blocked isocyanates

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.{{Cite journal|date=2001-11-01|title=Multistep chemistry in thin films; the challenges of blocked isocyanates|url=https://www.sciencedirect.com/science/article/pii/S0300944001001886|journal=Progress in Organic Coatings|language=en|volume=43|issue=1–3|pages=131–140|doi=10.1016/S0300-9440(01)00188-6|issn=0300-9440|last1=Wicks|first1=Douglas A.|last2=Wicks|first2=Zeno W.}} The shelf lives when stored at ambient temperature have been reported as good.{{Cite journal|last1=S.Rolph|first1=Marianne|last2=J.Markowska|first2=AnnaL.|last3=N.Warriner|first3=Colin|last4=K.O'Reilly|first4=Rachel|date=2016|title=Blocked isocyanates: from analytical and experimental considerations to non-polyurethane applications|journal=Polymer Chemistry|language=en|volume=7|issue=48|pages=7351–7364|doi=10.1039/C6PY01776B|doi-access=free}} 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.

A number of blocking agents maybe employed but a common one is Methylethyl ketone oxime (MEKO). Caprolactam is also used.{{Cite web|title=Blocked Isocyanate - an overview {{!}} ScienceDirect Topics|url=https://www.sciencedirect.com/topics/engineering/blocked-isocyanate|access-date=2021-08-25|website=www.sciencedirect.com}}

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.{{Cite web|title=Caprolactam And Phenol Blocked MDI Adhesion Promoter 45% Dispersion of Blocked Isocyanate Caprolactam And Phenol|url=https://www.glorisochem.com/rubber-fabric-adhesion-promoters/caprolactam-and-phenol-blocked-mdi-adhesion.html|access-date=2021-08-25|website=Gloriaful Industry Co., Limited}}{{Cite web|title=Development Of Superior Blocked High-Performance Prepolymers Utilizing Low Monomer Technology|url=https://www.coatingsworld.com/issues/2020-01-01/view_technical-papers/development-of-superior-blocked-high-performance-prepolymers-utilizing-low-monomer-technology/|access-date=2021-08-25|website=Coatings World}} 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.{{Cite journal |last1=Lee |first1=Jung Min |last2=Subramani |first2=Sankaraiah |last3=Lee |first3=Young Soo |last4=Kim |first4=Jung Hyun |date=2005-10-01 |title=Thermal decomposition behavior of blocked diisocyanates derived from mixture of blocking agents |url=https://doi.org/10.1007/BF03218476 |journal=Macromolecular Research |language=en |volume=13 |issue=5 |pages=427–434 |doi=10.1007/BF03218476 |s2cid=53052466 |issn=2092-7673}}

Different blocking agents for isocyanates have different unblocking temperatures.{{Cite web|title=Blocked Isocyanates and Their Usage in Plastic Coatings Applications|url=https://www.pcimag.com/articles/85231-blocked-isocyanates-and-their-usage-in-plastic-coatings-applications?v=preview|access-date=2021-09-09|website=www.pcimag.com|language=en}}

• Sodium bisulfite {{val|85|u=degC}}{{Cite journal|last1=Zhou|first1=Zhengfa|last2=Lv|first2=Hongxiang|last3=Wang|first3=Xiaowen|last4=Ren|first4=Fengmei|last5=Xu|first5=Weibing|date=2013|title=Deblocking of the water-soluble isophorone diisocyanate blocked by sodium bisulfite and its application|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/app.38238|journal=Journal of Applied Polymer Science|language=en|volume=128|issue=1|pages=597–599|doi=10.1002/app.38238|issn=1097-4628}}
• Diethyl malonate = {{val|110|u=degC}}{{Cite journal|last1=Kostyk|first1=Barry W.|last2=Wicks|first2=Zeno W.|date=1979|title=Malonic ester-blocked aliphatic isocyanates|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/pol.1979.170170814|journal=Journal of Polymer Science: Polymer Chemistry Edition|language=en|volume=17|issue=8|pages=2423–2428|doi=10.1002/pol.1979.170170814|bibcode=1979JPoSA..17.2423K|issn=1542-9369}}
• 3,5-Dimethylpyrazole = {{val|115|u=degC}}
• MEKO = {{val|135|u=degC}}
• Phenol = {{val|150|u=degC}}
• Caprolactam = {{val|170|u=degC}}

Catalysts also have a role in the unblocking of blocked isocyanates.{{Cite web|last=Blank|first=Werner|title=Advances in Catalysis for Organic Coatings|url=http://www.wernerblank.com/pdfiles/paper31.pdf|url-status=live|website=King Industries|archive-url=https://web.archive.org/web/20030321025657/http://www.wernerblank.com:80/pdfiles/paper31.pdf |archive-date=2003-03-21 }} 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.

Apart from uses in coatings and adhesives they are also used to enhance the performance of polyester tire cord.{{Cite journal|last=Schroeter|first=A.|date=2004|title=Enhance the treatment of Polyester cord by using a combination of phenolic - and Caprolactam-blocked Isocyanates|url=https://trid.trb.org/view/749524|journal=Tire Technology International|issn=0969-7217}} Other uses include Powder coatings, Coil Coatings, Cationic Electrocoating and primers.{{Cite web|last=Blank|first=Werner, J.|title=Catalysis of blocked isocyanates with non tin catalysts.|url=http://www.wernerblank.com/pdfiles/paper18ppt.pdf|url-status=live|website=King Industries|archive-url=https://web.archive.org/web/20030321112814/http://www.wernerblank.com:80/pdfiles/paper18ppt.pdf |archive-date=2003-03-21 }} 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.{{Cite book|last=Fink|first=Johannes Karl|url=https://doi.org/10.1016/C2015-0-00518-4|doi=10.1016/c2015-0-00518-4|title=Petroleum Engineer's Guide to Oilfield Chemicals and Fluids, second edition|publisher=Elsevier|year=2015|isbn=978-0-12-803734-8|edition=2|pages=653–693|chapter=18.6|doi=10.1016/c2015-0-00518-4}} The use of sodium bisulfite as a blocking agent has also allowed them to be used in waterborne resins such as PUDs.{{Cite journal|last1=Zhou|first1=Zhengfa|last2=Lv|first2=Hongxiang|last3=Wang|first3=Xiaowen|last4=Ren|first4=Fengmei|last5=Xu|first5=Weibing|date=2013|title=Deblocking of the water-soluble isophorone diisocyanate blocked by sodium bisulfite and its application|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/app.38238|journal=Journal of Applied Polymer Science|language=en|volume=128|issue=1|pages=597–599|doi=10.1002/app.38238|issn=1097-4628}} Similarly, vanillin maybe used as an isocyanate blocker and is actively being researched. It is then used in single component waterborne coatings.{{Cite journal |last1=Knospe |first1=Philipp |last2=Reichmann |first2=René |last3=Gutmann |first3=Jochen S. |last4=Dornbusch |first4=Michael |date=2023-03-01 |title=Vanillin as low-temperature isocyanate-blocking agent and its use in one-component aqueous coatings |url=https://doi.org/10.1007/s11998-022-00696-w |journal=Journal of Coatings Technology and Research |language=en |volume=20 |issue=2 |pages=501–520 |doi=10.1007/s11998-022-00696-w |s2cid=254358393 |issn=1935-3804|doi-access=free }}

• Isocyanate
• Polyurethane
• Polyurethane dispersion

{{reflist}}

• {{Cite web|title=Polymer Science: A Comprehensive Reference {{!}} ScienceDirect|url=https://www.sciencedirect.com/referencework/9780080878621/polymer-science-a-comprehensive-reference|access-date=2021-08-26|website=www.sciencedirect.com}}

Category:Organic chemistry

Category:Isocyanates