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Why the PET “Coke bottle” burst during filling?

At the beginning of this year, the National Development and Reform Commission and the Ministry of Ecology and Environment jointly issued the most stringent “Plastic Prohibition Order” to formulate a clear timetable, and to implement policies and measures for plastic reduction and recycling throughout the entire process of production, circulation, use, recycling, and disposal of plastic products. link. The establishment and effective operation of a reliable recycling system is the foundation of a circular economy!

The plastic recycling symbol originated in the United States, which divides plastics into different grades, a triangle symbol, usually at the bottom of plastic containers. There are numbers from 1 to 7 inside the triangle, and each number represents a different material.

“01” PET—polyester (polyethylene terephthalate)

It is often used to make plastic bottles such as mineral water bottles and carbonated beverage bottles. It is a food-grade safe plastic material. Generally, no plasticizers or other additives are added. The transparency of the freshly produced products can reach more than 90%. Recycling can be used to make polyester rope and clothing. Usually the highest heat resistance is 65°C, the cold resistance is down to -20°C, it is severely deformed above 80°C, and basically melts when the high temperature reaches 140°C.

There are generally four bottle types of PET used in beverage bottles: carbonated bottles, aseptic bottles, hot-fill bottles, and water bottles. Among them, carbonated bottles are easy to crack the bottom for inflated drinks, and the other three are generally not inflated so the bottom is not easy to crack.

In crystalline polymers, both crystalline and non-crystalline parts exist, and the crystalline parts are dispersed in the non-crystalline regions, and the two cannot be separated from each other. Crystallinity refers to the degree of crystallization, that is, the percentage of the mass or volume of the crystalline part to the mass or volume of the whole.

With the increase of crystallinity, the hardness, tensile strength and elastic modulus will also increase, but the elongation at break and impact strength will decrease accordingly. The study found that the crystallinity of the bottle samples with cracked bottom was higher than that of the intact bottle samples. Due to the increase of crystallinity, the tensile strength increases, but the elongation and impact strength decrease, the toughness is poor, and the ductility is poor. Therefore, the bottom of the carbonated bottle is prone to cracking when the bottom of the carbonated bottle is filled with air and the crystallinity is high.

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