Polyethylene terephthalate (sometimes written as polyethylene terephthalate), usually abbreviated as PET, PETE, or the old PETP or PET-P, is the most common thermoplastic polymer resin in the polyester family , Used in clothing fibers, containers for liquids and food, thermoforming materials in manufacturing, and engineering resins combined with glass fibers.

1. Application

Plastic bottles made of PET polyester are widely used for soft drinks. For some special bottles, such as those designated for beer containers, PET polyester is sandwiched with an additional layer of polyvinyl alcohol (PVOH) to further reduce its oxygen permeability.

Biaxial PET polyester film ("polyester film" is one of its well-known trade names) can be aluminum-plated on its surface by vapor deposition, which can reduce its permeability and make it reflective and opaque Sex. These properties are useful in many applications, including flexible food packaging and insulation materials (such as space blankets). Due to its high mechanical strength, PET polyester film is often used in the tape field, such as tape carriers or pressure-sensitive tape backing.

Non-oriented PET polyester sheet can be thermoformed to make packaging trays and blister packaging. [4] If crystalline PET polyester is used, trays can be used for quick-frozen meals because they can withstand both freezing temperatures and oven baking temperatures. Both amorphous PET polyester and PET polyester films are transparent to the naked eye. Coloring dyes can be easily blended into PET polyester sheet.

When filled with glass particles or fibers, it becomes significantly harder and more durable.

PET polyester is also used as a substrate for thin-film solar cells.

Polyester (a trademark converted from polyethylene terephthalate) can also be spliced ​​into a bell-shaped top rope to prevent wear when the rope passes through the ceiling.

Since the end of 2014, PET polyester has been used as the inner liner material of IV composite high-pressure gas cylinders. Compared with the polyethylene used in the early days, PET polyester has much better oxygen barrier properties. [5]

PET polyester can be used as 3D printing filaments, and can also be used as 3D printing PETG plastic.

2. Physical properties

PET polyester is a colorless semi-crystalline resin in its natural state. Depending on the processing method, PET polyester can be semi-rigid to rigid and very light. It can block gas and moisture very well, and also block alcohol (need additional isolation treatment) and solvents. It is strong and impact resistant. When exposed to chloroform and certain other chemicals such as toluene, PET polyester turns white. [10]

In addition to polyester fibers, the upper limit of crystallinity of commercial PET polyester is about 60%. Transparent products can be obtained by rapidly cooling the molten polymer to below the glass transition temperature (Tg), forming an amorphous solid. Like glass, when the melt cools, if the molecules do not have enough time to arrange in an orderly crystalline manner, amorphous PET polyester will form. At room temperature, the molecules are frozen in place, but if they are heated above the glass transition temperature (Tg), the molecules absorb enough heat energy, they will start to move again, nucleating and growing the crystals. This process is called solid phase crystallization.

When cooled slowly, the molten polymer will form a more crystalline material. When crystallized from an amorphous solid, this material forms spherulites containing many small crystallites instead of forming a large single crystal. When light passes through the boundary between the microcrystalline and amorphous regions, scattering occurs. This scattering means that crystalline PET polyester is opaque and white in most cases. Wire drawing is one of the few industrial processes that can produce nearly monocrystalline products.

3. Intrinsic viscosity

One of the most important characteristics of PET polyester is its intrinsic viscosity. 

The intrinsic viscosity of the material is obtained by extrapolating to the reduced viscosity when the solution concentration approaches zero, and is measured in deciliters/gram (dℓ/g). Intrinsic viscosity depends on the length of its polymer chain, and can be expressed without units due to extrapolation to zero concentration. The longer the polymer chain, the more entanglements between the chains, the higher the viscosity. The average chain length of a particular batch of resin can be controlled during the polycondensation process.

The intrinsic viscosity range of PET polyester: 

Fiber grade:

0.40–0.70 Textiles
0.72–0.98 Industrial and tire cord
Film grade:

0.60-0.70 BoPET (biaxial PET polyester film)
0.70–1.00 Thermoformed sheet
Bottle grade:

0.70-0.78 water bottle (flat)
0.78–0.85 carbonated soft drinks
Monofilament, engineering plastic

1.00–2.00

4. Dry treatment

PET polyester is easy to absorb moisture, which means it can absorb moisture from the surrounding environment. However, when this "moist" PET polyester is heated, it will hydrolyze and reduce its elasticity. Therefore, the resin must be dried before it enters the molding machine for processing. The drying treatment of PET polyester before entering the processing equipment is completed by using a desiccant or dryer.

In the dryer, the hot dry air is pumped into the bottom of the hopper filled with resin to flow upward through the pellets, removing moisture on the way. The hot and humid air leaves the top of the hopper and flows through the aftercooler first because it is easier to remove moisture from cold air than from hot air. The generated humid and cold air then passes through the desiccant layer. Finally, the dry and cold air leaving the desiccant layer is heated again in the heating furnace, and returns in the closed loop through the same process. Generally, the residual moisture content in the resin must be less than 50 parts per million (parts by weight) before processing. The residence time in the dryer should not be shorter than four hours. This is because a temperature higher than 160°C is required to complete the drying process in less than 4 hours, at which temperature the particles begin to hydrolyze inside before they are dried.

PET polyester can also be dried in a compressed air resin dryer. The compressed air dryer does not reuse dry air. Dry, heated compressed air circulates through polyester pellets as in a dryer, and then releases to the atmosphere

5.Degradation

Various types of degradation occur during processing of PET polyester. The main possible degradation modes include hydrolysis and thermal oxidation, the latter may be the most important. When PET polyester degrades, several things happen: discoloration, chain scission leading to molecular weight reduction, formation of acetaldehyde, and cross-linking (the formation of "gels" or "fish eyes"). The discoloration is caused by various chromophores formed after long-term heat treatment at high temperatures. This becomes a problem when the optical requirements for polymers are very high, such as in packaging applications. Thermal and thermal oxidative degradation leads to poor processing characteristics and performance of materials.

One way to alleviate this situation is to use copolymers. Comonomers such as CHDM or isophthalic acid lower the melting temperature and reduce the crystallinity of PET polyester (especially important when the material is used in plastic bottle manufacturing). Therefore, the resin can be plastically molded at a lower temperature and a smaller force. This helps prevent degradation and reduces the acetaldehyde content in the finished product to an acceptable (ie insignificant) level. Another way to improve polymer stability is to use stabilizers, mainly antioxidants, such as phosphites. Recently, improving the stability of materials at the molecular level through nanostructured chemistry has also been considered.

6. Plastic bottle processing equipment

There are two basic molding methods for PET PET bottles, one-step molding method and two-step molding method. In the two-step molding method, two independent machines are required. The first machine uses injection molding to obtain a preform, which is similar to a test tube, with the cap thread already in place. The body of the tube is significantly thicker because it will be expanded into its final shape using stretch blow molding in the second step.

In the second step, the preform is rapidly heated and then expanded to the two mold halves to form the final shape of the bottle. Preforms (uninflated bottles) themselves can now also be used as sturdy and unique containers; in addition to storing novel candies, some Red Cross branches also distribute them to houses as part of the "Bottle of Life" program It is mainly used to store medical history so that it can be provided to emergency rescuers.

In the one-step molding method, the entire process from raw materials to finished containers is carried out on one machine, which makes it particularly suitable for molding non-standard shapes (custom molded products), including jars, flat ovals, flask shapes, etc. . Its biggest advantage is that it reduces the occupied space, reduces the amount of product processing and energy consumption, and the visual quality is much higher than the two-step system.