about Oil and Alkyd Paints mechanisms of oxidative drying.pptx

mechanisms of oxidative drying of oil and alkyd paints

1. Oil and Alkyd Paints Paints Drying Through Oxidation

2. Table of Content Oxidative Drying Drying Mechanisms Driers Bodied Oils Oil paints and varnishes Alkyd resins Alkyd finishers 2

3. Oxidative Drying Complex process involving oxygen action on drying oils; not fully unraveled despite decades of research Induction period : almost always drying starts after a delay Role of substances: Anti-oxidants → delay or inhibit drying Metal soaps (driers) → greatly accelerate drying Oxygen uptake during drying: Linseed oil can absorb ~40% of its own weight in oxygen About half is retained in the dried film Products formed: Hydroperoxides, volatile decomposition products, and continued chemical changes during aging (enhanced by UV light) 3

4. Non-conjugated oils (e.g., linseed): Slow drying (120 hrs without driers at 25 °C) With driers → dries in ~2.5 hrs Addition of catalytic amounts of certain metal soaps (driers) dried in 2hrs & 15 min Hydroperoxides form during oxygen uptake Volatile decomposition products are generated during the drying process of a film and continue to be produced throughout its life. Both the drying and subsequent chemical changes during aging are influenced by exposure to natural ultraviolet (UV) light. Conjugated oils (e.g., tung oil): Oils containing fatty acids with conjugated double bonds, dry much faster than non-conjugated unsaturation Faster natural drying (48–72 h without driers) With driers → dries in ~1.5 h Hydroperoxides form only after film sets Autoxidation: Term used for these processes Involves mild reaction of atmospheric oxygen with oils 4

5. Drying Mechanisms Drying mechanisms differ between conjugated and non-conjugated oils Stronger evidence exists for non-conjugated oils (common in modern oxidative coatings) Early studies lacked modern instrumentation → understanding is still limited 5

6. Challenges in Understanding Mechanisms Complexity of oils: Mixtures of fatty acids with natural inhibitors Pure samples difficult to obtain Film-related factors: Oxygen transport into film Release of by-products Reaction depends on surface availability and film depth Free radical involvement: Highly reactive, short-lived species Detected only indirectly 6

7. Non-Conjugated Oils Contain linoleic and linolenic acids All oils also contain some oleic acid Drying mechanism: Formation of hydroperoxides Hydroperoxides undergo further reactions → cross-linking (network formation) or scission (chain splitting). 7

8. Initial Reaction Pathway Reaction with oxygen/free radicals occurs without loss of unsaturation For linoleic acid first reaction products are : Hydroperoxides form at 9 or 13 position . Reaction involves double-bond shift . That double bond converts from cis → trans 8

9. 9

10. Another explanation for that mechanism i nvolves an existing double bond and the adjacent CH₂ group The CH₂ next to the double bond is called the allylic position (*) 10

11. Driers, heat, or light decompose hydroperoxides → free radicals Free radicals abstract hydrogen atoms from allylic carbons. This n ew radicals can rearrange into conjugated forms. Allylic CH₂ between two double bonds, is the most reactive site. Oxygen adds to these radical sites → peroxy radicals. Peroxy radicals abstract more hydrogens → chain reaction; generates further radical sites for oxygen attachment Result: increased hydroperoxide concentration. In linolenic acid, this mechanism gives four possible hydroperoxide structures . 11

12. 12

13. Autoxidation begins with radical formation (hydrogen abstraction at weak sites) The free radical nature of these reactions explains: Induction period → time needed to build hydroperoxides & remove natural antioxidants by reaction before any chain reaction can commence Effect of antioxidants → act like free radical inhibitors, delaying chain reactions During early drying, chief method of cross-linking is by direct combination of free radical sites on different oil molecules Links formed: Peroxy bonds Ether bonds Carbon–carbon (C–C) bonds (most common). 13

14. Chains can be linked as shown below, with ether or C-C links most numerous 14

15. Link types: Ether links → dominant in room-temperature curing C–C links → most common in stoved (heated) films Peroxy links → low concentration but stable in polymeric structures Free radicals can also undergo decomposition or scission Aldehydes, ketones and carboxylic acids are all known to be decomposition products Hexanal is a principal oxidation by-product of linoleate oxidation, one of the causes of the acrid smell during drying 15

16. Paint Drying Mechanism Conjugated oils and Synthetic Alternatives

17. Conjugated Oils Conjugated oils react with oxygen, involving free radicals This reaction leads to the loss of unsaturation and the formation of cross-links between molecules Firstly, Oxygen attacks on conjugated double bonds, forming diradicals These diradicals further react with other double bonds, resulting in cross-linking The cross-linking enhances the stability and functionality of the oil films 17

18. Synthetic Alternatives (Allyl Ethers) Compounds bearing an allyl‑O‑ group can oxidize Form hydroperoxides at allylic positions Position next to double bond and ether oxygen promotes reaction Hydroperoxide fate with cobalt drier Breakdown → radicals → dimerization; can form acrolein (odorous/toxic) 18

19. Hydroperoxides break down with cobalt drier, causing dimerization. This process can further produce acrolein, which is toxic and has a bad smell. Production of acrolein makes these compounds less suitable as drying oil alternatives. Hydroxyperoxide 19

20. DRIERS Paint Drying Mechanism

21. Driers: What They Do Driers are metal soaps with an acid portion that provides solubility in oil medium Eg:-Synthetic acids- octanoic acid commonly used in drier formulations Naphthenic acids- rarely used Driers are typical additive materials in paints and coatings They are typically present in quantities of less than 1% of the total formulation 21

22. Primary (oxidative) Driers Primary driers act as true catalysts in the drying process They contain metals with variable valency , which can change oxidation states The lower valency state of the metal is more stable but can be oxidized to a higher valency during the drying process Cobalt and manganese - commonly used oxidative driers . Roles of driers Increase oxygen absorption Decompose peroxides to radicals 22

23. Secondary and Through Driers Secondary and through driers belong to a group of metals including lead , zirconium , calcium , and cerium These driers assist in the drying of the lower layers of the paint film The exact mechanisms of how they work are not fully understood , but it likely involves the interaction of the metals with carboxyl and possibly hydroxyl groups in the film-forming material Lead , previously common, is now avoided due to its toxicity 23

24. Primary driers mechanism How driers promote oxygen uptake is uncertain, but the decomposition of hydroperoxide by primary driers occurs as follows: The cobalt is oxidized and reduced over and over again. 24

25. Cobalt promotes rapid drying at the surface of the film than in the lower layers If the surface dries faster than the lower layers, it can cause buckling or 'shrivelling' when the lower layers harden and contract To prevent this, secondary driers are added with cobalt to accelerate the drying of the lower layers or the bulk of the film Calcium driers speed up the loss of unsaturation in the film, aiding in uniform drying Proper balance between primary and secondary driers prevents shrivelling Manganese , though less prone to surface bias than cobalt, is also used in combination with secondary driers to promote even drying 25

26. Aluminium alkoxide derivative driers Aluminium alkoxide derivatives were introduced as through driers in the late 1970s The general formula is Al-OR with substituent groups -OR, R' , and R“ These groups can react with hydrogen atoms , particularly in carboxyl and (less readily) hydroxyl groups of alkyds 26

27. This reaction bonds several alkyd molecules to a single aluminium atom , eliminating volatile by-products such as ROH, HR', and HR" from the drying film The process achieves cross-linking through the film, improving the film's structure Careful formulation is needed, especially maintaining the carboxyl ratio at 1.0 or less to ensure stability in the can These driers are particularly beneficial in thick films , providing improvement through drying 27

28. Air-drying decorative paints In typical air-drying decorative paints , a combination of cobalt , calcium , and zirconium is commonly used as driers Aluminium driers are used in special circumstances or with difficult pigment combinations For high solids coatings , which have longer oil lengths and lower molecular weights , the same combination of driers is used, but aluminium often replaces zirconium due to its special advantages In emulsified alkyds , the same driers — cobalt , calcium , and zirconium — are most commonly employed Formulation challenges include ensuring that the driers remain in the dispersed phase 28

29. Oil Paints and Varnishes: A Primer on Formulation and Performance 29

30. What are Resins? Resins are a broad class of organic compounds used to enhance the properties of paint, acting as binders and modifiers. They are typically hard, non-volatile substances that are either naturally occurring or synthetic. Natural Resins: A key example is rosin , which consists principally of abietic acid . Synthetic Resins: These are produced via polymerization. A simple example is petroleum resins , which are formed by the polymerization of unsaturated hydrocarbons found in petroleum. 30

31. The Role of Oils as Film-Formers Early Paints used oils, primarily linseed oil , as the sole film-former. These are known as drying oils . The oils harden through a process of oxidation and polymerization, resulting in a tough, solid film. Drawbacks: Simple oil films are prone to slow drying, have poor water resistance, and are susceptible to mildew. Modern Improvements: To overcome these limitations, modern paints often use modified oils or alkyds , which are formulated to improve drying speed and overall durability. 31

32. Resin Classifications: Terpenes & Coumarone-Indene Terpene Resins: Made by the polymerization of terpenes . This process causes them to lose most of their unsaturation. Coumarone-Indene Resins: These resins are produced by the polymerization or copolymerization of coumarone and indene. The attached aromatic rings give them slight unsaturation. 32

33. Phenolic Resins Part I: Novolac Formation: Phenolic resins are a class of thermosetting plastics formed by the reaction of a substituted phenol with formaldehyde. Novolac Resin: This specific type is produced using a ratio of 0.5-1.0 molecules of formaldehyde to one molecule of phenol, with an acid catalyst . Structure: Novolac is a short, linear polymer that requires an external cross-linker to cure. 33

34. Phenolic Resins Part II: Resole Formation: Unlike novolac , resole resins are produced with a higher proportion of formaldehyde and are catalyzed by an alkaline catalyst . Structure: Resole is a highly reactive, branched polymer that can be heat-cured without an external cross-linker. The use of an alkaline catalyst allows for a slower, more controllable reaction, making it easier to stop the process before cross-linking occurs . 34

35. Phenolic Resin Modification and Applications Modified Phenolic Resins: When the para -position of the phenol ring is blocked (e.g., by a tertiary butyl group), the resulting resin is soluble in common solvents. Modification with rosin can be used to improve solubility in oils and increase the resin's molecular weight. Applications: Oleoresinous Varnishes: A traditional and widespread use. Chemical-Resistant Coatings: Often used in finishes that require high durability. Wire Enamels: Provides insulation and protection for wiring. 35

36. "Oil Length" Definition: Oil length is the weight ratio of oil to resin in a varnish. It is a critical factor in determining the final properties of the product. Classification: Long Oil (3-5 parts oil to 1 part resin): Slowest drying but provides the best outdoor durability and flexibility . Medium Oil (1.5-3 parts oil to 1 part resin): A balance of properties. Short Oil (1-1.5 parts oil to 1 part resin): Dries quickly and has the highest hardness and chemical resistance, with properties dominated by the resin. 36

37. The Effects of Resins on Paint Properties Drying Time: Resins accelerate the drying process. Unlike oils, they are hard in their normal state and only require solvent evaporation. Film Properties: Resins help to harden the film and improve gloss. Resistance: Phenolic, coumarone-indene, and terpene resins provide improved water and chemical resistance. Aesthetic Drawback: These resins can give paint a poor initial color and may aggravate yellowing of the film as it ages. 37

38. Rosin Modification of Phenolic Resins Method: The reaction of rosin with a modified phenolic resin provides an alternative way to improve its solubility. Mechanism: The carboxylic group of the rosin can be esterified with a polyhydric alcohol (e.g., pentaerythritol ) to increase the resin's molecular weight and overall stability. Result: This modification creates a product with enhanced performance, often used in durable varnishes and coatings. [Blank space for the chemical reaction showing the modification with rosin] 38

39. Alkyd Resins 39

40. What are Alkyd Resins? Complex oil-modified polyester that serves as the film-forming agent in some paints and clear coatings These improve the properties of paints like drying, gloss, and durability Their behavior depends mainly on the type of oil and how much oil is used Polyester resins made from alcohols + acids Name “Alkyd” = Alcohol + Acid 40

41. Why Are They Important? Make paints dry faster Give hard, glossy films Improve durability & outdoor resistance Used in house paints, industrial finishes, lacquers 41

42. Oil Length Concept The amount of oil changes the properties of the paint Oil length = % of oil in resin - Long oil (>60%) : Flexible, durable, house paints - Medium oil (45–60%): Balanced properties - Short oil (<45%) : Harder, faster drying, industrial coatings Generally m ore oil length leads to more flexible but slower the drying process 42

43. How They Are Made 1. Oil + Glycerol → Monoglyceride (heat with catalyst) 2. Add Phthalic Anhydride → Polyester chain (alkyd resin) Reaction tracked by: - Acid value - Viscosity Final resin MW: 1000 – 5000 43

44. Types & Modifications Drying alkyds (linseed, soya oils) → Hard, durable Non-drying alkyds (castor oil) → Flexible, plastic-like Modifications: - Polyethylene glycol → water-dispersible paints - Polyamide → thixotropic (flow control) - Styrene/acrylics → faster drying process 44

45. Uses Decorative paints (gloss, outdoor durability) Industrial coatings (machinery, furniture) Lacquers & varnishes Benefits: Gloss retention, less cracking, long-lasting 45

46. Thank You 46