Bonding session - breaking the grip at Innovations in Adhesives and Sealant Technology

Materials World magazine
7 Jan 2013

What do composite-metal joining, plasma toothbrushes and gecko feet have
in common? They all played a part in Innovations in Adhesives and
Sealant Technology.

Divorce is never straightforward. To separate two materials that have been joined for so long – to remove that bond – is so much harder in practice than in theory.  

Debonding was one of several subjects discussed in the London-based event on 6 December. Researcher Yuchen Lu, of Oxford Brookes University, UK, was certainly concerned about broken marriages. He outlined various methods to loosen the grip that adhesives have on cars.  

According to Lu and his colleagues, almost two billion automobiles will clog the world’s streets by 2050. With scrappage numbers set to triple in the next 40 years, easier disassembly will become more important.  

Existing methods of removing automobile adhesives, including solvent immersion and mechanical disassembly, need improving. He advocated tailoring adhesive formulations, such as the ElectRelease epoxy adhesive developed by Massachusettsbased EIC Laboratories, which separates from a metal when an electrical current is passed across the epoxy-metal interface.  

Similarly, in the MagSilica product developed by Evonik, based in Essen, Germany, iron oxide particles are embedded during processing. When an external magnetic field is applied, the particles heat up to make disbanding easier.  

He added that organisations have even turned to lizard-kind to solve the reversible bonding conundrum, with the soles of gecko feet inspiring micro-patterned carbon nanotubes arrays.  

Dan Graham, Research Engineer at UK defence company BAE Systems, in Bristol, was concerned with keeping joints stuck together. He noted that existing joining techniques have their shortcomings. Adhesives are sensitive to preparation, hot and wet climates and degradation over time, while mechanical fixings are heavier and introduce fibre damage.  

BAE has developed a hybrid joining technique to incorporate the best of both methods. He explained, ‘The technique combines adhesive bonding with enhanced mechanical interlocking associated with an array of macroscale projections or pins that bridge the bondline.’  

The pin array was engaged in a dry pre-form and infused with the resin in a vacuum-assisted resin transfer moulding process. He added, ‘The composite component was cured and simultaneously formed a co-cured bond reinforced by the metallic pins embedded through the thickness of the laminate.’  

Graham claimed the technique improves strength, toughness and damage tolerance of joints, and could lead to lower mass structures. ‘We’re looking to use it for next generation frigates,’ he said. ‘I can see this being used for the automotive industry, especially for crash structures.’  

One facet of the adhesives and sealants industry that outsiders would be forgiven for forgetting is cleaning treatment, but as Dr Gary Critchlow of Loughborough University, UK, said, ‘the optimal surface for adhesion is a clean surface. If you get that wrong, everything [else] seems to go pear-shaped.’  

He noted that surface cleaning is a well-documented subject area, noting one book that cited 4,500 references. While there are many treatment options from which to choose, Ewen Kellar, of technology research company TWI, based in Cambridge, UK, extolled the benefits of one such treatment method – plasma.  

An advantage of plasma treatment is that it negates the need for messy processes that require contact. ‘If you could do everything in the gas phase, it has an awful lot of advantages,’ he said, mentioning its utility in pre-treatment for printing, coating and bonding applications.  

He outlined several interesting new applications for a technology that ‘frazzles bacteria but leaves surfaces intact’. He showed slides of plasma removing eczema from an empurpled (human) foot. A plasma scalpel is under development, and dentists could soon use plasma to skim the biofilm from teeth.  

Thanks to a pre-treatment method for a better adhesive surface, we may yet live to see a day where children look forward to brushing their teeth. Who wouldn’t, with a flaming plasma toothbrush?