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A sticky wonderland – Physics World
FABRICATION AND PROCESS FEATURE
A sticky wonderland
20 Mar 2018
This article was first published in the March 2018 issue of Physics World
Adhesives are everywhere, from the aerospace industry to the simple but infamous Post-it Note.
Alaina G Levine visits adhesive-giant 3M Company’s main US innovation centre to find out more about
the physics involved
(Courtesy: 3M)
My first thought as I stood in a narrow corridor filled with bright displays, music and booming, cheery
announcements was that I had been transported to Walt Disney World, circa 1986. I was, in fact, at the
headquarters of a $30bn, multinational materials science and adhesives corporation – 3M Company
(3M), the inventors of the Post-it Note. I was here in St Paul, Minnesota, US, to learn about 3M’s
contributions to the science and innovation of adhesives and tapes. I had been expecting scientists –
not an amusement park for sticky stuff.
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The visit began normally enough. In the vast entryway of the 3M Innovation Center, I was greeted by
Stefanie Giese-Bogdan, a communications manager with 3M. Inviting me to follow her, she took me
down the tunnel-like corridor, before closing the door to leave us standing in silent darkness. Then, the
walls, which were covered in huge screens, flashed on to give a sleek multimedia presentation on the
history and current work at 3M.
When the intro was over, double doors swung open at the other end of the tunnel. Ahead of me, in a
cavernous void, I could just make out a few chairs set up like a planetarium, letting you lean back and
gaze at the ceiling. Grinning mischievously, Giese-Bogdan suggested I take the middle chair on the
second row for the best view. As I leaned back another presentation began, this time projected on the
curved ceiling. And then suddenly, like a true Disney experience, spotlights flipped on all around me.
I was staring at the huge 3M World of Innovation – a massive room at the heart of the 3M Innovation
Center, where 3M technologists meet customers to discuss how to incorporate 3M inventions into their
products. “Innovation starts with imagination,” says Giese-Bogdan, who has a PhD in analytical
chemistry, as we walked around the room’s 27 stations, each devoted to a specific 3M core technology.
“At 3M we combine imagination with collaboration and communication. We solve problems by making
uncommon connections of technologies and by applying science to life.”
The Industrial Adhesives and Tapes Division at 3M is the largest department in the 116-year-old firm,
with more than 6100 employees worldwide, as well as research and development (R&D) centres in 10
countries and manufacturing locations in 26 nations. Adhesives are one of 46 core technologies that
3M puts into its products, but because adhesives are useful in so many sectors – from the aerospace
and automotive industries to medical equipment and electronics – they are found in more than 30,000
of the company’s products. And then of course there is the retail and consumer side, with your friendly
neighbourhood Post-it Notes and Scotch brand tapes.
The 3M World of Innovation, which is not open to the public, is a showroom and playground. It’s where
3M invites its customers to brainstorm with scientists and discuss how each technology might benefit
them. The customers get to play, experiment and discover. They witness demos, learn about new
innovations and, together, the 3M specialists and the customers try to solve problems. This is where my
tale of tape truly unrolled, as I messed around at an adhesives station and Giese-Bogdan, as one of the
hosts of the exhibition, explained to me how the innovation game is played. Like the time the Minnesota
Zoo called and explained it had a whale with a wound and needed a special bandage to stick to its slimy
lip. In fact, the solution the 3M researchers came up with proved so valuable and flexible that it became
incorporated into an entire product line of human bandages.
The science of sticky
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To find out more about the science of adhesives, I sat down with Cristina Thomas, senior technical
leader in corporate R&D. Thomas has a PhD in chemical engineering but considers herself a polymer
physicist because much of her technical career has been in the field of computational materials
modelling. “The world around us uses many more adhesives than we realize, because we often need to
bond things to surfaces,” she says. Even the room we’re in uses them, as Thomas points to the carpet,
ceiling and walls. They are in the room’s electronics, inside the display and even holding the microchips
in place. The building is kept energy efficient by film stuck to the windows and stretchable adhesives –
3M Command Strips –hold up the posters. These specialist products can bond to a surface and then
once stretched again can be removed without any residue.
An adhesive is a substance capable of holding materials together by surface attachment. Generally
made of polymers, modern adhesives allow things to be stuck together without needing to create
discontinuities, such as holes, in the substrate materials. They work by creating the same strong
molecular forces that hold materials together normally. But as polymer adhesive scientist Anthony
Pagliuca from 3M points out on the company’s wesbite, to do so an adhesive must first “wet out” the
substrate – it needs to flow over and cover the surface uniformly to maximize the contact area. The
extent to which a liquid can wet out depends on surface energy and, for an adhesive to be effective, its
surface energy must be equal to or lower than that of the substrate. Adhesion then occurs via
interactions such as hydrogen bonding, mechanical interlocking and chemical bonds.
A brief history of 3M
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Staple without staples 3M’s Post-it Note is a stationery must-have. (Courtesy: iStock/Rawpixel)
3M was founded in 1902 as a small-scale mining venture under the name Minnesota Mining and
Manufacturing Company. But the founders’ original goal, of mining one type of mineral from one
mine, turned out to be neither feasible nor sustainable. So they looked at other materials and
products and quickly found that they could harness science and engineering to develop and
improve products across multiple sectors. Early products from the 1920s included the first
waterproof sandpaper and masking tape, which launched the company’s interests in adhesives
and tapes. Over the years, 3M has grown into a firm employing 90,000 people, and with 60,000
products used in homes, businesses, schools, hospitals and more, touching most industries on
Earth. One-third of 3M’s sales come from products that were invented within the past five years.
One product that 3M is especially well known for is the ubiquitous Post-it Note, which famously
was discovered almost by accident. In 1968 Spencer Silver, a 3M scientist, was busily
researching adhesives in the laboratory. In the process, he discovered something special: an
adhesive that stuck lightly to surfaces but didn’t bond tightly to them. “It was part of my job as a
researcher to develop new adhesives, and at that time we wanted to develop bigger, stronger,
tougher adhesives,” said Silver. “This was none of those.”
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What Silver discovered was something called microspheres, which retain their stickiness but with
a “removability characteristic”, allowing attached surfaces to peel apart easily. He started sharing
his adhesive with other 3M scientists, trying to discover a problem that the adhesive could solve.
Almost six years later Art Fry, another 3M researcher, realized that Silver’s microspheres could
serve as an adhesive for a bookmark-type product that can be placed on paper then removed and
re-stuck somewhere else. Fry figured out how to manufacture it by 1977, got the green light from
management in 1978 and the Post-it Note was officially launched in the US in 1980, and in
Europe and Canada in 1981.
The rest is history – Post-it Notes hit the world by storm, and 3M continues to expand the product
line, most recently shifting to plant-based adhesives for all its Post-it Notes. Other recent 3M
milestones include the company earning its 100,000th patent in 2014, and unveiling a new,
state-of-the-art, $150m R&D laboratory on its Minnesota campus in 2015. But Post-it Notes
remain one of 3M’s most highly visible product lines and brands.
Adhesion scientists consider three factors when they design their products. First, there is the adhesion
mechanism – how the adhesive sticks to and interacts with the various surfaces. Second, the scientists
have to identify and examine the forces that will act on the adhesive-containing product while it is being
used. These forces, such as shear forces or peel forces, impact the performance of the adhesive and
can affect its integrity and function. Finally, there’s the durability, which includes examining how the
environment will influence the effectiveness of the adhesive.
Given these factors, it is not surprising to learn that physics plays a huge part in thinking about and
designing effective adhesives and tapes. After all, the basic mechanical properties of the adhesives
have to be precisely calculated and tested and clarified. Indeed, there is an entire building of labs at 3M
dedicated to just this task. It has every piece of equipment you’d expect in a corporate analytics and
materials-processing facility.
Managing how polymers flow and understanding their stability during and after application are important
aspects of developing both structural and pressure-sensitive adhesives. How will the adhesive be
deployed onto a surface? What is the application mechanism? What is the curing mechanism? Some
adhesives have to stay sticky while they hold two surfaces together, whereas others become solid or
foamy upon curing. Knowing the adhesive’s function and the environment in which it will operate
determines how to distribute it on a surface and how strong the chemical bond should be between the
adhesive and that surface. Viscosity and elasticity are characteristics of adhesives that matter greatly
too. Specialist engineers are needed to design, for example, adhesive storage cartridges and the nozzles
from which they are ejected in large-scale industrial systems.
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Materials science also plays a role in formulating adhesives and the substrates holding them, like tapes.
Depending on the application, adhesives or tapes may need to provide other properties in addition to
just good bonding performance. For example, 3M makes a pavement marking tape with a specific
optical performance to help motorists. These tapes have a layered structure in which the adhesive is on
the side contacting the road while reflective beads are embedded in the top layer so that drivers can see
the markings under various environmental conditions. “The adhesive is a mechanism to deliver
information to the driver via an additional aspect of physics, which is optics,” notes Thomas.
Sticky solutions Alaina G Levine plays at 3M’s World of Innovation. (Courtesy: Alaina G Levine)
Moreover, with adhesives and adhesion science embedded in many of 3M’s products, the sticky science
is often combined with other technologies. For example, 3M can borrow from one technology, such as its
tape formulations, combine it with cross-linking chemistry and create a strong water-based adhesive.
For one project, 3M scientists infused viscoelastic foam with adhesives to create strong bonding tape
ideal for use in the car industry. They also took ordinary elastomeric materials and developed a
proprietary high-strength matrix so manufacturers can create more impact-resistant products. Adhesive
experts, meanwhile, dipped into the science of microreplication – one of 3M’s biggest scientific assets.
The microreplication process involves melting plastic pellets and squeezing them into rolls of plastic film
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to create microscopic sculptures on the surface. There can be thousands of features per square
centimetre, arranged uniformly, that can change the physical, optical and chemical properties of the
surface. One application where 3M scientists have used the technology is to make road signs. Here, the
film is typically adhered to aluminium substrates, and thousands of tiny prisms reflect a car’s headlights
back to the driver, resulting in signs and markers on roads appearing significantly brighter than normal.
Adhesives also have to withstand a spectrum of environmental stimuli – anything that could impact the
function or durability needs to be addressed. Those pavement markings, for example, are exposed to
water, oil, atmospheric compounds, such as pollutants, and many other chemicals and concoctions.
They have to stay in place and be able to withstand constant shocks, vibrations, and movement from
vehicles. Adhesives additionally have to cope with changes in pressure, temperature and a host of
other variables.
In one of 3M’s many labs, I met Aaron Hedegaard, a chemical engineer who studies the viscosity and
elasticity of different formulations of adhesives to quantify their strength and stickiness. He does this by
smearing a sample of an adhesive on a rotational rheometer and measures its stiffness, dissipation
factor, and its response to being heated and chilled, even to temperatures well below a typical
Minnesota winter. “I develop new test methods, not just running the standard tests,” he says. “I am
trying to push the boundaries to make the next standard test.”
3M's SEALS
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A scientist’s seal 3M’s experts work with customers to create their ideal adhesive. (Courtesy: 3M/J Rolfes)
When designing adhesives for customers, 3M experts ask questions based on the SEALS
acronym:
S: Substrate – What is the nature of the substrate?
E: Environment – What is the bonding environment? What environment will the bond be
subjected to (internal/external, high/low temperatures, chemicals, salt)?
A: Application –What are you doing with your component? What application characteristics do
you need in terms of speed of cure, open time and rheology from the adhesive?
L: Load – What are the stresses on the joint in type, magnitude and direction?
S: Size – For industrial adhesives, how many units are you producing – per month, per quarter or
per year?
Overcoming physics
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Physics breakthroughs have impacted 3M adhesives in fascinating ways, according to Thomas. To a
chemist, a polymer is a macro-molecule composed of repeating smaller molecules or units, but people
now understand that these long molecules can be approximated as chains and so their behaviour can
be explored and explained using statistical methods from physics.
“We can treat polymers using simplified models where the molecules are represented by individual units
or beads that replace the group of atoms,” says Thomas. Macromolecules within the adhesion polymers
can then be investigated using physics principles, enabling scientists to understand their behaviour
under certain conditions. “If I’m doing a phase separation when it’s a polymeric system, it’s going to
behave differently than if I am separating smaller molecules – this is something that really advanced the
adhesion science field.”
Depending on temperature, some adhesives behave like glass (solid-like) and some like rubbers (fluidlike), so Thomas and her team draw ideas and knowledge from complex systems too. When you
“chemically cure” an adhesive with the aim of converting it into a solid, the adhesive is going through a
chemical reaction and, once cured, it is able to provide high strength and resistance to temperature,
humidity or chemical exposure. Depending on whether you cure with light, heat or another mechanism,
you are enhancing the performance of the adhesive due to the formation of an adhesive network that
behaves like a glassy polymer.
But just because there is a lot of physics in adhesives doesn’t mean that 3M scientists are shackled to
it. Back in the 3M World of Innovation, Giese-Bogdan showed me the Multi-layer Optical Film – a silvery,
reflective piece of film that is used with a special tape. “If you are tilting a reflective surface there is an
angle at which it is no longer reflective – that angle is called the Brewster’s angle,” she explains. “The
Multi-layer Optical Film does not have a Brewster’s angle, it is reflective at any angle.” The film is used to
make light shafts in buildings and, since Brewster’s angle is not a consideration, it can be placed around
bends. It can also be used in, for example, electronics – from phones to TVs – to save energy, and as a
parabolic mirror to direct light onto solar cells. It is one of the many product examples that Giese-Bogdan
has ready to showcase in the World of Innovation. “How often can you say you beat the laws of physics?”
she teases. “Well we did. We beat Brewster’s angle.”
The 3M Innovation Center gave me a glimpse into the amazing world of adhesives, and as I left, my takehome message was that adhesives are sticking around and expanding their reach, strength, and
diversity of use. “Many of the things that we see every day have adhesives or result from the use of
adhesion science. There is a lot of physics, of fundamental understanding behind that,” says Thomas.
“Even things we take for granted every day, such as a marking on the road [are bonded by adhesives].”
A huge diversity of good adhesives allows lorries to go over those tapes but also allows Post-it Notes to
be reused, and allows medical tapes to cover fragile or sensitive skins, as well as countless other
applications for tapes and adhesives across the globe.
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Enjoy the rest of the March 2018 issue of Physics World in our digital magazine or via the Physics
World app for any iOS or Android smartphone or tablet. Membership of the Institute of Physics
required
Alaina G Levine is a science writer, keynote speaker, science career consultant and author
of Networking for Nerds (Wiley, 2015) based in Tucson, Arizona, US, @AlainaGLevine
Copyright © 2018 by IOP Publishing Ltd and individual contributors
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