The production technology of polyether polyol is becoming more and more mature, and each company has its own proprietary technical expertise. For ordinary polyether polyols represented by propylene oxide glycol, its industrialization can be basically divided into three steps: raw material pretreatment, polymerization and post-treatment. The key points of these three steps are as follows.
1. Raw material pretreatment
In ring opening polymerization, in order to ensure the reaction speed, molecular weight control and product color unsaturated of polyether polyol synthesis, in addition to the requirement of high purity of polymerization monomer, the catalyst and initiator should also avoid bringing water, aldehyde, oxygen, polyhydroxy sugar and other trace impurities into the reaction system. Therefore, raw materials should be pretreated before reaction.
Propylene oxide is a colorless liquid with low boiling point. The boiling point is only 34 ℃. Volatile gas has ether odor, which is volatile, flammable and toxic. Pay attention to protection during operation. Due to the asymmetry of propylene oxide, acid catalysis leads to ring opening and isomerization. Compared with propylene oxide, ethylene oxide is more flammable, volatile and explosive. Safety protection measures shall be strictly implemented during use.
Many impurities in the raw materials will cause unnecessary side reactions to the polymerization, which will affect the smooth progress of the reaction and the product quality. For example, the presence of aldehydes can act as polymerization inhibitors in polymerization like free radicals. Therefore, the content of aldehyde in raw materials needs to be strictly controlled. The aldehyde content of propylene oxide raw materials of foreign companies is generally lower than 50mg/L, and Sanyo Chemical Company of Japan even requires the aldehyde content to be lower than 10mg/L. However, the acetaldehyde content in domestic industrial propylene oxide is about 400mg/L, far behind the foreign control indicators. Free radical like impurities that act as polymerization inhibitors in polymerization include chlorides, which can terminate the growth of normal polymerization chains.
——OK+R’CI→——OR’+KCI
Water in raw materials is also an important impurity, which will have two adverse effects in ring opening polymerization. First, trace water can be used as initiator to react with epoxy monomer, and the amount of initiator in the specified formula can be increased, thus reducing the design molecular weight of the synthetic polyol.
Secondly, water is the chain terminator, which will also affect the normal process of ring opening polymerization.
——OK+H2O→——OH+K++OH-
Therefore, the moisture content of bulk epoxy raw materials shall be controlled below 0.05%.
In addition, in the process of raw material pretreatment, the common method is to premix the initiator and catalyst to generate metal hydrocarbon oxides, and then add them to the polymerization reactor after vacuum dehydration.
2. Ring opening polymerization
Ring opening polymerization using epoxy compounds as raw materials is an exothermic reaction. During polymerization, 2100kJ and 1500kJ heat will be released per kilogram of ethylene oxide and propylene oxide, so the reaction heat needs to be discharged in time. In addition, oxygen will produce oxidation and polymerization inhibition on polymerization, so it is necessary to introduce dry nitrogen into the polymerization unit for replacement before reaction to ensure that the reaction is carried out without oxygen. At the same time, dry nitrogen needs to be introduced continuously in the reaction process to produce high-quality polyether polyols.
3. Post treatment process
The crude polyether polyol produced by polymerization needs post-treatment. Post treatment mainly includes neutralization, adsorption, dehydration, filtration, rectification and other units. After a series of post-treatment, polyether polyols not only neutralized and filtered the remaining catalyst impurities, but also removed other harmful impurities by adsorption, dehydration, distillation and other processes, thus obtaining polyether products with low water content, light color, no odor, good performance and uniform performance. Domestic and foreign manufacturers attach great importance to the refining of polyether and have made many beneficial explorations. The basic refining methods are as follows. In actual production, various combined refining methods are often used according to specific conditions.
(1) In the production of polyether polyols, inorganic acid or organic acid is used to neutralize polymerization. For systems using alkaline earth metal hydroxide as catalyst (such as calcium hydroxide, barium hydroxide, strontium hydroxide, etc.), it is rarely used to neutralize organic acids as neutralizers, because the salts they produce are not easy to remove through filtration. Organic acids are commonly used in industry as neutralizers, such as adipic acid, malonic acid, succinic acid, tartaric acid, etc. For the use of alkali metal hydroxide as catalyst system, it is recommended to use inorganic acid as neutralizer, such as hydrochloric acid, phosphoric acid, dilute sulfuric acid, etc., because the salt produced by neutralization is stable, the particles are large, and it is easy to separate. The inorganic acid commonly used in industry is phosphoric acid, which is relatively weak in acidity and less corrosive to equipment.
(2) The adsorption operation is to remove the residual catalyst plasma and colored substances in polyether polyols. Common adsorbents include magnesium silicate, aluminum silicate, active clay, active carbon, molecular sieve, diatomite, neutral carbon, etc. Generally speaking, the adsorption of this system can reach equilibrium within a period of time. If the contact time is too long, the color of polyether will become darker. Therefore, the adsorption operation time is generally 30-40 minutes. In the adsorption process, if the temperature is too high, oxidation will occur; If the temperature is too low, the viscosity of the system will become larger, causing difficulty in adsorption. Therefore, the adsorption operation temperature is generally about 100 ℃.
(3) In industrial production, ion exchange usually adopts neutralization adsorption method, but special polyethers with high requirements can also be treated by ion exchange method. This method can not only effectively remove metal ions in polyether polyols, but also remove colored and toxic substances in polyethers. This method can be combined with the traditional neutralization adsorption method to produce high quality polyether polyols. However, the ion exchange process is relatively complex and expensive. At present, it is mainly used in laboratories and small-scale production of special polyether polyols with high requirements in medicine, food, cosmetics, etc.
(4) The purpose of filtration is to remove the salt generated after catalyst neutralization. The traditional equipment is plate and frame filtration, which has low efficiency and high labor intensity. Now more advanced round screen filtration equipment is used. During the filtration process, appropriate filter aids should be added according to the molecular weight and function of polyether to reduce the viscosity of system materials, improve the filtration efficiency and reduce the residue of products in the filter cake. There are special filter aids abroad. Of course, if an appropriate amount of methanol, ethanol, isopropanol and other solvents are added to the material system to reduce the viscosity, the filtration operation will also be smooth.
(5) In order to obtain polyether products with low impurity content, light color and narrow molecular weight distribution, rectification adjustment procedures are required. Polyether distillation is carried out under reduced pressure of about 100 ℃ to remove trace water and other impurities in polyether. Falling film evaporator is widely used because the ether bond in polyether products is easy to be oxidized and broken. The materials are continuously transported to the high-speed centrifugal dispersion tray at the upper part of the evaporator, and the materials are centrifugally thrown to the kettle wall to form a liquid film drop. Under certain temperature and pressure drop conditions, the removal of impurities in the product can be controlled by controlling the falling film height. In order to avoid the oxidation of polyether products, 0.05%~0.5% di tert butyl p-cresol and other antioxidants, as well as butyl hydroxytoluene, amine antioxidants (such as phenothiazine) and other complexes should be added. In order to meet the needs of polyurethane synthesis reaction, polyether polyol products must not be alkaline. In general, polyether products need to be tested for acidity and alkalinity. If necessary, weak acid organic acid shall be added properly for adjustment to make it a uniform weak acid product. Generally, the pH value of polyether polyols should be controlled between 6 and 7. In many applications, soluble metal salts are not allowed in polyether polyols. The content of potassium and sodium ions is lower than 100mg/kg, which can only meet the requirements of hard foam. For special soft foam, even less than 5mg/kg.
