New silicone resins open up a wider range of applications

This paper introduces the properties of two important silicone resins used in the preparation of heat resistant coatings. While such resins are usually cured by heat, the new resins can be cured by a catalyst at room temperature. The resin offers many advantages, including low viscosity and the ability to coat large components.

The chemical structure of organosilicone resin and organosilicone hybrid resin determines its excellent performance which cannot be achieved by other resins. These substrates have been used as MAJOR components in many industrial coatings, from weather-resistant, chemical-resistant building protective coatings to heat-resistant coatings.

Silicone resin is more used for high temperature resistant coatings because of its higher silicone content and better high temperature resistance than silicone hybrid resin. High temperature resistant coatings are mainly used in exhaust systems, industrial ovens, grilles and combustor, and must have both corrosion resistance, weather resistance and excellent thermal stability. This type of coating is usually applied to the surface of steel with a dry film thickness of 20 ~ 25μm. According to their different chemical structures, silicone resins can have the following special properties:

> Thermal stability

> weatherability

> Resilient even at low temperatures

> Chemical resistance to aromatic and aliphatic solvents

> Low surface tension

> hydrophobicity, surface activity

> Anti adhesion and surface smoothness

Two major types of heat resistant silicone resins

The solvent, liquid resin and emulsion silicone resins used in high temperature resistant coatings are mainly methyl silicone resin and methyl - phenyl silicone resin. Coatings made from silicone resins containing only phenyl groups are thermoplastic and are intended for niche use, not large-scale use. Methyl silicone resin is polymethyl siloxane, the content of organic groups is the lowest. The varnish is prepared and its long-term heat resistance is between 180 and 200 ° C, but this is not common. The temperature stability of the paint can be increased to 600 ° C by adding inorganic pigments, such as aluminum powder, mica or iron oxide black.

Prolonged exposure to high temperatures usually results in complete oxidation of the methyl group, leaving behind the SiO2 skeleton. This chemical similarity to silica structure may partly explain the inorganic character of the resin. Commercially, methyl silicone resins are mainly solvent-based products.

Thus, the resin retains the following characteristics of polymethylsiloxane:

> Higher hardness

> lower thermoplasticity

> Poor compatibility with pigments

> Good compatibility with inorganic products and mineral materials

> has limited compatibility with organic compounds

> Good early water resistance even if only partially cured

> hydrophobicity after crosslinking

In addition to the methyl group, the phenyl content of methyl-phenyl silicone resins usually exceeds 20%. The phenyl groups in these resins increase the long-term heat resistance of the resin to 200 ~ 250℃. In addition, the addition of inorganic pigments may increase the heat resistance (depending on the formulation) to 650 ° C.

The compatibility with organic compounds (such as resins or co-substrates) has been significantly improved. The improvement of blend compatibility means that methyl-phenyl silicone is often used as the starting point for synthesizing hybrid silicone. However, these methyl-phenyl silicone resins are not easily compatible with methyl silicone resins because of the large polarity difference between them. In general, methyl-phenyl silicone resins are supplied in the form of aromatic solvent-based resins.

Thermocuring and room temperature crosslinking systems

Methylsilicone and methyl-phenyl silicone resins can generally be divided into two types: traditional thermocuring systems (curing at high temperatures in an oven to form the final film) and new versatile room temperature curing systems.

In traditional heat curing systems, physical drying occurs first, meaning that the solvent evaporates from the coating formulation. Heat is then applied to cross-link the resin molecules. In contrast, room temperature curing systems require no heating. Both physical drying and chemical crosslinking occur at room temperature.

Chemical crosslinking is initiated by adding a catalyst in the presence of moisture in the environment, without heating. FIG. 1 shows various curing conditions and curing processes. To accelerate the curing of a room temperature curing system in the presence of ambient moisture, an appropriate catalyst must be added, such as catalyst 1(TnBT) or a mixture of catalyst 1 and catalyst 2(tetramethylguanidine TMG). The chemical structures of these catalysts are shown in Figs. 2 and 3.

In the mixed catalyst, catalyst 1 acts as a Lewis acid to participate in the reaction, forming a chemical bond with the polymer, and catalyst 2 acts as a strong base to speed up the reaction. Both catalysts are miscible with each other and can be dissolved in xylene. The addition amount is 0.5% ~ 6% of the solid content of silicone resin.

To achieve complete cross-linking, it must be recognized that ambient moisture is the key because water is required for the alkoxy groups in room temperature cured silicone resins to hydrolyze, and only after hydrolysis can the condensation reaction between silanol groups occur.

Therefore, the curing mechanism of the film is a hydrolyzation-condensation reaction process (FIG. 4), which requires water (moisture in the air) and does not require high temperature, which is necessary for traditional thermocuring systems. The key structural differences between the two base systems are in the density and molecular weight of the functional groups (FIG. 5).

The molecular weight of methyl-silicone and methyl-phenyl silicone systems that require high temperature curing in an oven is substantially higher than that of organosilicone cured at room temperature. In addition, the density of alkoxy or silanol functional groups is very low. To obtain a high hardness, fully cross-linked coating, it is usually necessary to heat cure such silicone resins for 30 min at approximately 250°C.

Advantages of room temperature curing system

The silicone resin cured at room temperature has many alkoxy functional groups and low molecular weight. The low molecular weight makes the product very low viscosity, which makes it very good construction properties, such as spraying. Similarly, the system has a very high content of active substances, can prepare high solid coating system with very low VOC content.

Typically, the alkoxy group content in room temperature cured methylsilicone resins is about 15% to 30%(mass fraction), and the active substance content in commercially available products is up to 100%. In the field of methyl-phenyl silicone resins, recent developments in the catalysis of hydrolysis/condensation reactions have made it possible to use room temperature crosslinked silicone resins on a large scale.

A new type of methyl-phenylsiloxane resin has a methoxy content of 15% to 20%(mass fraction) and an active substance content of 90%(solvent: xylene). It is worth noting that its viscosity is low, about 130mPa · s, in the process of coating production only need to add a small amount of solvent, can achieve flexible coating formulation design. Another advantage is that the initial smoke generation is very low.

Due to the regulatory requirements in some application fields, new high solid organosilicone resins have been developed using derivatives of the ethoxy function. There is a kind of this kind of resin ethoxy content is 18% ~ 25%(mass fraction), the active substance content is 95%(solvent: propylene glycol methyl ether acetate), its viscosity is particularly low, only about 50 mPa · s, especially suitable for the coating system with very low solvent content.

In general, methyl-phenyl silicone resin curing films have very good adhesion, good flexibility and excellent compatibility with organic components. The content of methyl siloxane is 30% ~ 40%(mass fraction), and the content of active substance is 100%. Due to the extremely low viscosity (about 10mPa · s), there is little need to add solvent to the formulation. The smoke produced during the initial drying process is minimal and can be neglected. The hardness of the cured film is very high, and it shows good color stability and strong hydrophobicity. The advantages of room temperature drying are obvious. Because in the case of high temperature curing, the size of the object to be coated will be limited by the size of the oven.

With ROOM TEMPERATURE CURING SILICONE RESIN, EVEN LARGE OBJECTS (larger than oven size) can be COATED WITH HIGH TEMPERATURE RESISTANT SILICONE resin coatings. This opens up a wider field for the application of high temperature resistant coatings. However, it should be noted that a large number of alcohols are released during curing of these resins.

Last but not least, the energy consumption of room temperature curing silicone resins is significantly lower than that of thermal curing systems.

Summary of the latest advances in silicone resins

The fundamental reason for the successful use of silicone resins in the field of high temperature coatings is their unique properties. Room temperature curing system is more popular than traditional heat curing system.

The energy required for baking is saved by using catalyst for curing at room temperature and ambient moisture. The size of the object to be painted is not limited by the size of the oven, thus opening up a wider range of applications, especially in industry.

Conventional heat-cured silicone resins produce significantly less smoke and VOC during curing, meeting the growing demand for more environmentally friendly systems.

"The requirement for moisture content in the environment is very low."

Two questions for Marco Heuer

A growing number of heat resistant silicone coatings can be cured at room temperature. What are the limitations of using them, and under what circumstances is it still necessary to use heat-cured silicone resins?

Heat resistant silicone coatings cured at room temperature take a long time to cure completely. Therefore, for large objects such as industrial mufflers or chemical installations, the use of room temperature curing silicone paint has great advantages. But for small, mass-produced items that use heat-resistant silicone coatings, such as automotive exhaust systems, where production flux is critical, this situation is typical for areas where heat-curing coatings are needed.

What are the requirements for ambient moisture content for room temperature curing, and can this paint be used in desert areas?

The demand for ambient moisture is very low. Solidification can be achieved even in desert-like areas. In areas with high humidity, complete curing is not a problem. Film thickness is an important factor, as higher film thickness means it takes longer to cure completely.