Introduction
Brominated Polystyrene, referred to as BPS, is a brominated product of polystyrene (PS). It is an additive polymer brominated flame retardant with high bromine content and low toxicity. It has excellent heat resistance and impact resistance, no blooming, no migration, etc. It can be used for flame retardant treatment of high-temperature resistant resins such as nylon, polyester, and ABS [1].
Brominated polystyrene does not release carcinogens such as dioxins when burned and is a substitute for polybrominated diphenyl ether (PBDPO) flame retardants. Currently, countries around the world are actively researching production methods of brominated polystyrene with high thermal stability and high cost performance. The thermal stability of brominated polystyrene is related to the content of α-alkyl bromide (or chloride) in brominated polystyrene. The higher the content, the lower the thermal stability of brominated polystyrene [2].
1 Traditional synthesis methods and existing problems
There are two traditional synthesis methods of brominated polystyrene. One is a process route in which styrene is brominated first and then polymerized, and the other is a route in which polystyrene is directly brominated. The former can avoid the formation of carbon chains. Substitution of hydrocarbon groups prevents chain scission, but the process route is long and involves too many chemical units, so the cost is high [3]. At present, the production of brominated polystyrene is mainly based on the latter, and its molecular weight is determined by polystyrene. Commercially available polystyrene can be directly used as raw material to produce high molecular weight brominated polystyrene [4].
Both of the above process routes require the use of brominating agents, which are highly toxic and require excessive use in order to enhance the flame retardant effect, causing great harm to the human body and increasing production costs. In addition, due to the strong corrosiveness of the bromide agent, high requirements are placed on production equipment, and the safety risks of intermittent operation are greater.
2 Advantages of Microchannel Reactor Synthesis
The microchannel reactor has good mass transfer and heat transfer effects, can realize fully automatic material inlet and outlet control, and has a variety of corrosion-resistant materials to choose from. In view of the above characteristics of the microchannel reactor, the continuous production of brominated polystyrene can be achieved, reducing the use of brominating agents and catalysts, greatly improving the working environment, and improving production safety and efficiency.
This article uses polystyrene as the raw material, bromine chloride as the brominating agent, and chloride as the catalyst to synthesize brominated polystyrene. With yield, color, and bromine content as the main inspection indicators, the reaction temperature, materials The influence of factors such as ratio and catalyst dosage on the reaction determines the optimal process conditions for preparing brominated polystyrene. Figure 1 shows the synthesis process route of brominated polystyrene.
Figure 1 Synthesis process route of brominated polystyrene
3 Experimental Part
3.1 Experimental raw materials
Polystyrene (PS), bromine chloride (homemade), concentrated hydrochloric acid, concentrated sulfuric acid, sodium hydroxide (CP grade), dichloroethane (CP grade).
3.2 Experimental equipment
Shandong Haomai Chemical Technology Co., Ltd. produces silicon carbide microchannel reactors, PTFE and 316L metal feed pumps, and hot and cold integrated machines.
3.3 Analysis and detection methods
Thermogravimetric analysis (TGA), nuclear magnetic resonance spectroscopy (NMR), molecular weight and distribution detection (GPC), melting point test, trace moisture test.
3.4 Experimental process
Set up the experimental device, clean the feed system and reaction system with absolute ethanol, and then clean the reactor with dehydrated 1,2-dichloroethane. Weigh a certain amount of polystyrene, add dehydrated 1,2-dichloroethane, heat and stir to dissolve, cool to 10-15°C after dissolution, and set aside. Take a certain amount of bromine chloride solution in an ice bath and set aside. According to the material ratio in the experimental plan, the reaction materials are fed at a certain flow rate. After the material is discharged stably, samples are taken for detection and analysis. Adjust the reaction temperature, material ratio and catalyst dosage according to the reaction effect until the reaction effect meets the requirements. The experimental device is shown in Figure 2.
Figure 2 Experimental device diagram
3.5 Experimental post-processing
Let the reaction solution stand for separation, retain the lower solution, and wash the lower solution 2 to 3 times with deionized water. Add saturated NaHCO3 solution to the above-mentioned washed solution to adjust the pH to neutral, and then wash it with deionized water several times to remove impurity ions in the product solution as much as possible. After the above organic phase is diluted 2 times with the reaction solvent dichloroethane, it is added dropwise to hot water above 90°C, the solvent dichloroethane is evaporated, and the brominated polystyrene product precipitates in the water phase. The temperature is maintained at 82 to 85°C during the process. In order to remove the solvent in the product, boil it for another hour after the dropwise addition. The brominated polystyrene product in the flask was washed, suction filtered, and dried in a vacuum oven at 110°C. The brominated polystyrene preparation process flow is shown in Figure 3.
Figure 3 Brominated polystyrene preparation process flow
3.6 Analysis and Discussion
In order to optimize the best process conditions, experiments on reaction temperature, material ratio, catalyst dosage and other factors were carried out during the experiment.
3.6.1 Effect of reaction temperature
Under the condition that other factors remain unchanged, only the reaction temperature is changed, and the optimal reaction temperature is determined by evaluating the product yield and color. The experimental results are shown in Table 1.
Table 1 Effect of reaction temperature
As can be seen from Table 1, when the temperature is low, the product yield is low, but the color is better. When the temperature reaches above 10°C, the product yield basically remains unchanged; but the product color turns yellow as the temperature increases. , this is due to the increase in reaction temperature�, the catalyst activity is increased and the reaction is violent. Considering the product yield and color, the reaction temperature is preferably 5 to 10°C.
3.6.2 Influence of the dosage of brominating agent
Under the condition that other factors remain unchanged, only the material ratio is changed. By evaluating the product yield and stability, the optimal material ratio is selected. The experimental results are shown in Table 2.
Table 2 Influence of material ratio
As can be seen from Table 2, when the molar ratio of bromine chloride to polystyrene is lower than 2.6:1, the bromine content will decrease significantly. The decrease in bromine content will greatly reduce the cost performance of brominated polystyrene. . Because an important advantage of brominated polystyrene over polybrominated polystyrene is its high bromine content. When the proportion of bromine chloride is high, the bromine content of the product can reach more than 69%, according to experimental results. Therefore, it is more appropriate to choose a molar ratio of 2.6, which reduces the usage of brominating agent by about 20% compared with traditional processes.
2.6.3 Effect of catalyst dosage
When the reaction temperature was 5-10°C and the material ratio (n BrCl:n Ps) was 2.6:1, the effect of catalyst dosage on bromine content was investigated. The experimental results are shown in Table 3.
Table 3 Effect of catalyst dosage
As can be seen from Table 3, when the catalyst dosage reaches 1.8%, as the catalyst content increases, the increase in bromine content is no longer significant. At this time, the bromine content has reached 69.76%, which meets the requirements for brominated polyphenylene. Ethylene requirements. At this time, adding more catalysts will not only increase the production cost, but also use excessive amounts of catalysts, which will also make the reaction more violent and the local temperature too high, resulting in the generation of a large amount of free radicals, thereby increasing the main chain bromination and reducing product stability. This is detrimental to the entire production process, so it is more reasonable to choose 1.8% catalyst dosage.
4 Conclusion
(1) Silicon carbide microchannel reactor can be used for the preparation of industrial flame retardants when the reaction temperature is 5~10°C, the material ratio (n BrCl:n Ps) is 2.6:1, and the catalyst dosage is 1.8% , the bromine content of the product can reach more than 69%, meeting the requirements for industrial use.
(2) Compared with traditional production processes, the use of silicon carbide microchannel reactors can reduce the amount of brominating agent and catalysis
The dosage improves the working environment and saves production costs.
(3) Use a continuous method to produce environmentally friendly and efficient flame retardants with high bromine content and high thermal stability, which will reduce the cost of waste
Dependence on exports, it is of great significance to promote the upgrading of domestic flame retardant products and protect the environment and human health.
References
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