Polyester is an important synthetic fiber material, with the core step in its production process being the polycondensation reaction. Polycondensation is a process where monomer molecules react to form polymers and small molecule by-products (usually water or methanol). For the synthesis of polyester, the most common monomers are terephthalic acid (PTA) or dimethyl terephthalate (DMT) and ethylene glycol (EG).
Reaction Steps:
- Transesterification Reaction (if using DMT): First, DMT reacts with ethylene glycol in a transesterification reaction to produce the monomer bis(hydroxyethyl) terephthalate (BHET) and methanol. This reaction typically occurs at 200-250℃ and requires a catalyst (such as antimony compounds).
- Polycondensation Reaction: Next, BHET undergoes further polycondensation to form long-chain poly(ethylene terephthalate) (PET) and water as a by-product. This reaction usually takes place at a high temperature of 250-280℃ and under vacuum or an inert gas atmosphere to remove the water produced, driving the reaction towards higher molecular weight polymers.
Reaction Mechanism:
In the polycondensation reaction, hydroxyl (-OH) and carboxyl (-COOH) or ester (-COOR) groups undergo dehydration or alcohol removal reactions to form ester bonds (-COO-). During this process, the monomers gradually link into chains, forming linear polymers.
Reaction Control:
Controlling the polycondensation reaction is crucial for the properties of the final product. Reaction temperature, time, catalyst amount, and conditions (such as vacuum level or nitrogen flow) all affect the molecular weight and physical properties of the polyester. For instance, higher reaction temperatures and longer reaction times help form higher molecular weight polyester but may also lead to degradation and side reactions.
Industrial Applications:
By controlling the polycondensation reaction, polyester materials with different properties can be produced to meet various application needs. For example, high molecular weight polyester is typically used for products requiring high strength and toughness, such as plastic bottles and engineering plastics, while low molecular weight polyester is used for making fibers and films.
