Can you distinguish the two materials of PET and PBT?

PET and PBT are collectively referred to as thermoplastic polyester, or saturated polyester.

The chemical structures of PET and PBT are very similar, but there are still some differences.

PET is mainly used for fiber, often called synthetic fiber polymer, used for textile production of polyester fiber. Nowadays, the non-fiber uses of PET have increased, mainly polyester for bottles. A small part of PET can be used as engineering plastics after glass fiber reinforced modification. The so-called modification is a method of changing the original nature or form of a substance through physical or chemical means.

PBT is mainly for non-fiber use, and belongs to one of the five general engineering plastics. Fiber use is not the main one for PBT.

The two names are only one word apart

The Chinese name of PET is Polyethylene Terephthalate (PET for short), which is a PMT method of transesterification of dimethyl terephthalate and ethylene glycol, or high-purity terephthalic acid and ethylene glycol. Esterification first synthesizes bishydroxyethyl terephthalate and then undergoes polycondensation reaction to produce PTA.

PET is a crystalline saturated polyester, a milky white or light yellow, highly crystalline polymer with a smooth and shiny surface. Polyethylene terephthalate is a common resin in daily life, and it is also the most important type of thermoplastic polyester, commonly known as polyester resin.

The Chinese name of PBT is polybutylene terephthalate (PBT), which belongs to the polyester series and is a polycondensate of terephthalic acid and 1,4-butanediol.

The names of polyethylene terephthalate and polybutylene terephthalate are only one word apart, only divided into D and B, but they have different characteristics.

Structural characteristics of PET and PBT

The molecular chain structure of PBT and PET is similar, and many properties are the same, except that the main molecular chain of PET has changed from two methylene groups of PBT to four, so the molecules are more flexible and the processing performance is better.

The same characteristics: They all contain rigid phenylene groups, polar ester groups, and flexible methylene groups. Generally speaking, they exhibit greater rigidity, making them have a higher glass transition temperature and a higher melting temperature. The ester groups make The polymer has a certain degree of water absorption and hydrolysis. As a thermoplastic linear polymer, its regular structure gives it crystallinity. Both PET and PBT are milky white crystalline solids or colorless and transparent amorphous solids.

Different characteristics: PBT has strong flexibility, small intermolecular force, and strong crystallization ability.

Performance characteristics of PET and PBT

1. The mechanical properties of PET and PBT. PBT has outstanding strength and toughness, and the main application forms of unreinforced polymers are films and fibers. Its mechanical properties are affected by the size, shape and orientation of its crystallinity. For example, the tensile strength of an unoriented film can reach 175 to 176 MPa; after stretching and orientation, the tensile strength can reach 280 MPa. PBT is inferior to PET in tensile strength, compressive strength, bending strength, modulus, etc., and its toughness is worse than PET. However, after glass fiber reinforcement, the mechanical properties are greatly improved, and the reinforcement effect exceeds that of glass fiber reinforced POM, PC, PPO, PA and many other engineering plastics.

2. The thermal properties of PET and PBT. The glass transition temperature of PET is not high, only 75°C; the glass transition temperature of PBT is lower, only about 30°C. The heat distortion temperature of unreinforced PET and PBT is relatively low, at 55～88℃. After glass fiber reinforcement, the heat distortion temperature of the two is greatly increased, which can reach 200 ~ 245 ℃. After glass fiber reinforcement, the short-term heat resistance of PBT is obviously improved, so that PET and PBT have better heat resistance. Engineering plastics with high cost performance are widely used.

3. Electrical properties of PET and PBT. PET and PBT have good electrical properties, and the electrical properties are slightly reduced when the temperature rises, and the change of the electric field frequency has little effect on the dielectric properties of the polymer. Unreinforced PBT has a dielectric constant of 3.1 to 3.3, and is often used to make optical cable sheaths; reinforced PBT has a dielectric constant of 3.3 to 3.8, and is often used to make insulating bases for electronic appliances.

4. Other properties of PET and PBT. PET and PBT contain ester groups, strong acids and alkalis can cause hydrolysis, and water vapor can also cause hydrolysis, but it can be used for a long time in water below 60℃; stable to hydrofluoric acid and organic acids; to non-polar solvents such as hydrocarbons , Gasoline, lubricating oil, etc. are very stable, not affected by polar solvents such as acetone, chloroform, trichloroethylene, acetic acid, methanol, ethyl acetate, etc. at room temperature. Benzyl alcohol, nitrobenzene, and tricresol can dissolve PET and PBT. PET and PBT have excellent weather resistance, light aging resistance is better than PA, POM, etc., and also have better oxygen barrier properties. However, when PET resin is used as a non-fiber application resin, it has the disadvantages of slow crystallization speed, long molding cycle, large molding shrinkage, and poor dimensional stability, and needs to be modified.

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