Physical properties of ion exchange resins

Commonly used gel-type Ion exchange resins appear as transparent or translucent spheres, while macroporous resins appear as milky white or opaque spheres. Colors range from yellow, white, and reddish brown. High-quality resins have a high sphericity, are crack-free, have a uniform color, and are free of impurities.

Gel-type Ion exchange resins Particle size (in mm) is generally 0.3-1.2 mm (equivalent to 50-16 mesh), with an effective particle size (d10) of 0.36-0.61 mm and a uniformity coefficient (K) of 1.22-1.66. The effective particle size is the sieve aperture diameter through which 10% of the resin particles pass and 90% are retained. The uniformity coefficient is the ratio of the sieve aperture diameters (d60) to (d90) through which 60% of the particles pass, i.e., K = d60/d90. The uniformity coefficient is generally greater than 1; the closer it is to 1, the more uniform the particle size composition. Resin particle size significantly affects exchange rate, water flow resistance, and backwashing. Large particle size results in slower exchange rates and lower exchange capacity; small particle size results in greater water flow resistance; uneven particle size, with small particles trapped in the pores of larger particles, increases water flow resistance and hinders backwashing. Therefore, the particle size should be appropriate and evenly distributed.

Density, unit: g/cm³. Resin density is generally expressed as wet apparent density (bulk density) in a hydrated state and wet true density.

① Wet apparent density, unit: g/cm³. Wet apparent density is the mass of wet resin packed per unit volume and is used to calculate the amount of resin required in the exchange container. Wet apparent density = wet resin mass / wet resin bulk volume. The wet apparent density of various commercial resins is approximately 0.6-0.86 g/cm³. |

② Wet true density, unit: g/cm³. Wet true density is the density of the resin particles after they have absorbed water. Wet true density = wet resin mass / wet resin particle volume. Note that the volume of the resin particles in the above formula does not include the volume of the pores between the particles. Wet true density is generally 1.04-1.3 g/cm³. Typically, it is 1.3 g/cm³ for cation exchange resins and 1.10 g/cm³ for anion resins. Wet true density is used to determine the backwash intensity of the resin bed. Furthermore, in a mixed resin bed, wet true density is also related to the stratification of the resin after backwashing. Anion exchange resins are light and will be in the upper layer after backwashing, while cation exchange resins are heavy and will be in the lower layer after backwashing. During use, the density of the resin decreases slightly due to the shedding of groups and the breakage of chains within the resin backbone.

|

Gel-type Ion exchange resins Moisture content (unit: %). Moisture content refers to the mass fraction of water contained in the wet resin (after it has fully absorbed and expanded in water) and is generally around 50%. Moisture content is primarily determined by the resin's degree of crosslinking, the type and number of active groups, and other factors. The lower the degree of crosslinking, the larger the pores in the resin, and the higher the moisture content.

Swelling (unit: %). The change in resin volume caused by changes in conditions such as water absorption or transformation is called swelling. Swelling occurs when ions released by active groups upon contact with water hydrate to form hydrated ions, thereby expanding the cross-linked mesh. The volume increase of dry resin after contact with a solvent is called the absolute swelling degree, while the volume change of wet resin when it switches from one ionic form to another is called the relative swelling degree, also known as the transition swelling rate. Absolute swelling degree = (volume before swelling - volume after swelling) / volume before swelling. Relative swelling degree (or transition swelling rate) = (volume before transition - volume after transition) / volume before transition. The lower the resin's cross-linking degree, the more easily the active groups ionize, the greater the exchange capacity, and the greater the swelling degree. The larger the hydration radius of the exchangeable ions on the resin and the lower the electrolyte concentration in water, the greater the Gel-type Ion exchange resinss swelling degree. The order of swelling for strongly acidic cation resins and strongly basic anion resins in different ionic forms is: cations: H+ > Na+ > NH4+ > K+ > Ag+; anions: OH-> HCO3- ≈ CO32-> SO42-> Cl-. The swelling rate for styrene-based cation exchange resins from RNa to RH (expressed as RNa→RH) is approximately 5%-10%, while the swelling rate for styrene-based anion exchange resins from RCI to ROH is approximately 10%-20%. Acrylic-based weakly acidic cation exchange resins have a very high swelling rate, approximately 60%-70% for RweakH→RweakNa. Because all resins swell to a certain degree, space must be reserved when designing the exchange container. Resins with high transformation expansion rates are susceptible to aging due to repeated expansion and contraction during use.

Porosity and Specific Surface Area: Currently used D001x14-20 series Gel-type Ion exchange resins have an average pore diameter of 10-15.4 nm, a porosity (the pore volume per unit resin particle) of 0.09-0.21 mL/g, and a specific surface area of 16-36.4 m²/g (dry). Gel-type resins have a specific surface area of less than 1 m²/g.

Degree of crosslinking, measured in %, refers to the proportion of crosslinker used in the resin's manufacture. For example, styrene-based resins are polymerized using styrene as a monomer and divinylbenzene as a crosslinker. The degree of crosslinking refers to the mass fraction of divinylbenzene in the resin. The degree of crosslinking affects many resin properties. A higher degree of crosslinking increases the mechanical strength of the resin and reduces its resistance to swelling in water. Changes in the degree of crosslinking can alter properties such as the exchange capacity, water content, swelling capacity, and mechanical strength of the resin. The cross-linking degree of ion exchange resin used for water treatment should be 7% to 10%. At this time, the average pore size in the resin grid is 2 to 4 mm. Mechanical strength Mechanical strength reflects the ability of the resin to maintain the integrity of the particles. The resin will break when it is subjected to impact, collision, friction and swelling during use. Therefore, the resin should have sufficient strength and the annual loss of the resin is required to be less than 3% to 7%. Heat resistance Various resins have a certain operating temperature range. If the upper limit is exceeded, the resin will undergo thermal decomposition. As low as 0°C, the water in the resin will freeze, causing the particles to break. The storage and use temperature of the resin is usually controlled to be 5 to 40°C. (10) Conductivity Dry resin is not conductive, while wet resin can conduct electricity due to the dissociated ions.