Organically modified bentonite clays, or organoclays, have been used commercially as thickeners and flow-control additives in solvent-based systems for more than 40 years.
Organoclays also enable a formulator to build unique rheological characteristics into myriad products to meet stringent consumer preferences. Although demand for organoclay-based thixotropes has decreased somewhat in recent years, the clays are still prevalent in nail lacquer, antiperspirant, color-cosmetic and skin-care products ," contributing critical thixotropic flow properties. (Thixotropes set on standing, thin out when stirred and then set again after the movement is stopped.)
Organoclays are recognized widely for their ability to generate robust gels efficiently, suspend pigments and active ingredients and stabilize emulsions ," then thin down with increased shear when sprayed, brushed or physically applied to the human body. Every year approximately 1,000 metric tons (MTs) of organoclay powders make their way into the global personal-care and cosmetics markets.
Organoclays also find use in oil fields as additives in fluids for well drilling and enhanced oil recovery. More recently, organoclays have been added to animal feeds to remove harmful mycotoxins to accelerate livestock growth and improve the general health of the animals.
Steam Injection Perfected
Modifications to the steam-injection manifold design added flex joints at key points and eliminated sharp bends in the piping to avoid stress fractures from excessive vibration.
When performance counts
The level of performance required of an organoclay depends on the application. A modest-performing organoclay, for example, often is used in the manufacture of traffic paints and drilling fluids. In traffic paints, organoclays function primarily as anti-settling agents. The paints must maintain pigment suspension during storage. Other characteristics such as ease of dispersion, flow properties and gloss effects are of less importance.
On the other hand, high-performance organoclays are needed for end-use products such as printing ink, industrial coatings, architectural coatings and cosmetics. These applications require more from a rheological additive than simple thickening. They require high purity, consistency, efficiency and/or ease of dispersion.
Special processing techniques are needed to achieve these desired traits and to create a high-performance organoclay product. Although higher in cost on a per-pound basis, a high-performance organoclay reduces the amount of additives needed in many applications and, in turn, can reduce manufacturing expenses.
In many cases, the key to producing organoclays with performance characteristics suitable for these higher-quality products is the introduction of steam injection in the manufacturing process.
Organoclays result from the reaction of bentonite clay with a quaternary ammonium salt. Bentonite is a clay mineral with a platelet-like structure with exchangeable cations on the platelets' top and bottom surfaces. These platelets are arranged in stacks, and bentonite clay exists in its natural state as agglomerates of many of these stacks. To facilitate the reaction of the fatty quaternary amines with bentonite clay, it is necessary not only to break apart these agglomerates, but also to expose as many individual clay platelets as possible prior to reaction.
The cations present in bentonite clay are primarily sodium, potassium, calcium or magnesium. The clay's dispersibility and reactivity are dependent on the most prevalent cation.
Sodium bentonite ," with sodium as the most prevalent cation ," is used to produce high-quality organoclays. It has the most swelling capacity, is most easily dispersed and is most conducive to ion exchange.
Preparation before the steam
High-quality organoclays are produced in an aqueous environment. The first step in the manufacturing process is preparation of a bentonite-clay water slurry. The clay is mixed in water at a concentration of 4 percent to 6 percent in the presence of a phosphate dispersant.
Raw bentonite clay, as received from the mine, might contain anywhere from 5 percent to 60 percent non-clay impurities, depending on the individual deposit and the mining techniques employed. Removal of these non-clay impurities is essential and can be accomplished through sedimentation, hydrocyclone treatment or high-speed centrifugation.
Following the cleaning step, sometimes referred to as beneficiation, the clay is ready to react with the fatty quaternary amine. Once the bentonite clay and quaternary amine are combined, an ion-exchange reaction takes place almost immediately, and the organophilic clay coagulates. This coagulated organoclay then can be separated from the water, dried and processed to a fine powder.
A patent improvement
Sad-Chemie has been producing organoclays since the early 1970s. From the beginning, the company used steam injection of the bentonite clay slurry to improve organoclay performance.
Edwin T. Clocker later confirmed the concept in a 1976 patent. The patent described the use of steam injection to subject clay slurries to high temperatures (300 Degrees F to 470 Degrees F) and pressures (100 pounds per square inch gauge [psig] to 500 psig). These slurries then are flashed to atmospheric pressure to "explode" the agglomerated clay particles. Clocker showed that these highly dispersed clay slurries then could be combined with organic cations to generate superior organoclays.
The methods described by the Clocker patent, however, are not cost-effective because they specify high temperatures and pressures. These requirements result in a steam/clay ratio significantly higher than the ratio required by the Sad-Chemie process.
Over the years, Sad-Chemie has continued to optimize the efficiency of the steam-injection process. Steam-injection assemblies were replaced with steam ejectors that not only treat the clay, but also serve to pump the clay slurry between tanks. Clay solids were increased to reduce the steam cost per pound of product. The solids increase was initiated after research scientists at the company demonstrated that benefits derived from steam ejection are independent, within a moderate range, of solids concentration.
Sad-Chemie's first steam ejectors often were disabled by stress fractures caused by excessive vibration. The company eliminated these by improving steam-ejector manifold design and the start-up procedure. Modifications to the manifold design included flex joints at key points and the elimination of sharp bends in the piping.
Sad-Chemie improved the start-up by establishing a method to minimize pressure at the ejector-system discharge end. The rate of steam flow is increased gradually at start-up with the manifold discharge valves completely open. This procedure prevents shock effects as the hot steam first comes into contact with the relatively cold ejectors and associated piping. After the ejectors and associated piping reach a steady temperature, the steam flow then is increased quickly to the desired flow rate.
Steam delaminates the clay, increasing the number of clay platelets available for cation exchange during the coagulation step. Organoclay performance is enhanced as the steam increases the clay hydration, making the clay more dispersible and reactive. Increased clay hydration can result from cavitation effects as high-energy molecular collisions and rapid contraction-and-expansion transitions take place within the steam ejectors.
Organoclay performance can be enhanced significantly through the use of a single steam-ejection stage. A recommended steam-to-clay ratio is approximately 1.5. Higher-performance products can be created through the use of two steaming stages. Many people believe higher-performance products also benefit from a holding time of about 40 minutes to 80 minutes between the two stages.
Temperature greatly affects the performance of steam ejectors. If the slurry temperature is too high, vacuum suction will decrease significantly, and severe process vibration could result. If the slurry temperature is too low, the steam-to-clay ratio will not be high enough to bring about beneficial effects.
Cleaned and Steamed
Techniques such as steam injection optimize the dispersion of the clay platelets to enhance reaction efficiency.
What will it cost?
A typical steam ejection system used at Sad-Chemie is equipped with three ejectors made from stainless steel, six flex elements (two per ejector to allow for proper movement), a vortex meter for measuring steam flow rate, various valves and a flanged header. The header is made with smoothly curved piping.
This design helps to minimize pressure drop and system vibration. Construction costs for a well-designed steam ejector system typically range from $7,500 to $10,000, depending on specific process requirements.
Drucker is plant manager and Mattingly is senior research chemist for Sad-Chemie Inc.'s Performance Additives Div. in Louisville, Ky. Contact them at [email protected] and [email protected], respectively.