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Ever
tried to paint on top of silicone? After a few hours, the paint will
peel off. Annoying. Silicone is a so-called low surface energy polymer,
well known from flexible baking forms: A synthetic material that has an
extremely low adhesion or “stickiness”. Teflon is similarly non-sticky
and well known from frying pans. Researchers of Kiel University
(Germany) have now developed the first technology which is capable of
joining these two “unjoinable” materials. The technology applies passive
nano-scaled crystal linkers as internal staples. The nano staples open
up solutions to a large number of technical challenges, for example in
medical engineering. The work carried out within the DFG-funded
Collaborative Research Center 677 “Function by Switching” was published Aug. 24, 2012 in the scientific journal Advanced
Materials.
A new piece of technology
“If
the nano staples make even extreme polymers like Teflon and silicone
stick to each other, they can join all kinds of other plastic
materials”, says Professor Rainer Adelung. Adelung is leading the
functional nano materials group at the Institute of Materials Science in
Kiel and lead the research project from the materials science side. The
new technology of joining materials without chemical modifications can
be used, according to Adelung, in a variety of everyday life and high
tech applications. The technique is easy to use and does not need
expensive equipment or material.
Microscopic staples
The
linkers are micro and nano scaled crystals made of zinc oxide. They are
shaped like tetrapods, where four legs protrude from the point of
origin. Large-scale tetrapods are known for their ability to interlock
and form strong bonds, for example in coastal protection.
Stapling from the inside
During
the joining process, the zinc oxide crystals are sprinkled evenly onto a
heated layer of Teflon. Then, a layer of silicone is poured on top. In
order to join the materials firmly, they are then heated to 100 C
for less than an hour. “It’s like stapling two non-sticky materials from
the inside with the crystals: When they are heated up, the nano
tetrapods in between the polymer layers pierce the materials, sink into
them, and get anchored”, explains Xin Jin, the first author of the
publication, who is currently working on her PhD thesis. Her colleague
and supervisor, Dr. Yogendra Kumar Mishra, explains the adhesive
principle: “If you try to pull out a tetrapod on one arm from a polymer
layer, the shape of the tetrapod will simply cause three arms to dig in
deeper and to hold on even firmer.”
Stapling is better than gluing
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In
high technology businesses such as medical engineering, there is a
strong demand for innovative ways to make polymers, particularly
silicone, stick to other materials, for example to further develop
breathing masks, implants or sensors. Medical applications require
materials that are absolutely non-harmful, i.e. biocompatible. Many
joining methods involve chemical reactions, which may change the
polymers’ properties and can cause injurious or even toxic effects on
organisms. The tetrapod stapling, on the contrary, is a purely
mechanical process. Therefore the Kiel team assumes it to be
biocompatible.
As strong as sticky tape
With
the tetrapod staples, the scientists have achieved a stickiness—the
so-called peel strength—of 200 Newtons per meter, which is similar to
peeling sticky tape off glass. “The stickiness we have achieved with the
nano tetrapods is remarkable, because as far as we could verify, no one
has ever made silicone and Teflon stick to each other at all”, says
co-author Lars Heepe, PhD student from the Zoological Institute of Kiel
University, who precisely measured the adhesion and described what the
stapled material looks like on the microscopic scale. “Measuring
adhesion quantitatively is not as easy as it looks, precise experiments
have to be carried out in order to prove the function of the linkers and
rule out all errors“, says Professor Stanislav Gorb, leading the group
Functional Morphology and Biomechanics.
A joint, interdisciplinary effort
Three
research groups from different backgrounds combined their expertise in
material science, chemistry and biomechanics in this study within the
Collaborative Research Center 677 “Function by Switching” (CRC 677). For
Rainer Adelung and his colleagues, this study is not the end of the
project: “We are feeding our results directly into both practical
applications as well as further fundamental research.” The scientists’
local business partner nanoproofed GmbH is currently developing a
product for paintings on top of silicone. In the framework of CRC 677,
the staples are the basis for developing biomimetic adhesives, for which
adhesion can be switched on and off by light of different colours.
Joining the un-joinable: Adhesion between low surface energy polymers using tetrapodal ZnO linkers
Source: Kiel University
