What is the difference between PET and PBT?

In the plastic classification, the code of PET is No. 1, which has a wide range of functions:

Can be spun into polyester fiber, ie polyester;

Can be made into film for recording, video, film and other substrates, insulation film, product packaging, etc.;

As a plastic can be blown into a variety of bottles, such as Coke bottles, mineral water bottles, etc.;

Can be used as electrical components, Bearings, gears, etc.

PBT and PET molecular chain structure is similar, most of the properties are the same, but the molecular backbone is changed from two methylene groups to four, so the molecules are more supple and the processing performance is better.

Polybutylene terephthalate has similar properties to other thermoplastic polyesters, and its chemical and physical properties are similar: the strength of polybutylene terephthalate compared with polyethylene terephthalate (PET) The stiffness is lower, but the resistance to impact is better, and the glass transition temperature is also lower. Both PET and PBT are sensitive to hot water above 60 °C. Both plastics are UV sensitive and therefore cannot be used outdoors for long periods of time, but by adding additives, they help the two plastics to withstand UV rays.

PET

Polyethylene terephthalate

Typical application range

Automotive industry (structural devices such as mirror boxes, electrical components such as headlight reflectors, etc.), electrical components (motor housings, electrical couplings, relays, switches, internal components of microwave ovens, etc.). Industrial applications (pump housings, hand tools, etc.).

Injection molding process conditions

Drying treatment: Drying before processing is necessary because PET is more hygroscopic. It is recommended to dry at 120~165 °C for 4 hours. The required humidity should be less than 0.02%.

Melting temperature: for non-filling type: 265 ~ 280 ° C; for glass filling type: 275 ~ 290 ° C.

Mold temperature: 80~120 °C.

Injection pressure: 300~1300bar.

Injection speed: Higher injection speeds can be used without causing embrittlement.

Runners and gates: All conventional types of gates can be used. The gate size should be 50~100% of the thickness of the plastic part.

Chemical and physical properties

The glass transition temperature of PET is about 165 ° C, and the crystallization temperature of the material is 120-220 ° C. PET has a strong hygroscopicity at high temperatures. For glass fiber reinforced PET materials, bending deformation is also very likely to occur at high temperatures. The degree of crystallization of the material can be increased by adding a crystal strengthening agent. Transparent articles processed with PET have gloss and heat distortion temperatures. Special additives such as mica can be added to the PET to minimize bending deformation. If a lower mold temperature is used, a transparent article can also be obtained using a non-filled PET material.

PBT

Polybutylene terephthalate

Typical application range

Household appliances (food processing blades, vacuum cleaner components, electric fans, hair dryer housings, coffee utensils, etc.), electrical components (switches, motor housings, fuse boxes, computer keyboard keys, etc.), automotive industry (heatsink window , body panels, wheel covers, door and window components, etc.).

Injection molding process conditions

Drying treatment: This material is easily hydrolyzed at high temperatures, so drying treatment before processing is very important. It is recommended to dry in air at 120 ° C, 6 ~ 8 hours, or 150 ° C, 2 ~ 4 hours. The humidity must be less than 0.03%. If drying with a hygroscopic dryer, the recommended conditions are 150 ° C for 2.5 hours.

Melting temperature: 225~275°C, recommended temperature: 250°C.

Mold temperature: 40 to 60 ° C for unreinforced materials. The cooling channels of the mold are well designed to reduce the bending of the plastic parts. The loss of heat must be fast and even. It is recommended that the mold cooling channel has a diameter of 12 mm.

Injection pressure: medium (up to 1500 bar).

Injection speed: The injection speed should be as fast as possible (because PBT solidifies quickly).

Runners and gates: Round runners are recommended to increase pressure transfer (empirical formula: runner diameter = plastic thickness + 1.5mm). Various types of gates can be used. Hot runners can also be used, but care should be taken to prevent leakage and degradation of the material. The gate diameter should be between 0.8 and 1.0*t, where t is the thickness of the plastic part. If it is a submerged gate, the recommended minimum diameter is 0.75mm.

Chemical and physical properties

PBT is one of the toughest engineering thermoplastics. It is a semi-crystalline material with excellent chemical stability, mechanical strength, electrical insulation properties and thermal stability. These materials are very stable under a wide range of environmental conditions. PBT has a weak hygroscopic property. The tensile strength of the non-reinforced PBT is 50 MPa, and the tensile strength of the PBT of the glass additive type is 170 MPa. Excessive glass additives will cause the material to become brittle. PBT; crystallization is very rapid, which will cause bending deformation due to uneven cooling. For materials with a glass additive type, the shrinkage in the process direction can be reduced, but the shrinkage in the vertical direction to the process is essentially indistinguishable from that of the conventional material. Generally, the material shrinkage rate is between 1.5% and 2.8%. Materials containing 30% glass additives shrink between 0.3% and 1.6%. The melting point (225 ° C) and high temperature deformation temperature are lower than PET materials. The Vicat softening temperature is approximately 170 °C.

Polybutylene terephthalate is commonly used in the electrical engineering of houses, mainly for insulation purposes; it is also used in the manufacture of automobiles to make plug connectors. This material is also used in everyday household items, such as showers or iron handles. Polybutylene terephthalate can be drawn into silk and used to make brushed bristles for toothbrushes or make-up.