What are Adhesives
An adhesive is generally defined as a material which can stick two materials and keep them attached together with sufficient adhesive force. 
Some formal definitions of adhesive are as below:
- Britannica Encyclopaedia: Adhesive, any substance that is capable of holding materials together in a functional manner by surface attachment that resists separation. 
- Merriam-Webster Dictionary: A material tending to adhere or cause adherence. 
- IUPAC: It defines adhesion as a phenomenon in which interfaces between phases or components are maintained by intermolecular forces, chain entanglements, or both, across the interfaces. 
History of Adhesive:
The table below enumerates various historical developments in the field of adhesives. [5-7]
|Year||Development in the Field of Adhesives|
|~ 2 lakh years ago||Glue obtained from birch-bark-tar used to stick stone arrow heads to the shaft|
|~ 4 thousand years ago||Use of animal blood and protein as binder for cave paintings|
|~ 3500 BC||Egyptians developed the art of making boiling glue|
|~ 2000 BC||Use of gelatin glue for furniture manufacturing|
|~ 1000 BC||Made adhesive from animal skins in China, for lacquering|
|~ 200 BC||First literature: Making and using animal glue|
|~ 1300 AD||Aztecs used blood for construction work|
|1700||Industrial-scale production of glues by boiling, in The Netherlands|
|1750||The first patent on glue issued in Britain – Fish glue|
|1830||Used natural rubber for making adhesives|
|1922||Patent to BASF: Solvent soluble urea-formaldehyde resins|
|1931||Production of polychloroprene started|
|1936||P. Castan (Switzerland) invented epoxy resin|
|1940||IG Farben awarded patent for acrylate-based adhesives|
|1943||First use of phenolic resin – polyvinyl acetates aircraft manufacture.|
|1953||V. Krieble (USA): Anaerobically curing adhesives|
|1958||First cyanoacrylate adhesive|
|1967||First heat-resistant polyimide adhesive|
|1968||Moisture curing polyurethane adhesive|
|1980||Reactive hotmelt adhesive|
|1984||Anisotropic, conducting adhesive|
|1993||Aerobically curing adhesive|
|2000||Detachable adhesive systems responsive to change in temperature, stress, voltage and/or pH etc|
Global Adhesives Market [11-12]
- Market value in 2019: US$ 53 billion
- Expected Market Value: US$ 85.7 billion by 2030 at a CAGR of 4.5%
- Less Capital Intensive
- Highly fragmented
- Highly competitive
- Asia-Pacific Region: Dominating share & Fastest growing
- Major global players:
- Ashland Inc.
- Avery Dennison Corporation
- BASF SE
- Bostik SA
- H. B. Fuller
- Henkel AG & Company KGaA
- Pidilite Industries Limited
- Sika AG
- The 3M Company
- The Dow Chemicals Company
India Adhesives Market [13-15]
- Market value in 2020: US$ 1.5 billion
- Expected Market Value: US$ 17 billion by 2026 at a CAGR of 8.07%
- Major Usage Area: Packaging and Construction
- Dominated by four top players holding about 85% of the market share
- Major Indian players:
- Pidilite Industries
- Henkel AG & Co
- Arkema Group (Bostik India)
- H.B. Fuller Company
Classification of Adhesives
Classification of adhesives is quite complex and is based on various parameters; however, one of the methods of adhesive classification is:
- One part adhesive
- Heat activated
- UV / Light curing
- Moisture curing
- Pressure sensitive
- Two-part adhesive
Till recently, there had been no literature available on ‘electrocuring’ adhesives. [8-10]
Voltaglue: Voltage Activated Adhesives
- Voltaglue is the brainchild of a group of researchers from Nanyang Technological University, Singapore lead by Dr Terry Steele, Assistant Professor of Materials Science and Engineering at the University.
- Voltage-activated adhesives, also called as VoltaGlue by the scientists who developed them, are a type of stimuli responsive materials; wherein, carbenes are grafted on the surface of dendrimers, which hooks onto nearby carbene based dendrimer surfaces on application of an electric charge. 
- In this methodology, strength of the adhesive can be increased or decreased by changing the time duration of voltage application, which dictates the number of carbenes getting hooked. The developers of this adhesion technique call it “electrocuring.”
- In this technique, they have following benefits over the traditional adhesives:
- Control on reaction kinetics
- Improved pot time even after mixing all the compositions
- No heating or cooling requirement
- No need of activators such as initiators, preservatives, and even see-through surfaces for photoactivating adhesives
- Requirement of simple electronic interface
- PAMAM-g-diazirine was synthesized by reacting amine groups on the surface of polyamidoamine (PAMAM) with aryl-diazirine, and then dissolved in a buffer solution. This solution is designated by them as Voltaglue; which possesses ability to cross-link on application of electric voltage.
- DC curing method:
- Adhesion strength: 20 to 55 kPa
- Voltage electric fields requirement: >2 V/mm
- Electrocuring progresses from cathode to anode
- AC curing method:
- Adhesion strength: 25 to 85 kPa
- Curing time: 5 to 10 min
- Two-sided electrocuring migration from the cathode to the anode as current alternates
- The team in collaboration with Indian Institute of Technology, Kanpur, also developed a catechol based Voltaglue; wherein PAMAM conjugates that graft catechols as electron/proton donor(s) paired with a reducible acceptor(s) will be voltage activated for quinone-mediated crosslinking and adhesion, providing Voltaglue with a lap shear strength of about 50 kPa, and making the process single step curing instead of its traditional two-part curing methodology.
- The same researchers also invented a methodology to provide a quick and minimally invasive way of tears and holes closure in broken blood vessels by utilising Voltaglue applied balloon catheter and activating using a retractable electrode. [16-20]
- Is it substrate specific adhesive? What to do if the substrate is not compatible with the adhesive?
- Can a battery-operated device be created for the adhesive curing, like we have small UV bulb devices for UV curing the adhesive?
- How would curing be performed if the blood vessels are near very sensitive organs such as the heart, brain or spinal cord? Hope, it will not have secondary repercussions.
- Can a technique be developed to make normal adhesive electrocuring?
- Can addition of electrically conducting fillers prove to be of any benefit?
- Can a thermoplastic adhesive be developed with ‘electro-application’ properties?
Dear Readers, do go through the above literature and let me know your viewpoints in the Comments section.
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- Onusseit Hermann Dr, (2012), Adhesive Technology – Basic Principles, Beuth, DIN Deutsches Institüt für Normung e.V., Henkel AG & Co. KGaA
- Singh, Manisha, et al. “Synergistic voltaglue adhesive mechanisms with alternating electric fields.” Chemistry of Materials 32.6 (2020): 2440-2449.
- Singh, M.; Webster, R. D.; J. Steele, T. W. Voltaglue Electroceutical Adhesive Patches for Localized Voltage Stimulation. ACS Appl. Bio Mater. 2019, 2, 2633−2642.
- Singh, M.; Nanda, H. S.; O’Rorke, R. D.; Jakus, A. E.; Shah, A. H.; Shah, R. N.; Webster, R. D.; Steele, T. W. J. Voltaglue Bioadhesives Energized with Interdigitated 3D-Graphene Electrodes. Adv. Healthcare Mater. 2018, 7, No. e1800538.
- Ping, J.; Gao, F.; Chen, J. L.; Webster, R. D.; Steele, T. W. Adhesive curing through low-voltage activation. Nat. Commun. 2015, 6, No. 8050