Pigment dispersion process and the structure and function of dispersant2016.05.26 09:19
Coatings are widely used in our daily life, to the coated parts have the function of protection, decoration and special effect. Coating is mainly composed of film-forming substances ( Resin, fillers, additives and solvent (or water). Unlike dyes, pigments are usually insoluble in the medium used, in most cases In the form of aggregation. In order to get good tinting strength, hiding power and chromaticity and so on, must open the pigment of aggregate and remained stable. If the paint No good dispersion, many defects can occur, such as: flocculation, loss of light and color deviation, floating color/hair flower, precipitation, etc. This paper introduces the pigment and The structure of the dispersing agent, pigment and dispersant in the role of the dispersion.
1 the morphology and particle size of pigment
Most in the paint, paint is made into slurry using. Pigment particle size distribution is very wide range, generally from 0.05 mu m to 1 mm. But the table Ming, when the pigment particle size between 0.05 ~ 0.50 microns, which has the best of tinting strength, gloss, hiding power and weather resistance, etc.
Paint manufacturing as the smallest particle of matter formed in the process is called the original particles, it exist in single crystal or a group of crystal, size is very small. The original particle Between surface and physiognomy and formation of the briquette, known as aggregate, aggregate more closely, generally difficult to spread it into the original particle dispersion equipment. while The original particles and aggregates together by van der Waals force, the formation of large granular pigment clumps, known as the agglomeration. Between the particle agglomeration to point, Edge, contact Angle, small particles inter-atomic forces, by mechanical force to spread it into the original particles or aggregates.
We hope to become the original particle suspension after dispersing pigments (ideal), at least also aggregate and the suspension of the original particles. But the system Trend from high energy recovery to the low-energy state, therefore with fine particle size distribution of pigment easily gathered into flocculation body. Structurally, the flocculation body and agglomeration Body very similar, but the flocculation body color in between is resin solution rather than air. The presence of dispersing agent can prevent or slow flocculation.
2 pigment dispersion process
Pigment dispersion process by wetting, dispersion and stability of three steps. Wetting process, air and moisture on the surface of the paint by resin solution Replace, solid/gas two-phase (pigments/air) is converted to a solid/liquid two phase (pigments/resin solution). Dispersant especially low molecular model of wetting dispersant To speed up the process of wetting. Dispersion process, rely mainly on mechanical energy function (shock and shear force), state reunion of pigment were broken into smaller particles To become fragmented (uniform). The third step is stable, the process of dispersant used to maintain the stability of pigment decentralized state, stop runaway flocculation And based on the paint surface adsorption type and molecular structure of the binder, prompted the suspension stability state. Dispersant especially polymeric dispersant The stability of the pigment particles plays a big role.
3 the action of dispersing agent
Dispersant in paint production has played a very important role. The stability of the dispersion system can avoid many problem of coating and paint film ills, if match Reasonable, appropriate to add dispersant can effectively reduce the cost, improve the coating performance.
3.1 improve gloss, increasing flow leveling effect
A test shows that the same formula, using different dispersants, concluded that there will be obvious difference of paint film burnish. For example, in a black volume Wood coatings system, three different dispersants 20 ?gloss of paint film is 69.7%, 81.5% and 69.7% respectively. If adopted by the scattered Agent is inappropriate, flocculation after becoming coarse, the luster is similar to the role of extinction powder, will naturally affect luster.
3.2 to prevent floating color hair flower
To prevent the floating color hair flower, is known as one of the effect of dispersant. Choose appropriate dispersant, tank floating color hair flower or film Floating color hair flower, can be improved or eliminated. Figure 3 shows the use of different dispersants of blue paint with white paint evenly mixed by the same proportion The results of tank floating color hair flower. Also some dispersants, SN - 2004, for example, is controlled flocculation of dispersant, some molecular chain containing carboxy Base groups, and dye affinity at the same time, also can form hydrogen bond with each other, so that the stability of pigment particles, to achieve the effect of prevention floating color hair flower.
3.3 to improve the tinting strength
Pigment dispersion and stability better, the tinting strength significantly strengthened. Figure 4 shows the use of different dispersants dispersed black slurry, according to the phase with white paint Mixes with the proportion of the coating as a result, can see clearly the difference between the tinting strength.
3.4 to reduce the viscosity, increased pigment loading quantity
Use appropriate dispersant, the paste viscosity will be significantly reduced. This will increase the pigment loading capacity, improve production efficiency. Figure 5 is mining With different dispersants dispersed red paste viscosity contrast.
3.5 reduce flocculation, increase construction and usability
Many people in the assessment of dispersant, do research experiment. If dispersant collocation is improper, refers to the research of regional areas and refers to the research of Ming Show off color, pigment tinting strength will decline after flocculation, it brings to the color and paint construction difficult. Use flow board test, can also paste was observed Flocculation or not.
3.6 to prevent back to coarse, increase storage stability
If you choose not suitable dispersant, paste return coarse or color change due to flocculation, are not you want. In addition to the resin system Outside, dispersants is one of the important factors affecting paint storage stability. Poor dispersion paste happens to return coarse, sedimentation and tinting strength decreased.
3.7 increase the color and color saturation
Dispersant of the role of the most easily ignored for everyone. There are instances show that the different the same pigment dispersant dispersion, the hue and saturation Will have obvious difference. Generally speaking, the better the dispersion, the higher the saturation will be (if it is carbon black, it is the better the dispersion, the L value is lower, the higher the blackness ).
3.8 increase transparency (organic pigments) or hiding power (inorganic pigment)
For aluminum powder paint, will hope that the higher the transparency of the paste, the better. And for plain coloured lacquer, again want to paste the covering power of the higher the better. In fact this Related to the pigment particle size. The data shows, in addition to the refractive index, pigment particle size distribution is another important factor of transparency. Particle size increases, the scattering light can Increased, until a maximum, and then began to decline. The ability of the scattering light enhances the paint covering power, scattering maximum when the most powerful, Increase particle size covering power declines. And when the pigment particle size is less than a certain value, as the particle size of decline, increased transparency. Dispersant can't change Variable pigment itself characteristic, but can control the pigment particle size distribution, achieve more ideal color effect.
3.9 improve the grinding efficiency, reduce production costs
Wetting function of wetting dispersant make paint on the surface of the air and water can more quickly was replaced by the liquid of grinding media, at the same time so that dispersed phase The pigment of primary particles and keep the separation of control. All of these improve the grinding efficiency, reduces the energy consumption and manpower cost.
3.10 to prevent subsidence
Many people take it for granted that anti-settling must depend on anti-settling agent, actually this is one-sided. Good grinding paste if produce flocculation, pigment particles , density increased, would quickly settlement. Choose the appropriate dispersant paste can add less don't even need to add anti-settling agent. The author did a 70% Titanium pigment resin paste, its viscosity is very low (91.1 mPa s., shear rate is 10 s - 1), did not join any anti-settling agent, after 15 d thermal storage ( Placed 40 ℃) and room temperature for 3 months, no any precipitation. Dispersant anti-settling effect obviously on the storage stability of coatings is also helpful.
4 the types of basic structure and dispersant
Dispersant is also called the wetting dispersant, it besides has wet function, its reactive end can adsorption on the surface of the paint that crushed into small particles, on the other End solvation have enter into an adsorption layer (adsorption base, the more the longer the chain link, the adsorption layer thickness), electrical charge repulsion (coating) or entropy is investing Force (solvent coating), make the pigment particles dispersed suspension in have been for a long time, avoid again flocculation, thus to ensure that the paint system made of storage stability .
There are many kinds of dispersants, preliminary estimates, existing in the world has more than 1000 with dispersion effect is material. Now according to its structure to distinguish, can be divided into the following Seven kinds of types.
4.1 anionic wetting dispersant
Mostly by nonpolar negatively charged oil-wet hydrocarbon chain part of hydrophilic groups and polarity. The ends of the two groups are in molecular , form the asymmetric hydrophilic oil substructure. Its varieties are: sodium oleate C17H33COONa, carboxylic acid salt, sulfate salt (R - O - SO3Na), Sulfonate (R - SO3Na), etc. Anionic dispersant good compatibility, and is widely used in water-based coatings and printing ink. Multiple carboxylic acid polymers, etc. Can also be Used for solvent paint, and widely used as a dispersant controlled flocculation mode.
4.2 cationic wetting dispersant
Wrong polarity baseband positively charged compounds, are mainly amine salt and quaternary ammonium salt, pyridine Weng salt, etc. Cationic surfactant adsorption ability is strong, the carbon black , all kinds of iron oxide, organic pigment dispersion effect is good, but should pay attention to its react with a base of carboxyl, note also don't and anionic dispersed Agent used at the same time.
4.3 nonionic wetting dispersant
In water ionization, without charge, in the paint surface adsorption is weak, mainly used in coating of drainage. Mainly divided into the ethylene glycol and polyols Type, lower surface tension and improve the wettability. Cooperate with anionic dispersant used as wetting agent and emulsifier, widely used in water-based paste, water Coatings and printing ink.
4.4 type amphoteric wetting dispersant
Is composed of anionic and cationic compounds. Typical applications of phosphate salt type polymer. This kind of polymer acid value is higher, May affect adhesion between the layers.
4.5 type electrically neutral wetting dispersant
Molecular size of the anionic and cationic organic groups in the basic equal, the molecule is neutral, but it has a polarity. Such as oil, amino acid ester C18H35NH3OOCC17H33 etc all belong to this type, is widely applied in the coating.
4.6 polymeric dispersant
Polymeric dispersant is most commonly used, stability is the best. Polymeric dispersant is divided into many f within multiple ester polyols - ethylene imine block altogether Polymer dispersant, acrylate polymer dispersant, polyurethane or polyester polymer dispersant etc, because of their head and resin anchor group Adsorption of twisting, on the other side and pigment particles pack again, thus the storage stability is good.
4.7 controlled free radical type super dispersant
Using the latest controlled radical polymerization technology (CFPP), can make the structure of the dispersant is more neat. The commonly used methods are: GTP, ATRP (made Atom transfer radical polymerization), RAFT (reversible addition fragmentation chain transfer controlled radical polymerization, including C - RAFT and S - RAFT, etc.), NMP, SFRP (stable free radical polymerization), TEMPO, and so on. By adopting the controlled radical polymerization technique, can make the dispersing agent of relative molecular mass distribution is more concentrated , anchor groups also more concentrated, more efficient.
To fully play a role in the system of fillers in coatings, the application of the dispersant and rational choice is a must. And before the dispersed pigments, must again To understand the structure of the pigment and dispersant. Technical personnel or management personnel will think a lot of coating, adding dispersant increases cost, but it's not . Stable fillers slurry system, which can avoid a lot of follow-up questions. From the perspective of the result of comprehensive, screening of dispersant and add right amount reasonably, is Can save the cost, and greatly improve the performance of the coating.