a substance from PET bottles. The outer surface of bottles becomes also rough because of copying the rough mold surface. After all. the appearance of bottles gets partially hazy. It results in the loss of the commercial value of the bottles. This phenomenon was analyzed by using the experimental device. The phenomenon was successfully reproduced by the heat-set of the biaxially oriented PET sheets with the experimental device of the aluminum block. As the number of the heat-set times increases, the deposit accumulates. The substance was determined to be a cyclic trimer (Cy 3) by the use of liquid chromatography. The mechanism of the adherent phenomena could be elucidated by a series of experiments using the monomer; BHET (Bishydroxyethyl terephthalate) and the oligomer; Cy 3. BHET was adhered to the aluminum block in the liquid phase at the early stage. Cy 3 was then adhered to the aluminum block with the existence of BHET as an adhesive. BHET evaporates or transforms into the polymer under the environment of the higher temperature. The polymer is then deformed to Cy 3, which is stable even exposed to the environment of the higher temperature. Therefore only Cy 3 is substantially observed on the hot aluminum block after the long period of time.