Hydrophobic / Hydrophilic surface
A hydrophobic surface is one
that repels moisture. In contrast, a hydrophilic surface is one that absorbs
moisture. The potential of a surface to absorb or repel moisture is based on
many factors, including: temperature, relative humidity, material homogeneity,
and static electricity. Surface roughness is also a major factor; the rougher
the surface, the higher the spreading rate or attraction for water. The
smoother the surface, the more repellent it is to moisture.
Contact angle measurement
A simplistic method to
determine the level of surface protection is to observe the formation of large,
closely spaced water droplets, which demonstrates that moisture is effectively
prevented from being absorbed into the surface profile. The classification of
water droplets on a hard surface is known as a water break test and is used to
check surface cleanliness.
The size, shape, and height
of the water droplets are measured by their contact angle. Contact angle is
figured by a straight line that starts at the base of the droplet and travels
along its outer surface to the break-off point. The measurement between this
line and the surface determines the contact angle.
Water Spheres
Sphere’s or beading is a surface interaction between water and the paint surface; dust or dirt
particulates, oxidation, environmental pollutants, and etc. interfere with the
surface tension, thereby disrupting the spheres. Water spheres that are left to
evaporate on a car will lead to water marks; if there is acid or alkali
contamination in the water.
Dependent upon
the water source; ‘clean rainwater has no harmful effects; but cognizant that
water spheres whatever its aesthetic value doesn’t equate to surface
protection. The average unprotected water-based paint absorbs approximately a
pint of water; dependent upon what’s in the water this may be harmful.
Pros
– the aesthetics of
water spheres are sought after by many detailers. Many detailers believe that
if there is water spheres on a paint surface the LSP is providing protection.
Nothing could be further from the truth; water spheres are primarily due to
surface tension
Cons – dependent
upon the water source; ‘clean rainwater has no harmful effects, acid rain is
inert until heat is added and then it becomes corrosive, the water from a
standard household supply usually contains minerals like calcium (Ca) magnesium
(Mg) and alkaline. The beads have a very small surface area, so the sun will
increase the surface temperature very rapidly once these form a ‘bead’
reactivity ([:
the chemical reactions of two or more substances that interact with each other])
starts (moisture, heat and an acid or alkaline) and once they are dried they
will cause etching (a shallow indentation) of the paint surface
Be cognizant
that (a) Dust and road soil will also have a
negative impact on ‘water spheres’; this is often mistaken as ‘wax / sealant
failure. (b) Durability can fluctuate dependent upon
environmental conditions and the products used between the application of a
wax/sealant and the use of quick detailers (QD), car wash concentrate that
contains a wax, spray wax, etc.)
Surface Tension
Surface Tension
The elastic tendency of liquids which
makes them acquire the least surface area possible, so they pull themselves together
to form a spherical shape (water bead)
At liquid-air interfaces, surface
tension results from the greater attraction of liquid molecules to each other
(due to cohesion) than to the molecules in the air (due to adhesion). The net
effect is an inward force at its surface that causes the liquid to behave as if
its surface were covered with a stretched elastic membrane. Thus, the surface
becomes under tension from the imbalanced forces, which is probably where the
term "surface tension" came from. Because of the relatively high
attraction of water molecules for each other, water has a high surface tension compared
to that of most other liquids.
Do water spheres equate
to durability or to actual surface protection?
Durability [: able to exist for a
long time without significant deterioration] [1]
Spheres are a
surface interaction between water and the paint surface protection. Dust or
dirt particulates, oxidation, environmental pollutants, and etc. interfere with
the surface tension, thereby disrupting the spheres
Most waxes
will bead until enough surface tension is created, and then it will sheet as
the volume of water increases. Spheres of rain water on the surface of a waxy
surface, such as an automobile is due to surface tension. Water adheres weakly
to wax and strongly to itself, so water clusters into drops. Surface tension
gives them their near-spherical shape, because a sphere has the smallest
possible surface area to volume ratio.
Nearly all wax
and sealant products exhibit water spheres or sheeting initially (in fact so
does a clean paint surface without an applied protection) this is due solely to
surface tension, once the protection breaks down (abrasion from water, road
dirt/grime and other airborne pollutants) it will cause a reduction in the
surface tension and the spheres will revert to its former level
Slickness is also not an
indicator of protection; as it comes from the (silicone) polymers and oils,
once applied and exposed to the environment they start to evaporate /
deteriorate. If your goal is maximum protection, don't use water spheres/sheeting
as an indicator; while it’s true that it is a visual indicator that a wax /
sealant previously applied is still present it does not guarantee that it’s
actually providing protection.
Hydrophobic spheres on a
paint surface is aesthetically pleasant, and many people equate spheres with
protection; there is a long standing myth “that
as long as a paint surface maintains spheres it’s being protected from the
elements” (much like the myth that “the more soap produces suds the better
the cleaning effect”) There are many things that will affect the ‘shape’ of a
water bead on a surface; since gravity, surface tension and the surface energy
of the coating all play a part.
To test your wax / sealant
you must measure the water spheres of your paint (height, contact angle and
diameter) without any polish/wax applied. Next, measure the water spheres of
your paint (height, contact angle and diameter) within 24 hours after initially
applying your polish/wax.
This is your starting point.
This will also be the gauge for determining the water spheres (longevity,
duration and changes) for that specific product. As the water beads start to
diminish (get wider and shallower and loses contact angle), the polish/wax and
its film protection factor is going away, Once the water spheres is the same as
before you apply your product, the polish/wax and its protection are gone
Dust and road grime will
have a very negative effect on surface spheres than any other factor. The effect of this is often misconstrued as
wax failure when this isn't the case.
Durability
varies depending on various environmental factors, including but not limited
to:
•
Product is applied to a clean, cool and dry surface
•
Product was allowed to cross-link and form a molecular bond with
the paint system
•
No oils or water (including moisture, dew, etc.) was introduced
to the surface before it has formed a molecular bond
•
What the environmental conditions (ambient temperature etc.) are
where the vehicle is stored
•
The kind of airborne contaminants the vehicle is regularly
exposed to
•
Frequency of washing and kind of car wash solution used (i.e.
detergent strength, etc.)
Water spheres
[: convex beads that have a small, tight symmetrical
shape due to cohesion] [1]
Although you cannot equate a
products sphere ability to protection and durability, if an applied product
continues to `bead' water, one wash after another, then that would prove that
whatever it is that is causing high surface tension is not washing off.
How can you tell when a paint surface
protection has diminished to a point that it is no longer being protected?
Scientific
explanation
•
Water (H2O) is a polar molecule, composed of two hydrogen
(H2) atoms bonded to a single oxygen (O2) atom. Water
molecules adhere to each other, this is called cohesion.
•
Water molecules also can be attracted to other substances, such
as metal or dirt, especially if they have some static charge on them, this is
called adhesion.
•
Some substances are not attracted to water, and even repel it.
These include oils, fats and waxes; these are called non-polar substances.
•
When water falls on an un-waxed paint surface, the forces of
adhesion and cohesion are almost in equilibrium, and the water spreads out
A wax or sealant, when
applied properly to a clean paint surface, fills in the larger surface fissures
and layers the whole surface. The chemical structure of the wax prevents water
from penetrating to the surface of the car. Because the wax itself is
hydrophobic (literally repels water), the forces of adhesion are much less than
the forces of cohesion. So, water is more likely to bead higher and rounder
than on a surface without wax / sealant
Non-scientific explanations
a)
If the paint surface feels dry (your
hand or a cloth drags), it’s an indication that there’s nothing left between
you and the paint finish. Glazes, waxes and polymer’s create a finish with less
friction (surface tension) than the paint itself.
b)
A suggestion from a polymer product
manufacturer [To test your wax / sealant you must measure the water spheres of
your paint (height, contact angle and diameter) without any polish/wax applied.
Next, measure the water spheres
of your paint (height, contact angle and diameter) within 24 hours after
initially applying your polish/wax. This is your starting point. This will also
be the gauge for determining the water spheres (longevity, duration and
changes) for that specific product.
As the water beads start to diminish (get wider and shallower
with a diminished s contact angle), the polish/wax and its film protection
factor is going away, Once the water spheres is the same as before you apply
your product, the polish/wax and its protection are gone] [Sal Zaino]
Conclusion- water spheres is indicative but not conclusive proof of
protection
c) Indications that the products durability may be diminishing-
(contact angle varies) when the water beads become noticeably larger in
diameter with a flat, concave or an irregular shape usually indicate that the
surface tension of the wax or sealant is diminishing. Or when dust, dirt or bug
residue becomes more difficult to wipe off with a quick detailing spray are
indications that it may be time to renew the protection
d) Slickness- slide a micro fibre towel across a horizontal
surface to see how much resistance there is, if there has been a significant
reduction from what you experienced previously the durability is probably
diminishing
e) Sheeting (hydrophilic) - the self-cleaning (sheeting) ability
of the hydrophilic polymer seems to be much better than the hydrophobic organic
wax (spheres) effect, as it may accelerate the oxidation when drying after
rain.
There are some disadvantages
to water spheres (hydrophobic) as opposed to the sheeting effect (hydrophilic)
of a polymer, when they are dried by
ambient temperature they cause ‘spotting’ (if the rain contains calcium it will
leave a white residue) The other is there could be over a pint of liquid
trapped within the beads over the paint film surface area, if they contain acid
from industrial fall out (IFO) this could increase the time the acidic solution
remains on the paint surface compared to ‘water sheeting’.
The spheres have a very small
surface area, so the sun will increase the surface temperature very rapidly; many chemical compounds react to slight heating and an oxidizing process. Now
you have acid + water + oxygen + ozone + heat; all of which equates to
reactivity, which produces a highly concentrated acidic solution, causing
concave indentations (acid etching) to the paint surface
Any product can be
reformulated with active surface agents (surfactants) either ionic (‘sheeting’)
or non-ionic (‘spheres’) that alters the surface tension and causes water to ‘sheet’
or ‘bead’ to satisfy consumer demand.
But if a product beads on initial
application and after a period of time starts to sheet water (or vice-versa) it
is normally indicative that the wax/sealant protection has diminished.
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.
I would appreciate it if you would share these articles
as it helps other detailers further their knowledge.
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