summary
1. Wetting and dispersing agent
2. Defoamer
3. Thickener
4. Film-forming additives
5. Other additives
Wetting and dispersing agent
Water-based coatings use water as a solvent or dispersion medium, and water has a large dielectric constant, so water-based coatings are mainly stabilized by the electrostatic repulsion when the electric double layer overlaps.
In addition, in the water-based coating system, there are often polymers and non-ionic surfactants, which are adsorbed on the surface of the pigment filler, forming steric hindrance and stabilizing the dispersion. Therefore, water-based paints and emulsions achieve stable results through the joint action of electrostatic repulsion and steric hindrance. Its disadvantage is poor electrolyte resistance, especially for high-priced electrolytes.
1.1 Wetting agent
Wetting agents for waterborne coatings are divided into anionic and nonionic.
The combination of wetting agent and dispersing agent can achieve ideal results. The amount of wetting agent is generally a few per thousand. Its negative effect is foaming and reducing the water resistance of the coating film.
One of the development trends of wetting agents is to gradually replace polyoxyethylene alkyl (benzene) phenol ether (APEO or APE) wetting agents, because it leads to the reduction of male hormones in rats and interferes with endocrine. Polyoxyethylene alkyl (benzene) phenol ethers are widely used as emulsifiers during emulsion polymerization.
Twin surfactants are also new developments. It is two amphiphilic molecules linked by a spacer. The most notable feature of twin-cell surfactants is that the critical micelle concentration (CMC) is more than an order of magnitude lower than that of their “single-cell” surfactants, followed by high efficiency. Such as TEGO Twin 4000, it is a twin cell siloxane surfactant, and has unstable foam and defoaming properties.
1.2 Dispersant
Dispersants for latex paint are divided into four categories: phosphate dispersants, polyacid homopolymer dispersants, polyacid copolymer dispersants and other dispersants.
The most widely used phosphate dispersants are polyphosphates, such as sodium hexametaphosphate, sodium polyphosphate (Calgon N, product of BK Giulini Chemical Company in Germany), potassium tripolyphosphate (KTPP) and tetrapotassium pyrophosphate ( TKPP).
The mechanism of its action is to stabilize electrostatic repulsion through hydrogen bonding and chemical adsorption. Its advantage is that the dosage is low, about 0.1%, and it has a good dispersion effect on inorganic pigments and fillers. But also there are deficiencies: the one, along with the raising of pH value and temperature, polyphosphate is easily hydrolyzed, causes long-term storage stability bad; Incomplete dissolution in medium will affect the gloss of glossy latex paint.
1 Phosphate dispersant
Phosphate ester dispersants stabilize pigment dispersions, including reactive pigments such as zinc oxide. In gloss paint formulations, it improves gloss and cleanability. Unlike other wetting and dispersing additives, the addition of phosphate ester dispersants does not affect the KU and ICI viscosity of the coating.
Polyacid homopolymer dispersant, such as Tamol 1254 and Tamol 850, Tamol 850 is a homopolymer of methacrylic acid.
Polyacid copolymer dispersant, such as Orotan 731A, which is a copolymer of diisobutylene and maleic acid. The characteristics of these two types of dispersants are that they produce strong adsorption or anchoring on the surface of pigments and fillers, have longer molecular chains to form steric hindrance, and have water solubility at the chain ends, and some are supplemented by electrostatic repulsion to achieve stable results. To make the dispersant have good dispersibility, the molecular weight must be strictly controlled. If the molecular weight is too small, there will be insufficient steric hindrance; if the molecular weight is too large, flocculation will occur. For polyacrylate dispersants, the best dispersion effect can be achieved if the degree of polymerization is 12-18.
Other types of dispersants, such as AMP-95, have a chemical name of 2-amino-2-methyl-1-propanol. The amino group is adsorbed on the surface of the inorganic particles, and the hydroxyl group extends to the water, which plays a stabilizing role through steric hindrance. Due to its small size, steric hindrance is limited. AMP-95 is mainly a pH regulator.
In recent years, the research on dispersants has overcome the problem of flocculation caused by high molecular weight, and the development of high molecular weight is one of the trends. For example, the high molecular weight dispersant EFKA-4580 produced by emulsion polymerization is specially developed for water-based industrial coatings, suitable for organic and inorganic pigment dispersion, and has good water resistance.
Amino groups have a good affinity for many pigments through acid-base or hydrogen bonding. The block copolymer dispersant with aminoacrylic acid as the anchoring group has been paid attention to.
2 Dispersant with dimethylaminoethyl methacrylate as anchoring group
Tego Dispers 655 wetting and dispersing additive is used in waterborne automotive paints not only to orient the pigments but also to prevent the aluminum powder from reacting with water.
Due to environmental concerns, biodegradable wetting and dispersing agents have been developed, such as EnviroGem AE series twin-cell wetting and dispersing agents, which are low-foaming wetting and dispersing agents.
Defoamer
There are many kinds of traditional water-based paint defoamers, which are generally divided into three categories: mineral oil defoamers, polysiloxane defoamers and other defoamers.
Mineral oil defoamers are commonly used, mainly in flat and semi-gloss latex paints.
Polysiloxane defoamers have low surface tension, strong defoaming and antifoaming capabilities, and do not affect gloss, but when used improperly, they will cause defects such as shrinkage of the coating film and poor recoatability.
Traditional water-based paint defoamers are incompatible with the water phase to achieve the purpose of defoaming, so it is easy to produce surface defects in the coating film.
In recent years, molecular-level defoamers have been developed.
This antifoaming agent is a polymer formed by directly grafting antifoaming active substances on the carrier substance. The molecular chain of the polymer has a wetting hydroxyl group, the defoaming active substance is distributed around the molecule, the active substance is not easy to aggregate, and the compatibility with the coating system is good. Such molecular-level defoamers include mineral oils — FoamStar A10 series, silicon-containing — FoamStar A30 series, and non-silicon, non-oil polymers — FoamStar MF series.
This molecular-scale defoamer uses a supergrafted star polymer as an incompatible surfactant and has achieved good results in waterborne coating applications. The Air Products molecular-grade defoamer reported by Stout et al. is an acetylene glycol-based foam control agent and defoamer with both wetting properties, such as Surfynol MD 20 and Surfynol DF 37.
In addition, in order to meet the needs of producing zero-VOC coatings, there are also VOC-free defoamers, such as Agitan 315, Agitan E 255, etc.
thickener
There are many kinds of thickeners, currently commonly used are cellulose ether and its derivatives thickeners, associative alkali-swellable thickeners (HASE) and polyurethane thickeners (HEUR).
3.1. Cellulose ether and its derivatives
Hydroxyethyl cellulose (HEC) was first produced industrially by Union Carbide Company in 1932, and has a history of more than 70 years.
At present, the thickeners of cellulose ether and its derivatives mainly include hydroxyethyl cellulose (HEC), methyl hydroxyethyl cellulose (MHEC), ethyl hydroxyethyl cellulose (EHEC), methyl hydroxypropyl Base cellulose (MHPC), methyl cellulose (MC) and xanthan gum, etc., these are non-ionic thickeners, and also belong to non-associated water phase thickeners. Among them, HEC is the most commonly used in latex paint.
3.2 Alkali-swellable thickener
Alkali-swellable thickeners are divided into two categories: non-associative alkali-swellable thickeners (ASE) and associative alkali-swellable thickeners (HASE), which are anionic thickeners. Non-associated ASE is a polyacrylate alkali swelling emulsion.
3.3. Polyurethane thickener and hydrophobically modified non-polyurethane thickener
Polyurethane thickener, referred to as HEUR, is a hydrophobic group-modified ethoxylated polyurethane water-soluble polymer, which belongs to non-ionic associative thickener.
HEUR is composed of three parts: hydrophobic group, hydrophilic chain and polyurethane group.
The hydrophobic group plays an association role and is the decisive factor for thickening, usually oleyl, octadecyl, dodecylphenyl, nonylphenol, etc.
However, the degree of substitution of hydrophobic groups at both ends of some commercially available HEURs is lower than 0.9, and the best is only 1.7. The reaction conditions should be strictly controlled to obtain a polyurethane thickener with a narrow molecular weight distribution and stable performance. Most HEURs are synthesized by stepwise polymerization, so commercially available HEURs are generally mixtures of broad molecular weights.
In addition to the linear associative polyurethane thickeners described above, there are also comb-like associative polyurethane thickeners. The so-called comb association polyurethane thickener means that there is a pendant hydrophobic group in the middle of each thickener molecule. Such thickeners as SCT-200 and SCT-275 etc.
When adding a normal amount of hydrophobic groups, there are only 2 end-capped hydrophobic groups, so the synthesized hydrophobically modified amino thickener is not much different from HEUR, such as Optiflo H 500, see Figure 3.
If more hydrophobic groups are added, such as up to 8%, the reaction conditions can be adjusted to produce amino thickeners with multiple blocked hydrophobic groups. Of course, this is also a comb thickener.
This hydrophobic modified amino thickener can prevent the paint viscosity from dropping due to the addition of a large amount of surfactants and glycol solvents when color matching is added. The reason is that strong hydrophobic groups can prevent desorption, and multiple hydrophobic groups have strong association.
Post time: Dec-26-2022