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​High speed dispersion mixer for inks and coatings industry
- Aug 06, 2018 -

This article outlines the different types of High speed dispersion mixer used in inks and coatings production processes. These mixers can perform a series of processing tasks that sometimes overlap. Discussed in this article is how to choose the right mixer.

With thermochromism, fluorescence, color change, rust resistance or other new properties, modern coatings and inks may contain new raw materials with unique combinations, but fundamentally they are still formulations that rely on good mixing. Different degrees of agitation mixing and dispersion of a product affect its color, gloss, electrical conductivity, stability, adhesion, cure rate, weatherability and other performance characteristics. Therefore, there is a need for good commercial awareness to optimize the mixing process. Even traditional products and sophisticated processes benefit from a re-strategic assessment of agitation operations.

A variety of dispersing tools are used in the production of paints and inks. Some of these technologies are relatively new solutions to ancient process problems. The choice of mixer is based on a number of factors including viscosity curve, shear requirements, order of addition and throughput.

High speed dispersion

High-speed dispersers machine, formerly known as high-speed dissolvers, are standard heavy-duty machines used in the ink and coatings industries. It is an economical and relatively simple mixing equipment component that primarily functions to add powder to the liquid, separating the loose agglomerates to obtain an acceptable level of dispersion, and then grinding.

Blade tips operate at speeds of up to approximately 5,000 ft/min, and high-speed disperser-open disc blades create intense turbulence in low-viscosity materials. It also produces a unique vortex so that dry materials can be wetted quickly after they are added. The dispersing blades can be centered or off center, depending on the depth of the vortex (the eccentric vanes produce a smaller vortex that reduces air retention). As the material thickens or increases in volume, the blade speed is adjusted to maintain the vortex and velocity of the material turnover. Some of the basic guidelines that are commonly followed during the selection and operation of high-speed dispersers are:

• Normal working viscosity range: like water to about 50,000 centipoise (CP);

• Motor specifications: 1 horsepower (HP) per 10 gallons of product;

• Dispersing blade diameter: about 1/3 of the diameter of the container;

• Dispersing blade position: 0.5 blade diameter from the bottom of the vessel - 1.5 blade diameters below the liquid surface;

• Full volume of the mixing vessel: at least 30% larger than the batch to provide sufficient rise height to meet the rise in liquid level during mixing.

While many manufacturers still rely on high-speed dispersers for pre-mixing operations, many manufacturers have begun to combine more efficient agitators, the function of which is no longer simply to wet the powder. It is based on the fact that if the premixer is able to disperse the agglomerates as close as possible to the required specifications - in other words, to mimic the early stages of grinding - it can reduce the number of grindings of the mill, even allowing for smaller Grinding media to grind finer solids. This provides an opportunity to significantly increase efficiency and reduce cycle times. Some alternatives to these mixers include a new generation of high shear mixers and ultra high shear mixers.

High shear mixer

High shear mixers are rotor/stator mixers for more challenging solid-liquid dispersions and emulsions that cannot be adequately processed. A typical feature of this type of mixing device is that the four-bladed rotor rotates at high speed within the stationary stator. When the blade rotates, the material is continuously drawn into the agitating head, and the gas is discharged at a high speed through the opening portion of the stator. The resulting hydraulic shear energy promotes rapid homogenization, depolymerization and emulsification. Typical rotor tip speeds are 3000 to 4000 feet per minute.

Due to the limitation of the stator, the mixer provides a higher shear rate, but has less pumping capacity than an open disc type serrated type vane disperser. For the same reason, the viscosity limit is also low - about 10,000 to 20,000 centipoise.

Another difference is that the rotor/stator mixer has both a structure that can be used for mass production and a structure that can be used for on-line (continuous) production. Online high shear mixers behave like centrifugal pumps. It does not automatically initiate self-priming and requires static pressure (gravity feed) or positive pressure (pump feed) to add material to the stirred tank. In most cases, when the agitator is typically located on the ground or platform below the level of the recirculation tank, the product can be delivered to the rotor/stator assembly by gravity. Standard in-line mixers are easy to move mobile products, even with a viscosity of up to about 10,000 centipoise, depending on the shear thinning characteristics of the formulation. With an auxiliary pump, medium viscosity coatings, premixes and intermediates can be processed in a high shear mixer. For large stirred tanks (>500 gallons), online structures tend to be more practical than batch rotor/stator mixers or other high speed mixers that require high power motor drive and relatively large blades to create sufficient circulation to enter from the top. On the other hand, the online high shear mixer allows the product to circulate through a 2,500 gallon stirred tank as easily as in a 25 gallon stirred tank. In fact, with the right piping, an in-line mixer can serve multiple batch tanks of different sizes at the same time. A typical installation method is to use a simple valve to flow directly to the downstream finished area or to switch from one raw material source to another instantaneously.

When it is necessary to quickly add large quantities of powder to a liquid, or when a solid that is difficult to disperse takes a long time to fully be added, it is necessary to consider a high shear mixer with a built-in device that can add powder under the liquid surface. . The ejector-based powder injection system, while effectively removing floating solids, provides more precise control during agitation and mixing, but is often subject to clogging and intensive equipment maintenance issues. In addition, these systems require experienced operators to handle the difficult tasks often encountered, namely the balance of performance of three separate units in series: injectors, pumps and agitators. High shear mixers with a powder-injected monolithic construction are more reliable and easier to handle.

High shear mixer with SLIM technology

The Rose High Shear Mixer with Solid/Liquid Injection Manifold (SLIM) technology provides a way to inject powder under the surface, which is a more user-friendly and more versatile system. Easy to maintain mixer. A key advantage of the SLIM structure is that it does not require a centrifugal pump or ejector to create a pumping operation for powder injection.

SLIM has a unique rotor/stator that produces a powerful vacuum to pump powder and directly spray powder into the high shear zone of the mixer. Since solids and liquids are combined in an intensive mixing zone, although agglomeration and "fisheye" are not completely eliminated, they can be greatly reduced. In the SLIM process, the powder can be transported to the batch tank in a way that reduces the “dust” (releases the gas particles into the mixing zone), which makes cleaning easier and faster. This can be done by using a hose and a rod-shaped attachment – the flexible hose is connected to the solid inlet port and the stainless steel rod is immersed in a bulk bag or container at the other end for easy inhalation of light powder without much Dust environment. Solids with less problems can be added to the hopper and quickly inhaled by gravity.

SLIM technology is used in both batch and online configurations, making it easy to retrofit most existing processes. It typically handles solids up to 70% depending on the application and operates over a wide range of viscosities: from water to viscosity up to 10,000 centipoise during powder injection.

Multi-axle mixer

A multi-axle mixer with double or more independently driven agitators in series is a powerful system for ultra-high shear mixing and laminar flow over a wide range of viscosities: viscosity range from Watery to hundreds of thousands of centipoise. 

The simplest construction is a double-axle mixer featuring a low speed anchor and high speed serrated dispersing blades. The wings of the anchor agitator typically include an adjustable doctor blade to wipe the bottom and side walls of the container. In addition to increasing production, more stringent temperature control is possible.

Another typical configuration is a three-axis mixer that includes another rotor/stator assembly. The structure is very popular with formulators, and the final particle size distribution is critical. As discussed earlier, the use of serrated blades to add powder to a liquid results in an acceptable level of dispersion, but with stronger shearing machinery such as a stator/rotor agitator, a finer, more uniform particle size is typically obtained. distributed.

SLIM technology is an option for Rose's three-axis mixers with a working capacity of 4 gallons or more. The liquid base must be below 10,000 centipoise during powder injection, but the viscosity of the product may continue to rise after all solids are added. The later stages of the mixing cycle may only rely on an anchor stirrer, and if the product flows through the rotor/stator assembly, the chance of dispersion is limited.

In addition to improvements in viscosity and heat transfer, multi-axis mixers have advantages over single-axis units in that they are closed systems that provide the advantage of vacuum mixing. Treatment under vacuum eliminates unwanted air voids generated during agitation under atmospheric conditions, but it is also important to help develop certain formulations into higher densities with better performance. These properties are improved. The shearing effect and the result of contact with different parts.

Planet mixer

Specialty coatings and inks have very high viscosity peaks (more than 1 million centipoise), preferably better with planetary dispersers and dual planetary mixers. These machines have two or more blades that can rotate about their respective axes and also rotate around the mixing vessel. In other words, all the mixers continue to advance into the batch and will always be in contact with the new product.

For very challenging formulations, a dual planetary disperser can be used. The disperser consists of two dispersing shafts (each with two serrated blades) and two planetary agitators.

On the other hand, the conventional double planetary mixer (DPM) is an ideal mixer for the melting and kneading of semi-solid materials or high-viscosity slurries. Although not considered a high-speed mixer, the DPM is equipped with two identical agitators that increase the level of shear when the batch reaches a fairly high viscosity. In this very robust machine, a common processing technique is high viscosity mixing to ensure a satisfactory solid dispersion (from 2 million centipoise to as high as about 6 million centipoise), followed by a paint adjustment step until The loop ends.

in conclusion

For ink and coatings manufacturers, there are a variety of High speed dispersion to choose from. Knowing the difference between them is a very important step when choosing the right blender for your process.