Polyethylene terephthalate (PET)

Polyethylene terephthalate (PET)

August 23, 2024

wanplas

Polyethylene terephthalate (PET), with the chemical formula (C10H8O4)n, is obtained by ester exchange of dimethyl terephthalate with ethylene glycol or esterification of terephthalic acid with ethylene glycol to synthesize dihydroxyethyl terephthalate, followed by polycondensation. It is a crystalline saturated polyester, a milky white or light yellow, highly crystalline polymer with a smooth and glossy surface. It is a common resin in life and can be divided into APET, RPET and PETG.

It has excellent physical and mechanical properties in a wide temperature range, the operating temperature can reach 120°C, and it has excellent electrical insulation. Even under high temperature and high frequency, its electrical properties are still good, but its corona resistance is poor, and its creep resistance, fatigue resistance, friction resistance and dimensional stability are all very good.

Introduction

Name	polyethylene terephthalate (PET for short)
Aliases	Polyethylene terephthalate
CAS Number	25038-59-9
Melting point	250-255°C

Polyethylene terephthalate is the most important type of thermoplastic polyester, commonly known as polyester resin. It is made by ester exchange of dimethyl terephthalate and ethylene glycol or esterification of terephthalic acid and ethylene glycol to synthesize dihydroxyethyl terephthalate, and then polycondensation reaction. Together with PBT, it is collectively called thermoplastic polyester, or saturated polyester.

In 1946, the UK published the first patent for the preparation of PET. In 1949, the British ICI Company completed the pilot test. However, after the US DuPont Company purchased the patent, it established a production facility in 1953 and was the first in the world to achieve industrial production. In the early days, PET was almost always used for synthetic fibers (commonly known as polyester and Dacron in China). Since the 1980s, PET has made breakthrough developments as an engineering plastic, and nucleating agents and crystallization accelerators have been developed one after another. Currently, PET and PBT are thermoplastic polyesters and have become one of the five major engineering plastics.

PET is divided into fiber-grade polyester chips and non-fiber-grade polyester chips. ① Fiber-grade polyester is used to make polyester staple fibers and polyester filaments, and is the raw material for polyester fiber companies to process fibers and related products. Polyester is the largest variety of chemical fiber. ② Non-fiber-grade polyester is also used in bottles, films, etc., and is widely used in the packaging industry, electronics, medical care, construction, automobiles and other fields. Among them, packaging is the largest non-fiber application market for polyester, and it is also the fastest growing field for PET.

It is widely used as fiber, and engineering plastic resin can be divided into two categories: non-engineering plastic grade and engineering plastic grade. Non-engineering plastic grade is mainly used for bottles, films, sheets, baking-resistant food containers, etc.

PET is a milky white or light yellow, highly crystalline polymer with a smooth and shiny surface. It has excellent physical and mechanical properties in a wide temperature range, and its operating temperature can reach 120°C. It has excellent electrical insulation. Even at high temperature and high frequency, its electrical properties are still good, but its corona resistance is poor, and its creep resistance, fatigue resistance, friction resistance and dimensional stability are all very good. PET has ester bonds, which will decompose under the action of strong acids, strong alkalis and water vapor. It is resistant to organic solvents and has good weather resistance. The disadvantages are slow crystallization rate, difficult molding, high molding temperature, long production cycle and poor impact resistance. Generally, its processability and physical properties are improved by reinforcement, filling, blending and other methods. The effect of glass fiber reinforcement is obvious, and the rigidity, heat resistance, chemical resistance, electrical properties and weather resistance of the resin are improved. However, the disadvantage of slow crystallization rate still needs to be improved, and nucleating agents and crystallization promoters can be added. Adding flame retardants and flame retardant dripping agents can improve the flame retardancy and self-extinguishing properties of PET.

Advantages

It has good mechanical properties, the impact strength is 3 to 5 times that of other films, and it has good folding resistance.
Resistant to oil, fat, dilute acid, dilute alkali and most solvents.
It can be used for a long time in the temperature range of 55-60℃, and can withstand high temperature of 65℃ and low temperature of -70℃ for short-term use, and its mechanical properties are slightly affected by high and low temperatures.
The permeability of gas and water vapor is low, and it has excellent gas, water, oil and odor resistance.
High transparency, can block ultraviolet rays, and has good gloss.
Non-toxic, odorless, hygienic and safe, can be directly used for food packaging.

Performance

PET is a milky white or light yellow highly crystalline polymer with a smooth and shiny surface. It has good creep resistance, fatigue resistance, abrasion resistance and dimensional stability, low wear and high hardness, and has the greatest toughness among thermoplastics: good electrical insulation performance, little temperature influence, but poor corona resistance. It is non-toxic, weather-resistant, chemical-resistant, low water absorption, resistant to weak acids and organic solvents, but not resistant to hot water immersion and alkali.

PET resin has a high glass transition temperature, slow crystallization speed, long molding cycle, long molding cycle, large molding shrinkage, poor dimensional stability, brittle crystallized molding, and low heat resistance.

Through the improvement of nucleating agent, crystallizing agent and glass fiber reinforcement, PET has the following characteristics in addition to the properties of PBT.

The heat deformation temperature and long-term use temperature are the highest among thermoplastic general engineering plastics.
Because of its high heat resistance, the reinforced PET is immersed in a 250°C solder bath for 10 seconds with almost no deformation or discoloration, making it particularly suitable for the preparation of electronic and electrical parts for soldering.
The bending strength is 200MPa, the elastic modulus is 4000MPa, the creep and fatigue resistance are also very good, the surface hardness is high, and the mechanical properties are similar to those of thermosetting plastics.
Since the price of ethylene glycol used to produce PET is almost half that of butanediol used to produce PBT, PET resin and reinforced PET are the lowest priced among engineering plastics and have a high cost-effectiveness.

To improve the performance of PET, PET can be alloyed with PC, elastomers, PBT, PS, ABS, and PA.

PET (enhanced PET) is mainly processed by injection molding, and other methods include extrusion, blow molding, coating and secondary processing methods such as welding, sealing, machining, vacuum coating, etc. It must be fully dried before molding.

Polyethylene terephthalate is obtained by ester exchange of dimethyl terephthalate with ethylene glycol or by esterification of terephthalic acid with ethylene glycol to synthesize dihydroxyethyl terephthalate, and then polycondensation reaction. It is a crystalline saturated polyester with an average molecular weight of (2-3)×10 4 and a weight average to number average molecular weight ratio of 1.5-1.8. Glass transition temperature 80℃, Martin heat resistance 80℃, heat deformation temperature 98℃ (1.82MPa), decomposition temperature 353℃. It has excellent mechanical properties, high rigidity, high hardness, low water absorption, and good dimensional stability. It has good toughness, impact resistance, friction resistance, and creep resistance. It has good chemical resistance, soluble in cresol, concentrated sulfuric acid, nitrobenzene, trichloroacetic acid, chlorophenol, and insoluble in methanol, ethanol, acetone, and alkanes. The operating temperature is -100～120℃, and the bending strength is 148-310MPa.

Water absorption	0.06%-0.129%
Impact strength	64.1-128J/m
Rockwell hardness	M 90-95
Elongation	1.8%-2.7%

Applications

In the classification of plastics, PET is number 1 and has a wide range of uses:

The main applications are in the electronic and electrical fields: electrical sockets, electronic connectors, rice cooker handles, TV yokes, terminal blocks, circuit breaker housings, switches, motor fan housings, instrument mechanical parts, banknote counter parts, electric irons, and accessories for induction cookers and ovens; flow control valves, carburetor covers, window controllers, pedal transmissions, and switchboard covers in the automotive industry; gears, blades, pulleys, pump parts in the mechanical industry, as well as wheelchair bodies and wheels, lampshade housings, lighting housings, drain pipe joints, zippers, watch parts, and sprayer parts.

In addition:

Can be spun into polyester fiber, namely polyester.
It can be made into thin films for use as substrates for recording, video recording, movie films, insulating films, product packaging, etc.
As a plastic, it can be blown into various bottles, such as cola bottles, mineral water bottles, etc.
Can be used as electrical parts, bearings, gears, etc.

Processing methods

Polyester is made by polycondensation of terephthalic acid and ethylene glycol, following the general law of linear polycondensation. To produce polyester, two synthetic technologies have been developed: transesterification and direct esterification.

(1) Ester exchange method or indirect esterification method

This is a traditional production method, which consists of three steps: methyl esterification, ester exchange, and final polycondensation. The purpose of methyl esterification is to facilitate the refining and purification of dimethyl terephthalate.

① Methylation: Terephthalic acid reacts with a slightly excess amount of methanol to first esterify into dimethyl terephthalate. After evaporating water, excess methanol, methyl benzoate and other low-boiling substances, pure dimethyl terephthalate is obtained by distillation.

② Transesterification: At 190~200℃, cadmium acetate and antimony trioxide are used as catalysts to make dimethyl terephthalate and ethylene glycol (molar ratio of about 1:2.4) undergo transesterification to form polyester oligomers. Methanol is distilled off to ensure that the transesterification is complete.

③ Final polycondensation: At a temperature higher than the melting point of polyester, such as 283°C, antimony trioxide is used as a catalyst to cause self-condensation or ester exchange of ethylene terephthalate. By means of reduced pressure and high temperature, the by-product ethylene glycol is continuously distilled out to gradually increase the degree of polymerization.

In the methyl esterification and transesterification stages, the ratio of the two groups is not considered. In the final polycondensation stage, according to the distillation amount of ethylene glycol, the ratio of the two groups is naturally adjusted, gradually approaching the amount of equal substances, slightly overdosing ethylene glycol, blocking both ends of the molecule, and reaching the predetermined degree of polymerization.

(2) Direct esterification method

After the purification technology of terephthalic acid is solved, this is the preferred economic method. Terephthalic acid and excess ethylene glycol are first esterified at 200°C to form low-polymerization degree (such as X=1~4) polyethylene terephthalate, and then finally polycondensed at 280°C to form a high-polymerization degree final polyester product ( n =100~200). This step is the same as the indirect esterification method.

As the degree of polycondensation increases, the viscosity of the system increases. In engineering, it is more advantageous to carry out the polycondensation in two reactors. The first stage of pre-polycondensation: 270℃, 2000~3300Pa. The second stage of final polycondensation: 280~285℃, 60~130Pa.

Forming

PET can be processed by injection molding, extrusion, blow molding, coating, bonding, machining, electroplating, vacuum metallization, and printing. The following mainly introduces two types.

1. Injection molding:

Temperature setting: Nozzle: 280~295℃, front section 270~275℃, middle forging 265~275℃, rear section 250-270℃; screw speed 50~100 rpm, mold temperature 30~85℃, non-crystalline mold below 70℃, back pressure 5-15KG.

Try to use a dehumidifying dryer, material pipe temperature 240~280℃, injection molding temperature 260~280℃, drying temperature 120~140℃, time required 2~5 hours.

2. Film grade:

First, pre-dry the PET resin slices to prevent hydrolysis, and then extrude amorphous thick slices through a T-die at 280°C in an extruder, and quench them through a cooling drum or coolant to keep them in an amorphous state for stretching and orientation. The thick slices are then biaxially stretched by a tenter to form PET film. Longitudinal stretching is to preheat the thick slice to 86~87°C, and stretch it about 3 times along the plane extension direction of the thick slice at this temperature, so that it can be oriented to increase the crystallinity to a higher temperature: transverse stretching preheating temperature 98~100°C, stretching temperature 100~120°C stretching ratio 2.5~4.0, heat setting temperature 230~240°C. The film after longitudinal and transverse stretching also needs to be heat-set to eliminate the film deformation caused by stretching and make a film with good thermal stability.