Paint & Coatings Industry - November 2008 - (Page 66)

Self-Crosslinkable Thermosetting Water-Dilutable Effect of Monomer Ratio on Film Properties mportant properties that are desired in goodquality coatings include hardness, scratch resistance, antiblock resistance, resistance against household chemicals and grease, outdoor durability and UV-resistance, flexibility and toughness.1 These properties can be controlled by the use of acrylics in the coating formulation. They can provide a range of properties to the applied films due to the wide range of available monomers.2 The incorporation of two different acrylic-type monomers, having diverse physical and/or chemical properties, in the same polymer molecule in varying proportions leads to the formation of new acrylic copolymers with great scientific and commercial importance.3 To obtain optimal properties, high-performance coatings require crosslinking, as the crosslinking improves film toughness, provides solvent resistance and reduces dirt pick-up.4 That’s why crosslinkable or thermosetting acrylics (TSAs) are increasingly used in a variety of surface coating applications, since they are designed to react chemically after application.5 Generally, TSAs are stoving-type systems, in which the final film is obtained after heating at a particular temperature. When TSA resins are stoved, the reacTABLE 1 | Different types of TSA systems. Pendant Group O || —C—OH (Carboxyl) O || —C—OCH2CH2OH (Hydroxyl) —CH—CH2 (Glycidyl) \O/ O || —C—NH-CH2OH (Methylol) O || —C—NH-CH2OR (Alkyl methylol ) I tive groups from the functional acrylic monomer units take part in crosslinking reactions, which convert the low-molecular-weight polymer to a high-molecularweight film-forming material. On the basis of their crosslinking mechanism, TSAs can be sub-classified into two main groups. (1) One–pack TSA systems: In these systems, the use of external crosslinkers is not needed because the two types of pendant side groups present in a molecule crosslink with themselves intra-molecularly in the presence of trace amounts of catalyst, above a threshold temperature (e.g., TSA resin containing glycidyl and methylol groups). (2) Two-pack TSA systems: For these systems the use of an external crosslinker is needed because the polymers by themselves are not capable of undergoing chemical reactions through the pendant chemical groups. Therefore, the addition of an external agent – a crosslinker – is required. The crosslinker must be at least difunctional.6 Table 1 shows five TSA systems that are commercially exploited today.7 Industrially, TSAs are formulated in several ways and can be grouped conveniently into two classes: solventborne TSAs (where organic solvent is used for binder dissolution) and waterborne TSAs (where water is used for binder dissolution). Solventborne TSAs have a particular prominence in the area of industrial coatings, where performance is essential.4 Waterborne TSA systems for surface coating applications are growing rapidly and expanding vigorously, which may be attributed to the fact that regulations on all the four E’s, i.e., emission, environment, ecology and energy, could be restored.8 In the present work, a series of TSA resins were prepared by using 2-hydoxyethyl methacrylate (HEMA) and acrylic acid (AA) as monomers in varying molar ratios and butanol as solvent in the presence of benzoyl peroxide (initiator) at 60 °C. The prepared TSA resins were made water-soluble by neutralizing with ammonia solution. The coating films were then Monomer Type Acrylic or methacrylic acid Hydroxyl alkyl acrylate/ methacrylate Catalyst Basic Crosslinking Agent Epoxies Acidic Alkoxyaminos Carboxylic polymers, Aminos Glycidylacrylate/ Acidic methacrylate or basic Methylol acrylamide Etherified methylol acrylamide Acidic Epoxies, or basic Alkoxyaminos Acidic or basic Alkyds By Ganesh Datt Bajpai, Raju Shukla and Pramod Kumar Tripathi, Department of Oil and Paint Technology | H.B. Technological Institute, Kanpur, India 64 NOV EMBER 2008 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM

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Paint and Coatings Industry - November 2008 Contents Viewpoint Industry News Calendar of Events Company News Mergers and Acquisitions Names in the News HiTone Resins for Enhanced Pigment Affinity of Powder Coatings New Water-Based Interior Coatings Combine Innovation and Quality With Respect for the Environment Novel, UV-Curable Nanotechnology-Enabled Coatings Highly Weather-Resistant Radiation-Curable Monomers and Oligomers for Exterior Applications Konica Minolta — Sensing Success for Viracon Self-Crosslinkable Thermosetting Acrylics for Use in Water-Dilutable Coatings Paint and Criminal Forensic Science Supplier and Application/Curing Showcases Materials Watch Classifieds Advertiser Index

Paint & Coatings Industry - November 2008

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