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Urea-formaldehyde

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Urea-formaldehyde (UF), also known as urea-methanal, so named for its common synthesis pathway and overall structure,[1] is a nontransparent thermosetting resin or polymer. It is produced from urea and formaldehyde. These resins are used in adhesives, plywood, particle board, medium-density fibreboard (MDF), and molded objects. In agriculture, urea-formaldehyde compounds are one of the most commonly used types of slow-release fertilizer.[2]

UF and related amino resins are a class of thermosetting resins of which urea-formaldehyde resins make up 80% produced worldwide. Examples of amino resins use include in automobile tires to improve the bonding of rubber, in paper for improving tear strength, and in molding electrical devices, jar caps, etc.[3]

History

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UF was first synthesized in 1884 by Dr Hölzer, who was working with Bernhard Tollens, neither of whom realized that the urea and formaldehyde were polymerizing.[4]

In the following years a large number of authors worked on the structure of these resins.

In 1896, Carl Goldschmidt investigated the reaction further. He also obtained an amorphous, almost insoluble precipitate, but he did not realize that polymerization was occurring; he thought that two molecules of urea were combining with three molecules of formaldehyde. In 1897 Carl Goldschmidt patented the use of UF-resins as a disinfectant. General commercialisation followed this and in the following decades, more and more applications were described in the literature.[5]

In 1919, Hanns John (1891–1942) of Prague, Czechoslovakia, obtained the first patent for UF resin in Austria.[6]

Urea-formaldehyde was object matter of judgment via the European Court of Justice (now CJEU) of 5 February 1963, Case 26–62 Van Gend & Loos v Netherlands Inland Revenue Administration.[7]

Properties

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Urea-formaldehyde resin's attributes include high tensile strength, flexural modulus, high heat-distortion temperature, low water absorption, mould shrinkage, high surface hardness, elongation at break, and volume resistance. It has a refractive index of 1.55.[8]

Chemical structure

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The chemical structure of UF polymer consists of [(O)CNHCH2NH]n repeat units. In contrast, melamine-formaldehyde resins feature NCH2OCH2N repeat units. Depending on the polymerization conditions, some branching can occur. Early stages in the reaction of formaldehyde and urea produce bis(hydroxymethyl)urea.

Two steps in formation of urea-formaldehyde resin

Production

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About 20 million metric tons of UF are produced annually. Over 70% of this production is then put into use by the forest-products industry for bonding particleboard, MDF, hardwood plywood, and laminating adhesive.

General uses

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A range of objects made from UF

Urea-formaldehyde is pervasive. Urea-formaldehyde is widely utilized due to its inexpensive cost, quick reaction time, high bonding strength, moisture resistance, lack of color, and resistance to abrasion and microbes[9]. Examples include decorative laminates, textiles, paper, foundry sand molds, wrinkle-resistant fabrics, cotton blends, rayon, corduroy, etc. It is also used as wood glue. In the wood industry, it is utilized as a thermosetting adhesive to bond wood to create plywood and particleboard. It is also used as wood glue. UF was commonly used when producing electrical appliances casing (e.g. desk lamps). Foams have been used as artificial snow in movies. Urea-formaldehyde is widely used in agriculture as a slow-release fertilizer, which release small amounts of the active ingredient over time.[10]

Agricultural use

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Urea-formaldehyde compounds are a widely used as slow-release sources of nitrogen in agriculture.[2] The rate of decomposition into CO2 and NH
3 depends on the length of the urea-formaldehyde chains and it relies on the action of microbes found naturally in most soils.[11] The activity of these microbes, and the rate of ammonia release, is temperature-dependent. The optimum temperature for microbe activity is around 70–90 °F (21–32 °C).[12]

Foam insulation

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Urea-formaldehyde insulation

Urea-formaldehyde foam insulation (UFFI) commercialisation dates to the 1930s as a synthetic insulation with thermal conductivity of 0.0343 to 0.0373 W/m⋅K,[13] equating to U values for 50 mm thickness of between 0.686 W/m2K and 0.746 W/m2K or R-values between 1.46 m2K/W and 1.34 m2K/W (0.26 °F⋅ft2⋅h/BTU and 0.24 °F⋅ft2⋅h/BTU for 1.97-inch thickness).

UFFI is a foam with similar consistency to shaving cream, that is easily injected or pumped into voids. It is normally made on site using a pump set and hose with a mixing gun to mix the foaming agent, resin, and compressed air. The fully expanded foam is pumped into areas in need of insulation. It becomes firm within minutes, but cures within a week. UFFI is generally found in homes built or retrofitted from the 1930s to the 1970s, often in basements, wall cavities, crawl spaces and attics. Visually, it looks like oozing liquid that has been hardened. Over time, it tends to vary in shades of butterscotch, but new UFFI is a light yellow colour. Early forms of UFFI tended to shrink significantly. Modern UF insulation with updated catalysts and foaming technology have reduced shrinkage to minimal levels (between 2 and 4%). The foam dries with a dull matte colour with no shine. When cured, it often has a dry and crumbly texture.

Formaldehyde emissions

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Agricultural emissions

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Emissions from UF-based fertilizer application have been found to temporarily increase localized atmospheric formaldehyde concentration[10] and contribute to tropospheric ozone.[14] Application of UF fertilizers in greenhouses has been found to cause significantly higher air formaldehyde concentrations within the building.[10]

Conditions impacting emission levels

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Environmental conditions, such as temperature and humidity, can impact the levels of formaldehyde released from urea-formaldehyde products. Exposure to higher humidity and higher temperatures can both significantly increase the amount of formaldehyde emissions from UF products, such as wood-based panel boards.[15]

Reducing emissions

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Due to concerns of free formaldehyde emissions and environmental pollution from urea-formaldehyde products, there have been effective efforts to lower the formaldehyde content in UF resins.[16] A lower molar ratio of formaldehyde decreases the emission of free formaldehyde from UF products. There is a significant decrease in formaldehyde emissions from UF-based particleboard from F/U molar ratio of 2.0 to 1.0. The German standard for UF resins require the F/U molar ratio to be below 1.2. The U.S. NPA standard is an F/U molar ratio below 1.3.[17]

Health concerns

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Health effects occur when UF-based materials and products release formaldehyde into the air. Generally, no health effects from formaldehyde are seen when air concentrations are below 1.0 ppm. The onset of respiratory irritation and other health effects, and even increased cancer risk, begin when air concentrations exceed 3.0–5.0 ppm.

Health concerns led to banning of UFFI in the U.S. state of Massachusetts,[18][19] and Connecticut in 1981.[20] In 1982, the U.S. Consumer Product Safety Commission banned UFFI nationwide,[21] but this ban was reversed in 1983.[22][23] UFFI was banned in Canada in 1980, which remains in effect.[24]

See also

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References

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  1. ^ Uses Of Formaldehyde
  2. ^ a b Guo, Yanle; Zhang, Min; Liu, Zhiguang; et al. (2018). "Modeling and Optimizing the Synthesis of Urea-formaldehyde Fertilizers and Analyses of Factors Affecting these Processes". Scientific Reports. 8 (1). Springer Science and Business Media LLC: 4504. Bibcode:2018NatSR...8.4504G. doi:10.1038/s41598-018-22698-8. ISSN 2045-2322. PMC 5852125. PMID 29540771.
  3. ^ H. Deim; G. Matthias; R. A. Wagner (2012). "Amino Resins". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a02_115.pub2. ISBN 978-3527306732.
  4. ^ See:
    • Tollens, B. (1884) "Ueber einige Derivate des Formaldehyds" (On some derivatives of formaldehyde), Berichte der Deutschen Chemischen Gesellschaft, 17 : 653–659. On page 659, Tollens mentions in passing: "… , aus Harnstoff und Formaldehyd hat dagegen Dr. Hölzer ein festes, schwer lösliches Derivat erhalten." (… from urea and formaldehyde, on the other hand, Dr. Hölzer obtained a solid, almost insoluble derivative.)
    • B. Tollens (1896) "Ueber den Methylen-Harnstoff" (On methylene-urea), Berichte der deutschen chemischen Gesellschaft, 29 (3) : 2751–2752. Neither Hölzer nor Tollens realized that the urea and formaldehyde were polymerizing.
    In 1896, Carl Goldschmidt investigated the reaction further. He also obtained an amorphous, almost insoluble precipitate, but he did not realize that polymerization was occurring; he thought that two molecules of urea were combining with three molecules of formaldehyde:
    • Goldschmidt, Carl (1896) "Ueber die Einwirkung von Formaldehyd auf Harnstoff" (On the effect of formaldehyde on urea), Berichte der Deutschen Chemischen Gesellschaft, 29 (3) : 2438–2439.
    • Goldschmidt, C. (1897) "Ueber die Einwirkung von Formaldehyd auf Harnstoff," Chemiker-Zeitung, 21 (46) : 460.
    Goldschmidt had suggested that the reaction might be used to measure urea, so in 1897, Hermann Thoms (1859–1931) of Berlin investigated the reaction further: H. Thoms (1897) "Über Harnstoffbestimmung mittelst Formaldehyds" (On the determination of urea via formaldehyde), Berichte der Deutschen Pharmaceutischen Gesellschaft, 7 : 161–168. On page 168, Thoms suggested that urea and formaldehyde might be forming a polymer: "(vielleicht auch ein Polymeres dieser Zusammensetzung)" (perhaps also a polymer of this composition).
  5. ^ Meyer, Carl: Urea Formaldehyde resins: 1979: Addison-Wesley
  6. ^ See:
    • H. John "Verfahren zur Herstellung von Kondensationsprodukten aus Formaldehyd und Harnstoff bzw. Thioharnstoff oder anderen Harnstoffderivaten" (Process for the production of condensation products from formaldehyde and urea or thiourea or other urea derivatives), Austrian Patent 78,251, October 9, 1919.
    • H. John, "Process for the manufacture of condensation products of formaldehyde and carbamide or carbamide derivatives," Great Britain Patent 151,016, January 16, 1922.
    • Hanns John, "Manufacture of aldehyde condensation product capable of technical utilization," U.S. Patent 1,355,834, October 19, 1920.
  7. ^ Van Gend & Loos v Netherlands Inland Revenue Administration
  8. ^ Brady, George S.; Clauser, Henry R.; Vaccari, A. John (1997). Materials Handbook (14th ed.). New York, NY: McGraw-Hill. ISBN 978-0-07-007084-4.
  9. ^ Moubarik, Amine; Mansouri, Hamid Reza; Pizzi, Antonio; Allal, Ahmed; Charrier, Fatima; Badia, Miguel Angel; Charrier, Bertrand (2013-01-01). "Evaluation of mechanical and physical properties of industrial particleboard bonded with a corn flour–urea formaldehyde adhesive". Composites Part B: Engineering. 44 (1): 48–51. doi:10.1016/j.compositesb.2012.07.041. ISSN 1359-8368.
  10. ^ a b c Salthammer, Tunga; Gunschera, Jan (2021-01-01). "Release of formaldehyde and other organic compounds from nitrogen fertilizers". Chemosphere. 263: 127913. Bibcode:2021Chmsp.26327913S. doi:10.1016/j.chemosphere.2020.127913. ISSN 0045-6535. PMID 32822931. S2CID 221240156.
  11. ^ Hayatsu, M (2014). "A novel function of controlled-release nitrogen fertilizers". Microbes and Environments. 29 (2): 121–2. doi:10.1264/jsme2.me2902rh. PMC 4103517. PMID 25047661.
  12. ^ Pietikäinen, Janna; Pettersson, Marie; Bååth, Erland (2005-03-01). "Comparison of temperature effects on soil respiration and bacterial and fungal growth rates". FEMS Microbiology Ecology. 52 (1): 49–58. Bibcode:2005FEMME..52...49P. doi:10.1016/j.femsec.2004.10.002. ISSN 0168-6496. PMID 16329892.
  13. ^ Weijing Yuan; Decai Li; Yunwen Shen; Yang Jiang; Yanhua Zhang; Jiyou Gu; Haiyan Tan (30 May 2017). "Preparation, characterization and thermal analysis of urea-formaldehyde foam". RSC Advances. 7 (58). Royal society of Chemistry: 36223–36230. Bibcode:2017RSCAd...736223Y. doi:10.1039/C7RA06052A.
  14. ^ Kaiser, J.; Wolfe, G. M.; Bohn, B.; Broch, S.; Fuchs, H.; Ganzeveld, L. N.; Gomm, S.; Häseler, R.; Hofzumahaus, A.; Holland, F.; Jäger, J.; Li, X.; Lohse, I.; Lu, K.; Prévôt, A. S. H. (2015-02-06). "Evidence for an unidentified non-photochemical ground-level source of formaldehyde in the Po Valley with potential implications for ozone production". Atmospheric Chemistry and Physics. 15 (3): 1289–1298. Bibcode:2015ACP....15.1289K. doi:10.5194/acp-15-1289-2015. hdl:20.500.11850/99826. ISSN 1680-7316.
  15. ^ Frihart, Charles; Wescott, James; Chaffee, Timothy; Gonner, Kyle (2012). "Formaldehyde Emissions from Urea-Formaldehyde– and No-Added-Formaldehyde–Bonded Particleboard as Influenced by Temperature and Relative Humidity". Forest Products Journal.
  16. ^ Ayrilmis, Nadir; Lee, Young-Kyu; Kwon, Jin Heon; Han, Tae-Hyung; Kim, Hyun-Joong (2016-02-15). "Formaldehyde emission and VOCs from LVLs produced with three grades of urea-formaldehyde resin modified with nanocellulose". Building and Environment. 97: 82–87. Bibcode:2016BuEnv..97...82A. doi:10.1016/j.buildenv.2015.12.009. ISSN 0360-1323.
  17. ^ Meyers, George E. (May 1984). "How mole ratio of UF resin affects formaldehyde emission and other properties" (PDF). Forest Products Journal. 34 (5). Forest Products Research Society: 35–41 – via USDA.
  18. ^ Elinson, Lynn (1984). "Health Policy and Epidemiology: The Scientific Evidence for a Ban of Urea-Formaldehyde Foam Insulation". Journal of Public Health Policy. 5 (4): 513–537. doi:10.2307/3342418. JSTOR 3342418. PMID 6526937. S2CID 19201091.
  19. ^ "Toxic Chemicals & Pesticides". McGregor Legere & Stevens, PC.
  20. ^ "FACT SHEET: UREA FORMALDEHYDE FOAM INSULATION (UFFI)" (PDF). Connecticut Department of Public Health. August 2015. Retrieved 23 November 2023.
  21. ^ "CPSC Bans Urea Formaldehyde Foam Insulation (UFFI)". U.S. Consumer Product Safety Commission. March 1, 1982. Retrieved 23 November 2023.
  22. ^ "CPSC Asks For Rehearing In UFFI Case". U. S. Consumer Product Safety Commission. May 5, 1983. Retrieved 23 November 2023.
  23. ^ "Ban On UFFI Lifted". U.S. Consumer Product Safety Commission. August 25, 1983. Retrieved 23 November 2023.
  24. ^ "Canadian Prohibition of Urea Formaldehyde Foam Insulation". Health Canada. September 2, 2010. Retrieved 23 November 2023.
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