Waterborne Coatings Research polymer dispersant

Study: Local Liyuan Jie Wang Feng Liu Hongying Zhang

Abstract: This paper summarizes the aqueous polymer dispersions for coating the structural characteristics of pigment particles in the adsorption behavior and Research methods, commonly used type, and dispersion stability of polymer dispersant performance factors, and future direction of its development.

Keywords: waterborne coatings; polymer dispersant; adsorption; dispersion; stability
Abstract: The paper briefly summarized the character of polymeric dispersants, adsorption of dispersants onto pigment surface, main kinds of dispersants and the effect on disperse stability. The development trend of polymeric dispersants was also pointed out.

Keywords:: waterborne coating; polymeric dispersant; adsorption; dispersion; stability

In paint and other chemical Industry, pigment dispersion is an important part of paint manufacturing Technology. In order to paint the organic and inorganic pigments are uniform and stable dispersion, frequent use of dispersants. Dispersant in the paint storage, paint operations, coating formation, coating performance, and so has an important role.

While traditional dispersant in aqueous dispersion medium to display the role of effective dispersion and stability, but because they are pigment particles in the adsorption is not very strong, easy to desorption from the particle surface, resulting in re-aggregation of particles to be dispersed or precipitation system poor long-term stability of film appearance, hardness and resistance to aging and other properties have a negative effect. To overcome the limitations of traditional dispersant, in recent years the development and application of polymer dispersant. polymer dispersant is the molecular weight thousands of surface-active than the polymer, is essentially a surface active agent. The concept is that 80 early in the 20th century, first proposed by the mid-80s and related products introduced into the application stage. polymer dispersant pigment dispersion has a remarkable effect, especially on the wettability, stability has a considerable effect on the paint application performance has greatly improved and has become a new generation of highly efficient dispersant.

Present it is only ICI, DuPont, Sun Chemical, KVK and a few large companies several internationally renowned production of such products (mainly ICI's Solsperse series) [1], tight blockade of the production Technology, products sold to monopoly . of the polymer dispersant of a late start in the 90's have an overview of the polymer dispersions reported. [2] In recent years, China has also developed a number of polymer dispersant species, such as the NBZ-3, DA -50, WL series, but the result is not satisfactory, the product has not serialized. on the molecular weight distribution and chain dispersant selected groups less basic Research, product performance and foreign similar products, there is still a big gap .

A water-based coating polymer dispersant on the particle surface structure and adsorption
1.1 Dispersant for Waterborne Coatings Polymer molecular structure
Polymer dispersant by electrostatic repulsion and steric hindrance of obstacles to maintain the role of pigment particles in the medium of performance, uniform and stable dispersion system in order to make and meet performance requirements, adsorbed on the particle surface of the molecular structure of polymer dispersant need the following two characteristic: �� and solid particle surface to form a solid combination of the anchor segment, such as-COOH,-COO-,-NR2,-NR3 +,-SO32-,-PO43-,-OH, etc.; �� in the dispersion medium in a certain solvent chain length, such as polyethylene glycol. with the traditional formulation of surfactant dispersant compared to the structure of the anchor chain group and the solvent replaces the surfactant hydrophilic groups and lipophilic groups .

1.2 Hyperdispersant adsorption mechanism of the particle surface
Dispersant adsorbed on the dispersed phase wetting and dispersing properties of their shows premise. In the water system, the anchor end of the general adsorption on the surface of particles, which interact with the particles and the type of anchor groups and the surface properties of particles related. dispersions of solid particles and the binding force between the main are the following:
1.2.1 + on
For the strong polar surface inorganic particles, when the particle surface charge and functional groups hyperdispersant with opposite charge, the anchor polymer dispersant and particle surface groups can be the strong polar groups to form ion pairs with up, polymer dispersant adsorbed on the particle surface, shown in Figure 1A. Also, if the pH of the particle surface and anchoring the opposite group, ions can be formed.

1.2.2 Hydrogen Bonding
Most do not charge points of organic pigments, inorganic pigments on the surface as strong polarity, reactivity is not as high inorganic pigments, it is generally not form ion pairs of anchor form. But because of its molecular structure may contain hydrogen bond donors or by body, such as ester, carbonyl, and ether, etc., hence, have the ability to form hydrogen bonds, polymeric dispersants can be anchored on the pigment surface by hydrogen bonds. As low hydrogen bond energy, a single hydrogen bond is difficult to ensure adequate adsorption strength, each polymer dispersant molecules need to include multiple anchor groups (see Figure 1B), should adopt the block or comb-like molecular structure.

1.2.3 Surface Treatment of pigment dispersion
Some parts of organic pigments and carbon black surface completely non-polar or polar low, do not have the anchor for hyperdispersant active groups, so no matter what kind of super-dispersant use, no significant dispersion. At this point need to use the surface synergist (see Figure 1C). This is a pigment with derivatives of polar groups, its molecular structure and physical and chemical properties of dispersed pigment to be very similar, so it tightly through the van der Waals forces between molecules adsorbed in the organic pigment surface, through its molecular structure to the polar groups of polymer dispersant adsorption of Anchorage to provide active sites. Through this "synergistic effect", polymeric dispersants for organic pigments can produce a very effective wetting and stability.


(A. polar particle surface in a single point of ion adsorption; B. adsorption by multi-point hydrogen bonding; C. synergist by surface adsorption in non-polar.)
1.3 The adsorption behavior of polymer dispersant method
In order to fully characterize the surface of the pigment dispersant polymer adsorption, need to determine three parameters: �� adsorption amount �� (mg • m2 or mol • m2); �� direct integration with the pigment surface segment score p; �� adsorption layer thickness ��h .

1.3.1 Measurement of adsorption isotherms
Can be measured before and after adsorption system, the concentration of polymer dispersant to calculate the surface of the pigment dispersant adsorption ��:

1.3.3 adsorbed layer thickness ��h
Can be used ultracentrifugation, dynamic light scattering, micro electrophoresis method, by testing before and after adsorption of dispersant radius of the pigment particles adsorbed on the pigment surface to determine the thickness of polymer dispersant.

2 of polymer dispersant performance factors
Has proposed three kinds of dispersion and stability mechanism to explain the stabilization of polymer dispersions role: double-layer theory (DLVO theory) [3], space and stability mechanism [4], to do stabilization mechanisms [5], of polymer dispersant performance mainly in the following five factors:
2.1 Structure of dispersant
Polymer dispersant by the lipophilic and hydrophilic base, of which common lipophilic and aromatic base, alkyl aryl, hydrocarbon chain, and other non-polar groups; common hydrophilic groups are carboxyl, sulfonate, hydroxyl, Amino-and long polyether chain. different pro-oil - a combination of hydrophilic get a wide variety of dispersing agent, and because of their different types of dispersants with different chemical structure, and the combination of pigment particles approach the size of both binding different. At present, many dispersing agents contain aromatic rings, the use of pigment molecules with the aromatic ring plane ��-�� form a strong bond, so that the two firmly together. pigment - dispersing agent - water between the three whether the stability of the force is dispersed particle determinants of dispersant and water only enough affinity, only have good solubility, the polymer chains can be fully extended in water to form an effective three-dimensional barrier. Under this premise, , pro-oil-side binding with the pigment ions, the more favorable dispersion stability. However, if too hydrophilic dispersant, the relative weakening of its lipophilic and even the dispersant from the particle surface off and fail to disperse purposes. So when the synthetic polymer dispersions, hydrophilic monomer content is not too high. usually have a certain water-soluble dispersant case, the stronger the hydrophobicity, dispersion better.

2.2 dispersing agent with neutralizer
Carboxylic acid or acid-based polymer does not dissolve in water, neutralizer in which you want to join, so dissociation is COO-or SO32-, water-soluble polymer that has the. Early use of NaOH, KOH as a neutralizer , sodium and potassium ions are hygroscopic and remain in the system will affect the water resistance of the coating. after the switch to ammonia, but in the polymer and the salt will gradually after the release of ammonia, so that the water-soluble dispersant reduced dispersion variation. volatile lower alcohol amine as neutralizing agent can ensure the dispersion stability, without affecting the coating properties.

Links to Research Papers Download http://www.hi138.com 2.3 pH value of the dispersion system
System pH value is not the same time, the dissociation degree of carboxyl different, and this will affect the dispersing agent and water interaction between the media. Anionic polymer generally pH 8 ~ 9, it can be fully dissolved; cationic polymers pH4 ~ 5 when dissolved better. system pH, ionic strength will affect the particle electric double layer thickness and �� potential, the electrostatic repulsion between particles also changes, affecting system stability.

2.4 The amount of dispersing agent [6]
Too small amount of dispersant, a polymer chain may link two or more different pigment particles, only act as a "bridge" role, resulting in flocculation. At the same time, less than the amount of dispersant to the particle surface still not covered by part of the existence of dispersant. These are not covered by part of the interaction, to reduce the surface energy is concentrated, thereby reducing the dispersion stability of the system. dispersant use excessive, the dissolved particles in the medium part of the tangle with each other , also cause flocculation. dispersant further increases, there are plenty of curl in the shape of the distribution of dispersing agent around the particles, but also play a dispersion. but at this time dispersants and the binding force between particles was too weak to disperse less stable.

2.5 dispersant molecular weight and molecular weight distribution
Polymer chain containing at least more than 65 carbon atoms, can play an effective role to steric hindrance. [7] dispersing agent chain is too short, the traditional surfactants is similar to not provide sufficient steric repulsion; molecular weight too much will happen "bridging" effect. In general, the extension of the polymer in the solvent dispersant chain containing 50 to 100 carbon atoms, the molecular weight of 103 ~ 104. The dispersant polymer molecular weight distribution of the physical, chemical, mechanical properties and solution properties are large. low molecular weight polymer and the molecular weight distribution than the distribution of narrow width of the polymer, as a pigment dispersant effective. broad molecular weight distribution of polymers in solution occurred "classification" effect, that is, a molecular weight of a dispersant adsorbed on the particle surface more, and the rest remains in the system.

3 water-based paint pigment dispersant of the type of polymer
Water-based paint, in addition to water-soluble paint, the pigment and resin are dispersed in aqueous media. Adsorbed on the surface of the polymer pigment dispersant by electrostatic repulsion, steric hindrance effect of the particle dispersion stability. Currently used in the coating process Most of polycarboxylate dispersant dispersants, non-ionic dispersing agent (such as polyoxyethylene derivatives, polyethylene adjacent polyvinylpyrrolidone, etc.) Research is seldom used.

Polycarboxylate polymer dispersant by the Carboxyl vinyl monomer (eg acrylic acid, maleic anhydride, etc.) together or with other monomers, after esterification with a soda and alcohol by [8 ~ 12]. S . Creutz et al [8] with isobutylene amine and amine dimethylaminoethyl methacrylate as raw materials, synthesized using different polymerization block or random copolymer dispersant for water-based coating of organic or inorganic pigment dispersion of iron red, blue and orange copper phthalocyanine as the color of the pyrrole pigments are very effective. Zhujin Li et al [12], maleic anhydride, methyl methacrylate monomer by solution polymerization of polycarboxylic acid polymer dispersant , when one mole percent of maleic anhydride was 25.5%, the dispersion agent on the pigment red 22 in aqueous medium dispersion is good, can effectively reduce the pigment particles in the flocculation.

Styrene - maleic anhydride copolymer (SMA) is a more recent study of the polymer, its molecular plane containing pigments can form strong ��-�� bond of the benzene ring, and after hydrolysis of sterically hindered formation of carboxyl acid anion, it can be used water-based paint system. [13,14] Tianan Li et al [13] to butanone as solvent by solution free radical polymerization a series of homogeneous polymer SMA, using the polymer for water-based paint system The ultra-fine pigment dispersant has good dispersion dispersion, the viscosity, particle size, surface tension, are up to the requirements of ink jet printing. Xu Yanli, who first with the styrene monomer and maleic anhydride used for the suspension polymerization of the copolymer, and then to alcohol as the esterifying agent to esterification, will be transformed into carboxylic acid sodium salt. that kind of dispersant can effectively improve the phthalocyanine blue pigment wetting, flow and in the water The dispersion stability, the structure shown in Figure 2. [14]

4 Outlook
Polymer dispersant because of its excellent dispersion and stability on the role of pigment in water-based paint was widely used, but there are still some shortcomings, such as: the film is easy to absorb moisture in the polycarboxylic acid sodium salt, organic pigments on the role of poor, so good performance should be developed for non-ionic dispersants. From the analysis of molecular structure, block or graft copolymer than the currently used random copolymer is more appropriate. In addition, the pigment dispersant of selectivity, only water-based inorganic or organic pigments, and the need to study the properties of outstanding universal polymer dispersant.

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