The secret of efficient conductive electrostatic powder coatings lies in conductive carbon fibers?

Electrostatic powder coatings have the characteristics of environmental protection, high safety performance, relatively low cost, easy to coat, etc.
, and at the same time have the additional special function of anti-static, which has been widely used
in electronic information, chemical safety, coal explosion-proof and other fields.

Anti-static powder coatings are also very suitable for home appliances, small household appliances, power distribution cabinets and other civilian fields
because of their aesthetics and simplicity.

Conventional anti-static powder coatings are usually composed of 35% to 55% film-forming resin, 10% to 30% conductive materials, 15% to 30% fillers and 1% to 5% additives, and the mixture is prepared by melting mixed extrusion, tablet crushing, crushing (grinding) and other steps to obtain on-line coatingol.
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of the coating.

The addition of conductive materials is usually added in the production process of powder coatings, which not only makes the utilization efficiency of conductive materials not high, but also has a great impact on
the basic properties of the final coating due to some problems such as high oil absorption values of conductive materials.

The conductive materials used for traditional anti-static powder coatings are mainly carbon black, metal materials, etc
.
Carbon black is not only polluting and troublesome to operate, but also greatly reduces the impact strength, adhesion and wear resistance of the powder coating itself because of its high dosage
.

Not only that, the powder coating using carbon black as a conductive material cannot be toned (can only be made black), which greatly reduces the decorative effect of the paint;

When metal materials are added to antistatic powder coatings as conductive materials, although they can provide richer colors, their relative density is larger and the price is high, which greatly increases the production cost
of coatings.

In this study, the polyester/epoxy hybrid system is used as the main film former, and conductive carbon fiber is added to the basic formula of powder coatings by using conductive carbon fiber as a conductive material component, and a powder coating
with high efficiency conductive static discharge is prepared.

Formulation raw materials

The principle of powder coating conducts static electricity

According to the research summary of predecessors, the current principle interpretation of the electrostatic conduction of powder coating coatings mainly includes three major theories: infinite mesh chain theory, thermodynamic theory and competition theory
.
The details are as follows:

01

The infinite mesh chain theory was proposed by F.
Butch in 1972, and its main point is that when the concentration of metal particles in the polymer system reaches a certain critical value, the metal particles will act like a bridge, so that the free electron carrier "queues" from one end of the polymer to the other end, so that the insulator becomes a semiconductor or conductor, and the quality of the conductive grid chain composed of conductive particles in the coating film determines its ability
to conduct electrostatics.

02

Competition theory believes that the conductive motor theory of conductive powder coatings is the result
of the competition between three mechanisms: conductive channel, tunnel effect, and field-induced emission.

Since some of the conductive particles in the coating can contact each other to form a chain-like conductive channel, this is the conductive channel mechanism;

The tunneling effect and field-induced emission mechanism believe that the conductivity of the coating is not conducted by direct contact with the electric particles, but by the migration of electrons between particles due to thermal vibration or internal electric field.

Under the condition that the content of conductive material is low and the applied voltage is low, the spacing of conductive particles is larger, and the probability of forming a chain-like conductive channel is small, and the main role in this time is the tunneling effect
.

Under the condition of low content of conductive material and high applied voltage, the internal electric field between conductive particles is very strong, and electrons will have a great chance to leap to the film-forming interface layer and transition to the adjacent conductive particles to generate current, at which time the field-induced emission mechanism plays a major role
.

There are many factors that affect the static conductivity of doped type conductive powder coatings, and there are three main factors:

First, it is the type and characteristics of the film-forming main body, such as the degree of crystallization, the type of structure, the relative molecular mass, the surface tension and viscosity, etc.
;

Second, is the type and properties of conductive materials, such as the size, dosage, quality stability and microscopic surface characteristics of conductive particles;

Third, it is the influence of the production process in the preparation process of powder coatings, such as the type of extruder, production temperature, melting and mixing effect, etc
.

03

Thermodynamic theory believes that the interface effect formed between the film forming of the coating body and the conductive filler has a great influence on the formation of conductive circuits in doped systems.

In the preparation process of conductive electrostatic powder coating, the free surface of the conductive filler particles becomes a wet interface, and an interface layer is formed between the matrix and the filler, and the system generates excess interface energy, and with the increase of conductive filler, the critical concentration is a parameter related to the interface energy of the system.

When the interface energy surplus of the system reaches a certain level, the conductive particles begin to form a conductive network, which is macroscopic manifested as a sudden decrease in the resistivity of the system, which is a good explanation of the electrostatic discharge and conduction process
of the antistatic powder coating with carbon black as the conductive material.

Comparison of the drainage effect of different coating appearances

In general, according to the appearance effect of the coating, the coating can be divided into leveling, sand, orange and so on
.
According to the basic formula material, the epoxy/polyester hybrid system is used as the film-forming body, and the same proportion of conductive carbon fiber is added, and the powder coating of the leveling, sand-grain and orange type is prepared for the test comparison of the surface resistivity, and the results are shown in
Table 2.

The test results show that whether it is a planar coating or a textured coating, adding a certain amount of conductive carbon fiber, the coating can conduct static electricity
.

The coating surface formed by the orange type coating is uneven and the resistivity is small, which may be due to the fact that the carbon fiber particles are affected by the Bernard vortex effect during the melting and curing process of the film, and the conductive particles formed are more tightly
bridged.

Effects of conductive carbon fiber particle size

Within a certain range; The larger the particle size (long), the easier it is for the short-cut fiber dispersion process to form a tighter network structure; If the particle size is too small, it will lead to a longer spacing between adjacent conductive fiber particles, and the bridge chain structure of conductive particles will be sparse
.

According to the basic formula, 50 mesh, 100 mesh and 250 mesh conductive carbon fiber are selected to prepare three groups of wrinkled coatings, and the test results are shown in Table 3
.

The coatings of the 3 groups of samples were magnified by microscope, and the surface condition was shown in
Figure 1.

It can be seen from Figure 1 that under the condition of a certain amount of addition, the larger the particle size of the conductive carbon fiber, the tighter the conductive network structure chain formed in the coating, the lower the surface resistivity of the coating, and the better
the electrostatic conduction effect.

Effects of conductive carbon fiber dosage

The resistivity of conductive carbon fiber is 1.
4× 10-3 Ω·cm, and the conductivity is strong
.
The effect of different dosages on the resistivity of the coating is shown in Table 4
.

As can be seen from Table 4, under certain conditions, the surface resistivity decreases
with the increase of carbon fiber.
However, considering the electrostatic spraying coating process and cost of powder coatings, it is recommended that the amount of conductive carbon fiber should not exceed 1.
6%.

Influence of the production process

In the production process of powder coatings, the melt extrusion process of the extruder is an important process
to ensure that the components in the coating formulation are fully mixed and even.

Usually classified according to the number of screws of the extruder, it is generally divided into single screw extruder and twin screw extruder
.
According to the same formulation, the sample of the powder coating with a single screw extruder and a twin screw extrusion mechanism was prepared in this study, and the surface condition of the resulting sample coating was detected by a microscope, as shown in
Figure 2.

From the perspective of Figure 2, under the same conditions, compared with the twin screw extruder, the dual screw extrusion mechanism prepared sample coating single screw extrusion mechanism prepared sample coating single screw extruder production prepared by the conductive static powder coating bridge section is longer, the formation of conductive network is more obvious
.

This is due to the fact that during the extruder extrusion process, the shear force generated by the rotational movement of the twin screw extruder is more destructive to the conductive carbon fiber particles, and the proportion of carbon fiber particles being crushed is higher
.
Therefore, it is better
to use a single screw extruder in the production process of preparing a powder coating that conducts static conduction.

epilogue

The powder coating prepared by adding conductive carbon fibers as conductive material components to powder coating formulations has the effect
of efficiently conducting static electricity.

At the same time, due to the excellent conductive properties of conductive carbon fibers, the amount of addition in the product formula is less, which overcomes the shortcomings of the production operation and high cost caused by the high proportion of conductive materials used in the production and preparation process of traditional anti-static powder coatings
.
Compared with traditional antistatic powder coatings, the powder coating products in this study have a better price-performance ratio
.