New silicone resins open up a wider range of applications

Oct 10, 2019

This article describes the properties of two important silicone resins used in the preparation of heat resistant coatings. Generally, such resins need to be cured by heating, and the novel resin can be cured by a catalyst at room temperature. This resin has many advantages, including low viscosity and the ability to coat large parts.
The chemical structure of silicone resins and silicone hybrid resins determines that it has excellent properties not achieved by other resins. These binders have been a major component of many industrial coatings, from weatherable, chemically resistant architectural protective coatings to high temperature coatings.
Silicone resins are mostly used in high temperature resistant coatings because of their higher silicone content and superior high temperature resistance than silicone hybrid resins. High temperature resistant coatings are mainly used in exhaust systems, industrial ovens, grills and combustion chambers, and must have both corrosion resistance and 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 to 25 μm. Depending on its chemical structure, silicone resins can have the following special properties:
>thermal stability
>Weather resistance
> Maintains flexibility even at low temperatures
>Chemical resistance to aromatic solvents and aliphatic solvents
> low surface tension
>Water repellency, surface activity
>Anti-stick and surface slip
 Two types of heat resistant silicone resins
   Solvent-based, liquid-resin and emulsion-type silicone resins used in high-temperature coatings are mainly methyl silicone resins and methyl-phenyl silicone resins. Coatings made from silicone resins containing only phenyl groups are thermoplastic and can only be used in niche areas and are not suitable for a wide range of applications. The methyl silicone resin is polymethylsiloxane with the lowest content of organic groups. It is prepared as a varnish with a long-term heat resistance of between 180 and 200 ° C, but this is not common. The temperature stability of the paint can be increased to 600 ° C by adding an inorganic pigment such as aluminum powder, mica or iron oxide black.
Long-term exposure to high temperatures usually leads to complete oxidation of the methyl group, leaving the SiO2 skeleton. This chemical similarity to the silica structure can partially illustrate the inorganic character of such resins. Commercially, methyl silicone resins are mainly supplied in solvent-based products.
Therefore, the resin retains the following characteristics of polymethylsiloxane:
>High hardness
>Low thermoplastic
> Poor compatibility with pigments
>Good compatibility with inorganic and mineral materials
>Limited compatibility with organic compounds
>Good early water resistance even if only partially cured
>Water-repellent after cross-linking
The methyl-phenyl silicone resin has a phenyl content of usually more than 20% in addition to a methyl group. The phenyl group in these resins increases the long-term heat resistance of the resin to 200 to 250 °C. In addition, the addition of inorganic pigments may also increase the heat resistance (related to the formulation) to 650 °C.
The compatibility with organic compounds such as resins or co-binders has been significantly improved. An improvement in blend compatibility means that methyl-phenyl silicone resins are often used as a starting point for the synthesis of hybrid silicone resins. However, these methyl-phenyl silicone resins are not easily blended and compatible with methyl silicone resins because of the large difference in polarity between the two. In general, methyl-phenyl silicone resins are supplied in the form of aromatic solvent-based resins.
Thermal curing and room temperature crosslinking system
Methyl silicone resins and methyl-phenyl silicone resins are generally classified into two types: conventional heat curing systems (high temperature curing in an oven to form a final coating film) and a novel multi-purpose room temperature curing system.
Traditional heat curing systems first undergo physical drying, which means that the solvent evaporates from the coating formulation. Subsequently, heating is performed to cause crosslinking of the resin molecules. In contrast, room temperature curing systems do not require heating. Physical drying and chemical crosslinking occur simultaneously at room temperature.
Chemical crosslinking is initiated by the addition of a catalyst in the presence of moisture in the environment without heating. Figure 1 shows a schematic of various curing conditions and curing processes. In order to accelerate the solidification of the room temperature curing system in the presence of ambient moisture, it is necessary to add a suitable catalyst such as catalyst 1 (tetra-n-butyl titanate, TnBT) or a mixture of catalyst 1 and catalyst 2 (tetramethylguanidiniumTM). The chemical structures of these catalysts are shown in Figures 2 and 3.
In the mixed catalyst, Catalyst 1 participates in the reaction as a Lewis acid to form a chemical bond to the polymer, and Catalyst 2 accelerates the reaction rate as a strong base. Both catalysts are miscible and can be dissolved in xylene. The amount added is 0.5% to 6% of the solid content of the silicone resin.
In order to achieve complete cross-linking, it must be recognized that environmental moisture is the key, because water must be present in order to hydrolyze the alkoxy groups in the room temperature curing silicone resin, and the condensation reaction between the silanol groups can occur only after hydrolysis.
          Therefore, the curing mechanism of the coating film is a hydrolysis-condensation reaction process (Fig. 4), the reaction requires water (moisture in the air), high temperature is not required, and high temperature is a necessary condition of the conventional heat curing system. The key structural differences between the two binder systems are the density and molecular weight of the functional groups (Figure 5).
A methyl silicone resin and a methyl-phenyl silicone resin system which require high temperature curing in an oven have a molecular weight much higher than that of a room temperature cured silicone resin. Further, the functional group density of the alkoxy group or silanol of the baked resin is also very low. In order to obtain a high hardness, fully crosslinked coating, it is usually necessary to heat cure such a silicone resin at about 250 ° C for 30 min.
Advantages of room temperature curing system
The room temperature curing silicone resin has many alkoxy functional groups and low molecular weight. The low molecular weight allows the product to have a very low viscosity, which results in very good application properties such as sprayability. Also, the system has a very high active substance content and can produce a high solids coating system with a very low VOC content.
Generally, the content of the alkoxy group in the room temperature-cured methyl silicone resin is about 15% to 30% (mass fraction), and the content of the active material in the commercially available product is as high as 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 in a wide range.
A novel methyl-phenylsiloxane resin has a methoxy content of 15% to 20% (mass fraction) and an active content of 90% (solvent: xylene). It is worth noting that its viscosity is low, about 130mPa·s, and only a small amount of solvent can be added in the paint production process to achieve flexible coating formulation design. Another advantage is that the smoke generated during the early stages of baking is very low.
Due to regulatory requirements in certain applications, the development of new high solids silicone resins was prepared using ethoxy-functional derivatives. One such resin has an ethoxy group content of 18% to 25% (mass fraction) and an active material content of 95% (solvent: propylene glycol methyl ether acetate), and its viscosity is particularly low, only about 50 mPa·s, especially Suitable for coating systems with very low solvent content.
Generally, a cured coating film of a methyl-phenyl silicone resin has characteristics such as very good adhesion, good flexibility, and excellent compatibility with an organic component. The methylsiloxane has a methoxy group content of 30% to 40% (mass fraction) and an active material content of 100%. Since the viscosity is extremely low (about 10 mPa·s), it is almost unnecessary to add a solvent to the formulation. The amount of smoke generated during the initial drying process is minimal and can be ignored. The cured coating film has a very high hardness and exhibits good color stability and strong water repellency. The advantage of drying at room temperature is obvious. Because in the case of high temperature curing, the size of the article to be coated is 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 silicone resin coatings. This opens up a broader field for the application of high temperature resistant coatings. However, it should be noted that a large amount of alcohol compounds are released during the curing of these resins.
Finally, it is also important that the room temperature curing silicone resin consumes significantly less energy than the heat curing system.
Summary of the latest advances in silicone resins
The fundamental reason why silicone resins have been successfully used in the field of high temperature resistant coatings in the industry is their unique properties. Room temperature curing systems are more popular than conventional heat curing systems.
The curing is achieved by using a catalyst at room temperature and ambient moisture, saving the energy required for baking. The size of the object to be coated is not limited by the size of the oven, thus opening up a wider range of applications, especially in the industrial sector.
Traditional heat-curing silicone resins have significantly reduced smoke and VOC levels when cured, meeting the growing demand for more environmentally friendly systems.
“The requirements for moisture content in the environment are very low.”
2 questions about Marco Heuer
More and more heat resistant silicone coatings can be cured at room temperature. What are the limitations of using them, and under what circumstances do you still need to use thermosetting silicone resin?
It takes a long time for the room temperature curing heat resistant silicone coating to fully cure. Therefore, for large objects such as industrial mufflers or chemical plants, the use of such room temperature curing silicone coatings has great advantages. However, for mass production of small items using heat-resistant silicone coatings (such as automotive exhaust systems), throughput is critical, a typical area where thermal curing coatings are required.
What is the requirement for moisture content in the environment at room temperature, can this coating be used in desert areas?
The demand for moisture in the environment is very low. Curing can be achieved even in desert-like areas. In areas with high humidity, there is no problem with complete curing. Film thickness is an important factor, and high film thickness means it takes longer to fully cure.