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 inline scratches introduced, for example, by car wash machines. These micro scratches are only 0.2µ 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

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 re-flow 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

Test Panel

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 generous 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 de-ionized 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 1990s

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

This 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 coat systems

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) (W 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 its€™s single stage paint, if not its€™as a base coat-clear coat paint

PPG Enviracryl ® Powder Clearcoat - 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 Paint 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, that 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€™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)

Mohs Comparative Hardness Scale

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 care€™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

·         Catalysed polyurethane: 9H

·         Catalysed modified acrylic polyurethane: 4H

·         Water-based polyurethane: 3H

·         Water-based urethane/isocyanate catalyst: 2H

·         Low-VOC lacquer: 3H

·         Low-VOC catalysed lacquer: 2H (24 hours)

·         Urethane/nitrocellulose lacquer: F (24 hours)

Pencil hardness test is only one of many tests (see also Bibliography) that are done to evaluate a coating's performance. Other tests are abrasion, reverse impact resistance, direct impact resistance, cross-hatch adhesion, oxidation, gloss retention, UV resistance, yellowing, blistering, drying times, chemical/solvent resistance (using both the rubbing and spot/time tests), salt spray resistance, humidity resistance, acid and caustic resistance, the VOC and HAP contents, and so on.

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, TiO2 white is, by far, the most important one. The photo catalytic activity of TiO2leads to a rapid degradation of the organic binder matrix, so the surface of the TiO2particles has to be covered by an inorganic coating to prevent the matrix from being photo oxidized. This is done by applying layers of SiO2and Al2O3

There are two types of pigments used in paint formulation

1.      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.

2.      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.

Bibliography

1.           Royal Society of Chemistry (RSC) Library & Information Centre

2.           Glossary of Chemical Terms - Faculty of Chemical Technology

3.           SpecialChem4 Polymers

4.           Macromolecular Chemistry and Physics

5.           European Coatings Handbook 2nd Edition, by Brock, Groteklaes, Mischke

6.           Bayer Material Science, Automotive eNewsletters (Coating, Adhesives and Specialties)

7.           American Society for Testing and Materials (ASTM) publications - http://www.astm.org/Standard/standards-and-publications.html

8.           ASTM D1014 - 09 Standard Practice for Conducting Exterior Exposure Tests of Paints and Coatings on Metal Substrates

The information in this article is based on the current status of the technical development as well as our experience with the products.