Silicones in the coatings industry

Most of the additives used in coatings are surfactants, which achieve the effect

Silicone additives for coating systems are divided from the chemical composition, mainly including silane and silicone oil two categories, as shown in Figure 1 paint online coatingol.

1 Silicone additive used as a leveling agent for coatings

The structure of the leveling agent for coatings is generally composed

Silicone leveling agents are an important category of coating leveling agents, in general, low relative molecular mass (<5 000) polydimethylsiloxane can be used as a leveling agent, but because of its poor compatibility, the use is more limited, the leveling effect is not good enough, can not be applied to high-grade systems<b10> 。 At present, the silicone leveling agent widely used in coatings is a variety of modified silicone oil, according to the different types of modification, there are mainly polyether modified silicone oil, polyester modified silicone oil, phenyl modified silicone oil, alkyl modified silicone oil, phenyl alkyl co-modified silicone oil and alkyl - polyether co-modified silicone oil, etc.

The basic structure of polyether-modified silicones is shown in formula (1

Where the values of m and n determine the content of the siloxane segment in the structure, the smaller the m/n value, the higher the corresponding silicon content; The x and y values determine the relative molecular mass and hydrophilicity of the polyether segment (x indicates the number of propylene oxide units, which is a hydrophobic unit, y represents the number of ethylene oxide units, is a hydrophilic unit, when x >y, the polyether segment is mainly hydrophobic, x <y, the polyether segment is mainly hydrophilic), the values of m, n, x, y determine the performance of the corresponding polyether-modified silicone fluid, and also determine their use<b10>.

If the leveling agent is required to have a good feel and anti-shrinkage hole ability, it usually requires a higher silicon content and a longer siloxane segment length, which can be achieved by increasing the m and n values; When the leveling agent is required to have good defoaming ability, a higher silicon content and propylene oxide content are required, and the y value can be reduced by increasing the values of m, n, x to meet the requirements; When it is necessary to improve the safety of the use of additives and reduce the risk of varnish film malpractice, a relatively low silicon content is required, which can be achieved by increasing the value of m and reducing n; When it is necessary to use for aqueous systems, due to the need for a certain degree of water solubility, it is necessary to have a less high silicon content and a higher content of hydrophilic segment ethylene oxide, which can be achieved by increasing the values of m and y

When the leveling agent needs to be used in high-temperature systems (such as baking paint), due to the low decomposition temperature of polyether, it is easy to decompose during use, resulting in some drawbacks on the surface of the paint film, such as poor recoating, shrinkage, etc.

With the development of technology, there have recently been some leveling agents that combine silicone and acrylate, which combine the advantages of both, and have excellent pre-leveling and post-leveling effects

2 Silicone additives used as defoamers for coatings

As a defoamer for coatings, it is often necessary to have a lower surface tension than the application system and is incompatible
with the application system.
Due to their low polarity structure, silicones are not compatible with most coating resin systems and are widely used as raw materials
for coating defoamers.

In general, dimethicone with a relatively large molecular mass (10 000 ~ 100 000) can be used as a coating defoamer, and at present, such defoamers are widely used in
heavy antiseptic and other systems.
In order to improve the defoaming efficiency, some silica can also be added as a synergistic component
.
In addition, due to the high defoaming efficiency of silicone, it has a good effect at a very low amount of addition, in order to facilitate addition, the silicone defoamer is usually made into a lower concentration of solution to meet the needs
of actual production.

In the solvent-based coating system, in order to enhance the defoaming capacity of the coating, the fluorocarbon modified silicone will be used, by introducing a fluorocarbon segment with lower compatibility and surface tension in the silicone backbone, further reducing the compatibility of the coating with the system, greatly improving the defoaming effect, usually the longer the fluorocarbon chain is introduced, the better the defoaming effect, this type of defoamer is usually a high-end variety
of silicone defoamer.

In waterborne coating systems, due to the high polarity of waterborne resins, conventional dimethicones have limited compatibility with the system and are difficult to directly use as defoamers
.
Polyether modification of dimethicones is often required, especially in high-light systems
.
Generally, the higher the degree of polyether modification, the better the compatibility, but the defoaming effect will also decrease, generally need to choose the appropriate proportion of modification according to the difficulty of defoaming of the system, under the condition of meeting the defoaming capacity, the lower the silicon content, the better
.

Water-based silicone defoamer according to its own state, divided into concentrated and emulsion type 2 categories, concentrated defoamer has strong foam inhibition ability, but is not easy to disperse, in use need of strong shear; Emulsion-type defoamer is obtained by emulsifying a concentrated defoamer, which greatly improves the convenience of use and can be introduced into the
system by simply stirring.

3 Silicone substrate wetting agent

In the water-based system, because the surface tension of the water is too large, it is difficult to wet the substrate (especially the low surface tension substrate) quickly, so in the waterborne coating system, the use of the substrate wetting agent is particularly important
.
Silicone substrate wetting agents are widely used
in waterborne coating systems due to their wide applicability.

Formula (2) is the structure of a typical wetting agent for the substrate of
an aqueous system.

As can be seen from formula (2), the substrate wetting agent is actually a special class of polyether-modified silicone oils, and its relative molecular mass is usually very small
.
Because the rapid wetting of the substrate is a dynamic process, and according to the principle of physical chemistry, the principle of action of the wetting agent is to reduce the surface tension of the system by rapid migration to the surface of the system, so as to achieve the effect
of wetting the substrate.
According to the view of molecular kinematics, the migration rate of molecules depends to a large extent on the relative molecular mass, and the smaller the relative molecular mass, the faster
the migration rate.
Therefore, polyether-modified silicone oil with relatively small molecular weight is often used as a substrate wetting agent for aqueous systems
.

Further studies have found that polyether-modified silicone oil used as a substrate wetting agent is used in aqueous systems, and due to its special structure [see formula (3)], the silicone segments in it can be fully extended to extend a large amount of -CH3 outward, thereby greatly reducing the surface tension
of the system.
Usually, polyether-modified silicone oil with high relative molecular weight can only reduce the surface tension to 25 ~ 26 N / m in aqueous systems, while the wetting agent of the substrate of small molecules can usually reduce the surface tension to less than 22 N / m, and the lowest can be up to 20 ~ 21 N / m
.
Such a low surface tension makes water-based coatings with substrate wetting agents wet most low surface tension substrates, and even wet PTFE substrates
.
At the same time, because the substrate wetting agent can reduce the surface tension of the water-based system to a very low level, the substrate wetting agent can give the system good anti-shrinkage pore performance and atomization effect
during spraying construction.

Due to the relatively small molecular mass of the substrate wetting agent, its silicone segments are very short, so it does not give a good slip to the surface of the paint film (usually slip comes from longer silicone segments
).
When there is a requirement for the slip of the paint film, the substrate wetting agent is usually used with a silicone leveling agent with a high relative molecular mass, which is also the difference
between the substrate wetting agent and the ordinary silicone leveling agent.

4 Silicone additives used as wear-resistant additives

For systems that require wear resistance, in addition to increasing the crosslinking density from the resin point of view, the addition of additives is also a common means, and the use of silicone additives is a commonly used method
.
Common are ultra-high relative molecular mass silicone microspheres, ultra-high relative molecular weight silicone emulsions and silicone nano-modifiers
.

Ultra-high relative molecular mass wear-resistant silicone microspheres have a slight crosslinking structure of the high relative molecular mass polysiloxane dispersion, microscopic microsphere-shaped, after adding to the system, will eventually be arranged in a directional arrangement on the surface of the paint film, forming a layer of polysiloxane film, thereby providing excellent slip and surface wear resistance for
the system.

In the waterborne coating system, in addition to silicone microspheres, ultra-high relative molecular mass silicone emulsion is also a commonly used class of wear-resistant additives, they are usually polymerized by silicone emulsion, the internal phase viscosity is extremely high, the relative molecular mass can reach millions, added to the system after the film formation, can form a layer of ultra-high relative molecular mass silicone thin layer on the surface, thereby improving the wear effect of the paint film
.

Recently, someone has developed a special structure of silicone wear-resistant additives, the chemical composition of which is cage-shaped polysesquisiloxane, the structure as shown in formula (4), which is a low-poly cage silicone compound, wherein the R group can be a variety of reactive groups, such as hydroxyl, epoxy, acrylic double bond, etc.
, to add it to the system, its reactive group can participate in the resin curing reaction, thereby introducing an inorganic siloxane-like structure in the system, which can improve the flexibility and wear resistance of the system at the same time
。

5 Used as a silicone additive for coating resin modification

Silicone materials usually have some special properties, such as surface smooth, wear-resistant, weather-resistant, low-temperature and tough and other properties, through some silicone materials with reactive functional groups as modification additives, silicone segments can be introduced into the resin system, thereby introducing the excellent properties of silicone materials into the coating system, as shown
in Figure 2.

Silicone materials for resin modification have a variety of different functional groups, usually silicone polymers with reactive groups at both ends
.
Silicone materials with carbon hydroxyl groups usually have good reaction ability with polyurethane materials and can be used for the modification of polyester or polyurethane resins; Silicone materials with epoxy groups can be used for the modification of epoxy resins; Silicone materials with acrylate double bonds at both ends can be used for the modification of photocuring resins, and silicone materials with acrylate double bonds can also be used for the synthesis reaction of silicone modified emulsions; Silicone materials with amino groups can be used as the synthesis of silicone-modified polyimide resins for some high-end uses; Silicone materials with carboxyl groups can be used as modifications for powder coating resins and participate in reactions
.

6 Silicone additives used as stain-resistant additives

Coating systems applied in some special areas (e.
g.
3C) usually have surface stain resistance requirements
.
Stain resistant additives are mostly silicone additives of various special structures, as shown in formula (5), which usually contain some silicone groups R that can react with the system, and have some silicone segments that can be stretched on the surface, and it is these stretched silicone segments that provide excellent stain resistance
.

When R is hydroxyl, it can be used for polyurethane systems and amino paint systems, suitable for solvent-borne systems when the hydroxyl content is low, and waterborne systems when the hydroxyl content is high; When R is an acrylate double bond, it can be used in light-curing systems; When the R group is epoxy, it can be used in epoxy systems
.

7 Silicone additive used as pigment filler dispersant

In order to enable the pigment to be evenly dispersed and prevent settling, dispersants are indispensable
.
Structurally, the dispersant contains a pigment-friendly group and a solvation segment
.
Silicone compounds are rationally designed to produce compounds
that can be used as dispersants in coating systems.
Common silane coupling agents can be used as dispersants for fillers, and sometimes in order to cope with higher requirements, special structures can be designed to achieve better results
.

Formula (6) is a schematic diagram
of an organosilicon compound used as a dispersant for aqueous coatings.

Since the usual pigment filler surface (especially inorganic pigment filler) adsorbs moisture in the air or its own structure, it contains a certain hydroxyl group, when the silicone compound is added to the aqueous coating system, the organosilane in it will hydrolyze to produce the corresponding silicon hydroxyl group, the resulting silicon hydroxyl group can be further condensed with the hydroxyl group on the surface of the pigment, so as to be firmly connected to the pigment surface, generating a molecular adsorption layer on the surface of the pigment filler, coupled with the entropy stabilization provided by the hydrophilic solvation chain segment contained therein, It can provide a stabilizing effect for the dispersion of pigment fillers in aqueous systems, so it is especially suitable for the dispersion of some inorganic pigment fillers that are difficult to disperse, especially nano-scale inorganic pigment fillers (such as nano iron oxide, nano zinc oxide, nano silicon oxide).

The mechanism of action is shown in Figure
3.

8 Silicone additives used as adhesion promoters

Silane adhesion accelerators have been widely used in solvent-based coating systems with substrates of glass, ceramics, zinc metal, iron metal and some plastic systems
.
The principle of action is shown in Figure
4.

As shown in Figure 4, the alkoxy group in the silane reacts with a small amount of water in the system to generate a silicon hydroxyl group, and the silicon hydroxyl group reacts with the hydroxyl group on the surface of the substrate to form a solid adsorption layer, while the R group contained in the silane contains a reactive group (amino, epoxy, methacryloyl, etc.
) can be crosslinked with the resin system during the crosslinking process, providing a strong bridging effect between the resin and the substrate, thereby greatly improving adhesion
.

The same as the solvent-based coating system, in the waterborne coating system, when applied to glass, ceramics and some metal and plastic substrates, silane can still be used as an adhesion promoter, but unlike the solvent-based system, because the water-based system uses water as the dispersion medium, the silane adhesion accelerator that is usually suitable for solvent-based systems is added to the waterborne system, and it will fail quickly because the hydrolysis speed is too fast, so it can only be added
at the construction site when it is used.

According to the characteristics of the water-based coating system, silanes
suitable for water-based coating systems have been developed.
By adjusting the size of the R group in the silane structure, the hydrolysis rate of the silane is affected by the steric hindrance effect of the R group, so that the hydrolysis cycle of the silane can be synchronized with the service cycle of the waterborne coating, and the storage stability
of the waterborne coating is improved.

9 Silicone additive used as a diplomatic joint agent for waterborne coatings

Waterborne coatings, especially one-component waterborne polyurethane systems, are usually inferior to crosslinked two-component polyurethane systems
in terms of performance because there is no crosslinking.
In order to improve the performance of the use of one-component waterborne polyurethane, a post-crosslinking agent, such as azidine, is often added to the formulation of aqueous polyurethane coatings, and acridine is used to react with carboxyl groups in waterborne polyurethanes to provide a certain degree of crosslinking, thereby greatly improving the final performance of the coating, such as water resistance and solvent resistance
.

However, due to the considerable toxicity of aziridine, it is limited in use, secondly, the crosslinking speed of aziridine is too fast and can only be used as a two-component system, and third, the aziridinyl crosslinking agent cannot improve the adhesion
of waterborne polyurethane to the substrate.

To this end, a variety of new diplomatic agents of different structures have been developed, of which silicone diplomatic agents are one of them, and its principle of action is shown in
Figure 5.

Commonly used silicone crosslinker generally contains epoxy groups, in the paint film drying process, the epoxy group in the diplomatic joint agent can be open loop reaction with the carboxyl group in the aqueous polyurethane, resulting in chemical bond connection, in addition to its own crosslinking reaction, thereby obtaining a three-dimensional crosslinking structure, forming an organic - inorganic hybrid system, thereby improving the water resistance and chemical resistance of the paint film and other properties
.

10 Conclusion

Changes in the silicon content and relative molecular mass of silicone molecules can change the compatibility range of additives and coating systems, thereby affecting the dispersion state
of additives in the system.
According to the corresponding performance requirements of different coating systems such as leveling, hand feel, recoating, wetting, etc.
, the type of silicone additives can be reasonably screened to obtain the ideal coating effect
.
The introduction of more special functional groups such as organic fluorine structures and uniquely designed molecular structures such as star shapes will add more functional additive products
in the field of application.