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Selection principle and characteristics of leveling agent for powder coatings

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Characteristics and application of 1.1 leveling agent

Powder coating is mainly used for surface finishing and chemical protection materials, but has a different point of powder coatings and solvent paint general large surface tension and high melt viscosity is very high. Therefore, to provide powder coating and substrate with good wetting and leveling, the system must be added to a special additive to complete, the additive is our commonly used leveling agent.

The application of leveling agent for powder coatings has been developed for many years. In general, their main role is to prevent the film defects (such as shrinkage, fisheye) and orange (orange peel) to reduce. At the same time, the advantage of leveling agent is to improve the dispersion of the pigment and the effect of degassing in the curing process, increase the liquidity and so on. Good dispersion effect to paint on the dispersion effect of the system, enhances the uniformity of color and reduce the processing time, in addition, leveling agent in promoting the flow at the same time, can also prevent the flow of defects is not required, examples are given in the following table:


Selection principle and characteristics of leveling agent for powder coatings

Function and performance

Result

Improve surface tension

Increased wettability and film coverage

Improved surface flow

To reduce shrinkage, fish eye, orange peel, pinhole

Improve the degassing effect

Removal of gas and air bubbles

Improve the dispersion of pigment

Higher pigment content under the same melt viscosity

The addition of an acrylate polymer in powder coatings can improve the surface tension properties of coatings. The polymer is added to the molten resin surface (but not completely) to form a monolayer in the curing process to obtain the equilibrium surface chemical potential. When the acrylate polymer reaches the surface, the polar skeleton is still retained in the molten resin, while the alkyl is escaping into the surrounding environment, providing a more stable equilibrium condition. At this time, the surface tension tends to be balanced due to the large concentration of molecules arranged in the direction of the whole surface.

Nowadays, acrylate oligomers or copolymers are widely used as main leveling agents. The silicone - acrylic blends and pure silicon resin because of the pollution is too strong now rarely used. But there are some silicone polymer modified, such as leveling agent of polysiloxane polyether and polyester modified for powder coatings can improve the powder flow properties without causing pollution. These modifications are commonly used in transparent coating systems. Fluorocarbon polymer powder as leveling agent can be provided when the ratio of surface tension, the silicone resin with lower consumption is lower, but the use is not widespread, mainly because of the high price and the solvent, is not usually in the form of solid state for sale. The biggest advantage of fluorocarbon leveling agent is good recoatability, but very effective surface painting is difficult for those, such as substrate containing oil or other low surface tension of the metal surface coating and so on pollution. There are a number of high molecular weight thermoplastic polymer powder coating can also be effective in preventing defects, such as polyethylene, such as Ding Jizhi. Because the molecules of high melting point and difficult to disperse, so a lot of time will cause the orange peel. So it has been replaced by acrylic polymers. However, there are a few applications in some functional coatings (corner covers).

Because of the important active substances leveling agent is high viscous material, directly in the production of the use is not convenient, so people tend to use it into the 10%~15% resin to parent content (master batch) or they will form a free flowing powder adsorption on silica with high surface volume, usually the effective content of this free flow the powder can reach 60-70%. The use of master batch advection agent has the advantage of dispersion, and the use of Fumed Silica powder due to the high content of small amount of use, so good dispersion is necessary. Due to the excessive Silica will often lead to fogging transparent paint, so the use of Master batch is better, especially for the preparation of high transparent coatings more meaningful, there is specially provided for the transparent powder leveling agent exclusive sale.

Leveling mechanism of 1.2 leveling agent

The effect of leveling agent is mainly used to eliminate film surface defects, reduce the occurrence of orange peel, film forming and flow in the thermosetting powder coating is mainly determined by the two main factors, generally called surface tension and melt viscosity, surface tension is the main driving force to promote the flow of. The only resistance in the given baking conditions is the viscosity of the melt coating. Therefore, strict leveling agent should be referred to as surface tension regulator.

The leveling agent is not completely compatible with the resin and the filler. In the process of leveling, the leveling agent migrates to the surface and the substrate and the film to form a monolayer to provide uniform surface tension. Leveling and solvent type coating powder coating has a characteristic not exactly the same, the solvent based coatings for surface tension with volatile solvent coating surface will form a high, and with the continuous evaporation surface will increase the viscosity of the solvent, with the heat also reduces the temperature of the surface, so the surface tension will cause the difference of temperature difference and viscosity difference, these factors lead to the imbalance of the coating film, in order to maintain the balance of system will produce under the impetus, prompted a coating by continuous flow until the final viscosity stop the flow. At this time, the surface tension difference disappears, and finally the surface is flat. In fact in the irregular film on the equilibrium flow form in the hexagonal network edge connected, also known as Benard cell (Bernard Eddy) source is located in the central cell, and the paint is sinking to the edge of the honeycomb. Because of the convection caused by eddy current small lattice center uplift, if the liquidity difference of the coating after curing, will see the orange peel phenomenon. Generally, this is related to the surface tension viscosity and wettability of the coating. When the viscosity of the coating is low and the film is thick, it is not obvious when the viscosity increases and the film thickness decreases. As follows:

 

 

In the case of powder coating, the viscosity of the system is very large because of the absence of solvent. This is caused by the viscosity and temperature force can be ignored, while the surface tension will become an important driving force to promote the film flow, so when the temperature and viscosity of the resin when the surface tension plays a leading role.
The flow of powder coatings is generally divided into two processes. The first is the fusion of powder particles to form a continuous film, followed by a continuous film from the process of leveling.

 

In the process of continuous film coating from particles, as shown below (Frenkel) and dodge in theory.



Frenkel believes that between two particle fusion time (T) and particle radius (R), the contact area between the particle radius (X), the viscosity of the material (µ) and surface tension (γ) on
(X/R)2 = 3γt/3Rµ
Nix and Dodge have different interpretations, they think that the surface tension is the only factor of fusion and leveling for a fixed powder coating, and the viscosity of the system is blocked. According to this hypothesis, the two particles are fused together and the radius of curvature of the particle is positively proportional to the surface tension. The equation is as follows
t∝μRc/γ
This phenomenon has also been confirmed in theory, although the model is based on the description of a single circular particle, in fact, the real particle is not exactly the same as the ideal model (particle size is not the same). But it can be determined that the higher the melting viscosity, the longer the time needed for the fusion between particles. The Nix and Dodge equations can explain the phenomenon better.
The viscosity of the powder also affects the flow of the film in the second stage of the powder, and the Orchard equation is described. The equation was originally used to explain the flow of liquid coatings, but also for the description of the powder flow. The equation describes the flow time and film thickness (h), melt viscosity (μ), surface tension (γ), wavelength (λ), wave height (αt ,α0) to give the following equation:
ln(αt/α0)= -(16π4h3/3λ4)∫0t(γ/μ)dt
It is assumed that the viscosity of the powder coating does not change with time at a given temperature (for most thermoplastic powders), and the integral formula of the orchard equation can be expressed as:


It is assumed that the viscosity of the powder coating does not change with time at a given temperature (for most thermoplastic powders), and the integral formula of the orchard equation can be expressed as
t= 3λ4μ㏑(αt/α0)/16π4h3γ
Therefore, Orchard indicates that the lower melt viscosity is necessary to obtain better leveling effect, which is the same as that of the first stage of the fusion process. However, too low melt viscosity will lead to the occurrence of flow phenomenon. Therefore, in order to ensure good leveling effect must be properly balanced. At the same time, it can be seen that the surface force in the two stages of the powder
ln(αt/α0)= -(16π4h3/3λ4)∫0t(γ/μ)dt
It is assumed that the viscosity of the powder coating does not change with time at a given temperature (for most thermoplastic powders), and the integral formula of the orchard equation can be expressed as
t= 3λ4μ㏑(αt/α0)/16π4h3γ
Therefore, Orchard indicates that the lower melt viscosity is necessary to obtain better leveling effect, which is the same as that of the first stage of the fusion process. However, too low melt viscosity will lead to the occurrence of flow phenomenon. Therefore, in order to ensure good leveling effect must be properly balanced. At the same time, it can be seen that the surface driving force in the two stages of the powder. The larger surface tension contributes to the occurrence of fusion and leveling.
Two we can see the peace process to obtain good fusion and leveling is necessary to improve the surface tension of the film flow from above, however, leveling coating is required between coating and substrate has good wettability, and wettability depends mainly on between the coating and substrate surface tension. Here we describe a simple example (see figure below)
Two we can see the peace process to obtain good fusion and leveling is necessary to improve the surface tension of the film flow from above, however, leveling coating is required between coating and substrate has good wettability, and wettability depends mainly on between the coating and substrate surface tension. Here we describe a simple example (see figure below)


We will drop a drop of water in a glass plate, it will be found that the oval shaped droplets attached to the glass plate is not completely wetting on the substrate. If we add a little drops of water on the droplet surface active agent, then we will soon find it drops on the glass plate completely spread out, that reduces the surface tension of the water wetting and the glass plate is very good. This principle is the same as that of the coating on the substrate. Therefore in order to increase the wettability of the coating and the substrate must be added to reduce the surface tension. Below we give the analysis and interpretation of Young's equation.
When the solid-liquid contact surface will form a contact angle can be used to describe the relationship of Young's equation:

νsvsllvcosθ

among:νsv――Surface tension between solid and gas

νsl――Surface tension between solid and liquid

νlv――The surface tension between the liquid and gas. The contact angle

From here we can see that the contact angle of the smaller the better wetting effect

Only when v SV, V sl+ V lvcos theta can occur in Young's equation of wetting and spreading. Sometimes in order to facilitate people with spreading efficiency of S= V SV - V sl- V Lv to express

cosθ=(νsv sllv

thereforeS=νlv(cosθ-1)

Because the cos theta can never be greater than 1, the equation S= V LV (COS theta -1) that may not happen completely wetting, only in the extreme case of theta = 0, the spreading efficiency will be equal to zero. From one hand, the contact angle is smaller, the S has a smaller negative, i.e., angular spread better. From the equation of COS theta = (V - SV V SL / V LV) can be seen in order to reduce the values of surface tension, V LV coating as small as possible, and the surface tension of the substrate should be as large as possible, there are no pollutants and low surface tension.

Relationship between surface tension and defects in 1.3 powder coatings
Shrinkage occurs in powder coating defects, fisheye, loss of light and other phenomena often exist, which is the paint workers most concerned about one of the most headaches, because in the solvent type coating film-forming material with low viscosity, so the defects are often due to surface tension gradient due to the difference (moving from low the surface tension to the region of high surface tension). This phenomenon, also known as the maragony effect, can be mathematically described by the Fink-Jensen equation. The material flow is given in the equation (q) and film thickness (H), viscosity () and the relationship between the surface tension of the V

q=h2ν/2µ

It can be seen from the above that in order to better control the surface defects, thicker coating film, higher viscosity and equilibrium surface tension are of great importance.
However, scholtens, van der Linde and Tiemersma-Thoone believe that the maragony effect does not have an effect on the defects of powder coatings, and they believe that defects are caused by inadequate wetting of the melt coating and substrate.
Some experiments showed that the wettability of the substrate will improve with the increase of temperature, this is because the interfacial tension between the coating and substrate is reduced (V SL) according to Young's equation, the spreading efficiency becomes large.
De Lange can change the surface tension of epoxy and polyester powder coatings were found in the test of adding acrylic acid additives, in two cases with acrylic acid additive epoxy polyester powder and its contact angle were 600 and 350. When the addition of 0.25% acrylic acid additive is used, the contact angle is decreased obviously, but the defect can not be eliminated completely. When the concentration was 0.5%, the defect was completely eliminated. With the continuous increase, will have a positive impact convection flat, and reduce the orange peel effect. But the experiment also found that with the increase of acrylic additive and the contact angle decreased, could rise, and also found with smaller contact angle system will occur defects. Therefore, the generation of defects is very complex, is the result of a variety of factors.

1.4 conclusions
The Nix and Dodge equations quantitatively give the relation between powder particles in the first phase of fusion. The Orchard equation describes the flow field of the continuous film after fusion. The two processes show that the high surface tension promotes the flow, but on the other hand, the low surface tension also increases the wettability, reduces the defect generation, and the low melt viscosity also promotes the better flow. Therefore, in practice, we should consider two opposite factors to balance the final performance. The best way to solve the problem is to add proper amount of leveling agent. Owing to the difference of the polymerization monomer and polymerization method, there is a possibility of incompatibility between different leveling agents. In order to ensure good wetting performance. The leveling agent should be dispersed in the powder system, and it is necessary to have good dispersion in the production process. Low surface tension of the pollutants will seriously affect the surface properties of the coating. Therefore, in the process of production or processing to avoid the introduction of low surface tension into the paint, such as oil, water vapor, silicone oil, etc..
Powder system is different in different surface tension, also has the possibility of interference between each other, with high surface tension in the actual production in the arrangement of varieties to low surface tension in order to reduce the production of varieties of cross contamination phenomenon.
1.5 matters needing attention
From the above discussion we can see that the leveling agent in practical application should pay attention to add volume and the choice of varieties, the first leveling agent of any breed has a problem of optimal dosage, little use will produce flow in place of the phenomenon, but the excessive use can not only produce better flow and redress the deterioration of flow (from above two in the process of flat flow can add the appropriate amount of necessary, we all know that) it leveling agent is liquid viscous after excessive use will reduce the glass transition temperature of the powder and the powder stick is not easy to flow, so when you found in the actual in the application by adding excessive leveling agent has no effect when should find other factors rather than more leveling agent.
Secondly, the convection leveling agent selection must be careful, leveling agent of different monomers has certain compatibility, which is caused by the surface tension difference, the most typical phenomenon is the mutual cross contamination of shrinkage hole. Generally low surface tension of the material will have a high surface tension of the interference of objects, such as the use of a class of silicone powder made of the flow of the agent will be the production of ordinary acrylic leveling agent interference. In addition to the powder mixed with water oil and other foreign bodies with low surface tension will also have small pits phenomenon, surface tension difference of materials is very large, can be regarded as the extreme form of contraction, while the volcano is serious, when they overlap will form a mesh structure, this phenomenon is most common in the two component system. Generally speaking, the pollution level will change with the concentration of pollutants. At low pollution concentrations, we can find an isolated small contraction point, but when the concentration increases, these small shrinkage points become more and more until they dissolve each other. These defects are, in any case, due to the diffusion of low surface tension substances on the surface of another product. In general, a single particle is less likely to be noticed. However, the defects caused by the diffusion effect can be easily seen by the naked eye. Some components in room temperature is very stable but reach a certain temperature will lead to a sharp decline with the viscosity to produce local surface tension and finally the formation of pits or shrinkage, as some containing amide wax powder will be seriously polluted not containing amide wax powder.
Polyacrylate polymer wetting regulator is now commonly used is a balance of surface tension, it is a methyl methacrylate and butyl acrylate polymerization, the polymer can cross the surface tension between the balance of different powder effective difference caused by this is due to the surface tension of acrylic polymer in the baking process migration to coating surface the formation of single molecular layer made of different components tends to be uniform. But the application of this method is conditional, when the two components of the surface tension is so different that even add the interference of wetting enhancer nor equilibrium surface tension caused by a difference when the shrinkage occurs, so the wetting enhancer is only an auxiliary means of scientific design formula is the main. The commonly used levels of contamination are as follows:

Least sensitive-----

 

 

 

 

 

 

Most sensitive-----------

 

 

 

acrylic acid

Polyurethane / polyester blend system
Dull epoxy
Polyester /TGIC
High gloss epoxy
Low light mixed type
High light mixed type
Hammer lines and wrinkles

 

This provides a scientific basis for our production, in the actual production must take into account the various varieties of powder pollution sensitivity. If you can produce acrylic acid and production of no light and high light epoxy hybrid powder, can arrange the production according to the following sequence, producing high light mixed reproduction light epoxy acrylic acid production at last, so you can make the cross contamination to a minimum, because of pollution from the sensitivity for acrylic acid contamination resistance of the strongest.
1.6 quality evaluation method of leveling agent
Strictly speaking, leveling agent and no unified evaluation standard, this is because the product positioning of each manufacturer and characteristics of different decisions, but we can summarize some commonly recognized parameters to constrain suppliers in order to provide them with products can meet our requirements.
We need to consider the following aspects in terms of liquid leveling agent:
Viscosity, test standards can be carried out according to GB/T1723-1993,
Solid content, according to GB/T1725-1989,
The flash point, according to GB3536-83, open cup method
In addition to these numbers, we must perform the following tests to determine the product choice, this is because each monomer process in the production process of our factory leveling agent for use in the different additives and this will lead to the polymer structure and the molecular link between the degree of polymerization, difference of wetting and the degree of leveling effect, especially in the import flow leveling agent in the extremely obvious is the most obvious mutual incompatibility.
The factors that we need to consider in order to absorb the flow of silica or other carriers is the carrier If the choice of silica then its fineness and dispersion effects? What is the effective content of this leveling agent? How can dispersion flow freely? These factors can affect the dispersion and final surface state of the leveling agent in the system.
For the resin used as a carrier of masterbatch, we need to understand what is the carrier, the effective content and whether there is a special additive, for heavy metal containing such products in the selection must be careful.

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