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Synthesis of Polymers for Automotive Coatings using Living Radical Polymerization

Project Background
Co-PI: Dr. Robin Hutchinson, Queen's University, Chemical Engineering

North American regulations are a dominant force driving change in the basic nature of coatings resins: the volatile organic content (VOC) must be reduced from 1990 levels of 480 g/L of paint to below 300 g/L by 2010. Low molecular weight highly functionalized polymer and oligomer solutions at 60 to 80 weight percent solids have replaced high molecular weight, non-functional polymer solutions at 30 to 40 weight percent solids as key components in acrylic coatings formulas (1). The oligomeric chains form a high-MW polymer network on the surface to be coated via reaction of the functional groups (e.g.; hydroxyl or epoxy) with an added cross-linking agent. The base resins are made via high-temperature solution free-radical semibatch polymerization, with monomers (styrene, acrylates, methacrylates) and initiator fed continuously over several hours. Composition control is especially important during production of the new generation of low-MW materials; with an average chain-length of less than 50 monomeric units, it is essential that all chains contain sufficient functionality to participate in the crosslinking reactions needed to form a durable and tough coating.

Living radical polymerization (LRP) offers the potential for major advances in the manufacture of polymeric materials through its control of polymer microstructure - narrowing of molecular weight distributions (MWD), controlled composition distribution along the chain, and targeted placement of functional groups and branchpoints. In the coatings industry, this could translate to a variety of advantages such as higher solids contents (and therefore lower VOC levels) and lower required levels of high-cost functional monomers. These advantages are more likely to be realized commercially if LRP chemistry can be employed using semibatch process technology. Thus in this work we compare the production of low MW polystyrene via nitroxide mediated polymerization (NMP) in a semibatch reactor system to current free-radical technology. The experimental study is supported and guided by insights gained through computer simulation.

Research

Nitroxide-mediated polymerization is being explored as an alternative to free-radical chemistry for production of solvent-borne polymers (Mn<6000) used in automotive coatings. Through the production of polymer with decreased polydispersity and more defined composition and chain structure, it may be possible to increase polymer content in solution and decrease the required levels of higher-cost functional monomers. Results for styrene polymerization mediated by 4-hydroxy-TEMPO, combined with insight gained through computer simulation, illustrate some of the technical challenges that must be overcome to maintain the current semibatch industrial process technology, including efficient nitroxide usage at the high concentrations required during startup, and maintaining sufficient reaction rate to match current batch times.

The production of low-MW polymer by living-radical chemistry in semibatch procedures introduces some unique challenges. With a target Mn of 2000-5000 g/mol in a solution containing 70 wt% polymer, the concentration of chains (and therefore controlling agent) will be 0.1-0.25 mol/L at the end of the semibatch reaction. For a perfectly controlled system, all of these chains must be initiated at the start of the semibatch process when the volume is much lower (cf. Figure 1).

 

figure 1

Figure 1.