High efficient dispersion in lacquer paint production

High efficient dispersion in lacquer and paint production

5:21 AM, 16th April 2018
High efficient dispersion in lacquer and paint production
ystral Conti-TDS.

Dissolvers were invented in the middle of the last century and have survived in lacquer and paint production until today. They have too many disadvantages, limitations and negative effects on the final product.

A new and advanced dispersion technology has started to displace traditional High Speed Dissolvers in Lacquer and paint production. This new technology enables enor­mous reductions in production costs and process times. Additional effects are a higher degree of dispersion, improved quality and raw material savings. Finally the energy consumption is reduced to about 30 percent compared to the old dissolver technology.

By Dr Ing Hans-Joachim Jacob

Dispersion is the most important process in lacquer and paint production and has the highest influence on the quality of the final product. The better the dispersion the higher is the efficiency of the dispersed ingredients in the final coating. Traditional high speed dissolvers are simple free rotating discs with bent teeth at their circumference. Due to the wide distance between the rotating dissolver disc and the static wall of the vessel the shear gradient is very low: just about 50 reciprocal seconds.

To create shear forces with low shear rate a dissolver needs a high viscous liquid or at least a shear thickening or dilatant rheology. Lacquers and paints have exactly the opposite rheology: they are low viscous, shear thinning and thixotropic. A dissolver cannot create significant shear in a low viscous lacquer or paint. To shear with a dissolver the viscosity has to be increased extremely. For that reason the dissolver process starts with just a part of the liquids but all the thickeners to maximize the viscosity.

This is completely paradox since the most important ingredients which have to be dispersed are very fine powders (pigments, silica, fillers and extenders). Making a liquid extra high viscous before adding finest powders is absurd. A high viscous liquid will not get into smallest capillaries and cannot wet fine particles as fast and complete as a low viscous liquid. The results are stable agglomerates, quality issues, extra-long dispersing and milling times, heat generation and a waste of energy.

In other industries dissolvers have disappeared since years. Now the dispersion technology in lacquer and paint production has started to change as well – worldwide.

Conti-TDS Technology

The new Technology is based on an inline dispersing machine which is able to induct and disperse powders into liquids under high speed and vacuum. The machine is installed outside the process tank and recirculates the liquid with high speed. Additional pumps are not required.

Picture 1: Conti-TDS with process tank and big-bag station

The core of the machine is the dispersing chamber equipped with high shear tools. Exactly in the high-shear dispersing zone the machine creates a very strong vacuum. With this vacuum it inducts powder directly from bags, bag tipping stations, hoppers, drums, containers and big bags into the liquid. No vacuum tank or vacuum pump is required which pulls uncontrolled amounts of powder or solvent vapor out of the liquid into filters or exhaust systems like in a vacuum dissolver. The induction is dust-free because 100% of all the powder goes into the liquid. Dust pollution of the working area is completely avoided.

The powder is inducted into the recirculating liquid loop and arrives in the vessel as a liquid dispersion. No dust appears above the liquid surface; no partially wetted agglomerates are built up above the liquid surface, a common problem with dissolvers. The infamous powder crusts that usually build up inside a dissolver tank above the surface and then fall into the liquid and reduce product quality never occur when using this system.

Picture 2: ystral Conti-TDS

The mixing and dispersing chamber of the Conti-TDS has three connections: a liquid inlet (blue arrow), a powder inlet (orange) and a product outlet (purple, Pictures 2 and 3).

Liquid and powder are introduced into the machine from opposite sides and leave the chamber in the middle. That means the machine pulls liquid and powder from two different directions, mixes and disperses them, and pumps the final dispersion back to the vessel.

Liquid and powder cannot get in contact with each other before they reach the dispersing zone. This way uncontrolled agglomerations are avoided completely.

The dispersion of the powder into the liquid is always done under equal, constant and controlled conditions, independent from the speed of the operator. The product quality is absolutely reproducible, even for different batch sizes.

Picture 3: Wetting and Dispersing Zone

Constant quality has been one of the biggest problems with dissolvers. Particle size, Color strength, viscosity, air content, gloss and film formation are influenced by the way how the operator has added the powder. The quality changed from batch to batch, from operator to operator. Slower powder addition caused longer process times, higher temperatures, lower viscosities. Faster addition caused more agglomerates and further post processing time. It means longer shear, higher temperatures and lower viscosity as well. Dissolver batches always had to be quality checked – adjusted – checked and adjusted again. These steps took time and blocked production capacity during waiting for the QC results.

The Conti-TDS produces absolutely constant qualities. Because of the much faster process and the not required post-adjustment. One Conti-TDS can always replace a number of dissolvers.

The high vacuum inside the dispersing zone has two important reasons:

1. to induct the powder directly into the liquid

2. to wet and disperse every single particle.

Every powder contains air - lots of air. The powder particles are touching each other, but there is always air between the particles, even inside their capillaries, even inside agglomerates. The amount of air in the powder is always underestimated. A heavy powder like titanium dioxide contains at least 75 volume-% air. A very light powder like fumed silica contains up to 98 percent air.

Air is compressible. It compresses under pressure but expands under vacuum. Under the high vacuum inside the dispersing zone it expands up to 20 times compared to its normal volume. All the air between the single powder particles expands. The distance between the single powder particles increases in the same range. Particles that have touched each other initially become separated during their passage through the dispersing zone.

To reach this fluidization and separation no additional air is required, just the existing air under the influence of vacuum. Because of the wide distances between the particles in the dispersing zone, liquid easily gets around them. The vacuum allows the liquid to completely wet each particle from all directions.

The dispersion takes place in the relatively small rotor-stator-rotor zone under maximum vacuum, turbulence and shear. During the shear zone passage the effective liquid surface is increased some million times. This way the available liquid surface during powder wetting is larger than the powder surface. This is the ideal precondition for complete 100 percent wetting and immediate dispersion. Every single particle is wetted completely before it leaves the shear zone.

The air, which came in together with the powder, is separated from the dispersion due to the centrifugal forces outside the dispersing zone. It leaves the product in the process tank as big bubbles.

Picture 4: Particle size distribution titanium dioxide after induction, 30, 60 and 120 seconds dispersion

Picture 4 shows a typical particle size distribution for the induction and dispersion of titanium dioxide. Immediately after induction the result is better than reference samples produced with Dissolver and Mill. The medium particle size is already 0.46 µm. 99,9 percent of the particles are below 2 µm. Not any oversized particle is visible. The distribution is very narrow.

After 120 sec additional dispersion the medium particle size goes down to 0.38 µm. 100 percent are below 2µm and 98 percent are below 1µm.

Titanium dioxide is the most important pigment in lacquer, paint and ink production. It covers more than 59 percent of the global pigment demand. The prices for titanium dioxide are rising since 2009. Companies which are using the Conti-TDS for titanium dioxide are reporting about enormous savings. They save time, energy and raw material because of the better dispersion.

Picture 5: Skidded unit with one Conti-TDS-5 between two process tanks

Installation and Operation

The machine is installed in a recirculation loop with a process vessel. This way it operates independently from vessel size and filling level. To double the capacity with minimum additional costs one machine is often installed between two vessels (picture 5). You need just one machine, one control system and one powder addition point to double the production output.

During production in one vessel the other vessel is discharged from the last batch and filled with the liquid for the next batch already. In the average 50 seconds after finishing the powder induction the required particle size is reached and the product is ready to transfer. When the batch is ready to discharge the machine automatically switches over to the second tank and starts the process there. This way it is possible to produce up to 20.000 kg finished paint per hour. This way a continuous and uninterrupted production is possible.

The system in picture 5 shows a skidded unit with two tanks of about 2.200 liters volume. Bigger systems with up to 2 x 25.000 liters or more are available as well, but often they are not faster. Limitation is the speed of the powder handling. High effective powder handling and BigBag discharge systems are delivered in combination with the Conti-TDS. If the powder comes mainly in big bags the system on picture 5 has already an optimum size.

In areas with high flexibility and a wide range of batch sizes and formulations - like in the production of wood coatings or customer-specific industrial coatings - the two vessels system has a totally different target. In this case the two vessels are made in different sizes to cover a wide range of batch sizes. With such a “Two-Size Double-Tank System” it is possible to produce batches in a size range from 1 : 30 - for example batches from 100 to 3000 liters in the same system.

Picture 6: Two-Size Double-Tank System

The process tanks have sample points, but it is not necessary to check every single batch; the quality is constant. The foot print of such a skid is very small: just 2 x 5 meters. Today flexibility is getting more and more important. With such a system it’s possible to produce very effectively different formulations and variable batch sizes.

All Conti-TDS systems have an integrated and paint industry approved automatic cleaning unit. To keep the process system always clean every batch starts with a cleaning step. A part of the process liquid is added through the cleaning heads and keeps the tank and all relevant parts clean – even if the following batch is the same product. Additional cleaning with extra liquid is possible if there is a product change between incompatible materials or different colours.

Inside the process tanks there are high effective jet stream mixers installed which are moving and turning the liquid vertically. These mixers provide an instantaneous homogeneity. This is important because of the high speed powder addition. Simple agitators with mainly horizontal rotation are not able to mix the liquid homogenously and fast enough.

Economic Advantages

Additional to the technical advantages there are obvious economic advantages with this new technology as well. The production costs of paint are typically reduced to below 10 percent compared to the dissolver process. Additionally, a significant reduction in raw materials is possible because of better dispersion or higher color strength. A reduction of the production times to below 20 percent is typical. Consequently, energy savings down below 35 percent are reported as well. Generally the Conti-TDS guarantees higher product quality, much higher flexibility in the production and a much higher level of safety.

Author:  Dr - Ing. Hans-Joachim Jacob, Process - and Application Engineering at ystral gmbh.

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