Many coatings used in industrial, automotive, and architectural markets rely on organic coating resins. In addition to providing adhesion, durability, flexibility, and gloss properties, these resins serve as the film-forming component that binds other constituents like pigments and additives together into a uniform system. This system is the coating that provides protection and visual improvements to the substrate.
One key method for creating these resins is through free radical polymerization, in which resin polymers are formed by combining free radical intermediates through thermodynamically favored addition reactions. Generally, the monomers that can be polymerized through this technique are alkenes or functionally substituted alkenes, such as acrylic monomers. A few examples of acrylic monomers and other monomers that polymerized through free radical polymerization are as follows:
To begin the free radical polymerization process, an initiator must first be introduced to the monomers. When activated, the initiator will serve as a source of free radicals that will bond to and open alkene monomers, in turn producing another free radical. The newly formed initiator-monomer bonded free radical will then continue to open and bond with another monomer, repeating the process to propagate and create polymers. Since the initiator plays a critical role in polymerization, the right initiator must be used.
Vazo™ Free Radical Polymerization Initiators
Vazo™ is a class of azo compounds that produce free radical polymerization initiators, which are highly efficient and easy to use for providing free radical initiation to generate polymer resins used in coatings. As these compounds have a central linking group of N2, there is a thermodynamically favorable driving force to liberate N2 gas from the compound, allowing radical initiators to be easily formed via thermal or photolytic decomposition. The general chemical decomposition scheme of Vazo™ into N2 gas and radical initiators is shown below.
Vazo™ does not react with solvents, dyes, pigments, or redox-sensitive agents, and non-water-soluble grades are not pH sensitive with respect to decomposition. This makes Vazo™ easier to use in various systems, given that there are fewer reaction condition requirements. Additionally, since decomposition is controlled by temperature rather than by other chemical species, Vazo™ has first-order reaction kinetics that is highly predictable.
Key Vazo™ Benefits in Free Radical Polymerization for Coating Resins
Several factors specifically make Vazo™ the best choice for initiators to manufacture polymer resins used in coatings. Owing to the highly controlled decomposition mechanism of Vazo™, as discussed, polymers can be created that have specific molecular weights, narrow polydispersity, and lower viscosity.
Another critical property of Vazo™ for creating polymers used in coatings is that it does not readily abstract hydrogen from growing polymer chains. During the propagation stage of polymerization, other free radical polymerization initiators like organic peroxides can abstract hydrogen and leave an unexpected open site that causes propagation in an uncontrolled fashion, leading to prematurely cross-linked polymers. An example of this is illustrated below.
Non-Vazo™ (Organic Peroxide) Radical
Uncontrolled Polymer Propagation
As Vazo™ does not readily abstract hydrogen, it can ensure that the intended resin polymer is produced and polymer chain defects are reduced. This makes Vazo™ especially useful for specialty and novel polymer coating applications. In addition, since Vazo™ lacks oxygen bonds, opportunities for chain scission and premature "unzipping" of polymers are minimized, preventing premature film degradation and the associated yellowing of coatings that may occur with organic peroxide initiators.
Lastly, with environmental and clean air regulations being implemented, there have been recent free radical polymerization techniques developed to produce low-VOC resins that necessitate the use of an azo initiator. Examples include catalytic chain transfer (CCT) and reversible additional fragmentation chain transfer (RAFT) techniques.
How to Select and Use Vazo™ Compounds for Coating Resins
In general, Vazo™ grades are selected based on the reaction medium (solvent) and desired reaction temperature. Vazo™ grades are available in low-to-high reaction temperature range options, as well as solvent and water-soluble varieties. The number associated with each grade is a rough guideline for a 10-hour half-life temperature, where a suitable reaction temperature range may be 10°C to 30°C above or below the grade number. Lower initiation temperature grades of Vazo™ are useful where low color formation is a priority, as in clear coatings.
The following table outlines grades that can be considered for producing polymers for coating resins, by application:
Preferred initiator for high temperature polymerizations in a wide variety of
Vazo™ 56 WSP
Water soluble for use in emulsion polymerizations
Vazo™ 68 WSP
Highly versatile azo compound: soluble in both aqueous and organic systems
Usage rates of Vazo™ range from 0.01 to 4 wt% (by monomer weight), with a median use rate of 0.5 to 1.5 wt% (by monomer weight). Vazo™ may be used in bulk, solution, suspension, or emulsion-free radical polymerizations as the sole initiator. It can even be combined with other initiator types if desired, as Vazo™ does not undergo free radical induced decomposition in comparison to other initiators like organic peroxides.
Many industries rely on high-quality resins to formulate coatings. These coatings include those used on architectural buildings, marine vehicles and structures, automobiles, and even packaging for consumer goods. Free radical polymerization is a widely utilized technique to create polymer resins for coatings, and this polymerization technique requires the use of a free radical initiator. Vazo™ free radical initiators provide the following features:
Non-reactivity with other components in coating formulations, such as solvents, dyes, redox-sensitive agents, and other additives
Decomposition based on temperature, rather than dependent on other solution constituents
Greater control over coating polymer structure formation, with fewer chain defects and premature cross-linking
Minimized chain-scission and film degradation, leading to less yellowing in coatings
Variety of grade options for suitability in a range of solvents and reaction temperatures
These features benefit resin manufacturers with an initiator that is easier to use, will form precisely the polymers desired for each coating application, and enables the production of high-quality polymers.
Contact us below to request a quote on Vazo™ for use in your coating resin polymerization process.