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دیسپرس پیگمنت:انتخاب صحیح عامل دیسپرس کننده و تر کننده
Bright, vivid and durable colors are one of the most important part of the coatings’ aesthetic definition. Formulators are using pigments in order to bring this color in the paint. But keeping stable these solid particles in a liquid is a complex mechanism which consists in wetting, dispersing and stabilizing the solid particles in the system. This guide will help you to understand this mechanism in order to select the best product for your formulation. Selecting the right wetting and dispersing agent can enhance your paint’s quality, but also lower your production cost !
Dispersion Process and Dispersing Agents
The dispersion process is crucial. Indeed, it will influence many parameters such as the appearance (color strength, transparency, gloss...) but also the paint physical properties (rheology, stability flocculation…). Keeping problems at this stage can dramatically reduce the final paint quality. Understanding this process is a great help to prevent many coatings defects. Mastered Wetting, Dispersing and Stabilizing are the keys for high quality coatings.
The 3 stages of the pigment dispersion
The formulator introduces the solid pigments into the liquid phase. In this phase, air and moisture entrapped at the pigment surface are displaced to the grinding medium liquid phase. The pigment / air interfaces become pigment / liquid interface. To proceed, the liquid needs to wet the pigment surface (please visit the “Wetting Agent Guide” for more information on the wetting theory).
Wetting & dispersing additives have an influence on this stage, by modifying the surface tension at the interface they can help and accelerate the pigment wetting.
The grinding equipment (dissolvers, grinding mills, sand mills…) introduce mechanical energy in the system. This shear forces energy breaks and separates the pigment agglomerates in smaller particles. This new created surface is wetted by the grinding medium liquid. The dispersing energy (related to the dispersing time) is an extremely important parameter to reach an optimal separation of all pigment agglomerates to the ideal primary pigment particles.
Wetting & dispersing agents lower the forces between the pigment particles and help in crushing the agglomerates in smaller particles.
Naturally the newly obtained small particles have a tendency to re-agglomerate. It is the “flocculation”. Uncontrolled aggregates and flocculates have poor influence on the paint quality and can reduce the paint stability, reduce the color strength and gloss, or change the paint’s rheology.
Wetting & dispersing agents will prevent this unwanted effect and provide a long-term stabilization.
Dispersing Agent Chemistries
In addition to the characteristics of the 3 stages, we must take in consideration the influence of the grinding liquid medium :
The pigment dispersion process results in three important stages : Wetting, Dispersing, Stabilizing. A correct Wetting & Dispersing agent should fulfill the requirement for all these three stages.
There are many wetting agent manufacturers in the coatings market, nevertheless we can classify them as Conventional Wetting and dispersing agents.
Mainly low molecular weight, they are based on polyesters, polyamides, polyglycols and fatty acid chemistry (FAME). They have as general characteristics:
They have an excellent compatibility and excellent water dispersibility, and offers many alternatives to replace the APEO products (alkylphenol ethoxylated). Reducing the surface tension improves the wetting process.
Modified Fatty Acid type
Acting like emulsifier, they are giving excellent results for universal pigment concentrates.
Phosphoric Acid Ester
Polyether and polyester structure, excellent for inorganic material dispersion
Polymeric Wetting and Dispersing Agents
The classification of polymeric types wetting and dispersing agents is based on their anchoring mechanism, their chemical structure (polyacrylic, polyurethane, copolymer…) and their molecular weight. This type is also influenced by the polymer design (linear, branched, star designed) and the polymerization process (controlled polymerization process types offer high performances products but are also more expensive).
As polymer designed, polymeric wetting and dispersing agent can offer very high quality:
Polyacrylic acid based
Usually lower in molecular weight (and also in cost) in comparison with the other structures, they are particularly recommended in waterborne coatings to increase the pigment load of inorganic material. Very nice cost effective product. Ammonium and sodium salt are typical products for latex paints.
Excellent for the millbase viscosity reduction. As a consequence, enhance the pigment load and reduce the dispersing time. The flexibility of this structure (backbone, branched chains, anchoring groups) allows the design of various structures for many solventborne and solvent free systems.
They have similar properties to the polyurethane ones. Higher in molecular weights, they can offer a better compatibility where the polyurethane structure is not ok. Suitable structure for waterborne and solventborne systems.
CPT : Controlled Polymerization Technology / Living Chain Growth
This polymerization process allows the manufacturer to make very fine adjustment on the polymer chain, which is not the case with the classical step-growth process (condensation polymerization is a random process).
Wetting and dispersing agent polymerized with this process are very similar batch to batch, which is not the case of classical condensation where the molecular weight can vary significantly from one batch to the other. Very effective but more expensive products.
Select Wetting and Dispersing Agent for your End use Formulation
Selecting the best wetting and dispersing agent for a system may look complicated first, but many clues can orientate our choice. Then, through a serial of simple lab tests, it will be possible to select the best one.
The selected Wetting & Dispersing agent must be effective at the 3 steps of the dispersion process, or at least not have any unwanted negative effects.
General Comparison between Conventional and Polymeric Type
Wetting and Dispersing Demand
Usually, organic pigments have higher oil absorption than inorganic pigments. This will have a direct consequence on the additive demand, and of course on the formulation cost.
In order to select the best dosage, test should be done using the recommended dosage, then ¼ more and ¼ less, and compare the results.
Dispersing Agent According to End Use
This table summarizes the efficiency and cost of each type. It is not limited, but gives some orientations.
Selecting the right wetting and dispersing agent is a compromised based on many parameters. First, the system itself (waterborne or solvent borne). Then the pigment (organic, mineral, fine, rough, transparent…). And finally, the end-use application. In some formulation, changing the wetting & dispersing agent is a really positive choice, enhancing the paint quality. Products from the CPT offer excellent results, but is the cost reasonable in the considered formulation ? The right choice will be based on these test results of course, but also on the specification to reach, not only in terms of paint quality, but also economically.
Testing the Wetting & Dispersing agent
The wetting & dispersing agent has a significant influence on the paint properties. It has a direct impact on the particle size, and then, its efficiency can be evaluated by checking the right parameters.
To complete its validation, the wetting & dispersing agent must follow a serial of laboratory tests :
Compatibility with the system
Mix the wetting & dispersing agent with the system, without pigments. It should be perfectly compatible with the other formulation component. If not, try adjusting the pH or the polarity.
After the dispersion, make a simple poor-out of a small amount of paint diluted (10-20% in solvent or water).
Pigment shock results of a poor pigment stabilization.
Make a simple draw down and check the quality of the application : Color strength, transparency, gloss, general aspect. Incompatible wetting and dispersing agent can lead to many defects like seeding.
Rub out (for color mix, or pigment concentrates in a base paint)
In order to check the flooding, a simple rub out test can be done. After short drying time when the film is nearly dry, with the finger rub a part of the paint surface. The color should be the same as the unrubbed part.
Paint samples are stored at cold temperature (-5°C to 5°C) and high temperature (40°C to 60°C) for one or two weeks and the previous tests are realized, then the results are compared with the original ones and the sample stayed on the shelves.
A perfect product should not show any significant variation regarding the storage conditions.
By Vincent Makala