Urushiol is an odourless, mostly colorless to watery yellow oily sap found in plants such as poison oak, poison ivy, and poison sumac, in their leaves, stems, and even roots, which is secreted when these parts get damaged. Interestingly, its presence is observed in above plants even after their death upto a period of 5 years! Urushiol is so strong an allergen that every human on earth can get a rash just by 7.5 ml of its quantity. Such a rash is called as poison ivy rash. Certain symptoms of a poison ivy rash are redness, itching, swelling, blisters, difficulty in breathing (if the smoke is inhaled).
It can prove very dangerous if not washed within one hour of its body contact as it gets metabolized to form quinone derivatives, which can form covalent complexes with skin proteins such as keratin leading to them appear as foreign materials to the body’s immune system, which subsequently attacks it.
Chemically, Urushiol is a type of catechol, having a ring of six carbon atoms, with alcohol groups attached to two of them and then a chain of hydrocarbon trailing from another carbon atom. Interestingly, Urushiol was used in traditional Chinese medicines and was found to possess good antioxidant, antimicrobial, and anticancer properties. Conversely, it can cause allergen reaction, as it is sensitive to oxidation and polymerization which limits its potential as a therapeutic agent.
For over 6000 years, Japanese, Chinese and Korean crafts-men and furniture-makers have used the sap of urushi tree as a glue as well as varnish, which is tapped off from the trees trunk similar to that of natural latex. It creates a solid, shiny coating on oxidation and polymerization when painted in multiple thin layers under specific conditions of heat and humidity.
Moreover, urushi based lacquer, produced by oxidation and polymerization of urushiol, are found to be temperature resistant up to a temperature of 300°C also resistant to major acids, alkalis and alcohols, but has very poor UV stability. The material loses its allergen characteristics if the lacquer coating is properly cured.
Some of the recent research works dealing in urushiol-based adhesives are:
- Fan et al. copolymerized urushiol with acrylamide and fabricating adhesive hydrogels, combining the effects of hydrophobic/hydrophilic moieties, antifreezing agent, and adhesive catechol groups. The said adhesive hydrogel possessed improved mechanical strength (tensile stress: ∼0.12 MPa with strain of ∼1100%, toughness: ∼72 kJ/m3, compression stress: ∼6.72 MPa at strain of 90%) and adhesion properties, to various dry/wet biological/polymeric/ceramic/metallic substrates at temperatures ranging from −45 to 50 °C, which may reach up to 160, 425, and 275 N/m, respectively. The hydrogel adhesive was found to be suitable for use in extremely cold or hot environments and even in underwater conditions.
- Je and Won studied the curing and adhesion of urushiol in the presence of Fe+3, wherein pH and thickness were found to have major effect of adhesives performance. Adhesion strength of 1.4 kgfcm−2 was determined under acidic conditions.
- Yang et al. prepared latex using emulsion polymerization using methyl methacrylate and butyl acrylate and urushiol (added upto 6%), showing improved adhesion strength, hardness, contact angle and thermal stability with increased addition of urushiol.
- Jung et al. produced thermally cured degradable natural urushi based adhesive using a reversible polymer (in strong acid) additive which was in turn prepared by thermally treating a hemiaminal dynamic covalent network organo‐gel (synthesized by reaction between formaldehyde and poly(propylene glycol) bis(2‐aminopropyl ether) in 1‐methyl‐2‐pyrrolidinone).
- Watanabe et al. synthesized thermally cross-linked urushi thin film using iron(II) acetate, which possessed superior thermal processability and mechanical properties.
- Isni et al. adhesive by adding urushiol in epoxy resin and its curing in 24 hrs showing improved transparency. One of the sample prepared by mixing urushiol:epoxy in the ratio of 6:4 showed brightnes, transparency and tensile shear strength similar to that of Araldite AY103-1/HY956.
Some of the points to be discussed further could be:
- Are these materials economically viable?
- How to determine the allergen property of the adhesive prepared using urushiol and its stability over time?
- Can the adhesive be used for any generic applications or is limited to certain ones wherein body contact is not happening?
- What is the production rate of urushiol and can it hamper the materials cost and sustained usage?
- Is there any safer alternative to urushiol or can it be converted to a safer derivative by modification before making an adhesive from it?
- Even if the adhesive is found to be safe, will the stigma of it containing an allergen affects the adhesives market acceptability?
Dear Readers, do go through the above literature and let me know your viewpoints in the Comments section.
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- Chengzhang Wang, Yuanfeng He, Hao Zhou, Ran Tao, Hongxia Chen, Jianzhong Ye, Yusi Zhang, “Preparation and Characterization of Urushiol Methylene Acetal Derivatives with Various Degrees of Unsaturation in Alkyl Side Chain”, International Journal of Polymer Science, vol. 2015, Article ID 843290, 6 pages, 2015. https://doi.org/10.1155/2015/843290
- Chapter Fourteen – Toxins and Venoms: Progress in Molecular Biology and Translational Science Volume 112, 2012, Pages 373-415. https://doi.org/10.1016/B978-0-12-415813-9.00014-3
- Fan, Xianmou, et al. “An Antifreezing/Antiheating Hydrogel Containing Catechol Derivative Urushiol for Strong Wet Adhesion to Various Substrates.” ACS Applied Materials & Interfaces 12.28 (2020): 32031-32040. https://doi.org/10.1021/acsami.0c09917
- Je, Hyeondoo, and Jongok Won. “Natural urushiol as a novel under-water adhesive.” Chemical Engineering Journal 404 (2020): 126424. https://doi.org/10.1016/j.cej.2020.126424
- Yang, Jianhong, et al. “Preparation in presence of urushiol and properties of acrylate latex with interparticle bridges.” Journal of Coatings Technology and Research 15.4 (2018): 819-830. https://doi.org/10.1007/s11998-017-0023-6
- Jung, Jiyoon, et al. “Degradable Natural Lacquer (Urushi) Adhesives Using a Reversible Polymer Based on Hemiaminal Dynamic Covalent Networks.” ChemistrySelect 3.23 (2018): 6665-6670. https://doi.org/10.1002/slct.201800849
- Watanabe, Hirohmi, Aya Fujimoto, and Atsushi Takahara. “Characterization of catechol‐containing natural thermosetting polymer “urushiol” thin film.” Journal of Polymer Science Part A: Polymer Chemistry 51.17 (2013): 3688-3692. https://doi.org/10.1002/pola.26770
- Kim, Eun Kyung, Sun Ah Ahn, and Sungyoon Jang. “A study on transparency and characteristics of natural adhesives made of urushiol and glue.” Journal of Conservation Science 31.2 (2015): 115-123. https://doi.org/10.12654/JCS.2015.31.2.04