The elastomers of casting polyurethane can be divided into two categories: TDI system MDI system. For many years, TDI dosage has been in the leading position in polyurethane materials. Since the 1980s, especially after the 1990s, the consumption of MDI has been developing rapidly. In some special industries, the demand of MDI greatly exceeds that of TDI. This is because MDI volatilization is small, which is conducive to environmental protection. MDI system products have superior flame retardancy, water resistance high temperature resistance. Therefore, MDI only penetrates into the traditional TDI market, but also is widely used in rim technology. However, due to its obvious shortcomings, MDI affects its application direction: first, MDI must be equipped with heating kettle jacketed pipe in the production of polyurethane materials, so that it can be melted transported, which is as simple convenient as TDI. Secondly, MDI should be stored under 4 ℃ in a dry sealed way to reduce the production of MDI dimer. Even so, dimers appear in long-term storage. The second state of MDI does appear when it is fused. This only affects the proportion of raw materials, reduces the performance of materials, but also blocks the pipeline metering pump sometimes.
As early as the 1970s, foreign countries began to modify MDI, trying to develop products like TDI, that is, liquid at room temperature, stable storage convenient transportation. There are three ways to solve this problem: one is to increase the proportion of 2,4 '- MDI isomers in MDI composition, which can prevent the precipitation of 4,4' - MDI improve its storage stability. The other is to make part of bldi react with diols polyols to form prepolymers to achieve the purpose of modification. The third is heating MDI under catalytic condition to keep it in melting state make it have no precipitation phenomenon at room temperature, so as to achieve the purpose of modification. Shanxi Institute of chemical industry began to work on the modification of MDI in the 1980s, some progress has been made since 1990s. However, due to various reasons, it is limited to the experimental stage has been applied to production.
Until 2000, the solution of environmental protection problems based on TDI system promoted the breakthrough development of MDI system, gradually applied to the actual production process, which reduced the production cost improved the production efficiency. Since 2010, the cumulative output value has reached more than 6 million yuan, which has brought higher income for Shanxi Chemical Research Institute. The emphasis of this paper is on the comparative selection of modified MDI catalysts.
Phosphine is an unsaturated heterocyclic compound containing phosphorus. It is a highly efficient catalyst with short catalytic time less amount of catalyst. It is easy to obtain high quality modified MDI.
(1) At present, 3-methyl-1-ethyl-3-phosphine-1-oxide is the most commonly used effective catalyst.
(2) Preparation: firstly, alkanes aromatics react with phosphorus trichloride to form dichloroalkyl phosphorus, then react with tetracarboxydiene (such as butadiene isopentyl ethylene) to form phosphine oxide. After MDI was prepared with phosphine catalyst, the residual phosphine still had catalytic activity, which would make MDI in unstable state during storage, resulting in the decrease of NCO mass fraction increase of viscosity. Therefore, chemical inerting method physical adsorption method were used for inerting deactivation of MDI.
(3) Catalyst dosage range: 0.1xl0 ~ ~ 10xl0 (based on MDI dosage). Due to the small amount of catalyst, diluent such as tetrachloroethylene toluene can be prepared in advance for accurate rapid dispersion.
(4) The main characteristics include: low temperature short time of modification reaction; less amount of catalyst; easy to obtain high quality modified products.
2 organometallic compounds
Organometallic compounds can be divided into organotin compounds, organometallic esters metal complexes. Organotin compounds are the most important.
2.1 organotin compounds organotin compounds can be divided into four valence two valence catalysts. The typical representative organotin catalysts are dibutyltin diethyl dilaurate dibutyltin; the bivalent organotin catalysts are stannous oxalate, stannous oleate stannous octanoate. Catalyst dosage range: 0.001% ～ 1.0%. It is characterized by simple operation belongs to medium efficiency catalyst.
2-2 organometallic ester is mainly represented by alkyl titanate, which is a medium effect catalyst. Catalyst dosage range: 0.01% ～ 1.0%. The synthesis method is simple easy to operate.
2.3 carbon based metal complex, which is composed of transition metal elements nitric oxide through bonding. Common catalysts are: four carbon based iron nickel, five carbon based iron calcium. It is characterized by relatively simple synthesis. It belongs to medium efficiency catalyst.
3.1 structure of phosphite: (RO), P. Catalyst dosage range: 0.1% 10%. Characteristics: first, the catalytic effect is low, but the catalytic activity product quality are relatively stable. Because phosphite has antioxidant thermal stability. Second, it has the advantages of wide source, low cost, convenient operation no inerting treatment. Third, the preparation process is relatively simple.
3.2 structural formula of phosphate: (RO) Po. Catalyst dosage range: 0.1% 3%. Features: simple operation, low catalytic activity, no inerting treatment, simple synthesis process stable quality of modified products.
4. Organic phosphorus organic phosphorus includes phosphorus phosphorus oxide.
4.1 phosphorus mainly includes triethyl phosphorus triphenyl phosphorus. Catalyst dosage range: 0.1% ～ 3.0%, belonging to medium efficiency catalyst.
4.2 structural formula of phosphorus oxide: RPO. Mechanism: firstly, phosphorimide intermediate is produced phosphorus oxide one isocyanate group, then reacts with another isocyanate group to form compound containing carbodiimino group, the catalyst is reduced at the same time. The addition of 0.001% ～ 0.01% is a medium efficiency catalyst. It is characterized by less addition needs inerting treatment.
There are three types of organic arsenide: triaryl arsenic, trialkyl arsenic oxide polystyrene containing organic arsenic. Catalyst dosage range: 0.1% ～ 1%, belonging to medium efficiency catalyst. Its characteristics are: low reaction temperature, short time, easy separation of catalyst the product, good product quality, stable storage, suitable for continuous production operation.
The heterocyclic compounds are mainly 2-ethyl-1,3-dimethyl-1,3,2-dicarboxy-1,3,2-dicarbonyl-2-oxide, which is prepared n, n '- dimethylenediamine ethylphosphonic acid dichloride. Catalyst dosage range: 0.1% ～ 1.0%, belonging to medium efficiency catalyst.
Nitrogen compounds represent catalysts: carbamate group, amide group other compounds. The addition amount: 0.25% ～ 5.0%; characteristics: low catalytic effect, high reaction temperature, long time, a small amount of precipitation, the storage of modified products is stable, the process is simple, no inerting.
Conclusion in conclusion,
Several kinds of modified MDI catalysts are discussed in this paper. Among them, the highest catalytic efficiency is phospholine, the medium efficiency is organometallic compounds, organic arsenic compounds, nitrogen phosphorus heterocyclic compounds phosphorus compounds. However, phosphates nitrogenous compounds were the least effective. In practical work, we can choose the catalyst according to the quality requirements of modified MDI, the source cost of catalyst, the difficulty of operation, usually choose high-efficiency phosphor catalysts. Its advantages are low liquefaction temperature, short reaction time high product quality. The disadvantage is that the preparation process of the catalyst itself is complex, the inerting treatment is also needed. Phosphates are widely used in the production of low quality products because of their wide sources low price.