Chemistry:Blocked isocyanates

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Short description: Class of chemical compounds

Blocked isocyanates are organic compounds that have their isocyanate functionality chemically blocked to control reactivity.[1] They are the product of an isocyanate moiety (nearly always a di-isocyanate) and a suitable blocking agent. It may also be a polyurethane prepolymer that is NCO terminated but this functionality has also been chemically reacted with a blocking agent. They are usually used in polyurethane applications but not always.[2] They are extensively used in industrial applications such as coatings, sealants and adhesives.[3][4][5]

Overview

A blocked isocyanate can be added to materials that would normally react with the isocyanate such as polyols. They do not react at normal ambient room temperature. A formulation containing a blocked isocyanate is a single component material (and thus usually considered more convenient) but reacts like a two-component product but will not react until heated to the temperature required for activation which varies depending on the blocking agent.[6] The shelf lives when stored at ambient temperature have been reported as good.[7] On heating up to the activation temperature, cure can be as short as 20 minutes depending on the blocking agent and other formulation parameters.

Blocked isocyanates are particularly useful in products with a dual cure mechanisms. As an example, Ultraviolet light initiates the polymerization of an acrylate based polymer that contains hydroxy groups on the polymer backbone. The system would also contain an isocyanate blocked with a malonate. This is now a one-component system. When heat is applied, the polymerization is initiated. Higher temperatures then unblock the isocyanate, allowing the cure and crosslinking of the urethane.

Blocking agents

A number of blocking agents maybe employed but a common one is Methylethyl ketone oxime (MEKO). Caprolactam is also used.[8]

When blocked, there is no isocyanate (NCO) functionality, so it is much easier to disperse the species in water if the desire is to produce waterborne resins.[9][10] One of the key reasons different blocking agents are used apart from chemical properties is that they unblock at different temperatures. MEKO has a fairly low unblocking temperature and is thus in fairly common usage. A mixture of blocking agents maybe used to optimize properties.[11]

Unblocking temperatures

Different blocking agents for isocyanates have different unblocking temperatures.[12]

Catalysts

Catalysts also have a role in the unblocking of blocked isocyanates.[15] As with regular isocyanates organometallic compounds and tertiary amines may lower the unblocking temperature. Tin compounds, such as dibutyltin dilaurate, dibutyltin diacetate and other metal compounds are effective deblocking catalysts.

Uses

Apart from uses in coatings and adhesives they are also used to enhance the performance of polyester tire cord.[16] Other uses include Powder coatings, Coil Coatings, Cationic Electrocoating and primers.[17] Blocked isocyanates have also been used in tertiary oilfield recovery techniques. A blocked isocyanate is pumped into the rock/geological formation and then an organic compound with an active hydrogen is also pumped down. The result is a polymeric gel assisting with oil recovery.[18] The use of sodium bisulfite as a blocking agent has also allowed them to be used in waterborne resins such as PUDs.[19] Similarly, vanillin maybe used as an isocyanate blocker and is actively being researched. It is then used in single component waterborne coatings.[20]

See also

References

  1. ↑ Wicks, Zeno W. (1975-03-01). "Blocked isocyanates" (in en). Progress in Organic Coatings 3 (1): 73–99. doi:10.1016/0300-9440(75)80002-6. ISSN 0300-9440. https://dx.doi.org/10.1016/0300-9440%2875%2980002-6. 
  2. ↑ S.Rolph, Marianne; J.Markowska, Anna L.; N.Warriner, Colin; K.O'Reilly, Rachel (2016). "Blocked isocyanates: from analytical and experimental considerations to non-polyurethane applications" (in en). Polymer Chemistry 7 (48): 7351–7364. doi:10.1039/C6PY01776B. 
  3. ↑ "Blocked Isocyanates: OEM -Industrial Coatings". https://www.lpultd.com/wp-content/uploads/2021/02/Blocked-Isocyanates.pdf. 
  4. ↑ Jones, R. (2008-03-01). "Cross-linking organic coatings with blocked isocyanates". Transactions of the IMF 86 (2): 75–79. doi:10.1179/174591908X264464. ISSN 0020-2967. https://doi.org/10.1179/174591908X264464. 
  5. ↑ Subramani, S.; Park, Young-Jun; Cheong, In-Woo; Kim, Jung-Hyun (2004). "Polyurethane ionomer dispersions from a blocked aromatic-diisocyanate prepolymer" (in en). Polymer International 53 (8): 1145–1152. doi:10.1002/pi.1516. ISSN 1097-0126. https://onlinelibrary.wiley.com/doi/abs/10.1002/pi.1516. 
  6. ↑ Wicks, Douglas A.; Wicks, Zeno W. (2001-11-01). "Multistep chemistry in thin films; the challenges of blocked isocyanates" (in en). Progress in Organic Coatings 43 (1–3): 131–140. doi:10.1016/S0300-9440(01)00188-6. ISSN 0300-9440. https://www.sciencedirect.com/science/article/pii/S0300944001001886. 
  7. ↑ S.Rolph, Marianne; J.Markowska, AnnaL.; N.Warriner, Colin; K.O'Reilly, Rachel (2016). "Blocked isocyanates: from analytical and experimental considerations to non-polyurethane applications" (in en). Polymer Chemistry 7 (48): 7351–7364. doi:10.1039/C6PY01776B. 
  8. ↑ "Blocked Isocyanate - an overview | ScienceDirect Topics". https://www.sciencedirect.com/topics/engineering/blocked-isocyanate. 
  9. ↑ "Caprolactam And Phenol Blocked MDI Adhesion Promoter 45% Dispersion of Blocked Isocyanate Caprolactam And Phenol". https://www.glorisochem.com/rubber-fabric-adhesion-promoters/caprolactam-and-phenol-blocked-mdi-adhesion.html. 
  10. ↑ "Development Of Superior Blocked High-Performance Prepolymers Utilizing Low Monomer Technology". https://www.coatingsworld.com/issues/2020-01-01/view_technical-papers/development-of-superior-blocked-high-performance-prepolymers-utilizing-low-monomer-technology/. 
  11. ↑ Lee, Jung Min; Subramani, Sankaraiah; Lee, Young Soo; Kim, Jung Hyun (2005-10-01). "Thermal decomposition behavior of blocked diisocyanates derived from mixture of blocking agents" (in en). Macromolecular Research 13 (5): 427–434. doi:10.1007/BF03218476. ISSN 2092-7673. https://doi.org/10.1007/BF03218476. 
  12. ↑ "Blocked Isocyanates and Their Usage in Plastic Coatings Applications" (in en). https://www.pcimag.com/articles/85231-blocked-isocyanates-and-their-usage-in-plastic-coatings-applications?v=preview. 
  13. ↑ Zhou, Zhengfa; Lv, Hongxiang; Wang, Xiaowen; Ren, Fengmei; Xu, Weibing (2013). "Deblocking of the water-soluble isophorone diisocyanate blocked by sodium bisulfite and its application" (in en). Journal of Applied Polymer Science 128 (1): 597–599. doi:10.1002/app.38238. ISSN 1097-4628. https://onlinelibrary.wiley.com/doi/abs/10.1002/app.38238. 
  14. ↑ Kostyk, Barry W.; Wicks, Zeno W. (1979). "Malonic ester-blocked aliphatic isocyanates" (in en). Journal of Polymer Science: Polymer Chemistry Edition 17 (8): 2423–2428. doi:10.1002/pol.1979.170170814. ISSN 1542-9369. Bibcode: 1979JPoSA..17.2423K. https://onlinelibrary.wiley.com/doi/abs/10.1002/pol.1979.170170814. 
  15. ↑ Blank, Werner. "Advances in Catalysis for Organic Coatings". http://www.wernerblank.com/pdfiles/paper31.pdf. 
  16. ↑ Schroeter, A. (2004). "Enhance the treatment of Polyester cord by using a combination of phenolic - and Caprolactam-blocked Isocyanates". Tire Technology International. ISSN 0969-7217. https://trid.trb.org/view/749524. 
  17. ↑ Blank, Werner, J.. "Catalysis of blocked isocyanates with non tin catalysts.". http://www.wernerblank.com/pdfiles/paper18ppt.pdf. 
  18. ↑ Fink, Johannes Karl (2015). "18.6". Petroleum Engineer's Guide to Oilfield Chemicals and Fluids, second edition (2 ed.). Elsevier. pp. 653–693. doi:10.1016/c2015-0-00518-4. ISBN 978-0-12-803734-8. https://doi.org/10.1016/C2015-0-00518-4%7Cdoi=10.1016/c2015-0-00518-4. 
  19. ↑ Zhou, Zhengfa; Lv, Hongxiang; Wang, Xiaowen; Ren, Fengmei; Xu, Weibing (2013). "Deblocking of the water-soluble isophorone diisocyanate blocked by sodium bisulfite and its application" (in en). Journal of Applied Polymer Science 128 (1): 597–599. doi:10.1002/app.38238. ISSN 1097-4628. https://onlinelibrary.wiley.com/doi/abs/10.1002/app.38238. 
  20. ↑ Knospe, Philipp; Reichmann, René; Gutmann, Jochen S.; Dornbusch, Michael (2023-03-01). "Vanillin as low-temperature isocyanate-blocking agent and its use in one-component aqueous coatings" (in en). Journal of Coatings Technology and Research 20 (2): 501–520. doi:10.1007/s11998-022-00696-w. ISSN 1935-3804. https://doi.org/10.1007/s11998-022-00696-w. 

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