|
Sun Fading Paint Degradation Measured
by William
Rice
The UV
radiation resistance, of an industrial protective paint or
coating, to sunlight degradation is one of the most
important performance issues of critical concern to buyers
of industrial protective coatings. How the effects of that
UV exposure are measured in industrial protective paints or
coatings is the subject of this article.
Probably
the most obvious industrial protective paint or coating
performance issue is sunlight fade resistance.This is
separated into color loss, chalking and loss of gloss. What
ability does the industrial paint or coating have to resist
UV sunlight from decomposition of the paint's or coating's
pigments? How does the industrial protective paint or
coating keep the pigments from becoming a lighter color?
Most importantly, how does the industrial protective paint
or coating preserve the aesthetic appearance of the surface
to which it is applied?
Color Loss
Or Fade
The extent
of color loss of a protective coating depends on five basic
factors: the amount of solar sunlight UV radiation exposure,
the type of pigment (whether architectural, industrial, or
automotive grade), the pigment's color, the type of resin
or resin polymers and the types of coating additives used
(such as ultraviolet light absorbers UVA's, hindered amine
light stabilizers HALS, or nano-particles of titanium
dioxide or zinc oxide) in formulating the industrial
protective paint or coating. Maximum colorfast pigment
combined with optimum UV stable resin(s) or polymers is the
objective. (See article
Sun Also Destroys).
How Color
Loss Is Measured
Three
factors go into what makes up color; the chroma, light, and
hue. Chroma is how intense that color is. Lightness refers
to the degree of lightness or darkness of the color. And,
Hue is what color it is perceived to be. The change in color
is measured by a mathematical equation and is expressed as a
change in what is termed the "Delta E". Most warranties on
architectural and industrial coatings have a color
difference of a certain Delta E to be covered. Most people
(not color blind) are able to tell a color difference of
Delta E 2 to 3 if the samples are held up side to side. A
color difference ranked as Delta E 4 to 5 are easy to
remember because they are so extreme. The problem is most
warranties only apply if the Delta E change is 12 to 15.
This is like a fire engine red fading to pink. Remember that
the next time you are considering a product that has a "no
fade" warranty. Read very carefully what the warranty really
says in the fine type.
Chalking
After long
term exposure to UV sunlight solar radiation, industrial
protective paints or coatings will develop a loose white
substance called chalk. Chalk is what's left behind when
some of the resin and pigment are broken down by UV sunlight
solar radiation exposure. The "white" chalk makes the
underlying color look lighter and even dull. It's quite easy
on first examination to mistake chalking (which is somewhat
reversible) for true fading (which is permanent) color loss.
But, washing and wiping the architectural or industrial
coating's surface will remove the chalking and restore a
good deal of the color. Rating the degree of chalking is a
numerical standard of 2, 4, 6, 8 or 10 with 10 being no
chalk wiping off.
Note: One
excellent way of determining whether a surface is clean
enough for a industrial protective paint or coating to be
applied to it, is to wipe a clean white cloth across the
surface and if no color comes off, it is clean. Obviously,
if the surface is white, the cloth should be of a darker
color with no white coming off. For our restoration and
protection clearcoat, it's ready for the faded surface
that's left to be restored. (See
Vivilon Application Instructions).
Gloss
Retention
Gloss has
been defined as the ability to reflect without any
scattering of light. This reflecting Gloss retention on a
architectural or industrial protective coating is measured
naturally (I kid you not, as Jack Paar used to say) by an
instrument called a glossometer or gloss meter. These gloss
meters read gloss at 85, 60 and 20 degrees by shining at the
surface a known amount of light and then measuring the
reflectance. Light obeys the law of reflection, that the
angle of reflection equals the angle of incidence. Thus, a
light shined at a 20 degree angle (angle of incidence to the
surface) will be reflected at the same angle. A light that
reflects according to the law of reflection is called
specular reflection (because of its mirror like nature) as
opposed to light which is scattered and called a diffuse
reflection. There are a number of reasons for a diffuse
reflection. One of them, applicable to coatings, is the
scattering caused by a rough surface or rough surface
profile (see
Why Paints Fade
and
How To Restore Gelcoat
for coating
context).
In the
diagram below, the different angles that gloss is measured
at are depicted. The 85º is for measuring low gloss (satin)
or semi-gloss finishes. A 60º angle measures high gloss
materials like paints or coatings on metal. And, a 20º angle
measures super high gloss like coatings on polished metal.
To put all this in perspective, have you ever looked down
the side of a truck or boat almost parallel to the surface.
That is what an 85º measurement is like. And, looking down
the side did you see what looked like a quite shiny (glossy)
surface that wasn't there when you instead looked at the
truck or boat almost head on (perpendicular or almost 20º)
and saw no shine or gloss. This is why each finish has a
different gloss reading to accurately measure that coating.
The test gloss numbers would simply lose their meaning if a
super high gloss was measured by an 85ª test or vice versa.
Hopefully,
you know have a better idea of how a protective industrial
paint or coating's ability to withstand UV radiation
degradation from weathering is measured.
|
