For a long time, the fiber industry has known that PTT has excellent fiber properties. In a patent in 197 1, it is found that PTT fiber has lower modulus and better bending performance than PET, and is more suitable for making fiber fillers and carpets than PET. Ward and others compared the mechanical properties of three kinds of polyester fibers and found that PTT did have better elastic recovery and lower elastic modulus than PET and PBT. These two properties are very necessary for making fabrics with good hand feeling and carpets with good elastic recovery, so the chemical and fiber industries have never given up developing low-cost PDO technology and PTT. As early as 1970s, Shell Chemical Company produced PDO through acrolein route, which set off a small climax of PTT polymerization and application research. Although the manufacturing cost of PDO has been reduced, it is not enough to adapt to the development of commercial polyester, which leads to the termination of PTT research.
In the late 1980s, Shell and Degussa made breakthroughs in two different PDO production technologies: Degussa reduced the cost of manufacturing PDO by acrolein route, improved the purity and met the polymerization requirements; Shell has developed a synthetic route for hydroformylation of carbon monoxide and H2 with ethylene oxide (EO). Hydroformylation technology and easy availability of ethylene oxide raw materials have enhanced its core competitiveness. 1995, Shell announced the commercialization of PTT and established an 80kt/a PDO factory in Geismar, Louisiana. Subsequently, DuPont announced the transformation of a ready-made polyester factory in Kingston, North Carolina, to produce PTT from Degussa, and cooperated with Genecore to develop a potential and cheaper glycerol fermentation biological route to produce PDO. Half a century after the invention of PTT, it finally joined the ranks of PET and PBT and became a brand-new variety of commercial polyester.
1 1, 3- propanediol
1, 3- propanediol (CAS: 504-63-2) is a colorless and transparent liquid with a boiling point of 2 14℃. There are three synthetic routes, namely acrolein hydration, ethylene oxide formylation and biological fermentation.
1. 1 acrolein hydration method
Degussa Company in Germany has developed a new low-cost process for producing 1, 3- propanediol from propylene. The reaction steps are as follows: (1) propylene reacts with oxygen under the action of antimony oxide or other metal oxide catalysts to generate acrolein; (2) Acrolein is hydrated with water under the action of acid catalyst or chelating ion exchanger to prepare 3- hydroxypropionaldehyde (3-HPA); (3) Under the action of Ni catalyst or Pt and Ru catalyst, 3-HPA is hydrogenated with aldehyde group to prepare 1, 3- propanediol. The main reaction formula is shown in figure 1. The key technology of synthesizing PDO by acrolein hydration is the hydration conversion rate and selectivity of acrolein. The performance of hydration catalyst and hydration process determines the advanced process route. However, acrolein has a strong stimulating effect on respiratory tract and has some defects in environmental friendliness.
Degussa once built a 2kt/a pilot plant of 1 3- propanediol in Antwerp, and then built a 50kt/a industrial plant in 1996. All the products were used to produce PTT.
50℃
CH2 = CHCHO+H2O——→HO-ch 2ch2 CHO
10 MPa 3 HPA
50~ 125℃
Wang Xuesong et al. studied the isothermal weightlessness behavior at six temperatures of 304℃, 309℃, 365, 438+04℃, 365, 438+09℃, 324℃ and 336℃ by thermogravimetric analysis (TG). With the increase of decomposition temperature, the degradation weight loss rate is accelerated, and the coke yield is reduced at the same decomposition time. The maximum weight loss rate method and equal conversion method were used to process isothermal weight loss data. The apparent activation energy of PTT decomposition was 20 1kJ/mol and 192kJ/mol, respectively. The decomposition reaction is a first-order reaction, and the pre-exponential factors Ln(Z) are 36min- 1 and 34min- 1+ respectively. These kinetic data are very close to the results obtained by Kissinger's kinetic treatment method (E= 192kJ/mol, N= 1.0, Ln(Z)=37min- 1).
4 research prospects
To sum up, the special structure (molecular chain Z spiral arrangement) gives PTT many excellent properties. PTT fiber overcomes the disadvantages of strong rigidity of PET fiber, poor dyeing performance and flexibility of PBT fiber and easy deformation. It has good chemical resistance of PET, high resilience and dirt resistance like nylon, and is a good material for textile fibers and carpets. If we want to develop PTT in China, we should study the production of PDO while developing the application of PTT products. Among several production methods of PDO, the author suggests that ethylene oxide method should be the main method to form independent intellectual property rights, and industrial production should be carried out as soon as possible, and then the research on biological fermentation method should be strengthened.
PDO first appeared in DuPont's patent. Using carbohydrates such as glucose or starch as raw materials, glycerol was first fermented and then contacted with a single microorganism under suitable fermentation conditions to prepare PDO. The single microorganism used contains active dehydratase or diol dehydratase, and the introduction of this enzyme catalyst is the key to this process. 1995, DuPont's patent first proposed a new process for the synthesis of 1, 3- propanediol by biological fermentation.
Subsequently, Kato et al. also realized the biosynthesis of 1, 3- propanediol by using recombinant biological enzymes encoding glycerol -3- phosphate deoxygenase, glycerol -3- phosphatase, glycerol dehydratase and 1, 3- propanediol oxidoreductase. Duane-Coleman and others invented an improved method for producing 1, 3- propanediol from organisms with various carbon sources by using DNA containing various dehydratase proteins.
The research on the preparation of 1, 3- propanediol by biological fermentation in China is quite active, and some of them have achieved results. Professor Andy Lau, director of Tsinghua University Institute of Applied Chemistry, undertook the national key scientific and technological project "1,3- propanediol two-step fermentation production" on February 28th, 2003, and the experimental scale reached100t/a.
The main difficulties in the production of 1, 3- propanediol by biological fermentation developed by DuPont and Genecore are the improvement of yield and the selectivity of strains, so there is no report on the industrial production of 1, 3- propanediol by biological fermentation.
Synthesis of 2 PTT
PTT is obtained by melt polymerization of PDO with PTA or DMT, and its chemical structure is shown in Figure 3 (omitted). In the polyester industry, it is also called 3GT. G and T represent diol and terephthalic acid respectively, and the number before G represents the number of methylene groups in diol. The commercial name of shell PTT is Corterra? Polymer, and DuPont's business name is DuPontTMSorona? Polymer.
2. PTT synthesis by1DMT method
The main reaction process of synthesizing PTT by DMT method is shown in Figure 4 (omitted).
Kawase et al. first proposed to synthesize PTT through DMT route. Using tetrabutyl titanate as transesterification catalyst and polycondensation catalyst, the transesterification reaction was carried out in the range of 160-220℃, and PTT was obtained by polymerization at 250℃ in vacuum. Doerr et al. proposed that cobalt acetate and tetrabutyl titanate were used as transesterification catalysts, butyl stannic acid was used as polycondensation catalyst, and toner and tridecyl phosphite were added as stabilizers to improve the hue of PTT. Traub et al. also proposed that tributyl phosphite as stabilizer can improve the hue of PTT and reduce the content of carboxyl end groups. In a word, under the molar ratio of 1.2-2.2, the transesterification reaction was carried out at 140-220℃, and then the polymerization of PTT was carried out under vacuum at 250-270℃ with titanium tin compound as polycondensation catalyst.
2.2 synthesis of PTA
Schimdt et al. proposed that in the synthesis of PTT by direct esterification, esterification was carried out under pressure, antimony triacetate and titanium-silicon precipitated oxide were used as polycondensation catalysts, PTT was polymerized in vacuum at 257-265℃, and phosphoric acid and cobalt acetate were used as stabilizers, which could reduce the formation of acrolein and allyl alcohol. Kuo et al. proposed B.
Because the melting point of PTA > is 300℃, the solubility in PDO is poor, so the direct esterification process is best carried out in the presence of "mother liquor", and the reaction is carried out at 70- 150kPa and 250-270℃ 100- 140min. Mother liquor is the melt of PTT oligomer with polymerization degree of 3-7, and it is the first batch of reaction medium left in the reaction kettle to increase the solubility of PTA. Esterification is PTA autocatalytic reaction. After reaching the required degree of polymerization, 50%-60% of oligomer melt is transferred to a polycondensation kettle, and (0.5- 1.5)× 104 titanium butoxide or (1.0-2.5) × is added at 260-275℃. Use vacuum degree < 0. 15kPa to remove polycondensation by-products until the polymer viscosity reaches 0.7-0.9 dl/g. The reaction process is shown in Figure 5 (omitted).
2.3 PTT solid-state polycondensation
In order to obtain higher molecular weight PTT, melt-condensed chips can be thickened in solid state at 180-2 10℃ under vacuum or inert gas atmosphere. Ben Duh studied the solid-state viscosification of PTT with intrinsic viscosity of 0.445-0.660dL/g at 200-225℃, and treated it with a modified second-order kinetic model, that is, the total viscosification rate was-DC/DT = 2K >. A(C-Cad), where: C- total end group concentration, t- solid thickening time, ka- apparent reaction rate constant, Cai- apparent active end group concentration. The results show that PTT and PET have the same solid-phase tackifying mechanism. Apparent reaction rate constant ks and apparent active end group concentration Cai are two important parameters that affect the solid-phase viscosity increase, in which ka increases with the increase of viscosity increasing temperature and intrinsic viscosity of prepolymer, while Cai decreases with the increase of viscosity increasing temperature and intrinsic viscosity of prepolymer. That is to say, the solid-phase tackifying rate increases with the increase of tackifying temperature and intrinsic viscosity of prepolymer, and the apparent activation energy is about 52kJ/mol.
2.4 side reactions in PTT synthesis
Because the polycondensation process of PTT is carried out at high temperature, in addition to the main reaction to produce PTT, there will be side reactions such as ester bond breakage between the end groups of macromolecular chains and within macromolecular chains, which will lead to yellowing of products and decrease of melt viscosity.
24. 1 etherification reaction
Like DEG produced in PET synthesis, in the polycondensation process, PDO forms dipropylene glycol (DPG)*** and aggregates in PTT molecular chain. Acid PTA process further aggravated the formation of DPG, and the aggregation of DPG reduced the melting point of polymer and affected the dye uptake of fiber. The generation of DPG is shown in Figure 6 (omitted).
Thermal degradation reaction
The mechanism of thermal oxidative degradation and thermal degradation of PTT is similar to that of PET. Under the action of thermal motion, the ester bond in the sub-chain undergoes McClafferty rearrangement (as shown in Figure 7), and the oxygen atom on the ester carbonyl group attracts the hydrogen atom on the β -methylene group to form a six-membered ring transition state, which is decomposed into terminal carboxyl group and terminal allyl group. Allyl-terminated groups are further cracked into allyl alcohol, which is oxidized to acrolein in the presence of oxygen, and allyl alcohol also easily reacts with PDO to produce acrolein. Therefore, as a result of thermal degradation, the content of terminal carboxyl groups in the polymer increases, and the by-product acrolein increases, and the generated double bonds are easy to cross-link and deepen the color tone of the polymer.
2.4.3 Formation of cyclic dimer
Like PET, the lower hydroxyl group of PTT is bitten back in the molten state, and the ester bond is transesterified, resulting in cyclic oligomer. The difference is that the cyclic oligomers of PET are mostly cyclic trimers, while the cyclic oligomers of PTT are mostly cyclic dimers.
The melting point of cyclic dimer of PTT is 25 1-254℃, accounting for about 2.5%-3.0% in the molten polymer, which is more volatile than cyclic trimer of PET.