Basic Knowledge of  Polyethylene Terephthalate

The molecular structure of  Polyethylene Terephthalate plastic is highly symmetrical and has a certain degree of crystallinity, giving it excellent film-forming and molding properties.  Polyethylene Terephthalate plastic possesses good optical performance and weather resistance, and amorphous  Polyethylene Terephthalate plastic has excellent optical transparency.  Polyethylene Terephthalate has good creep resistance, fatigue resistance, wear resistance, and dimensional stability, with low abrasion and high hardness. It also has the highest toughness among thermoplastic plastics, with good electrical insulation properties that are minimally affected by temperature, though it has poor corona resistance. It is non-toxic, weather-resistant, chemically stable, with low water absorption, resistant to weak acids and organic solvents, but not resistant to hot water immersion or alkalis.  Polyethylene Terephthalate resin appears as a milky white semi-transparent or colorless transparent material with a relative density of 1.38 and a light transmittance of 90%.  Polyethylene Terephthalate is considered a medium-barrier material, with an oxygen transmission rate of 50-90 cm3?mm/(m2?d?MPa) and a carbon dioxide transmission rate of 180 cm3?mm/(m2?d?MPa). Its water absorption rate is 0.6%, indicating relatively high water absorption.[1]

Polyethylene Terephthalate film has very high tensile strength, comparable to aluminum foil, nine times that of HDPE film, and three times that of PC and PA films. Reinforced  Polyethylene Terephthalate has low creep, excellent fatigue resistance (better than reinforced PC and PA), good wear resistance, and good friction resistance. The mechanical properties of  Polyethylene Terephthalate are less affected by temperature.

Pure  Polyethylene Terephthalate has low heat resistance, but this significantly improves after reinforcement, with mechanical properties at 180°C better than PF laminate, making it one of the best heat-resistant thermoplastic engineering plastics.  Polyethylene Terephthalate has good heat-aging resistance, with a brittle temperature of -70°C and retains some toughness at -30°C.  Polyethylene Terephthalate is not easy to burn, with a yellow flame and dripping. Although  Polyethylene Terephthalate is a polar polymer, its electrical insulation is excellent, even at high frequencies. However, its corona resistance is poor, making it unsuitable for high-voltage insulation; its electrical insulation is affected by temperature and humidity, with humidity having a greater impact.  Polyethylene Terephthalate contains ester bonds and is not resistant to water, acids, and alkalis under high temperature and steam conditions.  Polyethylene Terephthalate is stable to organic solvents such as acetone, benzene, toluene, trichloroethane, carbon tetrachloride, and oils. It also has high resistance to some oxidizing agents like hydrogen peroxide, sodium hypochlorite, and potassium dichromate.  Polyethylene Terephthalate has excellent weather resistance and can be used outdoors for extended periods.

Applications of  Polyethylene Terephthalate

Polyethylene Terephthalate can be divided into two major categories based on its usage: fiber and non-fiber. The former is used to produce polyester staple fibers and polyester filaments, serving as raw material for polyester fiber companies to process fibers and related products (polyester being the most produced type of synthetic fiber). The latter includes films, containers, and engineering plastics.[2]

Fiber Applications

In the early stages of development,  Polyethylene Terephthalate was primarily used to produce synthetic fibers (accounting for about 70% of  Polyethylene Terephthalate consumption).  Polyethylene Terephthalate is also used to manufacture insulating materials, magnetic tape substrates, film or photographic film bases, and vacuum packaging. Another major non-fiber application of  Polyethylene Terephthalate is the production of hollow containers for beverages, food, etc. Polyester, known for its high fiber strength and good wearing performance, is currently the highest-produced synthetic fiber. Staple fibers can be blended with cotton, wool, or linen to make textiles for clothing or home furnishings, while filaments can be used for clothing or industrial purposes, such as filter cloth, tire cord fabric, parachutes, conveyor belts, and safety belts.

Non-Fiber Applications

Apart from fibers,  Polyethylene Terephthalate is mainly used in three categories: films and sheets, bottles, and engineering plastics. Mainly used for packaging materials such as food, pharmaceuticals, and non-toxic, sterile sanitary packaging; high-end packaging for textiles, precision instruments, and electronic components; substrates for audio tapes, video tapes, photographic film, movie film, disks, CDs, and magnetic cards; capacitor films, flexible printed circuit boards, and membrane switches.  Polyethylene Terephthalate films feature transparency, oil resistance, aroma retention, hygiene reliability, and a wide temperature range of use (suitable for both high-temperature sterilization and frozen packaging). However, they have poor heat-sealing properties and must be combined with other films (heat-sealing layers), and their relatively higher price compared to general plastic films limits their packaging applications.[3]

Since  Polyethylene Terephthalate can easily obtain a nearly amorphous, highly transparent, and easy-to-stretch state through rapid cooling,  Polyethylene Terephthalate can be used to produce biaxially oriented packaging films or high-strength, highly transparent stretch blow-molded bottles from amorphous preforms.  Polyethylene Terephthalate can also be directly extruded or blow-molded into non-stretch hollow containers.  Polyethylene Terephthalate bottles have high transparency and good barrier properties, making them suitable for use in fresh-keeping packaging materials, such as for beer, liquor, carbonated drinks, edible oil, food, condiments, pharmaceuticals, cosmetics, and health products.

Modification of  Polyethylene Terephthalate

1. Filling modification is one of the most direct and effective ways to comprehensively enhance material properties by using inorganic components that are completely different from the polymer matrix. 2. Currently, research on nanoparticle-modified  Polyethylene Terephthalate composites is quite mature. Ke et al. used layered clay to modify  Polyethylene Terephthalate and obtained  Polyethylene Terephthalate/clay nanocomposites through intercalation polymerization. The results showed that when the clay content was 5 wt%, the heat distortion temperature of the composite material increased by about 20°C to 50°C compared to pure  Polyethylene Terephthalate, and the modulus of the composite doubled compared to  Polyethylene Terephthalate. 3. Compared to nanoparticles, micron-sized glass fiber (GF) has outstanding advantages in cost and controllability, making it widely used in filling modification of polymer materials. 4. Two or more polymers, including  Polyethylene Terephthalate, are blended in appropriate proportions under specific temperature and shear stress conditions to form polymer alloys or blends with new properties. The compatibility between polymers is the key to the preparation of such polymers. 5.  Polyethylene Terephthalate and PE have significant differences in chemical structure and are incompatible. Simple binary blending of the two shows that to improve the impact resistance of  Polyethylene Terephthalate through polymer blending, compatibilization is necessary to enhance compatibility. In HDPE/ Polyethylene Terephthalate blends, the impact strength improves when EVA and EAA are added. When  Polyethylene Terephthalate is blended with PP, the resulting alloy combines the advantages of both, improving properties, such as  Polyethylene Terephthalate enhancing the heat resistance of PP, and PP reducing the sensitivity of  Polyethylene Terephthalate to moisture. Without compatibilizers,  Polyethylene Terephthalate/PP blends exhibit weak interfacial bonding and poor mechanical properties.

References

[1] Dhaka, V.; Singh, S.; Singh, J. "Occurrence, toxicity and remediation of polyethylene terephthalate plastics. A review". Environmental Chemistry Letters, 20(3), pp.1777-1800.

[2] Datye, K.V.; Raje, H.M.; Sharma, N.D. "POLY(ETHYLENE-TEREPHTHALATE) WASTE AND ITS UTILIZATION - A REVIEW". Resources and Conservation, 11(2), pp.117-141.

[3] Damayanti and Wu, H.S. "Strategic Possibility Routes of Recycled PET". Polymers, 13(9).