PBAT, or Poly (butyleneadipate-co-terephthalate), is a copolymer of butylene adipate and butylene terephthalate. It has both the characteristics of PBA (polybutylene adipate) and PBT (polybutylene terephthalate). It has both good mechanical properties, high ductility and elongation at break, and excellent biodegradability, is a fully biodegradable plastic.
Biodegradable plastics are plastics that degrade into small molecules or low-molecular compounds such as carbon dioxide and water under natural conditions such as soil and sand.
Degradable plastics are mainly divided into photodegradable plastics, biodegradable plastics, and photobiodegradable plastics. Photodegradable plastics need sufficient light to degrade, which brings great limitations to production, so the promotion of photodegradable plastics is not good.
There are dozens of types of biodegradable plastics. The varieties that can be mass-produced and industrially produced are mainly: polyhydroxy fatty acid ester synthesized by microbial fermentation, chemically synthesized polylactic acid (PLA), polycaprolactone, diol dicarboxylate Acid aliphatic polyester (PBS), aliphatic/aromatic copolyester, CO2/epoxy compound copolymer (APC), polyvinyl alcohol (PVA), etc., natural polymer starch-based plastics and their biodegradable plastic blends materials, plastic alloys, etc.
#01 How to prepare PBAT
Using adipic acid (AA), terephthalic acid (PTA), and butanediol (BDO) as monomers to synthesize poly(adipic acid/butylene terephthalate) in a certain proportion, the focus is on strict control during the process reaction esterification method, esterification time, polycondensation temperature, stabilizer, etc.
These key factors can directly affect the synthesis process and ultimately affect the performance of the product. In order to ensure the preparation of good PBAT resin, the best process conditions required are: co-esterification reaction method, esterification time is 185 min, and polycondensation temperature is 248~250℃.
#02 PBAT preparation process
There are three esterification methods for the preparation of PBAT: co-esterification, partial esterification and tandem esterification. The preparation flow chart and main process parameters are shown in Figure 1~Figure 3.
#03 PBAT operating characteristics
(1) In order to reduce the occurrence of side reactions of BDO, reduce the generation of tetrahydrofuran (THF), and reduce the consumption of raw materials, the entire esterification reaction is carried out under vacuum conditions, which reduces the esterification reaction temperature and also reduces energy consumption.
(2) Due to the low activity of the raw materials, a high-performance catalyst is used; because this catalyst has the characteristics of easy hydrolysis and deactivation, the original method of adding the catalyst above the liquid surface to the addition method below the liquid surface has solved the above problems.
(3) During the polycondensation process, the generated oligomers are easily carried into the spray system along with the vacuum gas phase pipeline, causing system blockage. To this end, a cyclone separation collection system is installed on the gas phase pipeline, and the generated oligomers are collected and captured by a cyclone separator, and the tail gas enters the BDO spray circulation system.
(4) Although the degree of side reactions can be reduced during the esterification reaction, THF and water are inevitably present in the esterification reaction. Because THF is of low toxicity, but its high concentration is very harmful to the human body, if it is directly discharged into sewage treatment, it will cause harm to the bacteria in the sewage treatment system.
To solve this problem, a THF recovery device was installed. The THF and water are separated and processed. After the recovery device is processed, the mass fraction of THF can reach more than 99.95%, which can be used for direct sales. The mass fraction of THF in the waste water is controlled at about 0.05%. The waste water is sent to the stripping tower for stripping. The THF and other organics are extracted from the waste water and sent to the gas boiler through a pipeline for combustion.
(5) Due to the low glass transition temperature of PBAT, such as the use of traditional water-drawing strand cutting, it is easy to cause sticking, entanglement, and unstable production. For this reason, the method of pelletizing with underwater die head is adopted to control the forming and crystallinity of the pellets through the length of the underwater conveying pipeline and the temperature of pelletizing water; at the same time, due to the easily degradable characteristics of PBAT, the pellets produced are transported to The drying tower performs drying, and then flushes with nitrogen for protection and packaging.
(6) In view of PBAT’s characteristics of difficult molecular chain growth, high viscosity, poor fluidity, etc., a company adopts its exclusive patented vertical liquid phase thickening kettle, which can increase the dynamic viscosity of the material from the current 350 Pa·s to 1000 Above Pa·s, the molecular chain increases, and at the same time, because of the high viscosity, the screw discharge method is required to ensure the stability of production.
(7) Since THF steam is produced during the production of PBAT, an axial fan is installed in some areas of the polycondensation device to enhance the ventilation capacity of the device, and at the same time, a combustible and harmful gas detection and alarm device is set. Personal protective masks and Safety equipment such as breathing apparatus; THF recovery device adopts a fully open concrete frame structure to prevent harmful gas from accumulating.
#04 PBAT development status and application prospects
As an environmentally friendly material with good performance, PBAT can reduce petroleum resource consumption and alleviate “white pollution”, which is of great significance to sustainable development.
Currently, the global PBAT market is in strong demand. Due to the high price of PBAT, China has good raw material production conditions. PBAT products are mainly used for export, with high industry concentration, large potential market and good industrialization prospects.
Co-rotating Twin Screw Extruder
Co-rotating twin-screw extruder: Due to the co-rotating twin-screw has the opposite speed at the meshing point, one screw pulls the material into the meshing gap, and the other screw pushes the material out of the gap, resulting in material from one screw turns to the other screw and advances in a “∞” shape.