Just the Facts: Knowing Strong Base Anion Resin Types
Deciding which type of strong base anion (SBA) resin to use in a deioinizing application does not have to be a guessing game. The following article reviews chemical property differences that exist in SBA resin and how these differences affect resin performance.Deciding which type of strong base anion (SBA) resin to use in a deioinizing application does not have to be a guessing game. In fact, selecting the correct SBA resin for your application can be simple as long as you know the facts about the resin and the circumstances in which they will be employed. The following article reviews chemical property differences that exist in SBA resin and how these differences affect resin performance.
Ion Exchange Resin Chemistry
In the hydroxide form, SBA resins are strongly basic enough to remove both strong acids such as hydrochloric and sulfuric acids and weak acids such as silicic and carbonic acids from the effluent of a hydrogen form strong acid cation (SAC) exchanger where neutral salts are converted to corresponding acids. In other words, SBA resins remove all anionic contaminants present. As a comparison, weak base anion resins (WBA) remove only contaminants such as sulfate and chloride, which are strong acids, and will not remove contaminants such as silica and carbon dioxide. This is not necessarily a negative aspect of a WBA resin if the product water need not be free from silica and carbon dioxide. Indeed, this can be a positive aspect of WBA resin given that WBA resins typically have greater capacities for mineral acids and higher regeneration efficiencies than SBA resins.
Resin Manufacturing & Functionalization
Ion exchange resins are copolymers typically made of styrene and divinyl benzene (DVB), which have a tremendous surface area. The styrene acts as the backbone or chain and the DVB act as the crosslinker. The porous beads that form are referred to as "gel-type" resin. Ion exchange sites are not only on the surface of the bead but throughout the bead. The amount of DVB crosslinker affects the resin’s physical strength, resistance to temperature and oxidative degradation, selectivity and capacity. Cation and anion resins are made similarly with the major difference being the functionalization of the copolymer.
SAC resins are fuctionalized with sulfuric acid producing a sulfonic functional group (-SO3–H+). The stationary functional group is the sulfonate group (-SO3–) and the mobile exchangeable cation is hydrogen (H+).
Adding functional groups to SBA resins involves two steps: chloromethylation followed by amination. The resulting stationary functional group is a quaternary ammonium group and the mobile exchangeable anion is hydroxide (OH–). There are two chemicals that generally are used for the amination step, each producing a different type of SBA resin with different chemical properties. Type I SBA resins are aminated with trimethylamine. Type II SBA resins, developed after the SBA Type I, are aminated with dimethyl-ethanolamine.
Acrylic can be used with DVB instead of styrene to produce acrylic SBA resins (they are functionalized differently from styrenic resins). In fact, the functional group is part of the polymer backbone.
We commonly have the three types of SBA resins in our arsenal to choose from when designing an ion exchanger: Type I, Type II and acrylic. They differ in chemical make up and, therefore, behave differently. No single SBA resin is the best choice for every application. Each type has its advantages and disadvantages according to the circumstances of the application.
Type I SBA resins provide lower TOC effluents than Type II and acrylic SBA resins for a number of reasons. First, Type I resins remove natural organic matter (NOM) better than acrylics and Type II resins because they are more strongly basic. NOMs tend to be weakly ionized; therefore, the most strongly basic resin removes them best. Second, Type I resins are the most chemically stable of the three types. Hence, TOC throw from a Type I is less than Type II and acrylics. Also, the amines that are thrown from Type I resins due to degradation are cationic and can be removed by a mixed bed or cation polisher. In contrast, the degradation products from Type II resins are not ionized. In ultrapure applications where TOC is a concern (for example, in a semiconductor manufacturing mixed-bed polisher), Type I resins are your best choice.
Type II and acrylic SBA resins resist organic fouling by NOMs better than Type I SBA resins because Type II and acrylics are less basic. In other words, organics slip through Type II and acrylic SBA resins during the service cycle. Actually, as Type II resins age, they become more resistant to organic fouling because their strong base sites are converted to weak base sites. The nature of the acrylic backbone allows for reversible removal of organic matter. For the same reason, Type Is are preferred for organics removal because they are prone to organic fouling. Type Is are more strongly basic than acrylics and Type II resins; therefore, Type I resins load more organics during service and are not efficiently removed during regeneration.
In general, Type II and acrylic SBA resins have better regeneration efficiencies and higher operating capacities than Type I SBA resins due to the nature of their functional groups. This is particularly true when the combined concentration of CO2 and silica in the feed water is less than 25 percent of the total anion concentration. Type I and Type II SBA resins age differently. For instance, Type II resins, as they age, have a higher percentage of strong base sites being converted to weak base sites than in Type I SBA resins. The different manner in which SBA resins age, in itself, is not necessarily good or bad. It will depend on the feed water, effluent requirements and end-point.
Anion resins have a distinct odor, especially SBA resins in the hydroxide form. High pH increases the degradation of SBA resins and, therefore, increases its odor. The degradation product responsible for the odor off of a Type I SBA resin is amine and has a strong fishy smell. The amine is cationic; therefore, it can be removed by cation resin. The degradation products of Type II SBA resins are alcohols and aldehydes and have less objectionable odors than Type I resins. SBA resins throw fewer odors in mixed beds because of lower pH. In the case of mixed beds that use Type I resin, the cation resin also removes the amine.
High temperature degrades the functional groups of all SBA resins. Type I resins are the most thermally stable followed by Type II with acrylics being the least temperature resistant. All the SBA resins are more susceptible to temperature degradation in the hydroxide form than in the chloride form.
In applications demanding the lowest silica levels over many regeneration cycles, regeneration temperature is extremely important. Silica is not removed from anion resin during regeneration in the same manner as ordinary ions. To remove silica from the SBA resin during regeneration, the silica has to de-polymerize, solubilize, ionize and elute. The first two steps are relatively slow and require energy. As a result, longer contact time and elevated temperature during regeneration help to remove silica from resin.
When achieving low silica leakages is the goal, Type I SBA resins are the resin of choice because they can be regenerated at the higher temperatures necessary to elute silica from resin.
The ion exchange application and the resin’s properties determine the type of SBA resin selected.