With the rapid development of the national economy, the quality requirements for epoxy resins in the industrial sectors such as electrical insulation and electronic components have become increasingly stringent. In order to ensure the reliability of insulation grades and electrical properties, these industrial departments are required to increase the purity of epoxy resins and reduce the content of organic chlorine therein. To this end, a large number of patents and research results abroad each year introduce how to reduce the organic chlorine content of epoxy resin and improve the purity of the epoxy resin.
Where does the organic chlorine from the epoxy resin come from? As we all know, the common epoxy resin is derived from the condensation of dibasic (or polyhydric) phenols with epichlorohydrin with the participation of alkalis. Due to the complexity of the reaction, both the main reaction and the by-product reaction exist in both the etherification stage and the dehydrochlorination stage. At the same time, due to the large viscosity of the reaction system, there are mesophases, and it is also difficult to make the reaction completely complete.
As a result of the incomplete reaction, the chlorohydrin ether remains in the finished resin. Also known as "hydrolyzed chlorine" or "saponified chlorine" compounds. As a result of the reaction, the formation of a compound known as "non-hydrolyzed chlorine", "fatty chlorine" or "bound chlorine" is formed.
The reason for the formation of organochlorines has been studied and it is advantageous to adopt different methods to reduce the organic chlorine content of epoxy resins. The principles of the methods used in the comprehensive patent literature are mostly based on trying to reduce or suppress the reaction and increase the degree of dehydrochlorination reaction. The specific review is as follows:
In the polycondensation stage, changing the ratio of primary and secondary reactions assumes that the concentration of alkali in the polycondensation reaction has a great influence on the progress of the primary and secondary reactions. Increasing the concentration of the alkali in the reaction system facilitates the main reaction. Therefore, there are certain requirements for the water in the reaction system. In addition to controlling the concentration of alkali, it is also necessary to control the raw materials involved (especially recycled epichlorohydrin) and the water generated in the reaction within a certain range. The method described in the document is to use a 50% NaOH aqueous solution as a dehydrochlorination agent, and azeotrope vacuum dehydration at the same time as adding alkali, so that the alkali concentration in the system is always maintained in a concentrated state. The epoxy resin produced by such a method has a molecular weight of 400-600 and an organic chlorine value of not more than 0.02%.
Some on the basis of the above, further studies using inert dipolar solvents to inhibit the part of the reaction will occur. When dimethyl amide, dimethyl sulfoxide, and dimethyl sulfone are used as solvents, the selectivity of the reaction can be improved, and the secondary reaction can be controlled within the minimum limit. The epoxy resin thus prepared has a hydrolyzed chlorine content of about 0.02%. However, the solvent used here is expensive and has limited resources, which limits the industrial application of this method.
Later, a method called "intermediate phase transfer method" to produce low chlorine content epoxy resin was developed. It is in the polycondensation reaction, the use of mesophase catalysts, the reaction is transferred from one phase (usually the aqueous phase) to the other phase (the organic phase or the upper phase) of the transfer to the smooth realization of the reaction. It can increase the conversion of chlorohydrin ethers under relatively easy conditions. In the polycondensation process, polyethylene glycol (molecular weight = 400) dissolved in methylbutyl ketone was used as a mesophase transfer catalyst, and a concentrated NaOH aqueous solution was used as a dehydrochlorination agent at 110Â°C. The epoxy resin thus obtained contained hydrolyzed chlorine. 0.0017%, combined with 0.041% of chlorine.
There are also methods for the supplementary dehydrochlorination by means of mesophase catalysts, indicating the use of 48% NaOH aqueous solution as a dehydrochlorination agent in the polycondensation reaction, with participation of benzyltriethylammonium chloride and linear or cyclic etherate, at 80 The reaction at Â°C resulted in a phenolic polyepoxide resin containing 0.027% hydrolyzed chlorine (0.035 without benzyltriethylammonium chloride).
Further improvement of the dehydrochlorination reaction in the post-treatment These methods mainly involve the selection of the appropriate solvent and the amount of the dehydrochlorination agent. Because here is how the oligomers that have been polycondensed can be further treated to reduce the organic chlorine content.
As suggested in the method, a two-stage process is used to reduce the hydrolyzed chlorine content. In the first stage, the epoxy resin is dissolved in an organic solvent (toluene or acetone), 0.5 to 10 moles of alkali metal bromide or iodide is added per mole of epoxy resin, and the mesophase catalyst is 2 parts by weight of the epoxy resin. -15% of the alcohol was put into the reactor and processed at 70-90Â°C. In the second stage, the mixture is treated with an aqueous solution of an alkali metal hydroxide in an amount of 1 to 5 moles per mole of the epoxy resin for 0.5 to 5 hours at a temperature of 40 to 60C. The content of hydrolyzed chlorine in the epoxy resin thus treated can be reduced from 0.041% to 0.0003%.
It is also possible to dissolve a relatively high content of epoxy resin in methyl ethyl ketone and treat it with a concentrated aqueous NaOH solution. Using this method, an epoxy resin with a chlorine content of 0.35% can be treated as a resin having a hydrolyzed chlorine content of 0.004% and a chlorine content of 0.067%.
In the above example, the finished epoxy resin is dissolved in a suitable solvent, and necessary post-treatment is performed to reduce the amount of chlorine. In fact, the semi-finished product can be condensed to determine its organic chlorine content through an intermediate test, and then the amount of additional base is determined according to its chlorine content. This facilitates the production of low-chlorine content epoxy resins.
In fact, the data is that the epoxy oligomer with 0.88% chlorine content was measured in a 1-5% concentrated aqueous alkali solution and reacted in a hydrophobic organic solvent at 80Â°C for 2 hours in the presence of a quaternary ammonium salt. . The thus obtained epoxy oligomer contained 0.03% of hydrolyzed chlorine. The data also describes the method of reducing the chlorine content to 0.02% by processing the epoxy oligomer with chlorine content of 0.6% with a 0.2-2% alkaline solution at 100Â°C for 30 minutes.
According to the results of a large number of patent data studies, various types of solvents can be selected considering different post-treatment methods. Hydrophobic organic solvents (such as aromatic hydrocarbons: benzene, toluene, xylene) are generally used, which ensure that the residual moisture can be azeotropically evaporated after dehydrochlorination is completed. And these solvents are also suitable for mesophase catalysts. Acetone is a slightly more polar solvent than hydrocarbons. Using it as a solvent facilitates the dehydrochlorination process. The data describes the dehydrochlorination with aqueous alkali solution in methyl ethyl ketone to produce an epoxy resin with a hydrolyzed chlorine content of 0.041%. Under the same conditions, when toluene was used as a solvent, the hydrolyzed chlorine content of the prepared epoxy resin was 0.156%. When ketones are used as the solvent, acetone, methyl ethyl ketone, methyl isobutyl ketone or mixtures of ketones and hydrocarbons are commonly used. It is also possible to use relatively polar fatty alcohols as solvents such as n-butanol, isopropanol and the like. The data describes that the dehydrochlorination is carried out in a n-butanol solvent with a concentrated alkaline solution at 60Â° C. The product obtained has 0.01% of hydrolyzed chlorine. The patent considers that a high purity product can be obtained by using a mixed solvent. For example, when methyl ethyl ketone:toluene:dimethyl sulfoxide=5:5:1, the hydrolyzed chlorine content can be obtained by reacting with a 50% KOH solution at 90Â° C. for 3 hours. It is 0.0008% epoxy resin combined with 0.0206% chlorine.
A large number of data indicate that the dehydrochlorination agent is generally NaOH, and in some cases KOH. Usually OH:Cl=(1.05-1.2):1, only a large excess of base is used in certain situations, for example, in the case of supplemental dehydrochlorination, the excess of alkali is 5-10 times.
Some data indicate that the formation of sodium chloride during the reaction did not slow the dehydrochlorination process. In contrast, a series of patents indicate that dehydrochlorination can be performed with the participation of certain inorganic salts. For example, the method described in the document is achieved by adding alkali metal halides such as NaCl, KCl, and LiCl to the reaction system. In dehydrochlorination, using acetone as a solvent and 50% NaOH as a dehydrochlorination agent under the participation of sodium chloride at a reaction temperature of 65Â°C, the hydrolyzable chlorine can be reduced to 0.028% (without the participation of sodium chloride. Under this test condition, the epoxy resin hydrolyzed chlorine content was 0.046%). In addition to the addition of the above salts, tests have also been conducted for the addition of phosphorus, boron, carboxylic acids, or their salts. These salts are typically added in an amount of (0.001-0.5) moles per mole of NaOH. Addition of these compounds during the test can reduce the hydrolyzed chlorine content of the processed epoxy oligomers from 0.1-5% to < 0.02%.
Maintaining a certain amount of water in the reaction system is extremely important for the dehydrochlorination reaction. It is believed that this can serve to reduce the hydrolysis reaction and maintain a high concentration of alkali. In the reaction process, methyl ethyl ketone was used as a solvent, 40% NaOH aqueous solution was used as a dehydrochlorination agent, and the reaction was carried out at 80Â° C. while continuously distilling water so that the water content in the reaction system did not exceed 3%. The hydrolyzed chlorine content was 0.002% and the combined chlorine was 0.052%.
There are many patent documents on reducing the organic chlorine content of epoxy resins in foreign countries, which shows that foreign countries attach great importance to this aspect of research work. From the practice of epoxy resin devices that have been introduced in China, it can be seen that the hydrolysis of chlorine in these devices can reach â‰¤0.03%, which can meet the requirements of electrical insulation and electronic components industry. Although the introduction of devices involves technical secrets, the methods described in a large number of patent documents are still very valuable. The key is to solve the flaws in engineering.
Source: 21st Century Fine Chemicals Network
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