Silicone Resins: Heat Resistant Coatings

Heat resistant coatings are used frequently in industrial coating applications that can protect the substrate at high temperatures (more than 180 °C). As a result, these are the primary choice of coatings to protect ovens, kettles, pipelines, exhausts, heat exchangers, chimneys, auto engines and stoves.

Fig 01: Heat resistant coatings used in infrastructure (the metallic paint)

Silicone Resin

Standard heat-resistant paint formulations can tolerate heat only up to 150-200 °C and frequently use a variety of organic resins as binders, including epoxy, polyester, alkyd and acrylic. Such paint formulations must include silicone resins as an essential ingredient in order to boost their capability for heat resistance.
Silicone resins have numerous important feature besides heat resistance, such as great weather ability, superior dielectric and water repellency capabilities. This improves durability and the reliability of the coatings.

Silicone resin reaction process: The polycondensation type reaction.

Silicone resins can be produced by hydrolyzing mixtures of chlorosilanes or alkoxy silanes to form highly reactive silanol groups:

Reactive silanol initially condense to form oligomer siloxane structures:

Further condensation occurs to form three-dimensional siloxane lattices. Driving the condensation by applying heat and catalysts increases the molecular weight and improves physical properties.

The most significant monomer in silicone resin technology by volume is methyl-trichlorosilane.  Silicone resins are often produced by combining multiple raw materials to incorporate multiple functional groups. Methyl silicone resins (R=methyl) have a high degree of hardness and cure quickly, but also exhibit brittleness, poor pigmentability, and poor compatibility with organic resins.The phenyl silicone (R=phenyl) based resins have great thermal stability, good pigmentability, and increased compatibility with organic resins, but it also produces products that are much more cross-linked and have lower thermoplasticity than those having methyl groups. A majority of silicone resins are of a mixed methyl phenyl type. The performance of the product can be determined by the methyl to phenyl molar ratio.

Types of silicone resins:

Methyl silicone resin:

A type of silicone resin having a completely methyl-based organic substituent group that produces hard films with high moisture resistance, dielectric characteristics, water repellency, dirt repellancy, UV resistance and releasing qualities. Coating made of this alone can resist temparatures of 400 C.

Methyl/Phenyl silicone resin:

The organic substituent group in these resins, which consists of methyl and phenyl groups, gives them great heat resistance, mechanical strength, water/UV resistant, and gloss. Heat resist up-to 650 °C

Organic resin modified silicone resin:

The silanol functionality can be reacted with hydroxyl groups on organic resins (e.g., polyesters, epoxy, alkyds, etc.) to form silicone-organic hybrid resins with performance improvements proportionate to the level of siloxane modification. They form a coating with the advantages of organic resins (such as mechanical properties, curing profile, economy and adhesion)
While silicone resins can enhance the paint’s thermal, chemical, and UV radiation resistance, a combination of silicone and organic binders will also improve several performance characteristics and physical features like making air dry acrylics, higher corrosion resistant epoxies and tougher alkyds.

Thermal properties of silicone resin:

The thermal behaviour of the silicone resin was analysed by thermo gravimetric analysis at a heating rate of 10°C/min from room temperature to 800°C under an air. In the examples below,

  • Sample A : 100% Methyl silicone resin compound.
  • Sample B : 50% Methyl 50% phenyl silicone resin compound.
  • Sample C : High MW 25% Methyl 75% phenyl silicone resin in solvent.

Thermal degradation behaviour of all samples in air were summarized in Table 1.

Table 01: Thermal degradation data by TGA analysis.

T10% represented the temperature at which the mass loss was 10 wt% of the system. Tmax represented the temperature of the maximum mass loss rate of the system.
The Td (temperature degradation) increased as the silicone resin’s phenyl content increased due to the silicone resin’s rising molecular weight. This could have prevented the -Si-O and -Si-Me groups from breaking down.
The temperatures of the maximum degradation rate (MRD) of silicone resin containing phenyl (Sample B & C) were lower than those of the silicone resin without phenyl (Sample A). This indicated that the thermal decomposition rate of the silicone resin slowed down with the addition of the content of the phenyl group moiety.

Silicone resin : Binders for heat resistant paint

For many years coatings based on methyl/phenyl silicone resins have proven to be one of the most effective and reliable products within the high heat and protective coatings segments. Fillers inorganic characters along with the methyl/phenyl modification provide outstanding long term thermal stability, attractive corrosion protection, weather resistance and good mechanical properties. In general, silicone resins for high heat applications can be categorized into two sub-groups: i) Heat curing and ii) Air curing systems.
Metal alkoxide catalysts were utilized in the moisture-curable air cure system to induce crosslinking. The silicone resin’s hydroxyl groups and moisture in the environment can react with the catalyst to crosslink the polymer.
The methyl silicone resin has a tremendous potential for heat resistance up to 400 °C, however methyl phenyl silicone resin performs better at higher temperatures up to 650 °C. Based on the formulas, modified polyester silicone resin and epoxy silicone resin can withstand heat up to 350-550 °C.

Summarized data of silver heat resistant paint having silicone resin used as binders.

Table 02: silver heat resistant paint test data

Fig: 02 Sample A – Methyl silicone resin metallic paint formulation – quenching at 450°C pass other 500°C and 600°C failed

Fig 03 Sample B – Methyl phenyl silicone resin metallic paint formulation– quenching at 450°C/500°C/600°C pass

Fig 04 Sample C – High phenyl silicone resin metallic paint formulation – quenching at 450°C /500°C /600°C pass

In conclusion, the TGA and metallic paint testing results demonstrated that changing the methyl and phenyl ratio of silicone resin improves the thermal stability. The UV resistance and corrosion resistance are extremely good of heat resistant coating. The silicone resin’s outstanding durability and substrate adhesion are demonstrated by the adhesion test that done after the quenching test. These resins are excellent for many various types of protective coatings.

—- Elkay Chemicals Pvt. Ltd. India.

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