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Alkyd

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Analkydis apolyesterresinmodified by the addition offatty acidsand other components.[1]Alkyds are derived frompolyolsand organic acids includingdicarboxylic acidsorcarboxylic acid anhydrideandtriglyceride oils.The termalkydis a modification of the original name "alcid", reflecting the fact that they are derived fromalcoholandorganic acids.[2]The inclusion of a fatty acid confers a tendency to form flexible coatings. Alkyds are used inpaints,varnishesand in moulds forcasting.They are the dominant resin or binder in most commercial oil-based coatings. Approximately 200,000 tons of alkyd resins are produced each year.[3]The original alkyds were compounds ofglycerolandphthalic acidsold under the name Glyptal.[4]These were sold as substitutes for the darker-coloredcopalresins, thus creating alkydvarnishesthat were much paler in colour. From these, the alkyds that are known today were developed.

Structure of an idealized alkyd resin derived fromglycerolandphthalic anhydride

Manufacture

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Main article:Drying oil

Alkyd resins may be classified as drying (including semi drying) and nondrying. Both types are typically produced from dicarboxylic acids or anhydrides, such asphthalic anhydrideormaleic anhydride,andpolyols,such astrimethylolpropane,glycerine,orpentaerythritol.[5]Alkyds are asynthetic resinand used in items such as paints. They are not the same asresinderived from natural sources such as plants.

For the drying resins, triglycerides are derived from polyunsaturated fatty acids(often derived from plant andvegetable oils,e.g.linseed oil). These drying alkyds are cured by theoxygenin air. The drying speed and the nature of the coatings depends on the amount and type of drying oil employed (more polyunsaturated oil means faster reaction in air) and presence of catalysts, the so-calledoil drying agents.These catalysts aremetal complexesthatcatalyzecrosslinking of the unsaturated sites.Cobaltsalts are particularly effective and widely used. However, because of the carcinogenicity of cobalt, its use in alkyds is being researched to phase it out.[6]

Alkyd resins are produced in two processes: thefatty acidprocess and thealcoholysisormonoglycerideprocess. Higher-quality, higher-performance alkyds are produced in the fatty acid process, in which the composition of the resulting resin may be more precisely controlled. In this process, anacid anhydride,a polyol and an unsaturated fatty acid are combined and cooked together until the product has achieved a predetermined level of viscosity.Pentaerythritolbased alkyds are made this way. More economical alkyd resins are produced from the alcoholysis or glyceride process, in which end-product quality control is not as paramount. In this process, raw vegetable oil, high in unsaturated component, is combined with additional polyol and heated to causetransesterificationof thetriglyceridesinto a mixture of mono- and diglyceride oils. Soybean oil is often used.[7]Acid anhydride is added to the resulting mixture to build the molecular weight of the resin into roughly the same product as with the fatty acid process. However, the alcoholysis process produces a more randomly oriented structure. To remove the water produced as a by-product and to increase the reaction rate, surplus phthalic anhydride is added. Water is thus removed with the unreacted acid by heating the bulk to a specific temperature. The reaction is not as controllable as would be desired, so a new process was introduced in whichxyleneis added to produce anazeotropewith the water. This gives greater control at a lower temperature and also produces resins at a lower viscosity, useful in making high-solids paints. This is known as the AZO process. In both cases, the resulting product is a polyester resin to whichpendantdrying oil groups are attached. At the conclusion of each process the resin is purified, diluted in solvent and sold topaintandvarnishmakers.[8]

Metal casting

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Alkyd or oil-urethane binders are used incastingfor the creation ofsand-based moulds. The alkyd resin is mixed with apolymericisocyanateand a metallic drier, which speeds up the reaction.[9]Unlike other no-bake mould technologies, the process yields no toxic fumes, but the moulds need more air-curing time.[10]

Coating types

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Alkyd resins are usually classed as long oil,[11]medium oil and short oil.[12]These terms represent the oil length in the resin.[13]Alkyds are also modified withphenolic resin,styrene,vinyl toluene,acrylic monomers (to make them dry more quickly) andisocyanatesto produce a polyurethane modified alkyd. Urethane alkyds are manufactured by reacting the OH groups residual on the alkyd with NCO groups from anisocyanateoftenTDI.[14]By adding certain modifying resins, it is possible to producethixotropicalkyds for decorative use such as non-drip paints. The latest alkyds are short oil resins in which the oil length is shortened by use of a polymeric chain stopper usually a monobasic acid such asbenzoic acidorpara-tert-butylbenzoic acid(Alkydal M 48). These have a better controlled molecular weight distribution and better durability. Alkyds for decorative use have extra oil cooked in to lengthen them and to make them more durable. Short oil resins used in stoving enamels are made from non-drying saturated oils or fatty acids. These usually have much higher hydroxyl and acid values to be able to react with thehydroxyl groupsof the amino resins. These mixtures are usually stabilized with amines to prevent gelling on storage.

Because the major components of an alkyd coating, i.e. fatty acids and triglyceride oils, are derived from low cost renewable resources, the cost of alkyd coatings has remained very low despite the ever-increasing cost ofpetroleum,which is the predominant raw-material source of most other coatings such asvinyls,acrylics,epoxiesandpolyurethanes.[15]Typical sources of drying oils for alkyd coatings aretung oil,linseed oil,sunflower oil,safflower oil,walnut oil,soybean oil,fish oil,corn oil,dehydratedcastor oil(in which dehydration transforms certain of its fatty acids’ single bonds into double bonds, some of them conjugated, creating a semi-drying oil) andtall oil(resinous oil by-product from pulp and paper manufacturing). Non-drying/plasticizer resins are made from castor, palm, coconut oils and Cardura[16][17](the proprietary name of a synthetic ester of versatic acid). Dehydrated castor oil was at one time the only oil permitted in resin manufacture in India; no edible oils were allowed.[citation needed]

They may be used to formulate flame-retardant coatings.[18]

Hybrids

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As with many resin systems and coatings, alkyds may be hybridized with other resin technologies. One example is acrylated alkyds.[19][20][21][22]Although urethane alkyds are in effect a hybrid, novel technologies include manufacturing a moisture-curable polyurethane alkyd.[23]Grafting of silicon based materials onto alkyds has also been researched.[24]As part of an effort to use more recycled materials, alkyds have been produced that are made from scrapPETbottles.[25][26]

Waterborne alkyds

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Main article:Waterborne resins

There has been a general trend worldwide to formulate resins and coatings that are waterborne rather than formulated with solvent as waterbased materials are perceived to be environmentally friendly. Waterborne alkyds have thus been made available too.[27][28]One method is to acrylic-modify the alkyd to make it water-reducible.[29]Synthesis techniques of acrylic-modified water-reducible alkyds to improve corrosion performance have likewise been studied.[30]

References

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  1. ^Alkyd Resins,Organic Chemistry, vol. 29, Elsevier, 1977, pp. 140–170,doi:10.1016/b978-0-12-618502-7.50010-6,ISBN978-0-12-618502-7,retrieved2021-05-14
  2. ^Elliott, W. T. (1993),"Alkyd Resins",Surface Coatings: Volume 1 Raw Materials and Their Usage,Dordrecht: Springer Netherlands, pp. 76–109,doi:10.1007/978-94-011-1220-8_5,ISBN978-94-011-1220-8,retrieved2021-05-14
  3. ^Frank N. Jones. "Alkyd Resins".Ullmann's Encyclopedia of Industrial Chemistry.Weinheim: Wiley-VCH.doi:10.1002/14356007.a01_409.ISBN978-3527306732..Published online: 15 January 2003.
  4. ^"Properties of Alkyd Resins".polymerdatabase.Retrieved2021-05-14.
  5. ^The Chemistry and Processing of Alkyd Resins.Monsanto Chemical Company. 1952.ASIN BOOOHKKBB6
  6. ^Halstead, Joshua (April 2023)."Expanded Applications and Enhanced Durability of Alkyd Coatings Using High-Performance Catalysts".CoatingsTech.20(3): 45–55 – via American Coatings Association.
  7. ^"Reichhold | Coatings Products".reichhold.Retrieved2021-05-14.
  8. ^"Additives for Coatings" J. H. Bielman, Ed. Wiley-VCH, 2000, Weinheim.ISBN3-527-29785-5.
  9. ^"Cobalt Drier for Paints | Cobalt Cem-All®".Borchers.Retrieved2021-05-15.
  10. ^C. W. Ammen.Metalcasting(New York: McGraw-Hill Professional, 1999) p. 36.ISBN978-0-07-134246-9
  11. ^"SERKYD S63W70 | Long Oil Alkyd Resin | Serkim".serkimresin.Retrieved2021-05-14.
  12. ^"Properties of Alkyd Resins".polymerdatabase.Retrieved2021-05-14.
  13. ^"Alkyd resins for paints".Polynt.Retrieved2021-05-14.
  14. ^[1],"Polyurethane-modified alkyd resin", issued 1977-10-05
  15. ^"Basics of Alkyd Resin Technology".Chemical Dynamics.2016-03-29.Archivedfrom the original on 2021-05-14.
  16. ^Herszenhaut, Marcelo."Novel Path to Hydroxyl-Functional Emulsions".Paint & Coatings Industry.Retrieved26 February2024.
  17. ^"Cardura™ E10P Glycidyl Ester".SpecialChem.Retrieved26 February2024.
  18. ^Abd El-Wahab, H.; Abd El-Fattah, M.; Abd El-Khalik, N.; Kazlauciunas, Algy (2015-01-01)."Synthesis and performance of new modified reactive flame-retardant alkyd resin based on tetrabromophthalic anhydride as varnish for surface coatings".Journal of Coatings Technology and Research.12(1): 97–105.doi:10.1007/s11998-014-9615-6.ISSN1935-3804.Archivedfrom the original on 2023-03-14.Retrieved2023-03-08.
  19. ^Chardon, Fabien; Denis, Maxinne; Negrell, Claire; Caillol, Sylvain (2021-02-01)."Hybrid alkyds, the glowing route to reach cutting-edge properties?".Progress in Organic Coatings.151:106025.doi:10.1016/j.porgcoat.2020.106025.ISSN0300-9440.
  20. ^Hofland, Ad (2012-04-01)."Alkyd resins: From down and out to alive and kicking".Progress in Organic Coatings.Alkyds for the 21st Century.73(4): 274–282.doi:10.1016/j.porgcoat.2011.01.014.ISSN0300-9440.
  21. ^Jowkar-Deriss, Mehrnoush; Karlsson, Ola J. (2004-09-24)."Morphologies and droplet sizes of alkyd–acrylic hybrids with high solids content".Colloids and Surfaces A: Physicochemical and Engineering Aspects.245(1): 115–125.doi:10.1016/j.colsurfa.2004.07.003.ISSN0927-7757.
  22. ^Dziczkowski, Jamie; Soucek, Mark D. (2010-09-01)."A new class of acrylated alkyds".Journal of Coatings Technology and Research.7(5): 587–602.doi:10.1007/s11998-009-9237-6.ISSN1935-3804.Archivedfrom the original on 2023-03-14.Retrieved2023-03-09.
  23. ^Naik, R. B.; Malvankar, N. G.; Mahato, T. K.; Ratna, D.; Hastak, R. S. (2014-07-01)."Novel moisture-cured hyperbranched urethane alkyd resin for coating application".Journal of Coatings Technology and Research.11(4): 575–586.doi:10.1007/s11998-013-9561-8.ISSN1935-3804.Archivedfrom the original on 2023-03-14.Retrieved2023-03-08.
  24. ^Chakraborty, Ruby; Thatte, Mrunal; Soucek, Mark D. (2009-12-01)."A new approach to graft siloxanes to alkyds".Journal of Coatings Technology and Research.6(4): 471–481.doi:10.1007/s11998-008-9155-z.ISSN1935-3804.Archivedfrom the original on 2023-03-14.Retrieved2023-03-14.
  25. ^Civan Çavuşoğlu, Ferda; Acar, Işıl (2023-03-01)."Synthesis of PET-based urethane-modified alkyd resins from depolymerization intermediates of post-consumer PET bottles: coating properties and thermal behaviors".Journal of Coatings Technology and Research.20(2): 741–761.doi:10.1007/s11998-022-00705-y.ISSN1935-3804.
  26. ^Spasojević, P. M.; Panić, V. V.; Džunuzović, J. V.; Marinković, A. D.; Woortman, A. J. J.; Loos, K.; Popović, I. G. (2015-07-16)."High performance alkyd resins synthesized from postconsumer PET bottles".RSC Advances.5(76): 62273–62283.doi:10.1039/C5RA11777A.ISSN2046-2069.
  27. ^"Synthesis of Waterborne alkyd resins".September 2017.Archivedfrom the original on 2021-05-14.
  28. ^US Patent 5,137,965 1992
  29. ^Büyükyonga, Özge Naz; Akgün, Nagihan; Acar, Işıl; Güçlü, Gamze (2017-01-01)."Synthesis of four-component acrylic-modified water-reducible alkyd resin: investigation of dilution ratio effect on film properties and thermal behaviors".Journal of Coatings Technology and Research.14(1): 117–128.doi:10.1007/s11998-016-9835-z.ISSN1935-3804.
  30. ^Aynali, Figen; Sakar, Gürcan; Kocyigit, Elif Suna; Kades, Alper (2023-11-01)."Synthesis of acrylic-modified water-reducible alkyd resin: improvement of corrosion resistance in painting formulations".Journal of Coatings Technology and Research.20(6): 2007–2017.doi:10.1007/s11998-023-00795-2.ISSN1935-3804.
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