Full Name Breakdown of the Four Materials:
• PBS: Polybutylene succinate
• PBSA: Polybutylene adipate succinate
• PBAT: Polybutylene terephthalate adipate
• PBST: Polybutylene terephthalate adipate
The diol monomer for all four copolyesters is 1,4-butanediol (BDO). It is the optimal choice across six dimensions: molecular structure, degradation performance, mechanical properties, processing, industrial raw materials, and bio-based pathways. It has irreplaceable advantages over ethylene glycol (EG), propylene glycol (PDO), pentylene glycol (PTD), and hexanediol (HDO).
I. Balanced Molecular Chain Length: Balancing Flexibility, Crystallinity, and Biodegradability (The Core Factor)
BDO is a straight-chain 4-carbon diol (-CH₂CH₂CH₂CH₂-), with a perfectly balanced chain length:
1. Compared to ethylene glycol (EG, 2 carbons), EG has a very short chain, extremely rigid molecular chains, excessively high polyester crystallinity, high brittleness, and very low elongation at break; the molecules are tightly packed, making it difficult for water molecules to penetrate, significantly slowing down hydrolysis/biodegradation and failing to meet national standards for biodegradability; furthermore, PET has a melting point of 260℃, and processing temperatures are too high, making it prone to thermal degradation and yellowing. BDO has two more methylene groups, resulting in a larger free volume in the chain segments, allowing the molecular chain to twist freely, giving the material high toughness and high elongation (PBAT elongation can reach over 560-800%, suitable for blown agricultural films and packaging bags).
2. Compared to 1,3-propanediol (PDO, 3 carbons), odd-carbon diols cause spatial dislocation of molecular chains, disordered crystallization, and excessively low melting points and poor thermal stability; they also result in numerous polycondensation side reactions, making it difficult to achieve high weight-average molecular weight and insufficient mechanical strength. (Note: PDO-based polyesters such as PTT have commercial applications in engineering plastics, but they are inferior to BDO-based systems in terms of the flexibility-degradation balance required for biodegradable polyesters.)
3. Compared to 1,5-pentanediol/1,6-hexanediol (5/6 carbons), the carbon chains are too long, the molecules are too soft, and the materials are prone to creep and severe heat deformation; the melting point is too low, limiting the operating temperature range; the raw material cost is higher, and the long alkyl chains are too hydrophobic, making it difficult for microbial enzymes to attack the ester bonds, significantly reducing the degradation rate.
4. BDO's 4-carbon even-numbered straight chain: The chain segments are sufficiently flexible, solving the problems of brittleness and difficulty in film formation in fatty polyesters; the chain regularity is moderate, allowing for controllable crystallization, ensuring dimensional stability at room temperature and providing a certain degree of heat resistance; the intermolecular spacing is moderate, allowing water and lipases to easily penetrate the main chain to break ester bonds, achieving complete biodegradation.
II. Degradation Performance Matched to Aliphatic-Aromatic Copolyester Systems The four types of materials are essentially random copolyesters of aliphatic diacids (succinic acid/adipic acid) + optional aromatic diacids (PTA) + BDO:
1. The aliphatic segments (succinic acid, adipic acid) are the degradation sites, with BDO connecting the acid units, and flexible spacing allowing the ester bonds to be fully exposed;
2. If short-chain diols are used, the aromatic segments rigidly stack and encapsulate the aliphatic ester bonds, preventing enzyme access and resulting in incomplete degradation;
3. BDO-based polyesters can be completely degraded into CO₂, H₂O, and biomass in industrial composting, seawater, and soil environments within 6–12 months, meeting degradation standards such as GB/T38082 and EN13432;
4. Appropriate chain length balances weather resistance/hydrolytic stability: They are not prone to premature hydrolysis and breakage during daily use, and degrade rapidly after disposal, balancing service life and environmental requirements.
III. Perfectly Adaptable Mechanical and Thermal Properties for General Plastic Applications
1. Melting Point Range Compatibility: PBS melts at approximately 115℃, PBAT at approximately 120℃, with processing temperatures of 150–180℃, overlapping with the processing windows of PE and PP. Existing blown film, injection molding, and extrusion equipment requires minimal modification. Glycol-based polyesters, on the other hand, have melting points above 250℃, resulting in high energy consumption and susceptibility to thermal decomposition.
2. Stiffness-Toughness Balance: PBS boasts high crystallinity and strength, making it suitable for injection molding of food containers and modified fillers. PBAT, through random copolymerization, reduces crystallinity and offers ultra-high toughness, primarily used in films. Both rely on the flexible segments of BDO to achieve a non-brittle, tear-resistant profile.
3. Excellent Low-Temperature Performance: BDO provides extremely low glass transition temperatures (Tg) (PBS -34℃, PBAT around -30℃), preventing hardening and cracking at sub-zero temperatures, making it suitable for agricultural mulch films and cold chain packaging.
IV. Polymerization Process Friendly, Facilitating Industrial Production of High Molecular Weight Polyesters
1. Matched Esterification/Polycondensation Reaction Activity: The primary hydroxyl groups at both ends of BDO exhibit stable reactivity, matching the esterification rates of succinic acid, adipic acid, and PTA, allowing for the controlled synthesis of high molecular weight (100,000–150,000) polyesters. Short-chain ethylene glycol is prone to volatilization at high temperatures, requiring significant excess feed and increasing production costs.
2. Controllable Side Reactions: The cyclization of BDO to tetrahydrofuran (THF) at high temperatures is a controllable byproduct, which can be suppressed industrially using negative pressure and catalysts. Separating propylene glycol and hexanediol byproducts is more costly.
3. Smooth Polycondensation and Devolutation: The polymerization process removes excess BDO. The boiling point and volatilization rate are compatible with existing polyester vacuum polycondensation production lines. Domestic PBT and PBAT production lines can share BDO storage, transportation, and distillation facilities.
V. Mature Raw Material Supply, Supporting Both Petroleum-Based and Bio-Based Routes
1. Mature Petroleum-Based Industrial Chain: BDO is a core raw material for PBT and PTMEG spandex, with ample global and domestic production capacity (5.58 million tons), stable commodity prices, and a complete supporting distillation and transportation system. Propylene glycol (100,000 tons) and pentanediol have smaller dedicated production capacities and higher procurement costs.
2. Fully Bio-Based Biodegradable Plastics: The bio-fermentation of succinic acid is mature; succinic acid can be hydrogenated to produce bio-based BDO. Using bio-based BDO + bio-based succinic acid/adipic acid, PBS/PBSA/PBAT with 100% bio-carbon content can be produced, meeting EU and domestic bio-based material subsidy and certification requirements. This is a key advantage that other glycols cannot achieve on a large scale.
VI. Unified Monomers Bring Industry Universality (Industry Standardization Choice)
Using BDO as the diol monomer across the entire industry offers the following advantages:
1. Raw material storage, transportation, catalysts, and process parameters are universal; factories can switch between PBS/PBAT/PBSA production capacity without changing the core monomer;
2. Downstream modification and blending (PBAT+PLA filling) performance data are mutually accessible, resulting in lower formulation development costs;
3. The performance range of end products (membranes, injection molding, spinning) is unified, facilitating standardization for national standards and degradation certification.
