Clear
Coats Scratch Resistant
During recent years, scratch resistance has become a major area
of clear coat R&D and test method development in the laboratories of the
automotive coating suppliers, the automotive industry itself, and many research
facilities. Especially on dark colour shades, smaller or bigger scratches
become very visible. Marring is the English term used to describe the fine
scratches introduced, for example, by car wash machines. These micro scratches
are only 1–2µ wide and a few hundred nanometres deep. With these dimensions
being in the order of the wavelength of light, the observer will not see the
scratches, but will only see the light scattering at the scratches. In many
cases, these scratches are just surface deformations and level out over time
and temperature (re-flow).
Under the impact of keys or shopping carts, the clear coat
surface starts to break, material is ablated, and a recovery in the form of
reflow effects is no longer possible. These film properties are best tested by
the nano indenter method
Car wash scratches by scanning tunnelling microscope (source:
DuPont). From the nano scratch experiment, and in an earlier stage, also from
micro scratch experiments, the resistance to plastic flow and the fracture limit
(load at which fracture starts) can be determined.
It is observed that 1K clear coats typically show a higher
resistance to plastic flow (they are ‘harder’), while 2K clear coats commonly
show higher fracture resistance as they are more resistant to hard impact
scratches. Scratch resistance can be related to higher cross-link density and
elasticity of the polymer network
Carbon Fibre Clear Coat (GM Corvette)
Along with
the front splitter, the roof of the ZR1 is also made of visible carbon fibre.
One of the problems with using unpainted carbon fibre for body work is that the
resin oxidizes over time, turning yellow and cracking just like the old plastic
rear windows on convertibles. To overcome this, GM materials engineers put a
lot effort into developing an additive for the clear coat that could be applied
to the carbon to preserve it for the life of the car. The ZR1 is the first car
ever to have a carbon fibre clear coat that will last the life of the car.
The chemical
additive costs $60,000 a gallon and the clear coat with the additive mixed in
comes to $2,000 a gallon. The visible carbon isn't the only such material on
this car. The entire front clip is made of the same material, although those
parts are painted.
Paint
Density
Low cross-link densities raise
the viscosities 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
Nano technology Coatings
Are applied
as a final coating over a traditional clear coat, and has an approximate
thickness of 7.5 µ (0.5 Mil) this top layer also contains the paint systems UV
protection. On Mercedes -Benz models that have CeramiClear paint there is a ‘C’
prefix to the three number paint codes. If the code reads"C040" you
have black nanotech paint
This was the
reasoning behind CeramiClear® – [PPG
Industries Inc (2004) ceramic clear coat is unmatched in scratch, surface
marring and acid-etching resistance as well as gloss retention. Test results
have shown that after a period of six months gloss retention shows an 80% improvement
and acid-etching resistance is 50% better compared to that of a non-ceramic
clear coat. They have also developed an aftermarket refinishing clear coat to
provide approximately the same level of protection as the OEM ceramic clear
coat.]
DuPont's SupraShield™,
PPG's Optech™ and CeramiClear™ Clear Coat are
all binary clear coat compositions using ceramic particles designed to offer
long-term scratch resistance, gloss and durability and is applied as a final
coating over a traditional clear coat, CeramiClear™ has an approximate thickness of 7.5 µ (0.5
Mil) this top layer also contains the paint systems UV protection
PPG
CeramiClear™ Clear Coat now comes in two commercial versions:
CeramiClear, a two-component clear coat, and Certech, a single component clear
coat.
Working with PPG, BASF and Mercedes Benz, in 2003 Menzerna developed special
polishes for use on PPG CeramiClear™ Clear Coat for removing scratches, swirls and paint
defects Beyond superior abrasives, Menzerna has pioneered the development of
polishes designed specifically for the hard clear coats, like those used by General Motors on the Corvette and PPG CeramiClear™
Clear Coat being used by Ferrari and Mercedes- Benz.
Super Intensive and Nanotechnology Polish are currently used by Mercedes - Benz in Germany on their
production line to remove swirls and over-spray incurred during the painting
process. Some OEM paint specification require a dense (hard) clear coat paint- Audi, BMW, Nissan and VW and others
Modern automotive paint systems of whatever density pose unique
challenges to the detailer, to determine the correct polish/and pad
combination, machine methodology. This requires a ‘test panel’ to see how the
selected polish set-up responses to the paint, and wither it will remove the
surface scratches
Select a ‘typical’ 18 x 18 – inch area -one that represents
the type of defects that you want to remove and starting with the least
abrasive polish and / pad combination and then proceed until the defects are
removed. This is the polish / pad combination you’ll need to use. Selecting the
correct pad / polish combination for the vehicles paint / defects can take just
as long as the paint correction process
Powder coating
This type of coating
that is applied as a free-flowing, dry powder. The main difference between a
conventional liquid paint and a powder coating is that the powder coating does
not require a solvent to keep the binder and filler parts in a liquid
suspension form.
The coating is
typically applied electrostatically and is then cured under heat to allow it
to flow and form a "skin". The powder may be a thermoplastic or a
thermoset polymer. It is usually used to create a hard finish that is tougher
than conventional paint. Once installed, maintaining the initial appearance of
a powder coating is a simple matter.
Powder coated
surfaces are more resistant to chipping, scratching, fading, and wearing than
other finishes. Most dirt and oil will come off with simple soap and water.
Powder Clear coat technology (PCC) The 2K liquid clear coat production line, the
ability of powder to apply with no sag and run behaviour results in a film
build varying only within a few microns between horizontals and verticals. This
attribute produces a homogeneous and consistent appearance over the entire
body. (Some BMW 3, 5-, and 7-series bodies
It is very important to be able to recognise the various
finishes and materials used by OEM’s as they all require different
methodologies and products for proper care and maintenance. Their paint finish
type will also vary between single stage, metallic, base coat clear coat, and
Tri-coat pearl paint finishes, which all vary in density (hardness)
Do not try to guess what the colour is, as even trained professionals
are unable to do that. Some colours may appear to be the same, but they are, in
fact, very different. Find a paint dealer with a spectrophotometer and ask them
about matching the colour. To assure that you pick the right colour for your
vehicle, it is important to find the "paint colour code". Every
vehicle has a paint colour code, but it is located in different places, depending
on the make/year/model
Other automotive
cleaning products also work well .Do not use strong solvents or other harsh
chemicals. Powder coat is more resistant to
these types of chemicals but may dull glossy finishes. The road dirt and grime
which builds up on surfaces from time to time contains moisture and salts which
will adversely affect the powder coating and must be removed
Polishing can be done
with a micro fibre towel and polishing compound (Menzerna PO 85) and a Lake County (LC) White or Black
foam pad to restore scratched surfaces
Solvents
used in Paint
Solvents hold the pigment molecules in suspension and help
provide an even distribution as they are transported through the air in a
liquid form and onto the surface.
Once the paint reaches the surface, the solvents outgas
(evaporate) leaving the paint to become solid and dry. The solvent carrier
system is used to suspend pigment agents, urethane, polyurethane, and other
molecules that will be left behind once the solvents evaporate. Solvents also
facilitate the paint to remain viscous (thin): that is they thin out the liquid
solution to be less like oil and more like water.
A viscous liquid is easier to atomize as it comes out of the
paint gun tip, and the more solvent that is
present in the liquid: the longer it will take to begin to become
sticky/tacky, and the longer it will take to dry. The longer it takes for the
paint to dry, the flatter the paint will end up as the drops have plenty of
time to settle into a flat surface.
Before the volatile
organic compounds (VOC) regulations came into force it was common for paints to
have 75%+ solvent content (that is, 75% or more of the solvent would evaporate
into the air) now to be compliant with VOC regulations paint is limited to a maximum 50% VOC content
or less.
South Coast California Air Quality District (AQMD) now requires
that multi-stage clear coats have a 250g/L (2.1 lb/gal.) VOC emissions rating
or better AQMD, Rule 1151 went into effect July 01, 2008, with complete
conversion expected by December 31, 2008. Canada's new regulation matches
California's, but won't be required until 2010.
Single-stage clear coats will require 340 g/L (2.8 lb. /gal.)
emission rating in California, 420 g/L (3.5 lb. /gal.) in Canada. WBCC falls
into the single application category of 1.2 lb. /gal. VOC, well below EPA,
California and Canada's standards
Paint
Systems
Water-based automotive paint - are essentially low solids paints (up to 60%
waterborne solvent), but they are legal because deionized water is used as the
solvent, as opposed to volatile organic compounds (VOC).in reality,
water-based (waterborne) automotive paint finishing is actually not that new;
PPG water-based paint has been used by some OEMs since the early 90s
Switching to waterborne from solvent-based automotive
refinishing paints is becoming more popular in the U.S. for health and
environmental reasons, as well as stricter environmental regulations in
California and Canada. Original equipment manufacturers (OEMs) and collision
repair shops in Europe and some OEMs in the United States already use
waterborne paints extensively
Lacquer-based auto paint - popular between
the mid 1920s and 1960s, and is still available today, though it has become
illegal in certain areas. Lacquer paint has better colour characteristic than
most any paint particularly in monotone solid colours. Lacquer can be sanded to
a much smoother finish than any other paint. Lacquer paint is cheap and goes on
easy for the inexperienced painter, plus it provides a nice high gloss.
However, it also
chips easily being a relatively “soft” paint, and it doesn’t stand up well to
UV and chemicals, making it a short-lived paint job. Acrylic lacquer is still
made by DuPont, under the name Lucite; this is the original acrylic lacquer
Nitrocellulose lacquer - was used as a finish on older (50 and 60’s) automobile
upholstery leather and exterior paint was finished with Nitrocellulose lacquer
and required oils for it to remain flexible otherwise it would crack. These
types of finishes are what Leatherequi and Connelly Hide Food were formulated
for. This is also the origin of ‘feeding’ paint / leather comes from. It was
also used on Guitars for most of the 20th century and is still used on some
current applications.
Manufactured by DuPont, the paint was also used on automobiles sharing the same colour
codes as many guitars including Fender and Gibson brands, although it fell out of favour for a number of reasons:
pollution, and the way the lacquer yellows and dehydrates , causing its surface
finish to crack over time. It was replaced with acrylic lacquer in the 50's.
Acrylic lacquer is still made by DuPont, under the name Lucite; this is the
original acrylic lacquer
Enamel paints - dry to a hard shell making them tougher than lacquer paints.
Professional shops bake on enamel paint in heated bays or "ovens,"
but enamels are also available in aerosol cans and for use with spray guns.
Enamels, while tougher than lacquer, do not lay down as easily as lacquer
paint, making them harder to apply. Some enamel colours require a clear
topcoat, known as a two-stage system, while others can be used alone, referred
to as a single-stage system.
Urethane paints - are newer than enamels, are more expensive, but they lay down
easily like lacquer while having the toughness of enamels. This auto paint
requires three products (3 Pack Paint): the colour, a reducer to thin the
colour to the right viscosity for the spray gun, and a catalyst used to accelerate
drying time.
1. Primer - the coating layer
that joins a corrosion-protected substrate to the topcoat. It functions to smooth out surface
irregularities, improve stone-chip performance, and helps to protect the
substrate from visible and UV light.
2. Basecoat - the coating layer
that provides colour and aesthetic effects.
Basecoat must provide uniform appearance initially and for many years
without fading. Modern requirements for
appealing vehicles often require micas, aluminium flakes and other effect
pigmentation. These can be incorporated
in one basecoat layer or in a separate layer
3. Clear coat - the coating layer
that forms the last interface to the environment. It carries the biggest part of the
technological performance and must be able to resist environmental etch, bird
droppings, car wash machines and other outside influences. To improve performance against all these
influences, OEM coating systems move from colour topcoats to basecoat/clear
coat systems.
The clear coat in combination with the basecoat forms the
automotive topcoat, which gives the vehicle its appearance. One and two component clear coats (1K and 2K)
are applied over both solvent and water-based basecoats.
Single stage paint system - until 1970 most cars were painted with solid
colour paint as the only top coat layer
The problem with single stage paints is pigment oxidation.
Clear coat paint was
first used on production cars in the US in the early ‘80s. While initially these coatings were based on
alkyd resins and were not very durable, later they came to be based on
thermoplastic acrylic enamels, which had slightly better outdoor durability.
At the same time, aluminium pigments were used to give a
metallic effect, but its durability was not sufficient, which then led to the
introduction of base coat–clear.
It proved difficult to get the metallic to lay evenly and get
decent gloss with minimum orange peel. Since most of the metallic floats close
to the surface sanding and polishing can cause problems with single stage metallic.
Some OEM vehicles still use single stage paint (notably; White, Red and Black)
and rarely other colours
Base Coat Clear Coat – two stage paint; base (colour) coat
and clear coat were adopted as an automotive industry standard in 19982, clear coat paint was
originally used to protect metallic paints and provide depth of colour. They
are applied over the primer surfacer and covered by the clear coat layer to
protect it from the environment.
There exist three main base coat systems in the paint shops of
the automotive industry worldwide: medium solids (MS) high Solids (HS) Water-based
(waterborne) (WB) paints are essentially low solids paints (up to 60% waterborne
solvent), but they are legal because deionized water is used as the solvent, as opposed to volatile organic compounds (VOC). North America predominantly uses HS,
whereas water-based clear coat is the preferred technology in Europe
Paint Test- using a cleaner wax and a clean micro fibre towel on a clean
paint surface, lightly abrade the surface, if the towel shows the paint colour
it’s single stage paint, if not it’s a base coat-clear coat paint
PPG Enviro Acrylic ®
Powder Clear coat - is an environmental friendly technology, as it does not emit
any organic solvent during its application. Powder clear coat exhibit:
hydrophobicity of the paint film, which results in: lower water permeability,
the higher cross-link density is better, scratch resistance is higher, this can
be related to higher cross-link density and elasticity of the polymer network
and better UV durability of the resin matrix.
Besides this, powder clear coat has the following other
advantages: same paint film thickness and similar appearance on horizontals and
verticals.
No other technology in OEM coatings can offer direct recycling
and ‘zero waste’ operation, although optimized e-coat operations come close.
One-hundred percent solids of the paint require lower flow rates at the
applicators and help to improve transfer efficiency to >90%. While the
average usage of clear coat is 3 kg per car for medium solids and 2 kg per car
for HS, it is 1.5 kg per car for powder clear coat. It remains a future target
to reduce the paint film build of powder clear coat from the current level of
65µm in 2007 to 40–50µm commonly used with liquid clear coat. UV-powder clear
coat (see Section 6.5.5) may be a way to achieve this target.
Metallic – also called polychromatic, finely shredded
aluminium flakes are a component of the base colour. This type of paint is
used on the majority of new automobiles (only red, black, and white
are available as solid colours) Metallic’s nearly always consist of a
base coat with a clear lacquer or urethane top coat for protection against the
metal flakes oxidizing and to provide extra gloss.
Tri-coat paint system - comprising a colour coat (very often white) mica pearl coat
has a tint that causes the light to be reflected slightly different and finally
a clear coat
Four stage paint - Lamborghini is one of the only manufacturers to utilize a 4
stage paint finish with a tinted clear coat
Pearlescent- a translucent paint with a finely ground silicate material
(Mica) that has the distinctively soft and luminescent visual quality of a
pearl, the colour of which appears to vary depending on the viewing angle and
the light available, this paint can be tinted shades other than pearl white
Mechanical
Properties
Tensile strength [: quantifies how much stress the material will
endure before suffering permanent deformation]
The urethane used for automobile paint is classified as a semi-solid
, micro structured permeable membrane, due to its micro-fissures, or pores’,
being a thermosetting polymer it remains flexible, whilst retaining its tensile
strength, to enable it to expand and contract to follow temperature
fluctuations (elongation). Some chemicals penetrate in easily, others stay on
top dependent upon molecule size.
Hardness
[In materials science, hardness
is the characteristic of a solid material expressing its resistance to
permanent deformation. There are three principal operational definitions of
hardness: Scratch hardness, Indentation hardness, Rebound or dynamic hardness]
Paint is graded by its hardness or density by a durometer; one
of several measures of the hardness of a material. Durometer,
like many other hardness tests, measures the depth of an indentation in the
material created by a given force. Clear coats can be made with harder or
softer durometer and that will also determine its scratch resistance.
The enamel paint finishes on vehicles from the 50’s and 60’s era
were as tough as porcelain. But rightly due to environmental concerns, those
high percentage petroleum based paints have been generally superseded,
resulting in the softer water-based paint finishes of today and the unavoidable
orange-peel seen on many new and re-painted vehicles.
Today’s paints, unfortunately, rank somewhere near the bottom of
the scale of hardness, especially single coat black / red paint the exception
being white single stage and PPG CeramiClear™ Clear Coat ®, when compared to all the materials your
paint can possibly come in contact with (always bear that in mind). Scratch
resistance can be related to higher cross-link density and elasticity of the
polymer network (See also Pencil
(Scratch) Hardness)
An adaptation of that hardness scale (1 - 10);
1.
Talc = 1
2.
Carbon Black [black paint pigmentation] = 2
3.
Glass = 6
4.
Titanium dioxide [white paint pigmentation] = 7
5.
Diamond =10
Hard and soft are both relative terms; you can scratch the hard
surface of a vehicles paint with a soft towel by the application of enough
pressure. Both pressure and mechanical stress are defined as force per unit
area. These two forces are the subject of Newton's third law of motion; the law
of reciprocal actions [: to every action
there is an equal and opposite reaction]
How can a 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. 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).
This can best be summed up as: your car’s paint finish, though
it may feel hard, is actually rather soft. Nearly everything else that comes in
contact with it, though it may feel soft, is harder by comparison and will
scratch it with enough pressure – sometimes just the slightest of pressure.
Densities
of Clear Coats
Compiling a chart that lists the consistency of automotive paint
can only at best be a rough guide, as this would necessitate listing by;
manufacturer, OEM assembly plant, model year, colour, new or aged, paint
specification, and etc. To ascertain paint hardness requires extensive
experience working on a wide range of vehicles to have a point of reference.
Paint can be very complex and yet really interesting; in polymer
chemistry, when a synthetic polymer is said to be cross-linked, it usually
means that the entire bulk of the polymer has been exposed to the cross-linking
method.
The resulting modification of mechanical properties depends
strongly on the cross-link density. This is how tight or dense the paint matrix
becomes after is it fully cured, think of a paint matrix like that of chicken wire
mesh, a small mesh (dense) larger mesh..., I think you get the analogy. Hard
paint has a dense cross-link density, whereas soft paint has a less dense
cross-link density.
In polymer chemistry, when a
synthetic polymer is said to be "cross-linked", it usually means that
the entire bulk of the polymer has been exposed to the cross-linking method.
The resulting modification of mechanical properties depends strongly on the
cross-link density.
Paint density - Low
cross-link densities raise the viscosities 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
There are other factors that will have an effect; the
composition of the clear coat used (single, duel component, or powder) this
generally reflects the trade off the OEM is prepared to accept between scratch
resistance and gloss level considered acceptable, oven drying time and its
temperature, the relevant age (i.e. how long ago was the paint applied) spot
panel repairs (refinish) that are carried out either at the assembly plant or
the rail head or port of entry. Less dense (soft) paint could also be caused if
it was polished before the paint had time to fully cure
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 ; is by using the least aggressive pad / polish
combination followed, if necessary by an incrementally more aggressive approach
until you find what works
Pencil
(Scratch)
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.
Pencil
Hardness for Common Coatings
·
Catalyzed polyurethane: 9H
·
Catalyzed modified acrylic polyurethane: 4H
·
Water-based polyurethane: 3H
·
Water-based urethane/isocyanate catalyst: 2H
·
Low-VOC lacquer: 3H
·
Low-VOC catalyzed lacquer: 2H (24 hours)
·
Urethane/nitrocellulose lacquer: F (24 hours)
Pigmentation
(Colour)
Systems
[: pigment is a
component of paint; a colouring material, most usually a finely ground powder
which does not dissolve. It is suspended in a liquid solvent carrier to become
the colouring agent in paint]
Utilizing high solids-low volatile organic compounds (VOC) the
pigmented coatings, an acrylic and polyurethane resin binder system is used to
improve flexibility, fastness and surface adhesion to the leather, then two or
three water-based pigmented base coat applications a clear water-based top coat
is paints matrix system.
Among the inorganic pigments, TiO2TiO2 white is, by far,
the most important one. The photo catalytic activity of TiO2 leads to
a rapid degradation of the organic binder matrix, so the surface of the TiO2 particles
has to be covered by an inorganic coating to prevent the matrix from being
photo oxidized. This is done by applying layers of SiO2 and Al2O3
There are two types of pigments used in paint formulation
·
Prime Pigments - these pigments are
what provide the primary coloration of the paint whether white or a colour
shade, quality primary pigments are usually expensive.
·
Extender Pigments - these pigments are
the filler used in paint pigment. They do not hide as well as primary pigments
and have a significant impact on the overall characteristics and performance of
paint, including hiding, durability, scrub ability and retention of colour.
Extender pigments have historically been made from clay, silica, talc, chalk,
and to assist in mildew prevention, zinc oxide.
To be of real practical use, a subject like automotive detailing requires a great deal of research, and updating as new products become available. The advent of materials like detailing clay, micro fibre technologies and finely milled micro diminishing abrasives, suitable for ceramic nanotechnology paints are examples of why it’s so important to monitor the industries new products, chemical technologies and ideas that are constantly being introduced, as are the techniques for applying them, hence all of the in-depth articles will be up-dated and revised on a regular basis
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 these articles are informative. They are based on the current status of technical development as well as my experience with the products.
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.
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
Copyright © 2002 - 2015 TOGWT® (Established 1980) all rights reserved
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