Improving Glass Quality:
Beyond Hydrophobicity: The Value of "Sliding Angle" and the
Myth of "Self-Cleaning" Glass
By Guillermo Seta
Over the last decade, protective surface coatings with water repellent qualities, also known as
hydrophobic or easy-to-clean (E2C) coatings, have been suffering a transformation by the form
and manner in which they have been supplied to the glass industry, as well as the perception by
both the consumer and the glass fabricator and/or glass shop.
From hydrophobic coatings and Contact Angle (as a "static dimension") to hydrophilic coatings
and self-cleaning glass and the NEW way of understanding what I call "functional architecture"
by assessing the value of Sliding Angle (as a "dynamic dimension"), I invite you to know some
essential facts one should know while looking at protective coatings.
Background
In order to better understand such interesting, and even intriguing transformation, and the
various developmental stages that have taken place globally, one must also understand the
reasons, market needs, and some of the leading suppliers involved, which I will refer to as
"players" - dominating the field in the highly-competitive glass industry. Many players have
surfaced and many others have disappeared, transformed, and/or merged, due to the
marketing need of the moment, whether driven by price or simply copying an existing
successful business model of another supplier. Such transformation has occurred in front of our
eyes over the last 10 years or so and, in many cases, it has been sheer copy cats without any
regard to quality or science whatsoever.
The concept of a hydrophobic coating in the US Flat Architectural Glass Industry was first
brought to the market, in any significant way, during the late 90's by Diamon-Fusion
International (DFI). DFI not only brought the concept of an E2C hydrophobic but also
oleophobic (oil-repellent) as well as its enhancement with additional properties of added
scratch and impact resistance, UV stability, optical clarity, increased electrical insulation, among
the main attributes of such 'enhanced hydrophobicity' as it was called by DFI back in the early
to mid 2000's.
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What makes a hydrophobic coating functional?
As its name denotes, one of the first and foremost core properties of any hydrophobic coating
that one should know is precisely how hydrophobic, or water-repellent, the coating makes the
surface. Why? Simply because the more hydrophobic the surface becomes, the less water will
spread out and deposit its minerals and contaminants on
the glass surface. The fewer deposits, the less cleaning is
required, and when cleaning is needed the spots are easier
to remove from the very smooth surface of this type of
hydrophobic coating. Less water on the surface also
reduces the moisture bacteria needs to grow. The treated
surface can now be easily cleaned with no more than
water, creating an additional environmental positive impact, a 'green' attribute, by eliminating
the need for harsh chemicals as cleaning agents since they are no longer required. The
measurement that defines the degree of water-repellency of a surface is (before or after being
treated with a hydrophobic coating) called “Contact Angle”, also called "wetting angle", which
is the angle formed by the material's surface and the tangent of the interface between the fluid
(i.e. water) and the environment at the liquid/solid (i.e. glass surface) point of contact. See
graphic on the right (typical Contact Angle of an untreated glass surface).
In core practical terms, however, the Contact Angle is simply an indicator of wettability, so the
smaller the Contact Angle, the easier to wet the surface is, and then by contrast, the larger the
Contact Angle is, the harder to wet the surface is, as shown on the graphic below.
You may then ask yourself the following two questions:
a) Is the single measurement of “Contact Angle” the best way to define a protective coating's
performance?
b) What else defines quality as an easy-to-clean surface?
These are probably two of the most relevant questions one can ask about any hydrophobic
easy-to-clean coating and perhaps the MOST HIDDEN fact by most coating suppliers, as they
truly uncover the truth behind the quality of the product. Yet, coating suppliers will typically
evade these critical considerations with all sorts of misleading statements and false claims. The
unequivocal and irrefutable scientific FACT is that performance not necessarily determined by
the initial Contact Angle measurement after the glass surface is treated but rather how it will
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perform over time or, in other words, how will the Contact Angle behaves with the normal tear
and wear once the surface is exposed to the different environments. Most hydrophobic easy-toclean coatings have an initial Contact Angle anywhere from as low as 75 degrees to as high as
120 degrees. The ones that are on the low end, especially under 90 degrees, are not interested,
for obvious reasons, in disclosing any measurement. These suppliers downplay or completely
disregard any value to Contact Angle measurements, as they do not want to lose any
“marketing edge” vs. any higher Contact Angle coating. You may hear a supplier calling this
issue “the war of the bubbles” and dismissing it as irrelevant. As I’ve shown, this is not at all
true - it does matter and you should know why in order to make a smart decision when
choosing a coating for your own use, whether you are the owner of a building, business or
simply for your own household or automotive use. In fact, their dismissal of this critical
measurement should tell you all you need to know about their coating.
So while Contact Angle is important, it is more important to know how the Contact Angle
measurement varies over time. If your coated surface still remains at a relatively high Contact
Angle AFTER a significant period of time, let’s say a few years (without the need to routinely reapply, which is the case with coatings that do not have a PERMANENT BOND), then you are in
the presence of a high-performance coating. The analogy that I often draw out of this question
is the following: if you are purchasing a computer, wouldn’t you want to know how much
memory and what size hard drive it has, as they will determine the overall performance of your
computer? Likewise, not having any information about the Contact Angle, from a purely
scientific or empirical perspective, is like not knowing the size of the hard drive in your
computer. This is simply a matter of accuracy and disclosure of scientific facts that DO affect
your everyday life by having access to improved performance. Then, taking the analogy a step
further, wouldn't you want to know the speed of the processor or how fast you can perform
your tasks with that same computer? This relevant 'speed' which determines your degree of
productivity to a great extent, is precisely what the "Sliding Angle" represents, as it will be
explained. One should know the difference between mediocre, good and excellent protective
coatings to enhance your glass surfaces and keep it cleaner for longer periods of time thus
reducing the required maintenance.
Brief historical context
Pilkington came in the mid 2000's with its Pilkington Activ™ hydrophilic (water-attracting)
coating product, introducing for the very first time, worldwide, the new concept of "selfcleaning" glass; and here lies, in my personal view, the biggest myth ever produced in the glass
industry. Why? Because as brilliant of an idea as this concept truly was, it was (and has been
since) equally misleading and flawed at many levels, as I have been saying since the day it was
first introduced. I call it a "lab coating", meaning that it can only produce its conceptual value
and intended functional qualities at a lab session where all the variables can be manipulated
and mechanically controlled. After more than 10 years, I have yet to see any building,
anywhere, treated with any such self-cleaning product that would truly clean itself even under
a torrential rain. However, if any producer or manufacturer of any such self- cleaning glass has
supplied to any building anywhere in the world that can show and field prove this self-cleaning
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feature and capability, I urge and invite them to share such unique application and specific
geographical location - as I'm yet to find it myself. Much to my surprise, many other players
followed suit over the next 10 years, mostly all glass manufacturers that had the need to
introduce something new, yet easy to bundle to increase sales volumes.
Why is it then that the self-cleaning glass concept failed so badly?
(Or at least not produced the high initial expectations of exponential increase in sales?)
The so-called self-cleaning glass then consisted of a hydrophilic coating with photocatalyctic
properties fused into the glass at high temperatures during the manufacturing process. This
type of coating typically reacts with the sun’s ultraviolet light and water (rain or induced water
spray). Thus, these two elements (sunlight and water) must be present at the exact same time
for the coating to be effective, creating the advertised sheeting action so that water can easily
rinse the loosened dirt away. Due to its integral characteristics, there are many limitations to
this concept of "self-cleaning", such as the following: glass must receive both sunlight and
water in order to be "activated" (become functional as intended), it is not applicable to all kinds
of glass, cannot be applied in the field but rather at manufacturing level only (making repair or
replacement a lot more expensive), and it’s typically a lot more expensive than hydrophobic
coatings. By contrast, most hydrophobic coatings require no ‘activation’.
There are other theories that refer to the "self-cleaning" glass differently - as those coatings
with very high Contact Angles where the mechanical action required is virtual unnecessary and
no accumulation of contaminants occurs. These theories are also hard to support, as I believe
the misconception of self-cleaning is contrary to the reality and expectations of any end-user.
For any glass to possibly be considered self-cleaning it would have to have a Contact Angle close
to 180 degrees to then create a “rolling effect” (as shown in graphic below) so that no action
will be required for any material, water or otherwise, to automatically roll off the treated glass
hence never sticking at any point, leaving it as if nothing had ever touched the surface. Such
ultra-hydrophobic theoretical concept is virtually the only way that one may reasonably claim
the glass is truly self-cleaning. Anything beyond this concept is disingenuous and misleading,
and therefore lacks credibility.
Ball “Rolling Effect” of a 180 deg. Contact Angle
So the answer to my somewhat rhetorical question of why self-cleaning glass is failing is very
simple: because one can mislead with a descriptive name for only so long, and facts end up
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speaking by themselves. In other words, a self-cleaning surface that does not actually self-clean
is an oxymoron, period. Furthermore, this is the reason why most, if not all, such self-cleaning
coatings have been a monumental failure as they have not been adopted in any significant
fashion anywhere in the world.
As opposed to the unrealistic expectations of “self-cleaning” glass, Easy-to-Clean hydrophobic
coatings essentially create a surface that repels the water as well as the minerals and
contaminants it carries. Supporters claim that by repelling this water, fewer drops can stick to
the glass and dry there (depositing its cargo) - thus, fewer spots. When it comes time to
cleaning the glass, those spots can’t stick as easily to the smooth surface and so they are easier
to remove. Thus the treated surface becomes easy-to-clean, very similar to the concept of a
Teflon-coated cooking pan.
Contact Angle Image: Treated Glass with Diamon-Fusion® nanocoating vs. Untreated Glass
What is "Sliding Angle"?
Why is this NEW concept in Architectural Glass so valuable?
While Sliding Angle is not necessarily a new concept, its historical use has been traditionally
limited to the automotive industry. The introduction of Sliding Angle into the Flat Glass
Architectural Industry is in my view, of paramount importance as this "dynamic dimension" (in
addition to the already known "static dimension" of Contact Angle) truly completes the whole
picture and much better defines overall performance since it adds the processor speed to the
size of the hard drive - going back to my aforementioned computer analogy.
Why do I mention speed? Because such "slickness" then provides the speed (which defines the
angle) at which water can travel making the treated surface that much faster while being
absent of water and all other contaminants that would otherwise stick. While the Contact
Angle, as a static dimension, will increase gravity for water to move faster with less "tail", the
Sliding Angle, as a dynamic dimension, will improve the quality of the glass by making it easy-toclean.
Sliding Angle may also be called "roll-off" angle and has its own practical meaning and
usefulness. It is defined as the angle between the sample surface and the horizontal plane at
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which the liquid drop starts to slide off the sample surface under gravity influence. In its
simplest explanation, however, Sliding Angle measures the “slickness” of a surface – whether
coated or not.
The intrinsic value of Sliding Angle when it comes to architectural glass has to do with the fact
that the material choices in architectural designs, including slope, ought to consider rain drops
rolling off in order to maintain the integrity of the glass and cosmetic beauty (of the building
facade) while minimizing glass erosion, a relevant but yet underestimated issue. The angles of
the surfaces relative to the ground should be at least larger than the water Sliding Angles on
the surface of the roofing materials. Then, the Sliding Angle absolutely determines how easyto-clean the building facade becomes: the lower the angle the easier to clean the glass
surface is. Coatings with Sliding Angles in the low 20 degrees are most effective when it comes
to the easy-to-clean feature, thus saving time and material cost during cleaning cycles,
providing a very tangible return-on-investment for the coating.
Sliding Angle is measured by placing a drop of water - the size of which is determined by the
testing standard - at one end of the sample of coated glass. Then the sample is tilted until the
drop of water begins sliding. The tilt of the glass at that very instant is the Sliding Angle. The
lower the Sliding Angle the slicker the surface. The slicker the surface, the less elements (such
as water and contaminants) can stick. As it measures how easy or difficult it is for things to stick
to the surface, Sliding Angle therefore can be that credible proxy to measure “easy-to-clean”
that the industry needs. So the lower the Sliding Angle of a surface, the easier it is to clean.
On the graphic above, a solid surface is tilted in order to measure the water droplets on the
surface as they slide.
Conclusion
A hydrophobic coating provides glass not only a water-repellant surface but most importantly
an easy-to-clean surface.
Seeking the best combination of a high and durable Contact Angle with a low and durable
Sliding Angle will unquestionably offer the best value on Architectural Glass, and therefore the
most cost-effective solution to protect your investment (i.e. commercial building), with best
return (ROI) and most significant costs savings, including water savings and best alternative for
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a cleaner and greener environment. As explained, the slicker the surface, the less elements
(such as water and contaminants) can stick to it. Such slickness or smoothness effectively
imparts the velocity (which defines the angle) at which water can travel making the treated
surface that much easier to clean while being absent of water and all other contaminants that
would otherwise stick.
While the Contact Angle, as a static dimension, will increase gravity for water to move faster
with less "tail", the Sliding Angle, as a dynamic dimension, will improve the quality of the glass
by making it easy-to-clean as pollutants and contaminants will find a harder barrier to stick under such smoother surface. It can then be extrapolated as follows:
The lower the Sliding Angle and the higher the Contact Angle, the faster the water (and other
contaminants) will travel over the treated hydrophobic surface, hence the faster and easier
the surface will be cleaned AND the less water shall be required for cleaning and maintaining
such treated glass as well.
Therefore, collectively speaking, the lowest possible Sliding Angle combined with the highest
possible Contact Angle, while both angles remain durable over time, will unquestionably
provide the highest glass performance, and improved quality overall, while enhancing both
beauty and functionality to all architectural glass.
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