VOLUME 16 ISSUE 1
WHAT’S HAPPENING AT MISSOURI S&T (FORMERLY UMR):
SHORT COURSE DATES
We will be offering "Introduction to Paint Formulation" Oct 21-25 (Fall 2019). This course is intended to give the person a fundamental knowledge of how to approach a starting formulation and troubleshoot it. This course involves both lecture and laboratory work.
For more information see our web site at http://coatings.mst.edu and to register contact us at mstformulation@mst.edu or call 573-341-4419. **These courses are held on the Rolla Campus**
ONLINE SHORT COURSE
We are offering "Introduction to the Coating Systems" online short course. This course is targeted for automotive and aviation type OEM companies. This self-paced seminar will cover the painting system from the composition of paints to the evaluation of the dry film. The pigments, resin, solvents and additives will be discussed including their influence on the coatings performance. Color measurement, surface profile, and other evaluation criteria will be related to composition. The importance of surface preparation and other manufacturing criteria will show the system complexity and each step's importance.
We are offering "Surface Defects: Elimination from Human and Process Contaminants" online short course. This course addresses many of the issues in prevention and minimization of defects. The course covers the defects caused by the coatings process, as well as human issues, including personal care product causes. Several of the surface defects are discussed – from basic principles and real world automotive and aircraft examples. The highly practical approach of this course will greatly aid the personnel involved in the painting operation to reduce and systematically approach issues.
EMPLOYMENT TAB
Anyone wanting to have job opening listed, please contact us at (573) 341-4419 or e-mail: mstformulation@mst.edu . You can also write to us at Missouri S&T Coatings Institute, BOM #2, 651 W. 13th St., Rolla, MO 65409-1020. Our web site is http://coatings.mst.edu
TECHNICAL INSIGHTS ON COATINGS SCIENCE
Anti-skinning agents
Ashish Zore, Graduate student, Missouri University of Science and Technology
Alkyd chemistry is one of the most widely used chemistries worldwide in the field of surface coating and ink technology. For the period 2019-2023, the market for alkyd is expected to grow at a CAGR (compound annual growth rate) of over 3% [1]. Alkyds are polyester-based materials modified with oil or a fatty acid which can be drying, semi-drying or non-drying type. Drying and semi-drying type crosslink through oxidation in presence of a metal drier like cobalt, zirconium, etc. These driers are essential for the proper balance of surface and through-drying. Unfortunately, they are also responsible for one of the commonly faced problems in an alkyd system - skinning i.e. formation of dry resin skin on the surface of the stored paint. The formation of skin occurs from the same drier catalyzed polymerization process that causes drying of coating film. The skin can be often removed in one piece, leaving the remaining paint usable, but sometimes the skin may break up into pieces, requiring filtering of the paint. In order to prevent this unintended drying of the resin during storage, anti-skinning agents are used. Only a small amount of anti-skinning agent is required to prevent skin formation, typically 0.2-0.6% on vehicle solids in most paint [2]. Adding too much may slow down the drying time, especially in case of non-volatile anti-skinning agents.
Phenolic anti-skinning agents are more readily oxidized than unsaturation present in the alkyd, and therefore react with the free radicals generated during the oxidative polymerization process. This prevents the crosslinking from taking place during storage. Some anti-skinning agents also form complexes with the drier and slow down its activity [3]. The amount of anti-skinning agent is very important as it should be enough to prevent crosslinking during storage but after application drying should be as fast as possible. The amount is especially crucial in case of non-volatile type, where drying is significantly affected by excessive addition.
The most common types of anti-skinning agents are oximes, substituted phenols or quinones. Volatile anti-skinning agents are only effective in closed containers as they evaporate readily in open air. Due to the volatile nature they evaporate immediately after application of the coating thereby having little or no effect on the drying time. The use of these volatile type anti-skinning agents are more prevalent in paints where closed in can storage is the required. Methyl ethyl ketoxime is one of the most commonly used anti-skinning agent for paint. Cyclohexanone oxime is less volatile and is used mainly in printing inks [4]. The mode of action of MEKO is based on its ability to form complexes with the metal drier like cobalt and manganese. The complex formed is quite stable in the liquid coating and makes the catalyst inactive for autoxidation polymerization. When the coating is applied to the substrate, the MEKO evaporates and the complex with the metals is broken, leaving the free metal for catalysis of the polymerization reaction [5].
Non-volatile anti-skinning agents, also referred to as antioxidants, are substituted phenolics or quinones. Hydroquinone is an example of a very strong antioxidant that can severely inhibit oxidation. Non-volatile anti-skinning agents should always be added with care, as even the slight excess addition can have a significant effect on drying of the coating after application. They are mainly used in printing inks to induce overnight duct stability, especially hydroquinone. The use of such strong antioxidants extends the drying time of the ink but most inks are applied as thin films on adsorbent substrate so slower drying does not lead to any severe set off effects [4]. The phenol prevents the skinning problem by reacting with the oxygen containing free radicals that are formed during the metal catalyzed autoxidation process.
ROO· + Ar-OH ----> ROOH + Ar-O·
RO· + Ar-OH ----> ROH + Ar-O·
Where ROOH is a peroxide, ROH is an alcohol, ROO· & RO· are the radical formed during the metal catalyzed oxidation reaction and Ar-OH is phenol antioxidant.
The antioxidant competes with the autoxidation reaction of the organic functionality of the alkyd resin involved in the autoxidation process
ROO· + RH ----> ROOH + R·
RO· + RH ----> ROH + R·
Where RH is the alkyd resin.
An effective antioxidant must have a reaction speed much higher than that of the autoxidation. The A· radical formed should not abstract hydrogen from other molecules, i.e. it should not initiate chain extension [5].
The market for anti-skinning agents in paints is expected to grow in the following years. The market is set to reach USD 0.97 billion dollars by 2024 at a CAGR of 5.1% as forecasted by Goldstein Research analyst [6]. Some of the global players in anti-skinning agent market include the following companies: Borchers, Dura Chemicals, Gelest Inc., OMG Americas, Polyrheo Inc., GSFC Ltd., Arkema and Troy Corporation. Based on the type of agent used, oximes hold the largest share i.e. 45% of the global anti-skinning agent market. This is due to the growing application of methyl ethyl ketoxime (MEKO) MEKO in printing inks, industrial wood, pigment dispersions, and general industrial end-uses to prevent skinning problems. Asia pacific (APAC) holds a 40% market share of the anti-skinning agent market in 2016 due to the growing GDP and rapid urbanization. This has resulted in huge demand for the anti-skinning agents from the expanding construction industry, new and improved infrastructure development and the growing decorative paint business [6].
Reference:
- Global Alkyd Resin Market 2019-2023, Technavio research, www.technavio.com.
- Handbook of additives
- Sasol Servo BV; "Recent Developments in Anti-skinning Agents for Air-drying Paints" Report 6.01E; 2003.
- Owen D.J.; "Printing Inks for Lithography"; SITA Technology 1990.
- Johan Bieleman, Additives for Special Functions, 2000, Wiley
- Goldstein Research, Category: Chemicals, Minerals & Materials, Global Anti-skinning Agent Market, 185, 2016