Recycling of Water-Based Adhesive Achieved by Changing pH
A research team has developed a reversible, water-based glue that has
good adhesion in the neutral pH range, but can be detached again in
strongly acidic or alkaline environments. As the team writes in the
journal Angewandte Chemie, the novel adhesive system, which is
based on electrostatic interactions, has bond strengths somewhere
between those of structural adhesives and pressure-sensitive adhesives.
The new adhesive also bonds “difficult” surfaces such as water-repellent
polypropylene.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Structural adhesive systems, such as two-component adhesives, form
chemical bonds when they react and cannot be separated again along the
adhesive interface. Pressure-sensitive adhesives such as Scotch tape, on
the other hand, can be cleanly removed by pulling. Adriana Sierra
Romero, Katarina Novakovic, and Mark Geoghegan of Newcastle University
in the UK have now found an alternative route to achieving adhesion and
separation: charge interactions promote structural bonding along the
interface, which can be neutralized and thus dissolved again.
To obtain the charge interaction, the team developed two separate
water-based polymer dispersions to be applied to surfaces. In both
dispersions, the base polymer was a copolymer composed of the
inexpensive, commercially available components styrene and butyl
acrylate. For one dispersion, the researchers coated the particles with
the surfactant lauryl sulfate and polymerized acrylic acid, which
together provide a negative charge in the neutral to alkaline pH range.
For the other dispersion, they coated the particles with the
polysaccharide chitosan, which contains positively charged amino groups
in neutral or acidic environment.
Both polymer dispersions formed sticky coatings on a variety of
surfaces. The researchers then observed that, when brought into contact,
the coated surfaces stuck tightly together due to the electrostatic
interactions between the positive and negative charges within the films.
This was even true in humid or wet environments, which usually have a
detrimental effect on water-based adhesives. However, when the pH was
adjusted to one extreme or the other, by adding either a strong acid or
base, the negative or positive charges within the glue were neutralized,
and the adhesion disappeared.
The team say that the pH-sensitive adhesive system could serve as a
novel, and recyclable, middle ground between structural adhesives with
fixed chemical bonds and peel-off adhesive films that bond using
physical interactions. They also emphasize that their novel
electrostatic glue adheres well to highly water-repellent polypropylene
surfaces, which are otherwise difficult to treat with aqueous adhesive
systems. Finally, the team propose integrating a bio-based material from
soybean oil into the base polymer, taking a further step toward
environmentally friendly, recyclable bonding systems.
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About the Author
Dr. Mark Geoghegan
is the Roland Cookson Professor of Engineering Materials at Newcastle
University, Newcastle, UK. His research interests cross the field of
soft matter, with interests in the structure and formation of thin
polymer films and interfaces, adhesin, diffusion semiconducting
polymers, and water-based “soft” nanotechnology.
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