The Role of Cross-linking in Ion Exchange Resins
By: C. F. ‘Chubb’ Michaud, CWS-VI
If it ain’t broke…One of the most frequent questions I get is, “How do I know when it is time to change my softener resin?” The simple answer is, “If it ain’t broke, don’t fix it.” If the system is working to your satisfaction, then it’s working well enough. We don’t replace resins because they are old. We replace them because the no longer work. The longer answer, however, is that you would have to have the resin analyzed. This involves taking the system apart and pulling a representative sample of the resin. I say representative because a sample from the top of the bed only will give you a false indication of early failure. The top couple of inches of any resin bed is simply a staging area for broken resin and debris that will be washed out on the next backwash cycle. Use a small PVC pipe to pull a sample from at least eight inches down into the bed. It will take the services of a knowledgeable laboratory to make a moisture retention and capacity determination to help judge the serviceability of the resin. Not just any old lab can do this work in a reliable manner; there are some tricks of the trade. The test, however, may cost upwards of $150 (USD), which is hardly worth the expense on residential systems of one or two cubic feet and is particularly the case if you decide not to change the resin as a result. A simpler and less expensive test is to examine the resin yourself (once you can get at it).
What I normally look for in older resin is broken and swollen beads or beads that look coated or off color (which may indicate fouling). Broken beads are a normal part of aging and still have usable capacity. Resins often start out with some broken beads so their presence should not set off an alarm (typical new resin specifications only claim about 90-percent whole, perfect beads). Small bead fragments (fines) are what we are looking for. Fines (fragments smaller than 60 mesh) are generally backwashed out of the softener during periodic regenerations. If allowed to accumulate due to inadequate backwashing, their presence would show up as pressure drop. If you are not happy with the water flow in the house and have an older softener, it may be time to change the resin—sight unseen. Poor flow is the surefire sign that the resin has gone south. If you can pull a sample of resin from the softener, examine it under a simple microscope or magnifying glass. Resin beads are perfect little spheres ranging from 0.3 to 1.2 mm. Broken and cracked beads can readily be seen.
Now that you have a resin sample, how do you inexpensively test for oxidation? Simply take a pinch of resin between your thumb and forefinger and try to crush it by squeezing and rubbing it really hard. If it mashes very readily, it is over the hill. If it mashes with considerable effort, you have about a year or so left. If you can’t mash it at all, it is probably serviceable for at least another few years and you just saved yourself $150.
I find it best to simply ask the customer about the performance of his system. Is it removing sufficient hardness? Has the hardness of the feed water changed? Is the flow adequate for the job? Have you changed the salt setting? Does it make enough soft water between cycles? Have you changed your water consumption patterns? Maybe the kids are off to college or moved back in with grandkids. A few questions will save an enormous amount of guesswork. The thing is…even if the resin is 15 years old, there is no need to change it if it is working and still providing satisfactory results.
How are resins made?
The most common modern softening resins are made from styrene and cross-linked with divinyl benzene (often referred to as S/DVB). The DVB is what holds the resin together. Styrene is another name for vinyl benzene (see sidebar and Figure 2) so the divinyl benzene is simply a styrene with a reactive vinyl (-HC=CH2) on both ends (see Figure 3). As resin ages, it oxidizes. When it oxidizes, the cross-linking unzips and it swells, takes on moisture and softens. It can crack; cracks lead to breakage and breakage leads to fines, which causes the pressure drop to increase, with a resulting poor flow if they are allowed to accumulate. Fines eventually backwash out of the vessel, resulting in resin loss and lower operating capacity. Lower capacity means more frequent regenerations. This wastes salt and it wastes water. If you used to regenerate every four days and now it’s every three, or if you used to use six pounds (2.72 kilos) of salt per cubic foot and now it takes eight (3.62 kilos), your resin is probably in need of replacement.
Figure 3. The nomenclature for S/DVB
Both S and DVB are in liquid forms (called monomers) and they are insoluble in water. When combined and agitated, the mixture of monomer and water forms tiny droplets of monomer suspended in the water phase, similar to the phenomenon one observes when shaking a bottle of oil and vinegar salad dressing. While maintaining proper agitation of the catalyzed mixture and raising the temperature, the mix polymerizes (reacts) as vinyl linkages break and rejoin with a neighbor, and the liquid droplets become tiny, hard plastic spheres known as copolymer (beads). This process is basic to both cation and anion resins, although the components may vary. The copolymer is washed, dried and screened and later functionalized into an ion exchange resin.
When polymerized styrene is reacted with sulfuric acid, a sulfonate group attaches to the backbone of the styrene (Figure 5). The sulfonation process renders the styrene largely water soluble and it swells with water and provides the active exchange site for ion exchange to take place. If the resin is in the hydrogen form, it serves as part of a de-ionization system by removing all of the cationic (positive charge) ions from the feed stream, replacing them with H+. If it is converted to the sodium or potassium form, it becomes a softener and will now replace only the divalent ions with the stronger double charge (i.e., calcium and magnesium) and is now a softener. The strands of polystyrene are held together by DVB. The level of DVB plays an important role in how that particular resin performs (kinetically), how well it resists oxidation and how long it lasts in service. Strong acid cation (SAC) exchange resins can range from 2-20 percent in DVB content.
How much cross-linking is enough?
Since the level of cross-linking directly influences how much plastic and how much water is contained in the resin (Table 1), one must keep in mind that it is the functionalized plastic portion of the resin that actually does the job. The water phase provides the pathways for ionic diffusion, so there must be a balance between where ions go and how they get there. Cross-linking also impacts capacity for this reason.
Think of ion exchange resin as a large hotel with thousands of rooms. The hallways have to be large enough to move in furniture and guests or the rooms go to waste. Likewise, they should not be so large as to waste floor space. Economy also plays a role. DVB is several times more expensive than is styrene. This explains the higher cost of the higher cross-linked products.
In regions of the world where chlorination is rare or non-existent, lower (six percent) DVB resins are often used. They are cheaper, kinetically faster and have adequate selectivity so as to do the job of softening or de-mineralizing. Most softening resins used on municipal waters in the US are 8-percent cross-linked but in some areas where the water has a higher chlorine content (above 1 ppm), 10-percent cross-linked resins are becoming more popular. In extreme cases, where measurable chlorine is over 3 ppm, macroporous resins with up to 15-percent cross-linking may be necessary to offer protection from oxidation. To carry our large hotel analogy a bit further, a macro resin is like an extremely large motel with access to the rooms from the outside of the building and lots of parking lots. Accessibility to the site is based on the resin having an extremely large surface area with lots of physical open pores (driveways) and very tight and largely inaccessible hallways. It is always suggested that you remove excess (anything beyond 1 ppm is excessive) chlorine prior to a softener.
Higher cross-linking means higher selectivity
The value of higher cross-linking contributes many factors that should be considered when choosing a resin. To recap, higher cross-linking means higher density with more plastic and less water (higher capacity). Higher cross-linking produces a tougher matrix that is more resistant to both chemical (oxidation) and physical breakdown (strength). Drawbacks include higher costs and slower kinetics (not a good thing for cold water). Although higher cross-linked resins have a higher total capacity1, the kinetics in colder water may actually result in a lower operating capacity than a lower cross-linked resin and prove to be an incorrect choice. A customer once told me that using a 10-percent cross-linked softener resin in 36oF (2oC) well water resulted in an effluent that did not have that usual ‘slickery’ feel. At that temperature, the softener was sized too small (and therefore, too slow) to completely soften the water, resulting in a constant bleed of about 1.5 to 2 gpg hardness—just enough to not feel like soft water, but sufficiently soft so as to provide the conveniences of treated water.
In addition to physical and chemical integrity, ion exchange resins exhibit higher ionic strength when reactive sites get closer together; they have a stronger attraction for the ions they remove (see Table 2). This table shows relative selectivity for H+ ion at a constant value of 1.0 and the relative higher selectivity for other ions as a function of cross-link level. Note the difference between the selectivity of sodium (Na+) and lead (Pb++) at 4, 8 and 16 percent DVB, respectively. This would indicate that a softening resin is not only an excellent choice for removing lead for drinking water, but that higher cross-linking is better yet. The flip side of this advantage is the drawback of regenerating the lead back off the resin using NaCl salt. One time use of resin favors higher cross-linking. Regenerability favors lower cross-linking. Got copper? Softeners remove copper very well. Can you spot the disadvantage of using potassium brine (KCl) in a softener to remove copper? How about for a high magnesium (Mg++) hard water?
Resins in service are subject to oxidation from chlorine or other oxidants (i.e., chloramine, ozone, peroxide, permanganate) in feed water. Oxidizers eventually take their toll on the cross-linker in the resin and cause it to break down—thus allowing the resin to unzip. As the effective level of cross-linker decreases, the resin swells and becomes softer. Reactive sites become further spaced out and selectivity of the resin drops, giving rise to higher hardness leakages and lower capacity. Resin swelling often masks that resin has been lost from the vessel. This is misleading when an operator uses the resin level of a vessel in service to determine resin losses and predicted capacity.
Transition metals such as copper and iron (often found in raw water or picked up from the plumbing by raw water) will catalyze the oxidation process, causing resin to fail much earlier than might be predicted. These metals are picked up by the resin and often will precipitate within the beads if dissolved oxygen is present in the feed water or the brine used for regeneration is high in pH. Testing determined that typical sodium chloride (NaCl) brines used for regeneration are generally alkaline with a high enough pH to precipitate iron and copper, leading to fouling and premature failure. The pH of the regenerant brine should be no higher than 7.2. Citric acid can be added to the brine tank to lower the pH of the brine solution (about one-half cup per bag of salt). KCl brines are generally just below pH 7. When installing a softener with all new copper plumbing, or in a new property with chloramine in the feed water, you WILL have a copper problem. In addition, chloramine is generally found at higher levels than traditionally used for chlorine. Rapid resin degradation will be a problem.
Effects of resin aging
We2 were able to simulate resin oxidation in the lab using a strong peroxide solution catalyzed with iron. This process unzipped the DVB cross-link in resin and swelling from moisture gain and other properties could be determined. Increased moisture could be equated to age and swelling (see Table 3). Note that total dry capacity didn’t really change because the reactive group on softening resin is very strong and resistant to degradation. Wet capacity, traditionally measured in meq/ml or Kgr/cu ft (this is the volumetric capacity) fell off over 40 percent in the equivalent of 13 years. Capacity loss was due to swelling, which means there are fewer reactive sites in a given volume. Resin density dropped as well. It was easier for smaller beads and fines to be lifted and washed out of the column. Whole beads will disappear as well, if the upper distributor is not screened. Also note that the aged resin underwent much more severe swelling when changing form from Ca to Na (>10 percent). That placed an additional stress on the resin, which hastened its physical break down.
Table 3. Effects of aging on the physical properties of resin.
I have often told customers that as long as cation resin is still there, it acts as a softener. But to what level of performance? Again, with a little assistance from the lab, the effects of aging on the properties of softening resin were measured and compared to that of new resin (see Table 4).
Lab regenerations were with the equivalent of 6 lbs./cu. ft. (100 gm/l) of NaCl. Note that the capacity dropped from 19.4 Kgr/cu. ft. to 15.6, a drop of 20 percent ( if all the resin stayed in the vessel)! Referring to Table 3, the volumetric drop in capacity for this resin went from 1.94 meq/ml to1.13 meq/ml, equaling a loss of just over 40 percent. During the service run, leakage increased to 17.3 ppm as CaCO3, an increase of nearly 30 percent. Individual bead strength deteriorated by 76 percent due to oxidation and the resulting pressure drop nearly doubled.
Table 4. Effects of aging on the performance of resin
Predicting the useful life of resin
Is resin life predictable? Yes, it is. Since cross-linking directly affects the moisture holding properties of resin, determining the percent of moisture gives an insight as to the effective cross-linking level of the resin. From this data, we can estimate the current performance characteristics and determine the useful life remaining (see Figure 6). To predict resin life at the outset, divide the number 10 by the ppm of chlorine to determine the expected years of service for an 8-percent resin. At 0.5 ppm, this is 20 years. At 2 ppm, it is only five years. A 10-percent resin will give double the life and a 15 percent will double that again. (After living in the same location for over 40 years, in which a new softener was purchased upon moving in, I have re-bed the system twice with an average life of 12 years on an 8 percent resin. Each time, I had the lab do a moisture determination on the spent resin and it was between 65 and 68 percent, which falls right on the curve in Figure 6.)
Figure 6. Useful resin life versus effective cross-linking
The functionality of a cation softening resin is very stable and does not deteriorate appreciably with age. Resin oxidation, however, will cause the resin to de-crosslink and swell, which results in a lower capacity per unit volume of resin. In addition, oxidized resin is weaker both physically and chemically, which requires more frequent regeneration. This means more salt down the drain and more water consumed in regeneration. A 25-percent loss in operating capacity could mean using an extra 200 pounds (90.7 kilos) of salt per year along with an extra 25 regeneration cycles and up to 2,000 extra gallons (267 BVs) of water down the drain. Given the marginal performance of older resins, one can easily justify resin replacement after about 10 years. Commercial and industrial systems that regenerate as often as twice a day can justify replacement in eight years or less.
I have often referred to The Periodic Table of the Elements as a teaching aid to explain the behavior of certain chemical elements and ions. Water chemistry is almost entirely inorganic chemistry in that the dissolved ions in water are referred to as mineral. They occur naturally and are derived from stone (natural); they were never living. Organic chemistry, on the other hand, is mostly derived from living or once living things and is a relatively modern branch of the science. Organic chemistry involves and almost infinite combination of carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), joined by a variety of chemical bonds, which lend an endless compliment of properties to the current and yet-to-be-discovered world of organic chemistry.
Being a more recent discovery, organic compounds tend to be more structured in their nomenclature with almost none named in honor of anything or anybody. Earlier discoveries might be named for their source or their appearance but the oft-confusing names are simply a strung-together combination of their building blocks.
Styrene is derived from petroleum and is a combination of benzene (C6H6) and ethylene (C2H4), which is catalytically dehydrated to form the styrene. Styrene is so called because it was found to occur naturally in the sap of the styrax tree. The chemical dehydration (removal of hydrogen) of the ethylene group produces a special reactive group we know as a vinyl group, which is common to styrene, acrylics, polyethylene, polypropylene and vinyl (polyvinyl chloride or PVC). Styrene is also known by its formal name as vinyl benzene. The vinyl group undergoes a nifty coupling reaction during polymerization (Figure 4). This vinyl reaction is the basis for many modern plastics.
- Michaud, C. F., Defining Ion Exchange Capacity, WC&P, March, 2011
- Fries, W., Unpublished research data on resin oxidation, Purolite Co, Philadelphia, PA, 1999.
- Michaud, C. F., Oxidation: Oxidizers, Age and Softening Resins, WC&P, August, 2000
- Michaud, C. F., Brodie, D., Ion Exchange Resin-Methods of Degradation, WC&P, January, 1990.
C. F. ’Chubb’ Michaud is the Technical Director and CEO of Systematix Company of Buena Park, CA, which he founded in 1982. He has served as chair of several sections, committees and task forces with WQA, is a Past Director and Governor of WQA and currently serves on the PWQA Board, chairing the Technical and Education Committees. Michaud is a past recipient of the WQA Award of Merit, PWQA Robert Gans Award and a member of the PWQA Hall of Fame. He can be reached at (714) 522-5453 or via email at AskChubb@aol.com