Synthesis of Acrylic Ion Exchange Resin
Acrylic ion exchange resin has the advantages of high exchange capacity and good resistance to organic pollution. It is widely used and its output is only inferior to that of styrene resin. Its main variety is weak acid ion exchange resin with carboxyl group. There are also many varieties and specifications of weakly alkaline and strongly alkaline resins with amide structure. This kind of anion exchange resin does not use chloromethyl ether, which is more toxic and expensive, in production, so it has been paid more and more attention by producers.
1、 Weak acid cation exchange resin
Spherical crosslinked copolymers can be obtained by suspension copolymerization of methyl acrylate or methyl methacrylate with divinylbenzene. Macroporous copolymers can also be made by adding an appropriate amount of pore forming agent into the monomer mixture. The method is basically the same as that of suspension copolymerization of styrene divinylbenzene. Because acrylate has certain water solubility, saturated salt water is generally used as the dispersion medium in suspension copolymerization to reduce the solubility of acrylate in the water phase.
The cross-linked copolymer of acrylate is hydrolyzed into weak acid resin.
It is difficult to hydrolyze the cross-linked copolymer of methyl methacrylate. High exchange capacity can only be obtained by hydrolysis under rather severe conditions.
The volume expansion rate of weak acid resin is large when it is transformed from P-COOH to P-COONa. Increasing the degree of crosslinking is bound to reduce the exchange capacity. In order to overcome this contradiction, the resin can be made into a low crosslinking interpenetrating structure, which can not only maintain high exchange capacity (up to 12 meq/g), but also reduce the variable expansion rate. However, the interpenetrating resin needs to be polymerized twice, which is still troublesome in production.
In the copolymerization of acrylate and divinylbenzene, there is also heterogeneity of crosslinking structure due to different monomer activities. In the late stage of copolymerization, long polyacrylate chain segments may be generated, and even some linear acrylic homopolymers may be generated, which may be the reason why such resins are prone to agglomeration in use. Many studies have been carried out on the copolymerization of acrylate and other crosslinking agents in an attempt to improve the performance of acrylic weakly acidic resin. The crosslinkers studied include ethylene glycol dimethacrylate (EGDM), allyl itaconic acid (DAI), allyl trimeric isocyanate (TAIC), and allyl methacrylate (AMA). When these crosslinkers are used to prepare weak acid resin, not only the reactivity with acrylate should be considered, but also the stability of hydrolysis should be considered. The results show that DAI and AMA are better crosslinkers.
DAI has two ester groups, which is suitable for preparing polar adsorption resin. It is more suitable to be used as crosslinking agent of weak acid resin. In addition to cross-linking, it also has carboxyl group, so the weak acid resin made by copolymerization and hydrolysis of DAI and methyl acrylate has high exchange capacity (up to 11.6meq/g, 4.5meq/ml), good mechanical strength, no agglomeration in use and other excellent properties.
The free radical polymerization activity of TAIC is low, and it can be mixed with divinylbenzene with high activity. As a cross-linking agent of acrylic weak acid resin, it can also obtain a resin with high exchange capacity and good mechanical strength.
Weak acid resin can be directly obtained by copolymerization of acrylic acid or methacrylic acid with crosslinking agent.
Because methacrylic acid can be dissolved in hot water, spherical resin can not be obtained by conventional suspension polymerization method. To minimize the solubility of monomer in water, the following measures can be taken:
(1) Saturated salt water is used as dispersion medium.
(2) Add non-polar solvent (such as benzene, toluene, heptane, petroleum ether, etc.) to the monomer.
(3) Reduce the reaction temperature as much as possible (e.g. 70 ℃). In this way, a weak acid resin with good performance can be obtained, but it is easy to bond when in use.
The mutual solubility of methacrylic acid and divinylbenzene is not very good. When the degree of crosslinking is high, the product may be uneven in structure. A weak acid resin with good properties can be obtained by using allyl methyl retenoate and divinylbenzene as the mixed crosslinking agent.
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