Pencil Hardness
The Pencil Hardness Test
provides a simple method to test the scratch hardness of coatings. In this
test, pencils in a range of 6B to 8H hardness-grade are used. Pencils graded
using this system is used to measure the hardness and resistance of varnishes
and paints. The resistance of a coating (also known as its pencil hardness) is
determined as the grade of the hardest pencil that does not mark the coating
when pressed firmly against it at a 45-degree angle.
Select a pencil and make a
line about one-inch-long on the paint sample. If the pencil leaves a scratch,
then take the next softer pencil and do the same thing. The number of the first
pencil that you use after you have made a scratch in the coating that doesn't
leave a mark is considered the "pencil hardness" of the coating.
(There are some coatings that are so hard that even the 9H pencils will not
scratch them. All of those coatings get a 9H pencil hardness rating to
designate their hardness.)
Scratch
resistance - surface roughness is the
major factor; not hardness, the rougher the surface, the higher the coefficient
of friction and the more the surface is prone to scratches when a towel or some
other object is dragged across its surface
Automotive paint surfaces contain
microscopic peaks and valleys, much like the profile of a mountain range. These
irregularities are known as capillary structures; there may be millions of
these defects per square inch. Such a surface has a high coefficient of
friction and is relatively easy to scratch
Scratch resistance - can be related to
higher cross-link density and elasticity of the polymer network. Low cross-link densities raise
the viscosity of the polymer (Soft). Intermediate cross-link densities
transform gummy polymers into materials that have elastomeric properties and
potentially high strengths. Very high cross-link densities (Hard) can cause
materials to become very rigid with a high gloss
The scratch hardness of organic paints / coatings
is greatly influenced by factors such as the nature of the metal substrate, the
thickness of the coating and the temperature and humidity of the surroundings.
Little information is available about the effect of these factors on the
scratch hardness and how this property is affected during the course of
weathering.
Tests were conducted using a power-driven
automatic Erichsen scratch hardness tester, on paint coatings air-dried for 48
hours and then aged in the laboratory for various periods over a span of 300 days.
It is found that some coatings which showed a fairly low scratch hardness
during initial testing attained a significantly greater hardness during the
course of ageing. The thickness of the coating had no significant influence on
the hardness during the early periods of air-drying. However, with aged
coatings, scratch hardness was observed to increase significantly with the
thickness of the coating.
Dense paint is very
resistant to correction; some paints will correct with a single pass and look
incredible, ready for wax even. Others will correct just as easily but leave a
hazy appearance that needs to be cleaned up with a less aggressive polish and a
soft pad.
The best way to detail
paint, wither it is hard or soft (dense or less dense); is by using the least
aggressive pad / polish combination followed, if necessary by an incrementally
more aggressive approach until you find what works.
•
Dense (Hard)
paint - paint requires a more aggressive abrasive polish and takes more time to
correct. But a dense (hard) paint
makes it easier to get that perfect gloss. PPG's Optech™ or CeramiClear™
respond better to polishes / compounds formulated for this type of paint
density.
•
Less dense (Soft)
paint- responds to abrasives more readily and makes polishing easier, but it’s
easier to induce surface marring.
•
Water-based paint is generally more porous and less dense
(softer)
•
Solvent-based clear coat paint is usually denser (harder) than a
water-based paint and not as porous
As always generalizing
carries a degree of risk as a panel(s) may have been re-painted. Before
commencing polishing do a test panel on the car, once you have achieved the
desired results with your selected polish / pad combination proceed
So how can a dense (hard) clear coat be
so easily scratched?
Force acts through a body
that has a surface area; if the surface area is really small while maintaining
an equal force, the pressure becomes astronomical and the object under pressure
capable of penetrating the surface of an otherwise tough material. (Newton's third law of motion)
That’s why a micro fine
thread that is twice as fine as silk and a 100 times finer than a human hair,
in an otherwise soft towel will scratch your paint. And the same reason a
mosquito can penetrate a rhino hide with its proboscis (stinger).
Relevant Articles
“Hard and Soft Paints” - http://togwt1980.blogspot.co.uk/2015/10/hard-soft-paint-correction.html
“The
Science behind Coatings” - http://togwt1980.blogspot.co.uk/2015/10/the-science-behind-coatings.html
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