Development and application of the hottest polyest

Development and application of polyester plastic PET bottles (Part 1)

in 1950, DuPont chemical company of the United States developed a synthetic fiber for the first time with "poly (p-phenylene terephthalate) as its own intellectual property and technical standard system for building environmental friendly and high-performance surface materials". Polyethylene terephthalate is commonly known as "polyester fiber", code named pet. At present, PET packaging containers on the market are mainly plastic containers made of PET resin through blow molding. It mainly depends on the development of bottle grade PET resin with high viscosity and high grade, as well as the development of biaxial stretch blow molding technology. The practicality of bi-directional stretch blow molding PET bottles has only a history of more than 20 years. However, due to its excellent physical and mechanical properties, good economy and many outstanding advantages of environmental friendly automobile lightweight, such as the optimization design of automobiles, the good adaptability of alloy materials and non-metallic materials, PET bottles have become an extremely attractive rookie in the field of packaging containers. At present, China has formed a production capacity of one billion bottles per year

1 molecular structure and physical properties of PET resin

the macromolecular chain in the microstructure of PET resin is formed by the repetition of multiple chain links of ethylene terephthalate, which has good regularity and provides better crystallization performance. Its molecular chain is relatively rigid (there is benzene ring in the main chain), and the force between molecules is strong (with strong polarity), which gives PET resin good rigidity, strength and good barrier performance to non-polar gases and other substances

pet resin is a typical crystalline polymer, but its crystallization speed is slow. Through crystallization treatment, the properties of pet products can be greatly adjusted, such as the discovery of loose kinetic energy, to meet the needs of application. Compared with crystalline resins such as polyethylene and polypropylene, pet macromolecular chain is more rigid (with phenylene structure), so the crystallization speed is much slower

pet resin itself has good thermal stability, but it is easy to degrade in the presence of water. As a result of degradation, the molecular weight of pet will decrease significantly, and the intrinsic viscosity and mechanical strength of products will also decrease correspondingly. Therefore, the moisture content must be eliminated as much as possible before forming. It is generally considered that the moisture content should be less than 0.005%, at least less than 0.01%

2 PET blow molding container

2.1 pet biaxial tensile blow molding

due to the stretching treatment of PET resin in the molding process, the polymers in the product have a good directional arrangement. Therefore, in addition to the general advantages inherent in PET resin, the product has significantly improved mechanical strength and many physical properties, thus showing a series of excellent characteristics

(1) high mechanical strength

the mechanical strength of biaxially stretched PET bottles is not only higher than that of PVC containers, but also significantly higher than that of PET containers produced by direct extrusion blow molding

(2) low material consumption and light weight

due to the high mechanical strength and excellent physical properties of biaxially stretched PET bottles, it can adopt a thinner wall thickness to reduce consumption, so as to reduce the cost of PET bottles, improve their economic competitiveness, and help reduce transportation costs

(3) good chemical resistance

mainly manifested in oil resistance, organic drug resistance, acid resistance, etc. in addition, it has moderate alkali resistance

(4) excellent appearance

biaxially stretched PET bottles have good transparency, low haze, glass like transparency and gloss; In addition, pet has good colouring performance and can be made into various colors needed in use

(5) good hygienic performance

biaxially stretched PET bottles have become widely recognized in the world today, and can be used to package food, drugs and other commodities. It is a non-toxic packaging material with high reliability

(6) waste biaxially stretched PET bottles are easy to handle.

there are mainly methods such as resin preparation by degradation, recycling granulation, incineration and heat recovery

2.2 PET bottle biaxial stretch blow molding method

Pet biaxial stretch blow molding has a variety of practical industrial methods. As far as the manufacturing method of parison is concerned, it can be divided into extrusion method and injection method. Among the injection methods, there are one-step method (hot blank method) and two-step method (cold blank method). The comparison is shown in Table 1. However, no matter which molding method is adopted, the biaxial tensile PET bottle needs to be made into parison in advance, and then stretch blow molding under appropriate conditions to produce the final product. Injection stretch blow molding, the parison is made by injection, the dimensional accuracy is relatively easy to control, and the sealing of the bottle mouth is relatively easy to ensure. Therefore, this molding method is widely used. At present, pet stretch blow molding is basically injection stretch blow molding

pet stretch blow molding mainly includes three basic parts: parison injection molding, parison reheating, and stretch blow molding. Now it is briefly introduced as follows:

Table 1 Comparison Project between hot billet method and cold billet method

Project

forming method

hot billet method

cold billet method

characteristics of production operation

the process of injection, drawing and blowing is carried out on one machine, which consumes less energy and is difficult to control

the parison preparation and stretch blow molding process are completed on two independent machines, and the energy consumption is relatively high

delivery condition of parison