Precautions for storing TDI

Properties of TDI: TDI is short for toluene diisocyanate, and it exists in two isomers: 2,4-toluene diisocyanate and 2,6-toluene diisocyanate. With the molecular formula C9H6N2O2 and a molecular weight of 174.16, it is a very important raw material in organic chemical manufacturing. Most TDI products are mixtures of two isomers, and these two isomers can give rise to three types of TDI, namely TDI-65/35, TDI-80/20, and TDI-100 (2,4-isomer/2,6-isomer). TDI is a colorless to pale yellow liquid with an irritating odor; its color deepens when exposed to light, and its boiling point is 251°C. Relative density: 1.22±0.01 (25°C); freezing point: 3.5–5.5°C (TDI-65); 11.5–13.5°C (TDI-80). Flash point: 132°C (closed cup). Vapor density 6.0, vapor pressure 1.13 Pa (20°C). The flammable range of the vapor-air mixture is 0.9% to 9.5%. Insoluble in water, soluble in propane, ethyl acetate, toluene, etc. It is highly reactive and can cross-link resins with hydroxyl groups at their ends, such as polyesters (condensates of diacids and polyols) and polyethers (polyhydroxy polyethers with 2, 3, 4, 6, etc., hydroxyl groups); it reacts very easily with water to produce carbon dioxide gas. It reacts easily with compounds containing reactive hydrogen atoms: amines, water, alcohols, acids, and bases. In particular, it undergoes uncontrolled reactions with sodium hydroxide and tertiary amines, releasing large amounts of heat. Reacting with water to produce carbon dioxide is one of the key reactions in the manufacturing process of polyurethane foam plastics, and moisture exposure should be avoided. The polymerization reaction is very slow at room temperature, but it can self-polymerize to form dimers when heated above 45°C or in the presence of a catalyst. It can react with strong oxidizing agents; it will catch fire when exposed to heat, open flames, or sparks, and upon heating it releases cyanides and nitrogen oxides. TDI is a basic raw material for manufacturing polyurethanes. Polyurethane is widely used in the production of polyurethane foam plastics and polyurethane elastomers (which are extensively used in solid tires, tapes, roller linings, oil seals, gaskets, mold pads, encapsulation of electrical components, adhesives, and coatings), as well as in paints and adhesives. It can also be utilized in films made from vinyl polymers, in the surface treatment of natural rubber, as additives to enhance the chemical resistance of paints, and in textile processing.

TDI is a Class C flammable liquid and does not require explosion protection. TDI uses foam fire suppression; water cannot be used. TDI is a highly toxic chemical, and its management requires the \"five doubles\" principle. Toxic gas detection and alarm devices should be installed. Forced exhaust and emergency ventilation are required 12 times; the emergency exhaust should be interlocked with toxic gas detection alarms.

Organic isocyanates are an important class of compounds that are widely used in the polyurethane industry, the coating industry, as well as in high-molecular materials such as dyes and pesticides. The production of isocyanates has attracted significant attention from developed countries around the world, with its output increasing year by year. Therefore, research and development in the isocyanate industry hold great practical significance in our country. The isocyanate product series mainly includes TDI (toluene diisocyanate), MDI (diphenylmethane diisocyanate), PAPI (polymeric methylphenyl isocyanate), PI (phenyl isocyanate), IPDI (isophorone diisocyanate), ODI (octadecyl isocyanate), and others. In the traditional isocyanate synthesis industry, all the aforementioned products are synthesized using the phosgene method. The phosgene-based synthesis process has a long route, is technically complex, involves expensive raw materials and high equipment costs. Moreover, phosgene is highly toxic, causing severe environmental pollution. Since the 1970s, with the growing awareness of environmental protection, improving the synthesis processes for isocyanates and developing methods for producing isocyanates using gaseous substances have become highly active areas of research in the chemical industry. The main advances in this field are presented below. 1 One-step method: Taking the synthesis of TDI as an example, the reaction equation is as follows: It is very difficult to synthesize isocyanates through a one-step reaction. According to the latest reports, using Pd(4-Mepy)2Cl2(CO)x as the main catalyst (where Mepy refers to methylpyridine) and FeCl3 as a co-catalyst can significantly improve selectivity. At 200°C and 8.5 MPa, after reacting for 1.5 hours, the reaction was nearly completely converted, with a selectivity of 70.9% for TDI; 5.7% of the by-products consisted of monoisocyanate. 2 Two-step method: Analysis of the reaction mechanism of the one-step method shows that it is difficult for the N atom to remain in the —NCO state; relatively speaking, —NCOOCH3 is a more stable product in the reaction system. Therefore, some researchers have proposed a two-step method in which methoxycarbonylation is carried out first in the reaction to prepare carbamates, which are then thermally decomposed to yield isocyanates; significant progress has been made in this approach. The specific approaches can be divided into amino group oxidation carbonylation and nitro group reduction carbonylation. 2.1 Oxycarbonylation synthesis of amines Taking PI as an example, the reaction equation for the oxycarbonylation of aniline in the first step is as follows: The main by-product of this reaction is diphenylurea, which can undergo auto-alkolysis to form a carbamate in the presence of methanol; therefore, it does not affect the outcome of the reaction. Research on catalysts for this reaction is very active. The several types that have been widely reported along with their reaction efficiencies are listed below: Table 1 Performance of catalysts for the oxidative carbonylation of aniline. Catalyst, Temperature/°C, Reaction time/h, Conversion/%, Selectivity/%: Pd(tpp)/NaI: 180, 3, 100, 83; Pd/C/NaI: 170, 2, 100, 92; RhI3·3H2O: 170, 2, 62.9, 100. *tpp: tetraphenylporphyrin dianion. Experimental studies show that the presence of iodine in the catalyst can improve reaction selectivity. However, when the halogen concentration is too high, it reduces the conversion rate of the amine. Increasing the aniline concentration or CO pressure both facilitates the reaction. The effect of O2 pressure on the reaction is complex, with extreme values. It is also reported that under similar catalysts and reaction conditions, the conversion rate of toluenediamine is only 20%–40%, with a selectivity for dicarbamates approaching 60%; the main by-products are monocarbamates and similar compounds. The second step is the thermal decomposition of the carbamate, with the reaction equation as follows: This step is a rapid reaction that proceeds easily and to a high degree of completion; it can take place even in the absence of a catalyst. Generally speaking, although the two-step method is simple in procedure, its reaction mechanism is relatively complex, making it difficult to predict the products. This poses certain challenges for its application, especially with heterocyclic isocyanate products such as TDI. 2.2 Reduction and carbonylation of nitro groups Taking PI as an example, the first step in the reduction and carbonylation of nitro groups is as follows: Regarding the catalysts for this reaction, all elements from Group VIII transition metals have been reported; in addition, metal elements such as Pt have also been mentioned in patents. Among them, compounds of Pd, Ru, and Rh exhibit the most effective catalytic activity. 2.2.1 Ru group: (1) Using 2 as a catalyst, at 150°C and 7×103 kPa, the reaction is carried out for 6 hours, yielding an efficiency of 80%. (2) Using ruthenium carbonyl complexes as catalysts, such as Ru3(CO)12, Ru3(CO)9, Ru6(CO)16, etc., among which Ru3(CO)12 yields the best results (with NEt4+Cl- as co-catalyst and toluene as solvent): at 170°C and 6×103 kPa, after 5 hours of reaction, the yield is 93%. 2.2.2 Pd class: (1) Keggin-type heteropoly cationic palladium catalyst (molecular formula PdCl2-H4PVMo11O40): at 170°C and 4.1 MPa, after 3 hours of reaction, the conversion rate was 99.3% with a selectivity of 92%. (2) Pd/C catalyst, 3,4,7,8-tetramethylphenanthroline, and 2,4,6-trifluoromethanoic acid as co-catalysts: at 180°C and 4.0 MPa, after 2 hours of reaction, the conversion rate was 99% with a selectivity of 96.6%. (3) The Pd/BaSO4 catalyst also performs well under similar conditions. For all the catalysts mentioned above, the main by-product of the reaction is aniline. Studies on the reaction mechanism show that the azene intermediate is the intermediate product and the main pathway of the reaction. Increased pressure favors the formation of —NCO, while an increase in temperature helps improve the conversion rate but reduces selectivity. In terms of catalysts, only divalent transition metal ions exhibit significant catalytic activity, such as Pd2+, Ru2+, Rh2+, Fe2+, etc. When reacting ***benzene, the conversion rate of the starting material exceeds 95%, but the selectivity **decreases, and the reaction products are quite complex. The second step is the thermal decomposition of the carbamate; as mentioned earlier, isocyanate is obtained after this thermal decomposition. 3 Three-Step Method 3.1 DMC (Dimethyl Carbonate) Method To better control the range of reaction products, the carbonylation process can be carried out first, after which the methoxycarbonyl group is introduced into the desired structure to complete the synthesis of the carbamate. Taking PI as an example, the first step is to synthesize DMC: DMC is a non-toxic chemical. This reaction uses copper as a catalyst, produces no by-products, and is a typical environment-friendly reaction. Step 2: Methylcarbamoylation of the amine. This reaction is catalyzed by oxides or carbonates of Pb. At a temperature of 160°C, after a 1-hour liquid-phase reaction, the conversion rate of aniline can reach 96%, with a carbamate yield of 95%, indicating a high degree of selectivity. The third step is the thermal decomposition of carbamates to produce isocyanates, and this reaction is the same as the one mentioned earlier. 3.2 Aniline—*** coupling method: Taking the preparation of MDI as an example, the first step is the synthesis of DPU (diphenylurea). The reaction is carried out using diethyl palladium as a catalyst, and a higher yield of DPU can be obtained when aniline is used in excess. At 120°C and 41×103 kPa, after 4 hours of reaction, the yield of DPU can exceed 90%. The second step is the methanol decomposition by DPU; this reaction can take place without a catalyst. The third step is the thermal decomposition of the carbamate, and this reaction is similar to the one described earlier. This method uses readily available raw materials, operates under mild reaction conditions, has a clear synthetic route, and produces few side reactions. However, since this process uses diphenylurea as an intermediate, it limits the applicability of this method, and it cannot be used for diisocyanates with complex structures such as TDI. South Korea’s LG company uses formaldehyde as an intermediate to carry out the synthesis of MDI; the specific reaction pathway is as follows: This reaction employs a solid acid as a catalyst, and the reaction product is diphenylmethyl urethane, which can be converted into MDI through thermal decomposition. The production cost of this method is slightly higher than that of the phosgene method. In short, among the various methods for synthesizing isocyanates via gas-phase processes, the simpler the steps, the more complex the reaction itself becomes, and the more difficult it is to carry out the reaction. By proceeding through the growth reaction pathway step by step, the feasibility of carrying out the reaction can be greatly improved. The DMC method can be applied to the synthesis of various isocyanates; it features a clear synthetic route, few by-products, and mild reaction conditions. Moreover, the methanol produced in the second and third steps can be recovered and reused to produce MDC. Although the production cost is slightly higher than that of the phosgene method, it is environmentally friendly, and this approach holds good prospects for industrial application. Moreover, our country already possesses dimethyl carbonate technology, which holds the potential to enable the environmentally friendly synthesis of certain isocyanate products in the near future, while also allowing for the development of production methods for different isocyanate products. Properties of toluene diisocyanate (TDI) and its hazards to human health: Toluene diisocyanate (TDI) has two isomers: 2,4-toluenediisocyanate and 2,6-toluenediisocyanate. Toluenediisocyanate is a water-white or pale yellow liquid with a strong, irritating odor. It has the tendency to accumulate in the human body and can have a latent effect; it causes severe irritation to the skin, eyes, and respiratory tract. Inhaling high concentrations of toluenediisocyanate vapor can lead to bronchitis, bronchopneumonia, and pulmonary edema ； Contact between liquids and the skin can cause dermatitis. Contact of liquids with the eyes can cause severe irritation, and if left untreated, it may lead to permanent damage. Prolonged exposure to toluene diisocyanate can cause chronic bronchitis. Those allergic to p-toluenediisocyanate may experience asthma, wheezing, shortness of breath, and coughing.

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