Paint & Coatings Industry - March 2008 - (Page 35)

hyd hydroxyl, carboxyl, amino, mercapto, isocyanato and sily silyl, etc. These resins are generally supplied as 50 80% solids, and have to be diluted during application to f form a uniform film. T These organic polymers have different methods of deg degradation, which are related to the bond strength and stability of the hydrocarbon backbone. Typical bon bond strengths are approximately 85 kcal/mole for the C-C bond and approximately 87 kcal/mole for the C-O bond. However, the replacement of carbon wit with silicon (Si-O-Si) increases the bond strength to app approximately 110 kcal/mole.4 Therefore, the incorpor poration of siloxane-based materials will increase the dur durability of organic polymer-based technologies. In add addition the inherent tendency of siloxanes to have low viscosity enables coating manufacturers to formu mulate coatings requiring less solvent and, in some cas cases, siloxanes can be used as reactive diluents. The first coating applied onto metal substrates is designed to protect the surface from corrosion and is typically an inorganic (or organic) zinc-rich primer. These technologies rely on cathodic corrosion protection from the zinc and are generally coated with an epoxy sealer used to form a barrier. Unfortunately, these coatings are not UV resistant and require topcoating with an organic UV-resistant coating.5 The types of topcoats used have typically been silicone alkyds, urethanes and condensation-curable polysiloxanes. These systems generally contain solvents, which are required for application; or, in the case of the condensation-curable siloxane technologies, will typically give off VOCs during curing (hydrolysis). In addition, urethanes used as topcoats are considered to be potentially toxic due to the possibility of minute amounts of free isocyanate, and they are generally formulated with a significant amount of solvent (see Table 1). Several applications have moved away from using urethanes to a greener technology such as those based on siloxanes. The polysiloxanes have been used to form coatings based on an interpenetrating network (IPN). These polysiloxanes utilize the alkoxy functionality on the silicone to homopolymerize and crosslink with silanes, which require a moisture curing mechanism. These compositions also require a high degree of alkoxy functionality on the silanes and/ or polysiloxane components, which, after curing, will have a VOC (Table 1) due to the generation of alcohol as a byproduct (Figure 1). In addition, these coatings will tend to wrinkle and/or crack if a good through cure is not achieved due to the continuation of hydrolysis after exposure to moisture and heat (sunlight). The byproducts are normally methanol; however in some cases ethanol, propanol or butanol can be present. The silanes are generally organofunctional and will react with organic resins as a secondary mechanism for curing. Improvements in polysiloxane technology utilize the durability of the siloxane network without relying on alkoxy functionality as the main crosslinking mechanism. The newer siloxane technologies have incorporated organic functional groups such as epoxy or amines onto the backbone of the polymer, utilizing addition curing mechanisms for crosslinking.6 Reducing the amount of condensation and increasing the addition reaction mechanism reduces the level of cracking that occurs after exposure to sunlight and moisture. Film Formation Understanding silicone polymer film formation is important in order to build the foundation for understanding how to use these types of polymers and more importantly to highlight their potential weaknesses. This next section focuses on the crosslinking mechanisms (hydrolysis, homopolymerization, copolymerization and addition reactions) of silicone-containing moieties and how they are used to form coatings. The alkoxy functional group on silanes or silicone polymers will react with water under ambient conditions to form silanol groups and generate alcohol as a byproduct (Figure 1). The silanol-functional siloxanes will homopolymerize to form oligomers and/ or ultimately high-molecular-weight silicone resins. Generally moisture is provided by the atmosphere and, therefore, in thick films there can be a problem with Figure 1 | Alkoxy functional group reaction with water. Alkoxy functional silicone 2S i OC H 3 + HOH Polysiloxane S i O S i + 2HOC H 3 Polysiloxane S i O S i + HOH Silanol functional silicone Si ity. OH + S i OH Figure 2 | Alkoxy or silanol-functional siloxane reaction with hydroxyl functional- Alkoxy functional silicone and polyol Organopolysiloxane Si OC H 3 + HO C H 2R S i O C H 2R + HOC H 3 35 PA I N T & C O A T I N G S I N D U S T R Y

Table of Contents Feed for the Digital Edition of Paint & Coatings Industry - March 2008

Viewpoint Industry News Calendar of Events Company News Mergers and Acquisitions Names in the News Marketplace Quarterly Stock Watch Reaching Strict VOC Requirements With Outstanding Durability for Industrial Maintenance and Marine Coatings A Unique Fungicide for Wood-Filled Plastic deSigns of the Times: Slacking Off! From Rocks to Nanos Shear-Free Dispersion Process Fluorinated Oxetane Oligomers as Versatile Intermediates for Polymer Modification The Effects of TPO Composition on Adhesion and Proposed Chemical Mechanism Supplier Showcase Color Management Materials Watch Products Classifieds Advertiser Index

Paint & Coatings Industry - March 2008

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