Silicone Resins Beyond the Millennium

Coating manufacturers and raw material suppliers have been adhering to regulations set by the EPA and have converted their customers from conventional formulations with high VOCs to high-solids formulations with significantly lower VOCs. The next step for companies that are being proactive and eliminating VOCs entirely is the conversion of their customers to powder coatings or solvent-free systems. One segment that is converting to high-solids and powder coatings is the high-temperature-resistant coatings market. Because of this, the segment has been growing at a rate greater than 10% every year over the last five years. The majority of the growth has been from liquid coating markets; however, significant growth has stemmed from new product applications and the ceramics market.

The customer transition from liquid to powder is complicated and several roadblocks must be addressed. The high-temperature-resistant market is no exception. The use of silicone polymers in this market posed an additional set of hurdles, which delayed the conversion from the late 1980s to the early ‘90s. Silicones by nature have a low softening point and are not compatible with organic polymers. The former would limit the quantity of silicone incorporated into an organic system and therefore limit the high temperature performance of the product. The problem with certain silicone resins and their compatibility limited their use and market acceptance. The re-engineering of silicone technology to increase the softening point and use the current composition enables formulators to develop technologies for the next generation.

Background

The use of silicone resins in powder coatings began in the late 1980s, and the technology became commercial in the mid ‘90s (see sidebar). During this time, formulations were developed using conventional silicone technology and were composed of relatively low levels of silicone. Silicone resins increase the high temperature resistance of coatings and, because of this, they were limited to applications with medium- to high-temperature-resistance such as barbecue grills, smokers, and cookware applications. In the late ‘90s, silicone manufacturers focused on developing new technologies that would enable formulators to maximize silicone content in powder coatings. This would improve the performance of current technology and open new markets to the possibility of using powder. Some of the new applications include coatings for lighting fixtures (whites), appliances (ovens, stoves, etc.), muffler and exhaust systems. During the last five years, the majority of the high-temperature-resistant powder coatings market has been supplied by U.S. manufacturers, with very little supplied by European manufacturers.

Figure 1 / Structural Units of Silicones

Review

Silicone manufacturers describe different polymers by the types of monomers used during synthesis. Silicon is a tetravalent atom, which forms four sigma bonds in its ground state, and is considered reactive through the oxygen bond (see Figure 1). If one oxygen is bonded to the silicon atom, it is referred to as a monofunctional, or “M” unit; silanes containing two silicon-oxygen bonds are difunctional, or “D” units. “T” units are trifunctional monomers, and “Q” represents a quadrifunctional silane.

In addition, silicon has a high affinity for oxygen and will form a high energy Si-OSi bond (108–110Kcal/mole), whereas the bond energy of the C-CC is 82–85 Kcal/mole and the C-OC bond energy is 84–87 Kcal/mole. Therefore, resins used in powder coatings are mainly composed of “T” units, which maximize the number of Si-O bonds. There may be some “D” units present to increase flexibility. Since the Si-OSi bond energy is approximately 20% higher than its organic counter part, the incorporation of a high degree of Si-O (siloxy) units will increase the heat- and UV resistance of the system.

Silanes could also be divided into a classification system that is based on the types of organic substituents present, which are represented by an “R” in Figure 1. The presence of octyl groups make them ideal for use as water repellents, and amino groups make them ideal for use as adhesion promoters. Thermal oxidation of silicone resins occurs through the oxidation of these organic substituents. The type and concentration of certain substituents will determine how the material will perform in different environments. Silicone resins with a high phenyl content are ideally suited for applications that require heat resistance. The phenyl substituent has a half-life, when exposed to 250°C (482°F), of 100,000 hours or 11.4 years (see Table 1). This makes phenyl silicone resins ideal for applications that require good gloss retention and flexibility at high temperatures. The methyl substituent has a half-life of 10,000 hours or 1.14 years, and the propyl substituent’s half life is seven hours. The methyl substituent is used in applications that require increased hardness. The organic substituent will also affect the compatibility of the silicone polymer with organic resins. Substituents with greater than three carbon atoms will increase compatibility of the silicone resin, and anything less would be generally incompatible with organics. Methyl silicone resins are incompatible with organic polymers; however, if used by themselves will form an excellent product.

Figure 2 / Polysiloxane Modification

Incorporation

The modification of coatings with silicone polymers can be achieved by either pre-reacting or post blending the silicone with organic polymers. There are four basic types of reactions that occur with silicone resins: alkoxy/water, alkoxy/hydroxyl, silanol/silanol and silanol/hydroxyl. The pre-reaction method is normally carried out by a resin manufacturer, and involves reacting an alkoxy or silanol functional resin with a hydroxyl functional organic resin (see Figure 2). This reaction requires heat and, if the silicone is alkoxy functional, will require distillation of an alcohol. However, the blending method uses conventional equipment to mix the silicone resin and other coating raw materials together.

Figure 3 / Polysiloxane Modification

In this case, the silicone is silanol functional and once applied to the substrate will homopolymerize to form Si-O-Si (see Figure 3) only after curing. It will also react with hydroxyl functional organic resins to form a thermoset system.

Figure 4 / Model of Silicone Coating after Curing at 200°C

Film Formation

Film formation begins with coating application, which can be done by either tribo or corona spray. After application, the coating passes through an oven set to cure at ~220°C (420°F). The curing process will begin with polymers going through a melt phase, enabling the reactive groups on the polymer to become mobile with enough energy to crosslink (see Figure 4). The silanol groups on the silicone will crosslink with other silanol groups (homopolymerize) to form the Si-O-Si matrix. The silanol groups will also react with hydroxyl groups on neighboring organic polymers to form a thermoset system. This is in addition to the reactions occurring with the organic binder (TGIC/polyester, GMA acrylic/phenol, epoxy/polyester or epoxy/cresol/novolac, etc.).

Figure 5 / Model of Silicone Coating after Pyrolysis (400°C)

The next phase of this process occurs when the product is exposed to its service temperature. If this is greater than 400°C (752°F) the film will go through a pyrolysis step (see Figure 5). During this step, the organic binder will be oxidized and the organic subtituents are also oxidized to form a mixture of CO₂, CO and other oxidized organic compounds and water. In theory, the only components remaining will be the oxidized silicone resin, inorganic pigments, and substrate. Therefore, if there is not enough silicone resin in the formulation then the system will fail or severely chalk.

Figure 6 / Model of Silicone Coating after Pyrolysis (400°C)

New Product Developments

The next generation of powder coatings will push the envelope on high temperature performance by maximizing the level of silicone resin. Complete oxidation of the film will leave an inorganic skeleton composed of the oxidized silicone resin and inorganic pigments, as illustrated in Figure 6.

Therefore, by increasing the level of silicone resin and decreasing the level of organic resin, the level of oxidation that occurs after pyrolysis will be decreased. Generally, silicone resins have a lower softening point than organic resins. The old line of products offered by silicone manufacturers had softening points of 35–55°C (95–131°F), and, therefore, could only be used in small quantities. This limited their market use to mid-range high-temperature-resistant applications. Increasing the softening point to 65–80°C (149–176°F) makes them more resistant to sintering and thereby makes it possible to formulate a coating with high levels of silicone and little to no organic resin.

Figure 7 / Gloss Retention after Heat Exposure

The incorporation of a phenyl silicone resin with a softening point of 78°C (172.4°F) into an epoxy cresol novolac system (see Table 2) resulted in excellent gloss retention (>91%) after being exposed to 260°C for 50 hours (see Figure 7).

Figure 8 / Gloss Retention after QUV-B (313 nm)

Placing this same formulation in a QUV-B (313nm) cabinet resulted in a gloss retention of 56% and no chalking after 196 hours (see Figure 8).

Increasing the softening point of methylphenyl silicone resins makes it possible to develop white or light-colored coatings that are resistant to yellowing after exposure to high temperatures. The organic resins and organic substituents will begin to yellow after being exposed to elevated temperatures due to incomplete oxidation of the organic components. Modification of the silicone resin to increase the methyl-to-phenyl ratio increases the inorganic character. The incorporation of a methylphenyl silicone resin with a softening point of 74°C (165°F) made it possible to formulate white powder coatings, which do not contain organic binders (see Table 3).

Figure 9 / DE of White Formulation

This coating was subjected to 250°C (482°F) for 200 hours and had a Delta E of only 1.14 (see Figure 9).

Figure 10 / Cross Hatch of White Formulation

These coatings had good crosshatch adhesion after exposure to elevated temperatures (see Figure 10). This opens up new markets and sets the stage for pursuing light colored appliance, lighting fixture white and cookware applications.

Applications

All results are based on powders, which were applied at a 20–40 (Greek Mu) (0.8–1.6mils) film thickness. The powder particle size was from 10 to 60 (Greek Mu). The coatings were applied using a Walther Pilot spray system (Tribo) and cured at 220–250°C (428-480°F) for 30 minutes. Films with higher film thickness will crack and delaminate after exposure to high temperatures. Coatings with high film thickness will also tend to form bubbles from trapped gases (water). The addition of Benzoin or Oxymelt A4 will aid in degassing of these coatings.

Powder Preparation

The raw materials were mixed in a Mixaco mixer for 3 minutes at 800 rpm and 20°C (68°F). The mixture was extruded through a single screw extruder (BUSS) with the first zone set at 60°C (140°F) and the second zone set at 100°C (212°F). The torque was 40% and the screw speed was set at 150 rpm.

The extrudate was ground into a powder using a Hosokawa high-speed mill set at 12,000 rpm. Fumed silica was added with a setting of 1,400 rpm. The fumed silica helps to increase free flow and fluidization, as well as decrease the tendency of the powder to sinter.

Conclusion

The increase in regulations set by the EPA has forced coatings manufacturers to focus new product developments on powder coatings. Silicone manufacturers have, in turn, focused on developing a new generation of products, which will enable formulators to develop high-performance products.

The next generation of products consist of a high level of phenyl or methylphenyl substituents and have a high softening point. The high phenyl content increases organic compatibility and heat resistance. Increasing the softening point will make it possible to formulate powder coatings with high levels of silicone. These new formulations will have excellent gloss stability and non-yellowing characteristics. The modification of organic systems with silicone will also increase UV resistance. These added benefits will make it possible to convert new markets to powder and improve the performance of existing markets.