Surface energy [: defined as the excess energy at
the surface of a material compared to the bulk or the work per unit area done
by the force that creates the new surface]
How can a surface have energy?
At first sight this is not an
unreasonable question. Energy is defined as the capacity to do work and if we
take the example of the average automobile panel, it is difficult to find
evidence that it is engaged in any form of work.
The situation becomes clearer when we
spray water droplets on a cars roof, part of which has been wax polished. The
droplets that land on the polished areas will form discrete near-spherical
droplets. This is due to the surface tension of the water
The water droplets that land on the
unpolished panel behave differently. They tend not to form droplets but to
spread out to form a thin film. In other words the surface tension forces that
hold the water droplets together have been overcome. It takes energy to
overcome the surfaces tension forces and this energy has to come from
somewhere. In fact it comes from the surface of the panel and more specifically
from the forces that hold the molecules of the panel’s material together.
A panel which has been polished using a
hydrocarbon wax will have a surface rich in hydrocarbon molecules. The forces
that hold hydrocarbons together are much weaker that the forces that act
between water molecules and consequently water on a hydrocarbon surface remains
in the droplet form.
An unpolished surface will have at its
surface a complex mixture of molecules made from carbon hydrogen and oxygen and
(unlike hydrocarbons) there will be a significant proportion of polar groups
(e.g. O-H) present. The forces of attraction between polar molecules are
stronger than those between non-polar hydrocarbon molecules and in this example
they are sufficiently strong to overcome the surface tension forces of water
and cause the droplets to spread out and form a film.
It is common in the coatings industry to
refer to low energy and high energy surfaces. Polyethylene and polypropylene
are examples of low energy surfaces. The forces between the hydrocarbon
molecules that make up the polymers are weak and consequently polar liquids
tend to form droplets on the surface rather than spread out.
It is difficult to coat low energy
surfaces but fortunately there are numerous ways of converting low energy into
high energy surfaces. All the methods aim to form oxygen containing species at
the surface and this oxidation can be achieved by exposure to ultraviolet
radiation, plasma or corona discharge or by flame or acid treatment.
Surface energy is quantified in terms of
the forces acting on a unit length at the solid-air or the solid-liquid
interface. The units of measurement are exactly the same as for surface
tension.
Contact Angle
The definitions of surface tension and
surface energy have involved consideration of the behaviour of liquids in
contact with solids and the formation of droplets or thin films. One convenient
way of quantifying this behaviour is to measure the angle θ formed by the
liquid-solid and the liquid-liquid interfaces:-
If θ is greater than 90° the liquid
tends to form droplets on the surface. If θ is less than 90° the liquid tends
to spread out over the surface and when the liquid forms a thin film, θ tends
to zero.
Surface energy + Adhesion + Contact angle
Adhesion
Adhesion [:
is the tendency of dissimilar particles or surfaces to cling to one another
(cohesion refers to the tendency of similar or identical particles/surfaces to
cling to one another)]
The numerical difference between the
surface tension of a coating and the surface energy of a substrate has a
profound effect on the way in which the liquid coating flows out over the
substrate and on the strength of the adhesive bond between the substrate and
the dry film.
If the surface tension of the coating is
greater than the surface energy of the substrate then the coating will not
spread out and form a film. As we increase the surface energy of the substrate,
we can reach a stage where the coating will spread out and form a film but,
when dry, has poor adhesion. Further increases in the surface energy of the
substrate will result in easier wet-film formation and better dry-film
adhesion.
It is important to emphasise that
surface energy is only one aspect governing the complex phenomenon that we
refer to as adhesion. Adhesive testing involves the application of force to
remove the coating from the substrate. The intention is to measure the force
needed to overcome the forces of adhesion between coating and substrate. In
practice however, the cohesive strength of the coating and of the substrate
both have an effect on how easy it is remove the coating. In fact there is a
supportable case for saying that there is no such thing as a true adhesive
failure since, at the molecular level, all failures are cohesive failures of
the coating or the substrate.
References
1. PRA Coatings Technology Centre
Publications
Relevant Articles
1. The Science behind Coatings – http://togwt1980.blogspot.co.uk/2015/01/the-science-behind-coatings.html
Relevant Articles
1. The Science behind Coatings – http://togwt1980.blogspot.co.uk/2015/01/the-science-behind-coatings.html
2. Is your paint coating still viable? - http://togwt1980.blogspot.co.uk/2015/06/is-your-paint-coating-still-viable.html
Always be
willing to learn; because the more you learn, the more you’ll realize what you
don’t know.
It is said that knowledge is power, with the caveat that
it includes access to a reliable information sources. I would like to think that these articles become an asset to
anyone who is new to detailing and to professional’s alike, as well as industry
experts who seek to advance their knowledge.
I hope the
article are informative.
By having some understanding of the ‘What’ and ‘Why’ as well as the ‘How’ along with a little science to help you understand how the chemicals we use react, you can achieve the results you desire.
By having some understanding of the ‘What’ and ‘Why’ as well as the ‘How’ along with a little science to help you understand how the chemicals we use react, you can achieve the results you desire.
I would appreciate it if you would share
these articles as it helps other detailers further their knowledge.
This was a tough subject to explain, so
I hope this makes sense and helps you. As always if you have questions, I’ll do
my best to answer; bear in mind the only stupid questions is the one that was
unasked. Questions and/ or constructive comments are always appreciated
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