April 5, 2023 Longchang Chemical

1. What is the use of levelling agents as a common coating aid?

The main function of paint is to decorate and protect. If flow and levelling defects occur, they not only affect the appearance, but also undermine the protective function. The formation of shrinkage holes and pinholes can lead to discontinuity in the paint film, which can reduce the protective properties of the paint. In the process of construction and film formation, some physical and chemical changes will occur, these changes and the nature of the paint itself will significantly affect the flow and leveling of the paint.

After the coating is applied, new interfaces will appear, typically the liquid/solid interface between the coating and the substrate and the liquid/air interface between the coating and the air. If the interfacial tension at the liquid/solid interface between the paint and the substrate is higher than the critical surface tension of the substrate, the paint will not be able to spread on the substrate and will naturally produce leveling defects such as fish eyes and shrinkage.

The evaporation of solvents during the drying process leads to differences in temperature, density and surface tension between the surface and the interior of the paint film, which in turn leads to turbulent movements within the film, resulting in so-called swirls. Swirls can lead to orange peel; in systems containing more than one pigment, swirls are also likely to lead to puffing and blooming if there is some difference in the motility of the pigment particles, and to filigree on vertical surfaces.

Insoluble particles are sometimes produced during the drying of the paint film, which can lead to the formation of surface tension gradients, often resulting in the formation of shrinkage in the paint film. For example, in cross-linked curing systems, where the formulation contains more than one resin, insoluble particles may be formed from less soluble resins as the solvent evaporates during the drying process. Also, in formulations containing surfactants, if the surfactant is incompatible with the system, or if its concentration changes as the solvent evaporates during drying leading to changes in solubility and the formation of incompatible droplets, a surface tension difference may also form. All of these may lead to the creation of shrinkage holes.

The presence of external contaminants during the construction and film formation of a coating may also lead to leveling defects such as shrinkage and fish eyes. These contaminants are usually oil, dust, paint mist, water vapour etc. from the air, construction tools and substrates.

The nature of the paint itself, such as working viscosity and drying time, can also have a significant effect on the *final leveling of the paint film. Too high an application viscosity and too short a drying time will usually produce a poorly levelled surface.

Therefore, it is necessary to help the paint to obtain a good leveling by adding levelling agents, by making some changes in the paint during construction and film formation and by adjusting the paint properties.

 

For a definite formulation system, a suitable levelling agent should be selected according to the nature of the formulation and the performance that the levelling agent is expected to achieve.

1.1. Solvent-based paint systems

In primer and midcoat formulations, acrylic levelling agents are usually used. If degassing and substrate wettability are required, it is advisable to choose

medium molecular weight or high molecular weight acrylic levelling agents. In primers, if stronger substrate wettability is required, silicone levelling agents and modified acrylic levelling agents (such as fluorine modified acrylic levelling agents and phosphate modified acrylic levelling agents) that significantly reduce surface tension can be considered, and if silicone levelling agents and fluorine modified acrylic levelling agents have side effects such as stabilising bubbles and affecting interlayer adhesion, phosphate modified acrylic levelling agents should be used.

In topcoat and transparent paint formulations, the relatively high requirements for the appearance of the film, generally can choose low molecular weight acrylic levelling agent, which will obtain good leveling, in the film is not easy to produce fog shadow. In cross-linked curing systems, acrylic levelling agents with reactive functional groups are often used to obtain better levelling and improve the physical and chemical properties of the film. Silicone levelling agents are necessary if better flow is required or if slip and scratch resistance are needed, in which case it is * preferable to use silicone levelling agents in combination with acrylic levelling agents.

It should be noted that silicone levelling agents provide levelling properties while effectively reducing the tendency of the coating to sag when applied on vertical surfaces. In addition, in metal flash paint formulations, silicone levelling agents should be used with caution, as they may lead to uneven alignment of aluminium flake pigments and uneven paint film colour.

1.2、Powder coating system

The levelling process of powder coatings is divided into two stages. ** The first stage is the melting of the powder particles and the second stage is the melting and flowing of the powder particles into a flat paint film. Powder coatings do not contain solvents and do not produce a surface tension gradient during the film formation process, the levelling is more related to the wetting of the substrate.

Powder coatings often use acrylic levelling agents. If the levelling agent is in liquid form, it is usually pre-made into a master batch before use. There are also acrylic levelling agents made into powders, specifically for powder coatings, which are liquid acrylic levelling agents adsorbed onto silica powders, and some lower grade levelling agents adsorbed with calcium carbonate.

If the powder coating needs to be smooth and anti-hanging, we should use silicone levelling agent, there are already made of powder specifically for powder coatings of silicone levelling agent. The use of silicone levelling agents should be noted to avoid the formation of shrinkage holes.

1.3、Water-based coating system

Water-based coating system is divided into water-soluble system and latex system.

In the water-soluble system, the surface tension of the system needs to be strongly reduced, * commonly used is silicone levelling agent and fluorocarbon compounds

These are used in the same way as they are used in solvent-based systems. Of course, if a truly flat surface is required, acrylic levelling agents for waterborne systems are necessary.

For latex systems, the film formation mechanism is completely different and the viscosity does not change with the evaporation of the solvent. The use of levelling agents in the formulation will potentially improve the substrate wetting of the paint and acrylic levelling agents can improve the film flatness, but the main flow properties of the paint are more often controlled and adjusted by the addition of rheology control agents.

2. Why is it necessary to add some levelling agent to a paint?

The presence of external contaminants during the application and film formation process of a paint can also lead to leveling defects such as shrinkage and fish eyes. These contaminants are usually oil, dust, paint mist, water vapour etc. from the air, construction tools and substrates.

The nature of the paint itself, such as working viscosity and drying time, can also have a significant effect on the *final leveling of the paint film. Too high an application viscosity and too short a drying time will usually produce a poorly levelled surface.

Therefore, it is necessary to help the paint to obtain a good leveling by adding a levelling agent, by making some adjustments to the paint during construction and film formation and to the properties of the paint.

3. Why do water-based levelling agents thicken?

In the chemical industry, levelling agents are indispensable because they achieve a highly homogeneous, glossy, smooth surface. We all see technicians mixing water-based levelling agents into natural thickeners, but with a good degree of moistening. A rheology additive is defined as some additive that changes the rheology of a fluid, for example from a Newtonian fluid to a plastic, pseudo-plastic fluid, or vice versa. These additives may be polymers, natural thickeners, inorganic or organic solvents, or a variety of organic or inorganic electrolytes or dispersants.

Levelling agents are used to achieve a highly homogeneous, glossy and smooth surface when inks, pastes, coatings, etc. are applied to the surface of materials. These levelling agents are mainly highly effective surface, surfactant and polymer electrolytes, whose mechanism of action is to disperse, break the foam, dilute, make the paint or ink highly fluid (without changing the solids content of the paint or ink) and have good surface wettability (both The use of these levelling agents can significantly reduce pinholes and the crater effect on the surface of printed parts.

4. The controlled nature of acrylate levelling agents is achieved by changing the molecular weight

The controlled nature of acrylate levelling agents is achieved by changing the molecular weight. Homopolymer compatibility is not as good as copolymer, such as homopolymer acrylic acid is usually poorly compatible with epoxy, polyester, polyurethane and other resins used in coatings, if they are mixed with physical methods will form a poor surface state of the non-light coating film, so acrylic homopolymer is not very suitable for leveling agents. The ideal levelling agent is a copolymer, which can be a terpolymer, or a modified copolymer, and only the copolymer can change the polymer’s *ness and glass temperature through different monomers.
Usually the number average molecular weight of acrylate levelling agents is controlled between 6000-20000, the molecular weight distribution is relatively narrow, the glass flower temperature is controlled below -20°C and the surface tension is below 25-26mN/m. This compatibility limited acrylic copolymer is considered a good levelling agent.
Acrylic levelling agents can be homopolymers or copolymers; linear structures or those with branched chains; random copolymers or block copolymers.

 

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